JPH05151630A - Overwritable magneto-optical recording disk having 2nd m layer - Google Patents

Abstract

PURPOSE:To prevent the degradation in C/N when reproduction is repetitively executed by providing a 2nd M layer having the relatively smaller MsHc product than the MsHc product of the M layer on the M layer and reducing the film thickness of the 2nd M layer smaller from the inner periphery toward the outer periphery. CONSTITUTION:The magneto-optical recording disk which includes at least two layers; the memory layer (M layer) having perpendicular magnetic anisotropy and a writing layer (W layer) having the perpendicular magnetic anisotropy exchange bonded thereto and can be overwritten simply by modulating a beam is provided with the magnetic layer (2nd M layer) having the Ms.Hc product at room temp. than the Ms.Hc product of the M layer and having the perpendicular magnetic anisotropy on the M layer and is decreased in the film thickness of the 2nd M layer successively smaller toward the outer periphery. (Ms is saturation magnetization and Hc is coercive force). As a result, the degradation in C/N when the reproduction is repetitively executed is prevented and the drastic degradation in the recording sensitivity is simultaneously prevented on the outer peripheral side of the disk.

Description

Detailed Description of the Invention

[0001]

INDUSTRIAL APPLICABILITY The present invention irradiates a laser beam while modulating the direction and intensity of a recording magnetic field Hb while modulating the pulse according to the binary information to be recorded.
Thus, the present invention relates to a magneto-optical recording disk (medium) capable of overwriting.

Overwriting refers to the act of recording new information without erasing the previous information. In this case, when reproduced, the previous information should not be reproduced.

[0003]

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. 2, in magneto-optical recording, a laser beam (L) is focused on a perpendicularly magnetized 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. 3 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.

By the way, in order to reuse the recorded medium, (1) initialize the medium again ^* initialize it with the device ^* , or
Alternatively, (2) it is necessary to add an erasing head similar to the recording head to the recording device, or (3) it is necessary to erase the recorded information in advance by using the recording device or the 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, the technological progress is remarkable, and the intensity of the irradiation 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 (in this specification, this recording auxiliary layer is referred to as a writing layer Writing layer or W layer) composed of a magnetic thin film capable of perpendicular magnetization, and both layers are exchange-coupled. In addition, an overwritable multi-layered magneto-optical recording medium is used which is 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.

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 the 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
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 exchange coupling force σ _W adjusting layer ... Hereinafter, this layer is abbreviated as C layer) may be present between the M layer and the W layer. For the C layer, refer to 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, the double sign ± indicates the case of the A (antiparallel) type medium described later in the upper stage, and the case of the P (parallel) type medium described later in the lower stage. 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, an initial auxiliary magnetic field Hini.) Is applied to the medium before recording, only the magnetization of the W layer can be aligned in the “A direction”. That is, "initialization". In the following explanation, "A
For convenience, the “direction” is indicated by an upward arrow ↑ on the surface of the present specification, and the “reverse A direction” is indicated by a downward arrow ↓. And Hi
Even if ni. disappears, the magnetization ↑ of the W layer is maintained as it is without being re-inverted by the equation 4.

When only the magnetization of the W layer is aligned to "A direction" ↑ by the external means before recording, a "initialized" state --- is shown conceptually. Become 7. The direction of magnetization in the M layer in FIG. 7 represents the information recorded up to that point. In the following description, since it has nothing to do with the direction of magnetization of the M layer, when this is shown by X and simplified, FIG. 7 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 and becomes "inverse A direction" ↓ magnetization (state 2 in FIG. 8).

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 8.
State 3 (P type) or state 4 (A type).

The change of state due to 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 it is impossible to turn the Hb on and off at a high speed (short time). Therefore, it is unavoidable and remains as it was during the high temperature cycle.

However, since H _C2 is still large, H _C2
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 state is the state 6 (P type) or the state 7 (A type) in FIG.

The change of 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. Please refer to FIG. 9 and FIG. In FIG. 9 and FIG. 10, the directions of magnetization formed at room temperature or when returning to room temperature are drawn for the P-type and A-type media, respectively.

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, if the M layer is irradiated with linearly polarized light, the reflected light contains information, 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 amorphous or crystalline ferrimagnetic material having no Curie point without compensation temperature, and amorphous or crystalline ferrimagnetic material having both compensation temperature and 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 in} which the M layer is magnetically coupled to the W layer in place of T _{C1 in} the first embodiment, and W in place of T _C2.
Using a temperature T _S2 at which the layer inverts at Hb, it is described 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 ………… Equation 6 H _C1 > H _C2 + | HD ₁ − (± HD ₂ ) | …… Equation 7 H _C1 > HD ₁ ……………………………………… Formula 8 H _C2 > H _D2 ─ …………………………………… Formula 9 H _C2 + H _D2 <| Hini. | <H _C1 ± H _D1 ── Formula 10 In the above formula, the double sign ± 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 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. 11 for the P type, and state 7 and state 8 in FIG. 11 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 _LminS . In other words,
The minimum temperature at which the domain wall in state 6 or state 8 disappears is T _LminS
Is.

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 deviating from the irradiation position, and then returning to room temperature. This FIG. 11 state 9 (P type) or state 11
(A type) is state 6 (P type) or state 7 in FIG.
It is the same as (A 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 T _Lmin on the way from room temperature to T _C1 , at that 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. 8 is reached. 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 vector of the dotted line and the vector of the solid line, 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. And the magnetization vector (open arrow) of the alloy in each state is shown corresponding to (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 the TM vector or RE vector of a certain alloy composition is larger, the alloy composition is named as the one having 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 types of the medium of the basic invention are shown in FIG.
It can be classified into the four quadrants shown in. In FIG. 14, the intersection of the coordinates represents the compensation composition of both layers. P mentioned above
The type belongs to the 1st quadrant and the 3rd quadrant shown in FIG. 14, 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,
In addition, the recording media are classified into 9 classes shown in Table 1 according to whether they have a compensation temperature or not.

[0047]

[Table 1]

[Explanation of Class 1-1] The recording medium of Class 1 shown in Table 1 (P type, 1 quadrant, Type 1)
The overwrite principle will be described in detail with reference to the medium No. 1-1 belonging to the category. This medium No. 1-1 has the following formula 1.
1: 2 _{T R <T comp.1 <T L} <T H ≦ T C1 ≦ T c2 and the formula 11: with the relationship _T comp.2 <T _C1. 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. 15 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.

[0050]

[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 wa
ll 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 the expressions 13 to 14.

[0052]

[Equation 2]

[0053]

[Equation 3]

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

[0055]

[Equation 4]

By 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 changed to state 1 or state 2 under the influence of Hini. 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 of 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

[0058]

[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 ---- A low-level laser beam is irradiated and the medium temperature rises to T _comp.1 or higher. Then, from P type to A
Move to type. Then, although the directions of the RE and TM spins of the M layer do not change, the magnitude relationship between the strengths 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 moves to state 3 in FIG. 16, and the mark in state 2 changes to state 4.
Move to.

Following the irradiation of the laser beam, the medium temperature eventually reaches T _Lmin . Then, Equation 2 and Equation 10
3 is satisfied at the same time. As a result, even if Hb ↑ exists, the mark in state 4 in FIG. 16 transits to state 5. On the other hand,
The mark in the state 3 in FIG. 16 is kept as it is because 2 of 10 and 3 of equation 10 are satisfied at the same time even if Hb ↑ is present. That is, from the state 3 to the same state in FIG.
It only goes to 6 state 5.

In this state, when the laser beam deviates from the spot region, 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 of the state 5 in FIG. 16 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.

[0063] ----- When the laser beam of the high-temperature cycle ----- high level is irradiated, the medium temperature is increased to the 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

[0065]

[Equation 6]

(1) or shown in

[0067]

[Equation 7]

(2) or

[0069]

[Equation 8]

The two expressions shown in either of (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, the directions of the spins of RE and TM do not change, but 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

[0071]

[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 relation 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 “reverse A direction” ↓. This state is state 11 in FIG.

Eventually, the temperature of the medium drops from the temperature in the 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.

[0074]

[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 description of the basic invention, an external means called an initial auxiliary magnetic field (Hini.) Was used. However, in the basic invention, the external means is Hi
It is not limited to ni. 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 selection invention.

The state 1 of FIG. 17 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, 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 enough to 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 called 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. 17, 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 and reaches _TLmin , 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 state 5 begins to drop, and finally the mark goes to state 1 via 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 ------------- When the laser beam is irradiated to the marks in the state 1 and the state 2 to raise the temperature, the state 5 is changed as described above. After that, the state 6 is reached. 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 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 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 an 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 the state 2 is formed through the 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.

[0088]

As described above, in order to prevent the information in the M layer from being erased by the "initialized" magnetization of the W layer, the medium has an inequality: H _C1 > σ at room temperature. _It is necessary to satisfy _w / 2M _S1 t ₁ (equation 13). However, C / N ratio, the laser beam intensity during reproduction When P _R, is proportional to the product of P _R of the square root and contrast (.theta.k). Therefore, C / N
To increase the ratio, it is necessary to increase P _R.
However, if P _R is increased, the temperature of the magnetic layer rises,
H _C1 decreases. Then, there is a risk that Expression 13 is not satisfied. Therefore, in order to increase the margin of P _R , Equation 13
It suffices to expand the temperature range that satisfies the above condition to the highest temperature possible. When the margin of P _R becomes wider, continuous reproduction in which the temperature of the medium increases can be executed.

On the other hand, the medium has the inequality: H _C1 <σ _w / 2M _S1 t ₁ (in order to make the magnetization of the M layer follow the predetermined orientation controlled by the W layer at the temperature T _L where the low temperature cycle occurs. It is necessary to satisfy 2) of Expression 13. From the viewpoint of the laser beam drive circuit, the minimum temperature T at which the high temperature cycle is executed
It is preferable that the distance between _Hmin (in _{terms of} beam intensity, P _Hmin ) and T _L (in terms of beam intensity, P _L ) is wide. This wide range is called the overwrite power margin. Assembly adjustment becomes easy.To widen the overwrite power margin, the temperature range satisfying the condition (2) of Equation 13 should be expanded to the lowest temperature.

However, if the temperature range satisfying the equation 13 is expanded to the highest temperature possible, the temperature range satisfying the equation 13-2 becomes narrow, and conversely, the temperature range satisfying the equation 13-2 is reduced to the lowest temperature. When expanded, the temperature range in which Expression 13 is satisfied becomes narrower. As a result of the latter, the P _R margin decreases, so that the C / N ratio gradually decreases when repeatedly reproduced. This is called deterioration or deterioration of "reliability of continuous reproduction".

To widen the P _R margin, Equation 1
The thickness (t ₁ ) of the M layer is increased in accordance with 3. But,
Then, the overwrite power margin will be narrowed. Therefore, the beam intensity fluctuates slightly (± 5%
Even if it is within), good overwrite characteristics cannot be obtained. For example, the bit error rate at the time of overwriting increases.

Therefore, the present inventor initially found that a magnetic layer having a perpendicular magnetic anisotropy (hereinafter referred to as “Ms (saturation magnetization) × Hc (coercive force)”) on the M layer at room temperature has a smaller product. Second M
Layer), thereby increasing the apparent thickness of the M layer. Thus, a wide overwrite power margin and a wide P _R margin was achieved. However, this medium has a drawback that the recording sensitivity is seriously reduced on the outer peripheral side of the disc.

An object of the present invention is to prevent the deterioration of "reliability of continuous reproduction" and the serious deterioration of recording sensitivity on the outer peripheral side of the disk at the same time.

[0094]

Therefore, the present invention includes at least two layers, an M layer having perpendicular magnetic anisotropy and a W layer having perpendicular magnetic anisotropy exchange-coupled with the M layer, and modulates a laser beam. In the magneto-optical recording disk capable of overwriting only by providing the magnetic layer, a magnetic layer (second M layer) having a perpendicular magnetic anisotropy with a smaller Ms.Hc product at room temperature than the M layer is provided on the M layer, and Provided is a medium characterized in that the thickness of a magnetic layer is reduced toward the outer periphery.

[0095]

According to the present invention, Ms H is formed on the M layer relatively to the M layer.
_{Since the} second M layer having a small _C product is provided and the film thickness of the second M layer is thinned from the inner circumference to the outer circumference,
It was possible to widen the P _R margin while sufficiently suppressing the decrease in the power margin. As a result, good overwrite characteristics can be obtained. In addition, there is no reduction in sensitivity at the outer circumference. In order to improve the sensitivity on the outer circumference, a method of reducing the film thickness of the W layer on the outer circumference side may be considered. However, this method is not a good idea because the size of the initial auxiliary magnetic field Hini. Is inversely proportional to the film thickness of the W layer, and a large Hini. Is required on the outer peripheral side.

As another method for ensuring the P _R margin and improving the sensitivity on the outer peripheral side, a method of adding a thermal diffusion layer (for example, a metal film) under the W layer or I layer is also available. Conceivable. However, this method increases the diffusion of heat, so that when the mark is viewed in the lengthwise direction, the criticality of the front end and the rear end is blurred and the edge becomes unclear. This causes a decrease in C / N ratio. Further, when the mark is viewed in the width direction, the mark becomes thick and extends to the adjacent track. This causes a problem of erasing information in adjacent tracks and a "problem called crosstalk". These problems become remarkable when a mark length recording system for writing a long mark is adopted.

In the present invention, the M layer and the W layer, and optionally the C layer (σ _W adjusting layer), the S layer and the I layer.
The layers are selected according to the basic invention or the selection invention. The second M layer, which is a feature of the present invention, is preferably composed of a transition metal-heavy rare earth alloy. The thickness of the 2nd M layer is 200-800 Å
A degree is preferable. However, it is necessary to reduce the thickness within this range as it goes to the outer circumference.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited thereto.

[0099]

Example 1 (1) First, a glass substrate (G) having a diameter of 130 mm and a thickness of 1.2 mm, and an ultraviolet curable resin layer (PP) having a thickness of about 100 μm.
A 2P substrate on which is formed is prepared. On this resin layer (PP), from the inner circumference side (radius r = 30 mm position) to the outer circumference side (r = 60 mm)
A large number of grooves are formed in a spiral shape up to the (mm position). Groove dimensions are: width 0.5 μm, pitch 1.6 μm, depth
It is 400Å. (2) Prepare a 6-element RF sputtering device, and set the 2P substrate in the chamber of the device. The chamber of the apparatus is once evacuated to a vacuum degree of 7 × 10 ⁻⁷ Torr. Or less, and then Ar gas is introduced at 5 × 10 ⁻³ Torr. And the deposition (d
eposition) Sputtering is performed at a speed of about 2Å / sec.

First, using a Si target, N ₂ gas was introduced into the chamber in addition to Ar gas for reactive sputtering, and silicon nitride (first protective layer) was deposited on the resin layer to a thickness of 700 Å. To form. Next, the introduction of N ₂ gas is stopped, and sputtering is carried out in a 5 × 10 ⁻³ Torr. Ar gas using a GdFeCo alloy target. This makes the first
A second M layer made of a perpendicular magnetic film of Gd ₁₉ Fe ₅₇ Co ₂₄ (Note: subscript numbers are atomic%; the same applies below) is formed on the protective layer. At the time of sputtering, a film thickness correction plate was placed on the target to incline the film thickness of the second M layer. This second layer has a film thickness of 500Å on the inner peripheral side and 200Å on the outer peripheral side.

Then, while maintaining the vacuum state, 5 × 10 ⁻³
Simultaneous sputtering is performed using two Tb targets and FeCo alloy targets in Torr. Ar gas. Thereby, Tb ₂₃ Fe ₇₂ Co ₅ having a film thickness t ₁ = 200Å is formed on the second M layer.
Forming an M layer of the perpendicular magnetization film. Then, while maintaining the vacuum state, the second GdFe was added in Ar gas of 5 × 10 ⁻³ Torr.
Sputtering is performed using a Co alloy target alloy. As a result, Gd ₃₅ Fe ₅₂ Co with a film thickness of 200 Å is formed on the M layer.
_A C layer composed of ₁₃ perpendicularly magnetized films is formed.

Subsequently, while maintaining the vacuum state, 5 × 10 ⁻³
Sputtering is performed using a DyFeCo alloy target as the sixth target in Ar gas of Torr. Thus, a W layer made of a perpendicularly magnetized film of Dy ₃₀ Fe ₃₅ Co ₃₅ having a film thickness t ₂ = 400Å is formed on the C layer. Finally, in the same manner as the first protective layer, silicon nitride (second protective layer) is formed on the W layer to a thickness of 700Å.

Thus, an overwritable magneto-optical recording disk was obtained. A cross section of the disk is shown in FIG. [Evaluation test 1] Permanent magnet giving initial auxiliary magnetic field Hini. = 4 kOe and recording magnetic field Hb = 300 Oe in "reverse A direction" ↓
Using a magneto-optical recording and reproducing device equipped with a permanent magnet that gives
Rotate the disk at 3600 rpm (constant angular velocity). A track A at the innermost peripheral position (r = 30 mm) and a track Z at the outermost peripheral position (r = 60 mm) of the rotating medium are irradiated with a laser beam (wavelength 830 nm). The laser beam modulates between the high level P _H and the low level P _L according to the first reference information. The first standard information is the duty ratio = 50%,
It is a signal with a frequency of 7.0 MHz. Low level is P _L = 3.0 mW
And then, the high level is set to P _H = 6.0 mW. The values of mW are all values on the magnetic film (hereinafter the same). Than this,
The first reference information should have been recorded on both tracks. After that, reproduction is performed by irradiating a laser beam of P _R = 1.0 mw to obtain the C / N ratio. The same thing is repeated with increasing P _H by 1.0 mW. The test continues until P _H = 12 mW.
The obtained results are shown in "vertical axis C / N ratio, the horizontal axis is P _H graph (FIG. 18)." Also, separately, 2 in the playback signal
The P _H value (“P _H ”) at which the next harmonic is the lowest is determined. [Evaluation Test 2] using the second harmonic obtained in Test 1 is the lowest "P _H", Test 1 similarly to track A,
Record in Z. Then, a laser beam of P _R = 2.0 mw is irradiated and the reproduction is repeated up to 10 ⁶ times to obtain the change in C / N ratio. The results are shown in FIG.

From the result of FIG. 18, for example, the value of P _H reaching 30 dB is the outer peripheral side (r = 60 mm) and the inner peripheral side (r = 30 mm).
There is not much difference. This means that "P _H " on the outer peripheral side is
Means that only slightly larger than the "P _H" of the inner peripheral side. From this, it is understood that the recording sensitivity on the outer peripheral side is not so much decreased and is good. Further, from the result of FIG. 19, it is known that “reliability of continuous reproduction” is high on both the inner and outer peripheral sides.

[0105]

Example 2 In the disk of this example, the second M layer is Dy ₂₃ Fe.
_The same disk as in Example 1 except that it is ₅₄ Co ₂₃ . This disk was also subjected to evaluation tests 1 and 2,
As shown in FIGS. 20 and 21, almost the same results were obtained.

[0106]

[Comparative Example 1] The thickness of the second M layer is 400Å from the inner circumference to the outer circumference.
A disk was manufactured in the same manner as in Example 1 except that (fixed) was used. When this disc was also subjected to evaluation tests 1 and 2, the results shown in FIGS. 22 and 23 were obtained. Figure 2
From the result of No. 3, it can be seen that the "reliability of continuous reproduction" is high on both the inner and outer peripheral sides. However, from the result of FIG. 22, the value of P _H reaching 30 dB was found to be at the outer circumference side (r = 60 mm) and the inner circumference side (r = 30 m).
There is a large difference in m). This means that "P _H " on the outer peripheral side
But it means that severely than "P _H" on the inner circumference side larger. From this, it can be seen that the recording sensitivity on the outer peripheral side is significantly lowered. This is the value of the outer peripheral side (r = 60 mm), for example, reaches 50 dB P _H is become known also because it is necessary 11 mW. In Example 1 (FIG. 18), the value is 9 mW.

[0107]

[Comparative Example 2] The second M layer is not provided and the M layer (TbFeCo)
A disk was manufactured in the same manner as in Example 1 except that the film thickness of was set to 300Å (constant). When this disc was also subjected to evaluation tests 1 and 2, the results shown in FIGS. 24 and 25 were obtained. From the results of FIG. 24, it can be seen that the recording sensitivity on the outer peripheral side is improved as compared with Comparative Example 1. However, it can be seen that the “reliability of continuous reproduction” on the inner peripheral side (r = 30 mm) is low.

[0108]

According to the present invention, it is possible to prevent a decrease in "reliability of continuous reproduction" on the inner and outer peripheral sides of the disc and a serious decrease in recording sensitivity on the outer peripheral side. Therefore, the overwrite characteristics of the entire disc are good.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing a cross-sectional structure of an overwritable magneto-optical recording disk according to an embodiment of the present invention.

FIG. 2 is a conceptual diagram illustrating a recording principle of a magneto-optical recording method.

FIG. 3 is a conceptual diagram for explaining the reproducing principle of the magneto-optical recording method.

FIG. 4 is a waveform diagram of a laser beam when overwriting according to the basic invention.

FIG. 5 is a waveform diagram of a laser beam when overwriting with two beams according to the basic invention.

FIG. 6 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. 7 is a conceptual diagram showing the magnetization directions of the M layer and the W layer.

FIG. 8 is an explanatory diagram showing changes in the magnetization directions of the M layer and the W layer.

FIG. 9 is an explanatory diagram showing how the magnetization directions of the M layer and the W layer change as a result of a low temperature cycle and a high temperature cycle for a P type medium. Both show the state at room temperature.

FIG. 10 is an explanatory diagram showing how the magnetization directions of the M layer and the W layer change as a result of a low temperature cycle and a high temperature cycle for an A type medium. Both show the state at room temperature.

FIG. 11 is an explanatory diagram showing changes in the magnetization directions of the M layer and the W layer.

FIG. 12 is an explanatory diagram for comparing a vector indicating a sublattice magnetization of a rare earth (RE) atom (solid arrow) with a vector indicating a sublattice magnetization of a transition metal (TM) atom (dotted arrow). Is.

[Fig. 13] is a vector of the sublattice magnetization (solid arrow and dotted arrow) and a vector indicating the magnetization direction of the alloy (white arrow).
It is explanatory drawing which shows the relationship with.

FIG. 14 is an explanatory diagram illustrating that, when the M layer and the W layer are respectively divided into RE-rich and TM-rich, overwritable media are divided into four categories (1 quadrant to 4 quadrants). is there.

[Fig. 15] is a overwriteable magneto-optical recording medium No.
3 is a graph showing the relationship between coercive force and temperature for 1-1 M layer and W layer.

FIG. 16 is a conceptual diagram showing how the magnetization directions of the M layer and the W layer change as a result of a low temperature cycle and a high temperature cycle for the medium of medium No. 1-1.

FIG. 17 is an explanatory diagram for explaining the overwrite principle of the overwritable magneto-optical recording medium having a four-layer film structure according to the selected invention.

FIG. 18 is a graph showing the recording sensitivity of the disc of Example 1.

[Fig. 19] is a graph showing "reliability of continuous reproduction" with respect to the disc of Example 1.

FIG. 20 is a graph showing the recording sensitivity of the disc of Example 2.

FIG. 21 is a graph showing “reliability of continuous reproduction” with respect to the disc of Example 2.

FIG. 22 is a graph showing the recording sensitivity of the disc of Comparative Example 1.

FIG. 23 is a graph showing “reliability of continuous reproduction” for the disc of Comparative Example 1.

FIG. 24 is a graph showing the recording sensitivity of the disc of Comparative Example 2.

FIG. 25 is a graph showing “reliability of continuous reproduction” for the disc of Comparative Example 2.

[Explanation of symbols for main parts]

L ………… Laser beam Lp …… Linearly polarized light B ₁ …… Mark (or mark) having magnetization in “A direction” B ₀ …… Mark (or mark) having magnetization in “reverse A direction” S ……… Substrate MO ...... Perpendicular magnetic film (magneto-optical recording layer) G ………… Glass substrate PP ………… UV curable resin layer G + PP = 2P substrate SiN …… First protective layer or second protective layer Second M… Second memory Layer M ………… Memory layer C ………… σw adjustment layer W ………… Lighting layer

Claims (1)

[Claims] 1. A magneto-optical device which comprises at least two layers, a memory layer having perpendicular magnetic anisotropy and a writing layer exchange-coupled with the magnetic layer having perpendicular magnetic anisotropy, and which can be overwritten simply by modulating a laser beam. In the recording disc, Ms.Hc is formed on the memory layer at room temperature from the memory layer.
A disk comprising a magnetic layer having a perpendicular magnetic anisotropy having a small product, and having a film thickness of the magnetic layer made thinner toward the outer periphery. However, Ms is saturation magnetization and Hc is coercive force.