Magnetic-optical recording media and apparatus

Abstract: In a magneto-optical recording system for recording information with bits having upward magnetization and having downward magnetization on a recording layer lf a magneto-­ optical recording medium, having four magnetic layers exchange-coupled with each other. A laser beam directed onto the medium is modulated to heat the magneto-optical recording medium to two values, for "High" writing and for "Low" writing. During "High" writing, the first to third layers lose magnetization because their Curie temperatures are exceeded, and the direction of the magnetization of the first layer, which is the recording layer is determined mainly by the magnetostatic coupling from the the fourth layer. During "Low" writing, the first and the third layers lose their magnetization as their Curie temperatures are exceeded, and the direction of the magnetization of the first layer is determined by the exchange coupling from the second layer, which in turn is dependent on the exchange coupling from the fourth layer.

Description: This invention relates to a magneto-optical recording method and media that are overwritable by optical modulation, and apparatus utilizing such a method and media. In the accompanying drawings Fig.10 shows a structure of a magneto-optical recording apparatus as previously proposed by the present applicants, for instance in EP-A1-0382859, which falls under Art. 54(3) EPC and is not relevant to the question of inventive step. In the drawing, 1 is a magneto-optical recording medium, 2 is a semiconductor laser, 3 is a beam splitter, 4 is a reproducing apparatus, and 5 is an external magnetic field generating device. The structure of the magneto-optical recording medium in the invention of our earlier application is also shown in Fig. 11. In the magneto-optical recording medium, as shown in Fig. 11, the following, for example, are formed on a substrate by a method such as sputtering: Adjacent magnetic layers are here coupled by exchange forces. The first magnetic layer is the recording layer for recording and storing information. The second magnetic layer, third magnetic layer, and fourth magnetic layer do not function as information media but are additional layers that enable direct overwriting by optical modulation. The fourth magnetic layer is an initializing layer, the sublattice magnetization of which does not reverse under heating due to laser illumination within the operating range and acts in opposition to the bias magnetic field. The third magnetic layer is a control layer for shielding the exchange force from the fourth magnetic layer at high temperatures. Next the operation will be explained. [0] Initialization process After formation of the layers of the magneto-optical recording medium, out of the 2 4 sublattice magnetic moment configurations, the recording medium is put first and once only into the two magnetization states shown in Fig. 12 in which the magnetic moments of the transition-metal (TM) sublattices of the second magnetic layer, the third magnetic layer, and the fourth magnetic layer point down. After recording, these two states will be the recorded states. Outlined arrows in the drawings will hereinafter indicate magnetisation, arrows appearing therewithin or alone will indicate the magnetic moment of the transition-metal sublattice, dotted line will indicate the state in which a magnetic wall exists between magnetic layers, and a short horizontal bar will indicate the state in which ferro-magnetisation is lost because of heating above the Curie temperature. The bias magnetic field Hb is generated so that in the vicinity of the Curie temperature of the second magnetic layer the sublattice magnetisation direction of the second magnetic layer that will result under the influence of the bias magnetic field will be opposite to the sublattice magnetisation of the fourth magnetic layer. If the magnetisation of the fourth magnetic layer is orientated downward and the fourth magnetic layer is formed of a material having a compensation temperature below its Curie temperature, then the bias magnetic field is generated upward as in the illustrated example. If the magnetisation of the fourth layer is orientated downward and the fourth magnetic layer is formed of a material not having a compensation temperature below its Curie temperature, the bias magnetic field Hb is generated downward. If not explicitly stated otherwise,the word magnetisation will hereinafter denote the magnetisation of the transition-metal sublattice. (1) Low-temperature operation. When the laser output is raised higher than in reproduction and the magnetic layers in the focused spot are heated to the vicinity of the Curie temperature of the first magnetic layer, as shown in Figure 13, the sublattice magnetisation direction of the second magnetic layer remains unchanged, and the sublattice magnetization of the second magnetic layer is transferred to the first magnetic layer, so the first magnetic layer becomes magnetized in the down direction. The third magnetic layer and the fourth magnetic layer play no particular role in this process, and even if the magnetization of the third magnetic layer is lost. it is magnetized in the same direction again by the exchange coupling with the fourth magnetic layer, giving the initial "0" state. [2] High-temperature operation When the temperature is raised to the vicinity of the Curie temperature of the second magnetic layer as shown in Fig. 14, the first magnetic layer and third magnetic layer are demagnetized, but the magnetization direction of the fourth magnetic layer does not change. The second magnetic layer receives no exchange forces from the first magnetic layer and the third magnetic layer, but is magnetized by the external magnetic field in the up direction. When the temperature falls below the Curie temperature of the first magnetic layer, the sublattice magnetization of the second magnetic layer is transferred to the first magnetic layer and the first magnetic layer becomes magnetized in the up direction. When the temperature falls below the Curie temperature of the third magnetic layer, the sublattice magnetization of the third magnetic layer aligns with the sublattice magnetization of the fourth magnetic layer in the down direction, then the temperature falls further, and the sublattice magnetization of the second magnetic layer aligns with the sublattice magnetization of the fourth magnetic layer via the third magnetic layer, in the down direction, giving the initial "1" state. In this way, optically modulated direct overwriting is possible by modulating only the laser light intensity. A similar recording medium and system is disclosed in EP-A3-0258978 and in EP-A-0382859 which is prior art by virtue of Article 54(3) EPC. Although prior-art magneto-optical recording media structured as above enable direct overwriting, the external magnetic field generating device is an obstacle to the further reduction of the size of the magneto-optical recording apparatus. Another problem is that the external magnetic field generating device is disposed on the opposite side of the magneto-optical recording medium from the optical head. If double-sided media are used, it is not possible to place the optical head on the opposite side of the magneto-optical recording medium. Accordingly the present invention provides a method of magneto-optically recording bits of information onto a medium comprising: a first magnetic layer with perpendicular magnetic anisotropy; a second magnetic layer disposed on this first magnetic layer and coupled to said first magnetic layer by an exchange force; a third magnetic layer disposed on this second magnetic layer and coupled to said second magnetic layer by an exchange force; and a fourth magnetic layer disposed on this third magnetic layer and coupled to said third magnetic layer by an exchange force; the Curie temperature of the layers of said magneto-optical recording medium satisfying: Tc4 > Tc1, Tc4 > Tc3, Tc2 > Tc1, Tc2 > Tc3, and the magnetisation of the first magnetic layer being not reversed by magnetic reversal of the second magnetic layer on cooling from below Tc1 to room temperature, and wherein the sublattice magnetisation directions of the second magnetic layer, the third magnetic layer and the fourth magnetic layer are aligned in first, second and third predefined directions respectively at room temperature, said predefined directions each being either the up direction or the down direction, said method comprising the steps of: (1) directing a laser beam onto said medium, said laser beam being modulatable to heat said magneto-optical recording medium to at least two values according to the binary information to be recorded; (2) heating a region of said magneto-optical recording medium to the higher of said two values, whereby the magnetisation is weakened at the heated region, and as a result a magnetic field opposite to the net magnetisation direction of the surrounding regions is thereby generated in the heated region; (3) magnetising the second magnetic layer such that the sublattice magnetisation direction of the second layer is in the direction opposite to said first predefined direction, on the initial cooling of the heating region; (4) cooling the magneto-optical recording medium to room temperature, during which process the first layer is magnetised by exchange coupling with the second layer, below Tc3 the third layer is magnetised by exchange coupling with the fourth layer, and subsequently the sublattice magnetisation direction of the second layer is reversed by exchange coupling with the third layer, without reversing the magnetisation of the first layer, thereby at room temperature the sublattice magnetisation directions of the second, third and fourth layers are realigned with the first, second and third predefined directions respectively, wherein that magnetising the second layer in step (3) is done by the opposite magnetic field generated in step (2). The invention also provides a magneto-optical recording system for recording bits of information in upward and downward magnetisations on a recording layer of a magneto-optical recording medium, said system comprising: a magneto-optical recording medium comprising: a first magnetic layer with perpendicular magnetic anisotropy; a second magnetic layer disposed on this first magnetic layer and coupled to said first magnetic layer by an exchange force; a third magnetic layer disposed on this second magnetic layer and coupled to said second magnetic layer by an exchange force; and a fourth magnetic layer disposed on this third magnetic layer and coupled to said third magnetic layer by an exchange force; the Curie temperature of the layers of said magneto-optical recording medium satisfying: Tc4 > Tc1, Tc4 > Tc3, Tc2 > Tc1 Tc2 > Tc3, and the magnetisation of the first magnetic layer being not reversed by magnetic reversal of the second magnetic layer on cooling from below Tc1 to room temperature, and wherein the sublattice magnetisation directions of the second magnetic layer, the third magnetic layer and the fourth magnetic layer are aligned in first, second and third predefined directions respectively at room temperature, said predefined directions each being either the up direction or the down direction; the system further comprising means for directing a laser beam onto the medium; means for modulating said beam to heat said magneto-optical recording medium to at least two values according to binary information to be recorded; and said system being adapted to carry out the above method when recording bits of information in one of the upward or downward magnetisation directions of the first layer. The second magnetic layer preferably comprises a layer of magnetic material having a compensation temperature below its Curie temperature, and the fourth magnetic layer preferably comprises a layer of magnetic material of which the magnetisation existing in the same direction as the transition-metal sublattice magnetisation is reduced as the temperature rises. The second magnetic layer preferably comprises a layer of magnetic material, the rare-earth (RE) sublattice magnetisation of which is dominant at room temperature and which does not have a compensation temperature, and the fourth magnetic layer is preferably formed of a magnetic material of which the magnetisation existing in the same direction as the rare-earth sublattice magnetisation is reduced as the temperature rises. The invention further provides a magneto-optical recording medium for use with the above method and/or the above system, wherein the first, second, third and fourth magnetic layers are of compositions and thicknesses substantially according to any one of the media 1-6 given in the following table: Similar media are described in the parallel applications EP 90 121 472.6, published as EP-A-0 428 946, and EP 90 311 320.7, published as EP-A-0 428 271. It is advantageous if the magneto-optical recording medium is adhesively joined to a read-only optical recording medium, or a write-once optical recording medium, thereby to form a composite recording medium. Thus, the invention is concerned with a magneto-optical recording system for recording information with bits having upward magnetisation and having downward magnetisation on a recording layer of a magneto-optical recording medium, having four magnetic layers exchange-coupled with each other. A laser beam directed onto the medium is modulated to heat the magneto-optical recording medium to two values, for "High" writing and for "Low" writing. During "High" writing, the first to third layers lose magnetisation because their Curie temperatures are exceeded, and upon cooling, the second layer is magnetised by a stray magnetic field mainly consisting of the magnetic field from the fourth layer. The magnetisation of the second layer is transferred to the first layer when the first layer cools below its Curie temperature, by exchange coupling. After this transfer to the first layer, the magnetisation of the second layer is reversed due to exchange coupling from the fourth layer through the third layer, to which the magnetisation of the fourth layer is transferred, when the third layer cools below its Curie temperature. The direction of magnetisation of the second layer as determined by the exchange coupling is opposite to the direction of the magnetisation as determined by the stray magnetic field. Even when the magnetisation of the second layer is reversed, the first layer is not reversed due to its high coercivity. Thus, the first layer attains magnetization in a first direction. During "Low" writing, the first and the third layers lose their magnetization as their Curie temperatures are exceeded, but the second layer does not lose its magnetisation, and its magnetization is retained, and transferred to the first layer when the first layer cools, so the first layer attains a magnetization in the second direction, opposite to the first direction. Thus, direct overwriting is possible by binary modulation of the emission intensity of the recording laser beam, without external provision of a magnetic field generating device. The invention will be further described by way of non-limitative example with reference to the accompanying drawings, in which:- Fig. 1 is a structural drawing illustrating an embodiment of the invented magneto-optical disk medium. Fig. 2A is an explanatory drawing illustrating the low-temperature recording operation. Fig. 2B is an explanatory drawing illustrating the high-temperature recording operation. Fig. 3 is an explanatory drawing illustrating the magnetization distribution of the fourth magnetic layer. Figs. 4 to 10 are structural drawings of magneto-optical recording apparatus according to embodiments of the invention. Fig. 11 is a structural drawing illustrating a prior-art magneto-optical disk medium. Fig. 12 is an explanatory drawing for explaining initialization. Fig. 13 is an explanatory drawing illustrating the low-temperature recording operation. Fig. 14 is an explanatory drawing illustrating the high-temperature recording operation. Embodiment 1 Fig. 1 shows a sectional view of a magneto-optical recording medium of a first embodiment of this invention, which is identical to the prior-art example except for the fourth magnetic layer, and which has similar functions caused by exchange forces for low-temperature operation. The recording medium comprises, for example, the following, formed on a glass substrate by a process such as sputtering. Adjacent magnetic layers are here coupled by exchange forces. In the embodiment under consideration, the first, third and fourth magnetic layers are TM rich, while the second magnetic layer is RE rich below its compensation temperature which is close to the Tc1, but is TM rich above the compensation temperature. The fourth magnetic layer is an orientation layer which produces, when it is heated, a stray magnetic field acting on the second magnetic layer in a direction opposite to the direction in which the exchange force from the fourth magnetic layer acts. Next the operation will be described. [0] Initialization process The initialization process after formation of the layers of the magneto-optical recording medium is the same as in the prior art example. [1] Low-temperature operation Low-temperature operation is as shown in Fig. 2A and is basically the same as in the prior-art example. [2] High-temperature operation When the temperature is raised to the vicinity of the Curie temperature of the second magnetic layer as shown in Fig. 2B, the first magnetic layer and the third magnetic layer are demagnetized, but the magnetization direction of the fourth magnetic layer does not change. The second magnetic layer receives no exchange forces from the first magnetic layer and the third magnetic layer, but is magnetized in the up direction by stray magnetic field, mainly consisting of magnetostatic coupling with the fourth magnetic layer. Other contribution of the stray magnetic field which acts on the second magnetic layer is a leakage magnetic flux from the neighboring regions of the same second magnetic layer. It is also possible to utilize the magnetic field leaking out of the optical head, particularly, the coils for driving an objective lens for focusing and tracking. It is advantageous if magnetic field leaking out of the optical head is in the same direction as the magnetostatic coupling force from the fourth magnetic layer. When the temperature falls below the Curie temperature of the first magnetic layer, the sublattice magnetization of the second magnetic layer is transferred to the first magnetic layer and the first magnetic layer becomes magnetized in the up direction. When the temperature falls below the Curie temperature of the third magnetic layer, the sublattice magnetization of the third magnetic layer aligns with the fourth magnetic layer in the down direction, and when the temperature falls further, the sublattice magnetization of the second magnetic layer aligns with the sublattice magnetization of the fourth magnetic layer via the third magnetic layer, in the down direction, giving the initial "1" state. A feature of this invention is that it makes use of magnetostatic coupling with the fourth magnetic layer. In a magneto-optical recording medium that is locally heated in high-temperature operation, the magnetization distribution of the fourth magnetic layer is as in Fig. 3, creating in effect an upwardly oriented magnetic field. Thus no external magnetic field is needed because its function is fulfilled by the orientation layer. In this way, optically modulated direct overwriting is possible by modulating only the laser light intensity. In magneto-optical recording media having the above-mentioned dielectric layer, magnetic layers, and protective layer on a substrate with grooves spaced at 1.6µ m intervals, a CN ratio of 45dB, with no remanent unerased signal, was obtained by writing a signal with 0.76µ m bit length on a signal with 2µ m bit length at a linear speed of 11m/sec with a peak laser power of 13mW and bottom power of 5mW. Embodiments 2 to 6 Tables 1 and 2 show embodiments 2 to 6. Using four-element RF magnetron sputtering apparatus, one places an Fe target having Tb and Co chips, an Fe target having Gd and Co chips, an Fe target having Tb chips, and a Co target having Tb chips, and sets in the chamber of the apparatus a glass substrate 1.2mm thick and 86mm in diameter, that is pregrooved at a 1.6µ m pitch. After evacuation of the chamber of the apparatus to a vacuum of 8 x 10 -7 Torr or less, Ar gas is introduced at 3mTorr, and sputtering is performed at a deposition rate of substantially 100 angstroms per minute. This creates a first magnetic layer of TbFeCo 800 angstroms thick on the substrate. Next a second magnetic layer of GdDyFeCo 1500 angstroms thick, a third magnetic layer of TbFe 100 angstroms thick, and a fourth magnetic layer of TbCo 400 angstroms thick are created in similar fashion, the vacuum condition being maintained all the while. Each of the magnetic layers is a perpendicularly magnetized layer, and adjacent layers are coupled by an exchange force. The lower table indicates the recording and reproduction characteristics of a 0.76µ m bit-length signal on a 2µ m bit-length signal under linear velocity conditions of 11m/sec for various numbers of chips on the targets and for various sputtering times. Direct overwriting was confirmed in this case with no observed remanent unerased signal. No. 1ST MAG. LAYER 2ND MAG. LAYER 3RD MAG. LAYER 4TH MAG. LAYER 2 Tb 23 Fe 72 Co 5 Gd 5 Dy 22 Fe 65 Co 8 Tb 20 Fe 80 Tb 18 Co 82 800 angstroms 2000 angstroms 800 angstroms 2000 angstroms 3 Tb 23 Fe 72 Co 5 Gd 5 Dy 22 Fe 65 Co 8 Tb 20 Fe 80 Tb 18 Co 82 800 angstroms 1500 angstroms 100 angstroms 1000 angstroms 4 Tb 23 Fe 72 Co 5 Gd 5 Dy 22 Fe 65 Co 8 Tb 20 Fe 80 Gd 8 Tb 10 Co 82 800 angstroms 1200 angstroms 100 angstroms 800 angstroms 5 Tb 23 Fe 72 Co 5 Gd 5 Dy 22 Fe 65 Co 8 Tb 20 Fe 80 Tb 18 Co 82 800 angstroms 1200 angstroms 100 angstroms 800 angstroms 6 Tb 23 Fe 77 Gd 5 Dy 22 Fe 65 Co 8 Tb 15 Dy 5 Fe 80 Tb 18 Co 82 1000 angstroms 1200 angstroms 100 angstroms 800 angstroms No. Peak Power (mW) Bottom Power (mW) Signal-to-Noise Ratio (dB) 2 16.5 7.0 40.9 3 14.0 6.5 42.0 4 13.0 4.5 43.1 5 11.5 4.5 45.4 6 10.0 4.0 41.1 Each of the magnetic layers here may itself comprise multiple sublayers: for example, before the first magnetic layer is formed, an exchange-coupled magnetic layer with a large magneto-optic effect may be provided. An improvement in CNR by 1dB was observed when the following were formed on a glass substrate by a method such as sputtering: Dielectric layer: SiN X 650 angstroms Reproduction layer: Tb 15 Fe 70 Co 15 100 angstroms First magnetic layer: Tb 22 Fe 69 Co 9 800 angstroms Second magnetic layer: Gd 8 Dy 17 Fe 60 Co 15 1500 angstroms Third magnetic layer: Tb 16 Fe 84 200 angstroms Fourth magnetic layer: Tb 20 Co 80 800 angstroms Protective layer: SiN X 700 angstroms Magnetic layers, dielectric layers, oxide layers, and the like can also be created between the magnetic layers to control the exchange forces. The magnetic layers may also comprise ferrimagnetic materials such as GdFe, GdTbFeCo, TbDyFeCo, NdDyFeCo, DyCo, TbHoFeCo, and DyHoCo. Embodiment 7 Two single-sided media were adhesively joined by a two-part epoxy-based resin. When magneto-optical recording media of the first embodiment were joined together, for example, in a magneto-optical recording medium with a protective layer, a CN ratio of 45dB, with no remanent unerased signal, was obtained by optical modulation of a 0.76µ m bit-length signal on a 2µ m bit-length signal at a linear velocity of 11m/sec with a peak laser power of 14mW and bottom laser power of 5mW. Adhesion may also be accomplished with a thermoplastic resin, thermosetting resin, or the like, and what is joined may be a read-only optical disk, a write-once optical disk, or the like. The media may also have shapes other than disk shapes: for example, they may have a card shape or the like. Embodiment 8 Fig. 4 shows a structural drawing of a magneto-optical recording apparatus. In the drawing, 1 is a magneto-optical recording medium, 2 is a semiconductor laser, 3 is a polarizing beam splitter, and 4 is a reproducing apparatus. The semiconductor laser is adapted for binary modulation of the beam intensity of the recording laser beam according to the information to be recorded, with output exceeding the laser beam intensity during reproduction. The magneto-optical recording medium is in the form of a disk having recording tracks 1a in spiral or circular form, and is rotated about is axis, by a motor 6, so that the disk is moved relative to the laser beam. Using the magneto-optical recording medium of the first embodiment, a CN ratio of 45dB, with no remanent unerased signal, was obtained by optical modulation of a 0.76µ m bit-length signal on a 2µ m bit-length signal at a linear velocity of 11m/sec with a peak laser power of 13mW and bottom laser power of 5 mW. Embodiment 9 Fig. 5 shows a structural drawing of a magneto-optic recording apparatus. In the drawing, 1 is a magneto-optical recording medium, 2 is a semiconductor laser, 3 is a beam splitter, and 4 is a reproducing apparatus. The laser beam emitted by the semiconductor laser can be generated in a pulse pattern suitable for high-temperature operation, comprising a pulse train of twenty times the maximum frequency of the recorded information, and a pulse pattern suitable for low-temperature operation, comprising a pulse train of ten times the maximum frequency of the recorded information. Identical values were employed for respective constituent pulses at this time. Using the magneto-optical recording medium of the first embodiment, overwriting with no remanent unerased signal was possible by optical modulation of a 0.76µ m bit-length signal on a 2µ m bit-length signal at a linear velocity of 11m/sec with a peak laser power of 15mW. Embodiment 10 Fig. 6 shows a structural drawing of a magneto-optical recording apparatus. In the drawing, 1 is a magneto-optical recording medium, 2-1 and 2-2 are semiconductor lasers, 3 is a polarizing beam splitter, 4 is a reproducing apparatus, and 5 is a beam splitter. The semiconductor lasers 2-1 and 2-2 have a wavelength of 830nm and maximum continuous emitted output of 16mW, and are focused onto adjacent positions on the magneto-optical recording medium. One of the lasers generates constant laser output suitable for low-temperature operation, while the other laser generates laser output that attains the temperature necessary for high-temperature operation only for the information "1." In this case direct overwriting was possible using the magneto-optical recording medium of embodiment 1 with the maximum laser output on the surface of the recording medium reduced by 3mW, to 10mW. Embodiment 11 Fig. 7 shows a structural drawing of a magneto-optical recording apparatus. In the drawing, 1 is a magneto-optical recording medium, 2-1 and 2-2 are semiconductor lasers, 3 is a polarizing beam splitter, 4 is a reproducing apparatus, and 5 is a beam splitter. The semiconductor laser 2-1 is for recording, having a wavelength of 830nm and maximum continuous emitted output of 20mW, while 2-2 is for reproducing, having a wavelength of 780nm and maximum continuous emitted output of 4mW, adapted for tracking the laser spot of 2-1, enabling verification to be performed simultaneously with overwriting. In this way data transfer rates during recording were made substantially equal to those during reproducing. Embodiment 12 Fig. 8 shows a structural drawing of a magneto-optical recording apparatus. In the drawing, 1 is a magneto-optical recording medium, 2-1 and 2-2 are identical semiconductor lasers with a wavelength of 830nm and maximum continuous emitted output of 20mW, 3 is a polarizing beam splitter, 4 is a reproducing apparatus, and 5 is a beam splitter. The laser beams are focused onto adjacent tracks on the magneto-optical recording medium and are adapted for simultaneous recording. Here the magneto-optical recording medium is moved, but lasers can also be arranged in an array or matrix for recording and reproduction without moving the magneto-optical recording medium. It is also possible to place in the optical path an optical element, such as a liquid crystal, the transmittance of which can be spatially varied. Embodiment 13 Fig. 9 shows a structural drawing of a magneto-optical recording apparatus. In the drawing, 1 is a magneto-optical medium, 2-1 and 2-2 are semiconductor lasers disposed on both sides of the optical recording medium, 3 is a polarizing beam splitter, and 4-1 and 4-2 are reproducing apparatuses. The optical recording medium of embodiment 7 is employed as an optical recording medium, and the semiconductor laser 2-2 and reproducing apparatus 4-2 can reproduce read-only optical disks. As described above, in a recording medium with four exchange-coupled magnetic layers, this invention provides the orientation layer with two opposing functions, an exchange force and a magnetostatic coupling force, enabling direct overwriting without any device for generating an external magnetic field.

Claims: A method of magneto-optically recording bits of information onto a medium (1) comprising: a first magnetic layer with perpendicular magnetic anisotropy; a second magnetic layer disposed on this first magnetic layer and coupled to said first magnetic layer by an exchange force; a third magnetic layer disposed on this second magnetic layer and coupled to said second magnetic layer by an exchange force; and a fourth magnetic layer disposed on this third magnetic layer and coupled to said third magnetic layer by an exchange force; the Curie temperature of the layers of said magneto-optical recording medium satisfying: Tc4 > Tc1, Tc4 > Tc3, Tc2 > Tc1, Tc2 > Tc3, and the magnetisation of the first magnetic layer being not reversed by magnetic reversal of the second magnetic layer on cooling from below Tc1 to room temperature, and wherein the sublattice magnetisation directions of the second magnetic layer, the third magnetic layer and the fourth magnetic layer are aligned in first, second and third predefined directions respectively at room temperature, said predefined directions each being either the up direction or the down direction, said method comprising the steps of: (1) directing a laser beam onto said medium, said laser beam being modulatable to heat said magneto-optical recording medium to at least two values according to the binary information to be recorded; (2) heating a region of said magneto-optical recording medium to the higher of said two values, whereby the magnetisation is weakened at the heated region, and as a result a magnetic field opposite to the net magnetisation direction of the surrounding regions is thereby generated in the heated region; (3) magnetising the second magnetic layer such that the sublattice magnetisation direction of the second layer is in the direction opposite to said first predefined direction, on the initial cooling of the heating region; (4) cooling the magneto-optical recording medium (1) to room temperature, during which process the first layer is magnetised by exchange coupling with the second layer, below Tc3 the third layer is magnetised by exchange coupling with the fourth layer, and subsequently the sublattice magnetisation direction of the second layer is reversed by exchange coupling with the third layer, without reversing the magnetisation of the first layer, thereby at room temperature the sublattice magnetisation directions of the second, third and fourth layers are realigned with the first, second and third predefined directions respectively, wherein that magnetising the second layer in step (3) is done by the opposite magnetic field generated in step (2). A magneto-optical recording system for recording bits of information in upward and downward magnetisations on a recording layer (1a) of a magneto-optical recording medium (1), said system comprising: a magneto-optical recording medium (1) comprising: a first magnetic layer with perpendicular magnetic anisotropy; a second magnetic layer disposed on this first magnetic layer and coupled to said first magnetic layer by an exchange force; a third magnetic layer disposed on this second magnetic layer and coupled to said second magnetic layer by an exchange force; and a fourth magnetic layer disposed on this third magnetic layer and coupled to said third magnetic layer by an exchange force; the Curie temperature of the layers of said magneto-optical recording medium satisfying: Tc4 > Tc1, Tc4 > Tc3, Tc2 > Tc1 Tc2 > Tc3, and the magnetisation of the first magnetic layer being not reversed by magnetic reversal of the second magnetic layer on cooling from below Tcl to room temperature, and wherein the sublattice magnetisation directions of the second magnetic layer, the third magnetic layer and the fourth magnetic layer are aligned in first, second and third predefined directions respectively at room temperature, said predefined directions each being either the up direction or the down direction; the system further comprising means (2) for directing a laser beam onto the medium; means for modulating said beam to heat said magneto-optical recording medium to at least two values according to binary information to be recorded; and said system being adapted to carry out the method according to claim 1 when recording bits of information in one of the upward or downward magnetisation directions of the first layer. The method according to claim 1 or system according to claim 2 wherein; said second magnetic layer comprises a layer of magnetic material having a compensation temperature below its Curie temperature, and said fourth magnetic layer comprises a layer of magnetic material of which the magnetisation existing in the same direction as the transition-metal sublattice magnetisation is reduced as the temperature rises. The method according to claim 1 or system according to claim 2, wherein; said second magnetic layer comprises a layer of magnetic material, the rare-earth sublattice magnetisation of which is dominant at room temperature and which does not have a compensation temperature, and said fourth magnetic layer is formed of a magnetic material of which the magnetisation existing in the same direction as the rare-earth sublattice magnetisation is reduced as the temperature rises. A magneto-optical recording medium for use with the method of claim 1 and/or the system of claim 2, wherein the first, second, third and fourth magnetic layers are of compositions and thicknesses substantially according to any one of the media 1-6 given in the following table: A method, system or medium as set forth in any one of the preceding claims, wherein said magneto-optical recording medium (1) is adhesively joined to a read-only optical recording medium, or a write-once optical recording medium.

Inventors: FUKAMI TATSUYA, NAKAKI YOSHIYUKI, TAGUCHI MOTOHISA, TOKUNAGA TAKASHI, TSUTSUMI KAZUHIKO, FUKAMI, TATSUYA, NAKAKI, YOSHIYUKI, TAGUCHI, MOTOHISA, TOKUNAGA, TAKASHI, TSUTSUMI, KAZUHIKO, FUKAMI, TATSUYA, C/O MITSUBISHI DENKI K. K., NAKAKI, YOSHIYUKI, C/O MITSUBISHI DENKI K. K., TAGUCHI, MOTOHISA, C/O MITSUBISHI DENKI K. K., TOKUNAGA, TAKASHI, C/O MITSUBISHI DENKI K. K., TSUTSUMI, KAZUHIKO, C/O MITSUBISHI DENKI K. K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1990
Main classification: G11B11
IPC codes: G11B13, G11B11