Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-12-28
2004-07-13
Rickman, Holly (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S690000, C428S900000, C427S131000, C204S192100, C204S192200
Reexamination Certificate
active
06761982
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium used in, for example, peripheral devices of calculators, or magnetic disk apparatuses for recording of image and sound data; a process and apparatus for producing the magnetic recording medium; and a magnetic recording and reproducing apparatus incorporating the magnetic recording medium.
BACKGROUND OF THE INVENTION
As recording density of magnetic recording media is increased, there have been proposed reduction in noise and enhancement of resolution by means of, for example, micronization or magnetic isolation of magnetic grains in a magnetic layer, or thinning of a magnetic layer.
However, when magnetic grains are micronized or magnetically isolated, or when a magnetic layer is thinned, the size of the magnetic grains is reduced, and therefore, thermal stability (i.e., thermal decay resistance) of the resultant magnetic recording medium tends to be deteriorated. The term “thermal decay” refers to a phenomenon in which recording bits become unstable and recorded data are lost. In a magnetic recording and reproducing apparatus, thermal decay is manifested in the form of reduction in reproduction output of recorded data with passage of time.
Hitherto, typical substrates for producing magnetic recording media are non-magnetic metallic substrates formed from, for example, an aluminum alloy. Usually, a hard film formed from NiP or similar material is provided on such a non-magnetic metallic substrate in order to harden its surface, then the surface of the substrate is subjected to texturing, and the substrate is used for producing a magnetic recording medium.
Texturing is a process for forming irregularities on the surface of a substrate along a predetermined direction (usually in a circumferential direction) of the substrate. When the surface of a substrate undergoes texturing, the crystal orientation of an undercoat layer and a magnetic layer, which are formed on the substrate, is enhanced, and the magnetic anisotropy of the magnetic layer is enhanced, and thus magnetic characteristics, such as thermal stability, of a magnetic recording medium can be enhanced.
In recent years, instead of a metallic substrate formed from aluminum or similar metal, a non-metallic substrate formed from material such as glass or ceramic has been widely used as a substrate for producing a magnetic recording medium. Such a non-metallic substrate has an advantage that head slap does not easily occur in the substrate, due to high hardness of the substrate. Furthermore, from the viewpoint of glide height characteristics, such a non-metallic substrate is advantageously used, because of its excellent surface smoothness.
However, a non-metallic substrate such as a glass substrate encounters difficulty in undergoing texturing, and involves problems that the magnetic anisotropy of a magnetic layer becomes unsatisfactory, and thermal stability is inclined to be deteriorated.
In order to solve such problems, there has been proposed formation of a hard film which can be easily textured on a non-metallic substrate formed from material such as glass or ceramic.
For example, Japanese Patent Application Laid-Open (kokai) No. 5-197941 discloses a magnetic recording medium including a non-metallic substrate coated, by sputtering, with a NiP film serving as a hard film which is easily textured.
A magnetic recording medium including a hard film formed on a non-metallic substrate is produced by the following process: the hard film is formed on the substrate in a film formation apparatus such as a sputtering apparatus; the substrate is temporarily removed from the apparatus and subjected to texturing by use of a texturing apparatus; the resultant substrate is again placed in the film formation apparatus; and then an undercoat layer and a magnetic layer are formed on the substrate.
However, in the case of the aforementioned conventional magnetic recording medium including a non-magnetic metallic substrate such as an aluminum substrate or a non-metallic substrate such as a glass substrate, when a hard film formed from NiP, which is provided on the substrate, is subjected to texturing, the magnetic anisotropy of a magnetic layer can be enhanced but the surface smoothness of the medium tends to be lowered because of surface irregularities of the hard film. Consequently, glide height characteristics are deteriorated, and attainment of high recording density becomes difficult. In addition, the production process for the magnetic recording medium includes complicated production steps, resulting in high production costs.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide a magnetic recording medium which exhibits excellent magnetic characteristics such as thermal stability and excellent glide height characteristics and which is easily produced.
Another object of the present invention is to provide a process and apparatus for producing the magnetic recording medium easily.
A further object of the present invention is to provide a magnetic recording and reproducing apparatus incorporating the magnetic recording medium exhibiting excellent magnetic characteristics such as thermal stability.
The present inventors have found that the thermal stability of a magnetic recording medium can be considerably enhanced when an orientation-determining layer is formed between a non-magnetic substrate and a non-magnetic undercoat layer, the orientation-determining layer has a crystal structure in which columnar fine crystal grains are inclined at an angle in a radial direction, the ratio of a coercive force in a circumferential direction of a magnetic layer (Hcc) to a coercive force in a radial direction of the magnetic layer (Hcr); i.e., Hcc/Hcr, is more than 1, and the magnetic layer includes a plurality of magnetic films and has a structure such that antiferromagnetic bonding can be formed between the magnetic films. The present invention has been accomplished on the basis of this finding.
Accordingly, the present invention provides a magnetic recording medium comprising a non-magnetic substrate, an orientation-determining layer for causing the non-magnetic undercoat layer to have a predominant plane of (200), a non-magnetic undercoat layer, a magnetic layer, and a protective layer, in order, wherein the non-magnetic undercoat layer has a bcc structure; the orientation-determining layer has a crystal structure in which columnar fine crystal grains are inclined in a radial direction; the ratio of a coercive force in a circumferential direction of the magnetic layer (Hcc) to a coercive force in a radial direction of the magnetic layer (Hcr); i.e., Hcc/Hcr, is more than 1; and the magnetic layer includes a plurality of magnetic films having an hcp structure and a predominant orientation plane of (110) plane, and permits antiferromagnetic bonding to be formed therebetween.
In the magnetic recording medium of the present invention, since the orientation-determining layer having a crystal structure in which columnar fine crystal grains are inclined in a radial direction is provided, the ratio of a coercive force in a circumferential direction of the magnetic layer (Hcc) to a coercive force in a radial direction of the magnetic layer (Hcr); i.e., Hcc/Hcr, is more than 1. Thus the magnetic anisotropy of the magnetic layer in a circumferential direction can be enhanced and crystal magnetic anisotropy constant (Ku) can be enhanced. Consequently, magnetic characteristics, such as thermal stability, coercive force, and S/N ratio of recorded/reproduced signals, can be enhanced.
In addition, in the present invention, due to antiferromagnetic bonding between magnetic films, magnetic films other than a primary magnetic film of largest coercive force assume an apparent non-magnetized state; or the primary magnetic film assumes a state in which apparent magnetization of the primary magnetic film is reduced in an amount corresponding to the magnetization of magnetic films other than the primary magnetic film.
Therefore, the volume
Sakai Hiroshi
Sakawaki Akira
Rickman Holly
Showa Denko Kabushiki Kaisha
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