GB2302980A

GB2302980A - High density magnetic recording medium

Info

Publication number: GB2302980A
Application number: GB9612880A
Authority: GB
Inventors: Isao Kobayashi; Takashi Ishii; Koji Uchida
Original assignee: Kao Corp
Current assignee: Kao Corp
Priority date: 1995-07-06
Filing date: 1996-06-19
Publication date: 1997-02-05
Anticipated expiration: 2016-06-19
Also published as: GB9612880D0; GB2302980B

Abstract

A non-magnetic support has a plurality of protuberances 12. A magnetic layer 20 comprises a matrix of separate dots 18 of a material having an average crystal grain size a in the range 2-30 nm. The spacing x between the dots is between 0.2 nm and 30 nm and the dots are in the size range 30-400 nm. This arrangement is said to give reduced noise during high density recording.

Description

MAGNETIC RECORDING MEDIUM This invention relates to a magnetic recording medium and more particularly to a magnetic recording medium having reduced noise during high-density recording.

The following means (1) to (5) have been mainly taken to reduce the noise of magnetic recording media: (1) Magnetic Layer To increase the Cr concentration in the magnetic layer-forming alloy including CoCr or to add Ta, B, an oxide, etc. to the alloy.

(2) Under Layer To control the column structure of an under layer for reduction of the average crystal grain size of the magnetic layer or to control the crystal orientation of an under layer for control of the orientation within the c-axis plane'.

(3) Heating Temperature To heat the support or to conduct heat treatment to accelerate segregation of a nonmagnetic substance, e.g., Cr, on magnetic crystal boundaries there by to weaken the magnetic force among magnetic crystal grains.

(4) Ar Gas Pressure To form a magnetic layer by sputtering under a high Ar gas pressure to provide physical gaps among magnetic crystal grains thereby to weaken the magnetic force among magnetic crystal grains.

(5) Multilayered Magnetic Layer To provide a nonmagnetic intermediate layer between magnetic layers thereby to weaken the magnetic force among magnetic crystal grains.

Japanese Patent Application Laid-Open 7-161025 discloses a magnetic recording medium aiming at reduction of noise in high-density recording.

Although noise is reduced by using a combination of means (1) to (5) above or the means described in the above-identified patent application, further reduction of noise has been demanded in high-density recording.

Accordingly, an object of the invention is to provide a magnetic recording medium having further reduced noise in high-density recording.

The inventors of the present invention have found that noise in recording at high density (particularly 80 kfci or higher) can be reduced markedly by forming a plurality of protuberances on the surface of a nonmagnetic support and forming a magnetic layer having a specific structure on the nonmagnetic support having the protuberances, as compared with a magnetic recording medium having no protuberances.

The present invention has been completed based on the above finding, by providing a magnetic recording medium comprising a nonmagnetic support having a plurality of protuberances on the surface thereof, and a magnetic layer provided above the nonmagnetic support, the magnetic recording medium being characterized in that: it has a high recording density of 80 kfci or higher; and the average crystal grain size a of the magnetic layer, the average circle-equivalent diameter b of the protuberances, and the distance x between adjacent magnetic layers satisfy the following relationships.

2 nm S a < 30 hm 30 nm S b < 400 nm b > 2a 0.2 nm < x < 30 nm The present invention thus provides a magnetic recording medium which has markedly reduced noise in high-density recording, as compared with a magnetic recording medium having no protuberances.

The magnetic recording medium of the present invention is suitable for use as a magnetic drum or a magnetic tape, etc., and especially suitable for use as a magnetic disc, for example, a hard disc.

Fig. 1 is a schematic view showing the structure of the magnetic recording medium according to the first embodiment of the present invention.

Fig. 2 is a schematic view showing the structure of the magnetic recording medium according to the second embodiment of the present invention.

Fig. 3 is a schematic view showing the structure of the magnetic recording medium according to the third embodiment of the present invention (corresponding to Fig. 1).

Fig. 4 is a schematic view showing the structure of the magnetic recording medium according to the fourth embodiment of the present invention.

Fig. 5 is a schematic view showing the structure of the magnetic recording medium according to the fifth embodiment of the present invention (correspond ing to Fig. 4).

Preferred embodiments of the magnetic recording medium according to the present invention will be described below in detail by referring to the accompanying drawings.

Fig. 1 is a schematic view showing the structure of a magnetic recording medium according to the first embodiment of the present invention.

The magnetic recording medium 10 according to the embodiment shown in Fig. 1 comprises a nonmagnetic support 14 having a plurality of protuberances 12 on the surface thereof, and a magnetic layer 16 provided above the nonmagnetic support 14. In the magnetic recording medium 10 shown in Fig. 1, the average crystal grain size a of the magnetic layer, the average circle-equivalent diameter b of the protuberances, and the distance x between adjacent magnetic layers satisfy the following relationships, and the magnetic recording medium 10 has a high recording density of 80 kfci or higher.

2 nm S a S - < 30 nm 30 nm S b S 400 nm b > 2a 0.2 nm < x < 30 nm Further, a plurality of the protuberances 12 in the magnetic recording medium 10 are continuous in the planar direction of the nonmagnetic support 14 as shown in Fig. 1. In other words, a plurality of the protuberances 12 form a continuous aggregate.

One way to form unevenness on the support of a magnetic recording medium is disclosed, e.g. , in Japanese Patent Application Laid-Open 3-73419. However, what is intended by providing unevenness in this patent application is to improve the CSS (Contact Start and Stop) durability of a magnetic recording medium but not to reduce noise.

Preferred details of the magnetic recording medium 10 of Fig. 1 follow. The nonmagnetic support 14 preferably is an aluminum support, an NiP-plated aluminum alloy support, a tempered glass support, a crystallized glass support, a ceramic support, a silicon alloy support, a titanium support, a titanium alloy support, a plastic support, a carbon support, or a support made of a composite of any two or more of these materials. In particular, a carbon support, especially a glassy carbon support, is advantageous for size reduction in diameter and thickness, excellent in heat resistance, and has electrical conductivity, and is therefore preferred in the present invention.

The plurality of protuberances 12 formed on the surface of the nonmagnetic support 14 satisfy the above-described relationships and are continuous in the planar direction of the nonmagnetic support 14. As far as these requirements are met, the method of forming the protuberances 12 is not particularly restricted.

For example, the protuberances 12 may be formed integrally with the nonmagnetic support 14 or be independently formed either directly on the nonmagnetic support or indirectly on the nonmagnetic support via one or more layers.

More specifically, the protuberances 12 integral with the nonmagnetic support 14 can be formed by, for example, chemically etching the surface of the nonmagnetic support 14. In using a carbon support as nonmagnetic support 14, the protuberances 12 can be formed by oxidizing the surface of the carbon support.

On the other hand, independent protuberances 12 can be provided on the nonmagnetic support 14 having a prescribed average surface roughness Ra by a wet plating technique such as electroless plating; physical vapor deposition (hereinafter abbreviated as PVD) such as vacuum evaporation, ion plating or sputtering; or chemical vapor deposition (hereinafter abbreviated as CVD). The protuberances 12 formed by these techniques preferably comprise a thin film of a metal or an alloy.

Materials constituting the protuberances 12 include metals having a low-melting point, such as Ag, Al, Au, Cu, Sn and In, and alloys containing these metals.

Silica [Si(O)] is particularly preferred. It is also particularly preferred to use an A1-M alloy, in which M is a metal capable of forming a carbide (hereinafter M will be referred to as "carbide-forming metal"), from the standpoint of ease of formation and improvement in close fixation of the protuberances 12. For further improvements in ease of formation and close fixation of the protuberances 12, a carbon support as nonmagnetic support 14 having thereon the protuberances 12 made of an Al-M alloy, wherein M is as defined above, formed by PVD is preferred. The carbide-forming metals include Si, Cr, Ta, Ti, Zr, Y, Mo, W and V. The thickness of the protuberances 12 (i.e., corresponding to the height of the top of the protuberances 12) is preferably 5 to 60 nm including 10, 20, 30, 40 and 50 nm and all ranges therebetween.

As described above, the protuberances 12 have an average circle-equivalent diameter b falling within a range of 30 nm ' b S 400 nm. If the average diameter b is less than 30 nm, it is difficult to form thereon a magnetic layer comprising discontinuous convexed portions. If it exceeds 400 nm, the spacing loss between the recording medium and a magnetic head would be too large. A preferred range of the average diameter b is represented by an inequality of 50 nm < b < 300 nm, with the range of 50 nm < b < 200 nm being further preferred and including 70, 90, 120, 150, 170 and 190 nm and all ranges therebetween.The average diameter b can be measured by, for example, cutting the nonmagnetic support 14 having formed thereon the protuberances 12 to a prescribed shape and observing its image under an atomic force microscope (hereinafter abbreviated as AFM).

As stated above, a plurality of the protuberances 12 in the magnetic recording medium 10 according to the embodiment shown in Fig. 1 are continuous in the planar direction of the nonmagnetic support 14. That is, a plurality of the protuberances are adjacent to each other on the surface of the nonmagnetic support 14 with substantially no discontinuous area among the protuberances. Accordingly, where the protuberances 12 comprise an Al-M alloy, for example, formed on the surface of the nonmagnetic support 14, the surface of the nonmagnetic support 14 is totally covered with the Al-M alloy so that substantially no portions of the surface of the nonmagnetic support 14 are exposed.

This is preferred regardless of the protuberance material.

While the average surface roughness Ra of the protuberances 12 is not particularly limited, it is preferably 1 to 30 nm for achieving a high recording density while minimizing the spacing with a magnetic head. A more preferred Ra is 1 to 20 nm including 2, 5, 10, 15 and 18 nm and all ranges therebetween. The maximum height Rmax of the protuberances 12 is not particularly limited either, but is preferably 10 to 150 nm, more preferably 20 to 100 nm, including 30, 40, 50, 60, 70, 80 and 90 nm and all ranges therebetween, for ensuring high durability.The average surface roughness Ra and maximum height Rmax of the protuberances can be measured in the same manner as for the measurement of the average diameter b i.e., by cutting the nonmagnetic support 14 having formed thereon the protuberances 12 to a prescribed shape and observing an AFM image of the cut piece.

The magnetic layer 16 which is formed above the nonmagnetic support 14 having the protuberances 12 will now be described with reference to Fig. 1.

As shown in Fig. 1, the magnetic layer 16 is formed above the nonmagnetic support 14 on which the protuberances 12 have been formed, and the magnetic layer 16 has unevenness corresponding to the shape of the protuberances 12. As shown in Fig. 1, the magnetic layer 16 comprises an aggregate made up of a plurality of the discontinuous convexed portions 18, which corresponds to the protuberances 12, in the planar direction of the nonmagnetic support 14. In other words, there are substantially discontinuous areas between every adjacent convexed portions 18 so that the individual convexed portions 18 are physically isolated from each other. Because the magnetic layer 16 comprises an aggregate of such discontinuous convexed portions 18, adjacent convexed portions 18 only interfere with each other with a very weak magnetic force.

As a result, the noise of the magnetic recording medium is reduced. It is essential for the magnetic layer 16 to have an average crystal grain size a falling within a range of 2 nm < a < 30 nm as hereinafter described.

A magnetic layer having such a specific average crystal grain size can be formed with ease by, for example, selecting a metal (or an alloy) which forms the magnetic layer, or controlling the conditions of the formation of the magnetic layer, which is within the skill of the ordinary worker in this art.

The distance x between adjacent magnetic layers (i.e., the distance between adjacent convexed portions 18) falls within a range of 0.2 < x < 30 nm including 0.5, 1, 2, 5, 10, 15, 20, 25 and 28 nm and all ranges therebetween. If x is less than 0.2 nm, the magnetic force binding between adjacent convexed portions increases, tending to incur an increase in noise. If x exceeds 30 nm, the number of magnetic fluxes per unit area is reduced, tending to reduce the output. The distance x preferably falls within a range of 0.2 nm I x # 10 nm, more preferably 0.2 nm # x # 5 nm. The distance x can be measured by, for example, observing the sectional and in-plane transmission electron microscope (TEM) images of a magnetic recording medium having the magnetic layer 16. The range of the distance x can be adjusted by controlling the shape of the protuberances 12.

The average crystal grain size a of the magnetic layer 16 falls within a range of 2 nm < - ' < - 30 nm, preferably 2 nm < - a < 20 nm including 3, 5, 10, 15 and 18 nm and all ranges therebetween. If the average crystal grain size a is less than 2 nm, the magnetic crystal grains are apt to have instable magnetism. An average crystal grain size a exceeding 30 nm is disadvantageous for high-density recording. The term "average crystal grain size " as used herein means an average grain size of the magnetic crystal grains 20 in the individual convexed portions 18 which form the magnetic layer 16. Accordingly, each convexed portion 18 is an aggregate composed of a plurality of the magnetic crystal grains.In the magnetic recording medium 10 shown in Fig. 1, the magnetic crystal grains 20 of the magnetic layer 16 have a column structure similar to those of a conventional magnetic layer formed by thin film deposition techniques such as PVD, and the average grain size of the magnetic crystal grains 20 corresponds to the diameter of the transverse cross section of the column structure. The average crystal grain size a can be measured by, for example, observing an AFM image or HR-SEM (high resolution scanning electron microscope) image.

The average crystal grain size a of the magnetic layer 16 and the average diameter b of the abovementioned protuberances 12 have the relationship of b 2 2a. As previously stated, each of the average crystal grain size a and the average diameter b should fall within their respective specific range and, at the same time, they are correlated with each other. Because of the above relationship between them, the present invention produces an advantageous effect that the noise occurring in high-density recording is reduced, as compared with magnetic recording media having no protuberances 12.

The advantageous effects of the present invention are particularly pronounced in high-density recording media of 80 kfci or higher. When the present invention is applied particularly to high-density recording media of 90 kfci or higher, especially 100 kfci or higher, the degree of noise reduction in high-density recording is high, as compared with magnetic recording media in which the magnetic layer comprises a continuous layer.

The magnetic layer 16 can be formed by a known thin film deposition technique, for example, PVD such as vacuum evaporation, sputtering or ion plating, or CVD. Materials which can be preferably used to constitute the magnetic layer 16 include Co magnetic alloys predominantly comprising Co, e.g., CoCr, CoNi, CoCrX, CoCrPtX, CoSm, CoSmX, CoNiX, and CoWX (wherein X is at least one metal selected from the group consisting of Ta, Pt, Au, Ti, V, Cr, Ni, W, La, Ce, Pr, Nd, Pm, Sm, Eu, Li, Si, B, Ca, As; Y, Zr, Nb, Mo, Ru, Rh, Ag, Sb and Hf; provided that the composition does not have a combination of the same X elements), and ferrite magnetic materials, e.g., Ba-ferrite, Sr-ferrite, 7ferrite, and Co-y-ferrite. These materials may be used either individually or as a combination of two or more thereof.

The second to fifth embodiments of the magnetic recording media of the present invention will be described by referring to Figs. 2 through 5.

Fig. 2 is a schematic view showing the structure of the magnetic recording medium according to the second embodiment of the present invention. Fig. 3 is a schematic view showing the structure of the magnetic recording medium according to the third embodiment (corresponding to Fig. 1). Fig. 4 is a schematic view showing the structure of the magnetic recording medium according to the fourth embodiment of the present invention. Fig. 5 is a schematic view showing the structure of the magnetic recording medium according to the fifth embodiment of the present invention (corresponding to Fig. 4). The description in terms of the second to fifth embodiments does not enter details with respect to the particulars common to the first embodiment, and the explanation given to the first embodiment can apply appropriately.The same reference numerals as used in Fig. 1 are given to the same members in Figs. 2 to 5.

In the magnetic recording medium 10 of the embodiment shown in Fig. 2, a layer 22 comprising a metal M2 (wherein M2 is a carbide-forming metal) is provided on the nonmagnetic support 14, and a plurality of the protuberances 12 which are continuous in the planar direction of the nonmagnetic support 14 is provided on the layer 22. Also, an amorphous layer 24 is provided on the protuberance 12, and a first under layer 26 and a second under layer 28 are provided on the amorphous layer 24 in this order. Further, the magnetic layer 16 is provided on the second under layer 28, and a protective layer 30 and a lubricant layer 32 are provided on the magnetic layer 16 in this order.

Preferred details of the magnetic recording medium 10 of the embodiment shown in Fig. 2 follow.

The layer 22 comprising a metal M2, which is provided on the nonmagnetic support 14, serves to make the surface profile of the protuberances 12 more suitable.

That is, the layer 22 is formed with fine unevenness on its surface, and the protuberances 12 formed on the layer 22 will have their own surface profile combined with the unevenness of the layer 22. As a result, the surface profile of the protuberances 12 become more suitable.

The metal M2 constituting the layer 22 has carbide forming capability. Examples of suitable metals M2 are the same as those enumerated as the metal M used to form the protuberances 12. Particularly preferred metals M2 are Ti, Si, Cr or Zr, or alloys containing Ti, Si, Cr or Zr. The layer 22 preferably has a thickness of 5 to 200 nm.

In Fig. 2 the amorphous layer 24 preferably contains an element of Groups 3B to 5B and comprises, for example, B, C, Si, P, etc. The amorphous layer 24 functions to eliminate the influence of the surface condition of the protuberances 12 upon the upper layers. The amorphous layer 24 thus facilitates designing of the material of the protuberances 12 independently of the materials for control of the magnetic layer (i.e., materials of the under layers hereinafter described) Taking the above-mentioned function of the amorphous layer 24 into consideration, it is preferable to provide the amorphous layer 24 between the protuberances 12 and the magnetic layer 16. It is still preferred for further improvement in the squareness ratio to provide the amorphous layer 24 directly on the protuberances 12 (especially on those comprising an Al M alloy).

The amorphous layer 24 is not particularly limited in method of formation. For example, the amorphous layer comprising amorphous carbon can be formed by PVD using a carbon material as a target.

While crystalline carbon materials such as graphite may be used as carbon material, a glassy carbon material is preferred. It is also preferable that the carbon material contains a carbide, nitride, boride or oxide of Si, Ti, W, Zr, Cr, Nb, Mo, Ta, Al, etc. or ceramic particles of BN, B4C, etc. The amorphous layer 24 preferably has a thickness of 5 to DO nm.

The first under layer 26 and the second under layer 28 which are provided on the amorphous layer 24 in this order are described below. The first under layer 26 is provided in order to further reduce noise of the recording medium. It is preferably made up of Ti or a Ti alloy. The second under layer 28 is provided in order to improve the static magnetic characteristics of the magnetic layer 16. To this effect the second under layer 28 is preferably made of those materials whose lattice constant is approximate (preferably +10%) to that of the material constituting the magnetic layer 16. For improvement in static magnetic characteristics and noise reduction, it is particularly preferred that the second under layer 28 be composed of Cr or a binary alloy containing Cr. Examples of suitable Cr-containing binary alloys are CrTi, CrMo, CrW, CrNb, CrSi, CrCo and CrTa.The first under layer 26 and the second under layer 28 preferably have a thickness of 5 to 200 nm and 5 to 150 nm, respectively.

Both the first under layer 26 and the second under layer 28 can be formed by, for example, PVD.

The protective layer 30 which is provided on the magnetic layer 16 is described below. The protective layer 30 is preferably formed of a material having high mechanical strength from the standpoint of wear resistance. More specifically, it is preferable to use at least one material selected from the group consisting of an oxide of a metal, e.g., Al, Si, Ti, Cr, Zr, Nb, Mo, Ta or W (e.g., silicon oxide or zirconium oxide); a nitride of the metal (e.g., boron nitride); a carbide of the metal (e.g., silicon carbide and tungsten carbide); carbon (e.g., diamond-like carbon); and boron nitride. Of these materials those preferred are carbon, silicon carbide, tungsten carbide, silicon oxide, zirconium oxide, boron nitride, and composite materials thereof. Carbon, especially diamond-like carbon or glassy carbon, is particularly preferred.

When the protective layer is formed by using carbon as a target in thin film deposition, hydrogen gas or a hydrocarbon gas (e.g., methane) may be incorporated to form a protective layer comprising hydrogenated carbon.

The protective layer 30 preferably has a thickness of 5 to 25 nm. The protective layer 30 can be formed by, for example, PVD.

The lubricant layer 32 which is provided on the protective layer 30 is described below. The lubricant layer 32 is used for improvement in running properties and durability of the magnetic recording medium. It can be formed by, for example, applying a lubricant to a thickness of about 5 to 100 A by a coating technique such as spin coating, dip coating or spray coating, or by vapor phase polymerization of a fluorocarbon compound and oxygen at a molar ratio of 1/0.5 to 1/100 under a vacuum condition (especially photo assisted CVD using an excimer laser). Suitable lubricants include fluorocarbon polymers. The fluorocarbon polymers include those having a polar group in the molecule thereof, those having no polar group in the molecule thereof, and combinations thereof.

The fluorocarbon polymers having a polar group in the molecule thereof preferably include perfluoropolyethers having a number average molecular weight of 2000 to 4000 and containing an aromatic ring or a hydroxyl group at the terminal thereof. More specifically, those having a -(CF2CF2O)n-(CF2O)m- skeleton with an aromatic ring or a hydroxyl group at the terminal and having a number average molecular weight of 2000 to 4000 are preferred. Fomblin AM 2001 and Fomblin Z-dol, both available from Augimont Co., can be mentioned as specific examples.

The fluorocarbon polymers having no polar group in the molecule preferably include perfluoropolyethers having a number average molecular weight of 2000 to 10000 and containing a perfluoroalkyl group at the terminal thereof. More specifically, those represented by formula: CF3-(CF2CF2O)n-(CF2O)m-CF3 and having a number average molecular weight of 2000 to 10000 are preferred. Fomblin Z03 available from Augimont Co. may be mentioned as a specific example.

In the magnetic recording medium 10 according to the third embodiment shown in Fig. 3, a plurality of the protuberances 12 are distributed on the nonmagnetic support 14 to form substantially discontinuous islands.

That is, the individual protuberances 12 are substantially discontinuous on the surface of the nonmagnetic support 14 to form a so-called island structure.

Accordingly, where, for example, the protuberances 12 formed on the nonmagnetic support 14 comprise an Al-M alloy, the surface of the nonmagnetic support 14 has areas covered with the islands of the Al-M alloy and uncovered (i.e., exposed) areas. In this case, the distance d between adjacent protuberances preferably satisfies the relationship 0 < d < b. If the distance d exceeds b, the favorable effect of noise reduction would be lessened. A preferred distance d between adjacent protuberances 12 is 2 to 200 nm including 20, 40, 60, 80, 100, 120, 140, 160 and 180 nm and all ranges therebetween. The protuberances 12 can be formed by, for example, PVD or CVD in a manner similar to those of the first and second embodiments.

In the magnetic recording medium 10 according to the fourth embodiment shown in Fig 4, a plurality of the protuberances 12 are distributed on the nonmagnetic support 14 in the planar direction of the support to form substantially discontinuous islands as in the third embodiment. The amorphous layer 24 is provided on the protuberances 12. The first under layer 26 and the second under layer 28 are provided on the amorphous layer 24. The magnetic layer 16 is provided on the second under layer 28, and the protective layer 30 and the lubricant layer 32 are provided on the magnetic layer 16 in this order. The same materials as used in the second embodiment for forming these layers can be used in the fourth embodiment.In this embodiment, it is particularly preferable for further improvement in the squareness ratio that the protuberances 12 comprise Si(O) and the amorphous layer 24 be provided directly on the protuberances 12.

The magnetic recording medium 10 according to the fifth embodiment shown in Fig. 5 is structurally similar to the magnetic recording medium according to the fourth embodiment shown in Fig. 4. The difference resides in that the distance d between adjacent protuberances 12 is larger than that of the embodiment shown in Fig. 4. It goes without saying that d in the fifth embodiment falls within the above-mentioned range with respect to b. Accordingly, in the areas between the protuberances 12, the magnetic support 14 is directly covered with the amorphous layer 24, on which the first under layer 26, the second under layer 28, the magnetic layer 16, the protective layer 30, and the lubricant layer 32 are formed in the order listed. In this embodiment, too, the distance x between adjacent magnetic layers (the distance between adjacent convexed portions of the magnetic layer) falls within a range of 0.2 < x < 30 nm.

The magnetic layer of the magnetic recording medium of the third ta fifth embodiments is preferably an aggregate comprising discontinuous convexed portions corresponding to the protuberances 12 and/or discontinuous flat areas arranged in the planar direction of the nonmagnetic support 14.

While the present invention has been described by referring to its preferred embodiments shown in Figs. 1 through 5, it should be understood that the present invention is not construed as being limited to these embodiments and various changes and modifications can be made therein.

For example, it is possible to omit the first under layer 26. In this case, the second under layer 28, the magnetic layer 16, the protective layer 30, and the lubricant layer 32 are successively formed on the amorphous layer 24 which is directly formed on the protuberances 12. It is also possible to omit the amorphous layer 24, in which case the first under layer 26 is directly formed on the protuberances 12, and the second under layer 28, the magnetic layer 16, the protective layer 30 and the lubricant layer 32 are successively formed thereon. Otherwise, both the amorphous layer 24 and the first under layer 26 may be omitted, in which case the second under layer 28, the magnetic layer 16, the protective layer 30, and the lubricant layer 32 are successively formed on the protuberances 12.While the protective layer is present between the convexed portions of the magnetic layer in the embodiments shown in Figs. 2, 4 and 5, a nonmagnetic material other than the protective layer may be present there.

The present invention will now be illustrated in greater detail with reference to non-limiting Examples.

EXAMPLE 1 A magnetic recording medium having the structure shown in Fig. 2 was prepared as follows.

An amorphous carbon support having a density of 1.5 g/cm3 and a diameter of 2.5" was abraded to have an average surface roughness Ra of 0.5 to 1.0 nm. After precision cleaning, the carbon support was set in a batch processing type sputtering apparatus having 4 cathodes.

A Ti target, an Al target, an Si target, and a C target each having a diameter of 5" and a thickness of 3 mm were put on the cathodes one by one. After setting the carbon support in the sputtering apparatus, the sputtering chamber was evacuated to 3 x 10-7 Torr.

Argon gas was introduced into the sputtering chamber to 5 mTorr, the carbon support was heated by a heater to 200"C, and sputtering was carried out to form the following layers in the order listed.

(1) Ti Layer Material: Ti Bias voltage applied to the support: -100 V Deposition thickness: 50 nm (2) Protuberance-forming Layer Material: AlSi (Si = 10 at%) Bias voltage applied to the support: -100 V Deposition thickness: 30 nm (3) Amorphous Layer Material: C Bias voltage applied to the support: -100 V Deposition thickness: 20 nm After formation of the amorphous layer, the argon gas feed was ceased, and the carbon support was allowed to cool to below 50 C. Then, nitrogen gas was introduced into the chamber, and the carbon support having formed thereon the layers (1) to (3) was taken out.

A Ti target, a Cr target, a CoCr12Pt8B4 (at%) target, and a C target each having a diameter of 5" and a thickness of 3 mm were put on the cathodes of the sputtering apparatus one by one. After setting the carbon support in the sputtering apparatus again, the chamber was evacuated to a degree of vacuum of below 3 x 10 7 Torr. Argon gas was introduced into the chamber to 5 mTorr, the carbon support was heated by a heater to 240"C, and sputtering was carried out to form the following layers in the order listed.

(4) First Under Layer Material: Ti Bias voltage applied to the support: -100 V Deposition thickness: 50 nm (5) Second Under Layer Material: Cr Bias voltage applied to the support: -100 V Deposition thickness: 40 nm (6) Magnetic Layer Material: CoCr12Pt8B4 (at%) Bias voltage applied to the support: -100 V Deposition thickness: 30 nm (7) Protective Layer Material: C Bias voltage applied to support: -100 V Deposition thickness: 15 nm After formation of the protective layer, the argon gas feed was stopped, and the carbon support was allowed to cool to below 50 C. Nitrogen gas was introduced into the chamber, and the carbon support having formed thereon the layers (1) to (7) was taken out.

The tape varnish treatment was conducted and then a lubricant Fomblin AM2001 (produced by Augimont Co.) was applied to the protective layer to a thickness of 2 nm to obtain a magnetic recording medium having the structure shown in Fig. 2.

The resulting magnetic recording medium was evaluated by making measurements as follows. The results obtained are shown in Table 1.

Average Diameter b. Average Rouahness Ra and Maximum Height Rmax of Protuberances: The carbon support having formed thereon layers (1) and (2) (protuberances) was cut to a size of about 8 x 8 mm, and measurements were taken over the area of 10 x 10 iim on the AFM image of the cut piece.

Averae Crvstal Grain Size a and Distance x between Convexed Portions in Magnetic Laver: The average crystal grain size a was obtained from the AFM image and HR-SEM image of the carbon support having formed thereon layers (1) through (6).

The distance x between the convexed portions was obtained by measurement on the TEM image of the cut area of the carbon support having formed thereon layers (1) to (6).

Static Magnetic Characteristics and Noise of Magnetic Recording Medium: The magnetic recording medium was cut to a size of about 8 x 8 mm, and the static magnetic characteristics were measured with a vibrating sample type magnetometer (VSM) at the maximum applied magnetic field of 10 kOe. The noise of the magnetic recording medium was measured by using an inductive head, a record reproduction evaluation system Guzik, an oscilloscope, and a spectrum analyzer.

EXAMPLE 2 The magnetic recording medium obtained in Example 1 was evaluated by making measurements in the same manner as in Example 1 except for increasing the recording density to 100 kfci (the minimum recording bit length: 254 nm). The results obtained are shown in Table 1 below.

EXAMPLE 3 A magnetic recording medium was prepared in the same manner as in Example 1, except for increasing the deposition thickness of AlSilO (at%) for the protuberance-forming layer to 60 nm. The same measurement as in Example 1 was conducted. The results obtained are shown in Table 1.

EXAMPLE 4 A carbon support was placed in a heat treating oven. The oven was evacuated to a degree of vacuum of below 3 x 10-7 Torr, and a mixed gas of argon and oxygen was then introduced therein to 30 mTorr. The inner temperature was raised to 1000"C by means of an infrared heater and maintained at that temperature for 10 minutes. Thereafter, the introduction of the argon oxygen mixed gas was stopped, and the oven temperature was dropped to below 100"C. Nitrogen gas was introduced into the oven, and the carbon support thus having formed thereon integral protuberances was taken out.

Thereafter, the respective layers which correspond to those in Example 1 were successively formed on the protuberances of the carbon support in the same manner as in Example 1 to obtain a magnetic recording medium.

The same measurement as in Example 1 was conducted.

The results obtained are shown in Table 1.

COMPARATIVE EXAMPLE 1 A magnetic recording medium was prepared in the same manner as in Example 1 except that the protuberance-forming layer was not formed. The same measurement as in Example 1 was conducted. The results obtained are shown in Table 1.

COMPARATIVE EXAMPLE 2 The magnetic recording medium obtained in Comparative Example 1 was evaluated in the same manner as in Example 1 except for increasing the recording density to 100 kfci (the minimum recording bit length: 254 nm). The results obtained are shown in Table 1.

COMPARATIVE EXAMPLE 3 A magnetic recording medium was prepared in the same manner as in Example 1, except for increasing the deposition thickness of AlSilO (at%) for the protuberance-forming layer to 100 nm. The same measurement as in Example 1 was conducted. The results obtained are shown in Table 1.

COMPARATIVE EXAMPLE 4 The magnetic recording medium obtained in Comparative Example 3 was evaluated in the same manner as in Example 1 except for increasing the recording density to 100 kfci (the minimum recordin bit length: 254 nm).The results obtained are shown in Table 1. Table 1

Hc Brt a b Ra Rmax Recording x Noise Density (Oe) (Gum) (nm) (nm) (nm) (nm) (kfci) (nm) ( Vrms) 1 2010 245 23.2 172 6.5 76 90 3 1.7 2 2010 245 23.2 172 6.5 76 100 3 2.9 Examples 3 2032 235 24.1 332 16.3 141 100 10 3.4 4 2005 247 22.8 285 12.6 120 100 7 3.2 1 2042 252 23.8 - 1.1 - 90 - 2.0 Comparative 2 2042 252 23.8 - 1.1 - 100 - 4.1 Examples 3 2003 240 23.5 734 25.5 185 90 23 3.1 4 2003 240 23.5 734 25.5 185 100 23 4.8 As is apparent from the results shown in Table 1 above, the magnetic recording media of Examples 1 to 4 according to the present invention show remarkable noise reduction as having a noise level of 2.9 to 3.4 pVrms at a recording density of 100 kfci (minimum recording bit length: 254 nm). To the contrary, it is seen that the noise level of the magnetic recording media having no protuberances (Comparative Examples 1 and 2) and the magnetic recording media whose protuberances have an extremely large average diameter b (Comparative Examples 3 and 4) is as high as 4.1 to 4.8 pVrms at the same recording density.

EXAMPLE 5 A magnetic recording medium having the structure shown in Fig. 4 was prepared as follows.

An amorphous carbon support having a density of 1.5 g/cm3 and a diameter of 2.5" was abraded to have an average surface roughness Ra of 0.5 to 1.0 nm. After precision cleaning, the carbon support was set in an electron beam vacuum evaporation apparatus. To begin with, protuberances were formed according to the following procedure.

Silicon having been degassed in a vacuum arc degasifier was used as an evaporation deposition material. The carbon support was set in the evaporation chamber, the silicon was put on the hearth, and the chamber was evacuated to a degree of vacuum of below 5 x 10'8 Torr. The temperature of the carbon support was set at 100 C. Then, the silicon evaporation deposition material was heated by an electron beam under high vacuum for removal of adsorbed gas with the shutter in the chamber closed so as to prevent deposition of the vapor on the carbon support. After degassing, the chamber was again evacuated to below 5 x 10-8 Torr, a mixed gas of argon and oxygen was introduced into the chamber to 1 x 10-6 Torr, and evaporation deposition of Si was started.The Si deposition thickness (thickness of protuberances) was measured with a corrected film thickness monitor. The deposition conditions are shown below.

(1) Protuberances Material: Si(O) Deposition thickness: 5 nm When the protuberances formed reached a prescribed thickness, the shutter was closed to terminate evaporation deposition: The mixed gas feed was ceased, and the chamber was cooled with water for more than 1 hour. Nitrogen gas was introduced into the chamber, and the carbon support having protuberances was taken out.

Then, a batch processing type sputtering apparatus having 4 cathodes was used. A Ti target, a Cr target, and a CoCr12Pt8B4 (at%) target and a C target each having a diameter of 5" and a thickness of 3 mm were put on the cathodes one by one. After setting the above carbon support with protuberances in the sputtering chamber, the chamber was evacuated to below 3 x 10 7 Torr. Argon gas was introduced into the chamber to 5 mTorr, the carbon support was heated by a heater to 240"C, and sputtering was carried out to form the following layers in the order listed.

(2) Amorphous Layer Material: C Bias voltage applied to the support: -100 V Deposition thickness: 20 nm (3) First Under layer Material: Ti Bias voltage applied to the support: -100 V Deposition thickness: 50 nm (4) Second Under Layer Material: Cr Bias voltage applied to the support: -100 V Deposition thickness: 40 nm (5) Magnetic Layer Material: CoCr12Pt8B4 (at%) Bias voltage applied to the support: -100 V Deposition thickness: 30 nm (6) Protective Layer Material: C Bias voltage applied to the support: -100 V Deposition thickness: 15 nm After formation of the protective layer, the argon gas feed was stopped, and the carbon support was allowed to cool to below 50"C. Nitrogen gas was introduced into the chamber, and the carbon support having formed thereon the layers (1) to (6) was taken out.

The tape varnish treatment was conducted and then a lubricant Fomblin AM2001 (produced by Augimont Co.) was applied to the protective layer (6) to a thickness of 2 nm to obtain a magnetic recording medium having the structure shown in Fig. 4.

The resulting magnetic recording medium was evaluated by making measurements in the same manner as in Example 1. Additionally, the distance d between adjacent protuberances was measured. The results obtained are shown in Table 2 below. For the measurement of the distance d between adjacent protuberances, the carbon support having formed thereon the protuberances was cut to a size of about 8 x 8 mm, and measurement was taken over an area of 10 x 10 Rm of the AFM image of the cut piece.

EXAMPLE 6 The magnetic recording medium obtained in Example 5 was evaluated in the same manner as in Example 5 except for increasing the recording density to 100 kfci (the minimum recording bit length: 254 nm).

The results obtained are shown in Table 2.

EXAMPLE 7 A magnetic recording medium was prepared in the same manner as in Example 5 except for changing the deposition thickness of the Si(O) layer (protuberances) to 10 nm. The same measurement as in Example 5 was conducted. The results obtained are shown in Table 2.

EXAMPLE 8 The magnetic recording medium obtained in Example 7 was evaluated in the same manner as in Example 5 except for increasing the recording density to 100 kfci (the minimum recording bit length: 254 nm).

The results obtained are shown in Table 2.

COMPARATIVE EXAMPLE 5 A magnetic recording medium was prepared in the same manner as in Example 5 except that evaporation deposition of Si for protuberance formation was not carried out. The same measurement as in Example 5 was conducted. The results obtained are shown in Table 2.

COMPARATIVE EXAMPLE 6 The magnetic recording medium obtained in Comparative Example 5 was evaluated in the same manner as in Example 5 except for increasing the recording density to 100 kfci (the minimum recording bit length: 254 nm). The results obtained are shown in Table 2.

COMPARATIVE EXAMPLE 7 A magnetic recording medium was prepared in the same manner as in Example 5 except that the support temperature during formation of protuberances was set at 260"C. The same measurement as in Example 5 was conducted. The results obtained are shown in Table 2.

COMPARATIVE EXAMPLE 8 The magnetic recording medium obtained in Comparative Example 7 was evaluated in the same manner as in Example 5 except for increasing the recording density to 100 kfci (the minimum recording bit length: 254 nm). The results obtained are shown in Table 2. Table 2

Hc Brt a b d Ra Rmax Recording x Noise Density (Oe) (Gum) (nm) (nm) (nm) (nm) (nm) (kfci) (nm) ( Vrms) 5 2024 243 23.2 86 61 4.5 62 90 1 1.5 6 2024 243 23.2 86 61 4.5 62 100 1 2.3 Examples 7 2008 238 24.1 120 23 9.1 96 90 3 1.8 8 2008 238 24.1 120 23 9.1 96 100 3 2.8 5 2042 252 23.8 - - 1.1 - 90 - 2.0 Comparative 6 2042 252 23.8 - - 1.1 - 100 - 4.1 Examples 7 2012 248 23.5 72 135 5.2 70 90 1 2.2 8 2012 248 23.5 72 135 5.2 70 100 1 4.3 As is apparent from the results in Table 2 above, the magnetic recording media of Examples 5 to 8 according to the present invention show remarkable noise reduction as having a noise level of 2.3 to 2.8 pVrms at a recording density of 100 kfci (minimum recording bit length: 254 nm). To the contrary, it is seen that the noise level of the magnetic recording media having no protuberances (Comparative Examples 5 and 6) and the magnetic recording media in which protuberances are arranged at an extremely large distance d (Comparative Examples 7 and 8) is as high as 4.1 to 4.3 pVrms at the same recording density.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims

CLAIMS:

1. A magnetic recording medium comprising a nonmagnetic support having a plurality of protuberances on the surface thereof, and a magnetic layer provided above the nonmagnetic support, the magnetic recording medium being characterized in that: it has a high recording density of 80 kfci or higher; and the average crystal grain size a of the magnet ic layer, the average circle-equivalent diameter b of the protuberances, and the distance x between adjacent magnetic layers satisfy the following relationships.

2 nm S a 5 30 nm 30 nm < b # 400 nm b > 2a 0.2 nm S x # 30 nm

2. The magnetic recording medium according to claim 1, wherein the maximum height of the protuberances Rmax is 10 to 150 nm.

3. The magnetic recording medium according to claim 1, wherein the protuberances comprises a thin film of a metal or an alloy.

4. The magnetic recording medium according to claim 1, wherein the plurality of protuberances are continuous in the planar direction of the nonmagnetic support.

5. The magnetic recording medium according to claim 4, wherein the protuberances comprise an Al-M alloy wherein M is a metal capable of forming a carbide.

6. The magnetic recording medium according to claim 1, wherein the plurality of protuberances are distributed to form substantially discontinuous islands.

7. The magnetic recording medium according to claim 6, wherein the distance d between adjacent protuberances satisfies the relationship of 0 < d S b.

8. The magnetic recording medium according to claim 1, wherein the protuberances are formed integrally with the nonmagnetic support.

9. The magnetic recording medium according to claim 1, wherein the protuberances are independently formed either directly on the nonmagnetic support or indirectly on the nonmagnetic support via one or more layers.