Magneto-optical recording medium and reproducing device

Details Magneto-optical recording medium and reproducing device Magneto-optical recording medium and reproducing device

1999-12-07

2002-02-12

Kiliman, Leszek (Department: 1773)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Physical dimension specified

C428S336000, C428S690000, C428S690000, C428S690000, C428S900000

Reexamination Certificate

active

06346322

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to magneto-optical recording media, such as magneto-optical disks, magneto-optical tapes and magneto-optical cards, and a reproducing device for reproducing information on such a magneto-optical recording medium.
BACKGROUND OF THE INVENTION
A magneto-optical recording medium has been practically used as a rewritable optical recording medium. Information is recorded on and reproduced from a magneto-optical recording medium with a converged light beam emitted from a semiconductor laser. However, the magneto-optical recording medium has such a drawback that the reproduction properties deteriorate when the diameter of a recording bit as a recording-use domain and the interval of the recording bits become smaller with respect to the diameter of the light beam.
When the diameter of the recording bit and the interval of the recording bits become smaller with respect to the diameter of the light beam, a recording bit adjacent to a target recording bit enters into the diameter of the light beam converged on the target recording bit. As a result, individual recording bits can not be read out separately and the reproduction properties deteriorate.
A structure for solving the above drawback of the magneto-optical recording medium is proposed in “High-Density Magneto-Optical Recording with Domain Wall Displacement Detection” (Joint Magneto-Optical Recording International Symposium/International Symposium on Optical Memory 1997 Technical Digest, Tu-E-04, p. 38,39). In this magneto-optical recording medium, the first, second and third magnetic layers are layered in this order. The first magnetic layer is made of a perpendicularly magnetized film having a relatively small wall coercivity and a relatively large wall mobility compared with those of the third magnetic layer in the vicinity of a readout temperature. The Curie temperature of the second magnetic layer is set lower than the Curie temperatures of the first and third magnetic layers. According to this structure, even when the recording bit diameter and the recording bit interval are small, individual recording bits can be read out separately without lowering the readout signal level, by moving the domain wall into a region where the temperature has risen by the irradiation of a light beam.
A method of reproducing information on the magneto-optical recording medium with the above-described structure will be explained with reference to
FIG. 10. A
first magnetic layer
101
, a second magnetic layer
102
and a third magnetic layer
103
are layered in an exchange coupled state. Denoting the Curie temperatures of the first, second and third magnetic layers in the laminated state by Tc
101
, Tc
102
and Tc
103
, respectively, Tc
101
and Tc
102
satisfy the relationship Tc
102
<Tc
101
. In
FIG. 10
, the arrows show the direction of transition metal magnetic moments of the respective magnetic layers. Here, magnetic domains have already been recorded in the third magnetic layer
103
, and an upwardly oriented magnetic domain and a downwardly oriented magnetic domain are present alternately in a repeated manner.
When a reproduction-use light beam
106
is irradiated and converged on such a magneto-optical recording medium from the first magnetic layer
101
side, the second magnetic layer
102
has a region heated to a temperature equal to or higher than its Curie temperature. In a region having a temperature lower than the Curie temperature, the magnetic domain information in the third magnetic layer
103
is copied to the first magnetic layer
101
through the second magnetic layer
102
by the exchange coupling. In other words, the upward transition metal magnetic moment at the front part of a region
110
irradiated with the light beam is copied as it is from the third magnetic layer
103
to the first magnetic layer
101
.
On the other hand, in the region heated to a temperature equal to or higher than the Curie temperature of the second magnetic layer
102
(the region located behind a light beam
106
by a movement of the medium such as a rotation of a disk substrate), since the exchange coupling between the first magnetic layer
101
and third magnetic layer
103
is cut off by the second magnetic layer
102
, the domain wall in the first magnetic layer
101
is readily movable.
When the information in the third magnetic layer
103
is copied as it is to the first magnetic layer
101
, a domain wall
107
is essentially formed. However, in a region where the second magnetic layer
102
has been heated to a temperature equal to or higher than its Curie temperature, since the domain wall in the first magnetic layer
101
is readily movable, the domain wall
107
moves to the most stable location. Here, considering a fact that the domain wall energy density decreases with an increase in temperature, the domain wall
107
moves to a location where the temperature is increased most by the irradiation of the light beam
106
, and forms a domain wall
108
.
Thus, in the magneto-optical recording medium of the above-described structure, since the domain wall can be moved by the characteristic of the second magnetic layer
102
, the recording domain of the third magnetic layer
103
can be enlarged in the first magnetic layer
101
. Therefore, even when the recording domain is reduced, it is possible to increase the amplitude of the readout signal from the first magnetic layer
101
, thereby allowing readout of signals of a cycle less than the diffraction limit of light.
However, in the above-mentioned reproduction method, there are two types of domain movements, i.e., a domain movement from the front part and a domain movement from the rear part. Hence, there is a problem that a single domain is read out twice. Referring now to
FIGS. 11 and 12
, the following description will explain this point.
FIG. 11
shows a state in which an independent magnetic domain
109
formed in the third magnetic layer
103
is present at the front part of the light beam
106
, the third magnetic layer
103
and first magnetic layer
101
are exchange coupled at the position of the independent magnetic domain
109
, and the upward moment is copied to the first magnetic layer
101
. In
FIG. 11
, the shaded portion of the second magnetic layer
102
is a region X where the second magnetic layer
102
is heated to its Curie temperature or a higher temperature.
In the state shown in
FIG. 11
, the domain wall
107
moves to the position of the domain wall
108
to enlarge the magnetic domain, and a readout magnetic domain
111
with an upward moment is formed in the region
110
irradiated with the light beam
106
. Therefore, a large readout signal amplitude is obtained. When the medium (magneto-optical recording medium) is moved relatively to the light beam
106
from the state shown in
FIG. 11
, a downward moment of the third magnetic layer
103
is copied to the first magnetic layer
101
upon passage of the independent magnetic domain
109
through the region X, and the moment in the readout magnetic domain
111
is also oriented downward.
Further, when the medium is moved into a state shown in
FIG. 12
, i.e., the independent magnetic domain
109
is located at the rear end of the region X of the second magnetic layer
102
, the upward moment of the independent magnetic domain
109
in the third magnetic layer
103
is copied to the first magnetic layer
101
, and a domain wall
107
′ moves to the position of a most stable domain wall
108
′. Thus, a readout magnetic domain
112
with an upward moment exists in the region
110
irradiated with the light beam
106
.
As described above, the independent magnetic domain
109
is read out once when it is located at the front end of the region X where the second magnetic layer
102
is heated to its Curie temperature or above by the irradiation of the light beam (in the state shown in FIG.
11
), and read out again when it is located at the rear end of the region X (in the state shown in FIG.
12
). This phenomenon is noticeable in a relatively

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