Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse... – Magnetic field and light beam
Reexamination Certificate
2000-03-31
2003-02-11
Dinh, Tan (Department: 2653)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
Magnetic field and light beam
C369S013540
Reexamination Certificate
active
06519211
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magneto-optical recording disk onto which information is written or deleted using a rising temperature due to irradiation of laser light and from which a recorded signal is read using a magneto-optical effect. The present invention also relates to a method and device for reproducing information recorded on the magneto-optical recording disk.
2. Description of the Related Art
In magneto-optical recording, a portion of a magnetic film of a magnetic-optical recording medium is locally heated to a Curie point or compensation composition temperature or higher by irradiation of laser light. The heated portion is magnetized in the direction of an external magnetic field, thereby forming a recording magnetic domain where an information signal is recorded. The magnetic film onto which the information signal is recorded is also referred to as a recording magnetic film (or simply recording film).
Among such magneto-optical recording methods for the magneto-optical recording medium is a magnetic field modulation recording method. In this method, the temperature of an overall recording magnetic film is increased by irradiation of laser light. An external magnetic field having a modulated direction in accordance with a recording signal is applied to a given portion of the recording magnetic film. The recording signal is thermomagnetically recorded on the given portion. This is called a magnetic field modulation recording method. Alternatively, laser light having a modulated intensity in accordance with a recording signal is irradiated onto a given portion of a recording magnetic film. The temperature of the given portion is increased so that the recording signal is thermomagnetically recorded onto the given portion. This is called an optical modulation recording method.
In a conventional magneto-optical recording medium, when the size of the recording magnetic domain is smaller than or equal to the diameter of a reproducing light spot, recording magnetic domains at the front and rear side of the recording magnetic domain which is a target to be reproduced are included in the reproducing light spot (i.e., a detection range). Interference of the adjacent recording magnetic domains causes a decrease in the reproduced signal, whereby the S/N ratio is reduced or the reproduction signal is not output.
A magneto-optical recording and reproducing method using magnetic super-resolution as shown in
FIGS. 1A and 1B
is a proposed technique to solve such a problem (see Nikkei Electronics, No. 539, Oct. 28, 1991). This magneto-optical recording and reproducing method will be briefly described below.
As shown in a cross-sectional view of
FIG. 1B
, a magneto-optical recording medium
60
includes a reproducing magnetic film
63
, a transcribing magnetic film
64
A, an intermediate film
64
, and a recording magnetic film
65
which are successively provided on a substrate (not shown). An arrow X in
FIG. 1B
indicates a moving direction along a track of the magneto-optical recording medium
60
. An upward arrow
61
indicates a magnetic field for recording and reproduction. A downward arrow
62
indicates an initial magnetic field.
FIG. 1A
is a plan view illustrating a part of a track of the magneto-optical recording medium
60
.
As shown in
FIGS. 1A and 1B
, when reproducing information, a reproducing light spot
67
is formed along the track. When laser light is irradiated onto the rotating magneto-optical recording medium
60
, the temperature distribution of a magnetic film structure including the reproducing magnetic film
63
and the transcribing magnetic film
64
A are not rotation symmetrical around the center of the circular reproducing light spot
67
. Specifically, a region
70
which has been irradiated by the reproducing light spot
67
has a high temperature greater than or equal to the Curie temperature Tc of the transcribing magnetic film
64
A (the region
70
is referred to as a high temperature region
70
). A crescent-shaped region
72
, which is positioned at the left side of the reproducing light spot
67
and outside the high temperature region
70
, has an intermediate temperature (the region
72
is referred to as an intermediate temperature region
72
). A region
71
which is positioned at the right side of the intermediate temperature region
72
and within the reproducing light spot
67
has a low temperature (the region
71
is referred to as a low temperature region
71
).
Assuming that a signal (information) is already thermomagnetically recorded as a recording magnetic domain
69
on the recording magnetic film
65
, the transcribing magnetic film
64
A is strongly exchange-coupled with the reproducing magnetic film
63
. The intermediate film
64
is provided in such a manner that the magnetic domain wall becomes stable when the magnetization direction of the reproducing magnetic film
63
is in agreement with the magnetization direction of the recording magnetic film
65
.
The reproducing operation of the magneto-optical recording medium
60
thus constructed will be described below.
The reproducing magnetic film
63
initially has the same magnetization direction as that of the initializing magnetic field
62
. Upon reproduction, laser light for reproduction is irradiated to a range between X1 and X2 shown in FIG.
1
B. The laser light forms the reproducing light spot
67
on the rotating magneto-optical recording medium
60
. This causes an increase in temperature of the rotating magneto-optical recording medium
60
, resulting in a temperature distribution shown in
FIG. 1A
(i.e., the temperature region
70
,
71
, and
72
). The coercive force of the reproducing magnetic film
63
is decreased due to the temperature increase. Exchange-coupling with the recording magnetic film
65
is therefore dominant in the intermediate temperature region
72
, so that the magnetization of the reproducing magnetic film
63
is directed to the magnetization direction of the recording magnetic film
65
.
In the high temperature region
70
having a temperature of Tc or higher, the magnetization of the transcribing magnetic film
64
A disappears in some portions thereof. Exchange-coupling between the reproducing magnetic film
63
and the recording magnetic film
65
is cut off at these portions, so that the magnetization of the reproducing magnetic film
63
is directed to the magnetization direction of the reproducing magnetic field
61
. Accordingly, the low and high temperature regions
71
and
70
within the reproducing light spot
67
masks the recording magnetic domains
69
. Only from a recording magnetic domain
69
X positioned in the intermediate temperature region
72
is information read as a reproduced signal.
With the above-described method, even when a single recording magnetic domain
69
has a size smaller than the diameter of the reproducing light spot
67
, there occurs substantially no interference by recording magnetic domains
69
ahead of and behind the single recording magnetic domain
69
. It is therefore possible to reproduce information stored in high density.
There is, however, a drawback with the above-described magneto-optical recording medium
60
as it needs the initializing magnetic field
62
for initially directing the magnetization of the reproducing magnetic film
63
in a single direction.
Japanese Laid-Open Publication No. 5-81717 proposes a magneto-optical recording medium
80
having a structure shown in
FIGS. 2A and 2B
which does not need the initializing magnetic field.
As shown in a cross-sectional view of
FIG. 2B
, the magneto-optical recording medium
80
includes a reproducing magnetic film
83
and a recording magnetic film
85
on a substrate (not shown). An arrow X represents a moving direction along a track of the magneto-optical recording medium
80
. As is different from the magneto-optical recording medium
60
shown in
FIGS. 1A and 1B
, an in-plane magnetization film is used as the reproducing magnetic film
83
in the magneto-optic
Murakami Motoyoshi
Nishikiori Keiji
Dinh Tan
Matsushita Electric - Industrial Co., Ltd.
Snell & Wilmer LLP
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