Magnetic recording medium

Abstract: There is disclosed a magnetic recording medium comprising a non-magnetic substrate, a soft magnetic layer formed on said substrate with or without a non-­ magnetic sublayer interposed therebetween, and a perpen­ dicular magneto-anisotropic recording layer laminated on said soft magnetic layer with or without a non-­ magnetic intermediate layer therebetween; said soft magnetic layer composed of the materials whose satura­ tion magnetization is higher than at least that of said recording layer, and whose thickness is in the range of 50 to 1,000 Å.

Description: BACKGROUND OF THE INVENTION Field of Industrial Utilization This invention relates to a magnetic recording medium. More particularly, it relates to a magnetic recording medium which shows a high output in a broad range of recording density. Discussion of Related Art The increasing demand for high-density recording has prompted the researchers to make earnest studies on various new recording systems. Especially in the field of magnetic recording, the vertical magnetic recording system is deemed as one of the most promising high-density recording methods and enthusiastic studies are being made on this system. As the material of recording media used for such vertical magnetic recording, there have been proposed Co-Cr alloy film, which is most highly prized, Co-O film, Fe-M-O films (M representing Sn, Ge, etc.), and the films in which Fe or other elements have been electrodeposited in fine pores of anodized aluminum, and the results of many tests and experiments have verified their excellent high-density recording performance. The vertical recording system using such magnetic media is indeed excellent in high-density recording characteristics as mentioned above, but on the other hand this system has the problem that it is low in low-density recording output as compared with the conventional longitudinal recording system. This is attributable to the fact that a strong demagnetizing field is generated by the free magnetic pole formed on the rear side of the film when the vertical recording medium is magnetized, and such strong demagnetizing field acts to attenuate recording megnetization. Therefore, it should be an effective measure against said problem to interpose a soft magnetic layer between the vertical magnetized recording layer and the substrate so as to reduce the activity of free magnetic pole at the interface adjoining the soft magnetic layer. This concept has already been disclosed in USP No. 4,210,946, and further studies are being made on this double-layer film recording medium. This type of recording medium, however, had some difficult problems. Firstly, when a vertical magnetically anisotropic recording layer such as a Co-Cr film is formed on the soft magnetic layer such as a permalloy layer, the crystal orientation of the Co-Cr film would be disturbed. Secondarily, as disclosed by Uesaka et al in Journal of Applied Physics, 57 , 3925 (1985), spike noise is produced in the soft magnetic layer to make it unable to obtain a good S/N ratio. Thirdly, in case the double-layer film recording medium is used in combination with a ring head, the head magnetic field in the vertical direction becomes broad due to the presence of the soft magnetic layer, resulting in the deteriorated high-density recording performance. Summary of the Invention The present invention has for its object to solve the problem of low reproducing output of the conventional vertical recording media and to provide a perpendicular magnetic recording medium which shows a high reproducing output in the broad range of recording density. JP-A-57-117122 discloses a magnetic recording medium in accordance with the prior art portion of claim 1 and which has the disadvantage of low reproducing output referred to above. The present inventors found that said object could be attained by providing a magnetic recording medium as defined in claim 1. In the specification, the term "in-plane magnetisation film" is defined as the magnetic film whose in-plane squareness ratio is larger than that of perpendicular direction. Brief Description of the Drawings Figures 1 and 2 are partial schematic illustrations showing the sectional structures of the embodiments of magnetic recording medium according to this invention. Figure 3 is a schematic drawing for illustrating the state of magnetisation in a magnetic recording medium according to this invention. Figure 4 is a schematic drawing for illustrating the state of magnetisation in a conventional double-layer film magnetic recording medium. Figure 5 is a characteristic diagram showing the Ni-Fe soft magnetic layer thickness dependency of coercive force Hc⊥ in the vertical direction of Co-Cr film and ΔΘ₅₀. Figure 6 is a partial schematic sectional view of a magnetic recording medium in an embodiment of this invention where a protective film is provided. Detailed Description of the Preferred Embodiments In the present invention an Ni-Fe (permalloy) film is used as the material showing soft magnetic characteristics for forming the soft magnetic layer. The thickness of the soft magnetic layer is preferably in the range of 50 to 500 Å. A non-magnetic intermediate layer with a thickness not exceeding 300 Å may be interposed between said soft magnetic layer and vertical magneto-anisotropic recording layer. Such non-magnetic intermediate layer may be composed of a single element selected from B, C, Ge, Mo, Si, Os, Ru, Re, Ta, Ti and W or an alloy, oxide or nitride of these elements. Also, a non-magnetic sublayer is provided between the non-magnetic substrate and said soft magnetic layer. The thickness of said non-magnetic sublayer is variable over a wide range according to the type of the non-magnetic substrate and the purpose of use of the recording medium. For instance, in the case of hard discs, the non-magnetic sublayer thickness is less than 10 µm, but generally it is less than 5000 Å. The magnetic recording medium according to this invention may have the structures such as shown in Figures 1 and 2. Figure 1 illustrates a magnetic recording medium having a non-magnetic sublayer. In this recording medium, a non-magnetic sublayer 2 is laminated on a non-magnetic substrate 1, and a soft magnetic layer 3 and a perpendicular magneto-anisotropic recording layer 5 are laminated successively on said non-magnetic sublayer 2. Shown in Figure 2 is the structure of a magnetic recording medium having both a non-magnetic sublayer and a non-magnetic intermediate layer. In this recording medium, a non-magnetic sublayer 2 is laminated on a non-magnetic substrate 1, and a soft magnetic layer 3, a non-magnetic intermediate layer 4 and a vertical magneto-anistropic recording layer are further laminated in that order on said non-magnetic sublayer 2. The reasons why the magnetic recording medium of this invention can yield a high reproducing output will be explained below with reference to FIG. 3 although the explanation is based on conjecture since the accurate mechanism is not yet completely elucidated. When the vertical magneto-anisotropic recording layer 5 is magnetized as shown in the drawing, a strong stray field is generated especially in the magnetization transition region. This accordingly regulates magnetization of the soft magnetic layer 3 in the magnetization transition region, inducing a closing tendency of the magnetic fluxes in said region notwithstanding the small thickness of the soft magnetic layer. Thus, although residual magnetization in the transition region increases sharply with decrease of the demagnetizing field, residual magnetization is reduced in the area around the bit center since the free magnetic pole is not so much lost in such area. In view of the fact that reproducing output is governed by the amount of residual magnetization in the magnetization transition region, it can readily be imagined that high residual magnetization, namely high reproducing output would be obtained in this recording medium even if the soft magnetic layer is small in thickness. The state of magnetization in a conventional double-layer film recording medium is described below with reference to FIG. 4. When a soft magnetic sublayer 6 is interposed between the non-magnetic substrate 1 and the perpendicular magneto-anisotropic recording layer 5, the magnetic fluxes are closed as indicated by arrows in FIG. 4 and the free magnetic pole generated on the rear side of the recording layer completely disappears due to the presence of said sublayer 6, so that residual magnetization in the bits is almost uniformly recovered as a whole and there occurs no residual magnetization enhancing action in the specific region alone. In this case, ideally speaking, the soft magnetic sublayer 6 is required to have a sufficient flux density and thickness to allow passage of the magnetic fluxes leaking from the vertical recording layer 5. Therefore, it can be simply reasoned that the higher the saturated magnetic flux density of the soft magnetic material used and the greater the thickness of its layer, the more desirable for attaining said object, that is, for obtaining higher reproducing output. Based partly on such concept, the thickness of the soft magnetic sublayer in the conventional double-layer film recording media has been set to a rather high level. However, the experiments conducted by the present inventors disclosed that when the thickness of the soft magnetic sublayer is increased, the perpendicular magneto-anisotropic recording layer formed thereon is deteriorated in its properties and also the noise from the soft magnetic sublayer is increased, while the reproducing output reaches a saturation when the thickness of the soft magnetic layer becomes greater than a certain fixed value. It was thus confirmed that the thickness of the soft magnetic sublayer should not be made greater than a certain level from the viewpoint of reproducing output of the recording medium; any increase of the layer thickness above said level only leads to a deterioration of the perpendicular magnetization characteristics of the recording layer, resulting in a reduced high-density recording performance, and an increase of noise level, and no noticeable merit is obtained. This fact is reflected in the specification of the soft magnetic layer in the magnetic recording medium of this invention, that is, an excellent high-density recording medium with a high output level can be obtained when saturation magnetization of said soft magnetic layer is higher than that of at least the perpendicular magneto-anisotropic recording layer and the thickness of said soft magnetic layer is in the range of 50 to 1,000 Å. Saturation magnetization of the soft magnetic layer needs to be only slightly higher than that of the perpendicular recording layer; any greater difference of saturation magnetization between the two layers provides no significant difference in reproducing output improving effect. When the thickness of the soft magnetic layer is less than 50 Å, reproducing output is reduced. This is probably because a continuous film can not be formed with such a thickness and the film has a so-called island structure. When the layer thickness exceeds 1,000 Å, the noise level increases although the output is saturated, and when a polycrystal film such as a Co-Cr perpendicular magneto-anisotropic film is formed on said layer, the perpendicular orientability of the principal axis of crystal is greatly disturbed. This is probably for the reason that when the thickness becomes greater than 1,000 Å, the soft magnetic layer comes to have its own crystallinity so strongly as to disturb the perpendicular orientability of the perpendicular magneto-anisotropic polycrystal oriented film. The preferred range of thickness of the soft magnetic layer is from 50 to 500 Å. It is not definitely known why a reduction of noise can be attained by drastically decreasing the thickness of the soft magnetic layer as compared with the conventional layer thickness of several thousand Å, but it is considered that this has a close relation with the state of the magnetic domain wall as evidenced from the following fact: in the films that created a large noise, there was observed the presence of Bloch wall almost unexceptionally, whereas in the soft magnetic films having a thickness of 50 to 500 Å (the preferred range of film thickness in the present invention), there were observed the other magnetic domain walls than Bloch wall, namely Neel wall and wedge-type wall. When a non-magnetic intermediate layer is interposed between the soft magnetic layer and the perpendicular magneto-anisotropic recording layer, the crystal orientability of said perpendicular recording layer is improved. This effect is intensified as the thickness of the soft magnetic layer is reduced. However, when the thickness of the soft magnetic layer exceeds 500 Å, it becomes difficult to restore the orientability of the perpendicular recording layer to a preferred level unless the thickness of the non-magnetic intermediate layer is increased to a substantial extent. Investigations of the influence of such non-magnetic intermediate layer on reproducing output level showed that no large drop of output occurs if the thickness of said intermediate layer is less than 300 Å. As for the material of said non-magnetic intermediate layer, the best material to be used is decided according to the material of the soft magnetic layer formed beneath said intermediate layer and the material of the perpendicular magneto-anisotropic recording layer formed on said intermediate layer, but the use of a single element selected from B, C, Ge, Mo, Si, Os, Ru, Re, Ta, Ti and W or an alloy, oxide or nitride of any combination of said elements could produce an excellent orientation controlling layer. The magnetic recording medium of this invention has the problem that the soft magnetic layer, because of its small thickness, tends to become saturated with the magnetic flux generated from the magnetic head, therefore the recording sensitivity is slightly lowered as compared with the conventional double-layer film recording medium, but such problem is a matter of no significance for practical use. The magnetic recording medium of this invention is not suited for a magnetic head of the type that sharpens the recording magnetic field like an auxiliary magnetic pole exciting type thin-film head, but in application to other types of magnetic head, especially bulk type head, bulk type ring head or one-side access type thin-film head, owing to the nature of the soft magnetic layer that it tends to saturate, there is caused no disturbance of head magnetic field due to the presence of soft magnetic layer and it is possible to realize excellent perpendicular magnetic recording even to a high-sensitivity recording region like a perpendicular magnetic recording single-layer medium. The Co-Cr perpendicular magneto-anisotropic layer provided on the soft magnetic layer is preferably one in which the Cr content is 15 to 25 at% and whose thickness is in the range of 500 to 3000 Å. The reason for defining the Cr content in the range of 15 to 25 at% is that when it is less than 15 at%, the crystallographic orientation of the Co-Cr film is deteriorated to suppress coercive force in the perpendicular direction, and when the Cr content exceeds 25 at%, the saturation magnetisation of the film is reduced to induce a drop of reproducing output. Also, when the thickness of the Co-Cr vertical magneto-anisotropic layer is less than 500 Å, the reproducing output of the recording medium is reduced, and when it exceeds 3000 Å, it becomes impossible to effect recording to the bottom of the recording layer due to the limit of capacity of the magnetic head and the overwrite characteristics are deteriorated. In the magnetic recording medium of this invention, a non-magnetic sublayer is interposed between the non-magnetic substrate and the soft magnetic layer. An effect of interposition of such non-magnetic sublayer is to shut off the gas produced from the substrate surface during formation of the soft magnetic layer to improve its magnetic properties. Also, in case of using permalloy as the material of the soft magnetic layer and Co-Cr as the material of the vertical magneto-anisotropic recording layer, when a Ti layer having an hcp structure is provided as a base of permalloy, the (111) plane of permalloy grows on the c plane of the Ti base, which facilitates the growth of c plane of Co-Cr on said (111) plane to improve c-axis orientability of the Co-Cr layer. The non-magnetic sublayer, soft magnetic layer, non-magnetic intermediate layer and perpendicular magneto-anisotropic recording layer in the magnetic recording medium of this invention can all be formed by a commonly used deposition method such as vapor deposition. Of course, other film forming methods are also usable. Vapor deposition referred to herein is a technique for depositing a material or a compound thereof in the form of vapor or ionized vapor on a substrate in a gaseous atmosphere or a vacuum. This technique includes the following methods: vacuum deposition, sputtering, ion plating, ion beam deposition, cluster ion beam deposition, CVD, and plasma polymerization. As the non-magnetic substrate used in the magnetic recording medium of this invention, there can be employed a film of a high-molecular material such as polyimide, polyethylene terephthalate, etc., a plate of glass, ceramic or a metal such as aluminum, anodized aluminum, brass, etc., a Si single crystal plate, a Si single crystal plate having its surface subjected to a thermal oxidation treatment, and the like. If necessary, this non-magnetic substrate may be provided with a grinding sublayer such as a nickel-phosphorus alloy layer or an anodized aluminum treated layer for conducting planer grinding or texturing. The magnetic recording media provided according to this invention include magnetic tapes or magnetic discs having a substrate of a synthetic resin film such as polyester film, polyimide film, etc., magnetic discs or magnetic drums using as substrate a disc or drum made of a synthetic resin film, aluminum plate, glass plate, etc., and various other forms of magnetic medium having a structure designed to move sliding on a magnetic head. Examples The magnetic recording medium of this invention will be described more particularly below by showing the embodiments thereof. Example 1 On a 50 µm thick polyimide film substrate were vacuum deposited Ti, Ni₉₁-Fe₁₉ and Co₇₇-Cr₂₃ successively to the thicknesses of 300 Å, 0-2,000 Å and 2,100 Å, respectively. The film forming operation was conducted under a vacuum of 2 x 10⁻⁵ to 4 x 10⁻⁶ Torr (2.66 x 10⁻³ to 5.33 x 10⁻⁴ Pa) by adjusting the substrate temperature to 250°C. Saturation magnetization of the soft magnetic layer was 830 G and that of the perpendicular recording layer was 360 G. The magnetic properties of the Co-Cr film of each of the thus prepared specimens were determined from the Kerr effect, and the c-axis orientability of the hcp structure of the Co-Cr film was determined from the half-value span ΔΘ₅₀ of the rocking curve of the (002) plane. FIG. 5 shows the Ni-Fe film thickness dependency of coercive force Hc⊥ in the perpendicular direction of the Co-Cr film and ΔΘ₅₀. It is seen that ΔΘ₅₀ increases while Hc⊥ lowers when the thickness of the Ni-Fe film reaches about 1,000 Å. A 3.5 inch disc was punched out from each of said specimens and subjected to an evaluation of recording and reproduction by using an amorphous ferrite composite type ring head with a gap length of 0.19 µm. The results of evaluation of output S at a recording density of 10 kfci, noise level and recording density D₅₀ at which the output is reduced by half were shown in Table 1 below. Both output (S) and noise level (N) were given as 0 dB when the Ni-Fe film thickness was 0 Å. Ni-Fe film thickness (Å) S (dB) N (dB) D₅₀ (kFCI) 0 0 0 165 50 +3.8 +0.08 171 150 +3.9 +0.08 167 300 +3.9 +0.12 170 500 +4.0 +0.15 160 700 +4.0 +0.6 138 1000 +3.8 +3.5 95 2000 +3.5 +6.0 60 From the results shown in Table 1, it is understood that a high output and high D₅₀ can be realised when the Ni-Fe film thickness is in the range of 50 to 500 Å, but the noise level elevates when the film thickness exceeds 500 Å. Example 2 On a 50 µm thick polyimide film substrate were vacuum deposited Ti, Ni₉₁-Fe₁₉, Ge and Co₇₉-Cr₂₁ successively to the thicknesses of 300 Å, 100 Å, 0-1000 Å and 1600 Å, respectively, at a substrate temperature of 250°C under a vacuum of 2 x 10⁻⁵ to 4 x 10⁻⁶ Torr (2.66 x 10⁻³ to 5.33 x 10⁻⁴ Pa). Saturation magnetisation of the soft magnetic layer was 830 G and that of the perpendicular recording layer was 490 G. A 3.5" disc was punched out from each of the thus prepared specimens and subjected to an evaluation test of recording and reproduction by using an amorphous ferrite composite type ring head (gap length: 0.21 µm). The results are shown in Table 2 below. In the table, the output level was given as 0 dB when the Ge film thickness was 0. Ge film thickness (Å) S (dB) D₅₀ (kFCI) 0 0 145 50 +0.8 168 100 +1.1 166 300 +1.0 165 500 +0.9 170 1000 -2.5 163 As seen from the results shown in Table 2, the output is greatly lowered when the thickness of the Ge film constituting the non-magnetic intermediate layer exceeds 300 Å. As described above, in the magnetic recording medium of this invention, saturation magnetization of the soft magnetic layer is higher than that of the magneto-anisotropic perpendicular recording layer, and by selecting the thickness of the soft magnetic layer from within the range of 50 to 1,000 Å, it is possible to phenomenally increase the reproducing output without impairing the high-density recording performance. Further, in the present invention, a protective layer composed of B or a B-based compound is provided on the magnetic recording surface, and on said protective layer is provided a lubricant layer composed of a perfluoropolyether having at its molecular chain end a 5-member, 6-member or 7-member heterocyclic ring having at least one N and also containing therein at least one double bond. Such protective and lubricant layers can greatly improve the durability and corrosion resistance of the film type magnetic recording media. Said protective layer can be formed by depositing B or a B-based compound by an ordinary vapor deposition method such as vacuum deposition or sputtering. The thus formed thin film is compact and has good continuity even though it is extremely small in thickness, and such film has no "island structure" which is seen in the films made of other materials. The presence of such compact and continuous protective film on the surface of the magnetic recording layer prevents the ambient gas from entering the recording layer to significantly improve corrosion resistance of the recording medium. B and B-based compounds have a nature that they can be easily formed into a thin film with high compactness and good continuity, so that they are suited for forming an excellent anticorrosive protective film. The film made of such material, beside its anti-corrosive effect, has an additional advantage that it is highly proof against damage by external forces owing to its high hardness. This property plays a very important role for protection of the magnetic layer, but at the same time it derives the problem that the film, because of its high hardness, tends to damage the magnetic head on which the film slides. In order to avoid such damage of the magnetic head, it needs to provide a lubricant layer on the surface of the protective layer of the recording medium to preclude direct contact of the recording medium with the magnetic head. The present inventors have made researches for a lubricant having best compatiblity with the protective layer made of B or a B-based compound and, as a result, found that the damage of the magnetic head can be minimized when using as lubricant a perfluoropolyether having its molecular chain end terminated with a 5-member, 6-member or 7-member heterocyclic ring having at least one N and also containing at least one double bond. This fact is considered attributable to the strong adsorption of said terminal functional group of the lubricant on the surface of the protective layer made of B or a B-based compound to insure long-time stable lubrication of the medium surface. FIG. 6 shows the basic structure of the magnetic recording medium according to this invention. B or its carbide or nitride is preferably used as the material of the protective layer 2. The thickness of this layer is preferably in the range of 30 to 1,000 Å for the reason that a thickness less than 30 Å can not provide the desired dynamic strength to the protective layer while a thickness exceeding 1,000 Å may badly deteriorate the recording and reproducing characteristics of the recording medium due to a too much spacing loss. It is also desirable that the thickness of the lubricant layer 1 is not greater than 1,000 Å. When its thickness exceeds 1,000 Å, the lubricant layer tends to become sticky to the magnetic head to hinder the sliding movement of the recording medium on the magnetic head. In the drawing, numeral 3 denotes a magnetic recording layer, 4 a substrate, and 5 a soft magnetic layer.

Claims: A magnetic recording medium comprising a non-magnetic substrate, a soft magnetic layer of Ni-Fe (permalloy) formed on said substrate therebetween, and a Co-Cr perpendicular magneto-anisotropic recording layer laminated on said soft magnetic layer with or without a non-magnetic intermediate layer therebetween, characterised in that a non-magnetic sublayer comprising a Ti-layer having a hcp structure is interposed between the substrate and the soft magnetic layer, said soft magnetic layer has a saturation magnetisation higher than that of said recording layer, and the thickness of said soft magnetic layer is in the range of 50 to 1000 Å. A magnetic recording medium according to claim 1 or 2, wherein the thickness of said soft magnetic layer is in the range of 50 to 500 Å. A magnetic recording medium according to any preceding claim, wherein said perpendicular magneto-anisotropic recording layer is composed of a Co-Cr alloy with a Cr content of 15 to 25 at%, and the thickness of said layer is in the range of 500 to 3000 Å. A magnetic recording medium according to any preceding claim, wherein the thickness of the non-magnetic intermediate layer is 300 Å or less. A magnetic recording medium according to any preceding claim, wherein said non-magnetic intermediate layer is composed of a single element selected from B, C, Ge, Mo, Si, Os, Ru, Re, Ta, Ti and W or an alloy, oxide or nitride of said elements.

Inventors: FUJIWARA HIDEO, KITAKAMI OSAMU, OGAWA YOICHI, FUJIWARA, HIDEO, KITAKAMI, OSAMU, OGAWA, YOICHI
Applicants: HITACHI MAXELL, HITACHI MAXELL LTD.
Application year: 1988
Main classification: G11B5
IPC codes: G11B5