Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
2002-02-21
2003-06-24
Resan, Steven A. (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S336000, C428S690000, C428S690000, C428S690000, C428S690000
Reexamination Certificate
active
06582815
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium capable of high density recording, in particular, reproduced with an MR head.
BACKGROUND OF THE INVENTION
Magnetic recording media are widely employed in which a magnetic layer comprised of a ferromagnetic iron oxide, Co-modified ferromagnetic iron oxide, CrO
2
, ferromagnetic alloy powder or the likes dispersed in binders is coated on a nonmagnetic support. Recording wavelengths have tended to shorten as recording densities have increased in recent years. The problems of self-demagnetization loss during recording and thickness loss during reproducing, where a thick recording layer results in low output, have become substantial. For this reason, recording layers are made thin. However, when a magnetic layer about equal to or less than 2 &mgr;m is directly coated on a support, the nonmagnetic support tends to affect the surface of the magnetic layer, and electromagnetic characteristics and dropout tend to deteriorate.
One way of solving this problem is to employ a simultaneous multilayer coating method to apply a nonmagnetic layer as a lower layer and then apply a thin magnetic coating liquid of high concentration (Japanese Unexamined Patent Publication (KOKAI) Showa Nos. 63-191315 and 63-187418). These inventions have permitted marked improvement in yield and made it possible to achieve good electromagnetic characteristics. However, magnetic recording media with further high density are required.
Inductive heads that record and reproduce by magnetic inductance have conventionally been employed as magnetic heads. However, in recent years, MR heads employing changes in magnetic resistance based on the level of magnetization of the medium have become popular in the field of high-density digital recording, primarily in hard disks, and are continuing to gain ground in the fields of high-density recording tapes, flexible disks, and the like.
Substantial research is being conducted and numerous papers written on the various problems of magnetic recording with MR heads. When reproducing with MR heads, since MR heads are highly sensitive, a medium with low noise is better suited than a medium with high output. Thus, various techniques of reducing the magnetic interaction between magnetic materials have been disclosed. For example, several techniques are known in which, in the case of thin films, a nonmagnetic layer is sandwiched between multilayered magnetic layers, magnetic materials are formed in pores of a nonmagnetic film or, in the case of vapor deposited tapes, CoO is formed on the surface of magnetic particle crystals.
In particulate media, which is the technical field of the present application, the dilution of the magnetic layer with nonmagnetic materials to lower the ratio of volume of magnetic powder to the volume of magnetic layer has been proposed, as in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 11-73640. Basically, this is a specific technique of blocking magnetostatic coupling of magnetic material with nonmagnetic material or antiferromagnetic substances. However, in the course of diluting the magnetic layer with nonmagnetic material, unless the magnetic material and nonmagnetic material are uniformly dispersed and aggregation of nonmagnetic material is prevented, magnetic pinholes occur, DC demagnetization noise (modulation noise) increases, and bit output amplitude fluctuation occurs, resulting in an increased error rate.
The present inventors conducted extensive research into improving the aptitude with MR heads, further developed the idea of Japanese Unexamined Patent Publication (KOKAI) Heisei No. 11-73640, and discovered a specific medium where, in the course of diluting the magnetic layer with nonmagnetic material, the magnetic material and nonmagnetic material are uniformly dispersed and the nonmagnetic material does not form aggregates. Thus, a medium with low AC demagnetization noise and DC demagnetization noise (modulation noise) was discovered.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a magnetic recording medium having good electromagnetic characteristics with MR heads and good production properties, and more specifically, to provide a magnetic recording medium affording good recording and reproduction performance at short recording wavelengths and good yields.
To achieve the above-stated objects, the present inventors conducted extensive research into magnetic layer structures and magnetic characteristics, resulting in the present invention.
That is, the present invention relates to a magnetic recording medium comprising a nonmagnetic layer comprising a nonmagnetic powder and a binder and a magnetic layer comprising a ferromagnetic metal powder, a binder and a nonmagnetic powder provided in this order on a flexible nonmagnetic support,
wherein said magnetic layer has an average thickness ranging from 0.01 to 0.2 &mgr;m, said ferromagnetic metal powder has a mean major axis length of less than 100 nm and a mean minor axis length of less than 20 nm, said nonmagnetic powder comprised in the magnetic layer is a granular or acicular nonmagnetic powder, the ratio of the mean particle size or minor axis length of said nonmagnetic powder to said mean minor axis length is higher than 0.2 and equal to or less than 2.5, and the maximum surface area of magnetic pinholes is equal to or less than 50% of a bit surface area.
Further, the present invention relates to a method of manufacturing the magnetic recording medium of claim 1 comprising coating a coating liquid for a nonmagnetic layer comprising a nonmagnetic powder and a binder and a coating liquid for a magnetic layer comprising a ferromagnetic metal powder, a binder and a nonmagnetic powder in this order on a flexible nonmagnetic support,
wherein a liquid comprising said ferromagnetic metal powder and said binder and a liquid comprising said nonmagnetic powder and said binder are independently kneaded and dispersed until the dispersibility of said ferromagnetic metal powder and said nonmagnetic powder are optimized, and two liquid obtained are mixed and dispersed immediately prior to coating to obtain said coating liquid for a magnetic layer.
The manufacturing method further desirably comprises a step of, once the nonmagnetic layer coating liquid and magnetic layer coating liquid have been applied to form coatings, imparting to the coatings an orientation of a magnetic field intensity of 0.4 to 1.5 times the Hc of the above-mentioned ferromagnetic metal powder in the longitudinal direction.
In the magnetic recording medium of the present invention, since the magnetic powder particles are in a magnetically separated state, the particle noise, that is, AC demagnetization noise, can be reduced, as well as the state in which the nonmagnetic powder and magnetic powder are present in the magnetic layer is rendered uniform, and fluctuation in the level of magnetization is inhibited, thereby reducing DC demagnetization noise (modulation noise). That is, when reproducing the magnetic recording medium of the present invention with a MR head, a high C/N ratio is achieved. Further, since the magnetic powder is diluted with a nonmagnetic powder, reducing the level of residual magnetization in the magnetic layer, thereby saturation of the MR head can be avoided.
The present invention will be described here.
MR heads yield higher reproduction output than inductive heads. In conventional inductive heads, considerable examination in which the residual magnetization of the magnetic layer was pushed up has been conducted. However, it is known that in MR heads, high residual magnetization is not necessarily related to improvement of performance since noise is high.
The reason that the mean thickness &dgr; of the magnetic layer is specified as falling within the range of 0.01 to 0.2 &mgr;m in the present invention is as follows. When the mean thickness &dgr; of the magnetic layer is excessively thin, the magnetization level is excessively low and it becomes impossible to detect the signal due to overall
Ejiri Kiyomi
Naoe Koji
Fuji Photo Film Co. , Ltd.
Resan Steven A.
Stroock & Stroock & Lavan LLP
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