Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2000-07-28
2003-10-07
Resan, Stevan A. (Department: 1773)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S212000, C428S332000, C428S690000, C428S690000, C428S900000, C369S013420, C369S013430, C369S013440, C369S013450, C369S013460, C369S013530
Reexamination Certificate
active
06630252
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magneto-optical recording medium. In particular, it relates to a magneto-optical recording medium of the type which shows improved sensitivity to the data-recording magnetic field and also exhibits improved readout C/N characteristics.
2. Description of the Related Art
Conventionally, various kinds of magnetic media that can be written to repeatedly (such as hard disks, floppy disks, magneto-optical disks and tapes) have been widely used for storing data. Among them, magneto-optical disks (simply called “MO disks” hereinafter) are advantageous in data retention since they will deteriorate (demagnetize) more slowly than the other kinds of magnetic media mentioned above.
As is known, a technique called “Magnetically-Induced Super Resolution (MSR)” may be used for realizing a high-density data-reproducing (readout) performance. According to this technique, data can be read out from an MO disk in which minute recording marks are arranged along the tracks by a pitch which is smaller than the diameter of the spot of the laser beam irradiated on the MO disk.
An MSR technique is disclosed in JP-A-7(1995)-244877 for example. This document teaches the use of “double-mask RAD (Rear Aperture Detection)” for reading data from an MO disk. By this method, cross talk between adjacent tracks can be minimized.
Referring to
FIGS. 9-11
of the accompanying drawings, the double-mask RAD method and the typical layer structure of an MO disk used for implementing this method will be described below.
As shown in
FIG. 9
, the conventional MO disk includes a readout layer
31
, an intermediate layer
32
and a data-recording layer
33
. These layers, which are made of a rare earth-transition metal amorphous alloy, are supported by a transparent resin substrate
20
made of e.g. polycarbonate. Thought not illustrated, the MO disk includes additional layers made of SiN (silicon nitride) for protection purposes. Laser beams, emitted on the side of the transparent substrate
20
for erasing, recording or reading data, will pass through the substrate
20
and strike on the readout layer
31
.
The readout layer
31
has a transition metal magnetization-dominant composition (hereinafter called “TM-rich” composition). The direction of magnetism of the layer
31
is oriented perpendicularly to this layer. The intermediate layer
32
has a rare earth magnetization-dominant composition (hereinafter called “RE-rich” composition). At room temperature, the direction of magnetism of the intermediate layer
32
is oriented longitudinally thereof, whereas at higher temperatures, the direction is perpendicular. This means that the direction of magnetism of the intermediate layer
32
, which is longitudinal at room temperature, is changed to be perpendicular at a certain temperature higher than the room temperature. The data-recording layer
33
has a TM-rich composition, and its direction of magnetism is perpendicular.
When the Curie temperatures of the three layers
31
,
32
and
33
are Tc
1
, Tc
2
and Tc
3
, respectively, the following relations hold:
Tc
2
<Tc
1
and Tc
2
<Tc
3
.
Further, when the coercivities of the readout layer
31
and the data-recording layer
33
are Hc
1
and Hc
3
, respectively, the following relations hold:
Hc
3
>Hc
1
.
The deletion of recording marks formed on the magneto-optical disk
10
is performed in the following manner. First, as shown in
FIG. 10
, the readout layer
31
is irradiated with a laser beam when the MO disk is held in a downward, data-deleting magnetic field. Thus, the irradiated area on the layer
31
and portions of the three layers
31
-
33
adjacent to the irradiated area are heated up to a temperature above the the Curie temperature Tc
3
. In this state, the direction of the magnetic domains on the data-recording layer
31
is aligned with the direction of the data-deleting magnetic field.
Then, the heated portions of the three layers
31
-
33
are brought away from the laser beam to cool down to the room temperature. Consequently, the direction of the cooled magnetic domains on the intermediate layer
32
becomes longitudinal of the layer
32
(horizontal in the figure). Thereafter, the readout layer
31
and the data-recording layer
33
are magnetically bonded to each other by weak force. Thus, all of the magnetic domains of the respective layers
31
and
33
are aligned in one direction (downward in FIG.
10
). This means that the previous data stored in the data-recording layer
33
has been deleted.
Once the previous data is deleted, new data can be written to the MO disk by a Light Intensity Modulation (LIM) method. Specifically, as shown in
FIG. 11
, the readout layer
31
is irradiated with a laser beam when the MO disk is put in an upward, data-recording magnetic field. According to the LIM method, the intensity of the laser beam is modulated in accordance with the data to be recorded, while the MO disk is kept in the data-recording magnetic field. Thus, only when the intensity of the laser beam is high, the direction of the irradiated magnetic domains on the recording magnetic layer
33
is aligned with the upward data-recording magnetic field. In this manner, appropriate recording marks are produced.
When the laser-heated region on the MO disk is brought away from the laser beam, it cools down to the room temperature. As a result, the direction of the cooled magnetic domain of the intermediate layer
32
is horizontally directed, thereby rendering the readout layer
31
and the data-recording layer
33
magnetically bonded to each other with weak force.
In the above state, the direction of the magnetic domains of the readout layer
31
can be aligned in one direction upon application of an external magnetic field which is stronger than the above-mentioned weak magnetic force bonding the readout layer
31
to the data-recording layer
33
.
Instead of the LIM method described above, Magnetic Field Modulation (MFM) may be used for writing data to the MO disk. According to this method, the direction of the applied magnetic field is modulated (reoriented upward and downward repeatedly) in accordance with the data to be recorded, while the readout layer
31
is being irradiated with a laser beam. In this manner, the direction of the laser-irradiated magnetic domains is aligned with the direction of the applied magnetic field.
The thus recorded data is read out from the MO disk in the following manner. As shown in
FIG. 9
, the readout layer
31
is irradiated with a laser beam S, while the laser-irradiated region is put in a downward, data-readout magnetic field. In the low-temperature region (hatched region) corresponding to a front portion of the laser beam S, the intermediate layer
32
and the data-recording layer
33
are only weakly bonded to each other, so that the direction of the magnetic domains of the intermediate layer
32
is aligned with the externally applied readout magnetic field. Then, due to the exchange interaction between the intermediate layer.
32
and the readout layer
31
, the direction of the magnetic domains of the readout layer
31
is directed upward. In this manner, the direction of the magnetic domains of the data-recording layer
33
is masked (front mask) by the readout layer
31
.
The temperature of the high-temperature region (crosshatched region) corresponding to a rear portion of the laser beam S is higher than the Curie temperature of the intermediate layer
32
, thereby breaking the exchange interaction between the intermediate layer
32
and the readout layer
31
. Thus, the direction of the magnetic domains of the readout layer
31
is aligned with the direction of the externally applied readout magnetic field. Thus, the direction of the magnetic domains of the data-recording layer
33
is masked (rear mask).
In the intermediate-temperature region between the low-temperature region and the high-temperature region (i.e., the region between the hatched region and the crosshatched region), the exchange interaction betwe
Bernatz Kevin M.
Fujitsu Limited
Greer Burns & Crain Ltd.
Resan Stevan A.
LandOfFree
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