Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse... – Magnetic field and light beam
Reexamination Certificate
2000-04-18
2002-07-16
Dinh, Tan (Department: 2653)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
Magnetic field and light beam
Reexamination Certificate
active
06421304
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a data reproducing apparatus and method handling a magneto-optical recording medium which has a magnetic three-layer film consisting at least of a displacement layer, a switching layer and a memory layer so formed that, in any region where the temperature of the magnetic film exceeds the Curie temperature of the switching layer, displacement of a magnetic wall is generated in the displacement layer to dimensionally enlarge the effectively recorded magnetic domain. More particularly, the present invention relates to a data reproducing apparatus and method capable of detecting generation of a magnetic wall displacement from a differential signal of a reproduced signal or a difference signal thereof in the time base direction and detecting data by the use of such detection output, hence performing proper reproduction of the data at a sufficiently low bit error rate even if any sudden DC level variation peculiar to a DWDD mode is caused in the reproduced signal.
There are known magneto-optical recording media employed as rewritable high-density recording media. And among such magneto-optical recording media, one type is attracting notice recently. It has a magnetic three-layer film consisting at least of a displacement layer, a switching layer and a memory layer, wherein a magnetic wall displacement of the displacement layer is generated in any region where the magnetic film temperature is rendered higher than the Curie temperature of the switching layer, so that the size of an effectively recorded magnetic domain is enlarged. A reproduction method handling such a magneto-optical recording medium is termed a DWDD (Domain Wall Displacement Detection) mode. According to this DWDD mode, a very large signal can be reproduced also from a tiny recording domain of a period below the optical limit resolution of a light beam, whereby a high density is attainable without the necessity of changing the wavelength of light, the numerical aperture NA of an objective lens and so forth.
Now a further detailed explanation will be given on such DWDD mode.
As shown in
FIG. 9A
, a magneto-optical recording medium
10
has a three-layer film of switched connection consisting of a displacement layer
11
, a switching layer
12
and a memory layer
13
formed in this order. The memory layer
13
is composed of a perpendicular magnetizing film indicating a great magnetic wall reluctance. The displacement layer
11
is composed of another perpendicular magnetizing film indicating a small magnetic wall reluctance and having a high magnetic wall displaceability. The switching layer
12
is composed of a magnetic layer whose Curie temperature Ts is lower than those of the displacement layer
11
and the memory layer
13
. Each arrow
14
in the individual layers denotes the direction of atomic spin. A magnetic wall
15
is formed in the boundary between regions where the atomic spin directions are mutually reverse.
If the surface of a recording film is locally heated by the use of a reproducing light beam (laser beam)
16
, there is formed a distribution of temperature T as shown in
FIG. 9B
, and accordingly a distribution of magnetic wall energy density &sgr; is formed as shown in FIG.
9
C. Since the magnetic wall energy density &sgr; generally becomes lower in accordance with a temperature rise, the distribution is such that the magnetic wall energy density &sgr; becomes minimum at the position of the highest temperature. As a result, a magnetic wall driving force F(x) for displacement toward the high temperature side, where the magnetic wall energy density &sgr; is low, is generated as shown in FIG.
9
D.
FIG. 9E
shows the positional relationship between a spot
16
P of the light beam
16
and a region
17
whose temperature is higher than the Curie temperature Ts of the switching layer
12
.
In any area of the medium
10
where the temperature is lower than the Curie temperature Ts of the switching layer
12
, the magnetic layers are mutually in switched connection, so that even when the magnetic wall driving force F(x) due to the above-described temperature gradient is applied, it is checked by the great magnetic wall reluctance of the memory layer
13
to eventually cause no displacement of the magnetic wall
15
. However, in any area of the medium
10
where the temperature is higher than the Curie temperature Ts, the switched connection between the displacement layer
11
and the memory layer
13
is cut off, so that the magnetic wall
15
of the displacement layer
11
having a small magnetic wall reluctance is rendered displaceable by the magnetic wall driving force F(x) due to the temperature gradient. Consequently, upon entrance of the magnetic wall
15
into the connection cut-off region beyond the position of the Curie temperature Ts with scanning of the medium
10
by the light beam
16
, then the displacement layer
12
begins to be displaced toward the higher temperature side of the magnetic wall
15
.
Whenever any of the magnetic walls
15
formed at intervals corresponding to the recorded signal on the medium
10
passes the position of the Curie temperature Ts with scanning of the medium by the light beam
16
, there occurs a displacement of the magnetic wall
15
of the displacement layer
11
. Since the effectively recorded magnetic domain is enlarged dimensionally by such displacement, it becomes possible to reproduce a very large signal as well even from tiny recorded domains of a period below the optical limit resolution of the light beam
16
.
As the light beam
16
scans the medium
10
at a fixed speed, the above-described magnetic wall displacement is generated at a temporal interval corresponding to the spatial interval of the recorded magnetic walls
15
. The generation of such magnetic wall displacement can be detected as a change in the polarization plane of the reflected light of the light beam (laser beam)
16
.
As shown by broken lines in
FIG. 9A
, a magnetic wall displacement is generated from the rear of the region
17
as well, so that the signal due to such magnetic wall displacement from the rear is superimposed as a ghost signal on the reproduced signal due to the magnetic wall displacement from the front. Although an explanation on this ghost signal is omitted here, the problem arising therefrom can be solved by properly contriving the application of a reproducing magnetic field or the recording film.
The DWDD type magneto-optical disk apparatus mentioned above is substantially similar in structure to any general magneto-optical disk recording/reproducing apparatus.
FIG. 10
shows a partial structure of a conventional reproducing section in such an apparatus. A reproduced signal S
MO
obtained from an optical head (not shown) is supplied to an equalizer circuit
21
where the frequency characteristic thereof is compensated. A reproduced signal S
MO
, obtained after such frequency characteristic compensation is supplied to a binary coding circuit
22
, which then converts the input signal into a binary signal S
2
.
The binary signal S
2
outputted from the binary coding circuit
22
is supplied to a data detection circuit
23
and a PLL (phase-locked loop) circuit
24
. In the PLL circuit
24
, a clock signal CLK synchronized with the leading and trailing edges of the binary signal S
2
is produced, and then the clock signal CLK is supplied to the data detection circuit
23
. Subsequently in the data detection circuit
23
, data are detected from the binary signal S
2
by the use of the clock signal CLK and then are outputted as reproduced data PD.
On a DWDD type magneto-optical disk, signals are recorded in such a manner that data bit strings are first converted into, e.g., RLL modulated bits and then are processed in NRZI (Non-Return to Zero Inverted) mode where a portion with data inversion is expressed as
1
while a portion without data inversion is expressed as
0
. In this case, the data detection circuit
23
converts, e.g., NRZI data into NRZ data, whereby RLL modulated data are obtained as
Fukumoto Atsushi
Kai Shinichi
Yoshimura Shunji
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