Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
2002-05-30
2003-12-16
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, C428S690000
Reexamination Certificate
active
06663955
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a coating type magnetic recording medium (i.e., a magnetic recording particulate medium) having high recording density, and particularly to a magnetic recording medium for high-density recording having a magnetic layer and a substantially non-magnetic lower layer, and containing ferromagnetic hexagonal ferrite in an uppermost layer.
BACKGROUND OF THE INVENTION
In the field of magnetic disks, 2-MB MF-2HD floppy disks using Co-modified iron oxide have come to be normally installed in personal computers. However, the capacity of data to be handled has today been rapidly increased, and accordingly, that capacity has become insufficient. It has therefore been desired that the capacity of floppy disks is increased.
Also in the field of magnetic tapes, with the recent spread of minicomputers, personal computers and office computers in work stations, the intensive research of magnetic tapes for recording computer data as external storage media (so-called backup tapes) has been conducted. Inputting the magnetic tapes for such applications to practical use, improvement in recording capacity has been eagerly desired, for an increase in recording capacity and miniaturization, coupled with miniaturization of computers and an increase in information processing ability.
Conventionally, magnetic recording media have been widely used in which a non-magnetic support is coated with a magnetic layer formed by dispersing iron oxide, Co-modified iron oxide, CrO
2
, a ferromagnetic metal powder or a hexagonal ferrite powder in a binder. Of these, the hexagonal ferrite powder is known to be excellent in high-density recording characteristics. However, when inductive heads which have been mainly used in a system using flexible media are used, the fine hexagonal ferrite fine powder is low in saturation magnetization, resulting in failure to obtain a sufficient output. However, magnetic resistance type heads (i.e., Magneto Resistive Head: MR head) employed in hard disks begin to be also used in removal recording using flexible media as described above.
It is known that the MR heads are high in sensitivity, so that even the use of the hexagonal ferrite fine powder provides a sufficient reproduction output and a reduction in noise by which hexagonal ferrite is characterized gives high C/N ratio. For example, Japanese Patent Application (Laid-Open) No. 302243/1998 discloses an example of reproduction with an MR head using a fine barium ferrite (BaFe) powder.
However, in the fine hexagonal ferrite powder as known herein, thermal stability of magnetization is deteriorated with a decrease in the size of magnetic particles. This phenomenon becomes worse in a magnetic recording medium used in a system reproducing a recording signal having a recording density of 1 Gbit/inch
2
or more, and it has been expected that this problem is solved in the magnetic recording medium side.
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) good in electromagnetic characteristics, particularly remarkably improved in C/N ratio in a high-density recording region, excellent in productivity, low in the price, and low in noise and excellent in high-density characteristics in a recording reproduction system in which an MR head is combined.
According to the present invention, there is provided a magnetic recording medium comprising a support and a magnetic layer formed thereon which mainly comprises a ferromagnetic powder and a binder, wherein the magnetic recording medium is used in a magnetic recording reproduction system in which a recording signal is reproduced with a magnetic resistance type head (MR head), the ferromagnetic powder is a hexagonal magnetic ferrite powder having an average tabular diameter of from 10 to 40 nm, and the magnetic layer has a coercive force of 159 kA/m (2000 Oe) or more and has at most less than 1% of a component which performs magnetic flux revolution in a region of 80 kA/m (1000 Oe) or less in a differential curve of a remanence curve.
Preferred embodiments of the present invention are as follows:
(1) The magnetic recording medium as described in the above item (1), wherein the average particle volume of the hexagonal magnetic ferrite powder is from 1000 to 10000 nm
3
, and the existing ratio of particles having a tabular diameter of less than 10 nm is 10% or less;
(2) The magnetic recording medium as described in the above item (1), wherein the magnetic layer is from 0.03 to 0.20 &mgr;m; and
(3) The magnetic recording medium as described in the above item (1), wherein a substantially non-magnetic layer is provided between the magnetic layer and the support.
DETAILED DESCRIPTION OF THE INVENTION
Although there is no particular limitation on a recording head for recording a signal on the magnetic recording medium of the present invention, an electromagnetic induction type thin film magnetic head such as an MIG head is suitably used. The hexagonal ferrite fine particles used in the magnetic layer of the present invention are tabular in form, and are required to control the average tabular diameter within the range of from 10 to 40 nm. Further, the coercive force of the magnetic layer is 159 kA/m (2000 Oe) or more, and the component which performs magnetic flux revolution in a region of 80 kA/m (1000 Oe) or less in a differential curve of a remanence curve is regulated little, thereby decreasing the influence of thermal fluctuation to improve the C/N ratio in high-density recording.
That is to say, in the magnetic recording medium of the present invention, the component which performs magnetic flux revolution in a region of 80 kA/m (1000 Oe) or less in a differential curve of a remanence curve is controlled so as to give at most less than 1%. Here, the remanence curve is drawn by applying a magnetic field of −800 kA/m (−10 kOe) in the direction parallel to a magnetic surface to saturate the magnetic layer, then, making the magnetic field zero, applying a magnetic field having the same distance in the reverse direction (for example, 8 kA/m (100 Oe)), followed by making the magnetic field zero, measuring the residual magnetic flux density (Br) for each case and the difference thereof, and plotting the difference of Br as the y-axis and the applied magnetic field as the x-axis. Further, “less than 1%” means the ratio of the area surrounded by a differential curve corresponding to 80 kA/mor less with the applied magnetic field as the X-axis and dy/dx as the Y-axis and the X-axis to the whole area of the differential curve and the X-axis.
In the present invention, as means for obtaining the above-described differential curve, it is preferred that a fine particle component present as a mixture in the hexagonal magnetic ferrite powder used in the magnetic layer is removed, for example, that the existing ratio of the particles having a tabular diameter of less than 10 nm in the hexagonal magnetic ferrite powder used is reduced to 10% or less.
Such means for removing the fine particle component include centrifugal separation.
In contrast with the present invention, even when the hexagonal magnetic ferrite powder has an average tabular diameter ranging from 10 to 40 nm, and the component which performs magnetic flux revolution in a region of 80 kA/m (1000 Oe) or less in a differential curve of a remanence curve (hereinafter referred to as the magnetic flux revolution component) is at most less than 1%, a coercive force of the magnetic layer of less than 159 kA/m (2000 Oe) results in failure to improve the C/N ratio. Further, even when the magnetic layer has a coercive force of 159 kA/m (2000 Oe) or more, and the hexagonal magnetic ferrite powder has an average tabular diameter ranging from 10 to 40 nm, 1% or more of the magnetic flux revolution component results in failure to improve the C/N ratio and overwrite erasing rate.
Further, in contrast with the present invention, even when the magnetic layer has a coercive force of 159 kA/m
Fuji Photo Film Co. , Ltd.
Resan Stevan A.
Sughrue & Mion, PLLC
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