GB2179486A

GB2179486A - Magnetic recording medium

Info

Publication number: GB2179486A
Application number: GB08619797A
Authority: GB
Inventors: Joichiro Ezaki; Yoshiaki Ito; Haruyuki Morita
Original assignee: TDK Corp
Current assignee: TDK Corp
Priority date: 1985-08-22
Filing date: 1986-08-14
Publication date: 1987-03-04
Also published as: GB2179486B; FR2586496A1; GB8619797D0; JPS6243819A; DE3628225A1

Abstract

A magnetic recording medium comprises a glass substrate polished to a roughness not greater than 100 ANGSTROM and preferably less than 50 ANGSTROM . A magnetic thin film containing iron oxide as a main component is formed on the polished surface of the glass substrate. With the magnetic recording medium preferably in the form of a magnetic disk, the floating height of the magnetic head of a magnetic disk apparatus can be reduced while ensuring a stabilized state thereof, whereby a high density recording can be accomplished in a satisfactory manner.

Description

SPECIFICATION Magnetic recording medium The present invention in general relates to a magnetic recording medium. More particularly, the invention concerns a magnetic recording medium imparted with an improved surface property (e.g. smoothness or flatness) so as to be used advantageously in a magnetic disk storage apparatus.

In accompaniment to the tendency of magnetic disk storage apparatus being developed with more and more increased storage capacity, a sputtered type magnetic disk (i.e. a disk having a magnetic recording medium film formed through sputtering technique) attracts attention because of possibility of recording at an increased density when compared with a coated type magnetic disk (i.e. a disk having a magnetic recording film formed through a coating technique). In the case of the coated type magnetic disk, the magnetic recording thin film has a relatively greater thickness on the order of 1 to 2 Am. Accordingly, the surface property of the magnetic recording film of the coated type magnetic disk is substantially immune to the influence of the surface property of the substrate carrying the magnetic film.In contrast, the magnetic thin film or layer of the sputtered type magnetic disk is as thin as on the order of 0.5 am or less. Thus, the surface property or smoothness of the magnetic thin film or layer of the sputtered type magnetic disk is extremely susceptible to the influence of the surface property of the substrate.

Under the circumstance, various attempts have been made to improve the surface property of the magnetic thin film of the sputtered type magnetic disk by using a substrate which is excellent in respect to the surface property such as smoothness or flatness. With the structure of the sputtered type magnetic disk thus realized, the floating height of magnetic head (i.e. gap between the head and the disk surface) can be reduced, which in turn means that the recording density can be correspondingly increased.

In a typical sputtered type magnetic disk, the substrate is formed of an aluminium alloy plate metallized with a Ni-P layer in thickness of ca. 50 am and having the surface polished. In another example of the sputtered type magnetic disk, the substrate is formed of an aluminium alloy plate having a surface deposited with a hardened alumite layer of ca. 2 am in thickness through anodization, the alumite surface being subsequently polished. In the substrates thus prepared, the surface roughness (Rmax) on the order of 0.15 ,um can be attained.

When a magnetic thin film such as, for example, of a composition containing an alloy Co-Ni as a main component, a layer of Cr is first formed on the substrate in a thickness of ca. 500 A through sputtering, which is followed by deposition of a magnetic thin film containing an Co-Ni alloy as a main component in a thickness of ca. 1000 A. Finally, a protecting lubrication film of C or the like is formed thereon in a thickness of ca. 200 A. The surface roughness (Rmax) of the magnetic medium film realized in this way is on the order of 0. 1 5 am, reflecting the surface property of the substrate.In case the magnetic thin film containing iron oxide as a main component is to be formed, a target containing Fe as a main component is sputtered in the atmosphere of A+02 to form on the substrate a sputtered film containing a-Fe2O3 as a main component in a thickness of ca. 2000 A. The resulting product is heated to a temperature of ca. 300 CC in a reducing atmosphere to convert the film containing a-Fe203 as a main component to a film containing Fe3O4 as a main component. The latter is again heated at a temperature of 300 C in an oxidizing atmosphere to be converted to a film containing a-Fe 202 as a main component. Finally, a protective lubrication film is formed to realize the magnetic medium thin film.The surface roughness of the magnetic film is also on the order of 0.15 am, reflecting the surface property of the substrate.

The hitherto known magnetic disks described above suffer various drawbacks. In the case of the substrate which is formed of the aluminum alloy plate metalized with the Ni-P film and having the surface polished, the surface of the aluminum alloy plate has to undergo an activation treatment in precedence to the metallization with Ni-P, making thus the substrate fabrication process much complicated. Besides, the process succeeding to the activation treatment involves more than a half of the cost for manufacturing the substrate, which is thus rendered very expensive. It is further noted that when the Ni-P metallization layer is heated to a temperature of 150 0C or higher, the Ni-P layer is crystallized to exhibit magnetic property.Consequently, this type substrate can not be used when a heating treatment is required as with the case of forming a magnetic iron oxide film.

On the other hand, the substrate formed of aluminum alloy and coated with the alumite film suffers a drawback that cracks are likely to be produced upon heat treatment because of difference in the coefficient of thermal expansion between the aluminum alloy and the alumite film. Under the circumstance, the heating temperature employed in forming the magnetic iron oxide film is limited to ca. 300 CC. Additionally, the alumite film is of a porous structure having a large number of pores. Consequently, when the magnetic thin film layer is formed on this substrate, magnetic defects are likely to occur at the the locations of these pores, to another disadvantage. Finally, the surface property as realized is not to be satisfied in view of the fact that the roughness (Rmax) is on the order of 0.15 Am.

An object of the present invention is to eliminate the drawbacks of the prior art and provide a magnetic recording medium exhibiting an improved surface property.

In view of the above and other objects which will become more apparent as description proceeds, it is proposed according to a general aspect of the present invention that a magnetic thin film for magnetic recording is formed on a glass substrate of which surface is finished through an ultra-high precision processing so that the surface roughness (Rmax) can not be greater than 100 A.

In a preferred embodiment of the present invention, the magnetic recording medium can be further improved by forming the magnetic thin film for the magnetic recording on a glass substrate undergone an ultra-high precision surface treatment so that the surface of the glass substrate has a surface roughness (Rmax) which does not exceed 100 A (more preferably 50 A). At least a portion of the surface of the substrate may preferably be reinforced.

The magnetic recording medium which includes the glass substrate undergone the ultra-high precision surface treatment and preferably having the surface reinforced at least partially, as stated above, can be advantageously employed in the fabrication of magnetic disks and others for attaining the magnetic recording at an increased recording density with an enhanced fidelity.

In applications where the magnetic recording medium is transported or driven at a high speed, the reinforced glass substrate may be advantageously employed in the fabrication of the magnetic recording medium such as a magnetic disk or the like. The surface finishing treatment to realize the desired surface property such as surface smoothness can be accomplished simply through polishing, which can ensure a facilitated and inexpensive fabrication of the glass substrate and hence the magnetic recording medium. Further, it is noted that upon forming of the magnetic thin film on the glass substrate, there arises no need for forming an underlying film or layer.In other words, the magnetic layer of a material of oxide series can be directly formed on the glass substrate without giving rise to any problems such as occurrence of cracks as with the case of the alumite film, notwithstanding of heat treatment of the glass substrate. Other numerous excellent effects of the magnetic recording medium according to the invention will be apparent as the description proceeds.

The above and other objects, features and advantages of the present invention will be fully understood upon reading the description which follows. The description makes reference to the accompanying drawings.

Figure 1 is a view for graphically illustrating distribution of stresses resulting from surface reinforcement of a glass substrate; and Figures 2a to Fig. 2f are sectional views showing schematically glass substrates whose surfaces are reinforced at least partially, wherein reinforced regions are indicated by hatched areas, respectively.

In the first place, it should be mentioned that the glass substrate having a surface undergone an ultra-high precision surface treatment so that the surface roughness does not exceed 100 A and more preferably 50 A and reinforced at least partially has never been employed in a magnetic recording medium such as magnetic disk or the like destined to be used in dynamic magnetic recordings. This may be explained by a fear of insecurity against mechanical shocks or by the fact that no suitable means for accomplishing the ultra-high precision polishing is available. In any case, there have heretofore been known no approaches or proposals as to the use of a glass substrate as the substrate for the dynamic recording medium.

After a series of intensive studies and experiments in an effort to find the possibility of finishing the surfaces of various substrates with an ultra-high precision such that the surface roughness (Rmax) is less than 100 A, the inventors of the present application have discovered that the aimed surface property can be accomplished through mechanochemical polishing of a glass substrate. More specifically, the surface property having a roughness of less than 100 A can be realized by polishing a glass substrate by using a polishing liquid containing colloidal silica by making use of the polishing action thereof ascribable to chemical reaction with the glass plate in addition to the mechanical polishing action.It has been surprisingly found that the glass substrate having a surface finished with ultra-high precision such that the surface roughness (Rmax) is less than 100 A can be obtained without difficulty through the polishing mentioned above. When a magnetic thin film is formed on the glass substrate thus prepared, the magnetic recording medium is also imparted with an extremely smooth surface, reflecting the surface precision of the glass substrate.

In general, in a magnetic disk apparatus, the recording density can be effectively increased by decreasing the floating height or level of a magnetic head. In this connection, it will be readily understood that the magnetic disk having poor surface property employed in a decreased floating height of the magnetic head will result in the chipping-off of the magnetic medium material and/or destruction of the magnetic head due to contact with protrusions possibily present in the surface of the magnetic recording medium. In other words, the surface property such as smoothness, flatness or the like is an important factor for realizing the stabilized floating state of the magnetic head as well as the reduction of the floating height thereof.

Since the magnetic recording medium according to the invention is composed of a magnetic recording thin film formed on the surface of a substrate finished with ultra-high precision such that the surface roughness (Rmax) is not greater than 100 A, it is possible to make the magnetic head float in a stable state at a reduced height of 0. 1 to 0.2 um. Further, when the magnetic thin film for the magnetic recording is formed on the glass substrate having the surface finished so that the roughness (Rmax) thereof is not greater than 50 A, the floating height or level of the magnetic head (i.e. the gap between the substrate surface and the magnetic head) can be reduced to 0.1 ,um or less, to a great advantage.

The materials which can be used for the glass substrate in carrying out the present invention may be substantially any of conventional glass materials including borosilicate glass, aluminosilicate glass, quartz glass, titanium silicate glass and others. It is however preferred that the glass material includes no crystalline components. When the glass material including crystalline component is used, the grain boundary portion will be polished at a higher rate because of a relatively feeble strength of the grain boundary, making it difficult or impossible to finish the substrate surface with the desired ultra-high precision.

In the case of the substrate composed of an aluminum alloy plate metalized with the layer of Ni-P or alumite layer, restriction is imposed to the heating temperature, as described hereinbefore. In contrast, the glass substrate according to the teaching of the invention can be heated up to ca. 400so and thus can be used advantageously particularly when the heat treatment is required as in the case of forming the magnetic film of iron oxide.

In view of the fact that the magnetic disk is usually rotated at a high speed of 3600 rpm in the magnetic disk apparatus, the magnetic disk is required to have a sufficient strength to withstand the rotation. Although the use of the glass substrate in the magnetic disk intended to such application has presented a problem in respect to the strength, the inventors of the present application have discovered in the course of experimentation and examination that the glass substrate can be imparted with adequate strength to be used as the substrate of the magnetic disk by reinforcing at least a part of the surface of the glass substrate.

In general, the reinforcing of the surface portion of the glass substrate can be accomplished by replacing ions in the surface region of the glass substrate by ions of large size at a temperature lower than the glass transition temperature. As a method of the ion substitution, the glass substrate material may be immersed in a molten potassium nitrate pool heated at a temperature of 450 UC. Through the ion substitution treatment, compressive stress is distributed remarkably in the surface region, as illustrated in Fig. 1, whereby the surface region of the glass substrate is correspondingly reinforced. The thickness of the layer or region in which the compressive stress is produced can be regulated to 150 um to 200,um by controlling the ion substitution temperature and the duration of immersion in the pool of molten potassium nitrate.

The surface region to be reinforced in a magnetic disk, for example, may be an inner edge portion, an inner peripheral portion, an outer edge portion, an outer peripheral portion, combinations thereof or the whole surface portion.

In the following, the invention will be described in more detail in conjunction with examples thereof.

Example 1 As a material of the glass substrate, a plate of aluminosilicate glass was used which was of a disk-like configuration having an outer diameter of 130 mm, an inner diameter of 40 mm and a thickness of 1.9 mm. The glass plate was polished by using a polishing liquid containing colloidal silica for ten minutes through mechanochemical polishing. The surface roughness (Rmax) as attained was go A. Formed on the glass plate or substrate was an a-FeO2 film of 2000 A in thickness by sputtering a target of iron in the atmosphere of ArQO (mixed at a ratio of 50 Y0 and under a vacuum of 5X10 3 Torr).The resulting product was reduced in a flow of hydrogen gas at a temperature of 360 CC for 2 hours to form a film of FQO# which was then oxidized in the air at a temperature of 310 CC to be converted to a a-Fe,O4. Thus, a magnetic disk was realized which has a surface roughness (Rmax) of 100 A.

The finished magnetic disk was placed in a disk apparatus to examine the floating stability thereof. To this end, the floating height of a magnetic head was decreased by reducing the peripheral speed of the magnetic disk. It was found that the magnetic disk can be maintained in a stabilized floating state even at the floating level of 0. 1 5 jtm.

Example 2 A glass plate of the same material as in the case of the Example 1 was polished through the mechanochemical polishing for 20 minutes to fabricate a substrate whose surface roughness was 40 A. Formed on the substrate was an a-Fe,O, film through same process mentioned above in conjunction with the Example 1 to realize a magnetic disk whose surface roughness was 45 A.

The magnetic disk thus prepared was examined as to the floating stability of the magnetic head through the same procedure as with the case of the Example 1. It was found that the magnetic head can be stably supported in the floating state down to a height or level of 0.08 ,um.

Example 3 As a material for the glass substrate, an aluminosilicate glass plate was used which was of a disk-like configuration having an outer diameter of 305 mm, an inner diameter of 40 um and a thickness of 1.9 mm. The glass plate was subjected to surface reinforcing treatment in the manners illustrated in Figs. 2a to 2f. To this end, the portion which is not to be reinforced was covered with a mask and the substrate was immersed in molten potassium nitrate at a temperature of 450 0C for ten minutes. The reinforced substrates and a non-reinforced substrate for the control were polished by using a polishing liquid containing colloidal silica for ten minutes. All the substrates exhibited substantially same surface roughness (Rmax) of 90 A.

These substrates were subjected to rotating fracture test in which each substrate was rotated at a high speed for evaluating the strength of the substrate with a rotational speed at which the fracture occurred. The maximum rotational number amounted to 20000 rpm. The test was performed on 20 specimens for each substrate. The results of the test were listed in the Table 1. As will be seen in the table, the strength of the substrate reinforced only partially according to the teaching of the invention as in the case of the examples illustrated in Figs. 2a to 2c is 1.5 times as high as that of the non-reinforced substrate. Accordingly, in actual applications where the rotational speed is usually 3600 rpm, there arises no problem as to the strength of the substrate and hence the magnetic disk.In contrast, in the case of the non-reinforced substrate specimens, some were found to be fractured at the rotational speed on the order of 5000 rpm, giving rise to a problem when employed in actual applications. In the case of the substrate specimens reinforced in the manners illustrated in Figs. 2d and 2f, no destruction or fracture occurred even at the maximum rotational speed of 20000 rpm, assuring thus a satisfactory strength of the disk.

Each of the substrates prepared according to the invention and illustrated in Figs. 2a to 2f was formed with an a- FQO3 film in a thickness of 2000 A through sputtering of a target of iron in the atmosphere of Ar+02 (mixed at a ratio of 50% and under vacuum of 5X10 3 Torr).

The product was next reduced in a gas flow of hydrogen at a temperature of 360 0C for 2 hours to form a film of Fe2O4 which was subsequently oxidized in the air at a temperature of 310 0C for 3 hours to form a a-Fe203. The magnetic disks were thus completed. All of these magnetic disks exhibited substantially same surface roughness on the order of 100 A.

These magnetic disks were placed in a magnetic disk apparatus to examine the floating stability of the magnetic head. To this end, the peripheral speed of the magnetic disk was decreased to thereby lower the floating level of the magnetic head. It was found that the magnetic head can be supported in the stabilized floating state even when the floating height was decreased down to 0.15 um.

TABLE 1

1Nonreinforced I Reinforced Disk Disk a musk Disk a| Disk | Disk c | Disk d | Disk e | Disk f I I I I Average Fracture 1 1 I I *11 *11 *1 Rotational Speed 1 9865 1 157251 149301 178751 200001 200001 20000 Minimum Fracture I I I *11 *11 *1 Rotational Speed I 5630 1 132801 125201 149001 200001 200001 20000 I I 1 1568 947 9271 13281 Notes: Rotational speed in rpm.

In the case identified by *1, no fracture occurred.

20 specimens were employed for each of the disks.

Example 4 As with the case of the example 3, the surface reinforced glass plates a, b, c, d and f illustrated in Figs. 2a to 2f, respectively, were subjected to the mechanochemical polishing for 20 minutes to form the substrates. All of the substrates thus prepared were substantially of a same surface roughness on the order of 40 A.

The a-Fe203 film was formed on each of these substrates through the same process as in the case of the example 1 to complete the magnetic disks all of which exhibited a substantially same surface roughness on the order of 45 A.

These magnetic disks were examined as to the floating characteristic of the magnetic head in the same manner as in the case of the example 1. It has been ascertained that the magnetic head is supported in the stabilized floating state at as low a level as 0.08 ,um above the disk.

As will now be appreciated from the foregoing description, the magnetic recording medium according to the present invention which can enjoy an excellent surface smoothies$ allows the floating height (gap between the disk surface and the magnetic head) to be reduced significantly while ensuring an increased recording density.

Although the invention has been described in conjunction with what is presently believed preferable, it will be understood that many modifications, variations and adaptations. may occur to those skilled in the art without departing from the spirit and scope of the invention. Accordingly, the invention is never restricted exactly to the disclosure made herein.

Claims

1. A magnetic recording medium, comprising a glass substrate having a surface roughness not greater than 100 A on which a magnetic thin film for magnetic recording is formed.

2. A magnetic recording medium according to claim 1, wherein said glass substrate has a surface roughness not greater than 50 A.

3. A magnetic recording medium according to claim 1 or 2, wherein said magnetic thin film contains iron oxide as a main component.

4. A magnetic recording medium according to any of claims 1 to 3, wherein said glass substrate is formed of a material selected from a group consisting of borosilicate glass, aluminosilicate glass, quartz glass and titanium silicate glass.

5. A magnetic recording medium, comprising a glass substrate having a surface reinforced at least partially on which a magnetic thin film for magnetic recording is formed.

6. A magnetic recording medium according to claim 5, wherein said glass substrate has a surface roughness not greater than 100 A.

7. A magnetic recording medium according to claim 5 or 6, wherein said magnetic thin film contains iron oxide as a main component.

8. A method of manufacturing a magnetic recording medium which comprises a glass substrate having a surface roughness not greater than 100 A on which a magnetic thin film for magnetic recording is formed, including steps of: polishing the surface of said glass substrate to said surface roughness through a mechanochemical polishing process; and forming said magnetic thin film on said polished surface of said magnetic recording medium through a sputtering process.

9. A method according to claim 8, further comprising the step of reinforcing at least partially the surface of said substrate by immersing the substrate in molten potassium nitrate.

10. A magnetic recording medium substantially as hereinbefore described with reference to the accompanying drawings.

11. A method of manufacturing a magnetic recording medium substantially as hereinbefore described with reference to the accompanying drawings.

12. Any novel subject matter or combination including novel subject matter herein disclosed in the foregoing Specification or Claims and/or shown in the drawings, whether or not within the scope of or relating to the same invention as any of the preceding claims.