Magnetic iron oxide particles and method of producing the same

Abstract: Magnetic iron oxide particles, suitable for use in magnetic recording media, comprise: - spindle-shaped magnetite ( FeO x.Fe₂O₃, 0 < x ≦ 1) particles having a major axis diameter of from 0.1 to 0.3 µm, an axial ratio (major axis diameter: minor axis diameter) of notl ess than 7:1 and a print-through characteristic of not less than 45 dB; or - spindle-shaped maghemite particles having a major axis diameter of from 0.1 to 0.3 µm, an axial ratio (major axis diameter: minor axis diameter) of not less than 7:1 and a print-through characteristic of not less than 53 dB. The particles may contain zinc and may have their surfaces modified by Co.

Description: The present invention relates to magnetic iron oxide particles which have a spindle shape and a substantially uniform particle size and shape, and which are substantially free of dendrites, have a large axial ratio (major axis diameter : minor axis diameter) and have an excellent print-through characteristic, to a method of producing the same, and to a magnetic recording medium comprising the same. The particles of the present invention are magnetic iron oxide particles having a spindle particle-shape, a substantially uniform particle size and shape, and are substantially free of dendrites. These particles are referred to simply as "spindle-shaped magnetic iron oxide particles" hereinunder. With the development of smaller-sized and lighter-weight magnetic recording apparatus, the necessity for a recording medium such as a magnetic tape and a magnetic disk having a higher performance has been increasing more and more. In other words, a magnetic recording medium is required to have high recording density, high sensitivity and high output characteristics. The properties in which magnetic iron oxide particles are required to have in order to satisfy said demands on a magnetic recording medium are a small particle size, high coercive force and excellent dispersibility. To state this more concretely, it is necessary that the magnetic iron oxide particles have as high a coercive force as possible in order to provide a magnetic recording medium with a high recording density, a high sensitivity and a high output. This fact is obvious from, for example, the statements "Since the improvement of magnetic tapes has been directed toward a high sensitivity and a high output, emphasis has been laid on enhancing the coercive force of acicular γ-Fe₂O₃ particles, ..." on page 310 of DEVELOPMENT OF MAGNETIC MATERIAL AND TECHNOLOGY OF IMPROVING DISPERSIBILITY OF MAGNETIC POWDER (1982), published by K.K. Sogo Gijutsu Center, and "It is known that there is a relationship between the particle size of acicular γ-Fe₂O₃ and the noise of a magnetic recording tape, and if the particle size become finer, the noise of magnetic recording tape lower" on page 312 of the same publication. DEVELOPMENT OF MAGNETIC MATERIAL AND TECHNOLOGY OF IMPROVING DISPERSIBILITY OF MAGNETIC POWDER also states on page 312 that "The condition for high-density recording in a coating type tape is that the tape is capable of maintaining high output characteristics with a low noise with respect to a short-wave signal". To satisfy this condition, it is necessary that the magnetic recording medium has both high coercive force (Hc) and large residual magnetization (Br), and is in the form of a thin coated film. As seen from the above, it is really necessary that the magnetic recording medium has both high coercive force (Hc) and large residual magnetization (Br) in order to obtain a high recording density and to satisfy these conditions, and accordingly the magnetic iron oxide particles are required to have a high coercive force, excellent dispersibility in a vehicle and a high orientation and high packing density in the coated film. The residual magnetization (Br) in a magnetic recording medium depends upon the dispersibility of the magnetic iron oxide particles in the vehicle and the orientation and packing density of the magnetic iron oxide particles in the coated film. In order to improve these properties, the magnetic iron oxide particles dispersed in the vehicle are required to have as large an axial ratio (major axis diameter : minor axis diameter) as possible, a uniform particle size and no inclusion of dendrites. As is well known, the magnitude of the coercive force of magnetic iron oxide particles depend upon the configurational anisotropy, crystalline anisotropy, strain anisotropy, exchange anisotropy, or the interaction thereof. Acicular magnetite particles and acicular maghemite particles which are used as magnetic iron oxide particles at present produce a relatively high coercive force by utilizing the anisotropy derived from their shapes, namely, by increasing the axial ratio. Known acicular magnetite particles are obtained by reducing starting material goethite particles at 250 to 400°C in a reducing gas such as hydrogen, and known acicular maghemite particles are obtained by further oxidizing the thus-obtained magnetite particles at 200 to 300°C in air. As described above, magnetic iron oxide particles which have a substantially uniform particle size, are substantially free of dendrites, and have a large axial ratio are now in the strongest demand. In order to obtain magnetic iron oxide particles provided with these properties, it is necessary that the starting material goethite particles have a substantially uniform particle size, are substantially free of dendrites and have a large axial ratio. As a method of producing goethite particles, which are the starting material, a method of producing acicular goethite particles by oxidizing a solution containing ferrous hydroxide particles which is obtained by adding more than an equivalent of an alkaline solution to a ferrous salt solution, the oxidation being carried out by blowing an oxygen-containing gas into the solution containing ferrous hydroxide particles at not higher than 80°C at a pH of not less than 11 [JP-B-39-5610 (1964)], and a method of producing spindle-shaped goethite particles by oxidizing an aqueous solution containing FeCO₃ which is obtained by reacting an aqueous ferrous salt solution with an alkali carbonate, the oxidation being carried out by blowing an oxygen-containing gas into the aqueous solution containing FeCO₃ [JP-A-50-80999 (1975)], are, for example, conventionally known. As the magnetic iron oxide particles having a high coercive force, so-called Co-doped magnetic iron oxide particles and so-called Co-coated magnetic iron oxide particles are conventionally known. The coercive force of these magnetic iron oxide particles increases with an increase in the Co content. The Co-doped magnetic iron oxide particles are obtained by producing Co-containing goethite particles by adding a Co salt when the starting material goethite particles are produced, and reducing the thus-obtained Co-containing goethite particles to produce Co-containing magnetite particles or further oxidizing the magnetite particles, if necessary, to produce Co-containing maghemite particles. The Co-coated magnetic iron oxide particles are obtained using the magnetite particles or maghemite particles obtained by reducing or further oxidizing, if necessary, the starting material goethite particles as a precursor and coating the surfaces of the precursor particles with a Co compound. Co-doped magnetic iron oxide particles have a high coercive force, but they are disadvantageous because, since Co diffuses in the crystals, the distribution of the coercive force is enlarged, thereby making the magnetic iron oxide particles thermally and temporally unstable. In contrast, Co-coated magnetic iron oxide particles are thermally and temporally stable. There is no end to the recent demand for the improvement of the properties of magnetic iron oxide particles. In addition to uniform particle size, absence of dendrites, high coercive force, thermal and temporal stability and large axial ratio, an improvement regarding the capacity of transferring a recording signal to the opposing magnetic layer, the so-called a print-through characteristic, is strongly demanded. The print-through characteristic has a tendency to deteriorate as the magnetic iron oxide particles become finer, especially when the particle size is not more than 0.3 µm. ELECTRONIC TECHNIQUE (1968), vol. 10, published by Nikkan Kogyo Shimbun, discloses on page 51 that "... it is known that the transferring effect has an unfavorable tendency to be deteriorated in proportion to the lowering of a noise level due to the reduction in the particle size...". This tendency is a serious problem at present when finer and finer magnetic iron oxide particles are used with a demand for high recording density, high sensitivity and high output characteristic. In addition, in order to improve the print-through characteristic of the Co-coated magnetic iron oxide particles, it is necessary that the particle size distribution of the precursor particles is as uniform as possible and that the axial ratio is as large as possible. To satisfy these conditions, the starting material goethite particles are also required to have as uniform particle size distribution as possible and as large an axial ratio as possible. Furthermore, demand for conservation in resources and energy used has become increasingly stronger, and it is also demanded that magnetic iron oxide particles and industrially and economically produced advantageously. As described above, magnetic iron oxide particles which have fine and uniform particle size, no inclusion of dendrites, thermal stability which does not change with time, a large axial ratio and excellent print-through characteristics are now in the strongest demand. If the method disclosed in JP-B-39-5610 (1964) is adopted for producing the starting material goethite particles, acicular goethite particles having a large axial ratio, particularly, not less than 10:1 are produced, but dendrites are included therein. The particles are not of a uniform size, and the print-through characteristic of the magnetic iron oxide particles obtained by using these goethite particles is unsatisfactory. On the other hand, if the method disclosed in JP-A-50-80999 (1975) is adopted for producing the starting material goethite particles, spindle-shaped particles having a uniform particle size and including no dendrites are produced. However, the axial ratio is at most about 7:1, in other words the production of particles having a large axial ratio is difficult, and this phenomenon becomes prominent as the major axis diameter of the particles produced is reduced. The print-through characteristics of the magnetic iron oxide particles and Cocoated magnetic iron oxide particles obtained by using these goethite particles are also unsatisfactory. Various attempts have conventionally been carried out to increase the axial ratio of spindle-shaped goethite particles. For example, there is a method of reducing the gas flow velocity at which an oxygen-containing gas is blown into a suspension containing FeCO₃ which is obtained by reacting an aqueous ferrous salt solution with an aqueous alkali carbonate to about 0.1 to 2.0 cm/s, as disclosed in JP-A-59-232922 (1984). According to this method, when the major axis is about 0.5 µm, the axial ratio is about 10:1; when the major axis is about 0.3 µm, the axial ratio is about 8:1; and when the major axis is as small as about 0.05 µm, the axial ratio also becomes as small as about 5:1. The axial ratio cannot therefore be said to be sufficiently large. In the example in JP-A-62-158801 (1987), spindle-shaped goethite particles having an axial ratio of 10:1 are obtained. These particles, however, are obtained by making the concentration of iron as low as about 0.2 mol/ℓ, and the axial ratio also cannot be said to be sufficiently large. Spindle-shaped magnetic iron oxide particles are also disclosed in EP-A-160,496. For these reasons, a process for obtaining spindle-shaped magnetic iron oxide particles or Co-coated magnetic iron oxide particles which have a substantially uniform particle size, are substantially free of dendrites, have a large axial ratio and which have an excellent print-through characteristic is now strongly demanded. The present inventors have found that (1) spindle-shaped goethite particles may be produced by ageing at a temperature of 40 to 60°C for 50 to 100 minutes an aqueous solution containing FeCO₃ which is obtained by reacting an aqueous alkali carbonate with an aqueous ferrous salt solution in a non-oxidizing atmosphere, the amount of aqueous alkali carbonate being 1.5 to 3.5 times the equivalent based on Fe²⁺ in the aqueous ferrous salt solution, and blowing an oxygen-containing gas into the suspension containing FeCO₃ for oxidation at a reaction temperature of from 40 to 70°C and a pH of from 7 to 11. The thus-obtained spindle-shaped goethite particles or hematite particles obtained by baking these goethite particles are then coated with a material having a sintering preventing effect and reduced under heating in a reducing gas, or further oxidized, if necessary, thereby obtaining spindle-shaped magnetic iron oxide particles which have a substantially uniform particle size, which are substantially free of dendrites and which are of magnetite ( FeO x·Fe₂O₃, 0<x≦1) having a print-through characteristic of not less than 45 dB or spindle-shaped maghemite particles having a print-through characteristic of not less than 53 dB, both of which have a major diameter of 0.1 to 0.3 µm and an axial ratio of not less than 7:1; (2) spindle-shaped goethite particles containing zinc may be produced by adding a zinc compound in advance to any of the aqueous alkali carbonate, the aqueous ferrous salt solution, the suspension containing FeCO₃ and the suspension containing FeCO₃ in the course of ageing prior to the oxidation step of blowing the oxygen-containing gas thereinto, and the thus-obtained zinc-containing goethite particles or hematite particles obtained by baking these goethite particles are reduced under heating in a reducing gas, or further oxidized, if necessary, thereby obtaining spindle-shaped zinc-containing magnetite particles having a print-through characteristic of not less than 45 dB or spindle-shaped zinc-containing maghemite particles having a print-through characteristic of not less than 53 dB, both of which have a major diameter of 0.1 to 0.3 µm and an axial ratio of not less than 8:1; (3) a mixed dispersion obtained by mixing an aqueous dispersion of the thus-obtained spindle-shaped maghemite containing or not containing zinc with at least an aqueous Co salt solution and an aqueous alkaline solution and having a pH of not less than 11 may be heated at a temperature of 50 to 100°C, thereby obtaining spindle-shaped magnetic iron oxide particles with the surfaces thereof modified by 0.5 to 15.0 atomic% of Co based on Fe and Co; and (4) 1 to 50% of an aqueous alkali hydroxide based on the aqueous alkali carbonate may be added to any of the aqueous alkali carbonate, the suspension containing FeCO₃ and the suspension containing FeCO₃ in the course of ageing prior to the oxidation step which is carried out at a reaction temperature of from 30 to 70°C and a pH of 7 to 11 and which consists of blowing the oxygen-containing gas thereinto so that the total amount of aqueous alkali carbonate and aqueous alkali hydroxide becomes 1.1 to 2.5 times the equivalent based on Fe²⁺ in the aqueous ferrous salt solution and the ageing is carried out at a temperature of 30 to 60°C for 10 to 100 minutes to produce spindle-shaped goethite particles, and the thus-obtained goethite particles or hematite particles obtained by baking these goethite particles are coated with a material having a sintering preventing effect, and reduced under heating in a reducing gas, or further oxidizing the product, if necessary, thereby obtaining spindle-shaped magnetic iron oxide particles. The present invention has been achieved on the basis of these findings. The present invention provides magnetic iron oxide particles which have a substantially uniform particle size, which are substantially free of dendrites, which have a print-through characteristic Q₁ of not less than 45 dB wherein Q₁ is calculated from the formula: Q₁ = 40 x (0.2 - A) + B in which A is the measured major axis diameter in µm and B is the measured print-through characteristic and which consist essentially of spindle-shaped magnetite ( FeO x .Fe₂O₃, 0 ≦ x ≦ 1) particles having a major axis diameter of from 0.1 to 0.3 µm and an axial ratio (major axis diameter: minor axis diameter) of not less than 7:1, which spindle-shaped magnetite particles are coated with a material having a sintering preventing effect. The above magnetite particles may contain zinc and have an axial ratio of not less than 8:1. The present invention also provides magnetic iron oxide particles which have substantially uniform particle size, which are substantially free of dendrites, which have a print-through characteristic Q₁ of not less than 54 dB wherein Q₁ is calculated from the formula: Q₁ = 40 x (0.2 - A) + B in which A is the measured major axis diameter in µm and B is the measured print-through characteristic and which consist essentially of spindle-shaped maghemite particles having a major axis diameter of from 0.1 to 0.3 µm and an axial ratio (major axis diameter: minor axis diameter) of not less that 7:1, which maghemite particles are coated with a material having a sintering preventing effect. The above maghemite particles may contain zinc and have an axial ratio of not less than 8:1. The above spindle-shaped magnetite particles, which may or may not contain the zinc, may have surfaces which are modified by from 0.5 to 15.0 atomic% of Co based on the total amount of Fe and Co, and the resulting magnetic iron oxide particles have a coercive force of from 3.82 x 10⁴ to 7.96 x 10⁴ Am⁻¹ (480 to 1000 Oe) and a print-through characteristic Q₃ of not less than 52 dB, wherein Q₃ is calculated from the formulae: Q₃ = 40 x (0.2 - A)+Q₂ and Q₂ = (700 - C)x0.02 + B in which A and B are as defined above and C is the measured coercive force in Oe. The above spindle-shaped maghemite particles, which may or may not contain the zinc, may have surfaces which are modified by from 0.5 to 15.0 atomic% of Co based on the total amount of Fe and Co, and the resulting magnetic iron oxide particles have a coercive force of from 3.82 x 10⁴ to 7.96 x 10⁴ Am⁻¹ (480 to 1000 Oe) and a print-through characteristic Q₃ of not less than 57.0 dB, wherein Q₃ is calculated from the formulae: Q₃ = 40 x (0.2 - A)+Q₂ and Q₂ = (700 - C) x 0.02 + B in which A and B are as defined above and C is the measured coercive force in Oe. The present invention also provides a process for producing magnetic magnetite iron oxide particles as defined above, which process comprises: (i) ageing an aqueous suspension of FeCO₃, obtained by reacting an aqueous alkali carbonate with an aqueous ferrous salt solution, in a non-oxidizing atmosphere at a temperature of from 40 to 60°C for from 50 to 100 minutes, the amount of the aqueous alkali carbonate being from 1.5 to 3.5 times equivalent of the amount of Fe²⁺ in the aqueous ferrous salt solution; (ii) blowing a molecular oxygen-containing gas into the suspension containing said FeCO₃ at a reaction temperature of from 40 to 70°C and a pH of from 7 to 11, thereby oxidizing the FeCO₃ to produce spindle-shaped goethite particles; (iii) coating said spindle-shaped goethite particles or spindle-shaped hematite particles obtained by baking said spindle-shaped goethite particles with a material having a sintering preventing effect; and (iv) reducing by heating in a reducing gas said coated particles. In the above process for producing magnetite particles: a zinc compound may be added to a solution of said aqueous alkali carbonate, to said aqueous ferrous salt solution or to said suspension of FeCO₃ prior to step (ii) so that spindle-shaped goethite particles containing zinc are obtained in step (ii); and said spindle-shaped goethite particles containing zinc or spindle-shaped hematite particles containing zinc obtained by baking said spindle-shaped goethite particles containing zinc are reduced under heating in a reducing gas, thereby obtaining spindle-shaped magnetite particles containing zinc as defined above. The magnetite particles obtained by the process defined above, which particles may or may not contain zinc, may be further oxidized to obtain maghemite particles as defined above. In the above processes for preparing magnetite or maghemite particles, which may or may not contain zinc, an aqueous dispersion of the resulting spindle-shaped particles may be mixed with at least an aqueous Co salt solution and an aqueous alkaline solution so that the resulting dispersion has a pH of not less than 11; and the said dispersion is heated to a temperature of 50 to 100°C, thereby obtaining Co-modified spindle-shaped particles as defined above. The present invention additionally provides a process for producing spindle-shaped magnetite or maghemite magnetic iron oxide particles as defined above, which process comprises: (a) ageing a suspension of FeCO₃, obtained by reacting an aqueous alkali carbonate with an aqueous ferrous salt solution, in a non-oxidizing atmosphere at a temperature of from 30 to 60°C for from 10 to 100 minutes and blowing a molecular oxygen-containing gas into said suspension of FeCO₃ at a reaction temperature of from 30 to 70°C and a pH of from 7 to 11 thereby oxidizing the FeCO₃ to produce spindle-shaped goethite particles, wherein from 1 to 50% of an aqueous alkali hydroxide based on the amount of said aqueous alkali carbonate is added to a solution of said aqueous alkali carbonate or to said suspension of FeCO₃ prior to oxidation of the FeCO₃ so that the total amount of aqueous alkali carbonate and aqueous alkali hydroxide is 1.1 to 2.5 times equivalent to the amount of Fe²⁺ in said aqueous ferrous salt solution; (b) coating said spindle-shaped goethite particles or spindle-shaped hematite particles obtained by baking said spindle shaped goethite particles, with a material having a sintering preventing effect; (c) reducing by heating in a reducing gas said coated particles, thereby producing spindle-shaped magnetite particles; and (d) if desired, oxidizing said spindle-shaped magnetite particles to produce spindle-shaped maghemite particles. In the above process, in order to produce magnetite or maghemite particles containing zinc as defined above: a zinc compound is added to a solution of said aqueous alkali carbonate, to said aqueous ferrous salt solution or to said suspension of FeCO₃ prior to oxidation of the FeCO₃, thereby obtaining spindle-shaped goethite particles containing zinc; said spindle-shaped goethite particles containing zinc or spindle-shaped hematite particles containing zinc obtained by baking said spindle-shaped goethite particles containing zinc are reduced under heating in a reducing gas, thereby obtaining spindle-shaped magnetite particles containing zinc; and if desired, said spindle-shaped magnetite particles are oxidized thereby obtaining spindle-shaped maghemite particles containing zinc. In the above processes, in order to produce magnetite or maghemite particles containing Co, and which may or may not contain zinc, as defined above: an aqueous dispersion of the resulting spindle-shaped particles is mixed with at least an aqueous Co salt solution and an aqueous alkaline solution so that the resulting dispersion has a pH of not less than 11; and the said dispersion is heated to a temperature of from 50 to 100°C, thereby obtaining spindle-shaped magnetite or maghemite particles with their surfaces modified by from 0.5 to 15.0 atomic% of Co based on the total amount of Fe and Co. The present invention further provides a magnetic recording medium in which the magnetic material consists essentially of magnetic iron oxide particles as defined above or which have been produced by a process as defined above. In the accompanying drawings: Fig. 1 shows the relationship between the major axis diameter and the measured print-through characteristic of maghemite particles; Figs. 2 and 3 show the relationship between the measured coercive force and the measured print-through characteristic of magnetite particles and maghemite particles, respectively; Figs. 4(a) and 4(b), 5(a) and 5(b) show the relationship between the correction value of the print-through characteristic at 700 Oe and the major axis diameter of Co-modified magnetite particles and Co-modified maghemite particles, respectively; Figs. 6(a) and 6(b), 7(a) and 7(b) show the relationship between the amount of zinc sulfate present and the major axis diameter, and the relationship between the amount of zinc sulfate present and the axial ratio, respectively, of spindle-shaped goethite particles; Figs. 8 to 12 are electron micrographs (x 30000) of the particle structures of the spindle-shaped goethite particles obtained in Example 1, Example 5, Comparative Example 1, Comparative Example 5 and Comparative Example 6, respectively; Figs. 13 and 14 are electron micrographs (x 30000) of the particle structures of the spindle-shaped magnetite particles obtained in Examples 17 and 21, respectively; Figs. 15 to 17 are electron micrographs (x 30000) of the particle structures of the spindle-shaped maghemite particles obtained in Example 25, Example 29 and Comparative Example 23, respectively; Figs. 18 and 19 are electron micrographs (x 30000) of the particles structures of the spindle-shaped Co-modified magnetite particles obtained in Examples 37 and spindle-shaped Co-modified maghemite particles obtained in Example 41, respectively; Figs. 20 and 21 are electron micrographs (x 30000) of the particle structures of the spindle-shaped goethite particles obtained in Examples 49 and 52, respectively; Figs. 22 to 25 are electron micrographs (x 30000) of the particle structures of the mixed particles of the spindle-shaped goethite particles and granular-shaped magnetite particles obtained in Comparative Examples 37 to 40, respectively; Figs. 26 and 27 are electron micrographs (x 30000) of the particle structures of the spindle-shaped magnetite particles obtained in Examples 65 and 68, respectively; Figs. 28 and 29 are electron micrographs (x 30000) of the particle structures of the spindle-shaped maghemite particles obtained in Example 73 and Example 76, respectively; and Figs. 30 and 31 are electron micrographs (x 30000) of the particle structures of the spindle-shaped Co-modified magnetite particles obtained in Examples 84 and the spindle-shaped Co-modified maghemite particles obtained in Example 89, respectively. Fig. 1 shows the relationship between the major axis diameter and the measured print-through characteristic of maghemite particles. In Fig. 1, the straight line A represents the characteristics of the spindle-shaped maghemite particles in accordance with the present invention, A′ represents the characteristics of the spindle-shaped maghemite particles obtained by an economical resources - energy reaction, B represents the characteristics of the acicular maghemite particles obtained by the conventional method described in JP-B-39-5610 (1964) and C represents the characteristics of the spindle-shaped maghemite particles obtained by the conventional method described in JP-A-50-80999 (1975). As is clear from Fig. 1, the spindle-shaped maghemite particles of the present invention have an excellent print-through characteristic. The magnetite particles of the present invention show a similar tendency to the straight line A in Fig. 1, and also have excellent print-through characteristics. The print-through characteristic is strongly influenced by the magnitude of the coercive force, which is higher than 480 to 500 Oe. Figs. 2 and 3 show the relationship between the measured coercive force and the measured print-through characteristic of magnetite particles and maghemite particles, respectively, obtained from many experiments carried out in the present invention. It is clear from Figs. 2 and 3 that there is a constant relationship between the coercive force and the print-through characteristic. Figs. 4(a) and 5(a) show the relationship between the correction value of the print-through characteristic at 700 Oe and the major axis diameter of Co-modified magnetite particles and Co-modified maghemite particles, respectively, and Figs. 4(b) and 5(b) show the relationship between the correction value of the print-through characteristic at 700 Oe and the major axis diameter of Co-modified magnetite particles and Co-modified maghemite particles, respectively, obtained by the economical resources - energy reaction. In these figures, the straight lines A, B and C represent the characteristics of the magnetic iron oxide particles with the surfaces modified by Co, the Co-modified magnetic iron oxide particles of the straight lines A, B and C being produced by modifying as the respective precursors the spindle-shaped magnetic iron oxide particles obtained the present invention; the acicular magnetic iron oxide particles obtained by the conventional method described in JP-B-39-5610 (1964); and the spindle-shaped magnetic iron oxide particles obtained by the conventional method described in JP-A-50-80999 (1975), by Co in the same way as in later described Example 1. It is clear from Figs. 4(a), 4(b), 5(a) and 5(b) that there is a constant relationship between the correction value of print-through characteristic at 700 Oe and the major axis diameter. The print-through characteristic of the Co-modified magnetic iron oxide particles of the present invention is excellent. Figs. 6(a) and 7(a) show the relationship between the amount of zinc sulfate present and the major axis diameter and the relationship between the amount of zinc sulfate present and the axial ratio, respectively, of spindle-shaped goethite particles obtained during the process of the present invention. The ordinates represent the major axis diameter and the axial ratio, respectively, of the spindle-shaped goethite particles obtained when the amount of zinc sulfate present is 0 to 10.0 atomic % of zinc based on the amount of Fe under the reaction conditions in later-described Example 5, and the abscissa represents the amount of zinc sulfate present. Figs. 6(b) and 7(b) show the relationship between the amount of zinc sulfate present and the major axis diameter and the axial ratio, respectively, of spindle-shaped goethite particles obtained by an economical resources - energy reaction. The ordinates represent the major axis diameter and the axial ratio, respectively, of the spindle-shaped goethite particles obtained when the amount of zinc sulfate present is 0 to 10.0 atomic % of zinc based on the amount of Fe under the reaction conditions in later-described Example 52, and the abscissa represents the amount of zinc sulfate present. As is obvious from Figs. 6(a), 6(b), 7(a) and 7(b), the influence of the existence of zinc sulfate on the major axis diameter of the spindle-shaped goethite particles is small, while the axial ratio has a tendency to increase more as the amount of zinc sulfate present increases. Consequently, it is considered that a zinc compound regulates the growth of the spindle-shaped goethite particles in the minor axis direction. A method of ageing a suspension containing FeCO₃ in a non-oxidizing atmosphere is disclosed in JP-B-59-48768 (1984). This is, however, a method of obtaining spindle-shaped goethite particles having a uniform particle size by treating a suspension containing FeCO₃ at room temperature for 120 to 240 minutes in a non-oxidizing atmosphere, which has been produced by using 1.06 times as much aqueous alkali carbonate as Fe²⁺. This method is completely different from the present invention which is aimed at producing spindle-shaped particles having a large axial ratio. The axial ratio of the spindle-shaped goethite particles obtained by the method disclosed in JP-B-59-48768 (1984) is about 4 in "Examples 1 and 2". As examples of an aqueous ferrous salt solution used in the present invention there are aqueous ferrous sulfate and aqueous ferrous chloride. As examples of an aqueous alkali carbonate used in the present invention there are aqueous solutions of sodium carbonate, potassium carbonate and ammonium carbonate. The amount of aqueous alkali carbonate used in the present invention is 1.5 to 3.5 times equivalent based on Fe²⁺ in the aqueous ferrous salt solution. If it is less than 1.5 times equivalent, the particle size of the spindle-shaped goethite particles obtained becomes non-uniform and the particles aggregate, thereby deteriorating the dispersibility. On the other hand, if it is more than 3.5 times equivalent, the axial ratio decreases more as the amount of aqueous alkali carbonate added increases, so that it is difficult to obtain spindle-shaped goethite particles having a large axial ratio. In addition, use of a large amount of expensive aqueous alkali carbonate is uneconomical. As an aqueous alkali hydroxide used in a method of producing spindle-shaped goethite particles by economical resources - energy reaction during the process of the present invention, aqueous sodium hydroxide and aqueous potassium hydroxide may be exemplified. An aqueous alkali hydroxide may be added to any of the aqueous alkali carbonate, the suspension containing FeCO₃ and the suspension containing FeCO₃ in the course of ageing prior to the oxidation step of blowing the oxygen-containing gas thereinto. In any case, the economical resources - energy reaction is possible. The amount of alkali hydroxide added is 1 to 50 normaly based on the amount of alkali carbonate. If it is less than 1%, the economical resources - energy reaction is difficult. On the other hand, if it is more than 50%, granular-shaped magnetite particles disadvantageously contaminate the spindle-shaped goethite particles. The total amount of aqueous alkali carbonate and aqueous alkali hydroxide used in the present invention is 1.1 to 2.5 times equivalent to the amount of Fe²⁺ in the aqueous ferrous salt solution and it is possible to reduce the mixing ratio of an aqueous alkaline solution to Fe. If it is less than 1.1 times equivalent, granular-shaped magnetite particles disadvantageously contaminate the spindle-shaped goethite particles. On the other hand, if it is more than 2.5 times equivalent, the amount of expensive alkali used is uneconomically increased. The ageing step in the present invention is carried out in an inert atmosphere by blowing an inert gas such as N₂ gas into a solution under agitation with the gas or mechanical agitation. The ageing temperature of the suspension containing FeCO₃ in the present invention is 40 to 60°C. If the temperature is lower than 40°C, the axial ratio becomes small. In other words, it is difficult to obtain spindle-shaped goethite particles having a large axial ratio. Even large temperature is higher than 60°C, it is possible to obtain spindle-shaped goethite particles having a large axial ratio, but to raise the ageing temperature more than necessary is pointless. The ageing time of the suspension containing FeCO₃ in the present invention is 50 to 100 minutes. If the aging time is less than 50 minutes, it is difficult to obtain spindle-shaped goethite particles having a large axial ratio. Even if the ageing time is more than 100 minutes, it is possible to obtain spindle-shaped goethite particles having a large axial ratio, but to prolong the ageing time more than necessary is pointless. The ageing temperature of the suspension containing FeCO₃ in a method of producing spindle-shaped goethite particles by economical resources - energy reaction in the present invention is 30 to 60°C. If the temperature is lower than 30°C, the axial ratio becomes small. In other words, it is difficult to obtain spindle-shaped goethite particles having a large axial ratio. The ageing time is 10 to 100 minutes. If the ageing time is less than 10 minutes, it is difficult to obtain spindle-shaped goethite particles having a large axial ratio. That is, it is possible to obtain spindle-shaped goethite particles having a large axial ratio even if the ageing temperature is lowered by about 10°C and the ageing temperature is shortened by about 40 minutes by a method of producing magnetic iron oxide particles with economical resources - energy reaction in comparison with a method using an aqueous alkali carbonate alone. Examples of the zinc compound used in the present invention are zinc sulfate and zinc chloride. The amount of zinc compound present is 0.3 to 10.0 atomic% (calculated as Zn) based on the amount of Fe in the aqueous ferrous salt solution. If it is less than 0.3 atomic%, it is difficult to obtain spindle-shaped goethite particles having a large axial ratio. Even if it is more than 10.0 atomic%, it is possible to obtain spindle-shaped goethite particles having a large axial ratio, but the magnetization value of the magnetic iron oxide particles obtained by reducing under heating the goethite particles, or further oxidizing the product, if necessary, is lowered. In consideration of the axial ratio of the spindle-shaped goethite particles, the amount of zinc compound used is preferably 0.5 to 8.0 atomic%. Almost the whole amount of zinc compound added is contained in the spindle-shaped goethite particles produced, as shown in the later Examples. Since the zinc compound affects in the axial ratio of the spindle-shaped goethite particles produced, it is necessary that the zinc compound is present prior to the oxidation of the suspension containing FeCO₃ by blowing an oxygen-containing gas thereinto. The zinc compound is therefore added to any of the aqueous alkali carbonate, the aqueous ferrous salt solution, the suspension containing FeCO₃ and the suspension containing FeCO₃ in the course of ageing prior to the oxidation step. Adding the zinc compound to the suspension containing FeCO₃ in the course of ageing is the most effective. The reaction temperature in the oxidation process in the present invention is 40 to 70°C. If the temperature is lower than 40°C, it is difficult to obtain spindle-shaped goethite particles. On the other hand, if the temperature is higher than 70°C, granular hematite particles contaminate the spindle-shaped goethite particles. The reaction temperature in the oxidation process in the method of producing spindle-shaped goethite particles with the economical resources - energy reaction is 30 to 70°C. If the reaction temperature is lower than 30°C, it is difficult to obtain spindle-shaped goethite particles having a large axial ratio. The pH in the oxidation process in the present invention is 7 to 11. If the pH is less than 7 or more than 11, it is difficult to obtain spindle-shaped goethite particles. The oxidation in the present invention is carried out by blowing an oxygen-containing gas such as air into the solution under agitation with the gas or mechanical agitation. In the present invention, it is possible to add a metal other than Fe, such as Co, Ni, Cr, Zn, Aℓ or Mn. These are conventionally generally added in the generation of goethite particles to improve various properties of magnetic iron oxide particles. In this case, it is also possible to obtain spindle-shaped goethite particles having a large axial ratio. As the starting material in the present invention, not only the spindle-shaped goethite particles produced but also spindle-shaped hematite obtained by dehydrating the goethite particles by an ordinary method and highly-densified spindle-shaped hematite obtained by heating the goethite particles at 250 to 800°C in a non-reducing atomosphere are usable. The reduction under heating in a reducing agent and oxidation in the present invention can be carried out by an ordinary method. If the starting material particles are coated with a material having a sintering preventing effect such as Si, Aℓ and P compounds by a known method prior to the reduction under heating so as to prevent sintering between the particles, the maintenance of the particle shape and the axial ratio of the starting material particles is facilitated. Co-modification of precursor particles in the present invention can be carried out by an ordinary method. For example, the Co-modification is carried out by heating a mixture obtained by mixing an aqueous dispersion of the precursor particles with at least an aqueous Co salt solution and an aqueous alkaline solution and having a pH of not less than 11 at a temperature of 50 to 100°C, as described in, e.g., JP-B-52-24237 (1977), JP-B-52-24238 (1977), JP-B-52-36751 (1977) and JP-B-52-36863 (1977). As the precursor particles in the present invention, the spindle-shaped magnetite particles, the spindle-shaped maghemite particles, the spindle-shaped magnetite particles containing zinc and the spindle-shaped maghemite particles containing zinc may be exemplified. The aqueous Co salt solution may contain an aqueous Fe(II) salt, if necessary. Examples of the aqueous Co salt solution in the present invention are aqueous solutions of cobalt sulfate, cobalt chloride and cobalt nitride. Examples of the aqueous Fe(II) salt solution are aqueous solutions of ferrous sulfate, ferrous chloride and ferrous nitride. As the aqueous alkali solution used for Co-modification in the present invention, aqueous sodium hydroxide and aqueous potassium hydroxide are usable. The temperature for Co-modification in the present invention affects in the treating time. If the temperature is lower than 50°C, it is difficult to produce the Co-modified magnetite particles or maghemite particles, and even if they are produced, treatment for a very long time is required. The amount of Co for Co-modification in the present invention is 0.5 to 15.0 atomic% (calculated as Co) based on the amount of Fe and Co. If it is less than 0.5 atomic%, the improvement of the coercive force of the spindle-shaped magnetite particles produced is unsatisfactory. Even if it is more than 15.0 atomic%, it is possible to obtain an improvement in the coercive force of the spindle-shaped magnetite particles or maghemite particles produced, but modification more than necessary is pointless. Almost the whole amount of Co added is utilized for modification of the surfaces of the magnetic iron oxide particles. In consideration of the coercive force of the spindle-shaped magnetite particles or maghemite particles, the amount of Co added is preferably 2.0 to 13.0 atomic%. The Co-modified surfaces of the magnetic iron oxide particles obtained in the present invention may be coated with a hydroxide or an oxide such as one of Aℓ, Si or Zn. These are generally used for improving the dispersibility in a binder. The spindle-shaped magnetic iron oxide particles according to the present invention, which have a uniform particle size and shape, are substantially free of dendrites, and have a large axial ratio and an excellent print-through characteristic (the correction print-through characteristic value is not less than 45.0 dB, in the case of magnetite particles, and not less than 53.0 dB, preferably not less than 54.0 dB in the case of maghemite particles), and are suitable as magnetic iron oxide particles having a high recording density, a high sensitivity and a high output, which are now in the strongest demand. The spindle-shaped magnetic iron oxide particles with the surfaces thereof modified by Co according to the present invention, which have a fine particle size, a high coercive force and an excellent print-through characteristic (the correction value of print-through characteristic is not less than 52.0 dB, preferably not less than 53.0 dB in the case of magnetite particles, and not less than 57.0 dB, preferably not less than 58.0 dB in the case of maghemite particles), are suitable as magnetic iron oxide particles having a high recording density, high sensitivity and a high output, which are now in the strongest demand. According to the method of producing spindle-shaped magnetic iron oxide particles with the economical resources - energy reaction of the present invention, it is possible to produce spindle-shaped magnetic iron oxide particles which have a large axial ratio and an excellent print-through characteristic (the correction value of print-through characteristic is not less than 45.0 dB in the case of magnetite particles, and not less than 53.0 dB, preferably not less than 54.0 dB in the case of maghemite particles) industrially and economically advantageously. The economical resources - energy reaction is capable of enhancing the productivity because it is possible to react at a high concentration, to reduce the amount of the aqueous alkaline solution, and to reduce the amount of energy used in the ageing process. According to the method of producing spindle-shaped magnetic iron oxide particles with the surfaces thereof modified by Co with the economical resources - energy reaction of the present invention, it is possible to produce spindle-shaped goethite particles having a large axial ratio. The economical resources - energy reaction is capable of enhancing the productivity because it is possible to react at a high concentration, to reduce the amount of alkaline solution, and to reduce the amount of energy in the ageing process. Accordingly, it is possible to produce spindle-shaped magnetic iron oxide particles with the surfaces thereof modified by Co industrially and economically advantageously which have a fine particle size, a large axial ratio, a high coercive force and an excellent print-through characteristic (the correction value of the print-through characteristic is not less than 52.0 dB, preferably not less than 53.0 dB in the case of magnetite particles, and not less than 57.0 dB, preferably not less than 58.0 dB in the case of maghemite particles). The present invention is further explained in the following Examples. Examples The major axis diameter and the axial ratio in each of the following Examples and Comparative Examples are expressed as average values of the values obtained by measuring them in electron microphotographs. The print-through characteristic is expressed by the correction value Q₁ of the print-through characteristic at a major axial diameter of 0.2 µm obtained by inserting the measured value of print-through characteristic and the major axial diameter in the following formula (1), which is obtained from the straight line A in Fig. 1: Q₁ = 40 x (0.2 - A) + B The print-through characteristic of the spindle-shaped magnetite particles or maghemite particles with the surfaces thereof modified by Co in the present invention is expressed by the correction value Q₃ of print-through characteristic at a coercive force of 700 Oe and a major axis diameter of 0.2 µm by inserting the measured values of print-through characteristic and coercive force into the following formula (2) so as to obtain the correction value Q₂ at a coercive force of 700 Oe, and inserting the thus-obtained correction value Q₂ and the major axis diameter into the following formula (3) which is obtained from the straight lines A in Figs. 4(a), 4(b), 5(a) and 5(b) showing the relationship between the correction value of print-through characteristic Q₂ and the major axis diameter: Q₂ = (700 - C) x 0.02 + B Q₃ = 40 x (0.2 - A) + Q₂ In formulae (1) to (3), Q₁ is the correction value (dB) of print-through characteristic at a major axis diameter of 0.2 µm, Q₂ is the correction value (dB) of the print-through characteristic at a coercive force of 700 Oe, Q₃ is the correction value (dB) of the print-through characteristic at a coercive force of 700 Oe and a major axis diameter of 0.2 µm, A is the major axis diameter (µm), B is the measured print-through characteristic and C is the measured a coercive force. The print-through characteristic was measured in accordance with the method described in JOURNAL OF THE JAPAN SOCIETY OF POWDER AND POWDER METALLURGY (1979) (published by Funtai Fummatsu Yakin Kyokai Corporation) Vol. 26, No. 4, page 149 and REPORT OF TECHNICAL RESEARCHES OF DENSHI TSUSHIN GAKKAI (published by Denshi Tsushin Gakkai Corporation) MR77-27, page 2. The magnetic iron oxide particles charged into a cylindrical container 6 mm in diameter and 5 mm in height were retained in a magnetic field of 50 Oe at 60°C for 80 minutes for magnetization. After the magnetic iron oxide particles were cooled to room temperature, the residual magnetization (Irp) was measured. A DC magnetic field was then applied to the sample, and the saturated residual magnetization (Irs) was obtained. The print-through characteristic was calculated from the following formula as the measured value: Measured value of Print-through characteristic P.T. (dB) = -20 log Irp/Irs. Production of Spindle-Shaped Goethite Particles Examples 1 to 8 Comparative Examples 1 to 6 Example 1 Into a reaction vessel which was maintained in a non-oxidizing atmosphere by blowing N₂ gas thereinto at a rate of 3.4 cm /second, 704 ℓ of an aqueous solution of 1.16 mol/ℓ of Na₂CO₃ was charged. The solution was mixed with 296 ℓ of an aqueous ferrous sulfate solution containing 1.35 mol/ℓ of Fe²⁺ (the content of Na₂CO₃ corresponds to 2.0 times equivalent based on Fe) to produce FeCO₃ at 47°C. The suspension containing FeCO₃ was maintained at 47°C for 70 minutes while continuously blowing N₂ gas thereinto at a rate of 3.4 cm /second. Thereafter, air was blown into the suspension containing FeCO₃ at a rate of 2.8 cm / second at 47°C for 5 hours, thereby obtaining yellowish brown precipitated particles. The pH of the suspension during the aeration was 8.5 to 9.5. A part of the suspension containing the yellowish brown precipitated particles was filtered out, washed with water, dried and pulverized by an ordinary method. The thus-obtained yellowish brown precipitated particles were goethite particles as determined by X-ray flourescent analysis. As is clear from the electron micrograph (x 30000) in Fig. 8, the goethite particles were spindle-shaped particles which had an average major axis diameter of 0.30 µm and an axial ratio of 12.6:1, and were substantially free of dendrites. 3000 g of the paste (equivalent to about 1000 g of the spindle-shaped goethite particles) obtained by filtering out the suspension containing the spindle-shaped goethite particles and washing it with water was suspended in 60 ℓ of water. The pH of the suspension was 9.7. To the suspension, 300 mℓ (equivalent to 2.0 wt% based on the spindle-shaped goethite particles) of an aqueous solution containing 20 g of sodium hexametaphosphate was added and the mixture was stirred for 30 minutes. 10% acetic acid was added to the mixture so that the pH of the suspension became 5.8. The spindle-shaped goethite particles were filtered out by a filter press and dried to obtain the spindle-shaped goethite particles coated with the P compound. Examples 2 to 4, Comparative Examples 1 to 5 Spindle-shaped goethite particles coated with a P compound, an Si compound or both compounds thereof were produced in the same way as in Example 1 except that the flow rate of N₂ gas, the kind, concentration, amount and mixing ratio of the aqueous alkali carbonate, the kind, concentration and amount of the aqueous Fe²⁺ solution and the temperature in the production of FeCO₃; the flow rate of N₂ gas, the temperature and the time in the ageing process; the temperature, the air flow rate and the reaction time in the oxidation process; and the kind and amount of solution in the coating process were varied. The main conditions for production and the properties of the goethite particles obtained are shown in Tables 1 and 2. The spindle-shaped goethite particles obtained in Examples 2 to 4 all had a uniform particle size and shape, and were substantially free of dendrites. In Example 3, Ni-containing goethite particles [the Ni content was 0.49 atomic% (calculated as Ni/Fe)] were produced by adding 0.5 atomic% of NiSO₄ (calculated as Ni/Fe) in the production of FeCO₃. In the spindle-shaped goethite particles obtained in Comparative Example 1, the particle size was non-uniform and aggregates were produced, as shown in the electron micrograph (x 30000) of Fig. 10. The electron micrographs (x 30000) of the spindle-shaped goethite particles obtained in Comparative Example 5 are shown in Figs. 11. The axial ratios of the spindle-shaped goethite particles obtained in Comparative Example 5 were small. Example 5 Into a reaction vessel which was maintained in a non-oxidizing atmosphere by blowing N₂ gas thereinto at a rate of 3.4 cm /second, 600 ℓ of an aqueous solution of 1.35 mol/ℓ of Na₂CO₃ was charged. The solution was mixed with 300 ℓ of an aqueous ferrous sulfate solution containing 1.35 mol/ℓ of Fe²⁺ (the content of Na₂CO₃ corresponds to 2.0 times equivalent based on Fe) to produce FeCO₃ at 47°C. The suspension containing FeCO₃ was maintained at 47°C for 60 minutes while continuously blowing N₂ gas thereinto at a rate of 3.4 cm /second, and 5.0 ℓ of an aqueous zinc sulfate solution containing 3.0 atomic% of Zn based on Fe was added. The mixture was held for 10 minutes more. Thereafter, air was passed into the aged suspension containing FeCO₃ at a rate of 2.8 cm /second at 47°C for 6.0 hours, thereby obtaining yellowish brown precipitated particles. The pH of the suspension during the aeration was 8.5 to 9.5. A part of the suspension containing the yellowish brown precipitated particles was filtered out, washed with water, dried and pulverized by an ordinary method. The thus-obtained yellowish brown precipitated particles were goethite particles as determined by X-ray (fluorescent) analysis. As is clear from the electron micrograph (x 30000) in Fig. 9, the goethite particles were spindle-shaped particles which had an average major axis diameter of 0.29 µm and an axial ratio of 17.0 and were substantially free of dendrites. The zinc content was 3.0 atomic% based on Fe. 3000 g of the paste (equivalent to about 1000 g of the spindle-shaped goethite particles) obtained by filtering out the suspension containing the spindle-shaped goethite particles and washing it with water was suspended in 60 ℓ of water. The pH of the suspension was 9.8. To the suspension, 20 g (equivalent to 2.0 wt% based on the spindle-shaped goethite particles) of sodium silicate (water glass No. 3) was added and the mixture was stirred for 60 minutes. 10% acetic acid was added to the mixture so that the pH of the suspension became 5.8. The spindle-shaped goethite particles were filtered out by a filter press and dried to obtain the spindle-shaped goethite particles coated with the Si compound. The properties of the spindle-shaped goethite particles obtained are shown in Table 2. Examples 6 to 8, Comparative Example 6 Spindle-shaped goethite particles were produced in the same way as in Example 5 except that the kind, concentration, and amount of aqueous alkali carbonate, the kind, concentration and amount of the aqueous Fe²⁺ solution and the temperature in the production of FeCO₃; the temperature and the time in the ageing process; the kind and amount of Zn compound and timing for adding it; the temperature, the air flow rate and the reaction time in the oxidation process; and the kind and amount of solution in the coating process were varied. The main conditions for production and the properties of the goethite particles obtained are shown in Tables 1 and 2. As determined by observation through an electron microscope, the spindle-shaped goethite particles obtained in Examples 6 to 8 all had a uniform particle size and were substantially free of dendrites. The electron micrographs (x 30000) of the spindle-shaped goethite particles obtained in Comparative Example 6 are shown in Fig. 12. The axial ratio of the spindle-shaped goethite particles obtained in Comparative Example 6 was small. Production of Spindle-Shaped Hematite Particles Examples 9 to 16 Comparative Examples 7 to 12 Example 9 800 g of the spindle-shaped goethite particles coated with the P compound which had been obtained in Example 1 were heated to 600°C in air to obtain spindle-shaped hematite coated with the P compound. As determined by observation through an electron microscope, the particles had an average major axis diameter of 0.24 µm and an axial ratio of 11.1:1. Examples 10 to 16, Comparative Examples 7 to 12 Spindle-shaped hematite particles were obtained in the same way as in Example 9 except that the kind of the spindle-shaped goethite particles coated with a P compound, an Si compound or both compounds thereof and the heating temperature were varied. The main conditions for production and the properties of the hematite particles obtained are shown in Table 3. Production of Spindle-Shaped Magnetite Particles Examples 17 to 24 Comparative Examples 13 to 18 Example 17 1000 g of the spindle-shaped hematite particles obtained in Example 9 were charged into a 13-ℓ retort reducing vessel, and H₂ gas was blown into the particles at a rate of 1.0 ℓ/minute while rotating the vessel to reduce the hematite particles at 300°C for 3 hours, thereby obtaining spindle-shaped magnetite particles. The electron micrograph (x 30000) of the spindle-shaped magnetite particles obtained is shown in Fig. 13. As is clear from the electron micrograph, the spindle-shaped magnetite particles were had an average major axis diameter of 0.23 µm and an axial ratio of 10.2:1, and a uniform particle size, and were substantially free of dendrites. When the magnetic characteristics were measured, the coercive force was 350 Oe, the saturated magnetization was 81.5 emu/g and the correction value of print-through characteristic Q₁ was 46.8 dB. Examples 18 to 24, Comparative Examples 13 to 18 Spindle-shaped magnetite particles were obtained in the same way as in Example 17 except that the kind of the hematite particles and the reducing temperature were varied. The main conditions for production and the properties of the hematite particles obtained are shown in Table 4. As determined by observation through an electron microscope, all of the spindle-shaped magnetite particles obtained in Examples 18 to 24 had a uniform particle size and were substantially free of dendrites. The electron micrograph (x 30000) of the spindle-shaped magnetite particles obtained in Example 21 is shown in Fig. 14. Production of Spindle-Shaped Maghemite Particles Examples 25 to 32 Comparative Examples 19 to 24 Example 25 600 g of the spindle-shaped magnetite particles obtained in Example 17 were oxidized at 270°C for 30 minutes to obtain spindle-shaped maghemite particles. As determined by observation through an electron microscope, the spindle-shaped maghemite particles obtained had an average major axis diameter of 0.23 µm and an axial ratio of 10.2:1, a uniform particle size, and were substantially free of dendrites. When the magnetic characteristics were measured, the coercive force was 390 Oe, the saturated magnetization was 72.0 emu/g and the correction value of print-through characteristic Q₁ was 55.8 dB. The electron micrograph (x 30000) of the spindle-shaped maghemite particles obtained in Exmaple 25 is shown in Fig. 15. Examples 26 to 32, Comparative Examples 19 to 24 Spindle-shaped maghemite particles were obtained in the same way as in Example 25 except that the kind of the spindle-shaped magnetite particles was varied. The main conditions for production and the properties of the maghemite particles obtained are shown in Table 5. As determined by observation through an electron microscope, all of the spindle-shaped maghemite particles obtained in Examples 26 to 32 had a uniform particle size and shape, and were substantially free of dendrites. The electron micrographs (x 30000) of the spindle-shaped maghemite particles obtained in Example 29 and Comparative Example 23 are shown in Figs. 16 and 17, respectively. Production of Spindle-Shaped Co-Modified Magnetite Particles Examples 33 to 40 Comparative Examples 25 to 30 Example 33 100 g of spindle-shaped magnetite particles obtained in Example 17 were charged into 1.0 ℓ of water with 0.056 mol of cobalt and 0.113 mol of ferrous iron was dissolved therein by using cobalt sulfate and ferrous sulfate while preventing the inclusion of air as much as possible, and dispersed until the dispersion became a fine slurry. Into the dispersion, 163 mℓ of 18-N aqueous NaOH solution was poured and water was further added so as to form 1.3 ℓ of a dispersion in which the hydroxyl concentration was 2.0 mol/ℓ. The temperature of the dispersion was raised to 100°C and it was stirred for 10 hours. Thereafter, the slurry was taken out, washed with water, filtered out and dried at 60°C to obtain spindle-shaped Co-modified magnetite particles. As determined by observation through an electron microscope, the spindle-shaped magnetite particles had the same configuration and particle size as the precursor, namely, the spindle-shaped magnetite particles had an average major axis diameter of 0.23 µm and an axial ratio of 9.2:1. When the magnetic characteristics were measured, the coercive force was 729 Oe, the saturated magnetization as was 82.5 emu/g and the corrected value of print-through characteristic Q₃ was 55.2 dB. The particles contained 3.82 atomic% of cobalt based on the amount of Fe and Co. Examples 34 to 40, Comparative Examples 25 to 30 Spindle-shaped magnetite particles modified by Co or Co and Fe(II) were obtained in the same way as in Example 33 except that the kind of the precursor, the amounts of Co added, Fe(II) added and NaOH added, the hydroxyl concentration and the temperature were varied under conditions that the amount of precursor spindle-shaped magnetite particles was 100 g and the whole volume of the dispersion was 1.3 ℓ. The main conditions for production and the properties of the particles obtained are shown in Table 6. The electron micrograph (x 30000) of the spindle-shaped magnetite particles modified by Co and Fe(II) obtained in Example 37 is shown in Fig. 18. Production of Spindle-Shaped Co-Modified Maghemite Particles Examples 41 to 48 Comparative Examples 31 to 36 Example 41 100 g of spindle-shaped maghemite particles obtained in Example 25 were charged into 1.0 ℓ of water with 0.048 mol of cobalt and 0.107 mol of ferrous iron was dissolved therein by using cobalt sulfate and ferrous sulfate while preventing the inclusion of air as much as possible, and dispersed until the dispersion became a fine slurry. Into the dispersion, 53 mℓ of 18-N aqueous NaOH solution was poured and water was further added so as to form 1.3 ℓ of dispersion in which the hydroxyl concentration was 0.5 mol/ℓ. The temperature of the dispersion was raised to 100°C and it was stirred for 10 hours. Thereafter, the slurry was taken out, washed with water, filtered out and dried at 60°C to obtain spindle-shaped Co-modified maghemite particles. As is clear from the electron micrograph (x 30000) shown in Fig. 19, the spindle-shaped maghemite particles had the same configuration and the particle size as the precursor, namely, the spindle-shaped maghemite particles had an average major axis diameter of 0.23 µm and an axial ratio of 9.1:1. When the magnetic characteristics were measured, the coercive force was 508 Oe, the saturated magnetization σs was 76.3 emu/g and the correction value of print-through characteristic Q₃ was 60.4 dB. The particles contained 3.41 atomic% of cobalt based on the amount of Fe and Co. Examples 42 to 48, Comparative Examples 31 to 36 Spindle-shaped maghemite particles modified by Co or Co and Fe(II) were obtained in the same way as in Example 41 except that the kind of the precursor, the amounts of Co added, Fe(II) added and NaOH added, the hydroxyl concentration and the temperature were varied under conditions that the amount of precursor spindle-shaped maghemite particles was 100 g and the whole volume of the dispersion was 1.3 ℓ. The main conditions for production and the properties of the particles obtained are shown in Table 7. Production of Spindle-Shaped Goethite Particles Examples 49 to 56 Comparative Examples 37 to 42 Example 49 Into a reaction vessel which was maintained in a non-oxidizing atmosphere by blowing N₂ gas thereinto at a rate of 3.4 cm /second, 580 ℓ of an aqueous solution of 1.32 mol/ℓ of Na₂CO₃ and 20.0 ℓ of an aqueous solution of 13.5 mol/ℓ of NaOH (which corresponds to 17.6% based on Na₂CO₃) were added. The solution was mixed with 400 ℓ of an aqueous ferrous sulfate solution containing 1.5 mol/ℓ of Fe²⁺ (the total content of Na₂CO₃ and NaOH corresponds to 1.5 times equivalent based on Fe²⁺, and the concentration of Fe²⁺ is equivalent to 0.60 mol/ℓ) to produce an Fe ² ⁺ containing precipitate at 45°C. The suspension containing the Fe ² ⁺ containing precipitate was maintained at 45°C for 50 minutes while continuously blowing N₂ gas thereinto at a rate of 3.4 cm /second. Thereafter, air was blown into the suspension of the Fe ² ⁺ containing precipitate at a rate of 4.5 cm /second at 47°C for 5.5 hours, thereby obtaining yellowish brown precipitated particles. The pH of the suspension during the aeration was 8.5 to 10.0. A part of the suspension containing the yellowish brown precipitated particles was filtered out, washed with water, dried and pulverized by an ordinary method. The thus-obtained yellowish brown precipitated particles proved to be goethite particles as determined by X-ray (fluorescent) analysis. As is clear from the electron micrograph (x 30000) in Fig. 20, the goethite particles were spindle-shaped particles which had an average major axis diameter of 0.29 µm and an axial ratio of 12.0:1, and were substantially free of dendrites. 3000 g of the paste (equivalent to about 1000 g of the spindle-shaped goethite particles) obtained by filtering out the suspension containing the spindle-shaped goethite particles and washing it with water was suspended in 60 ℓ of water. The pH of the suspension was 9.7. To the suspension, 300 mℓ (equivalent to 1.5 wt% based on the spindle-shaped goethite particles) of an aqueous solution containing 15 g of sodium hexametaphosphate was added and the mixture was stirred for 30 minutes. After adding 10 g of sodium silicate (water glass No. 3) (equivalent to about 1.0 wt% based on the spindle-shaped goethite particles) and stirring the resultant mixture for 60 minutes, 10% acetic acid was added to the mixture so that the pH of the suspension became 5.8. The spindle-shaped goethite particles were filtered out by a filter press and dried to obtain the spindle-shaped goethite particles coated with the P compound and the Si compound. Examples 50 and 51, Comparative Examples 37, 38 and 41 Yellowish brown particles were obtained in the same way as in Example 49 except that the kind, concentration, amount and mixing ratio of aqueous alkali carbonate, the kind, concentration, amount, mixing ratio and timing for addition of the aqueous alkali hydroxide, the kind, concentration, amount and reaction concentration of the aqueous Fe²⁺ solution and the mixing temperature in the production of an Fe ² ⁺ containing precipitate or FeCO₃; the temperature and the time in the ageing process; the temperature, the air flow rate and the reaction time in the oxidation process; and the kind and amount of solution in the coating process were varied. The main conditions for production and the properties of the goethite particles obtained are shown in Tables 8 and 9. All of the spindle-shaped goethite particles obtained in Examples 50 and 51 had a uniform particle size and were substantially free of dendrites. The particles obtained in Comparative Examples 37 and 38 were spindle-shaped goethite particles and had dendrites, as determined from the electron micrographs (x 30000) of Figs. 22 and 23, respectively, and by X-ray (fluorescent) analysis. Example 52 Into a reaction vessel which was maintained in a non-oxidizing atmosphere by blowing N₂ gas thereinto at a rate of 3.4 cm /second, 558 ℓ of an aqueous solution of 0.968 mol/ℓ of Na₂CO₃ and 42.0 ℓ of an aqueous solution of 6.5 mol/ℓ of NaOH (which corresponds to 25.3% based on Na₂CO₃) were added. The solution was mixed with 300 ℓ of an aqueous ferrous sulfate solution containing 1.5 mol/ℓ of Fe²⁺ (the total content of Na₂CO₃ and NaOH corresponds to 1.5 times equivalent based on Fe²⁺, and the concentration of Fe²⁺ is equivalent to 0.50 mol/ℓ) to produce an Fe ² ⁺ containing precipitate at 45°C. The suspension containing the Fe ² ⁺ containing precipitate was maintained at 50°C for 70 minutes while continuously blowing N₂ gas thereinto at a rate of 3.4 cm /second. An aqueous zinc sulfate solution was then added to the suspension so that the suspension contained 3.0 atomic% of zinc based on Fe²⁺ and the suspension was held for 10 minutes. After the ageing, air was blown into the suspension of the Fe ² ⁺ containing precipitate at a rate of 4.0 cm/second at 50°C for 6.7 hours, thereby obtaining yellowish brown precipitated particles. The pH of the suspension during the aeration was 8.7 to 9.8. A part of the suspension containing the yellowish brown precipitated particles was filtered out, washed with water, dried and pulverized by an ordinary method. The thus-obtained yellowish brown precipitated particles were goethite particles as determined by X-ray (fluorescent) analysis. As is clear from the electron micrograph (× 30000) in Fig. 21, the goethite particles were spindle-shaped particles which had an average major axis diameter of 0.29 µm and an axial ratio of 17.6:1, and were substantially free of dendrites. The zinc content was 3.0 atomic % zinc based on the amount of Fe. 3000 g of the paste (equivalent to about 1000 g of the spindle-shaped goethite particles) obtained by filtering out the suspension containing the spindle-shaped goethite particles and washing it with water was suspended in 60 ℓ of water. The pH of the suspension was 9.8. After adding 20 g of sodium silicate (water glass No. 3) (equivalent to about 2.0 wt% based on the spindle-shaped goethite particles) to the suspension and stirring the resultant mixture for 60 minutes, 10% acetic acid was added to the mixture so that the pH of the suspension became 5.8. The spindle-shaped goethite particles were filtered out by a filter press and dried to obtain the spindle-shaped goethite particles coated with the Si compound. Examples 53 to 56, Comparative Examples 39, 40 and 42 Yellowish brown particles were obtained in the same way as in Example 52 except that the kind, concentration, amount and mixing ratio of the aqueous alkali carbonate, the kind, concentration, amount, mixing ratio and timing for addition of the aqueous alkali hydroxide, the kind, concentration, amount and reaction concentration of the aqueous Fe²⁺ solution, and the mixing temperature in the production of an Fe ² ⁺ containing precipitate or FeCO₃; the temperature and the time in the ageing process; the kind, amount and timing for adding a zinc compound, if any; the temperature, the air flow rate and the reaction time in the oxidation process; and the kind and amount of solution in the coating process were varied. The main conditions for production and the properties of the goethite particles obtained are shown in Tables 8 and 9. All of the spindle-shaped goethite particles obtained in Examples 53 to 56 had a uniform particle size and were substantially free of dendrites. The particles obtained in Comparative Examples 39 and 40 were spindle-shaped goethite particles and had dendrites, as determined from the electron micrographs (× 30000) of Figs. 24 and 25, respectively, and by X-ray (fluorescent) analysis. Production of Spindle-Shaped Hematite Particles Coated with Sintering Inhibitor Examples 57 to 64 Comparative Examples 43 and 44 Example 57 800 g of the spindle-shaped goethite particles coated with the P compound and the Si compound obtained in Example 49 were heated to 600°C in air to obtain spindle-shaped hematite coated with the P compound and the Si compound. As determined by observation through an electron microscope, the particles had an average major axis diameter of 0.22 µm and an axial ratio of 8.4:1. Examples 58 to 64, Comparative Examples 43 and 44 Spindle-shaped hematite particles were obtained in the same way as in Example 57 except that the kind of the spindle-shaped goethite particles and the heating temperature were varied. The main conditions for production and the properties of the hematite particles obtained are shown in Table 10. Production of Spindle-Shaped Magnetite Particles Examples 65 to 72 Comparative Examples 45 to 46 Example 65 600 g of the spindle-shaped hematite particles obtained in Example 57 were charged into a 1.0-ℓ retort reducing vessel, and H₂ gas was blown into the particles at a rate of 0.2 ℓ/minute while rotating the vessel to reduce the hematite particles at 330°C for 3 hours, thereby obtaining spindle-shaped magnetite particles. The electron microphotograph (× 30000) of the spindle-shaped magnetite particles obtained is shown in Fig. 26. As is clear from the electron microphotograph, the spindle-shaped magnetite particles had an average major axis diameter of 0.20 µm, an axial ratio of 7.2:1 and a uniform particle size, and were substantially free of dendrites. When the magnetic characteristics were measured, the coercive force was 365 Oe, the saturated magnetization as was 80.6 emu/g and the correction value of print-through characteristic Q₁ was 45.8 dB. Examples 66 to 72, Comparative Examples 45 and 46 Spindle-shaped magnetite particles were obtained in the same way as in Example 65 except that the kind of the hematite particles and the reducing temperature were varied. The main conditions for production and the properties of the hematite particles obtained are shown in Table 11. The electron micrograph (× 30000) of the spindle-shaped magnetite particles obtained in Example 68 is shown in Fig. 27. Production of Spindle-Shaped Maghemite Particles Examples 73 to 80 Comparative Examples 47 and 48 Example 73 600 g of the spindle-shaped magnetite particles obtained in Example 65 were oxidized at 270°C for 30 minutes to obtain spindle-shaped maghemite particles. As is clear from the electron micrograph (× 30000) shown in Fig. 28, the spindle-shaped maghemite particles obtained had an average major axis diameter of 0.20 µm and an axial ratio of 7.1:1 and a uniform particle size, and were substantially free of dendrites. When the magnetic characteristics were measured, the coercive force was 400 Oe, the saturated magnetization as was 70.5 emu/g and the correction value of print-through characteristic Q₁ was 54.0 dB. Examples 74 to 80, Comparative Examples 47 and 48 Spindle-shaped maghemite particles were obtained in the same way as in Example 73 except that the kind of the magnetite particles was varied. The main conditions for production and the properties of the maghemite particles obtained are shown in Table 12. The electron micrographs (× 30000) of the spindle-shaped maghemite particles obtained in Example 76 is shown in Fig. 29. Production of Spindle-Shaped Co-Modified Magnetite Particles Examples 81 to 88 Comparative Examples 49 and 50 Example 81 100 g of spindle-shaped magnetite particles obtained in Example 65 were charged into 1.0 ℓ of water with 0.085 mol of cobalt dissolved therein by using cobalt sulfate while preventing the inclusion of air as much as possible, and dispersed until the dispersion became a fine slurry. Into the dispersion, 226 mℓ of 18-N aqueous NaOH solution was poured and water was further added so as to form 1.3 ℓ of a dispersion in which the hydroxyl concentration was 3.0 mol/ℓ. The temperature of the dispersion was raised to 100°C and it was stirred for 10 hours. Thereafter, the slurry was taken out, washed with water, filtered out and dried at 60°C to obtain spindle-shaped Co-modified magnetite particles. As determined by observation through an electron microscope, the spindle-shaped Co-modified magnetite particles had the same configuration and the particle size as the precursor, namely, the spindle-shaped magnetite particles had an average major axis diameter of 0.20 µm and an axial ratio of 6.9:1. When the magnetic characteristics were measured, the coercive force was 760 Oe, the saturated magnetization σs was 77.0 emu/g and the correction value of print-through characteristic Q₃ was 53.9 dB. The particles contained 6.15 atomic % of cobalt based on the amount of Fe and Co. Examples 82 to 88, Comparative Examples 49 and 50 Spindle-shaped magnetite particles modified by Co or Co and Fe(II) were obtained in the same way as in Example 81 except that the kind of the precursor, the amounts of Co added, Fe(II) added and NaOH added, the hydroxyl concentration and the temperature were varied under conditions such that the amount of precursor spindle-shaped magnetite particles was 100 g and the whole volume of the dispersion was 1.3 ℓ. The main conditions for production and the properties of the particles obtained are shown in Table 13. The electron micrograph (× 30000) of the spindle-shaped magnetite particles modified by Co and Fe(II) obtained in Example 84 is shown in Fig. 30. Production of Spindle-Shaped Co-Modified Maghemite Particles Examples 89 to 96 Comparative Examples 51 and 52 Example 89 100 g of spindle-shaped maghemite particles obtained in Example 73 were charged into 1.0 ℓ of water with 0.153 mol of cobalt dissolved therein by using cobalt sulfate while preventing the inclusion of air as much as possible, and dispersed until the dispersion became a fine slurry. Into the dispersion, 378 mℓ of 18-N aqueous NaOH solution was poured and water was further added so as to form 1.3 ℓ of a dispersion in which the hydroxyl concentration was 5.0 mol/ℓ. The temperature of the dispersion was raised to 100°C and it was stirred for 10 hours. Thereafter, the slurry was taken out, washed with water, filtered out and dried at 60°C to obtain spindle-shaped Co-modified maghemite particles. As is clear from the electron micrograph (× 30000) shown in Fig. 31, the spindle-shaped Co-modified maghemite particles had the same configuration and the particle size as the precursor, namely, the spindle-shaped maghemite particles and had an average major axis diameter of 0.20 µm and an axial ratio of 6.5:1. When the magnetic characteristics were measured, the coercive force was 912 Oe, the saturated magnetization σs was 67.9 emu/g and the correction value of print-through characteristic Q₃ was 59.5 dB. The particles contained 10.88 at% of cobalt based on the amount of Fe and Co. Examples 90 to 96, Comparative Examples 51 and 52 Spindle-shaped maghemite particles modified by Co or Co and Fe(II) were obtained in the same way as in Example 89 except that the kind of the precursor, the amounts of Co added, Fe(II) added and NaOH added, the hydroxyl concentration and the temperature were varied under conditions such that the amount of precursor spindle-shaped magnetite particles was 100 g and the whole volume of the dispersion was 1.3 ℓ. The main conditions for production and the properties of the particles obtained are shown in Table 14.

Claims: Magnetic iron oxide particles which have a substantially uniform particle size, which are substantially free of dendrites, which have a print-through characteristic Q₁ of not less than 45 dB wherein Q₁ is calculated from the formula: Q₁ = 40 x (0.2 - A) + B in which A is the measured major axis diameter in µm and B is the measured print-through characteristic and which consist essentially of spindle-shaped magnetite ( FeO x .Fe₂O₃, 0 ≦ x ≦ 1) particles having a major axis diameter of from 0.1 to 0.3 µm and an axial ratio (major axis diameter: minor axis diameter) of not less than 7:1, which spindle-shaped magnetite particles are coated with a material having a sintering preventing effect. Magnetic iron oxide particles according to claim 1, wherein the spindle-shaped magnetite particles contain zinc and have a said axial ratio of not less than 8:1. Magnetic iron oxide particles which have a substantially uniform particle size, which are substantially free of dendrites, which have a print-through characteristic Q₁ of not less than 53 dB wherein Q₁ is calculated from the formula: Q₁ = 40 x (0.2 - A) + B in which A is the measured major axis diameter in µm and B is the measured print-through characteristic and which consist essentially of spindle-shaped maghemite particles having a major axis diameter of from 0.1 to 0.3 µm and an axial ratio (major axis diameter: minor axis diameter) of not less than 7:1, which maghemite particles are coated with a material having a sintering preventing effect. Magnetic iron oxide particles according to claim 3, whrein the spindle-shaped maghemite particles contain zinc and have a said axial ratio of not less than 8:1. Magnetic iron oxide particles according to claim 1 or 2 which consist essentially of the spindle-shaped magnetite particles whose surfaces are modified by from 0.5 to 15.0 atomic % of Co based on the total amount Fe and Co, which magnetic iron oxide particles have a coercive force of from 3.82 x 10⁴ to 7.96 x 10⁴ Am⁻¹ (480 to 1000 Oe) and a print-through characteristic Q₃ of not less than 52 dB wherein Q₃ is calculated from the formulae: Q₃ = 40 x (0.2 - A) + Q₂ and Q₂ = (700 - C) x 0.02 + B in which A and B are as defined in claim 1 and C is the measured coercive force in Oe. Magnetic iron oxide particles according to claim 3 or 4 which consist essentially of the spindle-shaped maghemite particles whose surfaces are modified by from 0.5 to 15.0 atomic % of Co based on the total amount of Fe and Co, which magnetic iron oxide particles have a coercive force of from 3.82 x 10⁴ to 7.96 x 10⁴ Am⁻¹ (480 to 1000 Oe) and a print-through characteristic Q₃ of not less than 57 dB wherein Q₃ is calculated from the formulae: Q₃ = 40 x (0.2 - A) + Q₂ and Q₂ = (700 - C) x 0.02 + B in which A and B are as defined in claim 3 and C is the measured coercive force in Oe. A process for producing magnetic iron oxide particles as defined in claim 1, which process comprises: (i) ageing an aqueous suspension of FeCO₃, obtained by reacting an aqueous alkali carbonate with an aqueous ferrous salt solution, in a non-oxidizing atmosphere at a temperature of from 40 to 60°C for from 50 to 100 minutes, the amount of the aqueous alkali carbonate being from 1.5 to 3.5 times equivalent of the amount of Fe²⁺ in the aqueous ferrous salt solution; (ii) blowing a molecular oxygen-containing gas into the suspension containing said FeCO₃ at a reaction temperature of from 40 to 70°C and a pH of from 7 to 11, thereby oxidising the FeCO₃ to produce spindle-shaped goethite particles; (iii) coating said spindle-shaped goethite particles or spindle-shaped hematite particles obtained by baking said spindle-shaped goethite particles with a material having a sintering preventing effect; and (iv) reducing by heating in a reducing gas said coated particles. A process according to claim 7, wherein: a zinc compound is added to a solution of said aqueous alkali carbonate, to said aqueous ferrous salt solution or to said suspension of FeCO₃ prior to step (ii) so that spindle-shaped goethite particles containing zinc are obtained in step (ii); and said spindle-shaped goethite particles containing zinc or spindle-shaped hematite particles containing zinc obtained by baking said spindle-shaped goethite particles containing zinc are reduced under heating in a reducing gas, thereby obtaining spindle-shaped magnetite particles containing zinc as defined in claim 2. A process according to claim 7 or 8, wherein the resulting spindle-shaped magnetite particles are further oxidized, thereby obtaining spindle-shaped maghemite particles as defined in claim 3 or 4. A process according to any one of claims 7 to 9, wherein: an aqueous dispersion of the resulting spindle-shaped particles is mixed with at least an aqueous Co salt solution and an aqueous alkaline solution so that the resulting dispersion has a pH of not less than 11; and the said dispersion is heated to a temperature of 50 to 100°C, thereby obtaining Co-modified spindle-shaped particles as defined in claim 5 or 6. A process for producing magnetic iron oxide particles as defined in claims 1 or 3, which process comprises: (a) ageing a suspension of FeCO₃, obtained by reacting an aqueous alkali carbonate with an aqueous ferrous salt solution, in a non-oxidizing atmosphere at a temperature of from 30 to 60°C for from 10 to 100 minutes and blowing a molecular oxygen-containing gas into said suspension of FeCO₃ at a reaction temperature of from 30 to 70°C and a pH of from 7 to 11, thereby oxidizing the FeCO₃ to produce spindle-shaped goethite particles, wherein from 1 to 50% of an aqueous alkali hydroxide based on the amount of said aqueous alkali carbonate is added to a solution of said aqueous alkali carbonate or to said suspension of FeCO₃ prior to oxidation of the FeCO₃ so that the total amount of aqueous alkali carbonate and aqueous alkali hydroxide is 1.1 to 2.5 times equivalent to the amount of Fe²⁺ in said aqueous ferrous salt solution; (b) coating said spindle-shaped goethite particles or spindle-shaped hematite particles obtained by baking said spindle shaped goethite particles, with a material having a sintering preventing effect; (c) reducing by heating in a reducing gas said coated particles, thereby producing spindle-shaped magnetite particles; and (d) if desired, oxidizing said spindle-shaped magnetite particles to produce spindle-shaped maghemite particles. A process according to claim 11 for producing magnetic iron oxide particles as defined in claim 2 or 4, wherein: a zinc compound is added to a solution of said aqueous alkali carbonate, to said aqueous ferrous salt solution or to said suspension of FeCO₃ prior to oxidation of the FeCO₃, thereby obtaining spindle-shaped goethite particles containing zinc; and said spindle-shaped goethite particles containing zinc or spindle-shaped hematite particles containing zinc obtained by baking said spindle-shaped goethite particles containing zinc are reduced under heating in a reducing gas, thereby obtaining spindle-shaped magnetite particles containing zinc; and if desired, said spindle-shaped magnetite particles are oxidized thereby obtaining spindle-shaped maghemite particles containing zinc. A process according to claim 11 or 12 for producing magnetic iron oxide particles as defined in claim 5 or 6, wherein: an aqueous dispersion of the resulting spindle-shaped particles is mixed with at least an aqueous Co salt solution and an aqueous alkaline solution so that the resulting dispersion has a pH of not less than 11; and the said dispersion is heated to a temperature of from 50 to 100°C, thereby obtaining spindle-shaped particles with their surfaces modified by from 0.5 to 15.0 atomic % of Co based on the total amount of Fe and Co. A magnetic recording medium in which the magnetic material consists essentially of magnetic iron oxide particles as defined in any one of claims 1 to 6 or which have been produced by a process as claimed in any one of claims 7 to 13.

Inventors: FUJITA KAZUHIRO, HARADA TOSIHARU, ISOAI MASARU, TAKEDOI ATSUSHI, TANIHARA MAMORU, FUJITA, KAZUHIRO, HARADA, TOSIHARU, ISOAI, MASARU, TAKEDOI, ATSUSHI, TANIHARA, MAMORU
Applicants: TODA KOGYO CORP, TODA KOGYO CORP.
Application year: 1989
Main classification: G11B5
IPC codes: G11B5