Acicular hematite particles and magnetic recording medium

Abstract: Acicular hematite particles having an average major axis diameter of from 0.004 to 0.295 µm, a geometrical standard deviation of the minor axis diameter of not more than 1.35 and a pH of not less than 8, which particles contain not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 .

Description: The present invention relates to acicular hematite particles and a magnetic recording medium, and more particularly, to acicular hematite particles suitable as non-magnetic particles for a non-magnetic undercoat layer of a magnetic recording medium which exhibits a low light transmittance, an excellent smooth surface and a high strength, and can be prevented from being deteriorated in magnetic properties due to the corrosion of magnetic acicular metal particles containing iron as a main component which are dispersed in a magnetic recording layer thereof; a non-magnetic substrate for a magnetic recording medium provided with a non-magnetic undercoat layer containing the acicular hematite particles; and a magnetic recording medium comprising the non-magnetic substrate and a magnetic recording layer containing magnetic acicular metal particles containing iron as a main component. With a development of miniaturized, lightweight video or audio magnetic recording and reproducing apparatuses for long-time recording, magnetic recording media such as a magnetic tape and magnetic disk have been increasingly and strongly required to have a higher performance, namely, a higher recording density, higher output characteristic, in particular, an improved frequency characteristic and a lower noise level. Various attempts have been made at both enhancing the properties of magnetic particles and reducing the thickness of a magnetic recording layer in order to improve these properties of a magnetic recording medium. The enhancement of the properties of magnetic particles is firstly described. The required properties of magnetic particles in order to satisfy the above-described demands on a magnetic recording medium are a high coercive force and a large saturation magnetization. As magnetic particles suitable for high-output and high-density recording, magnetic acicular metal particles containing iron as a main component, which are obtained by heat-treating acicular goethite particles or acicular hematite particles in a reducing gas, are widely known. Although magnetic acicular metal particles containing iron as a main component have a high coercive force and a large saturation magnetization, since the magnetic acicular metal particles containing iron as a main component used for a magnetic recording medium are very fine particles having a particle size of not more than 1 µm, particularly, 0.01 to 0.3 µm, they easily corrode and the magnetic characteristics thereof are deteriorated, especially, the saturation magnetization and the coercive force are decreased. Therefore, in order to maintain the characteristics of a magnetic recording medium which uses magnetic acicular metal particles containing iron as a main component as magnetic particles, over a long period, it is strongly demanded to suppress the corrosion of magnetic acicular metal particles containing iron as a main component as much as possible. A reduction in the thickness of a magnetic recording layer is described. Video tapes have recently been required more and more to have a higher picture quality, and the frequencies of carrier signals recorded in recent video tapes are higher than those recorded in conventional video tapes. In other words, the signals in the short-wave region have come to be used, and as a result, the magnetization depth from the surface of a magnetic tape has come to be remarkably small. With respect to short wavelength signals, a reduction in the thickness of a magnetic recording layer is also strongly demanded in order to improve the high output characteristics, especially, the S/N ratio of a magnetic recording medium. This fact is described, for example, on page 312 of Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder , published by Sogo Gijutsu Center Co., Ltd. (1982), "··· the conditions for high-density recording in a coated-layer type tape are that the noise level is low with respect to signals having a short wavelength and that the high output characteristics are maintained. To satisfy these conditions, it is necessary that the tape has large coercive force Hc and residual magnetization Br, ··· and the coating film has a smaller thickness. ···". Development of a thinner film for a magnetic recording layer has caused some problems. Firstly, it is necessary to make a magnetic recording layer smooth and to eliminate the non-uniformity of thickness. As well known, in order to obtain a smooth magnetic recording layer having a uniform thickness, the surface of the base film must also be smooth. This fact is described on pages 180 and 181 of Materials for Synthetic Technology-Causes of Friction and Abrasion of Magnetic Tape and Head Running System and Measures for Solving the Problem (hereinunder referred to as " Materials for Synthetic Technology " (1987), published by the Publishing Department of Technology Information Center, "··· the surface roughness of a hardened magnetic coating film depends on the surface roughness of the base film (back surface roughness) so largely as to be approximately proportional, ···, since the magnetic coating film is formed on the base film, the more smooth the surface of the base film is, the more uniform and larger head output is obtained and the more the S/N ratio is improved." Secondly, there has been caused a problem in the strength of a base film with a tendency of the reduction in the thickness of the base film in response to the demand for a thinner magnetic coating film. This fact is described, for example, on page 77 of the above-described Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, "··· Higher recording density is a large problem assigned t the present magnetic tape. This is important in order to shorten the length of the tape so as to miniaturize the size of a cassette and to enable long-time recording. For this purpose, it is necessary to reduce the thickness of a base film ···. With the tendency of reduction in the film thickness, the stiffness of the tape also reduces to such an extent as to make smooth travel in a recorder difficult. Therefore, improvement of the stiffness of a video tape both in the machine direction and in the transverse direction is now strongly demanded. ···" There is no end to a demand for a higher performance in recent magnetic recording media. Since the above-described reduction in the thickness of a magnetic recording layer and a base film lowers the durability of the magnetic recording medium, an improvement of the durability of the magnetic recording medium is in strong demand. This fact is described in Japanese Patent Application Laid-Open (KOKAI) No. 5-298679, "··· With the recent development in magnetic recording, a high picture quality and a high sound quality have been required more and more in recording. The signal recording property is, therefore, improved. Especially, finer and higher-density ferromagnetic particles have come to be used. It is further required to make the surface of a magnetic tape smooth so as to reduce noise and raise the C/N. ··· However, the coefficient of friction between the magnetic recording layer and an apparatus during the travel of the magnetic recording tape increases, so that there is a tendency of the magnetic recording layer of the magnetic recording medium being damaged or exfoliated even in a short time. Especially, in a videotape, since the magnetic recording medium travels at a high speed in contact with the video head, the ferromagnetic particles are apt to be dropped from the magnetic recording layer, thereby causing clogging on the magnetic head. Therefore, an improvement in the running durability of the magnetic recording layer of a magnetic recording medium is expected.···" The end portion of a magnetic recording medium such as a magnetic tape, especially, a video tape is judged by detecting a portion of the magnetic recording medium at which the light transmittance is large by a video deck. If the light transmittance of the whole part of a magnetic recording layer is made large by the production of a thinner magnetic recording medium or the ultrafine magnetic particles dispersed in the magnetic recording layer, it is difficult to detect the portion having a large light transmittance by a video deck. For reducing the light transmittance of the whole part of a magnetic recording layer, carbon black or the like is added to the magnetic recording layer. It is, therefore, essential to add carbon black or the like to a magnetic recording layer in the present video tapes. However, addition of a large amount of non-magnetic particles such as carbon black impairs not only the enhancement of the magnetic recording density but also the development of a thinner magnetic recording layer. In order to reduce the magnetization depth from the surface of the magnetic tape and to produce a thinner magnetic recording layer, it is strongly demanded to reduce, as much as possible, the quantity of non-magnetic particles such as carbon black which are added to a magnetic recording layer. It is, therefore, strongly demanded that the light transmittance of a magnetic recording layer should be small even if the carbon black or the like which is added to the magnetic recording layer is reduced to a small amount. From this point of view; improvements in the magnetic recording medium are now in strong demand. There has, also, been pointed out such a problem that the magnetic acicular metal particles containing iron as a main component dispersed in the magnetic recording layer are corroded in a passage of time after the production thereof, so that magnetic properties of the magnetic recording medium are considerably deteriorated. Various efforts have been made to improve the substrate for a magnetic recording layer with a demand for a thinner magnetic recording layer and a thinner base film. A magnetic recording medium having at least one undercoat layer (hereinunder referred to "non-magnetic undercoat layer") comprising a binder resin and non-magnetic particles such as hematite particles which are dispersed therein, on a base film has been proposed and put to practical use (Japanese Patent Publication (KOKOKU) No. 6-93297 (1994), Japanese Patent Application Laid-Open (KOKAI) Nos. 62-159338 (1987), 63-187418 (1988), 4-167225 (1992), 4-325915 (1992), 5-73882 (1993), 5-182177 (1993), 5-347017 (1993), 6-60362 (1994), etc.) The above-described magnetic recording media composed of a base film and a non-magnetic undercoat layer composed of a binder resin and non-magnetic particles dispersed therein and formed on the base film, have a small light transmittance and a high strength, but the durability and the surface smoothness thereof are inconveniently poor. This fact is described in Japanese Patent Application Laid-Open (KOKAI) No. 5-182177 (1993), "··· Although the problem of surface roughness is solved by providing a magnetic recording layer as an upper layer after forming a thick non-magnetic undercoat layer on the base film, the problem of the abrasion of a head and the problem of durability are not solved and still remain. This is considered to be caused because a thermoset resin is usually used as a binder of the non-magnetic undercoat layer, so that the magnetic recording layer is brought into contact with a head or other members without any cushioning owing to the hardened non-magnetic undercoat layer, and a magnetic recording medium having such a non-magnetic undercoat layer has a considerably poor flexibility." Therefore, it has been strongly required to improve surface smoothness of the non-magnetic undercoat layer. Hitherto, it has been attempted to enhance a dispersibility of acicular hematite particles as non-magnetic particles by directing attention to the particle size distribution of major axis diameters of the particles (Japanese Patent Application Laid-open (KOKAI) No. 9-170003(1997), and the like). That is, in the above Japanese Patent Application Laid-open (KOKAI) No. 9-170003(1997), there has been described a method of heat-treating acicular goethite particles or acicular hematite particles produced by heat-dehydrating the acicular goethite particles, at a temperature of not less than 550°C, thereby obtaining high-density acicular hematite particles having a good geometrical standard deviation of a major axis diameter. However, as shown in Comparative Examples hereinafter, the obtained acicular hematite particles are deteriorated in a geometrical standard deviation of the minor axis diameter, so that the dispersibility of the particles is still unsatisfactory. Accordingly, with a recent tendency of reducing the thickness of the magnetic recording layer and non-magnetic base film, there have been most demanded acicular hematite particles-having a uniform particle size, which are suitable as non-magnetic particles for a non-magnetic undercoat layer having a smooth surface and a high mechanical; and a magnetic recording medium provided with a non-magnetic undercoat layer containing the acicular hematite particles, which exhibits a low light transmittance, an excellent smooth surface, a high mechanical strength and an excellent durability, and which can be prevented from being deteriorated in magnetic properties due to the corrosion of magnetic acicular metal particles containing iron as a main component contained in a magnetic recording layer. However, such acicular hematite particles and magnetic recording medium capable of satisfying these requirements have not been obtained yet. Further, in recent years, for purposes of short-wavelength recording and high-density recording, it has been inevitably necessary to use magnetic acicular metal particles containing iron as a main component, having a high coercive force and a large saturation magnetization. However, since the magnetic acicular metal particles containing iron as a main component are fine particles, the magnetic acicular metal particles containing iron as a main component tend to be corroded, resulting in deterioration in magnetic properties thereof. Therefore, it has been demanded to provide a magnetic recording medium in which magnetic properties thereof are not deteriorated and can be maintained for a long period of time. As a result of the present inventors' earnest studies, it has been found that (a) by heat-treating specific acicular goethite particles at a specific temperature and then heat-dehydrating the thus-treated acicular goethite particles to obtain specific acicular hematite particles, or (b) by subjecting specific acicular hematite particles to a acid-dissolving treatment under specific conditions to obtain specific acicular hematite particles, and then heating the acicular hematite particles obtained by the method (a) or (b) in an alkaline suspension, there can be obtained high-purity acicular hematite particles which exhibit a uniform particle size, especially wherein the geometrical standard deviation nears 1.0, and which are useful as non-magnetic particles for a non-magnetic undercoat layer of a magnetic recording medium which can be prevented from being deteriorated in magnetic properties due to the corrosion of magnetic acicular metal particles containing iron as a main component contained in a magnetic recording layer. The present invention has been attained on the basis of the finding. It is an object of the present invention to provide acicular hematite particles which are useful as non-magnetic particles for a non-magnetic undercoat layer of a magnetic recording medium exhibiting an excellent surface smoothness, and have a uniform particle size, especially are excellent geometrical standard deviation of a minor axis diameter thereof. It is another object of the present invention to provide acicular hematite particles suitable as non-magnetic particles for a non-magnetic undercoat layer of a magnetic recording medium which can exhibit a low light transmittance, a smooth surface and a high strength, and can be prevented from being deteriorated in magnetic properties due to the corrosion of magnetic acicular metal particles containing iron as a main component contained in a magnetic recording layer thereof, and a magnetic recording medium provided with a non-magnetic undercoat layer containing the acicular hematite particles. It is an other object of the present invention to provide acicular hematite particles suitable as non-magnetic particles for a non-magnetic undercoat layer having a low light transmittance, a smooth surface and a high, and a magnetic recording medium which can exhibit a low light transmittance, a smooth surface, a high strength and an excellent durability, and can be prevented from being deteriorated in magnetic properties due to the corrosion of magnetic acicular metal particles containing iron as a main component contained in a magnetic recording layer thereof. To accomplish the aims, in a first aspect of the present invention, there are provided acicular hematite particles having an average major axis diameter of from 0.004 to 0.295 µm, an aspect ratio (average major axis diameter/average minor axis diameter) of from 2:1 to 20:1, a geometrical standard deviation of the major axis diameter of not more than 1.50, a geometrical standard deviation of the minor axis diameter of not more than 1.35 and a pH of not less than 8, which particles contain not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 . In a second aspect of the present invention, there are provided acicular hematite particles having an average major axis diameter of from 0.004 to 0.295 µm, an aspect ratio (average major axis diameter/average minor axis diameter) of from 2:1 to 20:1, a geometrical standard deviation of the major axis diameter of not more than 1.50, a geometrical standard deviation of the minor axis diameter of not more than 1.35 and a pH of not less than 8, which particles contain not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 , and which have a coat on at least a part of the surface thereof, comprising at least one hydroxide or oxide selected from hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of from 0.05 to 50% by weight, calculated at Al or SiO 2 , based on the weight of said acicular hematite particles. In a third aspect of the present invention, there are provided acicular hematite particles having an average major axis diameter of from 0.004 to 0.295 µm, an aspect ratio (average major axis diameter/average minor axis diameter) of from 2:1 to 20:1, a geometrical standard deviation of the major axis diameter of not more than 1.50, a geometrical standard deviation of the minor axis diameter of not more than 1.35 and a pH of not less than 8, which further comprise aluminum within the particles in an amount of from 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles, and which particles contain not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 . In a fourth aspect of the present invention, there are provided acicular hematite particles having an average major axis diameter of from 0.004 to 0.295 µm, an aspect ratio (average major axis diameter/average minor axis diameter) of from 2:1 to 20:1, a geometrical standard deviation of the major axis diameter of not more than 1.50, a geometrical standard deviation of the minor axis diameter of not more than 1.35 and a pH of not less than 8, which further comprise aluminum within the particles in an amount of from 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles, and which particles contain not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 , and which have a coat on at least a part of the surface thereof, comprising at least one hydroxide or oxide selected from hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of from 0.05 to 50% by weight, calculated at Al or SiO 2 , based on the weight of said acicular hematite particles. In a fifth aspect of the present invention, there are provided acicular hematite particles having an average major axis diameter of from 0.004 to 0.295 µm, an aspect ratio (average major axis diameter/average minor axis diameter) of from 2:1 to 20:1, a geometrical standard deviation of the major axis diameter of not more than 1.50, a geometrical standard deviation of the minor axis diameter of not more than 1.35, a BET specific surface area of 35.9 to 180 m 2 /g and a pH of not less than 8, which particles contain not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 . In a sixth aspect of the present invention, there are provided acicular hematite particles having an average major axis diameter of from 0.004 to 0.295 µm, an aspect ratio (average major axis diameter/average minor axis diameter) of from 2:1 to 20:1, a geometrical standard deviation of the major axis diameter of not more than 1.50, a geometrical standard deviation of the minor axis diameter of not more than 1.35, a BET specific surface area of 35.9 to 180 m 2 /g and a pH of not less than 8, which particles contain not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 , and which have a coat on at least a part of the surface thereof, comprising at least one hydroxide or oxide selected from hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of from 0.05 to 50% by weight, calculated at Al or SiO 2 , based on the weight of said acicular hematite particles. In a seventh aspect of the present invention, there are provided acicular hematite particles having an average major axis diameter of from 0.004 to 0.295 µm, an aspect ratio (average major axis diameter/average minor axis diameter) of from 2:1 to 20:1, a geometrical standard deviation of the major axis diameter of not more than 1.50, a geometrical standard deviation of the minor axis diameter of not more than 1.35, a BET specific surface area of 35.9 to 180 m 2 /g and a pH of not less than 8, which further comprise aluminum within the particles in an amount of from 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles, and which particles contain not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 . In an eighth aspect of the present invention, there are provided acicular hematite particles having an average major axis diameter of from 0.004 to 0.295 µm, an aspect ratio (average major axis diameter/average minor axis diameter) of from 2:1 to 20:1, a geometrical standard deviation of the major axis diameter of not more than 1.50, a geometrical standard deviation of the minor axis diameter of not more than 1.35, a BET specific surface area of 35.9 to 180 m 2 /g and a pH of not less than 8, which further comprise aluminum within the particles in an amount of from 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles, and which particles contain not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 , and which have a coat on at least a part of the surface thereof, comprising at least one hydroxide or oxide selected from hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of from 0.05 to 50% by weight, calculated at Al or SiO 2 , based on the weight of said acicular hematite particles. In a ninth aspect of the present invention, there are provided acicular hematite particles having an aspect ratio (average major axis diameter/average minor axis diameter) of from 2:1 to 20:1, a geometrical standard deviation of the major axis diameter of not more than 1.50, a geometrical standard deviation of the minor axis diameter of not more than 1.30, a BET specific surface area of from 40 to 150 m 2 /g, an average major axis diameter of from 0.01 to 0.2 µm and a pH of not less than 8, which particles contain not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 . In a tenth aspect of the present invention, there are provided acicular hematite particles having an aspect ratio (average major axis diameter/average minor axis diameter) of from 2:1 to 20:1, a geometrical standard deviation of the major axis diameter of not more than 1.50, a geometrical standard deviation of the minor axis diameter of not more than 1.30, a BET specific surface area of from 40 to 150 m 2 /g, an average major axis diameter of from 0.01 to 0.2 µm and a pH of not less than 8, which particles contain not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 , and which have a coat on at least a part of the surface thereof, comprising at least one hydroxide or oxide selected from hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of from 0.05 to 50% by weight, calculated at Al or SiO 2 , based on the weight of said acicular hematite particles. In an eleventh aspect of the present invention, there are provided acicular hematite particles having an aspect ratio (average major axis diameter/average minor axis diameter) of from 2:1 to 20:1, a geometrical standard deviation of the major axis diameter of not more than 1.50, a geometrical standard deviation of the minor axis diameter of not more than 1.30, a BET specific surface area of from 40 to 150 m 2 /g, an average major axis diameter of from 0.01 to 0.2 µm and a pH of not less than 8, which further comprise aluminum within the particles in an amount of from 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles, and which particles contain not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 . In a twelfth aspect of the present invention, there are provided acicular hematite particles having an aspect ratio (average major axis diameter/average minor axis diameter) of from 2:1 to 20:1, a geometrical standard deviation of the major axis diameter of not more than 1.50, a geometrical standard deviation of the minor axis diameter of not more than 1.30, a BET specific surface area of from 40 to 150 m 2 /g, an average major axis diameter of from 0.01 to 0.2 µm and a pH of not less than 8, which further comprise aluminum within the particles in an amount of from 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles, and which particles contain not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 , and which have a coat on at least a part of the surface thereof, comprising at least one hydroxide or oxide selected from hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of from 0.05 to 50% by weight, calculated at Al or SiO 2 , based on the weight of said acicular hematite particles. In a thirteenth aspect of the present invention, there is provided a magnetic recording medium comprising: a non-magnetic base film; a non-magnetic undercoat layer on said non-magnetic base film, said undercoat layer comprising a binder resin and acicular hematite particles having an average major axis diameter of from 0.004 to 0.295 µm, an aspect ratio (average major axis diameter/average minor axis diameter) of from 2:1 to 20:1, a geometrical standard deviation of the major axis diameter of not more than 1.50, a geometrical standard deviation of the minor axis diameter of not more than 1.35 and a pH of not less than 8, which particles contain not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 ; and a magnetic coating film comprising a binder resin and magnetic acicular metal particles containing iron as a main component. In a fourteenth aspect of the present invention, there is provided a magnetic recording medium comprising: a non-magnetic base film; a non-magnetic undercoat layer on said non-magnetic base film, said undercoat layer comprising a binder resin and acicular hematite particles having an average major axis diameter of from 0.004 to 0.295 µm, an aspect ratio (average major axis diameter/average minor axis diameter) of from 2:1 to 20:1, a geometrical standard deviation of the major axis diameter of not more than 1.50, a geometrical standard deviation of the minor axis diameter of not more than 1.35 and a pH of not less than 8, which particles contain not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 , and which have a coat on at least a part of the surface thereof, comprising at least one hydroxide or oxide selected from hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of from 0.05 to 50% by weight, calculated at Al or SiO 2 , based on the weight of said acicular hematite particles; and a magnetic coating film comprising a binder resin and magnetic acicular metal particles containing iron as a main component. In a fifteenth aspect of the present invention, there is provided a magnetic recording medium comprising: a non-magnetic base film; a non-magnetic undercoat layer on said non-magnetic base film, said undercoat layer comprising a binder resin and acicular hematite particles having an average major axis diameter of from 0.004 to 0.295 µm, an aspect ratio (average major axis diameter/average minor axis diameter) of from 2:1 to 20:1, a geometrical standard deviation of the major axis diameter of not more than 1.50, a geometrical standard deviation of the minor axis diameter of not more than 1.35 and a pH of not less than 8, which further comprise aluminum within the particles in an amount of from 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles, and which particles contain not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 ; and a magnetic coating film comprising a binder resin and magnetic acicular metal particles containing iron as a main component. In a sixteenth aspect of the present invention, there is provided a magnetic recording medium comprising: a non-magnetic base film; a non-magnetic undercoat layer on said non-magnetic base film, said undercoat layer comprising a binder resin and acicular hematite particles having an average major axis diameter of from 0.004 to 0.295 µm, an aspect ratio (average major axis diameter/average minor axis diameter) of from 2:1 to 20:1, a geometrical standard deviation of the major axis diameter of not more than 1.50, a geometrical standard deviation of the minor axis diameter of not more than 1.35 and a pH of not less than 8, which further comprise aluminum within the particles in an amount of from 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles, and which particles contain not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 , and which have a coat on at least a part of the surface thereof, comprising at least one hydroxide or oxide selected from hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of from 0.05 to 50% by weight, calculated at Al or SiO 2 , based on the weight of said acicular hematite particles; and a magnetic coating film comprising a binder resin and magnetic acicular metal particles containing iron as a main component. In a seventeenth aspect of the present invention, there is provided a non-magnetic substrate comprising: a non-magnetic base film; and a non-magnetic undercoat layer on said non-magnetic base film, said undercoat layer comprising a binder resin and acicular hematite particles having an average major axis diameter of from 0.004 to 0.295 µm, an aspect ratio (average major axis diameter/average minor axis diameter) of from 2:1 to 20:1, a geometrical standard deviation of the major axis diameter of not more than 1.50, a geometrical standard deviation of the minor axis diameter of not more than 1.35 and a pH of not less than 8, which particles contain not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 . In an eighteenth aspect of the present invention, there is provided a non-magnetic substrate comprising: a non-magnetic base film; and a non-magnetic undercoat layer on said non-magnetic base film, said undercoat layer comprising a binder resin and acicular hematite particles having an average major axis diameter of from 0.004 to 0.295 µm, an aspect ratio (average major axis diameter/average minor axis diameter) of from 2:1 to 20:1, a geometrical standard deviation of the major axis diameter of not more than 1.50, a geometrical standard deviation of the minor axis diameter of not more than 1.35 and a pH of not less than 8, which particles contain not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 , and which have a coat on at least a part of the surface thereof, comprising at least one hydroxide or oxide selected from hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of from 0.05 to 50% by weight, calculated at Al or SiO 2 , based on the weight of said acicular hematite particles. In a nineteenth aspect of the present invention, there is provided a non-magnetic substrate comprising: a non-magnetic base film; and a non-magnetic undercoat layer on said non-magnetic base film, said undercoat layer comprising a binder resin and acicular hematite particles having an average major axis diameter of from 0.004 to 0.295 µm, an aspect ratio (average major axis diameter/average minor axis diameter) of from 2:1 to 20:1, a geometrical standard deviation of the major axis diameter of not more than 1.50, a geometrical standard deviation of the minor axis diameter of not more than 1.35 and a pH of not less than 8, which further comprise aluminum within the particles in an amount of from 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles, and which particles contain not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 . In a twentieth aspect of the present invention, there is provided a non-magnetic substrate comprising: a non-magnetic base film; and a non-magnetic undercoat layer on said non-magnetic base film, said undercoat layer comprising a binder resin and acicular hematite particles having an average major axis diameter of from 0.004 to 0.295 µm, an aspect ratio (average major axis diameter/average minor axis diameter) of from 2:1 to 20:1, a geometrical standard deviation of the major axis diameter of not more than 1.50, a geometrical standard deviation of the minor axis diameter of not more than 1.35 and a pH of not less than 8, which further comprise aluminum within the particles in an amount of from 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles, and which particles contain not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 , and which have a coat on at least a part of the surface thereof, comprising at least one hydroxide or oxide selected from hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of from 0.05 to 50% by weight, calculated at Al or SiO 2 , based on the weight of said acicular hematite particles. The present invention is explained in more detail below. First, the acicular hematite particles for a non-magnetic undercoat layer of a magnetic recording medium according to the present invention are described. The acicular hematite particles according to the present invention have substantially the same acicular shape as that of particles to be treated. The "acicular" shape may include not only a needle shape but also a spindle shape or a rice-ball shape. In addition, the lower limit of an aspect ratio of the particles (= average major axis diameter : average minor axis diameter, hereinafter referred to merely as "aspect ratio") is 2:1, preferably 3:1. With the consideration of dispersibility of the particles in vehicle, the upper limit of the aspect ratio is 20:1, preferably 10:1. When the aspect ratio is less than 2:1, it may be difficult to obtain a coating film having a sufficient strength. On the other hand, when the aspect ratio is more than 20:1, the particles may be entangled with each other in vehicle, thereby causing a tendency that the dispersibility thereof may be deteriorated or the viscosity thereof may be increased. The upper limit of the average major axis diameter of the acicular hematite particles according to the present invention is 0.295 µm. The lower limit of the average major axis diameter of the acicular hematite particles according to the present invention is 0.004 µm. If the upper limit of the average major axis diameter exceeds 0.295 µm, the surface smoothness of the coating film formed using such particles may be impaired because the particle size is large. On the other hand, if the lower limit of the average major axis diameter is less than 0.004 µm, the dispersion in the vehicle may be difficult because of the increase of the intermolecular force due to the fine particles. With the consideration of the dispersibility in the vehicle and the surface smoothness of the coating film, the upper limit thereof is preferably 0.275 µm, more preferably 0.200 µm, still more preferably 0.100 µm, and the lower limit thereof is preferably 0.008 µm, more preferably 0.010 µm, more preferably 0.020 µm. The upper limit of the average minor axis diameter of the acicular hematite particles according to the present invention is usually 0.147 µm. The lower limit of the average minor axis diameter of the acicular hematite particles according to the present invention is usually 0.002 µm. If the upper limit of the average minor axis diameter exceeds 0.147 µm, the surface smoothness of the coating film formed using such particles may be impaired because the particle size is large. On the other hand, if the lower limit of the average minor axis diameter is less than 0.002 µm, the dispersion in the vehicle may be difficult because of the increase of the intermolecular force due to the fine particles. With the consideration of the dispersibility in the vehicle and the surface smoothness of the coating film, the upper limit thereof is preferably 0.123 µm, more preferably 0.100 µm, still more preferably 0.050 µm, and the lower limit thereof is preferably 0.004 µm, more preferably 0.005 µm, more preferably 0.010 µm. The upper limit of the BET specific surface area (S BET ) of the acicular hematite particles according to the present invention is usually 180m 2 /g. The lower limit of the BET specific surface area (S BET ) of the acicular hematite particles according to the present invention is usually 35.9 m 2 /g. If the upper limit thereof is more than 180 m 2 /g, the dispersion in the vehicle may be difficult because of the increase of the intermolecular force due to the fine particles. On the other hand, if the lower limit thereof is less than 35.9 m 2 /g, the acicular hematite particles may be coarse particles or large particles produced by sintering a particle and between particles, which are apt to exert a deleterious influence on the surface smoothness of the coating film. With the consideration of the dispersibility in the vehicle and the surface smoothness of the coating film, the upper limit thereof (S BET ) is preferably 160 m 2 /g, more preferably 150 m 2 /g and the lower limit thereof (S BET ) is preferably 38 m 2 /g, more preferably 40 m 2 /g. The upper limit of the geometrical standard deviation of the major axis diameter of the acicular hematite particles according to the present invention is not more than 1.50. If the upper limit of the geometrical standard deviation of the major axis diameter exceeds 1.50, the coarse particles existent sometimes exert a deleterious influence on the surface smoothness of the coating film. With the consideration of the surface smoothness of the coating film, the upper limit thereof is preferably 1.45, more preferably not more than 1.40. From the point of view of industrial productivity, the lower limit thereof is preferably 1.01. The upper limit of the geometrical standard deviation of the minor axis diameter of the acicular hematite particles according to the present invention is not more than 1.35. If the upper limit of the geometrical standard deviation of the minor axis diameter exceeds 1.35, the coarse particles existent sometimes exert a deleterious influence on the surface smoothness of the coating film. With the consideration of the surface smoothness of the coating film, the upper limit thereof is preferably 1.33, more preferably not more than 1.30, still more preferably not more than 1.28. From the point of view of industrial productivity, the lower limit thereof is preferably 1.01. The acicular hematite particles for a non-magnetic undercoat layer of a magnetic recording medium according to the present invention have a high degree of densification. When the degree of densification is represented by a ratio value of the specific surface area (S BET ) measured by a BET method to the surface area (S TEM ) calculated from the major axis diameter and the minor axis diameter which were measured from the particles in an electron micrograph of the acicular hematite particles (hereinafter referred to merely as "S BET /S TEM value"), the S BET /S TEM value is 0.5 to 2.5. When the S BET /S TEM value is less than 0.5, although the acicular hematite particles are highly densified, the particle diameter thereof may be increased due to sintering in each particle or between particles, so that a coating film formed using these particles, may not have a sufficient smooth surface. On the other hand, when the S BET /S TEM value is more than 2.5, the degree of densification of the particles is insufficient, so that many pores tend to be formed on the surface and inside of the particle, resulting in insufficient dispersibility of the particles in vehicle. With the consideration of the smooth-surface of the coating film and the dispersibility in the vehicle, the S BET /S TEM value is preferably 0.7 to 2.0, more preferably 0.8 to 1.6. The pH value of the acicular hematite particles when suspended in an aqueous solution (concentration: 50 g/litre), is not less than 8. If it is less than 8, the magnetic acicular metal particles containing iron as a main component contained in the magnetic recording layer formed on the non-magnetic undercoat layer are gradually corroded, which leads to a deterioration in the magnetic characteristics. In consideration of a corrosion preventive effect on the magnetic acicular metal particles containing iron as a main component, the lower limit of the pH value of the acicular hematite particles is preferably not less than 8.5, more preferably not less than 9.0. The upper limit of the pH value of the acicular hematite particles is preferably 11, more preferably 10.5. The content of soluble sodium salt in the acicular hematite particles is not more than 300 ppm (calculated as Na). If it exceeds 300 ppm, the magnetic acicular metal particles containing iron as a main component contained in the magnetic recording layer formed on the non-magnetic undercoat layer are gradually corroded, thereby causing a deterioration in the magnetic characteristics. In addition, the dispersion property of the acicular hematite particles in the vehicle is easily impaired, and the preservation of the magnetic recording medium is deteriorated and chalking is sometimes caused in a highly humid environment. In consideration of a corrosion preventive effect on the magnetic acicular metal particles containing iron as a main component, the content of soluble sodium salt in the acicular hematite particles is preferably not more than 250 ppm, more preferably not more than 200 ppm, even more preferably not more than 150 ppm. From the point of view of industry such as productivity, the lower limit thereof is about 0.01 ppm. The content of soluble sulfate in the acicular hematite particles is not more than 150 ppm (calculated as SO 4 ). If it exceeds 150 ppm, the magnetic acicular metal particles containing iron as a main component contained in the magnetic recording layer formed on the non-magnetic undercoat layer are gradually corroded, thereby causing a deterioration in the magnetic properties. In addition, the dispersion property of the acicular hematite particles in the vehicle is easily impaired, and the preservation of the magnetic recording medium is deteriorated and chalking is sometimes caused in a highly humid environment. In consideration of a corrosion preventive effect on the magnetic acicular metal particles containing iron as a main component, the content of soluble sodium salt in the acicular hematite particles is preferably not more than 70 ppm, more preferably not more than 50 ppm. From the point of view of industry such as productivity, the lower limit thereof is about 0.01 ppm. With the consideration of the durability of the magnetic recording medium having a non-magnetic undercoat layer containing such acicular hematite particles, it is preferable acicular hematite particles contain aluminum present substantially uniformly within the particle in an amount of 0.05 to 50 % by weight (calculated as Al) based on the total weight of the acicular hematite particles. When the aluminum content exceeds 50 % by weight, although a magnetic recording medium having the non-magnetic undercoat layer containing such acicular hematite particles has a sufficient durability, the durability-improving effect becomes saturated, so that it is meaningless to add aluminum more than necessary. From the point of view of durability-improving effect of a magnetic recording medium and industrial productivity, the aluminum content therein is preferably 0.1 to 30 % by weight, more preferably 0.2 to 20 % by weight (calculated as Al) based on the total weight of the particles. Various properties of the acicular hematite particles which contain aluminum within the particle, such as aspect ratio, average major axis diameter, average minor axis diameter, BET specific surface area, geometrical standard deviation of major axis diameter, geometrical standard deviation of minor axis diameter and degree of densification are approximately equivalent in values to those of the acicular hematite particles in which aluminum is not contained within the particle. The resin adsorptivity of the acicular hematite particles which contain aluminum within the particle according to the present invention is usually not less than 65 %, preferably not less than 68 %, more preferably not less than 70%. The upper limit thereof is preferably 95 %. At least a part of the surface of the acicular hematite particle which may contain aluminum within the particle according to the present invention may be coated with at least one selected from the group consisting of a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon and an oxide of silicon. When the acicular hematite particles which are coated with the above-described coating material are dispersed in a vehicle, the treated particles have an affinity with the binder resin and it is more easy to obtain a desired dispersibility. The amount of the hydroxide of aluminum, the oxide of aluminum, the hydroxide of silicon or the oxide of silicon used as the coating material is usually not less than 50 % by weight, preferably 0.01 to 50 % by weight (calculated as Al and/or SiO 2 ) based on the total weight of the particles. If it is less than 0.01 % by weight (calculated as Al and/or SiO 2 ) based on the total weight of the particles, the dispersibility-improving effect by coating therewith may be insufficient. If the amount exceeds 50 % by weight (calculated as Al and/or SiO 2 ) based on the total weight of the particles, the dispersibility-improving effect by coating therewith becomes saturated, so that it is meaningless to add a coating material more than necessary. From the point of view of dispersibility in the vehicle and industrial productivity, the more preferable amount of coating material is 0.05 to 20 % by weight (calculated as Al and/or SiO 2 ) based on the total weight of the particles. Various properties of the acicular hematite particles which are coated with above-mentioned coating material, such as aspect ratio, average major axis diameter, average minor axis diameter, BET specific surface area, geometrical standard deviation of major axis diameter, geometrical standard deviation of minor axis diameter and degree of densification are approximately equivalent in values to those of the acicular hematite particles in which the surfaces thereof is not coated with the above-mentioned coating material. The process for producing the acicular hematite particles according to the present invention is exemplified as follows. The acicular hematite particles for a non-magnetic undercoat layer of a magnetic recording medium according to the present invention are produced by the following processes (I) and (II). (I) The acicular hematite particles according to the present invention can be obtained by subjecting acicular hematite particles to be treated to acid-dissolving treatment under specific conditions. The acicular hematite particles to be treated can be produced by various methods. Examples of these methods may include a method of directly producing the hematite particles by a wet process; a method of producing akaganeite particles (β-FeOOH) and then heat-dehydrating the akaganeite particles; or the like. As an ordinary production method, there may be industrially preferably used a method of producing acicular goethite particles as a precursor of the acicular hematite particles by the following wet process and then heat-dehydrating the obtained acicular goethite particles. An ordinary method of producing acicular goethite particles as one of precursors of the acicular hematite particles is described below. As described hereinafter, the acicular goethite particles can be produced by passing an oxygen-containing gas through a suspension containing ferrous precipitates such as hydroxides of iron or iron carbonate which are obtained by reacting a ferrous salt with either alkali hydroxide, alkali carbonate or mixed alkali composed of alkali hydroxide and alkali carbonate. Acicular goethite particles are produced by an ordinary method: (A) a method of producing needle-shaped goethite particles comprising oxidizing a suspension having a pH value of not less than 11 and containing colloidal ferrous hydroxide particles which is obtained by adding not less than an equivalent of an alkali hydroxide solution to an aqueous ferrous salt solution, by passing an oxygen-containing gas thereinto at a temperature of not higher than 80°C; (B) a method of producing spindle-shaped goethite particles comprising oxidizing a suspension containing FeCO 3 which is obtained by reacting an aqueous ferrous salt solution with an aqueous alkali carbonate solution, by passing an oxygen-containing gas thereinto after aging the suspension, if necessary; (C) a method of producing spindle-shaped goethite particles comprising oxidizing a suspension containing precipitates containing iron which is obtained by reacting an aqueous ferrous salt solution with an aqueous alkali carbonate solution and an alkali hydroxide solution; by passing an oxygen-containing gas thereinto after aging the suspension, if necessary; (D) a method of growing needle-shaped seed goethite particles comprising oxidizing a ferrous hydroxide solution containing colloidal ferrous hydroxide particles which is obtained by adding less than an equivalent of an alkali hydroxide solution or an alkali carbonate solution to an aqueous ferrous salt solution, by passing an oxygen-containing gas thereinto, thereby producing needle-shaped seed goethite particles, adding not less than an equivalent of an alkali hydroxide solution to the Fe 2+ in the aqueous ferrous salt solution, to the aqueous ferrous salt solution containing the needle-shaped seed goethite particles, and passing an oxygen-containing gas into the aqueous ferrous salt solution; (E) a method of growing needle-shaped seed goethite particles comprising oxidizing a ferrous hydroxide solution containing colloidal ferrous hydroxide particles which is obtained by adding less than an equivalent of an alkali hydroxide solution or an alkali carbonate solution to an aqueous ferrous salt solution, by passing an oxygen-containing gas thereinto, thereby producing needle-shaped seed goethite particles, adding not less than an equivalent of an aqueous alkali carbonate solution to the Fe 2+ in the aqueous ferrous salt solution, to the aqueous ferrous salt solution containing the needle-shaped seed goethite particles, and passing an oxygen-containing gas into the aqueous ferrous salt solution; and (F) a method of growing needle-shaped seed goethite particles comprising oxidizing a ferrous hydroxide solution containing colloidal ferrous hydroxide particles which is obtained by adding less than an equivalent of an alkali hydroxide solution or an alkali carbonate solution to an aqueous ferrous salt solution, by passing an oxygen-containing gas thereinto, thereby producing needle-shaped seed goethite particles and growing the obtained needle-shaped seed goethite particles in an acidic or neutral region. Elements other than Fe such as Ni, Zn, P and Si, which are generally added in order to enhance various properties of the particles such as the major axis diameter, the minor axis diameter and the aspect ratio, may be added during the reaction system for producing the goethite particles. The acicular goethite particles obtained have an average major axial diameter of usually 0.005 to 0.4 µm, an average minor axial diameter of usually 0.0025 to 0.20 µm, a geometrical standard deviation of the major axis diameter of not more than 1.70, a geometrical standard deviation of the minor axis diameter of 1.37 to 1.50 and a BET specific surface area of about usually 50 to 250 m 2 /g, and contain soluble sodium salts of usually 300 to 1500 ppm (calculated as Na) and ordinarily soluble sulfates of 150 to 3000 ppm (calculated as SO 4 ). Alternatively, the acicular hematite particles to be treated which contain aluminum therewithin in an amount of 0.05 to 50 % by weight (calculated as Al) based on the total weight of the particles, can be produced by containing aluminum in the acicular goethite particles by preliminarily adding an aluminum compound upon the above-mentioned production reaction of the acicular goethite particles. Further, in the production reaction of the acicular goethite particles which contain aluminum within the particle, an aluminum compound may be added to at least one solution selected from suspensions containing a ferrous salt, alkali hydroxide, alkali carbonate, mixed alkali composed of alkali hydroxide and alkali carbonate, or ferrous precipitates such as hydroxides of iron or iron carbonate. It is preferred that the aluminum compound is added to an aqueous ferrous salt solution. As the above-mentioned aluminum compounds, there may be used aluminum salts such as aluminum acetate, aluminum sulfate, aluminum chloride or aluminum nitrate, alkali aluminates such as sodium aluminate, alumina sol, aluminum hydroxide or the like. The amount of the aluminum compound added is 0.05 to 50 % by weight (calculated as Al) based on the total weight of the particles. The obtained acicular goethite particles usually have an average major axial diameter of ordinarily 0.005 to 0.4 µm, an average minor axial diameter of ordinarily 0.0025 to 0.20 µm, a geometrical standard deviation of the major axis diameter of not more than 1.70, a geometrical standard deviation of the minor axis diameter of 1.37 to 1.50 and a BET specific surface area of about ordinarily 50 to 250 m 2 /g, contain aluminum in an amount of usually 0.05 to 50 % by weight (calculated as Al) based on the total weight of the particles, and contain soluble sodium salts of usually 300 to 1500 ppm (calculated as Na) and soluble sulfates of usually 100 to 3000 ppm (calculated as SO 4 ). Next, there is described a process for producing acicular hematite particles to be treated , which may substantially uniformly containing aluminum within the particle. The acicular hematite particles which may substantially uniformly contain aluminum within the particle may be produced by heat-dehydrating the above-mentioned acicular goethite particles which may substantially uniformly contain aluminum within the particle. The temperature of the heat-dehydration is preferably 550 to 850°C to obtain high-density acicular hematite particles which may substantially uniformly contain aluminum within the particle. Especially, in the case where the-heat-dehydration is conducted at an elevated temperature as high as not less than 550°C, it is preferred that the surface of the acicular goethite particle which may substantially uniformly contain aluminum within the particle, is coated with an anti-sintering agent prior to the heat-dehydration, as is well known in the art. As the sintering preventive, sintering preventives generally used are usable. For example, phosphorus compounds such as sodium hexametaphosphate, polyphospholic acid and orthophosphoric acid, silicon compounds such as #3 water glass, sodium orthosilicate, sodium metasilicate and colloidal silica, boron compounds such as boric acid, aluminum compounds including aluminum salts such as aluminum acetate, aluminum sulfate, aluminum chloride and aluminum nitrate, alkali aluminate such as sodium aluminate, alumina sol and aluminum hydroxide, and titanium compounds such as titanyl sulfate may be exemplified. The amount of the sintering preventive applied onto the surface of the acicular goethite particle is about 0.05 to 10 % by weight based on the total weight of the particles, though the amount is varied depending upon kinds of sintering preventives used, pH value of the alkali aqueous solution or various conditions such as heat-treating temperature or the like. The acicular goethite particles coated with a sintering preventive have the BET specific surface area (S BET ) of usually about 50 to 250 m 2 /g. The coating treatment using a sintering preventive is composed of the steps of: adding a sintering preventive to an aqueous suspension containing the acicular goethite particles, mixing and stirring the resultant suspension, filtering out the particles, washing the particles with water, and drying the particles. Meanwhile, as the acicular hematite particles to be treated, there may be preferably used high-density acicular hematite particles which may substantially uniformly contain aluminum within the particle. In the case of low-density acicular hematite particles, many dehydration pores are present within particles or on surfaces thereof. Therefore, upon subjecting the particles to the acid-dissolving treatment, the dissolution is initiated from the dehydration pores, so that the particle shape can be no longer maintained, resulting in deteriorated dispersibility of the obtained particles. In order to obtain high-density acicular hematite particles which can maintain a particle shape of the acicular goethite particles, it is preferred that the acicular goethite particles which may substantially uniformly contain aluminum within the particle, are first heat-treated at a temperature as low as not less than 250°C and less than 550°C to form low-density acicular hematite particles which may contain aluminum which is present within the particle, and then the low-density hematite particles are heat-treated at an elevated temperature as high as 550 to 850°C. If the temperature for heat-treating the goethite particles is less than 250°C, the dehydration reaction takes a long time. On the other hand, if the temperature is not less than 550°C, the dehydration reaction is abruptly brought out, so that it is difficult to retain the shapes because the sintering between particles is caused. The low-density acicular hematite particles obtained by heat-treating the acicular goethite particles at a low temperature are low-density particles having a large number of dehydration pores through which H 2 O is removed from the acicular goethite particles and the BET specific surface area thereof is about 1.2 to 2 times larger than that of the acicular goethite particles as the precursor. The low-density acicular hematite particles obtained by heat-treating the acicular goethite particles coated with a sintering preventive at a temperature of not less than 250°C and less than 550°C, have an average major axis diameter of usually 0.005 to 0.30 µm, an average minor axis diameter of usually 0.0025 to 0.15 µm, a geometrical standard deviation of a major axis diameter of usually not more than 1.70, a geometrical standard deviation of a minor axis diameter of usually 1.37 to 1.50 and a BET specific surface area (S BET ) of usually about 70 to 350 m 2 /g, and contain ordinarily soluble sodium salts of usually 300 to 1500 ppm (calculated as Na) and ordinarily soluble sulfates of usually 100 to 3000 ppm (calculated as SO 4 ). The low-density acicular hematite particles containing aluminum which is present within the particle, obtained by heat-treating the acicular goethite particles coated with a sintering preventive at a temperature of not less than 250°C and less than 550°C, have an average major axis diameter of usually 0.005 to 0.30 µm, an average minor axis diameter of usually 0.0025 to 0.15 µm, a geometrical standard deviation of a major axis diameter of usually not more than 1.70, a geometrical standard deviation of a minor axis diameter of usually 1.37 to 1.50 and a BET specific surface area (S BET ) of usually about 70 to 350 m 2 /g, and containing aluminum in an amount of 0.05 to 50 % by weight (calculated as Al) based on the total weight of the particles, and contain soluble sodium salts of usually 500 to 3000 ppm (calculated as Na) and soluble sulfates of usually 300 to 4000 ppm (calculated as SO 4 ). The low-density acicular hematite particles which may substantially uniformly contain aluminum within the particle, are then heat-treated at a temperature of usually 550 to 850°C, preferably 550 to 800°C to obtain a high-density acicular hematite particles which may substantially uniformly contain aluminum within the particle. If the heat-treating temperature is less than 550°C, since the densification may be insufficient, a large number of dehydration pores may exist within and on the surface of the acicular hematite particles, so that the dispersion in the vehicle may become insufficient. Further, when the non-magnetic undercoat layer is formed from these particles, it may be difficult to obtain a coating film having a smooth surface. On the other hand, if the temperature exceeds 850°C, although the densification of the acicular hematite particles may be sufficient, since sintering is caused on and between particles, the particle size may increase, so that it may be difficult to obtain a coating film having a smooth surface. The BET specific surface area of the high-density acicular hematite particles having the average major axis diameter of usually 0.005 to 0.30 µm, the average minor axis diameter of usually 0.0025 to 0.15 µm, the geometrical standard deviation of a major axis diameter of usually not more than 1.70 and the geometrical standard deviation of a minor axis diameter of usually 1.37 to 1.50, is usually about 35 to 180 m 2 /g. The high-density acicular hematite particles contain soluble sodium salts of usually 500 to 4000 ppm (calculated as Na) and soluble sulfates of usually 300 to 5000 ppm (calculated as SO 4 ). The BET specific surface area of the high-density acicular hematite particles having the average major axis diameter of usually 0.005 to 0.30 µm, the average minor axis diameter of usually 0.0025 to 0.15 µm, the geometrical standard deviation of a major axis diameter of usually not more than 1.70 and the geometrical standard deviation of a minor axis diameter of usually 1.37 to 1.50, and containing aluminum within the particle, is usually about 35 to 180 m 2 /g. The high-density acicular hematite particles contain soluble sodium salts of usually 500 to 4000 ppm (calculated as Na) and soluble sulfates of usually 300 to 5000 ppm (calculated as SO 4 ), and contain aluminum in amount of 0.05 to 50 % by weight (calculated as Al) based on the total weight of the particles. The high-density acicular hematite particles which may substantially uniformly contain aluminum within the particle, are diaggregated by a dry-process, and formed into a slurry. The coarse particles thereof contained in the slurry are then deagglomerated by a wet-process. In the wet-deagglomeration, ball mill, sand grinder, colloid mill or the like is used until the coarse particles having a particle size of at least 44 µm are substantially removed. That is, the wet-pulverization is carried out until the amount of the coarse particles having a particle size of not less than 44 µm becomes to usually not more than 10 % by weight, preferably not more than 5 % by weight, more preferably 0 % by weight based on the total weight of the particles. If the amount of the coarse particles having a particle size of not less than 44 µm is more than 10 % by weight, the effect of treating the particles in an acid-dissolving treatment at the next step is not attained. Next, there is described a process for producing acicular hematite particles having a specific geometrical standard deviation of the minor axis diameter. The acicular hematite particles may be produced by subjecting a water suspension of acicular hematite particles as particles to be treated to an acid-dissolving treatment at an acid concentration of not less than 1.0 N, a pH value of not more than 3.0 and a temperature of 20 to 100°C so as to dissolve 5 to 50 % by weight of the acicular hematite particles based on the total weight of acicular hematite particles present in the water suspension, followed by washing with water; adding an aqueous alkali solution to the water suspension containing residual acicular hematite particles so as to adjust the pH value of the water suspension to not less than 13; and then heat-treating the water suspension at a temperature of 80 to 103°C, followed by filtering, washing with water and drying. First, the acicular hematite particles to be treated with acid, are described below. The particle shape of the hematite particles to be treated, is an acicular shape. The "acicular" shape may include not only a needle shape but also a spindle shape or a rice ball shape. In addition, the aspect ratio of the particles is usually 2:1 to 20:1. The average major axis diameter of the acicular hematite particles to be treated, is usually 0.005 to 0.3 µm, preferably 0.02 to 0.28 µm. The average minor axis diameter of the acicular hematite particles to be treated, is usually 0.0025 to 0.15 µm, preferably 0.01 to 0.14 µm. The geometrical standard deviation of the major axis diameter of the acicular hematite particles to be treated, is usually not more than 1.70, and the geometrical standard deviation of the minor axis diameter of the acicular hematite particles to be treated, is usually 1.37 to 1.50. The BET specific surface area (S BET ) of the acicular hematite particles to be treated, is usually 35 to 180 m 2 /g. The acicular hematite particles to be treated contain soluble sodium salts of usually 500 to 4000 ppm (calculated as Na) and soluble sulfates of usually 300 to 5000 ppm (calculated as SO 4 ). The acicular hematite particles to be treated, may contain aluminum in an amount of usually 0.05 to 50 % by weight (calculated as Al) which is present within the particle. Next, the acid-dissolving treatment of the acicular hematite particles to be treated is described in detail. The concentration of the acicular hematite particles in the water suspension is usually 1 to 500 g/liter, preferably 10 to 250 g/liter. When the concentration of the acicular hematite particles in the water suspension is less than 1 g/liter, the amount of the particles treated is too small, which is industrially disadvantageous. On the other hand, when the concentration of the acicular hematite particles in the water suspension is more than 500 g/liter, it becomes difficult to evenly subject the particles to the acid-dissolving treatment. As the acids, there may be used any of sulfuric acid, hydrochloric acid, nitric acid, sulfurous acid, chloric acid, perchloric acid and oxalic acid. With the consideration of corrosion or deterioration of a container in which a high-temperature treatment or a dissolving treatment is conducted, economy or the like, sulfuric acid is preferred. The concentration of the acid is usually not less than 1.0 N, preferably not less than 1.2 N, more preferably not less than 1.5 N. The upper limit of concentration of the acid is preferably 5 N. When the acid concentration is less than 1.0 N, the dissolution of the acicular hematite particles requires an extremely long period of time and is, therefore, industrially disadvantageous. Upon the acid-dissolving treatment, the initial pH value is usually not more than 3.0, preferably not more than 2.0, more preferably not more than 1.0. With the consideration of the dissolving time or the like, it is industrially preferred that the pH value is not more than 1.0. When the pH value is more than 3.0, the dissolution of the acicular hematite particles requires an extremely long period of time and is, therefore, industrially disadvantageous. The temperature of the water suspension is usually 20 to 100°C, preferably 50 to 100°C, more preferably 70 to 100°C. When the temperature of the water suspension is less than 20°C, the dissolution of the acicular hematite particles requires an extremely long period of time and is, therefore, industrially disadvantageous. On the other hand, when the temperature of the water suspension is more than 100°C, the dissolution of the particles proceeds too rapidly, so that it becomes difficult to control the dissolving treatment. Further, in that case, the dissolving treatment requires a special apparatus such as autoclave, resulting in industrially disadvantageous process. Incidentally, in the case where the acicular hematite particles to be treated have a relatively large average major axis diameter, i.e., in the range of 0.05 to 0.30 µm, it is preferred that the dissolving treatment is conducted under the hard conditions, e.g., at a pH value of not more than 1.0 and a temperature of 70 to 100°C. On the other hand, in the case where the acicular hematite particles to be treated have a relatively small average major axis diameter, i.e., in the range of 0.005 to 0.05 µm, it is preferred that the dissolving treatment is conducted under the soft conditions, e.g., at a pH value of 1.0 to 3.0 and a temperature of 20 to 70°C. The dissolving treatment with acid may be conducted until the amount of the acicular hematite particles dissolved reaches 5 to 50 % by weight, preferably 10 to 45 % by weight, more preferably 15 to 40 % by weight based on the total weight of the acicular hematite particles to be treated. When the amount of the acicular hematite particles dissolved is less than 5 % by weight, the fine particle components may not be sufficiently removed by the dissolving treatment. On the other hand, when the amount of the acicular hematite particles dissolved is more than 50 % by weight, the particles as a whole may be finely divided and the dissolving loss is increased, resulting in industrially disadvantageous process. Incidentally, the aqueous solution in which iron salts are dissolved in the above dissolving treatment, is separated from the slurry by filtration. From the standpoint of the reuse of resources, the thus-separated iron salts may be used as a raw material of the ferrous salt used for the production of acicular goethite particles. After completion of the above acid-dissolving treatment with acid, the acicular hematite particles remaining in the water suspension is filtered, washed with water and dried. As the water-washing methods, there may be used those ordinarily used in industrial fields, such as a washing method by decantation, a washing method conducted according to a diluting process by using a filter thickener, a washing method of feeding water through a filter press, or the like. (II) The acicular hematite particles according to the present invention are produced by heat-treating the specific acicular goethite particles which are produced by the aforementioned method, at a temperature of 100 to 200°C, whereby superfine goethite particles are adhered to the surface of the acicular goethite particles, and heat-treating at a temperature of 550 to 850°C. As the acicular goethite particles as precursor, acicular goethite particles having an average major axis diameter of usually 0.01 to 0.25 µm and a geometrical standard deviation of a minor axis diameter of usually 1.37 to 1.50, are used. Especially, acicular goethite particles having an average major axis diameter of usually 0.01 to 0.25 µm, an average minor axis diameter of usually 0.05 to 0.17 µm, a geometrical standard deviation of a major axis diameter of usually not more than 1.70, a geometrical standard deviation of a minor axis diameter of usually 1.37 to 1.50, and a BET specific surface area (S BET ) of usually 50 to 250 m 2 /g, containing soluble sodium salts of usually 500 to 1500 ppm (calculated as Na) and soluble sulfates of usually 150 to 3000 ppm (calculated as SO 4 ), are preferable. Also, the acicular goethite particles which substantially uniformly contain aluminum within the particle as starting particles, have an average major axis diameter of usually 0.01 to 0.25 µm and a geometrical standard deviation of a minor axis diameter of usually 1.37 to 1.50, are used. Especially, acicular goethite particles having an average major axis diameter of usually 0.01 to 0.25 µm, an average minor axis diameter of usually 0.05 to 0.17 µm, a geometrical standard deviation of a major axis diameter of usually not more than 1.70, a geometrical standard deviation of a minor axis diameter of usually 1.37 to 1.50, and a BET specific surface area (S BET ) of usually 50 to 250 m 2 /g, and containing aluminum in amount of 0.05 to 50 % by weight (calculated as Al) based on the total weight of the particles and containing soluble sodium salts of usually 500 to 1500 ppm (calculated as Na) and soluble sulfates of usually 150 to 3000 ppm (calculated as SO 4 ), are preferable. When the heating temperature is less than 100°C, it may be difficult to absorb a sufficient amount of the superfine goethite particles into the acicular goethite particles, thereby failing to obtain particles having a uniform particle size. On the other hand, when the heating temperature is more than 200°C, the acicular goethite particles is heat-dehydrated under such a condition that the superfine goethite particles still remain therein. As a result, the sintering between the particles is disadvantageously caused, thereby also failing to obtain particles having a uniform particle size. With the consideration of industrial productivity or the like, the heat-treating temperature is preferably 120 to 200°C. The heat-treating time is preferably 5 to 60 minutes. The acicular goethite particles obtained by heat-treating at a temperature of 100 to 200°C, have an average major axis diameter of usually 0.011 to 0.26 µm, and a geometrical standard deviation of minor axis diameter of usually not more than 1.30. Especially, it is preferred that the acicular goethite particles obtained by heat-treating at a temperature of 100 to 200°C, have an average major axis diameter of usually 0.011 to 0.26 µm, an average minor axis diameter of usually 0.0055 to 0.13 µm, a geometrical standard deviation of major axis diameter of usually not more than 1.50, a geometrical standard deviation of minor axis diameter of usually not more than 1.30, and a BET specific surface area (S BET ) of usually 50 to 250 m 2 /g, and contain soluble sodium salts of usually 300 to 1500 ppm (calculated as Na) and soluble sulfates of usually 150 to 3000 ppm (calculated as SO 4 ). Also, it is preferred that the acicular goethite particles which substantially uniformly contain aluminum within the particle as starting particles and are obtained by heat-treating at a temperature of 100 to 200°C, have an average major axis diameter of usually 0.011 to 0.26 and a geometrical standard deviation of a minor axis diameter of usually not more than 1.30, and containing aluminum in amount of 0.05 to 50 % by weight (calculated as Al) based on the total weight of the particles. Especially, the acicular goethite particles which substantially uniformly contain aluminum within the particle as starting particles and are obtained by heat-treating at a temperature of 100 to 200°C, have an average major axis diameter of usually 0.011 to 0.26, an average minor axis diameter of usually 0.0055 to 0.13 µm, a geometrical standard deviation of a major axis diameter of usually not more than 1.50, a geometrical standard deviation of a minor axis diameter of-usually not more than 1.30, and a BET specific surface area (S BET ) of usually 50 to 250 m 2 /g, and containing aluminum in amount of 0.05 to 50 % by weight (calculated as Al) based on the total weight of the particles, and contain soluble sodium salts of usually 300 to 1500 ppm (calculated as Na) and soluble sulfates of usually 150 to 3000 ppm (calculated as SO 4 ). The obtained acicular goethite particles are heat-treated at 550 to 850°C, thereby producing acicular hematite particles. Incidentally, as the acicular hematite particles according to the present invention, high-density acicular hematite particles produced by heat-treating acicular goethite particles at 100 to 200°C, heat-dehydrating the acicular goethite particles at 250 to 500°C, thereby obtaining low-density acicular hematite particles, and then heat-treating the obtained low-density acicular hematite particles at a temperature as high as 550 to 850°C, are preferable. Next, the high-purification treatment of the acicular hematite particles is explained. The high-density acicular hematite particles having a specific geometrical standard deviation of a minor axis diameter, which may substantially uniformly contain aluminum within the particle, are diaggregated by a dry-process, and formed into a slurry. The coarse particles thereof contained in the slurry are then deagglomerated by a wet-process. In the wet-deagglomeration, ball mill, sand grinder, colloid mill or the like is used until the coarse particles having a particle size of at least 44 µm are substantially removed. That is, the wet-pulverization is carried out until the amount of the coarse particles having a particle size of not less than 44 µm becomes to usually not more than 10 % by weight, preferably not more than 5 % by weight, more preferably 0 % by weight based on the total weight of the particles. If the amount of the coarse particles having a particle size of not less than 44 µm is more than 10 % by weight, the effect of treating the particles in a high-purification treatment at the next step is not attained. The acicular hematite particles for a non-magnetic undercoat layer of a magnetic recording medium according to the present invention can be produced by adding an aqueous alkali solution to a water suspension containing acicular hematite particles so as to adjust the pH value of the water suspension to not less than 13, and then heat-treating the water suspension at 80 to 103°C, followed by filtering, washing with water and drying. Upon the addition of the aqueous alkali solution, it is preferred that a filter cake obtained by filtering and water-washing the water suspension containing the acicular hematite particles, is preliminarily dispersed in water, thereby forming a water suspension again, or that the water suspension containing the acicular hematite particles is preliminarily washed with water by a decantation method. The concentration of the alkaline suspension containing the acicular hematite particles is preferably 50 to 250 g/liter. As the above-mentioned aqueous alkali solution, there may be used aqueous solutions containing alkali hydroxides such as sodium hydroxide, potassium hydroxide, calcium hydroxide or the like. If the pH value of the aqueous alkali solution containing the acicular hematite particles is less than 13, it is difficult to effectively remove the solid crosslinking caused by the sintering preventive which exists on the surfaces of the acicular hematite particles, so that it is difficult to wash out the soluble sodium salt, soluble sulfate, etc. existing within and on the surfaces of the particles. The upper limit of the pH value is about 14. In consideration of the effect of removing the solid crosslinking caused by the sintering preventive which exists on the surfaces of the acicular hematite particles, the effect of washing out the soluble sodium slat, soluble sulfate, etc., and the effect of removing the alkali which adheres to the surfaces of acicular hematite particles in the process of treatment with the aqueous alkali solution, the preferable pH value is in the range of 13.1 to 13.8. The heating temperature in the aqueous alkali suspension is preferably 80 to 103°C, more preferably 90 to 100°C. If the temperature is lower than 80°C, it is difficult to effectively remove the solid crosslinking caused by the sintering preventive which exists on the surfaces of the acicular hematite particles. If the heating temperature exceeds 103°C, although it is possible to effectively remove the solid crosslinking, since an autoclave is necessary or the treated solution boils under a normal pressure, it is not advantageous from the point of view of industry. The acicular hematite particles heat-treated in the aqueous alkali suspension are thereafter filtered out and washed with water by an ordinary method so as to remove the soluble sodium salt and soluble sulfate which are washed out of the interiors and the surfaces of the particles and to remove the alkali adhered to the surfaces of the acicular hematite particles in the process of treatment with the aqueous alkali solution, and then dried. As the method of washing the acicular hematite particles with water, a method generally industrially used such as a decantation method, a dilution method using a filter thickener and a method of passing water into a filter press is adopted. If the soluble sodium salt and soluble sulfate which are contained within the acicular hematite particles are washed out with water, even if soluble sodium salt and soluble sulfate adhere to the surfaces when the-surfaces of the acicular hematite particles are coated with a coating material, the soluble-sodium salt and soluble sulfate can be easily removed by water washing. The acicular hematite particles for a non-magnetic undercoat layer of a magnetic recording medium according to the present invention, may be coated with at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon, if required. In order to coat the acicular hematite particles, an aluminum compound and/or a silicon compound is added to an aqueous suspension containing the acicular hematite particles, and mixed under stirring. After mixing and stirring, the pH value of the mixed solution is adjusted by using an alkali or acid, if necessary. The acicular hematite particles thus coated with at least one selected from the group consisting of a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon and an oxide of silicon are then filtered out, washed with water, dried and pulverized. They may be further deaerated and compacted, if necessary. As the aluminum compound for the coating, the same aluminum compounds as those described above as the sintering preventive are usable. The amount of aluminum compound added is usually 0.01 to 50 % by weight (calculated as Al) based on the total weight of the particles. If the amount is less than 0.01 % by weight, the dispersibility-improving effect in the vehicle may be insufficient. On the other hand, if the amount exceeds 50 % by weight, the coating dispersibility-improving effect becomes saturated, so that it is meaningless to add an aluminum compound more than necessary. As the silicon compound, the same silicon compounds as those described above as the sintering preventive are usable. The amount of silicon compound added is 0.01 to 50 % by weight (calculated as SiO 2 ) based on the total weight of the particles. If the amount is less than 0.01 % by weight, the improvement of the dispersibility in the vehicle may be insufficient. On the other hand, if the amount exceeds 50.00 % by weight, the coating dispersibility-improving effect becomes saturated, so that it is meaningless to add an silicon compound more than necessary. When both an aluminum compound and a silicon compound are used, the amount thereof used is preferably 0.01 to 50 % by weight (calculated as Al and SiO 2 ) based on the total weight of the particles. Next, the magnetic recording medium according to the present invention is described. The magnetic recording medium according to the present invention comprises: a non-magnetic base film; a non-magnetic undercoat layer formed on the non-magnetic base film, comprising a binder resin and the acicular hematite particles; and a magnetic coating film formed on the non-magnetic undercoat layer, comprising a binder resin and magnetic particles. Firstly, the non-magnetic substrate having the non-magnetic undercoat layer according to the present invention is described. The non-magnetic substrate according to the present invention comprises: a non-magnetic base film; and a non-magnetic undercoat layer formed on said non-magnetic base film, comprising a binder resin and the acicular hematite particles. The non-magnetic substrate of the present invention is produced by forming a coating film on the base film and drying the coating film. The coating film is formed by applying a non-magnetic coating composition which contains the acicular hematite particles, a binder resin and a solvent, to the surface of the base film. As the base film, the following materials which are at present generally used for the production of a magnetic recording medium are usable as a raw material: a synthetic resin such as polyethylene terephthalate, polyethylene, polypropylene, polycarbonate, polyethylene naphthalate, polyamide, polyamideimide and polyimide; foil and plate of a metal such as aluminum and stainless steel; and various kinds of paper. The thickness of the base film varies depending upon the material, but it is usually about 1.0 to 300 µm, preferably 2.0 to 200 µm. In the case of a magnetic disc, polyethylene terephthalate is usually used as the base film, and the thickness thereof is usually 50 to 300 µm, preferably 60 to 200 µm. In a magnetic tape, when polyethylene terephthalate is used as the base film, the thickness thereof is usually 3 to 100 µm, preferably 4 to 20 µm; when polyethylene naphthalate is used, the thickness thereof is usually 3 to 50 µm, preferably 4 to 20 µm; and when polyamide is used, the thickness thereof is usually 2 to 10 µm, preferably 3 to 7 µm. As the binder resin used in the present invention, the following resins which are at present generally used for the production of a magnetic recording medium are usable: vinyl chloride-vinyl acetate copolymer, urethane resin, vinyl chloride-vinyl acetate-maleic acid copolymer, urethane elastomer, butadiene-acrylonitrile copolymer, polyvinyl butyral, cellulose derivative such as nitrocellulose, polyester resin, synthetic rubber resin such as polybutadiene, epoxy resin, polyamide resin, polyisocyanate, electron radiation curing acryl urethane resin and mixtures thereof. Each of these resin binders may contain a functional group such as -OH, -COOH, -SO 3 M, -OPO 2 M 2 and -NH 2 , wherein M represents H, Na or K. With the consideration of the dispersibility of the particles, a binder resin containing a functional group -COOH or -SO 3 M is preferable. The mixing ratio of the acicular hematite particles with the binder resin is usually 5 to 2000 parts by weight, preferably 100 to 1000 parts by weight based on 100 parts by weight of the binder resin. As the solvents, there may be used methyl ethyl ketone, toluene, cyclohexanone, methyl isobutyl ketone, tetrahydrofuran, a mixture of these solvents or the like. The total amount of the solvent used is 50 to 1,000 parts by weight based on 100 parts by weight of the acicular hematite particles. When the amount of the solvent used is less than 50 parts by weight, the viscosity of the non-magnetic coating composition prepared therefrom becomes too high, thereby making it difficult to apply the non-magnetic. coating composition. On the other hand, when the amount of the solvent used is more than 1,000 parts by weight, the amount of the solvent volatilized during the formation of the coating film becomes too large, thereby rendering the coating process industrially disadvantageous. It is possible to add a lubricant, a polishing agent, an antistatic agent, etc. which are generally used for the production of a magnetic recording medium to the non-magnetic undercoat layer. The thickness of the non-magnetic undercoat layer obtained by applying a non-magnetic coating composition on the surface of the base film and drying, is usually 0.2 to 10.0 µm, preferably 0.5 to 5.0 µm. If the thickness is less than 0.2 µm, not only it is impossible to ameliorate the surface roughness of the non-magnetic substrate but also the strength is insufficient. If the thickness is more than 10 µm, it is difficult to reduce the thickness of the magnetic recording medium. In case of using the acicular hematite particles as defined in first aspect as non-magnetic particles, the non-magnetic substrate according to the present invention has a gloss (of the coating film) of usually 189 to 300%, preferably 194 to 300%, more preferably 198 to 300%; a surface roughness Ra (of the coating film) of usually 0.5 to 9.6 nm, preferably 0.5 to 9.1 nm, more preferably 0.5 to 8.6 nm; a Young's modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of usually 120 to 160, preferably 121 to 160. In case of using the acicular hematite particles as defined in second aspect as non-magnetic particles, the non-magnetic substrate according to the present invention has a gloss (of the coating film) of usually 191 to 300%, preferably 196 to 300%, more preferably 201 to 300%; a surface roughness Ra (of the coating film) of usually 0.5 to 9.4 nm, preferably 0.5 to 8.9 nm, more preferably 0.5 to 8.4 nm; a Young's modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of usually 122 to 160, preferably 124 to 160. In case of using the acicular hematite particles as defined in third aspect as non-magnetic particles, the non-magnetic substrate according to the present invention has a gloss (of the coating film) of usually 191 to 300%, preferably 196 to 300%, more preferably 201 to 300%; a surface roughness Ra (of the coating film) of usually 0.5 to 9.3 nm, preferably 0.5 to 8.8 nm, more preferably 0.5 to 8.3 nm; a Young's modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of usually 122 to 160, preferably 124 to 160. In case of using the acicular hematite particles as defined in fourth aspect as non-magnetic particles, the non-magnetic substrate according to the present invention has a gloss (of the coating film) of usually 193 to 300%, preferably 198 to 300%, more preferably 203 to 300%; a surface roughness Ra (of the coating film) of usually 0.5 to 9.1 nm, preferably 0.5 to 8.6 nm, more preferably 0.5 to 8.1 nm; a Young's modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of usually 124 to 160, preferably 126 to 160. In case of using the acicular hematite particles as defined in ninth aspect as non-magnetic particles, the non-magnetic substrate according to the present invention has a gloss (of the coating film) of usually 193 to 300%, preferably 198 to 300%, more preferably 203 to 300%; a surface roughness Ra (of the coating film) of usually 0.5 to 9.0 nm, preferably 0.5 to 8.5 nm, more preferably 0.5 to 8.0 nm; a Young's modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of usually 124 to 160, preferably 126 to 160. In case of using the acicular hematite particles as defined in tenth aspect as non-magnetic particles, the non-magnetic substrate according to the present invention has a gloss (of the coating film) of usually 195 to 300%, preferably 200 to 300%, more preferably 205 to 300%; a surface roughness Ra (of the coating film) of usually 0.5 to 8.8 nm, preferably 0.5 to 8.3 nm, more preferably 0.5 to 7.8 nm; a Young's modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of usually 126 to 160, preferably 128 to 160. In case of using the acicular hematite particles as defined in eleventh aspect as non-magnetic particles, the non-magnetic substrate according to the present invention has a gloss (of the coating film) of usually 196 to 300%, preferably 201 to 300%, more preferably 206 to 300%; a surface roughness Ra (of the coating film) of usually 0.5 to 8.7 nm, preferably 0.5 to 8.2 nm, more preferably 0.5 to 7.7 nm; a Young's modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of usually 126 to 160, preferably 128 to 160. In case of using the acicular hematite particles as defined in twelfth aspect as non-magnetic particles, the non-magnetic substrate according to the present invention has a gloss (of the coating film) of usually 198 to 300%, preferably 203 to 300%, more preferably 208 to 300%; a surface roughness Ra (of the coating film) of usually 0.5 to 8.5 nm, preferably 0.5 to 8.0 nm, more preferably 0.5 to 7.5nm; a Young's modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of usually 128 to 160, preferably 130 to 160. The magnetic recording medium according to the present invention can be produced by applying a magnetic coating composition containing the magnetic particles, a binder resin and a solvent, on the non-magnetic undercoat layer, followed by drying, to form a magnetic recording layer thereon. As the magnetic particles used in the present invention, magnetic particles containing iron as a main component are usable. The magnetic acicular metal particles containing iron as a main ingredient used in the present invention comprises iron and at least one selected from the group consisting of Co, Al, Ni, P, Si, Zn,. Ti, B, Nd, La and Y. Further, the following magnetic acicular metal particles containing iron as a main component may be exemplified. 1) Magnetic acicular metal particles containing iron as a main component comprises iron; and cobalt of usually 0.05 to 40 % by weight, preferably 1.0 to 35 % by weight, more preferably 3 to 30 % by weight (calculated as Co) based on the weight of the magnetic acicular metal particles containing iron as a main component. 2) Magnetic acicular metal particles containing iron as a main component comprises iron; and aluminum of usually 0.05 to 10 % by weight, preferably 0.1 to 7 % by weight (calculated as Al) based on the weight of the magnetic acicular metal particles containing iron as a main component. 3) Magnetic acicular metal particles containing iron as a main component comprises iron; cobalt of usually 0.05 to 40 % by weight, preferably 1.0 to 35 % by weight, more preferably 3 to 30 % by weight (calculated as Co) based on the weight of the magnetic acicular metal particles containing iron as a main component; and aluminum of usually 0.05 to 10 % by weight, preferably 0.1 to 7 % by weight (calculated as Al) based on the weight of the magnetic acicular metal particles containing iron as a main component. 4) Magnetic acicular metal particles containing iron as a main component comprises iron; cobalt of usually 0.05 to 40 % by weight, preferably 1.0 to 35 % by weight, more preferably 3 to 30 % by weight (calculated as Co) based on the weight of the magnetic acicular metal particles containing iron as a main component; and at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10 % by weight, preferably 0.1 to 7 % by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component. 5) Magnetic acicular metal particles containing iron as a main component comprises iron; aluminum of usually 0.05 to 10 % by weight, preferably 0.1 to 7 % by weight (calculated as Al) based on the weight of the magnetic acicular metal particles containing iron as a main component; and at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10 % by weight, preferably 0.1 to 7 % by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component. 6) Magnetic acicular metal particles containing iron as a main component comprises iron; cobalt of usually 0.05 to 40 % by weight, preferably 1.0 to 35 % by weight, more preferably 3 to 30 % by weight (calculated as Co) based on the weight of the magnetic acicular metal particles containing iron as a main component; aluminum of usually 0.05 to 10 % by weight, preferably 0.1 to 7 % by weight (calculated as Al) based on the weight of the magnetic acicular metal particles containing iron as a main component; and at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10 % by weight, preferably 0.1 to 7 % by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component. 7) Magnetic acicular metal particles containing iron as a main component comprises iron; cobalt of usually 0.05 to 40 % by weight, preferably 1.0 to 35 % by weight, more preferably 3 to 30 % by weight (calculated as Co) based on the weight of the magnetic acicular metal particles containing iron as a main component; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10 % by weight, preferably 0.1 to 7 % by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component. 8) Magnetic acicular metal particles containing iron as a main component comprises iron; aluminum of usually 0.05 to 10 % by weight, preferably 0.1 to 7 % by weight (calculated as Al) based on the weight of the magnetic acicular metal particles containing iron as a main component; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10 % by weight, preferably 0.1 to 7 % by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component. 9) Magnetic acicular metal particles containing iron as a main component comprises iron; cobalt of usually 0.05 to 40 % by weight, preferably 1.0 to 35 % by weight, more preferably 3 to 30 % by weight (calculated as Co) based on the weight of the magnetic acicular metal particles containing iron as a main component; aluminum of usually 0.05 to 10 % by weight, preferably 0.1 to 7 % by weight (calculated as Al) based on the weight of the magnetic acicular metal particles containing iron as a main component; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10 % by weight, preferably 0.1 to 7 % by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component. 10) Magnetic acicular metal particles containing iron as a main component comprises iron; cobalt of usually 0.05 to 40 % by weight, preferably 1.0 to 35 % by weight, more preferably 3 to 30 % by weight (calculated as Co) based on the weight of the magnetic acicular metal particles containing iron as a main component; at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10 % by weight, preferably 0.1 to 7 % by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10 % by weight, preferably 0.1 to 7 % by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component. 11) Magnetic acicular metal particles containing iron as a main component comprises iron; aluminum of usually y 0.05 to 10 % by weight, preferably 0.1 to 7 % by weight (calculated as Al) based on the weight of the magnetic acicular metal particles containing iron as a main component; at least one selected from the group consisting of Nd, La and Y of ordinarily 0.05 to 10 % by weight, preferably 0.1 to 7 % by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10 % by weight, preferably 0.1 to 7 % by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component. 12) Magnetic acicular metal particles containing iron as a main component comprises iron; cobalt of usually 0.05 to 40 % by weight, preferably 1.0 to 35 % by weight, more preferably 3 to 30 % by weight (calculated as Co) based on the weight of the magnetic acicular metal particles containing iron as a main component; aluminum of usually 0.05 to 10 % by weight, preferably 0.1 to 7 % by weight (calculated as Al) based on the weight of the magnetic acicular metal particles containing iron as a main component; at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10 % by weight, preferably 0.1 to 7 % by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10 % by weight, preferably 0.1 to 7 % by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles containing iron as a main component. The iron content in the particles is the balance, and is preferably 50 to 99 % by weight, more preferably 60 to 95 % by weight (calculated as Fe) based on the weight of the magnetic acicular metal particles containing iron as a main component. The magnetic acicular metal particles containing iron as a main component comprising (i) iron and Al; (ii) iron, cobalt and aluminum, (iii) iron, aluminum and at least one rare-earth metal such as Nd, La and Y, or (iv) iron, cobalt, aluminum and at least one rare-earth metal such as Nd, La and Y is preferable from the point of the durability of the magnetic recording medium. Further, the magnetic acicular metal particles containing iron as a main component comprising iron, containing aluminum of 0.05 to 10 % by weight (calculated as Al) which are present within the particle, and optionally containing at least one rare-earth metal such as Nd, La and Y, are more preferable. With respect to the existing position of aluminum of usually 0.05 to 10 % by weight (calculated as Al) based on the weight of the magnetic acicular metal particles containing iron as a main component, it may be contained only in the core and inside portions, or in the surface portion of the magnetic acicular metal particles containing iron as a main component. Alternatively, aluminum may be approximately uniformly contained in the magnetic acicular metal particles containing iron as a main component from the core portion to the surface. An aluminum-coating layer may be formed on the surfaces of the particles. In addition, any of these positions may be combined. In the consideration of the effect of improving the surface property of the magnetic recording layer or the durability of the magnetic recording medium, magnetic acicular metal particles containing iron as a main component uniformly containing aluminum from the core portion to the surface and coated with an aluminum-coating layer are more preferable. When the content of aluminum is less than 0.05 % by weight (calculated as Al), the adsorption of the resin to the magnetic acicular metal particles containing iron as a main component in the vehicle may not be said sufficient, so that it may be difficult to produce a magnetic recording layer or a magnetic recording medium having a high durability. When the content of aluminum exceeds 10 % by weight, the effect of improving the durability of the magnetic recording layer or the magnetic recording medium is observed, but the effect is saturated and it is meaningless to add aluminum more than necessary. Furthermore, the magnetic characteristics of the magnetic acicular metal particles containing iron as a main component may be sometimes deteriorated-due to an increase in the aluminum as a non-magnetic component. The existing amount of aluminum of the magnetic acicular metal particles containing iron as a main component is preferably 0.1 to 7 % by weight. It is still more preferable to produce a magnetic recording medium of the present invention using the magnetic acicular metal particles containing iron as a main component containing Al and a rare-earth metal such as Nd, La and Y therein, because the magnetic recording layer or magnetic recording medium produced is apt to have a more excellent durability. Especially, magnetic acicular metal particles containing iron as a main component containing Al and Nd therein are the even more preferable. The magnetic acicular metal particles containing iron as a main component comprising iron and aluminum within the particles are produced, as is well known, by adding an aluminum compound at an appropriate stage during the above-described process for producing acicular goethite particles to produce acicular goethite particles containing aluminum at desired positions of the particles, and heat-treating, at a temperature of 300 to 500°C, the acicular goethite particles or the acicular hematite particles containing aluminum at desired positions within the particles which are obtained by dehydrating the acicular goethite particles. The magnetic acicular metal particles containing iron as a main component coated with aluminum are produced by heat-treating, at a temperature of 300 to 500°C, the acicular goethite particles coated with an oxide or hydroxide of aluminum, or the acicular hematite particles coated with the oxide or hydroxide of aluminum which are obtained by dehydrating the acicular goethite particles. The magnetic particles containing iron as a main component used in the present invention have an average major axial diameter of usually 0.01 to 0.50 µm, preferably 0.03 to 0.30 µm, an average minor axial diameter of usually 0.0007 to 0.17 µm, preferably 0.003 to 0.10 µm, and an aspect ratio of usually not less than 3 : 1, preferably not less than 5 : 1. The upper limit of the aspect ratio is usually 15 : 1, preferably 10 : 1 with the consideration of the dispersibility in the vehicle. The shape of the a magnetic particles containing iron as a main component may have not only acicular but also spindle-shaped, rice ball-shaped, cubic-shaped, plate-like shaped or the like. The geometrical standard deviation of the major axis diameter of the magnetic particles used in the present invention is preferably not more than 2.50. If it exceeds 2.50, the coarse particles existent sometimes exert a deleterious influence on the surface-smoothness of the magnetic recording layer. From the point of view of industrial productivity, the lower limit of the geometrical standard deviation of the major axis diameter is preferably 1.01. As to the magnetic properties of the magnetic acicular metal particles containing iron as a main component used in the present invention, the coercive force is usually 800 to 3500 Oe, preferably 900 to 3500 Oe, more preferably 1000 to 3500 Oe, and the saturation magnetization is usually preferably 90 to 170 emu/g, preferably 100 to 170 emu/g, more preferably 110 to 170 emu/g with the consideration of the properties such as high-density recording. With the consideration of the durability of the magnetic recording medium, it is preferred to use magnetic acicular metal particles containing iron as a main component, which contain aluminum, as magnetic particles. The resin adsorptivity of such magnetic acicular metal particles containing iron as a main component used in the present invention is usually not less than 65%, preferably not less than 68%, more preferably not less than 70%. As the binder resin for the magnetic recording layer, the same binder resin as that used for the production of the non-magnetic undercoat layer is usable. The mixing ratio of the magnetic acicular metal particles containing iron as a main component with the binder resin in the magnetic recording layer is usually 200 to 2000 parts by weight, preferably 300 to 1500 parts by weight based on 100 parts by weight of the binder resin. As the solvents, the same solvent as that used for the production of the non-magnetic undercoat layer is usable. The total amount of the solvent used is 65 to 1,000 parts by weight based on 100 parts by weight of the magnetic particles. When the amount of the solvent used is less than 65 parts by weight, the viscosity of the magnetic coating composition prepared therefrom becomes too high, thereby making it difficult to apply the magnetic coating composition. On the other hand, when the amount of the solvent used is more than 1,000 parts by weight, the amount of the solvent volatilized during the formation of the coating film becomes too large, thereby rendering the coating process industrially disadvantageous. It is possible to add a lubricant, a polishing agent, an antistatic agent, etc., which are generally used for the production of a magnetic recording medium to the magnetic recording layer. The thickness of the magnetic recording layer obtained by applying the magnetic coating composition on the surface of the non-magnetic undercoat layer and dried, is usually in the range of 0.01 to 5.0 µm. If the thickness is less than 0.01 µm, uniform coating may be difficult, so that unfavorable phenomenon such as unevenness on the coating surface is observed. On the other hand, when the thickness exceeds 5.0 µm, it may be difficult to obtain desired signal recording property due to an influence of diamagnetism. The preferable thickness is in the range of 0.05 to 1.0 µm. The magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe; a squareness {residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95; a gloss (of the coating film) of usually 192 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.8 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 120 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 µm -1 . In case of using the acicular hematite particles as defined in the first aspect as non-magnetic particles, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95; a gloss (of the coating film) of usually 192 to 300 %, preferably 197 to 300 %; a surface roughness Ra (of the coating film) of usually not more than 9.8 nm, preferably 0.5 to 9.3 nm, ; more preferably 0.5 to 8.8 nm, a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 120 to 160, preferably 122 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 µm -1 , preferably 1.20 to 2.00 µm -1 . In case of using the acicular hematite particles as defined in the second aspect as non-magnetic particles, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95; a gloss (of the coating film) of usually 194 to 300 %, preferably 199 to 300 %; a surface roughness Ra (of the coating film) of usually not more than 9.6nm, preferably 0.5 to 9.1 nm, more preferably 0.5 to 8.6 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 122 to 160, preferably 124 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 µm -1 , preferably 1.20 to 2.00 µm -1 . In case of using the acicular hematite particles as defined in the first aspect as non-magnetic particles and magnetic acicular metal particles containing iron as a main component and aluminum which are present in and/or on the surface of the particle, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 194 to 300%, preferably 199 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.6 nm, preferably 0.5 to 9.1 nm, more preferably 0.5 to 8.6 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 122 to 160, preferably 124 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 µm -1 , preferably 1.20 to 2.00 µm -1 . As to the durability, the running durability thereof is usually not less than 22 minutes, preferably not less than 24 minutes. Also, the scratch resistance thereof is usually A or B, preferably A. In case of using the acicular hematite particles as defined in the second aspect as non-magnetic particles and magnetic acicular metal particles containing iron as a main component and aluminum which are present in and/or on the surface of the particle, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 196 to 300%, preferably 201 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.4 nm, preferably 0.5 to 8.9 nm, more preferably 0.5 to 8.4 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 124 to 160, preferably 126 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 µm -1 , preferably 1.20 to 2.00 µm -1 . As to the durability, the running durability thereof is usually not less than 23 minutes, preferably not less than 25 minutes. Also, the scratch resistance thereof is usually A or B, preferably A. In case of using the acicular hematite particles as defined in the third aspect as non-magnetic particles, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 194 to 300%, preferably 199 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.6 nm, preferably 0.5 to 9.1 nm, more preferably 0.5 to 8.6 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 122 to 160, preferably 124 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 µm -1 , preferably, 1.20 to 2.00 µm -1 . As to the durability, the running durability thereof is usually not less than 22 minutes, preferably not less than 23 minutes. Also, the scratch resistance thereof is usually A or B, preferably A. In case of using the acicular hematite particles as defined in the fourth aspect as non-magnetic particles, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 196 to 300%, preferably 201 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.4 nm, preferably 0.5 to 8.9 nm, more preferably 0.5 to 8.4 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 124 to 160, preferably 126 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 µm -1 , preferably 1.20 to 2.00 µm -1 . As to the durability, the running durability thereof is usually not less than 23 minutes, preferably not less than 24 minutes. Also, the scratch resistance thereof is usually A or B, preferably A. In case of using the acicular hematite particles as defined in the third aspect as non-magnetic particles and magnetic acicular metal particles containing iron as a main component and aluminum which are present in and/or on the surface of the particle, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 196 to 300%, preferably 201 to 300%; a surface-roughness Ra (of the coating film) of usually not more than 9.4 nm, preferably 0.5 to 8.9 nm, more preferably 0.5 to 8.4 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 124 to 160, preferably 126 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 µm -1 , preferably 1.20 to 2.00 µm -1 . As to the durability, the running durability thereof is usually not less than 23 minutes, preferably not less than 24 minutes. Also, the scratch resistance thereof is usually A or B, preferably A. In case of using the acicular hematite particles as defined in the fourth aspect as non-magnetic particles and magnetic acicular metal particles containing iron as a main component and aluminum which are present in and/or on the surface of the particle, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 198 to 300%, preferably 203 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.2 nm, preferably 0.5 to 8.7 nm, more preferably 0.5 to 8.2 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 126 to 160, preferably 127 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 µm -1 , preferably 1.20 to 2.00 µm -1 . As to the durability, the running durability thereof is usually not less than 24 minutes, preferably not less than 26 minutes. Also, the scratch resistance thereof is usually A or B, preferably A. In case of using the acicular hematite particles as defined in the ninth aspect as non-magnetic particles, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 196 to 300%, preferably 201 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.4 nm, preferably 0.5 to 8.9 nm, more preferably 0.5 to 8.4 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 124 to 160, preferably 126 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 µm -1 , preferably 1.20 to 2.00 µm -1 . In case of using the acicular hematite particles as defined in the tenth aspect as non-magnetic, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 198 to 300%, preferably 203 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.2 nm, preferably 0.5 to 8.7 nm, more preferably 0.5 to 8.2 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 126 to 160, preferably 128 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 µm -1 , preferably 1.20 to 2.00 µm -1 . In case of using the acicular hematite particles as defined in the ninth aspect as non-magnetic particles and magnetic acicular metal particles containing iron as a main component and aluminum which are present in and/or on the surface of the particle, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 198 to 300%, preferably 203 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.2 nm, preferably 0.5 to 8.7 nm, more preferably 0.5 to 8.2 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 126 to 160, preferably 128 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 µm -1 , preferably 1.20 to 2.00 µm -1 . As to the durability, the running durability thereof is usually not less than 24 minutes, preferably not less than 26 minutes. Also, the scratch resistance thereof is usually A or B, preferably A. In case of using the acicular hematite particles as defined in the tenth aspect as non-magnetic particles and magnetic acicular metal particles containing iron as a main component and aluminum which are present in and/or on the surface of the particle, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 200 to 300%, preferably 205 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.0 nm, preferably 0.5 to 8.5 nm, more preferably 0.5 to 8.0 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 128 to 160, preferably 130 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 µm -1 , preferably 1.20 to 2.00 µm -1 . As to the durability, the running durability thereof is usually not less than 25 minutes, preferably not less than 27 minutes. Also, the scratch resistance thereof is usually A or B, preferably A. In case of using the acicular hematite particles as defined in the eleventh aspect as non-magnetic particles, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 198 to 300%, preferably 203 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.2 nm, preferably 0.5 to 8.7 nm, more preferably 0.5 to 8.2 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 126 to 160, preferably 128 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 µm -1 , preferably 1.20 to 2.00 µm -1 . As to the durability, the running durability thereof is usually not less than 24 minutes, preferably not less than 26 minutes. Also, the scratch resistance thereof is usually A or B, preferably A. In case of using the acicular hematite particles as defined in the twelfth aspect as non-magnetic, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 200 to 300%, preferably 205 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.0 nm, preferably 0.5 to 8.5 nm, more preferably 0.5 to 8.0 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 128 to 160, preferably 130 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 µm -1 , preferably 1.20 to 2.00 µm -1 . As to the durability, the running durability thereof is usually not less than 25 minutes, preferably not less than 27 minutes. Also, the scratch resistance thereof is usually A or B, preferably A. In case of using the acicular hematite particles as defined in the eleventh aspect as non-magnetic particles and magnetic acicular metal particles containing iron as a main component and aluminum which are present in and/or on the surface of the particle, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 200 to 300%, preferably 205 to 300%; a surface roughness Ra (of the coating film) of usually not more than 9.0 nm, preferably 0.5 to 8.5 nm, more preferably 0.5 to 8.0 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 128 to 160, preferably 130 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 µm -1 , preferably 1.20 to 2.00 µm -1 . As to the durability, the running durability thereof is usually not less than 25 minutes, preferably not less than 27 minutes. Also, the scratch resistance thereof is usually A or B, preferably A. In case of using the acicular hematite particles as defined in the twelfth aspect as non-magnetic particles and magnetic acicular metal particles containing iron as a main component and aluminum which are present in and/or on the surface of the particle, the magnetic recording medium according to the present invention has a coercive force of usually 800 to 3500 Oe, preferably 900 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.86 to 0.95, preferably 0.87 to 0.95, a gloss (of the coating film) of usually 202 to 300%, preferably 207 to 300%; a surface roughness Ra (of the coating film) of usually not more than 8.8 nm, preferably 0.5 to 8.3 nm, more preferably 0.5 to 7.8 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of usually 130 to 160, preferably 132 to 160; and a linear adsorption coefficient (of the coating film) of usually 1.10 to 2.00 µm -1 , preferably 1.20 to 2.00 µm -1 . As to the durability, the running durability thereof is usually not less than 26 minutes, preferably not less than 28 minutes. Also, the scratch resistance thereof is usually A or B, preferably A. The magnetic recording medium according to the present invention shows an anti-corrosion property of not more than 10.0 %, preferably not more than 9.5 % when expressed by he percentage (%) of change in coercive force thereof, and also shows an anti-corrosion property of not more than 10.0 %, preferably not more than 9.5 % when expressed by the percentage (%) of change in saturation magnetic flux density thereof. The important feature of the present invention lies in such a fact that when the acicular hematite particles are acid-dissolving treatment, there can be obtained acicular hematite particles which have such a uniform particle size as being expressed by the geometrical standard deviation of major axis diameter of not more than 1.50 and the geometrical standard deviation of minor axis diameter of not more than 1.35, and are, in particular, excellent in particle size distribution of minor axis diameter thereof. The reason why the acicular hematite particles according to the present invention can exhibit a uniform particle size, is considered, such that by acid-treating acicular hematite particles in the strongly acidic solution, superfine hematite particles can be removed therefrom, whereby it is possible to obtain acicular goethite particles having a uniform particle size with respect to both major axis diameter and minor axis diameter thereof, because the amount of the superfine hematite particles be reduced. Further, another important feature of the present invention lies in such a fact that when the acicular goethite particles are heat-treated at a temperature of 100 to 200°C before subjecting to heat-dehydration treatment, there can be obtained acicular hematite particles which have such a uniform particle size as being expressed by the geometrical standard deviation of major axis diameter of not more than 1.50 and the geometrical standard deviation of minor axis diameter of preferably not more than 1.30, and are, in particular, excellent in particle size distribution of minor axis diameter thereof. The reason why the acicular hematite particles according to the present invention can exhibit a uniform particle size, is considered, such that by heat-treating acicular goethite particles at a temperature of 100 to 200°C, superfine goethite particles can be absorbed into the acicular goethite particles, whereby it is possible to obtain acicular goethite particles having a uniform particle size with respect to both major axis diameter and minor axis diameter thereof, and further since the amount of the superfine goethite particles be reduced, the sintering between the particles due to the presence of the superfine goethite particles is unlikely to occur upon the subsequent heat-dehydration treatment, whereby it is possible to obtain acicular hematite particles which still maintain the uniform particle size of the acicular goethite particles. In the magnetic recording medium according to the present invention, when the acicular hematite particles of the present invention are used as non-magnetic particles for non-magnetic undercoat layer, there can be obtained such a magnetic recording medium having a small light transmittance, a more excellent smooth surface and a high strength. The reason why the magnetic recording medium according to the present invention can exhibit a more excellent smooth surface, is considered by the present inventors, such that by the synergistic effect of such a uniform particle size as being expressed by the geometrical standard deviation of major axis diameter of not more than 1.50 and the geometrical standard deviation of minor axis diameter of not more than 1.35 which results in less amount of coarse or fine particles, and the BET specific surface area of 35.9 to 180 m 2 /g which results in less amount of dehydration pores in the particles or on the surfaces of the particles, the acicular hematite particles according to the present invention can exhibit a more excellent dispersibility in vehicle, so that the obtained non-magnetic undercoat layer can have a more excellent smooth surface. The another reason why the surface smoothness of the magnetic recording medium is excellent is considered as follows. Since it is possible to sufficiently remove the soluble sodium and the soluble sulfate, which agglomerate acicular hematite particles by firmly crosslinking, the agglomerates are separated into substantially discrete particles by washing the particles with water, so that acicular hematite particles having an excellent dispersion in the vehicle are obtained. It has been found that the deterioration in magnetic properties of the magnetic acicular metal particles containing iron as a main component and dispersed in the magnetic layer, is considerably influenced by the pH value of non-magnetic particles in the non-magnetic undercoat layer and by the content of soluble sodium salts and soluble sulfates contained in the non-magnetic particles. That is, as described above, the acicular goethite particles used as a precursor have been produced by various methods. However, in any method, in the case where ferrous sulfate is used as a main raw material for the production of the acicular goethite particles, a large amount of sulfate [SO 4 2- ] is necessarily present in a reacted slurry. In particular, when the goethite particles are produced in an acid solution, water-soluble sulfates such as Na 2 SO 4 are produced simultaneously, and the reaction mother liquor contains alkali metals such as K + , NH 4 + , Na + or the like, so that precipitates containing alkali metals or sulfates tend to be formed. The thus-formed precipitates are represented by the formula of RFe 3 (SO 4 ) (OH) 6 , where R is K + , NH 4 + or Na + . Such precipitates are composed of insoluble sulfur-containing iron salts and, therefore, cannot be removed merely by an ordinary water-washing method. The insoluble salts are converted into soluble sodium salts or soluble sulfates by the subsequent heat-treatment. However, the soluble sodium salts or the soluble sulfates cause the acicular hematite particles to be cross-linked with each other by the action of a sintering preventive which is added as an essential component for preventing the deformation of the acicular hematite particles and the sintering therebetween in the high-temperature treatment for high densification, and are strongly bonded to the inside or the surface of the acicular hematite particles. Thus, the agglomeration of the acicular hematite particles are further accelerated by the soluble sodium salts or the soluble sulfates. As a result, it becomes extremely difficult to remove especially such soluble sodium salts or soluble sulfates enclosed within the particles or the agglomerated particles, by an ordinary water-washing method. In the case where the acicular goethite particles are produced in an aqueous alkali solution using ferrous sulfate and sodium hydroxide, a sulfate (Na 2 SO 4 ) is simultaneously produced, and NaOH is contained in the reacted slurry. Since Na 2 SO 4 and NaOH both are water-soluble, it is considered that these compounds can be essentially removed by sufficiently washing the acicular goethite particles with water. However, in general, the acicular goethite particles are deteriorated in water-washing efficiency due to a low crystallizability thereof, so that even when the acicular goethite particles are washed with water by an ordinary method, water-soluble components such as soluble sulfates [SO 4 2- ] or soluble sodium salts [Na + ] still remain in the acicular goethite particles. Further, as described above, these water-soluble components cause the acicular hematite particles to be cross-linked with each other by the action of the sintering preventive, and are strongly bonded to the inside or the surface of the acicular hematite particles, thereby further accelerating the agglomeration of the acicular hematite particles. As a result, it also becomes extremely difficult to remove, especially, such soluble sodium salts or soluble sulfates enclosed within the particles or the agglomerated particles by an ordinary water-washing method. As described above, it is considered that when the high-density hematite particles containing the soluble sodium salts or the soluble sulfates which are strongly bonded to the inside or the surface of the particles and to the inside of the agglomerated particles through the sintering preventive, are wet-pulverized to deagglomerate coarse particles, then the pH value of the slurry containing the high-density hematite particles is adjusted to not less than 13 and the slurry is heat-treated at not less than 80°C, the aqueous alkali solution can be sufficiently immersed into the high-density hematite particles, so that the bonding force of the sintering preventive strongly bonded to the inside or the surface of the particles and to the inside of the agglomerated particles is gradually weakened, whereby the sintering preventive is released from the inside or the surface of the particles and the inside of the-agglomerated particles and simultaneously the water-soluble sodium salts or the water-soluble sulfates tend to be readily removed by washing with water. In addition, the reason why the deterioration in magnetic properties due to the corrosion of the magnetic acicular metal particles containing iron as a main component and dispersed in the magnetic recording layer, can be suppressed, is considered, such that the amount of water-soluble components such as the soluble sodium salts or the soluble sulfates which accelerate the corrosion of metals contained in the acicular hematite particles for non-magnetic undercoat layer, is reduced and the particles have a pH value as high as not less than 8, so that the corrosion of the acicular hematite particles containing iron as a main component can be prevented from proceeding. In the magnetic recording medium according to the present invention, when the acicular hematite particles containing aluminum in specific amount within the particle of the present invention are used as non-magnetic particles for non-magnetic undercoat layer, there can be obtained such a magnetic recording medium having a small light transmittance, a more excellent smooth surface, a high strength and a high durability. The reason why the strength and the durability of the magnetic recording medium is more enhanced is considered that by using the acicular hematite particles containing aluminum uniformly within the particle, the resin adsorptivity of the acicular hematite particles to the binder resin in the vehicles are enhanced due to the use of the above-described particles, as will be shown in later-described examples, the degree of adhesion of the acicular hematite particles in the non-magnetic undercoat layer or the non-magnetic undercoat layer itself to the base film is enhanced; and the increase in adsorptivity of the binder resin for the magnetic particles in the coating composition by using as magnetic particles, the magnetic acicular metal particles containing iron as a main component and further containing aluminum, which results in increase in adhesion between the magnetic particles in the magnetic recording layer and the non-magnetic undercoat layer or between the magnetic recording layer itself and the non-magnetic undercoat layer. As described in Examples hereinafter, in the case where the acicular hematite particles according to the present invention is used as non-magnetic particles for the non-magnetic undercoat layer, since these particles contain less amount of fine particle components and, therefore, can show an excellent dispersibility in vehicle, there can be obtained such a non-magnetic undercoat layer having an excellent strength and an excellent smooth surface. Further, in the case where a magnetic recording medium is produced by using the non-magnetic undercoat layer, there can be obtained such a magnetic recording medium which can exhibit a low light transmittance, an excellent smooth surface and a high strength, and whose magnetic properties can be prevented from being deteriorated due to the corrosion of the magnetic acicular metal particles containing iron as a main component and dispersed in the magnetic recording layer, since the pH value of the acicular hematite particles used as non-magnetic particles is low and the amount of the soluble sodium salts or the soluble sulfates contained is reduced. Therefore, the acicular hematite particles according to the present invention are suitable as non-magnetic particles for a non-magnetic undercoat layer of a high-density magnetic recording medium. As described above, the magnetic recording medium according to the present invention can exhibit a low light transmittance, a smooth surface and a high strength, and can be prevented from being deteriorated in magnetic properties due to the corrosion of the magnetic acicular particles containing iron as a main component and dispersed in the magnetic recording layer. Therefore, the magnetic recording medium according to the present invention is suitable as a high-density magnetic recording medium. Further, in the magnetic recording medium according to the present invention, in the case where specific acicular hematite particles are used as non-magnetic particles for the non-magnetic undercoat layer, there can be obtained such a non-magnetic undercoat layer having a high strength and an excellent smooth surface, because the acicular hematite particles used as non-magnetic particles contain less amount of fine particle components, and have a pH value as high as not less than 8 and an excellent dispersibility in vehicle due to less content of the soluble salts. Consequently, when the magnetic recording medium is produced by using the above-mentioned non-magnetic undercoat layer and using as magnetic particles for the magnetic recording layer, the magnetic acicular metal particles containing iron as a main component and further containing a predetermined amount of Al, there can be obtained such a magnetic recording medium which can exhibit a low light transmittance, an excellent smooth surface, a high strength and an excellent durability, and can be prevented from being deteriorated in magnetic properties due to the corrosion of the magnetic acicular metal particles containing iron as a main component and dispersed in the magnetic recording layer. Therefore, the magnetic recording medium according to the present invention is suitable as a high-density magnetic recording medium. EXAMPLES The present invention is described in more detail by Examples and Comparative Examples, but the Examples are only illustrative and, therefore, not intended to limit the scope of the present invention. Various properties were evaluated by the following methods. (1) The residue on sieve after the wet-pulverization was obtained by measuring the concentration of the slurry after pulverization by a wet-process in advance, and determining the quantity of the solid content on the sieve remaining after the slurry equivalent to 100 g of the particles content was passed through the sieve of 325 meshes (mesh size: 44 µm). (2) The average major axis diameter and the average minor axis diameter of the acicular particles are expressed by the average values of 350 particles measured in the photograph obtained by magnifying an electron micrograph (× 30000) by 4 times in the vertical and horizontal directions, respectively. (3) The aspect ratio is the ratio of the average major axis diameter and the average minor axis diameter. (4) The geometrical standard deviation for particle size distribution of the major axis diameter and minor axis diameter was obtained by the following method. The major axis diameters and minor axis diameters of the particles were measured from the magnified electron microphotograph in the above-mentioned (2). The actual major axis diameters and minor axis diameters of the particles and the number of particles were obtained from the calculation on the basis of the measured values. On logarithmico-normal probability paper, the major axis diameters or minor axis diameters were plotted at regular intervals on the abscissa-axis and the accumulative number of particles belonging to each interval of the major axis diameters or minor axis diameters was plotted by percentage on the ordinate-axis by a statistical technique. The major axis diameters or minor axis diameters corresponding to the number of particles of 50% and 84.13%, respectively, were read from the graph, and each geometrical standard deviation was measured from the following formulae: Geometrical standard deviation of the major axis diameter or minor axis diameter = { major axis diameter (µm) or minor axis diameter (µm) corresponding to 84.13 % under integration sieve}/{major axis diameter or minor axis diameter (geometrical average diameter) corresponding to 50% under integration sieve}. The more the geometrical standard deviation nears 1.0, the more excellent the particle size distribution of the major axis diameters and minor axis diameters of the particles. (5) The specific surface area is expressed by the value measured by a BET method. (6) The degree of densification of the particles is represented by S BET /S TEM value as described above. S BET is a specific surface area measured by the above-described BET method. S TEM is a value calculated from the average major axis diameter d cm and the average minor axis diameter w cm measured from the electron microphotograph described in (2) on the assumption that a particle is a rectangular parallellopiped in accordance with the following formula: STEM (m 2 /g) = {(4·d·w + 2w 2 )/(d·w 2 ·ρ p )} x 10 -4 wherein ρ p is the true specific gravity of the hematite particles, and 5.2 g/cm 3 was used. (7) The content of each of Al, Si, P, and Nd in and/or on the particle was measured according to JIS K0119 using "fluorescent X-ray spectroscopy device 3063 M" (manufactured by Rigaku Denki Kogyo Co., Ltd.). (8) The pH value of the particles was measured as follows. 5 g of a sample was weighed and placed in a 300-ml conical flask. 100 ml of boiling pure water was added into the flask and the contents thereof were-heated and maintained in a boiling condition for about 5 minutes. Thereafter, the flask was plugged and allowed to stand for cooling up to an ordinary temperature. After boiling pure water was added in such an amount corresponding to the weight loss and the flask was plugged again, the contents of the flask were shaken and mixed for one minute and then allowed to stand for 5 minutes, thereby obtaining a supernatant. The pH value of the thus obtained supernatant was measured according to JIS Z 8802-7, and the pH value of the particles was expressed by the measured value. (9) The content of soluble sodium salts and the content of soluble sulfates were determined by filtering the supernatant prepared for the above measurement of the pH value of the particles using a filter paper No. 5, and measuring amounts of Na + and SO 4 2- in the filtrate by an inductively coupled plasma atomic emission spectrometry device (manufactured by Seiko Denshi Kogyo Co., Ltd.). (10) The resin adsorptivity of the particles represents the degree at which a resin is adsorbed to the particles. The closer to 100 % the value obtained in the following manner, the firmer the resin adsorptivity to the particles surfaces in the vehicle and the more favorable. The resin adsorptivity Wa was first obtained. 20 g of particles and 56 g of a mixed solvent (27.0 g of methyl ethyl ketone, 16.2 g of toluene, and 10.8 g of cyclohexanone) with 2 g of a vinyl chloride-vinyl acetate copolymer having a sodium sulfonate group dissolved therein were charged into a 100-ml polyethylene bottle together with 120 g of 3 mm steel beads. The particles and the solvent were mixed and dispersed by a paint shaker for 60 minutes.Thereafter, 50 g of the coating composition was taken out, and charged into a 50-ml settling cylinder. The solid content was separated from the solvent portion by the centrifugalization at a rate of 10000 rpm for 15 minutes. The concentration of the solid resin content contained in the solvent portion was determined by a gravimetric method and the resin content existing in the solid portion was determined by deducting the obtained resin content from the amount of the resin charged as the resin adsorptivity Wa (mg/g) to the particles.The total quantity of separated solid content was taken into a 100 ml-tall beaker, and 50 g of a mixed solvent (25.0 g of methyl ethyl ketone, 15.0 g of toluene, and 10.0 g of cyclohexanone) was added thereto. The obtained mixture was to ultrasonic dispersion for 15 minutes, and the thus-obtained suspension was charged into a 50-ml settling cylinder. The solid content was separated from the solvent portion by centrifuging them at a rate of 10000 rpm for 15 minutes. The concentration of the solid resin content contained in the solvent portion was measured so as to determine the resin content dissolved from the resin which had been adsorbed to the particle surfaces into the solvent phase.The process from the step of taking the solid content into the 100 ml-tall beaker to the determination of the resin content dissolved into the solvent phase was repeated twice. The total quantity We (mg/g) of resin content dissolved into the solvent phase in the three cycles was obtained, and the value calculated in accordance with the following formula is expressed as the resin adsorptivity T(%): T(%) = [(Wa - We)/Wa] × 100. (11) The viscosity of the coating composition was obtained by measuring the viscosity of the coating composition at 25°C at a shear rate D of 1.92 sec -1 by using "E type viscometer EMD-R" (manufactured by Tokyo Keiki, Co., Ltd.). (12) The gloss of the surface of the coating film of each of the non-magnetic undercoat layer and the magnetic coating layer was measured at an angle of incidence of 45° by "glossmeter UGV-5D" (manufactured by Suga Shikenki, Co., Ltd.). (13) The surface roughness Ra is expressed by the average value of the center-line average roughness of the profile curve of the-surface of the coating film by using "Surfcom-575A" (manufactured by Tokyo Seimitsu Co., Ltd.). (14) The strength of the coating film was expressed the Young's modulus obtained by "Autograph" (produced by Shimazu Seisakusho Co., Ltd.). The Young's modulus was expressed by the ratio of the Young's modulus of the coating film to that of a commercially available video tape "AV T-120" (produce by Victor Company of Japan, Limited). The higher the relative value, the more favorable. (15) The magnetic properties of the magnetic particles and magnetic recording medium were measured under an external magnetic field of 10 kOe by "Vibration Sample Magnetometer VSM-3S-15 (manufactured by Toei Kogyo, Co., Ltd.)". (16) The change with the passage of time in magnetic properties of the magnetic recording medium due to the corrosion of the magnetic acicular metal particles containing iron as a main component, which are dispersed in the magnetic coating film, was determined as follows. The magnetic recording medium was allowed to stand at a temperature of 60°C and a relative humidity of 90 % for 14 days. The coercive force values and the saturation magnetization values of the magnetic recording medium before and after the keeping test were measured, and the difference between the measured values before and after the keeping test was divided by the value before the keeping test, thereby obtaining an amount of change in each magnetic property which was expressed by a percentage. (17) The light transmittance is expressed by the linear adsorption coefficient calculated by substituting the light transmittance measured by using "UV-Vis-Recording Spectrophotometer UV-2100" (manufactured by Shimazu Seisakusho, Ltd.) for the following formula. The larger the value, the more difficult it is for the magnetic recording medium to transmit light: Linear adsorption coefficient (µm -1 ) = {l n (l/t)}/FT wherein t represents a light transmittance (-) at λ = 900 nm, and FT represents thickness (µm) of the coating film used for the measurement. (18) The durability of the magnetic medium was evaluated by the following running durability and the scratch resistance. The running durability was evaluated by the actual operating time under the conditions that the load was 200 gw and the relative speed of the head and the tape was 16 m/s by using "Media Durability Tester MDT-3000" (manufactured by Steinberg Associates). The longer the actual operating time, the higher the running durability.The scratch resistance was evaluated by observing through the microscope the surface of the magnetic tape after running and visually judging the degree of scratching. Evaluation was divided into the following four ranks. A: No scratch B: A few scratches C: Many scratches D: Great many scratches (19) The thickness of each of the base film, the non-magnetic undercoat layer and the magnetic coating film constituting the magnetic recording medium was measured in the following manner by using "Digital Electronic Micrometer R351C" (manufactured by Anritsu Corp.) The thickness (A) of a base film was first measured. Similarly, the thickness (B) (B = the sum of the thicknesses of the base film and the non-magnetic undercoat layer) of a non-magnetic substrate obtained by forming a non-magnetic undercoat layer on the base film was measured. Furthermore, the thickness (C) (C = the sum of the thicknesses of the base film, the non-magnetic undercoat layer and the magnetic recording layer) of a magnetic recording medium obtained by forming a magnetic recording layer on the non-magnetic substrata was measured. The thickness of the non-magnetic undercoat layer is expressed by (B) - (A), and the thickness of the magnetic recording layer is expressed by (C) - (B). Example 1: <Production of spindle-shaped hematite particles> 1200 g of spindle-shaped goethite particles obtained by the above production method (B) of goethite particles using a ferrous sulfate aqueous solution and a sodium carbonate aqueous solution (average major axis diameter: 0.171 µm, average minor axis diameter: 0.0213 µm, aspect ratio: 8.0:1, geometrical standard deviation of major-axis diameter: 1.34, geometrical standard deviation of minor axis diameter: 1.39, BET specific surface area: 151.6 m 2 /g, content of soluble sodium salts: 701 ppm (calculated as Na), content of soluble sulfate: 422 ppm (calculated as SO 4 ), pH value: 6.7) were suspended in water so as to obtain a slurry, and the concentration of the solid content was adjusted to 8 g/liter. 150 liter of the slurry was heated to 60°C and the pH value of the slurry was adjusted to 10.0 by adding a 0.1-N aqueous NaOH solution. To the alkali slurry was gradually added 36 g of #3 water glass as a sintering preventive, and after the end of addition, the resultant mixture was aged for 60 minutes. The pH value of the slurry was then adjusted to 6.0 by adding a 0.1-N acetic acid solution. Thereafter, the particles were filtered out, washed with water, dried and pulverized by an ordinary method, thereby producing spindle-shaped goethite particles coated with an oxide of silicon. The silicon content was 0.78 % by weight (calculated as SiO 2 ). 1000 g of the spindle-shaped goethite particles obtained were charged into a stainless steel rotary furnace, and heat-dehydrated in the air at 350°C for 40 minutes while rotating the furnace, to obtain low-density spindle-shaped hematite particles. The thus obtained low-density spindle-shaped hematite particles had an average major axis diameter of 0.145 µm, an average minor axis diameter of 0.0193 µm, an aspect ratio of 7.5:1, a geometrical standard deviation of major axis diameter of 1.34, a geometrical standard deviation of minor axis diameter of 1.39, a BET specific surface area of 176.5 m 2 /g and a S BET /S TEM value of 4.15. The content of soluble sodium salt of the low-density spindle-shaped hematite particles was 1717 ppm (calculated as Na) and the content of soluble sulfate was 1011 ppm (calculated as SO 4 ). The pH value of the low-density spindle-shaped hematite particles was 6.3. 850 g of the low-density spindle-shaped hematite particles were then charged into a ceramic rotary furnace, and heat-treated in the air at 610°C for 30 minutes while rotating the furnace so as to fill in dehydration pores. The resultant high-density spindle-shaped hematite particles had an average major axis diameter of 0.137 µm, an average minor axis diameter of 0.0190 µm, an aspect ratio of 7.2:1, a geometrical standard deviation of major axis diameter of 1.35, a geometrical standard deviation of minor axis diameter of 1.40, a BET specific surface area (S BET ) of 55.3 m 2 /g and a S BET /S TEM value of 1.28. The silicon content was 0.85 % by weight (calculated as SiO 2 ). The content of soluble sodium salt of the high-density spindle-shaped hematite particles was 2626 ppm (calculated as Na) and the content of soluble sulfate was 3104 ppm (calculated as SO 4 ). The pH value of the high-density spindle-shaped hematite particles was 5.3 After 800 g of the high-density spindle-shaped hematite particles obtained were roughly pulverized by a Nara mill in advance, the obtained high-density spindle-shaped hematite particles were charged into 4.7 liter of pure water and deagglomerated by a homomixer (manufactured by Tokushu-kika Kogyo, CO., Ltd.) for 60 minutes. The slurry of the high-density spindle-shaped hematite particles obtained was then dispersed for 3 hours at an axial rotation frequency of 2000 rpm while being circulated by a horizontal SGM (Dispermat SL, manufactured by S.C. Adichem, Co., Ltd.). The high-density spindle-shaped hematite particles in the slurry remaining on a sieve of 325 meshes (mesh size: 44 µm) was 0 % by weight. <Dissolving treatment with acid> The slurry of the high-density spindle-shaped hematite particles obtained was mixed with water, thereby adjusting the concentration of the slurry to 100 g/liter. A 70 % aqueous sulfuric acid solution was added to 7 liter of the slurry under stirring so as to adjust the sulfuric acid concentration to 1.3 N and the pH value to 0.65. The slurry was then heated to 80°C under stirring, and was held for 5 hours at 80°C, thereby dissolving 29.7 % by weight of the spindle-shaped hematite particles based on the total weight of the spindle-shaped hematite in the slurry. The slurry was filtered to separate a filtrate (aqueous acid solution of iron sulfate) therefrom. The slurry from which the filtrate had been separated, was then washed with water by a decantation method and the pH value of the slurry was adjusted to 5.0. When the concentration of the slurry at this point was checked so as to ensure the accuracy, it was 68 g/liter. 2 liter of the obtained slurry was filtered through a Buchner filter, and pure water was passed until the electric conductivity of the filtrate became not more than 30 µs. The high-density spindle-shaped hematite particles were then dried by an ordinary method and pulverized so as to obtain the high-density spindle-shaped hematite particles. The high-density spindle-shaped hematite particles obtained had an average major axis diameter of 0.131 µm, an average minor axis diameter of 0.0181 µm, an aspect ratio of 7.2:1, a geometric standard deviation of major axis diameter of 1.35, a geometric standard deviation of major axis diameter of 1.33, a BET specific surface area (S BET ) of 58.1 m 2 /g and a S BET /S TEM value of 1.28. The content of soluble sodium salt of the high-density spindle-shaped hematite particles was 138 ppm (calculated as Na) and the content of soluble sulfate was 436 ppm (calculated as SO 4 ). The pH value of the high-density spindle-shaped hematite particles was 4.8. <Treatment of spindle-shaped hematite particles in an aqueous alkali solution> The concentration of the high-density spindle-shaped hematite particles in the slurry was adjusted to 50 g/litre, and 5 litre of the slurry was adjusted to pH value 13.6 by adding 6N-aqueous NaOH solution. The resulting slurry was then heated to 95°C under stirring, and was held for 3 hours at 95°C. The resultant slurry was then washed with water by a decantation method and the pH value of the slurry was adjusted to 10.5. The concentration of the slurry at this point was 98 g/litre. The high-density spindle-shaped hematite particles were filtered out from 1 liter of the obtained slurry through a Buchner filter, and the purified water was passed into the filtrate until the electric conductivity of the filtrate became not more than 30 µs. The high-density spindle-shaped hematite particles were then dried by an ordinary method and pulverized to obtain the target high-density spindle-shaped hematite particles. The high-density spindle-shaped hematite particles obtained had an average major axial diameter of not more than 0.131 µm, a minor axial diameter of 0.0181 µm, and a specific ratio of 7.2:1. The geometric standard deviation of major axial diameter was 1.35, the geometric standard deviation of minor axial diameter was 1.33, the BET specific surface (S BET ) was 57.8 m 2 /g, the S BET /S TEM value was 1.27. The content of soluble sodium salt of the high-density spindle-shaped hematite particles was 99 ppm (calculated as Na) and the content of soluble sulfate was 21 ppm (calculated as SO 4 ). The pH value of the high-density spindle-shaped hematite particles was 9.3. Example 2 <Production of non-magnetic substrate: Formation of non-magnetic undercoat layer on base film> 12 g of the high-density spindle-shaped hematite particles obtained in Example 1 were mixed with a binder resin solution (30 % by weight of vinyl chloride-vinyl acetate copolymer resin having a sodium sulfonate group and 70 % by weight of cyclohexanone) and cyclohexanone, and each of the obtained mixtures (solid content: 72 % by weight) was kneaded by a plast-mill for 30 minutes. Each of the thus-obtained kneaded material was charged into a 140 ml-glass bottle together with 95 g of 1.5 mm glass beads, a binder resin solution (30 % by weight of polyurethane resin having a sodium sulfonate group and 70 % by weight of a solvent (methyl ethyl ketone : toluene = 1 : 1)), cyclohexanone, methyl ethyl ketone and toluene, and the obtained mixture was mixed and dispersed by a paint shaker for 6 hours to obtain a non-magnetic coating composition. The viscosity of the obtained coating film composition was 367 cP. The thus-obtained non-magnetic coating composition containing the high-density spindle-shaped hematite particles was as follows: High-density spindle-shaped hematite particles 100 parts by weight Vinyl chloride-vinyl acetate copolymer resin having a sodium sulfonate group 10 parts by weight Polyurethane resin having a sodium sulfonate group 10 parts by weight Cyclohexanone 44.6 parts by weight Methylethyl ketone 111.4 parts by weight Toluene 66.9 parts by weight The non-magnetic coating composition obtained was applied to a polyethylene terephthalate film of 12 µm thick to a thickness of 55 µm by an applicator, and the coating film was then dried, thereby forming a non-magnetic undercoat layer. The thickness of the non-magnetic undercoat layer was 3.5 µm. The non-magnetic undercoat layer produced from the high-density spindle-shaped hematite particles as the non-magnetic particles had a gloss of 204 %, and a surface roughness Ra of 6.6 nm. The Young's modulus (relative value) thereof was 124. Example 3 <Production of magnetic recording medium: Formation of magnetic recording layer> 12 g of magnetic acicular metal particles containing iron as a main component (average major axis diameter: 0.120 µm, average minor axis diameter: 0.0154 µm, a geometrical standard deviation of major axis diameter of 1.37, aspect ratio: 7.8:1, coercive force value: 1896 Oe, saturation magnetization value: 133.8 emu/g, pH value: 9.7), 1.2 g of a polishing agent (AKP-30: trade name, produced by Sumitomo Chemical Co., Ltd.), 0.12 g of carbon black (#3250B, trade name, produced by Mitsubishi Chemical Corp.), a binder resin solution (30 % by weight of vinyl chloride-vinyl acetate copolymer resin having a sodium sulfonate group and 70 % by weight of cyclohexanone) and cyclohexanone were mixed to obtain a mixture (solid content: 78 % by weight). The mixture was further kneaded by a plast-mill for 30 minutes to obtain a kneaded material. The thus-obtained kneaded material was charged into a 140 ml-glass bottle together with 95 g of 1.5 mm glass beads, a binder resin solution (30 % by weight of polyurethane resin having a sodium sulfonate group and 70 % by weight of a solvent (methyl ethyl ketone : toluene = 1 : 1)), cyclohexanone, methyl ethyl ketone and toluene, and the mixture was mixed and dispersed by a paint shaker for 6 hours. Then, the lubricant and hardening agent were added to the mixture, and the resultant mixture was mixed and dispersed by a paint shaker for 15 minutes. The thus-obtained magnetic coating composition was as follows: Magnetic acicular metal particles containing iron as a main component 100 parts by weight Vinyl chloride-vinyl acetate copolymer resin having a sodium sulfonate group 10 parts by weight Polyurethane resin having a sodium sulfonate group 10 parts by weight Polishing agent (AKP-30) 10 parts by weight Carbon black (#3250B) 3.0 parts by weight Lubricant (myristic acid: butyl stearate = 1 : 2) 3.0 parts by weight Hardening agent (polyisocyanate) 5.0 parts by weight Cyclohexanone 65.8 parts by weight Methyl ethyl ketone 164.5 parts by weight Toluene 98.7 parts by weight The magnetic coating composition obtained was applied to the non-magnetic undercoat layer to a thickness of 15 µm by an applicator, and the magnetic recording medium obtained was oriented and dried in a magnetic field, and then calendered. The magnetic recording medium was then subjected to a curing reaction at 60°C for 24 hours, and thereafter slit into a width of 0.5 inch, thereby obtaining a magnetic tape. The thickness of the respective magnetic recording layer was 1.1 µm. The coercive force Hc of the magnetic tape produced by forming a magnetic recording layer on the non-magnetic undercoat layer was 1986 Oe, the squareness (Br/Bm) thereof was 0.87, the gloss thereof was 220 %, the surface roughness Ra thereof was 6.2 nm, the Young's modulus (relative value) thereof was 128, the linear absorption coefficient thereof was 1.24 µm -1 . The percentages of change in coercive force and saturation magnetization which represent an anti-corrosion property of the magnetic tape, were 4.3 % and 4.6 %, respectively. Example 4: <Production of magnetic recording medium: Formation of magnetic recording layer> The same procedure as defined in Example 3 was conducted except that the magnetic particles were changed to magnetic acicular metal particles (average major axis diameter: 0.115 µm, average minor axis diameter: 0.0145 µm, a geometric standard deviation of major axis diameter of 1.36, aspect ratio: 7.9:1, coercive force: 1909 Oe, saturation magnetization: 133.8 emu/g, aluminum content of 2.85 % by weight, calculated as Al, (1.26 % by weight, calculated as Al, of aluminum in the central portion; 0.84 % by weight, calculated as Al, of aluminum in the surface layer portion, and 0.75 % by weight, calculated as Al, of aluminum on the surface coating), Nd content of 0.12 % by weight, resin adsorptivity: 81.6 %), thereby producing a magnetic tape. The thickness of the magnetic coating film was 1.0 µm. The thus obtained magnetic tape had a coercive force Hc of 1980 Oe, a squareness (Br/Bm) of 0.88, a gloss of 223 %, a surface roughness Ra of 6.4 nm, a Young's modulus (relative value) of coating film of 128, a linear absorption coefficient of 1.24 µm -1 , a running durability of 28.9 minutes and a scratch resistance of A. The percentages of change in coercive force and saturation magnetization which represent an anti-corrosion property of the magnetic tape, were 4.6 % and 2.7 %, respectively. Example 5 <Production of spindle-shaped hematite particles> 1200 g of spindle-shaped goethite particles obtained by the above production method (B) of goethite particles using a ferrous sulfate aqueous solution, a sodium carbonate aqueous solution and aluminum sulfate aqueous solution, and containing aluminum in an amount of 1.12 % by weight (calculated as Al) based on the total weight of the particles, uniformly within the particles (average major axis diameter: 0.167 µm, average minor axis diameter: 0.0196 µm, aspect ratio: 8.5:1, geometrical standard deviation of major axis diameter: 1.32, geometrical standard deviation of minor axis diameter: 1.40, BET specific surface area: 165.3 m 2 /g, content of soluble sodium salts: 1821 ppm (calculated as Na), content of soluble sulfate: 2162 ppm (calculated as SO 4 ), pH value: 6.8) were suspended in water so as to obtain a slurry, and the concentration of the solid content was adjusted to 8 g/liter. 150 liter of the slurry was heated to 60°C and the pH value of the slurry was adjusted to 10.0 by adding a 0.1-N aqueous NaOH solution. To the alkali slurry was gradually added 24 g of #3 water glass as a sintering preventive, and after the end of addition, the resultant mixture was aged for 60 minutes. The pH value of the slurry was then adjusted to 6.0 by adding a 0.1-N acetic acid solution. Thereafter, the particles were filtered out, washed with water, dried and pulverized by an ordinary method, thereby producing spindle-shaped goethite particles which substantially uniformly contain aluminum within the particle, coated with an oxide of silicon. The silicon content was 0.52 % by weight (calculated as SiO 2 ) based on the total weight of the particles. 1000 g of the spindle-shaped goethite particles which substantially uniformly contain aluminum within the particle, obtained were charged into a stainless steel rotary furnace, and heat-dehydrated in the air at 350°C for 40 minutes while rotating the furnace, to obtain low-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle. The thus obtained low-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, had an average major axial diameter of 0.143 µm, an average minor axial diameter of 0.0191 µm, a geometric standard deviation of major axis diameter of 1.32, a geometric standard deviation of minor axis diameter of 1.40, an aspect ratio of 7.5:1, a BET specific surface area (S BET ) of 188.9 m 2 /g and a S BET /S TEM value of 4.40. The low-density spindle-shaped hematite particles contained soluble sodium salts of 1682 ppm (calculated as Na) and soluble sulfates of 976 ppm (calculated as SO 4 ). The aluminum content was 1.23 % by weight (calculated as Al), the pH value of the low-density spindle-shaped hematite particles was 6.1. The SiO 2 content thereof was 0.57 % by weight (calculated as SiO 2 ) based on the total weight of the particles. 850 g of the low-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, were then charged into a ceramic rotary furnace, and heat-treated in the air at 650°C for 30 minutes while rotating the furnace so as to fill in dehydration pores. The resultant high-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, had an average major axial diameter of 0.141 µm, an average minor axial diameter of 0.0192 µm, a geometric standard deviation of major axis diameter of 1.33, a geometric standard deviation of minor axis diameter of 1.41, an aspect ratio of 7.3:1, a BET specific surface area (S BET ) of 56.1 m 2 /g and a S BET /S TEM value of 1.31. The silicon content was 0.57 % by weight (calculated as SiO 2 ) and the aluminum content was 1.23 % by weight (calculated as Al). The content of soluble sodium salt of the high-density spindle-shaped hematite particles was 2538 ppm (calculated as Na) and the content of soluble sulfate was 2859 ppm (calculated as SO 4 ). The pH thereof was 5.6. After 800 g of the high-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, obtained were roughly pulverized by a Nara mill in advance, the obtained high-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, were charged into 4.7 liter of pure water and deagglomerated by a homomixer (manufactured by Tokushu-kika Kogyo, CO., Ltd.) for 60 minutes. The slurry of the high-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, obtained was then dispersed for 3 hours at an axial rotation frequency of 2000 rpm while being circulated by a horizontal SGM (Dispermat SL, manufactured by S.C. Adichem, CO., Ltd.). The high-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, in the slurry remaining on a sieve of 325 meshes (mesh size: 44 µm) was 0 % by weight. <Dissolving treatment with acid> The slurry of the high-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, obtained was mixed with water, thereby adjusting the concentration of the slurry to 100 g/liter. A 70 % aqueous sulfuric acid solution was added to 7 liter of the slurry under stirring so as to adjust the sulfuric acid concentration to 1.3N and the pH value to 0.58. The slurry was then heated to 80°C under stirring, and was held for 5 hours at 80°C, thereby dissolving 29.5 % by weight of the high density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, based on the total weight of the high-density spindle-shaped hematite particles in the slurry. The slurry was filtered to separate a filtrate (aqueous acid solution of iron sulfate) therefrom. The slurry from which the filtrate had been separated, was then washed with water by a decantation method and the pH value of the slurry was adjusted to 5.0. When the concentration of the slurry at this point was checked so as to ensure the accuracy, it was 68 g/liter. A part of the water-washed slurry obtained was separated and filtered through a Buchner filter, and pure water was passed until the electric conductivity of the filtrate became not more than 30 µs. The resultant particles were then dried by an ordinary method and pulverized so as to obtain the high-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle. The high-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, obtained had an average major axis diameter of 0.133 µm, an average minor axis diameter of 0.0182 µm, an aspect ratio of 7.3:1, a geometric standard deviation of major axis diameter of 1.34, a geometric standard deviation of minor axis diameter of 1.33, a BET specific surface area (S BET ) of 60.3 m 2 /g, a S BET /S TEM value of 1.34, an aluminum content of 1.23 % by weight (calculated as Al). The content of soluble sodium salt of the high-density spindle-shaped hematite particles was 131 ppm (calculated as Na) and the content of soluble sulfate was 424 ppm (calculated as SO 4 ). The pH value of the high-density spindle-shaped hematite particles was 4.9. <Treatment of high-density spindle-shaped hematite particles in an aqueous alkali solution> The concentration of the high-density spindle-shaped hematite particles in the slurry was adjusted to 50 g/litre, and 5 litre of the slurry was adjusted to pH value 13.6 by adding a 6N-aqueous NaOH solution. The resulting slurry was then heated to 95°C under stirring, and was held for 3 hours at 95°C. The resultant slurry was then washed with water by a decantation method and the pH value of the slurry was adjusted to 10.5. The concentration of the slurry at this point was 98 g/litre. The particles were filtered out from the obtained slurry of 1 liter through a Buchner filter, and the purified water was passed into the filtrate until the electric conductivity of the filtrate became not more than 30 µs. The particles were then dried by an ordinary method and pulverized to obtain the target high-density spindle-shaped hematite particles. The high-density spindle-shaped hematite particles obtained had an average major axial diameter of not more than 0.133 µm, a minor axial diameter of 0.0182 µm, and a specific ratio of 7.3:1. The geometric standard deviation of major axial diameter was 1.33, the geometric standard deviation of minor axial diameter was 1.34, the BET specific surface (S BET ) was 60.1 m 2 /g, the S BET /S TEM value was 1.33. The aluminum content was 1.23 % by weight (calculated as Al). The content of soluble sodium salt of the high-density spindle-shaped hematite particles was 97 ppm (calculated as Na) and the content of soluble sulfate was 20 ppm (calculated as SO 4 ). The pH value of the high-density spindle-shaped hematite particles was 9.4, and the resin adsorptivity thereof was 72.5 %. Example 6: <Production of non-magnetic substrate: Formation of non-magnetic undercoat layer on base film> The same procedure as defined in Example 2 was conducted except that the spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, obtained in Example 5 were used instead of the spindle-shaped hematite particles, thereby obtaining a non-magnetic undercoat layer. The thickness of the non-magnetic undercoat layer was 3.5 µm. The thus obtained non-magnetic undercoat layer had a gloss of 212 %, a surface roughness Ra of 6.1 nm, and a Young's modulus (relative value) of coating film of 127. Example 7: <Production of magnetic recording medium: Formation of magnetic recording layer> The same procedure as defined in Example 3 was conducted except that magnetic acicular metal particles containing iron as a main component and further containing aluminum in amounts of 1.24 % by weight (calculated as Al) at a central portion of particle; 0.83 % by weight (calculated as Al) at a surface portion thereof; and 0.94 % by weight (calculated as Al) in a coating layer formed on the surface of particle (average major axis diameter: 0.112 µm, average minor axis diameter: 0.0147 µm, geometrical standard deviation of major axis diameter: 1.36, aspect ratio: 7.6:1, coercive force: 1908 Oe, saturation magnetization: 136.3 emu/g, resin adsorptivity: 82.0 %) were used, thereby obtaining a magnetic coating composition. The thus obtained magnetic coating composition was applied onto the non-magnetic undercoat-layer obtained in Example 6 in the same manner as in Example 3, thereby forming a magnetic recording layer and producing a magnetic tape. The thickness of the magnetic coating film was 1.1 µm. The thus obtained magnetic tape had a coercive force Hc of 1983 Oe, a squareness (Br/Bm) of 0.87, a gloss of 234 %, a surface roughness Ra of 6.1 nm, a Young's modulus (relative value) of coating film of 131, a linear absorption coefficient of 1.24 µm -1 , a running durability of 29.2 minutes, and a scratch resistance of A. Changes in the coercive force and the saturation magnetic flux density Bm with passage time were 4.5 % and 2.6 %, respectively. Example 8: <Production of spindle-shaped hematite particles> 1200 g of spindle-shaped goethite particles obtained by the above production method (B) of goethite particles using a ferrous sulfate aqueous solution and a sodium carbonate aqueous solution (average major axis diameter: 0.0812 µm, geometrical standard deviation of major axis diameter: 1.53, average minor axis diameter: 0.0110 µm, geometrical standard deviation of minor axis diameter: 1.37, aspect ratio: 7.4:1, BET specific surface area: 168.9 m 2 /g, soluble sodium salt content: 1212 ppm (calculated as Na), soluble sulfate content: 1816 ppm (calculated as SO 4 ), pH value: 6.8) were suspended in water so as to obtain a slurry, and the concentration of the solid content was adjusted to 8 g/liter. 150 liter of the slurry was heated to 60°C and the pH value of the slurry was adjusted to 10.0 by adding a 0.1-N aqueous NaOH solution. To the alkali slurry was gradually added 36.0 g of #3 water glass as a sintering preventive, and after the end of addition, the resultant mixture was aged for 60 minutes. The pH value of the slurry was then adjusted to 6.0 by adding a 0.1-N acetic acid solution. Thereafter, the particles were filtered out, washed with water, dried and pulverized by an ordinary method, thereby producing spindle-shaped goethite particles coated with an oxide of silicon. The silicon content was 0.78 % by weight (calculated as SiO 2 ). The spindle-shaped goethite particles obtained were charged into a heat-treatment metal furnace, and heat-treated therein at 150°C for 30 minutes, thereby absorbing superfine goethite particles present within the spindle-shaped goethite particles, into the spindle-shaped goethite particles. The thus obtained spindle-shaped goethite particles were charged again into the heat treatment metal furnace, and heat-dehydrated therein at 320°C for 30 minutes, thereby obtaining low-density spindle-shaped hematite particles. The thus obtained low-density spindle-shaped hematite particles had an average major axis diameter of 0.0736 µm, a geometrical standard deviation of major axis diameter of 1.38, an average minor axis diameter of 0.0118 µm, a geometrical standard deviation of minor axis diameter of 1.16, an aspect ratio of 6.1:1, a BET specific surface area of 190.3 m 2 /g, a S BET /S TEM value of 2.70. The silicon content was 0.78 % by weight (calculated as SiO 2 ). The content of soluble sodium salt of the low-density spindle-shaped hematite particles was 1826 ppm (calculated as Na) and the content of soluble sulfate was 2512 ppm (calculated as SO 4 ). The pH value of the low-density spindle-shaped hematite particles was 6.1. 850 g of the low-density spindle-shaped hematite particles were then charged into a ceramic rotary furnace, and heat-treated in the air at 650°C for 30 minutes while rotating the furnace so as to fill in dehydration pores. The resultant high-density spindle-shaped hematite particles had an average major axis diameter of 0.0727 µm, a geometrical standard deviation of major axis diameter of 1.38, an average minor axis diameter of 0.0120 µm, a geometrical standard deviation of minor axis diameter of 1.17, an aspect ratio of 6.1:1, a BET specific surface area (S BET ) of 86.8 m 2 /g, a S BET /S TEM value of 1.25. The silicon content was 0.87 % by weight (calculated as SiO 2 ). The content of soluble sodium salt of the high-density spindle-shaped hematite particles was 2121 ppm (calculated as Na) and the content of soluble sulfate was 2832 ppm (calculated as SO 4 ). The pH value of the high-density spindle-shaped hematite particles was 5.8. <Treatment of high-density spindle-shaped hematite particles in an aqueous alkali solution> The concentration of the high-density spindle-shaped hematite particles in the slurry was adjusted to 50 g/litre, and 5 litre of the slurry was adjusted to pH value 13.4 by adding a 6N-aqueous NaOH solution. The resulting slurry was then heated to 95°C under stirring, and was held for 3 hours at 95°C. The resultant slurry was then washed with water by a decantation method and the pH value of the slurry was adjusted to 10.5. The concentration of the slurry at this point was 98 g/litre. The particles were filtered out through a Buchner filter, and the purified water was passed into the filtrate until the electric conductivity of the filtrate became not more than 30 µs. The particles were then dried by an ordinary method and 'pulverized to obtain the target high-density spindle-shaped hematite particles. The high-density spindle-shaped hematite particles obtained had an average major axial diameter of 0.0726 µm, a minor axial diameter of 0.0120 µm, and a specific ratio of 6.1. The geometric standard deviation of major axial diameter was 1.38, the geometric standard deviation of minor axial diameter was 1.17, the BET specific surface (S BET ) was 86.2 m 2 /g, the S BET /S TEM value was 1.24. The content of soluble sodium salt of the high-density spindle-shaped hematite particles was 86 ppm (calculated as Na) and the content of soluble sulfate was 32 ppm (calculated as SO 4 ). The pH value of the high-density spindle-shaped hematite particles was 9.0, and the resin adsorptivity thereof was 75.1 %. Example 9: <Production of non-magnetic substrate: Formation of non-magnetic undercoat layer on base film> The same procedure as defined in Example 2 was conducted except that the high-density spindle-shaped hematite particles obtained in Example 8 were used instead of the spindle-shaped hematite particles, thereby obtaining a non-magnetic undercoat layer. The thickness of the non-magnetic undercoat layer was 3.4 µm. The thus obtained non-magnetic undercoat layer had a gloss of 208 %, a surface roughness Ra of 6.4 nm, and a Young's modulus (relative value) of coating film of 134. Example 10: <Production of magnetic recording medium: Formation of magnetic recording layer> The same procedure as defined in Example 3 was conducted except that the magnetic particles were changed to magnetic acicular metal particles containing iron as a main component (average major axis diameter: 0.103 µm, average minor axis diameter: 0.0152 µm, a geometric standard deviation of major axis diameter of 1.38, aspect ratio: 6.8:1, coercive force: 1910 Oe, saturation magnetization: 136 emu/g), and that the non-magnetic undercoat layer obtained in Example 9 was used, thereby producing a magnetic tape. The thickness of the magnetic coating film was 1.0 µm. The thus obtained magnetic tape had a coercive force Hc of 1991 Oe, a squareness (Br/Bm) of 0.88, a gloss of 235 %, a surface roughness Ra of 5.8 nm, a Young's modulus (relative value) of coating film of 137, a linear absorption coefficient of 1.26 µm -1 , a running durability of 298 minutes and a scratch resistance of A. Changes in the coercive force and the saturation magnetic flux density Bm with passage time were 4.6 % and 3.8 %, respectively. Example 11: <Production of magnetic recording medium: Formation of magnetic recording layer> The same procedure as defined in Example 3 was conducted except that the magnetic particles were changed to magnetic acicular metal particles (average major axis diameter: 0.115 µm, average minor axis diameter: 0.0145 µm, a geometric standard deviation of major axis diameter of 1.36, aspect ratio: 7.9:1, coercive force: 1909 Oe, saturation magnetization: 133.8 emu/g, aluminum content of 2.85 % by weight, calculated as Al, (1.26 % by weight, calculated as Al, of aluminum in the central portion, 0.84 % by weight, calculated as Al, of aluminum in the surface layer portion, and 0.75 % by weight, calculated as Al, of aluminum on the surface coating), Nd content of 0.12 % by weight, resin adsorptivity: 81.6 %), and that the non-magnetic undercoat layer obtained in Example 9 was used, thereby producing a magnetic tape.. The thickness of the magnetic coating film was 1.0 µm. The thus obtained magnetic tape had a coercive force Hc of 1985 Oe, a squareness (Br/Bm) of 0.89, a gloss of 238 %, a surface roughness Ra of 5.6 nm, a Young's modulus (relative value) of coating film of 138, a linear absorption coefficient of 1.27 µm -1 , a running durability of not less than 30 minutes and a scratch resistance of A. Changes in the coercive force and the saturation magnetic flux density Bm with passage time were 2.8 % and 2.6 %, respectively. Example 12: <Production of spindle-shaped hematite particles> 1200 g of spindle-shaped goethite particles obtained by the above production method (B) of goethite particles using a ferrous sulfate aqueous solution, a sodium carbonate aqueous solution and aluminum sulfate aqueous solution (average major axis diameter: 0.0846 µm, geometrical standard deviation of major axis diameter: 1.49, average minor axis diameter: 0.0115 µm, geometrical standard deviation of minor axis diameter: 1.38, aspect ratio: 7.4:1, BET specific surface area: 161.6 m 2 /g, aluminum content: 2.12 % by weight (calculated as Al), soluble sodium salt content: 1168 ppm (calculated as Na), soluble sulfate content: 1721 ppm (calculated as SO 4 ), pH value: 6.0) were suspended in water so as to obtain a slurry, and the concentration of the solid content was adjusted to 8 g/liter. 150 liter of the slurry was heated to 60°C and the pH value of the slurry was adjusted to 10.0 by adding a 0.1-N aqueous NaOH solution. To the alkali slurry was gradually added 42.0 g of #3 water glass as a sintering preventive, and after the end of addition, the resultant mixture was aged for 60 minutes. The pH value of the slurry was then adjusted to 6.0 by adding a 0.1-N acetic acid solution. Thereafter, the particles were filtered out, washed with water, dried and pulverized by an ordinary method, thereby producing spindle-shaped goethite particles which substantially uniformly contain aluminum within the particle, coated with an oxide of silicon. The silicon content was 0.90 % by weight (calculated as SiO 2 ). The spindle-shaped goethite particles which substantially uniformly contain aluminum within the particle, obtained were charged into a heat treatment metal furnace, and heat-treated therein at 140°C for 30 minutes, thereby absorbing superfine goethite particles present within the spindle-shaped goethite particles, into the spindle-shaped goethite particles.. The thus obtained spindle-shaped goethite particles which substantially uniformly contain aluminum within the particle, were charged again into the heat treatment metal furnace, and heat-dehydrated therein at 340°C for 30 minutes, thereby obtaining low-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle. The thus obtained low-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, had an average major axis diameter of 0.0793 µm, a geometrical standard deviation of major axis diameter of 1.37, an average minor axis diameter of 0.0119 µm, a geometrical standard deviation of minor axis diameter of 1.23, an aspect ratio of 6.7:1, a BET specific surface area (S BET ) of 181.0 m 2 /g, a S BET /S TEM value of 2.60. The silicon content was 0.99 % by weight (calculated as SiO 2 ) and the aluminum content was 2.35 % by weight (calculated as Al). The content of soluble sodium salt of the low-density spindle-shaped hematite particles was 1465 ppm (calculated as Na) and the content of soluble sulfate was 1965 ppm (calculated as SO 4 ). The pH value of the low-density spindle-shaped hematite particles was 5.9. 850 g of the low-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, were then charged into a ceramic rotary furnace, and heat-treated in the air at 650°C for 30 minutes while rotating the furnace so as to fill in dehydration pores. The resultant high-density spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, had an average major axis diameter of 0.0753 µm, a geometrical standard deviation of major axis diameter of 1.37, an average minor axis diameter of 0.0122 µm, a geometrical standard deviation of minor axis diameter of 1.24, an aspect ratio of 6.2:1, a BET specific surface area (S BET ) of 83.8 m 2 /g, a S BET /S TEM value of 1.23. The silicon content was 1.00 % by weight (calculated as SiO 2 ) and the aluminum content was 2.35 % by weight (calculated as Al). The silicon content was 1.00 % by weight (calculated as SiO 2 ). The content of soluble sodium salt of the high-density spindle-shaped hematite particles was 1612 ppm (calculated as Na) and the content of soluble sulfate was 2101 ppm (calculated as SO 4 ). The pH value of the high-density spindle-shaped hematite particles was 5.7. <Treatment of high-density spindle-shaped hematite particles in an aqueous alkali solution> The concentration of the high-density spindle-shaped hematite particles in the slurry was adjusted to 50 g/litre, and 5 litre of the slurry was adjusted to pH value 13.8 by adding a 6N-aqueous NaOH solution. The resulting slurry was then heated to 95°C under stirring, and was held for 3 hours at 95°C. The resultant slurry was then washed with water by a decantation method and the pH value of the slurry was adjusted to 10.5. The concentration of the slurry at this point was 98 g/litre. The particles were filtered out through a Buchner filter, and the purified water was passed into the filtrate until the electric conductivity of the filtrate became not more than 30 µs. The particles were then dried by an ordinary method and pulverized to obtain the target high-density spindle-shaped hematite particles. The high-density spindle-shaped hematite particles obtained had an average major axial diameter of 0.0752 µm, a minor axial diameter of 0.0122 µm, and a specific ratio of 6.2. The geometric standard deviation of major axial diameter was 1.37, the geometric standard deviation of minor axial diameter was 1.24, the BET specific surface (S BET ) was 83.4 m 2 /g, the S BET /S TEM value was 1.22. The content of soluble sodium salt of the high-density spindle-shaped hematite particles was 63 ppm (calculated as Na) and the content of soluble sulfate was 21 ppm (calculated as SO 4 ). The pH value of the high-density spindle-shaped hematite particles was 9.1, and the resin adsorptivity thereof was 79.6 %. Example 13: <Production of non-magnetic substrate: Formation of non-magnetic undercoat layer on base film> The same procedure as defined in Example 2 was conducted except that the spindle-shaped hematite particles which substantially uniformly contain aluminum within the particle, obtained in Example 12 were used instead of the spindle-shaped hematite particles, thereby obtaining a non-magnetic undercoat layer. The thickness of the magnetic coating film was 3.5 µm. The thus obtained non-magnetic undercoat layer had a gloss of 218 %, a surface roughness Ra of 6.2 nm, and a Young's modulus (relative value) of coating film of 135. Example 14: <Production of magnetic recording medium: Formation of magnetic recording layer> The same procedure as defined in Example 3 was conducted except that the magnetic particles were changed to acicular magnetic metal particles (average major axis diameter: 0.110 µm, average minor axis diameter: 0.0146 µm, a geometric standard deviation of major axis diameter of 1.38, aspect ratio: 7.5:1, coercive force: 1943 Oe, saturation magnetization: 132 emu/g), and that the non-magnetic undercoat layer obtained in Example 13 was used, thereby producing a magnetic tape. The thickness of the magnetic coating film was 1.0 µm. The thus obtained magnetic tape had a coercive force Hc of 1989 Oe, a squareness (Br/Bm) of 0.88, a gloss of 238 %, a surface roughness Ra of 5.8 nm, a Young's modulus (relative value) of coating film of 136, a linear absorption coefficient of 1.27 µm -1 , a running durability of not less than 30 minutes and a scratch resistance of A. Changes in the coercive force and the saturation magnetic flux density Bm with passage time were 2.8 % and 2.1 %, respectively. Example 15: <Production of magnetic recording medium: Formation of magnetic recording layer> The same procedure as defined in Example 3 was conducted except that the magnetic particles were changed to magnetic acicular metal particles (average major axis diameter: 0.115 µm, average minor axis diameter: 0.0145 µm, a geometric standard deviation of major axis diameter of 1.36, aspect ratio: 7.9:1, coercive force: 1909 Oe, saturation magnetization: 133.8 emu/g, aluminum content of 2.85 % by weight, calculated as Al, (1.26 % by weight, calculated as Al, of aluminum in the central portion, 0.84 % by weight, calculated as Al, of aluminum in the surface layer portion, and 0.75 % by weight, calculated as Al, of aluminum on the surface coating), Nd content of 0.12 % by weight, resin adsorptivity: 81.6 %), and that the non-magnetic undercoat layer obtained in Example 13 was used, thereby producing a magnetic tape. The thickness of the magnetic coating film was 1.0 µm. The thus obtained magnetic tape had a coercive force Hc of 1990 Oe, a squareness (Br/Bm) of 0.89, a gloss of 243 %, a surface roughness Ra of 5.4 nm, a Young's modulus (relative value) of coating film of 138, a linear absorption coefficient of 1.26 µm -1 , a running durability of not less than 30 minutes and a scratch resistance of A. Changes in the coercive force and the saturation magnetic flux density Bm with passage time were 1.8 % and 1.6 %, respectively. <Kinds of acicular goethite particles> Precursor 1 to 6: Various properties of acicular goethite particles as a precursor of acicular hematite particles are shown in Table 1. Examples 16 to 22 and Comparative Examples 1 to 6: <Production of low-density acicular hematite particles> Low-density acicular hematite particles were obtained in the same way as in Example 1 except for varying the kind of acicular goethite particles as a precursor, the kind and amount added of sintering preventive, and heat-dehydration temperature and time. Incidentally, the particles obtained in Comparative Example 4 were acicular goethite particles. The main producing conditions and various properties are shown in Tables 2 to 3. Examples 23 to 29 and Comparative Examples 7 to 11: <Production of high-density acicular hematite particles> High-density acicular hematite particles were obtained in the same way as in Example 1 except for varying the kind of low-density acicular hematite particles, and heat-treating temperature and time for high densification. The main producing conditions and various properties are shown in Tables 4 to 5. Examples 30 to 36 and Comparative Examples 12 to 13: <Acid-dissolving treatment of acicular hematite particles> Acicular hematite particles were obtained in the same way as in Example 1 except for varying the kind of high-density acicular hematite particles, use or non-use of the wet pulverization, the acid concentration, the pH value of slurry, and the heating temperature and time. The main producing conditions and various properties are shown in Tables 6 to 7. Examples 37 to 43 and Reference Examples 1 to 2: <Heat-treatment of acicular hematite particles in alkaline suspension> The same procedure as defined in Example 1 was conducted except that kind of acicular hematite particles, pH value of slurry, heating temperature and heating time were varied, thereby obtaining acicular hematite particles. Main production conditions and various properties are shown in Tables 8 and 9. Example 44: <Surface-coating treatment of acicular hematite particles> The concentration of the slurry having a pH value of 10.5 which was obtained in Example 37 by washing the slurry heat-treated in the alkaline suspension, with water by a decantation method, was 50 g/liter. 4 liters of the water-washed slurry was heated again to 60°C, and then mixed with 74.1 ml of 1.0N sodium aluminate aqueous solution (corresponding to 1.0 % by weight calculated as Al based on the weight of the acicular hematite particles). After maintaining the slurry at that temperature for 30 minutes, the pH value of the slurry was adjusted to 8.0 using an aqueous acetic acid solution. Next, the slurry was sequentially subjected to filtration, washing with water, drying and pulverization in the same manner as in Example 1, thereby obtaining acicular hematite particles whose surfaces were coated with a hydroxide of aluminum. Main production conditions and various properties are shown in Tables 10 and 11. Examples 45 to 50: Acicular hematite particles coated with a coating material by an ordinary method were obtained in the same way as in Example 44 except for varying the kind of acicular hematite particles and the kind and amount of the coating material. The main producing conditions and various properties are shown in Tables 10 and 11, respectively. Examples 51 to 64, Comparative Examples 14 to 22 and Reference Examples 3 to 11: <Production of non-magnetic substrate: Formation of non-magnetic undercoat layer on non-magnetic base film> By using the acicular hematite particles obtained in Example 30 to 50, Comparative Examples 1, 3 and 7 to 13 and Reference Examples 1 to 2, non-magnetic undercoat layers were formed in the same way as in Example 2. The main producing conditions and various properties are shown in Tables 12 and 13, respectively. <Production of magnetic recording medium: Formation of magnetic coating film> Magnetic particles used for forming the magnetic recording layers and various properties thereof are shown in Table 14. Examples 65 to 78, Comparative Examples 23 to 31 and Reference Examples 12 to 20: Magnetic recording media were produced in the same way as in Example 3 except for varying the kind of non-magnetic substrate and the kind of magnetic particles. The main producing conditions and various properties are shown in Tables 15 and 16. Examples 79 to 92, Comparative Examples 32 to 40 and Reference Examples 21 to 29: Magnetic recording media were produced in the same way as in Example 4 except for varying the kind of non-magnetic substrate and the kind of magnetic particles. The main producing conditions and various properties are shown in Tables 17 and 18. <Kinds of acicular goethite particles which substantially uniformly contain aluminum within the particle > Precursor 7 to 13: Various properties of acicular goethite particles as a precursor of acicular hematite particles are shown in Table 19. Examples 93 to 99 and Comparative Examples 41 to 46: <Production of low-density acicular hematite particles which substantially uniformly contain aluminum within the particle> Low-density acicular hematite particles were obtained in the same way as in Example 5 except for varying the kind of acicular goethite particles as a precursor, the kind and amount added of sintering preventive, and heat-dehydration temperature and time. Incidentally, the particles obtained in Comparative Example 44 were acicular goethite particles. The main producing conditions and various properties are shown in Tables 20 to 21. Examples 100 to 106 and Comparative Examples 47 to 51: <Production of high-density acicular hematite particles which substantially uniformly contain aluminum within the particle> High-density acicular hematite particles were obtained in the same way as in Example 5 except for varying the kind of low-density acicular hematite particles, and heat-treating temperature and time for high densification. The main producing conditions and various properties are shown in Tables 22 to 23. Examples 107 to 113 and Comparative Examples 52 to 53: <Acid-dissolving treatment of acicular hematite particles which substantially uniformly contain aluminum within the particle> Acicular hematite particles were obtained in the same way as in Example 5 except for varying the kind of high-density acicular hematite particles, use or non-use of the wet pulverization, the acid concentration, the pH value of slurry, and the heating temperature and time. The main producing conditions and various properties are shown in Tables 24 to 25. Examples 114 to 120 and Reference Examples 30 to 31: <Heat-treatment of acicular hematite particles in alkaline suspension> The same procedure as defined in Example 5 was conducted except that kind of acicular hematite particles, pH value of slurry, heating temperature and heating time were varied, thereby obtaining acicular hematite particles. Main production conditions and various properties are shown in Tables 26 and 27. Example 121 <Surface coating of acicular hematite particles which substantially uniformly contain aluminum within the particle> The concentration of the slurry having a pH value of 10.5 which was obtained in Example 114 by washing the slurry heat-treated in the alkaline suspension, with water by a decantation method, was 50 g/liter. 4 liters of the water-washed slurry was heated again to 60°C, and then mixed with 74.1 ml of 1.0N sodium aluminate aqueous solution (corresponding to 1.0 % by weight calculated as Al based on the weight of the acicular hematite particles). After maintaining the slurry at that temperature for 30 minutes, the pH value of the slurry was adjusted to 8.0 using an aqueous acetic acid solution. Next, the slurry was sequentially subjected to filtration, washing with water, drying and pulverization in the same manner as in Example 5, thereby obtaining acicular hematite particles whose surfaces were coated with a hydroxide of aluminum. Main production conditions and various properties are shown in Tables 28 and 29. Examples 122 to 127: Acicular hematite particles coated with a coating material by an ordinary method were obtained in the same way as in Example 121 except for varying the kind of acicular hematite particles and the kind and amount of the coating material. The main producing conditions and various properties are shown in Tables 28 and 29. Examples 128 to 141, Comparative Examples 54 to 62 and Reference Examples 32 to 40: <Production of non-magnetic substrate: Formation of non-magnetic undercoat layer on non-magnetic base film> By using the acicular hematite particles obtained in Example 107 to 127 and Comparative Examples 41, 43 and 47 to 53 and Reference Examples 30 to 31, non-magnetic undercoat layers were formed in the same way as in Example 2. The main producing conditions and various properties are shown in Tables 30 and 31. Examples 142 to 155, Comparative Examples 63 to 71 and Reference Examples 41 to 49: <Production of magnetic recording medium: Formation of magnetic coating film> Magnetic recording media were produced in the same way as in Example 3 except for varying the kind of non-magnetic substrate and the kind of magnetic particles. The main producing conditions and various properties are shown in Tables 32 and 33. <Kind of acicular goethite particles> Goethite particles 1 to 2: Acicular goethite particles 1 and 2 as precursors having properties shown in Table 34 were prepared. Goethite particles 3 to 5: Acicular goethite particles 3 to 5 which were subjected to a sintering preventing treatment, were obtained in the same way as in Example 8 except for varying the kind of starting particles and the element and amount of sintering preventive. Various properties of the obtained acicular goethite particles are shown in Table 35. <Heat treatment> Goethite particles 6 to 9: Acicular goethite particles 6 to 9 were obtained in the same way as in Example 8 except for varying the kind of acicular goethite particles as precursors and the heat-treating temperature and time. The main producing conditions are shown in Table 36 and various properties of the obtained acicular goethite particles are shown in Table 37. <Production of low-density acicular hematite particles> Hematite particles 1 to 4: Low-density acicular hematite particles 1 to 4 were obtained in the same way as in Example 8 except for varying the kind of acicular goethite particles as precursors and the heat-dehydrating temperature and time. The main producing conditions are shown in Table 38 and various properties of the obtained low-density acicular hematite particles are shown in Table 39. <Production of high-density acicular hematite particles> Hematite particles 5 to 13: High-density acicular hematite particles were obtained in the same way as in Example 8 except for varying the kind of particles to be treated and the heat-treating temperature and time. The main producing conditions are shown in Table 40 and various properties of the obtained high-density hematite particles are shown in Table 41. Examples 156 to 159 and Reference Examples 50 to 51: <Heat-treatment of acicular hematite particles in alkaline suspension> The same procedure as defined in Example 1 was conducted except that kind of acicular hematite particles, pH value of slurry, heating temperature and heating time were varied, thereby obtaining acicular hematite particles. Main production conditions and various properties are shown in Tables 42 and 43. Examples 160 : <Surface coating treatment of acicular hematite particles> After 700 g of the high-density acicular hematite particles obtained in the Example 156 were roughly pulverized by a Nara mill in advance, the obtained particles were charged into 7 liters of pure water and diagglomerated by a homomixer (manufactured by Tokushu-kika Kogyo, CO., Ltd.) for 60 minutes. The slurry of the acicular hematite particles obtained was then dispersed for 6 hours at an axial rotation frequency of 2000 rpm while being circulated by a horizontal sand grinder (Dispermat SL, manufactured by S.C. Adichem, CO., Ltd.). The pH value of the obtained slurry containing the acicular hematite particles was adjusted to 4.0 by using a 0.1 N acetic acid aqueous solution. By adding water to the slurry, the concentration of the resultant slurry was adjusted to 45 g/liter. 10 liter of the slurry was re-heated to 60°C, and 500 ml (equivalent to 3.0 % by weight (calculated as Al) based on the acicular hematite particles) of a 1.0 mol/liter aqueous aluminum acetate solution was added to the slurry, and the mixture was held for 30 minutes. Thereafter, the pH value of the mixture was adjusted to 7.1 by using aqueous sodium hydroxide solution. After holding the mixture for 30 minutes, the particles were then filtered out, washed with water, dried and, thereby obtaining acicular hematite particles coated with a hydroxide of aluminum. The main producing conditions and various properties are shown in Tables 44 and 45. Examples 161 to 163 Acicular hematite particles coated with a coating material were obtained in the same way as in Example 160 except for varying the kind of acicular hematite particles to be treated, the pH value before coating, the kind and amount of additives, and the final pH value. The main producing conditions are shown in Table 44 and various properties of the obtained acicular hematite particles are shown in Table 45. Examples 164 to 171, Comparative Examples 72 to 76 and Reference Examples 52 to 53: <Production of non-magnetic substrate: Formation of non-magnetic undercoat layer on non-magnetic base film> By using the acicular hematite particles 9 to 13 and the non-magnetic particles obtained in Example 156 to 163 and Reference Examples 50 to 51, non-magnetic undercoat layers were formed in the same way as in Example 2. The main producing conditions and various properties of the obtained non-magnetic undercoat layers are shown in Table 46. Examples 172 to 183, Comparative Examples 77 and 81 and Reference examples 54 to 55: <Production of magnetic recording medium: Formation of magnetic coating film> Magnetic recording media were produced in the same way as in Example 3 except for varying the kind of non-magnetic substrate and the kind of magnetic particles. The main producing conditions and various properties of the obtained magnetic recording media are shown in Table 47. <Kind of acicular goethite particles which substantially uniformly contain aluminum within the particle> Goethite particles 10 to 11: Acicular goethite particles 10 and 11 as precursors having properties shown in Table 48 were prepared. Goethite particles 12 to 14: Acicular goethite particles 12 to 14 which were subjected to a sintering preventing treatment, were obtained in the same way as in Example 12 except for varying the kind of acicular goethite particles as precursors and the element and amount of sintering preventive. Various properties of the obtained acicular goethite particles are shown in Table 49. <Heat treatment> Goethite particles 15 to 18: Acicular goethite particles 15 to 18 were obtained in the same way as in Example 12 except for varying the kind of starting particles and the heat-treating conditions. The main producing conditions are shown in Table 50 and various properties of the obtained acicular goethite particles are shown in Table 51. <Production of low-density acicular hematite particles which substantially uniformly contain aluminum within the particle> Hematite particles 14 to 17: Low-density acicular hematite particles 14 to 17 were obtained in the same way as in Example 12 except for varying the kind of acicular goethite particles as precursors and the heat-dehydrating temperature and time. The main producing conditions are shown in Table 52 and various properties of the obtained low-density acicular hematite particles are shown in Table 53. <Production of high-density acicular hematite particles which substantially uniformly contain aluminum within the particle > Hematite particles 18 to 26: High-density acicular hematite particles were obtained in the same way as in Example 12 except for varying the kind of particles to be treated and the heat-treating temperature and time. The main producing conditions are shown in Table 54 and various properties of the obtained high-density acicular hematite particles are shown in Table 55. Examples 184 to 187 and Reference Examples 56 to 457: <Heat-treatment of acicular hematite particles in alkaline suspension> The same procedure as defined in Example 12 was conducted except that. kind of acicular hematite particles, use or non-use of heat-treatment in alkaline suspension, pH value of slurry, heating temperature and heating time were varied, thereby obtaining acicular hematite particles. Main production conditions and various properties are shown in Tables 56 and 57. Example 188: <Surface coating treatment of acicular hematite particles> After 700 g of the high-density acicular hematite particles obtained in the Example 184 were roughly pulverized by a Nara mill in advance, the obtained particles were charged into 7 liters of pure water and diagglomerated by a homomixer (manufactured by Tokushu-kika Kogyo, CO., Ltd.) for 60 minutes. The slurry of the acicular hematite particles obtained was then dispersed for 6 hours at an axial rotation frequency of 2000 rpm while being circulated by a horizontal sand grinder (Dispermat SL, manufactured by S.C. Adichem, CO., Ltd.). The pH value of the obtained slurry containing the acicular hematite particles was adjusted to 4.0 by using a 0.1 N acetic acid aqueous solution. By adding water to the slurry, the concentration of the resultant slurry was adjusted to 45 g/liter. 10 liter of the slurry was re-heated to 60°C, and 500 ml (equivalent to 3.0 % by weight (calculated as Al) based on the acicular hematite particles) of a 1.0 mol/liter aqueous aluminum acetate solution was added to the slurry, and the mixture was held for 30 minutes. Thereafter, the pH value of the mixture was adjusted to 7.0 by using aqueous sodium hydroxide solution. After holding the mixture for 30 minutes, the particles were then filtered out, washed with water, dried and, thereby obtaining acicular hematite particles coated with a hydroxide of aluminum. The main producing conditions and various properties are shown in Tables 58 and 59. Examples 189 to 191: Acicular hematite particles coated with a coating material were obtained in the same way as in Example 188 except for varying the kind of particles to be treated, the pH value before coating, the kind and amount of additives, and the final pH value. The main producing conditions are shown in Table 58 and various properties of the obtained acicular hematite particles are shown in Table 59. Examples 192 to 199, Comparative Examples 82 to 986 and Reference examples 58 to 59: <Production of non-magnetic substrate: Formation of non-magnetic undercoat layer on non-magnetic base film> By using the acicular hematite particles 22 to 26 and the non-magnetic particles obtained in Example 184 to 191, and Reference Examples 56 to 57, non-magnetic undercoat layers were formed in the same way as in Example 2. The main producing conditions and various properties of the obtained non-magnetic undercoat layers are shown in Table 60. Examples 200 to 211, Comparative Examples 87 and 91 and Reference examples 60 to 61: <Production of magnetic recording medium: Formation of magnetic coating film> Magnetic recording media were produced in the same way as in Example 3 except for varying the kind of non-magnetic substrate and the kind of magnetic particles. The main producing conditions and various properties of the obtained magnetic recording media are shown in Table 61. Examples and Comparative Examples Kind of low-density acicular hematite particles Heat treatment for high densification Temperature (°C) Time (min) Example 23 Example 16 680 60 Example 24 Example 17 690 30 Example 25 Example 18 720 20 Example 26 Example 19 660 60 Example 27 Example 20 640 30 Example 28 Example 21 680 60 Example 29 Example 22 730 30 Comparative Example 7 Comparative Example 2 680 20 Comparative Example 8 Comparative Example 4 670 30 Comparative Example 9 Comparative Example 5 450 60 Comparative Example 10 Comparative Example 6 850 20 Comparative Example 11 Comparative Example 6 680 30 Examples and Reference Examples Kind of acicular hematite particles subjected to acid-dissolving treatment Heat treatment in aqueous alkali solution pH value (-) Temperature (°C) Time (min) Example 37 Example 30 13.5 90 180 Example 38 Example 31 13.8 85 120 Example 39 Example 32 13.1 95 180 Example 40 Example 33 13.5 98 240 Example 41 Example 34 13.3 80 120 Example 42 Example 35 13.5 90 300 Example 43 Example 36 13.6 85 90 Reference Example 1 Example 30 9.5 90 180 Reference Example 2 Example 30 13.1 45 180 Examples and Comparative Examples Kind of low-density acicular hematite particles Heat-treatment for high densification Temperature (°C) Time (min) Example 100 Example 93 630 60 Example 101 Example 94 650 60 Example 102 Example 95 680 30 Example 103 Example 96 640 75 Example 104 Example 97 680 30 Example 105 Example 98 660 30 Example 106 Example 99 710 20 Comparative Example 47 Comparative Example 42 680 30 Comparative Example 48 Comparative Example 44 680 30 Comparative Example 49 Comparative Example 45 Comparative Example 50 Comparative Example 46 850 30 Comparative Example 51 Comparative Example 46 670 30 Examples and Reference Examples Kind of high-density acicular hematite particles Heat-treatment in aqueous alkali solution pH value (-) Temperature (°C) Time (min) Example 114 Example 107 13.6 90 180 Example 115 Example 108 13.5 95 180 Example 116 Example 109 13.2 85 240 Example 117 Example 110 13.4 85 180 Example 118 Example 111 13.5 80 300 Example 119 Example 112 13.3 95 120 Example 120 Example 113 13.2 90 240 Reference Example 30 Example 107 9.3 90 180 Reference Example 31 Example 107 13.1 43 180 Goethite particles Kind of Precursor Conditions of heat-treatment Atmosphere Temperature (°C) Time (min) Goethite particles 6 Goethite particles 3 Air 150 30 Goethite particles 7 Goethite particles 4 Air 180 30 Goethite particles 8 Goethite particles 5 Air 120 20 Goethite particles 9 Goethite particles 4 Air 80 30 Kind of particles to be treated Kind of goethite particles Conditions of heat-dehydration treatment Atmosphere Temperature (°C) Time (min) Hematite particles 1 Goethite particles 6 Air 320 20 Hematite particles 2 Goethite particles 7 Air 340 30 Hematite particles 3 Goethite particles 8 Air 350 20 Hematite particles 4 Goethite particles 4 Air 340 20 Kind of particles to be treated Kind of particles to be treated Conditions of high-temperature heat-treatment Atmosphere Temperature (°C) Time (min) Hematite particles 5 Hematite particles 1 Air 680 30 Hematite particles 6 Hematite particles 2 Air 630 40 Hematite particles 7 Hematite particles 3 Air 600 40 Hematite particles 8 Goethite particles 7 Air 650 20 Hematite particles 9 Goethite particles 1 Air 700 30 Hematite particles 10 Goethite particles 9 Air 630 30 Hematite particles 11 Hematite particles 4 Air 680 30 Hematite particles 12 Hematite particles 3 Air 450 30 Hematite particles 13 Hematite particles 3 Air 900 30 Examples and Reference Examples Kind of particles to be treated Heat-treatment in aqueous alkali solution pH value (-) Temperature (°C) Time (min) Example 156 Hematite particles 5 13.7 85 210 Example 157 Hematite particles 6 13.4 95 120 Example 158 Hematite particles 7 13.8 90 240 Example 159 Hematite particles 8 13.1 98 180 Reference Example 50 Hematite particles 5 8.9 80 180 Reference Example 51 Hematite particles 5 13.5 40 180 Kind of goethite particles Kind of precursor Conditions of heat-treatment Atmosphere Temperature (°C) Time (min) Goethite particles 15 Goethite particles 12 Air 130 40 Goethite particles 16 Goethite particles 13 Air 150 30 Goethite particles 17 Goethite particles 14 Air 180 20 Goethite particles 18 Goethite particles 13 Air 80 30 Kind of particles to be treated Kind of goethite particles Conditions of heat-dehydration treatment Atmosphere Temperature (°C) Time (min) Hematite particles 14 Goethite particles 15 Air 330 30 Hematite particles 15 Goethite particles 16 Air 350 30 Hematite particles 16 Goethite particles 17 Air 370 25 Hematite particles 17 Goethite particles 18 Air 340 30 Kind of hematite particles Kind of particles to be treated Conditions of heat-treatment Atmosphere Temperature (°C) Time (min) Hematite particles 18 Hematite particles 14 Air 670 20 Hematite particles 19 Hematite particles 15 Air 630 30 Hematite particles 20 Hematite particles 16 Air 610 40 Hematite particles 21 Goethite particles 16 Air 640 30 Hematite particles 22 Goethite particles 17 Air 650 30 Hematite particles 23 Goethite particles 18 Air 650 30 Hematite particles 24 Hematite particles 17 Air 650 30 Hematite particles 25 Hematite particles 16 Air 440 30 Hematite particles 26 Hematite particles 16 Air 910 30 Examples and Reference Examples Kind of particles to be treated Heat treatment in aqueous alkali solution pH value (-) Temperature (°C) Time (min) Example 184 Hematite particles 18 13.1 90 180 Example 185 Hematite particles 19 13.5. 95 120 Example 186 Hematite particles 20 13.6 98 150 Example 187 Hematite particles 21 13.8 85 240 Reference Example 56 Hematite particles 18 8.2 85 180 Reference Example 57 Hematite particles 18 13.4 40 180

Claims: Acicular hematite particles having an average major axis diameter of from 0.004 to 0.295 µm, an aspect ratio (average major axis diameter/average minor axis diameter) of from 2:1 to 20:1, a geometrical standard deviation of the major axis diameter of not more than 1.50, a geometrical standard deviation of the minor axis diameter of not more than 1.35 and a pH of not less than 8, which particles contain not more than 300 ppm of soluble sodium salt, calculated as Na and not more than 150 ppm of soluble sulfate, calculated as SO 4 . Acicular hematite particles according to claim 1, which have a coat on at least a part of the surface thereof, comprising at least one hydroxide or oxide selected from hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of from 0.05 to 50% by weight, calculated at Al or SiO 2 , based on the weight of said acicular hematite particles. Acicular hematite particles according to claim 1 or 2, which further comprise aluminum within the particles in an amount of from 0.05 to 50% by weight, calculated as Al, based on the weight of said acicular hematite particles. Acicular hematite particles according to claim 3, which have a resin adsorptivity of not less than 65%. Acicular hematite particles according to any one of the preceding claims, which have a BET specific surface area of 35.9 to 180 m 2 /g. Acicular hematite particles according to any one of the preceding claims, which have a ratio value of a BET specific surface area S BET to a specific surface area S TEM of 0.5:1 to 2.5:1 (the specific surface area S TEM being calculated from the major axis diameter and the minor axis diameter which were measured from the particles in an electron micrograph of the acicular hematite particles). Acicular hematite particles according to any one of the preceding claims, which have a geometrical standard deviation of the minor axis diameter of not more than 1.30, a BET specific surface area of from 40 to 150 m 2 /g and an average major axis diameter of from 0.01 to 0.2 µm. A magnetic recording medium comprising: a non-magnetic base film; a non-magnetic undercoat layer on said non-magnetic base film, said undercoat layer comprising a binder resin and acicular hematite particles as defined in any one of the preceding claims; and a magnetic coating film comprising a binder resin and magnetic acicular metal particles containing iron as a main component. A magnetic recording medium according to claim 8, wherein said acicular hematite particles have a coat on at least a part of the surface thereof, comprising at least one hydroxide or oxide selected from hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon in an amount of from 5 to 50% by weight based on the total weight of the acicular hematite particles. A magnetic recording medium according to claim 8 or 9, wherein said magnetic acicular metal particles contain aluminum in an amount of from 0.05 to 10% by weight, calculated as Al, based on the weight of said magnetic acicular metal particles. A magnetic recording medium according to any one of claims 8 to 10, which has a coating film gloss of from 192 to 300%, a coating film surface roughness Ra of not more than 9.8 nm and a coating film linear absorption of from 1.10 to 200 µm -1 . A magnetic recording medium according to any one of claims 8 to 11, in which the change in the coercive force is not more than 10.0% and the change in the saturation magnetic flux density is not more than 10.0%. A magnetic recording medium according to any one of claims 8 to 12, which has a coating film gloss of from 194 to 300%, a coating film surface roughness Ra of not more than 9.6 nm, a coating film linear absorption of from 1.10 to 2.00 µm -1 , and a running durability of not less than 22 minutes. A magnetic recording medium according to any one of claims 8 to 13 which has a coating film gloss of from 196 to 300%, a coating film surface roughness Ra of not more than 9.4 nm, and a coating film linear absorption of from 1.10 to 2.00 µm -1 . A magnetic recording medium according to any one of claims 8 to 14 which has a coating film gloss of from 194 to 300%, a coating film surface roughness Ra of not more than 9.6 nm, a coating film linear absorption of from 1.10 to 2.00 µm -1 , and a running durability of not less than 24 minutes. A non-magnetic substrate comprising: a non-magnetic base film; and a non-magnetic undercoat layer on said non-magnetic base film, wherein the non-magnetic undercoat layer is as defined in claim 8 or 9. A non-magnetic substrate according to claim 16 which has a coating film gloss of from 189 to 300% and a coating film surface roughness Ra of from 0.5 to 9.6 nm. A non-magnetic substrate according to claim 16 or 17 which has a coating film gloss of from 191 to 300% and a coating film surface roughness Ra of from 0.5 to 9.3 nm. A non-magnetic substrate according to any one of claims 16 to 18, which has a coating film gloss of from 193 to 300% and a coating film surface roughness Ra of from 0.5 to 9.0 nm.

Inventors: HAYASHI KAZUYUKI, IWASAKI KEISUKE, MORII HIROKO, TANAKA YASUYUKI, HAYASHI, KAZUYUKI, IWASAKI, KEISUKE, MORII, HIROKO, TANAKA, YASUYUKI
Applicants: TODA KOGYO CORP, TODA KOGYO CORPORATION
Application year: 1998
Main classification: C01G49
IPC codes: G11B5, C09C1, C01G49