Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse...
Reexamination Certificate
1998-06-19
2001-05-01
Dinh, Tan (Department: 2651)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
Reexamination Certificate
active
06226234
ABSTRACT:
This application claims the benefit under 35 U.S.C. §371 of prior PCT International Application No. PCT/JP96/03716 which has an International filing date of Dec. 20, 1996 which designated the United States of America, the entire contents of which are hereby incorporated by reference.
TECHNICAL FIELD
This invention relates to a magneto-optical recording medium and a reproducing method therefor. More particularly, the invention relates to a magneto-optical recording medium and a reproducing method therefor which can reproduce or read a fine recorded magnetic domain, much smaller than a reproducing light spot, in an enlarged form and which are suitable for high density recording.
BACKGROUND ART
A magneto-optical recording medium is capable of rewriting recorded information, has a large storage capacity, and is highly reliable as a recording medium. Thus, it begins to be put to practical use as a computer memory or the like. With increases in the amount of information and downsizing of equipment, however, a demand is growing for even higher density recording/reproducing technologies. To record information on a magneto-optical recording medium, a magnetic modulation method is used which irradiates the magneto-optical recording medium with constant laser light, and simultaneously applies a magnetic field of a polarity corresponding to a recording signal to a heated area of the medium. This method permits overwrite recording, and achieves high density recording, e.g., recording at the shortest mark length of 0.15 &mgr;m. An optical modulation recording system for irradiating the medium with light power-modulated in response to recording signals under a constant magnetic field applied has also found practical use.
To reproduce or read record marks recorded at a high density, an optical reproduction resolution or resolving power determined by the spot size or diameter of a reproducing light beam should be given particular attention. For example, it is impossible to discern and reproduce a fine mark with a magnetic domain length of 0.15 &mgr;m by the use of reproducing light with a spot diameter of 1 &mgr;m. As one of approaches for eliminating this restriction on reproduction resolving power by the optical spot diameter of reproducing light, a proposal has been made for magnetic super-resolution (MSR) as described in the Journal of Magnetics Society of Japan, Vol. 17, Supplement No. S1, pp. 201 (1993). This technique utilizes the phenomenon that when reproducing light is cast on a magneto-optical recording medium, a temperature distribution occurs in a magnetic film within a reproducing light spot. Because of this phenomenon, a magnetic mask is generated in the spot to decrease the effective spot diameter contributing to the reproduction or reading of a signal. The use of this technology can improve reproduction resolving power without reducing the actual reproducing light spot diameter. However, this method makes the effective spot diameter small by the action of the magnetic mask. Thus, the amount of light contributing to a reproduction output decreases, and a reproduction C/N ratio declines accordingly. Consequently, it becomes difficult to obtain a sufficient C/N ratio.
Japanese Patent Application Laid-Open No. 1-143041 discloses a reproducing method for a magneto-optical recording medium, which uses a magneto-optical recording medium having a first magnetic film, a second magnetic film and a third magnetic film connected together magnetically at room temperature, the Curie temperatures ohf the first, second and third magnetic films being T
C1
, T
C2
and T
C3
, respectively, with T
C2
>room temperature and T
C2
<T
C1
, T
C3
, the coercivity of the first magnetic film, H
C1
, being sufficiently small at a temperature in the vicinity of the Curie temperature T
C2
of the second magnetic film, and the coercivity of the third magnetic film, H
C3
, being sufficiently large compared with a required magnetic field in a temperature range from room temperature to a required temperature T
PB
higher than T
C2
,
whereby the recorded magnetic domains of the first magnetic film are enlarged to perform reproduction. This method utilizes a temperature rise of the medium upon irradiation with reproducing light to cut off the magnetic connection between the first and third magnetic films, and enlarges the magnetic domains of the first magnetic film by a demagnetizing field acting in the recorded magnetic domains in this condition and an externally applied magnetic field. With this technique, the second magnetic film used has a Curie temperature set at a lower value than the temperature of a readout area working during reproduction. The present invention, on the other hand, does not use a magnetic film with such magnetic characteristics.
It is an object of the present invention to solve the problems with the earlier technologies by a method different from the method described in Japanese Patent Application Laid-Open No. 1-143041, and provide a magneto-optical recording medium which gives reproduced signals at a sufficient C/N ratio even when fine magnetic domains are recorded, as well as a reproducing method for the signals.
DISCLOSURE OF THE INVENTION
According to a first aspect of the present invention, there is provided a magneto-optical recording medium having a magneto-optical recording film and an auxiliary magnetic film, and being adapted to reproduce a signal by magnetically transferring a recorded magnetic domain of the magneto-optical recording film to the auxiliary magnetic film upon irradiation with reproducing light, characterized in that:
the auxiliary magnetic film is a magnetic film of at least one layer which transforms from a plane-magnetized film into a perpendicular-magnetized film when exceeding its critical temperature, while the magneto-optical recording film is a perpendicular-magnetized film at a temperature not lower than room temperature;
whereby a larger magnetic domain than the recorded magnetic domain of the magneto-optical recording film can be transferred to the auxiliary magnetic film at the time of reproduction by virtue of the magnetic characteristics of the auxiliary magnetic film.
The magneto-optical recording medium of the present invention can be further classified into the following two types of magneto-optical recording media: The first type of magneto-optical recording medium, as illustrated in
FIGS. 2A and 2B
, has a structure in which a first auxiliary magnetic film
5
and a second auxiliary magnetic film
4
are sequentially laminated on a magneto-optical recording film
6
, the magneto-optical recording film
6
, the first auxiliary magnetic film
5
and the second auxiliary magnetic film
4
having magnetic characteristics such that when the Curie temperatures of the magneto-optical recording film, the first auxiliary magnetic film and the second auxiliary magnetic film are designated as T
C0
, T
C1
and T
C2
, respectively, and the critical temperatures of the first auxiliary magnetic film and the second auxiliary magnetic film are designated as T
CR1
and T
CR2
, respectively, a relationship expressed as room temperature <T
CR2
<T
CR1
<T
C0
, T
C1
, T
C2
is satisfied. The first auxiliary magnetic film
5
and the second auxiliary magnetic film
4
, as shown in
FIG. 3
, have magnetic characteristics such that each of them acts as a plane-magnetized film in a temperature range from room temperature to a certain critical temperature (T
CR
) higher than room temperature, but acts as a perpendicular-magnetized film at a temperature above T
CR
. The magneto-optical recording film
6
is a perpendicular-magnetized film at a temperature not lower than room temperature.
The principle of action (reproduction) of the first type of magneto-optical recording medium will now be described.
FIG. 2A
shows the state of magnetization of each layer before reproduction after writing recorded magnetic domains into the magneto-optical recording film
6
by the optical modulation recording system or the like. When this medium is irradiated with reproduc
Ohnuki Satoru
Ohta Norio
Shimazaki Katsusuke
Takao Hiroki
Watanabe Hitoshi
Birch & Stewart Kolasch & Birch, LLP
Dinh Tan
Hitachi Maxell Ltd.
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