Magneto-optical recording medium and reproducing device

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

1999-12-10

2002-03-05

Kiliman, Leszek (Department: 1773)

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Web or sheet containing structurally defined element or...

Physical dimension specified

C428S336000, C428S690000, C428S690000, C428S690000, C428S690000, C428S900000, C369S013010, C369S275500, C369S275400, C369S284000, C369S286000, C369S288000, C360S053000, C360S114050

Reexamination Certificate

active

06352765

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 been rased 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.
8
.
A first magnetic layer
110
, a second magnetic layer
120
and a third magnetic layer
130
are layered in an exchange coupled state. Denoting the Curie temperatures of the first through third magnetic layers in the laminated state by Tc
110
, Tc
120
and Tc
130
, respectively, Tc
110
and Tc
120
satisfy the relationship Tc
120
<Tc
110
. In
FIG. 8
, 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
130
, 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
104
is irradiated and converged on such a magneto-optical recording medium from the first magnetic layer
110
side, the second magnetic layer
120
has a region heated to a temperature equal to or higher than its Curie temperature. In a region having a temperature equal to or lower than the Curie temperature, the magnetic domain information in the third magnetic layer
130
is copied to the first magnetic layer
110
through the second magnetic layer
120
by the exchange coupling. In other words, the upward transition metal magnetic moment at the front part of a region
108
irradiated with the light beam is copied as it is from the third magnetic layer
130
to the first magnetic layer
110
.
On the other hand, in the region heated to a temperature equal to or higher than the Curie temperature of the second magnetic layer
120
(the region located behind the light beam
104
by a movement of the medium such as a rotation of a disk substrate), since the exchange coupling between the first magnetic layer
110
and third magnetic layer
130
is cut off by the second magnetic layer
120
, the domain wall in the first magnetic layer
110
is readily movable.
When the information in the third magnetic layer
130
is copied as it is to the first magnetic layer
110
, a domain wall
105
is essentially formed. However, in the region where the second magnetic layer
120
has been heated to a temperature equal to or higher than its Curie temperature, since the domain wall in the first magnetic layer
110
is readily movable, the domain wall
105
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
105
moves to a location where the temperature is increased most by the irradiation of the light beam
104
, a and forms a domain wall
106
.
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
120
, the recording domain in the third magnetic layer
130
can be enlarged in the first magnetic layer
110
. Therefore, even when the recording domain is reduced, it is possible to increase the amplitude of the readout signal from the first magnetic layer
110
, thereby allowing readout of signals of a cycle l 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. 9 and 10
, the following description will explain this point.
FIG. 9
shows a state in which an independent magnetic domain
107
formed in the third magnetic layer
130
is present at the front part of the light beam
104
, the third magnetic layer
130
and first magnetic layer
110
are exchange coupled at the position of the independent magnetic domain
107
, and the upward moment is copied to the first magnetic layer
110
. In
FIG. 9
, the shaded portion of the second magnetic layer
120
is a region X where the second magnetic layer
120
is heated to its Curie temperature or a higher temperature.
In the state shown in
FIG. 9
, the domain wall
105
moves to the position of the domain wall
106
to enlarge the magnetic domain, and a readout magnetic domain
109
with an upward moment is formed in the region
108
irradiated with the light beam
104
. Therefore, a large readout signal amplitude is obtained.
When the medium (magneto-optical recording medium) is moved relatively to the light beam
104
from the state shown in
FIG. 9
, a downward moment of the third magnetic layer
130
is copied to the first magnetic layer
110
upon passage of the independent magnetic domain
107
through the region X, and the moment in the readout magnetic domain
109
is also oriented downward.
Further, when the medium is moved into a state shown in
FIG. 10
, i.e., the independent magnetic domain
107
is located at the rear part of the region X of the second magnetic layer
120
, the upward moment of the independent magnetic domain
107
in the third magnetic layer
130
is copied to the first magnetic layer
110
, and a domain wall
105
′ moves to the position of a most stable domain wall
106
′. Thus, a readout magnetic domain
140
with an upward moment exists in the region
108
irradiated with the light beam
104
.
As described above, the independent magnetic domain
107
is read out once when it is located at the front part of the region X where the second magnetic layer
120
is heated to its Curie temperature or above by the irradiation of the light beam (in the state shown in FIG.
9
), and read out again when it is located at the rear part of the region X (in the state shown in FIG.
10
). This phenomenon is noticeable

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