Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
1999-10-29
2002-01-15
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S690000, C428S690000, C428S690000, C428S900000, C369S013010
Reexamination Certificate
active
06338911
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magneto-optical recording medium for recording/reproducing information with laser light by utilizing magneto-optical effects, and particularly relates to a magneto-optical recording medium in which each recording magnetic domain in a recording layer is extended and reproduced.
BACKGROUND OF THE INVENTION
A number of studies and examinations have been made to promote high-densification in magneto-optical recording by utilizing magneto-optical effects to achieve repeatedly-overwritable information recording media with further greater capacity.
In such magneto-optical recording media, arises a drawback in that reproduction characteristics deteriorate as the diameter of recording bits as recording-use magnetic domains and the distance of the recording bits become smaller relative to the beam diameter of light beam converged on a medium.
This stems from that each recording bit cannot be reproduced separately, since a beam spot of the light beam converged on one target recording bit falls also on adjacent recording bits.
To solve the foregoing problem, the configuration of a recording medium, the reproducing method, etc. have been innovated to increase the recording density, and as a result, the super high resolution reproduction, the magnetic domain extending reproduction utilizing the moving (displacement) of magnetic domain walls, and the like are proposed. Here, the super high resolution reproduction and the magnetic domain extending reproduction will be explained below.
First, super high resolution technology is explained as a technology to achieve high densification of a magneto-optical recording medium.
As shown in
FIG. 9
, according to the magnetostatic-coupling-type super high resolution technology disclosed by, for example, the Japanese Publication for Laid-Open Patent Application No. 40600/1998 (Tokukaihei 10-40600, Date of Publication: Feb. 13, 1998) and the Japanese Publication for Laid-Open Patent Application No. 150418/1994 (Tokukaihei 6-150418, Date of Publication: May 31, 1994), a medium has at least a reproduction layer
121
, a non-magnetic intermediate layer
112
, and a recording layer
104
in this order. Arrows in each magnetic layer represent directions of magnetizations. The reproduction layer
121
is prepared so as to be in an in-plain magnetization at room temperature and to be turned to a perpendicular magnetization state when temperature rises to above a certain critical temperature during reproduction. The exchange-coupling between the reproduction layer
121
and the recording layer
104
is broken by the presence of the non-magnetic intermediate layer
112
.
No magneto-optical signal is generated in the reproduction layer
121
at room temperature since the reproduction layer
121
is in an in-plain magnetization state, but during reproduction, heated by a projected beam, only a central part of the beam spot of the medium (a part behind the beam spot center in the moving direction of the beam spot when the beam spot moves at a high linear speed) exhibits a perpendicular magnetization, only from which magneto-optical signals are generated. Let the part be called “aperture,” and the reproduction layer
121
is magnetostatically coupled with the recording layer
104
only at the aperture area in accordance with a magnetic field generated by a recording magnetic domain in the recording layer
104
, so that a magnetization direction thereof is determined according to the recording magnetic domain. Therefore, by this scheme, a recording magnetic domain can be selected and read only from the area of the aperture, thereby allowing a structure of micro magnetic domains, or high densification, of the magneto-optical recording medium.
The foregoing scheme, however, has a drawback in that a reproduction signal quantity decreases as the recording magnetic domains become smaller in size.
On the other hand, the magnetic domain extending reproduction is a scheme of reproduction by extending small recording magnetic domains, which ensures that a reproduction signal quantity great enough can be obtained even with small recording magnetic domains. The following description will explain the foregoing scheme, while referring to FIG.
10
.
As shown in
FIG. 10
, magnetic films in a multi-layer structure are exchange-coupled thereby causing small recording magnetic domains in a recording layer
104
to be extended in a magnetic domain extending layer (displacement layer)
102
so that amplitudes of reproduction signals are increased, thereby resulting in that high-density recording is realized. This is disclosed by the Japanese Publication for Laid-Open Patent Application No. 114750/1995 (Tokukaihei 7-114750, Date of Publication: May 2, 1995). Incidentally, arrows in the layers represent directions of sub-lattice magnetizations of transition metals in the film, and in each layer are formed magnetic domain walls (Bloch walls)
110
, each being present between adjacent magnetic domains having magnetizations in opposite directions.
To realize the magnetic domain extending reproduction, the following requirements have to be satisfied.
1. The recording layer
104
stably maintains small magnetic domains in a temperature range from room temperature to a temperature for reproduction.
2. At least in the vicinity of a Curie temperature Tc
3
of the intermediate layer
103
, the recording layer
104
, the intermediate layer
103
, and the magnetic domain extending layer
102
are exchange-coupled.
3. The intermediate layer
103
loses magnetic order when the temperature becomes higher than the foregoing Curie temperature Tc
3
, thereby breaking the exchange-coupling from the recording layer
104
to the magnetic domain extending layer
102
in a temperature range above the Curie temperature Tc
3
.
4. At a region where the foregoing exchange-coupling is broken, a magnetic domain wall
110
moves (is displaced) relative to a domain transferred by the exchange-coupling, since the frictional force due to magnetic domain wall coercivity in the magnetic domain extending layer
102
is small and a magnetic domain wall energy gradient is generated by a temperature gradient. As a result, the magnetization in the foregoing region is directed in the same direction as that transferred by the exchange-coupling.
The following description will explain in more detail the magnetic domain extending reproduction disclosed in Tokukaihei 7-114750, while referring to FIG.
10
.
The moving of the magnetic domain wall is explained first.
A graph in the center of
FIG. 10
shows the temperature distribution in the center of a track when a laser is projected on the optical recording medium which shifts rightward relative to the laser. Here, since the disk as the recording medium moves at a high linear speed, a portion where the film temperature becomes highest appears behind a center of a beam spot in the moving direction of the beam spot relative to the disk.
Further, a graph below in
FIG. 10
shows distribution of a magnetic domain wall energy density &sgr;
2
in a radial direction in the magnetic domain extending layer
102
. Normally, the magnetic domain wall energy density &sgr;2 decreases as the temperature rises, becoming 0 at the Curie temperature or above. Therefore, in the case of the temperature gradient in the radial direction as shown in the graph in the center of
FIG. 10
, the magnetic domain wall energy density &sgr;2 decreases to a level corresponding to the Curie temperature, as shown in the graph below in the same figure.
Here, a force F
2
expressed by the following formula shown below is exerted to magnetic domain walls
110
in each layer at a position x in the radial direction:
F
2
=−
d
&sgr;2/
dx
Since the force F
2
is exerted so as to move the magnetic domain walls
110
toward where the magnetic domain wall energy is lower and the magnetic domain extending layer
102
has a smaller frictional force due to coercivity of the magnetic domain wall
110
compared with the other magnetic layers thereby having great
Iketani Naoyasu
Mieda Michinobu
Mori Go
Takahashi Akira
Kiliman Leszek
Sharp Kabushiki Kaisha
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