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United States Patent6125083
Nishimura , ; et al.September 26, 2000
Title
Magneto-optical recording method and medium comprising three layers, whose middle layer has a lower curie temperature than the other layers
Abstract
In a magneto-optical recording medium, a first magnetic layer for participating in reproduction of information is laminated on a substrate. A second magnetic layer for storing information is laminated on the first magnetic layer. A third magnetic layer having a Curie temperature lower than those of the first and second magnetic layers is disposed between the first magnetic layer and the second magnetic layer. A magnetization of a region of the first magnetic layer adjacent to a region of the third magnetic layer the temperature of which is above the Curie temperature of the third magnetic layer is aligned with the direction of magnetization of a region around the region of the first magnetic layer.
Inventors:Nishimura; Naoki (Tokyo, JP), Hiroki; Tomoyuki  (Zama, JP), Okada; Takeshi  (Tokyo, JP)
Assignee:Canon Kabushiki Kaisha (Tokyo, JP)
Appl. No.:982454
Filed:December 2, 1997
Foreign Application Priority Data
Jun 10, 1994 [JP] 6-128778
Jun 10, 1994 [JP] 6-128779
Jun 10, 1994 [JP] 6-128780
Aug 26, 1994 [JP] 6-201979
Aug 26, 1994 [JP] 6-201980
Sep 30, 1994 [JP] 6-236209
Oct 24, 1994 [JP] 6-258002
Current U.S. Class:369/13.46 
Field of Search:369/13,110,14,283,116,275.2 360/59,114 365/122 428/694ML,694MM,694EC
U.S. Patent Documents
5168482December 1992Aratani et al.
5208799May 1993Nakao et al.
5239524August 1993Sato et al.
5241520August 1993Ohta et al.
5367507November 1994Sato et al.
Foreign Patent Documents
0498461Aug., 1992EP
0536780Apr., 1993EP
0583720Feb., 1994EP
0586175Mar., 1994EP
393056Apr., 1991JP
393058Apr., 1991JP
4-255946Sep., 1992JP
4-255947Sep., 1992JP
4-271039Sep., 1992JP
6-124500May., 1994JP
6-180874Jun., 1994JP
62-175945Sep., 1987JP
62-175948Aug., 1987JP
94-16017Jul., 1994KR
Other References
Kaneko, M., et al., "IRISTER--Magneto-Optical Disk for Magnetically Induced SuperResolution," Proc. IEEE, vol. 32, No. 4, Apr. 1994, pp. 544-552. .
Kaneko, M., et al., "Multilayered Magneto-Optical Disks for Magnetically Induced Superresolution," Jpn. J. Appl. Phys., Part 1(31) No. 2, Feb. 22, pp. 568-575. .
Patent Abstract of Japan, vol. 17, No. 36, published Jan. 22, 1993, English Abstract of Japanese Patent No. 4-255946..~
Primary Examiner: Neyzari; Ali
Attorney, Agent or Firm:Fitzpatrick, Cella, Harper & Scinto
Parent Case Text


This application is a continuation of Appln. Ser. No. 08/487,706, filed Jun. 7, 1995, now abandoned.
Claims

What is claimed is:
1. A magneto-optical recording medium comprising:
a substrate;
a first magnetic layer laminated on said substrate, said first magnetic layer contributing to the reproduction of information;
a second magnetic layer laminated on said first magnetic layer for storing information; and
a third magnetic layer disposed between said first and second magnetic layers and having a Curie temperature lower than the Curie temperatures of said first and second magnetic layers,
wherein the following condition is satisfied among said magnetic layers at a temperature of the Curie temperature of said third magnetic layer and higher:
where H.sub.wb is the effective magnetic field due to the Bloch magnetic wall energy of a recording domain of said first magnetic layer, H.sub.d is a static magnetic field applied to the recording domain of said first magnetic layer, H.sub.c1 is the coercive force of said first magnetic layer, and H.sub.wi is the exchange-coupling force applied to the recording domain of said first magnetic layer from said third magnetic layer.

2. The magneto-optical recording medium as set forth in claim 1, wherein said first magnetic layer has a compensation temperature between room temperature and its Curie temperature.

3. The magneto-optical recording medium as set forth in claim 1, wherein said first magnetic layer is an in-plane magnetization film at room temperature and becomes a perpendicular magnetization film when increased in temperature.

4. The magneto-optical recording medium as set forth in claim 1, wherein said first magnetic layer is a perpendicular magnetization film over a range from room temperature to its Curie temperature.

5. The magneto-optical recording medium as set forth in claim 2, wherein the Curie temperature of said second magnetic layer is lower than the Curie temperature of said first magnetic layer.

6. The magneto-optical recording medium as set forth in claim 2, wherein the Curie temperature of said third magnetic layer is around the compensation temperature of said first magnetic layer.

7. The magneto-optical recording medium as set forth in claim 3, wherein a film thickness of said first magnetic layer is no less than 20 nm and no more than 100 nm.

8. The magneto-optical recording medium as set forth in claim 3, wherein a film thickness of said third magnetic layer is no less than 3 nm and no more than 30 nm.

9. The magneto-optical recording medium as set forth in claim 3, wherein said first magnetic layer is mainly formed of GdFeCo.

10. The magneto-optical recording medium as set forth in claim 3, wherein said third magnetic layer is mainly formed of one of GdFe and GdFeCo.

11. The magneto-optical recording medium as set forth in claim 3, wherein said third magnetic layer is an in-plane magnetization film at room temperature and becomes a perpendicular magnetization film when increased in temperature.

12. The magneto-optical recording medium as set forth in claim 11, wherein an in-plane anisotropy of said third magnetic layer at room temperature is greater than an in-plane anisotropy of said first magnetic layer at room temperature.

13. The magneto-optical recording medium as set forth in claim 1, wherein each of said first and second magnetic layers is formed of a ferrimagnetic rare earth-iron family element amorphous alloy.

14. The magneto-optical recording medium as set forth in claim 13, wherein said first magnetic layer is rare earth rich at room temperature and said second magnetic layer is iron family rich at room temperature, and vice versa.

15. The magneto-optical recording medium as set forth in claim 9, wherein said first magnetic layer has the following composition:
Gd.sub.x (Fe.sub.100-y Co.sub.y).sub.100-x, wherein 24.ltoreq.x.ltoreq.32 and 20.ltoreq.y.ltoreq.50.

16. The magneto-optical recording medium as set forth in claim 10, wherein said third magnetic layer has the following composition:
Gd.sub.x (Fe.sub.100-y Co.sub.y).sub.100-x, wherein 25.ltoreq.x.ltoreq.50 and 0.ltoreq.y.ltoreq.20.

17. The magneto-optical recording medium as set forth in claim 1, wherein said second magnetic layer contains one of TbFe, TbFeCo, DyFe and DyFeCo as a main component.

18. The magneto-optical recording medium as set forth in claim 17, wherein said second magnetic layer has the following composition:
Tb.sub.x (Fe.sub.100-y Co.sub.y).sub.100-x, wherein 14.ltoreq.x.ltoreq.33 and 14.ltoreq.y.ltoreq.45.

19. The magneto-optical recording medium as set forth in claim 6, wherein the compensation temperature Tcomp of said first magnetic layer and the Curie temperature T3 of said third magnetic layer satisfy the following relation:

20. The magneto-optical recording medium as set forth in claim 1, wherein a saturation magnetization Ms1 of said first magnetic layer satisfies the following relation at room temperature:

21. The magneto-optical recording medium as set forth in claim 1, wherein a saturation magnetization Ms2 of said second magnetic layer satisfies the following relation when being rare earth rich at room temperature:
and the following relation when being iron family rich at room temperature:

22. The magneto-optical recording medium as set forth in claim 1, wherein a saturation magnetization Ms3 of said third magnetic layer satisfies the following relation when being rare earth rich at room temperature:
and the following relation when being iron family rich at room temperature:

23. The magneto-optical recording medium as set forth in claim 1, wherein the Curie temperature T3 of said third magnetic layer satisfies the following relation:

24. The magneto-optical recording medium as set forth in claim 1, wherein a coercive force of said second magnetic layer is greater than a coercive force of said first magnetic layer.

25. The magneto-optical recording medium a set forth in claim 1, wherein said first magnetic layer is a perpendicular magnetization film at a temperature range from room temperature to the Curie temperature thereof and the coercive force of said third magnetic layer is greater than the coercive force of said first magnetic layer.

26. The magneto-optical recording medium as set forth in claim 1, wherein said third magnetic layer cuts off an exchange-coupling force working between said first and second magnetic layers at least at a temperature equal to or higher than room temperature.

27. The magneto-optical recording medium as set forth in claim 1, wherein said third magnetic layer is a perpendicular magnetization film over a range from room temperature to its Curie temperature.

28. An information reproducing method for reproducing information stored in a magneto-optical recording medium which comprises: a substrate; a first magnetic layer laminated on the substrate, and contributing to the reproduction of information; a second magnetic layer laminated on the first magnetic layer for storing information; and a third magnetic layer disposed between the first and second magnetic layers and having a Curie temperature lower than the Curie temperatures of said first and second magnetic layers, wherein the following condition is satisfied among the magnetic layers at a temperature of the Curie temperature of the third magnetic layer and higher, H.sub.wb -H.sub.d >H.sub.c1 +H.sub.wi where H.sub.wb is the effective magnetic field due to the Block magnetic wall energy of a recording domain of the first magnetic layer, H.sub.d is a static magnetic field applied to the recording domain of the first magnetic layer, H.sub.c1 is the coercive force of the first magnetic layer, and H.sub.wi is the exchange-coupling force applied to the recording domain of the first magnetic layer from the third magnetic layer, said method comprising the steps of:
irradiating a light spot;
raising the temperature of a high temperature region of the light spot to the Curie temperature of the third magnetic layer or higher to thereby orient the magnetization in the high temperature region of the first magnetic layer in one direction;
transferring information stored in said second magnetic layer to said first magnetic layer at least at a medium-temperature region inside the light spot; and
reproducing the information by detecting the magneto-optic effect of reflected light of the light spot.

29. The information reproducing method as set forth in claim 28, wherein, in said transferring step, the information stored in said second magnetic layer is transferred to said first magnetic layer via said third magnetic layer due to an exchange-coupling force.

30. The information reproducing method as set forth in claim 28, wherein, in said transferring step, the information stored in said second magnetic layer is transferred to said first magnetic layer due to a magnetostatic coupling force.

31. The information reproducing method as set forth in claim 28, further comprising the step of applying an external magnetic field for initializing magnetization in a low-temperature region within said light spot to align in one direction.

32. A magneto-optical recording medium comprising:
a substrate;
a first magnetic layer laminated on said substrate, said first magnetic layer contributing to the reproduction of information; and
a second magnetic layer laminated on said first magnetic layer for storing information, said second magnetic layer being magnetostatically coupled to said first magnetic layer,
wherein said first magnetic layer has a compensation temperature between room temperature and the Curie temperature of said second magnetic layer, and the following condition is satisfied among said magnetic layers at a temperature of said compensation temperature of said first magnetic layer and higher:
where H.sub.wb is the effective magnetic field due to the Bloch magnetic wall energy of a recording domain of said first magnetic layer, H.sub.st is a static magnetic field applied to the recording domain of said first magnetic layer from said second magnetic layer, H.sub.leak is a static magnetic field applied to the recording domain from a region other than the recording domain of said first magnetic layer, and H.sub.c1 is the coercive force of said first magnetic layer.

33. A magneto-optical recording medium according to claim 32, wherein a non-magnetic layer is provided between said first magnetic layer and said second magnetic layer.

34. An information reproducing method for reproducing information stored in a magneto-optical recording medium which comprises a substrate, a first magnetic layer laminated on the substrate and contributing to the reproduction of information; and a second magnetic layer laminated on the first magnetic layer for storing information an being magnetostatically coupled to the first magnetic layer, wherein the Curie temperature of the second magnetic layer, and the following condition is satisfied among the magnetic layers at a temperature of the compensation temperature of the first magnetic layer and higher, H.sub.wb +H.sub.st .+-.H.sub.leak >H.sub.c1, where H.sub.wb is the effective magnetic field due to the Block magnetic wall energy of a recording domain of the first magnetic layer, H.sub.st is a static magnetic field applied to the recording domain of the first magnetic layer from the second magnetic layer, H.sub.leak is a static magnetic field applied to the recording domain from a region other than the recording domain of the first magnetic layer, and H.sub.c1 is the coercive force of the first magnetic layer, said method comprising the steps of:
irradiating a light spot;
raising the temperature of a high temperature region of the light spot to the compensation temperature of the first magnetic layer or higher to thereby orient the magnetization in the high temperature region of the first magnetic layer in one direction;
transferring information stored in the second magnetic layer to the first magnetic layer at least at a medium-temperature region inside the light spot; and
reproducing the information by detecting the magneto-optical effect of reflected light of the light spot.
Description
BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magneto-optical recording medium, wherein information is recorded and reproduced using a laser beam, and more specifically, to a magneto-optical recording medium and a magneto-optical reproducing method, which are capable of realizing high-density recording and super-resolution reproduction.

2. Related Background Art

As a high-density recording system which is rewritable, a magneto-optical recording system has been receiving a lot of attention, wherein information is recorded by writing magnetic domains in a magnetic thin film using the thermal energy of a semiconductor laser beam, and the recorded information is read out using a magneto-optical effect. In recent years, the demand has been increasing to enhance the recording density of this magneto-optical recording medium for further increasing its storage volume.

The line recording density of an optical disc, such as, a magneto-optical recording medium, largely depends on a laser beam wavelength 1 in the reproducing optical system and the numerical aperture, N.A., of an objective lens. Specifically, since the diameter of a beam is determined when the reproducing light wavelength and the objective lens aperture number are determined, the shortest mark length which can be reproduced is limited to about .lambda./2 N.A.

On the other hand, the track density is mainly limited by crosstalk between adjacent tracks and depends on the diameter of the reproducing beam spot like the shortest mark length.

Accordingly, in order to realize higher-density recording with a conventional optical disc, it is necessary to shorten the laser beam wavelength in the reproducing optical system or increase the numerical aperture, N.A., of the objective lens. However, it is not easy to shorten the laser beam wavelength due to a drop in the efficiency of the element, generation of heat, and the like. On the other hand, when increasing the numerical aperture of the objective lens, the processing of the lens becomes difficult, and further, the distance between the lens and the disc becomes so short that a mechanical problem, such as, collision with the disc, occurs. In view of this, techniques have been developed to improve the structure of the recording medium and the information reading method so as to increase the recording density.

For example, in a magneto-optical reproducing method as disclosed in Japanese Patent Application Laid-open No. 3-93056, a medium structure as shown in FIGS. 1A to 1C has been proposed. FIG. 1A is a sectional view of an optical disc as an example of the super-resolution technique. A substrate 20 is normally formed of a transparent material, such as, glass or polycarbonate. On the substrate 20, an interference layer 34, a reproduction layer 31, an intermediate layer 32, a memory layer 33 and a protective layer 35 are laminated in the order named. An interference layer 34 is provided for enhancing the Kerr effect, and the protective layer 35 is provided for protecting the magnetic layers. Arrows in the magnetic layers each represent the direction of magnetization or atomic magnetic moment in the magnetic film. A light spot irradiates the medium having the reproduction layer, the intermediate layer and the memory layer to form a temperature distribution on the medium. In the temperature distribution, magnetic coupling between the reproduction layer and the memory layer at a high-temperature region is cut off by the intermediate layer having a low Curie temperature, and magnetization of the reproduction layer at the portion where the magnetic coupling was cut off, is aligned in one direction by an external magnetic field, so as to mask a portion of magnetic-domain information of the memory layer within the light spot. In this manner, a signal having a period equal to or smaller than the diffraction limit of light can be reproduced so as to improve the line recording density.

On the other hand, in super-resolution producing methods as disclosed in Japanese Patent Application Laid-open Nos. 3-93058 and 4-255946, a medium formed of a reproduction layer 31, an intermediate layer 32, and a memory layer 33 is used as shown in FIGS. 2A to 2C. Prior to reproducing information, magnetization of the reproduction layer 31 is aligned in one direction by an initializing magnetic field 21 so as to mask magnetic-domain information of the memory layer 33. Thereafter, a light spot 2 irradiates the medium to form a temperature distribution on the medium. In the temperature distribution, the initialized state of the reproduction layer 31 is held in a low-temperature region to form a front mask 4. On the other hand, in a high-temperature region where the temperature is equal to or higher than a Curie temperature Tc2 of the intermediate layer 32, magnetization of the reproduction layer 31 is forcibly oriented in a direction of the reproducing magnetic field 22 so as to form a rear mask 5. Only in a medium-temperature region, is the magnetic-domain information of the memory layer 33 transferred so as to reduce the effective size of the reproducing light spot. By this arrangement, a recorded mark 1 equal to or smaller than the diffraction limit of light can be reproduced so as to improve the line recording density.

On the other hand, in Japanese Patent Application Laid-open No. 6-124500, a magneto-optical recording medium structure has been proposed, as shown in FIGS. 3A to 3C, for providing a super-resolution technique to realize a recording density exceeding the optical resolution of the reproduced signal.

FIG. 3A is a sectional view of an optical disc as an example of the super-resolution technique. Arrows in the magnetic films each represent a direction of the iron family element sublattice magnetization in the film.

The memory layer 42 is a film formed of a material, such as, TbFeCo, DyFeCo or the like, having a large perpendicular magnetic anisotropy. Information is held in the memory layer 42 in the form of magnetic domains, which are directed upward or downward relative to a film surface. The reproduction layer 41 is an in-plane magnetization film at room temperature and becomes a perpendicular magnetization film when increased in temperature to Tl-mask.

When information reproducing light irradiates the disc having the foregoing medium structure from a side of the substrate 20, a temperature gradient at the center of the data track becomes as shown in FIG. 3C. When viewing this from the side of the substrate 20, an isotherm of Tl-mask exists in the light spot as shown in FIG. 3B. As described above, since the reproduction layer 41 is an in-plane magnetization film at a temperature lower than Tl-mask, it does not contribute to the Kerr effect (forming the front mask 4) at that portion so that the recorded magnetic domain held in the memory 42 is masked by the front mask 4. On the other hand, at a portion where a temperature is no less than Tl-mask, the reproduction layer 41 becomes a perpendicular magnetization film, and further, the direction of the magnetization becomes the same as the recorded information due to the exchange-coupling force from the memory layer 42. As a result, the recorded magnetic domain of the memory layer 42
is transferred only to an aperture portion 3, which is smaller than the size of the spot 2 so that super resolution is realized.

In the foregoing known super-resolution techniques, since the front mask 4 at the low-temperature region extends toward the adjacent tracks, those techniques aim to also improve the track density along with the line recording density.

However, in the method disclosed in Japanese Patent Application Laid-open No. 3-93056, although the resolution can be enhanced without reducing signal equality, it is necessary to apply the reproducing magnetic field. Further, in the methods disclosed in Japanese Patent Application Laid-opens Nos. 3-93058 and 4-255946 it is necessary to align the magnetization of the reproduction layer 31 in one direction prior to reproducing information so that an initializing magnet 21 for that purpose should be added to the conventional device. Further, in the super-resolution reproducing method disclosed in Japanese Patent Application Laid-open No. 6-124500, since only the front mask 4 is used, when expanding the mask region for enhancing the resolution, the position of the aperture 3 deviates from the center of the spot causing a deterioration in signal equality.

As described above, the conventional super-resolution reproducing methods include problems such that the resolution can not be increased to a sufficient level, the magneto-optical recording/reproduction apparatus is complicated in structure and is expensive and it is difficult to reduce the size thereof.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above-mentioned problems, and has as its object to provide a magneto-optical recording medium and a magneto-optical information reproducing method using the medium, which can reproduce a recorded mark equal to or smaller than the diffraction limit of light in high signal equality with a simple structure which does not require either an initializing magnetic field or a reproducing magnetic field upon reproduction.

In order to achieve the above object, there is provided a magneto-optical recording medium comprising:

a substrate;

a first magnetic layer laminated on the substrate for reproducing information;

a second magnetic layer laminated on the first magnetic layer for storing the information; and

a third magnetic layer disposed between the first and second magnetic layers and having a Curie temperature lower than Curie temperatures of the first and second magnetic layers,

wherein the direction of magnetization of a region of the first magnetic layer, the region being adjacent to a region of the third magnetic layer having a temperature equal to or higher than the Curie temperature of the third magnetic layer, is oriented in the direction of magnetization around the region of the first magnetic layer.

In order to achieve the above object, there is also provided an information reproducing method for reproducing information stored in a magneto-optical recording medium including:

a substrate;

a first magnetic layer laminated on the substrate for reproducing the information;

a second magnetic layer laminated on the first magnetic layer for storing the information; and

a third magnetic layer disposed between the first and second magnetic layers and having a Curie temperature lower than Curie temperatures of the first and second magnetic layers,

wherein the direction of magnetization of a region of the first magnetic layer, the region being adjacent to a region of the third magnetic layer having a temperature equal to or higher than the Curie temperature of the third magnetic layer, is oriented in the direction of magnetization around the region of the first magnetic layer,

the information reproducing method comprising the steps of:

irradiating a light spot;

increasing, in temperature, the third magnetic layer to near its Curie temperature in a high-temperature region within the light spot so as to orient the direction of magnetization, in the high-temperature region, of the first magnetic layer in the direction of magnetization of the first magnetic layer around the high-temperature region;

transferring the information stored in the second magnetic layer to the first magnetic layer at least at a medium-temperature region within the light spot; and

reproducing the information by detecting a magneto-optical effect of reflected light of the light spot.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C, 2A to 2C and 3A to 3C are diagrams showing conventional super-resolution methods, respectively;

FIG. 4 is a diagram showing a basic layer structure of magnetic layers of a magneto-optical recording medium according to a first embodiment of the present invention;

FIGS. 5A to 5C are diagrams showing one manner of operation of an information reproducing method for the magneto-optical recording medium according to the first embodiment of the present invention, wherein FIG. 5A is a diagram showing a mask region and an aperture region within a light spot on an upper surface of the medium, FIG. 5B is a diagram showing a magnetization direction state of each layer, and FIG. 5C is a diagram showing a temperature distribution in a track direction;

FIGS. 6A to 6C are diagrams showing another manner of operation of the information reproducing method for the magneto-optical recording medium according to the first embodiment of the present invention, wherein FIG. 6A is a diagram showing a mask region and an aperture region within a light spot on an upper surface of the medium, FIG. 6B is a diagram showing a magnetization direction state of each layer, and FIG. 6C is a diagram showing a temperature distribution in a track direction;

FIGS. 7A to 7C are diagrams for explaining a principle in which a high-temperature region in a light spot is masked in the magneto-optical recording medium according to the first embodiment of the present invention;

FIG. 8 is a diagram showing static magnetic fields Hleak, Hst and an effective magnetic field Hwb due to a Bloch magnetic wall energy, which are applied to a recorded magnetic domain transferred to a reproduction layer;

FIG. 9A is a diagram showing stable magnetization states for a layer structure of an anti-parallel type, wherein an exchange-coupling force and a magnetostatic coupling force are dominant, respectively;

FIG. 9B is a diagram showing stable magnetization states for a layer structure of a parallel type, wherein an exchange-coupling force and a magnetostatic coupling force are dominant, respectively;

FIGS. 10A to 10C are diagrams, respectively, showing temperature dependencies of saturation magnetizations with respect to GdFeCo having different compensation temperatures;

FIG. 11 is a diagram showing a composition dependency of a compensation temperature and a Curie temperature of GdFeCo;

FIG. 12 is a diagram showing an example of a temperature characteristic of a diamagnetic field energy 2Ms2 and a perpendicular magnetic anisotropy constant Ku of the reproduction layer of the magneto-optical recording medium according to the first embodiment of the present invention;

FIGS. 13A to 13C are diagrams showing examples of temperature characteristics of Ms of the respective magnetic layers of the magneto-optical recording medium according to the first embodiment of the present invention;

FIG. 14 is a diagram showing an example of a layer structure of the magneto-optical recording medium of the present invention;

FIGS. 15A and 15B are diagrams showing an interface magnetic wall;

FIG. 16 is a diagram showing the temperature dependency of the saturation magnetization of a GdFeCo reproduction layer;

FIG. 17 is a diagram showing the temperature dependency of the saturation magnetization of a TbFeCo memory layer;

FIG. 18 is a diagram showing the reproduction-layer-composition-x dependency of the saturation magnetization of the reproduction layer at the Curie temperature of an intermediate layer;

FIG. 19 is a diagram showing the reproduction-layer-composition-x dependency of C/N and energy (Ewb--Eleak--Est--Ecl) in Experimental Examples 7 to 10;

FIG. 20 is a diagram showing the reproduction-layer-composition-x dependency of C/N and energy (Ewb--Eleak--Est--Ecl) in Experimental Examples 11 to 14;

FIG. 21 is a diagram showing the reproducing power dependency of a carrier, noise, amplitude and a DC level;

FIG. 22 is a diagram showing the reproducing magnetic field dependency of C/N;

FIG. 23 is a diagram showing the reproducing power dependency of crosstalk;

FIG. 24 is a diagram showing a basic layer structure of a magneto-optical recording medium according to a second embodiment of the present invention;

FIG. 25A is a diagram showing a mask effect due to the film thickness of the reproduction layer;

FIG. 25B is a diagram showing an aperture effect due to the film thickness of the reproduction layer;

FIG. 26A is a diagram showing a mask effect due to the film thickness of the intermediate layer;

FIG. 26B is a diagram showing an aperture effect due to the film thickness of the intermediate layer;

FIG. 27 is a diagram showing the reproduction layer film-thickness dependency of C/N;

FIG. 28 is a diagram showing the reproduction layer film-thickness dependency of crosstalk;

FIG. 29 is a diagram showing the intermediate layer film-thickness dependency of C/N;

FIG. 30 is a diagram showing the intermediate layer film-thickness dependency of crosstalk;

FIG. 31 is a diagram showing the basic structure of a magneto-optical recording medium according to a third embodiment of the present invention;

FIG. 32 is a diagram showing the reproducing power dependency of carrier and noise;

FIG. 33 is a diagram showing the mark length dependency of C/N;

FIG. 34 is a diagram showing the reproducing power dependency of crosstalk;

FIG. 35 is a diagram showing the reproduction-layer-Gd-content dependency of C/N;

FIG. 36 is a diagram showing the reproduction-layer-Gd-content dependency of crosstalk;

FIG. 37 is a diagram showing the intermediate-layer-Gd-content dependency of C/N;

FIG. 38 is a diagram showing the intermediate-layer-Gd-content dependency of crosstalk;

FIG. 39 is a diagram showing the memory-layer-Tb-content dependency of C/N;

FIG. 40 is a diagram showing another structure of the magneto-optical recording medium of the third embodiment;

FIG. 41 is a diagram showing the recording power dependency of a carrier and noise when magnetic field modulation recording is performed relative to the medium of the third embodiment;

FIG. 42 is a diagram showing the basic structure of a magneto-optical recording medium according to a fourth embodiment of the present invention;

FIG. 43 is a diagram showing the reproducing power dependency of a carrier and noise;

FIG. 44 is a diagram showing the reproducing power dependency of a carrier, noise, amplitude, and a DC level;

FIG. 45 is a diagram showing the reproducing magnetic field dependency of C/N;

FIG. 46 is a diagram showing the mark length dependency of C/N;

FIG. 47 is a diagram showing the reproducing power dependency of crosstalk;

FIG. 48 is a diagram showing the relationship between C/N and the saturation magnetization of the reproduction layer;

FIG. 49 is a diagram showing the relationship between crosstalk and the saturation magnetization of the reproduction layer;

FIG. 50 is a diagram showing the relationship between C/N and the saturation magnetization of the intermediate layer;

FIG. 51 is a diagram showing the relationship between crosstalk and the saturation magnetization of the intermediate layer;

FIG. 52 is a diagram showing the relationship between the saturation magnetization and the compensation temperature of the reproduction layer;

FIG. 53 is a diagram showing the relationship between the saturation magnetization and the Curie temperature of the intermediate layer;

FIG. 54 is a diagram showing the relationship between the difference between the compensation temperature of the reproduction layer and a Curie temperature of the intermediate layer, and C/N;

FIG. 55 is a diagram showing the relationship between C/N and the saturation magnetization of the memory layer;

FIG. 56 is a diagram showing another structure of the magneto-optical recording medium of the fourth embodiment;

FIG. 57 is a diagram showing the recording power dependency of a carrier and noise when magnetic field modulation recording is performed relative to the medium of the fourth embodiment;

FIG. 58 is a diagram showing a basic layer structure of magnetic layers of a magneto-optical recording medium according to a fifth embodiment of the present invention;

FIGS. 59A to 59C are diagrams showing one manner of operation of an information reproducing method for the magneto-optical recording medium according to the fifth embodiment of the present invention, wherein FIG. 59A is a diagram showing a mask region and an aperture region within a light spot on an upper surface of the medium, FIG. 59B is a diagram showing the magnetization direction state of each layer, and FIG. 59C is a diagram showing the temperature distribution in a track direction;

FIGS. 60A to 60C are diagrams showing another manner of operation of an information reproducing method for the magneto-optical recording medium according to the fifth embodiment of the present invention, wherein FIG. 60A is a diagram showing a mask region and an aperture region within a light spot on an upper surface of the medium, FIG. 60B is a diagram showing the magnetization direction state of each layer, and FIG. 60C is a diagram showing the temperature distribution in a track direction;

FIG. 61 is a diagram showing one example of the layer structure of the magneto-optical recording medium according to the fifth embodiment of the present invention;

FIG. 62 is a diagram showing another example of the layer structure of the magneto-optical recording medium according to the fifth embodiment of the present invention;

FIGS. 63A and 63B are diagrams showing the basic layer structure of magnetic layers of a magneto-optical recording medium according to a sixth embodiment of the present invention;

FIGS. 64A to 64C are diagrams showing one manner of operation of an information reproducing method for the magneto-optical recording medium according to the sixth embodiment of the present invention, wherein FIG. 64A is a diagram showing a mask region and an aperture region within a light spot on an upper surface of the medium, FIG. 64B is a diagram showing the magnetization direction state of each layer, and FIG. 64C is a diagram showing the temperature distribution in a track direction;

FIGS. 65A to 65C are diagrams for explaining a principle in which a high-temperature region in a light spot is masked in the magneto-optical recording medium according to the sixth embodiment of the present invention;

FIG. 66 is a diagram showing static magnetic fields Hleak, Hst and an effective magnetic field Hwb due to a Bloch magnetic wall energy, which are applied to a recorded magnetic domain transferred to a reproduction layer; and

FIG. 67 is a diagram showing an example of the layer structure of the magneto-optical recording medium according to the sixth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(First Embodiment)

A magneto-optical recording medium according to a first embodiment of the present invention and an information reproducing method using the medium will be described in detail hereinbelow with reference to the accompanying drawings.

The magneto-optical recording medium of the present invention has, on a translucent substrate, at least three magnetic layers, that is, a first magnetic layer, a third magnetic layer having a Curie temperature lower than those of the first magnetic layer and a second magnetic layer, and the second magnetic layer being a perpendicular magnetization film, in the order named from a side of the substrate (FIG. 4). Hereinbelow, the first magnetic layer will be referred to as a reproduction layer, the second magnetic layer will be referred to as a memory layer and the third magnetic layer will be referred to as an intermediate layer.

The reproduction layer is a layer for reproducing magnetization information held in the memory layer. The reproduction layer is located closer to a light incident side of the medium as compared with the intermediate layer and the memory layer, and its Curie temperature is set to be higher than those of the intermediate layer and the memory layer for preventing deterioration of a Kerr rotation angle upon reproduction. Further, it is necessary that the coercive force of the reproduction layer is smaller than that of the memory layer. Preferably, the reproduction layer has a small magnetic anisotropy, and a compensation temperature between room temperature and the Curie temperature. Further, the magnetization manner of the reproduction layer is such that the reproduction layer is a perpendicular magnetization film at room temperature and between room temperature and the Curie temperature, or the reproduction layer is an in-plane magnetization film at room temperature and becomes a perpendicular magnetization film between room temperature and the Curie temperature. As a specific material of the reproduction layer, a material, for example, a rare earth-iron family amorphous alloy, such as, GdFeCo, GdTbFeCo, GdDyFeCo, NdGdFeCo or the like, mainly containing GdFeCo, is preferable since it has a high Curie temperature and a low coercive force and easily causes contraction of recorded magnetic domains in a high-temperature region, which is the prime aim of the present medium.

The intermediate layer is provided mainly for partly mediating and partly reducing or cutting off an exchange-coupling force from the memory layer to the reproduction layer. Accordingly, the intermediate layer is located between the reproduction layer and the memory layer and has a Curie temperature which is set to be higher than room temperature and lower than those of the reproduction layer and the memory layer. The Curie temperature of the intermediate layer is set to be high enough to mediate the exchange-coupling force from the memory layer to the reproduction layer at a low-temperature portion and a medium-temperature portion within a light spot, but low enough to cut off the exchange-coupling force at a highest-temperature portion within the light spot, and thus preferably, 80.degree. C. or higher and 220.degree. C. or lower, and more preferably, 110.degree. C or higher and 180.degree. C. or lower. When the reproduction layer has a compensation temperature between room temperature and the Curie temperature, the Curie temperature of the intermediate layer is set to a temperature within a range of, preferably, -100.degree. C. to +50.degree. C. relative to the compensation temperature, and more preferably, -80.degree. C. to +20.degree. C. relative to the compensation temperature. As a material of the intermediate layer, for example, a rare earth-iron family amorphous alloy, such as, TbFe, TbFeCo, GdFe, GdFeCo, GdTbFeCo, GdDyFeCo, DyFe, DyFeCo, TbDyFeCo or the like is preferable. A non-magnetic element, such as, Cr, Al, Si, Cu or the like may be added for lowering the Curie temperature. Further, when masking a low-temperature region by causing the reproduction layer to be an in-plane magnetization film at a low temperature, it is preferable that the in-plane magnetic anisotropy of the intermediate layer at room temperature is greater than that of the reproduction layer at room temperature, for example, the saturation magnetization Ms of the intermediate layer at room temperature is greater than that of the reproduction layer at room temperature, for strengthening the in-plane magnetic anisotropy of the reproduction layer at the low temperature.

The memory layer is a layer for storing recorded information and thus is required to stably hold the magnetic domains. As a material of the memory layer, a material which has a large perpendicular magnetic anisotropy and can stably hold a magnetization state, for example, a rare earth-iron family amorphous alloy, such as, TbFeCo, DyFeCo, TbDyFeCo or the like, garnet, a platinum family-iron family periodic structure film, such as, Pt/Co, Pd/Co or the like, or a platinum family-iron family alloy, such as, PtCo, PdCo or the like is preferable. An element, such as, Al, Ti, Pt, Nb, Cr or the like may be added to the reproduction layer, the intermediate layer and the memory layer for improving their corrosion resistance. For enhancing the interference effect and the protective performance, a dielectric layer formed of SiN.sub.x, A10.sub.x, TaO.sub.x, SiO.sub.x or the like may be provided in addition to the foregoing reproduction, intermediate and memory layers. Further, for improving thermal conductivity, a layer formed of Al, AlTa, AlTi, TlCr, Cu or the like and having good thermal conductivity may be provided. Further, an initialization layer whose magnetization is aligned in one direction for performing optical modulation overwriting may be provided. Further, auxiliary layers for recording assistance and reproducing assistance may be provided to adjust the exchange-coupling force or the magnetostatic coupling force. Moreover, a protective coat formed of the foregoing dielectric layer or a polymer resin may be added as a protective film.

Now, the recording/reproduction process of the present invention will be described hereinbelow.

First, magnetic domains are formed, according to a data signal, in the memory layer of the magneto-optical recording medium of the present invention. In a first recording method, recording is performed, after once erasing old information, by modulating the power of a laser while a magnetic field is applied in a recording direction. In a second recording method, new information is overwrite-recorded on old information by modulating the power of a laser while an external magnetic field is applied. In these optical modulation recording methods, by determining the intensity of the laser beam in consideration of the linear velocity of the recording medium so as to allow only a given region within the light spot to reach near the Curie temperature of the memory layer, a recorded magnetic domain equal to or smaller than a diameter of the light spot can be formed. As a result, a signal having a period equal to or smaller than the diffraction limit of light can be recorded. On the other hand, in a third recording method, overwrite-recording is performed by modulating an external magnetic field while a laser beam, having sufficient power to cause the temperature of the memory layer to be equal to or higher than its Curie temperature, irradiates the medium. In this case, a magnetic domain equal to or smaller than the diameter of the light spot can be formed by setting the modulation rate to be large depending on the linear velocity. As a result, a signal having a period equal to or smaller than the diffraction limit of light can be recorded.

As is clear from later-described mechanism, in order for the super resolution of the present invention to function stably, it is necessary that magnetization around a recorded mark is oriented in a direction is opposite to that of the mark.

In the first recording method which is the most popular, the laser power is held constant at a high power while a constant magnetic field is applied so as to initialize (erasing operation) magnetization of a track to be subjected to recording, and thereafter, in the state where the direction of the magnetic field is inverted, the laser power is modulated in intensity so as to form a desired recorded mark. At this time, when there is a portion around the recorded mark where the directions of the magnetization are random, noise is caused upon reproduction. For this reason, a region wider than the recorded mark is generally erased for enhancing a quality of a reproduced signal. Accordingly, since the magnetization around the recorded magnetic domains aligns without fail in a direction opposite to that of the magnetic domains, the super resolution of the present invention operates stably in this recording method.

In the second recording method, a medium having a structure as described in

Japanese Patent Application Laid-open No. 62-175948 (this medium has a write layer in which magnetization is aligned in one direction prior to recording, in addition to the memory layer for holding the recorded information) is used. Accordingly, an erasing operation in advance of recording is not required. On the other hand, when recording is effected on this medium, the laser intensity is modulated between Ph and PI (Ph>Pl) depending on information to be recorded, while a constant magnetic field is applied in a direction opposite to that of the write layer. When the medium is increased to a temperature Th corresponding to Ph, since Th is set to be substantially equal to Tc of the write layer, magnetization of the memory layer and the write layer is oriented in the direction of the external magnetic field so as to form the magnetic domain. On the other hand, when the medium is increased only to a temperature Tl corresponding to P1, the direction of magnetization becomes the same as that of the write layer. This process occurs regardless of the magnetic domain recorded in advance. It is assumed that a laser beam of Ph irradiates onto the medium. In this case, although a portion forming the recorded magnetic domain is increased to Th, the temperature distribution at this time extends two-dimensionally so that, even if the laser intensity is increased to Ph. there always occurs a portion around the magnetic domain where the temperature increases only to Tl. Accordingly, the portion having the opposite magnetization direction exists around the recorded magnetic domain so that the super resolution of the present invention also operates stably in this recording method.

Further, as another recording method, a magnetic field modulation recording can be cited, wherein the direction of the foregoing external magnetic field is changed alternately. In this recording method, magnetic field modulation is performed while a DC laser beam irradiates the medium at a high power. In order to record new information without a history of the magnetic domain recorded before, the width forming the magnetic domain should be always constant. Accordingly, in this case, some measure should be taken, or otherwise, there occurs a region around the recorded magnetic domain where the directions of the magnetization are random so that the super resolution of the present invention does not operate stably. Accordingly, when performing magnetic field modulation recording, it is necessary that the initialization is executed with a power greater than the normal recording power or the initialization of magnetization is performed extensively relative to both lands and grooves, prior to shipping the medium or the first recording.

Now, the reproducing method of the present invention will be described hereinbelow.

In the present invention, magnetic super resolution is realized by apparently and optically masking a partial region within the light spot without applying an external magnetic field. First, the magneto-optical recording medium and the magneto-optical reproducing method will be described with reference to the drawings, wherein a high-temperature region is formed with a rear mask and the other region is caused to be an aperture region, that is, the magnetization of the reproduction layer is such that the reproduction layer is a perpendicular magnetization film at room temperature and between room temperature and the Curie temperature. FIGS. 7A, 7B and 7C are diagrams showing a process, wherein the recorded magnetic domain of the reproduction layer transferred from the memory layer (hereinbelow simply referred to as "recorded magnetic domain") is contracted in the high-temperature region while the light spot moves. For brevity, in FIGS. 7A to 7C, the contracting process of only one recorded magnetic domain is shown. Further, in these figures, a rare earth-iron family ferromagnetic substance is used as a magnetic material, blank arrows 30 represent the whole magnetization, black arrows 31 represent the iron family sublattice magnetization, the reproduction layer 11 is an RE rich magnetic layer and the memory layer 13 is a TM rich magnetic layer. On the other hand, in FIGS. 5A to 5C, the whole image upon reproduction is shown along with the temperature distribution. The temperature distribution of the medium is shifted from the center of the light spot in a direction opposite to a moving direction of the light spot due to the limit of thermal conductivity. As shown in FIG. 7A, shortly after the light spot 2 has reached the recorded magnetic domain 1, the recorded magnetic domain 1 does not reach the high-temperature region 5. In addition to an effective magnetic field Hwi due to the exchange-coupling force from the memory layer 13, an effective magnetic field Hwb due to the Bloch magnetic wall energy and a static magnetic field Hd from the interior of the medium are applied to the recorded magnetic domain 1. Hwi works to stably hold the recorded magnetic domain 1 of the reproduction layer, while Hwb and Hd apply forces in directions to expand and contract the recorded magnetic domain. Accordingly, in order for the reproduction layer 11 to be stably transferred with the magnetization of the memory layer 13, a condition expressed by relation (1) should be satisfied before the recorded magnetic domain 1 reaches the high-temperature region 5.

A coercive force Hcl of the reproduction layer 11 is apparently increased due to the exchange-coupling force from the memory layer 13. Accordingly, relation (1) can be easily established to stably transfer the magnetization information of the memory layer 13 so that the recorded information can be reproduced accurately.

If the interface magnetic wall energy between the reproduction layer 11 and the memory layer 13 is .sigma.wi, the saturation magnetization of the recorded magnetic domain 1 of the reproduction layer 11 is Msl and the film thickness of the reproduction layer is hi, and Hwi is expressed by relation (2).

When the light spot further moves so that the recorded magnetic domain 1 enters the high-temperature region 5, Hwi reaches around the Curie temperature of the intermediate layer 12 so that .sigma.wi is rapidly decreased to diminish Hwi. Accordingly, the reproduction layer 11 returns to the state where the coercive force is small, to satisfy relation (3) so that a Bloch magnetic wall 8 of the recorded magnetic domain 1 easily moves.

If a Bloch magnetic wall energy is .sigma.wb and a radius of the recorded magnetic domain 1 of the reproduction layer 11 is r, Hwb is expressed by relation (4) and works in a direction to contract the recorded magnetic domain 1 (FIG. 8).

Accordingly, when Hwb-Hd becomes dominant in positive (sign is +) to satisfy relation (5), the recorded magnetic domain 1 is contracted.

In this manner, as shown in FIG. 7B, the recorded magnetic domain 1 is contracted and inverted when entering the high-temperature region 5, and as shown in FIG. 7C, the magnetization is all oriented in an erasing direction.

Specifically, as shown in FIGS. 5A to 5C, the reproduction layer 11 always becomes a perpendicular magnetization film oriented in the erasing direction at the high-temperature region 5 within the light spot 2, thus serving as an optical mask (rear mask 5). Accordingly, as shown in FIG. 5A, the light spot 2 is apparently narrowed to a region excluding the high-temperature region 5 and serving as the aperture region 3 so that the recorded magnetic domain (recorded mark) having a period equal to or smaller than the detection limit can be detected.

On the other hand, in the conventional super-resolution method, as described in Japanese Patent Application Laid-open No. 4-255947, a mask is formed using an external magnetic field Hr based on relation (6).

In the present invention, since the mask is formed by changing a magnitude of the effective magnetic field Hwb-Hd inside the medium instead of using the external magnetic field Hr. the external magnetic field is not necessary.

Now, the method for making Hwb-Hd dominant in positive at a high temperature will be described in further detail.

Hd in relation (5) is formed by a leakage magnetic field Hleak from the ambient erasing magnetization, a static magnetic field Hst from the magnetization of the memory layer 13 and the like, and is expressed by relation (7).

In relation (7), Hleak works in a direction to expand the recorded magnetic domain 1 as shown in FIG. 8. A first method to make Hwb-Hd dominant in positive in the high-temperature region is a method which diminishes Hleak, that is, reduces the magnetic field preventing inversion of the recorded magnetic domain 1. If the saturation magnetization of the reproduction layer 11 around the recorded magnetic domain to be made to disappear is Msl" and a radius of the recorded magnetic domain 1 is r, Hleak is roughly expressed by relation (8).

In relation (8), the radius r of the recorded magnetic domain and the film thickness hl of the reproduction layer can not be easily changed. Accordingly, it is necessary to diminish Msl". This is achieved by selecting a material for the reproduction layer, which has a compensation temperature between room temperature and the Curie temperature. Since the magnetization is reduced at the compensation temperature, Hleak can be diminished. An example will be described, wherein GdFeCo is used for the reproduction layer 11. FIGS. 10A to 10C respectively show temperature dependencies of Ms of GdFeCo having different compensation temperatures. Although the maximum temperature on the medium upon reproduction differs depending on the reproducing power, the maximum temperature shown in the figures reaches approximately 160.degree. C.-220.degree. C. in general, and the medium-temperature region is a region where the temperature is lower than the maximum temperature by about
20.degree. C.-60.degree. C. Accordingly, in case of FIGS. 10B and 10C, Msl" is large so that Hleak also becomes large. If a composition in which the compensation temperature exists between room temperature and the Curie temperature, is used for the reproduction layer 11, Ms in the medium-temperature and high-temperature regions is reduced to diminish Hd. When GdFeCo is used for the reproduction layer 11, since the compensation temperature largely depends on a composition of, particularly, a rare earth element (Gd) as shown in FIG. 11, it is preferable to set the Gd content to be 25 to 35% in case a magnetic layer mainly containing GdFeCo is used as the reproduction layer 11.

A second method is a method which makes Hst dominant in negative, that is, facilitates inversion of the recorded magnetic domain 1 by the static magnetic field Hst from the memory layer 13. In relation (7), when entering the high-temperature region from the exchange-coupling region, Hst is determined whether to work in a direction to contract the recorded magnetic domain 1 or work to hold the recorded magnetic domain 1, depending on whether the reproduction layer 11 and the memory layer 13
are of a parallel type or an anti-parallel type. The reason is as follows:

As shown in FIGS. 9A and 9B, the exchange-coupling force aligns in a direction of TM sublattice magnetization, where the exchange force is great, and the magnetostatic coupling force aligns in a direction of the whole magnetization. FIG. 9A shows the anti-parallel type, wherein the reproduction layer 11 is RE rich and the memory layer 13 is TM rich. In this case, when the intermediate layer 12 reaches around the Curie temperature to cut off the exchange coupling, the recorded magnetic domain 1 is caused to be inverted in magnetization due to the magnetostatic coupling force with the memory layer 13 (Hst becomes negative). To the contrary, in case of the parallel type as shown in FIG. 9B (in the figure, both the reproduction and memory layers are shown to be TM rich), the magnetostatic coupling force works in a direction to hold the exchange-coupling state (Hst becomes positive). Accordingly, for inverting the recorded magnetic domain 1, the composition of the anti-parallel type is desired.

Specifically, for example, both the reproduction layer 11 and the memory layer 13 may be set to be ferromagnetic, and kinds of the dominant sublattice magnetization in the reproduction layer 11 and the memory layer 13 may be set to be opposite to each other. For example, the reproduction layer 11 and the memory layer 13 are formed of rare earth (RE) iron family (TM) element alloys, and the reproduction layer 11 is arranged to be rare earth element sublattice magnetization dominant (RE rich), while the memory layer 13 is arranged to be iron family element sublattice magnetization dominant (TM rich) at room temperature. It is necessary that this anti-parallel composition is achieved at least at the temperature where the recorded magnetic domain 1 is contracted (in the foregoing medium-temperature region to high-temperature region 5).

A value of Hst can be roughly calculated using, on the assumption that the magnetic domain is cylindrical, the radius of the recorded magnetic domain 1, the distance from the magnetic domain of the memory layer 13 and the saturation magnetization Ms2 of the memory layer (see pages 40 and 41, Nagoya University doctoral thesis "Research about Rare Earth-Iron Family Amorphous Alloy Thin Film and Magnetism and Magneto-optical Effect of Composite Film thereof" 1985. 3 by Tadashi Kobayashi). Hst is proportional to the saturation magnetization Ms2 of the memory layer (relation 9).

Accordingly, it is preferable that Ms2 is set to be such a large value that does not cause deterioration in the stability of the recorded information or cause inversion of the erasing magnetization.

Further, the static magnetic field Hst from the memory layer 13 also works on the magnetization in the erasing direction. However, if the magnetization in the erasing direction is inverted by Hst, a magnetic wall is formed over an extensive range of the high-temperature region 5 so that the magnetic wall energy is largely increased. Accordingly, magnetization inversion does not occur, and the magnetization in the erasing direction is held. Thus, in the high-temperature region 5, a region is generated in which magnetization is always oriented in the erasing direction. This region becomes the rear mask 5. If a radius of the inverted magnetic domain is R, an effective magnetic field Hwb' of the Bloch magnetic wall energy in case of the erasing magnetization being inverted is expressed by relation (10).

Thus, a condition that the erasing magnetization is not inverted by Hst is expressed by relation (11).

Only one of the foregoing two methods, that is, the method of reducing Hleak and the method of increasing Hst at a negative side, may be used. On the other hand, if the two methods are used in combination, the super-resolution effect is realized to the greatest extent. As described above, by using the magneto-optical recording medium of the present invention, the magnetization can be oriented in a uniform direction in the high-temperature region 5 of the light spot upon reproduction without applying the external magnetic field so as to optically mask the magnetization of the memory layer 13.

Further, in the medium where the reproduction layer is a perpendicular magnetization film at room temperature and between room temperature and the Curie temperature, since the aperture region 3 extends over substantially all the region other than the high-temperature region 5, it is necessary that the reproduction layer 11 becomes a perpendicular magnetization film to a sufficient extent even in the low-temperature region, so as to stably transfer the magnetization of the memory layer 13. Accordingly, a material in which the magnetization is oriented in a perpendicular direction to a further extent as compared with the reproduction layer 11 (a material having a coercive force greater than the reproduction layer 11), for example, TbFe, DyFe, TbFeCo and DyFeCo, may be preferably used for the intermediate layer 12. By using such a material, the interface magnetic wall energy owl is increased so that the reproduction layer 11 can transfer the magnetization information of the

memory layer 13 stably due to the exchange-coupling force. Further, in case the reproduction layer 11 has a small perpendicular magnetic anisotropy, for example, even in case the reproduction layer, when alone, becomes an in-plane magnetization film, by using the intermediate layer in which the magnetization is oriented in the perpendicular direction to a further extent, to be laminated on the reproduction layer 11, the perpendicular magnetic anisotropy of the reproduction layer 11 is sufficiently increased to allow the aperture region to transfer the magnetization information of the memory layer 13 accurately.

The formation of the mask has been described above using the expressions containing the magnetic field. On the other hand, the formation of the mask can also be described using expressions containing energy. When, particularly, Ms is close to
0, the magnetic field, even largely applied to, does not effectively act on the recorded magnetic domain. Accordingly, it is preferable to describe it in terms of energy since the formation of the mask can be judged more precisely. For describing in terms of an energy relation, the foregoing definition and relation expressions of the magnetic field may be multiplied by 2Msl, respectively. Accordingly, relations (1), (2), (3), (4), (5), (7) and (8) are expressed by relations (12), (13), (14), (15), (16), (17) and (18), respectively. In the relations, Ewb represents a Bloch magnetic wall energy, Ed represents a static magnetic field energy from the interior of the medium applied to the Bloch magnetic wall of the recorded magnetic domain, Ewi represents an exchange-coupling energy with the second magnetic layer, and Ecl represents a coercive force energy of the first magnetic layer.

Further, the method has been described before, wherein the magnetization information of the memory layer 13 is optically masked only in the high-temperature region 5 within the light spot 2. Now, a method in which the low-temperature region is also masked in addition to the high-temperature region 5 so as to detect the magnetization information only in the medium-temperature region, that is, a magneto-optical recording medium and an information reproducing method in which the magnetization manner of the reproduction layer is such that the reproduction layer is an in-plane magnetization film at room temperature and makes the transition to a perpendicular magnetization film between room temperature and the Curie temperature, will be described hereinbelow. FIGS. 6A, 6B and 6C show a structure of the mask and the aperture region, the magnetization state and the temperature distribution for the medium having the present manner, respectively. In this case, a magnetic film which is an in-plane magnetization film at room temperature and becomes a perpendicular magnetization film at a high temperature, is used for the reproduction layer 11. An example of such a magnetic film will be explained hereinbelow. In general, in the case of a single-layer magnetic film, if the saturation magnetization is Ms and a perpendicular magnetic anisotropy energy is Ku, a main direction of its magnetization is known to be determined by an effective perpendicular magnetic anisotropy constant K defined by relation (19).

wherein, 2.pi.Ms.sup.2 represents a diamagnetic field energy.

When K is positive, the magnetic film becomes a perpendicular magnetization film. On the other hand, when K is negative, the magnetic film becomes an in-plane magnetization film. Accordingly, as shown in FIG. 12, the magnetic film which changes relation in magnitude between Ku and 2.pi.Ms.sup.2 depending on a temperature, is effective to make the transition from an in-plane magnetization film to a perpendicular magnetization film. In such a reproduction layer 11, relation (20) is established in the low-temperature region where a temperature is equal to or lower than a temperature Tl-mask at which the region (the aperture region 3 in FIG. 6A) reproducing the magnetization information of the memory 13 is reached. Accordingly, the low-temperature region becomes an in-plane magnetization film (front mask 4) to mask the magnetization information of the memory layer 13.

On the other hand, when the temperature T of the medium is increased, Ms is decreased to rapidly diminish 2.pi.Ms.sup.2. Accordingly, the relation in magnitude between 2.pi.Ms.sup.2 and Ku is reversed to satisfy relation (21).

Accordingly, an in-plane magnetization film makes the transition to a perpendicular magnetization film to form the aperture region 3. Further, at a temperature equal to or higher than Th-mask, the rear mask 5 is formed in the high-temperature region 5 as described before.

As shown in FIGS. 6A to 6C, in this method, the reproduction layer 11 becomes an in-plane magnetization film in the low-temperature region 4 and a perpendicular magnetization film in the high-temperature region 5 where the magnetization is always oriented in the erasing direction, so that both work as optical masks. Only the medium-temperature region of the reproduction layer 11 becomes a perpendicular magnetization film where a signal of the memory layer 13 is transferred due to the exchange coupling so that the medium-temperature region becomes the information detectable region (aperture region 3).

In this method, since the low-temperature region 4 is masked in addition to the high-temperature region 5, information on the adjacent tracks (grooves 6a, 6b in FIG. 6A) can also be masked. Thus, crosstalk is reduced to improve the track density. Further, as described above, in the method where the medium-temperature region is used as the detecting region, the aperture region 3 within the laser spot 2 becomes a narrow region sandwiched between the high-temperature region 5 and the low-temperature region 4. Further, even when the laser power fluctuates, the widths of the aperture region 3 do not change, but are held constant. Thus, even when the higher-density recording is performed, reproduction can be achieved satisfactorily with high resolution so that stable reproduction is realized even when a laser power fluctuation occurs. Further, in the present invention, since the detecting region is located near the center of the laser spot, a better C/N ratio can be expected.

As described before, FIGS. 10A to 10C respectively show temperature dependencies of Ms of GdFeCo having different compensation temperatures. Among them, in the composition where the compensation temperature is between room temperature and the Curie temperature as shown in FIG. 10A, since the saturation magnetization is reduced to 0 in a temperature range higher than room temperature and lower than the Curie temperature, an intersection is generated between the diamagnetic field energy and the perpendicular magnetic anisotropy constant so that the transition from an in-plane magnetization film to a perpendicular magnetization film occurs. On the other hand, in FIGS. 10B and 10C, such a transition does not occur. Accordingly, as a material of the reproduction layer 11, for example, a material having a compensation temperature between room temperature and the Curie temperature is preferable, and further, a material having a magnetic anisotropy which is smaller than the diamagnetic field energy
2.pi.Ms.sup.2 at room temperature, is preferable.

When laminating this magnetic film via the memory layer 13, the intermediate layer 12 and the like, Ku is apparently increased due to the action of the exchange-coupling force from the memory layer 13. Accordingly, a temperature at which the transition to a perpendicular magnetization film occurs, shifts to a lower temperature side as compared with the case of no such lamination. However, by setting the temperature of transition to the perpendicular magnetization in the single-layer state to a relatively high value, it can be arranged so that the magnetic film is an in-plane magnetization film at room temperature and becomes a perpendicular magnetization film when increased in temperature, even when the magnetic film is laminated with the memory layer 13.

In this case, a condition where the reproduction layer 11 becomes an in-plane magnetization film, is expressed by relation (20').

where Ew13 represents an energy for orienting the magnetization of the reproduction layer 11 in a perpendicular direction due to the exchange-coupling force from the memory layer 13.

On the other hand, a condition where the reproduction layer 11 becomes a perpendicular magnetization film, is expressed by relation (21').

The intermediate layer 12 may be in the form of a perpendicular magnetization film having a large perpendicular magnetic anisotropy. However, when the intermediate layer 12 having the large perpendicular magnetic anisotropy is laminated on the reproduction layer 11 being an in-plane magnetization film at room temperature, the interface magnetic wall tends to permeate into a side of the reproduction layer 11 as shown in FIG. 15A so that the magnetization information of the memory layer 13 can not be masked sufficiently. In view of this, it is preferable to use a magnetic layer for the intermediate layer 12, which has a perpendicular magnetic anisotropy small enough to work as a magnetic wall portion between the reproduction layer 11 and the memory layer 13 in a low-temperature region near room temperature as shown in FIG. 15B, and in other words, which has a large in-plane anisotropy. For working as a magnetic wall portion, a magnetic material having small magnetic wall energy, such as, GdFe or GdFeCo, may be preferably used as the intermediate layer. The fact that the in-plane anisotropy is large corresponds to the fact that K in relation (19) is a smaller value (K is a negative value and its absolute value is large). In order to make K of the intermediate layer 12 at room temperature smaller than K of the reproduction layer 11 at room temperature, when, for example, using a rare earth-iron family element alloy, such as, GdFe, GdFeCo or the like, the Ms of the intermediate layer
12 at room temperature is made greater than that of the reproduction layer 11 by increasing the content of a rare earth element (Gd). Further, it is also effective to diminish Ku by increasing the content of Co. Among them, the method of increasing Ms is preferable since Ms is reduced as the intermediate layer 12 approaches the Curie temperature so that the perpendicular anisotropy is increased in the aperture region. However, since the reproduction layer 11 does not become a perpendicular magnetization film to a sufficient extent in the aperture region when the in-plane anisotropy of the intermediate layer 12 is increased too much, the in-plane anisotropy thereof is increased to a degree which does not deteriorate the signal quality.

Quite naturally, the intermediate layer is arranged to reach the Curie temperature after the reproduction layer 11 becomes a perpendicular magnetization film and is exchange-coupled to the memory layer 13. In other words, it is necessary to arrange that the reproduction layer 11 is an in-plane magnetization film at room temperature and becomes a perpendicular magnetization film at least between room temperature and the Curie temperature of the intermediate layer 12. Examples of the temperature dependencies of Ms of the reproduction layer 11, the intermediate layer 12 and the memory layer 13 of such a medium are shown in FIGS. 13A to 13C, respectively. In the figures, a positive Ms represents a RE rich mixture, while negative Ms represents a TM rich mixture.

The present invention will be described in further detail by way of experimental examples. However, the present invention is not limited to these experimental examples.

First, a magneto-optical recording medium in which a reproduction layer is a perpendicular magnetization film at room temperature and between room temperature and the Curie temperature was prepared and evaluated, which will be described in the following Experimental Examples 1 and 2.

Experimental Example 1

Si, Gd, Tb, Fe and Co targets were attached to a DC magnetron sputtering apparatus, and a glass substrate having a diameter of 130 mm and a polycarbonate substrate with lands and grooves were fixed to a substrate holder which was set at a position separated from the respective targets by a distance of 150 mm. Thereafter, the interior of the chamber was evacuated by a cryopump to a high vacuum of 1.times.10.sup.-5 Pa or less. During the evacuation, Ar gas was introduced into the chamber to 0.4 Pa, and thereafter, an SiN interference layer of 900 .ANG. thickness, a GdFeCo reproduction layer of 400 .ANG. thickness, a TbFeCo intermediate layer of 50 .ANG. thickness, a TbFeCo memory layer of 350 .ANG. thickness and an SiN protective layer of 700 .ANG. thickness were formed in the order named, thus obtaining the magneto-optical recording medium of the present invention with a structure shown in FIG. 14. Upon formation of each SiN dielectric layer, N.sub.2 gas was introduced in addition to the Ar gas, and the SiN layer was formed by DC reactive sputtering, adjusting the mixing ratio of the Ar and N.sub.2 gases, so as to obtain a refractive index of 2.2.

The composition of the GdFeCo reproduction layer was Gd.sub.30 (Fe.sub.65 Co.sub.35).sub.70 and represented an RE rich layer at room temperature, an Ms of 196 emu/cc, a compensation temperature of 240.degree. C. and a Curie temperature of
300.degree. C. or more.

The composition of the TbFeCo intermediate layer was Tb.sub.18 (Fe.sub.97 Co.sub.3).sub.82 and represented a TM rich layer at room temperature, an Ms of -95 emu/cc and a Curie temperature of 135.degree. C.

The composition of the TbFeCo memory layer was Tb.sub.18 (Fe.sub.88 Co.sub.12).sub.82 and represented a TM rich layer at room temperature, an Ms of -120 emu/cc and a Curie temperature of 220.degree. C.

After recording a magnetic domain of a 0.78 .mu.m mark length in the magneto-optical recording medium, the magnetic domain was observed by a polarizing microscope under irradiation of a semiconductor laser beam of 830 nm. While increasing the laser power, it was confirmed that the recorded magnetic domain was contracted and the magnetization was oriented in an erasing direction at the center (high-temperature region) of the light spot at a certain laser power.

Subsequently, the recording/reproduction characteristic thereof was measured using this magneto-optical recording medium. The measurement was performed by setting the N.A. of an objective lens to be 0.55, a laser beam wavelength to be 780 nm, a recording power to be in a range of 7.sup.- 13 mW and a reproducing power to be in a range of 2.5.sup.- 3.5 mW, so as to provide the highest C/N ratio. The linear velocity was set to be 9 m/s. First, erasing was performed entirely on the medium, and thereafter, carrier signals of 5.8 MHz, 11.3 MHz and 15 MHz (corresponding to mark lengths 0.78 .mu.m, 0.40 .mu.m and 0.30 .mu.m, respectively) were recorded in the memory layer so as to examine the mark-length dependency of C/N.

Subsequently, crosstalk with the adjacent tracks (hereinafter referred to as "crosstalk") was measured. Specifically, after recording a signal of a 0.78 .mu.m mark length on the land as in the foregoing manner and measuring a carrier level Cl, a carrier level C2 was similarly measured upon tracking the adjacent groove where data had been erased, and the crosstalk was represented by a ratio (C2/C1). Since the experiment was performed on the assumption that data were recorded on both the land and groove, the effective track pitch was 0.8 .mu.m.

Both the C/N ratios and the crosstalk were measured without applying an initializing magnetic field and a reproducing magnetic field. Table 1 shows compositions and materiality values of each layer and the results of the C/N ratios and the crosstalk.

Experimental Example 2

Using the same apparatus and method as in Experimental Example 1, an SiN interference level of 900 .ANG. thickness, a GdFeCo reproduction layer of 400 .ANG. thickness, a DyFeCo intermediate layer of 60 .ANG. thickness, a TbFeCo memory layer of
350 .ANG. thickness and an SiN protective layer of

700 .ANG. thickness were formed on a polycarbonate substrate in the order named, thus obtaining the magneto-optical recording medium of the present invention with a structure shown in FIG. 14.

The composition of the GdFeCo reproduction layer was Gd.sub.28 (Fe.sub.65 Co.sub.35).sub.72 and represented a RE rich layer at room temperature, an Ms of 160 emu/cc, a compensation temperature of 180.degree. C. and a Curie temperature of
300.degree. C. or more.

The composition of the DyFeCo intermediate layer was Dy.sub.20 (Fe.sub.97 Co.sub.3).sub.80 and represented a TM rich layer at room temperature, an Ms of -80 emu/cc and a Curie temperature of 128.degree. C.

The composition of the TbFeCo memory layer was Tb.sub.18 (Fe.sub.88 Co.sub.12).sub.82 and represented a TM rich layer at room temperature, an Ms of -120 emu/cc and a Curie temperature of 220.degree. C.

Subsequently, using this magneto-optical recording medium, the mark-length dependency of C/N and the crosstalk were examined as in Experimental Example 1. The results are shown in Table 1.

Now, a magneto-optical recording medium in which a reproduction layer is an in-plane magnetization film at room temperature and becomes a perpendicular magnetization film between room temperature and a Curie temperature thereof was prepared and evaluated, which will be described in the following Experimental Examples 3, 4, 5 and 6.

Experimental Example 3

Using the same apparatus and method as in Experimental Example 1, an SiN interference layer of 900 .ANG. thickness, a GdFeCo reproduction layer of 400 .ANG. thickness, a GdFe intermediate layer of look thickness, a TbFeCo memory layer of 300
.ANG. thickness and an SiN protective layer of 700 .ANG. thickness were formed on a polycarbonate substrate in the order named, thus obtaining a sample with a structure shown in FIG. 14.

The composition of the GdFeCo reproduction layer was set to represent an RE rich layer at room temperature, an Ms of 218 emu/cc, a compensation temperature of 238.degree. C. and a Curie temperature of 300.degree. C. or more.

The composition of the GdFe intermediate layer was set to represent an RE rich layer at room temperature, an Ms of 475 emu/cc and a Curie temperature of 190.degree. C.

The composition of the TbFeCo memory layer was set to represent a TM rich layer at room temperature, an Ms of -150 emu/cc and a Curie temperature of 260.degree. C.

Subsequently, using this magneto-optical recording medium, the recording/reproduction characteristic was evaluated as in Experimental Example 1. The results are shown in Table 1.

Experimental Example 4

Using the same apparatus and method as in Experimental Example 1, an SiN interference layer of 900 .ANG. thickness, a GdFeCo reproduction layer of 450 .ANG. thickness, a GdFe intermediate layer of 80 .ANG. thickness, a TbFeCo memory layer of
320 .ANG. thickness and an SiN protective layer of 700 .ANG. thickness were formed on a polycarbonate substrate in the order named, thus obtaining a sample with a structure shown in FIG. 14.

The composition of the GdFeCo reproduction layer was set to represent a RE rich layer at room temperature, an Ms of 170 emu/cc, a compensation temperature of 190.degree. C. and a Curie temperature of 300.degree. c. or more.

The composition of the GdFe intermediate layer was set to represent a RE rich layer at room temperature, an Ms of 540 emu/cc and a Curie temperature of 165.degree. C.

The composition of the TbFeCo memory layer was set to represent a TM rich layer at room temperature, an Ms of -50 emu/cc and a Curie temperature of 240.degree. c.

Subsequently, using this magneto-optical recording medium, the recording/reproduction characteristic was evaluated as in Experimental Example 1. The results are shown in Table 1.

Experimental Example 5

Using the same apparatus and method as in Experimental Example 1, an SiN interference layer of 900 .ANG. thickness, a GdFeCo reproduction later of 380 .ANG. thickness, a GdFe intermediate layer of 120 .ANG. thickness, a TbFeCo memory layer of
350 .ANG. thickness and an SiN protective layer of 700 .ANG. thickness were formed on a polycarbonate substrate in the order named, thus obtaining a sample with a structure shown in FIG. 14.

The composition of the GdFeCo reproduction layer was set to represent a RE rich layer at room temperature, an Ms of 280 emu/cc, a compensation temperature of 290.degree. C. and a Curie temperature of 300.degree. C. or more.

The composition of the GdFe intermediate layer was set to represent a RE rich layer at room temperature, an Ms of 420 emu/cc and a Curie temperature of 195.degree. C.

The composition of the TbFeCo memory layer was set to represent a TM rich layer at room temperature, an Ms of -200 emu/cc and a Curie temperature of 220.degree. C.

Subsequently, using this magneto-optical recording medium, the recording/reproduction characteristic was evaluated as in Experimental Example 1. The results are shown in Table 1.

Experimental Example 6

Using the same apparatus and method as in Experimental Example 1, an SiN interference layer of 900 .ANG. thickness, a GdFeCo reproduction layer of 430 .ANG. thickness, a GdFeCo intermediate layer of 130 .ANG. thickness, a TbFeCo memory layer of 350 .ANG. thickness and an SiN protective layer of 700 .ANG. thickness were formed on a polycarbonate substrate in the order named, thus obtaining a sample with a structure shown in FIG. 14.

The composition of the GdFeCo reproduction layer was set to represent a RE rich layer at room temperature, an Ms of 250 emu/cc, a compensation temperature of 260.degree. C. and a curie temperature of 300.degree. C. or more.

The composition of the GdFeCo intermediate layer was set to represent a RE rich layer at room temperature, an Ms of 480 emu/cc and a Curie temperature of 176.degree. C.

The composition of the TbFeCo memory layer was set to represent TM rich at room temperature, an Ms of -240 emu/cc and a Curie temperature of 270.degree. C.

Subsequently, using this magneto-optical recording medium, the recording/reproduction characteristic was evaluated as in Experimental Example 1. The results are shown in Table 1.

Now, the known super-resolution magneto-optical recording medium was prepared, and evaluation thereof was performed in the same manner as in the foregoing experimental examples.

Comparative Example 1

First, a medium the same as that described in Japanese Patent Application Laid-open No. 3-93056 was prepared and evaluated.

Using the same film forming apparatus and method as in Experimental Example 1, an SiN interference layer of 900 .ANG. thickness, a GdFeCo reproduction layer of 300 .ANG. thickness, a TbFeCoAl intermediate layer of 100 .ANG. thickness, a TbFeCo memory layer of 400 .ANG. thickness and an SiN protective layer of 700 .ANG. thickness were formed on a glass substrate in the order named, thus obtaining the magneto-optical recording medium of Comparative Example 1.

The composition of the GdFeCo reproduction layer was set to represent a TM rich layer at room temperature, an Ms of -180 emu/cc and a Curie temperature of 300.degree. C. or more.

The composition of the TbFeCoAl intermediate layer was set to represent a TM rich layer at room temperature, an Ms of -160 emu/cc and a Curie temperature of 140.degree. C.

The composition of the TbFeCo memory layer was set to represent a TM rich layer at room temperature, an Ms of -150 emu/cc and a Curie temperature of 250.degree. C.

Subsequently, using this magneto-optical recording medium, the recording/reproduction characteristic was measured as in Experimental Example 1. In this case, however, upon reproduction, a reproducing magnetic field was applied to the medium in a perpendicular direction, by changing the magnitude of the reproducing magnetic field between 0 Oe, 200 Oe and 400 Oe. The results are shown in Table 1.

Comparative Example 2

Next, a medium the same as that described in Japanese Patent Application Laid-open No. 3-255946 was prepared and evaluated.

Using the same film forming apparatus and method as in Experimental Example 1, an SiN interference layer of 900 .ANG. thickness, a GdFeCo reproduction layer of 300 .ANG. thickness, a TbFeCoAl intermediate layer of 100 .ANG. thickness, a GdFeCo auxiliary layer of 160 .ANG., a TbFeCo memory layer of 400 .ANG. thickness and an SiN protective layer of 700 .ANG. thickness were formed on a glass substrate in the order named, thus obtaining the magneto-optical recording medium of Comparative Example 2.

The composition of the GdFeCo reproduction layer was set to represent a TM rich layer at room temperature, an Ms of -160 emu/cc and a Curie temperature of 300.degree. C. or more.

The composition of the TbFeCoAl intermediate layer was set to represent a TM rich layer at room temperature, an Ms of -160 emu/cc and a Curie temperature of 140.degree. C.

The composition of the GdFeCo auxiliary layer was set to represent a TM rich layer at room temperature, an Ms of -160 emu/cc and a Curie temperature of 280.degree. C. The composition of the TbFeCo memory layer was set to represent a TM rich layer at room temperature, an Ms of -150 emu/cc and a Curie temperature of 250.degree. C.

Subsequently, using this magneto-optical recording medium, the recording/reproduction characteristic was measured as in Experimental Example 1. In this case, however, prior to reproduction, an initializing magnetic field in a perpendicular direction was applied to the medium by changing a magnitude of the initializing magnetic field between 0 Oe, 1,000 Oe and 2,000 Oe, and a reproducing magnetic field was applied to the medium by changing the magnitude of the reproducing magnetic field between 0 Oe, 200 Oe and 400 Oe. The results are shown in Table 1.

Comparative Example 3

Next, a medium the same as that described in Japanese Patent Application Laid-open No. 6-124500 was prepared and evaluated.

Using the same film forming apparatus and method as in Experimental Example 1, an SiN interference layer of 900 .ANG. thickness, a GdFeCo reproduction layer of 400 .ANG. thickness, a TbFeCo memory layer of 400 .ANG. thickness and an SiN protective layer of 700 .ANG. thickness were formed on a glass substrate in the order named, thus obtaining the magneto-optical recording medium of Comparative Example 3.

The composition of the GdFeCo reproduction layer was set to represent a RE rich layer at room temperature, an Ms of 180 emu/cc, a compensation temperature of 240.degree. C. and a Curie temperature of 300.degree. C. or more.

The composition of the TbFeCo memory layer was set to represent a TM rich layer at room temperature, an Ms of 150 emu/cc and a Curie temperature of 250.degree. C.

Subsequently, using this magneto-optical recording medium, the recording/reproduction characteristic was measured as in Experimental Example 1. The results are shown in Table 1.

According to the measurement results of the foregoing Experimental Examples 1 to 6, particularly according to the measurement results with the short mark lengths, in any of the media, high C/N ratios were obtained with the short mark lengths without applying a reproducing magnetic field. Further, in the media where the reproduction layer is an in-plane magnetization film at room temperature and becomes a perpendicular magnetization film between room temperature and the Curie temperature, an improvement in C/N and the crosstalk was observed. On the other hand, in the medium of Comparative Example 1, a sufficiently high C/N ratio was not obtained without applying the reproducing magnetic field of 400 Oe. Further, the crosstalk showed the bad results. On the other hand, in the medium of Comparative Example 2, no improvement in C/N and the crosstalk were observed without applying a sufficient initializing magnetic field and a reproducing magnetic field. Further, in Comparative Example 3, a sufficiently high C/N ratio was not obtained.

Accordingly, in the magneto-optical recording medium of the present invention, the C/N ratio or both the C/N ratio and the crosstalk can be improved without applying the reproducing magnetic field or without applying both the initializing magnetic field and the reproducing magnetic field. Thus, the line recording density or both the line recording density and the track density can be improved.

Next, verification of the foregoing energy relation expressions (12) to (18) was performed in Experimental Examples 7 to 10 and 11 to 15 and Comparative Examples 4 to 8.

Experimental Example 7

Si, Gd, Tb, Fe and Co targets were attached to a DC magnetron sputtering apparatus, and a glass substrate having a diameter of 130 mm and a polycarbonate substrate with lands and grooves were fixed to a substrate holder, which was set at a position separated from the respective targets by a distance of 150 mm. Thereafter, the interior of the chamber was evacuated by a cryopump to a high vacuum of 1.times.10.sup.-5 Pa or less. During the evacuation, Ar gas was introduced into the chamber to 0.5 Pa, and thereafter, an SiN interference layer of 900 .ANG. thickness, a GdFeCo reproduction layer of 400 .ANG. thickness, a TbFeCo intermediate layer of 100 .ANG. thickness, a TbFeCo memory layer of 350 .ANG. thickness and an SiN protective layer of 700 .ANG. thickness were formed in the order named, thus obtaining the magneto-optical recording medium of the present invention with a structure shown in FIG. 14. Upon formation of each SiN dielectric layer, N.sub.2 gas was introduced in addition to the Ar gas, and the SiN layer was formed by DC reactive sputtering, adjusting the mixing ratio of the Ar and N.sub.2 gases, so as to obtain a refractive index of 2.1.

The composition of the GdFeCo reproduction layer was Gd.sub.x (Fe.sub.57 Co.sub.43).sub.100-x (the figure of a composition ratio represents an atomic ratio (%); hereinafter this definition will apply), and x was set to be 25%. Hereinafter, the polarity of a saturation magnetization will be described as being positive in case that the rare earth element sublattice magnetization is dominant and as being negative in the case that the iron family element sublattice magnetization is dominant.

The composition of the TbFeCo intermediate layer was Tb.sub.20 (Fe.sub.97 Co.sub.3).sub.80. A film of this composition was measured alone and represented an Ms3 of -120 emu/cc at room temperature and a Curie temperature of 155.degree. C. Since the medium of the present example is a medium of a front-aperture detection (FAD) type in which only the rear mask is formed, TbFeCo having a large perpendicular magnetic anisotropy is used for the intermediate layer so as to avoid formation of a front mask as much as possible.

The composition of the TbFeCo memory layer was Tb.sub.20 (Fe.sub.80 Co.sub.20).sub.80. A film of this composition was measured alone to observe the temperature dependency of a saturation magnetization Ms2, and represented an Ms2 of -240 emu/cc at room temperature and a Curie temperature of 250.degree. C. The temperature dependency of Ms2 is shown in FIG. 17.

Experimental Example 8

Subsequently, using the same apparatus and method as in Experimental Example 7, the magneto-optical recording medium of the present invention having a structure like that in Experimental Example 7 was prepared. The intermediate layer and the memory layer respectively had the same film thicknesses and compositions as those of the intermediate layer and the memory layer in Experimental Example 7. The reproduction layer also had the same film thickness as that in Experimental Example 7, but the composition thereof was changed. Specifically, x was set to 26% in Gd.sub.x (Fe.sub.57 Co.sub.43).sub.100-x. A film of this composition was measured alone to observe the temperature dependency of the saturation

magnetization Msl, and represented an Msl of 151 emu/cc at room temperature, a compensation temperature of 172.degree. C. and a Curie temperature of 300.degree. C. or more. The temperature dependency of Msl is shown in FIG. 16.

Experimental Example 9

Subsequently, using the same apparatus and method as in Experimental Example 7, the magneto-optical recording medium of the present invention having the same structure as that in Experimental Example 7 except for the composition of the reproduction layer, was prepared. Specifically, in the composition of the reproduction layer, x was set to 28% in Gd.sub.x (Fe.sub.57 Co.sub.43).sub.100-x. A film of this composition was measured alone to observe the temperature dependency of the saturation magnetization Msl, and represented an Msl of 236 emu/cc at room temperature, a compensation temperature of 225.degree. C. and a Curie temperature of 300.degree. C. or more. The temperature dependency of Msl is shown in FIG. 16.

Experimental Example 10

Subsequently, using the same apparatus and method as in Experimental Example 7, the magneto-optical recording medium of the present invention having the same structure as that in Experimental Example 7 except for the composition of the reproduction layer, was prepared. Specifically, in the composition of the reproduction layer, x was set to 31% in Gd.sub.x (Fe.sub.57 Co.sub.43).sub.100-x. A film of this composition was measured alone to observe the temperature dependency of the saturation magnetization Msl, and represented an Msl of 325 emu/cc at room temperature, a compensation temperature of 275.degree. C. and a Curie temperature of 300.degree. C. or more. The temperature dependency of Msl is shown in FIG. 16.

Comparative Example 4

Subsequently, using the same apparatus and method as in Experimental Example 7, the magneto-optical recording medium of Comparative Example 4, having a structure like that in Experimental Example 7, was prepared. The intermediate layer and the memory layer respectively had the same film thicknesses and compositions as those of the intermediate layer and the memory layer in Experimental Example 7. The reproduction layer also had the same film thickness as that in Experimental Example 7, but the composition thereof was changed. Specifically, x was set to 23% in Gd.sub.x (Fe.sub.57 Co.sub.43)100-x.

In the state where the magnetic films having the foregoing magnetic characteristics were laminated, it was examined whether a mask was formed in the high-temperature region. In case of the magnetostatic energy from the reproduction layer and the memory layer being dominant in the magnetostatic energy inside the medium, such as, in case of a magnetic layer other than the reproduction layer and the memory layer being relatively small in thickness, it is necessary, in order for the mask to be formed in the high-temperature region, that relation (22) is established based on the foregoing energy relation expressions.

First, energies applied to the Bloch magnetic wall of the recorded magnetic domain transferred to the reproduction layer (Bloch magnetic wall energy Ewb, static magnetic field energy Eleak from the reproduction layer, static magnetic field energy Est from the memory layer) were derived.

Since each term in relation (22) depends on temperature, each term is indicated relative to a temperature, for accuracy, so as to determine whether relation (22) is established. On the other hand, since Ewi is rapidly reduced when the intermediate layer reaches around the Curie temperature, it is frequent that a sign of inequality in relation (22) is established before the intermediate layer reaches the Curie temperature. In view of this, it was examined whether relation (22) was established at the Curie temperature of the intermediate layer. At this time point, Ewi can be regarded as O. For calculation, a Bloch magnetic wall energy Ewb of the reproduction layer and saturation magnetizations of the reproduction layer and the memory layer when the intermediate layer reaches the Curie temperature, are necessary. Accordingly, first, each of these materiality values was calculated. It is assumed that the reproduction layer and the memory layer lose the exchange-coupling force around the Curie temperature of the intermediate layer, i.e. about 155.degree. C. Values at this temperature were taken as the materiality values. A Bloch magnetic wall energy awb of the reproduction layer, when measured with the reproduction layer in the form of a single layer film, did not depend on the composition thereof within a range of this experiment and was about 1.9 erg/cc at about 155.degree. C. Further, Ms2 of the memory layer was derived to be -225 emu/cc from FIG. 17. On the other hand, Msl of the reproduction layer differed depending on the compositions of the reproduction layer and were derived to be values as shown in Table 3. Using these materiality values, the energies were calculated.

First, by substituting .sigma.wb=1.9 erg/cc and r=0.2 .mu.m into relation (15), Ewb=9.50.times.10.sup.4 erg/cc was obtained. Further, Msl, necessary for deriving Eleak, was obtained in the following manner. Specifically, Msl values corresponding to the respective reproduction layers were plotted. Since the precise measurement of Msl around the compensation composition was not easy, a value corresponding to Gd.sub.x (Fe.sub.57 Co.sub.43).sub.100-x (x=21%) alone was measured, and from this Msl and other Msl values, Msl values corresponding to x=23, 25 were plotted as shown in FIG. 18 so as to presume Msl values therefor. Using hl=30 nm and r=0.2 .mu.m, Eleak was obtained from relation (18). Further, Est is expressed by relation (23).

Accordingly, Hst was first calculated. Hst can be calculated from relation (24) in the simplified manner. In relations (24) and (25), a represents a radius of the recorded magnetic domain of the memory layer, h2 represents a film thickness of the memory layer and (r, .theta., z) represents coordinates of a measurement point, applied with a magnetic field Hst in a film-thickness direction, in a polar coordinate system having the origin, which is the center of an end surface, at a light-incident side, of the recorded magnetic domain in the memory layer, wherein r represents a distance in a radial direction, .theta. represents an angle and z represents a distance toward the light-incident side.

k(r/a, z/a, .theta.) and f(r, z, .theta.) are defined by relations (25) and (26), respectively.

The influence of a static magnetic field from a recorded magnetic domain of the memory layer other than a recorded magnetic domain of the memory layer just under a recorded magnetic domain of the reproduction layer being observed, is not so large. Accordingly, for simplification, relation (24) only deals with the recorded magnetic domain of the memory layer just under the recorded magnetic domain of the reproduction layer being observed. However, for further accuracy, it is better to calculate the magnetostatic energy from all the magnetization in the memory layer. This also applies to the calculation of Hleak defined by relation (8) in a simplified manner. As a result of calculating relation (24) using a calculator, Hst/(4.pi.Ms2)=0.15 was obtained in case of a diameter of the recorded magnetic domain being 0.4 nm (a=0.2 nm). Est was obtained using this value, Ms1 and Ms2. The results are shown in Table 3.

Further, a coercive force energy Ec is expressed by relation (27).

From the temperature dependencies of the saturation magnetization and the coercive force of the reproduction layer, Ec at 155.degree. C. depended on the composition of the reproduction layer only to a small extent in this experimental example, and thus were substantially 6.times.10.sup.4 erg/cc for any of the compositions.

These energy values are shown in Table 3.

As described before, in order for the mask to be formed in the high-temperature region, relation (22) should be established. Since Ewi=O, an expression Ewb--Eleak--Est--Ec for showing whether the recorded magnetic domain of the reproduction layer is contracted and inverted, is shown in FIG. 19 relative to the compositions x of the reproduction layer. According to FIG. 19, when x.gtoreq.25%, relation (28) was to be established so that it was expected that the recorded magnetic domain of the reproduction layer would be contracted and inverted, and thus the rear mask would be formed.

On the other hand, when x.ltoreq.24%, relation (29) was to be established so that it was expected that the rear mask would not be formed.

Next, the recording/reproduction characteristic was measured using this disc magneto-optical recording medium. The measurement was performed by setting the N.A. of an objective lens to be 0.55, the laser beam wavelength to be 780 nm, the recording power to be in a range of 7 to 13mW and the reproducing power to be 2.4 mW. The linear velocity was set to be 9 m/s, and no external magnetic field was applied upon reproduction. First, erasing was performed entirely on the medium, and thereafter, carrier signals of 5.8 MHz, 11.3 MHz and 15 MHz (corresponding to mark lengths 0.78 .mu.m, 0.40 pm and 0.30 .mu.m, respectively) were recorded in the memory layer so as to examine the mark-length dependency of C/N. When the recording of the mark length 0.78 .mu.m was performed, a C/N ratio of 48 dB or more was obtained for all the discs. On the other hand, when the recording of the mark length 0.30 .mu.m was performed, a C/N ratio of 35 dB or more was obtained for the media of Experimental Examples 7 to 10, while a C/N ratio of 20 dB or more was not obtained for the medium of Comparative Example 4.

C/N ratios at the mark length 0.40 pm are shown in FIG. 19 relative to the compositions of the reproduction layer, along with the energies. As seen in FIG. 19, when x.gtoreq.25% in the composition Gd.sub.x (Fe.sub.57 Co.sub.43).sub.100-x of the reproduction layer, the C/N ratio was 40 dB or more so that good values were obtained. On the other hand, when x=23%, the C/N ratio deteriorated. When comparing this with the foregoing energy relation, it is appreciated that this matches well the energy calculation results. This reveals that the medium of the present invention, satisfying the foregoing energy conditional expressions, shows an excellent reproduction characteristic.

Experimental Example 11

Next, the foregoing magneto-optical recording medium in which the reproduction layer is an in-plane magnetization film at room temperature and becomes a perpendicular magnetization film between room temperature and the Curie temperature, was prepared. First, Si, Gd, Tb, Fe and Co targets were attached to a DC magnetron sputtering apparatus, and a glass substrate having a diameter of 130 mm and a polycarbonate substrate with lands and grooves were fixed to a substrate holder which was set at a position separated from the respective targets by a distance of 150 mm. Thereafter, the interior of the chamber was evacuated by a cryopump to a high vacuum of 1.times.10.sup.-5 Pa or less. During the evacuation, Ar gas was introduced into the chamber to 0.5 Pa, and thereafter, an SiN interference layer of 900 .ANG. thickness, a GdFeCo reproduction layer of 400 .ANG. thickness, a GdFe intermediate layer of 100 .ANG. thickness, a TbFeCo memory layer of 350 .ANG. thickness and an SiN protective layer of 700 .ANG. thickness were formed in the order named, thus forming the magneto-optical recording medium of the present invention with a structure shown in FIG. 14. Upon formation of each SiN dielectric layer, N.sub.2 gas was introduced in addition to the Ar gas, and the SiN layer was formed by DC reactive sputtering, adjusting a mixing ratio of the Ar and N.sub.2 gases, so as to obtain a refractive index of 2.1.

The composition of the GdFeCo reproduction layer was Gd.sub.x (Fe.sub.58 Co.sub.42).sub.100-x, and x was set to be 27%. A film of this composition was measured alone to observe the temperature dependency of a saturation magnetization Msl (emu/cc), and represented Msl of 150 emu/cc at room temperature, a compensation temperature of 188.degree. C. and a Curie temperature of 300.degree. C. or more.

The composition of the GdFe intermediate layer was Gd.sub.37 Fe.sub.63. A film of this composition was measured alone and represented a saturation magnetization Ms3 of 420 emu/cc at room temperature and a Curie temperature of 198.degree. C. In the present medium, GdFe having a small perpendicular magnetic anisotropy and a large saturation magnetization is used for the intermediate layer so that the reproduction layer becomes an in-plane magnetization film around room temperature to a sufficient extent so as to form a front mask.

The composition of the TbFeCo memory layer was Tb.sub.20 (Fe.sub.80 CO.sub.20).sub.80. A film of this composition was measured alone to observe the temperature dependency of the saturation magnetization Ms2, and Ms2 was -240 emu/cc at room temperature and the Curie temperature was 250.degree. C. The temperature dependency of Ms2 is shown in FIG. 17.

Experimental Example 12

Subsequently, using the same apparatus and method as in Experimental Example 11, the magneto-optical recording medium of the present invention, having a structure like that in Experimental Example 11, was prepared. The intermediate layer and the memory layer, respectively, had the same film thicknesses and compositions as those of the intermediate layer and the memory layer in Experimental Example 11. The reproduction layer also had the same film thickness as that in Experimental Example 11, but the composition thereof was changed. Specifically, x was set to be 28% in Gd.sub.x (Fe.sub.58 CO.sub.42).sub.100-x. A film of this composition was measured alone to observe the temperature dependency of the saturation magnetization Msl, and represented an Msl of 200 emu/cc at room temperature, a compensation temperature of 205.degree. C. and a Curie temperature of 300.degree. C. or more.

Experimental Example 13

Subsequently, using the same apparatus and method as in Experimental Example 11, the magneto-optical recording medium of the present invention, having the same structure as that in Experimental Example 11, except for the composition of the reproduction layer, was prepared. Specifically, in the composition of the reproduction layer, x was set to 29% in Gd.sub.x (Fe.sub.58 CO.sub.42).sub.100-x. A film of this composition was measured alone to observe the temperature dependency of the saturation magnetization Msl, and represented an Msl of 240 emu/cc at room temperature, a compensation temperature of 225.degree. C. and a Curie temperature of 300.degree. C. or more.

Experimental Example 14

Subsequently, using the same apparatus and method as in Experimental Example 11, the magneto-optical recording medium of the present invention, having the same structure as that in Experimental Example 11, except for the composition of the reproduction layer, was prepared. Specifically, in the composition of the reproduction layer, x was set to 31% in Gd.sub.x (Fe.sub.58 CO.sub.42).sub.100-x. A film of this composition was measured alone to observe the temperature dependency of the saturation magnetization Msl, and represented an Msl of 310 emu/cc at room temperature, a compensation temperature of 260.degree. C. and a Curie temperature of 300.degree. C. or more.

Comparative Example 5

Subsequently, using the same apparatus and method as in Experimental Example 11, the magneto-optical recording medium of Comparative Example 5, having a structure like that in Experimental Example 11, was prepared. The intermediate layer and the memory layer, respectively, had the same film thicknesses and compositions as those of the intermediate layer and the memory layer in Experimental Example 11. The reproduction layer also had the same film thickness as that in Experimental Example 11, but the composition thereof was changed. Specifically, x was set to 25% in

Gd.sub.x (Fe.sub.58 CO.sub.42).sub.100-x. A film of this composition was measured alone to observe the temperature dependency of the saturation magnetization Msl, and represented an Msl of 51 emu/cc at room temperature, a compensation temperature of 150.degree. C. and a Curie temperature of 300.degree. C. or more.

Comparative Example 6

Subsequently, using the same apparatus and method as in Experimental Example 11, the magneto-optical recording medium of Comparative Example 6, having a structure like that in Experimental Example 11, was prepared. The intermediate layer and the memory layer, respectively, had the same film thicknesses and compositions as those of the intermediate layer and the memory layer in Experimental Example 11. The reproduction layer also had the same film thickness as that in Experimental Example 11, but the composition thereof was changed. Specifically, x was set to 26% in Gd.sub.x (Fe.sub.58 CO.sub.42).sub.100-x.

In the state where the magnetic films having the foregoing magnetic characteristics were laminated, a condition of formation of the mask in the high-temperature region, when the exchange-coupling force from the memory layer was lost in case of recording the mark length 0.4 .mu.m, was derived. It is assumed that the reproduction layer and the memory layer lose the exchange-coupling force at about 200.degree. C. Values at this temperature were taken as the materiality values. A Bloch magnetic wall energy .sigma.wb of the reproduction layer, when measured with the reproduction layer in the form of a single layer film, did not depend on the composition thereof in the present experimental example and was about 1.5 erg/cc at about 200.degree. C. Further, the saturation magnetization Ms2 of the memory layer was derived to be -180 emu/cc. On the other hand, the saturation magnetization Mel of the reproduction layer differed depending on the compositions of the reproduction layer and were derived to be values as shown in Table 4. Using these materiality values, the effective magnetic fields were calculated.

First, by substituting .sigma.wb=1.5 erg/cc and r=0.2 .mu.m into relation (15), Ewb=7.50.times.104 erg/cc was obtained. Eleak was obtained by using hl=30 nm, r=0.2 pm and Msl was derived corresponding to each of the reproduction layers in the same manner as in Experimental Examples 7 to 10. Est was calculated using Hst=0.15, Msl and Ms2 as described before.

From the temperature dependencies of the saturation magnetization and the coercive force of the reproduction layer, a coercive force energy Ec at about 200.degree. C. depended on the composition of the reproduction layer only to a small extent in this experimental example, and thus were substantially 6.times.10.sup.4 erg/cc for any of the compositions.

These energy values are shown in Table 4.

Further, an expression Ewb--Eleak--Est--Ec for showing whether the recorded magnetic domain of the reproduction layer is contracted and inverted, is shown in FIG. 20 relative to the compositions x of the reproduction layer. According to FIG. 20, when x>26%, relation (28) was to be established so that it was expected that the recorded magnetic domain of the reproduction layer