Magneto-optical storage media

Details Magneto-optical storage media Magneto-optical storage media

2002-03-19

2004-03-16

Neyzari, Ali (Department: 2655)

Dynamic information storage or retrieval

Storage or retrieval by simultaneous application of diverse...

Magnetic field and light beam

C369S013430, C369S013440, C428S690000

Reexamination Certificate

active

06707766

ABSTRACT:

FIELD OF THE INVENTION
The present invention refers to magneto-optical storage media for magneto-optically storing and reproducing information with a laser beam.
BACKGROUND OF THE INVENTION
Magneto-optical storage media, an application of magneto-optical effects, are increasing their storage density as a result of a variety of research and development projects to develop repeatedly rewritable information storage media with a large capacity.
The magneto-optical storage medium has a short-coming that reproduction properties deteriorate with a relative decrease in the diameter or interval of storage bits, which form magnetic domains for storage, to the diameter of the light beam focused on the medium.
This is because the diameter of the light beam focused on a target storage bit encompasses an adjacent storage bit, and the information stored on the individual storage bits cannot be separately reproduced.
To eliminate the short-coming, attempts have been made to improve storage density through working on the arrangement and reproduction technique of the storage medium. One of the proposed methods is an expanded magnetic domain reproduction system by means of displacement of magnetic walls.
Here, a reference is made to a prior art, Japanese Laid-Open Patent Application No. 6-290496/1994 (Tokukaihei 6-290496 published on Oct. 18, 1994; hereinafter will be referred to as Prior Art 1) disclosing an expanded magnetic domain reproduction technology by means of displacement of magnetic walls.
According to the technology, in a magneto-optical storage medium, high density storage is realized using reproduction signals having an increased amplitude, by coupling magnetic films that form a multi-layered structure through an exchange force, and increasing tiny storage magnetic domains in a recording layer
104
in size by means of a magnetic domain expansion layer
101
. FIG.
7
(
a
) shows such an arrangement. Note that arrows are drawn in some layers to denote the directions of sub-lattice magnetization of the transition metals composing the layers, and also that magnetic walls (Bloch walls)
110
are formed in the layers between such adjacent magnetic domains that the directions of their magnetization are different from each other by 180°. The layers in which no arrows are drawn are non-magnetic. Portions of the magnetic layers in which arrows are absent denote loss of ordered magnetization in them due to temperature elevated to the Curie temperature or even higher.
There are four principal requirements for the magneto-optical storage medium as follows:
1. The recording layer
104
should be provided so as to stably hold tiny magnetic domains in place at temperatures ranging from room temperature to temperatures reached during reproduction.
2. The recording layer
104
, the intermediate layer
102
, and the magnetic domain expansion layer
101
should be coupled through an exchange force at least in a proximity of the Curie temperature, T
C102
, of the intermediate layer
102
.
3. The intermediate layer
102
should lose ordered magnetization as its temperature rises past the Curie temperature T
C102
, cutting off the exchange coupling among the recording layer
104
, the intermediate layer
102
, and the magnetic domain expansion layer
101
above the Curie temperature T
C102
.
4. The magnetic domain expansion layer
101
should generate a low frictional force due to magnetic domain wall coercivity, and a temperature gradient should cause a magnetic wall energy gradient. Hence, the magnetic walls
110
move where the intermediate layer
102
functions so as to cut off the exchange coupling, with the portion to which magnetization is duplicated from a magnetic domain
104
a
in the recording layer
104
as an original. As a result, the magnetization in those regions become aligned to the same direction as that of the magnetic domain
104
a.
FIG.
7
(
b
) is a graph illustrating the distribution of temperature in the middle of a track of a disk moving to the right relative to the person observing as a result of projection of a laser beam to the magneto-optical storage medium. Here, the disk is moving at such a high linear velocity that temperature is highest downstream of the center of the beam spot with respect to the direction of the movement of the beam spot.
FIG.
7
(
c
) is a graph illustrating the distribution of the magnetic wall energy density &dgr;
101
in the magnetic domain expansion layer
101
in a circumferential direction. Typically, the magnetic wall energy density decreases with an increase in temperature, dropping to 0 above the Curie temperature. Therefore, when there is a temperature gradient in a circumferential direction as shown in FIG.
7
(
b
), the magnetic wall energy density &dgr;
101
decreases with high temperatures as shown in FIG.
7
(
c
).
The force, F
101
, exerted on the magnetic walls in the layers at position x along the circumference is given by the following expression:
F
101
=−d&dgr;
101
/dx
The force F
101
acts to move the magnetic walls to a lower magnetic wall energy level. The magnetic domain expansion layer
101
, in comparison to the other magnetic layers, generates a low frictional force due to wall coercivity, i.e., is likely to allow movement of the magnetic walls. Therefore, when the exchange force is no longer available from the intermediate layer
102
, the magnetic domain expansion layer
101
allows the force F
101
to move the magnetic walls to a lower magnetic wall energy level.
In FIG.
7
(
a
), prior to the projection of a laser beam to the disk, the three magnetic layers are coupled through an exchange force where temperature is equivalent to room temperature, while the magnetic domains stored in the recording layer
104
have been duplicated to the magnetic domain expansion layer
101
. Here, in each of the layers, there exist magnetic walls between such adjacent magnetic domains that have mutually reverse magnetization directions.
Where temperature has been raised to the Curie temperature, T
C102
, of the intermediate layer
102
or higher, the intermediate layer
102
loses magnetization, cutting off the exchange coupling between the magnetic domain expansion layer
101
and the recording layer
104
; therefore the magnetic domain expansion layer
101
can no longer hold the magnetic walls in place, allowing the magnetic walls to move toward a higher temperature portion according to the force F
101
exerted on the magnetic walls. Here, the magnetic walls move at a velocity sufficiently faster than does the medium. Therefore, the duplicate magnetic domains in the magnetic domain expansion layer
101
are larger in size than those stored in the recording layer
104
.
However, the medium described in Prior Art 1 entails following problems: since the exchange coupling from the recording layer
104
through the magnetic domain expansion layer
101
is cut off where temperature has risen to the Curie temperature, T
C102
, of the intermediate layer
102
or higher, the magnetic walls become movable in the magnetic domain expansion layer
101
, whereas a parasitic magnetic field generated by the storage magnetic domains of the recording layer
104
builds up an unignorable magnetostatic coupling force.
The magnetostatic force arising from the magnetic fields generated by the other magnetic layers and the like, as well as that arising from the magnetic moments of those magnetic layers per se, is ignorably small in comparison to the exchange force, since an exchange force arises from exchange of electrons between magnetic layers at their interface. However, when the exchange coupling is cut off as in the above case, the magnetostatic coupling force is no longer ignorable. According to a super-resolution technology disclosed in Japanese Laid-Open Patent Application No. 10-40600/1998 (Tokukaihei 10-40600; published on Feb. 13, 1998), Japanese Laid-Open Patent Application No. 6-150418/1994 (Tokukaihei 6-150418; published on May 31, 1994), and other documents, the magnetization direction of the reproduction layer is caused to con

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