JPH0522301B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to an overwritable magneto-optical recording method using a Curie point writing type magneto-optical recording medium that can be read using the magnetic Kerr effect. [Prior Art] Magneto-optical disks are known as erasable optical disk memories. Magneto-optical disks have advantages over conventional magnetic recording media using magnetic heads, such as high-density recording and non-contact recording and playback, but on the other hand, the recorded area must be erased before recording. It has the disadvantage that it cannot be magnetized in one direction (it must be magnetized in one direction). To compensate for this drawback, proposals have been made such as a method in which a recording/reproducing head and an erasing head are provided separately, or a method in which recording is performed while irradiating continuous laser beams while simultaneously modulating the applied magnetic field. [Problems to be Solved by the Invention] However, these methods have drawbacks such as the need for large-scale equipment and high cost, or the inability to perform high-speed modulation. The present invention eliminates the drawbacks of the above-mentioned conventional example, and provides magneto-optical recording that enables overwriting similar to magnetic recording media by simply adding a magnetic field generating means with a simple structure to the conventional device configuration. The purpose is to provide media and recording methods. [Means for Solving the Problems] The present invention capable of achieving the above object includes: a first magnetic layer having a Curie point T ₁ , a coercive force H ₁ , a thickness h ₁ and a saturation magnetization Ms _{1 ;} ,
a second magnetic layer having a coercive force H ₂ , a thickness h ₂ and a saturation magnetization Ms ₂ ; a Curie point T ₃ , a coercive force H ₃ and a thickness h ₃ ;
and an exchange-coupled four-layer structure consisting of a third magnetic layer having a saturation magnetization Ms ₃ and a fourth magnetic layer having a Curie point T ₄ , a coercive force H ₄ , a thickness h ₄ and a saturation magnetization Ms ₄ . A magneto-optical recording medium having a perpendicularly magnetized film on at least a substrate, the magnetic wall energy of the first and second magnetic layers, the domain wall energy of the second and third magnetic layers, and the magnetic wall energy of the third and fourth magnetic layers. Assuming that the domain wall energies are σw ₁₂ , σw ₂₃ , and σw ₃₄ in order, the above four magnetic layers meet the following conditions: T ₁ , T ₄ > T ₂ , T ₃ regarding the relationship of the Curie points of each magnetic layer. Regarding the relationship of coercive force of each magnetic layer, H ₂ > H ₄ > H ₁ , H ₃ Regarding the relationship of film thickness of each magnetic layer, h ₁ + h ₂ ≧250 Å h ₁ + h ₂ + h ₃ + h ₄ ≧ 600 Å of each magnetic layer Regarding the relationship between saturation magnetization, film thickness, coercive force, and domain wall energy, σw ₁₂ /2Ms ₁ h ₁ > H ₁ , H ₂ >σw ₁₂ /2Ms ₂ h ₂ , σw ₂₃ /2Ms ₂ h ₂ , H ₃ < Using a magneto-optical recording medium that satisfies σw ₂₃ /2Ms ₃ h ₃ , σw ₃₄ /2Ms ₃ h ₃ , H ₄ >σw ₃₄ /2Ms ₄ h ₄ , perform the following binary recording. This recording method is characterized by the following. (a) At a location different from the recording head on the medium, change the direction of magnetization of the second magnetic layer with coercive force H ₂ in a direction sufficient to magnetize the fourth magnetic layer with coercive force H ₄ in one direction. Magnetic field B of a magnitude that does not cause reversal
(b) Next, a bias magnetic field is applied by the recording head, and at the same time the direction of magnetization of the first and second magnetic layers is made uniform near the Curie point T ₂ of the second magnetic layer (at a temperature close to T ₂ ). By irradiating the medium with laser power sufficient to raise the temperature of the medium to a temperature at which the medium can be aligned in a stable direction with respect to the magnetization of the fourth magnetic layer, the first magnetic layer can be aligned without changing the direction of magnetization of the fourth magnetic layer. , the first type of preliminary recording in which the direction of magnetization of the second magnetic layer is aligned in a stable direction with respect to the fourth magnetic layer via the third magnetic layer, or the Curie point of the fourth magnetic layer is simultaneously applied while applying a bias magnetic field. Around T ₄ (T ₄
By irradiating the medium with enough laser power to raise the temperature of the medium to a temperature close to that at which the direction of magnetization of the fourth magnetic layer can be uniformly reversed,
Performing a second type of preliminary recording in which the direction of magnetization of the magnetic layer is reversed and simultaneously the first to third magnetic layers are magnetized in a stable direction with respect to the fourth magnetic layer, according to the signal, ( c) then by moving the medium to pass the pre-recorded bits through said magnetic field B;
For bits formed by the first type of preliminary recording, the magnetization direction of all the first to fourth magnetic layers remains unchanged, and for bits formed by the second type of preliminary recording, the magnetization direction of the fourth magnetic layer is Binary recording in which the direction of the magnetic field B is reversed to the same direction as the magnetic field B, and the directions of magnetization of the first and second magnetic layers remain unchanged. Here, regarding the direction of magnetization of the third magnetic layer,
I didn't go into details, but they are arranged like one of the following. If σw ₂₃ /2Ms ₃ h ₃ <σw ₃₄ /2Ms ₃ h ₃ , the third
The magnetization of the magnetic layer is always aligned in a stable direction with respect to the direction of magnetization of the fourth magnetic layer. Further, when σw ₂₃ /2Ms ₃ h ₃ >σw ₃₄ /2Ms ₃ h ₃ , the magnetization of the third magnetic layer is always aligned in a stable direction with respect to the direction of magnetization of the second magnetic layer. [Embodiments] The present invention will be described in detail below with reference to the drawings. FIGS. 1a and 1b are schematic cross-sectional views each showing an embodiment of the magneto-optical recording medium of the present invention. The magneto-optical recording medium shown in FIG. The magnetic layer 4 is laminated. a first magnetic layer 1, a second magnetic layer 2,
The Curie points of the third magnetic layer 3 and the fourth magnetic layer 4 are T ₁ , T ₂ , T ₃ , T ₄ in this order, and the coercive forces are H ₁ ,
H ₂ , H ₃ , H ₄ , film thicknesses are h ₁ , h ₂ , h ₃ , h ₄ in order, and saturation magnetizations are Ms ₁ , Ms ₂ , Ms ₃ , Ms ₄ in order. Domain wall energy of the first magnetic layer and the second magnetic layer,
The domain wall energy of the second magnetic layer and the third magnetic layer, the third
Assuming that the domain wall energies of the magnetic layer and the fourth magnetic layer are σw ₁₂ , σw ₂₃ , and σw ₃₄ in this order, the above four magnetic layers satisfy the following conditions and are exchange-coupled. For the Curie point of each magnetic layer, the relationship is T ₁ , T ₄ > T ₂ , T ₃ For the coercive force of each magnetic layer, the relationship is H ₂ > H ₄ > H ₁ , H ₃ Thickness of each magnetic layer For the relationship, h ₁ + h ₂ ≧250 Å, h ₁ + h ₂ + h ₃ + h ₄ ≧ 600 Å. For the saturation magnetization, film thickness, coercive force, and domain wall energy of each magnetic layer, the following relationship σw ₁₂ /2Ms ₁ h ₁ ＞H ₁ , H ₂ ＞σw ₁₂ /2Ms ₂ h ₂ , σw ₂₃ /2Ms ₂ h ₂ , H ₃ ＜σw ₂₃ /2Ms ₃ h ₃ , σw ₃₄ /2Ms ₃ h ₃ , H ₄ ＞σw ₃₄ /2Ms ₄ h ₄ However, usually T ₁ of the first magnetic layer 1 is 150 to 400
℃, H ₁ is 0.1 to 1 KOe, film thickness h ₁ is 50 to 300 Å, T ₂ of the second magnetic layer 2 is 70 to 200 ℃, H ₂ is 2 to 12 KOe,
The film thickness h ₂ is 50 to 300 Å, and the T ₃ of the third magnetic layer 3 is 0 to 300 Å.
200℃, _H3 is 0.1~1KOe, film thickness _h3 is 50~300Å,
_T4 of the fourth magnetic layer 4 is 100 to 300°C, _H1 is 0.5 to
4KOe and film thickness h ₄ are preferably in the range of about 50 to 600 Å. The main component of each magnetic layer can be one that exhibits perpendicular magnetic anisotropy and exhibits a magneto-optical effect.
Amorphous magnetic alloys of rare earth elements and transition metal elements are preferred. Examples include GdCo, GdFe, TbFe,
DyFe，GdTbFe，TbDyFe，GdTbFeCo，
Examples include TbFeCo and GdTbCo. Adjacent magnetic layers of the magneto-optical recording medium of the present invention are coupled by exchange force, the first magnetic layer 1 and the second magnetic layer 2 are coupled relatively strongly, and the second magnetic layer and the third magnetic layer The layers are relatively weakly bonded. Further, the third magnetic layer and the fourth magnetic layer are coupled relatively weakly. This means that the domain wall energy σw between strongly coupled magnetic layers is large, and the domain wall energy σw between weakly coupled magnetic layers is small, so that the σw of each magnetic layer is
When adjusting Ms, h, and H, the value of σw is optimized. The four magnetic layers 1, 2, 3, and 4 are arranged so as to satisfy the above relational expression [this means that the two types of magnetization states finally completed by recording (Fig.
f)], the film thickness of each layer,
Coercive force, magnitude of saturation magnetization, domain wall energy, etc. may be set. In FIG. 1b, which is another example of the magneto-optical recording medium of the present invention, 5 and 6 are four magnetic layers 1, 2, 3,
This is a protective film for improving the durability of No. 4 or for improving the magneto-optical effect. 7 is an adhesive layer for bonding the bonding substrate 8 together. By laminating layers 1 to 6 on the bonding substrate 8 and gluing them together, recording and reproduction can be performed on both sides. The recording process of the present invention will be described below using FIGS. 2 to 4. Before recording, the magnetization directions of the magnetic layers 1 to 4 may be parallel (same direction) and stable, or antiparallel (opposite directions) and stable between adjacent magnetic layers. However, since parallel adjacent magnetic layers are relatively strongly coupled, and antiparallel adjacent magnetic layers are relatively weakly coupled, the magnetization of the magneto-optical recording medium of the present invention is in the following state. It is preferable. The magnetizations of the first and second magnetic layers are parallel and stable, and the magnetizations of the second and fourth magnetic layers are antiparallel and stable. 35 in FIG. 3 is a magneto-optical disk having the configuration described above. For example, suppose that the magnetization state of a certain part of this magnetic layer is initially as shown in FIG. 2a. It is assumed that the magnetization of the third magnetic layer is parallel to the magnetization of the fourth magnetic layer and is in a stable state. The magneto-optical disk 35 is rotated by a spindle motor and passes through the magnetic field generating section 34 . At this time, if the magnitude of the magnetic field of the magnetic field generator 34 is set to a value between the coercive forces of the second magnetic layer 2 and the fourth magnetic layer 4 (the direction of the magnetic field is upward in this embodiment), as shown in FIG. As shown in , the fourth magnetic layer 4 is magnetized in a uniform direction,
The third magnetic layer 3, which is strongly coupled to the fourth magnetic layer, also
It is magnetized parallel to the magnetization of the fourth magnetic layer. On the other hand, the magnetization of the second magnetic layer 2 remains unchanged. Furthermore, the magnetization of the first magnetic layer 1, which is strongly coupled to the second magnetic layer 2, remains as it was at the beginning. Next, when the magneto-optical disk 35 rotates and passes the recording/reproducing head 31, a laser beam having two types (first type and second type) of laser power values is emitted by a signal from the recording signal generator 32. According to
Either power is used to irradiate the disk surface. The first type of laser power is enough to raise the temperature of the disk to around the Curie point of the second magnetic layer 2, and the second type of laser power is enough to raise the temperature of the disk to around the Kyrie point of the fourth magnetic layer 4. It is a power that can be heated. That is, in FIG. 4, which shows an outline of the relationship between the coercive force and temperature of both magnetic layers 2 and 4, the first type of laser power reaches around T ₂ and the second type laser power reaches around T ₄ of the disk. Can increase temperature. The temperature of the second magnetic layer 2 and the fourth magnetic layer 4 is raised to near the Curie point of the second magnetic layer by the first type of laser power, but bits stably exist in the fourth magnetic layer 4 at this temperature. Since it has a coercive force, by appropriately setting the bias magnetic field during recording, the direction of magnetization of each magnetic layer will change to the magnetization of the fourth magnetic layer as the temperature of the recording bit area decreases. Arrange in a stable direction. In other words, bits as shown in FIG. 2c are formed from any of the bits shown in FIG. 2b (first type preliminary recording). Here, setting the bias magnetic field appropriately means the following. That is, in the first type of preliminary recording, the magnetization of each of the first to third magnetic layers is subjected to a force (exchange force) that aligns them in a stable direction with respect to the direction of magnetization of the fourth magnetic layer 4, so that the original No bias magnetic field is required. However, the bias magnetic field is set in a direction that assists magnetization reversal of the fourth magnetic layer 4 in preliminary recording using the second type of laser power, which will be described later. Further, it is preferable for convenience to set the bias magnetic field to have the same magnitude and direction in preliminary recording with either the first type or the second type of laser power. From this point of view, it is preferable to set the bias magnetic field to the minimum size necessary for preliminary recording of the second type of laser power based on the principle shown below, and settings that take this into account are the appropriate setting as mentioned above. It is a setting. On the other hand, when the temperature of the fourth magnetic layer 4 is raised to near the Curie point using the second type of laser power (second type of preliminary recording), the fourth magnetic layer 4 is magnetized by the bias magnetic field set as described above. The direction of is reversed. Subsequently, the magnetization of the first to third magnetic layers also changes to the fourth magnetic layer 4.
Arrange in a stable direction. That is, Fig. 2b
A bit as shown in FIG. 2d is formed from either of these. In this way, each location on the magneto-optical disk is preliminarily recorded in the state shown in FIG. Become. Next, when the magneto-optical disk 35 is rotated and the pre-recorded bits c and d pass through the magnetic field generating section 34 again, the magnitude of the magnetic field of the magnetic field generating section 34 will be between the coercive forces of the magnetic layers 2 and 4 as described above. Therefore, the recording bit c remains in the state e (final recording state) without any change. On the other hand, the recording bit d becomes the state f (another final recording state) due to magnetization reversal in the third and fourth magnetic layers 3 and 4. In order for the state of the recorded bits of f to exist stably, the following relationship is required as described above. (B) H ₂ >σw ₁₂ /2Ms ₂ h ₂ ,σw ₂₃ /2Ms ₂ h ₂ H ₄ >σw ₃₄ /2Ms ₄ h ₄ (B) σw ₁₂ /2Ms ₁ h ₁ >H ₁ (C) H ₃ < σw ₂₃ /2Ms ₃ h ₃ , σw ₃₄ /2Ms ₃ h ₃ , (d) h ₁ +h ₂ ≧250Å, h ₁ +h ₂ +h ₃ +h ₄ ≧600Å (a) is necessary for the second magnetic layer and the fourth This is to ensure that the magnetic layer is in an unstable state. The reason for (b) is that the first and second magnetic layers are strongly coupled and the magnetization direction of the first magnetic layer is always aligned in a stable direction with respect to the magnetization of the second magnetic layer. (C) is necessary in order to always make the magnetization of the third magnetic layer stable with respect to the direction of magnetization of either the second or fourth magnetic layer. (D) is necessary because, as will be made clear in later examples, consideration is given to optimizing recording sensitivity and reproduction C/N ratio. The states e and f of the recorded bits are controlled by the laser power during recording and do not depend on the state before recording, so overwriting is possible. Recorded bits e and f can be reproduced by irradiating a reproduction laser beam and processing the reproduction light by a recording signal regenerator 33. As mentioned above, the magnitude (modulation degree) of the reproduced signal mainly depends on the magneto-optical effect of the first and second magnetic layers. In the present invention, since a material with a high Curie temperature (a material with a high magneto-optical effect) can be used for the first magnetic layer 1 on which the reproduction light is incident, recording with a large reproduction signal is possible in the present invention. [Examples] Example 1 A polycarbonate disc-shaped substrate with pregroove and preformat signals engraved thereon was set at a distance of 10 cm from the target in a sputtering device equipped with a 5-dimensional target source. Rotated. In argon, from the first target at a sputtering speed of 100 Å/min and a sputtering pressure of 5×10 ^-3 Torr.
ZnS was provided as a protective layer with a thickness of 900 Å. Next, in argon, a second target was sputtered at a sputtering speed of 100 Å/min and a sputtering pressure of 5×10 ^-3 Torr.
GdFeCoTi alloy was sputtered with a film thickness of 200 Å, T ₁
A first magnetic layer of Gd ₁₈ Fe ₅₅ Co ₂₄ Ti ₃ was formed at a temperature of approximately 350°C. H ₁ of the first magnetic layer itself was about 300 Oe or less, and the sublattice magnetization was larger in the transition metal. Next, under similar conditions, TbFe alloy was sputtered using a third target, with a film thickness of 150 Å and T ₂ =
A second magnetic layer of Tb ₁₈ Fee ₈₂ was formed at about 140°C.
The H ₂ of this second magnetic layer itself was 10 KOe, and the sublattice magnetization was larger in the transition metal. Next, under similar conditions, from the fourth target,
Sputter GdTbFe alloy, film thickness 100 Å, T ₃ = approx.
A third magnetic layer of Gd ₁₂ Tb ₁₂ Fe ₇₆ was formed at 160°C.
The H ₃ of this third magnetic layer itself is 100 to 300 Oe,
Sublattice magnetization was larger for rare earths. Next, under similar conditions, from the fifth target,
Sputter TbFeCo alloy, film thickness 200 Å, T ₄ = approx.
A fourth magnetic layer of Tb ₂₃ Fe ₇₂ Co ₅ was formed at 210°C.
The H ₄ of this fourth magnetic layer itself is 500 to 1500 Oe,
Sublattice magnetization was larger for rare earths. Next, ZnS was sputtered from the first target under the same conditions to form a 2000 Å thick ZnS layer as a protective layer. Next, the above substrate on which the film had been formed was bonded to a polycarbonate bonding substrate using a hot melt adhesive to produce a magneto-optical disk. This magneto-optical disk was set in a recording/reproducing device, and a laser beam with a wavelength of 830 nm focused to approximately 1 μ was transmitted at 2 MHz at a duty rate of 50% while passing through the magnetic field generator of 2KOe at a linear velocity of approximately 7 m/sec. Recording was performed using binary laser powers of 4 mW and 8 mW while modulating the laser power. The bias magnetic field was 150 Oe. After that, a 1mW laser beam was irradiated to reproduce the signal, and a binary signal could be reproduced. Next, an experiment similar to that described above was conducted on a magneto-optical disk that had been completely recorded. As a result, previously recorded signal components were not detected, confirming that overwriting is possible. Example 2 and Comparative Example Magneto-optical disk samples were fabricated using the same configuration and materials as in Example 1, with only the film thicknesses of the first to fourth magnetic layers being changed. For each sample,
The reflectance and Kerr rotation angle were measured. The results are shown in Table 1 below. Note that the reflectance was 23 to 24%, including reflection from the substrate surface (approximately 4%). Further, it is considered that the larger the value of the Kerr rotation angle, the larger the CN ratio of the reproduced signal.

【table】

〔Effect of the invention〕

As explained in detail above, a magneto-optical recording medium having four magnetic layers that satisfies the above-mentioned requirements is used, and during recording, a magnetic field generating section is provided at a separate position from the recording head, and a binary laser power is By recording with , overwriting became possible. Further, since the magnetic layer of the recording medium used in the recording method of the present invention, which is mainly used for reproduction, can be selected from materials with a high Kyrie point, that is, a material with a large magneto-optical effect, according to the present invention, as a result, Bits with a large reproduced signal can be obtained.

[Brief explanation of the drawing]

FIGS. 1a and 1b are diagrams each showing an example of the structure of a magneto-optical medium used in the present invention, and FIG. 2 shows the magnetization of magnetic layers 1, 2, 3, and 4 during the recording method of the present invention. A diagram showing the orientation, FIG. 3 is a conceptual diagram of the recording/reproducing device,
FIG. 4 is a schematic diagram showing the relationship between coercive force and temperature of the second magnetic layer 2 and the fourth magnetic layer 4. B...Transparent substrate with pregroove, 1, 2,
3, 4... Magnetic layer, 5, 6... Protective layer, 7... Adhesive layer, 8... Bonding substrate, 31... Recording/
Reproducing head, 32... Recording signal generator, 33...
...Recording signal regenerator, 34...Magnetic field generator, 35...
...Magneto-optical disk.

Claims (1)

[Claims] 1. A first magnetic layer having a Kyrie point T ₁ , a coercive force H ₁ , a thickness h ₁ and a saturation magnetization Ms ₁ , a Kyrie point T ₂ ,
a second magnetic layer having a coercive force H ₂ , a thickness h ₂ and a saturation magnetization Ms ₂ ; a Curie point T ₃ , a coercive force H ₃ and a thickness h ₃ ;
and an exchange-coupled four-layer structure consisting of a third magnetic layer having a saturation magnetization Ms ₃ and a fourth magnetic layer having a Curie point T ₄ , a coercive force H ₄ , a thickness h ₄ and a saturation magnetization Ms ₄ . A magneto-optical recording medium having a perpendicularly magnetized film on at least a substrate, the magnetic wall energy of the first and second magnetic layers, the domain wall energy of the second and third magnetic layers, and the magnetic wall energy of the third and fourth magnetic layers. A magneto-optical recording medium characterized in that the four magnetic layers are coupled so as to satisfy the following conditions, where the domain wall energies are σw ₁₂ , σw ₂₃ , and σw ₃₄ in order. The relationship between the Curie points of each magnetic layer is T ₁ , T ₄ > T ₂ , T ₃ The relationship between the coercive force of each magnetic layer is H ₂ > H ₄ > H ₁ , H ₃ The relationship between the thickness of each magnetic layer For h ₁ + h ₂ ≧250Å h ₁ +h ₂ +h ₃ +h ₄ ≧600Å For the relationship among the saturation magnetization, film thickness, coercive force, and domain wall energy of each magnetic layer, σw ₁₂ /2Ms ₁ h ₁ ＞H ₁ , H ₂ >σw ₁₂ /2Ms ₂ h ₂ , σw ₂₃ /2Ms ₂ h ₂ , H ₃ <σw ₂₃ /2Ms ₃ h ₃ , σw ₃₄ /2Ms ₃ h ₃ , H ₄ >σw ₃₄ /2Ms ₄ h _{4 2} Curie point T ₁ , a coercive force H ₁ , a thickness h ₁ and a saturation magnetization Ms ₁ of the first magnetic layer, and a Curie point T ₂ ,
a second magnetic layer having a coercive force H ₂ , a thickness h ₂ and a saturation magnetization Ms ₂ ; a Curie point T ₃ , a coercive force H ₃ and a thickness h ₃ ;
and an exchange-coupled four-layer structure consisting of a third magnetic layer having a saturation magnetization Ms ₃ and a fourth magnetic layer having a Curie point T ₄ , a coercive force H ₄ , a thickness h ₄ and a saturation magnetization Ms ₄ . A magneto-optical recording medium having a perpendicularly magnetized film on at least a substrate, the magnetic wall energy of the first and second magnetic layers, the domain wall energy of the second and third magnetic layers, and the magnetic wall energy of the third and fourth magnetic layers. Assuming that the domain wall energies are σw ₁₂ , σw ₂₃ , and σw ₃₄ in order, the above four magnetic layers meet the following conditions: T ₁ , T ₄ > T ₂ , T ₃ regarding the relationship of the Curie points of each magnetic layer. Regarding the relationship of coercive force of each magnetic layer, H ₂ > H ₄ > H ₁ , H ₃ Regarding the relationship of film thickness of each magnetic layer, h ₁ + h ₂ ≧250 Å h ₁ + h ₂ + h ₃ + h ₄ ≧ 600 Å of each magnetic layer Regarding the relationship between saturation magnetization, film thickness, coercive force, and domain wall energy, σw ₁₂ /2Ms ₁ h ₁ > H ₁ , H ₂ >σw ₁₂ /2Ms ₂ h ₂ , σw ₂₃ /2Ms ₂ h ₂ , H ₃ < It is characterized by recording the following binary values using _a magneto-optical recording medium that satisfies σw 23 /2Ms 3 h 3 , σw ₃₄ /2Ms ₃ h ₃ , H ₄ > _{σw 34} /2Ms _{4 h 4} recording method. (a) At a location different from the recording head on the medium, change the direction of magnetization of the second magnetic layer with coercive force H ₂ in a direction sufficient to magnetize the fourth magnetic layer with coercive force H ₄ in one direction. Magnetic field B of a magnitude that does not cause reversal
(b) Next, the recording head applies a bias magnetic field and at the same time irradiates the fourth magnetic layer with enough laser power to raise the temperature of the medium to near the Curie point T ₂ of the second magnetic layer. The first type of preliminary recording, in which the direction of magnetization of the first and second magnetic layers is aligned in a stable direction with respect to the fourth magnetic layer via the third magnetic layer, without changing the direction of magnetization, or the bias magnetic field is applied. By irradiating the medium with enough laser power to raise the temperature of the medium to near the Curie point T ₄ of the fourth magnetic layer, the direction of magnetization of the fourth magnetic layer is reversed and the magnetization of the first to third magnetic layers is simultaneously applied.
A second type of preliminary recording is performed in response to the signal, in which both the magnetic layers are magnetized in a stable direction with respect to the fourth magnetic layer, and (c) the medium is then moved to record the preliminary recorded information. By passing the bit through the magnetic field B,
For bits formed by the first type of preliminary recording, the magnetization direction of all the first to fourth magnetic layers remains unchanged, and for bits formed by the second type of preliminary recording, the magnetization direction of the fourth magnetic layer is Binary recording in which the direction of the magnetic field B is reversed to the same direction as the magnetic field B, and the directions of magnetization of the first and second magnetic layers remain unchanged.