Reproducing method and reproducing apparatus for magneto-optical recording medium Number:6,693,854 from the United States Patent and Trademark Office (PTO) owispatent External magnetic fields H(x) or reproducing light beams L(x) having different intensity patterns are applied to an identical recording position on a magneto-optical recording medium so that different pieces of information corresponding to the patterns are reproduced from the identical recording position. A function H(x) or L(x) is a password for obtaining the information. Only a person, who knows the function, can access specified information recorded on the magneto-optical recording medium. The function H(x) or L(x) is a function in which the magnetic field intensity or the light intensity is modulated with respect to the recording position x so that magnetic domains may be thinned out to perform reproduction at a specified cycle from continuous magnetic domains in a recording area. Other than the use for the purpose of security, reproduced C/N can be remarkably improved by performing reproduction n times while radiating a reproducing light beam P.sub.L /P.sub.H which is power-modulated so that the high power P.sub.H is applied at every cycle which is not less than n times the recording clock, to a specified magneto-optical recording medium capable of magnetic domain-magnifying reproduction.<H reproducing, Reproducing, Reproducing, met, 6693854.html, Patent, pto, 6693854

Title: Reproducing method and reproducing apparatus for magneto-optical recording medium

Abstract: External magnetic fields H(x) or reproducing light beams L(x) having different intensity patterns are applied to an identical recording position on a magneto-optical recording medium so that different pieces of information corresponding to the patterns are reproduced from the identical recording position. A function H(x) or L(x) is a password for obtaining the information. Only a person, who knows the function, can access specified information recorded on the magneto-optical recording medium. The function H(x) or L(x) is a function in which the magnetic field intensity or the light intensity is modulated with respect to the recording position x so that magnetic domains may be thinned out to perform reproduction at a specified cycle from continuous magnetic domains in a recording area. Other than the use for the purpose of security, reproduced C/N can be remarkably improved by performing reproduction n times while radiating a reproducing light beam P.sub.L /P.sub.H which is power-modulated so that the high power P.sub.H is applied at every cycle which is not less than n times the recording clock, to a specified magneto-optical recording medium capable of magnetic domain-magnifying reproduction.

Patent Number: 6,693,854 Issued on 02/17/2004 to Shimazaki, et al.

Inventors: Shimazaki; Katsusuke (Kitasouma-gun, JP), Imai; Susumu (Toride, JP), Ohta; Norio (Tsukuba-gun, JP), Awano; Hiroyuki (Noda, JP), Yoshihiro; Masafumi (Kitasouma-gun, JP), Watanabe; Hitoshi (Yuuki-gun, JP), Takao; Hiroki (Kitasouma-gun, JP)
Assignee: Hitachi Maxwell, Ltd. (Osaka, JP)

Appl. No.: 10/388,397

Filed: March 17, 2003

Related U.S. Patent Documents


Application Number	Filing Date	Patent Number	Issue Date
242907		6556516

Foreign Application Priority Data


Aug 27, 1996 [JP]			8-225520
Sep 19, 1996 [JP]			8-246777
Mar 28, 1997 [JP]			9-094899

Current U.S. Class: 369/13.05 ; 369/13.14

Current International Class: G11B 11/00 (20060101)

Field of Search: 369/13.02,13.07,13.27,116,13.14,13.39,47.28,59.12,53.2,47.53,44.39,53.36,288 428/696MM

References Cited [Referenced By]

U.S. Patent Documents


5062091	October 1991	Maeada et al.
5331612	July 1994	Murakami et al.
5394380	February 1995	Hasegawa
5471457	November 1995	Bakx et al.
6061307	May 2000	Shimazaki et al.
6101153	August 2000	Shimazaki et al.
6122228	September 2000	Shimazaki et al.

Foreign Patent Documents


A-1-143041	Jun., 1989	JP
A-6-259823	Sep., 1994	JP
A-7-192342	Jul., 1995	JP

Other References

Y Murakami et al., Super Resolution Readout of a Magneto-Optical Disk with an In-Plane Magnetization Layer, Proceedings of Magneto-Optical Recording International Symposium '92; J. Magn. Soc. Jpn., vol. 17, Supplement No. S1 (1993), pp. 201-204..

Primary Examiner: Neyzari; Ali
Attorney, Agent or Firm: Oliff & Berridge, PLC

Parent Case Text

This is a Division of application Ser. No. 09/242,907 filed Mar. 22, 1999 now U.S. Pat. No. 6,556,516 which is the National Stage of PCT/JP97/02984, filed Aug. 27, 1997. The entire disclosure of the prior application is hereby incorporated by reference herein in its entirety.

Claims

What is claimed:

1. A method for reproducing information recorded on a magneto-optical recording medium while applying an external magnetic field to the magneto-optical recording medium, the method comprising the step of: applying the external magnetic field including different application patterns to an identical recording area on the magneto-optical recording medium so that different pieces of information corresponding to the application patterns are reproduced from the identical recording area.

2. The method according to claim 1, wherein the application pattern is represented as a function H(x) of a magnetic field intensity with respect to a position x on the magneto-optical recording medium.

3. The method according to claim 2, wherein the function H(x) is a password to obtain information, and only a person, who knows the function H(x), is able to access specified information capable of being reproduced with the function H(x).

4. The method according to claim 1, wherein a specified information sequence is precoded and recorded beforehand on the magneto-optical recording medium so that the specified information sequence is reproduced when the function H(x) of the magnetic field intensity is used.

5. The method according to claim 1, wherein the different application patterns are obtained by changing at least one of an intensity of the magnetic field, a frequency of the magnetic field, a rising shape and a falling shape of a signal at a portion at which the external magnetic field is switched, and a timing of application of the magnetic field.

6. The method according to claim 1, wherein an amplitude of a reproduced signal differs as an intensity of the magnetic field applied during reproduction differs, and thus the different pieces of information are obtained.

7. The method according to claim 1, wherein an information sequence recorded on a specified portion of the magneto-optical recording medium is reproduced by applying the magnetic field having an application pattern capable of reproducing the specific portion.

8. The method according to claim 7, wherein the information sequence recorded on the specified portion of the magneto-optical recording medium is an information sequence which is discontinuously recorded in a track direction.

9. The method according to claim 8, wherein the information sequence recorded on the specified portion of the magneto-optical recording medium is an information sequence which is composed of code information recorded at every cycle that is at least twice as much as a recording clock.

10. The method according to claim 1, wherein information sequences, which are different from each other, are reproduced by applying an alternating magnetic field or a direct current magnetic field.

11. The method according to claim 1, further comprising the step of radiating a reproducing light beam including different radiation patterns onto the identical recording area on the magneto-optical recording medium so that different pieces of information corresponding to a combination of the radiation pattern of the reproducing light beam and the application pattern of the external magnetic field are reproduced from the identical recording area.

12. The method according to claim 11, wherein a magneto-optical recording medium, which comprises at least a reproducing layer and a recording layer in this order on a substrate, is used as the magneto-optical recording medium.

13. The method according to claim 11, wherein a magneto-optical recording medium, which comprises a magneto-optical recording film having perpendicular magnetization, and an auxiliary magnetic film that causes transition from an in-plane magnetizable film to a perpendicularly magnetizable film at a temperature exceeding a critical temperature Tcr, is used as the magneto-optical recording medium.

14. The method according to claim 11, wherein the magneto-optical recording medium comprises a magneto-optical recording film having perpendicular magnetization, and an auxiliary magnetic film which causes transition from an in-plane magnetizable film to a perpendicularly magnetizable film at a temperature exceeding a critical temperature Tcr, with a non-magnetic film intervening therebetween, and wherein a relationship of room temperature<Tcr<Tcomp<Tco<Tc holds concerning a Curie temperature Tco of the magneto-optical recording film and a Curie temperature Tc and a compensation temperature Tcomp of the auxiliary magnetic film.

15. The method according to claim 13, wherein: the information is reproduced by irradiating the magneto-optical recording medium with the reproducing light beam which is power-modulated to have at least two light powers of Pr.sub.1 and Pr.sub.2 while applying a DC magnetic field provided that the power Pr.sub.1 of the reproducing light beam is a power to transfer a recording magnetic domain in the magneto-optical recording film to the auxiliary magnetic film and magnify the magnetic domain, and the power Pr.sub.2 of the reproducing light beam is a power to reduce or extinguish the magnified magnetic domain, and wherein: the recording area of the magneto-optical recording medium is irradiated with the reproducing light beam having a Pr.sub.2 /Pr.sub.1 pattern modulated to radiate the power Pr.sub.2 of the reproducing light beam at every cycle which is n times (n>2) as much as a recording clock, and then the recording area is repeatedly irradiated (n-1) times with the reproducing light beam having a pattern which has the same cycle as that of the modulated Pr.sub.2 /Pr.sub.1 pattern and in which a timing for radiating the reproducing light beam of Pr.sub.2 is deviated by an amount of one clock from each preceding cycle, so that the information in the recording area is reproduced.

16. The method according to claim 15, wherein the recording area of the magneto-optical recording medium is irradiated with the reproducing light beam having a Pr.sub.2 /Pr.sub.1 pattern modulated to radiate the power Pr.sub.2 of the reproducing light beam at every cycle which is three times as much as the recording clock, the recording area is subsequently irradiated with the reproducing light beam having a pattern which has the same cycle as that of the modulated Pr.sub.2 /Pr.sub.1 pattern and in which the timing for radiating the reproducing light beam of Pr.sub.2 is deviated by an amount of one clock, and then the recording area is irradiated with the reproducing light beam having a pattern which has the same cycle as that of the modulated Pr.sub.2 /Pr.sub.1 pattern and in which the timing for radiating the reproducing light beam of Pr.sub.2 is deviated by an amount of two clocks, so that the information recorded in the recording area is reproduced.

17. The method according to claim 1, wherein an information sequence to be reproduced as at least two or more different information sequences, each of which is three-valued or higher multi-valued information, are obtained from the information recorded at an identical position on the identical magneto-optical recording medium.

18. The method according to claim 1, wherein an information sequence to be reproduced from the information recorded in the identical recording area on the identical recording medium is reproduced as a plurality of binary sequences having different (d, k) constraints.

19. The method according to claim 1, wherein a plurality of channels of information are reproduced by using the single magneto-optical recording medium.

20. The method according to claim 1, wherein a certain pattern is used to read information recorded in a specified recording magnetic domain, as information depending on magnetization information, and another pattern is used to read the information recorded in the specified recording magnetic domain, as information not depending on the magnetization information.

21. The method according to claim 1, wherein the method is carried out to reproduce the information by using magnetically induced super resolution.

22. The method according to claim 1, wherein the method is carried out to reproduce the information by using magnetic domain magnification.

23. A magneto-optical reproducing apparatus for reproducing recorded information by radiating a reproducing light beam onto a magneto-optical recording medium, the apparatus comprising: a magnetic field unit for applying an external magnetic field to the magneto-optical recording medium during reproduction; a means for selecting a specified external magnetic field application pattern from a plurality of external magnetic field application patterns; and a driving unit for driving the magnetic field unit in accordance with the selected external magnetic field application pattern, wherein the information recorded on the magneto-optical recording medium is reproduced while applying the magnetic field to the magneto-optical recording medium in accordance with the specified external magnetic field application pattern to make it possible to obtain specified information based on the pattern.

24. The magneto-optical reproducing apparatus according to claim 23, wherein the external magnetic field application pattern is a password to access the specified information recorded on the magneto-optical recording medium, the specified pattern is selected when a user inputs the password into the reproducing apparatus, and the specified information is reproduced by applying the external magnetic field in accordance with the pattern to the magneto-optical recording medium.

25. The magneto-optical reproducing apparatus according to claim 23, further comprising a storage unit for storing the plurality of external magnetic field application patterns of the external magnetic field.

26. The magneto-optical reproducing apparatus according to claim 23, wherein the reproducing apparatus has a recording function, and the reproducing apparatus further comprises an encoder for preceding the specified information before the information is recorded so that the specified information is reproduced in accordance with the application pattern.

27. The magneto-optical reproducing apparatus according to claim 23, further comprising a means for selecting a specified radiation pattern from a plurality of patterns of the reproducing light beam, wherein the information recorded on the magneto-optical recording medium is reproduced while applying the reproducing light beam and the external magnetic field to the magneto-optical recording medium in accordance with a combination of the specified radiation pattern and the specified external magnetic field application pattern to make it possible to obtain the specified information based on the combination of the specified patterns.

28. The magneto-optical reproducing apparatus according to claim 23, further comprising a function to reproduce at least two or more information sequences, each of which is a three-valued or higher multi-valued information sequence, from a recorded information sequence on the identical magneto-optical recording medium.

29. The magneto-optical reproducing apparatus according to claim 23, further comprising a function to reproduce the information as a plurality of binary sequences having different (d, k) constraints, from a recorded information sequence on the identical magneto-optical recording medium.

30. The magneto-optical reproducing apparatus according to claim 23, further comprising a function to reproduce the information recorded at an identical position on the identical recording medium, as different pieces of information, and record and reproduce the information in a plurality of channels.

31. The magneto-optical reproducing apparatus according to claim 23, wherein a certain information recorded on the recording medium is accessed by reproducing an information sequence recorded continuously on the magneto-optical recording medium while thinning out the information sequence.

32. The magneto-optical reproducing apparatus according to claim 23, wherein an information sequence recorded continuously on the magneto-optical recording medium is reproduced by rotating the magneto-optical recording medium a plurality of times.

Description

TECHNICAL FIELD

The present invention relates to a magneto-optical recording and reproducing method and a recording and reproducing apparatus therefor, to be carried out by using a magneto-optical recording medium. In particular, the present invention relates to a magneto-optical recording and reproducing method and a recording and reproducing apparatus therefor, based on the use of a novel reproducing technique which has not been used in any conventional magneto-optical recording and reproduction process.

BACKGROUND ART

In order to realize a more advanced high density process carried out on a magneto-optical recording medium, a technique attracts attention, in which the magneto-optical recording medium is irradiated with a reproducing laser beam while applying an external magnetic field during reproduction to obtain a reproduced signal.

The magnetically induced super resolution technique has been suggested, as described, for example, in Journal of Magnetic Society of Japan, Vol. 17, Supplement No. S1, p. 201 (1993). This technique improves the magnetic film of the magneto-optical recording medium, and it utilizes the occurrence of the temperature distribution in a light spot upon irradiation with a reproducing light beam. Thus, a magnetic mask is generated in the spot so that the effective spot diameter, which contributes to signal reproduction, is reduced. The use of this technique makes it possible to improve the reproducing resolving power without reducing the optical spot diameter.

Another technique for amplifying a minute reproduction signal has been also suggested, as disclosed, for example, in Japanese Patent Application Laid-Open No. 1-143041, in which the magnetic domain is magnified by applying an external magnetic field during reproduction. Japanese Patent Application Laid-Open No. 6-259823 also discloses a technique for amplifying the magnetic domain.

The use of the magnetically induced super resolution technique and the magnetic domain-magnifying reproduction technique makes it possible to mutually distinguish and reproduce a plurality of minute magnetic domains existing in the reproducing light spot. However, since such magnetic domains are subjected to high density recording, the recording clock cycle is short. When such signals are reproduced, it is feared that C/N is deteriorated due to occurrence of interference between waveforms. For this reason, a technique is further required to improve C/N when the magnetic domains, which are subjected to high density recording, are reproduced by using the techniques as described above.

In the case of the conventional techniques described above, the reproduced signal, which is obtained upon reproduction, corresponds to information recorded on the magneto-optical recording medium, either when the magnetically induced super resolution technique is used, or when the minute reproduced signal is amplified by applying the external magnetic field during reproduction to magnify the magnetic domain, because of the following reason. That is, it has been premised that the recorded information should be faithfully reproduced.

However, it is not necessarily demanded that all of recorded information is faithfully reproduced, depending on the use or application of an application system to be constructed in some cases, it is intended to obtain different pieces of reproduced information depending on the use and the purpose. For example, in a certain application system in relation to any security use, there may be a case in which only information, that is recorded on a specified portion of a magneto-optical recording medium, is intended to be reproduced when information recorded on the magneto-optical recording medium is reproduced. Similarly, in an application system in relation to any security use or any cipher description use, there may be a case in which it is intended to directly obtain a reproduced signal by converting information with a specified function, when information recorded on the magneto-optical recording medium is reproduced. Further, depending on the use of the application system, it is desired to perform direct reproduction while thinning out information at specified intervals, when information recorded on the magneto-optical recording medium is reproduced.

An object of the present invention is to provide a magneto-optical reproducing method and a reproducing apparatus therefor, for obtaining different pieces of reproduced information depending on the use and the purpose, without faithfully reproducing all of recorded information, when information recorded on a magneto-optical recording medium is reproduced. Another object of the present invention is to provide a magneto-optical reproducing method and a reproducing apparatus therefor, which make it possible to reproduce, with high C/N, information subjected to high density recording on a magneto-optical recording medium.

DISCLOSURE OF THE INVENTION

According to a first aspect of the present invention, there is provided a method for reproducing information recorded on a magneto-optical recording medium while applying an external magnetic field to the magneto-optical recording medium, the method comprising the step of: applying the external magnetic field including different application patterns to an identical recording area on the magneto-optical recording medium so that different pieces of information corresponding to the application patterns are reproduced from the identical recording area. The application pattern is represented as a function H(x) of the magnetic field intensity with respect to the position x on the magneto-optical recording medium. It is possible to construct a system in which the function H(x) is used as a password to obtain information, and only a person, who knows the function H(x), is able to access specified information capable of being reproduced with the function H(x).

According to a second aspect of the present invention, there is provided a method for reproducing information recorded on a magneto-optical recording medium while radiating a reproducing light beam onto the magneto-optical recording medium, the method comprising the step of: radiating the reproducing light beam including different radiation patterns onto an identical recording position on the magneto-optical recording medium so that different pieces of information corresponding to the radiation patterns are reproduced from the identical recording position.

According to a third aspect of the present invention, there is provided a magneto-optical reproducing apparatus for reproducing recorded information by radiating a reproducing light beam onto a magneto-optical recording medium, the apparatus comprising: a magnetic field unit for applying an external magnetic field to the magneto-optical recording medium during reproduction; a means for selecting a specified external magnetic field application pattern from a plurality of external magnetic field application patterns; and a driving unit for driving the magnetic field unit in accordance with the selected external magnetic field application pattern, wherein the information recorded on the magneto-optical recording medium is reproduced while applying the magnetic field to the magneto-optical recording medium in accordance with the specified external magnetic field application pattern to make it possible to obtain specified information based on the pattern.

According to a fourth aspect of the present invention, there is provided a magneto-optical reproducing apparatus for reproducing recorded information by radiating a reproducing light beam onto a magneto-optical recording medium, the apparatus comprising: a light source for radiating the reproducing light beam onto the magneto-optical recording medium during reproduction; a means for selecting a specified radiation pattern from a plurality of radiation patterns; and a driving unit for driving the light source in accordance with the selected specified pattern, wherein: the information recorded on the magneto-optical recording medium is reproduced while radiating the reproducing light beam onto the magneto-optical recording medium in accordance with the specified radiation pattern to make it possible to obtain specified information based on the pattern.

The principle of operation of the magneto-optical recording and reproducing method according to the present invention will be explained below. Recording is performed on the magneto-optical recording medium by magnetizing the recording layer in an area intended to perform recording, in a desired direction. Reproduction is performed by detecting the magneto-optical effect such as the Kerr rotation angle and the Faraday rotation angle. In an ordinary reproduction process, it is desired to stably obtain a reproduced signal faithfully corresponding to the recorded magnetization state. In order to stably obtain the reproduced signal faithfully corresponding to the recorded magnetization state, it is desired to obtain a reproduced signal having a stable shape, regardless of slight fluctuation of the radiation state of the laser beam to be radiated during the reproduction. When the external magnetic field is applied during the reproduction in accordance with, for example, the magnetically induced super resolution, it is desired to obtain a reproduced signal having a stable shape, regardless of slight fluctuation of the application state of the external magnetic field.

On the contrary, the present invention actively utilizes the change of the shape of the reproduced signal by changing the application pattern of the external magnetic field applied during the reproduction, the radiation pattern of the laser beam radiated during the reproduction, or the combination thereof. Even when an identical magnetization state is recorded on the recording layer of the magneto-optical recording medium, it is possible to obtain a plurality of reproduced signals having different shapes by changing the application pattern of the external magnetic field, the radiation pattern of the laser beam radiated during the reproduction, or the combination thereof, because of the following reason. That is, the magnetization state of the reading layer, which directly contributes to the reproduced signal, can be changed by changing the application pattern of the external magnetic field, the radiation pattern of the laser beam radiated during the reproduction, or the combination thereof. It is especially preferable to use a magneto-optical recording medium comprising a plurality of stacked layers including two layers of a recording layer and a reading layer, the magneto-optical recording medium being constructed such that the magnetization state of the reading layer depends on the magnetization state of the recording layer and the temperature distribution caused by irradiation with the reproducing laser beam. The use of such a magneto-optical recording medium makes it possible to obtain a plurality of reproduced signals having different shapes by changing the application pattern of the external magnetic field, the radiation pattern of the laser beam radiated during the reproduction, or the combination thereof. In other words, the present invention is preferably applied to perform the recording and the reproduction on the magneto-optical recording medium comprising a plurality of stacked layers including at least the recording layer and the reading layer, and the present invention is especially preferably applied to perform the recording and the reproduction on the magneto-optical recording medium capable of performing the magnetically induced super resolution or the magnification of the magnetic domain.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a magneto-optical recording and reproducing method according to the present invention described in a first embodiment.

FIG. 2 illustrates a magneto-optical recording and reproducing method according to the present invention described in a second embodiment.

FIG. 3 illustrates a magneto-optical recording and reproducing method according to the present invention described in a third embodiment.

FIG. 4 illustrates the magneto-optical recording and reproducing method according to the present invention described in the third embodiment, which is different from FIG. 3.

FIG. 5 illustrates the magneto-optical recording and reproducing method according to the present invention described in the third embodiment, which is different from FIGS. 3 and 4.

FIG. 6 illustrates the magneto-optical recording and reproducing method according to the present invention described in the third embodiment, which is different from FIGS. 3 to 5.

FIG. 7 illustrates the magneto-optical recording and reproducing method according to the present invention described in the third embodiment, which is different from FIGS. 3 to 6.

FIG. 8 exemplarily shows a representative structure of a recording medium preferably used in the present invention.

FIG. 9 exemplarily shows a representative structure of a recording medium preferably used in the present invention.

FIGS. 10A and B show an experimental result obtained when the magnetic domain is magnified on the recording medium shown in FIG. 8, respectively.

FIGS. 11A and B conceptually illustrate the magnetic characteristic of the recording medium shown in FIG. 8, respectively.

FIG. 12 exemplarily shows a representative structure of a recording medium preferably used in the present invention.

FIGS. 13A and B conceptually illustrate the magnetic characteristic of the recording medium described in a fourth embodiment (4) and shown in FIG. 12, respectively.

FIG. 14 conceptually illustrates the magnetic characteristic of the recording medium described in the fourth embodiment (4) and shown in FIG. 12, which is different from FIG. 13.

FIG. 15 exemplarily shows a representative structure of a recording medium preferably used in a fourth embodiment (5) and in the present invention.

FIGS. 16A and B conceptually illustrate the magnetic characteristic of the recording medium described in the fourth embodiment (5) and shown in FIG. 15, respectively.

FIG. 17 illustrates an arrangement of a recording and reproducing apparatus described in a fifth embodiment, the recording and reproducing apparatus comprising a magnetic field data selector for selecting a function H(x) corresponding to a magnetic field application pattern for reproduction.

FIG. 18 illustrates an arrangement of a recording and reproducing apparatus described in the fifth embodiment, the recording and reproducing apparatus comprising a magnetic field data selector for selecting a function H(x) corresponding to a magnetic field application pattern for reproduction, and a reproducing light beam data selector for selecting a function L(x) corresponding to an intensity pattern of a reproducing light beam.

FIG. 19 shows a timing chart for illustrating a magneto-optical recording and reproducing method based on the use of external magnetic fields 1 to 3 described in a sixth embodiment.

FIG. 20 shows the magnetic temperature characteristic concerning a magneto-optical recording layer and a magneto-optical reproducing layer of a magneto-optical recording medium described in a tenth embodiment.

FIG. 21 explains the process for reproducing the recording magnetic domain in the magneto-optical recording layer from the magneto-optical reproducing layer by irradiating the magneto-optical recording medium described in the tenth embodiment with the optically modulated reproducing light beam, wherein (a) illustrates the directions of the magnetic domains in the recording layer and the reproducing layer, and (b) illustrates the optically modulated reproducing light power.

FIG. 22 illustrates the principle of the extinguishment of the magnetic domain, wherein (a) illustrates the sub-lattice magnetization of the reproducing layer at a temperature less than the compensation temperature, and (b) illustrates the sub-lattice magnetization of the reproducing layer at a temperature above the compensation temperature.

FIG. 23 conceptually shows a stacked structure of a magneto-optical recording medium used for the reproducing method according to the present invention.

FIG. 24 shows a graph illustrating a temperature-dependent characteristic of the Kerr effect of an auxiliary magnetic film of the magneto-optical recording medium produced in the embodiment of the present invention.

FIG. 25 shows graphs (A) to (E) illustrating reproduced signal waveforms observed on an oscilloscope when the magneto-optical recording medium obtained in the tenth embodiment of the present invention is subjected to reproduction with continuous light beams having various reproducing powers.

FIG. 26 conceptually illustrates magnetization states (A) to (C) of the respective layers of the magneto-optical recording medium when the signal waveform shown in FIG. 25(A) is obtained.

FIG. 27 conceptually illustrates magnetization states (A) to (C) of the respective layers of the magneto-optical recording medium when the signal waveform shown in FIG. 25(C) is obtained.

FIG. 28 conceptually illustrates magnetization states (A) to (C) of the respective layers of the magneto-optical recording medium when the signal waveform shown in FIG. 25(E) is obtained.

FIG. 29 shows a timing chart illustrating a method for reproducing information by repeating application of a high power reproducing light beam at every three clock cycles under a DC magnetic field while deviating the phase by one clock three times by using the magneto-optical recording medium described in the tenth embodiment.

FIG. 30 shows a medium structure of a magneto-optical recording medium produced in an eleventh embodiment.

FIG. 31 shows the magnetic temperature characteristic of the magneto-optical recording medium shown in FIG. 30.

FIG. 32 illustrate the principle of reproduction on the magneto-optical recording medium produced in the eleventh embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments and Examples of the magneto-optical recording and reproducing method and the recording and reproducing apparatus therefor according to the present invention will be explained below with reference to the drawings. However, the present invention is not limited thereto.

First Embodiment

This embodiment is illustrative of the magneto-optical recording and reproducing method in which the information, which is recorded at a position x on an identical recording medium, can be reproduced as different pieces of information by changing the function H(x) corresponding to the application pattern of the external magnetic field applied when the information is reproduced and the function L(x) corresponding to the radiation pattern of the reproducing laser beam.

FIG. 1 conceptually illustrates the magneto-optical reproducing method according to the present invention. (A) depicts a recording magnetic domain sequence on a recording medium. (B) depicts a function H(x) corresponding to the application pattern of the external magnetic field applied during reproduction. (C) depicts a function L(x) corresponding to the reproducing laser beam radiation pattern. (D) depicts a signal amplitude waveform reproduced by using the function H(x) and the function L(x). (E) depicts a reproduced information sequence. H(x) represents the intensity of the external magnetic field applied at a position x on the recording medium, and it is a function of x. H(x) is defined, for example, by the intensity of the applied magnetic field such as the maximum intensity of the applied magnetic field and the minimum intensity of the applied magnetic field, the frequency of the applied magnetic field, the rising shape and the falling shape of the signal at the portion at which the external magnetic field is switched, and the timing of the applied magnetic field. H(x) can be changed by changing at least one of the foregoing factors. L(x) represents the intensity of the reproducing laser beam radiated at x, and it is a function of x. L(x) is defined, for example, by the laser beam intensity, the clock frequency for making synchronization with the radiation of the laser beam, the duty for radiating the laser beam, the rising shape and the falling shape of the radiation intensity of the laser beam, and the radiation timing of the laser beam. L(x) is changed by changing at least one of the foregoing factors.

As shown in (A), it is assumed that a binary or two-valued information sequence of "1", "0", "1", "0", "1" is recorded in recording magnetic domains 11, 12, 13, 14, and 15 of the magneto-optical recording medium respectively. Preferably, the recording magnetic domains are subjected to recording by using the magneto-optical modulation recording method in which the external magnetic field having the polarity modulated depending on the recording signal is applied while radiating the pulse light beam which is synchronized with the recording clock. However, it is allowable to perform recording based on the optical modulation method or the magnetic field modulation method provided that the magnetic domains can be formed in desired stable shapes. In this embodiment, the recording information is added to the central position of the recording magnetic domain by using the mark position recording method. As described later on, the magneto-optical recording medium to be used is a medium of the type in which information is reproduced while radiating the reproducing light beam and applying the magnetic field during the reproduction.

When the reproduction is performed, as shown in FIG. 1 (B), it is possible to apply, to the magneto-optical recording medium, the magnetic field represented by two types of functions H.sub.1 (x) and H.sub.2 (x). When the function H.sub.1 (x) is used, the magnetic field is applied in a direction of magnetization of the recording magnetic domain (polarity of H.sub.ch) regardless of the recording position x. On the other hand, when H.sub.2 (x) is used, the magnetic field is applied by changing the intensity of the magnetic field depending on the position of the recording magnetic domain. The reproducing light beam is used as shown in FIG. 1(C). That is, a pulse-shaped laser beam L(x), which is synchronized with the recording magnetic domain (recording clock), is radiated.

In the conventional magneto-optical recording and reproduction process, the reproducing magnetic field is applied as indicated by the function H.sub.1 (x) shown by a broken line in (B). That is, the reproducing magnetic field is applied in the direction of magnetization of the recording magnetic domain (polarity of H.sub.ch) regardless of the position of the recording magnetic domain. In this case, a binary information sequence "10101", is reproduced, and the information, which has been recorded in the recording magnetic domains, is faithfully reproduced.

On the contrary, the function H.sub.2 (x), which corresponds to the magnetic field pattern indicated by a solid line in FIG. 1(B), is designed such that the magnetic field is inverted into H.sub.fx having the polarity reverse to that of H.sub.ch only at the timing (position) at which x corresponds to the position of the recording magnetic domain 13. It is noted that the direction of the magnetization of the recording magnetic domain 13 is opposite to the direction of the reproducing magnetic field. Therefore, it is impossible to reproduce the magnetization information "1" of the recording magnetic domain 13 by using either the magneto-optical recording medium based on the use of the magnetic domain magnification-based reproduction process as shown in the fourth embodiment or the magneto-optical recording medium based on the use of the magnetically induced super resolution-based reproduction process as shown in the ninth embodiment as described later on. Accordingly, when the recording magnetic domains 11 to 15 are subjected to reproduction by using the magnetic field pattern in accordance with the function H.sub.2 (x), a reproduced signal of "100011" is obtained.

Therefore, when the identical recording magnetic domains are subjected to reproduction, the reproduced signal (waveform) differs depending on whether the function H(x) of the applied reproducing magnetic field is H.sub.1 (x) or H.sub.2 (x). Thus, it is possible to obtain different pieces of information. Otherwise, it is considered that the information, which does not depend on the magnetization state of the recording magnetic domain 13, is obtained when the function H.sub.2 (x) is used. In other words, it is considered that when the function H.sub.2 (x) is used, the recording magnetic domain can be subjected to reproduction to produce the information which does not depend on the magnetization state, at a certain position on the identical recording medium, while the recording magnetic domain can be subjected to reproduction to produce the information which depends on the magnetization state of the recording magnetic domain, at another position on the identical recording medium. This concept is used as follows. That is, for example, if the information located on the recording magnetic domain 13 is important for a user, the user can access the necessary information by using the function H.sub.1 (x) which is capable of reproducing the magnetization information of the recording magnetic domain. If the information located on the recording magnetic domain 13 is not important for another user, the recording area containing the recording magnetic domain 13 can be subjected to reproduction by using the function which fails to read the magnetization state of the recording magnetic domain 13 or the function H.sub.2 (x) which is used to read the magnetization state as the magnetization information in the erasing direction irrelevant to the direction of the magnetization. That is, the function H(x) can operate as a password to access the necessary information for each user. Accordingly, it is possible to obtain a plurality of pieces of information from the identical recording magnetic domain depending on the function H(x). As demonstrated in the embodiment described later on, the information can be ciphered by precoding the recording signal in accordance with the function H(x) upon the recording process. Thus, it is possible to construct a system which is effective to keep the secret information based on the use of the magneto-optical recording medium.

It is also preferable in the embodiment of the present invention that the sampling frequency, which is used when the binary information is obtained from the reproduced signal, is not coincident with the sampling frequency which is used during the recording. For example, the reproduction process may be performed by using a sampling frequency which is three times as much as the sampling frequency used when the recording process is performed. Alternatively, the reproduction process may be performed, in which the sampling frequency during the recording process is 1/3 of the recording clock.

The information to be reproduced may be reproduced not as binary or two-valued information but as three-valued or higher multi-valued information. In this procedure, a threshold value is set beforehand to perform reproduction so that the multi-valued information is obtained. The information may be multi-valued in accordance with the amplitude of the reproduced signal.

Further, it is appropriate to combine the sampling frequencies upon the recording and the reproduction of the information, and the selection of the number of steps to obtain the multi-valued information. By doing so, even if the waveform itself of the reproduced signal is identical, it is possible to obtain those which are different from each other as reproduced information sequences.

The foregoing description may be summarized into the following statements.

i) The information, which has been recorded at the identical position on the identical recording medium, can be obtained as the reproduced signals having different waveforms by changing H(x) or L(x) during the reproduction.

ii) Even if the reproduced signals have the identical waveform, the recorded information can be reproduced as different pieces of information by changing the process condition such as the preset value for the slice level and the sampling cycle.

iii) There are a variety of methods in which the information, which has been recorded at the identical position on the identical recording medium, is reproduced as various different pieces of information by utilizing i) and ii) described above in combination.

Explanation will be made for examples of the variety of methods in which the information, which has been recorded at the identical position on the identical recording medium, is reproduced. At first, a plurality of pieces of information can be reproduced from an identical recording position by changing the application pattern represented by H(x) to apply the magnetic field to the identical recording position on the recording medium. For example, it is assumed that H(x) is represented by three types of H1(x), H2(x), and H3(x). A plurality of different pieces of information can be reproduced from an identical recording position depending on the selection of any of them. In the example shown in FIG. 1, those used as H(x) are the two types of the magnetic field waveform as depicted by the broken line shown in FIG. 1(B) and the magnetic field waveform as depicted by the solid line shown in FIG. 1(B). However, H(x) may be a more complicated function. Similarly, a plurality of different pieces of information can be reproduced from an identical recording position by changing the laser beam radiation pattern represented by L(x) to apply the laser beam to the identical recording position on the recording medium. L(x) may be a complicated function. Further, a plurality of different pieces of information can be reproduced from one piece of recorded information located at an identical position on an identical recording medium by changing L(x) in addition to the change of H(x).

Furthermore, the information sequence may be reproduced from the information recorded at an identical position on an identical recording medium, as at least two or more different information sequences, each of which is the three-valued or higher multi-valued information. Moreover, taking notice of the (d, k) constraint which is known in the field of the coding technique, the present invention is also directed to the fact that the information sequence, which is reproduced from the information located at an identical position on an identical recording medium, is reproduced as a plurality of binary sequences having different (d, k) constraints.

Those usable as the recording medium in the specified embodiment of the present invention include, for example, the conventional magneto-optical recording medium, the recording medium capable of performing the magnetically induced super resolution, and the recording medium capable of magnifying the magnetic domain as demonstrated in the embodiment described later on. It is preferable to use the recording medium capable of magnifying the magnetic domain in view of the fact that a large reproduced signal amplitude can be obtained even when the recording magnetic domain is minute. Especially, in the embodiment of the present invention, it is preferable to use the recording medium capable of magnifying the magnetic domain in view of the fact that a large difference can be obtained in the reproduced signal amplitude depending on whether the information recorded in the recording magnetic domain is "0" or "1", when H(x)=H.sub.ch is applied. The recording medium capable of magnifying the magnetic domain will be explained in the fourth embodiment described later on.

The illustration concerning FIG. 1 assumes the mark position recording. However, the present invention can be also carried out in accordance with the mark edge recording by making appropriate alteration. It is a matter of course that the present invention is also applicable even when the magnetic domain length of the magnetic domain to be subjected to recording differs.

Second Embodiment

This embodiment specifically illustrates an example in which the reproduced signal is changed when H(x) is changed.

FIG. 2 conceptually illustrates this embodiment. FIG. 2 shows, from the top, the position on the magneto-optical recording disk based on the sample servo system, a reproducing clock, a reproducing laser beam pulse, a reproducing external magnetic field 1, a reproduced signal waveform obtained by using the pulse light beam and the reproducing external magnetic field 1, a reproducing external magnetic field 2, and a reproduced signal waveform obtained by using the pulse light beam and the reproducing external magnetic field 2. The position on the magneto-optical disk based on the sample servo system is roughly classified into those in a servo area and those in a recording area. Tracking pits and an embedded clock pit are arranged in the servo area. Magneto-optical recording magnetic domains are disposed in the recording area.

The reproducing clock is generated by utilizing a detection signal obtained from the illustrated embedded clock pit. In FIG. 2, H(x) corresponds to the reproducing external magnetic field 1 and the reproducing external magnetic field 2. L(x) corresponds to the reproducing laser beam pulse. L(x), which is used when the reproducing external magnetic field 1 is applied, is identical to that used when the reproducing external magnetic field 2 is applied. However, the reproducing laser beam pulse is modulated into two levels of a high power PH and a low power PL. The high power PH preferably has a duty of 10% to 60%. In this embodiment, the duty is 30%.

In FIG. 2, the reproducing external magnetic field 1 was obtained by alternately applying an external magnetic field H.sub.E having a polarity in the same direction as the magnetization direction of the recording magnetic domains (black magnetic domains in FIG. 2) and a magnetic field H.sub.S in an initializing direction (magnetization direction of white magnetic domains in FIG. 2) opposite thereto, at a cycle which was twice as much as the reproducing clock. (H.sub.E, H.sub.S shown in FIG. 2 correspond to H.sub.ch, H.sub.fx shown in FIG. 1.) The duty of the external magnetic field H.sub.E is preferably 5% to 45%. In this embodiment, the duty was adjusted to be 25%. On the other hand, the reproducing external magnetic field 2 was applied at the same frequency at the same intensity (H.sub.E, H.sub.S) with the same duty as those of the reproducing external magnetic field 1 except that the phase of the reproducing external magnetic field 1 was deviated in an amount of one cycle of the reproducing clock.

As shown in FIG. 2, it is understood that when the identical recording area on the magneto-optical recording medium is subjected to reproduction, the waveform of the reproduced signal, which is obtained when the reproduction is performed by using the reproducing external magnetic field 1, is different from that obtained when the reproduction is performed by using the reproducing external magnetic field 2. Also in this embodiment, different pieces of information can be obtained even when the waveform of the reproduced signal is identical, as described in ii) in the first embodiment. The fact described in ii) in the first embodiment is also applicable to this embodiment. Those usable as the recording medium in this embodiment include, for example, the conventional magneto-optical recording medium, the recording medium capable of performing the magnetically induced super resolution, and the recording medium capable of magnifying the magnetic domain. It is preferable to use the recording medium capable of magnifying the magnetic domain, in the same manner as in the first embodiment.

Third Embodiment

This embodiment is illustrative of another example of the magneto-optical recording and reproducing method in which information, which is recorded at an identical position on an identical recording medium, can be obtained as reproduced signals having different waveforms, by changing at least one of H(x) and L(x).

FIGS. 3 to 7 conceptually illustrate the reproducing method according to this embodiment. FIGS. 3 to 7 show, from the top, the position on the magneto-optical recording disk based on the sample servo system, a reproducing clock, a reproducing laser beam pulse, a reproducing external magnetic field, a reproduced signal waveform obtained by using a DC light beam and a DC external magnetic field, a reproduced signal waveform obtained by using the pulse light beam and the pulse magnetic field, a reproduced signal waveform after sample hold obtained by using the pulse light beam and the pulse magnetic field, and a sample hold pulse synchronized with the clock. For example, the clock synchronization, which utilizes the embedded clock pit, is performed in the same manner as in the case shown in FIG. 2.

In FIGS. 3 to 7, the pulse light beam and the pulse magnetic field are applied to the magneto-optical recording medium during the reproduction. However, the respective cases use different application patterns of the reproducing magnetic field and/or the reproducing light beam (different in the function H(x) or L(x) concerning the position X on the magneto-optical recording medium). As a result, although the magneto-optically recorded magnetic domain pattern (recorded information) is completely identical in all of the cases shown in FIGS. 3 to 7, the waveform of the reproduced signal and the reproduced signal waveform after the sample hold differ, because the application pattern of the reproducing magnetic field or the reproducing light beam is different.

A reproducing condition shown in FIG. 3 is the same as the reproducing condition shown in FIG. 2 except that the reproducing light beam and the reproducing magnetic field were applied at the same cycle as that of the reproducing clock. In a reproducing condition shown in FIG. 4, the high power pulse is thinned out so that the pulse laser of the high power P.sub.H is radiated at every three cycles of the reproducing clock, as compared with FIG. 3, in which L(x) is different. In FIG. 5, the high pulse magnetic field H.sub.E is thinned out so that the high pulse magnetic field H.sub.E is radiated at every three cycles of the reproducing clock, as compared with FIG. 3, in which H(x) is different. In FIG. 6, the reproducing light beam pulse is given at multiple levels, as compared with FIG. 3, in which L(x) is different. In FIG. 7, the pulse magnetic field is given at multiple levels, as compared with FIG. 3, in which H(x) is different. The waveform of the reproduced signal, which is obtained when the pulse light beam and the pulse magnetic field are used, is different in each of the cases, because H(x) and L(x) are different as described above. FIGS. 3 to 7 show the reproduced signal waveform obtained when the DC light beam and the DC magnetic field are used respectively. It is understood that the reproduced signal, which reflects the magnetization of each of the recording magnetic domains, is not obtained even when the reproduction is performed by using the DC light beam and the DC magnetic field. In the case of the use of the DC light beam and the DC magnetic field, there is no difference in reproducing condition. Therefore, the reproduced signal is coincident throughout FIGS. 3 to 7.

As understood from this embodiment, it is possible to obtain different pieces of information depending on H(x) and/or L(x) even when the identical recording magnetic domain pattern (recorded information) on the magneto-optical recording medium is subjected to reproduction by using the reproducing magnetic field and the reproducing light beam. Those usable as the recording medium in this embodiment include, for example, the conventional magneto-optical recording medium, the recording medium capable of performing the magnetically induced super resolution, and the recording medium capable of magnifying the magnetic domain. It is preferable to use the recording medium capable of magnifying the magnetic domain, in the same manner as in the first embodiment.

Fourth Embodiment (1)

Those usable as the recording medium in the first to third embodiments include, for example, the conventional magneto-optical recording medium, the recording medium capable of performing the magnetically induced super resolution, and the recording medium capable of magnifying the magnetic domain. It is preferable to use the recording medium capable of magnifying the magnetic domain, in view of the fact that the large amplitude of the reproduced signal can be obtained even in the case of the minute magnetic domain. Explanation will be made below in the fourth embodiments (2) to (5) for examples of the magneto-optical recording medium capable of magnifying the magnetic domain in an especially preferred manner.

Fourth Embodiment (2)

FIG. 8 shows an exemplary structure of the magneto-optical recording medium capable of magnifying the magnetic domain. The recording medium of this embodiment comprises, on a transparent substrate, a dielectric layer 42, a magnifying reproducing layer 43, a non-magnetic layer 44, an information-recording layer 45, and a dielectric layer 46 which are successively stacked. The minute magnetic domain, which is recorded on the information-recording layer 45, is transferred to the magnifying reproducing layer 43. When the reproducing magnetic field is applied, the magnetic wall is moved in the magnifying reproducing layer 43. Thus, the reproduction can be performed based on the use of the magnification of the magnetic domain. That is, when the magnifying reproducing magnetic field is applied in the same direction as that of the magnetization of the minute magnetic domain in a state in which the minute magnetic domain is transferred from the information-recording layer 45 to the magnifying reproducing layer 43, the magnetic wall is moved in the direction to magnify the magnetic domain, because the coercive force of the magnetic wall is small in the magnifying reproducing layer 43. Thus, a magnified magnetic domain is formed. As a result, a magnified mark (the magnetic domain magnified in the magnifying reproducing layer 43) is observed in the reproducing light spot. The minute magnetic domain appears while being magnified on the surface of the magneto-optical recording medium as described above. Therefore, the reproduced signal, which has a sufficient intensity, is obtained from the magnified magnetic domain. When the reducing reproducing magnetic field is applied in a direction opposite to that of the magnifying reproducing magnetic field after the magnified magnetic domain in the information-recording layer 45 is subjected to the reproduction, the magnified magnetic domain in the magnifying reproducing layer 43 is reduced. The direction of magnetization is the same as the direction of the magnetic field of the reducing reproducing magnetic field. The reducing reproducing magnetic field and the magnifying reproducing magnetic field as described above can be applied by using an alternating magnetic field. A reproduced signal, which is subjected to amplification for each of the minute magnetic domains, can be obtained by synchronizing the cycle of the alternating magnetic field with the recording clock.

With reference to FIG. 8, a perpendicularly magnetizable film, in which the coercive force of the magnetic wall is smaller than the reproducing magnetic field when the reproducing light beam is irradiated, may be used for the magnifying reproducing layer 43. The magnifying reproducing layer 43 can be composed of, for example, a rare earth transition metal alloy such as GdFe, GdFeCo, and GdCo, an alloy or an alternately stacked material composed of a Pd or Pt layer and a Co layer, or a magnetic member of oxide of garnet. The magnifying reproducing layer 43 i