Perpendicular magnetic films, multi-layered films suitable for conversion to perpendicular magnetic films and process for producing perpendicular magnetic films from said multi-layered films.

Abstract: A perpendicular magnetic film comprising a Co-containing γ-Fe₂O₃ film formed on a substrate, in which the plane (400) is predominantly oriented in parallel with said substrate, the molar ratio of Co to Fe is from 0.01:1 to 0.32:1, the spacing of said plane (400) is not more than 2.084 A, and the optical absorption coefficient at 700 nm is not more than 2.5 µm⁻¹.

Description: The present invention relates to a perpendicular magnetic film and a multi-layered film for a perpendicular magnetic film. More particularly, the present invention relates to a perpendicular magnetic film composed of Co-containing γ-Fe 2 O 3 which is suitable as a perpendicular magnetic recording material and which has an excellent oxidation resistance, an excellent corrosion resistance, a large coercive force, a large squareness, a large Faraday rotation angle  F and a small optical absorption coefficient α and, hence, a large figure of merit, and to a multi-layer film used as a precursor for such a perpendicular magnetic film. With recent remarkable demand for miniaturization and higher reliability of information processing devices and systems, perpendicular magnetic films as a high-density recording medium have been rapidly developed and put to practical use. A perpendicular magnetic film has an easy magnetization axis perpendicular to the film surface, in other words, perpendicular magnetic anisotropy, and it is required to have excellent coercive force Hc and squareness (residual magnetization σr/saturation magnetization σs). In addition, since the magnetic characteristics of a magnetic recording medium are deteriorated due to oxidation by the oxygen in the air, the perpendicular magnetic film is required to be stable against oxidation. When a perpendicular magnetic film is used as a medium for magneto-optical recording in which recording and reproduction of information is carried out by using an optical beam such as a laser beam, the medium is required to have excellent magneto-optical characteristics such as a Faraday rotation angle  F , an optical absorption coefficient a (when the ratio of the intensity I 1 of the light which has transmitted a material having a film thickness of t to the intensity I 0 of incident light, namely, the transmittance (I 1 /I 0 ) is assumed to be exp (-αt)) and a figure of merit (2|F|/α). With increasing demand for higherdensity recording, a growing tendency is to use recorded carrier signals having a high frequency, in other words, that the recorded carrier signals have a shorter wavelength. The wavelength of a light source which is used for magneto-optical recording at present is 830 nm or 780 nm, but for the purpose of higher-density recording in the future, semiconductor lasers having a wavelength of 670 nm, 640 nm, 532 nm, 490 nm, etc. are considered as potential light sources. In the case of magneto-optical recording, the diameter of a recorded bit is determined by the wavelength of a laser beam, and the shorter the wavelength, the smaller the bit diameter becomes. A material whose Faraday rotation angle has a large dependence on the wavelength in a short wavelength region of not-more than 780 nm, is therefore strongly demanded. An amorphous alloy thin film such as a Gd-Co film and a Tb-Fe film composed of a rare earth metal and a transition metal, a spinel oxide thin film such as a cobalt ferrite film (Japanese Patent Application Laid-Open (KOKAI) Nos. 51-119999 (1976), 63-47359 (1988), 3-17813 (1991), 3-188604 (1991) and 4-10509 (1992)); a magnetoplumbite oxide thin film such as a barium ferrite film (Japanese Patent Application Laid-Open (KOKAI) No. 62-267949 (1987)) and a substitutional garnet thin film have been proposed as perpendicular magnetic oxide thin films for magneto-optical recording. Among the above-described perpendicular magnetic films, the cobalt ferrite film which is typical of spinel oxides films are stable against oxidation because it is an oxide, it has a high coercive force due to the large crystalline magnetic anisotropy, and it has a large Faraday rotational angle  F in a short wavelength range in the vicinity of 800 nm and 500 nm. Owing to these magnetic characteristics a cobalt ferrite film is considered to be promising as a magneto-optical recording medium. As the process for producing a cobalt ferrite (CoFe 2 O 4 ) film, various methods such as a sputtering method, a vacuum evaporation method and a MOCVD method are known. Among these, a sputtering method is mainly adopted. The sputtering method, however, is disadvantageous in that although the easy magnetization axis of a cobalt ferrite (CoFe 2 O 4 ) film is in the plane (400), the easy magnetization axis is likely to be oriented at random or the plane (111) is likely to be oriented in parallel with the substrate, so that it is difficult to produce a perpendicular magnetic film and it is impossible to obtain a large coercive force and a large squareness. As a method for obtaining a cobalt ferrite (CoFe 2 O 4 ) film in which the plane (400) is predominantly oriented in parallel with the substrate, (1) the method described in Digits 9th Annual Conf. Magnetics Japan, p315, (2) the method described in Digits 13th Annual Conf. Magnetics Japan, p 246, and (3) the method described in Japanese Patent Application Laid-Open (KOKAI) No. 4-10509 (1992), for example, are known. The method (1) is a method of depositing Fe and Co ionized in an oxygen plasma on an MgAl 2 O 4 substrate or a silica glass substrate which is heated to a temperature of 500°C. Since it is necessary to maintain the substrate temperature at a high temperature such as not lower than 500°C during film formation, the productivity is poor. In addition, since the material of the substrate is limited, it is industrially and economically disadvantageous. The method (2) is a plasma-exciting MO-CVD method. Since it is necessary to maintain the substrate temperature of 300 to 400°C in a vacuum during film formation, the productivity is poor, which is industrially and economically disadvantageous. The method (3) is a method of annealing a multi-layered metal film obtained by laminating at least two layers of Co and Fe at a temperature of not lower than 500°C in an atmosphere containing oxygen. Since a high temperature is necessary, the material of the substrate is limited, which is disadvantageous both industrially and economically. Although a perpendicular magnetic film which is composed of a spinel oxide and which has an excellent oxidation resistance, an excellent corrosion resistance, a large coercive force, a large squareness, a large Faraday rotational angle  F and a small optical absorption coefficient a, and hence, a large figure of merit, is now in the strongest demand, none of the conventional magnetic thin films sufficiently meet these requirements. Since the cobalt ferrite (CoFe 2 O 4 ) film is an oxide, it is stable against oxidation, has a large coercive force due to the crystalline anisotropy which increases with the increase in the Co content, and a large Faraday rotational angle  F in the vicinity of 800 nm and 500 nm. However, the cobalt ferrite (CoFe 2 O 4 ) film is disadvantageous in that the optical absorption coefficient α at 700 nm is as large as about 3.5 to 7.0 µm -1 , so that the figure of merit (2| F |/α) is small when using a laser beam having a wavelength of not more than 700 nm. Especially, in order to maintain the substrate temperature at not less then 500°C, the heat resistance of the substrate itself is required, but the heat resistance of polycarbonate, epoxy resin, etc. which are generally used as a material of the substrate is insufficient. Since the material of the substrate is thus limited, it is disadvantageous both industrially and economically. A magnetic recording medium using an oxide layer as a primary layer has been proposed in Japanese Patent Application Laid-Open (KOKAI) Nos. 3-17813 (1991), 5-12765 (1993) and 5-166167 (1993). Japanese Patent Application Laid-Open (KOKAI) No. 5-166167 (1993) discloses a fixed magnetic disk obtained by forming a Co-Zn ferrite film having a spinel crystalline structure in which the plane (100) is predominantly oriented in parallel with the substrate on an NiO film having an NaCl-type crystalline structure. In the fixed magnetic disk, Co ferrite film is formed on an NiO primary layer so as to accelerate crystalline orientation in the plane (100), but the spacing (2.09 Å) of plane (100) of the NiO layer is smaller than the spacing (not less than 2.10 Å) of the plane (400) of the Co x Zn y Fe 3-x-y O 4 layer. When the lattice constant of a primary film is larger than that of a Co ferrite film, the perpendicular anisotropy is generally accelerated, while it is reduced when the lattice constant of a primary film is smaller than that of a Co ferrite film, as shown in Japanese Patent Application Laid-Open (KOKAI) Nos. 3-17813 (1991). The fixed magnetic disk disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 5-166167 (1993), therefore, has a small perpendicular anisotropy. A perpendicular magnetically anisotropic film of Co ferrite of formula Co 0.10 Fe 2.90 O 4 prepared on a glass disk substrate by plasma MO-EVD has also been disclosed in IEEE Translation Journal on Magnetics in Japan, 1991 vol 6 pp 765-770). Accordingly, the present invention aims at the industrially and economically advantageous production of a perpendicular magnetic film which is composed of a spinel oxide, and which has an excellent oxidation resistance, an excellent corrosion resistance, a large coercive force, a large squareness, a large Faraday rotational angle  F in a short wavelength region and a small optical absorption coefficient α, and hence, a large figure of merit, at a temperature of not higher than 500°C and as low as possible. As a result of various studies undertaken by the present inventors so as to attain the above aims, it has been found that a perpendicular magnetic film comprising a substrate and a Co-containing γ-Fe 2 O 3 film formed on the substrate in which the plane (400) is predominantly oriented in parallel with the substrate, the Co content is 0.01 to 0.32 mol based on 1 mol of Fe, the spacing of the plane (400) is not more than 0.2084 nm (2.084 Å), and the optical absorption coefficient at 700 nm is not more than 2.5 µm -1 , may be obtained by annealing at a temperature of 280 to 450°C a multi-layered film for a perpendicular magnetic film comprising a multiplicity of units formed on the substrate, each unit being composed of an Fe 3 O 4 layer and a CoO layer laminated with one on top of the other, in which the plane (400) is predominantly oriented in parallel with the substrate, one unit has a thickness of not more than 13 nm (130 Å) and the molar ratio of Co to Fe is between 0.01 and 0.32. The perpendicular magnetic film has not more than 2.5 µm -1 of an the optical absorption coefficient at 700 nm and shows an excellent oxidation resistance, an excellent corrosion resistance, a large coercive force, a large squareness, a large Faraday rotational angle  F and a small optical absorption coefficient α, and hence, a large figure of merit at 700 nm. On the basis of this finding, the present invention has been achieved. The present invention seeks to provide a perpendicular magnetic film which is composed of a spinel oxide and which has an excellent oxidation resistance, an excellent corrosion resistance, a large coercive force, a large squareness, a large Faraday rotational angle in a short wavelength region, a small optical absorption coefficient α, and hence, a large figure of merit (2| F |/α). The present invention further seeks to provide a multi-layered film as a precursor of a perpendicular magnetic film which is composed of a spinel oxide and which has an excellent oxidation resistance, an excellent corrosion resistance, a large coercive force, a large squareness, a large Faraday rotation angle in a short wavelength region, a small optical absorption coefficient α, and hence, a large figure (2| F |/α), in order to industrially produce the perpendicular magnetic film at a temperature of not higher than 500°C and as low as possible. In a first aspect of the present invention, there is provided a perpendicular magnetic film comprising a Co-containing γ-Fe 2 O 3 film formed on a substrate, in which the plane (400) of the Co-containing γ-Fe 2 O 3 film is predominantly oriented in parallel with said substrate, the molar ratio of Co to Fe is from 0.01:1 to 0.32:1, the spacing of said plane (400) is not more than 0.2084 nm (2.084 Å), and the optical absorption coefficient at 700 nm is not more than 2.5 µm -1 . In a second aspect of the present invention, there is provided a multi-layered film suitable for conversion to a perpendicular magnetic film according to the first aspect, which multi-layered film comprises a multiplicity of units formed on a substrate, each unit comprising an Fe 3 O 4 layer and a CoO layer laminated with one on top of the other, in which the plane (400) is predominantly oriented in parallel with said substrate, the thickness of each unit is not more than 13 nm (130 Å) and the molar ratio of Co to Fe is from 0.01:1 to 0.32:1. In a third aspect the present invention also provides a process in which the perpendicular magnetic film according to the first aspect, is obtained by annealing the multi-layered film as defined in the second aspect at a temperature of 280 to 450°C. In a fourth aspect of the present invention, there is provided a perpendicular magnetic film comprising: a primary layer of an NiO film formed on a substrate in which the plane (100) is predominantly oriented in parallel with said substrate; and a Co-containing γ-Fe 2 O 3 film formed on said primary layer, in which the plane (400) is predominantly oriented in parallel with said substrate, the molar ratio of Co to Fe is from 0.10:1 to 0.32:1, the spacing of said plane (400) is not more than 0.2084 nm (2.084 Å), and the optical absorption coefficient at 700 nm is not more than 2.5 µm -1 . In a fifth aspect of the present invention, there is provided a multi-layered film suitable for conversion to a perpendicular magnetic film according to the fourth aspect, which multi-layered film comprises a primary layer of an NiO film formed on a substrate, in which the plane (100) is predominantly oriented in parallel with said substrate, and a multiplicity of units formed on said primary layer, each unit being composed of an Fe 3 O 4 layer and a CoO layer laminated with one on top of the other, in which the plane (400) is predominantly oriented in parallel with said substrate, the thickness of each unit is not more than 80 nm (800 Å), and the molar ratio of Co to Fe is from 0.10:1 to 0.32:1. In a sixth aspect of the present invention, there is provided a process in which the perpendicular magnetic film according to the fourth aspect is obtained by annealing the multi-layered film as defined in the fifth aspect at a temperature of 280 to 450°C. In a seventh aspect of the present invention, there is provided a perpendicular magnetic film according to the first aspect, wherein said Co-containing γ-Fe 2 O 3 film formed on said substrate in which said plane (400) is predominantly oriented in parallel with said substrate, contains 0.01 to 0.25 mol of Mn based on 1 mol of Co. In an eighth aspect of the present invention, there is provided a multi-layered film suitable for conversion to a perpendicular magnetic film according to the seventh aspect, which multi-layered film comprises a multiplicity of units formed on a substrate, each unit comprising an Fe 3 O 4 layer and an oxide layer containing Co and Mn laminated with one on top of the other, in which the plane (400) is predominantly oriented in parallel with said substrate, the molar ratio of Mn to Co is from 0.01:1 to 0.25:1, the molar ratio of Co to Fe is from 0.01:1 to 0.32:1, and the thickness of each unit is not more than 13 nm (130 Å). In a ninth aspect of the present invention, there is provided a process in which a perpendicular magnetic film according to the seventh aspect, is obtained by annealing the multi-layered film as defined in the eighth aspect at a temperature of 280 to 450°C. In a tenth aspect of the present invention, there is provided a perpendicular magnetic film according to the fourth aspect, wherein said Co-containing γ-Fe 2 O 3 film formed on said primary layer, in which the plane (400) is predominantly oriented in parallel with said substrate, contains 0.01 to 0.25 mol of Mn based on 1 mol of Co. In an eleventh aspect of the present invention, there is provided a multi-layered film suitable for conversion to a perpendicular magnetic film, according to the tenth aspect, which multi-layered film comprises a primary layer of an NiO film formed on a substrate in which the plane (100) is predominantly oriented in parallel with said substrate; and a multiplicity of units formed on said primary layer, each unit comprising an Fe 3 O 4 layer and an oxide layer containing Co and Mn laminated with one on top of the other, in which the plane (400) is predominantly oriented in parallel with said substrate, the thickness of each unit is not more than 80 nm (800 Å), the molar ratio of Co to Fe is from 0.10:1 to 0.32:1 and the molar ratio of Mn to Co is from 0.01:1 to 0.25:1. In a twelfth aspect of the present invention, there is provided a process in which the perpendicular magnetic film according to the tenth aspect is obtained by annealing the multi-layered film as defined in the eleventh aspect at a temperature of 280 to 450°C. Figs. 1a to 1c show X-ray diffraction spectra of multi-layer films; Fig. 2 shows the relationship between the Faraday rotational angle of a perpendicular magnetic film and the measuring wavelength; Fig. 3 shows the relationship between the optical absorption coefficient of a perpendicular magnetic film and the measuring wavelength; Fig. 4 shows the relationship between the figure of merit of a perpendicular magnetic film and the measuring wavelength; Fig. 5 shows the magnetization curve of the perpendicular magnetic film obtained in Example 1; Fig. 6 shows the magnetization curve of the perpendicular magnetic film obtained in Example 3; Fig. 7 shows the magnetization curve of the perpendicular magnetic film obtained in Example 4; Fig. 8 shows X-ray diffraction spectrum of multi-layer films obtained in Example 7; Fig. 9 shows the magnetization curve of the perpendicular magnetic film obtained in Example 8; Fig. 10 shows X-ray diffraction spectrum of multi-layer films obtained in Example 9; Fig. 11 shows X-ray diffraction spectrum of multi-layer films obtained in Example 10; Fig. 12 shows the magnetization curve of the perpendicular magnetic film obtained in Example 10; A first embodiment of a perpendicular magnetic film according to the present invention is a Co-containing γ-Fe 2 O 3 film formed on a substrate in which the plane (400) is predominantly oriented in parallel with the substrate, the Co content is 0.01 to 0.32 mol, preferably 0.05 to 0.30 mol, more preferably 0.1 to 0.26 mol based on 1 mol of Fe, the spacing of the plane (400) is not more than 0.2084 nm (2.084 Å), preferably 0.2072 to 0.2082 nm (2.072 to 2.082 Å), and the optical absorption coefficient at 700 nm is not more than 2.5 µm -1 , preferably 0.5 to 2.2 µm -1 . If the molar ratio of Co to Fe is less than 0.01, it is difficult to obtain a spinel oxide film in which the plane (400) is predominantly oriented in parallel with the substrate. In order to facilitate the production of a spinel oxide film in which the plane (400) is predominantly oriented in parallel with the substrate, the molar ratio of Co to Fe is preferably not less than 0.05. On the other hand, if the molar ratio of Co to Fe exceeds 0.32, it is difficult to transform Fe 3 O 4 into γ-Fe 2 O 3 at a temperature of not higher than 450°C. In addition, the optical absorption coefficient increases, so that the figure of merit is lowered. When the spacing of the plane (400) of a perpendicular magnetic film exceeds 0.2084 nm (2.084 Å), the magnetostrictive anisotropy is small, so that it is difficult to obtain a perpendicular magnetic film having a large coercive force. The thickness of a perpendicular magnetic Co-containing γ-Fe 2 O 3 film according to the present invention is 50 to 500 nm (500 to 5,000 Å). The first embodiment of a perpendicular magnetic film according to the present invention is produced by annealing a multi-layered film which is composed of a multiplicity of units formed on a substrate, each unit being composed of an Fe 3 O 4 layer and a CoO layer laminated with one on top of the other, and in which the plane (400) is predominantly oriented in parallel with the substrate, the thickness of one unit is not more than 13 nm (130 Å), preferably 3 to 10 nm (30 to 100 Å), and the molar ratio of Co to Fe is between 0.01 and 0.32, preferably between 0.05 and 0.30, more preferably between 0.1 and 0.26, at a temperature of 280 to 450°C, preferably 290 to 400 °C. The annealing time is preferably 0.8 to 5 hours. The multi-layered film as the precursor of a perpendicular magnetic film of the present invention is composed of a multiplicity of units formed on a substrate, each unit being composed of an Fe 3 O 4 layer and a CoO layer laminated with one on top of the other, and is obtained by a well-known method such as a reactive sputtering method in which metal targets (Fe and Co) are sputtered in an oxidizing atmosphere, a direct method in which an oxidized film is directly formed from sintered targets of Fe 3 O 4 and Co 3 O 4 , and a reactive evaporation method in which metals (Fe and Co) are evaporated in an oxidizing atmosphere. As the material of the substrate usable in the present invention, a metal such as Al, an Al alloy and stainless steel; and a glass such as soda glass, borosilicate glass, barium borosilicate glass and aluminosilicate glass may be exemplified. The thickness of the substrate is 0.1 to 2 mm. In the multi-layered film of the present invention, the thickness of one unit is not more than 13 nm (130 Å), preferably 3 to 10 nm (30 to 100 Å), and the molar ratio of Co to Fe is between 0.01 and 0.32. It is impossible to obtain a perpendicular magnetic film aimed at in the present invention, in case of using a multi-layered film in which the thickness of one unit exceeds 13 nm (130 Å) or a multi-layered film in which the molar ratio of Co to Fe is less than 0.1 or more than 0.32. The multi-layered film of the present invention is composed of at least three units, preferably 5 to 150 units, each unit being composed of an Fe 3 O 4 layer and a CoO layer laminated with one on top of the other. Since the thickness of a perpendicular magnetic film is about not more than 5000 Å in practical use, the number of units is determined so as to produce a film of a desired thickness. The annealing temperature of the multi-layered film of the present invention is 280 to 450°C, preferably 290 to 400 °C. If the annealing temperature is lower than 280°C, the transformation of Fe304 into γ-Fe 2 O 3 or diffusion of Co is insufficient for obtaining a perpendicular magnetic film of the present invention. If the annealing temperature exceeds 450°C, the transformation of γ-Fe 2 O 3 into α-Fe 2 O 3 begins to be caused, so that it is impossible to produce a perpendicular magnetic film aimed at in the present invention. In annealing, the transformation temperature for transforming Fe 3 O 4 into γ-Fe 2 O 3 is lowered by adding Cu to the film, as described in, for example, Japanese Patent Application Laid-Open (KOKAI) No. 57-54309 (1982). It is possible to produce a similar effect by adopting this method in the present invention. Annealing may be carried transformed Fe 3 O 4 into γ-Fe 2 O 3 at a temperature of 240 to 320°C, and Co is then diffused at a temperature of 280 to 450°C. In this case, it is possible to suppress the production of a crack which is likely to be caused during the transformation of Fe 3 O 4 into γ-Fe 2 O 3 . What is the most important in the first embodiment is that the perpendicular magnetic film of the present invention is produced by annealing at a temperature of 280 to 450°C a multi-layered film, which is composed of a multiplicity of units formed on a substrate, each unit being composed of an Fe 3 O 4 layer and a CoO layer laminated with one on top of the other, and in which the plane (400) is predominantly oriented in parallel with the substrate, one unit is of not more than 13 nm (130 Å) in thickness, and the molar ratio of Co to Fe is between 0.01 and 0.32. A second embodiment of a perpendicular magnetic film of the present invention is composed of a primary layer of an NiO film formed on a substrate in which the plane (100) is predominantly oriented in parallel with the substrate, and a Co-containing γ-Fe 2 O 3 film formed on the primary layer in which the plane (400) is predominantly oriented in parallel with the substrate, the molar ratio of Co to Fe is between 0.10 and 0.32, preferably 0.10 to 0.26, the spacing of the plane (400) is not more than 0.2084 nm (2.084 Å), preferably 0.2072 to 0.2082 nm (2.072 to 2.082 Å), and the optical absorption coefficient at 700 nm is not more than 2.5 µm -1 , preferably 0.5 to 2.2 µm -1 . The thickness of the NiO layer as a primary layer is 50 to 200 nm (500 to 2,000 Å), and the thickness of the Co-containing γ-Fe 2 O 3 film is 50 to 500 nm (500 to 5,000 Å). The second embodiment of a perpendicular magnetic film is produced in the following methods. (i) An NiO layer as a primary layer in which the plane (100) is predominantly oriented in parallel with a substrate is formed on the substrate, and a Co-containing Fe 3 O 4 layer is formed on the primary layer. In the Co-containing Fe 3 O 4 layer, the molar ratio of Co to Fe is between 0.10 and 0.32, preferably between 0.10 and 0.26. The two-layered film obtained is annealed at a temperature of 280 to 450°C, preferably at 290 to 400°C, to form the perpendicular magnetic film composed of a Co-containing γ-Fe 2 O 3 film and NiO layer. The annealing time is preferably 0.8 to 5 hours. (ii) A multi-layered film composed of an NiO layer as a primary layer formed on a substrate in which the plane (100) is predominantly oriented in parallel with the substrate, and a multiplicity of units formed on the NiO layer, each unit being composed of an Fe 3 O 4 layer and a CoO layer laminated with one other, in which the plane (100) is predominantly oriented in parallel with the substrate, one unit of the laminated films composed of Fe 3 O 4 layers and CoO layers is of not more than 80 nm (800 Å) in thickness, preferably 3 to 50 nm (30 to 500 Å) in thickness, and the molar ratio of Co to Fe is between 0.10 and 0.32, preferably between 0.10 and 0.26 is annealed at a temperature of 280 to 450°C, preferably at 290 to 400°C. The annealing time is preferably 0.8 to 5 hours. The process for producing the perpendicular magnetic film from the multi-layered film described above is preferred in view of the perpendicular magnetic film having small grain size and a high S/N ratio. It is sometimes impossible to obtain a homogeneous Co-containing γ-Fe 2 O 3 layer by using a laminated film composed of an Fe 3 O 4 layer and a CoO layer in which the thickness of one unit exceeds 80 nm (800 Å), since Co ions do not diffuse into a γ-Fe 2 O 3 layer, sufficiently. Also, it is sometimes impossible to obtain a perpendicular magnetic film aimed at in the present invention by using a laminated film composed of an Fe 3 O 4 layer and a CoO layer, in which the molar ratio of Co to Fe is less than 0.10. The spacing of the plane (100) of the NiO layer as a primary layer of the second embodiment of a perpendicular magnetic film is 0.2090 nm (2.090 Å) and the spacing of the plane (400) of the Co-containing γ-Fe 2 O 3 layer is not more than 0.2084 nm (2.084 Å). This means that the provision of an NiO layer as a primary layer rather develops a tendency of increasing the perpendicular anisotropy as compared with the perpendicular anisotropy of a perpendicular magnetic film obtained by using ferrite having the spacing of the plane (400) larger than that of the NiO layer, as shown in Japanese Patent Application Laid-Open (KOKAI) No. 5-166167, in addition to the tendency of accelerating the crystalline orientation of the plane (400) of the Co-containing γ-Fe 2 O 3 layer. A third embodiment of a perpendicular magnetic film according to the present invention is a Co-Mn-containing γ-Fe 2 O 3 film formed on a substrate in which the plane (400) is predominantly oriented in parallel with the substrate, the molar ratio of Co to Fe is 0.01 to 0.32, preferably 0.10 to 0.26, the molar ratio of Mn to Co is 0.01 to 0.25, preferably 0.03 to 0.08, the spacing of the plane (400) is not more than 0.2084 nm (2.084 Å), preferably 0.2072 to 0.2082 nm (2.072 to 2.082 Å), and the optical absorption coefficient at 700 nm is not more than 2.5 µm -1 , preferably 0.5 to 2.2 µm -1 . The thickness of the Co-Mn-containing γ-Fe 2 O 3 film is 50 to 500 nm (500 to 5,000 Å). The third embodiment of a perpendicular magnetic film according to the present invention is produced by annealing a multi-layered film comprising a multiplicity of units formed on a substrate, each unit being composed of an Fe 3 O 4 layer and an oxide layer containing Co and Mn laminated with one on top of the other, in which the plane (400) is predominantly oriented in parallel with the substrate, the molar ratio of Mn to Co is between 0.01 and 0.25, preferably 0.03 and 0.08, the molar ratio of Co to Fe is between 0.01 and 0.32, preferably between 0.10 and 0.26, the thickness of one unit is not more than 13 nm (130 Å), preferably 3 to 10 nm (30 to 100 Å), at a temperature of 280 to 450°C, preferably 290 to 400°C. The annealing time is preferably 0.8 to 5 hours. A fourth embodiment of a perpendicular magnetic film of the present invention is composed of a primary layer of an NiO film formed on a substrate in which the plane (100) is predominantly oriented in parallel with the substrate and a Co-Mn-containing γ-Fe 2 O 3 film formed on the primary layer in which the plane (400) is predominantly oriented in parallel with the substrate, the molar ratio of Co to Fe is between 0.10 and 0.32, preferably between 0.10 and 0.26, the molar ratio of Mn to Co is between 0.01 and 0.25, preferably between 0.03 and 0.08, the spacing of the plane (400) is not more than 0.2084 nm (2.084 Å), preferably 0.2072 to 0.2082 nm (2.072 to 2.082 Å), and the optical absorption coefficient at 700 nm is not more than 2.5 µm -1 , preferably 0.5 to 2.2 µm -1 . The thickness of the NiO layer is 50 to 200 nm (500 to 2,000 Å), and the thickness of the Co-Mn-containing γ-Fe 2 O 3 film is 50 to 500 nm (500 to 5,000 Å). The fourth embodiment of a perpendicular magnetic film is produced in the following methods. (i) An NiO layer as a primary layer in which the plane (100) is predominantly oriented in parallel with a substrate is formed on the substrate, and a Co-Mn-containing Fe 3 O 4 layer is formed on the primary layer. In the Co-Mn-containing Fe 3 O 4 layer, the molar ratio of Co to Fe is between 0.10 and 0.32, preferably between 0.10 and 0.26. The two-layered film obtained is annealed at a temperature of 280 to 450°C, preferably at 290 to 400°C. The annealing time is preferably 0.8 to 5 hours. (ii) A multi-layered film composed of an NiO layer as a primary layer formed on a substrate in which the plane (100) is predominantly oriented in parallel with the substrate, and a multiplicity of units formed on the NiO layer, each unit being composed of an Fe 3 O 4 layer and an oxide layers containing Co and Mn layer laminated with one on top of the other, in which the plane (400) is predominantly oriented in parallel with a substrate, one unit of the laminated layers composed of Fe 3 O 4 layers and oxide layers containing Co and Mn is of not more than 80 nm (800 Å) in thickness, preferably 3 to 50 nm (30 to 500 Å) in thickness, the molar ratio of Co to Fe is between 0.10 and 0.32, preferably between 0.10 and 0.26, and the molar ratio of Mn to Co is between 0.01 and 0.25, preferably between 0.03 and 0.08, is annealed at a temperature of 280 to 450°C, preferably at 290 to 400°C. The annealing time is preferably 0.8 to 5 hours. The process for producing the perpendicular magnetic film from the multi-layered film described above is preferred in view of the perpendicular magnetic film having a small grain size and a high S/N ratio. The present inventors have confirmed that it is impossible to obtain a perpendicular magnetic film aimed at in the present invention either by using a multi-layered film in which the plane (222) is predominantly oriented in parallel with the substrate in case where the thickness of one unit composed of Fe 3 O 4 layer and the CoO layer exceeds 13 nm (130 Å) even if the molar ratio of Co to Fe is in the range of 0.01 to 0.32, or by using a multi-layered film in which the molar ratio of Co to Fe is out of the range of 0.01 to 0.32 even if the thickness of one unit composed of Fe 3 O 4 layer and CoO layer is not more than 13 nm (130 Å) and the plane (400) is predominantly oriented in parallel with the substrate, as shown in Comparative Examples 1 and 2 which will be described later and in Figs. 1a and 1b. The perpendicular magnetic film of the present invention has a coercive force as large as not less than 3 kOe, preferably not less than 3.5 kOe, more preferably not less than 5 kOe, and a squareness as large as not less than 0.7, preferably not less than 0.75. The reason why the perpendicular magnetic film of the present invention has a large coercive force is considered to be as follows. A conventional cobalt ferrite (CoFe 2 O 4 ) film produces a large coercive force by utilizing the crystalline magnetic anisotropy. In contrast, in the perpendicular magnetic film of the present invention, the plane (400) is predominantly oriented in parallel with the substrate, and the spacing of the plane (400) is not more than 0.2084 nm (2.084 Å), preferably 0.2072 to 0.2082 nm (2.072 to 2.082 Å) which is even smaller than 0.2086 nm (2.086 Å), which is the spacing of the plane (400) of γ-Fe 2 O 3 having the smallest spacing of all the spinel crystal structures mainly composed of Fe. From these facts, the present inventors consider that the magnetostrictive anisotropy due to the strain is imparted to the perpendicular magnetic film of the present invention in addition to the crystalline magnetic anisotropy. It has been confirmed that the contraction of (400) spacing of the perpendicular magnetic film of the present invention is caused not by the shrinkage or the like of the substrate but due to the film itself from the fact that the spacing of the film produced by transforming a monolayer Fe 3 O 4 film (200 nm (2000 Å)) in which the plane (222) is oriented in parallel with the same glass substrate as that of the perpendicular magnetic film of the present invention into a γ-Fe 2 O 3 film was 0.2407 nm (2.407 Å), which is approximately the same as 0.2408 nm (2.408 Å) of the bulk value. Although it is conventionally considered that by providing an NiO layer between a substrate and a recording layer, the perpendicular anisotropy is reduced, since the spacing of the plane (400) of Co-containing γ-Fe 2 O 3 layer is smaller than that of the plane (100) of NiO layer, the perpendicular anisotropy of a perpendicular magnetic film according to the present invention is rather increased. Accordingly, the squareness is also increased. In addition, it is possible to produce a recording layer not only by annealing a laminated film composed of a multiplicity of Fe 3 O 4 layers and CoO layers as a precursor, but also by annealing a Co-containing Fe 3 O 4 layer as a precursor. In the case of using a laminated film as a precursor, it is possible to make a large unit having a thickness of not more than 80 nm (800 Å), the production is facilitated and the productivity is greatly enhanced. If Mn as well as Co is contained in a recording layer, the squareness is increased. Especially, when an NiO layer is provided as a primary layer and a Co-Mn-containing γ-Fe 2 O 3 is formed thereon, the tendency of increasing the squareness is prominent. The perpendicular magnetic film of the present invention has as large a Faraday rotational angle as conventional cobalt ferrite (CoFe 2 O 4 ) film, and has such a small optical absorption coefficient a especially at a wavelength of 700 nm as 2.5 µm -1 , preferably 0.5 to 2.2 µm -1 , which results in a large figure of merit (2| F |/α) at 700 nm. This will be explained in the following with reference to drawings. Fig. 2 shows the relationship between the Faraday rotational angles of the perpendicular magnetic films produced in Examples 1, 3 and 4 which are described later and the wavelength of measuring light. In Fig. 2, the curves a, b and c represent the Faraday rotational angles of the perpendicular magnetic films produced in Examples 1, 3 and 4, respectively. As shown in Fig. 2, the Faraday rotational angle with respect to light wavelength changes with a change in the ratio of cobalt and iron. The smaller is the ratio of Co to Fe, the shorter the frequency is at which the Faraday rotational angle changes from a negative value to a positive value. In contrast, in the perpendicular magnetic film of the present invention, it is possible to freely control the Faraday rotational angle in correspondence with the light wavelength by varying the ratio of cobalt and iron. Fig. 3 shows the relationship between the optical absorption coefficients of perpendicular magnetic films according to the present invention and a conventional cobalt ferrite (CoFe 2 O 4 ) film, and the measuring wavelength of measuring light. In Fig. 3, the curves a, b and c represent the optical absorption coefficients of the perpendicular magnetic films produced in Examples 1, 3 and 4, respectively, and the broken line shows that of a conventional cobalt ferrite (CoFe 2 O 4 ) film. The optical absorption coefficient of a conventional cobalt ferrite (CoFe 2 O 4 ) film is extracted and plotted from the description at p 773 of IEEE Transactions on Magnetics MAG-vol 12 (1976). As shown in Fig. 3, the optical absorption coefficients of the perpendicular magnetic films of the present invention are much smaller than that of the conventional cobalt ferrite (CoFe 2 O 4 ) film. Fig. 4 shows the relationship between the figure of merits (2| F |/α) of perpendicular magnetic films according to the present invention and a conventional cobalt ferrite film, and the measuring wavelength of measuring light. In Fig. 4, the curves a, b and c represent the figure of merits of the perpendicular magnetic films produced in Examples 1, 3 and 4, respectively, and the broken line shows that of a conventional cobalt ferrite (CoFe 2 O 4 ) film. The figure of merit of a conventional cobalt ferrite (CoFe 2 O 4 ) film is obtained by incorporating the optical absorption coefficient shown in Fig. 3 into the general formula of the figure of merit described in IEEE Transactions on Magnetics MAG-vol 12 (1976) and plotting the calculated values. As shown in Fig. 4, the perpendicular magnetic film of the present invention has a small optical absorption coefficient and, hence, a large figure of merit. The perpendicular magnetic film of the present invention is therefore suitable as a magneto-optical recording medium. In the perpendicular magnetic film of the present invention, the spacing of the plane (400) is not more than 0.2084 nm (2.084 Å). As to the magnetic characteristics thereof, the saturation magnetization is not less than 270 emu/cc, preferably not less than 330 emu/cc, the coercive force is not less than 4.0 kOe, preferably not less than 4.5 kOe, and the squareness is not less than 0.7, preferably not less than 0.75. The optical absorption coefficient thereof at 700 nm is not more than 2.5 µm -1 , preferably 0.5 to 2.2 µm -1 , and the figure of merit at 700 nm is not less than 1.0, preferably not less than 2.0. In the perpendicular magnetic film of the present invention which uses an NiO layer as a primary layer, the spacing of the plane (400) is not more than 0.2084 nm (2.084 Å), the saturation magnetization is not less than 270 emu/cc, preferably not less than 326 emu/cc, the coercive force is not less than 3.0 kOe, preferably not less than 5.0 kOe, the squareness is not less than 0.75, preferably not less than 0.80, the optical absorption coefficient thereof at 700 nm is not more than 2.2 µm -1 , preferably not more than 1.9 µm -1 , and the figure of merit at 700 nm is not less than 1.0, preferably not less than 2.0. In the perpendicular magnetic film of the present invention comprising Co-Mn-containing γ-Fe 2 O 3 layer, the spacing of the plane (400) is not more than 0.2084 nm (2.084 Å), the saturation magnetization is not less than 270 emu/cc, preferably not less than 330 emu/cc, the coercive force is not less than 4.5 kOe, preferably not less than 5.0 kOe, the squareness is not less than 0.75, preferably not less than 0.80, the optical absorption coefficient thereof at 700 nm is not more than 2.2 µm -1 , preferably not more than 1.9 µm -1 , and the figure of merit at 700 nm is not less than 1.0, preferably not less than 2.0. In the perpendicular magnetic film of the present invention comprising Co-Mn-containing γ-Fe 2 O 3 layer which uses an NiO layer as a primary layer, the spacing of the plane (400) is not more than 0.2084 nm (2.084 Å), the saturation magnetization is not less than 270 emu/cc, preferably not less than 330 emu/cc, the coercive force is not less than 4.0 kOe, preferably not less than 5.0 kOe, the squareness is not less than 0.85, preferably not less than 0.90, the optical absorption coefficient thereof at 700 nm is not more than 2.2 µm -1 , preferably not more than 1.9 µm -1 , and the figure of merit at 700 nm is not less than 1.0, preferably not less than 2.0. Since the perpendicular magnetic film of the present invention comprises a Co-containing γ-Fe 2 O 3 film which has an excellent oxidation resistance, an excellent corrosion resistance, a large coercive force, a large squareness, a large Faraday rotation angle  F and a small optical absorption coefficient α, and hence, a large figure of merit, the film is especially suitable as a magneto-optical recording medium. According to a method of producing a perpendicular magnetic film of the present invention, it is not necessary to heat the substrate to not less than 300°C during film formation in a vacuum, and it is possible to produce a perpendicular magnetic film by annealing at a low temperature such as less than 500°C. The method of the present invention is therefore very advantageous both industrially and economically. Examples The present invention will be explained in more detail hereinunder with reference to the following examples and comparative examples. In the following examples and comparative examples, the magnetic characteristics were measured by using a "Vibration sample magnetometer VSM-3S-15", produced by Toei Kogyo K.K. The wavelength dependency of the Faraday rotation angle was measured by a "spectral Kerr Faraday measuring apparatus BH-M800", produced by Nihon Kagaku Engineering K.K. The crystalline orientation was examined by the X-ray diffraction spectrum obtained by using an X-ray diffraction apparatus, rotating anode type RIGAKU RU 300" (wavelength: 0.15418 nm (1.5418 Å), ordinary output 50 kv/240 nmA), produced by Rigaku Denki K.K. The optical absorption coefficient was expressed by the value measured by a multi-purpose recording spectrophotometer MPS 2000 produced by Shimazu Seisakusho Ltd. The figure of merit was expressed by the value calculated from the general formula 2| F |/α by inserting the measured value of the optical absorption coefficient thereinto. Example 1 In a high-frequency high-rate sputtering device SH-250H-T06(manufactured by Nippon Shinku Corp.) a glass substrate was placed with a distance of 80 mm from an Fe target and a Co target and the substrate temperature was held at a temperature of 280°C. Reactive sputtering was first carried out by using the Fe target in an atmosphere of argon and oxygen under a partial oxygen pressure of 0.015 Pa (0.11 mTorr) and the total pressure of 0.67 Pa (5 mTorr) at a deposition rate of 4 nm/min (40 Å/min), thereby forming a spinel Fe 3 O 4 film on the glass substrate to a thickness of 4 nm (40 Å) as a first layer. The shutter was next rotated to form a spinel CoO film to a thickness of 7 Å as a second layer by sputtering the Co target at a deposition rate of 4.2 nm/min (42 Å/min). The first layer and the second layer constituted one unit having a thickness of 4.7 nm (47 Å). The molar ratio of Co to Fe in one unit was 0.26. These operations were alternately repeated 100 times to obtain a laminated film composed of 50 layers of spinel Fe 3 O 4 films each having a thickness of 4 nm (40 Å), and 50 layers of spinel CoO films each having a thickness of 0.7 nm (7 Å). The making power at the time of film formation was 300 W on the Fe side and 300 W on the Co side. In the thus-obtained multi-layered film, the plane (400) was predominantly oriented in parallel with the substrate, as shown in the X-ray diffraction spectrum in Fig. 1(c). The multi-layered film was annealed in the air at a temperature of 450°C for one hour so as to oxidize Fe 3 O 4 into γ-Fe 2 O 3 and diffuse Co, thereby producing a Co-containing γ-Fe 2 O 3 film. As a result of the X-ray diffraction of the Co-containing γ-Fe 2 O 3 film obtained, the plane (400) was proved to predominantly orient in parallel with the substrate. The magnetization curve of the film is shown in Fig 5. In Fig. 5, the solid line a shows the magnetization curve of the film to which a magnetic field was applied in the perpendicular direction, and the broken line b shows the magnetization curve of the film to which a magnetic field was applied in the in-plane direction. As shown in Fig. 5, since the residual magnetization and the coercive force of the solid line a were larger than those of the broken line b , the film was recognized as a perpendicular magnetic film. As to the magnetic characteristics of the perpendicular magnetic film, the saturation magnetization was 340 emu/cc (4250 G), the coercive force was 5.4 kOe and the squareness was 0.75. The spacing of the plane (400) was 0.2082 nm (2.082 Å), the optical absorption coefficient at 700 nm was 1.9 µm -1 and the figure of merit at 700 nm was 4.0. Thus, the film was proved to be a good perpendicular magnetic film. Example 2 The multi-layered film obtained in the same way as in Example 1 was annealed in the air at a temperature of 400°C for one hour so as to oxidize Fe 3 O 4 into γ-Fe 2 O 3 and diffuse Co, thereby producing a Co-containing γ-Fe 2 O 3 film. As a result of the X-ray diffraction of the Co-containing γ-Fe 2 O 3 film obtained, the plane (400) was proved to predominantly orient in parallel with the substrate. From the observation of the magnetization curve of the film, it was found that the residual magnetization and the coercive force of the film to which a magnetic field was applied in the perpendicular direction were larger than those of the film to which a magnetic field was applied in the in-plane direction. Thus, the film was recognized as a perpendicular magnetic film. As to the magnetic characteristics of the perpendicular magnetic film, the saturation magnetization was 342 emu/cc (4300 G), the coercive force was 6.7 kOe and the squareness was 0.76. The spacing of the plane (400) was 0.2075 nm (2.075 Å), the optical absorption coefficient at 700 nm was 1.9 µm -1 and the figure of merit at 700 nm was 4.0. Thus, the film was proved to be a good perpendicular magnetic film. Example 3 Reactive sputtering was first carried out by using an Fe target in an atmosphere of argon and oxygen under a partial oxygen pressure of 0.015 Pa (0.11 mTorr) and the total pressure of 0.67 Pa (5 mTorr) at a deposition rate of 4 nm/min (40 Å/min), thereby forming a spinel Fe 3 O 4 film on the glass substrate which was held at 200°C to a thickness of 4 nm (40 Å) as a first layer. The shutter was next rotated to form a spinel CoO film to a thickness of 0.5 nm 5 Å as a second layer by sputtering the Co target at a deposition rate of 2.8 nm/min (28 Å/min). The first layer and the second layer constituted one unit having a thickness of 4.5 nm (45 Å). The molar ratio of Co to Fe in one unit was 0.19. These operations were alternately repeated 100 times to obtain a laminated film composed of 50 layers of spinel Fe 3 O 4 films each having a thickness of 4 nm (40 Å), and 50 layers of spinel CoO films each having a thickness of 0.5 nm (5 Å). The making power at the time of film formation was 300 W on the Fe side and 200 W on the Co side. In the thus-obtained multi-layered film, the plane (400) was predominantly oriented in parallel with the substrate. The multi-layered film was annealed in the air at a temperature of 400°C for one hour so as to oxidize Fe 3 O 4 into γ-Fe 2 O 3 and diffuse Co, thereby producing a Co-containing γ-Fe 2 O 3 film. As a result of the X-ray diffraction of the Co-containing γ-Fe 2 O 3 film obtained, the plane (400) was proved to predominantly orient in parallel with the substrate. The magnetization curve of the film is shown in Fig. 6. In Fig. 6, the solid line a shows the magnetization curve of the film to which a magnetic field was applied in the perpendicular direction, and the broken line b shows the magnetization curve of the film to which a magnetic field was applied in the in-plane direction. As shown in Fig. 6, since the residual magnetization and the coercive force of the solid line a were larger than those of the broken line b , the film was recognized as a perpendicular magnetic film. As to the magnetic characteristics of the perpendicular magnetic film, the saturation magnetization was 350 emu/cc (4400 G), the coercive force was 5.2 kOe and the squareness was 0.78. The spacing of the plane (400) was 0.2081 nm (2.081 Å), the optical absorption coefficient at 700 nm was 0.8 µm -1 and the figure of merit at 700 nm was 7.4. Thus, the film was proved to be a good perpendicular magnetic film. Example 4 Reactive sputtering was first carried out by using an Fe target in an atmosphere of argon and oxygen under a partial oxygen pressure of 0.015 Pa (0.11 mTorr) and the total pressure of 0.67 Pa (5 mTorr) at a deposition rate of 4 nm/min (40 Å/min), thereby forming a spinel Fe 3 O 4 film on the glass substrate which was held at a temperature of 160°C to a thickness of 6.2 nm (62 Å) as a first layer. The shutter was next rotated to form a spinel CoO film to a thickness of 0.4 nm (4 Å) as a second layer by sputtering the Co target at a deposition rate of 2.8 nm/min (28 Å/min). The first layer and the second layer constituted one unit having a thickness of 6.6 nm (66 Å). The molar ratio of Co to Fe in one unit was 0.09. These operations were alternately repeated 100 times to obtain a laminated film composed of 50 layers of spinel Fe 3 O 4 films each having a thickness of 6.2 nm (62 Å), and 50 layers of spinel CoO films each having a thickness of 0.4 nm (4 Å). The making power at the time of film formation was 300 W on the Fe side and 200 W on the Co side. In the thus-obtained multi-layered film, the plane (400) was predominantly oriented in parallel with the substrate. The multi-layered film was annealed in the air at a temperature of 350°C for one hour so as to oxidize Fe 3 O 4 into γ-Fe 2 O 3 and diffuse Co, thereby producing a Co-containing γ-Fe 2 O 3 film. As a result of the X-ray diffraction of the Co-containing γ-Fe 2 O 3 film obtained, the plane (400) was proved to predominantly orient in parallel with the substrate. The magnetization curve of the film is shown in Fig. 7. In Fig. 7, the solid line a shows the magnetization curve of the film to which a magnetic field was applied in the perpendicular direction, and the broken line b shows the magnetization curve of the film to which a magnetic field was applied in the in-plane direction. As shown in Fig. 7, since the residual magnetization and the coercive force of the solid line a were larger than those of the broken line b , the film was recognized as a perpendicular magnetic film. As to the magnetic characteristics of the perpendicular magnetic film, the saturation magnetization was 350 emu/cc (4400 G), the coercive force was 5.3 kOe and the squareness was 0.78. The spacing of the plane (400) was 0.2077 nm (2.077 Å), the optical absorption coefficient at 700 nm was 1.2 µm -1 and the figure of merit at 700 nm was 3.8. Thus, the film was proved to be a good perpendicular magnetic film. Example 5 A multi-layered film was produced in the same way as in Example 1 except that the laminated film was composed of 15 layers of spinel Fe 3 O 4 films and 15 layers of spinel CoO films (total thickness: 70 nm (700 Å)). The multi-layer film was annealed in the air at a temperature of 400°C for one hour so as to oxidize Fe 3 O 4 into γ-Fe 2 O 3 and diffuse Co, thereby producing a Co-containing γ-Fe 2 O 3 film. As a result of the X-ray diffraction of the Co-containing γ-Fe 2 O 3 film obtained, the plane (400) was proved to predominantly orient in parallel with the substrate. From the observation of the magnetization curve of the film, it was found that the residual magnetization and the coercive force of the film to which a magnetic field was applied in the perpendicular direction were larger than those of the film to which a magnetic field was applied in the in-plane direction. Thus, the laminated film was recognized as a perpendicular magnetic film. As to the magnetic characteristics of the perpendicular magnetic film, the saturation magnetization was 318 emu/cc (4000 G), the coercive force was 5.7 kOe and the squareness was 0.75. The spacing of the plane (400) was 0.2076 nm (2.076 Å), the optical absorption coefficient at 700 nm was 1.7 µm -1 and the figure of merit at 700 nm was 4.1. Thus, the film was proved to be a good perpendicular magnetic film in spite of the very thin film thickness. Example 6 A multi-layered film produced in the same way as in Example 4 was annealed in the air at a temperature of 300°C for 2 hours so as to oxidize Fe 3 O 4 into γ-Fe 2 O 3 and diffuse Co, thereby producing a Co-containing γ-Fe 2 O 3 film. As a result of the X-ray diffraction of the Co-containing γ-Fe 2 O 3 film obtained, the plane (400) was proved to predominantly orient in parallel with the substrate. From the observation of the magnetization curve of the film, it was found that the residual magnetization and the coercive force of the film to which a magnetic field was applied in the perpendicular direction were larger than those of the film to which a magnetic field was applied in the in-plane direction. Thus, the film was recognized as a perpendicular magnetic film. As to the magnetic characteristics of the perpendicular magnetic film, the saturation magnetization was 350 emu/cc (4400 G), the coercive force was 4.5 kOe and the squareness was 0.74. The spacing of the plane (400) was 0.2079 nm (2.079 Å), the optical absorption coefficient at 700 nm was 1.4 µm -1 . and the figure of merit at 700 nm was 3.7. Thus, the film was proved to be a good perpendicular magnetic film in spite of the very thin film thickness. Comparative Example 1 A multi-layered film was produced in the same way as in Example 1 except that the thickness of the spinel Fe 3 O 4 film was 13 nm (130 Å), that the thickness of the spinel CoO film was 2.4 nm (24 Å) (the molar ratio of Co to Fe was 0.28), that the thickness of one unit was 15.4 nm (154 Å), and that the film was composed of 30 layers of spinel Fe 3 O 4 films and 30 layers of spinel CoO films. The X-ray diffraction spectrum of the multi-layered film obtained is shown in Fig. la. As shown in Fig. la, the plane (222) was proved to predominantly orient in parallel with the substrate. By using the multi-layered film obtained, a spinel oxide film was produced in the same way as in Example 1. As a result of the X-ray diffraction of the spinel oxide film obtained, the plane (222) was proved to predominantly orient in parallel with the substrate. From the observation of the magnetization curve of the spinel oxide film, it was found that the residual magnetization and the coercive force of the film to which a magnetic field was applied in the in-plane direction were larger than those of the film to which a magnetic field was applied in the perpendicular direction. Thus, it was proved that the film was not a perpendicular magnetic film. Comparative Example 2 A multi-layered film was produced in the same way as in Example 1 except that the thickness of the spinel Fe 3 O 4 film was 3.6 nm (36 Å), that the thickness of the spinel CoO film was 1.7 nm (17 Å). (the molar ratio of Co to Fe was 0.69), that the thickness of one unit was 5.3 nm (53 Å), and that the film was composed of 60 layers of spinel Fe 3 O 4 films and 60 layers of spinel CoO films. The X-ray diffraction spectrum of the multi-layered film obtained is shown in Fig. 1b. As shown in Fig. 1b, the plane (400) was proved to predominantly orient in parallel with the substrate. By using the multi-layered film obtained, a spinel oxide film was produced in the same way as in Example 1. As a result of the X-ray diffraction of the spinel oxide film obtained, the plane (400) was proved to predominantly orient in parallel with the substrate. From the observation of the magnetization curve of the spinel oxide film, it was found that the residual magnetization and the coercive force of the film to which a magnetic field was applied in the in-plane direction were larger than those of the film to which a magnetic field was applied in the perpendicular direction. Thus, it was proved that the film was not a perpendicular magnetic film. Example 7 In a high-frequency high-rate sputtering device SH-250H-T06 (manufactured by Nihon Shinku Corp.) a glass substrate was placed with a distance of 80 mm from an Ni target at room temperature. Reactive sputtering was first carried out by using the Ni target in an atmosphere of argon and oxygen under a partial oxygen pressure of 0.015 Pa (0.11 mTorr) and the total pressure of 0.67 Pa (5 mTorr) at a deposition rate of 4 nm/min (40 Å/min), thereby forming an NaCl-type NiO film on the glass substrate to a thickness of 100 nm (1000 Å) as a primary layer. As a result of the X-ray diffraction of the NiO film obtained, the plane (100) was proved to predominantly orient in parallel with the substrate. A multi-layered film was produced on the NiO film in the same way as in Example 1 except that the thickness of the spinel Fe 3 O 4 film was 39 nm (390 Å), that the thickness of the spinel CoO film was 7.2 nm (72 Å) (the molar ratio of Co to Fe was 0.26), that the thickness of one unit was 46.2 nm (462 Å), and that the laminated film was composed of 5 layers of spinel Fe 3 O 4 films and 5 layers of spinel CoO films. The X-ray diffraction spectrum of the multi-layered film obtained is shown in Fig. 8. As shown in Fig. 8, the plane (400) was proved to predominantly orient in parallel with the substrate. The multi-layered film was annealed in the air at a temperature of 400°C for one hour so as to oxidize Fe 3 O 4 into γ-Fe 2 O 3 and diffuse Co, thereby producing a Co-containing γ-Fe 2 O 3 film having the NiO film as the primary layer. As a result of the X-ray diffraction of the Co-containing γ-Fe 2 O 3 film obtained, the plane (400) was proved to predominantly orient in parallel with the substrate. From the observation of the magnetization curve of the Co-containing γ-Fe 2 O 3 , the film was recognized as a perpendicular magnetic film. As to the magnetic characteristics of the perpendicular magnetic film, the saturation magnetization was 350 emu/cc (4400 G), the coercive force was 5.4 kOe and the squareness was 0.81. The spacing of the plane (400) was 0.2081 nm (2.081 Å), the optical absorption coefficient at 700 nm was 1.9 µm -1 . and the figure of merit at 700 nm was 4.0. Thus, the film was proved to be a good perpendicular magnetic film. Example 8 In a high-frequency high-rate sputtering device SH-250H-T06 (manufactured by Nihon Shinku Corp.) a glass substrate was placed with a distance of 80 mm from targets and the substrate temperature was held at a temperature of 280°C. Reactive sputtering was first carried out by using an Fe target in an atmosphere of argon and oxygen under a partial oxygen pressure of 0.015 Pa (0.11 mTorr) and the total pressure of 0.67 Pa (5 mTorr) at a deposition rate of 4 nm/min (40 Å/min), thereby forming a spinel Fe 3 O 4 film on the glass substrate to a thickness of 40 nm (400 Å) as a first layer. The shutter was next rotated to form an oxide layer containing Co and Mn (the molar ratio of Mn to Co was 0.08) to a thickness of 0.8 nm (8 Å) as a second layer by sputtering at a deposition rate of 4.2 nm/min (42 Å/min). The first layer and the second layer constituted one unit having a thickness of 4.8 nm (48 Å). The molar ratio of Co to Fe in one unit was 0.26. These operations were alternately repeated 100 times to obtain a laminated film composed of 50 layers of spinel Fe 3 O 4 films each having a thickness of 4 nm (40 Å), and 50 layers of oxide layers containing Co and Mn each having a thickness of 0.8 nm (8 Å). The making power at the time of film formation was 300 W on the Fe side and 300 W on the Co-Mn side. As a result of the X-ray diffraction of the thus-obtained multi-layer film, the plane (400) was proved to predominantly orient in parallel with the substrate. The multi-layered film was annealed in the air at a temperature of 450°C for one hour so as to oxidize Fe 3 O 4 into γ-Fe 2 O 3 and diffuse Co and Mn, thereby producing a Co-Mn-containing γ-Fe 2 O 3 film. As a result of the X-ray diffraction of the Co-Mn-containing γ-Fe 2 O 3 film obtained, the plane (400) was proved to predominantly orient in parallel with the substrate. The magnetization curve of the Co-Mn-containing γ-Fe 2 O 3 film is shown in Fig. 9. In Fig. 9, the solid line a shows the magnetization curve of the film to which a magnetic field was applied in the perpendicular direction, and the broken line b shows the magnetization curve of the film to which a magnetic field was applied in the in-plane direction. As shown in Fig. 9, since the residual magnetization and the coercive force of the solid line a were larger than those of the broken line b , the film was recognized as a perpendicular magnetic film. As to the magnetic characteristics of the perpendicular magnetic film, the saturation magnetization was 340 emu/cc (4250 G), the coercive force was 6.3 kOe and the squareness was 0.90. The spacing of the plane (400) was 0.2082 nm (2.082 Å), the optical absorption coefficient at 700 nm was 2.1 µm -1 and the figure of merit at 700 nm was 2.9. Thus, the film was proved to be a good perpendicular magnetic film. Example 9 In a high-frequency high-rate sputtering device SH-250H-T06 (manufactured by Nihon Shinku Corp.) a glass substrate was placed with a distance of 80 mm from an Ni target at room temperature. Reactive sputtering was first carried out by using the Ni target in an atmosphere of argon and oxygen under a partial oxygen pressure of 0.015 Pa (0.11 mTorr) and the total pressure of 0.67 Pa (5 mTorr) at a deposition rate of 4 nm/min (40 Å/min), thereby forming an NaCl-type NiO film on the glass substrate to a thickness of 100 nm (1000 Å) as a primary layer. As a result of the X-ray diffraction of the NiO film obtained, the plane (100) was proved to predominantly orient in parallel with the substrate. A multi-layered was formed on the NiO film in the same way as in Example 1 except that a spinel Fe 3 O 4 film having a thickness of 20 nm (200 Å) and an oxide layer containing Co and Mn (the molar ratio of Mn to Co was 0.08) and having a thickness of 4.1 nm (41 Å) constituted one unit having a thickness of 24.1 nm (241 Å), that the molar ratio of Co to Fe in one unit was 0.26, and that the laminated film was composed of 10 layers of spinel Fe 3 O 4 films each having a thickness of 20 nm (200 Å) and 10 layers of oxide layer containing Co and Mn having a thickness of 4.1 nm (41 Å). The X-ray diffraction spectrum of the multi-layered film obtained is shown in Fig. 10. As shown in Fig. 10, the plane (400) was proved to predominantly orient in parallel with the substrate. The multi-layered film was annealed in the air at a temperature of 400°C for one hour so as to produce a Co-Mn-containing γ-Fe 2 O 3 film having the NiO film as the primary layer. As a result of the X-ray diffraction of the film obtained, the plane (400) was proved to predominantly orient in parallel with the substrate. From the observation of the magnetization curve of the film, the film was recognized as a perpendicular magnetic film. As to the magnetic characteristics of the perpendicular magnetic film, the saturation magnetization was 346 emu/cc (4350 G), the coercive force was 6.3 kOe and the squareness was 0.95. The spacing of the plane (400) was 0.2081 nm (2.081 Å), the optical absorption coefficient at 700 nm was 2.1 µm -1 and the figure of merit at 700 nm was 2.9. Thus, the film was proved to be a good perpendicular magnetic film. Example 10 In a high-frequency high-rate sputtering device SH-250H-T06 (manufactured by Nihon Shinku Corp.) a glass substrate was placed with a distance of 80 mm from an Ni target and the substrate temperature was held at 25 to 100 °C. Reactive sputtering was first carried out by using the Ni target in an atmosphere of argon and oxygen under a partial oxygen pressure of 0.015 Pa (0.11 mTorr) and the total pressure of 0.67 Pa (5 mTorr) at a deposition rate of 4 nm/min (40 Å/min), thereby forming an NaCl-type NiO film on the glass substrate to a thickness of 100 nm (1000 Å) as a primary layer. As a result of the X-ray diffraction of the NiO film obtained, the plane (100) was proved to predominantly orient in parallel with the substrate. A monolayer of Co-containing Fe 3 O 4 film (the molar ratio of Co to Fe was measured to be 0.26 as a result of composition analysis) having a thickness of 200 nm (2000 Å) was formed on the NiO film by sputtering an Fe-Co alloy target in an atmosphere of argon and oxygen under a partial oxygen pressure of 0.015 Pa (0.10) mTorr and the total pressure of 0.67 Pa (5 mTorr) at a deposition rate of 4 nm/min (40 Å/min). The making power at the time of film formation was 300 W. The X-ray diffraction spectrum of the monolayer film obtained is shown in Fig. 11. As shown in Fig. 11, the plane (400) was proved to predominantly orient in parallel with the substrate. When the magnetization curve of Co-containing Fe 3 O 4 film was measured, the film was recognized as an in-plane magnetic film. The monolayer film was annealed in the air at a temperature of 400°C for 2 hours so as to produce a Co-containing γ-Fe 2 O 3 film having the NiO film as the primary layer. As a result of the X-ray diffraction of the film obtained, the plane (400) was proved to predominantly orient in parallel with the substrate. The magnetization curve of the Co-containing γ-Fe 2 O 3 film is shown in Fig. 12. In Fig. 12, the solid line a shows the magnetization curve of the film to which a magnetic field was applied in the perpendicular direction, and the broken line b shows the magnetization curve of the film to which a magnetic field was applied in the in-plane direction. As shown in Fig. 12, since the residual magnetization and the coercive force of the solid line a were larger than those of the broken line b , it was proved that the monolayer of Co-containing Fe 3 O 4 film was transformed into a perpendicular magnetic film (Co-containing Fe 3 O 4 was transformed into Co-containing γ-Fe 2 O 3 ) by annealing. As to the magnetic characteristics of the perpendicular magnetic film, the saturation magnetization was 358 emu/cc (4200 G), the coercive force was 5.4 kOe and the squareness was 0.82. The spacing of the plane (400) was 0.2082 nm (2.082 Å), the optical absorption coefficient at 700 nm was 1.9 µm -1 and the figure of merit at 700 nm was 4.0. Thus, the film was proved to be a good perpendicular magnetic film.

Claims: A perpendicular magnetic film comprising a Co-containing γ-Fe 2 O 3 film formed on a substrate, in which the plane (400) of the Co-containing γ-Fe 2 O 3 film is predominantly oriented in parallel with said substrate, the molar ratio of Co to Fe is from 0.01:1 to 0.32:1, the spacing of said plane (400) is not more than 0.2084 nm (2.084 Å), and the optical absorption coefficient at 700 nm is not more than 2.5 µm -1 . A perpendicular magnetic film comprising: a primary layer of an NiO film formed on a substrate, in which the plane (100) is predominantly oriented in parallel with said substrate, and a Co-containing γ-Fe 2 O 3 film formed on said primary layer, in which the plane (400) is predominantly oriented in parallel with said substrate, the molar ratio of Co to Fe is from 0.10:1 to 0.32:1, the spacing of said plane (400) is not more than 0.2084 nm (2.084 Å), and the optical absorption coefficient at 700 nm is not more than 2.5 µm -1 . A film according to claim 1 or 2 wherein said Co-containing γ-Fe 2 O 3 film contains 0.01 to 0.25 mol of Mn based on 1 mol of Co. A film according to claim 1, 2 or 3 wherein said Co-containing γ-Fe 2 O 3 film has a coercive force of not less than 3kOe. A film according to any one of the preceding claims wherein said Co-containing γ-Fe 2 O 3 film has a squareness of not less than 0.7. A multi-layered film suitable for conversion to a perpendicular magnetic film as defined in claim 1, which multi-layered film comprises a plurality of units formed on a substrate, each unit comprising an Fe 3 O 4 layer and CoO layer laminated one on top of the other, in which the plane (400) is predominantly oriented in parallel with said substrate, the thickness of each unit is not more than 13 nm (130 Å), and the molar ratio of Co to Fe is from 0.01:1 to 0.32:1. A multi-layered film suitable for conversion to a perpendicular magnetic film as defined in claim 2, which multi-layered film comprises a primary layer of an NiO film formed on a substrate, in which the plane (100) is predominantly oriented in parallel with said substrate; and a plurality of units formed on said primary layer, each unit comprising an Fe 3 O 4 layer and CoO layer laminated one on top of the other, in which the plane (400) is predominantly oriented in parallel with said substrate, the thickness of each unit is not more than 80 nm (800 Å), and the molar ratio of Co to Fe is from 0.10:1 to 0.32:1. A multi-layered film suitable for conversion to a perpendicular magnetic film, as defined in claim 3, which multi-layered film comprises a plurality of units formed on a substrate, each unit comprising an Fe 3 O 4 layer and an oxide layer containing Co and Mn laminated one on top of the other, in which the plane (400) is predominantly oriented in parallel with said substrate, the molar ratio of Mn to Co is from 0.01:1 to 0.25:1, the molar ratio of Co to Fe is from 0.01:1 to 0.32:1, and the thickness of each unit is not more than 13 nm (130 Å). A multi-layered film suitable for conversion to a perpendicular magnetic film as defined in claim 3, which multi layered film comprises a primary layer of an NiO film formed on a substrate, in which the plane (100) is predominantly oriented in parallel with said substrate; and a plurality of units formed on said primary layer, each unit comprising an Fe 3 O 4 layer and an oxide layer containing Co and Mn laminated one on top of the other, in which the plane (400) is predominantly oriented in parallel with said substrate, the thickness of each unit is not more than 80 nm (800 Å), the molar ratio of Co to Fe is from 0.10:1 to 0.32:1, and the molar ratio of Mn to Co is from 0.01:1 to 0.25:1. A process for producing a film as defined in any one of claims 1 to 5 which comprises annealing at 280 to 450°C a multi-layered film as defined in any one of claims 6 to 9.

Inventors: DOI TAKANORI, TAMARI KOUSAKU, DOI, TAKANORI, TAMARI, KOUSAKU
Applicants: TODA KOGYO CORP, TODA KOGYO CORP.
Application year: 1993
Main classification: G11B5
IPC codes: H01F10, G11B5, H01F41