Stock material or miscellaneous articles – Structurally defined web or sheet – Continuous and nonuniform or irregular surface on layer or...
Reexamination Certificate
2000-10-26
2003-10-28
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Continuous and nonuniform or irregular surface on layer or...
C428S212000, C428S409000, C428S690000, C428S690000, C369S013430, C369S013450
Reexamination Certificate
active
06638597
ABSTRACT:
RELATED APPLICATION DATA
The present application claims priority to Japanese Application No. P11-309153 filed Oct. 29, 1999, which application is incorporated herein by reference to the extent permitted by law.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a recording medium that is read by a laser beam utilizing a magneto-optical effect. More particularly, the present invention relates to a magneto-optical recording medium comprising at least a reproduction layer and a recording layer, on which a readout process based on magnetically induced super resolution is carried out utilizing a blue-violet laser beam, that is, a laser beam having a wavelength of 350 nm to 450 nm.
2. Description of the Related Art
There is a method for rewritably recording a data signal with high density using a magneto-optical recording means. This method comprises utilizing the thermal energy of a laser beam to initially cause a partial portion of a magnetic layer of a magneto-optical recording medium to be heated beyond the Curie temperature or the compensation temperature. The method also includes causing the coercive force in the heated portion to be decreased or extinguished, and then causing the direction of magnetization of the heated portion to be inverted into the direction of a recording magnetic field that is externally added to form an information magnetic domain, so as to execute the recording of the data signal.
The above-cited magneto-optical recording medium comprises a transparent substrate, such as a polycarbonate resin, having a main surface. The recording medium further comprises a plurality of layers sequentially laminated on the main surface of the substrate. The sequentially laminated layers comprise: a dielectric film that may be made of silicon nitride or aluminum nitride; a magnetic recording layer that may be made of an amorphous film of rare-earth transition-metal alloy and that is provided with an easy axis of magnetization in the vertical direction with regard to a film surface and that has a surpassing magneto-optical-effect characteristic. The plurality of layers further includes another dielectric film that may be made of silicon nitride or aluminum nitride; a reflection layer composed of aluminum, gold, or silver; and a protection layer, which may be made of an ultraviolet-ray-cured type resin.
By causing the above-cited magneto-optical recording medium to be exposed to a laser beam irradiated from the side of the transparent substrate, the above-cited information magnetic domain may be formed, such that the recording of the data signal is executed against the magnetic layer.
To reproduce the recorded data signal, the magneto-optical recording medium is exposed to a laser beam irradiated from the side of the above-cited transparent substrate to detect the actual rotating angle of the magnetized or polarized surface via a magneto-optical effect, such as the Kerr effect, which is generated in the information magnetic domain in the magnetic layer to execute reproduction of the recorded data signal.
The linear recording density of optical disks (such as the magneto-optical recording medium, a disc for digital audio recording, a disc for digital video recording or the like) is also determined by the signal-to-noise (S/N) ratio in the course of performing reproduction in most cases. And yet, the amount of the recorded data signal read under reproduction mode is dependent on the cyclic period of bit array of the recorded data signal, the wavelength (&lgr;) of the laser beam, and the numerical aperture (N.A.) of the objective lens.
More specifically, the bit cyclic period (“f”) is determined by the laser wavelength (&lgr;) of the reproduction optical system and also by an optical detection limit of the optical system. The optical detection limit may be an optical refraction limit determined by the numerical aperture (N.A.) of the objective lens. More precisely, the bit cyclic period (“f”) at the optical detection limit is defined by equation expressed by way of “f=&lgr;/(2 N.A.)”.
Accordingly, in order to realize higher density recording of a data signal in the magneto-optical recording medium, the laser wavelength (&lgr;) of the reproduction optical system may be shortened or the numerical aperture (N.A.) of the objective lens may be increased. However, within the current technology, improvement of the laser wavelength (&lgr;) of the laser beam and the numerical aperture (N.A.) of the objective lens are limited.
In recent years, modem technology has developed a semiconductor laser having a wavelength of around 400 nm. For example, a GaN semiconductor laser is capable of producing a blue-violet laser beam having a laser wavelength of 350 nm to 450 nm. On the other hand, a numerical aperture (N.A.) of an objective lens that is about 0.7 may be obtained by a resin mold.
In view of the above-described circumstances, there have been a number of studies, developments, and suggestions with regard to structures of a magneto-optical recording medium and methods for recording and reproducing such a medium.
As for the recording method, a so-called “mark length recording method” has been suggested. According to the “mark length recording method,” an information mark is recorded not by a method in which information is recorded according to the presence of a mark (i.e., the so-called “mark position recording method”), but by a method in which a linear density of the recording medium is achieved by varying the length of the mark to make an edge portion thereof to record the information.
Another recording method, “a laser pulse irradiation magnetic field modulating recording method,” has also been suggested. According to this recording method, a recording laser beam is irradiated so that a waveform thereof is made to be a pulse form, not a continuous waveform, in accordance with a phase of an external magnetic field. This method results in prevention of unnecessary expansion in a track direction of the recording medium, reduction of cross-write and cross-erase in adjacent tracks, and improvement in track density.
On the other hand, there have been suggested a number of “magnetically induced super resolution readout methods,” as a reproducing method.
In order to realize the above “magnetically induced super-resolution readout method”, a magneto-optical recording medium comprising at least a reproduction layer and a recording layer is introduced. This method comprises a process for causing the data signal recorded on the recording layer, which may have high coercive force, to be transferred onto the reproduction layer. In this method, a polarized surface is subject to rotation by a magneto-optical effect (e.g., Kerr effect) of the reproduction layer generated by a laser beam irradiated onto the reproduction layer. By detecting the rotation of the polarized surface, readout (reproduction) of the recorded data is executed. In this case, by using the reproducting laser beam to form a thermal distribution within a spot in the reproduction layer, a part of the recorded data signal is emerged in the spot of the reproduction layer in order that the optical reader can restrictively read a single information magnetic domain within the spot, thus making it possible to reproduce the information magnetic domain based on a cyclic period being less than the bit cyclic period at the above-described optical detection limit.
The magnetically induced super-resolution readout method described above is disclosed in Japanese Patent Application Publication Laid-Open No. HEISEI-1-143041/1989 and also in Japanese Patent Application Publication Laid-Open HEISEI-1-143042/1989, both being a basic application of the U.S. Pat. No. 5,018,119. The magneto-optical recording medium used for implementing the magnetically induced super-resolution readout method essentially comprises a reproduction (readout) layer, an intermediate layer, and a recording layer, which are magnetically coupled with each other at room temperature. When performing the magnetically induced super-resolut
Shimouma Takashi
Shinoda Masataka
Tanaka Yasuhito
Bernatz Kevin M.
Sonnenschein Nath & Rosenthal LLP
Sony Corporation
Thibodeau Paul
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