GB2071696A - Magneto-optical Recording Medium - Google Patents

Magneto-optical Recording Medium Download PDF

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Publication number
GB2071696A
GB2071696A GB8107897A GB8107897A GB2071696A GB 2071696 A GB2071696 A GB 2071696A GB 8107897 A GB8107897 A GB 8107897A GB 8107897 A GB8107897 A GB 8107897A GB 2071696 A GB2071696 A GB 2071696A
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GB
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Patent type
Prior art keywords
thin film
gd
tb
fe
recording medium
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
GB8107897A
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GB2071696B (en )
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Kokusai Denshin Denwa KK
Original Assignee
Kokusai Denshin Denwa KK
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
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Classifications

    • GPHYSICS
    • G11INFORMATION STORAGE
    • G11BINFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B11/00Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor
    • G11B11/10Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor using recording by magnetic means or other means for magnetisation or demagnetisation of a record carrier, e.g. light induced spin magnetisation; Demagnetisation by thermal or stress means in the presence or not of an orienting magnetic field
    • G11B11/105Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor using recording by magnetic means or other means for magnetisation or demagnetisation of a record carrier, e.g. light induced spin magnetisation; Demagnetisation by thermal or stress means in the presence or not of an orienting magnetic field using a beam of light or a magnetic field for recording by change of magnetisation and a beam of light for reproducing, i.e. magneto-optical, e.g. light-induced thermo-magnetic recording, spin magnetisation recording, Kerr or Faraday effect reproducing
    • G11B11/10582Record carriers characterised by the selection of the material or by the structure or form
    • G11B11/10586Record carriers characterised by the selection of the material or by the structure or form characterised by the selection of the material
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C45/00Amorphous alloys
    • C22C45/02Amorphous alloys with iron as the major constituent
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01FMAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F10/00Thin magnetic films, e.g. of one-domain structure
    • H01F10/08Thin magnetic films, e.g. of one-domain structure characterised by magnetic layers
    • H01F10/10Thin magnetic films, e.g. of one-domain structure characterised by magnetic layers characterised by the composition
    • H01F10/12Thin magnetic films, e.g. of one-domain structure characterised by magnetic layers characterised by the composition being metals or alloys
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01FMAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F10/00Thin magnetic films, e.g. of one-domain structure
    • H01F10/08Thin magnetic films, e.g. of one-domain structure characterised by magnetic layers
    • H01F10/10Thin magnetic films, e.g. of one-domain structure characterised by magnetic layers characterised by the composition
    • H01F10/12Thin magnetic films, e.g. of one-domain structure characterised by magnetic layers characterised by the composition being metals or alloys
    • H01F10/13Amorphous metallic alloys, e.g. glassy metals
    • H01F10/133Amorphous metallic alloys, e.g. glassy metals containing rare earth metals
    • H01F10/135Amorphous metallic alloys, e.g. glassy metals containing rare earth metals containing transition metals
    • H01F10/136Amorphous metallic alloys, e.g. glassy metals containing rare earth metals containing transition metals containing iron

Abstract

A magneto-optical recording medium of an amorphous Gd-Tb-Fe ternary alloy thin film having a magnetic easy axis perpendicular to the film surface. The atomic ratio of a combination of Gd and Tb is in the range of 15 to 35 atom%. This recording medium provides both for efficient recording and optical reproduction.

Description

SPECIFICATION Magneto-optical Recording Medium The present invention relates to a magnetooptical recording medium for use as for example, a magneto-optical memory or a magnetic recording display element. In particular, the invention pertains to a magnetic thin film recording medium which has a magnetic easy axis perpendicular to the film surface and permits information to be recorded thereon by forming an inverted magnetic domain of a circular or arbitrary configuration and the information to be read out by utilizing a magneto-optical effect such as the magnetic Kerr effect.

In a ferromagnetrc thin film which has a magnetic easy axis in a direction perpendicular to the film surface, it is possible to provide a small inverted magnetic domain which has a reverse magnetic moment from a uniform magnetization in the film surface which is uniformly magnetized with the south ot north pole magnetic field. By representing the presence and absence of the inverted magnetic domain by "1" and "0", such a ferromagnetic thin film can be used as a highdensity magnetic recording medium.On ferromagnetic thin film having such a property, a thin film of the type which has a large coercive force at room temperature and whose Curie point or magnetic compensation temperature is relatively close to room temperature permits recording of information thereon by forming the inverted magnetic domain at a desired position with a light beam by utilization of the Curie point or the magnetic compensation temperature.

Accordingly, such a thin film is now generally used as a beam addressable file memory.

Known ferromagnetic thin films which have a magnetic easy axis perpendicular to the film surface and can be used as beam addressable files include polycrystalline metal films represented by a Mn-Bi thin film, amorphous metal films as of Gd-Co, Gd-Fe, Tb-Fe, Dy Fe and so forth and compound single crystal thin films represented by GdlG. These ferromagnetic thin films respectively have such merits and demerits as referred to below.

The polycrystalline metal thin films in which information is written by making use of the Curie point, represented by the Mn-Bi thin film, are excellent as magnetic recording media in that they have as large a coercive force as several Kilo Oerstreds at room temperature, but they are defective in that to write in information calls for a large energy because of high Curie point (in case of the Mn8Bi thin film, Tc=3600C). Further, since they are polycrystalline, it is necessary to make a thin film of a stoichiometrical composition and the fabrication of such a thin film is technically difficult.The amorphous metal thin films of Gd Co and Gd-Fe in which information is written through utilization of the magnetic compensation point have advantages in that since they are amorphous, they can be formed on an arbitrary substrate and that the magnetic compensation temperature can be freely controlled to some extent by adding a small amount of impurity. But these amorphous metal thin films are defective in that the coercive force at room temperature is small (300 to 500 Oe), and that recorded information is unstable. In addition, even for the formation of thin films of such a small coercive force, it is necessary to control their composition to within about 1 atom %; accordingly, these films are also not favourable in terms of manufacture.

The compound single crystal thin films represented by GdlG have a serious defect in that they are very costly as compared with the other thin films. An amorphous Tb-Fe or Dy-Fe alloy thin film containing 15 to 30 atom % of Tb or Dy, which has been proposed as a new magnetic thin film recording medium free from the above said defects, has the following merits: (1) Since this amorphous alloy thin film has a magnetic easy axis perpendicular to the film surface and as large a coercive force as several Kilo-Oestreds at room temperature, high-density recording of information is possible and the recorded information is very stable.

(2) The amorphous alloy thin film has a large coercive force and permits writing therein a magnetic domain of a desired configuration.

(3) The amorphous alloy thin film has a large coercive force over a wide composition range, and the composition range which provides excellent properties for a magnetic medium is also wide.

Accordingly, no severe limitations are imposed on the thin film composition and the thin film is very easy to manufacture and good in terms of yield.

(4) Since the Curie point is as low as 1 200C in a case of the Tb-Fe thin film and 600C in a case of the Dy-Fe thin film, thermal write in utilizing the Curie point can be achieved with a small amount of energy.

However, the amorphous Tb-Fe and Dy-Fe alloy thin films have the following shortcomings.

With low Curie point, the energy for writing information may be small but the S/N ratio of the information read out by light is low.

An object of the present invention is to provide a magnetic thin film recording medium which is free from the defects of the prior art magnetic thin film magneto-optical recording media having a magnetic easy axis perpendicular to the film surface and from which a light reproduced output can be obtained efficiently.

In accordance with the present invention, there is provided a magneto-optical recording medium of an amorphous Gd-Tb-Fe ternary alloy thin film having a magnetic easy axis perpendicular to the film surface, characterized in that the atomic ratio of a combination of Gd and Tb is in the range of 1 5 to 35 atoms%.

Embodiments of the present invention will hereinafter be described by way of example and by comparison with the prior arts with reference to the accompanying drawings, in which: Fig. 1 shows the light reproducing characteristics of known amorphous alloy thin films; Fig. 2 shows light reproducing characteristics of a magnetic recording medium of the present invention and the known amorphous alloy thin films; and Fig. 3 shows waveforms of reproduced outputs of a prior art amorphous Gd-Fe thin film and a Gd-Te-Fe thin film of the present invention.

In Fig. 1 S (signal) and the S/N ratio for read out of an amorphous alloy thin film by light are shown as functions of the power of a laser used for irradiating the thin film. It appears from Fig. 1 that in terms of optical reproduction the Tb-Fe and Dy-Fe thin films which have good properties as recording media are inferior to the Gd-Fe thin film which is not an excellent recording medium.

This is a serious defect when the recording medium is used as a magneto-optical memory.

The magneto-optical recording medium of the present invention is an amorphous Gd-Tb-Fe alloy thin film which has a magnetic easy axis perpendicular to the film surface and a Curie point in the range of 120 to 2000C.

For providing sufficient magnetic anisotropy to orient magnetization in a direction perpendicular to the film surface, it is necessary to make the thin film amorphous and this condition is achieved by forming the thin film, by sputtering or vacuum evaporation, on a substrate held at a temperature below room temperature.Further, in order that the magnetization may be stably oriented in the direction perpendicular to the film surface, it is necessary that the film thickness be 1 00A or more and, in order to provide the Curie point in the range of 120 to 2000C as mentioned above, it is necessary that the Gd-Tb-Fe composition is (GdxTb,~x) Fe and restricted in the following range: 0.1 5 < YS0.35 0.001X11.00 The magneto-optical recording medium in this composition range is able to have a magnetic easy axis perpendicular to the film surface and permits very high-density recording.

Although the magneto-optical recording medium of the present invention has a relatively low Curie temperature of 120 to 2000C, it is characterized in that its light reproduced output utilizing the magnetic Kerr effect is larger than the light reproduced outputs obtainable with recording media of the Tb-Fe and Gd-Fe alloys which have substantially the same Curie temperature as the recording medium of the present invention.As seen from Fig. 2, however, the light reproduced output of the Gd-Tb-Fe recording medium, shown in comparison with the reproduction outputs of the Tb-Fe and Gd-Fe recording media, does not simply lie between the outputs of the Tb-Fe and Gd-Fe recording media but is far larger than either of them. In contrast, its magnetic properties, for example, its coercive force Hc and Curie temperature Tc are both imtermediate between those of the Gd-Fe and Tb-Fe recording media, as shown in Table 1 below.For instance, an amorphous alloy thin film of Gd.13 Tb.13Fe.74 has a coercive force Hc of 2 KOe and a Curie temperature Tc of 1 800C and permits the writing of information therein with substantially the same energy for heat recording on the Gd-Fe and TtHFe recording media and has the coercive force Hc of 2 KOe for retaining the recorded information, which is equal to the coercive forces of the TbFe and DyFe recording media Table 1 Composition Tc/ C HclOe Gd.2sFe.7s 1 90 70 Gd.17Tb.14Fe.7 1 90 400 Gd.10Tb.17Fe.73 183 780 Gd.13Tb.13Fe.74 1 80 2000 Tb.23Fe.77 120 2200 As has been described above, the magnetooptical recording medium of the present invention retains such features of the well-known amorphous Tb-Fe, Gd-Fe and Dy-Fe alloy thin films that it has a magnetic easy axis perpendicular to the film surface, a large coercive force at room temperature and a Curie point ciose to room temperature and is also easy to manufacture, and in addition, the magneto-optical recording medium of the present invention has the advantage that only its light reproduced output is larger than the output of any of the prior art recording media. Accordingly, by employing the magneto-optical recording medium of the present invention as a storage medium of a magneto-optical memory in which information is written by a light beam and from which the information is read out by making use of the Kerr effect, such as, for example, one which is called a beam addressable file memory, it is possible to realize an excellent memory device of high recording density and a good S/N ratio. The write means is not limited specifically to a light beam but may also be any means for supplying the energy necessary for producing an inverted magnetic domain, such as for example, a needle type magnetic head, a heat pen or an electronic beam.

Fig. 3 illustrates examples in which information was recorded by a magnetized needle on an amorphous Gd-Fe thin film and a Gd-Tb-Fe thin film of the invention and the information was read out therefrom utilizing the magnetic K-rr effect. The conditions for reading out the thin films were exactly the same, but the light reproduced output of the Gd-Tb-Fe thin film was very large as compared with the output of the Gd-Fe thin film. This indicates that the thin film of the present invention is excellent magneto optical recording medium.

The foregoing description has been given of the Gd-Tb-Fe ternary alloy film, but it is also effective to add a small amount of an impurity (for example, La, Y, Dy, Ho, Er, Bi, Cr, or Mo) for the purpose of further reducing the energy for recording.

Claims (5)

Claims
1. A magneto-optical recording medium of an amorphous Gd-Tb-Fe ternary alloy thin film having a magnetic easy axis perpendicular to the film surface, characterized in that the atomic ratio of a combination of Gd and Tb is in the range of 15 to 35 atom %.
2. A medium as claimed in Claim 1 wherein the amorphous Gd-Tb-Fe ternary alloy has a composition-(GdxTb1~x)yFe1-y in which 0.15 < Y < 0.35 and 0.001X11.00.
3. A medium as claimed in Claim 1 or 2 wherein the amorphous Gd-Tb-Fe ternary thin film contains a small amount of an impurity so as to reduce the energy required for recording.
4. A medium as claimed in Claim 3 wherein said impurity is selected from the group consisting of La, Y, Dy, Ho, Er, Bi, Cr and Mo.
5. A medium as claimed in any preceding claim wherein the thin film thickness is at least 1 0or.
GB8107897A 1980-03-12 1981-03-12 Magneto-optical recording medium Expired GB2071696B (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
JP3025180A JPS6032331B2 (en) 1980-03-12 1980-03-12

Publications (2)

Publication Number Publication Date
GB2071696A true true GB2071696A (en) 1981-09-23
GB2071696B GB2071696B (en) 1983-11-16

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Application Number Title Priority Date Filing Date
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JP (1) JPS6032331B2 (en)
GB (1) GB2071696B (en)
NL (1) NL8101143A (en)

Cited By (24)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE2913601A1 (en) * 1978-04-03 1979-10-04 Goodyear Tire & Rubber Method for determining the depth of a separation point in a mature
DE3309483A1 (en) * 1982-03-17 1983-09-29 Canon Kk Magneto-optical recording material
FR2543725A1 (en) * 1983-03-29 1984-10-05 Kokusai Denshin Denwa Co Ltd magneto-optical recording medium
EP0125536A2 (en) * 1983-05-11 1984-11-21 MOVID Information Technology, Inc. Thermo-magnetic recording materials supporting small stable domains
US4489139A (en) * 1981-01-14 1984-12-18 Sharp Kabushiki Kaisha Magneto-optic memory medium
US4569881A (en) * 1983-05-17 1986-02-11 Minnesota Mining And Manufacturing Company Multi-layer amorphous magneto optical recording medium
DE3604642A1 (en) * 1985-03-20 1986-10-02 Hitachi Ltd Magneto-optical recording medium
US4615944A (en) * 1983-05-17 1986-10-07 Minnesota Mining And Manufacturing Company Amorphous magneto optical recording medium
US4683176A (en) * 1983-08-15 1987-07-28 Nihonshinku Gijutsu Kabushiki Kaisha Optical magnetic recording member
US4693943A (en) * 1982-05-10 1987-09-15 Canon Kabushiki Kaisha Magnetooptical recording medium
US4721658A (en) * 1984-04-12 1988-01-26 Minnesota Mining And Manufacturing Company Amorphous magneto optical recording medium
US4734334A (en) * 1983-11-05 1988-03-29 Canon Kabushiki Kaisha Magneto-optical recording medium
US4799114A (en) * 1980-11-01 1989-01-17 Daidotokushuko Kabushiki Kaisha Thermomagnetic recording carrier and a method for thermomagnetic recording
US4822675A (en) * 1987-01-14 1989-04-18 Minnesota Mining And Manufacturing Company Stable magneto optic recording medium
US4833043A (en) * 1983-05-17 1989-05-23 Minnesota Mining And Manufacturing Company Amorphous magneto optical recording medium
US4838962A (en) * 1985-07-12 1989-06-13 Hitachi, Ltd. Magneto-optical recording medium
US4950556A (en) * 1987-10-26 1990-08-21 Minnesota Mining And Manufacturing Company Magneto-optic recording medium
US5019462A (en) * 1988-02-02 1991-05-28 Basf Aktiengesellschaft Sheet-like, multilayer magneto-optical recording material
DE3348423C2 (en) * 1982-05-10 1994-11-17 Canon Kk Use of an amorphous magnetic quaternary GdTbFeCo alloy for the manufacture of a magneto-optical recording layer
DE3348424C2 (en) * 1982-03-17 1996-06-27 Canon Kk Terbium:iron:cobalt alloy used as magneto:optic recording material
US5595805A (en) * 1991-07-08 1997-01-21 Sharp Kabushiki Kaisha Magneto-optical recording medium
US6001467A (en) * 1987-11-26 1999-12-14 Sharp Kabushiki Kaisha Magneto-optic memory medium
US6042954A (en) * 1996-04-22 2000-03-28 Sharp Kabushiki Kaisha Magneto-optical recording medium
US6139949A (en) * 1989-02-10 2000-10-31 Mitsubishi Denki Kabushiki Kaisha Magneto optical recording medium

Families Citing this family (14)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS6137765B2 (en) * 1980-04-16 1986-08-26 Fuji Photo Film Co Ltd
JPS5873030A (en) * 1981-10-27 1983-05-02 Kokusai Denshin Denwa Co Ltd <Kdd> Optical magnetic recording medium
JPH0234096B2 (en) * 1982-01-21 1990-08-01 Japan Broadcasting Corp
JPH0232690B2 (en) * 1982-05-10 1990-07-23 Canon Kk
JPH0410132B2 (en) * 1982-05-17 1992-02-24
JPS5917222A (en) * 1982-07-21 1984-01-28 Hitachi Ltd Manufacture of multilayer magnetic thin-film
JPH0118506B2 (en) * 1982-11-04 1989-04-06 Seiko Denshi Kogyo Kk
JPS59178628A (en) * 1983-03-30 1984-10-09 Canon Inc Information recording method
DE3773531D1 (en) * 1986-05-14 1991-11-14 Teijin Ltd Magneto-optical record carrier.
JP2680586B2 (en) * 1987-11-26 1997-11-19 シャープ株式会社 Magneto-optical storage medium
CA1333820C (en) * 1988-09-13 1995-01-03 Masahiko Sekiya Magneto-optical recording medium
CA2017284C (en) * 1989-07-04 1995-10-03 Kazutomi Suzuki Optical recording medium
JPH0746445B2 (en) * 1993-10-12 1995-05-17 ダイセル化学工業株式会社 And the magneto-optical recording disk, a method of manufacturing the same
US7076983B2 (en) 2001-03-16 2006-07-18 Nakayama Steel Works, Ltd. Apparatus and method for hot rolling

Cited By (28)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE2913601A1 (en) * 1978-04-03 1979-10-04 Goodyear Tire & Rubber Method for determining the depth of a separation point in a mature
US4799114A (en) * 1980-11-01 1989-01-17 Daidotokushuko Kabushiki Kaisha Thermomagnetic recording carrier and a method for thermomagnetic recording
US4489139A (en) * 1981-01-14 1984-12-18 Sharp Kabushiki Kaisha Magneto-optic memory medium
DE3309483C3 (en) * 1982-03-17 2003-02-27 Canon Kk A magneto-optical recording material and its use
DE3309483A1 (en) * 1982-03-17 1983-09-29 Canon Kk Magneto-optical recording material
DE3348424C2 (en) * 1982-03-17 1996-06-27 Canon Kk Terbium:iron:cobalt alloy used as magneto:optic recording material
DE3348423C2 (en) * 1982-05-10 1994-11-17 Canon Kk Use of an amorphous magnetic quaternary GdTbFeCo alloy for the manufacture of a magneto-optical recording layer
US4693943A (en) * 1982-05-10 1987-09-15 Canon Kabushiki Kaisha Magnetooptical recording medium
US5738950A (en) * 1982-05-10 1998-04-14 Canon Kabushiki Kaisha Magnetooptical recording medium
US4612068A (en) * 1983-03-29 1986-09-16 Kokusai Denshin Denwa Co., Ltd. Magneto-optical recording medium
FR2543725A1 (en) * 1983-03-29 1984-10-05 Kokusai Denshin Denwa Co Ltd magneto-optical recording medium
EP0125536A2 (en) * 1983-05-11 1984-11-21 MOVID Information Technology, Inc. Thermo-magnetic recording materials supporting small stable domains
EP0125536A3 (en) * 1983-05-11 1986-06-25 MOVID Information Technology, Inc. Thermo-magnetic recording materials supporting small stable domains
US4569881A (en) * 1983-05-17 1986-02-11 Minnesota Mining And Manufacturing Company Multi-layer amorphous magneto optical recording medium
US4833043A (en) * 1983-05-17 1989-05-23 Minnesota Mining And Manufacturing Company Amorphous magneto optical recording medium
US4615944A (en) * 1983-05-17 1986-10-07 Minnesota Mining And Manufacturing Company Amorphous magneto optical recording medium
US4683176A (en) * 1983-08-15 1987-07-28 Nihonshinku Gijutsu Kabushiki Kaisha Optical magnetic recording member
US4734334A (en) * 1983-11-05 1988-03-29 Canon Kabushiki Kaisha Magneto-optical recording medium
US4721658A (en) * 1984-04-12 1988-01-26 Minnesota Mining And Manufacturing Company Amorphous magneto optical recording medium
DE3604642A1 (en) * 1985-03-20 1986-10-02 Hitachi Ltd Magneto-optical recording medium
US4838962A (en) * 1985-07-12 1989-06-13 Hitachi, Ltd. Magneto-optical recording medium
US4822675A (en) * 1987-01-14 1989-04-18 Minnesota Mining And Manufacturing Company Stable magneto optic recording medium
US4950556A (en) * 1987-10-26 1990-08-21 Minnesota Mining And Manufacturing Company Magneto-optic recording medium
US6001467A (en) * 1987-11-26 1999-12-14 Sharp Kabushiki Kaisha Magneto-optic memory medium
US5019462A (en) * 1988-02-02 1991-05-28 Basf Aktiengesellschaft Sheet-like, multilayer magneto-optical recording material
US6139949A (en) * 1989-02-10 2000-10-31 Mitsubishi Denki Kabushiki Kaisha Magneto optical recording medium
US5595805A (en) * 1991-07-08 1997-01-21 Sharp Kabushiki Kaisha Magneto-optical recording medium
US6042954A (en) * 1996-04-22 2000-03-28 Sharp Kabushiki Kaisha Magneto-optical recording medium

Also Published As

Publication number Publication date Type
JPS56126907A (en) 1981-10-05 application
JPS6032331B2 (en) 1985-07-27 grant
GB2071696B (en) 1983-11-16 grant
NL8101143A (en) 1981-10-01 application

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PE20 Patent expired after termination of 20 years

Effective date: 20010311