Magnetic recording medium comprising a spinel magnetic film, medium comprising a multilayered film for the preparation of a spinel magnetic film and process for the preparation of a magnetic recording medium with a spinel magnetic film

Abstract: A perpendicular magnetic film comprising a spinel thin film which is provided on a substrate; which contains Fe as the main ingredient and further contains Co and Ni; which has a coercive force of less than 2.39 x 10⁵ Am⁻¹ (3000 Oe); the plane (400) thereof being predominantly oriented in parallel with the said substrate, the spacing of the plane (400) being not more than 0.2082 nm; the molar ratio of Co:Fe being from 0.005:1 to 0.32:1 and the molar ratio of Ni:Co being not less than 0.6:1.

Description: The present invention relates to a perpendicular magnetic film which is a spinel thin film formed on a substrate. The perpendicular magnetic film of the invention is thus a magnetic recording medium. More particularly, the present invention relates to a magnetic recording medium in which a spinel thin film is provided on a substrate, the spinel thin film being composed of a spinel thin film containing Fe as the main ingredient and further containing Co and Ni, which has an excellent oxidation resistance, an excellent anticorrosivity, and an appropriate coercive force for preventing magnetic saturation of a magnetic head which is widely used at present, particularly, a coercive force of less than 3000 Oe, and hence, which is suitable as a high-density magnetic recording medium. The present invention also relates to a process for producing such a magnetic recording medium and a multilayered film which is used as an intermediate in the production of the magnetic recording medium. With recent remarkable tendency for miniaturized and higher reliable information processing apparatuses and systems, magnetic recording media have increasingly been required to have a higher recording density. A perpendicular magnetic film as a magnetic recording medium which respond to such demand has been rapidly developed and put to practical use. Perpendicular magnetic films which are magnetized perpendicularly to the film surface are not only free from demagnetization but also capable of high-density recording. An alloy film such as a CoCr alloy film has conventionally been proposed as a perpendicular magnetic film. However, it is necessary to coat the surface of a CoCr alloy film with a carbon film having a thickness of about 100 to 200 Å in order to prevent the deterioration of the magnetization characteristics. As a result, the spacing (the distance between a magnetic head and the recording medium) loss is increased by the distance corresponding to the thickness of the carbon film, which is unsuitable for high-density recording. For this reason, a material of a perpendicular magnetic film is strongly required to be an oxide which is stable against oxidation. As to the coercive force of a perpendicular magnetic film, an appropriate coercive for preventing a magnetic saturation of a magnetic head which is widely used at present, particularly, a coercive force of less than 3000 Oe is required. The coercive force (Hc) of a magnetic recording medium has a close relationship with the performance of a magnetic head, as is well known. When the coercive force (Hc) of the magnetic recording medium is as high as more than 3000 Oe, the current at which a writing operation is performed becomes so high that the head core of a magnetic head which is widely used at present is magnetically saturated due to an insufficient saturation flux density (Bm). As a result, it is impossible to sufficiently magnetize the magnetic recording medium. A ferrite head is widely used in a magnetic recording and reading-out machine which corresponds to a magnetic recording medium having a coercive force of not more than 1000 Oe, while a head such as a Sendust head, an amorphous head and a thin-film head, whose head core is made of a material having a high saturation flux density, is used in a magnetic recording and reading-out machine which corresponds to a magnetic recording medium having a coercive force of more than 1000 Oe. As a perpendicular magnetic film for magnetic recording, an amorphous alloy film such as a CoCr alloy film and a CoPt alloy film, 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 magneto plumbite oxide thin film such as a barium ferrite film (Japanese Patent Application Laid-Open (KOKAI) No. 62-267949 (1987)) and the like have conventionally been proposed. Examples of the perpendicular magnetic recording films and processes for producing the same which have conventionally been disclosed are set forth below, concretely. (i) A process for producing a cobalt ferrite spinel film, which comprises forming a spinel film by sputtering onto a substrate of a low temperature in an oxidizing air by using an alloy target composed mainly of Co and Fe (Japanese Patent Application Laid-Open (KOKAI) No. 63-47359). In Japanese Patent Application Laid-Open (KOKAI) No. 63-47359, the description of "When a reactive sputtering is carried out in an oxidizing air by using an alloy target composed mainly of Co and Fe, a cobalt·ferrite spinel film represented by Co x Fe 3-x O 4 , (wherein 0.5 ≤ x ≤ 1.05) is formed on the substrate. In this case, even if the surface temperature is as low as about 200°C, a spinel film having a good crystallizability is obtained." is disclosed. (ii) A magnetic recording medium comprising a substrate, an under layer of a crystalline film having a spinel crystalline structure, and a magnetic recording layer of a spinel ferrite crystalline film, wherein the lattice constant of the under layer is larger than that of the magnetic recording layer (Japanese Patent Application Laid-Open (KOKAI) No. 3-17813). The description of "The under layer is composed of a material represented by the following general formula: AB 2 0 4 wherein A is a metal element of divalent ions and at least one selected from the group consisting of Mg, Mn, Co, Ni, Cu, Zn, Fe, etc., and B is a metal element of trivalent ions and at least one selected from the group consisting of Cr, In, Rh, Sc, Tl, Fe, etc. The magnetic recording layer 3 is composed of the material represented by the following general formula: AB x Fe 3-x O 4 wherein A and B are the same as in the above-mentioned general formula, and 0 ≤ x < 2." is disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 3-17813. (iii) An oriented film of oxide crystals represented by the following general formula: A x B 3-x O y wherein A is at least one selected from the group consisting of Mn, Co, Ni, Cu, Mg, Cr, Zn, Li and Ti; B is Fe or Al;, 0.5 ≤ x ≤ 2.0 and 2.5 ≤ y ≤ 4, wherein the ratio (I 111 /I 222 ) of the reflective peak intensity I 111 of the crystal face (111) and the reflective peak intensity I 222 of the crystal face (222) is less than 0.2 when the X-ray diffraction peak is indexed on the assumption that the film has a spinel crystalline structure(Japanese Patent Application Laid-Open (KOKAI) No. 3-188604). (iv) A process for producing a cobalt ferrite film comprising the steps of forming a multi-layer metal film by laminating at least two of Co layer and Fe layer on a substrate, and heat-treating the obtained multi-layer film in an air containing oxygen (Japanese Patent Application Laid-Open (KOKAI) No. 4-10509). There are the following description in Japanese Patent Application Laid-Open (KOKAI) No. 4-10509. "The total thickness of the Co layer and the Fe layer in the multi-layer metal film is not more than 100 Å. This is because if the total thickness of the Co layer and Fe layer exceeds 100 Å, it is difficult to produce a cobalt ferrite film having a large Kerr rotation angle.""Seven kinds of multilayers were formed on a glass substrate 3 (Coning 7059, produced by CONING) by sputtering in Ar while using a single Co target and a single Fe target under the following condition, as shown in Fig. 1.Sputtering condition: Total sputtering pressure: 2 mtorr Sputtering current: 0.2 A Substrate temperature: room temperature In each of the multilayers, the thickness ratio of a Co layer 1 and an Fe layer 2 was 1 : 2, and the total film thickness was constantly 2000 Å. These multilayers obtained were 1 layer of (Co/Fe = 660 Å/1340 Å), 2 layers of (Co/Fe = 330 Å/670 Å), 4 layers of (Co/Fe = 165 Å/335 Å), 8 layers of (Co/Fe = 82.5 Å/167.5 Å), 10 layers of (Co/Fe = 66 Å/134 Å), 20 layers of (Co/Fe = 33 Å/67 Å), and 40 layers of (Co/Fe = 17 Å/33 Å). Each of the multilayers was heat-treated in an electric oven in the air in the heat treatment pattern shown in Fig. 3. More specifically, the film was heated at a high raising rate until 300°C, and then heated at a raising rate of 100°C/hr until 500°C. The film was heated at 500°C for 2 hours, and was then gradually cooled. Thus, a cobalt ferrite film was formed on the glass substrate "Namely, the cobalt ferrite films disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 4-10509 are a multilayer (in-plane magnetic film) composed of a cobalt layer and a ferrite layer and a film (perpendicular magnetic film) composed of a cobalt·ferrite oxide. Each of the multi-layer metal films is heat-treated at a temperature of not lower than 500°C for not less than 2 hours, thereby obtaining a cobalt·ferrite oxide film. (v) An amorphous alloy film such as a Gd-Co film and a Tb-Fe film, composed of a rare earth metal and a transition metal (Japanese Patent Application Laid-Open (KOKAI) No. 51-119999) and a magneto-plumbite oxide thin film such as a barium ferrite film (Japanese Patent Application Laid-Open (KOKAI) No. 62-267949). Among the above-described perpendicular magnetic films, the cobalt ferrite films which are typical of spinel oxides are stable against oxidation, because they are oxides, and have a large crystalline magnetic anisotropy. Owing to these magnetic characteristics the cobalt ferrite films are considered to be promising as a perpendicular magnetic recording medium. As the process for producing a cobalt ferrite (CoFe 2 O 4 ) film, various methods such as sputtering method, vacuum evaporation method and MO-CVD method are known. However, although a perpendicular magnetic film which has an excellent oxidation resistance, an excellent anticorrosivity, and an appropriate coercive force for preventing a magnetic saturation of a head which is widely used at present is now in the strongest demand, none of the conventional magnetic thin films sufficiently meet these requirements. For example, a cobalt ferrite (CoFe 2 O 4 ) film produced by a sputtering method is disadvantageous in that although the easy magnetization axis of a cobalt ferrite (CoFe 2 O 4 ) film is in the plane (400), the plane (400) is likely to orient at random or the plane (111) is likely to orient in parallel with the substrate, so that it is difficult to produce a perpendicular magnetic film. As examples of 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 Proceedings of the 9-th Meeting of Magnetic Society of Japan 29PB-10 , (2) the method described in Proceedings of the 13-th Meeting of Magnetic Society of Japan , p 246, and (3) the method described in Japanese Patent Application Laid-Open (KOKAI) No. 4-10509 (1992) 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 heated to 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, in order to raise the substrate temperature to not lower than 500°C, the substrate itself is required to have a high heat-resistance. However, the heat-resistance of glass plastics or the like, which is widely used as a material of the substrate for a perpendicular magnetic recording medium, is insufficient. In this way, since the material of the substrate is limited, it is not advantageous either industrially or economically. The method (2) is a plasma-excitation MO-CVD method. Since it is necessary to maintain the substrate temperature at 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 multilayered metal film produced 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, as described above, which is disadvantageous industrially and economically. EP-A-122030 discloses a magnetic recording member comprising a substrate having formed on the surface thereof a substantially perpendicular incidence magnetic film comprising a composition of a magnetic metal and oxygen where the magnetic metal comprises one or more ferromagnetic metals and/or magnetic alloys thereof. The oxygen content of this magnetic film is not beyond 50 atm%. JP-A-62-103842 discloses a ferromagnetic film for magnetic recording, comprising an artificial lattice film having an alternating laminated structure in which 15-100Å thick metal films mainly composed of an Fe-Co alloy and 15-100Å thick spinel-type oxide films mainly composed of an Fe-Co alloy are laminated alternately on a substrate, such alternate lamination being repeated at least 5 times. Accordingly, it is required to eliminate the above described problems in the related art and to provide a magnetic recording medium having a magnetic film which is composed of an oxide having a spinel crystal structure, and which has an excellent oxidation resistance, an excellent anticorrosivity and an appropriate coercive force. Also, it is a technical problem to be solved of the present invention to provide a process for producing a magnetic recording medium of the invention at a temperature of lower than 500°C and as low as possible with industrial and economical advantages. As a result of the present inventors' studies, it has been found that by annealing a Co- and Ni-containing multilayered film provided on a substrate, the film comprising as one unit a magnetite [ FeO x · Fe 2 O 3 (0 < x ≤ 1)] layer which contains or does not contain Co and/or Ni and an oxide layer which contains Co and/or Ni (provided that the magnetite layer and the oxide layer are multilayered so as to contain both Co and Ni as a whole unit), which are formed on a substrate, the plane (400) of the multilayered film being predominantly oriented in parallel with the substrate, the thickness of one unit being not more than 300 Å, the molar ratio of Co to Fe being 0.005 to 0.32, and the molar ratio of Ni to Co being not less than 0.6, at a temperature of 280 to 450°C, the thus-obtained magnetic recording medium has an excellent oxidation resistance, an excellent anticorrosivity and an appropriate coercive force, for example, less than 3,000 Oe. On the basis of the finding, the present invention has been attained. It is an object of the present invention to provide a magnetic recording medium in which, provided on a substrate, is an oxide having a spinel crystal structure, which has an excellent oxidation resistance, an excellent anticorrosivity and an appropriate coercive force. To achieve this aim, in a first aspect of the present invention, there is provided a magnetic recording medium in which a spinel thin film is provided on a substrate, which film contains Fe as the main ingredient and further contains Co and Ni and which has a coercive force of less than 2.39 x 10 5 Am -1 (3000 Oe), a saturation magnetization of not less than 0.25T (200 emu/cc, 2500 G) and a squareness of not less than 0.20; the plane (400) thereof being predominantly oriented in parallel with the said substrate, the spacing of the plane (400) being not more than 0.2082 nm; the molar ratio of Co:Fe being from 0.005:1 to 0.32:1 and the molar ratio of Ni:Co being not less than 0.6:1. In a second aspect of the present invention, there is provided a Co- and Ni-containing multilayered film provided on a substrate, the film comprising a multiplicity of units, each unit composed of a magnetite [ FeO x · Fe 2 O 3 (0 < x ≤ 1)] layer which optionally contains Co, Ni or both Co and Ni and of an oxide layer which contains Co, Ni or both Co and Ni provided that the magnetite layer and the oxide layer are multilayered so as to contain both Co and Ni as a whole unit, the plane (400) thereof being predominantly oriented in parallel with said substrate, each unit having a thickness of not more than 30 nm, the molar ratio of Co:Fe being from 0.005:1 to 0.32:1, and the molar ratio of Ni:Co being not less than 0.6:1. In a third aspect of the present invention, there is provided a process for producing a magnetic recording medium of the invention which process comprises annealing the Co- and Ni-containing multilayered film of the invention at a temperature of 280 to 450°C. Figs. 1(a) to 1(c) show the X-ray diffraction spectra of multilayered films; Fig. 2 shows the magnetization curves of the perpendicular magnetic film obtained in Example 1; Fig. 3 shows the magnetization curves of the perpendicular magnetic film obtained in Example 2; Fig. 4 shows the magnetization curves of the perpendicular magnetic film obtained in Example 3; Fig. 5 shows the magnetization curves of the perpendicular magnetic film obtained in Example 4; Fig. 6 shows the magnetization curves of the perpendicular magnetic film obtained in Example 5; Fig. 7 shows the magnetization curves of the perpendicular magnetic film obtained in Example 6; and Fig. 8 shows the magnetization curves of the film obtained in Comparative Example 4. A perpendicular magnetic film according to the present invention is a spinel thin film formed on a substrate, i.e. a magnetic recording medium. The spinel thin film contains Fe as the main ingredient and further contains Co and Ni. The plane (400) of the film formed on a substrate is predominantly oriented in parallel with the substrate and the spacing of the plane (400) is not more than 0.2082 nm (2.082 Å), preferably 0.2070 to 0.2080 nm (2.070 to 2.080 Å). The molar ratio of Co:Fe in the thin film is 0.005:1 to 0.32:1, preferably 0.01:1 to 0.20:1 and the molar ratio of Ni:Co therein is not less than 0.6:1, preferably 1.0:1 to 5.0:1. The coercive force of the film is less than 2.39 x 10 5 Am -1 (3000 Oe), preferably from 0.955 x 10 5 to 1.99 x 10 5 Am -1 (1200 to 2500 Oe). In the present invention, the spacing of the plane (400) in the spinel thin film containing Fe as the main ingredient and further containing Co and Ni is not more than 0.2082 nm (2.082 Å). The spinel thin film of the present invention is a thin film wherein Fe 2+ in the magnetite which constitutes the multilayered film is completely oxidized into Fe 3+ , and also includes a thin film in which a slight amount of Fe 2+ in the magnetite remains. When the plane (400) of a film is not predominantly oriented in parallel with the substrate, or when the spacing of the plane (400) of a film exceeds 0.2082 nm (2.082 Å), the magnetization film does not constitute a perpendicular magnetic film. If the molar ratio of Co:Fe is less than 0.005:1, it is difficult to produce a perpendicular magnetic film. In order to facilitate the production of a spinel thin film in which the plane (400) is predominantly oriented in parallel with the substrate, the molar ratio of Co:Fe is preferably not less than 0.01:1. If the molar ratio of Co:Fe exceeds 0.32:1, it is difficult to oxidize a magnetite, in other words, to transform magnetite [ FeO x · Fe 2 O 3 (0 < x ≤ 1)] into γ-Fe 2 O 3 , or to diffuse the Co ions and Ni ions in the spinel oxide layer composed of Fe as the main ingredient at a temperature of not higher than 450°C. If the molar ratio of Ni:Co is less than 0.6:1, it is difficult to achieve a coercive force of less than 2.39 x 10 5 Am -1 (3000 Oe). In order to obtain a coercive force of not more than 1.99 x 10 5 Am -1 (2500 Oe), which is more preferable from the point of view of prevention of the magnetic saturation of a magnetic head, the molar ratio of Ni:Co is preferably not less than 1.0:1. The larger the Ni content, the smaller the coercive force is apt to become. With consideration for a preferable coercive force, the upper limit of the molar ratio of Ni:Co is not more than 5.0:1, preferably not more than 3.0:1. When the molar ratio of Ni:Co is 5.0:1, the coercive force is about 0.64 x 10 5 Am -1 (800 Oe), and when the molar ratio of Ni:Co is 3.0:1, the coercive force is about 1.03 x 10 5 Am -1 (1300 Oe). If the coercive force is more than 2.39 x 10 5 Am -1 (3000 Oe), it is impossible to prevent a magnetic saturation of a magnetic head which is widely used at present. From the point of view of prevention of the magnetic saturation of a magnetic head, the coercive force is preferably not more than 1.99 x 10 5 Am -1 (2500 Oe), more preferably 0.955 x 10 5 to 1.99 x 10 5 Am -1 (1200 to 2500 Oe). A magnetic recording medium according to the present invention can be produced by annealing at a temperature of 280 to 450°C, preferably 290 to 400°C, a Co- and Ni-containing multilayered film of the invention. A multilayered film according to the present invention can be produced by any of a reactive sputtering method in which a composite metal target (composed of an Fe target which contains or does not contain Co and/or Ni, and a metal target which contains one selected from Co, Ni, Co-Ni alloy, and Co and Ni) are sputtered in an oxidizing atmosphere; a direct method in which an oxidized film is directly formed from a sintered target of magnetite [ FeO x · Fe 2 O 3 (0 < x ≤ 1)] which contains or does not contain Co and/or Ni and a sintered target of an oxide which contains Co and/or Ni; and a reactive evaporation method in which metals (Fe which contains or does not contain Co and/or Ni, and one selected from the group consisting of Co, Ni, Co-Ni alloy, and Co and Ni) are evaporated in an oxidizing atmosphere. An example of layers which constitutes the unit in the Co- and Ni-containing multilayered film used in the present invention is as follows. (1) Co- and Ni-containing magnetite layer / Co oxide layer (2) Co- and Ni-containing magnetite layer / Ni oxide layer (3) Co- and Ni-containing magnetite layer / Co and Ni oxides layer (4) Co-containing magnetite layer / Ni oxide layer (5) Co-containing magnetite layer / Co and Ni oxides layer (6) Ni-containing magnetite layer / Co oxide layer (7) Ni-containing magnetite layer / Co and Ni oxides layer (8) Magnetite layer / Co and Ni oxides layer The Co oxide layer is typically composed of Co oxide having NaCl-type crystalline structure and the Ni oxide layer is typically composed of Ni oxide having NaCl-type crystalline structure. Also, the Co and Ni oxides layer is typically composed of Co and Ni oxides having NaCl-type crystalline structure. As the material of the substrate in the present invention, commercially available materials such as soda-lime glass are usable. In a multilayered film according to the present invention, the thickness of one unit is not more than 30 nm (300 Å), preferably 5 to 25 nm (50 to 250 Å), and the molar ratio of Co:Fe is 0.005:1 to 0.32:1. It is difficult to obtain a magnetic recording medium according to the present invention by using a multilayered film in which the thickness of one unit exceeds 30 nm (300 Å) or a multilayered film in which the molar ratio of Co:Fe is less than 0.005:1 or more than 0.32:1. The molar ratio of Ni:Co in the multilayered film of the present invention is at least 0.6:1. If it is less than 0.6:1, a coercive force of less than 2.39 x 10 5 Am -1 (3000 Oe) cannot be achieved. The multilayered film of the present invention is typically composed of at least three units. Each unit is composed of a magnetite [ FeO x · Fe 2 O 3 (0 < x ≤ 1)] layer which optionally contains Co and/or Ni and an oxide layer which contains Co and/or Ni (provided that the magnetite layer and the oxide layer are multilayered so as to contain both Co and Ni as a whole unit). The thickness of the spinel film is preferably 50 to 500 nm (500 to 5000 Å), more preferably 50 to 300 nm (500 to 3000 Å). The number of units in the multilayered film of the invention is therefore determined so as to produce a film of a desired thickness. The annealing temperature of the multilayered film of the present invention is 280 to 450°C, preferably 290 to 400°C. The annealing time is preferably 0.8 to 5 hours. If the annealing temperature is lower than 280°C, the oxidation of the magnetite [ FeO x · Fe 2 O 3 (0 < x ≤ 1)], i.e., the transformation of magnetite [ FeO x · Fe 2 O 3 (0 < x ≤ 1)] into γ-Fe 2 O 3 or the diffusion of Co and Ni in the spinel thin film composed of Fe as the main ingredient is insufficient for obtaining a magnetic recording medium of the present invention. If the annealing temperature exceeds 450°C, the transformation γ-Fe 2 O 3 into α-Fe 2 O 3 begins to take place so that it is impossible to produce a magnetic recording medium according to the present invention. In annealing, the transformation temperature for oxidizing the magnetite [ FeO x · Fe 2 O 3 (0 < x ≤ 1)] i.e. transforming magnetite [ FeO x · Fe 2 O 3 (0 < x ≤ 1)] into γ-Fe 2 O 3 can be lowered by adding Cu to the spinel thin 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 out in two stages: the magnetite [ FeO x · Fe 2 O 3 (0 < x ≤ 1)] is first oxidized ( FeO x · Fe 2 O 3 (0 < x ≤ 1) is first transformed into γ-Fe 2 O 3 ) at a temperature of 240 to 320°C, and Co and Ni are 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 oxidation of the magnetite (transformation of FeO x · Fe 2 O 3 (0 < x ≤ 1) into γ-Fe 2 O 3 ). What is the most important in the present invention is that the magnetic recording medium of the present invention can be produced by annealing a Co- and Ni-containing multilayered film composed of a multiplicity of units formed on a substrate, each unit composed of a magnetite [ FeO x · Fe 2 O 3 (0 < x ≤ 1)] layer which contains or does not contain Co and/or Ni and an oxide layer which contains Co and/or Ni (provided that the magnetite layer and the oxide layer are multilayered so as to contain both Co and Ni as a whole unit), wherein the plane (400) thereof is predominantly oriented in parallel with the substrate, each unit has a thickness of not more than 30 nm (300 Å), the molar ratio of Co:Fe is 0.005:1 to 0.32:1, and the molar ratio of Ni:Co is not less than 0.6:1. This has been confirmed from the fact that a magnetic recording medium according to the present invention cannot be obtained either by using a multilayered film in which the molar ratio of Co:Fe is between 0.005:1 and 0.32:1 but, the thickness of one unit composed of a magnetite [ FeO x · Fe 2 O 3 (0 < x ≤ 1)] layer which contains or does not contain Co and/Ni and an oxide layer which contains Co and/or Ni (provided that the magnetite layer and the oxide layer are multilayered so as to contain both Co and Ni as a whole unit) exceeds 30 nm (300 Å), so that the planes (311) and (222) are predominantly oriented in parallel with the substrate, or by using a multilayered film in which the thickness of one unit of a magnetite [ FeO x · Fe 2 O 3 (0 < x ≤ 1)] layer which contains or does not contain Co and/or Ni and an oxide layer which contains Co and/or Ni (provided that the magnetite layer and the oxide layer are multilayered so as to contain both Co and Ni as a whole unit) is not more than 30 nm (300 Å), but the molar ratio of Co:Fe is not in the range of 0.005:1 to 0.32:1, so that the plane (400) thereof is predominantly oriented in parallel with the substrate as shown in later-described Comparative Examples 3 and 4, and in Figs. 1(a) and 1(b). The coercive force of a conventional cobalt ferrite (CoFe 2 O 4 ) film is dependent upon the Co content, and the more the Co content therein increases, the more the coercive force thereof raises. On the other hand in the present invention, the coercive force is not largely dependent upon the Co content but is approximately determined by the molar ratio of Ni:Co. For this reason, it is possible to control the coercive force of the perpendicular magnetic film only by the molar ratio of Ni:Co and to obtain a desired coercive force with stability and good reproducibility. The reason why the coercive force is not largely dependent upon the Co content in the present invention is considered to be as follows. The coercive force of a conventional cobalt ferrite (CoFe 2 O 4 ) film is changed by utilizing the crystalline magnetic anisotropy. In contrast, in the present invention, the spacing of the plane (400) is not more than 0.2082 nm (2.082 Å), as shown in the later-described example, 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 which has the smallest spacing of the plane (400) in any other spinel crystal structure mainly composed of Fe. From these facts, it has been considered that the present invention benefits from magnetostrictive anisotropy due to the strain of the lattice in addition to the crystalline magnetic anisotropy due to cobalt. The small spacing achieved in the present invention is not caused by the shrinkage or the like of the substrate but is due to the film itself. That has been confirmed from the fact that the spacing of a film produced by transforming a monolayer Fe 3 O 4 film (200 nm (2000 Å)) in which the plane (222) thereof is oriented in parallel with the same glass substrate as that of the magnetic recording medium 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. The spinel thin film used in the present invention has a coercive force of less than 2.39 x 10 5 Am -1 (3000 Oe), preferably from 9.55 x 10 4 to 1.99 x 10 5 Am -1 (1200 to 2500 Oe). The spinel thin film used in the present invention has a saturation magnetization of not less than 200 emu/cc (2500 G). The spinel thin film used in the present invention has a squareness of not less than 0.20, preferably not less than 0.40. The magnetic recording medium of the present invention is excellent in oxidation resistance and anticorrosivity, because the magnetic film is composed of an oxide. The magnetic recording medium is suitable for high-density magnetic recording, because it comprises a spinel thin film composed of Fe as the main ingredient which has an appropriate coercive force, particularly, a coercive force of less than 2.39 x 10 5 Am -1 (3000 Oe). According to a process for producing a magnetic recording medium of the present invention, it is not necessary to heat the substrate to not lower than 300°C during film formation in a vacuum, and it is possible to produce a magnetic recording medium by annealing at a low temperature such as not higher than 500°C. The process 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 Vibration Sample Magnetometer VSM-3S-15", manufactured by Toei Kogyo K.K. Example 1 In a high-frequency high-rate sputtering device SH-250H-T06, manufactured by Nippon Shinku Kabushiki Kaisha, a glass substrate was placed with a distance of 80 mm from an Fe target and a Co-Ni target and the substrate temperature was held at 200°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 layer on the glass substrate to a thickness of 6 nm (60 Å) as a first layer. The shutter was next rotated to form a Co- and Ni-containing oxide layer (the molar ratio of Ni to Co is 1.1) to a thickness of 0.5 nm (5 Å) as a second layer by sputtering the Co-Ni 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 6.5 nm (65 Å). The molar ratio of Co to Fe in one unit was 0.06. These operations were alternately repeated 68 times to obtain a multilayered film composed of 34 Fe 3 O 4 layers each having a thickness of 6 nm (60 Å), and 34 Co- and Ni-containing oxide layers 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 300 W on the Co-Ni side. In the thus-obtained multilayered 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 obtained multilayered film was annealed in the air at a temperature of 350°C for 2 hours so as to oxidize Fe 3 O 4 into γ-Fe 2 0 3 and diffuse Co and Ni, thereby producing a spinel thin film containing Fe as the main ingredient and further containing Co and Ni. 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 thus-obtained spinel thin film is shown in Fig. 2. In Fig. 2, the solid line a shows the magnetization curve of the spinel thin film to which a magnetic field was applied in the perpendicular direction, and the broken line b shows the magnetization curve of the spinel thin film to which a magnetic field was applied in the in-plane direction. As shown in Fig. 2, since the residual magnetization and the coercive force of the solid line a were larger than those of the broken line b , the spinel thin film was recognized as a perpendicular magnetic film. As to the magnetic characteristics of the spinel thin film, the saturation magnetization was 260 emu/cc (3300 G), the coercive force was 1.75 x 10 5 Am -1 (2.2 kOe), and the squareness was 0.63. The spacing of the plane (400) was 0.2077 nm (2.077 Å). Example 2 In a high-frequency high-rate sputtering device SH-250H-T06, manufactured by Nippon Shinku Kabushiki Kaisha, a glass substrate was placed with a distance of 80 mm from an Fe target and a Co-Ni target and the substrate temperature was held at 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 layer on the glass substrate to a thickness of 5.4 nm (54 Å) as a first layer. The shutter was next rotated to form a Co- and Ni-containing oxide layer (the molar ratio of Ni to Co is 1.1) to a thickness of 0.6 nm (6 Å) as a second layer by sputtering the Co-Ni 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 6 nm (60 Å) The molar ratio of Co to Fe in one unit was 0.08. These operations were alternately repeated 68 times to obtain a multilayered film composed of 34 Fe 3 O 4 layers each having a thickness of 5.4 nm (54 Å), and 34 Co- and Ni-containing oxide layers each having a thickness of 0.6 nm (6 Å). The making power at the time of film formation was 300 W on the Fe side and 300 W on the Co-Ni side. In the thus-obtained multilayered film, the plane (400) was predominantly oriented in parallel with the substrate. The obtained multilayered film was annealed in the air at a temperature of 400°C for 1 hour so as to oxidize Fe 3 O 4 into γ-Fe 2 0 3 and diffuse Co and Ni, thereby producing a spinel thin film containing Fe as the main ingredient and further containing Co and Ni. 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 thus-obtained spinel thin film is shown in Fig. 3. In Fig. 3, the solid line a shows the magnetization curve of the spinel thin film to which a magnetic field was applied in the perpendicular direction, and the broken line b shows the magnetization curve of the spinel thin film to which a magnetic field was applied in the in-plane direction. As shown in Fig. 3, since the residual magnetization and the coercive force of the solid line a were larger than those of the broken line b , the spinel thin film was recognized as a perpendicular magnetic film. As to the magnetic characteristics of the spinel thin film, the saturation magnetization was 240 emu/cc (3000 G), the coercive force was 1.75 x 10 5 Am -1 (2.2 kOe), and the squareness was 0.64. The spacing of the plane (400) was 0.2076 nm (2.076 Å). Example 3 In a high-frequency high-rate sputtering device SH-250H-T06, manufactured by Nippon Shinku Kabushiki Kaisha, a glass substrate was placed with a distance of 80 mm from an Fe target and a Co-Ni target and the substrate temperature was held at 180°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 layer on the glass substrate to a thickness of 5.4 nm (54 Å) as a first layer. The shutter was next rotated to form a Co- and Ni-containing oxide layer (the molar ratio of Ni to Co is 1.1) to a thickness of 0.9 nm (9 Å) as a second layer by sputtering the Co-Ni 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 6.3 nm (63 Å) The molar ratio of Co to Fe in one unit was 0.12. These operations were alternately repeated 68 times to obtain a multilayered film composed of 34 Fe 3 O 4 layers each having a thickness of 5.4 nm (54 Å), and 34 Co- and Ni-containing oxide layers each having a thickness of 0.9 nm (9 Å). The making power at the time of film formation was 300 W on the Fe side and 300 W on the Co-Ni side. In the thus-obtained multilayered film, the plane (400) was predominantly oriented in parallel with the substrate. The obtained multilayered film was annealed in the air at a temperature of 400°C for 1 hour so as to oxidize Fe 3 O 4 into γ-Fe 2 0 3 and diffuse Co and Ni, thereby producing a spinel thin film containing Fe as the main ingredient and further containing Co and Ni. 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 thus-obtained spinel thin film is shown in Fig. 4. In Fig. 4, the solid line a shows the magnetization curve of the spinel thin film to which a magnetic field was applied in the perpendicular direction, and the broken line b shows the magnetization curve of the spinel thin film to which a magnetic field was applied in the in-plane direction. As shown in Fig. 4, since the residual magnetization and the coercive force of the solid line a were larger than those of the broken line b , the spinel thin film was recognized as a perpendicular magnetic film. As to the magnetic characteristics of the spinel thin film, the saturation magnetization was 210 emu/cc (2700 G), the coercive force was 1.67 x 10 5 Am -1 (2.1 kOe), and the squareness was 0.57. The spacing of the plane (400) was 0.2077 nm (2.077 Å). Example 4 In a high-frequency high-rate sputtering device SH-250H-T06, manufactured by Nippon Shinku Kabushiki Kaisha, a glass substrate was placed with a distance of 80 mm from an Fe target and a Co-Ni target and the substrate temperature was held at 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 layer on the glass substrate to a thickness of 8 nm (80 Å) as a first layer. The shutter was next rotated to form a Co- and Ni-containing oxide layer (the molar ratio of Ni to Co is 1.5) to a thickness of 0.9 nm (9 Å) as a second layer by sputtering the Co-Ni target at a deposition rate of 4.2 nm/min (4 2 Å/min). The first layer and the second layer constituted one unit having a thickness of 8.9 nm (89 Å). The molar ratio of Co to Fe in one unit was 0.07. These operations were alternately repeated 68 times to obtain a multilayered film composed of 34 Fe 3 O 4 layers each having a thickness of 8 nm (80 Å), and 34 Co- and Ni-containing oxide layers each having a thickness of 0.9 nm (9 Å). The making power at the time of film formation was 300 W on the Fe side and 300 W on the Co-Ni side. In the thus-obtained multilayered film, the plane (400) was predominantly oriented in parallel with the substrate. The obtained multilayered film was annealed in the air at a temperature of 400°C for 1 hour so as to oxidize Fe 3 O 4 into γ-Fe 2 O 3 and diffuse Co and Ni, thereby producing a spinel thin film containing Fe as the main ingredient and further containing Co and Ni. 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 thus-obtained spinel thin film is shown in Fig. 5. In Fig. 5, the solid line a shows the magnetisation curve of the spinel thin film to which a magnetic field was applied in the perpendicular direction, and the broken line b shows the magnetization curve of the spinel thin 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 spinel thin film was recognized as a perpendicular magnetic film. As to the magnetic characteristics of the spinel thin film, the saturation magnetization was 230 emu/cc (2900 G), the coercive force was 1.59 x 10 5 Am -1 (2.0 kOe), and the squareness was 0.61. The spacing of the plane (400) was 0.2074 nm (2.074 Å). Example 5 In a high-frequency high-rate sputtering device SH-250H-T06, manufactured by Nippon Shinku Kabushiki Kaisha, a glass substrate was placed with a distance of 80 mm from an Fe target and a Co-Ni target and the substrate temperature was held at 200 °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 layer on the glass substrate to a thickness of 5.6 nm (56 Å) as a first layer. The shutter was next rotated to form a Co- and Ni-containing oxide layer (the molar ratio of Ni to Co is 2.7) to a thickness of 0.7 nm (7 Å) as a second layer by sputtering the Co-Ni 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 6.3 nm (63 Å) The molar ratio of Co to Fe in one unit was 0.05. These operations were alternately repeated 68 times to obtain a multilayered film composed of 34 Fe 3 O 4 layers each having a thickness of 5.6 nm (56 Å), and 34 Co- and Ni-containing oxide layers 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-Ni side. In the thus-obtained multilayered film, the plane (400) was predominantly oriented in parallel with the substrate. The obtained multilayered film was annealed in the air at a temperature of 400°C for 1 hour so as to oxidize Fe 3 O 4 into γ-Fe 2 O 3 and diffuse Co and Ni, thereby producing a spinel thin film containing Fe as the main ingredient and further containing Co and Ni. 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 thus-obtained spinel thin film is shown in Fig. 6. In Fig. 6, the solid line a shows the magnetization curve of the spinel thin film to which a magnetic field was applied in the perpendicular direction, and the broken line b shows the magnetization curve of the spinel thin 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 spinel thin film was recognized as a perpendicular magnetic film. As to the magnetic characteristics of the spinel thin film, the saturation magnetization was 210 emu/cc (2700 G), the coercive force was 1.03 x 10 5 Am -1 (1.3 kOe), and the squareness was 0.48. The spacing of the plane (400) was 0.2075 nm (2.075 Å) Example 6 In a high-frequency high-rate sputtering device SH-250H-T06, manufactured by Nippon Shinku Kabushiki Kaisha, a glass substrate was placed with a distance of 80 mm from an Fe target and a Co-Ni target and the substrate temperature was held at 200°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 layer on the glass substrate to a thickness of 130 nm (130 Å) as a first layer. The shutter was next rotated to form a Co- and Ni-containing oxide layer (the molar ratio of Ni to Co is 1.5) to a thickness of 0.6 nm (6 Å) as a second layer by sputtering the Co-Ni target at a deposition rate of 3 nm/min (30 Å/min). The first layer and the second layer constituted one unit having a thickness of 13.6 nm (136 Å). The molar ratio of Co to Fe in one unit was 0.015. These operations were alternately repeated 30 times to obtain a multilayered film composed of 15 Fe 3 O 4 layers each having a thickness of 13 nm (130 Å), and 15 Co- and Ni-containing oxide layers each having a thickness of 0.6 nm (6 Å). The making power at the time of film formation was 300 W on the Fe side and 300 W on the Co-Ni side. In the thus-obtained multilayered film, the plane (400) was predominantly oriented in parallel with the substrate. The obtained multilayered film was annealed in the air at a temperature of 380°C for 2 hours so as to oxidize Fe 3 O 4 into γ-Fe 2 O 3 and diffuse Co and Ni, thereby producing a spinel thin film containing Fe as the main ingredient and further containing Co and Ni. 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 obtained magnetization curve of the thus-obtained spinel thin film is shown in Fig. 7. In Fig. 7, the solid line a shows the magnetization curve of the spinel thin film to which a magnetic field was applied in the perpendicular direction, and the broken line b shows the magnetization curve of the spinel thin 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 spinel thin film was recognized as a perpendicular magnetic film. As to the magnetic characteristics of the spinel thin film, the saturation magnetization was 280 emu/cc (3500 G), the coercive force was 1.59 x 10 5 Am -1 (2.0 kOe), and the squareness was 0.64. The spacing of the plane (400) was 0.2070 nm (2.070 Å). Example 7 In a high-frequency high-rate sputtering device SH-250H-T06, manufactured by Nippon Shinku Kabushiki Kaisha, a glass substrate was placed with a distance of 80 mm from an Fe target and a Co-Ni target and the substrate temperature was held at 200°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 layer on the glass substrate to a thickness of 20 nm (200 Å) as a first layer. The shutter was next rotated to form a Co- and Ni-containing oxide layer (the molar of Ni to Co is 1.5) to a thickness of 1.5 nm (15 Å) as a second layer by sputtering the Co-Ni target at a deposition rate of 3 nm/min (30 Å/min). The first layer and the second layer constituted one unit having a thickness of 21.5 nm (215 Å). The molar ratio of Co to Fe in one unit was 0.035. These operations were alternately repeated 18 times to obtain a multilayered film composed of 9 Fe 3 O 4 layers each having a thickness of 20 nm (200 Å), and 9 Co- and Ni-containing oxide layers each having a thickness of 1.5 nm (15 Å). The making power at the time of film formation was 300 W on the Fe side and 300 W on the Co-Ni side. In the thus-obtained multilayered film, the plane (400) was predominantly oriented in parallel with the substrate. The obtained multilayered film was annealed in the air at a temperature of 380°C for 3 hours so as to oxidize Fe 3 O 4 into γ-Fe 2 O 3 and diffuse Co and Ni, thereby producing a spinel thin film containing Fe as the main ingredient and further containing Co and Ni. 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. Since the residual magnetization and the coercive force of the thus-obtained spinel thin film represented by the magnetization curves of the spinel thin film to which a magnetic field was applied in the perpendicular direction were larger than those represented by the magnetization curves of the spinel thin film to which a magnetic field was applied in the in-plane direction, the spinel thin film was recognized as a perpendicular magnetic film. As to the magnetic characteristics of the spinel thin film, the saturation magnetization was 280 emu/cc (3500 G), the coercive force was 1.4 x 10 5 Am -1 (1.8 kOe), and the squareness was 0.64. The spacing of the plane (400) was 0.2078 nm (2.078 Å). Comparative Example 1 In a high-frequency high-rate sputtering device SH-250H-T06, manufactured by Nippon Shinku Kabushiki Kaisha, a glass substrate was placed with a distance of 80 mm from an Fe target and a Co-Ni target and the substrate temperature was held at 200°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 layer on the glass substrate to a thickness of 5.6 nm (56 Å) as a first layer. The shutter was next rotated to form a Co- and Ni-containing oxide layer (the molar ratio of Ni to Co is 0.24) to a thickness of 0.7 nm (7 Å) as a second layer by sputtering the Co-Ni 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 6.3 nm (63 Å). The molar ratio of Co to Fe in one unit was 0.15. These operations were alternately repeated 68 times to obtain a multilayered film composed of 34 Fe 3 O 4 layers each having a thickness of 5.6 nm (56 Å), and 34 Co- and Ni-containing oxide layers 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-Ni side. In the thus-obtained multilayered film, the plane (400) was predominantly oriented in parallel with the substrate. The obtained multilayered film was annealed in the air at a temperature of 400°C for 1 hour so as to oxidize Fe 3 O 4 into γ-Fe 2 O 3 and diffuse Co and Ni, thereby producing a spinel thin film containing Fe as the main ingredient and further containing Co and Ni. 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. Since the residual magnetization and the coercive force of the thus-obtained spinel thin film represented by the magnetization curves of the spinel thin film to which a magnetic field was applied in the perpendicular direction were larger than those represented by the magnetization curves of the spinel thin film to which a magnetic field was applied in the in-plane direction, the spinel thin film was recognized as a perpendicular magnetic film. As to the magnetic characteristics of the spinel thin film, the saturation magnetization was 290 emu/cc (3600 G), the coercive force was 3.5 x 10 5 Am -1 (4.4 kOe), and the squareness was 0.77. The spacing of the plane (400) was 0.2077 nm (2.077 Å) Comparative Example 2 In a high-frequency high-rate sputtering device SH-250H-T06, manufactured by Nippon Shinku Kabushiki Kaisha, a glass substrate was placed with a distance of 80 mm from an Fe target and a Co-Ni target and the substrate temperature was held at 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 layer on the glass substrate to a thickness of 5.6 nm (56 Å) as a first layer. The shutter was next rotated to form a Co- and Ni-containing oxide layer (the molar ratio of Ni to Co is 0.57) to a thickness of 0.7 nm (7 Å) as a second layer by sputtering the Co-Ni 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 6.3 nm (63 Å) The molar ratio of Co to Fe in one unit was 0.15. These operations were alternately repeated 68 times to obtain a multilayered film composed of 34 Fe 3 O 4 layers each having a thickness of 5.6 nm (56 Å) and 34 Co- and Ni-containing oxide layers 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-Ni side. In the thus-obtained multilayered film, the plane (400) was predominantly oriented in parallel with the substrate. The obtained multilayered film was annealed in the air at a temperature of 400°C for 1 hour so as to oxidize Fe 3 O 4 into γ-Fe 2 O 3 and diffuse Co and Ni, thereby producing a spinel thin film containing Fe as the main ingredient and further containing Co and Ni. 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. Since the residual magnetization and the coercive force of the thus-obtained spinel thin film represented by the magnetization curves of the spinel thin film to which a magnetic field was applied in the perpendicular direction were larger than those represented by the magnetization curves of the spinel thin film to which a magnetic field was applied in the in-plane direction, the spinel thin film was recognized as a perpendicular magnetic film. As to the magnetic characteristics of the spinel thin film, the saturation magnetization was 270 emu/cc (3400 G), the coercive force was 2.47 x 10 5 Am -1 (3.1 kOe), and the squareness was 0.70. The spacing of the plane (400) was 0.2077 nm (2.077 Å) Comparative Example 3 A multilayered film was produced in the same way as in Example 1 except that the thickness of the spinel Fe 3 O 4 film was 34.5 nm (345 Å), the thickness of the Co- and Ni-containing oxide layer (the molar ratio of Co to Fe was 0.02, and the molar ratio of Ni to Co was 2.0) was 1.5 nm (15 Å), the thickness of one unit was 36.0 nm (360 Å) and the multilayered film was composed of 5 Fe 3 O 4 layers each having a thickness of 34.5 nm (345 Å), and 5 Co- and Ni-containing oxide layers each having a thickness of 1.5 nm (15 Å). The X-ray diffraction spectrum of the multilayered film obtained is shown in Fig. 1(a). As shown in Fig. 1(a), the planes (311) and (222) were proved to predominantly orient in parallel with the substrate. By using the multilayered 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 planes (311) and (222) were 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 spinel film was not a perpendicular magnetic film. Comparative Example 4 A multilayered 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) the thickness of the Co- and Ni-containing oxide layer (the molar ratio of Co to Fe was 0.69, and the molar ratio of Ni to Co was 1.5) was 1.7 nm (17 Å), the thickness of one unit was 5.3 nm (53 Å), and the multilayered film was composed of 60 Fe 3 O 4 layers each having a thickness of 3.6 nm (36 Å), and 60 Co- and Ni-containing oxide layers each having a thickness of 1.7 nm (17 Å). The X-ray diffraction spectrum of the multilayered film obtained is shown in Fig. 1(b). As shown in Fig. 1(b), the plane (400) was proved to predominantly orient in parallel with the substrate. By using the multilayered 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 spinel film was not a perpendicular magnetic film. Reference Example 1 In a high-frequency high-rate sputtering device SH-250H-T06, manufactured by Nippon Shinku Kabushiki Kaisha, 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 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 layer on the glass substrate to a thickness of 4 nm (40 Å) as a first layer. The shutter was next rotated to form a CoO layer to a thickness of 0.7 nm (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 multilayered film composed of 50 Fe 3 O 4 layers each having a thickness of 40 Å, and 50 CoO layers 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 multilayered film, the plane (400) was predominantly oriented in parallel with the substrate. The obtained multilayered film was annealed in the air at a temperature of 450°C for 1 hour so as to oxidize Fe 3 O 4 into γ-Fe 2 O 3 and diffuse Co, thereby producing a spinel thin film containing Fe as the main ingredient and further containing Co. 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. Since the residual magnetization and the coercive force of the thus-obtained spinel thin film represented by the magnetization curves of the spinel thin film to which a magnetic field was applied in the perpendicular direction were larger than those represented by the magnetization curves of the spinel thin film to which a magnetic field was applied in the in-plane direction, the spinel thin film was recognized as a perpendicular magnetic film. As to the magnetic characteristics of the spinel thin film, the saturation magnetization was 340 emu/cc (4250 G), the coercive force was 4.30 x 10 5 Am -1 (5.4 kOe), and the squareness was 0.75. The spacing of the plane (400) was 0.2082 nm (2.082 Å).

Claims: A magnetic recording medium in which a spinel thin film is provided on a substrate; which film contains Fe as the main ingredient and further contains Co and Ni and which has a coercive force of less than 2.39 x 10 5 Am -1 (3000 Oe), a saturation magnetization of not less than 0.25T (200 emu/cc, 2500 G) and a squareness of not less than 0.20; the plane (400) thereof being predominantly oriented in parallel with the said substrate, the spacing of the plane (400) being not more than 0.2082 nm; the molar ratio of Co:Fe being from 0.005:1 to 0.32:1 and the molar ratio of Ni:Co being not less than 0.6:1. A medium according to claim 1, wherein the film has a thickness of from 50 to 500 nm. A medium according to claim 1 or 2, wherein the said coercive force is from 9.55 x 10 4 to 1.99 x 10 5 Am -1 (1200 to 2500 Oe). A medium according to any one of the preceding claims, wherein the molar ratio of Co:Fe is from 0.01:1 to 0.20:1. A medium according to any one of the preceding claims, wherein the molar ratio of Ni:Co is from 1.0:1 to 5.0:1. A medium in which a Co- and Ni- containing multilayered film is provided on a substrate, the film comprising a multiplicity of units, each unit composed of a magnetite [ FeO x . Fe 2 O 3 (O < X ≤ 1)] layer which optionally contains Co, Ni or both Co and Ni and of an oxide layer which contains Co, Ni or both Co and Ni provided that the magnetite layer and the oxide layer are multilayered so as to contain both Co and Ni as a whole unit, the plane (400) thereof being predominantly oriented in parallel with said substrate, each unit having a thickness of not more than 30 nm, the molar ratio of Co:Fe being from 0.005:1 to 0.32:1, and the molar ratio of Ni:Co being not less than 0.6:1. A medium according to claim 6, wherein the number of units is not less than three. A medium according to claim 6 or 7, wherein each unit has a thickness of from 5 to 25 nm. A process for producing a magnetic recording medium as defined in any one of claims 1 to 5, which process comprises annealing a Co- and Ni-containing multilayered film as defined in any one of claims 6 to 8 at a temperature of 280 to 450°C.

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