Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
2000-10-10
2003-02-11
Resan, Stevan A. (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S336000, C428S402000, C428S690000, C252S062580, C148S100000, C148S105000, C148S301000, C148S302000, C148S306000, C148S331000, C075S349000, C075S350000
Reexamination Certificate
active
06517934
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium, a magnetic powder, and a method for producing a magnetic powder. In particular, the present invention relates to a magnetic recording medium comprising a rare earth element-iron-boron magnetic powder, which is particularly suitable for use in ultra-high density recording, for example, a digital video tape, a backup tape of a computer, a large capacity floppy disc, etc.
BACKGROUND ART
Magnetic recording media are required to have a further increased recording density with the shift of a writing-reading system from an analog system to a digital system. In particular, when video tapes and backup tapes of computers, which face severe competition with hard discs or optical discs, cannot satisfy the above requirement, the continuance of the products may be endangered.
To satisfy the requirement to the increase the recording density, magnetic recording media comprising a thin film of a magnetic layer are proposed. However, so-called coating type magnetic recording media, which are produced by applying a magnetic paint containing a magnetic powder dispersed in a binder on a non-magnetic support, are superior to the thin metal film type ones in view of the productivity, and practical reliability such as corrosion resistance. Roughly speaking, the electromagnetic conversion characteristic of the coating type magnetic recording media has been improved by the improvement of magnetic powders and the improvement of production methods.
In connection with the improvement of the magnetic powders, the magnetic properties are year-by-year improved in conjunction with the miniaturization of the particle size to cope with the short-wavelength recording. Formerly, magnetic powders such as ferromagnetic iron oxide powder, cobalt-modified ferromagnetic iron oxide powder and chromium oxide powder, which are used for audio tapes or domestic video tapes, are mainly used, but recently acicular metal magnetic powders having a particle size of about 0.1 &mgr;m is proposed for the high density recording magnetic recording media.
To prevent the decrease of output due to the demagnetization in the short wavelength recording, a coercive force has been increased year-by-year, and the alloy of iron-cobalt achieved a coercive force of about 198.9 kA/m (see U.S. Pat. No. 5,252,380, JP-A-5-234064, JP-A-6-25702, JP-A-6-139553, etc.)
In connection with the improvement of the production methods of the magnetic recording media, the use of binders having various functional groups, the improvement of the dispersing technique of the above magnetic powders, and the improvement of the calendering method after the application process can remarkably increase the surface smoothness of the magnetic layers, and thus greatly contribute to the increase of the output in the short wavelength range (see U.S. Pat. Nos. 4,324,177, 4,952,444, JP-A-4-19815, etc.)
However, since the recording wavelength is shortened with the recent increase of the recording density, when the thickness of a magnetic layer is large, an output decreases only to 10 to 20% in the shortest recording wavelength range with the saturation magnetization or the coercive force of the conventional magnetic powders. In addition, since a very short recording wavelength is used, the influences of self-demagnetization loss in the course of writing and reading and thickness loss due to the thickness of the magnetic layer, which have not caused any problem, increase, and thus sufficient dissolution may not be attained. Such problems cannot be solved by the above-described improvement of the magnetic properties of the magnetic powders or the increase of the surface properties achieved by the production methods of the media. Thus, it is proposed to decrease the thickness of the magnetic layer.
In general, the effective thickness of the magnetic layer is about one third (⅓) of the shortest recording wavelength used in a system. For example, with the shortest recording wavelength of 1.0 &mgr;m, the thickness of the magnetic layer should be about 0.3 &mgr;m. Furthermore, with the miniaturization of a cassette, the whole thickness of the magnetic recording medium should be decreased to increase a recording capacity per unit volume. Consequently, the thickness of the magnetic layer should be decreased. In addition, to increase the recording density, the area of a writing magnetic flux, which is generated with a magnetic head, should be decreased, and thus the magnetic head is miniaturized. Therefore, the amount of the generated magnetic flux decreases. Accordingly, the magnetic layer should be made thin to cause complete reversal of magnetization with the minute magnetic flux.
When the thickness of the magnetic layer is decreased, the surface roughness of the non-magnetic support has some influence on the surface of the magnetic layer and thus the surface properties of the magnetic layer tend to deteriorate. Furthermore, when the thickness of a single magnetic layer is decreased, it may be contemplated to decrease the solid concentration of a magnetic paint or to decrease the amount of the magnetic paint applied. However, these methods cannot prevent defects formed in the course of application, or achieve the increase of the filling of the magnetic powder. Therefore, the strength of the coated film may deteriorate. Accordingly, to decrease the thickness of the magnetic layer by the improvement of the production methods of the magnetic recording media, a so-called simultaneous multiple layer coating method is proposed, which comprises providing an undercoat layer between a non-magnetic support and a magnetic layer, and applying a magnetic paint of the upper magnetic layer while the undercoat layer is still wet (see U.S. Pat. Nos. 4,863,793, 4,963,433, 5,645,917, 5,380,905, 5,496,607, etc.)
With such improvements of the coating methods, it becomes possible to thinly coat a magnetic layer having a thickness of about 1.0 &mgr;m, and such thin film-coating methods and the above-described improvement of the magnetic powders can solve the various problems such as the decrease of the output caused by the demagnetization, which is the essential problem of longitudinal recording.
However, in these days, the improvements of the magnetic powders and the production methods of the magnetic recording media reach the limits. In particular, in the case of the improvement of the magnetic powders, insofar as the acicular magnetic powder is used, the practical lower limit of the particle size is about 0.1 &mgr;m, because when t he particle size is less than about 0.1 &mgr;m, a specific surface area of the particle increases greatly, and thus not only the saturation magnetization decreases but also the dispersion of the magnetic powder in the binder becomes very difficult.
In connection with the coercive force, signals can be recorded on magnetic recording media having a very high coercive force because of the technical innovation of the magnetic heads. In particular, in the case of the longitudinal recording system, it is desirable to increase the coercive force to as high as possible to prevent the deterioration of the output due to the writing and reading demagnetization, insofar as the recorded signals can be erased with the magnetic head. Accordingly, the realistic and most effective method to increase the recording density of the magnetic recording media is to increase the coercive force of the media.
It is effective to further decrease the thickness of the magnetic layer to suppress the influence of the decrease of the output caused by the writing and reading demagnetization, which is the essential problem of the longitudinal recording. However, the thickness of the magnetic layer will reach the limit, insofar as the above-described acicular magnetic powder having a particle size of about 0.1 &mgr;m is used. The reason is as follows: the acicular particles are aligned in the plane direction of the magnetic recording medium on the average by longitudinal orientation, but some particles may be aligned in
Bernatz Kevin M.
Birch & Stewart Kolasch & Birch, LLP
Hitachi Maxell Ltd.
Resan Stevan A.
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