Magneto-optical recording medium and method

Abstract: A magneto-optical recording medium having a first magnetic layer and a second magnetic layer having a higher curie point and a lower coercive force than those of the first magnetic layer, which magneto-­ optical recording medium satisfying a condition of where H H is a coercive force of the first magnetic layer, H L is a coercive force of the second magnetic layer. M s is a saturation magnetization of the second magnetic layer, h is a film thickness of the second magnetic layer, and σ w is a magnetic wall energy between the first and second magnetic layers. The second magnetic layer contains an element which is not ferromagnetic at a room temperature. A recording method using such a medium comprises the steps of: (a) applying a first magnetic field large enough to magnetize the second magnetic layer in one direction but not large enough to reverse magnetization of the first magnetic layer, and (b) selectively effecting a first type of recording or a second type of recoridng depending on an infor­ mation signal, in the first type of recording, a light beam having a power large enough to heat the medium to a vicinity of a curie point of the first magnetic layer being irradiated while a bias of the opposite direction to the first magnetic field is applied so that the magnetization of the first magnetic layer adopts a direction stable to the second magnetic layer while the direction of magnetization of the second magnetic layer is kept unchanged; in the second type of recording, a light beam having a power large enough to heat the medium to a vicinity of a curie point of the second magnetic layer being irradiated while a bias of the opposite direction to said first magnetic field is applied so that the magnetization of the second magnetic layer is reversed and the first magnetic layer is reversed and the first magnetic layer is magnetized in a direction stable to the second magnetic layer.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to magneto-optical recording medium having a recording layer of two-layer magnetic film for recording information by radiation of light beam, and recording method using the same. Related Background Art Research and development of an optical memory element which uses a laser beam to realize a high density and large capacity memory have been rapidly done. Among others, a magneto-optical recording medium is advisable as a rewritable optical memory element. However, when information recorded on such a magneto-optical recording medium is to be rewritten, the information should be erased and then new information should be recorded. Namely, overwriting of information cannot be attained. For example, when information on a predetermined track on a magneto-optical disk is to be rewritten, the information on the track is erased in one revolution of the disk and then new information is written in the next revolution. As a result, a recording speed is low. In order to solve the above problem, separate record/reproduce head and erase head are provided in the prior art apparatus, or information is recorded while a magnetic field applied with a continuous laser beam is modulated. However, such an apparatus is of large scale and high cost or cannot attain high speed modulation. On the other hand, Japanese Laid-Open Patent Application No. 175948/1987 (corresponding to DE-OS No. 3,619,618) discloses a magneto-optical recording medium according to the preamble of claim 1 and a recording method using the same which enable over-writing like a magnetic recording medium by providing simple magnetic field generation means in the conventional apparatus. EP-A-0258978 (CANON), which was published after the present priority date and is thus only in the state of the art under Article 54(3), also discloses an overwriteable magnetic recording medium whereby the documents as for as supported by priority documents being filed before the priority date of the present applicant do not mention any of the features according to the characterising part of claim 1. EP-A-0257530 (SONY), which was published after the present priority date and is thus only in the state of the art under Article 54(3) likewise discloses an overwriteable magnetic recording medium. In all three cases, the medium comprises a pair of layers; a first primarily concerned with retaining data for readout and a second having a higher Curie temperature and lower coercive force, which applies a biasing magnetic field to the first at high temperatures. At room temperature, the coercive forces of both layers are larger than the magnetic coupling force exerted on each, so they can stably coexist either parallel or antiparallel. Both the layers comprise an alloy of a rare earth element and a transition metal. The present invention provides an improved medium of the above type in which the second layer is alloyed with an elements which does not exihibit ferromagnetism at ambient temperature, the non-ferromagnetic element being one or more of the group consisting of Cu, Ag, Ti, Mn, B, Pt, Si, Cr, Al and Ge. This reduces the exchange force acting on the second layer, enabling a lower coercive force material to be used. Claim 9 contains a method of manufacturing a medium according to the invention. Claim 10 contains a recording method using a medium according to the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a sectional view of one embodiment of a magneto-optical recording medium of the present invention, Fig. 2 shows a temperature characteristics of coercive forces of magnetic layers of the medium shown in Fig. 1, Fig. 3 shows magnetization status in a recording step using the medium of Fig. 1, Fig. 4 shows a conceptual view of a recording and reproducing apparatus which uses the medium of Fig. 1. Fig. 5 shows magnetization status in a recording step when another embodiment of recording medium is used, and Fig. 6 shows a sectional view of other embodiment of a magneto-optical recording medium of the present invention which has a protection layer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows a sectional view of one embodiment of the magneto-optical recording medium of the present invention. The magneto-optical recording medium of Fig. 1 has a first magnetic layer 2 and a second magnetic layer 3 laminated on a pre-grooved transparent substrate 1. The first magnetic layer 2 has a low curie point T L and a high coercive force H H , and the second magnetic layer 3 has a high curie point T H and a low coercive force H L . The terms high and low mean relative relations when both magnetic layers are compared. (The coercive forces are compared at a room temperature.) The relations are shown in Fig. 2. Usually, T L and H H of the first magnetic layer 2 are selected to 70 - 180°C and 3 - 10K Oe (240 - 800 K A/m), respectively, and T H and H L of the second magnetic layer are selected to 100 - 400°C and 0.5 - 2K Oe (40 - 160 K A/m), respectively. A principal composition of each of the magnetic layers is amorphous magnetic alloy of a rare earth element and a transition metal element which exhibits perpendicular magnetic anisotropy and magneto-optical effect. Examples are GdCo, GdFe, TbFe, DyFe, GdTbFe, TbDyFe, GeTbFeCo, TbFeCo and GdTbCo, etc. In a recording method which uses the magneto-optical recording medium of the present invention, the first magnetic layer 2 primarily contributes to the reproduction. Namely, the magneto optical effect exhibited by the first magnetic layer 2 is primarily utilized for the reproduction, and the second magnetic layer 3 plays an important role in recording information. In the two-layer film of the recording medium of the present invention, the following relation must be met: H H > H L > σ w 2M s h where M s is a saturation magnetization of the second magnetic layer 3, h is a thickness of the second magnetic layer 3, and σ w is a magnetic wall energy between the two magnetic layers. The above relation should be met in order for magnetization of bits at the end of recording (shown by 4f in Fig. 3) to exist stably. The thicknesses coercive forces, saturation magnetization of the magnetic layers (perpendicular magnetization layers) 2 and 3 and the magnetic wall energy may be appropriately selected to meet the above relation. Practically and specifically, the solution magnetization M s of the second magnetic layer is selected to be large, the film thickness h thereof is selected to be large or the magnetic wall energy σ w is selected to be small. However, when the film thickness h is large, a sensitivity of the magneto-optical recording medium is lowered. Further, when the saturation magnetization is large, the coercive force H L is lowered. Experimentarily, when H L is lower than 1K Oe (80000 A/m), it tends to be H L < σ w /2M s h. Accordingy, the best practical way is to reduce the magnetic wall energy σ w . As taught in EP-A-0258978, when an intermediate layer of a non-magnetic element is provided between the first magnetic layer 2 and the second magnetic layer 3, an exchange coaction through the intermediate layer is substantially reduced even if it is only of several tens Å thickness, and an apparent σ w is lowered. However, in actual, a magneto-optical recording medium having an appropriate σ w cannot be prepared with high reproducibility. In the experiment done by the inventor of the present invention, a non-magnetic element was added to the second magnetic layer. It was found that since the material which reduced the exchange coaction was dispersed in the layer, the exchange force acting on the second magnetic layer through the magnetic walls of the first and second magnetic layers was reduced as it was by the intermediate layer and that the exchange force acting on the second magnetic layer could be set with high reproducibility unlike the case where the intermediate layer was used. The non-magnetic element may be one which hardly affects to M s of the second magnetic layer when it is added thereto (that is, hardly lowers M s and currie point). Examples are one or more of Cu, Ag, Ti, Mn, B, Pt, Si and etc. Amount of addition is preferably 2 - 70 atomic % to the second magnetic layer, and more preferably 5 - 20 atomic %. The drop of the curie point due to the addition of the non-magnetic element is preferably held within 30°C. The drop of the Curie point is considered to be caused primarily by alloying of the added non-magnetic element and the rare earth element or transition metal of the second magnetic layer. Further, study was done to prevent such drop and it was found effective to separately provide an evaporation source for the rare earth element or transition metal element and an evaporation source for the non-magnetic element in forming the second magnetic layer and form the layer by a conventional sputtering method, ion beam vapor deposition or sputtering method, electron beam evaporation deposition method, or cluster beam evaporation deposition method while the substrate is rotated. With this method, the drop of the Curie point is small even if a large amount of non-magnetic element is added, and the apparent σ w can be reduced without adverse effect by the reduction of M s , that is, adverse effects to the record sensitivities and record status of the first and second magnetic layers. The reason therefor is considered that a laminated structure of the rare earth-transition metal element and the non-magnetic element is formed when viewed microscopically. The magnetic layers 2 and 3 are preferably exchange-coupled while taking a magnitude of an effective biasing magnetic field in a recording step and stability of binary record bits into account. The recording method using the magneto-optical recording medium of the present invention is explained with reference to Figs. 2 to 4. Fig. 3 shows directions of magnetizations of the magnetic layers 2 and 3 in the recording step, and Fig. 4 shows a block diagram of the recroding apparatus. Before recording, the stable directions of magnetizations of the magnetic layers 2 and 3 may be either parallel (same direction) or anti-parallel (opposite directions). In Fig. 3, the stable directions of magnetizations are parallel. In Fig. 4, numeral 9 denotes a magneto-optical disk having the above construction. It is assumed that initial magnetization of a portion of the magnetic layers is that shown by 4a in Fig. 3. The magneto-optical disk 9 is rotated by a spindle motor and passes through a magnetic field generation unit 8. If magnitude of the magnetic field of the magnetic field generation unit 8 is set between the coercive forces of the magnetic layers 2 and 3 (the direction of the magnetic field is upward in the present embodiment), the second magnetic layer 3 is magnetized in a uniform direction as shown by 4b in Fig. 3 while the magnetization of the first magnetic layer 2 is kept unchanged. When the magneto-optical disk 9 rotates and passes through a record/reproduce head 5, a laser beam of one of two laser powers is irradiated to the disk plane in accordance with a signal from a record signal generator 6. The first laser power is large enough to heat the disk to a vicinity of the curie point of first magnetic layer 2, and the second laser power is large enough to heat the disk to a vicinity of the curie point of the second magnetic layer. In Fig. 2 which shows the relation between the coercive forces of the magnetic layers 2 and 3 and a temperature, the first laser power can heat the disk to the vicinity of T L and the second laser power can heat the disk to the vicinity of T H . The first magnetic layer 2 is heated by the first laser power to the vicinity of the curie point. Since the second magnetic layer 3 has a coercive force at this temperature which allows stable existence of bits, the bits shown by 4c in Fig. 3 are formed from 4b of Fig. 3 by properly setting a recording biasing magnetic field. (First preliminary recording) The proper setting of the biasing magnetic field is explained below. In the first preliminary recording, a force (exchange force) to arrange the magnetization of the first magnetic layer 2 is applied in a direction stable to the magnetization of the second magnetic layer 3 (the same direction in the present embodiment) and hence the biasing magnetic field is not necessary in principle. However, in preliminary recording which uses the second laser power, the biasing magnetic field is not in a direction to assist the reversal of magnetization of the second magnetic layer 3 (the direction to impede the first preliminary recording). It is advisable that the magnitudes and directions of the biasing magnetic fields in the preliminary recordings by the first and second laser powers are same. In view of the above, it is advisable that the biasing magnetic field is set to a minimum level required for the preliminary recording by the second laser power by the following principle. The proper setting of the biasing magnetic field is done while taking those factors into consideration. The second preliminary recording is explained. When the disk is heated by the second laser power to the vicinity of the curie point of the second magnetic layer 3 (second preliminary recording), the magnetization of the second magnetic layer 3 is reversed by the biasing magnetic field which has been set in the manner described above. Then, the magnetization of the first magnetic layer 2 is arranged in a direction stable to the second magnetic layer 3 (the same direction in the present embodiment). The bits shown by 4d in Fig. 3 are formed from 4b of Fig. 3. In this manner, each area of the magneto-optical disk is preliminarily recorded into the state 4c or 4d of Fig. 3 by the biasing magnetic field and the first or second laser power selected in accordance with the signal. As the magneto-optical disk 9 is rotated so that the bits 4c and 4d of the preliminary recording again pass through the magnetic field generation unit 8, the recorded bits 4c do not change because the magnitude of the magnetic field of the magnetic field generation unit 8 has been set between the coercive forces of the magnetic layers 2 and 3. (Final record status). On the other hand, the recorded bits 4d are changed to 4f because the magnetization of the second magnetic layer 3 is reversed. (another final record status). In order for the recorded bit status of 4f to exist stably, the following relaiton should be met. σ w 2M s h < H L < H H where M s is the magnitude of saturation magnetization of the second magnetic layer 3, h is the thickness thereof, σ w is the magnetic wall energy between the magnetic layers 2 and 3, and σ w /2M s h is a strength of the exchange force acting on the second magnetic layer. The magnetic field having the magnitude of σ w /2M s h tends to change the direction of the second magnetic layer 3 to the direction stable to the magnetization of the first magnetization layer 2 (the same direction in the present embodiment). In order for the magnetization of the second magnetic layer 3 not to be reversed by this magnetic field, the coercive force H L of the second magnetic layer 3 should meet the relation of H L > σ w /2M s h. Since the recorded bit status 4e and 4f are controlled by the laser power in the recording step and do not depend on the status prior to the recording, the overwriting can be attained. The recorded bits 4e and 4f can be reproduced by irradiating a reproducing laser beam and processing a reproduced light by a record signal reproducing unit 7. In the embodiment of Fig. 3, the magnetization is stable when the directions of magnetization of the first magnetic layer 2 and the second magnetic layer 3 are same. A magnetic layer whose magnetization is stable when the magnetizations of the first and second magnetization layers are anti-parallel may also be used. Fig. 5 shows magnetization status in the recording step of such a magnetic medium. In Fig. 5, 10a - 10f correspond to 4a - 4f of Fig. 3. Fig. 6 shows a sectional view of another embodiment of the magneto-optical recording medium of the present invention. In Fig. 6, the like elements to those shown in Fig. 1 are designated by the like numerals and the explanation thereof is omitted. A protection layer 11 is provided between the substrate 1 and the first magnetic layer 2. Another protection layer 12 is provided on the second magnetic layer 3. A bonding substrate 14 is bonded thereto through an adhesive layer 13. This arrangement improves an anti-corrosion property of the medium. The protection layers preferably have a thickness of 200 Å or more. Material of the protection layers is fine non-magnetic material which is usually inorganic dielectric material such as Si₃N₄, SiC, ZnS, AℓN, SiO, Aℓ₂O₃, Cr₂O₃, Si or Ge etc. Example 1 A pregrooved and preformatted polycarbonate disk-shaped substrate was set in a sputtering device having three target sources with a distance to the target being set to 10cm, and the substrate was rotated. Air in the sputtering device was evacuated to 1 x 10⁻⁶ Torr (1,33 x 10⁻⁴ Pa) or lower, and a ZnS protection layer was formed to a thickness of 1000 Å by the first target in argon at a sputtering rate of 100 Å/min and a sputtering pressure of 5 x 10⁻³ Torr (6,66 x 10⁻¹ Pa). Then, a TbFe alloy was sputtered in argon by the second target at a sputtering rate of 100 Å/min and a sputtering pressure of 5 x 10⁻³ Torr (0,66 Pa) to form a first magnetic layer having a film thickness of 300 Å, T L of approximately 140°C and H H of approximately 10 KOe (800000 A/m) and composition of Tb₁₈Fe₈₂ with Fe element sub-lattice magnetization preference. Then, a TbFeCuCo alloy was sputtered in argon at a sputtering pressure of 5 x 10⁻³ Torr (0,66 Pa) to form a second magnetic layer having a film thickness of 500 Å, T H of approximately 200°C and H L of approximately 1 KOe and composition of Tb 18.4 Fe₅₆Co 5.6 Cu₂₀ with Tb element sub-lattice magnetization preference. Then, a ZnS protection layer having a thickness of 3000 Å was formed in argon by the first target at a sputtering rate of 100 Å/min and a sputtering pressure of 5 x 10⁻³ Torr (0,66 Pa). Then, the substrate was bounded to a polycarbonate bonding substrate by hot melt adhesive material to complete a sample of the magneto-optical disk. The magneto-optical disk was loaded to a recording and reproducing apparatus and rotated past a magnetic field generation unit of 2.5K Oe (200 000 A/m) at a linear speed of approximately 8 m/sec and information was recorded by laser powers of 4 mW and 8 mW while the laser beam having a wavelength of 830 nm focused to approximately 1 µm was modulated at 2 MHz with a duty factor of 50 %. The biasing magnetic field was at 100 Oe (8000 A/m). Then, the information was reproduced by irradiating a lsaer beam of 1.5 mW to reproduce a binary signal. The same experiment was done for the magneto-optical disk having information recorded on the entire surface. The signal component of the previously recorded signal was not detected and the overwriting was attained. Example 2 and Comparative Example A sample of magneto-optical disk having the same composition, film thicknesses and coercive forces as those of the Example 1 was formed in the same manner as that of the Example 1 except for the type and amount of the non-magnetic element to be added to the second magnetic layer being changed as shown in Table 1. In order to examine the stability of the recorded bits 4f of Fig. 3 for the samples including that of the Example 1, the magnitude of the magnetic field at which the magnetizations of the first and second magnetic layers are reversed were checked while the external magnetic field was applied. Then, the stability of the recorded bits 4f without application of the external magnetic field was examined. In table 1, o indicates stable and x indicate not stable. For each of the samples shown in Table 1, only the film thickness of the second magnetic layer was changed to prepare a new sample, and the film thickness of the second magnetic layer at which the recorded bits 45 started to be unstable was checked. The results are shown in Table 1. It is seen from Table 1 that in each of the samples in which the non-magnetic element was added to the second magnetic layer, the stability of the bits 4f is improved over that of the comparative example in which no such element was added. As a result, the film thickness of the second magnetic layer can be set to a small thickness and the recording sensitivity is improved. Example 3 A pregrooved and preformatted polycarbonate disk-shaped substrate was set in a sputtering device having three target sources with a distance to the target being set to 10 cm, and it was rotated at 15 rpm. Air in the sputtering device was evacuated to 1 x 10⁻⁶ Torr or less, and a ZnS protection layer was formed to a thickness of 1000 Å in argon by the first target at a sputtering rate of 100 Å/min and a sputtering pressure of 5 x 10⁻³ Torr (0,66 Pa). Then, a TbFe alloy was sputtered in argon by the second target at a sputtering rate of 100 Å/min and a sputtering pressure of 5 x 10⁻³ Torr (0,66 Pa) to form a first magnetic layer having a film thickenss of 300 Å, T L of approximately 140°C and H H of approximately 10K Oe and having composition of Tb₁₈Fe₈₂ with Fe element sub-lattice magnetization preference. Then, a TbFeCo alloy was sputtered in argon by the third target at a sputtering pressure of 5 x 10⁻³ Torr and Cu was simultaneously sputtered by the fourth target at a sputtering rate of 100 Å/min to form a second magnetic layer having a film thickness of 500 Å, T H of approximately 200°C and H L of approximately 1K Oe and having composition of Tb 18.4 Fe₅₆Co 5.6 Cu₂₀ with Tb element sub-lattice magnetization preference. Then, a ZnS protection layer was formed to a thickness of 2000 Å in argon by the first target at a sputtering rate of 100 Å/min and a sputtering pressure of 5 x 10⁻³ Torr (0,66 Pa). Then, the substrate was bounded to a polycarbonate bonding substrate by hot melt adhesive material to complete a sample of the magneto-optical disk. The sample was loaded to the recording and reproducing apparatus and the magnetic field generation unit of 2.5K Oe (200000 A/m) was moved at a linear velocity of approximately 8m/sec, and information was recorded by laser powers of 4mW and 8mW while the laser beam having a wavelength of 830 nm focused to approximately 1 µ was modulated at 2 MHz with a duty factor of 50 %. The biasing magnetic field was at 100 Oe (8000 A/m). Then, a laser beam of 1.5mW was irradiated to reproduce a binary signal. The same experiment was done for the magneto-optical disk having information recorded on the entire surface. The previously recorded signal component was not detected and the overwriting was attained. Example 4 Sampels of the magneto-optical disk having the same material, film thickness and coercive forces as those of the Example 3 were formed in the same manner as that of the Example 3 except for the types and amounts of non-magnetic elements to be added, the rotation speed of the substrate when the films are formed, and further except that the second magnetic layer was formed by one target for comparison purpose. In order to examine the stability of the recorded bits 4f of Fig. 3 for each of the samples including the Example 3, the magnitude of the magnetic field at which the magnetizations of the first and second magnetic layers are reversed was checked under the application of an external magnetic field. Then, the stability of the recorded bits 4f without application of the external magnetic field was examined. In Table 2, o indicates stable and x indicates not stable. For each of the sample shown in Table 2, only the film thickness of the second magnetic layer was changed to prepare a new sample, and the film thickness of the second magnetic layer at which the recorded bits 4f started to be unstable was checked. The results are shown in Table 2. It is seen from Table 2 that in the samples prepared by separate evaporation sources (sputtering sources) for the second magnetic layer for the magnetic material and the non-magnetic material, the drop of the Curie point is small and the stability of the bits 4f is high. In the samples 4-4, 4-8 and 4-12, the stabilities of bits 4f are x because the coercive force of the second magnetic layer was lowered. The effect of separation of the evaporation sources is remarkable for the material such as Cr or Al which can greatly lower the curie point when it is added to the transition metal.

Claims: A magneto-optical recording medium comprising a first magnetic layer (2) and a second magnetic layer (3) having a higher Curie point (TC) and, at room temperature, a lower coercive force (H L ) than those of the first magnetic layer (2), the layers being coupled magnetically together by magnetic exchange coupling forces, in which the coercive forces of both layers (2, 3) are higher than the exchange force acting on each so that, at room temperature and in the absence of an external field, the layers (2, 3) can exist stably in either parallel or anti-parallel magnetisation states, the first layer (2) and the second layer (3) each comprising an alloy of a rare earth element and a transition metal, characterised in that said second layer alloy is alloyed with an element which does not exhibit ferromagnetism at ambient temperature, the non-ferromagnetic element being one or more of the group consisting of Cu, Ag, Ti, Mn, B, Pt, Si, Cr, Al and Ge. A medium according to claim 1 wherein the non-ferromagnetic element makes up 2 - 70 atomic % of the second magnetic layer (3). A medium according to claim 2 wherein the amount of the non-ferromagnetic element is 5 - 20 atomic % of the second magnetic layer (3). A medium according to any preceding claim in which the first layer (2) and the second layer (3) each comprises an alloy selected from a group consisting of GdCo, GdFe, TbFe, DyFe, GdTbFe, TbDyFe, GdTbFeCo, TbFeCo and GdTbCo. A medium according to any preceding claim wherein the first magnetic layer (2) has a Curie point lying between 70 and 180°C, and the second magnetic layer (3) has a Curie point lying between 100 and 400°C, and being higher than that of said first layer (2). A medium according to any preceding claim, wherein the first magnetic layer (2) has a coercive force of 240-8000 KA/m (3 to 10 K Oe), and the second magnetic layer (3) has a coercive force of 40-160 KA/m (0.5 to 2K Oe). A magneto-optical recording disc comprising a medium according to any preceding claim. A medium according to any of claims 1-6 further comprising at least one protection layer (11, 12) comprising a non magnetic material. A method of manufacturing a magneto-optical recording medium according to claim 1 comprising the steps of depositing each of said layers in turn onto a substrate (1), the second magnetic layer being formed by simultaneously evaporation depositing rare earth element - transition metal material and non-ferromagnetic element material from separate, spaced evaporation sources. A recording method using a recording medium according to any one of claims 1-8 comprising the steps of: (a) applying (4b) a first magnetic field large enough to magnetize said second magnetic layer (3) in one direction but not large enough to reverse magnetization of said first magnetic layer (2), and (b) selectively effecting a first type of recording or a second type of recording depending on the power of a light beam controlled by an information signal; in said first type of recording (4c), using a light beam having a power large enough to heat said recording medium to the vicinity of the Curie point of said first magnetic layer (2) while a second field bias is applied so that the magnetization of said first magnetic layer (2) adopts a direction stable relative to said second magnetic layer (3) while the direction of magnetization of said second magnetic layer (3) is kept unchanged; and in said second type of recording (4d), using a light beam having a power large enough to heat said recording medium to the vicinity of a Curie point of said second magnetic layer (3) while a bias field is applied so that the magnetization of said second magnetic layer (3) is reversed and said first magnetic layer (2) is magnetized in a direction stable relative to said second magnetic layer (3).

Inventors: OSATO YOICHI, OSATO, YOICHI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1988
Main classification: G11B13
IPC codes: G11B11