Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse... – Magnetic field and light beam
Reexamination Certificate
2001-10-05
2003-06-10
Neyzari, Ali (Department: 2655)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
Magnetic field and light beam
C369S013350, C369S275400
Reexamination Certificate
active
06577560
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magneto-optic recording medium as well as a high density magneto-optic recording and reproducing method and device, in which the recording medium has a multilayered magnetic layer including a recording layer and a reproducing layer. By utilizing temperature distribution generated by a laser beam irradiation, a magnetic domain is transferred from the recording layer to the reproducing layer. By detecting the magnetic domain that is enlarged after a domain wall moves, a micro mark can be reproduced beyond a resolution of an optical system.
DESCRIPTION OF THE PRIOR ART
A magneto-optic recording medium is a removable recording medium having advantages of large record capacity, high reliability and good portability. Such a magneto-optic recording medium is used widely in various applications such as a data recording medium of a computer or a recording medium of image data. Along with the various applications, the market demands for high density and large capacity has been increasing year after year.
There are two general methods for increasing record density (i.e., for increasing record capacity) in a disk-like recording medium on which data are recorded along tracks. In a first method, a record mark is downsized so that bit density in a track can be increased. In a second method, track density is increased.
In the case of a magneto-optic recording medium (usually called a magneto-optic disk), the first method is restricted by a spot diameter of a light beam that is irradiated to the recording medium. In order to reproduce a bit that was recorded in a period smaller than a beam spot diameter, a light beam having less beam spot should be used. However, the beam spot diameter cannot be smaller than a value determined by a wavelength &lgr; of a light source and a numerical aperture NA of an object lens. In other words, a micro mark cannot be reproduced beyond the resolution of the optical system.
However, as described in Japanese unexamined patent publication No. 6-290496 for example, a magneto-optic recording and reproducing method is proposed, which enables reproduction of a micro mark beyond the resolution of the optical system. This magneto-optic recording and reproducing method uses a recording medium having a multilayered magnetic layer including a recording layer and a reproducing layer. Temperature distribution due to a laser beam irradiation generates the expansion of a magnetic domain copied from the recording layer to the reproducing layer. The magnetic domain is detected after being enlarged by a movement of a domain wall, so that a micro mark can be reproduced. The principle of this magneto-optic recording and reproducing method will be explained below.
FIG. 1
shows a structure of the magnetic layer of the magneto-optic recording medium that is used for the magneto-optic recording and reproducing method and the principle of the signal reproduction. As shown in
FIG. 1
a
, the magneto-optic recording medium
100
has a multilayered magnetic layer including a recording layer
101
having high coercivity of a domain wall, a switching layer
102
having a relatively low Curie temperature Ts and a reproducing layer
103
having low coercivity of a domain wall. When the magneto-optic recording medium
100
is irradiated with a laser beam LB, a temperature distribution TD is generated as shown by a graph in
FIG. 1
b
. The temperature is the highest at the center of the beam spot and lower as departed from the center. However, the temperature distribution becomes asymmetric in accordance with a movement direction (a rotation direction) DD of the magneto-optic recording medium
100
.
In the area where the temperature is lower than the Curie temperature Ts of the switching layer
102
, the recording layer
101
and the reproducing layer
103
have exchange coupling with each other via the switching layer
102
. Therefore, the domain wall movement is not generated in the reproducing layer
103
being hindered by the high domain wall coercivity of the recording layer
101
. However, in the area RC including the beam spot center where the temperature is higher than the Curie temperature Ts of the switching layer
102
, magnetization of the switching layer
102
is erased. As a result, the exchange coupling between the recording layer
101
and the reproducing layer
103
is cut off, and the movement DM of the domain wall to the high temperature side is generated in the reproducing layer
103
having low domain wall coercivity. Thus, the domain wall of the magnetic domain MS recorded on a track moves sequentially from the portion that reaches the Curie temperature Ts of the switching layer
102
to the center of the laser beam irradiation spot while the magneto-optic recording medium
100
rotates. This movement of the domain wall causes enlargement of the magnetic domain, which can be read by the laser beam LB.
As explained above, in the magneto-optic recording and reproducing method described in Japanese unexamined patent publication No. 6-290496, the recorded micro mark is reproduced not directly but indirectly. Namely, a magnetic domain, which was enlarged by the domain wall movement in the reproducing layer, is read. Thus, the micro mark can be reproduced beyond the resolution of the optical system.
However, the above-mentioned magneto-optic recording medium must have recording and reproducing tracks (also referred to simply as recording tracks) that are separated from neighboring tracks magnetically. For example, in one method, every other track of the recording tracks is irradiated with a high power laser beam to be heated, so that a magnetic partition area (a kind of track) having a different magnetic state is formed between the recording and reproducing tracks. In another method, a groove or a bank is formed between the recording and reproducing tracks so as to separate the tracks from each other spatially. In either method for making a magnetic partition area (a track), such a pseudotrack that does not contribute to information recording and reproducing directly may become an obstacle to the second method of realizing high density by shortening a track pitch.
There is another method for realizing high density of a magneto-optic recording medium as described in Japanese unexamined patent publication No. 6-76399, for example. In this multivalued recording method, an edge position of a record mark is shifted step by step from a predetermined reference position in accordance with record data.
FIG. 2
shows an example where an edge position of a record mark is shifted in four steps, so that four values (two bits) of information can be recorded in one record mark. Prepits of a magneto-optic recording medium are reproduced by an optical head, so that a reference clock is produced from the reproduced signal. This reference clock is used as a modulation signal for recording magnetic field.
As shown in
FIG. 2
, external magnetic field EM having a polarity alternating at a predetermined frequency in accordance with a reference clock is applied to a magneto-optic recording medium by a magnetic head. On the other hand, a laser pulse LP is irradiated at a timing of t
0
, t
1
, t
2
or t
3
in accordance with record data. Thus, an edge of a record mark WM is shifted in four positions corresponding to the four timings t
0
-t
3
. In addition, after a half period (T/2), the external magnetic field EM having the opposite polarity is applied while the laser pulse LP is irradiated again at the timing of t
0
, t
1
, t
2
or t
3
, so that a mark is recorded with an overlapping of the former mark.
By the above-mentioned multivalued recording method, bit density in a track is increased. However, if this recording method is combined with the above-mentioned domain wall movement type reproducing method, the recording tracks must be separated from each other magnetically as explained above. Namely, since the recording tracks RT and the magnetic partition areas (magnetic partition tracks) IT are arranged alternately as shown in
FIG. 3
, incre
Itakura Akihiro
Taguchi Masakazu
Greer Burns & Crain Ltd.
Neyzari Ali
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