MAGNETOOPTICAL RECORDING MEDIUM

Abstract: The multilayer magnetooptical recording medium has a data-recording layer and an initialising layer, and it enables light-modulated overwriting to be effected. The neighbouring layers are coupled together by an exchange force inclusive of an auxiliary layer. This is provided to efficiently transfer the data from the initialising layer onto the data-recording layer.

Description: Technical Field This invention relates to a magneto-optic recording medium that is directly overwritable by optical modulation, enabling new information to be written directly over old information. Background Art In Fig. 92, (a) is an oblique view of the main parts of a prior-art magneto-optic read-write device as shown, for example, in Preprints of the 34th Joint Congress of Applied Physics , Spring 1987, 28 P-Z L-3; (b) is a sectional view illustrating optical reading and writing of the recording medium; and (c) is a plot of the laser power variations for writing information in areas on the recording medium. In these drawings, 1 is a magneto-optic recording medium comprising a glass or plastic substrate 2, a first magnetic layer 3, and a second magnetic layer 4. An exchange coupling force acts between the first and second magnetic layers 3 and 4, tending to align their magnetization in the same direction. A laser beam is focused by an objective lens 5 onto a spot 6 on the information medium 1. The numeral 7 indicates areas in which the direction of magnetization in the first magnetic layer 3 is upward in Fig. 92 (b), this indicating the recording of binary "1" data. An initializing magnet 9 generates a magnetic field of substantially 4x10 5 AT/m (5000 oersteds) to initialize the second magnetic layer 4. A bias magnet 8 disposed facing the objective lens 5 with the Information medium 1 in between generates a magnetic field of substantially 1.6-4.8x10 4 AT/m (200 to 600 oersteds). In Fig. 92 (c) laser power is shown on the vertical axis and areas are indicated on the horizontal axis. The laser power is modulated to record the information "1" in the region R1 and the information "0" in the region R0. The dash-dot line in Fig. 92 (a) separates new data (DN) on the left from old data (DO) on the right. The operation will be explained next. The recording medium 1 is rotated in the direction of the arrows in Fig. 92 (a) and (b) by a support and driving mechanism not shown in the drawing. The first magnetic layer 3 has the same properties as the recording layer in the media used in general magneto-optic disks comprising, for example, Tb 21 Fe 79 . and here too it functions as a reading and writing layer. The second magnetic layer 4, called the auxiliary layer, comprises Gd 24 Tb 3 Fe 73 , for example, and provides the overwrite function, enabling new information to be written over old information in real time. The Curie temperatures Tc1 and Tc2 of the first and second magnetic layers 3 and 4, their room-temperature coercivities Hc1 and Hc2, and their room-temperature exchange coupling strengths Hw1 and Hw2 satisfy the following relations: Tc1 < Tc2 Hc1 - Hw1 > Hc2 + Hw2 First the reading of information recorded in the first magnetic layer 3 (the recording layer) will be explained. As shown in Fig. 92 (b), the first magnetic layer 3 is magnetized in the up direction to represent a "1" and in the down direction to represent a "0." When this information is read, the first magnetic layer 3 is illuminated by the beam spot 6, and the magnetic orientation of the first magnetic layer 3 in the beam spot 6 is transformed by the well-known optical Kerr effect to optical information, in which form it is detected. Fig. 93 indicates the temperature changes in the magnetic layers in the spot caused by the laser beam power, with A corresponding to the intensity of the laser beam that illuminates the recording medium 1 during reading. At this intensity the maximum temperature increase in the first and second magnetic layers 3 and 4 in the beam spot 6 does not attain the Curie temperatures Tc1 and Tc2 of these layers, so the illumination in the beam spot does not erase the direction of magnetization; that is, it does not erase the recorded information. Next the overwriting operation will be explained. The initializing magnet 9 in Fig. 92 generates a magnetic field of intensity Hini in the direction of the arrow b (up) in the drawing. This field Hini is related to the coercivity and exchange coupling strength of the first and second magnetic layers 3 and 4 as follows: Hc1 - Hw1 > Hini > Hc2 + Hw2 As a result, when the information medium 1 revolves in the direction of the arrow a in Fig. 92 (b), those parts of the second magnetic layer 4 that pass over the initializing magnet 9 are uniformly magnetized in the up direction, regardless of the magnetic alignment of the first magnetic layer 3. The first magnetic layer 3 itself is not affected at room temperature by the magnetic field of the initializing magnet or by the exchange coupling force exerted by the second magnetic layer 4, so it remains in its previous state. To write a "1," which means to magnetize the first magnetic layer 3 in the up direction, the laser beam is modulated to the intensity B in Fig. 93. The temperature in the beam spot 6 then rises above the Curie temperature Tc1 of the first magnetic layer 3, but does not reach the Curie temperature Tc2 of the second magnetic layer 4. Consequently, the first magnetic layer 3 loses its magnetization, while the second magnetic layer 4 retains the upward magnetic alignment given by the initializing magnet 9. As the disk turns and the area leaves the illumination of the beam spot 6, when the temperature of the first magnetic layer 3 falls below its Curie temperature Tc1, the magnetic alignment of the second magnetic layer 4 is transferred to the first magnetic layer 3, so that the first magnetic layer 3 becomes magnetized in the up direction, corresponding to a "1." To record a "0," which means to magnetize the first magnetic layer 3 in the down direction, the laser beam is modulated to the intensity C in Fig. 93. The temperature in the beam spot 6 then rises above both the Curie temperature Tc1 of the first magnetic layer 3 and the Curie temperature Tc2 of the second magnetic layer 4. Consequently, the first and second magnetic layers 3 and 4 both lose their magnetization. As the disk turns and the area leaves the illumination of the beam spot 6, when the temperature of the second magnetic layer 4 falls below its Curie temperature Tc2, the second magnetic layer 4 is magnetized in the down direction by the weak magnetic field applied in the direction of the arrow c (down) in Fig. 92 by the bias magnet 9. Moreover, when the temperature of the first magnetic layer 3 falls below its Curie temperature Tc1, the magnetic alignment of the second magnetic layer 4 is transferred to the first magnetic layer 3, so that the first magnetic layer 3 becomes magnetised in the down direction, corresponding to a "0". By the above overwriting operations, new information can be written over old information in real time by modulating the laser beam power between the values B and C in Fig 93 according to the binary codes "0" and "1" of the new information. Since the prior -art magneto-optic recording medium is structured as above, however, it has the problems that an initializing magnet with a strong magnetic field is required and the overall structure of the read-write apparatus is complex and large in size. The present invention is directed toward a solution of such problems, as highlighted above, an object being to obtain a magneto-optic recording medium that does not require an initializing magnet during writing or overwriting operations and can be easily overwritten. Another example of a prior -art read-write device is the subject of published European patent application No. 0 257 530 which relates to a thermo-magnetic recording method which has first and second magnetic thin films. In a first heating state, the layers are heated to a temperature higher that the Curie temperature of the first magnetic thin film but will not cause the inversion of the magnetic moment of the second magnetic thin film. In the second heating state layers are heated to a temperature which is higher than the Curie temperature of the first magnetic thin film but which is also high enough to cause the inversion of the magnetic moment of the second magnetic thin film. The heating condition is modulated according to an information signal to be recorded. EP-4-0258978 also relates to a magneto-optic recording medium One particular example of such a medium, shown in Figures 56 to 59, employs a pre-grooved translucent substrate with four magnetic layers superimposed thereon. EP-A-288069 similarly describes an example of a four-layer recording medium referred to as "Embodiment 1". Disclosure of the invention As is known from EP-A-0288069 the present invention relates to a magneto-optic recording medium comprising a first magnetic layer having perpendicular magnetic anisotropy; a second magnetic layer provided on the first magnetic layer and coupled to it by an exchange force a third magnetic layer provided on the second magnetic layer and coupled to it by an exchange force, and a fourth magnetic layer provided on the third magnetic layer and coupled to it by an exchange force, wherein the following relationships are satisfied: Tc1<Tc2, Tc3<Tc2, and Tc3<Tc4 where, Tc1: Curie temperature of first magnetic layer; Tc2: Curie temperature of second magnetic layer; Tc3: Curie temperature of third magnetic layer, Tc4: Curie temperature of fourth magnetic layer. In contrast to EP-A-0258978 and in accordance with the present invention Tc1<Tc4, the magnetization of the first magnetic layer is not reversed due to the reversal of the magnetization of the second magnetic layer when at room temperature: the following relationships are additionally satisfied at room temperature: Hc1>Hw1(2) + Hb and Hc4>Hw4(3) and there exists a temperature between room temperature and Tc1 or Tc3, whichever is lower, at which the following relationships are also satisfied: Hc2<Hw2(3)- Hw2(1) and Hc3<Hw3(4) - Hw3(2) where, Hc1: coercivity of first magnetic layer; Hc2: coercivity of second magnetic layer; Hc3: coercivity of third magnetic layer; Hc4: coercivity of fourth magnetic layer; Hwi(j): reversal field shift in i-th layer due to exchange coupling force between j-th layer and i-th layer; Hb: external magnetic field applied during writing in the direction of magnetization of the first magnetic layer at room temperature in the state in which sublattices of the first magnetic layer are aligned with sublattices of the second magnetic layer. Preferably Tc2≤Tc4. In accordance with a further feature of the invention, an interface control layer can be disposed at an interface between magnetic layers, for controlling the exchange force. The invention will now be described in more detail with reference to the following drawings. Brief Description of Drawings Fig. 1 (a) is an oblique view of the main parts of a magneto-optic recording apparatus usable with a magneto-optic recording medium, in accordance with the present invention and presented to introduce the principles underlying the present invention (b) is a partial sectional view illustrating optical reading and writing of a two-layer magneto-optic recording medium, and (c) is a plot of laser power variations for recording information in areas on the magneto-optic recording medium. Fig. 2 is a drawing illustrating the relationship between the temperature in the first magnetic layer of the two-layer magneto-optic recording medium and the reversal field. Fig. 3 is a drawing showing the specific structure of the two-layer magneto-optic recording medium. Fig. 4 shows the states of the layers of the two-layer magneto-optic recording medium during the writing of information. Fig. 5 illustrates the magnetization states of an alloy of a rare-earth metal and transition metal having a room-temperature compensation temperature. Fig. 6 (a) is a partial sectional view illustrating optical reading and writing of a three-layer magneto-optic recording medium again presented to introduce the principles underlying the present invention, and (b) is a plot of laser power variations for recording information in areas on the magneto-optic recording medium. Fig. 7 shows the states of the layers of the three-layer magneto-optic recording medium during the writing of information. Fig. 8 is an illustrative drawing showing the possibility of overwriting in a room-temperature environment. Fig. 9 is a drawing showing the specific structure of an inventive four-layer magneto-optic recording medium, in accordance with the present invention. Fig. 10 illustrates the states of the layers of an inventive four-layer magneto-optic recording medium in accordance with the present invention during the writing of information. Figs. 11 through 91 illustrate the conditions of inventive four-layer magneto-optic recording media in accordance with the present invention having layers of different compositions. Fig. 92 (a) is an oblique view of the main parts of a prior-art magneto-optic read-write apparatus, (b) is a partial sectional view illustrating optical reading and writing of the related magneto-optic recording medium, and (c) is a plot of laser power variations for recording information in areas on the magneto-optic recording medium. Figure 93 indicates the temperature changes in the magnetic layers in the spot caused by the laser beam power. Best Mode of Carrying Out the Invention A detailed description will be given of an overwritable magneto-optic recording medium that does not require an initializing magnet for writing and overwriting operations. The 4-layer magneto-optic recording medium in accordance with the present invention will later be described with reference to more simple 2-layer and 3 layer arrangements which are sufficient to introduce the principles necessary for understanding the operation of the 4-layer structure. In Fig. 1, (a) is an oblique view of the main parts of a magneto-optic recording apparatus comprising a magneto-optic recording medium, (b) is a partial sectional view illustrating optical reading and writing of a two-layer magneto-optic recording medium, and (c) is a plot of laser power variations for recording information in areas on the magneto-optic recording medium. In these drawings, 11 is a magneto-optic recording medium, 20 is a laser beam emitted by a beam-emitting device for illuminating the magneto-optic recording medium 11 and reading and writing information, 16 is a beam spot formed by focusing the laser beam 20 with an objective lens 5 so as to illuminate the magneto-optic recording medium 11, and 18 is a magnet that applies a magnetic field in a fixed direction to the part of the magneto-optic recording medium 11 illuminated by the beam emitting device. The magneto-optic recording medium 11 is rotationally driven in the direction of the arrow (a) in the drawings. In the arrangement which will subsequently be described, reference will be made to Figure 2 wherein the solid line indicates the temperature characteristic of the reversal field when the laser intensity is R 1 in Figure 1(c). The dashed line indicates the characteristic when the laser intensity is R 0 . when the laser intensity is R 1 , the temperature in the illuminated beam spot 16 rises to Tr 1 . When the laser intensity is R 0 , the temperature in the illuminated beam spot 16 rises to Tr 0 . An arrangement will be described in which the first and second magnetic layers comprise an alloy of rare-earth (RE) and transition-metal (TM) elements. When an RE-TM alloy is selected for both the first and second magnetic layers, the direction and magnitude of the externally observable magnetization of each alloy depends on the direction and magnitude of the magnetization magnetic moment per unit volume) of its TM and RE sublattices. Let the magnetic direction and magnitude of the TM sublattice be represented by a dotted line vector , the magnetic direction and magnitude of the RE sublattice by a solid-line vector ↑, and the magnetic direction and magnitude of the alloy as a whole by a double-line vector . The vector is expressed as the sum of the vector and the vector ↑. Due to an interaction between the TM sublattice magnetization and the RE sublattice magnetization in the alloy, the vector and the vector ↑ always point in opposite directions. When both are equal in magnitude, the sum of and ↑ or and ↓ is zero, so the magnetization of the alloy is zero (that is, the magnitude of the externally apparent magnetization is zero). The alloy composition that produces this zero value is called the compensation composition. At other compositions, the alloy has a magnetization equal in strength to the difference in strength between the two sublattice magnetizations, oriented ( or ) in the direction of the stronger vector. The magnetization of this vector is externally observable. For example, ↑ and ↓ gives , and ↑ and gives . When the strength of the RE sublattice magnetization vector exceeds the strength of the TM sublattice magnetization vector, the alloy is said to be RE-rich, and when the strength of the TM sublattice magnetization vector exceeds the strength of the RE sublattice magnetization vector, the alloy is said to be TM-rich. The compositions of the first and second magnetic layers can both be classified as RE-rich or TM-rich. Fig. 3 is a sectional view of a magneto-optic recording medium comprising such RE-TM alloys, in which 2 is the substrate, 21 is a first amorphous magnetic alloy layer of MFeCo (in which M represents Tb or Dy or a combination of both), and 22 is a second amorphous magnetic alloy layer of GdTbFe. A non-magnetic substance such as glass or plastic is used for the substrate 2. The MFeCo ternary first amorphous magnetic alloy layer 21 (which will be called the first magnetic alloy layer) has a composition of M x (Fe 1-y Co y ) 1-x , where 0.15 < x < 0.3 and 0 < y ≤ 0.50. The second amorphous magnetic alloy layer 22 (which will be called the second magnetic alloy layer) has a composition of (Gd 1-y Tb y ) x Co 1-x , where 0.15 < x < 0.35 and 0.3 ≤ y ≤ 1. Magnetic layers such as the above can be formed by methods such as sputtering or vacuum evaporation deposition. Specific examples employing such an arrangement will now be described in detail. Example 1 Ferrimagnetic layers as follows are formed on a substrate by sputtering. 1st magnetic alloy layer Tb 23 Fe 72 Co 5 5x10 -8 m (500 angstroms) 2nd magnetic alloy layer Gd 14 Tb 14 Co 72 1.5x10 -7 m (1500 angstroms) The magnetic layers are coupled by an exchange force. The Curie temperature of the first magnetic alloy layer 21 is substantially 180°C. The second magnetic alloy layer 22 has a reversal field of 79600 A/m (1000 oersteds) or more from room temperature to 250°C, so the second magnetic alloy layer 22 does not undergo magnetic reversal within the operating temperature range. At a temperature of substantially 150°C the exchange force exceeds the coercivity of the first magnetic alloy layer 21, the difference between the exchange force and the coercivity reaching a maximum value of substantially 79600 A/m (1000 oersteds.) The external magnet 18 generates a constant magnetic field of substantially 79600 A/m (1000 oersteds) oriented in the up direction. The second magnetic alloy layer 22 is magnetized uniformly in the up direction. The direction of the magnetic field generated by the external magnet 18 is opposite to that shown in the preceding set-up. The writing of information on a magneto-optic recording medium 23 of this composition will be explained with reference to Fig. 4. As shown at (1) or (5) in Fig. 4, the composition of the first magnetic alloy layer of Tb 23 Fe 72 Co 5 is the compensation composition at room temperature, so the net magnetization of the alloy is zero. The second magnetic alloy layer of Gd 14 Tb 14 Co 72 is RE-rich and has a net magnetization in the up direction. In this example, a "0" is recorded when the direction of magnetization of the TM sublattice of the first magnetic alloy layer 21 is down, and a "1" is recorded when the direction is up. The writing of a "1" is shown at (1) to (4) in Fig. 4. First, when a domain is illuminated by a laser beam spot of intensity R1 [see (c) in Fig. 1], its temperature rises to Tr1, exceeding the Curie temperature of the first magnetic alloy layer 21, so the first magnetic alloy layer 21 is demagnetized [(2) in Fig. 4]. When the rotation of the disk carries the domain out of the beam spot, the temperature of the first magnetic alloy layer falls. Hw1 - Hc1 reaches a maximum in the vicinity of Tp in Fig. 2, but the bias field Hb exceeds this value (|Hb| > Hw1 - Hc1), so the first magnetic alloy layer receives a net magnetization in the (up) direction of the bias field Hb. At this temperature, the first magnetic alloy layer is TM-rich. This is because, from Fig. 5, the compensation point is close to room temperature, making the alloy TM-rich at higher temperatures. Accordingly, the TM sublattice is magnetized in the up direction [(3) in Fig. 4]. When the temperature falls further to room temperature, the first magnetic alloy layer has the compensation composition [(4) in Fig. 4]. The recording of a "0" is shown at (5) to (8) in Fig. 4. First, when a domain is illuminated by a laser beam spot of intensity R0, its temperature rises to Tr0, [(6) in Fig. 4]. When the rotation of the disk carries the domain out of the beam spot, the temperature of the first magnetic alloy layer falls. As explained previously, in the vicinity of Tp in Fig. 2 (substantially 150°C), the magnetic alignment of the second magnetic alloy layer is transferred by the exchange force to the first magnetic alloy layer (Hb < Hw1 - Hc1), so the TM sublattice of the first magnetic alloy layer is magnetized in the down direction. [(7) in Fig. 4]. When the temperature falls further to room temperature, the first magnetic alloy layer has the compensation composition [(8) in Fig. 4]. Optically modulated direct overwriting is thus possible by modulating only the intensity of the laser beam. When a signal was recorded in this way with a linear speed of 6m/s and pit length of 0.8 to 5 micrometers, the field generated by the external magnet 18 being 1000 oersteds and the laser beam being modulated between a peak power of 16mW and bottom power of 5mW, an erasability characteristic of 25dB or greater was obtained. The signal was read with a laser power of 1.5mW. Example 2 to 8 It suffices for the coercivity of the second magnetic layer 14 to have a sufficiently large magnitude in the vicinity of the Curie temperature of the first magnetic layer 13. The magneto-optic information recording media shown in Table 1 were obtained by sputtering magnetic layers on a glass substrate by the same method as in the first example. 1ST MAGNETIC LAYER 2ND MAGNETIC LAYER COMPOSITION THICKNESS (Å) x10 -10 m COMPOSITION THICKNESS (Å) x10 -10 m MEDIUM 2 Tb 23 Fe 72 Co 5 500 Gd 15 Tb 14 Co 71 1500 MEDIUM 3 Tb 23 Fe 72 Co 5 400 Gd 14 Tb 14 Co 72 1500 MEDIUM 4 Tb 23 Fe 72 Co 5 400 Gd 14 Tb 16 Co 70 1500 MEDIUM 5 Tb 23 Fe 72 Co 5 500 Gd 14 Tb 14 Co 72 1300 MEDIUM 6 Tb 23 Fe 72 Co 5 400 Gd 14 Tb 14 Co 72 1300 MEDIUM 7 Dy 23 Fe 72 Co 5 500 Tb 30 Co 70 1500 MEDIUM 8 Tb 23 Fe 72 Co 5 500 Tb 33 Co 67 1500 When a magneto-optic recording apparatus using the other magneto-optic information recording media listed as examples of this invention in Table 1 was operated at a linear speed of 6m/s, light-modulated direct overwriting was possible as in the first example. Erasability characteristics of 20dB or greater were obtained as shown in Table 2, and characteristics of 23 to 35dB were obtained with optimum adjustement of the power. GENERATED FIELD ((Oe)) x79.6 A/m PEAK POWER (mW) BOTTOM POWER (mW) MEDIUM 2 1000±100 12.0 to 17.0 4.0 to 7.0 MEDIUM 3 1200±100 10.0 to 15.0 4.0 to 7.0 MEDIUM 4 1300±100 11.0 to 17.0 4.0 to 7.0 MEDIUM 5 1000±100 13.0 to 17.0 4.5 to 7.5 MEDIUM 6 1200±100 12.0 to 15.0 4.5 to 7.5 MEDIUM 7 800±100 9.0 to 17.0 3.5 to 7.5 MEDIUM 8 1200±100 12.0 to 17.0 4.0 to 8.0 Another arrangement of a magneto-optic recording medium has layers with the following composition and thickness. Example 9: first magnetic alloy layer Tb 23 Fe 67 Co 10 ((thickness: 5x10 -8 m (500 angstroms)) second magnetic alloy layer Gd 12 Tb 12 Co 76 ((thickness: 1.5x10 -7 m (1500 angstroms)) Good overwriting characteristics similar to those of example 1 were obtained. Example 10 Substrate: 1.2mm-thick plastic substrate TbFeCo ternary first amorphous magnetic alloy layer: Tb 23.6 (Fe 90 Co 10 ) 76.4 Thickness: 5x10 -8 m (500 angstroms) Room-temperature coercivity: Approx. 10K oersteds Curie temperature: 180°C GdTbCo ternary second amorphous magnetic alloy layer: (Gd 50 Tb 50 ) 24 Co 76 Thickness: 1.8x10 -7 m (1800 angstroms) Room-temperature coercivity: Approx. 1K oersted Curie temperature: ≥ 300°C After being initialized once at 796000 A/m(10.000 oersteds), the magneto-optic recording medium of example 10 was successfully overwritten 1000 times or more at a disk speed of 6m/s with the recording field held at 79600 A/m (1000 oersteds). The writing power of the laser beam was modulated between 15mW and 5mW at rates of 1MHz and 1.5MHz. Examples 11 to 15 Magneto-optic information recording media having first magnetic layers and second magnetic layers with the compositions listed in Table 3 but otherwise similar to example 10 were obtained and their overwritability by a single light beam was investigated as in example 10. Comparative examples 1 and 2 Magneto-optic information recording media having first magnetic layers and second magnetic layers with the compositions listed in Table 3 but otherwise similar to example 10 were obtained as comparative examples, and their overwritability by a single light beam was investigated as in example 10. 1ST MAGNETIC LAYER 2ND MAGNETIC LAYER OVERWRITABLE COMPOSITION THICKNESS (Å)x10 -10 COMPOSITION THICKNESS (Å)x10 -10 MEDIUM 11 Tb 23.6 (Fe 90 Co 10 ) 76.4 500 (Gd 65 Tb 35 ) 23.5 Co 76.5 1800 YES MEDIUM 12 Tb 22.4 (Fe 90 Co 10 ) 77.6 400 (Gd 70 Tb 30 ) 22.0 Co 76.0 1800 YES MEDIUM 13 Tb 24.0 (Fe 95 Co 5 ) 76.0 500 Tb 24.2 Co 75.8 1500 YES COMPARATIVE EXAMPLE 1 Tb 24.2 (Fe 95 Co 5 ) 75.8 500 (Gd 60 Tb 40 ) 15.0 Co 85.0 1800 NO COMPARATIVE EXAMPLE 2 Tb 25.1 (Fe 90 Co 10 ) 74.9 500 (Gd 56 Tb 44 ) 35.0 Co 65.0 1800 NO MEDIUM 14 Tb 27.3 (Fe 90 Co 10 ) 72.7 400 (Gd 60 Tb 40 ) 23.9 Co 76.1 1500 YES MEDIUM 15 Tb 23.6 (Fe 50 Co 50 ) 76.4 400 (Gd 50 Tb 50 ) 24.5 Co 75.5 1800 YES Next an overwritable magneto-optic recording medium requiring no initializing magnet and having a three-layer structure will be described. In Fig. 6, reference numerals identical to those in Fig. 1 denote elements similar to those in Fig. 1. The numeral 24 denotes a first magnetic layer, 25 denotes a second magnetic layer, and 26 denotes a third magnetic layer. In this arrangement, the first magnetic layer 24, the second magnetic layer 25, and the third magnetic layer 26 are all TM-RE alloys, and all layers are TM-rich. The first magnetic layer 24 and the second magnetic layer 25 are coupled by an exchange coupling force which aligns the direction of magnetization of the TM sublattices in the two layers. The second magnetic layer 25 and the third magnetic layer 26 are coupled by a similar exchange coupling force. The magnetization of the third magnetic layer 26 is initialized to the up direction in the drawing by, for example, an electromagnet. The first magnetic layer 24 is the recording layer which holds a sublattice magnetization (in this case the magnetization of the TM sublattice) representing the information "0" or "1." The second magnetic layer 25 and the third magnetic layer 26 are provided for the purpose of overwritability. The second magnetic layer 25 is particularly referred to as the auxiliary layer; its sublattice magnetization is transferred to the first magnetic layer 24. (That is, the sublattices of the first magnetic layer 24 are magnetically aligned with the sublattlces of the second magnetic layer.) This enables the first magnetic layer 24 to be magnetized in the desired direction. The third magnetic layer 26 is an initializing layer. The sublattice magnetic alignments of the third magnetic layer 26 are transferred to the second magnetic layer 25, whereby the sublattices of the second magnetic layer 26 can be magnetized in a fixed direction at room temperature. Specific characteristics of the first magnetic layer 24, the second magnetic layer 25, and the third magnetic layer 26 will be described next. The Curie temperatures Tc1, Tc2, and Tc3 of the layers satisfy the relationship: Tc1 < Tc2 < Tc3 In this arrangement all three layers are TM-rich, so the following conditions must be satisfied at room temperature. At room temperature: Hc1 - Hw1 (2) > 0 Hc3 - Hb - Hw3(2) > 0 In the interval from room temperature to Tc1 there exists a temperature at which the following condition is satisfied: Hc2 + Hw2(1) - Hw2(3) < 0 where, Hci: coercivity of the i-th magnetic layer Hwi(j): reversal field shift in i-th layer due to exchange coupling force between j-th layer and i-th layer Hb: field applied by the bias magnet 18 during writing Next the operation will be described. Reading is the same as in the prior art, so the description below pertains to overwriting. As the recording medium passes through the beam spot, it undergoes the environmental changes shown in Table 4. EXTERNAL FIELD A/m TEMPERATURE °C ENVIRONMENT I 0 ROOM TEMP. ENVIRONMENT II Hb ROOM TEMP. ENVIRONMENT III Hb ELEVATED TEMP. ENVIRONMENT IV Hb ROOM TEMP. ENVIRONMENT V 0 ROOM TEMP. Environment III obtains in the substantially 1-micrometer area of the beam spot 16. Environments II and IV obtain in the substantially 1-millimeter regions in front of and behind the beam spot. In environment III, the temperature of the magnetic layers in the beam spot 16 rises to a maximum temperature that depends on the laser intensity. Afterward, when the beam spot 16 has been passed, the temperature falls, returning to room temperature in a space of several tens of micrometers. Overwriting is enabled by binary modulation of the above-mentioned laser intensity. Writing with the stronger laser intensity R1 will be referred to as high writing, and the maximum temperature reached by the magnetic layers at this time will be denoted Thigh. Writing with the weaker laser intensity R0 will be referred to as low writing, and the maximum temperature reached by the magnetic layers at this time will be denoted Tlow. The processes of low writing and high writing will be described in detail with reference to the changes in magnetization in corresponding environments. Low writing Environment I: The first magnetic layer 24 has characteristics satisfying the above equation (1). Since the coercivity Hc1 is greater than the exchange coupling force Hw1(2) exerted by the second magnetic layer 25, the previous state of magnetization is preserved; the sublattices thus retain their up or down state according to the information written previously. As can be seen from equation (3), the coercivity Hc3 of the third magnetic layer 26 is sufficiently high that the pre-existing up-magnetization of its TM sublattice is retained. According to equation (2), the exchange coupling force between the second magnetic layer 25 and the third magnetic layer 26 is strong enough that the TM sublattice in the second magnetic layer 25 is magnetically aligned with the TM sublattice in the third magnetic layer 26; that is, it is magnetized in the up direction. There are accordingly two states in environment I: one in which "1" information is written in the first magnetic layer 24, which is the recording layer; and another in which the information "0" is written. If the upward state of magnetization of the TM sublattice in the first magnetic layer 24 is named the "1" state and the downward state is named the "0" state, the two states are as indicated at (1) in Fig. 7. In the drawings, indicates the magnetization of the TM sublattice, and ↑ indicates the magnetization of the RE sublattice, the length being proportional to the strength of magnetization. In all layers, is longer than ↑ because all magnetic layers in this arrangement are TM-rich. In the "1" state, the sublattices in the first magnetic layer 24 and the second magnetic layer 25 are magnetized in opposite directions, so their mutual states oppose the exchange coupling force; they are in an unstable state with an energy of σw12 at the interface between the two layers. This Instability is represented by cross-hatching in the drawing. The first magnetic layer 24 and the second magnetic layer 25 are able to maintain this unstable state because they satisfy the conditions in equations (1) and (2), respectively. Environment II: The difference from environment I is that a downward-oriented external magnetic field Hb is applied by the bias magnet 18. At (1) in Fig. 7, a somewhat unstable state exists only in the first magnetic layer 24 in the "0" state. Since Hb points down, however, It acts to preserve the direction of magnetization of the first magnetic layer 24 in the "0" state. The other layers are in a sufficiently stable state that their magnetization is not reversed by the external magnetic field Hb which has a strength of at most a few hundred oersteds. The state at (1) in Fig. 7 is therefore maintained in environment II. Environment III: The magnetic layers in the beam spot 16 illuminated with laser intensity R0 are heated to a maximum temperature of Tlow, which satisfies the relationship: Tc1 ≤ Tlow < Tc2 < Tc3 At this Tlow, the first magnetic layer 24 is demagnetized because it is at or above its Curie temperature Tc1. The Curie temperatures Tc2 and Tc3 of the second magnetic layer 25 and the third magnetic layer, however, are high enough that the state of magnetization in environment II is maintained. The magnetization state at this maximum temperature Tlow is accordingly as shown at (2) in Fig. 7. Upon leaving the beam spot 16, the magnetic layers rapidly cool. During the cooling process, at a point where the temperature of the first magnetic layer 24 has fallen a little below Tc1, spontaneous magnetization begins to appear in the first magnetic layer 24. The coercivity of the first magnetic layer 24 is sufficiently small at this temperature, and the exchange coupling force from the second magnetic layer 25, which has a sufficiently high Curie temperature, is comparatively large [Hc1 + Hb < Hw1(2)], that the sublattices of the first magnetic layer 24 are magnetically aligned with the sublattices of the second magnetic layer 25. This state of magnetization is maintained when cooling proceeds and the magnetic layers return to room temperature. Thus at the point of return to room temperature, the state of magnetization is as shown at (3) in Fig. 7. Environments IV and V: All magnetic layers in (3) in Fig. 7 are sufficiently stable that they remain in the same state in environment IV and environment V. Low writing therefore leaves the TM sublattice of the first magnetic layer magnetized in the "1" state, regardless of whether it was formerly in the upward-pointing "1" state or the downward-pointing "0" state. High recording Environments I and II are the same as environments I and II in low recording, at (1) in Fig. 7. Environment III: The magnetic layers in the beam spot 16 illuminated by the laser intensity R1 are heated to a maximum temperature of Thigh, which satisfies the relationships: Tc1 < Tc2 ≤ Thigh < Tc3 At temperature Thigh, both the first magnetic layer 24 and the second magnetic layer 25 lose their magnetization. The Curie temperature of the third magnetic layer 26, however, is high enough that the state of magnetization in environment II is maintained. The magnetization states at this maximum temperature Thigh are therefore as shown at (4) in Fig. 7. When the magnetic layers leave the beam spot 16, they rapidly cool. During the cooling process, at a point at which the temperature of the second magnetic layer 25 is slightly less than Tc2, the second magnetic layer 25 begins to undergo spontaneous magnetization. At temperatures sufficiently higher than Tc1, the coercivity of the second magnetic layer 25 and the exchange coupling force exerted by the third magnetic layer 26 are small enough that the main factor at this temperature is the bias field Hb. The second magnetic layer 25 is thus magnetized in the down direction in alignment with the bias field, as shown at (5) in Fig. 7. A state of instability therefore exists between the second magnetic layer 25 and the third magnetic layer 26, but the second magnetic layer 25 is supported by the relatively strong, downward-pointing force of the bias field [Hw2(3) - Hc2 < Hb], and the Curie temperature Tc3 of the third magnetic layer 26 is sufficiently high that its coercivity remains high enough to hold the upward direction of magnetization [Hc3 - Hb - Hw3(2) > 0]. As the temperature falls, the coercivity of the second magnetic layer 25 and the third magnetic layer 26 increases and they become more stable. When the temperature falls slightly below Tc1, the first magnetic layer 24 begins to undergo spontaneous magnetization, so just as in low writing, the sublattices of the first magnetic layer 24 are magnetically aligned with the sublattices of the second magnetic layer 25. This magnetic alignment remains unchanged as cooling proceeds to room temperature [at (6) in Fig. 7].Upon cooling to room temperature, the magnetic alignment of the sublattices of the third magnetic layer 26 is transferred to the second magnetic layer 25, which satisfies equation (2), as shown at (7) in Fig. 7, while the first magnetic layer 24, which satisfies equation (1), maintains its existing state of magnetization. Environments IV and V: The first magnetic layer 24 and the second magnetic layer 25, which satisfy equations (1) and (2), continue to hold the states of magnetization shown in Fig. 7. High recording therefore leaves the TM sublattice magnetization of the first magnetic layer in the "0" state, regardless of whether it was previously in the up-oriented "1" state or the down-oriented "0" state. Given binary information can therefore be overwritten without difficulty by modulation of the laser intensity between the two values R0 and R1 as shown above. The specific composition of the magnetic layers used is as follows. First magnetic layer Tb 21 Fe 74 Co 5 Second magnetic layer Dy 19 Fe 62 Co 19 Third magnetic layer Tb 20 Co 80 Good overwriting with a C/N ratio of 45dB was obtained by applying a downward-oriented magnetic field of 15920 A/m (200 oersteds) as the bias field. Fig. 8 shows the results (marked with circles) of measurements of the quantities on the left sides of equations (1) to (3) made in the vicinity of room temperature. These results indicate that overwriting is possible in the temperature range from 0°C to 50°C. In the arrangements shown above the first magnetic layer 24, the second magnetic layer 25, and the third magnetic layer 26 were all TM-rich, but other combinations are possible also. Table 5 lists examples for which overwriting has been confirmed. The direction of the bias field is for the case in which the third magnetic layer is magnetized in the up direction. In the column that indicates the type of each layer, the notation (RE) means that the RE sublattice is magnetically dominant at room temperature, but the TM sublattice is magnetically dominant at higher temperatures (above the compensation point). The conditions that permit reading and writing on a three-layer overwritable recording medium like the one described above are summarized as follows. At room temperature: where, Hc1: coercivity of first magnetic layer Hc2: coercivity of second magnetic layer Hc3: coercivity of third magnetic layer Hb : magnetic field applied during writing Hwi(j): reversal field shift in i-th layer due to exchange coupling force between j-th layer and i-th layer a = 1: when the second layer has a compensation temperature at room temperature or higher a = -1: when the second layer does not have a compensation temperature at room temperature or higher b = 1: when the first and second layers are both TM-rich, or both RE-rich b = -1: in other cases c = 1: when the second and third layers are both TM-rich, or both RE-rich c = -1: in other cases By providing one or more magnetic layers with lower Curie temperatures between the second magnetic layer and third magnetic layer of the above-described three-layer magneto-optic recording medium, reading and writing can be better stabilized and the C/N ratio can be improved. Fig. 9 shows a sectional view of the main parts of a magneto-optic recording medium according to this invention, in which 27, 28, and 29 denote a second magnetic layer, a third magnetic layer, and a fourth magnetic layer, respectively. The third magnetic layer is the newly added layer with respect to the 3-layer arrangements previously discussed. The fourth magnetic layer corresponds to the third magnetic layer in a three-layer medium. The recording medium 30 is formed by a method such as sputtering on a glass substrate and has a ferrimagnetic structure such as the following : 1st magnetic layer Dy 23 Fe 68 Co 9 5x10 -8 m (500 angstroms) [compensation composition (at room temperature)] 2nd magnetic layer Tb 25 Fe 60 Co 15 7x10 -8 m (700 angstroms) [RE-rich] 3rd magnetic layer Tb 16 Fe 84 2x10 -8 m (200 angstroms) [TM-rich] 4th magnetic layer Tb 30 Co 70 7x10 -8 m (700 angstroms) [RE-rich] Adjacent magnetic layers are coupled by an exchange force. The fourth magnetic layer 29 has a coercivity of 55700A/m (700 oersteds) or greater at temperatures from room temperature up to substantially 300°C, and does not undergo magnetic reversal due to heating by laser illumination within the range of operating conditions. The second magnetic layer has a compensation temperature equal to or greater than room temperature. The external magnet 18 generates a magnetic field of 15900A/m to 31900A/m (200 to 400 oersteds). The fourth magnetic layer 29 is initially uniformly magnetized in, for example, the up direction by a means such as exposing the recording medium -30 to a magnetic field exceeding the reversal field of the fourth magnetic layer 29. The external magnet 18 generates an upward-pointing field this time. The temperature characteristics of the layers and the magnetic characteristics interrelating the layers are as follows: Tc4 > Tc2 > Tc1 > Tc3 > (room temperature) -Hw1(2) + Hc1 - Hb > 0 (room temperature) Hw1(2) - Hc1 - Hb > 0 (reference temperature: a temperature between room temperature and Tc1) Hw2(3) - Hw2(1) + Hc2 + Hb > 0 (room temperature) Hw3(4) - Hw3(2) - Hc3 - Hb > 0 (a certain temperature between Tc3 and room temperature) -Hw4(3) + Hc4 > 0 (entire operating temperature range) The operation will be explained next. In low writing, the laser output is increased over the read-out power. The area in the beam spot 6 exceeds the reference temperature but does not reach the magnetic reversal temperature of the second magnetic layer 27 (the temperature at which the coercivity becomes smaller than the external field, causing the magnetization to reverse itself to align with the external field), so the TM and RE sublattices of the second magnetic layer 27 do not change their directions of magnetization. At the reference temperature, the magnetic alignment of the second magnetic layer 57 is transferred to the first magnetic layer 24, causing the magnetic alignment of the TM sublattice of the first magnetic layer 24 to point downward as shown at (1) in Fig. 10. At the reference temperature, the first magnetic layer is TM-rich. The third magnetic layer 28 and the fourth magnetic layer 29 make no special contribution to this operation. Even if the third magnetic layer 28 loses its magnetization, it will be remagnetized in the same fixed direction by the exchange force coupling it with the fourth magnetic layer 29. After this, when the layers leave the beam spot and cool to the vicinity of room temperature, the first magnetic layer 24 returns to the compensation composition (Fig. 9). In high writing, the temperature exceeds the magnetic reversal temperature of the second magnetic layer 27 but does not reach the magnetic reversal temperature of the fourth magnetic layer 29. The first and third magnetic layers lose their magnetization, but the magnetic alignments of the sublattices of the fourth magnetic layer 29 do not change. Since the temperature exceeds the compensation temperature of the second magnetic layer, the second magnetic layer becomes TM-rich. [(2) in Fig. 10] At its magnetic reversal temperature, the second magnetic layer 27 is not affected by exchange forces from the first and third magnetic layers, so it is upwardly magnetized in alignment with the field of the external magnet 18. [(3) in Fig. 10] In addition, the magnetic alignment of the second magnetic layer 27 is transferred to the first magnetic layer 24, so the the first magnetic layer is also magnetized in the up direction. Since the exchange force is weakest between the first magnetic layer 24 and the second magnetic layer 27, stronger between the second magnetic layer 27 and the third magnetic layer 28, and strongest between the third magnetic layer 28 and the fourth magnetic layer 29, and since the third magnetic layer 28 is below its Curie temperature at this time, the sublattices of the third magnetic layer 28 align with the sublattices of the fourth magnetic layer 29 [(5) in Fig. 10]. When the temperature falls further and the exchange forces increase, the sublattices of the second magnetic layer 27 are magnetically aligned via the third magnetic layer 28 with the sublattices of the fourth magnetic layer 29, thus returning to their initial state. [(6) in Fig. 10] In this way, light-modulated direct overwriting is possible by modulating only the laser intensity. On a magneto-optic recording medium comprising a substrate with grooves spaced at 1.6 micrometers, provided with a dielectric enhancement layer on which the above-described magnetic layers were formed and to which a magnetic field of 15900A/m (2000 oersteds) had been applied, a C/N ratio of 45dB and erasability characteristic of 40dB or better were obtained at a linear speed of 11m/s with a pit length of 0.76 micrometers. The laser beam was modulated between a peak power of 18mW and a bottom power of 7mW. Embodiment 2 Recording media were formed by sputtering magnetic layers onto a glass substrate as in the preceding embodiment. Completely light-modulated direct overwriting is possible on any of these recording media by optimization of the writing conditions. 1ST MAG. LAYER 2ND MAG. LAYER 3RD MAG. LAYER 4TH MAG. LAYER Dy 23 Fe 68 Co 9 4x10 -8 m (400 Å) Tb 25 Fe 60 Co 15 7x10 -8 m (700Å) Tb 16 Fe 84 2x10 -8 m (200 Å) Tb 30 Co 70 7x10 -8 m (700 Å) Dy 23 Fe 72 Co 5 5x10 -8 m (500 Å) Tb 25 Fe 60 Co 15 7x10 -8 m (700 Å) Tb 16 Fe 84 2x10 -8 m (200 Å) Tb 30 Co 70 7x10 -8 m (700 Å) Dy 23 Fe 68 Co 9 5x10 -8 m (500 Å) Tb 25 Fe 60 Co 15 7x10 -8 m (700 Å) Tb 16 Fe 84 1x10 -8 m (100 Å) Gd 10 Tb 20 Co 70 1.2x10 -7 m (1200 Å) Figs. 11 to 91 list all combinations of different types of layers, giving transition-metal and rare-earth-metal alloy compositions for each layer. The notation used in these drawings is defined below. TM : a transition-metal-rich alloy of transition and rare-earth metals, not having a compensation temperature between room temperature and the Curie temperature RE: a rare-earth-rich alloy of transition and rare-earth metals, having a compensation temperature between room temperature and the Curie temperature re: a rare-earth-rich alloy of transition and rare-earth metals, not having a compensation temperature between room temperature and the Curie temperature t : thickness (angstroms) Ms: saturation magnetic moment (emu/cc) Hc: coercivity (oersteds) Tc: Curie temperature (°C) Sw (or σ w ): interface magnetic barrier energy (erg/cm 2 ) Hwi: total exchange force exerted on the i-th layer Hwi(j): exchange force exerted on the i-th layer by the j-th layer [σ wij /(2|Msi|ti)] Both i and j are counted from the substrate side. ↑ : magnetic alignment of the TM sublattice : net magnetization; sum of the TM sublattice magnetization and the RE sublattice magnetization Tstor: all temperatures within the storage temperature range (ex: -10°C to 60°C) Tread: all temperatures in the range form the lowest temperature during use to the temperature reached during read-out TL: a temperature higher than the reading temperature but lower than Tc1, at which transfer of the magnetic alignment of the second magnetic layer to the first magnetic layer occurs Tini: all temperatures under the temperature conditions during use (or the temperature at a certain location not illuminated by the laser beam, provided for initialization) Tall: all temperatures in the operating temperature range (0°C and above) Tuse: the operating temperatures of the driving apparatus (0°C to 50°C) In each drawing, a1 indicates the state in which "0" information is recorded, and a2 indicates the state in which "1" information is recorded. In the a2 state, cross-hatching indicates a magnetic wall. In all of the preceding embodiments it is important to control the exchange forces between the magnetic layers. For this purpose, interface control layers may be inserted between the magnetic layers. For example, in a two-layer medium such as described for illustrating the present invention, ferrimagnetic layers of the following compositions were sputtered onto a glass substrate, in the following order. 1st magnetic layer Tb 23 Fe 72 Co 5 5x10 -8 m 500 angstroms Interface control layer Tb 26 Fe 70 Co 4 5x10 -9 m 50 angstroms 2nd magnetic layer Tb 30 Fe 70 1.5x10 -7 m 1500 angstroms The interface control layer was formed by increasing the argon gas pressure by a factor of about six in the sputtering process. This medium showed good overwriting characteristics at a linear speed of 6m/s with a peak power of 9mW to 17mW, a bottom power of 4mW to 7.5mW, and a bias field of 23900+/-637A/m (300 ±80 oersteds.) In a medium with four magnetic layers, the following ferrimagnetic layers and dielectric layer were formed by sputtering on a glass substrate: 1st magnetic layer Dy 23 Fe 68 Co 9 5x10 -8 m (500 angstroms) Interface control layer SiN x 1x10 -9 m (10 angstroms) 2nd magnetic layer Gd 13 Dy 12 Fe 60 Co 15 1.2x10 -7 m (1200 angstroms) 3rd magnetic layer Tb 16 Fe 84 2x10 -8 m (200 angstroms) 4th magnetic layer Tb 30 Co 70 7x10 -8 m (700 angstroms) Adjacent magnetic layers, including the layers separated by the interface control layer, are coupled by exchange forces as before. This medium showed good overwriting characteristics at a linear speed of 11m/s with a bias magnetic field of 15900A/m (200 oersteds,) when the laser power was modulated between a peak power of 18mW and bottom power of 7mW. The following, which include the above, can be used as the interface control layer. 1. The interface control layer can be formed by increasing the pressure of the gas used in the sputtering of the normal magnetic layers by a factor of five or more. 2. A dielectric comprising a nitride such as SiN or AlN, or an oxide such as SiO x can be used. 3. The normal magnetic layers can be formed by neutral sputtering using only argon. An interface control layer can be formed by reactive sputtering with admixture of a reactive gas such as oxygen or nitrogen. 4. A rare-earth (RE) - transition-metal (TM) layer in which the RE component is 30at% or greater can be formed under the normal sputtering gas pressure. 5. A non-magnetic metal such as Al or Cu can be used. 6. A magnetic layer with an axis of easy magnetization oriented in the plane of the disk can be used. An interface control layer can be formed by methods such as the above, but methods other than the methods shown here can also be used as long as they permit control of the exchange coupling force. Interface control layers that do not contribute to the essential operation can be added to a four-layer medium, between any of the magnetic layers. As a second method, the coercivities Hc4(A) and Hc4(B) of the two fourth layers (on the pair of substrates that will be joined together) are arranged so that |He(A)| > |Hc4(A)| > |He(B)| > |Hc4(B)|, where He(A) and He(B) are magnetic fields applied by an initial magnetic field applying means. After such a pair of substrates has been joined together, first the magnetic field applying means 43 applies a magnetic field He(A) exceeding the Hc4(A) of the A-side of the disk to align the magnetization of the A-side initializing layer, as in (a) in Fig. 96. Next a magnetic field He(B) that exceeds Hc4(B) but does not exceed Hc4(A) and is oriented oppositely to He(A) is applied as at (b) in Fig. 96 to align the magnetization of the initializing layer on the B-side. This method of initializing the media can also be carried out by reversing the disk during the process, as illustrated at (a) and (b) in Fig. 97. Fig. 98 shows the assembled disk that has been initialized at (a) in Fig. 97. It is also possible for a device to detect the A- and B-sides of an assembled disk in which the fourth layers on the two sides 40A and 40B are initialized in the same direction, to change the orientation of the bias magnetic field, and to perform the final signal processing by taking account of the fact that the orientation of the bit cells is changed. The magneto-optic recording media of this invention can be widely applied to enable high-density, high-speed recording of audio information, visual information, and computer data.

Claims: A magneto-optic recordimg medium comprising a first magnetic layer (24) having perpendicular magnetic anisotropy, a second magnetic layer (27) provided on this first magnetic layer (24) and coupled to said first magnetic layer (24) by an exchange force, a third magnetic layer (28) provided on this second magnetic layer (27) and coupled to said second magnetic layer (27) by an exchange force, and a fourth magnetic layer (29) provided on this third magnetic layer (28) and coupled to said third magnetic layer (28) by an exchange force, wherein the following relationships are satisfied. Tc1<Tc2, Tc3<Tc2, and Tc3<Tc4 where, Tc1: Curie temperature of first magnetic layer; Tc2: Curie temperature of second magnetic layer; Tc3: Curie temperature of third magnetic layer; Tc4: Curie temperature of fourth magnetic layer, characterised in that Tc1<Tc4, the magnetisation of said first magnetic layer is not reversed due to the reversal of the magnetisation of said second magnetic layer at room temperature; the following relationships are aditionally satisfied at room temperature, Hc1>Hw1(2) + Hb, and Hc4>Hw4(3) and there exists a temperature between room temperature and Tc1 or Tc3, whichever is lower, at which the following relationships are satisified: Hc2<Hw2(3) - Hw2(1) and Hc3<Hw3(4) - Hw3(2), where, Hc1: coercivity of first magnetic layer; Hc2: coercivity of second magnetic layer: Hc3: coercivity of third magnetic layer; Hc4: coercivity of fourth magnetic layer, and Hwi(j): reversal field shift in i-th layer due to exchange coupling force between j-th layer and i-th layer, Hb: external magnetic field applied during writing in the direction of magnetisation of the first magnetic layer at room temperature in the state in which sublattices of the first magnetic layer are alligned with sublattices of the second magnetic layer. The magneto-optic recording medium of claim 1, wherein Tc2≤Tc4. The magneto-optic recording medium of claim 1 or 2 having an interface control layer disposed at an interface between magnetic layers, for exchange force control.

Inventors: FUKAMI TATSUYA, NAKAKI Y MITSUBISHI, TAGUCHI MOTOHISA, TOKUNAGA TAKASHI, TSUTSUMI KAZUHIKO, FUKAMI, TATSUYA, NAKAKI, Y., TAGUCHI, MOTOHISA, TOKUNAGA, TAKASHI, TSUTSUMI, KAZUHIKO, FUKAMI, TATSUYA MITSUBISHI DENKI K.K., NAKAKI, Y. MITSUBISHI DENKI K.K., TAGUCHI, MOTOHISA MITSUBISHI DENKI K.K., TOKUNAGA, TAKASHI MITSUBISHI DENKI K.K. SANGYO, TSUTSUMI, KAZUHIKO MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1989
Main classification: G11B11
IPC codes: G11B11