MAGNETIC RECORDING MEDIUM

Abstract: A magnetic recording medium comprising a substrate, a ferromagnetic thin film formed on the substrate, and a protective film formed directly on the ferromagnetic thin film, and a process for producing such magnetic recording medium. The expression "formed directly" refers to the technique for forming the protective film on the ferromagne­ tic thin film so that the interface therebetween is free from contamination by, for instance, conducting the protective film formation under the vacuum used when forming the ferromagnetic thin film.

Description: BACKGROUND OF THE INVENTION This invention relates to a magnetic recording medium, more particularly, to a magnetic recording medium having an overcoated layer greatly conducive to the improvement of durability of the medium. In order to meet the growing request for high-density recording in recent years, efforts are being made for the development and practical use of so-called no-binder type magnetic recording medium made of a thin ferromagnetic metal film formed by vacuum evaporation, sputtering, ion-plating or other methods with no use of binder. The improvement of magnetic properties is of course an important requirement for realizing practical use of this type of magnetic recording medium, but the positive prevention of change in quality of the medium due to oxidation and other causes in use and the improvement of durability or the strength against frictional contact with the magnetic head are of equally as important factors for said purpose. Proposals have been made for preventing change in quality of the medium and improving the durability thereof, for example a method in which a non-oxidizable metal such as Rh, Au, Pt, Pd, Cr, Al, Pd-B, Ce-Sn, Ag-Cu or the like is deposited as a protective layer on a thin ferromagnetic metal film (Japanese Patent Laid-Open Nos. 40505/78 and 73932/80). Such method comprising formation of a metallic protective layer on a ferromagnetic thin film, however, was still unsatisfactory for providing the sufficient durability to the recording medium. US-A-4 411 963 discloses a magnetic recording medium having the layer structure set forth in the first part of claim 1, wherein the protective layer is made of nitrides or carbides of Si, Zr, Hf, Ti, Ta, W or Nb. A lubricant layer is formed on top of the protective film. The recording and protective layers are formed in a continuous process under vacuum, so that the interface between these layers is free of a reaction deposit of the elements constituting the recording film and free from contamination of foreign substances. DE-A-3 117 931 discloses a further magnetic recording medium in which a lubricant layer that may consist of or contain boron nitride is formed on a protective layer covering the magnetic recording film or directly on the recording film. The lubricant layer is produced by applying a dry solid lubricant agent powder and rubbing the powder under pressure onto the respective underlying layer. In this process, the surface of the underlying layer will be exposed to air and heavily contaminated. SUMMARY OF THE INVENTION An object of this invention is to provide a magnetic recording medium which is phenomenally improved in its durability owing to the increased strength of adhesion of the protective layer to the ferromagnetic thin film. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view illustrating schematically an example of construction of the apparatus used for carrying out the process for producing a magnetic recording medium in accordance with this invention. FIGs. 2 and 3 are sectional views showing schematically the sectional structures of the samples of magnetic recording medium produced in accordance with this invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to the production of a magnetic recording medium with high durability comprising forming a thin ferromagnetic metal film on a substrate by a vapor deposition method such as vacuum evaporation, sputtering or ion-plating, and further forming directly on said ferromagnetic film a protective layer by vapor deposition. The term "directly" is used here to refer to such a condition that the interface between the ferromagnetic film and the protective layer thereon is free of obvious reaction deposit of the elements constituting the ferromagnetic film (exclusive of the reaction deposit of the elements constituting the protective layer) and contamination by foreign substances, and such "direct" lamination can be achieved by, for instance, forming a protective layer on said ferromagnetic thin film under the vacuum used when forming said ferromagnetic film. The necessity of said direct formation of the protective layer on the ferromagnetic film is attributed to the following fact. When the thin ferromagnetic metal film formed by such method is taken out into the air or it is left in the film forming apparatus for a long time, the surface of the film tends to collect and adsorb oil and water vapors which cause oxidation or contamination of the film surface, and when a protective layer is formed thereon, the adhesion between said ferromagnetic film and protective layer is reduced under the influence of said oxidized layer or adsorbed materials, resulting in a poor durability of the product. (For instance, when Si, and Si alloy and an Si oxide are deposited on a thin ferromagnetic metal film and their microscopic strength of adhesion is compared, it is found that the Si and Si alloy deposits show a far higher strength of adhesion than the Si oxide. This is considered ascribable to the fact that the force of bonding to Fe, Co or Ni, is greatly reduced by the intervention of oxygen, nitrogen and other elements.) When a ferromagnetic metal film is deposited with an overcoating (protective) layer immediately after formation of said film, the overcoated layer material can be deposited on said film while its surface is still in a clean and active state, so that the strength of adhesion between them is maximized and naturally the durability of the product is improved. In this case, no oxidized layer exists at the interface between the ferromagnetic film and the overcoated layer. In case the substrate is heated when forming the ferromagnetic metal film, it is desirable to keep the substrate heated until the formation of the overcoat layer is completed, because such continuous heating prevents adsorption of residual gases in the film forming chamber and makes it possible to maintain the film surface active. The substrate temperature is preferably maintained in the range of 50-400°C which is suited for maximizing the adhesion strength without causing deterioration of magnetic properties of the ferromagnetic metal film. In case the film surface was contaminated, it is recommended to clean the film surface by an appropriate method such as ionic sputtering before forming the overcoating layer. The protective layer (overcoated layer) thickness is selected to meet the requirements that the desired protective effect is provided and that the magnetic record reading output is not reduced by spacing loss due to the gap between the magnetic recording medium surface and the magnetic head, and in view of this, the preferred protective layer thickness is in the range of 0.003-0.3 µm, more preferably 0.01-0.15 µm. As materials for said protective layer, those mentioned in claim 1 are used for the following reasons. The protective layer is required to fulfill the following requirements: it is resistant to corrosion; it can protect the magnetic layer as desired; it has enough lubricity against frictional contact with the magnetic head; and it allows the minimized film thickness (100 nm or less) for arresting the reduction of electro-magnetic conversion efficiency. The surface of a ferromagnetic thin metal film formed on a non-magnetic substrate usually has microscopic undulations. The protective layer is required to adhere firmly on such microscopic undulations, and it is desirable that such microscopic undulations be flattened by forming the protective layer thereon. The size of microscopic undulations is maximized in a perpendicular magnetic film composed of perpendicularly oriented columnar crystallites. For instance, in the case of perpendicular magnetic films made from Co-based alloy films, there exist on their surfaces the undulations with a period of 20-50 nm and a depth of 5-20 nm. In the area of the surface where the interface of microscopic crystallites is exposed, there even exist the depressions with a size of atomic order (1 nm or less). The protective layer formed on a ferromagnetic metal film having such microscopic undulations is required to adhere firmly on such undulate film surface with the protective layer material getting down to every nook and corner of the undulations. The experiments conducted by the present inventors showed that the use of B and B compounds as the protective layer material produces an especially excellent effect. This owes to the facts that the B atoms are significantly small in size in comparison with the atoms such as Fe, Ni and Co, constituting the ferromagnetic metal film and can easily make their way even into the microscopic depressions on the film surface, and that B have a good affinity for the metal atoms and can firmly adhere to a magnetic film. Further, the B type materials are dense and hard and can be worked into a smooth surface so that, in use for a magnetic recording medium, they prove especially excellent in wear resistance and lubricity for the magnetic head. Also, it is easy with these materials to form an amorphous-like layer on the surface of a magnetic film by sputtering or other means, and when a protective layer with a thickness on the order of 20-50 nm is formed with these materials on a magnetic film, such a protective layer shows a tendency to flatten the microscopic undulations on the magnetic film surface. Moreover, B has excellent corrosion resistance, and when a lubricant composed of an organic material is used for the purpose of further improving the durability, B shows better affinity for the organic material than other metals used for the protective layer. B compounds usable in this invention include B₄Si, B₆Si and B₄Si x C 1-x . Mixtures of these compounds are also usable. In this invention, use of amorphous material for the protective layer is desirable as such a material is found better suited for forming a homogeneous film, but crystalline material may be contained. The present invention also provides a magnetic recording medium having a protective film comprising two laminated filmy layers having different functions. The roles of the protective film provided on a ferromagnetic film are (1) to prevent the change in quality of the ferromagnetic film due to oxidation, etc., of the film, (2) to prevent or minimize the wear of the latter film when it moves sliding along the magnetic head, and (3) to minimize the friction between the film and the magnetic head in sliding movement. According to the present invention, a protective film that can well perform said roles is provided characteristically by laminating two filmy layers differing in the roles they play. The first layer is composed of a material which is effective for preventing the change in quality of the ferromagnetic film and has excellent wear resistance and the second layer is composed of a material which produces little friction on sliding contact with the magnetic head. Materials used for the first layer are given in claim 5. These materials are usually hard and highly resistant to wear. They are also capable of forming a dense film free of pinholes and further provide a high strength of adhesion to the ferromagnetic film. As the material for the second layer, Sn, Pb, In, Se, Te, Cu, Ag, Au or alloys containing one of these elements as principal constituent are used. These materials are usually soft and can play the role of a lubricant as the film made of such material intervenes between the first layer made of a hard material and the magnetic head also made of a hard material. The thickness of the protective film of said structure should be properly selected from the range of 0.003-0.5 µm, preferably 0.01-0.2 µm, by taking into considerations the essential conditions that the selected thickness should be able to provide a sufficient protective effect and to inhibit the reduction of the magnetic record reading output caused by spacing loss due to the gap between the recording layer surface of the magnetic recording medium and the magnetic head. It is required that the sum of the thickness of the first layer and the thickness of the second layer falls within said range. In the case of the two-layer structured protective film, it is desirable that the first layer is 1.5 to 5 times greater in thickness than the second layer. The ferromagnetic metal film in accordance with this invention is preferably formed from a Co-based alloy such as Co-Cr, Co-V, Co-Mo, Co-W, Co-Re, Co-O, Co-Cr-Rh, Co-Cr-Ru, Co-Ni-O, Co-Ni, Co-P, Co-B, Co-Si, Co-Y, Co-La, Co-Pr, Co-Ce, Co-Sm, Co-Mn, Co-Ni-P, Co-Ni-B, Co-Ni-Ag, Co-Ni-Nd, Co-Ni-Ce, Co-Ni-Zn, Co-Ni-Cu, Co-Ni-Hg, Co-Ni-W, Co-Ni-Re, Co-Mn-P, Co-Zn-P and the like or an Fe-based alloy such as Fe-Co, Fe-Ni, Fe-Si, Fe-Rh, Fe-V and the like by making a film thereof by a vapor deposition method such as evaporation, sputtering or ion plating. The thickness of the ferromagnetic metal film should be in the range of 0.03-5 µm, but a thickness in the range of 0.05-1 µm is preferred for realizing high-density magnetic recording. As the substrate material for the ferromagnetic metal film, there can be used high polymeric materials such as polyimide, polycarbonate, polyvinylidene chloride, polyethylene terephthalate, polyethylene naphthalate, cellulose acetate, polyamide and the like, metallic materials such as aluminum, stainless steel, brass and the like, as well as glass and ceramic. A filmy layer made of other material may be provided between the substrate made of a material such as mentioned above and the ferromagnetic metal film. For example, an intermediate thin layer of Cr may be provided for increasing the strength of adhesion between the substrate and the ferromagnetic metal film, or a thin layer of a soft magnetic material may be provided for improving the magnetic properties of the ferromagnetic metal film. The substrate may take the form of a disc, tape or drum. The present invention is further described below by way of the Examples thereof. Example 1 The metallising apparatus shown in FIG. 1 includes an infrared heater 2 and deposition metal sources 3, 4 and 5. By using a polyimide film as substrate 1, a magnetic recording medium of the structure shown in FIG. 2 was produced by sputtering. After evacuating the sample chamber of the sputtering apparatus to 5 x 10⁻⁵ Pa, Ar gas of 0.3 Pa was introduced into the chamber and a Co-20wt%Cr alloy was sputter deposited to form layer 14 with a thickness of 0.3 µm under the conditions of: high frequency output = 4 W/cm²; substrate temperature = 100°C. Then the sputter target was changed to B while maintaining the vacuum of the sample chamber and a 20 nm thick protective layer 15 was formed under the same conditions as described above. In the same way as described above but by using B₄Si and B₆Si targets in place of B target, there were produced samples of magnetic recording medium having the protective layers made of said materials. Also produced as comparative samples were samples having the similar structure with no protective layer or with a protective layer of Cr. A disc test piece was cut out from each of said samples and subjected to a durability test in the following way: each sample disc was set on a disc turning device and turned continuously at a speed of 1.5 m/s in sliding contact with the magnetic head under a load of 10 g, and the number of turns which the samples could endure until they became flawed was counted. A corrosion resistance test was also conducted by placing the sample in an environment of 60°C and 90% RH for one month and then examining the sample surface by an optical microscope. The results are shown comprehensively in Table 1. In the corrosion resistance test results, ○ indicates that no trace of corrosion was seen in the ferromagnetic metal film, ▵ indicates that a discoloration was seen under observation with the optical microscope, and X indicates that a discoloration was clearly seen with the naked eye. It was thus found that the magnetic recording medium having a protective film of B or a B-based compound has excellent durability and corrosion resistance. Examination by a scanning electron microscope of the surfaces of the samples having a 20 nm thick protective film of B or a B-based material and the samples having no protective film showed that the surfaces of the samples having a protective film were less in undulations and more smooth than the surfaces of the samples having no protective film. It was also noted that the durability enhanced proportionally to the thickness of the protective film, but the electromagnetic conversion efficiency deteriorated as the distance between the magnetic recording medium and the head increased. The range of protective film thickness which caused no deterioration of electromagnetic conversion efficiency and could maintain high durability was 10-80 nm, more preferably 15-30 nm. In this Example, a protective film of B or a B-type material was provided successively to the formation of the ferromagnetic metal film by using the same apparatus, and it was experimentally ascertained that, by so doing, the adhesive strength of the protective film is increased. When Co-based alloys other than Co-Cr, Fe-based alloys and Ni-based alloys were used as magnetic film material, durability and corrosion resistance were also improved by providing a protective film made of any of B, B₄Si and B₆Si. The similar effect was also seen when using B₄Si x C 1-x , mixture of B₄C and B₄Si. The foregoing discussion pertains to the case where both magnetic film and protective film were formed by sputtering, but the similar effect was observed when they were formed by vacuum evaporation. It was also found that both durability and corrosion resistance improve when using Al, glass and the like as the non-magnetic substrate. Example 2 A magnetic recording medium shown in FIG.3 was produced by providing a ferromagnetic film 11 consisting of a 0.5 µm thick permalloy film and an overlying 0.15 µm thick Co-20.5wt%Cr film on a disc-shaped oxidized aluminum substrate 1 of 100 mm in diameter and 2 mm in thickness, and a protective film was formed on said magnetic recording medium in the following procedure: said recording medium was placed in a continuous sputtering apparatus, and a 20 nm thick layer of B was formed as the first layer 16 of the protective film under the conditions of 4 W/cm² high frequency output and 120°C substrate temperature and then a 10 nm thick layer of Sn was formed thereon as the second layer 17 under the same conditions. Other samples having the similar structure were produced under the same conditions by using B₄Si and B-3wt%C in place of B as the first layer. As comparative samples, there were prepared samples having no protective film and the samples having a 10 nm thick Sn layer alone as protective film on the magnetic film. These samples were subjected to the same durability test as conducted in Example 1. The test results are shown in Table 2.

Claims: A magnetic recording medium comprising a substrate, a ferromagnetic thin film formed on the substrate, and a protective film formed on the ferromagnetic thin film, an interface of said ferromagnetic thin film and said protective film being free of a reaction deposit of elements constituting said ferromagnetic thin film, exclusive of a reaction deposit of elements constituting said protective film, and free from contamination by foreign substances, said ferromagnetic thin film being made of a material selected from the group consisting of Co-based alloys, Fe-based alloys and Ni-based alloys, characterized in that said protective film is made of at least one of the materials B, B₄Si, B₆Si and B₄Si x C 1-x . A magnetic recording medium according to claim 1, characterized by a lubricant layer made of an organic material on said protective film. A magnetic recording medium according to claim 1 or 2, characterized in that said protective film is made of an amorphous material. A magnetic recording medium according to any of claims 1 to 3, characterized in that said protective film has a thickness in the range of 0,003 to 0,3 µm, preferably in the range of 0,01 to 0,15 µm. A magnetic recording medium comprising a substrate, a ferromagnetic thin film formed on the substrate, and a protective film formed on the ferromagnetic thin film, an interface of said ferromagnetic thin film and said protective film being free of a reaction deposit of elements constituting said ferromagnetic thin film, exclusive of a reaction deposit of elements constituting said protective film, and free from contamination by foreign substances, said ferromagnetic thin film being made of a material selected from the group consisting of Co-based alloys, Fe-based alloys and Ni-based alloys, characterized in that said protective film comprises a first and a second layer, said first layer being made of at least one of the materials B, B₄Si, and B containing 3 wt.-% of C, and said second layer being made of Sn, Pb, In, Se, Te, Cu, Ag, Au, a Sn-based alloy, a Pb-based alloy, an In-based alloy, a Se-based alloy, a Te-based alloy, a Cu-based alloy, an Ag-based alloy, or an Au-based alloy. A magnetic recording medium according to claim 5, characterized in that said protective film has a thickness in the range of 0,003 to 0,5 µm, preferably in the range of 0,01 to 0,2 µm. A magnetic recording medium according to claim 5 or 6, wherein said first layer has a thickness of 1,5 to 5 times greater than a thickness of the second layer. The magnetic recording medium of any of claims 1 to 7, wherein said ferromagnetic thin film and said protective film are formed continuously without exposing said ferromagnetic thin film and said protective film to air and in maintaining said ferromagnetic thin film and said protective film in a vacuum. A magnetic recording medium according to any of claims 1 to 8, characterized in that said ferromagnetic thin film is made of a Co-Cr-alloy.

Inventors: ASADA, SEIICHI, FUTAMOTO, MASAAKI, HONDA, YUKIO, ISHIHARA, HEIGO, NISHIMURA, TAKASHI HITACHI DAINI KYOSHINRYO, YOSHIDA, KAZUETSU
Applicants: HITACHI MAXELL LTD., HITACHI, LTD.
Application year: 1985
Main classification: G11B5
IPC codes: G11B5