PERPENDICULAR MAGNETIC RECORDING MEDIUM AND METHOD OF MAKING SAME

Abstract: A magnetic recording medium suitable for use in a single-pole type recording system utilizing a perpendicular magnetization mode, the medium including a nonmagnetic base, a low coercive force layer on the base, and a magnetic recording layer overlying the low coercive force layer. The magnetic recording layer has an easy axis of magnetization perpendicular to the nonmagnetic base. The nonmagnetic base is composed of a material which is stable at a temperature of 220°C and higher. In accordance with the invention, the low coercive force layer is an iron-nickel alloy containing about 81 to 89.5 by weight nickel, the low coercive force layer resulting from vapor phase deposition of the alloy on the nonmagnetic base while the temperature of the base is at least 220°C.

Description: SPECIFICATION TITLE "PERPENDICULAR MAGNETIC RECORDING MEDIUM AND METHOD OF MAKING SAME" BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a perpendicular magnetic recording medium and a method of preparing the same wherein a low coercive force layer is deposited on a nonmagnetic base at elevated temperatures. Description of the Prior Art The most common type of magnetic recording medium used in computer memories or as a recording medium for video or audio signals makes use of horizontal recording or in-plane recording on a magnetic recording layer coated or otherwise deposited on a nonmagnetic substrate. This type of recording has the disadvantage that as the wavelength of the recording signals becomes shorter, i.e., the recording density becomes higher, there is a larger demagnetization occurring in the recording medium with the result that the magnetic flux density and hence the playback output are lowered. There has heretofore been suggested a perpendicular magnetic recording system in which recording takes place through magnetization along the thickness of the recording layer of the magnetic recording medium. In the perpendicular magnetic recording system, the shorter the recording wavelength, the lower the demagnetization. Thus, this type of system would be preferable to the in-plane magnetization where short wavelength, high density recording is required. One example of perpendicular recording and media used therein will be found in U.S. Patent No. 4,210,946 to Iwasaki et al. This patent describes a magnetic recording medium suitable for use in a single-pole type magnetic recording transducer operating in a perpendicular magnetization mode and comprising a nonmagnetic base on which there is supported a layer of low coercive force material. On top of the low coercive force material there is a magnetic recording layer which is formed of an alloy composed mainly of chromium and cobalt and has an easy axis of magnetization perpendicular to the surface of the magnetic recording layer. The thickness of the magnetic recording layer is no greater than 3.0 microns. The low coercive force material has a coercive force no greater than 1/5 of the coercive force of the magnetic recording layer, and the thickness of the low coercive force layer is greater than 0.1 micron. Another example of perpendicular recording record media will be found in U.S. Patent No. 4,287,544 to Lazzari. This patent describes a carrier comprising a nonmagnetic substrate on which there is applied a magnetic data layer of an anisotropic magnetic material having its axis of easy magnetization perpendicular to the surface of the nonmagnetic substrate. This data layer includes an additional layer of an anisotropic magnetic material whose axis of easy magnetization is parallel to the surface of the nonmagnetic substrate. An intermediate nonmagnetic, insulative coupling layer may be provided between the two layers. Another nonmagnetic sub-layer can be provided directly on the surface of the substrate to improve the adhesion of the additional layer to the substrate and to isolate it from the magnetic layer. This prior art has suggested a perpendicular magnetization recording layer of a Co-Cr alloy which is deposited on a nonmagnetic substrate, usually consisting of a polymer film. These double layer perpendicular magnetic recording media have a high permeability magnetic thin film or layer of an Fe-Ni alloy as the in-plane magnetization layer between the nonmagnetic substrate and the perpendicular magnetization layer for increasing the recording and/or reproduction efficiency. In the double layer perpendicular magnetic recording medium, the magnetic properties of the in-plane magnetization layer, i.e., the soft or high permeability thin magnetic layer, are critical. The Fe-Ni alloy film which is used as the high permeability magnetic film must be easily magnetized, that is, it must have a low coercive force Hc. Generally, it must be from 0.3 to 0.5 microns in thickness and must be capable of being produced by mass production techniques. If the coercive force Hc of the high permeability layer is high, the magnetic reluctance of the high permeability layer is increased so that the recording and reproducing efficiency of the resulting magnetic recording medium is lowered. Fe-Ni alloys with a low coercive force Hc are known in the art, such as the composition consisting of 21.5 weight percent of Fe and 78.5 weight percent of Ni. It has been found experimentally that when such a film 0.39 microns in thickness is prepared, the coercive force Hc is increased to 20 to 30 oersteds at a substrate temperature lower than 220 C and that for obtaining the film with a coercive force He less than 2 oersteds, it is necessary that the substrate temperature be adjusted to more than 260' C. The higher substrate temperature is not desirable since the plastic film used as a substrate must be extremely heat resistant, while the durability of the manufacturing apparatus is also lowered. To overcome the aforementioned difficulties concerned with the film thickness, it has also been suggested to use a film structure composed of Fe-Ni layers and an interposed T film for lowering the coercive force. This method is not particularly preferred because three evaporation steps are required, and the provision of the nonmagnetic Ti layer tend to lower themagnetic characteristics. For mass production, vacuum evaporation is preferred as a means of depositing high permeability magnetic thin films since a higher deposition speed can be obtained with vacuum evaporation than with sputtering. The deposition speed achieved with vacuum deposition is about 5000 A/sec which is about 30 times that achieved with, for example, a sputtering process which deposits at a rate of about 170 Atsec. Accordingly, one of the problems to be solved with a high permeability magnetic thin film used as the in-plane magnetic film is to lower the coercive force Hc while providing a process that is mass-producible. SUMMARY OF THE INVENTION The present invention seeks to meet the above-described requirements and to provide a method for the preparation of a high permeability magnetic thin film having superior characteristics as a magnetic layer, including a low coercive force Hc and hence to provide a method for the preparation of a perpendicular recording medium which permits the production of such a medium with improved recording and/or reproducing efficiency. The present inventors conducted research to determine possible causes for the high coercive force of an Fe-Ni film containing 21.5 weight percent of Fe and 78.5 weight percent of Ni, in order to develop a method for the preparation of an Fe-Ni film with reduced coercive force Hc. It has now been found that, with the Fe-Ni film of the above composition, a striped magnetic domain is produced in the case of a film above a predetermined thickness due to the presence of magnetic anisotropy perpendicular to the film surface originated from magnetoelastic anisotropy, thus increasing the coercive force of the Fe-Ni film. The magnetoelastic anisotropy K is related to the magnetostriction constant A and the internal stress f by the equation: EMI6.1 In order to reduce the magnetoelastic anisotropy K and thus the coercive force, it is necessary to reduce the constant N and the stress 6 . One of¯the methods of reducing the internal stress 6 is to elevate the substrate temperature. However, the high substrate temperature is not desirable for the reasons given above. On the other hand, the constant A depends on the alloy composition. Hence, one may expect to produce a film of lower coercive force by employing an Fe-Ni alloy material having a smaller magnetostriction constant x h. The present invention is based upon the ¯foregoing analysis and resides in a method of producing a perpendicular magnetic recording medium including the steps of evaporating a high permeability magnetic film or layer on a substrate while the substrate temperature is at least 220 C, the film or layer consisting essentially of an Fe-Ni alloy with an Ni content of 82 to 89.5 weight percent, the balance being essentially iron, and forming a perpendicular magnetic recording layer on the film thus deposited. By controlling the composition of the Fe-Ni alloy and the substrate temperature at the time of evaporation, an Fe-Ni alloy film with a low coercive force can be produced by a one-step operation. It will thus be seen that the present invention provides an effective method for forming a high permeability magnetic thin layer with an extremely low coercive force Hc and thus producing a perpendicular magnetic recording medium with an improved recording and/or reproducing 'efficiency by adjustment of the Ni content of the Fe-Ni alloy and also adjusting the substrate temperature employed during evaporation of the high permeability magnetic thin film or layer. The production efficiency is also improved by employing vacuum evaporation as the method of preparing the high permeability magnetic thin layer. BRIEF DESCRIPTION OF THE DRAWINGS A further description of the present invention will be made in conjunction with the attached sheets of drawings in which: FIG. 1 is a plot of coercive force versus nickel content of the Fe-Ni alloy used in the high permeability magnetic thin layer; FIG. 2 is a plot of coercive force against substrate temperature at the time of evaporation; FIG. 3 is a plot of coercive force of the high permeability magnetic thin film against film thickness; FIG. 4 is a diagram showing the relation between the composition of the Fe-Ni alloy and the magnetostriction cpmstamt x obtained by experiment; and FIG. 5 is a diagram showing the relation between crystal magnetic anisotropy K1 and the nickel content of the alloy. DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the present invention, a substrate composed of a nonmagnetic material such as a temperature-resistant ssmthetic resin, a ceramic substrate, or a nonmagnetic alloy, is arranged for vacuum evaporation by providing an evaporation source such as an Fe-Ni alloy ingot and a heating means such as a heater or an electron gun for heating the source. The particularly preferred substrate is a polyimict resin which can resist temperatures in excess of 220 0C and higher without losing its desirable physical properties. The substrate is positioned opposite to the evaporation source. The source is heated by the heating means for depositing the alloy material and forming the high magnetic permeability film on the substrate surface. The composition of the thin film of high permeability magnetic material and the substrate temperature at the time of deposition are critical, and it has been found that the coercive force can be lowered by adjusting these factors so as to be within predetermined ranges. According to our experimental work, it has been found that by controlling the Ni content in the Fe-Ni alloy of the high permeability magnetic layer, the coercive force Hc of the resulting high permeability magnetic layer can be lowered markedly. Thus, a magnetic thin film or layer of high magmetic permeability material 0.39 microns thick was prepared with various Ni contents in the Fe-Ni alloy while the substrate temperature at the time of deposition was maintained at 2200 C. The coercive force Hc of the resulting high permeability layer was less than 3 oersteds for Ni contents in the range from 82.0 to 89.5 weight percent and less than 2 oersteds for Ni contents in the range of 82.5 to 88.0 weight percent, as shown in FIG. 1. The coercive force Hc of the resulting high permeability magnetic thin film also becomes markedly lower by using an. elevated substrate temperature at the time of deposition. High permeability magnetic thin film samples 0.39 microns thick were prepared with the use of an Fe-Ni alloy evaporation source having 18 weight percent Fe and 82 weight percent Ni resulting Fe-Ni alloy film having 83.3 weight percent Ni and 16.7 weight percent Fe, and utilizing various substrate temperatures in the range from 100- to 3000C, It was found, as shown by curve (a) in FIG. 2, that the coercive force Hc of the resulting high permeability magnetic layer was markedly lower when the substrate temperature used was higher than 220'C, when it became less than 3 oersteds. Similarly, the coercive force Hc was rapidly lowered with an increase in the substrate temperature as shown by curve (b) in FIG. 2 for an Fe-Ni alloy containing 15.5 weight percent of Fe and 84.5 weight percent of Ni, and by a curve (c) in FIG. 2 for an Fe-Ni alloy containing 14 weight percent of Fe and 86 weight percent of Ni. In each case, the coercive force Hc could be lowered to less than 3 oersteds by using a substrate temperature of 2204C or above. When using an Fe-Ni alloy having an Fe content of 21.5 weight percent, and 78.5 weight percent Ni, the coercive force was lowered with an increase in the substrate temperature as shown by curve (d) in FIG. 2. However, to lower the coercive force to below 3 oersteds, it necessary to use a substrate temperature above 260' C. The coercive force Hc of the resulting high permeability magnetic film depends not only on nickel content and substrate temperature but on the film thickness, as shown in FIG. 3. It has been found that, with a substrate temperature set, for example, at 2600 C, the coercive force Hc is lowered gradually with an increase in film thickness as shown by curve A in FIG. 3. With lower substrate temperatures, the coercive force Hc becomes maximum for a film thickness of 0.39 microns as shown by curve B in FIG. 3 for a substrate temperature of 1500C and by a curve C in FIG. 3 for a substrate temperature of 1800C. It is apparent that, by setting the substrate temperature to achieve the desired coercive force Hc at a film thickness of 0.39 microns, as mentioned above, the coercive force Hc can be lowered sufficiently despite the difference in the film thickness. It will be accordingly understood that by using an Fe-Ni alloy with a nickel content in the range from 82 to 89.5 weight percent, and adjusting the substrate temperature at the time of deposition so as to be at least 2200 C, the coercive force Hc of the film can be controlled to a value of less than 3 oersteds. After the deposition of the layer of high permeability magnetic material by vacuum evaporation, a Ti thin film can be applied as well as the perpendicular magnetization recording layer by sputtering or vacuum evaporation. The process of vacuum evaporation is the most preferred when considering production efficiency although sputtering can also be used. The perpendicular magnetization recording layer is prepared by using methods or materials typically employed for this type of medium, for example, a Co-Cr alloy having 10 to 25 atomic percent of Cr with the balance being essentially Co. The result is the production of a perpendicular magnetization recording layer with an improved vertical orientation. A thin film of Ti may be used for improving the properties of the perpendicular magnetization recording V layer, if controlled to a film thickness of 100 to 500 A. 0 With a Ti thin film less than 100 A, the thin film can be formed only with difficulty and hence the effect of the Ti thin film as a base coat can be lowered. The Ti thin film may occasionally be dispensed with. The present invention may also be discussed in terms of the theoretically calculated magnetostriction constant, particularly the relationship between the magnetostriction constant h and the composition for which the film of a low coercive force may be obtained. The thin layer of high permeability magnetic material of the present invention is prepared in the form of an Fe-N alloy polycrystalline film. The magnetostriction constant of the polycrystalline film may be approximately calculated from the magnetostriction constant of the single crystal from the following equation: EMI13.1 The value of magnetostriction constant N obtained by our experiments is shown in FIG. 4. In this Figure, the curve I is a relation between the magnetostriction constnant R and a Ni content of the evaporation source while the curve II is a relation between the magnetoresistance constant x and a Ni content of the resulting Fe-Ni alloy film. It will be seen from FIG. 4 that the composition range which gives a film of reduced coercive force Hc corresponds to a composition range with a smaller magnetostriction constant. FIG. 5 shows the composition dependency of the crystal magnetic anisotropy K1. In FIG. 5, the curve III is derived from the data of Phys. Rev., Volume 89 (1953), page 64 by R.M. Bozorth and J.G.Walker, whereas the curve IV is derived from the data of the Rev. Mod. Phys., Volume 25 (1953), page 42 by R.M. Bozorth. It will be seen from FIG. 5 that the crystal magnetic anisotropy is important with conventional Fe-Ni alloys, and there is preferably employed a composition with an Ni content of 78.5 weight percent so that the crystal magnetic anisotropy is nearly equal to zero. The present invention will be explained more completely by reference to several specific examples. It should be noted, however, that these examples are given only for purposes of illustration and are not intended to limit the scope of the invention. Example 1 A polyimide film 25 microns in thickness was heated at 2200C by an infrared heater. On the film there was deposited an Fe-Ni alloy using an evaporation source having an Fe content of 17 weight percent and an Ni content of 83 weight percent under a vacuum of 2.0 x 10-6 Torr with a deposition speed of 13 A/sec to give an Fe-Ni alloy film 0.35 micron thick having an Fe content of 15.5 weight percent and a Ni content of 84.5 weight percent. A Ti film was evaporated to a thickness of 200 A on the Fe-Ni alloy film under a vacuum of 2.0 x 10 6 Torr and an evaporation rate of 14 A/sec. On the Ti film there was further deposited a Co-Cr alloy film to a thickness of 0.12 micron under a vacuum of 2.0 x 10-6 Torr and an evaporation o rate of 24 A/sec to give a sample tape. The coercive force Hc of the Fe-Ni film of the resulting sample tape was 0.9 oersted. Comparative Example 1 The procedure of Example 1 was followed except that the polyimide film was heated to 2000C. The coercive force Hc of the Fe-Ni film of the resulting sample tape was 29 oersteds. Example 2 A polyimide film 25 microns thick was heated to 220 C by an infrared heater. On the film there was deposited an Fe-Ni alloy uing an evaporation source with an Fe content of 18 weight percent and an Ni content of 82 weight percent under a vacuum of 2.0 x 10-6 Torr with an evaporation rate of 13 0 A/sec to give an Fe-Ni alloy film 0.35 micron thick having an Fe content of 16.7 weight percent and a Ni content of 83.3 weight percent. A Ti film was then evaporated to a thickness of 200 A on the Fe-Ni alloy film under a vacuum of 2.0 x 10-6 Torr and an evaporation rate of 14 A/sec. On the Ti film there was further deposited a Co-Cr alloy film having a thickness of 0.12 micron under a vacuum of 2.0 x 10-6 Torr with an evaporation rate of 24 A/sec to give a sample tape. The coercive force Hc of the Fe-Ni alloy film of the resulting sample tape amounted to 1.3 oersted. Example 3 A polyimide film 25 microns in thickness was heated to 2200C by an infrared heater. On the film there was deposited an Fe-Ni alloy using an evaporation source with an Fe content of 14 weight percent and an Ni content of 86 weight percent under a vacuum of 2.0 x 10-6 Torr and an evaporation rate of 13 /sec to give an Fe-Ni alloy film 0.35 micron thick having an Fe content of 11 weight percent and a Ni content of 89 weight percent. A Ti film was then deposited to a thickness of 200 A on the Fe-Ni alloy film under a vacuum of 2.0 x 10-6 Torr with an evaporation rate of 14 14/sec, On the Ti film there was further deposited a Co-Cr alloy film to a thickness of 0.12 micron under a vacuum of 2.0 x 10 6 Torr and an evaporation rate of 24 A/sec to yield a sample tape. The coercive force of the Fe-Ni alloy film of the resulting sample tape amounted to 2.7 oersteds. Comparative Example 2 The procedure of preceding Example 3 was repeated except that an Fe-Ni alloy having a content of 21.5 weight percent Fe and 78.5 weight percent Ni was used. The coercive force Hc of the Fe-Ni alloy film of the resulting sample tape amounted to 27 oersteds Comparative Example 3 The procedure of preceding Example 3 was followed except that the Fe-Ni alloy evaporation source had an Fe content of 12 weight percent and a Ni content of 88 weight percent resulting an Fe-Ni alloy film having an Fe content of 8.5 weight percent and a Ni content of 91.5 weight percent. The coercive force Hc of the Fe-Ni alloy film of the resulting sample tape amounted to 8.2 oersteds. It will thus be seen from the above Examples that the coercive force Hc can be markedly lowered by employing the improvements of the present invention. It will also be evident that various modifications can be made to the described embodiments without departing from the scope of the present invention.

Claims: WE CLAIM AS OUR INVENTION: 1. In a magnetic recording medium suitable for use in a single-pole type recording system utilizing a perpendicular magnetization mode wherein said medium includes a nonmagnetic base, a low coercive force layer on said base and a magnetic recording layer overlying said low coercive force layer, said magnetic recording layer having an easy axis of magnetization perpendicular to said nonmagnetic base, the improvement which comprises: providing a non-magnetic base of a material which is stable at a temperature of 220 C and higher, and utilizing as said low coercive layer an iron-nickel alloy containing about 82 to 89.5 weight percent nickel, said low coercive force layer resulting from vapor phase deposition of said alloy on said nonmagneticbase while the temperature of faid base base is at least 220eC. 2. A magnetic recording medium according to claim 1 wherein said low coercive force layer has coercive force not in excess of 5 oersteds. 3. A magnetic recording medium according to claim 1 which includes a layer of Ti between said low coercive force layer and said magnetic recording layer. 4. A magnetic recording medium according to claim 3 wherein said layer of Ti has a thickness between 100 and o 500 A. 5. A magnetic recording medium according to claim 1 wherein said nonmagnetic base is a polyimide resin. 6. A magnetic recording medium according to claim 1 wherein said nonmagnetic base is a ceramic material. 7. A magnetic recording medium according to claim 1 wherein said nonmagnetic base is a nonmagnetic alloy. 8. In a method for the production of a perpendicular magnetic recording medium wherein a low coercive force material is first applied to a nonmagnetic base followed by application of a magnetic recording layer thereover, the improvement which comprises the step of: vapor depositing a high permeability, low coercive force film on a nonmagnetic substrate while the temperature of the substrate is at least 2200 C, said film consisting essentially of an Fe-Ni alloy containing 82 to 89.5 by weight Ni. 9. The method of claim 8 wherein said depositing takes place under vacuum conditions. 10. The method of claim 8 wherein said nonmagnetic base is composed of a polyimide resin. 11. The method of claim 8 wherein said nonmagnetic base is composed of a ceramic material. 12. The method of claim 8 wherein said nonmagnetic base is a nonmagnetic alloy. 13. The method of claim 8 which includes the step of depositing a film of Ti over said low coercive force film. 14. A method according to claim 13 wherein the thickness of said film of Ti is in the range from 100 to o 500 A.

Inventors: KATAHIRA, TAKU C/O SONY CORPORATION, NAKAMURA, ETSUKO C/O SONY CORPORATION, TANAKA, TOSHIKAZU C/O SONY CORPORATION
Applicants: SONY CORPORATION
Application year: 1985
Main classification: G11B5
IPC codes: C23C14, G11B5, C22C19, H01F41