Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse... – Magnetic field and light beam
Reexamination Certificate
2001-02-27
2003-05-27
Dinh, Tan (Department: 2653)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
Magnetic field and light beam
C369S013540, C369S275400
Reexamination Certificate
active
06570826
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical recording medium, and more particularly to a magneto-optical recording medium allowing high-density recording and reproduction of information.
2. Description of the Related Art
A magneto-optical disk is known as a high-density recording medium, and an increase in its recording density is demanded with an increase in quantity of information. Increasing the recording density of the medium can be realized by shortening the space between adjacent recording marks. However, the reproduction of each recording mark is limited by the size of a light beam (beam spot) on the medium. In the case that the density of the recording marks is set so that only one recording mark is present inside the beam spot, an output waveform corresponding to “1” or “0” can be observed as a reproduced signal according to whether or not the recording mark is present inside the beam spot.
However, in the case that the density of the recording marks is increased so that a plurality of recording marks are present inside the beam spot, the reproduced output does not change irrespective of movement of the beam spot on the medium, so that the output waveform becomes linear and the presence or absence of recording marks cannot be distinguished. The reproduction of such small recording marks having a period shorter than the size of the beam spot may be effected by reducing the size of the beam spot. However, the size of the beam spot is limited by the wavelength A of light output from a light source and the numerical aperture NA of an objective lens, so that the spot size cannot be sufficiently reduced.
Recently commercially available is a magneto-optical disk drive adopting a reproducing method using a magnetically induced super-resolution (MSR) technique for reproducing a recording mark smaller than the size of the beam spot by the use of an existing optical system. The MSR is a reproducing method such that while one mark present inside the beam spot is being reproduced, another mark is masked to thereby increase a reproductive resolution. Accordingly, such an MSR medium requires at least a mask layer or reproducing layer for masking the other mark so that the one mark is reproduced during signal reproduction, in addition to a recording layer for recording marks.
A magneto-optical recording medium using a perpendicularly magnetized film as the reproducing layer is proposed in Japanese Patent Laid-open No. Hei 3-88156, for example. In the prior art described in this publication, however, an initial magnetic field of several kilooersteds is required for initialization of the reproducing layer. Accordingly, the disk drive using this magneto-optical recording medium cannot be reduced in size. A magneto-optical recording medium using a magnetic film having an axis of easy magnetization in a longitudinal direction at room temperature and an axis of easy magnetization in a perpendicular direction at a predetermined temperature or higher as the reproducing layer is proposed in Japanese Patent Laid-open No. Hei 5-81717, for example.
Further, an MSR medium having a reproducing layer, a recording layer, and an intermediate layer interposed between the reproducing layer and the recording layer is described in U.S. Pat. No. 6,020,079. In the MSR medium described in this U.S. Patent, a reproducing laser beam is directed onto the medium to form a temperature distribution composed of a low-temperature region, an intermediate-temperature region, and a high-temperature region inside the beam spot. The low-temperature region and the high-temperature region form a double mask, and a recorded mark is read from only the intermediate-temperature region. Since the double mask is formed by the low-temperature region and the high-temperature region, the intermediate-temperature region for reading a recorded mark can be greatly reduced in size, thereby allowing high-density recording and reproduction. Such a magneto-optical recording medium is referred to as a double-mask rear aperture detection (DRAD) type magneto-optical recording medium.
An MSR magneto-optical recording medium commercially available at present is either of a land recording type such that data is recorded on lands only or of a groove recording type such that data is recorded on grooves only. Attention has recently been given to a land/groove recording type such that data is recorded on both lands and grooves as recording tracks, so as to achieve higher-density recording and reproduction. In a magneto-optical recording medium adopting this land/groove recording type, a transparent substrate having a plurality of lands and a plurality of grooves alternately formed is used.
In the case that a magneto-optical recording film capable of performing MSR reproduction is formed on the land/groove substrate, there is a problem that a reproducing magnetic field required for MSR reproduction of recorded marks becomes large. This is considered to be due to the fact that the center distance between adjacent land and groove, i.e., the track pitch, is small, for example. Further, it is considered that the deposited condition of the magnetic film formed on the substrate is influenced by such a decrease in the track pitch. In particular, the deposited condition of the magnetic film tends to be influenced by a side wall of each groove (a portion between a horizontal surface of each land and a horizontal surface of its adjacent groove), so that an increase in required reproducing magnetic field is considered to be also due to the magnetic film formed at this side wall of each groove. This problem becomes more remarkable as the track pitch is more decreased in order to increase the recording density. In particular, an increase in reproducing magnetic field with a track pitch of 0.7 &mgr;m or less is a large problem. This tendency is remarkable particularly in reproduction on the grooves.
To provide such a large reproducing magnetic field, a large magnet must be installed in a magneto-optical disk drive, so that it is difficult to reduce the size of the magneto-optical disk drive. Further, a power consumption in the magneto-optical disk drive also becomes large. In a conventional land/groove recording type optical disk, the width of each land is substantially equal to the width of each groove in general. However, there have been proposed some examples of an optical disk with the width of each land being different from the width of each groove. For example, Japanese Patent Laid-open No. Hei 8-273202 discloses an optical disk with the width of each groove being greater or smaller than the width of each land, so as to reduce crosstalk and increase the amplitude of a cross track signal.
Further, Japanese Patent Laid-open No. Hei 9-231615 discloses an optical disk with the width of each groove being greater than the width of each land and the level difference between the lands and the grooves being 100 nm or more, so as to reduce thermal crosstalk between adjacent tracks. While each of the above-mentioned two publications discloses an optical disk with the width of each groove being greater than the width of each land, there is no recognition of the problem to be solved by the present invention such that an increase in reproducing magnetic field is caused by a decrease in the track pitch in a land/groove recording type magneto-optical recording medium.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a magneto-optical recording medium having recording tracks consisting of lands and grooves which can reduce a reproducing magnetic field.
In accordance with an aspect of the present invention, there is provided a magneto-optical recording medium capable of reproducing a recorded mark smaller in size than a beam spot, having recording tracks consisting of lands and grooves, said magneto-optical recording medium comprising a transparent substrate on which said lands and said grooves are alternately formed; a magnetic reproducing layer formed on said transparent substrate
Dinh Tan
Greer Burns & Crain Ltd.
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