A longitudinal magnetic recording medium

Abstract: It is disclosed that a longitudinal magnetic recording medium and apparatus has recording sections having longitudinal magnetic films (3) magnetically isolated in a recording direction. The magnetically isolated recording sections can record a single bit each so that the longitudinal magnetic recording medium can remove the irregular domain boundary of its own caused on boundaries of magnetization reversals in the recording direction. This can improve the signal-to-media noise ratio to a great extent in writing and reading.

Description: A LONGITUDINAL MAGNETIC RECORDING MEDIUM AND APPARATUS Background of the Invention The present invention relates to a longitudinal magnetic recording medium and apparatus and more particularly concerns a longitudinal magnetic recording medium and apparatus preferable for high density recording at a high signal-to-noise ratio. The longitudinal magnetic recording medium having a longitudinal magnetic film, unlike perpendicular magnetic recording media, involves a problem of high media noise of its own. Longitudinal magnetic recording medium has an irregular domain boundary caused by a strong demagnetizing field produced at a boundary of magnetization reversals in a recording direction. It is known that the irregular domain boundary is a primary cause of the media noise in a read-out operation as reported in the J. Appl. Phys., 53, (1982) pp2576-2578. It has been proposed to reduce the media noise that grain of the magnetic film should be made fine to improve magnetic anisotropy to increase coercive force as disclosed in the Japanese Patent Laid-Open 61-217925. It has also been attempted to do it that grain boundary of the magnetic film should be made clear with anneal as in disclosed in the Japanese Patent Laid-Open 62-117143. Summary of the Invention However, the method that the coercive force of the longitudinal magnetic film should be made high has the disadvantage that the irregular domain boundary cannot be removed substantially as the magnitude of the coercive force is limited by a material of the longitudinal magnetic film. If the coercive force of the longitudinal magnetic film is greater than 1 59 154 A/m (2 kOe), the magnetic field strength of a magnetic head is so low for the coercive force that the medium cannot be recorded saturately. It therefore involves a practical problem that the coercive force is limited in connection with the recording performance of the magnetic head. On the other hand, the method that the grain boundary of the magnetic film should be made clear has the disadvantage that magnetic isolation is insufficient as the magnetic grains are close. The irregular domain boundary cannot be theoretically made smaller than the size of the grain. US-A-4 802 050 discloses a method of making the magnetic isolation in a recording direction of a longitudinal magnetic film track and between tracks by non-magnetic layers. Patent Abstracts of Japan, vol. 7, no. 64 (P.183) (1209), 17.03.83 & JP-A-57 208627 discloses strip-shaped magnetic films with air isolation therebetween formed on a tape type plastic substrate. Proceedings VLSI and computer peripherals, May 1989, pages 1-12 to 1-15 describes a magnetic isolation only between adjacent tracks of magnetic discs. An object of the present invention is to provide a longitudinal magnetic recording medium in a way that the irregular domain boundary in a recording direction can be removed completely to reduce the media noise to a level as low as dc-erased noise, thereby improving the media signal-to-noise ratio to a great extent and achieving greater areal recording density than 500 Mb/6,45 cm² (/in²). The foregoing object is accomplished in accordance with aspects of the present invention by a longitudinal magnetic recording medium as claimed in claim 1. Also, it is desirable that the longitudinal magnetic recording medium has a plurality of concentric recording tracks, and the recording tracks should be magnetically isolated therebetween by air to remove an irregular domain boundary on an edge of the recording track. Further, it is desirable that wherein a plane shape of the recording section partitioned by isolating in the recording direction and also between recording tracks in the magnetic film should be a rectangular with its recording direction being shorter than a direction perpendicular to the recording direction. Further more, for the longitudinal magnetic film can be used a multilayered film such as a composite film having longitudinal magnetic films laminated on a perpendicular magnetic film. Moreover, the magnetic isolation areas of the longitudinal magnetic film can be formed in a way that the grooves are formed on a surface of nonmagnetic substrate so that they have ruggedness, and such nonmagnetic substrate having ruggedness can have the longitudinal magnetic film formed directly or through an underlayer thereon. Thus, the portion corresponding to the grooves can become the magnetic isolation areas. The protrusion or hill-like portion of the nonmagnetic substrate can have a longitudinal magnetic film having a coercive force not lower than 79 577 A/m (1 kOe) formed thereon, and the grooved portion thereof has a longitudinal magnetic film having a coercive force lower than 79 577 A/m (1 kOe) formed thereon. This allows recording a main signal on the protrusion or hill-like portion and a tracking signal and a servo signal on the grooved portion. A longitudinal magnetic recording apparatus can be constructed with use of the longitudinal magnetic recording medium described above. For the longitudinal magnetic recording film forming method for the above described longitudinal magnetic recording medium may be used a sputtering process, a vacuum evaporating process, or a plating process. Fig. 1 is an illustrative view for a magnetic disk made in a way that it has periodical grooves formed in a radial and circumferential direction to form recording section (hereinafter referred to as the magnetic film domains) isolated magnetically. The grooves can be formed by way of a lithographic technique using electron rays or X rays. A width of the groove or nonmagnetic area isolating adjacent magnetic film domains is a few nanometers to 5 µm, or preferably 30 nm to 1 µm. The figure shows the surface shape on only a single side of the magnetic disk, but similar surface shape may be formed on both sides. For the nonmagnetic substrate is available reinforced glass substrate, Ni-P plated aluminum substrate, or ceramic substrate. For the underlayer is available chrome alloy film such as Cr, Cr-Si, or Cr-Ti or Ge film. For the longitudinal magnetic film is available Co, Fe, or Ni base alloy. It may have a protective layer such as C, B, SiNx, or SiOx formed thereon. In order to improve the micro-structure an magnetic characteristics of the magnetic film, an underlayer can be provided, to control crystalline orientation of magnetic film, between the magnetic film and the nonmagnetic substrate. Further, the magnetic disk can have a spiral groove formed from its center to the circumference and have periodical grooves made in the recording direction to form magnetic film domains. In the longitudinal magnetic recording medium using the longitudinal magnetic film, the longitudinal magnetic film is magnetically isolated in the recording direction by the grooves, and the magnetically isolated recording sections have a single bit recorded as assigned each. This can magnetically isolate the boundary of magnetization reversals in the recording direction. It therefore can remove the irregular domain boundary caused on boundaries of magnetization reversals in the recording direction of the magnetic recording medium using the longitudinal magnetic film of its own. The removal of the irregular domain boundary which is a principal cause of the media noise can decrease the media noise to a level as low as the dc-erased noise so that the write/read signal-to-media noise ratio can be improved to a great extent. In addition, as discussed in the IEEE, Trans. Magnetic, MAG23, (1987) pp3690-3692, a plurality of concentric grooves can be formed on the magnetic disk so that the so-called discrete track which isolates the neighboring recording tracks can be formed to remove the irregular domain boundaries at edges of the recording tracks. As this irregular domain boundary also is thought as a cause of the media noise, removal of it can further improve the signal-to-media noise ratio in the writing and reading modes of operation. In the magnetic film domain formed by the magnetic isolation in the recording direction and the one between the recording tracks, the width of the magnetically isolated grooves can be made narrower and at the same time at least one of the lengths in the recording direction and track direction of the magnetic film domains can be made shorter so that the areal recording density can be controlled to increase. Brief Description of the Drawings Fig. 1 is an illustrative view for a magnetic disk in an embodiment of the present invention; Fig. 2 is a cross-sectional view of magnetic disks in embodiments 1 and 4 according to the present invention; Figs. 3 and 4 are cross-sectional views of magnetic disks in embodiments 2 and 3 according to the present invention; and Fig. 5 is a schematic view illustrative of a substrate surface of a magnetic disk in embodiment 5 according to the present invention. Detailed Description of the Preferred Embodiments [Embodiment 1] Fig. 2 is a cross-sectional structure in the vicinity of a surface of a longitudinal magnetic recording medium of an embodiment according to the present invention. The following describes the embodiment with reference to the figure. First, a substrate 1 of reinforced glass disk of 130 mm diameter and 1.5 mm thick has concentric grooves of 400 nm deep and 1.0 µm wide at intervals of 5.0 µm formed in its circumferential directions on each of surfaces thereof and grooves of 400 nm deep and 0.2 µm wide at intervals of 0.5 µm formed in its radial directions thereon by way of the photolithographic technique. Each of the surfaces, in turn, has a nonmagnetic underlayer 2, a magnetic film 3, and a protective layer 4 formed in a vacuum chamber using a dc magnetron sputtering system. The nonmagnetic underlayer 2 is formed of Cr film of 250 nm thick in a sputtering condition that substrate temperature is 200 °C, argon gas pressure is 0.7 Pa, and applied power density is 50 kW/m². The magnetic film 3 is formed of Co-10at%Cr-12at%Pt of 100 nm thick having an axis of easy magnetization in a longitudinal direction of the substrate 1 in the same condition. The protective layer 4 is formed of carbon of 20 nm thick in a sputtering condition that the substrate temperature is 100 °C, the argon gas pressure is 1.3 Pa, and the applied power density is 30 kW/m². This completes forming the longitudinal magnetic recording medium, or magnetic disk. [Reference Example 1] An alternative magnetic disk is formed in the same construction and the same sputtering conditions as in the embodiment 1 described above except that the reinforced glass disk of 130 mm diameter and 1.5 mm thick has no grooves formed in the radial directions on each of the surfaces. [Reference Example 2] Another alternative magnetic disk is formed in the same construction and the same sputtering conditions as in the embodiment 1 described above except that the reinforced glass disk of 130 mm diameter and 1.5 mm thick has no grooves formed in both the circumferential and radial directions on each of the surfaces. Table 1 shows results of evaluation of write/read characteristics of the magnetic disks obtained in the embodiment 1 and the reference examples 1 and 2 where spacing of a thin film head to the surface of the magnetic film is 0.22 µm. As can be clearly seen from the results, the embodiment 1 having the grooves formed in the both circumferential and radial directions on the reinforced glass substrate for the magnetic disk is improved in signal-to-medium noise ratio to a great extent as compared with the reference example 1 having the grooves formed in the circular directions alone and the reference example 2 having no grooves at all. [Embodiment 2] Fig. 3 is a cross-sectional structure in the vicinity of a surface of a longitudinal magnetic recording medium of another embodiment according to the present invention. As shown in the figure, a magnetic disk substrate 1 formed of reinforced glass of 95 mm diameter and 1.2 mm thick has a nonmagnetic underlayer 2, a magnetic film 3, and a protective layer 4 formed on each of surfaces thereof in a vacuum chamber using a dc magnetron sputtering system. The nonmagnetic underlayer 2 is formed of Cr film of 250 nm thick in a sputtering condition that substrate temperature is 200 °C, argon gas pressure is 0.7 Pa, and applied power density is 50 kW/m². The magnetic film 3 is formed of Co-10at%Cr-12at%Pt of 100 nm thick having an axis of easy magnetization in a longitudinal direction of the substrate 1 in the same condition. The protective layer 4 is formed of carbon of 20 nm thick in a sputtering condition that the substrate temperature is 100 °C, the argon gas pressure is 1.3 Pa, and the applied power density is 30 kW/m². Then, each of the surfaces has concentric grooves of 150 nm deep and 1.0 µm wide at intervals of 5.0 µm formed in its circumferential directions and grooves of 150 nm deep and 0.2 µm wide at intervals of 0.5 µm formed in its radial directions thereon by way of the photolithographic technique. This completes forming the longitudinal magnetic recording medium, or magnetic disk. [Reference Example 3] An alternative magnetic disk substrate 1 formed of reinforced glass of 95 mm diameter and 1.2 mm thick has a nonmagnetic underlayer 2, a magnetic film 3, and a protective layer 4 formed in a vacuum chamber using a dc magnetron sputtering system on both side thereof, and is formed in the same construction and the same sputtering conditions as in the embodiment 2 described above except that it has no grooves formed in the radial directions on both the surfaces. Table 1 shows results of evaluation of write/read characteristics of the magnetic disk obtained in the embodiment 2 and the reference example 3. As can be clearly seen from the results, the embodiment 2 having the grooves formed in the both circumferential and radial directions on the reinforced glass substrate for the magnetic disk is improved in signal-to-medium noise ratio to a great extent as compared with the reference example 3 having the grooves formed in the circumferential directions alone. [Embodiment 3] Fig. 4 is a cross-sectional structure in the vicinity of a surface of a longitudinal magnetic recording medium of another embodiment according to the present invention. As shown in the figure, a magnetic disk substrate 1 formed of reinforced glass of 95 mm diameter and 1.2 mm thick has a nonmagnetic underlayer 2 and a magnetic film 3 formed on each of surfaces thereof in a vacuum chamber using a dc magnetron sputtering system. The nonmagnetic underlayer 2 is formed of Cr film of 300 nm thick in a sputtering condition that substrate temperature is 200 °C, argon gas pressure is 0.7 Pa, and applied power density is 50 kW/m². The magnetic film 3 is formed of Co-12at%Cr-10at%Pt of 100 nm thick having an axis of easy magnetization in a longitudinal direction of the substrate 1 in the same condition. Then, each of the surfaces has concentric grooves of 150 nm deep and 1.0 µm wide at intervals of 5.0 µm formed in its circumferential directions and grooves of 150 nm deep and 0.2 µm wide at intervals of 0.5 µm formed in its radial directions thereon by way of the photolithographic technique. Further, each of the surfaces has a protective layer 4 is formed of carbon of 20 nm thick in a sputtering condition that the substrate temperature is 100 °C, the argon gas pressure is 1.3 Pa, and the applied power density is 30 kW/m², using the dc magnetron sputtering system. This completes forming the longitudinal magnetic recording medium, or magnetic disk. [Reference Example 4] An alternative magnetic disk is formed in the same construction and the same sputtering conditions as in the embodiment 3 described above except that the reinforced glass substrate 1 of 95 mm diameter and 1.2 mm thick has no grooves formed in both the circumferential and the radial directions on each of the surfaces. Table 1 shows results of evaluation of write/read characteristics of the magnetic disk obtained in the embodiment 3 and the reference example 4. As can be clearly seen from the results, the embodiment 3 having the grooves formed in the both circumferential and radial directions on the reinforced glass substrate for the magnetic disk is improved in signal-to-medium noise ratio to a great extent as compared with the reference example 4 having no grooves at all. Also, the magnetic disk in the embodiment 3 has higher corrosion resistance than the one in the embodiment 2 as walls of the grooves formed on the magnetic film are covered with the carbon. [Embodiment 4] In Fig. 2, a substrate 1 of reinforced glass disk of 95 mm diameter and 1.2 mm thick has a spiral groove of 200 nm deep and 1.0 µm wide at intervals of 5.0 µm formed from a center thereof to a circumference thereof on each of surfaces thereof and grooves of 200 nm deep and 0.2 µm wide at intervals of 0.5 µm formed in its radial directions thereon by way of the photolithographic technique. Each of the surfaces, in turn, has a nonmagnetic underlayer 2, a magnetic film 3, and a protective layer 4 formed in a vacuum chamber using a dc magnetron sputtering system. The nonmagnetic underlayer 2 is formed of Cr film of 250 nm thick in a sputtering condition that substrate temperature is 100 °C, argon gas pressure is 0.7 Pa, and applied power density is 50 kW/m². The magnetic film 3 is formed of Co-10at%Cr-12at%Pt of 100 nm thick having an axis of easy magnetization in a longitudinal direction of the substrate 1 in the same condition. The protective layer 4 is formed of carbon of 20 nm thick in a sputtering condition that the substrate temperature is 100 °C, the argon gas pressure is 1.3 Pa, and the applied power density is 30 kW/m². This completes forming the longitudinal magnetic recording medium, or magnetic disk. Results of evaluation of write/read characteristics of the magnetic disk obtained in the embodiment, as shown in Table 1, are high in the signal-to-medium noise ratio. [Embodiment 5] As shown in Fig. 5, a substrate 1 of reinforced glass disk of 95 mm diameter and 1.2 mm thick has a grooves of 250 nm deep and 1.0 µm wide at intervals of 5.0 µm formed in circumferential directions on each of surfaces thereof and grooves of 250 nm deep in radial directions thereof at central angles of 6.0 x 10⁻⁴ deg. by way of the photolithographic technique. In the Fig. 2, in turn, each of the surfaces has a nonmagnetic underlayer 2, a magnetic film 3, and a protective layer 4 formed in a same vacuum chamber using a dc magnetron sputtering system. The nonmagnetic underlayer 2 is formed of Cr film of 200 nm thick in a sputtering condition that substrate temperature is 100 °C, argon gas pressure is 0.6 Pa, and applied power density is 50 kW/m². The magnetic film 3 is formed of Co-10at%Cr-12at%Pt of 100 nm thick having an axis of easy magnetization in a longitudinal direction of the substrate 1 in the same condition. The protective layer 4 is formed of carbon of 20 nm thick in a sputtering condition that the substrate temperature is 100 °C, the argon gas pressure is 1.3 Pa, and the applied power density is 30 kW/m². This completes forming the longitudinal magnetic recording medium, or magnetic disk. Results of evaluation of write/read characteristics of the magnetic disk obtained in the embodiment, as shown in Table 1, are high in the signal-to-medium noise ratio. An alternative method for forming the grooves on the above mentioned magnetic disk substrate or the thin film formed thereon may be an electron beam lithography process or a mechanical cutting process. In writing and reading of the magnetic disks described in the embodiments 1, 2, 3, 4, and 5 above, the magnetic head is positioned in a way that a semiconductor laser is used to detect position of the groove or nonmagnetic area, or that a magnetic film of high coercive force is formed for recording a clock signal between the nonmagnetic substrate and the magnetic film for magnetic recording. Each of the magnetic films isolated by the groove has a single bit recorded thereon to allow self-clocking and writing/reading in an NRZ or NRZ-I coding technique. Alternatively, two or more ones of the isolated magnetic film domains can be made to record respectively in reversed directions in magnetization so that write/read can be made in a (1, 7), (2, 7) coding technique or a coding technique in which only the minimum distance of magnetization reversals is limited. A similar effect can be found by replacing the reinforced glass used as the nonmagnetic substrate in the above described embodiments by Ni-P plated aluminum alloy substrate or ceramic substrate. A similar effect can be found by replacing the magnetic film 3 used in the above described embodiments 1, 2, 3, 4 and 5 by Co-Cr-Ta, Co-Ni-Zr, or Co-Ni-Cr. As described so far, the advantages of the longitudinal magnetic recording media and apparatus of the present invention consist in particular in the fact that the signal-to-media noise ratio and the recording capacity can be increased to a great extent in the writing and reading modes of operation as compared with the conventional ones in the way that the nonmagnetic substrate for the magnetic recording media or the longitudinal magnetic film formed thereon for the longitudinal magnetic recording media has the grooves formed in the recording direction, or both in the recording direction and between the recording tracks to form the magnetically isolated magnetic film domain to remove the irregular domain boundary caused on the boundary of magnetization reversals.

Claims: A longitudinal magnetic recording medium having a non-magnetic substrate (1) and a longitudinal magnetic film (3) formed on the non-magnetic substrate (1) directly or through a non-magnetic underlayer (2), wherein the longitudinal magnetic film (3) has a recording section isolated magnetically in a recording direction and the magnetic isolation is made by air (5), characterized in that said non-magnetic substrate (1) is of disk-like shape and made of a material selected from the group consisting of reinforced glass, Ni-P plated aluminum and ceramic. A longitudinal magnetic recording medium according to claim 1, wherein the longitudinal magnetic recording medium has a plurality of concentric recording tracks, and the recording tracks are magnetically isolated therebetween by air (5). A longitudinal magnetic recording medium according to claim 2, wherein a plane shape of the recording section is a rectangular with its recording direction being shorter than a direction perpendicular to the recording direction. A longitudinal magnetic recording medium according to claim 3, wherein the longitudinal magnetic film (3) is a composite film having a longitudinal magnetic film laminated on a perpendicular magnetic film. A longitudinal magnetic recording medium according to claim 3, wherein the non-magnetic substrate (1) has grooves (5) formed on its surfaces, the grooves serving as the magnetic isolation areas. A longitudinal magnetic recording medium according to claim 5, wherein a protrusion portion of the non-magnetic substrate (1) has a longitudinal magnetic film (3) having a coercive force not lower than 79577 A/m (1 kOe) formed thereon, and a grooved portion (5) thereof has a longitudinal magnetic film (3) having a coercive force lower than 79577 A/m (1 kOe) formed thereon.

Inventors: AKAGI KYO, FUTAMOTO MASAAKI, KUGIYA FUMIO, MATSUDA YOSHIBUMI, MIYAMURA YOSHINORI, NAKAO TAKESHI, OUCHI YASUHIDE, SUZUKI MIKIO, TAKANO HISASHI, AKAGI, KYO, FUTAMOTO, MASAAKI, KUGIYA, FUMIO, MATSUDA, YOSHIBUMI, MIYAMURA, YOSHINORI, NAKAO, TAKESHI, OUCHI, YASUHIDE, SUZUKI, MIKIO, TAKANO, HISASHI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1991
Main classification: G11B5
IPC codes: G11B5