Process for manufacturing magnetic recording medium

Abstract

A manufacturing process of Co-Ni magnetic recording media by sputtering Co-Ni thin film layer on a substrate in Argon atomsphere including nitrogen and heating the thin film to evaporate nitrogen from the thin film to prepare the magnetic recording media appropriate for high density longitudinal recording, wherein on sputtering the substrate is maintained at a temperature of room temperature to 300.degree. C.

Description

DETAILED DESCRIPTION OF THE INVENTION

Field of the Invention

This invention relates to magnetic recording media, especially a rigid magnetic recording disk which is used for a longitudinal, that is in-plane, recording.

BACKGROUND OF THE INVENTION

As magnetic recording media on a rigid disk, there have been used .gamma.-hematite, Co-Pt, Co-Ni-Pt, and Co-Ni-P. The hematite media are coated with a resin on an aluminum or aluminum alloy substrate in a thin layer. The hematite media have been used for a long time in a lot of applications. But recently, strong demand for high recording density disk has occurred but the coated hematite media can not clear the demand. Instead of the coated hematite media, thin film media have been proposed for high recording density. The thin film media include plated media and sputtered media. The plated media, e.g. Co-Ni-P, have a disadvantage such as corrosion due to residual chemicals. Also, the corrosion causes an error of reading written data so that a very high density of recording cannot be accomplished. Consequently, plated thin film media for longitudinal recording have inherently high defect densities and poor corrosion resistance.

By contrast, sputtered thin film media have been expected to have a superior quality in every respect including: electrical characteristics, flyability, durability, wear, adhesion, defect density, corrosion resistance and reproducibility. As the sputtered thin film media, cobalt base alloys, e.g. Co-Ni-Pt, Co-Ni, have been proposed to be adapted for magnetic recording media for high longitudinal recording density. The Co-Ni-Pt is rather expensive because it contains platinum of about 10 at % which is an expensive element.

On the other hand, directly sputtered Co-Ni film tends to have its C-axes perpendicular to the sputtered plane so it is not appropriate to longitudinal recording.

For longitudinal recording media, magnetic recording media of sputtered thin film were proposed in some articles. According to Dr. H. Maeda, J. Appl. Phys. 53 (5) May 1982 p. 3735 "High coercivity Co and Co-Ni alloy films" and J. Appl. Phys. 53 (10) October 1982 p. 6941 "Effect of nitrogen on the high coercivity and microstructures of Co-Ni alloy films" and his patent application, Japanese laid-open patent 57-72307, cobalt metal or Co-Ni-alloy is sputtered in an atmosphere including nitrogen to form a thin film including nitrogen and then the thin film is heat-treated in vacuum or inert gas atmosphere to make it thin film magnetic recording media with good magnetic properties. Those papers teach that the sputtering process is required to be done on a substrate which is cooled by liquid nitrogen to obtain a sufficient nitrogen content in the sputtered thin film.

Like Dr. Maeda's experiment, when a glass plate or a quartz plate is used as the substrate, it is no problem to cool the substrate to liquid nitrogen temperature. The sputtering surface of the substrate is cooled to the temperature to make it easy to capture nitrogen in the sputtered film even though ion bombarding by sputtering causes to increase the temperature of the sputtering surface.

However, when an anodized aluminum plate is used as a substrate, the anodized layer is cooled to liquid nitrogen temperature on its one side which adheres to aluminum substrate and is heated on the other side by ion bombarding so the sputtered surface of the anodized layer is heated to a temperature above room temperature. The temperature difference between both the sides of anodized layer on aluminum substrate reaches to 100.degree. C. or more. Since the anodized layer has poor heat conductivity, this temperature difference results in cracks in the layer or peeling of the layer.

Further, the cooling of the substrate by liquid nitrogen causes high cost to manufacture a disk not only by precious liquid nitrogen consumption but also by requirement of complicated equipment and by impossibility of simultaneous sputtering of the two sides of the substrate.

BRIEF DESCRIPTION OF THE INVENTION

An object of the invention is to provide a process of manufacturing sputtered magnetic recording media having excellent magnetic properties.

Another object of the invention is to provide longitudinal magnetic recording media which is manufactured by sputtering on a substrate of a temperature of room temperature or more.

According to this invention, the objects are accomplished by a process comprising the steps of: sputtering with an alloy target on a non-magnetic substrate in atmosphere of inert gas including nitrogen, the non-magnetic substrate being maintained at a temperature of room temperature to 300.degree. C. and the alloy target having approximately the same composition as the composition of the final product, to obtain on the substrate a thin film including nitrogen, and heating the thin film to evaporate the nitrogen which has been absorbed in the thin film in the sputtering process and to form magnetic recording media which have a composition of Ni less than 35 atomic % and the balance substantially being Co and substantially a hexagonal close packed crystalline structure which C-axes are mainly in a recording media plane. This invention has a special advantage that the magnetic recording media are secured even to an anodized aluminum alloy substrate.

The thickness of the sputtered media is preferably 400 to 1000 angstrom. The substrate temperature is allowed to be from room temperature to 300.degree. C., preferably to 100.degree. C. from the point of productivity and ease of nitrogen absorption.

The heat-treatment after sputtering is accomplished at a temperature at which the nitrogen that has been absorbed in the sputtered film is evaporated from the film. The preferred temperature is 310.degree. C. to 500.degree. C.

DETAILED DESCRIPTION OF THE EMBODIMENTS

According to this invention, first a sputtered thin film including nitrogen of an appropriate content is prepared during sputtering process to make Co-Ni-N layer of amorphous structure or fine crystalline structure of grain size of about 50 to about 250 angstrom or mixture of such microcrystals and amorphous state. Then by crystallization and/or grain growth through heating, C-axes of hexagonal Co-Ni crystals are mainly oriented in the surface.

According to this invention, amorphous-like or microcrystal Co-Ni-N layer including nitrogen of 10 to some tens atomic % can be formed without special cooling of a substrate at sputtering deposition rate of 25 .ANG./min. to 170 .ANG./min., even at high speed sputtering deposition rate of 300 .ANG./min. to 2,000 .ANG./min. Heat-treatment after the sputtering results in magnetic recording media showing considerably better properties than the prior art. When the substrate was heated to more than 300.degree. C. on the sputtering, the nitrogen content in the sputtered layer was decreased so that coercivity after heat treatment was decreased and a magnetic thin film appropriate for high density magnetic recording was not obtained. When the substrate temperature is from room temperature to 100.degree. C., nitrogen is easily captured in the sputtered layer.

The composition of the media is not limited to cobalt metal and cobalt-nickel alloy, but Pt, Ru, Rh, Os, Ir, Re, Cr, Ti, Mo and/or W of less than about 10 atomic % can be added to cobalt or cobalt nickel alloy. These additives may improve corrosion resistance.

This invention will be explained by examples below.

The examples were carried out by magnetron sputtering but it is apparent from ion technicals that the similar results can be obtained by ion beam sputtering.

EXAMPLE I

By planar magnetron sputtering process, thin film layers were formed on anodized aluminum alloy substrates (3.9 wt% Mg-bal. Al) by the following conditions.

______________________________________Initial evacuation 1-2 .times. 10.sup.-6 TorrTotal atm. pressure 20 m TorrNitrogen content 50 vol. % in ArgonElectric power 1 KWTarget diameter 120 mmDistance between cathod 120 mmand substrateFilm thickness 800-1000.ANG.______________________________________

As a target, cobalt metal and Co-Ni alloy plates were used. Substrate temperature was controlled by an electric heater positioned in a substrate holder and measured by a thermocouple on the back of the substrate. Deposition rate of the thin film was adjusted by sealing the substrate. Nitrogen contents in the sputtered layer were determined by Auger electron microscope analysis.

From X-ray diffraction of the sputtered layers, it was recognized that all samples except the one of 350.degree. C. substrate temperature exhibited halo pattern diffraction spectra, that is, amorphous or microcrystal material's spectra, and some unknown spectra.

After 1 hour heat treatment at 400.degree. C. in vacuum, the thin films were measured with magnetic properties in a direction parallel to the film surface by vibrating magnetometer.

The experimental data are shown in Table 1 in which squareness S means a ratio of residual induction Br to magnetic flux density B at 3 KOe magnetic field strength.

Nitrogen content of all the films was less than 5 atomic % after 1 hour heat treatment at 400.degree. C. in vacuum. It was recognized that all of the films belonged to h.c.p. crystal except the sample of 350.degree. C. substrate temperature by X-ray diffraction and that C-axes were strongly oriented in the media plane.

When nickel content of the thin film composition exceeded 40 atomic %, f.c.c. phase appeared along with h.c.p. and the film is not appropriate for magnetic recording media because both squareness and coersivity were too low.

From the above experiments it is apparent that the media which were made on the substrate of less than 300.degree. C. showed enough squareness and were appropriate for magnetic recording media. Especially, the film made on a substrate

TABLE 1__________________________________________________________________________ NitrogenSputtering condition content Magnetic propertiesTarget Substrate Deposition before Coercive Residualcomposition temperature rate heating force induction Squareness(at %) (.degree.C.) (.ANG./min) (at %) Hc(Oe) Br(KG) S(Br/B3k)__________________________________________________________________________Metallic R.T. 300 15 400 11.3 0.75Co 100 300 15 400 11.5 0.75 200 200 16 400 10.8 0.70 300 200 12 350 10.2 0.70 350 200 7 150 8.0 0.55Co--15 Ni R.T. 300 15 1,050 10.5 0.78 100 300 16 1,050 11.6 0.83 200 200 15 650 10.5 0.75 300 200 13 600 10.8 0.75 350 200 5 200 9.0 0.65Co--25 Ni R.T. 300 16 1,100 11.0 0.83 100 300 15 1,100 10.5 0.85 200 200 13 700 10.0 0.80 300 200 13 700 9.5 0.80 350 200 9 300 7.2 0.60Co--35 Ni R.T. 200 14 750 10.0 0.75 100 200 16 800 10.0 0.75 200 200 13 450 9.5 0.73 300 200 11 400 9.0 0.73 350 200 7 250 6.6 0.55__________________________________________________________________________ of room temperature to 100.degree. C. showed high coersivity and suitable for high density magnetic recording. The films of Co-15% Ni and Co-25% Ni showed squareness and residual induction which are superior to the squareness of 0.75 and the magnetic flux density of 8000 G in prior art.

Magnetization of a film after heat treatment of 300.degree. C..times.1 hour is only 20 to 30% of one after heat treatment of 400.degree. C..times.1 hour. By another experiment it was found that magnetic properties of the media which were formed on an anodized aluminum alloy substrate and heat-treated at 310.degree. C. for an hour were approximately on the same level as that obtained with the media after 500.degree. C..times.1 hour heat treatment. 310.degree. C..times.1 hour heat treatment is enough to obtain excellent magnetic properties.

Heat treatment at a temperature of above 500.degree. C. may be applied, but when an anodized aluminum alloy substrate is used, such a high temperature heat treatment deforms the aluminum substrate and causes crack in the film or peeling-off the film. So, it is preferable that the heat treatment is done at a temperature between 310.degree. C. and 500.degree. C.

EXAMPLE II

By the same sputtering conditions as described in EXAMPLE I except for the following, thin film layers were prepared on anodized aluminum alloy substrate with anodized layer of about 13 .mu.m thick.

______________________________________Total atm. pressure 40 m TorrNitrogen content 75 vol. % in ArgonElectric power 1-2.5 KWElectrode distance 80 mmFilm thickness 400-600.ANG.______________________________________

As a result of increasing nitrogen content in the sputtering atmosphere, the nitrogen content in the sputtered layers in this EXAMPLE were rather large than those in EXAMPLE 1. But, during heat treatment at 320.degree. to 400.degree. C. for 1 hour almost all of nitrogen in the layer was evaporated and the nitrogen in the film after the heat treatment was less than 5 at %, that is, most samples contained only about 1% nitrogen.

Magnetic properties measured with different substrate temperatures are shown in Table 2. From the table, it is apparent that nitrogen content in sputtered layers depends on substrate temperature and the lower substrate temperature effects the higher nitrogen content. Even high speed deposition rate, e.g. 800 .ANG./min. and 2000 .ANG./min. did not affect magnetic properties.

TABLE 2__________________________________________________________________________ NitrogenSputtering condition content Magnetic propertiesTarget Substrate Deposition before Coercive Residualcomposition temperature rate heating force induction Squareness(at %) (.degree.C.) (.ANG./min) (at %) Hc(Oe) Br(KG) S(Br/B3k)__________________________________________________________________________Co--15 Ni R.T. 800 22 1,000 10.5 0.79 100 800 22 950 10.7 0.80 200 800 18 900 10.7 0.80 300 800 14 750 10.4 0.77 350 800 10 500 9.1 0.67Co--20 Ni R.T. 50 45 900 10.5 0.80 100 100 38 850 10.3 0.82Co--25 Ni R.T. 2,000 17 850 10.6 0.80 100 2,000 18 850 10.5 0.79 300 2,000 13 750 10.9 0.82 R.T 400 35 1,050 10.2 0.81 100 400 34 1,000 10.4 0.81 300 400 26 950 10.5 0.83__________________________________________________________________________

As apparent from the above examples, by sputtering on a non-magnetic substrate which is not cooled by liquid nitrogen but is maintained at a temperature of room temperature to 300.degree. C., preferably to 100.degree. C., considerably better magnetic properties were obtained than those obtained on a liquid nitrogen cooled substrate. Since this invention does not use liquid nitrogen, thin film forming is easy and causes big economic effect because of possibility of the simultaneous sputtering of both side of the disk and simple equipment for sputtering and no consumption of precious liquid nitrogen.

Claims

1. A process for manufacturing magnetic recording media comprising the steps of:

sputtering a metal alloy comprised of nickel and cobalt as a thin film onto a non-magnetic substrate consisting essentially of an anodized aluminum alloy in an inert gas containing nitrogen, said substrate being maintained at a temperature of from about room temperature to 300.degree. C. during said sputtering step thereby forming a nitrogen containing metal thin film comprised of nitrogen and said metal alloy on said anodized aluminum alloy substrate; and

heating said nitrogen containing thin film and said substrate to evaporate the nitrogen absorbed in said thin film during the sputtering step thereby forming a magnetic recording media with a metal alloy thin film on said substrate, said metal alloy thin film having a composition of less than 35 atomic % nickel with the balance being substantially cobalt, said metal alloy thin film having a hexagonal close-packed crystalline structure wherein the C-axes of the cobalt-nickel crystals forming said structure are in the plane of said recording media.

2. The process as set forth in claim 1, wherein the thin film is 400 to 1,000 .ANG. thick.

3. The process as set forth in claim 1, wherein the non-magnetic substrate is maintained at a temperature of room temperature to 100.degree. C.

4. The process of claim 1 wherein said heating step reduces the amount of nitrogen in said metal alloy thin film to about 1 atomic %.