Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
2002-09-06
2004-04-27
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...
C428S690000
Reexamination Certificate
active
06726986
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a coating type magnetic recording medium (i.e., a magnetic recording particulate medium) capable of high density recording, in particular, to a magnetic recording medium wherein magnetic particle distribution is reduced and electromagnetic characteristics in high density recording is improved.
BACKGROUND OF THE INVENTION
In the field of magnetic disc, a 2 MB MF-2HD floppy disc using Co-modified iron oxide has been generally loaded in a personal computer. However, along with the increase in the amount of data to be dealt with, the capacity thereof has become insufficient and the increase of the capacity of the floppy disc has been demanded.
In the field of magnetic tape also, with the prevalence of the office computer, such as minicomputers, personal computers and work stations, magnetic tapes for recording computer data as external storage media (a so-called backup tape) have been eagerly studied. For putting magnetic tapes for such usage to practical use, the improvement of recording capacity has been strongly demanded conjointly with the miniaturization of a computer and the increase of information processing performance (i.e., the increase of throughput) for achieving high capacity recording and the miniaturization.
Magnetic layers comprising an iron oxide, a Co-modified iron oxide, CrO
2
, a ferromagnetic metal powder, or a hexagonal ferrite powder dispersed in a binder, which are coated on a nonmagnetic support, have been conventionally widely used in magnetic recording media. Of these, hexagonal ferrite fine powders have been known to be excellent in high density recording characteristics, however, when inductive heads which have been mainly used in the system using flexible media are used, hexagonal ferrite fine powders are small in saturation magnetization, hence sufficient output cannot be obtained. However, a magneto-resistance head (an MR head) which is used in a hard disc has also come to be used in removable recording using flexible media as above.
Since the MR heads are high sensitivity, sufficient reproduction output can be obtained even when hexagonal ferrite fine powders are used, and it is known that a high C/N ratio can be obtained due to the reduction of noise which is the characteristic of hexagonal ferrite fine powders. For instance, an example of the reproduction with an MR head by using a barium ferrite (BaFe) fine powder is disclosed in JP-A-10-302243 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”).
Coercive force (Hc) measured in BaFe magnetic powders of generally from about 40 to about 400 kA/m (from 500 to 5,000 Oe) can be produced. Higher Hc is advantageous for high density recording but it is restricted by the capacities of recording heads. When saturation magnetization of a recording head exceeds 1.4 T (tesla), the coercive force is preferably 175 kA/m (2,200 Oe) or higher, but recording by the magnetic flux revolution is difficult when the coercive force is 400 kA/m (5,000 Oe) or higher. A high Hc part of 400 kA/m (5,000 Oe) or higher substantially does not contribute to recording and reproduction in the distribution of magnetization of a magnetic layer due to magnetic particle distribution, and the reduction of reproduction output and recording and the increase of noise are caused when high Hc parts increase.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a coating type magnetic recording medium (i.e., a magnetic recording particulate medium) which is excellent in electromagnetic characteristics, conspicuously improved in C/N ratio in high density recording region, excellent in productivity, and inexpensive, further, low in noise and excellent in high density characteristics in a recording and reproducing system integrated with an MR head.
The object of the present invention has been achieved by a magnetic recording medium comprising a support having thereon a magnetic layer containing a ferromagnetic powder and a binder as main components, wherein the ferromagnetic powder is a hexagonal ferrite powder having an average tabular diameter of from 10 to 40 nm, and the part which is subjected to magnetic flux revolution by the application of a magnetic field of 400 kA/m (5,000 Oe) or higher in the distribution of magnetization of the magnetic layer is less than 1%.
A preferred mode of the present invention is as follows.
(1) The magnetic recording medium is a magnetic recording medium for reproduction with a magneto-resistance type magnetic head (an MR head).
DETAILED DESCRIPTION OF THE INVENTION
Hexagonal ferrite powders used in a magnetic layer in the present invention are tabular particles and the tabular diameter of hexagonal ferrite powders should be restricted to 10 to 40 nm. Further, by restricting at least a part which is subjected to magnetic flux revolution by the application of a magnetic field of 400 kA/m (5,000 Oe) or higher (also called a high Hc part) in the distribution of magnetization of the magnetic layer, the influence of a high Hc part can be lessened, to thereby improve the reduction of reproduction output in high density recording and the increase of noises, as a result, a magnetic recording medium having a high C/N ratio and excellent overwriting aptitude can be provided.
A part subjected to magnetic flux revolution by the application of a magnetic field of 400 kA/m (5,000 Oe) or higher in the distribution of magnetization of the magnetic layer in the present invention means a part subjected to magnetic flux revolution by the applied magnetic field of 400 kA/m (5,000 Oe) or higher on the differential curve of Remanence curve. In the magnetic recording medium according to the present invention, magnetic characteristics are controlled so that the part is less than 1%, preferably 0.7% or less, and more preferably 0.5% or less. Remanence curve is drawn by applying a magnetic field of −800 kA/m (−10 kOe) in the direction horizontal to a magnetic plane of a magnetic layer, saturating the magnetic layer, making the magnetic field zero, applying a magnetic field of equal interval (e.g., 8 kA/m (100 Oe)) in the reverse direction, making the magnetic field zero, measuring residual magnetic flux densities (Br) and differences thereof, and taking the differences of Br as y-axis and the applied magnetic fields as x-axis. “Less than 1%” means the ratio of the same area of a differential curve corresponding to 400 kA/m (5,000 Oe) or larger of the whole area enclosed with the differential curve at large taking applied magnetic field as X-axis and dy/dx as Y-axis and X-axis.
A means for obtaining the above differential curve, i.e., a means for reducing high Hc parts, is not particularly restricted in the present invention. The means is to use hexagonal ferrite powder shaving less high Hc parts, specifically removing hexagonal ferrite powders having a tabular diameter of less than 10 nm by centrifugation and the like, and selecting and unifying the crystal structures and compositions of hexagonal ferrite powders.
Even when the average tabular diameter of hexagonal ferrite magnetic powders is in the range of from 10 to 40 nm, a C/N ratio and an overwriting erasure rate are not improved if a high Hc part is 1% or more. On the other hand, even when a part subjected to magnetic flux revolution is less than 1%, a C/N ratio is not improved if the average tabular diameter of hexagonal ferrite magnetic powders is higher than 40 nm.
Further, when a high Hc part is less than 1% and the average tabular diameter of hexagonal ferrite magnetic powders is less than 10 nm, it is difficult to obtain stable magnetic recording due to thermal fluctuation.
In addition, the coercive force of the magnetic layer in the present invention is preferably as large as possible within the range of satisfying the condition of a high Hc part, and it is generally 159 kA/m (2,000 Oe) or more, preferably from 159 to 400 kA/m (2,000 to 5,000 Oe), but the upper limit depends upon the system to which the magnetic recording medium is applied,
Fuji Photo Film Co. , Ltd.
Resan Stevan A.
Sughrue & Mion, PLLC
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