Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse... – Magnetic field and light beam
Reexamination Certificate
2000-07-11
2003-09-30
Dinh, Tan (Department: 2653)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
Magnetic field and light beam
C369S013280
Reexamination Certificate
active
06628574
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a reproducing method and a reproducing apparatus for a magneto-optical recording medium which make it possible to detect minute magnetic domains formed on the magneto-optical recording medium, with high resolving power and high S/N ratio. In particular, the present invention relates to a reproducing method and a reproducing apparatus for a magneto-optical recording medium which make it possible to perform reproduction individually in a magnified manner from a plurality of minute magnetic domains existing within a reproducing laser spot when magnetization information is reproduced from minute magnetic domains recorded on the magneto-optical recording medium.
BACKGROUND ART
A technique for performing recording and reproduction at a higher density is demanded in view of the increase in amount of information and the advance of the apparatus to acquire a compact size. The magneto-optical recording medium such as a magneto-optical disk is known as an optical memory having a large storage capacity on which information is rewritable. In order to record information on the magneto-optical recording medium, the magnetic field modulation method is used, in which a magnetic field having a polarity corresponding to a recording signal is applied to a portion at which the temperature is raised, while irradiating the magneto-optical recording medium with a laser beam. This method makes it possible to perform overwrite recording, in which high density recording has been achieved. For example, recording has been achieved with a shortest mark length of 0.15 &mgr;m. The optical modulation recording system has been also practically used, in which recording is performed by radiating a power-modulated light beam corresponding to a recording signal while applying a constant magnetic field.
However, even when the minute magnetic domain is formed by using the recording system as described above, the following problems arise during reproduction.
(1) Since the spot radius of the reproducing light beam is too large as compared with the size of the recording magnetic domain (recording mark), it is impossible to individually detect a plurality of magnetic domains existing within the reproducing light beam spot. That is, the reproducing resolving power is insufficient. For this reason, it is impossible to reproduce information from individual recording magnetic domains.
(2) Each of recording magnetic domains has a small areal size, and hence the reproduction signal has a small output. For this reason, the reproduction signal has low S/N.
The magnetically induced super resolution (MSR) technique, which has been suggested, for example, in
Journal of Magnetic Society of Japan
, Vol. 17, Supplement, No. S1, p. 201 (1993), is one of methods to solve the foregoing problem (1). A magneto-optical recording medium used for the magnetically induced super resolution generally comprises a reproducing layer for magnetically induced super resolution, an exchange force control layer, and an information-recording layer. When minute magnetic domains are contained in a reproducing light spot during reproduction, one of the magnetic domains is masked by utilizing the temperature distribution in the reproducing layer so that the other magnetic domain may be individually subjected to reproduction. As described above, in the magnetically induced super resolution, the reproducing resolving power is improved by narrowing the effective field of the radius of the light spot. However, the foregoing problem (2) cannot be solved even by using the magnetically induced super resolution technique, because the intensity of the reproduction signal from each of the magnetic domains does not change.
A reproducing apparatus has been also contrived in order to perform reproduction from recording domains having been subjected to high density recording. Such an apparatus is exemplified by those based on the optical super resolution technique in which a shielding element is inserted into an optical path so that a light-collecting spot which exceeds the diffraction limit of the laser beam is obtained by means of optical super resolution. This technique is discussed in detail in Yamanaka et al., “High Density Optical Recording by Super Resolution”,
Jan. J. Appl. Phys
., 2, Supplement 28-3, 1989, pp. 197-200. Also in this technique, a magnetic mask is generated in the spot by utilizing the fact that the temperature distribution is generated in a magnetic film at the inside of the reproducing light beam spot when an magneto-optical recording medium is irradiated with a reflected light beam. Thus, the effective spot radius, which contributes to the reproduction of the signal, is reduced.
The present inventors have disclosed, in Japanese Patent Application Laid-Open No. 8-7350, a magneto-optical recording medium comprising, on a substrate, a reproducing layer and a recording layer, which makes it possible to magnify and reproduce a magnetic domain transferred to the reproducing layer, by transferring the magnetic domain in the recording layer to the reproducing layer, and applying a reproducing magnetic field during reproduction. An alternating magnetic field is used as the reproducing magnetic field. That is, a magnetic field in a direction to magnify the magnetic domain and a magnetic field in a direction opposite thereto are applied alternately to magnify and reduce respective magnetic domains. The use of the magneto-optical recording medium makes it possible to solve the foregoing problem (2) and amplify the reproduction signal obtained from the magnetic domain. However, it is not easy to control the reproducing magnetic field which is used to magnify the magnetic domain in the reproducing layer. In this viewpoint, this technique requires further improvement. When the magnetic domain is transferred by the aid of the exchange coupling force, the magnification for the magnetic domain effected in the reproducing layer for transferring the magnetic domain thereto is restricted by the size of the magnetic domain in the recording layer. That is, the size of the magnetic domain cannot be magnified to be larger than that of the magnetic domain in the recording layer, at a portion of the reproducing layer on the side of the recording layer. The size of the magnetic domain increases as the separating distance from the recording layer becomes large. Therefore, the following problem arises as well. That is, in an area of the reproducing layer located just over the magnetic domain in the recording layer intended to be reproduced, magnetization is in a direction identical with that in the recording layer in the depth direction for all concerning magnetic domains, however, in an area deviated in the in-plane direction from the magnetic domain intended to be reproduced, a state tends to occur, in which magnetic domain portions having a direction identical with that of magnetization in the recording layer in the depth direction and magnetic domain portions having a direction different therefrom co-exist in a mixed manner.
The present inventors have further disclosed a magneto-optical recording medium and a reproducing method achieved by further developing the technique disclosed in Japanese Patent Application Laid-Open No. 8-7350, in WO 98/02877 and WO 98/02878 disclosed after the priority date of this application. In the reproducing method disclosed in the International Publications, an alternating magnetic field, which is applied during reproduction, is effectively controlled to reliably magnify and reduce the magnetic domain transferred to a magnetic domain-magnifying reproducing layer.
The present inventors have also disclosed, in International Publication WO 97/22969, a method for reproducing information by radiating two types of power-modulated reproducing light beams onto a magneto-optical recording medium comprising a magneto-optical recording film, a first auxiliary magnetic film and a second auxiliary magnetic film composed of a magnetic material which causes transition from in-plane magnetization to perpen
Awano Hiroyuki
Ohta Norio
Shimazaki Katsusuke
Watanabe Hitoshi
Yoshihiro Masahumi
Dinh Tan
Hitachi Maxell Ltd.
Oliff & Berridg,e PLC
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