Stock material or miscellaneous articles – All metal or with adjacent metals – Having magnetic properties – or preformed fiber orientation...
Reexamination Certificate
2002-04-12
2004-11-02
Bernatz, Kevin M. (Department: 1773)
Stock material or miscellaneous articles
All metal or with adjacent metals
Having magnetic properties, or preformed fiber orientation...
C428S637000, C428S668000, C428S678000, C428S336000, C428S690000, C428S690000, C428S690000, C369S013430, C369S031010, C369S013450, C369S013460
Reexamination Certificate
active
06811889
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magneto-optical recording medium, and more particularly to a magneto-optical recording medium in which data is recorded and erased using the increase in temperature of a recording layer due to heating by a laser beam and data is optically read out using a magneto-optical effect.
2. Description of the Related Art
Data is recorded in an magneto-optical recording medium by thermal magnetic recording. More specifically, a laser beam is irradiated onto a recording layer in the magneto-optical recording medium; as a result, the recording layer is locally heated to a Curie temperature or more. The irradiated region of the recording layer is magnetized in the direction of an external magnetic field to form a recorded magnetic domain. On the other hand, data is read out using a magneto-optical effect. That is, a weak laser beam is irradiated onto the recording layer. In this case, the power of the laser beam is so low to such a degree that data cannot be recorded and erased. Then, a polarization plane of light reflected from or transmitted through the recording layer is rotated in accordance with the recorded state (i.e., the direction of magnetization of the recorded magnetic domain) thereof. The data is read out by detecting this rotation.
As for a conventional thermal magnetic recording method, there are two methods: a magnetic field modulation recording method and a laser power modulation recording method. According to the magnetic field modulation recording method, a laser beam with a predetermined intensity is irradiated onto a recording layer to increase the temperature thereof, and the direction of an external magnetic field is modulated in accordance with a signal to be recorded. According to the laser power modulation recording method, a laser beam with its intensity modulated in accordance with a signal to be recorded is irradiated to the recording layer under the external magnetic field with a predetermined intensity. In particular, in order to increase the linear recording density in the longitudinal direction of a recording track, the magnetic field modulation recording method is excellent. The reason for this is that the length of the recorded magnetic domain is not limited to a spot size of a laser beam in the magnetic field modulation recording method.
Hereinafter, a conventionally proposed method for overwriting data by the laser power modulation recording method will be described.
FIG. 15
is a schematic cross-sectional view showing a magneto-optical recording medium. As shown in this figure, a recording layer includes a recording/readout magnetic film
151
and a supporting magnetic film
152
. The recording/readout magnetic film
151
is a perpendicular magnetic anisotropy film which has a high coercivity H
c1
and a low Curie temperature T
c1
. The supporting magnetic film
152
is a perpendicular magnetic anisotropy film which has a low coercivity H
c2
and a high Curie temperature T
c2
. These films are exchange-coupled with each other. Data is recorded in the recording layer by thermal magnetic recording, using an initializing magnetic field (H
i
)
153
and a recording magnetic field (H
b
)
154
which generate magnetic fields opposite to each other, and a laser beam
155
whose intensity is modulated in accordance with a signal to be recorded (e.g., J. Saito et al., Proc. Int. Symp. on Optical Memory, 1987, J P N, J. Appl. Phys., Vol. 26, Supplement 26-4 (1987), p. 155).
The recording/readout magnetic film
151
is for recording and reading out data, and the supporting magnetic film
152
is for assisting the recording of data into the recording/readout magnetic film
151
. These films are exchange-coupled with each other by a exchange-coupling force H
1-2
(H
2-1
) therebetween. Suppose that the magnitude of magnetization of the recording/readout magnetic film
151
and that of the supporting magnetic film
152
are M
1
and M
2
, the thicknesses thereof are t
1
and t
2
, and energy of a domain wall therebetween, if any, is &sgr;w, the exchange-coupling force H
1-2
seen from the recording/readout magnetic film
151
is represented by the following equation:
H
1-2
=&sgr;w/
2
M
1
t
1
and the exchange-coupling force H2-1 seen from the supporting magnetic film
152
is represented by the following equation:
H
2-1
=&sgr;w/
2
M
2
t
2
At room temperature, the following relationships are obtained: H
c1
>H
1-2
, H
c2
>H
2-1
, and H
c2
+H
2-1
<H
i
<H
c1
, and the magnetization direction of the supporting magnetic film
152
is aligned with a direction of the initializing magnetic field (H
i
)
153
.
Recording data in the magneto-optical recording medium with the above-mentioned structure will be described, in which a laser beam with a low-level intensity and a laser beam with a high-level intensity are used. In the case of using a laser beam at a low level, when irradiated with the laser beam, the temperature of the recording layer reaches the vicinity of the Curie temperature T
c1
of the recording/readout magnetic film
151
and the coercivity H
c1
thereof is lower than H
1-2
. Thus, the magnetization direction of the supporting magnetic film
152
(i.e., the direction of H
i
) in the vicinity of the Curie temperature T
c1
is transferred to the recording/readout magnetic film
151
by the exchange-coupling force H
1-2
. In the case of using a laser beam at a high level, when being irradiated with the laser beam, the temperature of the recording layer reaches the vicinity of the Curie temperature T
c2
of the supporting magnetic film
152
. Thus, the magnetization direction of the supporting magnetic film
152
is aligned with the direction of the recording magnetic field
154
(H
b
). Thereafter, in the course of cooling step, the magnetization direction of the supporting magnetic film
152
is transferred to the recording/readout magnetic film
151
by the exchange-coupling force H
1-2
.
As described above, data can be overwritten in the magneto-optical recording medium by these two operations.
In the conventional magneto-optical recording medium, when the length of a recorded magnetic domain to be read out becomes less than the spot size of a readout light, recorded magnetic domains adjacent to the recorded magnetic domain to be read out are within the range of the readout light. Consequently, readout signals based on these adjacent recorded magnetic domains are detected together with a readout signal based on the recorded magnetic domain to be read out. Therefore, an S/N ratio is decreased due to the signal interference of the readout signals.
In view of the above problem, a magneto-optical recording medium having a super resolution effect has been proposed (M. Ohta et al., Proceeding of Magneto-optical Recording International Symposium '91, J. Magn. Soc. J P N., Vol. 15, Supplement No. S1 (1991), p. 319). According to the super resolution effect, the spot size of readout light apparently becomes smaller. Readout of data by using this effect is called readout by magnetically induced super resolution. An exemplary structure of a magneto-optical recording medium for super resolution readout will be described with reference to
FIGS. 14A and 14B
.
FIG. 14A
is a top plan view of the magneto-optical recording medium, and
FIG. 14B
is a cross-sectional view thereof. In these figures, the reference numeral
141
denotes an initializing magnetic field H
i
,
142
a recording magnetic field H
r
,
143
readout light,
144
a readout light spot,
145
a recorded magnetic domain,
146
a region at a temperature of T
d
or more,
147
a readout magnetic film made of a perpendicular magnetic anisotropy film with a low coercivity H
c1
,
148
a recording magnetic film made of a perpendicular magnetic anisotropy film with a high coercivity H
c2
. The readout magnetic film
147
and the recording magnetic film
148
are exchange-coupled with each other by a exchange-coupling force H
1-2
(H
2-1
) to form a recording layer.
At room temperature, the c
Birukawa Masahiro
Fukamachi Yuuichi
Hino Yasumori
Kudoh Yoshihiko
Bernatz Kevin M.
Matsushita Electric - Industrial Co., Ltd.
Renner , Otto, Boisselle & Sklar, LLP
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