CA2010470C - Magneto-optic recording medium and manufacturing method - Google Patents

Abstract

A magneto-optic recording medium has a recording layer for storing information and an initializing layer which is uniformly magnetized in a fixed direction during the manufacturing process. The medium is directly overwritable by modulation of a laser beam between two power levels. In low writing, the recording layer is heated sufficiently to acquire the magnetization of the initializing layer by means of exchange coupling between the layers. In high writing, the recording layer is heated to a higher temperature, above its Curie temperature, and is magnetized in the opposite direction by an external bias magnet. Additional magnetic layers may be provided between the recording and initializing layers to facilitate high writing. Since the initializing layer is always magnetized in the same direction, no external initializing magnet is required.

Description

2010471~) MAGNETO-OPTIC RECORDING MEDIUM AND MANUFACTURING METHOD

BACKGROUND OF THE INVENTION
This invention relates to a magneto-optic recording medium, more particularly to a magneto-optic recording medium that permits direct overwriting without an initializing magnet, and to a method of manufacturing such a magneto-optic recording medium.
Magneto-optic media are advantageous for data storage due to their high area data density, removability, and erasability. Such media generally take the form of a disk comprising a transparent substrate coated with a magnetic layer. Information is stored as the direction of magnetization of bit cells in the magnetic layer, the up direction representing a "1," for example, and the down direction a "O."
Information is read by focusing a weak beam of polarized light from a laser onto a spot on the spinning disk and detecting the plane of polarization of the reflected light. Due to the well-known Kerr effect, the plane of polarization is rotated slightly to the right or left depending on the direction of magnetization of the bit cells. Information is written by raising the power of the laser beam so as to heat the magnetic layer above its Curie temperature, thereby causing it to lose its magnetization, -and applying an external magnetic field to remagnetlze the magnetic layer in the desired direction when it cools after leaving the beam spot.
To avoid restrictions associated with high-speed switching of magnetic fields, it is preferable that the writing be done by modulating the laser beam rather than magnetic field. One prior-art scheme is to make two passes over the track or sector to be written. In the first pass, the magnetic field is oriented in, for example, the down direction and the laser beam is left on to erase all bits to the "O" state. In the second pass the magnetic field is changed to the up orientation and the laser beam is modulated (switched on and off, for example) to change selected bits to the "1" state. This scheme is too slow for general applications, however, because it requires two rotations of the disk.
Alternative schemes that permit new information to be written directly over old information in a single rotation of the disk have been proposed. The recording medium employed in one of these schemes comprises a transparent substrate coated with a pair of magnetic layers: a recording layer having a comparatively low Curie temperature, and an auxiliary layer having a higher Curie temperature. Inf'ormation is represented by the magnetization of the recording layer. The two layers are 2~10470 exchange-coupled, but at room temperature the coercivity of the recording layer is high enough that the recording layer can retain its magnetization direction despite the exchange coupling with the auxiliary layer.
In-formation is written using two external magnets: a bias magnet and an initializing magnet. The bias magnet is disposed facing the spot il]uminated by the laser beam; the initializing magnet is disposed in front of this spot. The magnetic fields generated by the bias magnet and initializing magnet are oriented in opposite directions:
for example, the initializing field in the up direction and the bias field in the down direction. The initializing field is sufficiently strong to reverse the magnetization of the auxiliary layer, but not strong enough to reverse the magnetization of the recording layer. The bias field is comparatively weak.
To magnetize a bit cell in the up direction, the laser power is modulated to a level that heats the magnetic layers to a temperature above the Curie temperature of the recording layer, but below the Curie temperature of the auxiliary layer. As the bit cell passes over the initializing magnet, the auxiliary layer is magnetized in the up direction. When the bit cell enters the beam spot, the recording layer is demagnetized, but the auxiliary layer retains its up magnetization. When the bit cell leaves the Z0~0470 beam spot and the magnetic layers cool, the up magnetization of the auxiliary layer is transferred to the recording layer by the exchange coupling, this coupling being stronger than the weak bias field and the coercivity of the recording layer.
To magnetize a bit cell in the down direction, the laser beam is modulated to a higher power level that heats both magnetic layers above their Curie temperatures, so that both magnetic layers are demagnetized in the beam spot.
When they leave the beam spot and cool, both layers are magnetized in the down direction by the bias field.
A drawback of this scheme is that having two external magnets complicates the structure of the read-write mechanism.

SUMMARY OF THE INVENTION
It is accordingly an obJect of the present invention to provide a magneto-optic recording medium that can be directly overwritten without an initializing magnet.
A further ob~ect of this invention is to provide a method of manufacturing such a magneto-optic recording medium.
A magneto-optic recording medium comprises a transparent substrate, a recording layer for storing information, and an initializing layer which is uniformly , 201()4, ~
magnetized in a -fixed direction during the manufacturing process. The medium is directly overwritable by modulation of a laser beam between two power levels. In low writing, the recording layer is heated sufficiently to acquire the magnetization of the initializing layer by means of exchange coupling between the layers. In high writing, the recording layer is heated to a higher temperature, above its Curie temperature, and magnetized in the opposite direction by an external bias magnet. The temperature reached in high writing is not high enough for the initializing layer to lose its magnetization. Since the initializing layer always remains magnetized in the same direction, no external initializing magnet is required.
An auxiliary layer may be provided between the recording layer and initializing layer. Normally the auxiliary is magnetized in the same direction as the initializing layer, and in low writing this magnetization is transferred to the recording layer. In high writing, the auxiliary layer is magnetized in the opposite direction by the bias magnet, then this magnetization is transferred to the recording layer by an exchange coupling. Afterward, the initializing layer reverses the magnetization of the auxiliary layer.
A buffer layer with a low Curie temperature may be provided between the auxiliary layer and initializing layer, to ensure that the magnetization of the auxiliary layer is not reversed until after being transferred to the re~ording layer.
Alternatively, the initializing layer can be given a low Curie temperature, in which case it must be remagnetized by the bias magnet after writing.
Overwritability is enabled by relations between the Curie temperatures of the layers, and relationæ between the coercivities and eYch~nge coupling forces of the layers.
Interface control layers may be additionally provided to ~OII~LO1 the strength of exchange coupling.
In one aspect, the present invention provides a magnetic-optic recording medium comprising a first magnetic layer having perpendicular magnetic anisoL~y, a second magnetic layer provided on said first magnetic layer and coupled to said first magnetic layer by an exchange force, and a third magnetic layer provided on said second magnetic layer and coupled to said second magnetic layer by an exchange force, wherein the following relationships are satisfied: Tcl ~ Tc2 and Tc3 < Tc2, where, Tcl: Curie temperature of first magnetic layer; Tc2:
Curie temperature of second magnetic layer; and Tc3: Curie temperature of third magnetic layer; characterised in that the following relationships are additionally satisfied at room temperature: Hcl > Hw1(2) and Hc3 > Hw3(2), and there exists a temperature between room temperature and Tc3 at which the following relationship is satisfied: Hc2 < Hw2(3) - Hw2(1), where, Hcl: coercivity of the first magnetic layer; Hc2:
coercivity of the second magnetic layer; Hc3: coercivity of the third magnetic layer; and Hwi(j): reversal field shift in i-th - 2010~7~
layer due to exchange coupling force between j-th layer and i-th layer.
In another aspect, the present invention provides a method of recording information on a magneto-optic recording material according to the present invention, comprising the steps of:
i) applying an external magnetic field to part of a recording medium according to the present invention, ii) applying a laser beam to said part simultaneously with the application of the magnetic field, and iii) modulating the intensity of the beam ~epDnAing on a bit to be written at said part of the medium.
In yet another aspect, the present invention provides, in a method of manufacturing an overwritable magneto-optic recording medium having two halves, each half having a substrate and having a corresponding initializing layer, the initializing layers each having a magnetization which is not reversed in reading or writing, the steps of: adhering the halves of the recording medium each to the other to form a joined recording medium; applying a first magnetic field stronger than the coercivity of the initializing layers substantially perpendicular to the joined recording medium;
applying a second magnetic field weaker than the coercivity of one of the initializing layers but stronger than the coercivity of the other initializing layer and oriented oppositely to the first magnetic field substantially perpendicular to the joined recording medium, after applying the first magnetic field.

6a - 2010~70 BRIEF DESCRIPTION OF THE DRAWINGS
Figs. lA and lB show the structure of a novel two-layer magneto-optic recording medium and illustrate the direct overwriting operatioh.
Fig. 2 illustrates reversal field characteristics of the first magnetic layer of the medium in Figs. lA and lB.
Fig. 3 illustrates the saturation magnetization characteristic of a ferrimagnetic alloy having a compensation temperature.
Fig. 4A is a sectional view showing the structure of another novel two-layer magneto-optic recording medium.
Fig. 4B illustrates reversal field characteristics of the first magnetic layer of the medium in Fig. 4A.
Fig. 5 illustrates the overwriting of the medium in 6b Fig. 4A.
Fig. 6 shows the structure of a novel three-layer magneto-optic recording medium and illustrates the direct overwriting operation.
Fig. 7 illustrates the overwriting of the medium in Fig. 6 in more detail.
Fig. 8 illustrates magnetic characteristics of the medium in Fig. 6 that enable direct overwriting.
Fig. 9 is a sectional view showing the structure of a novel four-layer magneto-optic recording medium.
Fig. 10 illustrates the overwriting of the medium in Fig. 9.
Figs. 11 to 91 illustrate further novel four-layer magneto-optic recording media and give their magnetic characteristics.
Fig. 92A is a sectional view showing the structure of another novel three-layer magneto-optic recording medium.
Fig. 92B illustrates the saturation magnetization characteristics of the layers of the medium in Fig. 92A.
Fig. 93 illustrates the overwriting of the medium in Fig. 92A.
Fig. 94 illustrates a magneto-optic recording medium comprising a polycarbonate substrate, a dielectric layer, four magnetic layers, and a protective layer.
Fig. 95 illustrates a method of magnetizing the initializing layer of the medium in Fig. 94.
Fig. 96 illustrates a two-sided magneto-optical recording medium comprising two halves that have been magnetized as in Fig. 95, then joined together.
Fig. 97 illustrates a two-sided magneto-optical recording medium comprising two halves that have been joined together, then magnetized as in Fig. 95.
Fig. 98 illustrates another method of magnetizing the initializing layers of a two-sided magneto-optical recording medium.
Fig. 99 illustrates yet another method of magnetizing the initializing layers of a two-sided magneto-optical recording medium.

DETAILED DESCRIPTION OF THE INVENTION
Novel magneto-optic recording media that permit direct overwriting without an initializing magnet will be described with reference to Figs. 1 to 93. These media comprise a plurali~ty of magnetic layers, one of which is a recording layer and another of which is an initializing layer.
Overwriting is enabled by relationships among the Curie temperatures, coercivities, and exchange coupling forces of the magnetic layers, these relationships being the novel features of the invention. Examples of specific compositions of the magnetic layers will be given, but the 20104~0 scope of the invention is not limited to these examples.
Fig. lA is an oblique view of a novel magneto-optic recording medium and the magnetic and optical apparatus directly involved in reading and writing. The magneto-optic recording medium 1 is disk-shaped, and is rotationally driven in the direction of the arrow (a) by a driving mechanism 2. A beam 3 of polarized light emitted from a device such as a semiconductor laser 4 is focused by an ob~ective lens 5, forming a beam spot 7 on the magneto-optic recording medium 1. A bias magnet 9 located below the beam spot 7 generates a magnetic field ~Ib oriented in, for example, the down direction. Information is recorded as the magnetization direction of bit cells 11 in the magneto-optic recording medium 1. AdJacent bit cells having the same magnetization may fuse into a single magnetic domain, as illustrated in the drawing. As the magneto-optic recording medium 1 turns in the direction of the arrow (a), the beam 3 can be modulated between high and low power levels to alter the magnetization o~ the bit cells, thus writing new information directly over old.
Fig. lB is a sectional view of the magneto-optic recording medium 1, showing its structure and the overwriting process in more detail. The magneto-optic recording medium 1 comprises a substrate 13, a first magnetic layer 15, and a second magnetic layer 17. The 2010a~70 substrate 13 is transparent, comprising a material such as plastic or glass. The first magnetic layer 15, which is the recording layer, has perpendicular magnetic anisotropy;
e.g., it has an easy axis of magnetization oriented perpendicular to the plane of the disk. Information is represented by the direction of magnetization of the first magnetic layer 15, the up representing, for example, the binary information "1" and the down direction the binary information "O." The second magnetic layer 17, which is preferably thicker than the first magnetic layer 15, is the initializing layer and is uniformly magnetized in, for example, the up direction during the manufacturing process as will be explained later.
Information recorded in the first magnetic layer 15 is read by operating the beam 3 at a weak power level which does not generate signi~icant heating of the magnetic layers. Upon reflection from the first magnetic layer 15, the plane of polariza~ion of the light is rotated to the right or left according to the direction of magnetization of the first magnetic layer 15. The information stored in the first magnetic layer 15 is read by detecting the plane of polarization of the reflected light as in the prior art.
Information is written by modulating the beam 3 between two higher power levels, which heat the first magnetic layer 15 sufficiently to permit alteration of its magnetic alignment. At the higher of these two power levels, designated R1 in the drawings, the first magnetic layer 15 is magnetically aligned with the bias field Hb through a process that will be described later. At the lower of these two power levels, designated RO, the first magnetic layer 15 is magnetically aligned with the second magnetic layer 17.
These power levels and the resulting written information are illustrated at the lower left in Fig. lB.
Next the magnetic properties of the first magnetic layer 15 and the second magnetic layer 17 will be described.
Let Tcl be the Curie temperature and Hcl be the coercivity of the first magnetic layer 15, and let Tc2 be the Curie temperature and Hc2 be the coercivity of the second magnetic layer 17. These two layers are exchange-coupled by a force which tries to align their magnetization in the same direction. The exchange coupling can be described in terms of its effect on the reversal field, which is the minimum external field that must be applied to reverse the direction of magnetization o~ the layers. In the absence of the exchange coupling, the reversal field of the first magnetic layer 15 would be llcl and the reversal field of the second magnetic layer 17 would be E[c2. The exchange coupling shifts the reversal field of the first magnetic layer 15 by an amount Hw1(2), and shifts the reversal field of the second magnetic layer 17 by an amount l-lw2(1).

The compositions of the magnetic layers are selected so that the first magnetic layer 15 has a lower Curie temperature, that is, Tcl < Tc2 and at room temperature, Hcl > Hw1(2) and Elc2 > Elw2(1).

The reversal fields vary depending on temperature.
Fig. 2 illustrates these variations for the first magnetic layer 15 in two cases: one (on the right) in which the first magnetic layer 15 cools to room temperature after being heated to a temperature TrO slightly less than its Curie temperature Tcl, and one (on the left) in which the first magnetic layer cools to room temperature after being heated to a temperature Trl intermediate between Tcl and Tc2. TrO is the temperature to which the first magnetic layer 15 is heated by the beam 3 when the power level is RO, and Trl is the temperature to which the first magnetic layer 15 is heated when the power level is R1. Temperature is indicated on the horizontal axis in Fig. 2. External field strength is indicated on the vertical axis, with positive values corresponding to the up direction and negative values -to the down direction.
When the first magnetic layer 15 is heated to Trl it loses its magnetization, Hcl and Hw1(2) both becoming zero as shown by the curves on the left in Fig. 2. Then as the first magnetic layer 15 cools below Tcl, it is magnetized according to the applied external field and the exchange coupling force. The exchange force operates in the direction in which the second magnetic layer 17 is magnetized: the up direction in the drawings. The first magnetic layer 15 is magnetized in the up direction if the external field is above EIcl - IIw1(2), and in the down direction if the external field is below -IIcl - Hw1(2). At external fields between Hcl - Hw1(2) and -IIcl - Hw1(2), the direction of magnetization o~ the first magnetic layer 15 is left unchanged.
When the first magnetic layer 15 is heated to TrO the situation is similar except that the shift IIw1(2) in the reversal field is greater, as indicated by the curves on the right in Fig. 2.
The reason for the greater shift in reversal field after heating to TrO is as follows. Since the first magnetic layer 15 absorbs laser light, during laser illumination it is heated to a higher temperature than the second magnetic layer 17. After laser illumination, thermal transfer between the two layers gradually removes this temperature differential. When the first magnetic layer is heated to Trl by the higher laser power level, in cooling to the vicinity of Tp enough time elapses to substantially eliminate the temperature differential between the layers.
When the first magnetic layer 15 is only heated to TrO by the lower laser power, it reaches Tp more quickly and a large temperature differential remains. In the vicinity of Tp in Fig. 2, the second magnetic layer 17 is therefore at a higher temperature on the left side of Fig. 2 than on the right side. The exchange coupling is weaker at higher temperatures, hence the shift Ilw1(2) is smaller on the left side of Fig. 2 than on the right side.
Let Hrl be the minimum value attained by the curve Hcl - Elw1(2) on the left side in Fig. 2, and let HrO be the minimum value attained by this curve on the right side in Fig. 2. Thus HrO < ~Irl, both values being negative and both being attained in the vicinity of Tp. The strength Hb of the bias magnet 9 is set to a value substantially midway between HrO and llrl.
Since Hw1(2) becomes zero at Tcl, while Hc2 does not become zero until Tc2, it can be inferred that the condition Hc2 > Hw2(1) stated earlier is true not only at room temperature but at all temperatures up to Tc2. If Trl is sufficiently below Tc2 and Hc2 is sufficiently high, it will furthermore be true that Hc2 - Elw2(1) > iElbl.

at all temperatures up to Trl, ensuring that the second magnetic layer 17 never undergoes magnetic reversal.
The operations of writing "1" or "O" information by magnetizing the first magnetic layer 15 in the up or down direction can now be explained.
To magnetize a bit cell in the first magnetic layer 15 in the up direction, the beam 3 is modulated to the RO power level: this will be referred to as low writing. When the bit cell enters the beam spot 7 it is heated to the temperature TrO on the right Ln Fig. 2. When the temperature of the bit cell falls slightly below TrO, -~cl - Elw1(2) > llb so that the bit cell is magnetized in the down direction.
However, when the rotation of the disk carries the bit cell out of the beam spot, the bit cell cools. In the vicinity of Tp, -Z0104~0 Hcl - Hw1(2) < llb < O

so the bit cell is magnetized in the up direction. More specifically, the up magnetization of the second magnetic layer 17 is transferred to the first magnetic layer 15 by the exchange coupling, which overcomes both the coercivity of the first magnetic layer 15 and the bias field Hb. This magnetization is retained when the bit cell cools to room temperature, hence low writing leaves the recording layer aligned with the initializing layer, in the up direction.
To magnetize a bit cell in the first magnetic layer 15 in the down direction, the beam 3 is modulated to the R1 power level: this will be referred to as high writing.
When the bit cell enters the beam spot 7 it is heated to the temperature Trl on the left in Fig. 2. When the rotation of the disk carries the bit cell out of the beam spot and the bit cell cools below the Curie temperature Tcl of the first magnetic layer 15, at first Hb < -Hcl - Hw1(2), so the first magnetic layer 15 is magnetized in the down direction of the bias field Hb. During subsequent cooling, -Hcl - Hw1(2) < Hb < Hcl - Hw1(2), so the downward magnetization is retained as the bit cell cools to room temperature. Thus high writing leaves the recording layer aligned with the bias field, in the down direction.
The first and second magnetic layers can advantageously be made of amorphous, ferrimagnetic alloys of rare-earth (RE) and transition-metal (TM) elements. A ferrimagnetic alloy comprises RE and TM sublattices which are always magnetized in opposite directions. If the magnitude of the magnetization (magnetic moment per unit volume) of the TM
and RE sublattices is equal, the alloy is said to have the compensation composition, and its net magnetization, which is externally observable, is zero. The temperature at which this condition obtains is called the compensation temperature. If the magnetization of the RE sublattice is stronger, the alloy is said to be RE-rich, and has an externally observable net magnetization oriented in the same direction as the magnetization of the RE sublattice. If the magnetization of the TM sublattice is stronger, the alloy is said to be TM-rich, and has an externally observable net magnetization oriented in the same direction as the magnetization of the TM sublattice.
- Fig. 3 illustrates the saturation magnetization characteristics of a ferrimagnetic alloy having a 20104'70 compensation temperature. The saturation magnetization curve MS indicates the maximum possible net magnetization of the alloy. The MR curve indicates the corresponding magnetization of the rare-earth sublattice; the MT curve indicates the corresponding magnetization of the transition-metal sublattice. At low temperatures, the alloy is RE-rich. At the compensation temperature, the net magnetization is zero, hence the saturation magnetization is zero. At temperatures between the compensation temperature and Curie temperature, the alloy is TM-rich. In this range, the algebraic (signed) value of MS is indicated by a dashed line in Fig. 3, while the absolute value is indicated by a solid line. Above the Curie temperature, all magnetization disappears.
Both the first and second magnetic layers of a magneto-optic recording medium may be RE-TM alloys. More particularly, the first magnetic layer may be an alloy of iron, cobalt, and one or both of the rare-earth elements terbium and dysprosium, while the second magnetic layer may be an alloy of iron with the rare-earth elements gadolinium and terbium. Using the letter M to represent terbium or dysprosium or a combination of both, the compositions of the two layers should be as follows:
1st magnetic layer: Mx(Fel_yCoy)l-x 0.15 < x < 0.3, 0 < y < 0.50 2nd magnetic layer: (Gd1_yTby)XCOl_x 0.15 < x < 0.35, 0.3 < y < 1.
The Kerr rotation effect in an RE-TM alloy is due primarily to the TM sublattice, so it is the TM sublattice that stores the information "0" or "1." The exchange coupling of adJacent layers operates between like sublattices, tending to align the RE sublattice of the first magnetic layer with the RE sublattice of the second magnetic layer, and the TM sublattice of the first magnetic layer with the TM sublattice of the second magnetic layer.
Accordingly, the bias field Elb should be oriented in the direction opposite to the magnetization of the TM sublattice of the second magnetic layer.
Fig. 4A shows an example of a novel magneto-optic recording medium 19 of this type comprising a substrate 13, a first magnetic layer 21, and a second magnetic layer 23.
The two magnetic layers have the following composition and thickness:
1st magnetic layer: Tb23Fe72Co5 500 angstroms 2nd magnetic layer: Gd14Tb14Co72 1500 angstroms This magneto-optic recording medium will be referred to as medium 1.
In Fig. 4A and subsequent drawings, the magnetization of the RE sublattice is represented by a solid-line arrow, and the magnetization of the TM sublattice by a dashed-line -arrow. The lengths of the sublattice arrows represent the magnitude of magnetization. The net magnetization is represented by a large arrow enclosing the two sublattice arrows.
In Fig. 4A, at room temperature the first magnetic layer 21 has the compensation composition, as indicated by sublattice magnetization arrows of equal lengths. The net magnetization is zero, indicated by omission of the net magnetization arrow. The second magnetic layer 23 is RE-rich throughout the overwriting temperature range, and has a net magnetization in the up direction. The bias field Hb also points upward, opposite to the magnetization direction of the TM sublattice of the second magnetic layer 23, and has a strength of substantially 1000 oersteds.
At higher temperatures, the first magnetic layer 21 becomes TM-rich until losing its magnetization at its Curie temperature, which is substantially 180C. At temperatures in the neighborhood of 150C the reversal field shift Hw1(2) exceeds the coercivity Elcl of the ~irst magnetic layer 21, the difference between the two reaching a maximum value of approximately 1000 oersteds.
Fig. 4B shows the reversal field curves of the first magnetic layer 21 for cooling from Trl and TrO. The curves are similar to those in Fig. 2, except that the reversal ~ield shi~t is in the upward direction. Since the first magnetic layer 15 is TM-rich and the second magnetic layer 17 is RE-rich, the exchange coupling attempts to give them opposite net magnetizations, which is the reason that the shift Hw1(2) in Fig. 4A is opposite to the shift Hw1(2) in Fig. 2. Tp is substantially 150C. At room temperature the reversal field approaches infinity because the first magnetic layer 21 has the compensation composition; this feature enhances the data retention characteristics of the medium by making it immune to reversal by ambient magnetic fields.
The second magnetic layer 23 has a reversal field of 1000 oersteds or more from room temperature to 250C, and the net magnetization of the second magnetic layer 23 is in the same direction as the bias field ~Ib, so the second magnetic layer 23 does not undergo magnetic reversal within the operating temperature range.
The writing of information on this magneto-optic recording medium 19 will be described in detail with reference to Fig. 5.
High writing, which magnetizes the TM sublattice of the first magnetic layer 21 in the up direction in alignment with the bias field, is illustrated at (1) to (4) in Fig. 5.
The process starts at room temperature, at which the first magnetic layer 21 has the compensation composition, and may be magnetized in either direction as shown at (1) in Fig. 5.

The laser beam 3 is modulated to the power level R1. When a bit cell enters the beam spot 7 its temperature rises to Trl, exceeding the Curie temperature Tcl of the first magnetic layer 21, so the first magnetic layer 21 is demagnetized, as shown at (2) in Fig. 5. When the rotation of the disk carries the bit cell out of the beam spot and the temperature of the first magnetic layer 21 falls slightly below Tcl, the bias field llb gives the first magnetic layer 21 a net magnetization in the up direction, as can be seen from the graph on the left in Fig. 4A. Since the first magnetic layer 21 is TM-rich, the TM sublattice is magnetized in the up direction as shown at (3) in Fig. 5.
When the temperature fal]s further to room temperature, the sublattices retain their magnetization directions, as shown at (4) in Fig. 5.
Low writing, which magnetizes the TM sublattice of the first magnetic layer 21 in the down direction in alignment with the TM sublattice of the second magnetic layer 23, is illustrated at (5) to (8) in Fig. 5. The bit cell may be magnetized in either direction at (5) in Fig. 5. The laser beam 3 is modulated to the power level RO. When the bit cell enters the beam spot 7, its temperature rises to TrO
and the first magnetic layer 21 becomes TM-rich, as shown at (6) in Fig. 5. When the rotation of the disk carries the blt cell out of the beam spot and the bit cell cools, the ~ FB772 sublattice alignment of the second magnetic layer 23 is transferred by the exchange coupling to the first magnetic layer 21. This transfer occurs in the vicinity of Tp in the graph on the right in Fig. 4B. Thus the TM sublattice of the first magnetic layer 21 is magnetized in the down direction as shown at (7) in Fig. 5. This magnetization is retained when the temperature reaches room temperature and the first magnetic layer 21 returns to the compensation composition, as shown at (8) in Fig. 5.
Reading and writing tests of the magneto-optic recording medium 19 described above have been carried out with a linear speed of 6m/s, bit-cell length of 0.8 to 5 micrometers, and bias field Hb of 1000 oersteds.
Information was written by modulating the laser beam between a peak power R1 of 16mW and bottom power RO of 5mW. The information was read with a laser power of 1.5mW. An erasability characteristic of 25dB or greater was obtained.
Seven more magneto-optic recordlng media similar to the magneto-optic recording medium 19 (medium 1) described above, but differlng slightly in the thicknesses of the two magnetic layers and the composition of the second magnetic layer 23, were also tested at a linear speed of 6m/s. These media are listed as media 2 to 8 in Table 1. The substrate 13 was glass, and the layers were deposited by sputtering.

lST MAGNETIC LAYER 2ND MAGNETIC LAYER
COMPOSITION TEIICKRESS COMPOSITION TIIICKRESS

MEDIwM 2 Tb23Fe72C5 500 Gd1sTb14C71 1500 MEDIUM 3 Tb23Fe72C5 400 Gd14Tb14C72 1500 MEDIUM 4 Tb23Fe72C5 400 Gd14Tb16C70 1500 MEDIUM 5 Tb23Fe72C5 50 Gd14Th14C72 1800 MEDIUM 6 Tb23Fe72C5 400 Gdl4Tbl4co72 1800 MEDIUM 7 DY23Fe72C5 50 Tb30C70 1500 MEDIUM 8 Tb23Fe72C5 500 Tb33Co67 1500 Light-modulated direct overwriting was demonstrated for all the media listed in Table 1. Erasability was 20dB or greater. Carrier-to-noise characteristics of 23dB to 35dB
were obtained with optimum adJustment of the laser power and bias field, as listed in Table 2.

BIAS PEAK BOTTOM
FIELD POWER POWER
(Oe) (mW) (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 novel magneto-optic recording medium (medium 9) with magnetic layers of the following composition and thickness was also tested:
1st magnetic layer Tb23Fe67Colo 500 angstroms 2nd magnetic layer Gd12Tb12C76 1500 angstroms Direct overwriting was demonstrated with characteristics similar to those of medium 1 already described.
Yet another magneto-optic recording medium (medium 10) has been tested, with a plastic substrate and magnetic layers of the following compositions:
Substrate: 1.2mm-thick plastic disk 1st magnetic layer: Tb23.6(Fe90C10)76.4 500 angstroms Room-temperature coercivity: Approx. lOK oersteds Curie temperature: 180C
2nd magnetic layer: (Gd50Tb50)24Co76 1800 angstroms Room-temperature coercivity: Approx. lK oersted Curie temperature: > 300C
The second magnetic layer of this magneto-optic recording medlum was initialized in the manufacturing process by a field of 10,000 oersteds, as will be described later. The medium was then successfully overwritten 1000 times or more at a linear speed of 6m/s with a bias field of 1000 oersteds. The laser power was modulated between 15mW (R1) and 5mW (R0) at rates of lMHz and 1.5MTIz.

Direct overwriting has been demonstrated in six more magneto-optic media (media 11 to 15) having the compositions listed in Table 3 but otherwise similar to the preceding medium 10. For comparison, Table 3 also lists two media that could not be directly overwritten (examples 1 and 2).

lST MAGNETIC 2ND MAGNETIC OVER-WRIT-LAYER LAYER ABLE

COMPOSITIONTIIICK- COMPOSITIONTIIICK-NESS R NESS R

DIUM 11 Tb23.6(Fe90C10)76.4 500 (Gd65Tb35)23 5C76 5 1800 YES
DIUM 12 Tb22.4(Fe9ocolo)77.6 400 (Gd70Tb30)22 0C76 o 1800 YES
MEDIUM 13 Tb24.0(Fe95co5)76.o 500 T 24.2 75.8 1500 YES
COMPARATIVE
EXAMPLE 1 Tb24.2(Feg5C5)75.8 500 (Gd60Tb40)15 0C85 o I800 NO
COMPARATIVE
EXAMPLE 2 Tb25.1(Fe90Cl0)74.9 500 (Gd56Tb44)35 0C65 o 1800 NO
MEDIUM 14 Tb27 3(Eegocolo)72.7 400 (Gd60Tb40)23.9 76.1 MEDIUM 15 Tb23.6(Fe50C50)76.4 400 (Gd50Tb50)24 5Co75 5 1800 YES

Direct overwritability can be enhanced by providing, between the recording layer and the initializing layer, an auxiliary layer with a Curie temperature higher than the Curie ternperature of the recording layer and intermedlate between the temperatures reached in high writing and low writing. Such a three-layer magneto-optic recording medium will be described next wlth reference to Figs. 6 to 8.

-zoio470 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. The first magnetic layer 24 is the recording layer, the second magnetic layer 25 is the auxiliary layer, and the third magnetic layer 26 is the initializing layer. All three magnetic layers are TM-RE alloys, and all are TM-rich.
The arrows in Fig. 6 accordingly indicate both the direction of net magnetization of the layers and the direction of magnetization of their TM sublattices.
The third magnetic layer 26 is uniformly magnetized in, for example, the up direction during the manufacturing process. Since the third magnetic layer 26 is TM-rich, the bias field Hb is oriented in the down direction.
The first magnetic layer 24 and the second magnetic layer 25 are exchange-coupled by a force which attempts to align the direction of magnetization of the TM sublattices in the two layers. The second magnetic layer 25 and the third magnetic layer 26 are similarly exchange-coupled. The notation Hwi(j) will be used as before to denote the shift in the reversal field of the i-th layer due to the exchange coupling with the J-th layer. Tci will denote the Curie temperature and llci will denote the coercivity of the i-th layer.

2~104 ~ 0 The Curie temperatures Tcl, Tc2, and Tc3 of the layers satisfy the relationship:

Tcl < Tc2 < Tc3.

At room temperature, the following relationships are satisfied:

Hcl > Hw1(2) and Hc3 > Hw3(2).

Since Hw3(2) falls to zero at Tc2 and Tc2 < Tc3, the relationship Hc3 > EIw3(2) holds not only at room temperature but at all temperatures from room temperature up to Tc3.
The following relationship is satisfied at a certain temperature Tq equal to or greater than room temperature but less than Tcl:

Hc2 < IIw2(3) - EIw2(1).

This relationship, which can be rewritten as Hc2 + Hw2(1) - E~w2(3) < O, implies that at temperature Tq, the magnetization of the third magnetic layer 26 can be transferred to the second 2010~70 magnetic layer 25 by the exchange coupling, even if the first magnetic layer 24 is magnetized in the opposite direction. This transfer is opposed by the bias field Hb.
The bias field Hb should be selected so that at Tq, IHbl ~ 111c2 + E1w2(1) - Hw2(3)1 enabling the transfer to take place despite the bias field.
Other constraints on the bias field will be mentioned below.
Next the overwriting operation will be described. As a bit cell in the recording medium passes through the beam spot 7, it experiences the environments listed in Table 4.

EXTERNAL TEMPERATURE
FIELD

ENVIRONMENT I O ROOM TEMP.
ENVIRONMENT II 11b ROOM TEMP.
ENVIRONMENT III Hb TrO or Trl ENVIRONMENT IV Hb ROOM TEMP.
ENVIRONMENT V O ROOM TEMP.

In environments I and V, the blt cell is unaffected by the bias field 13b or the laser beam 3. In environments II
and IV, which obtain in substantially l-millimeter regions in front of and behind the beam spot 7, the bit cell feels the bias field Hb but is not heated by the laser beam 3. In environment III, which obtains in the substantially 1-micrometer area of the beam spot 7, the temperature of the magnetic layers in the beam spot 7 rises to a maximum value that depends on the laser power. Afterward, when the beam spot 7 has been passed, the temperature falls, returning to room temperature in a space of several tens of micrometers.
As before, overwriting is performed by modulating the laser power between a high value R1 and a lower value RO, heating the magnetic layers in the beam spot 7 to temperatures o-f Trl and TrO, respectively. High writing and low writing will be described in detail with reference to Fig. 7, showing the changes in magnetization in different environments.
Low writing, which aligns the TM sublattice of the first magnetic layer 24 with the TM sublattice of the third magnetic layer 26 in the up direction, is illustrated at (1) to (3) in Fig. 7.
In~environment I, since Hcl > Hw1(2) the first magnetic layer 24 retains its previous state of magnetization. This may be either the up state or the down state of the TM sublattice, depending on the previously 20~04~() recorded information. Both states are shown at (1) in Fig.
7. The TM sublattice of the third magnetic layer 26 is magnetized in the up direction. The TM sublattice of the second magnetic layer 25, having been previously aligned with the TM sublattice of the third magnetic layer 26 as will be described later, is also magnetized in the up direction.
If the first magnetic layer 24 is magnetized downward, the sublattices in the first magnetic layer 24 and the second magnetic layer 25 are magnetized in opposite directions, a state opposed by the exchange coupling. An unstable condition exists at the interface between the first magnetic layer 24 and the second magnetic layer 25, with a wall energy of u w12~ represented by cross-hatching in the drawing.
In environment II, the bit cell remains at room temperature. A downward-oriented external magnetic field Hb is applied by the bias magnet 18, but Hb is too small to reverse~the magnetization of any of the layers.
Specifically, Hb should be chosen so that at room temperature IHbl < IHcl + Hw1(2)l, IHbl ~ IHc2 - Hw2(1) + Elw2(3)l, and IHbl < IEIc3 + Elw3(2)l.

-~ FB772 201(~470 If the TM sublattice of the first magnetic layer 24 is oriented downward, the bias field llb helps to maintain that state, counteracting the instability at the interface with the second magnetic layer 25. The magnetic alignments shown at (1) in Fig. 7 are therefore retained in environment II.
In environment III, the bit cell enters the beam spot 7 and is heated to a maximum temperature TrO which satisfies the relationship:

Tcl < TrO < Tc2 < Tc3 The inequality Tcl < TrO need not be strictly fulfilled; TrO
may be slightly lower than Tcl. At TrO, the first magnetic layer 24 is substantially demagnetized because it is near or above its Curie temperature Tcl. The Curie temperatures Tc2 and Tc3 of the second magnetic layer 25 and the third magnetic layer 26, however, are high enough that these two layers retain the upward magnetization of their TM
sublattices, as shown at (2) in Fig. 7.
In environment IV, upon leaving the beam spot 7, 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 Tcl, spontaneous magnetization begins to appear in the first magnetic layer 24. In the neighborhood of a certain temperature Tp, the exchange coupling between the first magnetic layer 24 and the second magnetic layer 25 is strong enough to overcome the coercivity Hcl of the first magnetic layer 24 and the bias field Hb, so the sublattices of the first magnetic layer 24 are magnetically aligned with the sublattices of the second magnetic layer 25. Thus the TM sublattice of the first magnetic layer 24 is magnetized in the up direction as shown at (3) in Fig. 7.
In environment V, this state of magnetization, with all TM sublattices aligned in the up direction, is maintained at room temperature.
High writing, which aligns the TM sublattice of the first magnetic layer 24 with the bias field in the down direction, is illustrated at (1) and (4) to (7) in Fig. 7.
Environments I and II are the same as environments I
and II in low writing, as shown at (1) in Fig. 7.
In environment III, the bit cell enters the beam spot 7 and is heated to a maximum temperature Trl which satis~ies the relationships:

Tcl < Tc2 < Trl < Tc3 The inequality Tc2 < Trl need not be strictly fulfilled; Trl may actually be slightly lower than Tc2. At temperature Trl, both the first magnetic layer 24 and the second magnetic layer 25 are demagnetized. The Curie temperature of the third magnetic layer 26, however, is high enough that its net upward magnetization is maintained. The magnetization states at this maximum temperature Trl are therefore as shown at (4) in Fig. 7.
In environment IV, the bit cell leaves the beam spot 7 and rapidly cools. During the cooling process, at a point at which the temperature of the second magnetic layer 25 is slightly less ~han Tc2, the second magnetic layer 25 begins to undergo spontaneous magnetization. At temperatures sufficiently higher than Tcl, 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 bias field Hb is able to magnetize the second magnetic layer 25 in the down direction. Since the second magnetic layer 25 is TM-rich, its TM sublattice is oriented in the down direction, as shown at (5) in Fig. 7.
This state is unstable in that the second magnetic layer 25 and the third magnetic layer 26 are oppositely magnetized, but it is maintained as cooling proceeds down to the Curie temperature Tcl of the first magnetic layer 24.
The bias field Hb should be selected so as to satisfy the following conditions in this temperature range:

20104 ~ 0 Hw2(3) - Hc2 < Illbl < Ilc3 - Hw3(2) When the temperature falls slightly below Tcl, the first magnetic layer 24 begins to undergo spontaneous magnetization. Both the bias field Hb and the exchange coupling with the second magnetic layer 25 act to magnetize the TM sublattice of the first magnetic layer 24 in the down direction as shown at (6) in Fig. 7.
When the temperature falls to Tq, since Hc2 + Hw2(1) - Elw2(3) < llb < O, the exchange coupling between the second and third magnetic layers reverses the magnetization of the second magnetic layer 25, aligning it with the magnetization of the third magnetic layer 26.
At this temperature Tq, moreover, the coercivity of the first magnetic layer 24 llas become high enough that the exchange coupling with the second magnetic layer 25 cannot overcome the bias field llb and the coercivity Hcl of the first magnetic layer 24 and reverse the magnetization of the first magnetic layer 24. Accordingly, while the second and third magnetic layers 25 and 26 are aligned with their TM
sublattices oriented in the up direction, the TM sublattice of the ~irst magnetic layer 24 remains oriented in the down 20~0470 direction, as shown at (7) in Fig. 7.
In environment V, the magnetization state shown at (7) in Fig. 7 is maintained at room temperature.
An example of the composition of the magnetic layers of a three-layer medium as described above is:
First magnetic layer: Tb21Fe74C5 Second magnetic layer: DylgFe62col9 Third magnetic layer: Tb20C80 In tests, a medium with this structure demonstrated direct overwritability with a carrier-to-noise ratio of 45dB, using a downward-directed bias field of 200 oersteds.
Fig. 8 shows the results of measurements of the following quantities made in the vicinity of room temperature:

Hcl - Hw1(2) Hc2 + Hw2(1) - Hw2(3) Hc3 - Illbl - Hw3(2) Data points are indicated by circles. These results indicate that in the temperature range from 0C to 50C, the recording layer and initializing layer maintain their existing magnetizations, while the auxiliary layer is aligned with the initializing layer by the exchange coupling, even if the recording layer is oppositely magnetized. Direct overwriting is possible at ambient temperatures in this range.
In the three-layer medium described above all three layers were TM-rich, but it is also possible to use combinations of different types of layers. Some examples for which direct overwriting has been confirmed are listed in Table 5. The notation TM designates a TM-rich layer, RE
designates an RE-rich layer, and (RE) designates a layer that is RE-rich at room temperature but TM-rich at higher temperatures: that is, a layer having a compensation temperature above roorn temperature, but within the operating temperature range. The sign of the bias field Hb indicates whether Hb is directed up (positive) or down (negative).
The direction of Elb is given for the case in which the net room-temperature magnetization of the third magnetic layer is up.
The overwriting mechanism is the same in all cases. In low writing, the recording layer sublattices align with the auxiliary layer sublattices, which are already aligned with the initializing layer sublattices. In high writing, first the net magnetization of the auxiliary layer aligns with the bias fleld, causing its sublattices to align oppositely to those of the initializing layer. Then this sublattice alignment is transferred from the auxiliary layer to the recording layer, after whlch the auxillary layer reverses to align with the initializing layer.
The conditions necessary for direct overwriting on these three-layer media are summarized as follows.
At room temperature:

(a) IIcl - 0.5(ab + l)IIb - Hw1(2) > 0 (b) Hc3 + 0.5(ac - l)IIb - Hw3(2) > 0 There must also exist, between room temperature and Tcl, a temperature at which the -following condition is satisfied:

(c) Hc2 + 0.5(-a - l)Hb + Hw2(1) - IIw2(3) < 0 where, Hb: absolute value of the bias field a = 1: when the second layer has a compensation temperature above room temperature a = -1: when the second layer does not have a compensation temperature above roorn temperature b =~1: when the ~irst 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 The bias field and the net magnetization of the third magnetic layer should be oriented in the same direction when ac = 1, and in opposite directions when ac = -1. It is not necessary for the bias field to be oriented opposite to the TM sublattice of the third magnetic layer; if the second magnetic layer is RE-rich at all temperatures, as in rows No. 3 and 4 of Table 5, the bias field and the TM sublattice o~ the third magnetic layer should be oriented in the same direction.

201047~

UJ
~ , . . . . .
CS:
z C~
o o o o o o o o o U~ o ~ o ~ o ~ U~ U~
+ ~ ~ I t Q OC~ ~ Q O QC~ Q

Z
O o u~ c~
O O ~
o o o o a, ~ o a. a e) = ~s O O ~ ~ C~l c~C~ +
O ~ ~ O O ~ ~ O O ~ ~_ _~ ~ ~ C~ CO ~ C~ CS~ 1-- CO C'~ O O
C~ O O C~ ~ C S C~ C~ C~ C~:5 0 0 C l'_ CC

~ O ~_ o o a~ c~ o o o e~ a~

~ ~ ~ ~ c~ ~ o J _. _~

Z ~
~ ~ _ _ . _ z ~ ~ ~ ~ I_ ~ ~ O 1--20104 ~ ~) A constraint on the design of three-layer recording media as described above is that in high writing, the initializing layer should not reverse the magnetization of the auxiliary layer until after the magnetic alignment of the auxiliary layer has been transferred to the recording layer. A way to ensure that this constraint is satisfied is to provide another magnetic layer with a low Curie temperature between the initializing layer and the~auxiliary layer. The extra layer acts as a buffer layer by decoupling the auxiliary layer from the initializing layer.
Fig. 9 shows a sectional view of such a magneto-optic recording medium 30 comprising a glass substrate 13 and four magnetic layers. The first magnetic layer 24 is the recording layer, the second magnetic layer 27 is the auxiliary layer, the third magnetic layer 28 is the buffer layer, and the fourth magnetic layer 29 is the initializing layer. An example of the compositions and thicknesses of the layers is as follows:
1st magnetic layer Dy23Fe68co9 500 angstroms Comp.
2nd magnetic layer Tb25Fe60C15 700 angstroms (RE) 3rd magnetic layer Tbl6Fe84 200 angstromsTM
4th magnetic layer Tb30C70 700 angstromsRE
The first magnetic layer 24 has the compensation composition at room temperature, making it resistant to the effects of ambient magnetic fields, thus improving its data retentivity ZOlQ470 characteristics. The second magnetic layer 27 is RE-rich at room temperature, but has a compensation temperature, higher than room temperature, above which it becomes TM-rich. The third magnetic layer 28 is TM-rich. The fourth magnetic layer 29 is RE-rich and has a coercivity of 700 oersteds or greater at temperatures up to substantially 300C, so the fourth magnetic layer 29 does not undergo magnetic reversal within the range of operating conditions. The bias field Hb has a strength of 200 to 400 oersteds and is oriented in the same direction as the net magnetization of the fourth magnetic layer 29, upward in the drawing.
The Curie temperatures of these layers are related as follows, where as before Tci denotes the Curie temperature of the i-th layer:

Tc4 > Tc2 > Tcl > Tc3 > room temperature Adjacent magnetic layers are exchange-coupled. The exchange coupling is strongest between the third and fourth layers, less strong between the second and third layers, and still less strong between the first and second layers. The exchange forces and bias field are related at various temperatures as follows, where as before Hci denotes the coercivity of the i-th layer and Hwi(J) denotes the shift in the reversal field of the i-th layer due to the exchange coupling with the J-th layer.

-Hw1(2) + Hcl - Hb > O
(at room temperature) Hw1(2) - Hcl - Elb > O
(at a temperature Tp between room Temp. and Tcl) Hw2(3) - }lw2(1) - llc2 + llb > O
(at room temperature) Hw3(4) - Hw3(2) - llc3 - Hb > O
(at a temperature Tq between room Temp. and Tc3) -Hw4(3) + Hc4 > O
(in the entire operating temperature range) Overwriting of this medium will be explained next. Low writing, which aligns the TM sublattice of the first magnetic layer 24 in the same direction as the TM sublattice of the fourth magnetic layer 29, will be described with reference to Fig. 9. Before the bit cell to be written enters the beam spot, the TM sublattices of the second, third, and fourth magnetic layers are all oriented in the down direction as shown in Fig. 9. In the beam spot, the bit cell is heated to a temperature equal to or greater than Tp but not high enough for the second magnetic layer 27 to undergo magnetic reversal. As the first magnetic layer 24 cools through the vicinity of Tp, the magnetic alignment of 201047~) the second magnetic layer 27 is transferred to the first magnetic layer 24, magnetizing the TM sublattice of the first magnetic layer 24 in the down direction as shown in Fig. 9. This alignment is maintained as the bit cell cools to room temperature.
The temperature to which the bit cell is heated in low writing may exceed Tc3, in which case the third magnetic layer 28 will be temporarily demagnetized, but as the bit cell cools, the third magnetic layer 28 will be remagnetized in the same direction as before due to the exchange coupling with the second magnetic layer 27 and the fourth magnetic layer 29.
High writing, which aligns the TM sublattice of the first magnetic layer 24 with the bias field ~Ib, will be described with reference to Fig. 10. Before the bit cell enters the beam spot, the TM sublattices of the second, third, and fourth magnetic layers are all magnetized in the down direction, as at (1) in Fig. 10. The first magnetic layer 24 may be magnetized in either direction. In the beam spot, the bit cell is heated above the Curie temperatures of the first and third magnetic layers 24 and Z8, demagnetizing them as shown at (2) in Fig. 10. The heating also exceeds the compensation temperature of the second magnetic layer 27, causing it to become TM-rich.
The maximum temperature reached is high enough for -20104~70 magnetic reversal to occur in the second magnetic layer 27:
the bias field Elb switches the net magnetization of the second magnetic layer 27 to the up direction as shown at (3) in Fig. 10. Since the second magnetic layer 27 is TM-rich at this temperature, its TM sublattice is magnetized in the up direction. Since the first and third magnetic layers are both demagnetized, magnetic reversal of the second magnetic layer 27 is not opposed by any exchange coupling.
When the bit cell cools to the vicinity of Tp, the exchange coupling between the first and second magnetic layers aligns the TM sublattice of the first magnetic layer 24 in the up direction as shown at (4) in Fig. 10. Since the first magnetic layer 24 is TM-rich at this temperature, the bias field l~b assists in this process.
When the temperature of the bit cell drops below Tc3, in the vicinity of Tq, the third magnetic layer 28 acquires the magnetization of the fourth magnetic layer 29, so that its TM sublattice is aligned in the down direction as shown at (5) in Fig. 10. Finally, at room temperature the magnetic alignment of the third magnetic layer 28 is transferred to the second magnetic layer 27 as shown at (6) in Fig. 10, leaving its TM sublattice aligned downward.
Since the second magnetic layer 27 is RE-rich at room temperature, this alignment is assisted by the bias field Hb. The transfer of magnetization proceeds from the fourth magnetic layer to the third magnetic layer to the second magnetic layer, rather than in the opposite direction, because the exchange coupling is strongest between the third and fourth magnetic layers and weakest between the first and second magnetic layers.
A magneto-optic recording medium 30 as described above has been tested by reading and writing at a linear speed of llm/s with a bit-cell length of 0.76 micrometers, with a bias magnetic field of 200 oersteds, and the laser writing power being modulated between a peak power of 18mW (R1) and a bottom power of 7mW (R0). A carrier-to-noise ratio of 45dB and erasability characteristic of 45dB or better were obtained. The medium had a grooved glass substrate with grooves spaced 1.6 micrometers apart. A dielectric layer was deposited on the substrate, then the magnetic layers were formed by sputtering.
Direct overwriting has also been confirmed under optimized writing conditions for the four-layer media listed in Table 6. These media are similar to the medium described above, but differ slightly in the thickness or composition of some of the magnetic layers.

~ FB772 lST MAG. 2ND MAG. 3RD MAG. 4TH MAG.
LAYER LAYER LAYER LAYER
DY23Fe68Cg Tb2sFe60col5 Tbl6Fe84 Tb30C70 DY23Fe72C5 Tb25Fe60col5 Tbl6Fe84Tb30 70 DY23Fe68C9 Tb2sFe60cols Tbl6Fe84GdloTb2oco7o 500 R 700 R loo R 1200 R

Many other types of four-layer recording media are possible. Each of the magnetlc layers may be RE-rich, TM-rich, or of the type having a compensation temperature above room temperature, so in all there are 3 x 3 x 3 x 3 = 81 possible combinations.
Figs. 11 to 91 list examples of all these combinations, showing the composition and thickness o~ each layer, its magnetic properties, the direction of the bias field, and the conditions to be satisfied by the exchange coupling ~orces and bias field. The notation used in these drawings is explained below.
TM: a TM-rich alloy not having a compensation temperature between room temperature and the Curie temperature RE: an RE-rich alloy having a compensation temperature between room temperature and the Curie temperature re: a RE-rich alloy not having a compensation temperature between room temperature and the Curie temperature t: thickness (angstroms) Ms: saturation magnetization (emu/cc) Hc: coercivity (oersteds) Tc: Curie temperature (C) Sw (or ~ w) interface wall energy (erg/cm2) Hwi: total shift in reversal field of the i-th layer Hwi(~): shift in reversal field of the i-th layer due to exchange coupling with the J-th layer 1~ wij/(21Msilti)]
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 (example: -10C to 60C) Tread: all temperatures in the range form the lowest temperature during use to the temperature reached when the disk is read TL: a ~temperature higher than the reading temperature but lower than Tcl, at which transfer of the magnetic alignment of the second magnetic layer to the first magnetic layer occurs Tini: the operating temperature range of the read-write apparatus (0C to 50C), or a special temperature provided for aligning the second and third magnetic layers with the ` FB772 fourth magnetic layer at a location not illuminated by the laser beam Tall: all temperatures in the operating temperature range (0C and above) Tuse: the operating temperature range of the read-write apparatus (0C to 50C) al: state in which the TM sublattice of the recording layer is magnetized downward, storing a "O" for example a2: state in which the TM sublattice of the recording layer is magnetized upward, storing a "1" for example In the a2 state a wall exists between domains in the first and second magnetic layers, as indicated by cross-hatching.
In these media Tc3 ~ Tcl and Tc2 < Tc4, but these conditions are not always essential. It is only necessary for Tcl and Tc3 both to be less than Tc2 and Tc4, that is for:

Tcl c Tc2, Tc3 < Tc2, Tcl < Tc4, and Tc3 < Tc4.

The conditions preventing magnetic reversal of the first and fourth magnetic layers must be fulfilled at room temperature:

Hcl > Elw1(2) and ~Ic4 > l~w4(3).

20104~0 Since Tc3 < Tc4 and Elw4(3) becomes zero at Tc3, it can be inferred that Hc4 > Hw4(3) at all temperatures up to Tc4.
The conditions for magnetic reversal of the second and third magnetic layers need only be satisfied at a certain temperature in the range from room temperature to Tcl or Tc3, whichever is lower, these conditions being:

Hc2 < Hw2(3) - Hw2(1) and Hc3 < Hw3(4) - Hw3(2).

The bias field Hb should be small enough not to destroy any of these inequalities. The constraints on the bias field vary depending on the types of layers, as indicated in Figs. 11 to 91.
In the four-layer media described above the auxiliary layer was decoupled from the initializing layer during the transfer of magnetization from the auxiliary layer to the recording layer by a buffer layer disposed between the auxiliary layer and initializing layer. This decoupling can also be accomplished in a three-layer medium by using an initializing layer with a low Curie temperature, so that the initializing layer is demagnetized while the transfer from the auxiliary layer to the recording layer is taking place.
The Curie temperature conditions to be satisfied in such a medium are:

Tcl < Tc2 and Tc3 < Tc2.

The following conditions should be satisfied at room temperature:

Hcl > Hw1(2) and Hc3 > EIw3(2).

The following conditions should be satisfied at a certain temperature Tq in the range from room temperature to Tc3:

Hc2 < Hw2(3) - IIw2(1).

A three-layer magneto-optic recording medium satisfying these conditions will be described next with reference to Figs. 92A, 92B, and 93.
Fig. 92A shows a three-layer magneto-optic recording medium comprising a substrate 24 and three magnetic layers.
The first magnetic layer 31 is the recording layer, the second magnetic layer 32 is the auxiliary layer, and the third magnetic layer 33 ls the initializing layer. Their compositions are as follows:

1st magnetic layer Tb23(Feg2Co8)77 2nd magnetic layer Tb26(Fe85Co15)74 3rd magnetic layer Tb26Fe74 Fig. 92B shows the saturation magnetization -20io470 characteristics of the layers. The third magnetic layer 33 is RE-rich. The second magnetic layer 32 has a compensation temperature intermediate between room temperature and the Curie temperature Tcl of the first magnetic layer 31. The first magnetic layer 31 has a compensation temperature substantially equal to room temperature. The bias field Hb is directed upward.
In low writing, the bit cell to be written is heated to a temperature in the vicinity of Tcl and well below Tc2, such as the temperature TrO in Fig. 92B. In high writing the bit cell is heated to a temperature exceeding Tc2, such as the temperature Trl in Fig. 92B. High and low writing will be described with reference to Fig. 93.
High writing is illustrated at (1) to (6) in Fig. 93.
The initial state is shown at (1) in Fig. 93. The first magnetic layer 31 may be magnetized in either direction, depending on the previous recorded information. In the beam spot the bit cell is heated to the temperature Trl, demagnetizing all three magnetic layers as shown at (2) in Fig. 93. When the bit cell cools slightly below Tc2, the bias field Hb magnetizes the second magnetic layer 32 in the up direction. Since the second magnetic layer 32 is TM-rich at this temperature, its TM-sublattice is magnetized in the up direction as shown at (3) in Fig. 93. When the bit cell cools sli~htly below Tcl, the bias field Hb and the exchange 20104 ~ 0 coupling with the second magnetic layer 32 combine to magnetize the first magnetic layer 31, which is TM-rich at this point, in the same direction as shown at (4) in Fig.
93.
When the bit cell cools below Tcomp in Fig. 92B, the second magnetic layer 32 becomes RE-rich as shown at (5) in Fig. 93. Next, when the bit cell cools slightly below Tc3, the bias field Hb gives the third magnetic layer 33 a net magnetization in the up direction. Since the third magnetic layer 33 is RE-rich, its TM sublattice is magnetized in the down direction. As cooling proceeds, the third magnetic layer 33 retains this magnetization despite the opposing exchange force exerted by the second magnetic layer 32. The cooling curves of the third magnetic layer 33 are analogous to the curve shown on the left in Fig. 4B. When the condition:

Hc3 + llw3(2) > ~Ib > -Hc3 + Hw3(2) is satisfied, the previous magnetization is maintained.
When the temperature in the bit cell falls to the vicinity o~ Tq, the exchange coupling with the third magnetic layer 33 reverses the magnetization o~ the second magnetic layer 32, because 201Q4*0 Hc2 < Hw2(3) - Hw2(1).

This magnetic reversal is assisted by the bias field Hb.
The final result is shown at (6) in Fig. 93: the TM
sublattice of the first magnetic layer 31 is magnetized in the up direction, and the TM sublattices of the second magnetic layer 32 and the third magnetic layer 33 are aligned in the down direction. This state is maintained at room temperature.
Low writing is illustrated at (7) and (12) in Fig. 93.
Initially the second and third magnetic layers 31 and 32 are aligned as they were in high writing, with their TM
sublattices magnetized in the down direction as shown at (7) in Fig. 93. In the beam spot the bit cell is heated to the temperature TrO in the vicinity of Tcl, which is high enough to demagnetize the third magnetic layer 33 and possibly the first magnetic layer 31, but not so high that the bias field is able to reverse the magnetization of the second magnetic layer 32. If TrO < Tcl, the resulting state is shown at (8) in Fig. 93. If TrO > Tcl, the resulting state is shown at (9) in Fig. 93. In either case, as the bit cell cools, the exchange coupling with the second magnetic layer 32 aligns the first magnetic layer 31 as shown at (10) in Fig. 93, with its TM sublattice magnetized in the down direction.
As cooling proceeds below Tcomp2 in Fig. 92B, the -second magnetic layer 31 becomes RE-rich as shown at (11) in Fig. 93. When the bit cell cools below Tc3, the exchange coupling with the second magnetic layer 32 and the bias field combine to magnetize the RE-rich third magnetic layer 33 with its TM sublattice oriented downward. At room temperature the bit cell is left as shown at (12) in Fig.
93, with all three TM sublattices magnetized in the down direction.
A buffer layer can be added to the magneto-optic recording medium 40 described above to improve the magnetization of the initializing layer in high writing.
The buffer layer should be disposed between the auxiliary layer and the initializing layer, and should have a Curie temperature lower than that of the initializing layer. A
Dy23Fe77 layer with a thickness of 500 angstroms, for example, can advantageously be used as the buffer layer.
If this buffer layer is referred to as the fourth magnetic layer, bearing in mind that the fourth magnetic layer ls disposed between the second magnetic layer and the third magnetic layer, the Curie temperature conditions to be satisfied are:

Tcl < Tc2 and Tc4 < Tc3 < Tc2;

The following conditions should be satisfied at room temperature:

Hcl > Hw1(2) and llc3 > llw3(4).

The following conditions should be satisfied at a certain temperature Tq in the range from room temperature to Tc4:

Hc2 < Hw2(4) - Hw2(1) and Hc4 < Hw4(3) - Hw4(2).

Low writing of this medium is the same as for the three-layer medium described in Figs. 92A, 92B and 93. High writing is the same except than when the initializing layer is magnetized by the bias field Hb at a temperature slightly below Tc3, the buffer layer is above its Curie temperature Tc4. Magnetization of the initializing layer is therefore unopposed by any exchange coupling. The Curie temperature Tc4 of the buffer layer should be low enough that by the time Tc4 is reached, the coercivity Hc3 of the initializing layer is able to resist any exchange coupling effects that might be passed from the auxiliary layer to the initializing layer through the buffer layer.
In the vicinity of temperature Tq, the inequality conditions stated above ensure that, with proper setting of the bias field, the magnetization of the initializing layer will be transferred to the buffer layer and thence to the 20104 ~ 0 auxiliary layer, leaving all three layers with their sublattices aligned in the same direction.
In all of the media described so far, it is important to control the strength of the exchange coupling between the magnetic layers. For this purpose, interface control layers may be added between the magnetic layers. For example, an interface control layer may be added to a two-layer magneto-optic recording medium as follows:

1st magnetic layer Tb23Fe72Co5 500 angstroms Interface control layer Tb26Fe70co4 50 angstroms 2nd magnetic layer Tb30Fe70 1500 angstroms Recording media of this structure have been fabricated by sputtering the above layers onto a glass substrate. The interface control layer was formed by increasing the argon sputtering gas pressure by a factor of about six during the sputtering process. These media demonstrated good overwriting characteristics when tested at a linear speed of 6m/s with a peak writing power R1 of 9mW to 17mW, a bottom writing power R0 of 4mW to 7.5mW, and a bias field Hb of 300 +80 oersteds.
Four-layer media with a dielectric interface control have been fabricated as follows:

`~~ FB772 20104~0 1st magnetic layer Dy23Fe68C9500 angstroms Interface control layer SiNX10 angstroms 2nd magnetic layer Gd13DY12Fe60C151200 angstroms 3rd magnetic layer Tbl6Fe84200 angstroms 4th magnetic layer Tb30C70700 angstroms The ferrimagnetic layers and dielectric layer were formed by sputtering on a glass substrate. These media demonstrated good overwriting characteristics when tested at a linear speed of llm/s with a bias field Elb of 200 oersteds. The laser writing power was modulated between a peak power R1 of 18mW and bottom power R0 of 7mW.
The interface control layers in the above two-layer and four-layer media do not decouple the first and second magnetic layers; they only moderate the exchange coupling between these two layers, thereby reducing the instability at the interface between these two layers left after high writing. Such interface control layers can be added as required to any two-layer, three-layer, or four-layer media, between any of the magnetic layers.
Several methods of forming an interface control layer are listed below. This list of methods is not exhaustive;
any other method that permits control of the strength of the exchange coupling can be used.
1. An interface control layer can be formed by increasing `~ FB772 20~0470 the pressure of the sputtering gas by a factor of five or more, as in the two-layer medium described above.
2. A dielectric layer comprising an aluminum nitride or a silicon nitride, as in the four-layer medium described above, or an oxide such as SiOx can be deposited.

3. The active magnetic layers can be formed by sputtering using only argon. Interface control layers can be formed by reactive sputtering with admixture of a reactive gas such as oxygen or nitrogen.

4. An RE-TM a]loy with an RE component of 30at% or greater can be deposited under normal sputtering gas pressure as an interface control layer.

5. A paramagnetic or diamagnetic metal such as Al or Cu can be deposited as an interface control layer.

6. A magnetic layer with an axis of easy magnetization oriented in the plane of the disk can be used.
Finally, methods of manufacturing the novel magneto-optic recording media will be described with reference to Figs. 94 to 99. The novel step is a step of uniformly magnetizing the initializing layer, which retains the magnetization received in the manufacturing process and does not undergo magnetic reversal when the medium is written or read. The basic method comprises three steps:
(a) depositing a recording layer on a substrate;
(b) depositing the initializing layer on the recording ~OlD~70 layer;
(e) applying a magnetie field exeeeding the eoereivity of the initializing layer, oriented in a single direetion perpendieular to the initializing layer.
The deposition steps ean be earried out by well-known sputtering process. Magnetie layers in addition to the recording layer and initialization layer may also be deposited, as already described. Dielectric and protective layers may furthermore be added. An example will be given next.
Fig. 94 shows a magneto-optic recording medium comprising a polycarbonate substrate 2, a SiNX dieleetric layer 34, magnetie layers 35, 36, 37, and 38, and a proteetive layer 39. The first magnetie layer 35 is the reeording layer and the fourth magnetie layer 38 is the initializing layer. This magnetoeoptle reeording medium ean be manufaetured by sputtering the dielectric layer 34 onto the substrate 2, sputtering suceessive magnetie layers 35, 36, 37,~ and 38 onto the dielectric layer 34, then forming the protective layer 39 over the fourth magnetic layer 38.
The result at this point is a disk-shaped magneto-optic recording medium 40 in which the magnetie domains of the fourth layer 38 have random alignments.
With reference to Fig. 95, the recording medium 40 is now plaeed in a magnetie field lle generated by a deviee sueh zoio470 as an electromagnet 41. The field He is oriented in a single direction perpendicular to the initializing layer (the fourth layer 38), and its strength exceeds the room-temperature coercivity of the initializing layer, so it aligns the magnetic moments of the initializing layer in a single direction perpendicular to the plane of the recording medium 40. This alignment is retained after the application of the field He is terminated.
The magneto-optic recording medium shown in Fig. 94 and Fig. 95 is single-sided, but the same method can be used to manufacture two-sided magneto-optic recording media. First, a pair of magneto-optic recording media are manufactured by the method shown in Fig. 94 and Fig. 95. Next, with reference to Fig. 96, an adhesive such as an epoxy or hot-melt adhesive is applied to the protective layers 39 of the two media and they are Joined together to form a single disk. The two initializing layers (the fourth layers 38) in the disk are magnetized in opposite directions.
With reference to Fig. 97, it is also possible to Join the two halves of the disk together before applying the magnetic field to align the initializing layers. In this case both initializing ~ayers 38 will be magnetized in the same direction, so the magneto-optic read-write apparatus must be able to detect which side of the disk is up and switch the orientation of the bias field accordingly. The signal-processing circuits should be capable of taking account of the fact that the orientation of the bit cells has opposite signi~icance between the two sides. As an alternative, the signal-processing circuits may employ the edge-recording technique, in which information is represented not by the up or down orientation of bit cells, but by the presence or absence of a transition between the up and down states.
Fig. 98 illustrates another method of manufacturing a two-sided magneto-optic recording medium. First a pair of magneto-optic recording media comprising substrates, recording layers, initializing layers, and possibly other layers are fabricated as shown, for example, in Fig. 94.
The initializing layer in one of these media, labeled A in Fig. 98, has a first coercivity; the initializing layer in the other of the media, labeled B in Fig. 98, has a second coercivity lower than the first coercivity. Media A and B
are now ~oined together as illustrated in Fig. 98. Next, as shown at (a) in Fig. 98, a first magnetic field He(A) exceeding the first coercivity is applied in a single direction perpendicular to the media by a device such as an electromagnet 43, aligning both the A and B initializing layers in the same direction (left in the drawing). Then a second magnetic field lle~B) less than the first coercivity but exceeding the second coercivity is applied in a single 201()470 direction opposite to the direction of the first magnetic field, thus reversing the alignment of the B initializing layer while leaving the alignment of the A initializing layer unchanged. The result is a two-sided magneto-optic recording medium having initializing layers magnetized in opposite directions.
Instead of reversing the direction of the applied magnetic field as in Fig. 98, it is possible to reverse the orientation of the magneto-optic recording medium after application of the first magnetic field, as illustrated in Fig. 99.
The scope of this invention is not limited to the media and manufacturing methods shown in the accompanying drawings and tables, but includes many modifications and variations that will be apparent to one skilled in the art. In particular, magnetic layers of other compositions and thicknesses can be used, provided the stated conditions on their Curie temperatures, coercivities, and exchange coupli~g forces are satisfied.

Claims (16)

1. A magneto-optic recording medium comprising a first magnetic layer having perpendicular magnetic anisotropy and a second magnetic layer provided on said first magnetic layer and coupled to said first magnetic layer by an exchange force, wherein the following relationship is satisfied:
Tc1 < Tc2, wherein, Tc1: Curie temperature of first magnetic layer Tc2: Curie temperature of second magnetic layer, characterised in that the following relationship is satisfied at room temperature;
Hc1 > Hw1 + Hb where, Hc1: coercivity of first magnetic layer;
Hc2: coercivity of second magnetic layer;
Hw1: reversal field shift in first magnetic layer due to exchange force;
Hw2: reversal field shift in second magnetic layer due to exchange force;
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; and in the range of operating temperatures Hc2 > Hw2 + Hb where, Hb: external magnetic field applied during writing in the direction of magnetisation of the second magnetic layer in the state in which sublattices of the first magnetic layer are aligned with sublattices of the second magnetic layer.

2. The magneto-optic recording medium of claim 1, wherein the Curie temperature Tc2 of the second layer is not exceeded during writing of the recording medium.

3. A magneto-optic recording medium comprising a first magnetic layer having a perpendicular magnetic anisotropy, a second magnetic layer provided on said first magnetic layer and coupled to said first magnetic layer by an exchange force, and a third magnetic layer provided on said second magnetic layer and coupled to said second magnetic layer by an exchange force, wherein the following relationships are satisfied:
Tc1 < Tc2 < Tc3 where, Tc1: Curie temperature of first magnetic layer;
Tc2: Curie temperature of second magnetic layer;
Tc3: Curie temperature of third magnetic layer; and the following relationship is additionally satisfied at room temperature:
Hc1 > Hw1(2) characterised in that there exists a temperature between room temperature and Tc1 at which the following relationship is satisfied Hc2 < Hw2(3) - Hw2(1), where, Hc1: coercivity of first magnetic layer;
Hc2: coercivity of second magnetic layer;

Hc3: coercivity of third 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;
and in the range of operating temperatures:
Hc3 > Hw3(2) + Hb where, Hb: external magnetic field applied during writing in the direction of magnetisation of the third magnetic layer in the state in which sublattices of the third magnetic layer are aligned with sublattices of the second magnetic layer.

4. The magneto-optic recording medium of claim 3, wherein the Curie temperature Tc3 of the third layer is not exceeded during the writing of the recording medium.

5. A magneto-optic recording medium comprising a first magnetic layer having perpendicular magnetic anisotropy, a second magnetic layer provided on said first magnetic layer and coupled to said first magnetic layer by an exchange force, a third magnetic layer provided on said second magnetic layer and coupled to said second magnetic layer by an exchange force, and a fourth magnetic layer provided on said third magnetic layer and coupled to said third magnetic layer by an exchange force, wherein the following relationships are satisfied:
Tc1 < Tc2, Tc3 < Tc2, Tc1 < Tc4, 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; and Tc4: Curie temperature of fourth magnetic layer, characterised in that the magnetisation of said first magnetic layer is not revised due to the reversal of the magnetisation of said second magnetic layer 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 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, 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 of room temperature in the state in which sublattices of the first magnetic layer are aligned with sublattices of the second magnetic layer.

6. The magneto-optic recording medium of claim 5, wherein Tc2 Tc4.

7. A magnetic-optic recording medium comprising a first magnetic layer having perpendicular magnetic anisotropy, a second magnetic layer provided on said first magnetic layer and coupled to said first magnetic layer by an exchange force, and a third magnetic layer provided on said second magnetic layer and coupled to said second magnetic layer by an exchange force, wherein the following relationships are satisfied:
Tc1 < Tc2 and Tc3 < Tc2, where, Tc1: Curie temperature of first magnetic layer;
Tc2: Curie temperature of second magnetic layer; and Tc3: Curie temperature of third magnetic layer;
characterised in that the following relationships are additionally satisfied at room temperature:
Hc1 > Hw1(2) and Hc3 > Hw3(2), and there exists a temperature between room temperature and Tc3 at which the following relationship is satisfied:
Hc2 < Hw2(3) - Hw2(1), where, Hc1: coercivity of the first magnetic layer;
Hc2: coercivity of the second magnetic layer;
Hc3: coercivity of the third 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.

8. The magneto-optic recording medium of claim 7, wherein Tc3 Tc1.

9. The magneto-optic recording medium of claim 7, further comprising a fourth magnetic layer disposed between the second magnetic layer and the third magnetic layer and coupled to said second magnetic layer and said third magnetic layer by respective exchange forces, wherein the following relationships are satisfied:
Tc4 < Tc3 < Tc2 and Tc1 < Tc2, wherein, Tc4: Curie temperature of the fourth magnetic layer;
and the following relationships are additionally satisfied at room temperature:
Hc1 > Hw1(2) and Hc3 > Hw3(4) and there exists a temperature between room temperature and Tc4 at which the following relationships are satisfied:
Hc2 < Hw2(4) - Hw2(1) and Hc4 < Hw4(3) - Hw4(2).

10. The magneto-optic recording medium of any one of claims 1 to 9, having an interface control layer disposed at an interface between magnetic layers, for exchange force control.

11. A method of recording information on a magneto-optic recording material as claimed in claim 1 or 2, comprising the steps of:
i) applying an external magnetic field to part of a recording medium as claimed in claim 1 or claim 2, ii) applying a laser beam to said part simultaneously with the application of the magnetic field, and iii) modulating the intensity of the beam depending on a bit to be written at said part of the medium.

12. The method of claim 11, wherein the temperatures to which the first and second magnetic layers are beated during writing is below the Curie temperature Tc2 of the second magnetic layer.

13. A method of recording information on a magneto-optic recording material as claimed in claim 3 or 4, comprising the steps of:
i) applying an external magnetic field to part of a recording medium as claimed in claim 1 or claim 2, ii) applying a laser beam to said part simultaneously with the application of the magnetic field, and iii) modulating the intensity of the beam depending on a bit to be written at said part of the medium.

14. The method of claim 13, wherein the temperatures to which the first, second and third magnetic layers are treated during writing are below the Curie temperature Tc3 of the third magnetic layer.

15. A method of manufacturing an over-writable magneto-optic recording medium having initializing layers the magnetisation of which is not reversed in reading or writing; characterised in that;
two substrates having recording layers and initializing layers with different coercivities are joined, and thereafter a magnetic field stronger than the coercivity of one of the initializing layers is applied substantially perpendicular to the substrates, then a magnetic field weaker than the coercivity of said one of the initializing layers but stronger than the coercivity of the other initializing layer and oriented oppositely to the aforesaid magnetic field is applied substantially perpendicular to the substrates.

16. In a method of manufacturing an overwritable magneto-optic recording medium having two halves, each half having a substrate and having a corresponding initializing layer, the initializing layers each having a magnetization which is not reversed in reading or writing, the steps of:
adhering the halves of the recording medium each to the other to form a joined recording medium;
applying a first magnetic field stronger than the coercivity of the initializing layers substantially perpendicular to the joined recording medium;
applying a second magnetic field weaker than the coercivity of one of the initializing layers but stronger than the coercivity of the other initializing layer and oriented oppositely to the first magnetic field substantially perpendicular to the joined recording medium, after applying the first magnetic field.