Home
Patent Search
IMT Blog
REGISTER
|
SIGN IN
United States Patent
5132945
Osato , ; et al.
July 21, 1992
Title
Magnetooptical recording medium allowing overwriting with two or more magnetic layers and recording method utilizing the same
Abstract
A magnetooptical recording medium provided with a first magnetic layer and a second magnetic layer having a higher Curie point and a lower coercive force than the first magnetic layer and being exchange coupled with the first magnetic layer satisfies the relation: ##EQU1## wherein H.sub.H is the coercive force of the first magnetic layer; H.sub.L is the coercive force of the second magnetic layer; M.sub.2 is the saturation magnetization of the second magnetic layer; h is the thickness thereof; and .sigma..sub.w is the magnetic wall energy between the first and second magnetic layers and a method of recording information on the same.
Inventors:
Osato; Yoichi
(Yokohama,
JP
)
, Kawade; Hisaaki
(Atsugi,
JP
)
, Fujii; Eiichi
(Yokohama,
JP
)
, Kasama; Nobuhiro
(Yokohama,
JP
)
, Kobayashi; Tadashi
(Yokohama,
JP
)
Assignee:
Canon Kabushiki Kaisha
(Tokyo,
JP
)
Appl. No.:
475941
Filed:
January 30, 1990
Foreign Application Priority Data
Jul 08, 1986 [JP] 61-158787
Aug 16, 1986 [JP] 61-191202
Nov 05, 1986 [JP] 61-262034
Nov 25, 1986 [JP] 61-278566
Nov 25, 1986 [JP] 61-278567
Feb 02, 1987 [JP] 62-20384
Feb 03, 1987 [JP] 62-21675
Feb 04, 1987 [JP] 62-23993
Feb 06, 1987 [JP] 62-24706
Feb 06, 1987 [JP] 62-24707
Feb 10, 1987 [JP] 62-27083
Feb 10, 1987 [JP] 62-27982
Feb 23, 1987 [JP] 62-33736
Mar 10, 1987 [JP] 62-52897
Mar 16, 1987 [JP] 62-70279
Mar 26, 1987 [JP] 62-70273
Mar 26, 1987 [JP] 62-70274
Mar 26, 1987 [JP] 62-70278
Mar 26, 1987 [JP] 61-72559
Jun 18, 1987 [JP] 62-153108
Current U.S. Class:
369/13.38
369/13.49
369/13.51
360/59
365/122
Field of Search:
369/13,14 360/59,224,131 365/10,27,32,122
U.S. Patent Documents
3521294
July 1970
Treves
4059828
November 1977
Kobayashi et al.
4126494
November 1978
Imamura et al.
4198692
April 1980
Kobayashi
4556291
December 1985
Chen
4612587
September 1986
Kaneko et al.
4628485
December 1986
Tanaka et al.
4645722
February 1987
Katayama et al.
4701881
October 1987
Tanaka et al.
4771347
September 1988
Horimai et al.
4871614
October 1989
Kobayashi
Foreign Patent Documents
0217067
Apr., 1987
EP
0225151
Jun., 1987
EP
2110459
Jun., 1983
GB
3619618
Dec., 1986
DE
57-70653
May., 1982
JP
58-08045
Jun., 1983
JP
58-50639
Mar., 1983
JP
60-05404
Jan., 1985
JP
61-240453
Oct., 1986
JP
Other References
IEEE Transactions on Magnetics, vol. 22, No. 5 (Sep., 1986 931:3 .
Mizutani, Japanese Patent Abstracts, vol. 9, No. 86 (p.349). .
Tanaka, Japanese Patent Abstracts, vol. 6, No. 34 (p. 104). .
Tsumashima et al., IEEE Trans. on Mag. vol. Mag 17, No. 6, (Nov. 1981) pp. 2840-2842. .
Mizutani, Japanese Patent Abstracts, vol. 9, No. 86 (Apr., 1985) p. 349. .
Nippon Kogaku K. K., "Overwrite System of Mag. Op. Disk Sys.". .
Inter Symposium on Mag Op. 4/1987, 7 pages. .
Kobayashi, et al., Japanese Journal of Applied Physics, vol. 20, No. 11 (Nov. 1981), pp. 2089-2095..~
Primary Examiner:
Levy; Stuart S.
Assistant Examiner:
Nguyen; Hoa
Attorney, Agent or Firm:
Fitzpatrick, Cella, Harper & Scinto
Parent Case Text
This application is a continuation of application Ser. No. 071,190, filed Jul. 8, 1987 now abandoned.
Claims
What is claimed is:
1. A magnetooptical recording medium comprising a first magnetic layer and a second magnetic layer having a higher Curie point and a lower coercive force than those of said first magnetic layer and being exchange coupled with said first magnetic layer, characterized in that provided between said first magnetic layer and said second magnetic layer is an adjusting layer for adjusting a magnetic wall energy .sigma..sub.w between said first and second magnetic layers such that the magnetic wall energy .sigma..sub.w satisfies a relation:
wherein H.sub.H is the coercive force of the first magnetic layer; H.sub.L is the coercive force of the second magnetic layer; M.sub.s is the saturation magnetization of the second magnetic layer; and h is the thickness of said second magnetic layer.
2. A magnetooptical recording medium according to claim 1, wherein said adjusting layer is composed of a material selected from Ti, Cr, Al, Ni, Fe, Co, rare earth elements and transition metals and has a thickness in a range from 5 to 50 .ANG..
3. A magnetooptical recording medium according to claim 1, wherein said first and second magnetic layers are composed of alloys selected from GdCo, GdFe, TbFe, DyFe, GdTbFe, TbDyFe, GdFeCo, TbFeCo, GdTbCo and GdTbFeCo.
4. A magnetooptical recording medium according to claim 1, wherein the Curie points of said first and second magnetic layers are respectively in ranges from 70.degree. to 180.degree. C. and from 150.degree. to 400.degree. C.
5. A magnetooptical recording medium according to claim 1, wherein the coercive forces of said first and second magnetic layers are respectively in ranges from 3 to 100 KOe, and from 0.5 to 2 KOe.
6. A magnetooptical recording medium comprising a first magnetic layer and a second magnetic layer having a higher Curie point and a lower coercive force than those of said first magnetic layer and being exchange coupled with said first magnetic layer, characterized in that provided between said first magnetic layer and said second magnetic layer is an adjusting layer for adjusting a magnetic wall energy .sigma..sub.W between said first and second magnetic layers such that the magnetic wall energy .sigma.W satisfies a relation:
wherein H.sub.H is the coercive force of the first magnetic layer; H.sub.L is the coercive force of the second magnetic layer; M.sub.s is the saturation magnetization of the second magnetization layer; and h is the thickness of said second magnetic layer, said adjusting layer being composed of a magnetic material exhibiting surfacial magnetic anisotropy at room temperature and vertical magnetic anisotropy at temperatures close to the Curie point of said first magnetic layer.
7. A magnetooptical recording medium according to claim 6, wherein said first magnetic layer, second magnetic layer and adjusting layer are composed of alloys of rare earth element and transition metal, in which the first magnetic layer has a composition rich in the transition metal compared with the compensation composition, said second magnetic layer and adjusting layer have compositions both rich in the rare earth element compared with the compensation compositions.
8. A magnetooptical recording medium provided, in succession on a substrate, with a first magnetic layer with a high Curie point T.sub.H1 and a low coercive force H.sub.L1, a second magnetic layer with a lower Curie point T.sub.L2 and a higher coercive force H.sub.H2 compared with those of said first magnetic layer, and a third magnetic layer with a higher Curie point T.sub.H3 and a lower coercive force H.sub.L3 compared with those of said second magnetic layer; wherein said three magnetic layers are mutually so coupled as to satisfy conditions: ##EQU11## wherein .sigma..sub.w12 is the magnetic wall energy of the first and second magnetic layers; .sigma..sub.w23 is the magnetic wall energy of the second and third magnetic layers; h.sub.1, h.sub.2 add h.sub.3 are respective thicknesses of the first, second and third magnetic layers; and M.sub.s1, M.sub.s2 and M.sub.s3 are respectively saturation magnetizations of said layers.
9. A magnetooptical recording medium according to claim 8, wherein the Curie points and coercive forces of said first and third magnetic layers satisfy relations:
10. A magnetooptical recording medium according to claim 8, wherein the Curie points of said three magnetic layers are respectively in a range of 150.degree. to 400.degree. C. for T.sub.H1, 70.degree. to 200.degree. C. for T.sub.H2, and
100.degree. to 250.degree. C. for T.sub.H3.
11. A magnetooptical recording medium according to claim 8, wherein the coercive forces of said three magnetic layers are respectively in a range of 0.1 to 1 KOe for H.sub.L1, 2 to 10 KOe for H.sub.H2, and 0.5 to 4 KOe for H.sub.L3.
12. A magnetooptical recording medium according to claim 8, wherein said first, second and third magnetic layers are composed of alloys of rare earth element and transition metal, wherein the first and second magnetic layers have compositions both rich in the transition metal compared with the compensation composition, while the third magnetic layer has a composition rich in the rare earth element compared with the compensation composition.
13. A magnetooptical recording medium according to claim 8, wherein said first, second and third magnetic layers are composed of alloys selected from GdCo, GdFe, TbFe, DyFe, GdTbFe, TbDyFe, GdFeCo, TbFeCo, and GdTbCo.
14. An information recording process on a magnetooptical recording medium provided, in succession on a substrate, with a first magnetic layer with a high Curie point T.sub.H1 and a low coercive force H.sub.L1. a second magnetic layer with a lower Curie point T.sub.L2 and a higher coercive force H.sub.H2 compared with those said first magnetic layer, and a third magnetic layer with a higher Curie point T.sub.H3 and a lower coercive force H.sub.L3 compared with those of said second magnetic layer, wherein said three magnetic layers are mutually so coupled as to satisfy conditions: ##EQU12## wherein .sigma..sub.w12 is the magnetic wall energy of the first and second magnetic layers: .sigma..sub.w23 is the magnetic wall energy of the second and third magnetic layers; h.sub.1, h.sub.2 and h.sub.3 are respective thicknesses of the first, second and third magnetic layers; and M.sub.s1, M.sub.s2 and M.sub.s3 are respectively saturation magnetizations of said layers, comprising steps of: orienting the magnetization of the third magnetic layer in a predetermined direction while retaining the magnetization of the second magnetic layer; and
(b) selectively effecting either a first type recording by irradiating said medium with a light beam of a power for heating the medium close to the Curie point T.sub.L2 of the second magnetic layer while applying a bias magnetic field as a bias field, thereby orienting the magnetizations of the first and second magnetic layer in a stable direction with respect to the magnetization of the third magnetic layer while retaining the magnetization of the third magnetic layer, or a second type recording by irradiating the medium with a light beam of a power for heating the medium close to the Curie point T.sub.H3 of the third magnetic layer, thereby inverting the magnetization of said third magnetic layer and simultaneously orienting the magnetizations of the first and second magnetic layers in a stable direction with respect to the magnetization of the third magnetic layer, according to information signal.
15. A magnetooptical recording medium provided, in succession on a substrate, with a first magnetic layer with a Curie point T.sub.1 and a coercive force H.sub.1, a second magnetic layer with a Curie point T.sub.2 and a coercive force H.sub.2
and a third magnetic layer with a Curie point T.sub.3 and a coercive force H.sub.3. which comprises satisfying conditions:
(A) that each magnetic layer is composed principally of an amorphous alloy of rare earth element and transition metal;
(B) that
(C) that the first magnetic layer has a composition rich in the transition metal compared with the compensation composition and the second and third magnetic layers have compositions both rich in the rare earth element, or that the first magnetic layer has a composition rich in the rare earth element and the second and third magnetic layers have compositions both rich in the transition metal.
16. A magnetooptical recording medium according to claim 15, wherein the Curie points of said three magnetic layers are respectively in the ranges of 150.degree. to 400.degree. C. for T.sub.3, 70.degree. to 200.degree. C. for T.sub.1 and
90.degree. to 400.degree. C. for T.sub.2.
17. A magnetooptical recording medium according to claim 15, wherein the coercive forces of said three magnetic layers are respectively in the ranges of 0.1 to 1 KOe for H.sub.2, 2 to 10 KOe for H.sub.1, and 0.5 to 4 KOe for H.sub.3.
18. An information recording process on a magnetooptical recording medium provided, in succession on a substrate, with a first magnetic layer with a Curie point T.sub.1 and a coercive force H.sub.1, a second magnetic layer with a Curie point T.sub.2 and a coercive force H.sub.2 and a third magnetic layer with a Curie point T.sub.3 and a coercive force H.sub.3, and satisfying conditions:
(A) that each magnetic layer is composed principally of an amorphous alloy of rare earth element and transition metal;
(B) that H.sub.1 >H.sub.3 >H.sub.2 and T.sub.3 .gtoreq.T.sub.2 >T.sub.1 ; and
(C) that the first magnetic layer has a composition rich in the transition metal compared with the compensation composition while the second and third magnetic layers have compositions both rich in the rare earth element, or that the first magnetic layer has a composition rich in the rare earth element while the second and third magnetic layers have compositions both rich in the transition metal, comprising steps of:
(a) orienting the magnetization of the second and third magnetic layers in a predetermined direction while retaining the magnetization of the first magnetic layer; and
(b) selectively effecting either a first type recording by irradiating said medium with a light beam of a power for heating the medium close to the Curie point T.sub.1 of said first magnetic layer while applying a bias magnetic field as a bias field, thereby orienting the magnetization of the first magnetic layer in a stable direction with respect to the magnetizations of the second and third magnetic layer while retaining the magnetizations of the second and third magnetic layers, or a second type recording by irradiating the medium with a light beam of a power for heating the medium close to the Curie point T.sub.3 of the third magnetic layer thereby inverting the magnetization of said second and third magnetic layers and simultaneously orienting the magnetization of the first magnetic layer in a stable direction with respect to the magnetizations of the second and third magnetic layers, according to information signal.
19. A magnetooptical recording medium provided, on at least a substrate, with a quadraple-layered magnetic film consisting of a first magnetic layer with a Curie point T.sub.1, a coercive force H.sub.1, a thickness h.sub.1 and a saturation magnetization M.sub.s1, a second magnetic layer with a Curie point T.sub.2, a coercive force H.sub.2, a thickness h.sub.2 and a saturation magnetization M.sub.s2, a third magnetic layer with a Curie point T.sub.3, a coercive force H.sub.3, a thickness h.sub.3 and a saturation magnetization M.sub.s3, and a fourth magnetic layer with a Curie point T.sub.4, a coercive force H.sub.4, a thickness h.sub.4 and a saturation magnetization M.sub.s4, in which said magnetic layers are exchange-coupled, wherein said four magnetic layers are so coupled as to satisfy conditions:
(I) as for the Curie points of the magnetic layers:
(II) as for the coercive forces of the magnetic layers:
(III) as for the thicknesses of the magnetic layers:
(IV) as for the saturation magnetizations, thicknesses, coercive forces and magnetic wall energies of the magnetic layers: ##EQU13## wherein .sigma..sub.w12, .sigma..sub.w23 and .sigma..sub.w34 are magnetic wall energies respectively for the first and second magnetic layers, second and third magnetic layers, and third and fourth magnetic layers.
20. An information recording process on a magnetooptical recording medium provided, at least on a substrate, with a quadraple-layered magnetic film consisting of a first magnetic layer with a Curie point T.sub.1, a coercive force H.sub.1, a thickness h.sub.1 and a saturation magnetization M.sub.s1, a second magnetic layer with a Curie point T.sub.2, a coercive force H.sub.2, a thickness h.sub.2 and a saturation magnetization M.sub.s2, a third magnetic layer with a Curie point T.sub.3, a coercive force H.sub.3, a thickness h.sub.3 and a saturation magnetization M.sub.s3, and a fourth magnetic layer with a Curie point T.sub.4, a coercive force H.sub.4, a thickness h.sub.4 and a saturation magnetization M.sub.s4, in which said magnetic layer are so exchange-coupled as to satisfy following conditions:
(I) as for the Curie points of the magnetic layers:
(II) as for the coercive forces of the magnetic layers:
(III) as for the thicknesses of the magnetic layers:
(IV) as for the saturation magnetizations, thicknesses, coercive forces and magnetic wall energies of the magnetic layers: ##EQU14## wherein .sigma..sub.w12, .sigma..sub.w23 and .sigma..sub.w34 are magnetic wall energies respectively for the first and second magnetic layers, second and third magnetic layers, and third and fourth magnetic layers; comprising steps of:
(a) orienting the magnetization of the fourth magnetic layer in a predetermined direction while retaining the magnetization of the second magnetic layer; and
(b) selectively effecting either a first type recording by irradiating the medium with a light beam of a power for heating the medium close to the Curie point T.sub.2 of the second magnetic layer while applying a bias magnetic field as a bias field, thereby orienting, across the third magnetic layer, the magnetizations of the first and second magnetic layers in a stable direction with respect to the magnetization of the fourth magnetic layer while retaining the magnetization of the fourth magnetic layer, or a second type recording by irradiating the medium with a light beam of a power for heating the medium close to the Curie point T.sub.4 of the fourth magnetic layer thereby inverting the magnetization of said fourth magnetic layer and simultaneously orienting the magnetizations of the first, second and third magnetic layers in a stable direction with respect to the magnetization of said fourth magnetic layer, according to information signal.
21. A magnetooptical recording medium comprising:
a substrate;
a first magnetic layer formed on said substrate;
a second magnetic layer formed on said first magnetic layer, said second magnetic layer having a higher Curie point and a lower coercive force at room temperature than those of said first magnetic layer; and
a third magnetic layer provided between said first magnetic layer and said second magnetic layer, said third magnetic layer exhibiting surfacial magnetic anisotropy at room temperature and vertical magnetic anisotropy at temperatures close to the Curie point of said first magnetic layer.
22. A magnetooptical recording medium according to claim 21, wherein said first, second and third magnetic layers are composed of alloys of rare earth element and transition metal, and said first magnetic layer has a composition rich in the transition metal compared with a compensation composition, while said second and third magnetic layers have compositions both rich in the rare earth element compared with the compensation composition.
23. A magnetooptical recording medium according to claim 21, wherein the magnetic anisotropy of said third magnetic layer varies in a range from 50.degree. to 100.degree. C.
24. A magnetooptical recording medium according to claim 21, wherein said third magnetic layer is composed of an alloy of more than one element selected from Dy, Nb, Pr and Tb and more than one element selected from Co, Fe and Ni.
25. A magnetooptical recording medium according to claim 21, wherein said third magnetic layer is composed of rare earth ortho-ferrite or rare earth ortho-chromite.
26. A magnetooptical recording medium according to claim 21, wherein said first and second magnetic layers are composed of alloys selected from GdCo, GdFe, TbFe, DyFe, GdTbFe, TbDyFe, GdFeCo, TbFeCo, GdTbCo and GdTbFeCo.
27. A magnetooptical recording medium according to claim 21, wherein the Curie point of said first magnetic layer is in a range from 70.degree. to 180.degree. C. and the Curie point of said second magnetic layer is in a range from 150.degree. to 400.degree. C.
28. A magnetooptical recording medium according to claim 21, wherein the coercive force of said first magnetic layer is in a range from 3 to 10 KOe and the coercive force of said second magnetic layer is in a range from 0.5 to 2 KOe.
29. A magnetooptical recording medium according to claim 21, wherein a pregroove is provided on said substrate.
30. A magnetooptical recording medium according to claim 21, further comprising a protective layer provided on said second magnetic layer and between said first magnetic layer and said substrate.
31. A process for recording information on a magnetooptical recording medium which comprises a substrate, a first magnetic layer formed on the substrate, a second magnetic layer formed on the first magnetic layer and having a higher Curie point and a lower coercive force at room temperature than those of the first magnetic layer and a third magnetic layer provided between the first and second magnetic layers and exhibiting surfacial magnetic anisotropy at room temperature and vertical magnetic anisotropy at temperatures close to the Curie point of the first magnetic layer, said process comprising the steps of:
(a) orienting the magnetization of the second magnetic layer in a predetermined direction while retaining the magnetization of the first magnetic layer; and
(b) selectively effecting either a first type recording by irradiating the medium with a light beam of a power for heating the medium close to the Curie point of the first magnetic layer while applying a bias magnetic field, thereby orienting the magnetization of the first magnetic layer in a stable direction with respect to the magnetization of the second magnetic layer while retaining the magnetization of the second magnetic layer, or a second type recording by irradiating the medium with a light beam of a power for heating the medium close to the Curie point of the second magnetic layer thereby inverting the magnetization of the second magnetic layer and simultaneously orienting the magnetization of the first magnetic layer in a stable direction with respect to the magnetization of the second magnetic layer, according to information signal.
32. A magnetooptical recording medium comprising:
a substrate;
a first magnetic layer formed on said substrate;
a second magnetic layer formed on said first magnetic layer and having a lower Curie point and a higher coercive force at room temperature than those of said first magnetic layer, said second magnetic layer being exchange-coupled with said first magnetic layer; and
a third magnetic layer formed on said second magnetic layer and having a high Curie point and a lower coercive force at room temperature than those of said second magnetic layer, said third magnetic layer being exchange-coupled with said second magnetic layer and the exchange-couple force between said second and third magnetic layers being small compared with that between said first and second magnetic layers.
33. A magnetooptical recording medium according to claim 32, wherein the Curie point of said first magnetic layer is equal to or higher than that of said third magnetic layer.
34. A magnetooptical recording medium according to claim 32, wherein the coercive force of said first magnetic layer is equal to or lower than that of said third magnetic layer.
35. A magnetooptical recording medium according to claim 32, wherein the Curie points of said first, second and third magnetic layers are respectively in ranges from 150.degree. to 400.degree. C., from 70.degree. to 200.degree. C. and from
100.degree. to 250.degree. C.
36. A magnetooptical recording medium according to claim 32, wherein the coercive forces of said first, second and third magnetic layers are respectively in ranges from 0.1 to 1 KOe, from 2 to 10 KOe and from 0.5 to 4 KOe.
37. A magnetooptical recording medium according to claim 32, wherein said first, second and third magnetic layers are composed of alloys of rare earth metal and transition metal, said first and second magnetic layers have compositions rich in the transition metal compared with the compensation composition and said third magnetic layer has a composition rich in the rare earth compared with the compensation composition.
38. A magnetooptical recording medium according to claim 32, wherein said first, second and third magnetic layers are composed of alloys selected from GdCo, GdFe, TbFe, DyFe, GdTbFe, TbDyFe, GdFeCo, TbFeCo, GdTbCo and GdTbFeCo.
39. A process for recording information on a magnetooptical recording medium which comprises a substrate, a first magnetic layer formed on the substrate, a second magnetic layer formed on the first magnetic layer, having a lower Curie point and a higher coercive force at room temperature than those of the first magnetic layer and exchange-coupled with said first magnetic layer an a third magnetic layer provided on the second magnetic layer, having a higher Curie point and a lower coercive force at room temperature than those of the second magnetic layer and exchange-coupled with the second magnetic layer, said exchange-couple force between the second and third magnetic layers being small compared with that between the first and second magnetic layers, said process comprising the steps of:
(a) orienting the magnetization of the third magnetic layer in a predetermined direction while retaining the magnetization of the second magnetic layer; and
(b) selectively effecting either a first type recording by irradiating the medium with a light beam of a power for heating the medium close to the Curie point of the second magnetic layer while applying a bias magnetic field, thereby orienting the magnetizations of the first and second magnetic layers in stable directions with respect to the magnetization of the third magnetic layer while retaining the magnetization of the third magnetic layer, or a second type recording by irradiating the medium with a light beam of a power for heating the medium close to the Curie point of the third magnetic layer thereby inverting the magnetization of the third magnetic layer and simultaneously orienting the magnetizations of the first and second magnetic layers in stable directions with respect to the magnetization of the third magnetic layer, according to information signal.
40. A magnetooptical recording medium comprising:
a first magnetic layer;
a second magnetic layer exchange-coupled to said first magnetic layer, said second magnetic layer having a higher Curie point and a lower coercive force at room temperature than those of said first magnetic layer; and
a third magnetic layer provided between said first magnetic layer and said second magnetic layer, said third magnetic layer having a larger saturation magnetization than that of said first and second magnetic layers.
41. A magnetooptical recording medium according to claim 40, wherein said first, second and third magnetic layers are composed of amorphous alloys of rare earth element and transition metal element.
42. A magnetooptical recording medium according to claim 41, wherein said first magnetic layer is composed of a composition rich in one of the transition metal element and the rare earth element and said second and third magnetic layers are composed of the other of the transition metal element and the rare earth element.
43. A magnetooptical recording medium according to claim 41, wherein aid first and second magnetic layers are composed of alloys selected from GdCo, GdFe, TbFe, DyFe, GdTbFe, TbDyFe, GdFeCo, TbFeCo, GdTbCo and GcTbFeCo.
44. A magnetooptical recording medium according to claim 40, wherein the Curie points of said first, second and third magnetic layers are respectively in ranges from 70.degree. to 200.degree. C., from 150.degree. to 400.degree. C. and from
90.degree. to 400.degree. C.
45. A magnetooptical recording medium according to claim 40, wherein the coercive forces of said first, second and third magnetic layers are respectively in ranges from 2 to 10 KOe, from 0.5 to 4 KOe and from 0.1 to 1 KOe.
46. A magnetooptical recording medium according to claim 40, wherein the following conditions are satisfied: ##EQU15## wherein .sigma..sub.w13 is the magnetic wall energy between the first and third magnetic layers; .sigma..sub.w23 is the magnetic wall energy between the second and third magnetic layers; h.sub.1, h.sub.2 and h.sub.3 are respectively thicknesses of the first, second and third magnetic layers; and M.sub.s1, M.sub.s2 and M.sub.s3 are respectively saturation magnetizations of said layers.
47. A process for recording information on a magnetooptical recording medium which comprises a first magnetic layer, a second magnetic layer exchange-coupled to the first magnetic layer and having a higher Curie point and a lower coercive force at room temperature than those of the first magnetic layer and a third magnetic layer provided between the first and second magnetic layers and having a larger saturation magnetization than said those magnetic layers, said process comprising steps of:
(a) orienting the magnetization of the second magnetic layer in a predetermined direction while retaining the magnetization of the first magnetic layer; and
(b) selectively effecting either a first type recording by irradiating the medium with a light beam of a power for heating the medium close to the Curie point of the first magnetic layer while applying a bias magnetic field, thereby orienting the magnetization of the first magnetic layer in a stable direction with respect to the magnetization of the second magnetic layer while retaining the magnetization of the second magnetic layer, or a second type recording by irradiating the medium with a light beam of a power for heating the medium close to the Curie point of the second magnetic layer thereby inverting the magnetization of the second magnetic layer and simultaneously orienting the magnetization of the first magnetic layer in a stable direction with respect to the magnetization of the second magnetic layer, according to information signal.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetooptical recording medium provided with a recording layer composed of a magnetic film allowing information recording by irradiation with a light beam, and a recording method utilizing said recording medium.
2. Description of the Related Background Art
Optical memory devices utilizing laser beam are being actively developed in recent years as useful memories of high density and large capacity. In particular, magnetooptical recording is attracting attention as a rewritable recording method, and the magnetooptical recording media employed as a rewritable optical memory device.
FIG. 1 schematically illustrates a conventional apparatus for such magnetooptical recording, wherein a disk-shaped magnetooptical recording medium or magnetooptical disk 31, provided with a magnetic layer having an easy axis of magnetization perpendicular to the surface of said layer, is rotated by a spindle motor 32. An optical head 34, provided with a laser unit, an objective lens 34 etc., performs information recording by projecting a light beam 35 (turned on and off according to the information to be recorded) onto the disk 31 through the objective lens 33. A bias magnetic field is applied by an electromagnet 36 to an area of the disk 31 irradiated by the light beam. The optical head 34 is moved in the radial direction of the disk
31, thereby recording information in spiral or concentric patterns.
In conventional apparatus as shown in FIG. 1, the information recording and erasure are conducted in steps 30a to 30f shown in FIG. 2. At first, as shown in 30a, the magnetic layer 37, constituting the recording layer of the magnetooptical disk, is magnetized in a predetermined direction. Then, as shown in 36b, the magnetic layer 37 is irradiated by the light beam 35. The irradiated area is heated close to the Curie point of said magnetic layer 37 by absorption of the irradiating beam, thus causing a decrease in the coercive force. In this state the magnet 36 shown in FIG. 1 applies a bias magnetic field B' of a direction opposite to the aforementioned predetermined direction, whereby the magnetization in the area irradiated by the light beam is inverted. Thus, after having passed the position of irradiation, as shown by 30c, a record bit 38 having a direction of magnetization different from that in the surrounding area is formed. The information is recorded as a train of such record bits 38 or an information track.
For erasing the information recorded as in 30d, an unmodulated light beam 35 is projected while a bias magnetic field -B' of a direction opposite to that of the magnetic field at the recording is applied by the magnet 36 as shown in 30e, thereby heating the magnetic layer 37 again to a temperature close to the Curie point. Thus the magnetic layer 37 restores the magnetization aligned in the predetermined direction, thus returning to the state prior to the recording as shown in 30f.
The recorded information can also be reproduced by irradiating the magnetic layer 37 having record bits 38, with an unmodulated light beam of a reduced intensity insufficient for heating to the Curie point, and detecting the direction of polarization of the reflected or transmitted light beam by a known method utilizing magnetooptical effect.
However, in case of rewriting already recorded information, the conventional apparatus as explained above is incapable of so-called overwriting but requires a step of erasing followed by a step of new recording. Thus, in case of changing the information recorded in a track on a magnetooptical disk, it becomes necessary to erase the information of said track in a turn of the disk and to record the new information in a succeeding turn, and such operation inevitably results in a loss of recording speed.
In order to resolve such drawback there has already been proposed an apparatus equipped with a record/reproducing head and a separate erasing head, or an apparatus in which the recording is achieved by modulating the applied magnetic field while a continuous laser beam is projected, but such apparatus are associated with other drawbacks such as being bulky and expensive or incapable of high speed modulation.
On the other hand, in order to improve the recording sensitivity and the S/N ratio at reproduction in such magnetooptical recording medium, technology utilizing two mutually exchange-coupled magnetic layers is disclosed in the Japanese Patent laid-open No. 78652/1982, corresponding to the U.S. patent application Ser. No. 315,467 which is continued as Continuation-in-part No. 644,143, which is further continued as Continuation No. 908,934 now U.S. Pat. No. 4,799,114 issued Jan. 17, 1989. In addition to the above-mentioned applications, such magnetic layer of two-layered structure was described in "Magnetization Process of Exchange-coupled Ferrimagnetic Double-Layered Films", Kobayashi et al., Japanese Journal of Applied Physics, Vol. 20. No. 11, November 1981, P. 2089-2095 and "Thermomagnetic Writing on Exchange-coupled Amorphous Rare-Earth Iron Double-layer Films" Tsunashima et al., IEEE Transactions on Magnetics, Vol. MAG-17, No. 6, November 1981, P. 2940-2842.
However such exchange-coupled double-layered films are still incapable of overwriting and thus require an erasing step.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a magnetooptical recording medium and a recording method, which are free from the above-explained drawbacks and enable an overwriting operation as in the magnetic recording media, by merely attaching magnetic field generating means of a simple structure to the conventional apparatus.
The foregoing object can be achieved, according to the present invention, by a magnetooptical recording medium composed of a substrate, a first magnetic layer formed thereon, and a second magnetic layer exchange-coupled with said first magnetic layer and having a higher Curie point and a lower coercive force compared with those of said first magnetic layer, and satisfying a condition:
wherein M.sub.s is the saturation magnetization of the second magnetic layer, h is the thickness thereof, .sigma..sub.w is the magnetic wall energy between two magnetic layers, and H.sub.H and H.sub.L are coercive forces the first and second magnetic layers.
The information recording on said medium is conducted by a step of applying a first magnetic field of a magnitude enough for magnetizing said second magnetic layer but insufficient for inverting the direction of magnetization of said first magnetic layer, and a step of applying a bias magnetic field of a direction opposite to that of said first magnetic field and simultaneously projecting a light beam of a power enough for heating the medium close to the Curie point of the first magnetic layer thereby obtaining the recording of a first kind in which the magnetization of the first magnetic layer is aligned in a direction stable to the second magnetic layer while the direction of magnetization of the second magnetic layer is not changed, or applying said bias magnetic field and simultaneously projecting a light beam of a power enough for heating the medium close to the Curie point of the second magnetic layer thereby obtaining the recording of a second kind in which the direction of magnetization of the second magnetic layer is inverted and the first magnetic layer is simultaneously magnetized in a direction stable to the second magnetic layer.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a conventional magnetooptical recording apparatus;
FIG. 2 is a schematic view showing the recording and erasing processes utilizing a conventional magnetooptical recording medium;
FIG. 3 is a schematic cross-sectional view of a magnetooptical recording medium embodying the present invention;
FIG. 4 is a chart showing the temperature characteristic of the coercive force of the magnetic layer in the medium shown in FIG. 3;
FIG. 5 is a schematic view showing the state of magnetization in a recording process utilizing the medium shown in FIG. 3;
FIG. 6 is a schematic view of an apparatus for recording and reproduction with the medium of the present invention;
FIGS. 7A to 7D are charts showing B-H loops of the medium shown in FIG. 3;
FIG. 8 is a schematic view of an embodiment of the magnetooptical recording apparatus utilizing the medium of the present invention;
FIG. 9 is a chart showing the mode of modulation of the light beam irradiating the medium in the apparatus shown in FIG. 8;
FIG. 10A is a schematic view showing states of magnetization in the recording process utilizing the apparatus shown in FIG. 8;
FIG. 10B is a schematic view showing states of magnetization in the recording process utilizing another embodiment of the medium;
FIG. 11 is a chart showing the relation between the coercive force of the first magnetic layer and the magnetic field inducing noise increase;
FIGS. 12 and 13 are schematic cross-sectional views of embodiments of the magnetooptical recording medium provided with an adjusting layer for the magnetic wall energy;
FIG. 14 is a chart showing the relation between the thickness of the adjusting layer and the exchange force of the magnetic layers, in the medium shown in FIG. 13;
FIG. 15 is a chart showing the temperature characteristic of the effective bias magnetic field of the magnetic layers in the medium shown in FIG. 13;
FIGS. 16 and 17 are charts showing the temperature characteristic of the magnetic field required for orienting the magnetization of the adjusting layer in the perpendicular direction;
FIG. 18 is a chart showing the temperature characteristic of the coercive force of the magnetic layers in the medium shown in FIG. 3;
FIG. 19 is a chart showing the temperature characteristic of the coercive force and exchange force of the magnetic layers in the medium shown in FIG. 3;
FIGS. 20 and 21 are schematic views showing states of magnetization in the recording process utilizing the compensation temperature;
FIGS. 22 and 23 are charts showing the temperature characteristic of the coercive force, when the magnetic layer has the compensation temperature between the room temperature and the Curie temperature;
FIGS. 24, 25 and 26 are schematic cross-sectional views of embodiments of the magnetooptical recording medium of the present invention provided with a protective layer;
FIGS. 27 and 28 are schematic cross-sectional views of embodiments of the magnetooptical recording medium of the present invention utilizing triple-layered magnetic film;
FIGS. 29, 31 to 33 are charts showing states of magnetization in the recording process utilizing the medium shown in FIG. 27;
FIGS. 30 and 34 are charts showing the temperature characteristic of the coercive force of the magnetic layers in the medium shown in FIG. 27;
FIGS. 35 and 36 are schematic cross-sectional views of embodiments of the magnetooptical recording medium of the present invention utilizing quadraple-layered magnetic film;
FIG. 37 is a schematic view showing states of magnetization in the recording process utilizing the medium shown in FIG. 35;
FIG. 38 is a chart showing the temperature characteristic of the coercive force of the magnetic layers in the medium shown in FIG. 35;
FIGS. 39, 41, 42 and 43 are charts showing states of magnetization in the recording and erasing processes utilizing the medium shown in FIG. 3;
FIG. 40 is a chart showing the relation between the erasing laser power and the residual signal after erasure in the medium shown in FIG. 3;
FIG. 44 is a chart showing the relation between the recording laser power and the reproduced C/N ratio in the medium shown in FIG. 3; and
FIGS. 45 and 46 are schematic views showing variations of the magnetooptical recording apparatus utilizing the medium of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Now the present invention will be clarified in detail by embodiments thereof shown in the attached drawings.
FIG. 3 is a schematic cross-sectional view of an embodiment of the magnetooptical recording medium of the present invention. Said medium is composed of a translucent substrate 1 provided with guide grooves in advance (called "pregrooved"), and a a first magnetic layer 2 and a second magnetic layer 3 laminated thereon. The first magnetic layer 2 has a lower Curie point (T.sub.L) and a higher coercive force (H.sub.H), while the second magnetic layer 3 has a higher Curie point (T.sub.H) and a lower coercive force (H.sub.L). The terms "higher" and "lower" are defined through relative comparison of the two magnetic layers, the comparison of the coercive force being at the room temperature. This comparison is more detailedly illustrated in FIG. 4. It is generally desirable that the first magnetic layer 2 has T.sub.L in a range of 70.degree.-180.degree. C. and H.sub.H in a range of 3-10 KOe, and the second magnetic layer 3 has T.sub.H in a range of 150.degree.-400.degree. C. and H.sub.L in a range of 0.5-2 KOe.
The thickness, coercive force, saturated magnetization and magnetic wall energy of said magnetic layers 2, 3 are so selected that the two states of the finally recorded bit can exist in stable manner. The magnetic layers 2, 3 may be exchange-coupled or magnetostatically coupled although exchange-coupling is preferable in consideration of the magnitude of the effective bias magnetic field at the recording and the stability of the recorded binary bit.
In the magnetooptical recording medium of the present invention, the first magnetic layer 2 is principally related to the reproduction. The magnetooptical effect exhibited by said first magnetic layer 2 is principally utilized in the reproduction, while the second magnetic layer 3 plays an important role in the recording.
On the other hand, in the conventional exchange-coupled double-layered magnetic film mentioned above, the magnetic layer with a lower Curie point and a higher coercive force is principally related to the recording, and the magnetic layer with a higher Curie point and a lower coercive force is principally related to the reproduction. In such conventional exchange-coupled double-layered film, there stands a relationship: ##EQU2## among the saturation magnetization M.sub.s of the latter magnetic layer, film thickness h and magnetic wall energy .sigma..sub.w between two layers.
On the other hand, in the exchange-coupled double-layered film of the recording medium of the present invention, there is required a relation: ##EQU3## among the saturation magnetization M.sub.s of the second magnetic layer, film thickness h and magnetic wall energy .sigma..sub.w between two layers.
This condition (detailed later) is required for stabilizing the state of magnetization of the bit finally formed by recording, as shown by 4f in FIG. 5. Consequently the effective bias magnetic field, thickness, coercive force, saturation magnetization, magnetic wall energy etc. of the magnetic layers 2, 3 can be so determined as to satisfy the above-mentioned relation.
Each magnetic layer can be composed of a substance exhibiting a vertical magnetic anisotropy and a magnetooptical effect, preferably an amorphous magnetic alloy of a rare-earth element and a transition metal element such as GdCo, GdFe, TbFe, DyFe, GdTbFe, TbDyFe, GdFeCo, TbFeCo, GdTbCo or GdTbFeCo.
The following explanation of a recording process utilizing the above-explained magnetooptical recording medium, makes reference to FIG. 5, shows the states of magnetization of the magnetic layers 2, 3 in the steps of the recording process, while FIG. 6 schematically shows a recording apparatus. Prior to recording, the stable directions of magnetization of the magnetic layers 2, 3 may be mutually same or opposite. FIG. 5 shows a case in which said stable directions of magnetization are mutually same.
In FIG. 6, it is assumed that a part of a magnetooptical disk 9 of the above-explained structure has an initial magnetization as shown by 4a in FIG. 5. The magnetooptical disk 9, being rotated by a spindle motor, passes the position of a magnetic field generating unit 8, generating a magnetic field of which intensity is selected at a suitable value between the coercive forces of the magnetic layers 2, 3 (magnetic field being directed upwards in the present embodiment), whereby, as shown by 4b in FIG. 5, the second magnetic layer 3 is uniformly magnetized while the first magnetic layer 2 retains the initial magnetization state.
The rotated magnetooptical disk 9, in passing the position of a record/reproducing head 5, is irradiated by a laser beam having two power levels according to the signal from a recording signal generator 6. The first laser power level is enough for heating the disk to a temperature close to the Curie point of the first magnetic layer 2, while the second laser power level is enough for heating the disk to a temperature close to the Curie point of the second magnetic layer 3. More specifically, referring to FIG. 4 showing the relation between the temperature and the coercive forces of the magnetic layers 2, 3, the first laser power can heat the disk close to T.sub.L while the second laser power can heat it close to T.sub.H.
The first laser power heats the first magnetic layer 2 close to the Curie point thereof, but the second magnetic layer 3 has a coercive force capable of stably maintaining the bit at this temperature. Thus, through a suitable selection of the recording bias magnetic field, a record bit shown in 4c can be obtained, as a first preliminary recording, from either state in 4b in FIG. 5.
The suitable selection of the bias magnetic field means that in the first preliminary recording, such bias magnetic field is essentially unnecessary, since the first magnetic layer receives a force (exchange force) to arrange the magnetization in a direction stable to the direction of magnetization of the second magnetic layer 3, said directions being same in this case. However, said bias magnetic field is provided, in a second preliminary recording to be explained later, in a direction to assist the magnetic inversion of the second magnetic layer 3, namely in a direction prohibiting the first preliminary recording, and it is convenient to maintain said bias magnetic field in a same intensity and a same direction, both in the first and second preliminary recordings.
In consideration of the foregoing, the bias magnetic field is preferably selected at a minimum necessary intensity required for the second preliminary recording to be explained in the following, and such selection corresponds to the suitable selection mentioned above.
In the following there will be explained the second preliminary recording. This is achieved by heating the disk with the second laser power close to the Curie point of the second magnetic layer 3, whereby the direction of magnetization of the second magnetic layer 3 is inverted by the bias magnetic field selected as explained above, and the direction of magnetization of the first magnetic layer 2 is also arranged in a stable direction (same direction in the present case) with respect to the second magnetic layer 3. In this manner a bit as shown by 4d in FIG. 5 can be formed from either state shown in 4b.
Thus each area of the magnetooptical disk can have a preliminary record of the state 4c or 4d in FIG. 5, respectively by the first or second laser power corresponding to the input signal.
Then the magnetooptical disk 9 is further rotated and passes the position of the magnetic field generating unit 8, generating a magnetic field of which intensity is selected between the coercive forces of the magnetic layers 2, 3 as explained before, whereby the record bit 4c remains unchanged and assumes a final record state 4e, while the record bit 4d assumes another final record state 4f as the result of magnetic inversion of the second magnetic layer 3.
In order that the record 4f can stably exist, there is required the aforementioned relationship (1): ##EQU4## among the saturation magnetization M.sub.s of the second magnetic layer 3, film thickness h and magnetic wall energy .sigma..sub.w between the magnetic layers 2, 3. .sigma..sub.w /2M.sub.s h indicates the magnitude of the exchange force received by the second magnetic layer, or represents the magnitude of a magnetic field acting to rearrange the magnetization of the second magnetic layer 3 in a stable direction (same direction in the present case) with respect to the direction of magnetization of the first magnetic layer 2. Therefore, in order that the second magnetic layer 3 can retain its magnetization unchanged against said magnetic field, said layer should have a coercive force H.sub.L larger than the magnitude of said magnetic field (H.sub.L >.sigma..sub.w /2M.sub.s h).
Stated differently, in order that the bit can stably exist, following relations are required among the coercive forces H.sub.H, H.sub.L of the first and second magnetic layers and the effective bias magnetic fields H.sub.Heff, H.sub.Leff of said layers:
These relations will be explained in more detail in relation to FIGS. 7A to 7D. FIG. 7A is a chart showing the B-H loop, or the relation between the external magnetic field, in abscissa, applied to the first magnetic layer formed as a single layer, and the magnitude of the magnetization in said layer in ordinate. The chart indicates that, when the magnetic field is intensified in a direction of (+), the magnetization is aligned in a direction (+) or a direction (.uparw.) at an intensity H.sub.H, and, when the magnetic field is intensified in a direction (-), the magnetization is aligned in a direction (-) or (.dwnarw.) at an intensity -H.sub.H. FIG. 7B shows a similar B-H loop for the second magnetic layer formed as a single layer.
FIG. 7C shows a B-H loop of the first magnetic layer when the first and second magnetic layers are superposed with exchange-coupling and when said second magnetic layer is magnetized upward. In contrast to the case of single layer shown in FIG.
7A, an effective bias magnetic field H.sub.Heff is applied, facilitating to align the magnetization of the first magnetic layer with that of the second magnetic layer.
In order that the record bit 4f shown in FIG. 5 can stably exist, the state of a point A, where the direction of magnetization of the first magnetic layer is opposite to that of the second magnetic layer under a zero external field should be stable and should not transform to the state of a point B wherein said directions of magnetization of the magnetic layers are mutually same. For this reason there is required a condition H.sub.H -H.sub.Heff >0.
FIG. 7D shows a similar B-H loop of the second magnetic layer when the first and second magnetic layers are superposed with exchange-coupling and when said first magnetic layer is magnetized upwards.
In contrast to the case of single layer shown in FIG. 7B, an effective bias magnetic field H.sub.Leff is applied, facilitating to align the magnetization of the second magnetic layer with that of the first magnetic layer. In order that the record bit 4f in FIG. 5 can stably exist, the state of a point A, where the direction of magnetization of the first magnetic field is opposite to that of the second magnetic field under a zero external field should be stable and should not transform to the state of a point B wherein said directions are mutually same. For this reason there is required a condition H.sub.L -H.sub.Leff >0.
Either in the first or second magnetic layer, an inversion of magnetization from a stable state to an unstable state requires a magnetic field equal to the coercive force of the magnetic layer plus the exchange force, since such inversion has to be made against the exchange force acting on said layer.
On the other hand, an inversion from an unstable state to a stable state requires a magnetic field equal to the coercive force of the magnetic layer minus the exchange force, since the exchange force facilitates the inversion in this case.
Therefore, in order that the magnetization of the first magnetic layer is not inverted in the magnetic field generating unit 8 and the magnetization of the second magnetic layer is aligned to the direction of the magnetic field of said unit in any combination of the magnetized states, the external field B should be adjusted to an internal level if there stands a relation:
This is because the magnetic field required for inverting the magnetization of the second magnetic layer is larger than H.sub.L +H.sub.Leff when the first and second magnetic layers are in a stable state, and because the magnetic field not inducing the inversion of magnetization of the first magnetic layer should be smaller than H.sub.H -H.sub.Heff when the first and second magnetic layers are in a stable state.
An overwriting operation is therefore rendered possible, since the record bits 4e, 4f do not rely on the state prior to recording but only on the level of laser power at the recording. The record bits 4e, 4f can be reproduced by irradiation with a reproducing laser beam and processing with a signal reproducing unit 7.
FIG. 8 is a schematic view showing a more detailed embodiment of the recording apparatus shown in FIG. 6. In FIG. 8 there are shown a magnetooptical disk or recording medium 11 of a structure as shown in FIG. 3; a spindle motor 12 for rotating said disk 11; a clamper 13 for fixing the disk 11 on the rotating shaft of the motor 12; and an optical head 14 for projecting a light beam 15 onto the disk 11. Said optical head 14 is provided with a laser light source 16 composed for example of a semiconductor laser; a collimating lens 17; a beam splitter 18; an objective lens 19; a sensor lens 20; an analyzer 26 and a photodetector 21, and is radially moved by an unrepresented mechanism. Also the objective lens 19 moves in the axial direction and a direction perpendicular thereto to achieve so-called auto tracking (AT) and auto focusing (AF), according to control signals detected by the photodetector in an already known manner. The laser light source 16 is driven by a laser driver circuit 22
and emits a light beam 15 modulated in intensity between two non-zero values, according to the recording information entered from an input terminal 23, as will be explained later.
In a position opposed to the optical head 14 across the disk 11, there is provided first magnetic field generating means 24 to apply a bias magnetic field of a predetermined direction to an area of the disk 11 irradiated by the light beam 15. Also at a position distance by 180.degree. in the rotating direction of the disk 11, there is provided second magnetic field generating means 25 for applying a bias magnetic field of a direction opposite to said predetermined direction. Said first and second magnetic field generating means may be composed of electromagnets, but the use of permanent magnets is preferable for simplifying the apparatus and reducing the cost thereof, since the direction of magnetic fields need not be switched in the present invention.
In the following explained is the process of information recording with the apparatus shown in FIG. 8. The light beam 15 from the laser unit 16 is modulated, as shown in FIG. 9, between two non-zero levels P1 and P2, corresponding to binary recording signals "0" and "1". The light beam of the level P1 has an energy for heating the magnetic layers 2, 3 of the medium shown in FIG. 3 to the Curie temperature T.sub.L of the first magnetic layer, while that of the level P2 has an energy for heating said magnetic layers to the Curie temperature T.sub.H of the second magnetic layer. By the irradiation with such modulated beam, the magnetic layers of the disk 11 undergo changes in magnetization as shown in 10a-10h in FIG. 10A, thereby recording information.
In FIG. 10A, arrows in the first and second magnetic layers 2, 3 indicate the directions of magnetization, and the length of arrow indicates the magnitude of coercive force. An upward magnetized area, shown in 10a, is subjected to an upward bias magnetic field of -B2 by the second magnetic field generating means 25 as shown in 10b, and is then moved to the position of the optical head 14 by the rotation of the disk for irradiation with the modulated light beam 15. In response to the light beam of the power P1, the first magnetic layer 2 alone reduces its coercive force as shown in 10c but retains the upward magnetization as shown in 10d due to the magnetic interaction of the second magnetic layer 3, despite of the application of a bias magnetic field B1 by the first magnetic field generating means 24. On the other hand, in response to the light beam of the power P2, the magnetization is inverted downwards as shown in 10h by the application of the bias field B1.
On the other hand, in a downward magnetized area as shown in 10e, the second magnetic layer 3 alone inverts the magnetization by the application of a bias magnetic field -B2. Then, in response to the light beam of the level P1, the first magnetic layer 2 reduces the coercive force of the first magnetic layer 2 as shown in 10c, and inverts the magnetization upwards as shown in 10d by the magnetic interaction of the second magnetic layer 3. Also in response to the light beam of the power P2, both magnetic layer reduce the coercive force as shown in 10g, and retain the downward magnetization as shown in 10h by the application of the bias magnetic field B1.
As explained in the foregoing, the apparatus shown in FIG. 8 can determine the direction of magnetization solely according to the change in the power of the light beam, regardless of the initial magnetization of the magnetic film. Consequently the already recorded information need not be erased but can be rewritten by direct overwriting. The recorded information can be reproduced in conventional manner, by detecting the direction of magnetization of the first magnetic layer utilizing the magnetooptical effect. As an example, in the apparatus shown in FIG. 8, the recorded information can be reproduced as electrical signals, by causing the laser unit 16 to continuously emit a light beam of a power insufficient for heating the disk to the Curie point of the second magnetic layer and receiving the reflected light from the disk 11 by the photodetector 21 through the analyzer 26.
In the apparatus shown in FIG. 8, the magnetic fields B1, B2 applied to the magnetic layers respectively by the first and second magnetic field generating means are so selected as to satisfy the following relations:
wherein H.sub.H ' and H.sub.L ' are coercive forces of the first and second magnetic layers at a temperature T.sub.L, H.sub.H " and H.sub.L " are coercive forces at a temperature T.sub.H, and H.sub.o is the magnitude of the magnetic interaction between two layers.
In the foregoing description it is assumed the first and second magnetic layers 2, 3 are stable when the magnetizations thereof are in a same direction, but a similar process is applicable also when said magnetizations are stable when they are mutually oppositely directed. FIG. 10B illustrates states of magnetization in the recording process of such case, wherein 40a to 40f respectively correspond to 4a to 4f in FIG. 5.
EXAMPLE 1
A polycarbonate substrate with pregrooves and preformat signals was set in a sputtering apparatus with three targets, and was rotated at a distance of 10 cm from the target.
A ZnS protective layer of 1000 .ANG. in thickness was obtained by sputtering from a first target in argon gas, with a sputtering speed of 100 .ANG./min., and a sputtering pressure of 5.times.10.sup.-3 Torr. Then a TbFe alloy was sputtered from a second target, in argon gas, with a sputtering speed of 100.ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr to obtain a first magnetic layer of Tb.sub.18 Fe.sub.82 with a thickness of 500 .ANG., T.sub.L of about 140.degree. C. and H.sub.H of about 10 KOe.
Then a TbFeCo alloy was sputtered in argon gas at a sputtering pressure of 5.times.10.sup.-3 Torr to obtain a second magnetic layer of Tb.sub.23 Fe.sub.60 Co.sub.17 with a thickness of 500 .ANG., T.sub.H of ca. 250.degree. C. and H.sub.L of ca. 1 KOe.
Subsequently a ZnS protective layer of 3000 .ANG. in thickness was obtained by sputtering from the first target in argon gas, with a sputtering speed of 100 .ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr.
After forming these layers, the above-mentioned substrate was adhered to a polycarbonate substrate with hot-melt adhesive material to obtain a magnetooptical disk. Said disk wa mounted on a record/reproducing apparatus and was made to pass through, with a linear speed of 8 m/sec., the second magnetic field generating means for applying a field of 1500 Oe to the disk. The recording was conducted with a laser beam of a wavelength of 830 nm, concentrated to ca. 1 .mu.m and modulated in two levels of 4 and 8 mW, with a duty ratio of 50% and a frequency of 2 MHz. The bias magnetic field was 100 Oe. Binary signals could be reproduced by irradiation with a laser beam of 1.5 mW.
The above-explained experiment was conducted on a magnetooptical disk already recorded over the entire surface. The previously recorded signal components were not detected in the reproduction, and the possibility of overwriting was thus confirmed.
EXAMPLE 2
A magnetooptical disk was prepared in the same manner as in the Example 1, except that the second magnetic layer was composed of Tb.sub.23 Fe.sub.70 Co.sub.7 with T.sub.H =200.degree. C., H.sub.L =ca. 1 KOe and H.sub.Leff =ca. 300 Oe, and was subjected to recording and reproduction in the same manner as in the Example 1 except the use of a magnetic field generating field of ca. 2.5 KOe to obtain similar results as those in the Example 1.
The magnitude of H.sub.Heff in this case will be explained in the following in relation to FIGS. 7C and 7D.
Since H.sub.H -H.sub.Heff was larger than H.sub.L -H.sub.Leff, when the first and second magnetic layers were magnetized in a same direction and subjected then to an inverting magnetic field, the magnetization of the second magnetic layer was inverted at H.sub.L -H.sub.Leff =0.7 KOe so that H.sub.Leff could not be measured.
However, based on the condition H.sub.H -H.sub.Heff >H.sub.L -H.sub.Leff, a conclusion 4.3 KOe>H.sub.Heff from the conditions H.sub.H =5 KOe, H.sub.L =1 KOe and H.sub.Leff =0.3 KOe.
Also the H.sub.Heff was measured as ca. 1 KOe in an experiment in which a first magnetic TbFe layer of same composition and thickness was superposed with a second magnetic TbFeCo layer with a modified composition to increase H.sub.L -H.sub.Leff.
It was confirmed, for the above-mentioned coercive forces and exchange force, that the first and second magnetic layers satisfied the condition:
by which the second magnetic layer alone is magnetized in the direction of the magnetic field from the magnetic field generating unit 24.
EXAMPLE 3
A polycarbonate disk-shaped substrate with pregrooves and preformat signals was set in a sputtering apparatus with three targets, and was rotated at a distance of 10 cm from the target.
A protective SiC layer of 700 .ANG. in thickness was obtained by sputtering from a first target in argon gas, with a sputtering speed of 100 .ANG./min. and a sputtering pressure of 3.times.10.sup.-3 Torr. Then a GdTbFe alloy was sputtered from a second target, in argon gas, with a sputtering speed of 50 .ANG./min. and a sputtering pressure of 3.times.10.sup.-3 Torr to obtain a first magnetic layer of Tb.sub.8 Gd.sub.12 Fe.sub.80 with a thickness 200 .ANG., T.sub.L =ca. 160.degree. C. and H.sub.H =ca. 8 KOe.
Then a TbFeCo Cu alloy was sputtered in argon gas at a sputtering pressure of 3.times.10.sup.-3 Torr to obtain a second magnetic layer of Tb.sub.23 Fe.sub.50 Co.sub.15 Cu.sub.12 with a thickness of 400 .ANG., T.sub.H =ca. 180.degree. C. and H.sub.L =ca. 1 KOe.
Subsequently a Si.sub.3 N.sub.4 protective layer of 1200 .ANG. in thickness was obtained by sputtering from the first target in argon gas, with a sputtering speed of 70 .ANG./min. and a sputtering pressure of 3.times.10.sup.-3 Torr.
After forming these layers, the above-mentioned substrate was adhered to a polycarbonate plate with hot-melt adhesive material to obtain a magnetooptical disk. Said disk was mounted on a record/reproducing apparatus and was made to pass through, a with a linear speed of 8 m/sec., the second magnetic field generating means for applying a field of 1500 Oe to the disk. The recording was then conducted with a laser beam of a wavelength of 830 nm, concentrated to ca. 1 .mu.m and modulated in two levels of 4 and 8 mW, with a duty ratio of 50% and a frequency of 2 MHz. The bias field at the irradiated area was 100 Oe. Binary signals could be reproduced by irradiation with a laser beam of 1.5 mW.
The above-explained experiment was conducted on a magnetooptical disk already recorded over the entire surface. The previously recorded signal components were not detected in the reproduction, and the possibility of overwriting was thus confirmed.
In case the apparatus shown in FIG. 6 employs a permanent magnet as the magnetic field generating unit 8, the magnetooptical recording medium of the present invention is constantly exposed, in the recording and reproducing operations, to a magnetic field generated by said magnetic field generating unit. Even in the reproduction, the medium is subjected to a laser beam irradiation of an energy of ca. 1/3 to 1/10 of the energy at the recording, and may therefore reach a temperature of about 70.degree. C. at maximum in passing the magnetic field generating unit. Thus the magnetization of the first magnetic layer 2 may become inverted in repeated reproductions, for example of 10.sup.10 times. In order to avoid such inversion of magnetization, it is preferable, as shown in the following examples, to select the coercive force H.sub.H of the first magnetic layer and the magnetic field B applied by the magnetic field generating unit in such a manner as to satisfy a relation
0.2.times.H.sub.H -0.3>B (KOe), and, more prefearbly a condition H.sub.H >1.5 KOe. A further preferred condition is H.sub.H >5 KOe as will be explained in the following.
EXAMPLE 4
Samples of magnetooptical disks 4-1 to 4-12 were prepared in the same process as in the Example 1, with the same film thickness and the same structure except that the composition, coercive force H.sub.H and Curie point T.sub.L of the first magnetic layer 2 were changed.
Each disk of Examples 1 and 4-1 to 4-12 was mounted on a record/reproducing apparatus and was made to pass through, with a linear speed of 8 m/sec., the second magnetic field generating means for applying a field of 2 KOe. The recording was conducted with a laser beam of a wavelength of 830 nm, concentrated to ca. 1 .mu.m and modulated in two levels of 4 and 8 mW, with a duty ratio of 50% and a frequency of 2 MHz. The bias magnetic field at the recording head was 100 Oe. Then investigated was the change of the noise component in the reproduced signal, after 10.sup.10 reproductions from a same track by irradiation with a laser beam of 1.0 mW and with varied magnetic field B generated by the magnetic field generating unit 8.
Subsequently the temperature inside the apparatus was set at 30.degree., 45.degree. and 60.degree. C. and the intensity of magnetic field causing the increase of noise component in the reproduced signal was determined for each temperature, as summarized in Tab. 1.
TABLE 1 __________________________________________________________________________ B value (KOe) B value (KOe) B value (KOe) causing noise causing noise causing noise lst mag. H.sub.H T.sub.L increase at increase at increase at Example layer (KOe) (.degree.C.) 30.degree. C. 45.degree. C. 60.degree. C. __________________________________________________________________________ 1 Tb.sub.18 Fe.sub.82 10 140 8 6 3.2 4-1 Tb.sub.17 Fe.sub.83 8 140 6 4.7 2.6 4-2 Tb.sub.16
Fe.sub.84 6 140 4 3.1 1.5 4-3 Tb.sub.15 Fe.sub.85 4 140 2.2 1.5 0.5 4-4 Tb.sub.14.5 Fe.sub.85.5 3 140 1.5 1.0 0.3 4-5 Tb.sub.14 Fe.sub.86 2 140 1.2 0.5 0.1 4-6 Tb.sub.10 Gd.sub.7 Fe.sub.83 7 150 4.8 3.7 2.2 4-7 Tb.sub.10 Gd.sub.6 Fe.sub.84 5 150 3.3 2.3 1.0 4-8 Tb.sub.10 Gd.sub.5 Fe.sub.85 3.5 150 2.8 1.2 0.4 4-9 Tb.sub.10 Gd.sub.7 Fe.sub.80 Co.sub.3 7.5 165 5.4 4.3 2.5 4-10 Tb.sub.10 Gd.sub.7 Fe.sub.80 Co.sub.4 5.5 170 3.5 2.7 1.3 4-11 Tb.sub.15 Fe.sub.81 Co.sub. 4 4.5 160
2.5 1.8 0.6 4-12 Tb.sub.14.5 Fe.sub.82 Co.sub.3.5 3.5 160 1.8 1.2 0.4 __________________________________________________________________________
Tab. 1 indicates that the magnetic field B inducing the increase of noise component in the reproduced signal becomes smaller as the coercive force H.sub.H of the first magnetic layer becomes smaller, or as the temperature inside the apparatus becomes higher, and that this relationship is not affected by the composition of the first magnetic layer.
FIG. 11 illustrates the relation of H.sub.H value and the B value inducing the noise increase for these samples, based on the above-explained results.
From these relationships it is apparent that, in order to prevent the noise increase up to 60.degree. C., H.sub.H and B should at least satisfy a relation: 0.2.times.H.sub.H -0.3>B.
It is also apparent that the increase in noise takes placed even at a small value of B unless H.sub.H is at least equal to 1.5 KOe.
Since the temperature inside the apparatus does not exceed 60.degree. C. in practice, the increase in noise can be prevented even after prolonged reproduction if the above-mentioned conditions are satisfied.
However, in addition to the foregoing conditions, it is preferable to maintain H.sub.H at least equal to 5 KOe for the following two reasons:
(i) The minimum necessary value of B is equal to the sum of the coercive force H.sub.L of the second magnetic layer and the magnetic field acting on the second magnetic layer as the result of the exchange force, and is considered in the order of
1 to 2 KOe in practice. Thus, it will be apparent from FIG. 11 that the value of H.sub.H should be equal to or higher than 5 KOe in order to maintain the noises at a low level in continuous reproduction at a temperature up to 60.degree. C. and with the magnetic field B at 1-2 KOe.
(ii) FIG. 11 indicates the magnitude of the magnetic field B and the value of H.sub.H required for suppressing the noise at the selected value of B at each temperature inside the apparatus. For example, at a temperature 60.degree. C. and in a range H.sub.H <5 KOe, there is obtained a relation H.sub.H =5 (B+0.3). Thus, for an increase .DELTA.B of the magnetic field B generated by the magnetic field generating unit, there is required a corresponding increase 5.times..DELTA.B in the value of H.sub.H.
However, in a range of H.sub.H exceeding 5 KOe, the increase required for H.sub.H corresponding to a certain increase in the magnetic field B at a temperature of 30.degree.-60.degree. C. is smaller than that required in a range of H.sub.H below
5 KOe.
As explained in the foregoing, the magnetooptical recording medium of the present invention is required to satisfy the aforementioned relation (1): ##EQU5## For this purpose it is also very effective to adjust the magnetic wall energy between the magnetic layers. The adjustment of the magnetic wall energy can be achieved by following methods:
(I) adjustments of the composition of the magnetic layers;
(II) addition of a predetermined step in the preparation of the medium; and
(III) formation of an adjusting layer for the magnetic wall energy, between the magnetic layers;
which will be explained further in the following.
I. Adjustment of the composition of the magnetic layers
In a magnetooptical recording medium of the structure as shown in FIG. 3, the magnetic wall energy can be reproducibly reduced by forming one of the first and second magnetic layer 2, 3 with a composition richer in transition metals compared with the compensation composition, and forming the other layer with a composition richer in rare-earth elements. Such example is shown in the following.
EXAMPLE 5
A disk-shaped polycarbonate substrate with pregrooves and preformat signals was set in a sputtering apparatus with three targets, and was rotated at a distance of 10 cm from the target.
A protective Si.sub.3 N.sub.4 layer of 600.ANG. in thickness was obtained by sputtering from a first target in argon gas, with a sputtering speed of 40.ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr. Then a TbFe alloy was sputtered from a second target, in argon gas, with a sputtering speed of 100.ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr to obtain a first magnetic layer of Tb.sub.3 Gd.sub.16 Fe.sub.81, richer in Fe with respect to the compensation composition, with a thickness of 400 .ANG., T.sub.L =ca. 155.degree. C. and H.sub.H =ca. 8 KOe.
Then a TbFeCo alloy was sputtered in argon at a sputtering pressure of 5.times.10.sup.-3 Torr to obtain a second magnetic layer of Tb.sub.10 Dy.sub.13 Fe.sub.60 Co.sub.17, richer in Tb and Dy with respect to the compensation composition, with a thickness of 300 .ANG., T.sub.H =ca. 200.degree. C. and H.sub.L =ca. 1 KOe.
Subsequently a Si.sub.3 N.sub.4 protective layer of 1500 .ANG. in thickness was obtained by sputtering from the first target in argon gas, with a sputtering speed of 40 .ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr.
After forming these layers, the above-mentioned substrate was adhered to a polycarbonate plate with hot-melt adhesive material to obtain a magnetooptical disk. Said disk was mounted on a record/reproducing apparatus and was made to pass through, with a linear speed of 8 m/sec., a magnetic field generating unit for applying a field of 2.5 KOe. The recording was conducted with a laser beam of a wavelength of 830 nm, concentrated to ca. 1 .mu.m and modulated in two levels of 4 and 8 mW, with a duty ratio of 50% and a frequency of 2 MHz. The bias magnetic field at the irradiated area was 100 Oe, in a direction to invert the magnetization of the second magnetic layer. Binary signals could be reproduced by irradiation with a laser beam of 1.5
mW.
The above-explained experiment was conducted also on a magnetooptical disk already recorded over the entire surface. The previously recorded signal components were not detected in the reproduction, and the possibility of overwriting was thus confirmed.
EXAMPLE 6
In the Example 5, the first magnetic layer was richer in Fe while the second magnetic layer was richer in Tb and Dy in comparison with the compensation composition.
In the present example, there were prepared and evaluated samples of the magnetooptical disk, in which the first and second magnetic layers had same coercive forces as explained above, are composed of a combination of a composition richer in transition metals such as Fe and a composition richer in rare earth elements such as Tb or Dy.
A coercive force of 8 KOe for the first magnetic layer is achieved, in a transition metal-rich composition, by Tb.sub.3 Gd.sub.16 Fe.sub.81, or, in a coercive composition, by Tb.sub.3.3 Gd.sub.17.7 Fe.sub.77. Also a coercive force of 1 KOe for the second magnetic layer is achieved, in a transition metal-rich composition by Tb.sub.7.4 Dy.sub.9.6 Fe.sub.64.7 Co.sub.18.3, or, in a rare earth-rich composition, by Tb.sub.10 Dy.sub.13 Fe.sub.60 Co.sub.17.
Samples listed in Tab. 2 were prepared by selecting a transition metal-rich composition or a rare earth-rich composition mentioned above for the first and second magnetic layer, and selecting other materials and layer thicknesses same as those in the Example 5. These samples were then subjected to the record/reproducing experiment as in the Example 5. The results are shown in Tab. 2.
"TM" and "RE" respectively show a composition richer in the transition metals and a composition richer in the rare earth elements, compared to the compensation composition.
TABLE 2 __________________________________________________________________________ Evaluation of record bits 1st mag. layer 2nd mag. layer bit 4e in bit 4f in Example (TM or RE rich) (TM or RE rich) FIG. 5 FIG. 5 __________________________________________________________________________ 5 Tb.sub.3 Gd.sub.16 Fe.sub.81 (TM) Tb.sub.10 Dy.sub.13 Fe.sub.60 Co.sub.17 +RE) + 6-1 Tb.sub.3.3 Gd.sub.17.7 Fe.sub.79 (RE) Tb.sub.7.4 Dy.sub.9.6 Fe.sub.64.7 Co.sub.18.3 +TM) + 6-2 Tb.sub.3 Gd.sub.16 Fe.sub.81 (TM) Tb.sub.7.4 Dy.sub.9.6 Fe.sub.64.7 Co.sub.18.3 -TM) .+-. 6-3 Tb.sub.3.3 Gd.sub.17.7 Fe.sub.79 (RE) Tb.sub.10 Dy.sub.13 Fe.sub.60 Co.sub.17 -RE) .+-. __________________________________________________________________________
"+" indicates that the record bits are stable in the absence of external magnetic field and provide satisfactory reproduction signals; ".+-." indicates that the record bits are partially inverted or the reproduction signals are of insufficient quality; and "-" indicates that the record bits are unstable.
These results indicate that stable record bits are obtained only when one of the first and second magnetic layers is composed of the transition metal-rich composition and the other is composed of the rare earth-rich composition.
II. Addition of a predetermined step in the preparation of the medium
In the preparation of the magnetooptical recording medium as shown in FIG. 3, a medium satisfying the afore-mentioned relation (1) can be easily obtained by adding one of following steps after the formation of the first magnetic layer and before the formation of the second magnetic layer:
(A) a step of standing in an atmosphere of remaining gas or inert gas at 7.times.10.sup.-7 Torr for 5 minutes of longer;
(B) a step of standing in an atmosphere with a partial pressure, at least equal to 2.times.10.sup.-6 Torr, of a substance capable of reacting with a constituent element of the first or second magnetic layer or being chemically absorbed by said element; or
(C) a step of exposure to a plasma atmosphere of inert gas or a substance capable of reacting with a constituent element of the first or second magnetic layer or being chemically absorbed by said element.
The first and second magnetic layers can be formed by sputtering, or evaporation for example with electron beam heating.
Examples of the above-mentioned remaining gas are H.sub.2 O, O.sub.2, H.sub.2, N.sub.2 and low-molecular compounds consisting of C, H, N and O, and examples of the inert gas are Ar, He and Ne.
Examples of the gas capable of reacting with the constituent element of the first or second magnetic layer or being chemically absorbed by said element are H.sub.2 O, O.sub.2, H.sub.2, N.sub.2, H.sub.2 S, CS.sub.2 and CH.sub.4.
In the usual manufacturing process for the medium, after the formation of the first magnetic layer, the formation of the second magnetic layer is conducted immediately (for example within 1 minute) in a clean high-vacuum atmosphere. However the addition of one of the steps (A)-(C) modifies the exchange force, coercive force or stability of the magnetic layers, and a reproducible recording characteristic can be obtained through precise control of the process conditions, process time etc.
The effects of the steps (A)-(C) will be verified in the following examples.
EXAMPLE 7
A pregrooved and proformatted polycarbonate disk substrate was set in a sputtering apparatus with three targets, and was rotated at a distance of 10 cm from the target.
The apparatus was evacuated to 1.times.10.sup.-7 Torr, and a protective SiO layer of 1000 .ANG. in thickness was obtained by sputtering from a first target in argon gas, with a sputtering speed of 100 .ANG./min. and a sputtering pressure of
5.times.10.sup.-3 Torr. Then a TbFe alloy was sputtered from a second target, in argon gas, with a sputtering speed of 100 .ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr to obtain a first magnetic layer composed of Tb.sub.18 Fe.sub.82
with a thickness of 300.ANG., T.sub.L =ca. 140.degree. C. and H.sub.H =ca. 10 KOe. After the completion of sputtering, argon gas supply was continued for 30 minutes, with a pressure of 5.times.10.sup.-3 Torr in the sputtering chamber.
Then a TbFeCo alloy was sputtered in argon gas with a sputtering pressure of 5.times.10.sup.-3 Torr to obtain a second magnetic layer composed of Tb.sub.23 Fe.sub.70 Co.sub.7 with a thickness of 400.ANG., T.sub.H =ca. 200.degree. C. and H.sub.L =ca. 1 KOe.
Subsequently a SiO protective layer of 2000.ANG. in thickness was formed by sputtering from the first target in argon gas, with a sputtering speed of 100 .ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr.
After forming these layer, the above-mentioned substrate was adhered to a polycarbonate plate with hot-melt adhesive material to obtain a magnetooptical disk. Said disk was mounted on a record/reproducing apparatus and was made to pass through, with a linear speed of 8 m/sec., a unit for generating a magnetic field of 2.5 KOe. The recording was conducted with a laser beam of a wavelength of 830 nm, concentrated to ca. 1 .mu.m and modulated in two levels of 4 and 8 mW, with a duty ratio of 50% and a frequency of 2 MHz. The bias magnetic field was 100 Oe. Binary signals could be reproduced by irradiation of a laser beam of 1.5 mW.
The above-explained experiment was repeated also on a magnetooptical disk already recorded over the entire surface. The previously recorded signal components were not detected in the reproduction, so that the possibility of overwriting was thus comfirmed.
EXAMPLE 8 AND REFERENCE EXAMPLE
Samples of the magnetooptical disk were prepared in a process similar to that of the Example 7 but with varied conditions (atmosphere and pressure) in the step between the formations of the first and second magnetic layers, as listed in Tab. 3. Those marked with * are examples of the present invention, and others are reference examples.
In examples 8-30 to 8-33, a disk electrode of 20 cm in diameter was placed at 5 cm from the polycarbonate substrate, and a plasma treatment was conducted with a discharge power of 50 W, in the presence of various gasses listed in Tab. 3 at a pressure of 5.times.10.sup.-3 Torr in the sputtering chamber. In examples 8-5 to 8-14, the main valve of the vacuum pump was suitably closed to vary the remaining gas atmosphere.
Each sample was evaluated for the stability of the record bits 4f, shown in FIG. 5, in the absence of external magnetic field, through the measurement of an external magnetic field inducing the inversion of magnetization in the magnetic layers. "+" and "-" respectively indicate that the record bits are stable or unstable.
Also each samples was tested for recording and reproduction in the same manner as in the Example 7. "+" and "-" respectively indicate that the recording was satisfactorily or unsatisfactorily made.
TABLE 3 ______________________________________ Sample evaluation Vacuum Time Stability Record Example Atmosphere (Torr) (min) of bits 4f state ______________________________________ *Ex. 7 argon gas 5 .times. 10.sup.-3 30 + + 8-1 " "
1/4 - - *8-2 " " 2 - - *8-3 " " 5 + + 8-4 " " 15 + + 8-5 remaining gas 1 .times. 10.sup.-6 1/4 - - 8-6 " " 2 - - *8-7 " " 5 + + *8-8 " " 15 + + *8-9 " " 30 + + 8-10 remaining gas 3 .times. 10.sup.-6 1/4 - - 8-11 " " 2 - - *8-12 " " 5 + + *8-13 " " 15 + + *8-14 " " 30 + + *8-15 oxygen gas 3 .times. 10.sup.-6 1/2 + + *8-16 " " 2 + + *8-17 " " 5 + + *8-18 " " 15 + + *8-19 " " 30 + + *8-20 nitrogen gas 3 .times. 10.sup.-6 1/2 + + *8-21 " " 2 + + *8-22 " " 5 + + *8-23 " " 15 + + *8-24 nitrogen gas 3 .times. 10.sup.-6 30 + + *8-25 hydrogen gas 3 .times. 10.sup.-6 1/2 - - *8-26 " " 2 + + *8-27 " " 5 + + *8-28 " " 15 + + *8-29 " " 30 + + *8-30 argon plasma 3 .times. 10.sup.-3 1/12 + + *8-31 oxygen plasma " 1/12 + + *8-32 nitrogen plasma " 1/12 + + *8-33 hydrogen " 1/12 + + plasma ______________________________________
Results of the Example 7, Examples 8 and Reference Examples indicate that the recording by overwriting can be satisfactorily achieved by the magnetooptical recording medium prepared employing either one of the steps (A) to (C).
III. Formation of a magnetic wall energy adjusting layer between the magnetic layers
Formation of an adjusting layer 41, as shown in FIG. 12, between the first and second magnetic layers 2, 3 allows to arbitrarily regulate the magnetic wall energy therebetween, thereby obtaining a medium satisfying the aforementioned condition (1). More practically there may be employed a structure shown in FIG. 13, using a pregrooved substrate 1 and provided with protective layers 42, 43. In FIGS. 12 and 13, same components as those in FIG. 3 are represented by same numbers and will not be explained further.
Said adjusting layer 41 may be composed of a material not deteriorating the magnetic layers, for example Ti, Cr, Al, Ni, Fe, Co, rare earth element or a fluoride thereof.
The thickness of said adjusting layer 41 is suitably selected in consideration of the materials and thicknesses of the first and second magnetic layers, but is generally selected within a range of 5 to 50 .ANG..
EXAMPLE 9
A pregrooved and preformatted polycarbonate disk substrate was set in a sputtering apparatus with ternary targets, and was rotated at a distance of 10 cm from the target.
A ZnS protective layer of 1000 .ANG. in thickness was formed by sputtering from a first target in argon gas, with a sputtering speed of 100 .ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr. Then a TbFe alloy was sputtered from a second target, in argon gas, with a sputtering speed of 100 .ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr to obtain a first magnetic layer composed of Tb.sub.18 Fe.sub.82 with a thickness of 500 .ANG., T.sub.L =ca. 140.degree. C. and H.sub.H =ca. 5 KOe.
Then a Co adjusting layer was formed with a thickness of 10 .ANG., by sputtering in argon gas with a sputtering pressure of 5.times.10.sup.-3 Torr. Then TbFe and Co were simultaneously sputtered from second and third targets in argon gas with a sputtering pressure of 5.times.10.sup.-3 Torr to obtain a second magnetic layer composed of Tb.sub.15 Fe.sub.68 Co.sub.17 with a thickness of ca. 200 .ANG., T.sub.H =ca. 250.degree. C. and H.sub.L =ca. 2 KOe.
Subsequently a ZnS protective layer of 3000 .ANG. in thickness was formed by sputtering from the first target in argon gas, with a sputtering speed of 100 .ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr.
After forming these layers, the above-mentioned substrate was adhered to a polycarbonate plate with hot-melt adhesive material to obtain a magnetooptical disk. Said disk was mounted on a record/reproducing apparatus and was made to pass through, with a linear speed of 8 m/sec., a unit for generating a magnetic field of 2.5 KOe. The recording was then conducted with a laser beam of a wavelength of 830 nm, concentrated to ca. 1 .mu.m and modulated in two levels of 4 and 8 mW, with a duty ratio of 50% and a frequency of 2 MHz. The bias magnetic field was 100 Oe. Binary signals could be reproduced by irradiation with a laser beam of 1.5 mW.
The above-explained experiment was repeated with a magnetooptical disk already recorded over the entire surface. The previously recorded signal components were not detected in the reproduction, so that the possibility of overwriting was thus confirmed.
The adjusting layer was composed of Co in the foregoing embodiment, but it may also be composed of a magnetic material of which easy direction of magnetization is positioned longitudinally along the disk surface at room temperature but vertically to the disk surface at the recording temperature. The use of such material reduces the magnetic wall energy between the magnetic layers at room temperature and provides a larger exchange force to the magnetic layers at recording, thereby providing a magnetooptical recording medium enabling overwrite recording with a smaller bias magnetic field and superior in the stability of the record bits. Such structure will be shown in the following examples.
A sample for exchange force measurement was prepared by sputtering, on a slide glass, a first magnetic layer of Tb.sub.18 Fe.sub.82 of 500 .ANG. in thickness, then an adjusting layer of Fe or Tb.sub.25 Fe.sub.70 Co.sub.5 in various thicknesses, and a second magnetic layer of Tb.sub.22 Fe.sub.70 Co.sub.8 of 500 .ANG. in thickness. The first magnetic layer showed a coercive force of 12 KOe, with prevailing Fe sub-lattice magnetization, while the second magnetic layer showed a coercive force of
6 KOe, with prevailing Tb sub-lattice magnetization.
Each sample was subjected to the measurement of the external magnetic field inducing the inversion of magnetization of the first and second magnetic layers, in a VSM and in the presence of an external magnetic field. In a decreasing magnetic field, the samples showed an inversion of magnetization of the second magnetic layer into a stable (opposite) direction with respect to that of the first magnetic layer. The exchange force applied to the second magnetic layer was determined from such inversion-inducing magnetic field, as shown in FIG. 14, which indicates the exchange force on the second magnetic layer in ordinate and the thickness of the adjusting layer (Fe or TbFeCo) in abscissa.
As will be apparent from this chart, the exchange force is annulated even at a layer thickness of 70 .ANG. with a Fe layer having a longitudinal easy direction of magnetization. On the other hand, in case of the Tb.sub.25 Fe.sub.70 Co.sub.5
with a coercive force of ca. 300 Oe and with a vertical easy direction of magnetization, parallel to those of the first and second magnetic layer, the exchange force is still effective even at a layer thickness of 500 .ANG..
Therefore, the stability of record bits and the stable recording characteristic can be both obtained by forming, between the first and second magnetic layers, an adjusting layer of a material of which easy direction of magnetization is longitudinal at room temperature but is vertical at the recording temperature.
Temperature-dependent change of easy direction of magnetization is already known in substances showing spin rearrangement. For example, DyCo.sub.5, reported by M. Ohkoshi and H. Kobayashi in Physica, 86-88B (1977), p. 195-196, exhibits a change of the easy direction of magnetization from longitudinal to vertical in a temperature range of 50.degree.-100.degree. C. Similar results are known in compounds in which Dy is replaced by another rare earth element such as Nd, Pr or Tb, or in which Co is replaced by another transition metal such as Fe or Ni. Also Tsushima reported, in Oyo Buturi, 45, 10 (1976), p. 962-967, the spin rearrangement in rare earth orthoferrites and rare earth orthochromites. A suitable modification of the composition of these substances allows to achieve a change of the easy direction of magnetization from longitudinal to vertical state in the recording temperature range.
Also it is already known that a thin magnetic film has to satisfy a condition:
in order to have a magnetization vertical to the film surface, wherein M.sub.s is the saturated magnetization and H.sub.k is the uniaxial anisotropic magnetic field in said vertical direction.
Therefore, in order that the adjusting layer has the easy magnetization axis in the longitudinal direction at room temperature and in the vertical direction at the recording temperature range, it is desirable to select the Curie point of said adjusting layer in the vicinity of said recording temperature. Since M.sub.s shows a rapid decrease in the vicinity of the Curie point, a substance showing a relation H.sub.k <4.pi.M.sub.s at room temperature may show a relation H.sub.k .gtoreq.4.pi.M.sub.s in the recording temperature range. As the component vertical to the substrate surface increases by the magnetization of the adjusting layer, said magnetization is further oriented vertical to the substrate surface by the exchange forces from the first and second magnetic layers. The exchange forces H.sub.eff(1-2) and H.sub.eff(2-3) working on the adjusting layer respectively from the first and second magnetic layers can be represented by:
wherein M.sub.s2 is the saturation magnetization of the adjusting layer, h.sub.2 is the thickness thereof, and .sigma..sub.w12 .sigma..sub.w23 are magnetic wall energies respectively between the first magnetic layer and the adjusting layer and between the second magnetic layer and the adjusting layer.
Therefore, in order to orient the magnetization of the adjusting layer in a direction vertical to the film surface in the recording temperature range by means of said exchange forces H.sub.eff(1-2) and H.sub.eff(2-3), it is advantageous to select a small saturation magnetization M.sub.s and a thickness h.sub.2, as long as the easy direction of magnetization remains longitudinal at room temperature.
EXAMPLE 10
A pregrooved and preformatted polycarbonate disk substrate was placed in a sputtering apparatus with quaternary targets, and was rotated at a distance of 10 cm from the targets.
A Si protective layer of 500 .ANG. in thickness was sputtered from a first target, in argon gas, with a sputtering speed of 100 .ANG./min., and a sputtering pressure of 5.times.10.sup.-3 Torr. Then a GdTbFe alloy was sputtered from a second target, in argon gas, with a sputtering speed of 100 .ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr to obtain a first magnetic layer composed of Tb.sub.12 Gd.sub.10 Fe.sub.78 with a thickness of 300 .ANG., T.sub.L =ca. 150.degree. C. and H.sub.H =ca. 8 KOe. Fe was prevailing in the sub-lattice magnetization of the first magnetic layer.
Then a TbFeCo alloy was sputtered from a third target, in argon gas, with a sputtering speed of 100 .ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr to obtain an adjusting layer, composed of Tb.sub.35 Fe.sub.60 Co.sub.5 having a thickness of 200 .ANG. and a coercive force of almost zero at the Curie temperature of ca. 170.degree. C. The easy direction of magnetization of said adjusting layer was neither longitudinal nor vertical, and the external magnetic field required for orienting the magnetization in either direction was ca. 2.5 KOe.
Then a TbFeCo alloy was sputtered from a fourth target, in argon gas, with a sputtering speed of 100 .ANG./min and a sputtering pressure of 5.times.10.sup.-3 Torr to obtain a second magnetic layer composed of Tb.sub.24 Fe.sub.68 Co.sub.8 with a thickness of 300 .ANG., T.sub.H =ca. 180.degree. C. and H.sub.L =ca. 1.5 KOe. Tb was prevailing in the sub-lattice magnetization of said second magnetic layer.
Subsequently a Si protective layer of 1000 .ANG. in thickness was formed by sputtering from the first target, in argon gas, with a sputtering speed of 100 .ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr.
After forming these layers, the above-mentioned substrate was adhered to a polycarbonate plate with hot-melt adhesive material to obtain a magnetooptical disk.
The effective bias magnetic field, caused by the exchange force on the second magnetic layer, was almost zero in thus prepared magnetooptical disk, when measured in the same manner as in FIG. 14.
Said magnetooptical disk was mounted on a record/reproducing apparatus and was made to pass through, with a linear speed of ca. 8 m/sec., a unit generating a magnetic field of 2.5 KOe. The recording operation was then conducted with a laser beam of a wavelength of 830 nm, concentrated to ca. 1 .mu.m and modulated in two levels of 4 and 8 mW, with a duty ratio of 50% and a frequency of 2 MHz. The bias magnetic field was 100 Oe. Binary signals could be reproduced by irradiation with a laser beam of 1.5 mW.
The above-explained experiment was repeated with a magnetooptical disk already recorded over the entire surface. The previously recorded signal components were not detected in the reproduction, so that the possibility of overwriting was confirmed.
EXAMPLE 11
Samples of the magnetooptical disk were prepared with the same process and materials as in the Example 10, except that the material and thickness of the adjusting layer were varied.
The prepared samples were subjected to the measurements of the effective bias field caused by the exchange force on the second magnetic layer and the recording characteristic in the same manner as in the Example 10. The results are summarized in Tab. 4.
TABLE 4 __________________________________________________________________________ Effective bias Recording Adjusting Thickness of field on 2nd character- Example layer adjusting la. layer istic __________________________________________________________________________ 10 Tb.sub.35 Fe.sub.60 Co.sub.5 200 .ANG. 0 Oe Good 11-1 None -- 2000 Oe First-type record not possible 11-2 Fe.sub.70 Cr.sub.30 40 .ANG. 200 Oe Good 11-3 DyCo.sub.5
200 .ANG. 0 Oe Good 11-4 Sm.sub.0.7 Fe.sub.0.3 FeO.sub.3 150 .ANG. 300 Oe Good 11-5 Si 40 .ANG. 250 Oe First-type record sensitivity low, bit error rate high 11-6 Si 60 .ANG. 150 Oe First-type record not possible __________________________________________________________________________
The sample 11-1 was not provided with the adjusting layer. In this case stable record bits could not be formed because the effective bias magnetic field, required for orienting the magnetization of the second magnetic layer in a direction stable with respect to that of the first magnetic layer, was larger than the coercive force of the second magnetic layer.
The Fe.sub.70 Cr.sub.30 film employed in the sample 11-2 has a Curie point lower than 200.degree. C. and a longitudinal easy axis of magnetization. With a thickness not exceeding 30 .ANG., the magnetization was oriented in the vertical direction by the exchange forces from the first and second magnetic layers. Therefore the effective bias field on the second magnetic layer became larger and stable recording could not be achieved. On the other hand, with a thickness of 100 .ANG. or larger, vertical magnetization was not induced in the recording temperature range due to the excessively large saturation magnetization, so that the recording was not made due to the absence of the exchange force.
DyCo.sub.5 (magnetic transition point 50.degree.-80.degree. C.) and Sm.sub.0.7 Er.sub.0.3 FeO.sub.3 (magnetic transition point ca. 110.degree. C.) employed in the samples 11-3 and 11-4 provided satisfactory recording in a thickness range of
100-400 .ANG.. This result is based on a change of the easy axis of magnetization from the longitudinal direction to the vertical direction in the recording temperature range (50.degree.-150.degree. C.), despite of the fact that the effective bias field on the second magnetic layer is almost zero at room temperature.
Si employed in the samples 11-5 and 11-6 is not magnetic. With a thickness of 40-60 .ANG., the Si layer hinders the exchange coupling between the first and second magnetic layers, so that the measured effective bias field on the second magnetic layer was as small as 250-150 Oe. This effective bias field decreased with the rise of temperature, so that the first-type recording, in which the magnetization of the first magnetic layer is arranged in a direction stabler with respect to the magnetization of the second magnetic layer against the bias field was not possible at the recording temperature range.
FIG. 15 shows the effective bias fields on the first or second magnetic layer in ordinate, as a function of temperature in abscissa, measured on the sample of the Example 10.
The first magnetic layer receives no effective bias field up to 80.degree. C., but receives, from 90.degree. C., a bias field for orienting the magnetization of said first magnetic layer in a stable direction with respect to the second magnetic layer. Said bias field monotonously decreased above 90.degree. C. to reach zero at the Curie point of the first magnetic layer. The second magnetic layer received no bias field over the entire temperature range measured. These results coincide with the satisfactory recording characteristic of the magnetooptical disk of the Example 10.
Then a sample was prepared by sputtering an adjusting layer of Tb.sub.35 Fe.sub.60 Co.sub.5, employed in the Example 10, with a thickness of 1000 .ANG. on a slide glass, and by forming a Si.sub.3 N.sub.4 protective layer of 1000 .ANG. in thickness thereon. FIG. 16 shows the external magnetic field required to orient the magnetization of said Tb.sub.35 Fe.sub.60 Co.sub.5 layer into the vertical direction, in ordinate, as a function of temperature in abscissa, measured on the above-mentioned sample. The magnitude of said required external field decreases with the rise of temperature, and becomes about 500 Oe in the temperature range of 80.degree.-90.degree. C. where the first magnetic layer starts to receive a large bias field. In the sample of the Example 10, it is assumed that the magnetization of the adjusting layer is oriented, with the rise of temperature, in the vertical direction at the interface between the first magnetic layer and the adjusting layer owing to the exchange force at said interface, and the magnetization of the adjusting layer in the vicinity of the interface with the second magnetic layer is also oriented in the vertical direction in a temperature range of 80.degree.-90.degree. C., so that a large effective bias field emerges in this state between the first and second magnetic layers through the adjusting layer.
The above-mentioned adjusting layer, composed of a rare earth-transition metal alloy, can be optimized in composition, in consideration of the following factors:
(i) The rare earth-transition metal alloy shows a magnetic anisotropy in the vertical direction, when the rare earth element represents a range 12-28 atomic % in the rare earth and transition metal elements. Outside said range, the easy axis of magnetization is in the longitudinal direction or in the direction of surface, possibly because of following two reasons. Firstly, the condition H.sub.k .gtoreq.4.pi.M.sub.s for realizing a vertical magnetization cannot be satisfied, because of the large saturation magnetization M.sub.s, wherein H.sub.k being the vertical anisotropic magnetic field. Secondly, the vertical magnetic anisotropy in a rare earth-transition metal alloy film is caused by the coupling of the rare earth element and the transition metal element. The magnetic moment of the rare earth-transition metal element pair has a high probability of orientation in the vertical direction only in the above-mentioned percentage.
(ii) The rare earth-transition metal alloy employed in the adjusting layer, if rich in the rare earth element compared with the compensation composition, is increased in the rare earth content from a composition showing vertical magnetic anisotropy thereby increasing the saturation magnetization and facilitating the magnetization in the longitudinal direction. If the alloy is rich in the transition metal element compared with the compensation composition, the transition metal is increased further from the composition showing vertical magnetic anisotropy, thereby increasing the saturation magnetization and facilitating the magnetization in the longitudinal direction. If the curie point of the material is selected in the vicinity of the recording temperature, the saturation magnetization decreases in the vicinity of the recording temperature, thereby satisfying the condition H.sub.k .gtoreq.4.pi.M.sub.s for vertical magnetization. In this manner it is possible to orient the easy axis of magnetization in the surface direction at room temperature and in the vertical direction at the recording temperature.
The change of the easy axis of magnetization of the adjusting layer from the surface or longitudinal direction to the vertical direction was experimentally confirmed in the following manner.
Three samples were prepared by sputtering, on glass substrates, magnetic layers of Fe, Tb.sub.5 Gd.sub.5 Fe.sub.90 or Tb.sub.16 Gd.sub.16 Fe.sub.68 of a thickness of 500 .ANG. as an adjusting layer, under an argon pressure of 5.times.10.sup.-3
Torr. On each sample a second magnetic layer of Tb.sub.24 Fe.sub.74 Co.sub.6 of a thickness of 500 .ANG. was formed without breaking the vacuum, and a Si.sub.3 N.sub.4 protective layer of 700 .ANG. in thickness was formed thereon.
Each sample was subjected to the measurement of the external magnetic field required to orient the magnetization of the adjusting layer into the vertical direction, as a function of temperature.
Fe, Tb.sub.5 Gd.sub.5 Fe.sub.90 or Tb.sub.16 Gd.sub.16 Fe.sub.68 employed as the adjusting layer did not have a vertical easy axis of magnetization at room temperature.
FIG. 17 shows the external magnetic field required for vertical orientation in ordinate, as a function of the temperature in abscissa, obtained in said measurement.
The adjusting layer composed of Fe did not show orientation of the magnetization into the vertical direction, since the decrease of saturation magnetization is still small at 160.degree. C. The Tb.sub.5 Gd.sub.5 Fe.sub.90 and Tb.sub.16 Gd.sub.16
Fe.sub.68, both being rare earth-transition metal alloy and having the easy direction of magnetization also in the surface direction, show a significant decrease in magnetization because the Curie point is in a range of 100.degree.-200.degree. C. Thus the magnetization can be oriented in the vertical direction with a limited external magnetic field when heated to about 100.degree. C. Particularly Tb.sub.16 Gd.sub.16 Fe.sub.68, which is richer in the rare earth element compared to the compensation composition, shows easier orientation of the magnetization in the vertical direction with a smaller external field at a higher temperature, in comparison with Tb.sub.5 Gd.sub.5 Fe.sub.90 which is richer in the transition metal element.
Besides the required external field becomes smaller, in the temperature range of 70.degree.-80.degree. C., than the coercive force of the second magnetic layer. A measurement for identifying whether the external magnetic field required for inverting the magnetization of the adjusting layer is dependent on the direction of magnetization of the second magnetic layer, namely whether an exchange force exists between the adjusting layer and the second magnetic layer, clarified that no exchange force was present at room temperature but a bias field caused by an exchange force of ca. 200 Oe was present at 90.degree. and 110.degree. C.
Tb.sub.16 Gd.sub.16 Fe.sub.68, richer in the rare earth element than the compensation composition, shows an enhanced orientation of the easy direction of magnetization in the vertical direction at higher temperatures because of the following two reasons.
Firstly, it is empirically known that, in an exchange-coupled combination of the adjusting layer and the second magnetic layer, a stronger exchange force is obtained in a combination in which both layers are rich in the rare earth element or in the transition metal than in a combination in which one layer is rich in the rare earth element while the other is rich in the transition metal. Thus the adjusting layer is earily oriented in the vertical direction, since the second magnetic layer is a magnetic film vertically oriented to the film surface.
Secondly, the rare earth elements have lower Curie points in isolated state. Thus, in the rare earth-transition metal alloys, the rare earth element contributes more significantly to the decrease of magnetization at higher temperatures, if the composition is rich in the rare earth element. For this reason the compensation temperature is present above room temperature.
A composition not showing vertical easy direction of magnetization because of the excessively large magnetization of the rare earth element shows a decrease of magnetization of said rare earth element at higher temperatures, so that the magnetization is represented by the magnetizations of the rare earth element and the transition metal which originally have vertical magnetic anisotropy.
EXAMPLE 12
A pregrooved and preformatted polycarbonate disk substrate was placed in a sputtering apparatus with quaternary targets, and was rotated at a distance of 10 cm from the targets
A Si.sub.3 N.sub.4 protective layer of 700 .ANG. in thickness was formed by sputtering from a first target, in argon gas, with a sputtering speed of 100 .ANG./min , and a sputtering pressure of 5.times.10.sup.-3 Torr. Then a TbDyFeCo alloy was sputtered from a second target, in argon gas, with a sputtering speed of 100 .ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr to obtain a first magnetic layer of Tb.sub.15 Dy.sub.5 Fe.sub.76 Co.sub.4 with a thickness of 300 .ANG., T.sub.L =ca. 150.degree. C. and H.sub.H =ca. 10 KOe. Fe and Co atoms were prevailing in the sub-lattice magnetization of the first magnetic layer Then a TbGd Fe alloy was sputtered from a third target, in argon gas, with a sputtering speed of 100 .ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr to form an adjusting layer of Tb.sub.16 Gd.sub.16 Fe.sub.68 with a thickness of 200 .ANG., and Curie point of ca 160.degree. C. Said adjusting layer did not show a vertical easy direction of magnetization at room temperature, and the external magnetic field required for orienting the magnetization into the vertical direction was ca. 2 KOe at room temperature. Then a TbGdFeCo alloy was sputtered from a fourth target, in argon gas, with a sputtering speed of 100 .ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr to obtain a second magnetic layer of Tb.sub.20 Gd.sub.5 Fe.sub.67 Co.sub.8 with a thickness of 300 .ANG., T.sub.H =ca. 190.degree. C. and H.sub.L =ca. 1.8 KOe. Tb and Gd were prevailing in the sub-lattice magnetization of said second magnetic layer.
Subsequently a Si.sub.3 N.sub.4 protective layer of 800 .ANG. in thickness was formed by sputtering from the first target, in argon gas, with a sputtering speed of 100 .ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr.
After these layer formations, the above-mentioned substrate was adhered to a polycarbonate plate with hot-melt adhesive material to obtain a magnetooptical disk.
Then there was made a measurement, with VSM (vibrating magnetization measurer), of the