Stock material or miscellaneous articles – Surface property or characteristic of web – sheet or block
Reexamination Certificate
2002-03-13
2004-04-13
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Surface property or characteristic of web, sheet or block
C428S690000, C360S313000
Reexamination Certificate
active
06720075
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a magnetic recording medium, more particularly, to a magnetic recording medium capable of performing high-density recording by improving thermal stability of recording magnetization. Furthermore, a vertical magnetic recording method is one in which magnetic recording is carried out by applying recording magnetization in a direction vertical to a surface of a magnetic recording medium (an easy axis of magnetization), and one of the promising technologies for excellent magnetic recording methods supporting a recent trend of high density recording.
BACKGROUND ART
One example of a magnetic recording material will now be described, based on
FIG. 1
showing a configuration of principal elements of a common vertical magnetic recording medium
100
in the prior art. As shown in
FIG. 1
, for example, the vertical magnetic recording medium
100
has a structure formed by laminating a soft magnetic underlying layer
102
consisting of nickel-iron, etc., a crystal controlling layer
103
consisting of chromium (Cr) and titanium (Ti), etc. laid on for crystal controlling, a vertical magnetic recording layer
104
consisting of an alloy containing cobalt such as cobalt-chromium (Co—Cr), etc. in which magnetic recording is carried out, and a protecting layer
105
consisting of hard DLC (Diamond Like Carbon), etc., in order from the bottom, on a non-magnetic substrate
101
consisting of aluminum, etc.
Herein, the underlying layer
102
is a layer laid on for improving recording sensitivity and this layer is not an essential layer for the vertical magnetic recording medium
100
. Furthermore, in order to improve good crystallization and adhesion, a layer consisting of chromium or titanium, etc. may be formed before film formation of the respective magnetic layers.
In the vertical magnetic recording medium
100
, for attainment of both high density recording and decrease of noise level, miniaturization and homogenization of magnetic particle diameters and elimination (segregation) of magnetic interaction between magnetic particles in the vertical magnetic recording layer
104
, etc. are required and various investigations are made for them.
It is known that as the miniaturization and homogenization of the magnetic particle diameter and the elimination of the magnetic interaction between the magnetic particles in the vertical magnetic recording layer
104
are carried out, recording magnetization is destabilized by heat. Consequently, a design to produce the vertical magnetic recording medium
100
having large vertical coercive magnetic force Hc is required.
However, in the vertical magnetic recording medium
100
in the prior art, the vertical coercive magnetic force Hc was also decreased by heat causing a temperature rise.
This influence of the heat will now be described in detail. Since recording and reading (reproducing) are carried out in a vertical magnetic recording medium, it is designed so as to have, for example, approximately 2800 Oe as the maximum vertical coercive magnetic force Hc within a range in which magnetic recording is allowed.
FIG. 2
is a diagram showing a relationship between temperature (° C.) and vertical coercive magnetic force (Hc) with respect to a common vertical magnetic recording medium in the prior art. As clearly shown in the figure, the vertical coercive magnetic force decreases with temperature rise. Hence it is recognized that recording magnetization of a vertical magnetic recording medium is destabilized by temperature rise.
Also,
FIG. 3
is a diagram showing change of residual magnetization Mr with time at certain temperatures with respect to a vertical magnetic recording medium produced by eliminating the underlying layer
102
from the vertical magnetic recording medium
100
shown in FIG.
1
. Herein, the vertical axis shows relative change, provided that the residual magnetization Mr in the vertical direction from the vertical magnetic recording layer
104
in an early stage is one. When one second after beginning of a measurement is defined as the time standard and the obtained data are extrapolated, the appearance of the change with time is shown. The filled triangle indicates at room temperature RT (approximately 25° C.) and the filled dot indicates at 75° C. that is assumed to be the guaranteed temperature of the vertical magnetic recording medium. The dashed line indicates a tolerance of temperature for maintaining a function of the vertical magnetic recording medium.
As clearly shown in
FIG. 3
, although the residual magnetization Mr hardly decreases at the room temperature and there is no problem, the decrease of the magnetization become significant and dips below the tolerance at 75° C.
The decrease of magnetization in a medium caused by temperature rise or heat as described above is a phenomenon known as the thermal fluctuation magnetic after effect or the thermal magnetic relaxation.
That is, magnetization within a magnetic particle in a single magnetic domain is stabilized such that various magnetic energies represented by an anisotropic energy are minimized at lower temperature. Such condition of the magnetization is conceptually like a condition on which an inner part surrounded by magnetic barrier &Dgr;E is stabilized. Also, it is known that energy is added to a magnetization spin as thermal energy as temperature is raised and the magnetization comes to be in a disordered state as thermal energy kT (k is Boltzmann constant) is larger than the energy barrier &Dgr;E.
However, according to the statistical mechanics, even if the thermal energy is not so large, the thermal energy kT may randomly exceed the magnetic energy barrier &Dgr;E. The larger the thermal energy is, the smaller the thermal energy barrier &Dgr;E is, and the longer elapsed time is, the more the probability increases. Usually, if temperature is constant, the magnetic energy barrier &Dgr;E and the thermal energy kT are approximately constant. Hence, with respect to magnetization spins directed to one direction by means of magnetic recording, magnetization spins in the random state increase with time. Therefore, it seems that the magnetization decreases with time. This is a phenomenon referred to as the thermal magnetic relaxation.
In the case of the vertical magnetic recording medium, as the influence of the phenomenon is considered, the magnetic energy barrier &Dgr;E strongly depends on vertical coercive magnetic force Hc along the vertical direction, and the higher the vertical coercive magnetic force Hc is, the higher the magnetic energy barrier &Dgr;E is.
In the previously described vertical magnetic recording medium in the prior art, since temperature inside a magnetic recording medium driver rises, not only does thermal energy kT increase, but also a magnetic energy barrier &Dgr;E dependent on vertical coercive magnetic force Hc decreases due to a decrease of vertical coercive magnetic force and the phenomenon of thermal magnetic relaxation easily occurs at higher temperature.
As clearly seen from the previous illustration, in the vertical magnetic recording medium
100
in the prior art, the vertical coercive magnetic force Hc in the vertical magnetic recording layer
104
decreases with heat causing a temperature rise so that the magnetization condition is destabilized. Then, such decrease of the coercive magnetic force caused by heat is also problematic in a magnetic recording medium for another magnetic recording method.
DISCLOSURE OF THE INVENTION
Therefore, it is an object of the present invention to provide a magnetic recording medium capable of attaining high density using a magnetic material that is thermally stabilized by an increase of coercive magnetic force in a recording magnetic layer with temperature rise.
The object can be achieved by a magnetic recording medium comprising a recording magnetic layer in which magnetic recording is carried out, wherein the recording magnetic layer is set such that coercive magnetic force increases with temperature rise within an operating temperature range in which t
Matsumoto Koji
Tagawa Ikuya
Bernatz Kevin M.
Fujitsu Limited
Greer Burns & Crain Ltd.
Thibodeau Paul
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