JPH05234158A - Overwritable magneto-optical recording medium - Google Patents

Abstract

PURPOSE:To obviate the generation of crosstalks even when the above recording medium is formed to narrower tracks by providing a heat insulating layer formed of a material having a low thermal diffusion rate and a heat diffusion layer formed of a material having a high thermal conductivity in addition to an M layer and a W layer. CONSTITUTION:The heat insulating layer is provided directly or via a protective layer on the respective W layers or I layers of the media having a two-layered film structure consisting of the M layer and the W layer, a three-layered film structure consisting of the M layer, the W layer and the I layer and further, a four-layered film structure consisting of the M layer, the W layer, an S layer and the I layer. The heat diffusion layer is provided on the vicinity of the heat inslating layer. As a result, the recording medium is formed to narrow tracks by providing such a heat inslating layer, the heat transfer to the periphery of the marks, i.e., the marks on the adjacent tracks is suppressed and the generation of the crosstalks is obviated in spite of irradiation with a high- intensity laser beam even in the high-intensity cycle of a high-temp. cycle. The generation of the deformation and destruction of the marks is thus obviated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overwritable magneto-optical recording medium.

[0002]

2. Description of the Related Art Recently, an optical recording / reproducing method satisfying various requirements including high density, large capacity, high access speed, and high recording / reproducing speed, and a recording apparatus, reproducing apparatus and recording medium used therefor. Efforts are being made to develop. Among the wide range of optical recording / reproducing methods, the magneto-optical recording / reproducing method has a unique advantage that information can be recorded and then erased, and new information can be recorded again and again. For being full of the greatest attraction.

The recording medium used in this magneto-optical recording / reproducing method has a perpendicular magnetic layer (perpendicular magnetic layer or layers) consisting of one layer or multiple layers as a layer for recording. This magnetic film is made of, for example, amorphous GdFe or Gd
It consists of Co, GdFeCo, TbFe, TbCo, TbFeCo, and the like. The perpendicular magnetic film generally has concentric or spiral tracks, and information is recorded on the tracks. There are two types of tracks, explicit and implicit. [Explicit Track] The magneto-optical recording medium has a disk shape. In a disc having explicit tracks, information recording tracks are formed in a spiral or concentric shape when viewed in a direction perpendicular to the disc plane. There is a groove for tracking and separation between two adjacent tracks. Conversely, the space between the grooves is called a land. In reality, the land and groove are reversed on the front and back of the disc. Therefore, looking at the disk in the same way as the beam enters, the front side is called a groove and the back side is called a land. Since the perpendicularly magnetized film is formed on the groove and the land over the entire surface, the groove portion may be the track or the land portion may be the track. There is no particular relationship between the width of the groove and the width of the land.

In order to form such a land and groove,
Generally, a substrate has a land formed in a spiral or concentric shape on the surface and a groove sandwiched between two adjacent lands. A thin perpendicular magnetization film is formed on such a substrate. As a result, the perpendicular magnetic film has lands and grooves. [Mark] In the present specification, “upwar (upwar
d) ”or“ downward ”,
Orientation "and the other is defined as" reverse A orientation ". The information to be recorded is binarized in advance, and this information is "A direction".
Are recorded with two signals: a mark (B ₁ ) having a magnetization of _{No. 1} and a mark (B ₀ ) having a magnetization of “inverse A direction”. These marks B ₁ and B ₀ correspond to either one or the other of the digital signals 1 and 0, respectively. However, generally, the magnetization of the recorded track is aligned in the "reverse A direction" by applying a strong external magnetic field before recording. In the old sense, the act of aligning the directions of magnetization is "initialization ^* (init
ialize ^* ) ”. Then, a mark (B ₁ ) having "A direction" magnetization is formed on the track. The information is
It is expressed by the presence or absence of this mark (B ₁ ) and / or the mark length. The mark has been called a pit or a bit in the past, but is recently called a mark. [Principle of mark formation] In the formation of marks, the characteristic of the laser, that is, the excellent coherence in space and time (coheren
ce) is preferably used to focus the beam into a spot that is almost as small as the diffraction limit determined by the wavelength of the laser light. The narrowed light is applied to the track surface, and information is recorded by forming marks with a diameter of 1 μm or less on the perpendicular magnetization film. In optical recording, recording densities up to about 10 ⁸ marks / cm ² can theoretically be achieved. This is because the laser beam can be concentrated into a spot with a diameter as small as about its wavelength.

As shown in FIG. 4, in magneto-optical recording, a laser beam (L) is focused on a perpendicular magnetization film (MO) and heated. Meanwhile, a recording magnetic field (Hb) in the opposite direction to the initialized ^* direction is externally applied to the heated portion. Then, the coercive force Hc (coersivity) of the locally heated portion decreases and becomes smaller than the recording magnetic field (Hb). As a result, the magnetization of that portion is aligned in the direction of the recording magnetic field (Hb). In this way, the oppositely magnetized mark is formed.

Ferromagnetic materials and ferrimagnetic materials have different temperature dependences of magnetization and Hc. Ferromagnetic materials have Hc that decreases near the Curie point, and recording is performed based on this phenomenon. Therefore, it is referred to as Tc writing (Curie point writing). On the other hand, ferrimagnetic materials have a compensation temperature T below the Curie point.
_comp. , where the magnetization (M) becomes zero. On the contrary, Hc becomes very large near that temperature, and when it deviates from that temperature, Hc drops sharply. This lowered Hc is defeated by the relatively weak recording magnetic field (Hb). That is, recording becomes possible. This recording process is called T _comp. Writing (compensation point writing).

However, it is not necessary to stick to the Curie point or its vicinity and the compensation temperature. In short, recording is possible by applying a recording magnetic field (Hb) for compensating the lowered Hc to the magnetic material having the lowered Hc at a predetermined temperature higher than room temperature. However, the perpendicular magnetization film (Mc in the region where Hc in this region has the original high Hc) that has not reached a predetermined temperature higher than room temperature (M
Too high Hb that reverses the magnetization of (O) is not possible. [Principle of Reproduction] FIG. 5 shows the principle of information reproduction based on the magneto-optical effect. Light is an electromagnetic wave that has an electromagnetic field vector that is normally divergent in all directions on a plane perpendicular to the optical path. When the light is converted to linearly polarized light (L _p ) and is applied to the perpendicular magnetic film (MO), the light is reflected on its surface or transmitted through the perpendicular magnetic film (MO). At this time, the polarization plane rotates according to the direction of the magnetization M. This rotating phenomenon is caused by the magnetic Kerr effect or the magnetic Faraday.
y) Called the effect.

For example, if the plane of polarization of the reflected light is "A direction"
Rotation of θ k degrees with respect to magnetization causes rotation of −θ k degrees with respect to "reverse A direction" magnetization. Therefore, when the axis of the optical analyzer (polarizer) is set perpendicular to the plane inclined by -θk, the light reflected from the mark (B ₀ ) magnetized in the "reverse A direction" passes through the analyzer. I can't. On the other hand, the mark (B ₁ ) magnetized in “A direction”
The light reflected from is multiplied by (sin2θk) ² and passes through the analyzer, and is captured by a detector (photoelectric conversion means). As a result, the mark (B ₁ ) magnetized in the "A direction" appears brighter than the mark (B ₀ ) magnetized in the "reverse A direction", and generates a strong electric signal in the detector. Therefore, the electrical signal from this detector is
The information is reproduced because it is modulated according to the recorded information. However, to re-use a recorded medium (1) to reinitialize the medium ^* device initialization ^* either with, or (2) or an erase head having the same arrangement as a recording head in the recording apparatus, or (3) It is necessary to previously erase the recorded information by using a recording device or an erasing device as a pre-stage process. Therefore, in the magneto-optical recording method, it has hitherto been impossible to perform overwriting capable of recording new information on the spot regardless of the presence or absence of recorded information.

However, if the direction of the recording magnetic field Hb can be freely modulated between "A direction" and "reverse A direction" as required, overwriting becomes possible. However, it is impossible to modulate the direction of the recording magnetic field Hb at high speed. For example, when the recording magnetic field Hb is a permanent magnet, it is necessary to mechanically reverse the direction of the magnet. However, it is impossible to reverse the direction of the magnet at high speed. Even when the recording magnetic field Hb is an electromagnet, it is impossible to modulate the direction of a large-capacity current at such a high speed.

However, technological advances have been remarkable, and the intensity of the irradiating light beam should be recorded without modulating the intensity of the recording magnetic field Hb (including ON and OFF) or without modulating the direction of the recording magnetic field Hb. A magneto-optical recording method capable of overwriting, a magneto-optical recording medium capable of being overwritten, and an overwritable recording device also employed therein have been invented simply by modulating in accordance with binary information. A patent application has been filed (JP-A-62-175948 = DE3,619,61)
8A1 = US patent pending Ser. No. 453,255). Hereinafter, this invention is referred to as a "basic invention". [Explanation of Basic Invention] In the basic invention, "a recording / reproducing layer basically consisting of a magnetic thin film capable of perpendicular magnetization is used.
(In this specification, this recording / reproducing layer is referred to as a memory layer memory
layer or M layer) and a recording auxiliary layer reference layer composed of a magnetic thin film capable of perpendicular magnetization (in this specification, this recording auxiliary layer is a writing layer writing layer or W).
Layers) and both layers are exchange-coupled, and
An overwritable multi-layered magneto-optical recording medium in which only the magnetization of the W layer can be oriented in a predetermined direction without changing the magnetization direction of the M layer at room temperature is used.

Then, the information is represented and recorded by the mark having the "A direction" magnetization and the mark having the "reverse A direction" magnetization in the M layer (and also in the W layer in some cases).
In this medium, the W layer has an external means (for example, an initial auxiliary magnetic field Hin.
By i.), the direction of the magnetization can be aligned in the “A direction”. Moreover, at that time, the magnetization direction of the M layer is not reversed, and further, the magnetization direction of the W layer once aligned in the “A direction” is not reversed even when the exchange coupling force from the M layer is received. On the contrary, the magnetization direction of the M layer is not reversed even when receiving the exchange coupling force from the W layer aligned in the “A direction”.

The W layer has a lower coercive force H _C and a higher Curie point T _C than the M layer. According to the recording method of the basic invention, only the magnetization direction of the W layer of the recording medium is aligned in the “A direction” by external means before recording. This act is specifically referred to herein as "initialize". This "initialization" is unique to overwritable media.

Then, the medium is irradiated with a laser beam pulse-modulated according to the binarized information. The intensity of the laser beam has a high level P _H and a low level P _L , which correspond to the high level and low level of the pulse. This low level is higher than the reproduction level P _R that illuminates the medium during reproduction. As is already known, even when recording is not performed, the laser beam may be turned on at <very low level>, for example, to access a predetermined recording location on the medium. This <very low level> is also the same as or close to the reproduction level P _R. Therefore, for example, the output waveform of the laser beam in the basic invention is as shown in FIG.

Although not specified in the specification of the basic invention, in the basic invention, the recording beam is not a single beam but two beams which are close to each other, and is a low beam which does not modulate the preceding beam in principle. It is also possible to use a level laser beam (for erasing) and a high level laser beam (for writing) that modulates the trailing beam according to information. In this case, the trailing beam is pulse modulated between a high level and a base level (equal to or lower than the low level and may have zero output). The output waveform in this case is, for example, as shown in FIG.
Shown in.

A recording magnetic field Hb whose direction and intensity are not modulated acts on the medium irradiated with the beam. Hb is
It cannot be narrowed down to the same size as the irradiated portion of the beam (spot area), and the area on which Hb acts is much larger than the spot area. When illuminated by a low level beam, M is independent of the direction of magnetization of the previous mark.
One of the "A-oriented" mark (B ₁ ) and the "reverse A-oriented" mark (B ₀ ) is formed on the layer.

When a high level beam is irradiated, the other mark is formed on the M layer regardless of the magnetization direction of the previous mark. This completes the overwrite. In the basic invention, the laser beam is pulse-modulated according to the information to be recorded. However, this is also done in the conventional magneto-optical recording, and it should be recorded.
The means for pulse-modulating the beam intensity in accordance with the digitized information is a known means. For example, THE BELL SYSTEM TECHN
ICAL JOURNAL, Vol. 62 (1983), 1923 -1936. Therefore, given the required high and low levels of beam intensity, it is readily available with only minor modifications to conventional modulation means. For a person skilled in the art, such a modification would be easy given the high and low levels of beam intensity.

One of the features of the basic invention is that
There are high and low levels of beam intensity. That is, when the beam intensity is at a high level, the "A direction" magnetization of the W layer is reversed to "reverse A direction" by the recording magnetic field Hb or other external means (re
verse), and the "reverse A direction" magnetization of the W layer forms a mark having the "reverse A direction" magnetization (or "A direction" magnetization) in the M layer. When the beam intensity is low, the magnetization direction of the W layer is the same as in the "initialized" state, and
By the action of the layer (this action is transmitted to the M layer through the exchange coupling force), the M layer is magnetized “A direction” [or “reverse A direction”].
Magnetization] is formed.

In the present specification, ○○○ [or △△△]
With the expression, when XX outside [] is read first, XX outside [] will be read also in the following XX [or ΔΔ △ ]. On the contrary, ○○○
If you select and read the △△△ one in [] without reading, read the following △ ○ △ [or △△△ ] without reading ○○○. Should be read.

The medium used in the basic invention is roughly classified into a first embodiment and a second embodiment. In any of the embodiments, the recording medium has a multilayer structure including an M layer and a W layer. The M layer is a magnetic layer having a high coercive force and a low magnetization reversal temperature at room temperature. The W layer is a magnetic layer having a lower coercive force and a higher magnetization reversal temperature at room temperature than the M layer. Both the M layer and the W layer may themselves be composed of a multilayer film. In some cases, a third layer (for example, an adjustment layer for the exchange coupling force σ _W ) may be present between the M layer and the W layer.
See JP-A-64-50257 and JP-A-1-273248. Furthermore, there is no clear boundary between the M layer and the W layer, and it is possible to gradually change from one to the other.

In the first embodiment, the coercive force of the M layer is H _C1 ,
The W layer has H _C2 , the M layer has a Curie point T _C1 , the W layer has T _C2 , the room temperature is T _R , and the temperature of the recording medium when irradiated with a low-level P _L laser beam is T _L , When a high-level P _H laser beam is irradiated, it is T _H , and the coupling magnetic field received by the M layer is H _D1 (H _D1 is σ _W multiplied by the saturation magnetic moment M _S of the M layer and the film thickness t of the M layer. Is calculated by dividing the coupling magnetic field received by the W layer by H _D2 (where H _D2 is σ _W divided by the product of the W layer saturation magnetic moment M _S and the W layer thickness t). The recording medium is defined by the following formula 1
And satisfying formulas 2 to 5 at room temperature.

T _R <T _C1 ≈T _L <T _C2 ≈T _H ………… Equation 1 H _C1 > H _C2 + | HD ₁ − (± HD ₂ ) | ……… Equation 2 H _C1 > HD ₁ ……………………………………… Formula 3 H _C2 > H _D2 ………………………………… Formula 4 H _C2 + H _D2 <| Hini. | <H _C1 ± H _D1 ──Equation 5 In the above equation, the symbols “≈” are equal or almost equal (±
20 ° C). Further, in the above formula, regarding the composite ±, the upper row is for an A (antiparallel) type medium described later, and the lower row is for a P (parallel) type medium described later. The ferromagnetic medium belongs to the P type.

That is, the relationship between the coercive force and the temperature is generally represented by a graph as shown in FIG. The thin line represents that of the M layer, and the thick line represents that of the W layer. Therefore, when an external means such as an initial auxiliary magnetic field (Hini.) Is applied to this recording medium at room temperature, according to Equation 5, the magnetization direction of the M layer is not reversed and W
Only the magnetization of the layer is reversed. Therefore, when an action (for example, the initial auxiliary magnetic field Hini.) Is exerted on the medium before recording, only the magnetization of the W layer can be aligned in the “A direction”. That is, "initialization". In the following description, the “A direction” is indicated by an upward arrow ↑ and the “reverse A direction” is indicated by a downward arrow ↓ on the surface of the present specification for convenience. And Hin
Even if i. disappears, the magnetization ↑ of the W layer is maintained as it is without re-reversal according to the equation (4).

A conceptual representation of a state in which only the magnetization of the W layer is aligned in the "A direction" ↑ before recording by an external means --- "initialized" state --- 9 In FIG. 9, the magnetization direction ^* in the M layer represents the information recorded up to that point. In the following description, since there is no relation to the orientation, when this is shown by X and is simplified, FIG. 9 can be shown as state 1 in FIG.

Here, the medium temperature is raised to T _H by irradiating a high level laser beam. Then, T _H
Is higher than the Curie point T _C1 , the magnetization of the M layer disappears. Furthermore, since T _H is near the Curie point T _C2 , the magnetization of the W layer disappears completely or almost. Here, the recording magnetic field Hb of "A direction" or "reverse A direction" is applied according to the type of medium. Hb may be a stray magnetic field from the medium itself. In order to simplify the explanation, it is assumed that the recording magnetic field Hb in the "reverse A direction" ↓ is applied. Since the medium is moving, the irradiated portion is immediately moved away from the laser beam and cooled. When the temperature of the medium is lowered in the presence of Hb, the magnetization of the W layer is reversed according to Hb to become "inverse A direction" ↓ magnetization (state 2 in FIG. 10).

Then, the cooling is further advanced, and the medium temperature becomes T
_When it is slightly lower than _C1, the magnetization of the M layer appears again. In that case, due to the magnetic coupling (exchange coupling) force, the magnetization direction of the M layer is influenced by the W layer and becomes a predetermined direction. As a result, a "reverse A direction" ↓ mark (P type medium) or an "A direction" ↑ mark (A type medium) is formed in the M layer depending on the type of medium. This state is shown in Figure 1.
0 state 3 (P type) or state 4 (A type).

The change in state caused by this high level laser beam will be referred to as a high temperature cycle here. Next, the medium temperature is raised to T _L by irradiating the laser beam of low level P _L. Since T _L is near the Curie point T _C1, the magnetization of the M layer disappears at all or almost disappears. However, since the temperature is lower than the Curie point T _C2 , the magnetization of the W layer does not disappear. This state is shown as state 5 in FIG. Here, the recording magnetic field Hb is unnecessary, but Hb at a high speed (short time)
It is impossible to turn on and off. Therefore, it is unavoidable and remains as it was during the high temperature cycle.

However, since H _C2 is still large, H
The magnetization ↑ of the W layer is not reversed by b. Since the medium is moving, the irradiated portion is immediately moved away from the laser beam and cooled. As cooling progresses, magnetization again appears in the M layer. The direction of the appearing magnetization has a predetermined direction due to the influence of the W layer due to the magnetic coupling force. As a result, depending on the type of medium, a mark “A direction” ↑ (for P type medium) or a mark “reverse A direction” ↓ (for A type medium) is formed in the M layer. This magnetization does not change even at room temperature. This is the state 6 (P type) in Figure 10.
Alternatively, it is the state 7 (A type).

The change in state caused by the low level laser beam will be referred to as a low temperature cycle here. As described above, regardless of the magnetization direction of the M layer before recording, by selecting the high temperature cycle and the low temperature cycle, the mark of “reverse A direction” ↓ and the mark of “A direction” ↑ can be obtained. It can be freely formed on the M layer. That is, overwriting is possible by modulating the laser beam in a pulse shape between a high level (high temperature cycle) and a low level (low temperature cycle) according to information. See FIGS. 11 and 12. In FIG. 11 and FIG. 12, P type and A are respectively
All types of media are depicted as a result of room temperature or back to room temperature.

In the above description, the magnetic material composition in which there is no compensation temperature T _comp. Between the room temperature and the Curie point for both the M layer and the W layer has been described. However, if the compensating temperature T _comp. Exists, the direction of the magnetization is reversed when the compensating temperature T _comp. Is exceeded. In reality, the directions of the sublattice magnetizations of RE and TM do not change, but the magnitude relation is large. Is reversed, the direction of the magnetization as a whole (alloy) is reversed -----
The explanation becomes more complicated because the A and P types are reversed. In this case, the direction of the recording magnetic field Hb is also opposite to the direction ↓ in the description on the previous page when considering at room temperature. That is, Hb having the same direction as the magnetization direction ↑ of the “initialized” W layer is applied.

The recording medium is generally disc-shaped, and the medium is rotated during recording. Therefore, the recorded portion (mark) is again subjected to the action of external means such as Hini. After recording, and as a result, the magnetization of the W layer is aligned in the original “A direction” ↑. That is, the W layer is “initialized”. But,
At room temperature, the influence of the magnetization of the W layer does not affect the M layer,
Therefore, the recorded information is retained.

Therefore, when the M layer is irradiated with linearly polarized light, information is reproduced in the reflected light, so that the information is reproduced as in the conventional magneto-optical recording medium. The perpendicularly magnetized films forming the M layer and the W layer are amorphous or crystalline of a ferromagnetic material having no Curie point and having a Curie point.
It is selected from the group consisting of an amorphous or crystalline ferrimagnetic material having a Curie point without a compensation temperature and an amorphous or crystalline ferrimagnetic material having both a compensation temperature and a Curie point. The above description is for the first embodiment using the Curie point as the magnetization reversal temperature. On the other hand, the second embodiment utilizes Hc lowered at a temperature lower than the Curie point. The second embodiment uses the temperature T _S1 at which the M layer is magnetically coupled to the W layer in place of T _{C1 in} the first embodiment, and the temperature T _S2 at which the W layer inverts at Hb instead of T _C2. Is used in the same manner as in the first embodiment.

In the second embodiment, the coercive force of the M layer is H _C1 ,
Let W _C2 be that of the W layer, T _s1 be the temperature at which the M layer is magnetically coupled to the W layer, T _{S2 be} the temperature at which the magnetization of the W layer is reversed at Hb, T _{R be} the room temperature, and _{L be} the low level P _L. the temperature of the medium when irradiated with a laser beam T _L, it T _H when irradiated with a laser beam of high level P _H, the coupling magnetic field M layer receives H _D1 (H _D1 is the sigma _W of the M layer Saturation magnetic moment M _S
Calculated by the quotient divided by the product of M and the film thickness t of the M layer), and the coupling magnetic field received by the W layer is H _D2 (where H _D2 is σ _W is the W layer saturation magnetic moment M _S and the W layer thickness t). (Calculated by the quotient divided by the product of and), the recording medium satisfies the following expression 6 and satisfies the expressions 7 to 10 at room temperature. _{T R <T s1 ≒ T L} <T s2 ≒ T H ............... formula _{6 H C1> H C2 + |} H D1 - (± H D2) | ......... formula 7 H _C1> H _D1 ......... ………………………… Formula 8 H _C2 > H _D2 ─ ……………………………… Formula 9 H _C2 + H _D2 <| Hini. | <H _C1 ± H _D1 ── Formula 10 In the above formula, for composite ±, the upper row is A (antiparalle
In the case of the l) type medium, the lower row shows the case of the P (parallel) type medium.

In the second embodiment, at high temperature T _H , the magnetization of the W layer has not disappeared, but is sufficiently weak, and the magnetization of the M layer has disappeared or is sufficiently weak. The recording magnetic field Hb ↓ is sufficiently large even if a sufficiently weak magnetization remains in both the M layer and the W layer, so that the Hb ↓ can cause the magnetization direction of the W layer and, in some cases, the M layer to follow the Hb ↓. . This state is state 2 in FIG. Immediately thereafter, or when the laser beam is no longer emitted and cooling is allowed to proceed and the medium temperature falls below T _H or moves away from _H b, the W layer is σ _W.
To influence the M layer through the magnetic field to make the magnetization direction of the M layer follow a stable direction. As a result, the state becomes the state 3 (P type) or the state 4 (A type) in FIG.

On the other hand, at the low temperature T _L , the W layer is of course M.
The layers have not lost their magnetization. However, that of the M layer is relatively small. In this case, there are two types of mark states, that is, state 5 and state 6 in FIG. 13 for the P type, and state 7 and state 8 in FIG. 13 for the A type. In the states 6 and 8, the interface domain wall (indicated by a thick line ━) is formed between the M layer and the W layer, and the state is slightly unstable (metastable). State 1 indicates any of states 5 to 8. Just before the medium part in this state reaches the irradiation position of the laser beam,
Hb ↓ is applied. Nevertheless, this state 6 or state 8 is retained. Because the W layer has sufficient magnetization at room temperature, the magnetization is not reversed by Hb ↓. In addition, the memory layer in the state 8 whose direction is opposite to Hb ↓ has a larger exchange coupling force σ from the W layer than the influence of Hb ↓.
_Under the influence of W, the magnetization direction is maintained in the same direction as the W layer due to the P type.

Shortly thereafter, the state 6 or the state 8 is irradiated with a low level laser beam. Therefore, the medium temperature rises. Along with that, the coercive force of both layers decreases.
However, since the W layer has a high Curie point, the coercive force H
The decrease in _C2 is small, it does not lose to Hb ↓, and the magnetization direction "A direction" ↑ when "initialized" is maintained. On the other hand, although the M layer has a low Curie point, since the medium temperature is still lower than the Curie point T _{c1 of the} M layer, the coercive force H _C1 remains. However, since H _C1 is small, the effect of Hb ↓ on the W layer and the effect via the exchange coupling force σ _w from the W layer (in the case of the P type, the force to turn the same direction)
Receive. In this case, the latter is stronger, and two equations of 2: H _c1 + Hb <(σ _w / 2M _S1 t ₁ ) 3 of Equation 10: H _c2 + Hb> (σ _w / 2M _S2 t ₂ ) are obtained. At the same time satisfied. The lowest temperature at which these equations are satisfied simultaneously is called T _LS . In other words, the minimum temperature at which the domain wall in state 6 or state 8 disappears is T _LS .

As a result, the state 6 shifts to the state 9 and the state 8 shifts to the state 10. On the other hand, the state 5 without the domain wall is the same as the state 9, and the state 7 without the domain wall is also the same as the state 10. Therefore, the previous state (in the case of the P type, the state 5 or 6 or A In the case of the type, regardless of the state 7 or 8, the irradiation of the low level beam forms the mark of the state 9 (P type) or the state 10 (A type).

This state does not change even when the medium temperature is lowered by returning the irradiation of the laser beam to the mark or moving away from the irradiation position, and then returning to room temperature. This Figure 13 state 9 (P type) or state 10 (A
Type) is state 6 (P type) or state 7 (A in FIG. 10).
Type). It will be appreciated that this allows a low temperature cycle to be achieved without raising the medium temperature to the Curie point T _C1 of the M layer.

In fact, also in the case of the first embodiment in which the low temperature cycle is carried out at T _C1 or more, since the medium temperature passes through T _LS on the way from room temperature to T _C1 , at this time, in the case of the P type, the state 6 The transition from state 8 to state 9 occurs in the case of type A, and the transition from state 8 to state 10 respectively. After that, T _C1 is reached, and the state 5 shown in FIG. 10 is established. The above description has explained the magnetic material composition in which there is no compensation temperature T _comp. Between the room temperature and the Curie point for both the M layer and the W layer.
However, if the compensating temperature T _comp. Exists, the direction of the magnetization is reversed when the compensating temperature T _comp. Is exceeded, and the A and P types are reversed, and the explanation becomes complicated accordingly. Further, the direction of the recording magnetic field Hb is also opposite to the direction when considered at room temperature.

In both the first and second embodiments, the M layer and the W layer are made of a transition metal (eg Fe, Co) -heavy rare earth metal (eg G).
d, Tb, Dy, etc.) A recording medium which is an amorphous ferrimagnetic material selected from alloy compositions is preferable. Both the M layer and the W layer are a transition metal and a heavy rare earth metal (heavy metal).
When selected from the alloy composition, the direction and magnitude of the magnetization appearing externally as each alloy is the direction and magnitude of the sublattice magnetization of the transition metal atom (TM) inside the alloy and the heavy rare earth metal. It is determined by the relationship with the direction and size of the sublattice magnetization of the atom (RE). For example, the direction and magnitude of the sublattice magnetization of TM is represented by a vector indicated by a dotted arrow, and that of RE sublattice magnetization is represented by a vector indicated by a solid line arrow, and the direction and magnitude of magnetization of the entire alloy are outlined. It is represented by the vector indicated by the arrow. At this time, the white arrow (vector) is a dotted arrow (vector) and the solid arrow (vector)
Expressed as the sum of and. However, in the alloy, due to the interaction between the TM sublattice magnetization and the RE sublattice magnetization, the dotted arrow (vector) and the solid arrow (vector) always have opposite directions. Therefore, the sum of the dotted arrows (vectors) and the solid arrows (vectors) makes the alloy vector zero (that is, the magnitude of the magnetization appearing outside) when the strengths of the two are equal. The alloy composition when it reaches zero is called the compensation composition. For any other composition, the alloy has an intensity equal to the intensity difference of both sublattice magnetizations and has a hollow arrow (vector) with an orientation equal to the orientation of the larger vector.

Therefore, the magnetization vector of the alloy is written adjacent to the dotted line vector and the solid line vector, for example, as shown in FIG. The sublattice magnetization states of RE and TM are roughly classified into four states, which are shown in (1A) to (4A) of FIG. The magnetization vector of the alloy (white arrows) in each state is shown in correspondence with (1B) to (4B) of FIG. For example, if the RE vector is larger than the TM vector,
The state of sublattice magnetization is shown in (1A), and the magnetization vector of the alloy is shown in (1B).

When the strength of either the TM vector or the RE vector of a certain alloy composition is larger, the alloy composition takes the name of the larger strength and is XX rich, for example, RE.
Called to be rich. For both M and W layers,
It is divided into TM-rich composition and RE-rich composition.
Therefore, when the composition of the M layer is plotted on the ordinate and the composition of the W layer is plotted on the abscissa, the type of the medium of the basic invention as a whole is shown in FIG.
It can be classified into the four quadrants shown in. In FIG. 16, the intersection of the coordinates shows the compensation composition of both layers. P mentioned above
The type belongs to the 1st quadrant and the 3rd quadrant shown in FIG. 16, and the A type belongs to the 2nd quadrant and the 4th quadrant.

On the other hand, looking at the change of the coercive force with respect to the temperature change, there is an alloy composition having a characteristic that the coercive force temporarily increases to infinity and then drops before reaching the Curie point (temperature at which the coercive force is zero). .. The temperature corresponding to this infinity is called the compensation temperature (T _comp. ). At a temperature lower than the compensation temperature, the RE vector (solid arrow) is larger than the TM vector (dotted arrow), so it can be said to be TM rich, and vice versa at a temperature higher than the compensation temperature. Therefore, it can be said that the compensation temperature of the alloy having the compensation composition is at room temperature.

On the contrary, the compensation temperature does not exist between room temperature and the Curie point in the TM-rich alloy composition. Since the compensation temperature below room temperature is meaningless in magneto-optical recording, the compensation temperature in this specification means that it exists between room temperature and the Curie point. Classifying the presence or absence of the compensation temperature of the M layer and the W layer, the medium is type 1
Are classified into four types. In Type 1, both the M layer and the W layer have a compensation temperature. In the type 2, the M layer has no compensation temperature and the W layer has compensation temperature. Type 3 is M
The layer has a compensation temperature and the W layer has no compensation temperature. In Type 4, both the M layer and the W layer have no compensation temperature. One-quadrant media includes all four types. Therefore, for both the M layer and the W layer, it is divided into RE rich or TM rich,
Moreover, the recording media are classified into 9 classes shown in FIG. [Explanation of Class 1-1] Class 1 shown in FIG. 20 here
The overwrite principle will be described in detail by taking as an example the medium No. 1-1 belonging to the recording medium (P type, 1 quadrant, type 1).

[0044] The medium No.1-1, the following equation 11: 2 _{T R <T comp.1 <T L} <T H ≦ T C1 ≦ T c2 and Formula 11: the relationship _T comp.2 <T _C1 Have. In the present specification, “=” in “≦” means that they are equal or almost equal (± 20 ° C. or so). For the purpose of simplifying the explanation, the following explanation is T _H <T _C1 <T _c2
Those having a relationship of will be described. T _comp.2 is T
_It may be higher than, equal to, or lower than _L , but for the purpose of simplifying the explanation, in the following explanation, T _L <T _comp.2
And FIG. 17 is a graph showing the above relationship. The thin line shows the graph of the M layer, and the thick line shows the graph of the W layer.

At room temperature T _R , the magnetic field of the M layer is the initial auxiliary magnetic field Hin.
The condition that only W layer is inverted without being inverted by i.

[0046]

[Equation 1]

Equation 12 is expressed as This medium No.1-1
Satisfies Equation 12. However, H _C1 : Coercive force of M layer H _C2 : Coercive force of W layer M _S1 : Saturation magnet moment of M layer (saturation magnet)
ization) M _S2 : saturation magnetic moment of W layer t ₁ : thickness of M layer t ₂ : thickness of W layer σ _w : domain wall energy = exchange coupling force (interface w
all energy) At this time, the conditional expression of Hini. is expressed by Expression 15 shown in Expression 4. When the Hini. Disappears, the magnetizations of the M layer and the W layer are affected by the exchange coupling force. Nevertheless, the conditions under which the magnetizations of the M layer and W layer are maintained without being inverted are expressed by equations 13-14. This medium No. 1-1 satisfies equations 13-14.

[0048]

[Equation 2]

[0049]

[Equation 3]

The magnetization of the W layer of the recording medium satisfying the conditions of equations 12 to 14 at room temperature should be measured by the time immediately before recording.

[0051]

[Equation 4]

Hini. Which satisfies the expression 15 shown in FIG. At this time, the M layer remains in the previous recording state. This state is state 1 or state 2 in FIG.
Indicated by either. This will be described with reference to FIG. The states 1 and 2 are held until just before recording.
Then, the recording magnetic field Hb is applied in the “A direction” ↑.

It is difficult to limit the recording magnetic field Hb to the same range as the irradiation area (spot area) of the laser beam, as is the case with general magnetic fields. When the medium is a disk, once the recorded information (mark) is rotated once, it is affected by Hini.
Becomes After that, the mark passes near a laser beam irradiation region (spot region). At this time, the marks in state 1 and state 2 are affected by the approach to the recording magnetic field Hb applying means. In this case, if the magnetization direction of the M layer of the mark in the state 2 having the magnetization opposite to Hb is reversed by Hb, the information just recorded one rotation before disappears. The conditions that should not be so are

[0054]

[Equation 5]

It is represented by 2 of the equation 15 shown in The disk-shaped medium No. 1-1 must satisfy the condition (2) of conditional expression 15 at room temperature. Conversely, one condition that determines Hb is
It is shown by 2 in Expression 15. The marks in states 1 and 2 finally reach the spot area of the laser beam. There are two types of laser beam intensities, low level and high level, as in the basic invention.

----- Low Temperature Cycle ---- The laser beam of low level is irradiated, and the medium temperature rises to T _comp.1 or higher. Then, from P type to A
Move to type. Although the directions of the RE and TM spins of the M layer do not change, the magnitude relationship of the strength is (3A) in FIG.
To reverse (4A). Therefore, the magnetization of the M layer is as shown in FIG.
Invert from (3B) to (4B). As a result, the mark in state 1 shown in FIG. 18 shifts to state 3, and the mark in state 2 changes to state 4.
Move to.

Following the irradiation of the laser beam, the medium temperature eventually becomes T _LS . Then, the two in Equation 10 and the one in Equation 10
3 are satisfied at the same time. As a result, even if Hb ↑ is present, the mark in state 4 in FIG. 18 transits to state 5. On the other hand,
In FIG. 18, the mark of state 3 is maintained as it is, since 2 of 10 and 3 of formula 10 are satisfied at the same time even when Hb ↑ is present. In other words, the state 3 only changes to the same state as the state 5 in FIG.

In this state, when the laser beam deviates from the spot area, the medium temperature starts to drop. Medium temperature is T
_When it cools down below _comp.1, it will return to the original P type from A type. Then, the magnitude relationship between the RE spin and the TM spin of the M layer is reversed from (2A) to (1A) in FIG. for that reason,
The magnetization of the M layer is reversed from (2B) to (1B) in FIG. As a result, the mark in the state 5 in FIG. 18 shifts to the state 6 (the magnetization of the M layer is “A direction” ↑). This state 6 is maintained even when the medium temperature drops to room temperature. In this way, the mark of “A direction” ↑ is formed on the M layer.

_--High Temperature Cycle ---- When a high level laser beam is irradiated, the medium temperature rises to a low temperature T _L via T _comp.1 . As a result, the state 7 is the same as the state 5 in FIG. Irradiation with a high level laser beam further raises the medium temperature. When the medium temperature exceeds T _comp.2 of the W layer, the A type shifts to the P type. Although the directions of the RE and TM spins in the W layer do not change, the magnitude relationship between the strengths is (1A) to (2A) in FIG.
Reverse to. Therefore, the magnetization of the W layer is reversed from (1B) to (2B) in FIG. As a result, the magnetization of the W layer is
The direction will be ↓. This is state 8 in FIG.

However, since H _C2 is still large at this temperature, the magnetization of the W layer is not reversed by ↑ Hb. When the temperature further rises to T _H , the coercive force of the M layer and the W layer becomes small because the temperature is close to the Curie point. As a result, the medium is

[0061]

[Equation 6]

(1) or

[0063]

[Equation 7]

(2) or

[0065]

[Equation 8]

The two expressions (3) shown in (3) are simultaneously satisfied. Therefore, the magnetizations of both layers are reversed almost at the same time and follow the direction of Hb ↑. This state is state 9. In this state, when the laser beam deviates from the spot area, the medium temperature starts to drop. When the medium temperature becomes T _{comp. 2} or less, the P type shifts to the A type. Then, although the directions of the spins of RE and TM do not change, the magnitude relationship of the strength is reversed from (4A) to (3A) in FIG. Therefore, the magnetization of the W layer is reversed from (4B) to (3B) in FIG. As a result, the magnetization of the W layer becomes “inverse A direction” ↓.
This is state 10 in FIG. In state 10, the medium is

[0067]

[Equation 9]

4 of Expression 15 shown in is satisfied. Therefore, W
Even if Hb ↑ acts on the layer, it does not reverse. When the temperature of the medium further decreases from the temperature in this state 10,
_When it becomes _comp.1 or less, it returns from the A type to the original P type. Then, the magnitude relationship between the intensity of the RE spin and the intensity of the TM spin of the M layer is reversed from (4A) to (3A) in FIG. Therefore, the magnetization of the M layer is reversed from (4B) to (3B) in FIG.
As a result, the magnetization of the M layer becomes “inverse A direction” ↓. This state is state 11 in FIG.

Eventually, the temperature of the medium drops from the temperature in state 11 to room temperature. Since H _C1 at room temperature is sufficiently large (see 5 in Equation 15), the magnetization ↓ of the M layer is not inverted by ↑ Hb, and the state 11 is maintained.

[0070]

[Equation 10]

In this way, the mark of "inverse A direction" ↓ is formed on the M layer. [Explanation of Selected Invention] The above explanation has been made on the two-layer film of the M layer and the W layer. With such a two-layer film, overwriting is possible even with a multilayer film of three or more layers. In particular,
In the above description, the external means called the initial auxiliary magnetic field (Hini.) Is used. However, in the basic invention, the external means is Hin
It is not limited to i. In short, by the time before recording, the magnetization of the W layer is oriented in a predetermined direction, that is,
It just needs to be "initialized". Therefore, instead of Hini. As an external means, the initialization layer (intializing laye
An invention was made using r). Japanese magazine "OPTRONICS"
See No. 4, 1990, pages 227-231. More detailed information is WO / 90/2400 (PCT / JP89 / 863) published on Mar. 8, 1990. Hereinafter, this invention is referred to as a selected invention.

The state 1 of FIG. 19 shows the structure of the medium used in the selection invention. This medium is composed of a substrate and a magnetic film basically having a four-layer structure formed on the substrate. This magnetic film comprises, in order, an M layer made of a vertically magnetizable magnetic thin film, a W layer made of a vertically magnetizable magnetic thin film, and a switching layer (hereinafter S) made of a vertically magnetizable magnetic thin film.
A layer is abbreviated) and an "initializing" layer (hereinafter abbreviated as I layer) composed of a perpendicularly magnetizable magnetic thin film is basically formed of a four-layer structure (the S layer may be omitted).

In the International Patent Publication, the M layer is the first magnetic layer, the W layer is the second magnetic layer, the S layer is the third magnetic layer (see claim 3), and the I layer is It is called the fourth magnetic layer (see claim 3). At points other than the third term, the names of the third magnetic layer and the fourth magnetic layer are opposite to each other, which seems to be a mistaken writing. Also, the magazine "OPTRONIC"
In S ", the S layer is called the control layer.

In this four-layer structure medium, the M layer and the W layer are exchange-coupled with each other, and it is possible to direct only the magnetization of the W layer to a predetermined direction without changing the magnetization direction of the M layer at room temperature. You can
Moreover, the W layer and the I layer are S at a temperature below the Curie point of the S layer.
Exchange coupled through the layers. The I layer has the highest Curie point and does not lose its magnetization even when irradiated with a high level laser beam. The I layer always holds the magnetization in a predetermined direction, and this serves as a means for repeatedly "initializing" the W layer in preparation for the next recording each time recording is performed. Therefore, I
The layer is called the "initialization" layer.

However, in the course of the high temperature cycle (for example, near T _H ), the magnetization reversal of the W layer must occur, and in that case, the influence from the I layer must be small so that it can be ignored. It is advantageous that the exchange coupling force σ _w24 between the W layer and the I layer becomes smaller as the temperature becomes higher. However, even at T _H , if sufficient σ _w24 remains, an S layer is required between the W layer and the I layer. If the S layer is a non-magnetic material, σ _w24 is zero or very small. However, at some temperature below T _H to room temperature, σ _w24 must be large due to the “initialization” of the W layer. At that time, the S layer must give an apparently sufficiently large exchange coupling force between the W layer and the I layer. For that purpose, the S layer needs to be magnetic. Therefore, the S layer becomes a magnetic material at a relatively low temperature and becomes W.
Apparently sufficiently large exchange coupling force σ between layer I and layer I
_w24 is given, and at a relatively high temperature, it becomes a non-magnetic material and gives an apparent zero or very small exchange coupling force σ _w24 between the W layer and the I layer. Therefore, the S layer is referred to as the switching layer.

Next, the principle of the four-layer film overwrite will be described with reference to FIG. This description is a typical example, and there are other examples. For example, the explanation becomes more complicated if any one of the layers has T _comp. Between room temperature and the Curie point. In FIG. 19, white arrows indicate the magnetization directions of the layers. The state before recording is either state 1 or state 2. Focusing on the M layer, the state 1 is the “A-oriented” mark (B ₁ ), and the state 2 is the “reverse A-oriented” mark (B ₀ ).
And the domain wall between the M layer and the W layer (shown by a bold line ━)
, And is in a somewhat unstable state (metastable).

------- Low temperature cycle -----
When the laser beam is applied to the marks in state 1 and state 2 to raise the temperature, the magnetization of the S layer first disappears. Therefore, the state 1 shifts to the state 3, and the state 2 shifts to the state 4. When the temperature further rises to reach T _LS , the magnetization of the M layer becomes weak and the action via the exchange coupling force from the W layer becomes strong. As a result, the magnetization of the M layer in state 4 is reversed, and at the same time, the domain wall between layers disappears. This is state 5. Since the mark in state 3 originally has no domain wall between layers, the state is directly shifted to state 5.

Here, when the irradiation of the laser beam stops or moves away from the irradiation position, the temperature of the mark in the state 5 starts to drop, and then the mark goes to the state 1 via the state 3. This is the low temperature cycle. When the temperature further rises from the state 5 and exceeds the Curie point of the M layer, the magnetization disappears and the state 6 is established. Here, when the irradiation of the laser beam stops or moves away from the irradiation position, the temperature of the mark in state 6 starts to decrease, and eventually the Curie point of the M layer reaches a slightly lower temperature. Then, magnetization appears in the M layer. The direction of this magnetization is W
Stable to the magnetization direction of the W layer by the action of exchange coupling force from the layers (direction in which no domain wall is generated between layers)
Becomes Since the type is P type here, the state 5 is reproduced. The temperature drops further, and accordingly state 3 occurs, followed by the state 1 mark. This process is another example of a low temperature cycle.

----- High temperature cycle -----
If the marks in state 1 and state 2 in FIG. 19 are irradiated with a laser beam to raise the temperature, the state is changed to state 6 via state 5 as described above. When the temperature is further increased, the coercive force of the W layer is extremely reduced. Therefore, the magnetization is reversed by the recording magnetic field Hb ↓. This is state 8.

Here, when the irradiation of the laser beam is stopped or the laser beam is moved away from the irradiation position, the medium temperature starts to decrease.
Eventually, the medium temperature falls slightly below the Curie point of the M layer.
Then, magnetization appears in the M layer. The direction of this magnetization is
Due to the action via the exchange coupling force from the W layer, the orientation becomes stable with respect to the magnetization direction of the W layer (direction in which no domain wall is generated between layers). Here, since it is the P type, the state 9 appears.

When the temperature further decreases, magnetization appears in the S layer, and as a result, the W layer and the I layer are magnetically coupled (by exchange coupling force). As a result, the magnetization direction of the W layer is stable with respect to the magnetization direction of the I layer (direction in which no domain wall is formed between layers). Since it is of P type here, the magnetization of the W layer is inverted in the “A direction”, and as a result, an interface domain wall is generated between the M layer and the W layer. This state is maintained even at room temperature, and the mark of state 2 is generated.

This is the high temperature cycle. When the temperature further rises after the state 8 appears due to the recording magnetic field Hb ↓, the temperature eventually exceeds the Curie point of the W layer. Then, the state 7 appears. Here, when the irradiation of the laser beam stops or moves away from the irradiation position, the medium temperature starts to decrease. Eventually, the medium temperature falls slightly below the Curie point of the W layer. Then, magnetization appears in the W layer. The direction of this magnetization follows the direction of the recording magnetic field Hb ↓. As a result, the state 8 appears.

When the temperature is further lowered, the mark of state 2 is formed through state 9. This process is another example of a high temperature cycle. ---------- Overwrite -------- As mentioned above, the mark (B ₁ ) of the state 1 is formed in the M layer in the low temperature cycle and the M layer in the high temperature cycle regardless of the previous recording state. The mark (B ₀ ) of the state 2 is formed at. Therefore, overwriting becomes possible.

[0084]

When information is recorded on an overwritable magneto-optical recording medium, the intensity of a laser beam is converted into P _H (high level) and P _L (low level) to relate to new information. the mark is formed by P _H. P _L
If the laser beam intensity is too low during irradiation, the mark B ₀ shown in FIG. 4 may not be formed. B ₀
If no mark is formed, so-called unerased information of the previous information occurs, good overwrite characteristics cannot be obtained, and the mark shape becomes unclear, resulting in a high bit error rate (BER). Conversely, if P _L is set too high,
Since a mark B ₁ is formed due to a high temperature cycle, and accurate information cannot be reproduced, the BER also remains in this case.
Becomes higher. It refers to an acceptable range of the laser beam intensity of the P _L to the margins of the P _L, conventional medium, this margin is disadvantageously narrow. This has caused the following secondary problems. Currently, the setting accuracy of the semiconductor laser used for the light source is about ± 10%,
In addition, there is a variation of about ± 10% between laser solids at the time of manufacturing, and a variation of about ± 10% with the passage of time.
The intensity of the laser beam that reaches the M and W layers through the optical system due to the influence of dust and dust is ±
It fluctuates by about 10%. ± 1 between solids when manufacturing optical system
There is a variation of about 0%, and the intensity of the laser beam reaching the M layer and the W layer fluctuates by about ± 10%. If the margin of P _L is narrow due to these variations or fluctuations, the non-defective rate of the recording apparatus may decrease, or P outside the margin may be reduced.
As a result of irradiating the medium with _{L, the} information to be recorded cannot be recorded or the C / N decreases.

In order to solve these problems, in the overwritable magneto-optical recording medium, a material having a high thermal conductivity, such as Al, Ag,
Au, Cu or the like by providing a thermal diffusion layer, was found to enlarge the margin of the P _L. (Japanese Patent Application No. 2-22
5700) Currently, the track pitch in a recordable magneto-optical recording medium is about 1.6 μm, and the value of this pitch is ISO (International Organization For Standeriz).
a-tion International Organization for Standardization). However, there is a demand for increasing the recording capacity and increasing the recording density, and it is necessary to reduce the track pitch (narrow track) in the future. When the track is made narrower, the track-to-track spacing (groove width) becomes narrower from the current 0.4 μm to 0.2 μm, and the depth becomes shallower from the current 950 Å to 600 Å.

Therefore, the present inventor made a prototype of an overwritable magneto-optical recording medium having a narrow track and provided with a thermal diffusion layer, and attempted recording and reproduction. As a result, BER
It has been found that there is a problem in that C increases and C / N decreases. C / N is proportional to the square root of the laser beam intensity (P _R ) during reproduction and the rotation angle (± θk in FIG. 5) of the plane of polarization of the reflected light after P _R irradiation. Therefore, as one method of improving the C / N, there is a method of increasing the irradiation intensity (P _R ) of the laser beam during reproduction. However, if P _R is set too high, the temperature of the medium irradiated with the laser beam reaches the minimum temperature at which a low temperature cycle occurs, and if it exceeds this temperature, the information mark previously formed by the high temperature cycle disappears. It is expected that the information may not be reproduced correctly because of the fear that it will happen. In order to solve this, by providing a thermal diffusion layer similar to the one described above, heat diffuses in the thermal diffusion layer even if P _R is increased, so that the peripheral portion outside the mark formation region is thermally protected. It was devised to minimize the influence and prevent the mark from being deformed or destroyed. However, even in this case, it was found that the BER increases and the C / N decreases when the track is narrowed.

The object of the present invention is to make P _R higher during reproduction for the future narrowing of the track, and even if a laser beam of high intensity P _H is irradiated, the BER does not increase, and C /
An object is to obtain an overwritable magneto-optical recording medium in which N does not decrease.

[0088]

Information is recorded by irradiating an overwritable magneto-optical recording medium with a laser beam and forming a mark by the heat of the light. Figure 3
As shown in, the light intensity of the cross section cut perpendicularly to the beam traveling direction has almost Gaussian distribution, the center is highest, and the temperature of the irradiation position on the medium also has Gaussian distribution. The distribution is close. The mark is formed when a predetermined temperature (the position indicated by the line segment ab in FIG. 3) is exceeded. Therefore, the size of the mark becomes the size indicated by the line segment 31. As can be seen from FIG. 3, the temperature distribution widely spreads in the track width direction in the peripheral portion outside the mark formation region. One of the layers constituting the medium in the present invention,
A thermal diffusion layer is provided directly on the W layer or through a protective layer in order to increase the P _L margin and to prevent the adverse effect of heat on the peripheral portion when the P _R irradiation intensity during reproduction is further increased. It is provided. The present inventors have studied, by providing the heat diffusion layer, when forming a mark with a high temperature cycle (irradiated at P _H), form marks temperature of the medium surrounding the mark outside the mark formation region in the high-temperature cycle When the temperature is higher than the minimum required temperature and lower than the temperature at which the mark is formed in the high temperature cycle, the area is wider than when the thermal diffusion layer is not provided. When you do, the heat of the peripheral part is transmitted to the adjacent track,
Since so-called crosstalk occurs, which adversely affects the marks on the adjacent tracks by deformation and destruction, B
It was found that ER increased and noise increased.

As a result of earnest research, the present inventor has provided the above by providing a heat insulating layer made of a material having a low thermal diffusivity with the thermal diffusion layer directly or through the protective layer on the W layer or I layer. The inventors have found that such adverse effects are eliminated and have completed the present invention. Therefore, the present inventor firstly refers to “a memory layer (M layer) made of a perpendicularly magnetizable magnetic thin film, a writing layer (W layer) made of a perpendicularly magnetizable magnetic thin film, directly on the W layer, or a protective layer. The M layer and the W layer are exchange-coupled to each other via a heat diffusion layer adjacent to each other via a heat diffusion layer, and the magnetization direction of the M layer is not changed at room temperature and only the magnetization of the W layer is predetermined. A multi-layered magneto-optical recording medium capable of being oriented in the direction of optical modulation, wherein a heat insulating layer is provided between the W layer or the protective layer and the thermal diffusion layer (claim 1). )"I will provide a.

Further, secondly, "the memory layer (M layer)
And the lighting layer (W layer), adjacent to the W layer,
An initialization layer (I that always holds the magnetization in a predetermined direction)
Layer), a thermal diffusion layer adjacent to the I layer directly or via a protective layer, both layers of the M layer and the W layer, and both layers of the W layer and the I layer are exchange-coupled, In addition, in the optical modulation overwritable multilayer magneto-optical recording medium capable of directing only the magnetization of the W layer in a predetermined direction without changing the magnetization direction of the M layer at room temperature, the I layer or the protective layer and the heat A medium characterized in that a heat insulating layer is provided between the diffusion layer and the diffusion layer (claim 2).

Thirdly, "a memory layer (M layer) made of a vertically magnetizable magnetic thin film, a writing layer (W layer) made of a vertically magnetizable magnetic thin film, and a vertically magnetizable magnetic thin film adjacent to the W layer. A switching layer (S layer), an initialization layer (I layer) adjacent to the S layer, and a thermal diffusion layer adjacent to the I layer directly or via a protective layer, the M layer and the W layer. Both layers of the layers, and further both the W layer and the I layer are exchange-coupled, and it is possible to direct only the magnetization of the W layer to a predetermined direction without changing the direction of the magnetization of the M layer at room temperature. A multi-layered magneto-optical recording medium capable of overwriting optical modulation, characterized in that a heat insulating layer is provided between the I layer or the protective layer and the heat diffusion layer (claim 3).

Fourth, there is provided "a medium characterized in that the heat insulating layer is made of SiO ₂ or ZnS (claim 4)". Fifth, "the thickness of the heat insulating layer is 2 to 1 of the thickness of the heat diffusion layer.
A medium (claim 5) according to claim 1 or claim 2 or claim 3, characterized in that it is 0 times.

Sixth, “The thermal diffusivity of the heat insulating layer is 5 × 10 ¹¹
~ 8 × 10 ¹¹ [nm ² / sec] It is characterized by the above-mentioned.
Or the medium according to claim 2 or claim 3 (claim 6).

[0094]

The medium used in the present invention is an overwritable magneto-optical recording medium, and has a two-layer film structure including an M layer and a W layer, a three-layer film structure including an M layer, a W layer and an I layer, and A heat insulating layer is provided directly on the W layer or I layer of a medium having a four-layer film structure composed of an M layer, a W layer, an S layer and an I layer, respectively, or via a protective layer, and heat diffusion is adjacent to this layer. Provide layers. It is preferable that the thermal diffusion layer uses a material having a high thermal conductivity, for example, Al, Ag, Au, Cu or the like, and the heat insulating layer uses a material having a low thermal diffusivity such as SiO ₂ or ZnS. The thickness of the heat insulating layer is 2 to 10 times that of the heat diffusion layer.

Since the medium of the present invention is provided with a heat insulating layer, the heat conduction to the mark periphery, that is, the mark on the adjacent track is suppressed even if the track is narrowed and a high-intensity laser beam is irradiated in a high temperature cycle. Since crosstalk does not occur, the mark is not deformed or destroyed.
In addition, the combined use with the thermal diffusion layer makes it possible to further increase the laser intensity during reproduction and to obtain an overwritable magneto-optical recording medium in which crosstalk does not occur. Therefore, BER becomes low and C / N becomes high.

[0096]

EXAMPLE FIG. 1 shows the structure of a medium according to this example, and FIG. 2 shows the names of constituent substances and film thicknesses of the respective layers. A disk-shaped glass substrate having a diameter of 130 mm and a thickness of 1.2 mm is prepared as a substrate. Track pitch is 1.4 μm (previously 1.
A narrow track (6 μm) was used. Using RF magnetron sputtering equipment, TbFeCo with different composition
Two alloys are placed in the chamber as targets. Further, the above substrate is also placed in the chamber.

The inside of the chamber is evacuated to 7 × 10 ⁻⁷ Torr and the inside of the chamber is evacuated to a high vacuum. Then Ar gas 1.8 × 10 ^-3
Introduce Torr. The deposition rate is 3Å / sec. Under the above conditions, Tb ₂₂ Fe _{73 is used} as the first memory layer as shown in FIG.
Co ₅ , as a second lighting layer, Dy ₂₈ Fe ₄₀ Co ₃₂ ,
SiO ₂ was formed as the third heat insulating layer, and Al was formed as the fourth heat diffusion layer adjacent to the heat insulating layer. The above layers are
The respective film thicknesses were formed on the substrate. The thickness of the third layer is 80
The thickness of the 0th and fourth layers was 400 Å, the thickness of the thermal diffusion layer was 1, and the thickness of the heat insulating layer was 2.

[0098] Using the thus formed medium at the time of reproduction were measured with a laser beam intensity mark formed by previously P _H is destroyed (P _Rmax). In this embodiment, the medium is rotated at a linear velocity of 5 [m / sec],
The intensity of P _H with respect to the mark formed by the 9.0 mW, P
By changing the _R at 0.1mW interval between 2.0 mW ～3.4MW, by irradiating respective 10 ^five times to measure the change in the noise level, the intensity of the P _R where the noise level began to rise and the P _Rmax. As a result of this measurement, P _Rmax was 2.4 mW.

Next, P_RmaxP as in the measurement_HStrength of
The mark on the first track at a temperature of 9.0 mW.
To the first track against the second track next to it
Higher P than when the mark was formed_HOver 10.0 mW
Intensity of 0.2mW each 10³Times laser beam
Irradiation was performed to form marks. At this time, each P _H
Measure the noise level at and the noise level starts to rise
P_HStrength is P_ctAnd As a result of this measurement, P_ct
Was 12.4 mW.

The ratio of P _Rmax and P _ct , which are the results of these measurements, was 5.2.

[0101]

Comparative Example Unlike the medium used in the examples, the medium used here has a three-layer structure of an M layer, a W layer, and a thermal diffusion layer, and an overwritable medium without a heat insulating layer is used. P _Rmax and P _ct were measured under the same conditions as above. As a result, P _Rmax was 2.9 mW and P _ct was 11.0 mW. The ratio of P _Rmax and P _ct , which are the results of these measurements, is 3.8
Met.

When the heat insulating layer using SiO ₂ having a high thermal diffusivity in the example is provided, both P _Rmax and P _ct are increased as compared with the case where the heat insulating layer is not provided, and P _Rmax and P _ct The ratio was higher when the heat insulating layer was provided, and the margins of P _R and P _H were expanded.

[0103]

In the overwritable magneto-optical recording medium of the present invention, in addition to the M layer and the W layer, a heat insulating layer formed of a material having a low thermal diffusivity and a material having a high thermal conductivity are formed. A thermal diffusion layer is provided. Therefore, with respect to the medium, when forming a mark with a high intensity P _H, although the intensity of P _H is high, a intermittent illumination for the thermal diffusivity is present low heat insulating layer, heat is the heat diffusion layer Hardly reaches for that reason,
Crosstalk hardly occurs even when the track is narrowed.

In addition, P _R
Since the medium is continuously irradiated with heat, heat propagates through the heat insulation layer and reaches the heat diffusion layer, but this layer has a high thermal conductivity,
Since the diffusion of heat to the peripheral portion outside the mark formation region spreads over a wide range, P _R can be made higher. For these reasons, high strength without causing crosstalk be irradiated with laser beam, it is possible to further increase the P _R at the time of reproduction, to an improved C / N.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing a vertical cross section of an overwritable magneto-optical recording medium according to the present invention.

FIG. 2 is a diagram showing properties of each layer of an overwritable magneto-optical recording medium according to an example of the present invention.

FIG. 3 is a graph showing a distribution state of medium temperature at a laser beam irradiation position.

FIG. 4 is a conceptual diagram illustrating a recording principle of a magneto-optical recording method.

FIG. 5 is a conceptual diagram for explaining the reproducing principle of the magneto-optical recording method.

FIG. 6 is a waveform diagram of a laser beam when overwriting according to the basic invention.

FIG. 7 is a waveform diagram of a laser beam when overwriting with two beams according to the basic invention.

FIG. 8 shows M of an overwritable magneto-optical recording medium.
It is a graph which shows the relationship between coercive force and temperature about a layer and a W layer.

FIG. 9 is a conceptual diagram showing the magnetization directions of the M layer and the W layer.

FIG. 10 is an explanatory diagram showing changes in the magnetization directions of the M layer and the W layer.

FIG. 11 is a low temperature cycle for P-type media,
It is explanatory drawing which shows how the direction of the magnetization of M layer and W layer changes as a result of a high temperature cycle. Both show the state at room temperature.

FIG. 12 is a low temperature cycle for Type A media,
It is explanatory drawing which shows how the direction of the magnetization of M layer and W layer changes as a result of a high temperature cycle. Both show the state at room temperature.

FIG. 13 is an explanatory diagram showing changes in the magnetization directions of the M layer and the W layer.

FIG. 14 is an explanatory diagram for comparing a vector indicating a sublattice magnetization of a rare earth (RE) atom (solid arrow) and a vector indicating a sublattice magnetization of a transition metal (TM) atom (dotted arrow). Is.

FIG. 15 is an explanatory diagram showing the relationship between the vector of sublattice magnetization and the vector (open arrow) indicating the direction of magnetization of the alloy.

FIG. 16 is an explanatory diagram illustrating that, when the M layer and the W layer are divided into RE-rich and TM-rich, the overwritable medium is divided into four classifications (quadrants 1 to 4). is there.

FIG. 17 is an overwritable magneto-optical recording medium N.
o. 3 is a graph showing the relationship between coercive force and temperature for 1-1 M layer and W layer.

FIG. 18 is a conceptual diagram showing how the magnetization directions of the M layer and the W layer change as a result of the low temperature cycle and the high temperature cycle for the medium of medium No. 1-1.

FIG. 19 is an explanatory diagram for explaining the overwriting principle of an overwritable magneto-optical recording medium having a four-layer film structure according to the selected invention.

FIG. 20 is an explanatory diagram illustrating that when the medium of the basic invention is classified from various points of view, it is eventually classified into nine classes, class 1 to class 9.

[Explanation of symbols for main parts]

L ・ ・ ・ ・ Laser beam Lp ・ ・ ・ ・ ・ ・ Linear polarized light B ₁・ ・ ・ ・ Mark with magnetization in "A direction" B ₀・ ・ ・ ・ "Inverse A direction" Magnetized mark S ··· Substrate MO ··· Perpendicular magnetic film (magneto-optical recording layer) M, 14 ··· Memory layer W, 13 ··· Writing layer 11 ··· .... Thermal diffusion layer 12 ... Thermal insulation layer

Claims (6)

[Claims] 1. A memory layer (M layer) made of a vertically magnetizable magnetic thin film and a writing layer (W layer) made of a vertically magnetizable magnetic thin film, which are adjacent to the W layer directly or via a protective layer. It is composed of a thermal diffusion layer, both the M layer and the W layer are exchange-coupled, and the magnetization direction of the M layer is not changed at room temperature and only the magnetization of the W layer is directed in a predetermined direction. A multi-layered magneto-optical recording medium capable of optical modulation overwriting, wherein a heat insulating layer is provided between the W layer or the protective layer and the heat diffusion layer. 2. The memory layer (M layer), the writing layer (W layer), an initialization layer (I layer) adjacent to the W layer, which always holds a magnetization in a predetermined direction, and the I layer. The thermal diffusion layer is adjacent to the layer directly or via a protective layer, both the M layer and the W layer, and the W layer and the I layer are exchange-coupled and M at room temperature. In a multilayer magneto-optical recording medium capable of optical modulation overwriting, in which only the magnetization of the W layer is oriented in a predetermined direction without changing the magnetization direction of the layer, a layer between the I layer or the protective layer and the thermal diffusion layer is provided. A medium characterized in that a heat insulating layer is provided in the medium. 3. A memory layer (M layer) made of a vertically magnetizable magnetic thin film, a writing layer (W layer) made of a vertically magnetizable magnetic thin film, and a vertically magnetizable magnetic thin film adjacent to the W layer. A switching layer (S layer), an initialization layer (I layer) adjacent to the S layer, and a thermal diffusion layer adjacent to the I layer directly or via a protective layer, the M layer and the W layer Both layers, and further, both the W layer and the I layer are exchange-coupled, and the magnetization of the W layer can be oriented in a predetermined direction without changing the magnetization direction of the M layer at room temperature. A modulation-overwritable multi-layered magneto-optical recording medium, wherein a heat insulating layer is provided between the I layer or the protective layer and the heat diffusion layer. 4. The medium, wherein the heat insulating layer is made of SiO ₂ or ZnS. 5. The medium according to claim 1, 2 or 3, wherein the thickness of the heat insulating layer is 2 to 10 times the thickness of the thermal diffusion layer. 6. The thermal diffusivity of the heat insulating layer is 5 × 10 ¹¹ to 8 × 10.
The medium according to claim 1, 2 or 3, characterized in that it is ¹¹ [nm ² / sec].