Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2001-09-25
2004-11-23
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S212000, C428S409000, C428S690000, C428S690000, C428S690000, C428S690000, C369S013420, C369S013070
Reexamination Certificate
active
06821642
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application is related to Japanese Patent Application No. 2001-157148 filed in May 25, 2001, whose priority is claimed under 35 USC §119, the disclosure of which is incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magneto-optical recording medium, and particularly to a magneto-optical recording medium which can perform magnetically induced super resolution reproduction.
2. Description of the Related Art
In order to increase recording density, a magneto-optical recording medium has been developed which is provided with a recording mark having a mark length shorter than a spot diameter of a laser beam and formed at a period shorter than the spot diameter.
Particularly, a magnetically induced super resolution (MSR) reproduction method is proposed as a method of reproducing a recording mark smaller than a spot diameter of a beam.
In this method, a laser light for reproduction is irradiated while a magneto-optical disk in which a plurality of magnetic layers including a recording layer and a reproduction layer are stacked is rotated, so that a temperature distribution is produced in a circumferential direction of the magneto-optical disk, and a small recording mark is read out by using this temperature distribution. By this, resolution equivalent to the case where reproduction is substantially made with a light spot smaller than a spot diameter of the reproduction laser light can be obtained.
Besides, as a medium capable of reproducing recording marks recorded at a period shorter than a beam spot by using the magnetically induced super resolution, a magnetically induced super resolution medium constituted by three magnetic layers having predetermined characteristics is disclosed in Japanese Patent Unexamined Publication No. 2000-200448.
This medium is constituted by three magnetic layers of a reproduction layer, an intermediate layer, and a recording layer, and is a double mask magnetically induced super resolution medium (Double mask Rear Aperture Detection: D-RAD medium) in which a low temperature mask (called a front mask) is formed in a low temperature region of a temperature distribution formed in a beam spot, and a high temperature mask (called a rear mask) is formed in a high temperature region.
According to this medium, a recording mark having a length of 0.38 &mgr;m and formed on a land substrate of a track pitch of 0.9 &mgr;m can be reproduced by a reproduction magnetic field of 300 Oe or less.
Besides, in order to further increase the recording density of a recording medium, it is necessary to shorten a track pitch of the medium in a radius direction and to shorten a mark length of a recording mark.
As one method of shortening the track pitch to increase the density, there is a method in which a land groove substrate is used, and recording marks are formed on both a land and a groove.
Even if the land groove substrate is applied to the D-RAD medium, if it is a high density medium of about 2.3 GB/3.5 inches, it is possible to form a medium in which cross talk from an adjacent track is hardly generated.
However, in the case where the track pitch of the land groove substrate is shortened to further increase the capacity of a medium, there arises a problem that cross talk from an adjacent track can not be neglected and a reproduction power margin becomes narrower than a design value.
For example, in the case where a high density D-RAD medium of a track pitch of about 0.50 &mgr;m is reproduced by an optical system of an LD wavelength of 660 nm and NA=0.55, the cross talk from an adjacent track is generated to such a degree that it can not be neglected, which becomes a problem at a practical use level of recording and reproduction of data.
FIG.
12
and
FIG. 13
are graphs each explaining the dependency of cross talk of a conventional D-RAD medium upon reproduction power (Pr).
The vertical axis indicates a decibel value CNR (dB) equivalent to the amount of generated cross talk, and the horizontal axis indicates a reproduction power (mW).
FIG. 12
relates to a land groove substrate of a track pitch of 0.65 &mgr;m, and
FIG. 13
relates to a land groove substrate of a track pitch of 0.50 &mgr;m.
In the graphs, in the case where 8T marks having the shortest mark length of 0.300 &mgr;m are stored in a groove adjacent to a land, a decibel value of a signal leaking into the land by the cross talk is measured.
According to
FIG. 12
, even if the reproduction power Pr is raised from 3.6 mW to about 5.5 mW, the cross talk is 20 dB or less which does not become a problem in practical use.
Besides, when a region from Pr=3.8 mW as the rising (Prth) of CNR at which reproduction can be performed to a point at which the cross talk is 20 dB or less (to Pr=5.5 mW) is considered to be a reproduction power margin, in the case of
FIG. 12
, there is a reproduction power margin of ±18.3%.
On the other hand, in the land groove substrate of a track pitch of 0.50 &mgr;m shown in
FIG. 13
, the cross talk exceeds 20 dB when the reproduction power Pr is approximately 4.5 mW.
That is, high cross talk is observed at the high Pr side, and the influence of the cross talk becomes high even in a region of lower reproduction power. In
FIG. 13
, the reproduction power margin is decreased to about ±9.0%.
It is conceivable that the high cross talk is observed at the high Pr side like this because of the following mechanism.
FIG. 14
is an explanatory view of magnetization states of the respective layers at the time of reproduction of a conventional D-RAD medium.
FIG. 14
shows the directions of magnetization of a reproduction layer
51
, an intermediate layer
52
, and a recording layer
53
as three magnetic layers of the D-RAD medium.
In a state where a reproduction magnetic field
61
is applied from below, a light beam is irradiated to the magnetic layers from above.
FIG. 14
shows the magnetization states in the vicinity of a beam spot BS where the light beam is irradiated, and in the case where this medium is moved in the upper right direction in the drawing, a temperature distribution in the beam spot is divided into three regions (a low temperature region A, an intermediate temperature region B, and a high temperature region C).
With respect to the direction of magnetization, the up direction indicates a recording direction L
1
, and the down direction indicates an erasing direction L
2
.
As shown in
FIG. 14
, when the reproduction magnetic field
61
is applied in the recording direction L
1
, in the low temperature region A within the beam spot, all the magnetization of the intermediate layer
52
is directed in the external magnetic field direction L
1
, the magnetization of the reproduction layer
51
exchange-coupled to the intermediate layer
52
is directed in the erasing direction L
2
, and a low temperature mask A
1
is formed in the reproduction layer
51
. At this time, irrespective of the direction of magnetization of the recording layer
53
of the low temperature region, the directions of magnetization of the intermediate layer
52
are made uniform, and an interface magnetic wall
62
is produced between the magnetization of the recording layer
53
in the recording direction L
1
and the intermediate layer
52
.
On the other hand, in the high temperature region C within the beam spot, since the magnetization of the intermediate layer
52
reaches the Curie temperature, spontaneous magnetization disappears (expressed by a blank portion), and exchange-coupling force to the reproduction layer
51
is cut. Accordingly, in this high temperature region C, all the magnetization of the reproduction layer
51
is made uniform in the external magnetization direction L
1
, that is, in the reproduction layer
51
, all the spontaneous magnetization is directed in the recording direction L
1
, and a high temperature mask C
1
is formed.
Besides, in the intermediate temperature region B, a recording mark recorded in the recording layer
53
Bernatz Kevin M.
Fujitsu Limited
Greer Burns & Crain Ltd.
Thibodeau Paul
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