Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
2002-08-13
2004-03-30
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...
C428S329000, C428S690000, C428S690000
Reexamination Certificate
active
06713171
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a particulate magnetic recording medium for high-density recording. In particular, the present invention relates to a magnetic recording medium for high-density recording comprising a magnetic layer comprising a hexagonal ferrite powder and an substantially nonmagnetic lower layer.
BACKGROUND OF THE INVENTION
With the widespread popularity of office computers such as minicomputers, personal computers and workstations in recent years, considerable researches have been conducted into magnetic tapes (so-called “backup tapes”) for recording computer data as external memory devices. In the course of the practical development of magnetic tapes for such applications, particularly when combined with size reduction and increased information processing capability in computers, there has been a strong demand for increased recording capacity to achieve high-capacity recording and size reduction.
Magnetic recording media, in which a magnetic layer, comprising iron oxide, Co-modified iron oxide, CrO
2
, a ferromagnetic metal powder, and/or hexagonal ferrite powder dispersed in a binder, is coated on a nonmagnetic support are widely employed. Of these, hexagonal ferrite powder is known to have good high-density recording characateristics. Further, the use of an MR head in reproduction is known to yield a low noise level and a higher C/N ratio (Japanese Unexamined Patent Publication (KOKAI) Heisei No. 10-302243).
The use of hexagonal ferrite powders with good high-density recording characteristics such as barium ferrite powder in the magnetic layer in this fashion has drawn attention. However, since hexagonal ferrite powders such as barium ferrite powder are plate-shaped and have an easily magnetized magnetic domain perpendicular to the flat surfaces thereof, the plate-shaped particles tend to stack and magnetically couple, increasing noise. Thus, it is necessary to reduce noise and improve electromagnetic characteristics.
Accordingly, the object of the present invention is to provide a magnetic recording medium employing ferromagnetic hexagonal ferrite powder such as barium ferrite, having improved electromagnetic characteristics, particularly improved high-density recording characteristics, in which the C/N ratio in the high-density recording region is markedly improved.
The present inventors conducted extensive research for achieving a magnetic recording medium having a markedly improved electromagnetic characteristics, particularly the C/N ratio in high-density recording region, resulting in the discovery that by limiting the number of magnetic aggregates having a size and magnetic intensity equal to or greater than a prescribed level present in the magnetic layer to equal to or less than 100/2,000 &mgr;m
2
, it was possible to reduce noise and obtain a magnetic recording medium having good electromagnetic characteristics; the present invention was devised on this basis.
SUMMARY OF THE INVENTION
That is, the magnetic recording medium of the present invention is comprised of a nonmagnetic layer comprising a nonmagnetic powder and a binder and a magnetic layer comprising a ferromagnetic hexagonal ferrite powder and a binder in this order on a support, and is characterized in that the number of magnetic aggregates present in the magnetic layer that are equal to or higher than 0.5 &mgr;m in size and have a magnetic intensity equal to or greater than 20 times the average value of magnetization turbulence obtained from a magnetic force image obtained by a magnetic force microscope is equal to or less than 100/2,000 &mgr;m
2
.
The “magnetic aggregates” referred to here are those observed as specific points in magnetic force images obtained by observation by a magnetic force microscope (MFM) on the magnetic layer surface after DC erasure. They are caused by a larger magnetic flux only in the portion than in the other portions by large magnetic aggregates.
The “magnetic aggregates” the number of which is considered in the present invention are equal to or greater than 0.5 &mgr;m in size and have a magnetic intensity of 20 times or more the average value of magnetization turbulence obtained from a magnetic force image obtained by observation by a magnetic force microscope.
Here, the term “magnetization turbulence” denotes the nonuniformity of magnetization (unit) due to the magnetic aggregates. It can be obtained by measuring the size of the partial magnetic aggregates observed following DC erasure with an MFM. In the MFM, a magnet is employed as the probe in an atomic force microscope (AFM) and the magnetic force can be measured by the same method as in AFM. “Magnetization turbulence” is measured as follows.
A 40 &mgr;m square of DC-erased medium is measured with an MFM, and five visual fields are taken. The surface roughness Ra equation is applied to each of magnetic force image data, and the average magnetization turbulence value A (mean value of magnetization turbulence) is calculated. The number of “magnetic aggregates” with a magnetic intensity equal to or greater than 20 times average magnetization turbulence value A and having a size equal to or greater than 0.5 &mgr;m is counted. It is thus possible to obtain the number of “magnetic aggregates” per 2,000 &mgr;m
2
.
In the present invention, the number of magnetic aggregates present in the magnetic layer having a size equal to or greater than 0.5 &mgr;m and having a magnetic intensity equal to or greater than 20 times the average value of the magnetization turbulence obtained from a magnetic force image by a magnetic force microscope is equal to or less than 100/2,000 &mgr;m
2
.
When recording and reproducing at a linear recording density of 100 kfci (about 0.5 &mgr;m), at the size of magnetic aggregates equal to 0.5 &mgr;m, magnetization inversion reached the critical point and medium noise deteriorates substantially. Thus, the size of magnetic aggregates is set to equal to or greater than 0.5 &mgr;m in view of an effect on medium noise.
Further, in the present invention, the magnetic intensity of the magnetic aggregates the number of which is specified as being equal to or less than 100/2,000 &mgr;m
2
is set to “20 times the average value of magnetization turbulence obtained from a magnetic force image by magnetic force microscope”. This is because magnetic aggregates recognized as a phase shift when recording a recording signal are those having 20 times or more magnetic intensity, and magnetic aggregates with a intensity less than 20 times cause no problem during reproduction.
A large quantity of magnetic aggregates of the above-stated size and magnetic intensity causes an increase in medium noise and deterioration of electromagnetic characteristics, and makes a medium unsuited to high-density recording. Accordingly, the number of such magnetic aggregates is limited to equal to or less than 100/2,000 &mgr;m
2
, preferably equal to or less than 50/2,000 &mgr;m
2
, and more preferably, equal to or less than 30/2,000 &mgr;m
2
. The number of magnetic aggregates would ideally be 0/2,000 &mgr;m
2
, but the practical lower limit is about 5/2,000 &mgr;m
2
.
The magnetic aggregates in the magnetic layer result from the aggregation of a ferromagnetic hexagonal ferrite powder and abrasives contained in the layer. Accordingly, in order to adjust the number of magnetic aggregates equal to or greater than 0.5 &mgr;m in size and having an intensity of A×20 or more in the magnetic layer to equal to or less than 100/2,000 &mgr;m
2
, dispersion of the ferromagnetic hexagonal ferrite powder in the magnetic coating liquid is to be improved or aggregation of abrasives is to be reduced. To this end, the following methods can be employed:
(1) Dispersing the magnetic coating liquid comprising a ferromagnetic hexagonal ferrite powder and a binder is conducted for at least 10 hours, preferably 20 to 24 hours.
Conventionally, dispersion is conducted for 3 to 8 hours. In the present invention, the same conditions are employed as in the conventional method with the exception of the dispersion period, w
Doushita Hiroaki
Harasawa Takeshi
Ozawa Takako
Fuji Photo Film Co. , Ltd.
Resan Stevan A.
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