Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse... – Magnetic field and light beam
Reexamination Certificate
2000-07-28
2003-04-08
Neyzari, Ali (Department: 2655)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
Magnetic field and light beam
C369S013430, C428S690000
Reexamination Certificate
active
06545955
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to magneto-optical disks, tapes, cards, and other similar storage media used in conjunction with magneto-optical recording/reproduction devices, and their reproduction methods.
BACKGROUND OF THE INVENTION
Conventionally, magneto-optical storage media have been commercialized as rewritable optical storage media. Such a magneto-optical storage medium has a disadvantage that reproduction characteristics deteriorate when the diameters of, and intervals between, recording bits, i.e., recording magnetic domains, are reduced too much relative to the diameter of a spot formed on the magneto-optical storage medium by focusing a light beam projected by a semiconductor laser device.
This is because the light beam focused on the targeted recording bit encompasses adjacent recording bits within its coverage and fails to separately reproduce the individual recording bits.
To overcome the disadvantage, various magnetic super high resolution reproduction technologies have been developed using a magnetic multi-layer film. These magnetic super high resolution reproduction technologies reduce signal interference during reproduction by forming a magnetic masking area and thus forming a magnetic aperture smaller than the beam spot, thus enabling reproduction of signals whose cycles do not exceed diffraction limits of light.
Nevertheless, the magnetic super high resolution reproduction technologies have a problem that the strength of reproduced signals decreases with a decrease in the recording cycle for the magnetic recording domain, because the aperture also needs to be reduced in size.
To solve the problem, a method is suggested to enable magnetic domain expansion reproduction without applying a.c. external magnetic fields ((Magnetic Domain Expansion Readout with DC Lasers and DC Magnetic Fields [Magnetic Amplifying Magneto-Optical system, or MAMMOS]), an article from resumes for lectures in 44th Conference organized in spring 1997 by the Society of Applied Physics Researchers, 30a-NF-3, page 1068).
Now, referring to FIG.
17
through
FIG. 19
, a magneto-optical storage medium based on the foregoing method will be explained.
FIGS. 17 and 18
are plan and cross-sectional views schematically illustrating magnetization of the magneto-optical storage medium during reproduction.
FIG. 19
is a cross-sectional view showing the arrangement of a magneto-optical disk that is an application of the magneto-optical storage medium.
As shown in
FIG. 18
, the magneto-optical storage medium has a stack structure including a reproduction layer
201
, a supplementary reproduction layer
203
, and a storage layer
207
. The reproduction layer
201
and the supplementary reproduction layer
202
exhibit in-plane magnetization at room temperature, and changes to perpendicular magnetization when temperature is elevated by projection of a focused light beam
208
(light beam spot
208
′ in FIG.
17
). The storage layer
204
is constituted by a perpendicularly magnetized film, where magnetic information is stored in the form of directions of the magnetization in magnetic domains
209
and
210
.
The reproduction layer
201
is specified to change to perpendicular magnetization at a lower temperature than the supplementary reproduction layer
203
changes to perpendicular magnetization. Consequently, on heating by the light beam
208
, the magnetic domain
212
in which the reproduction layer
201
has changed to perpendicular magnetization grows larger than the magnetic domain
211
in which the supplementary reproduction layer
203
changes to perpendicular magnetization.
The direction of the magnetization in the magnetic domain
211
, in which the supplementary reproduction layer
203
has changed to perpendicular magnetization due to the heat of the light beam
208
, is determined by coupling to the storage layer
207
through exchange forces. Hence, the magnetic information in the storage layer
207
is duplicated to the supplementary reproduction layer
203
so that the direction of the auxiliary grating moment of the supplementary reproduction layer
203
conforms to that of the storage layer
207
.
Next, the magnetic information in the magnetic domain
211
, in which the supplementary reproduction layer
203
has changed to perpendicular magnetization, is duplicated to the reproduction layer
201
so that the direction of the transition metal (TM) moment of the reproduction layer
201
conforms to that of the supplementary reproduction layer
203
. Here, since the magnetic domain
212
, in which the reproduction layer
201
changes to perpendicular magnetization, grows larger than the magnetic domain
211
, in which the supplementary reproduction layer
203
changes to perpendicular magnetization, the magnetization of the supplementary reproduction layer
203
, i.e., the magnetization of the storage layer
207
, is expanded and duplicated to the reproduction layer
201
.
As described above, in the magneto-optical storage medium used in accordance with the aforementioned method, small magnetic domains in the storage layer
207
are expanded and duplicated to the reproduction layer
201
; therefore, high density storage is realized, and expansion of reproduction signals is enabled.
It should be noted that as shown in
FIG. 19
, typically, the foregoing magneto-optical storage medium includes the arrangement shown in
FIG. 18
, and further constitutes overlapping layers including a substrate
213
, a transparent dielectric protection layer
214
, and a protection layer
215
among others.
However, the storage layer
207
, the supplementary reproduction layer
203
, and the reproduction layer
201
are coupled together through exchange forces in the magneto-optical storage medium capable of reproducing magnetic domains by means of expansion in accordance with the aforementioned method.
Therefore, the transition from in-plane to perpendicular magnetization of the supplementary reproduction layer
203
and the reproduction layer
201
proceeds gradually with rising temperature, resulting in difficulties in improving reproduction resolution.
Further, in a vicinity of transition temperature Tp
201
at which the reproduction layer
201
changes to perpendicular magnetization, the supplementary reproduction layer
203
exhibits in-plane magnetization and is coupled to the reproduction layer
201
through exchange forces. The coupling interrupts the change of the reproduction layer
201
to perpendicular magnetization, ostensibly raising transition temperature Tp
201
. Consequently, the magnetic domain
212
formed in the reproduction layer
201
becomes smaller than when no coupling is established through exchange forces. In a vicinity of transition temperature Tp
203
at which the supplementary reproduction layer
203
changes to perpendicular magnetization, the reproduction layer
201
exhibits perpendicular magnetization and is coupled to the supplementary reproduction layer
203
through exchange forces. The coupling causes the supplementary reproduction layer
203
to start changing to perpendicular magnetization at a temperature below transition temperature Tp
203
at which the supplementary reproduction layer
203
desirably changes to perpendicular magnetization, ostensibly lowering transition temperature Tp
203
. Consequently, the magnetic domain
211
formed in the supplementary reproduction layer
203
becomes smaller than when no coupling is established through exchange forces.
When the magnetization in the storage layer
207
is duplicated to the supplementary reproduction layer
203
, the magnetic domain
211
is larger than the magnetic recording domain
209
, and therefore affected by magnetic domains surrounding the magnetic recording domain
209
, making it difficult to duplicate the magnetization to the supplementary reproduction layer
203
with high resolution. Further, the magnetization in the magnetic domain
211
in the supplementary reproduction layer is not sufficiently expanded and duplicated to the magnetic domain
212
,
Hirokane Junji
Iwata Noboru
Birch & Stewart Kolasch & Birch, LLP
Neyzari Ali
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