Stock material or miscellaneous articles – Circular sheet or circular blank – Recording medium or carrier
Reexamination Certificate
1999-03-24
2004-05-25
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Circular sheet or circular blank
Recording medium or carrier
C428S141000, C428S216000, C428S690000, C428S690000, C428S690000, C428S690000
Reexamination Certificate
active
06740383
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic recording medium and more particularly to a magnetic recording medium which enables high-density recording by virtue of a high coercive force and, particularly, can suppress an increase in noise or preferably can reduce noise, and also can improve a reproducing output and S/N ratio. The present invention also relates to a magnetic recording disk device, in brief, a magnetic disk device, for recording and reproducing information, using the magnetic recording medium of the present invention.
2. Description of Related Art
The development of information processing techniques has led to an increasing demand for an increase in the density of magnetic disk devices used in external storage devices for computers. Specifically, in the reproducing head of the magnetic disk devices, the use of a magnetoresistive head utilizing a magnetoresistor, wherein the electric resistance changes in response to the magnetic field intensity, that is, an MR head, instead of the conventional wound-type inductive thin film magnetic head has been proposed in the art. The MR head applies magnetoresistance, that is, the change in electric resistance produced in a magnetic material on application of an external magnetic field, to the reproduction of a signal on a recording medium and has features including a reproduction output margin that is several times larger than that of the conventional inductive thin film magnetic head, a low inductance and a large S/N ratio. Further, the use of an AMR (anisotropic magnetoresistive) head utilizing anisotropic magnetoresistance, a GMR (giant magnetoresistive) head utilizing giant magnetoresistance, and a spin valve GMR head of a practical type, besides the MR head, have also been proposed.
Further, in order to meet the demand for high density recording, a sufficient improvement in properties, to cope with the above MR head, AMR head, or GMR head (including spin valve head) has been demanded of the magnetic recording medium. In particular, low noise is required in the magnetic recording medium, in addition to a high coercive force Hc for high-density recording.
Hitherto, as is well-known in the art, the magnetic recording medium has been generally produced by depositing chrominium on a nonmagnetic substrate such as an aluminum substrate to form an underlayer, followed by depositing a cobalt-based alloy on the resulting chromium underlayer to form a magnetic recording layer.
Further, to obtain a reduced noise level, many changes, such as the addition of an additional element to the alloy of the magnetic recording layer, thereby breaking the magnetic interaction between the magnetic particles or reduction in the particle size of the magnetic particles in the magnetic recording layer, have been made to the magnetic recording medium. For example, Japanese Unexamined Patent Publication (Kokai) No. 63-148411 discloses a low noise and high density recording-type magnetic recording medium which is suitable for use in high density recording devices. The magnetic recording medium disclosed in this publication is characterized in that the Co/Ni-based alloy or Co/Cr-based alloy constituting the magnetic recording layer contains a third element added thereto, that is, any one of Ta, Mo and W or an alloy thereof. Japanese Unexamined Patent Publication (Kokai) No. 7-50008 discloses a magnetic recording medium which can simultaneously satisfy the requirements of a high coercive force and low noise. Specifically, the magnetic recording medium disclosed in this publication comprises a nonmagnetic substrate layer and a magnetic layer deposited through a nonmagnetic metal underlayer (thickness of 10 to 300 nm) of chromium or a chromium alloy on the nonmagnetic substrate layer, the magnetic layer being formed of an alloy containing Co, Cr, Pt, and at least one member selected from the group consisting of Nb, Hf, W, Ti, and Ta. According to this invention, not only a high coercive force of 1,610 to 1,750 Oe (Examples 1 to 7) but also low noise can be achieved. Similarly, Japanese Unexamined Patent Publication (Kokai) No. 7-50009 discloses a magnetic recording medium wherein a thin film medium of an alloy of 95 to 60 at % of Cr and 5 to 40 at % of at least one member selected from Mo and W is used as an underlayer for a single magnetic layer consisting of a CoCrPt alloy. This magnetic recording medium also can realize, simultaneously, a high coercive force and low noise. More specifically, the use of a Cr underlayer containing 28 at % of Mo results in about 10% reduction in noise as compared with the underlayer consisting of Cr alone. In the techniques disclosed in these publications, however, tBr (a product of the layer thickness t and the residual magnetic flux density Br of the magnetic recording layer) is not less than 270 G.&mgr;m, rendering these techniques unsatisfactory for the higher density recording expected in the future.
Furthermore, recently, since the magnetic disk devices are carried for business and other uses, a glass substrate is frequently used in place of the aluminum substrate to improve a resistance to shock of the devices. For example, Japanese Unexamined Patent Publication (Kokai) No. 5-197941 discloses a thin metallic layer-type magnetic recording medium characterized by depositing, on a nonmagnetic glass substrate, a nonmagnetic heat storage layer having good heat conductivity (Cr, Ti or CrTi alloy) and a NiP layer in the described order, followed by forming a magnetic recording layer of Co alloy through an applied Cr underlayer. According to this invention, since a nonmagnetic heat storage layer having good heat conductivity is deposited at a large thickness of 300 to 1500 Å under the-NiP layer, a crystallization of NiP due to sudden increase of the temperature of the NiP layer during heating of the same layer at a temperature of 250 to 300° C. on a heating device such as an IR heater can be prevented and, as a function thereof, an undesirable reduction of the coercive force of the Co alloy can also be prevented. Moreover, Japanese Unexamined Patent Publication (Kokai) No. 5-314471 discloses a substrate for use in a magnetic recording medium characterized by having a nonmagnetic NiP layer having a surface roughness, in terms of a maximum height (R max), of 500 Å or less, produced upon electroless plating, on a glass substrate having a surface roughness, in terms of a maximum height (R max), of 500 Å or less. According to this invention, a magnetic recording medium having excellent durability due to use of the glass substrate and excellent corrosion resistance due to application of the electroless plating layer can be provided.
In this connection, the inventors have measured the read/write performances for the above-discussed magnetic recording media and other conventional magnetic recording media using a glass substrate as the nonmagnetic substrate, and found that sufficiently high read/write performances cannot be attained when the recording density is the level required at present, that is, generally 1 Gb/in
2
or more. In other words, for these magnetic recording media, it becomes possible to obtain low noise but it is difficult to avoid drawbacks which are caused simultaneously, such as a reduction of reproducing output. It is therefore desirable to ensure a remarkable reduction of noise in the media, i.e., highly increased S/N ratio, while maintaining a high reproducing output.
The related prior art will be further described in connection with the above descriptions. In the magnetic recording medium, a nonmagnetic underlayer to be sandwiched between the nonmagnetic substrate and the magnetic recording layer is generally constituted from chromium or an alloy thereof. This is because, if the chromium-based material is used as the underlayer material, it can adjust a direction of easy magnetization in the overlying magnetic recording layer consisting of a cobalt-based alloy to an areal direction. In fact, it is well-known that the underlayer is pre
Ikeda Yasuji
Okamoto Iwao
Sato Kenji
Shinohara Masayoshi
Yamaguchi Kiyoshi
Bernatz Kevin M
Greer Burns & Crain Ltd.
Thibodeau Paul
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