JPH09293285A - Magneto-optical recording medium and its reproducing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain stable production and simultaneously a high signal level by providing the surface of a substrate with a reproducing layer consisting of an alloy magnetic material contg. rare earths and transition metal, intermediate layer, cutting layer and recording layer in this order and satisfying prescribed conditions. SOLUTION: The mask for the low-temp. part of this medium is formed by a change in the magnetic characteristics of the intermediate layer and the mask for the high-temp. part is formed as exchange bonding is cut off when the cutting layer exceeds a Curie temp. Consequently, the formation of the mask no longer depends upon the magnetic characteristics of the reproducing layer and the compsn. margin of the reproducing layer widens considerably. The higher signal level is obtd. particularly if the compsn. is so set that the magnetization of the reproducing layer in an aperture is made small. The Curie temp. of the cutting layer does not depend so much on the compsn. rate of the rare earth metals and transition metal of the cutting layer and, therefore, the stable formation of the mask of the high-temp. part is made possible in spite of a minor change in the compsn. of the cutting layer. The exchange bonding strength changes drastically near the Curie temp. and, therefore, the fluctuation of the mask diameter is small in spite of a minor change in the reproduction magnetic fields.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magneto-optical recording medium and a reproducing method thereof. More particularly, it relates to a super-resolution type magneto-optical recording medium capable of recording a large capacity and a reproducing method thereof.

[0002]

2. Description of the Related Art A magneto-optical recording medium has been put to practical use as a high-density, low-cost rewritable information recording medium. In particular, a medium using a recording layer of an amorphous alloy of a rare earth metal and a transition metal shows very excellent characteristics.

The magneto-optical disk is a recording medium having a very large capacity, and it is desired to further increase the capacity as the amount of information in society increases. If the recording density of the optical disk is normal,
It is determined by the size of the spot of the reproduction light. The size of the spot can be made smaller as the wavelength of the laser becomes shorter. Therefore, a study on making the wavelength of the laser shorter is under way, but it is very difficult.

On the other hand, in recent years, so-called super-resolution techniques which attempt to obtain a resolution higher than that determined by the wavelength of the laser by various means have been attempted. One of them is using a magneto-optical disk and using magnetically induced super resolution using the exchange coupling force between multilayer films.
solution or MSR).

As one form of MSR, a RAD (rear ape) for reproducing only a signal from a high temperature portion in a reproduction spot.
There is a method called rture detection). In RAD, the magnetization of the low temperature portion is made parallel to the film surface or is aligned in one direction by an external magnetic field to form the low temperature portion as a “mask area” that does not generate a signal having information, and the high temperature portion The signal reproduction is performed only in the high temperature portion by forming the "aperture area" in which the signal can be reproduced by the temperature change of the magnetic characteristics.

The effect of RAD is that the reproduction spot is not effectively reduced by limiting the reproduction area to a high temperature portion and the signal resolution is improved, and since the adjacent track also becomes a mask area, the signal from the adjacent track is The point is that almost no leakage (crosstalk) occurs. As one form of RAD, an in-plane magnetized film is composed of an exchange-coupled reproducing layer and a recording layer, and an intermediate layer between them, and the perpendicular magnetic anisotropy of the intermediate layer is small at room temperature and is not exchange-coupled with other layers. Therefore, it is proposed to use a medium having such a characteristic that the perpendicular magnetic anisotropy increases as the temperature rises.

In such a medium, since the exchange coupling force between the reproducing layer and the recording layer is weak at room temperature, if the coercive force of the reproducing layer is sufficiently small, the magnetization direction of the reproducing layer is made unidirectional by applying an appropriate external magnetic field. Can be aligned. When the temperature of the medium rises due to the heating of the reproducing light, the exchange anisotropy between the reproducing layer and the recording layer increases due to the increase in the magnetic anisotropy of the intermediate layer, so that the reproducing layer is not affected by the external magnetic field. Transfer the magnetic domain direction.

As a result, the reproduction signal is obtained only from the high temperature portion of the reproduction spot. If the composition of the reproducing layer is set to have a transition metal magnetization dominant or a slight rare earth magnetization dominant at room temperature, the effective magnetic field due to exchange coupling that the reproducing layer receives as the magnetization of the reproducing layer increases when the temperature further rises. H
As w becomes smaller, the magnetization of the reproducing layer is again aligned in one direction by the external magnetic field.

As a result, a reproduction method called a double mask is adopted in which a reproduction signal is generated only from a narrow temperature range sandwiched between a low temperature mask and a high temperature mask. Since the aperture area is further narrowed in the double mask, higher signal resolution can be obtained as compared with the case where there is one mask area. In addition to the MSR method, FAD that reproduces only the low temperature part
, Etc., but the double mask RA described above
D is a very excellent MSR system in that it has a high resolution of reproduced signals and a small crosstalk.

[0010]

In the double mask RAD method using the above-mentioned three magnetic layers, the composition of the intermediate layer and the reproducing layer is strictly controlled in order to generate a mask for a low temperature part and a mask for a high temperature part in a low magnetic field. Need to control. In particular, the mask formation temperature in the high temperature portion is sensitive to the composition of the reproducing layer and the intermediate layer.

Therefore, the composition range in which both the high temperature portion mask and the low temperature portion mask can be satisfactorily formed is very narrow. Further, since the magnetization change in the high temperature portion is relatively gradual with respect to the temperature, the mask diameter in the high temperature portion changes sensitively to the change in the external magnetic field during reproduction. Therefore, it is difficult to obtain a realistic composition margin and an external magnetic field margin.

Further, since the composition is such that the magnetization of the reproducing layer increases at a high temperature, there is a problem that the perpendicular magnetic anisotropy of the reproducing layer decreases near the mask and the signal from the aperture decreases.

[0013]

The present invention provides a wide composition margin for a reproducing layer in reproducing a double mask RAD,
This enables stable production and at the same time obtains higher signal levels. The gist of the present invention is:
A reproducing layer made of an alloy magnetic material containing a rare earth metal and a transition metal, an intermediate layer, a cutting layer and a recording layer are provided in this order on the substrate, and the coercive force of the recording layer at room temperature is larger than that of the reproducing layer. A magneto-optical recording medium is characterized in that the intermediate layer has a rare earth metal-dominant magnetization, and the Curie temperature of the cutting layer is lower than the Curie temperature of any other magnetic layer.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in more detail with reference to the drawings. FIG. 1 is an explanatory diagram of a magneto-optical recording medium of the present invention, and FIG. 2 is an explanatory diagram of a conventional magneto-optical recording medium. As shown in FIG. 1, the magneto-optical recording medium of the present invention has at least four magnetic layers of a reproducing layer, an intermediate layer, a cutting layer and a recording layer.

In the medium of the present invention, the magnetic anisotropy of the intermediate layer is low at a low temperature, and the exchange coupling force between the recording layer and the reproducing layer is small, so that the effective magnetic field Hw exerted by the domain wall on the reproducing layer by the exchange coupling. Is small. For this reason, if the coercive force Hc1 of the reproducing layer is sufficiently small, the magnetization of the reproducing layer is aligned in one direction under an appropriate external magnetic field to form a mask.

In the portion where the medium temperature is high, the exchange coupling between the reproducing layer and the recording layer is strengthened due to the improvement of the magnetic anisotropy due to the decrease of the magnetization of the intermediate layer, and the reproducing layer is not affected by the recording layer even under an external magnetic field. It becomes possible to transfer the magnetization direction and form an aperture. When the temperature further rises and exceeds the Curie temperature of the cutting layer, exchange coupling between the reproducing layer and the recording layer disappears, and the magnetization of the reproducing layer is aligned in one direction again by the external magnetic field to form a mask.

In this way, the double mask can be formed in the low temperature part and the high temperature part. That is, the mask in the low temperature portion is generated by the change in the magnetic characteristics of the intermediate layer, and the mask in the high temperature portion is generated because the cutting layer exceeds the Curie temperature and the exchange coupling force is cut. As a result, the magnetic properties of the reproducing layer do not depend on the generation of the mask, and the composition margin of the reproducing layer is significantly widened.

Particularly, if the composition is set so that the magnetization of the reproducing layer in the aperture becomes small, a higher signal level can be obtained as compared with the conventional method. Further, since the Curie temperature of the cutting layer does not depend so much on the composition ratio of the rare earth metal and the transition metal of the cutting layer, the mask in the high temperature portion can be stably formed even if the composition of the cutting layer changes to some extent.

Since the exchange coupling force changes rapidly near the Curie temperature, even if the reproducing magnetic field changes to some extent, the change in mask diameter is small. Hereinafter, desirable characteristics of each layer will be described in order. In the present invention, the reproducing layer is initialized at low temperature by the reproducing magnetic field Hr.

Therefore, if the coercive force of the reproducing layer is Hc1 and the effective magnetic field that the reproducing layer receives from the magnetization of the recording layer by the exchange coupling force is Hw1, it is necessary to satisfy the condition of Hr> Hc1 + Hw1 (A). is there. When the temperature further rises, the formula (A) is not established mainly with the increase of Hw1, and an aperture for transferring the magnetization direction of the recording layer is formed. To form the aperture, it is sufficient to satisfy the condition of Hr <Hw1 + Hc1 (B).

If the transition temperature Tt1 for forming the aperture is too low, the formation of the low temperature mask is insufficient, and if it is too high, the formation of the aperture is insufficient. A preferable transition temperature is in the range of 50 ° C or higher and 160 ° C or lower, and more preferably 70 ° C or higher and 140 ° C or lower.

Of course, the transition temperature is also a function of Hr, but in an ordinary drive, it is at most about 200 (Oe) to 800 (Oe) due to the demand for miniaturization, so the transition at any point of this magnetic field strength. It is sufficient that the temperature is within the above temperature range. Since Hr is desired to be as small as possible, it is preferable that the transition temperature is within the above temperature range when Hr is 200 (Oe). The transition temperature Tt2 at which the temperature further rises and becomes a high temperature mask again from the aperture for transferring the information from the recording layer to the reproducing layer is preferably 120 ° C. or higher and 250 ° C. or lower. More preferably, it is 150 ° C. or higher and 230 ° C. or lower.

This temperature is substantially determined by the Curie temperature of the cutting layer. Since the exchange coupling force disappears at this time, the condition of Hr> Hc1 ... (C) may be satisfied in order to form a high temperature mask.

In the conventional RAD method in which the cutting layer is not provided, since the exchange coupling force still remains at this time, the expression (A) must be satisfied for mask generation, and the expression (B) is satisfied within a sufficient temperature range. The conditions required to satisfy both the requirement and the requirement are extremely strict. However, in the method of the present invention, the simple condition of the expression (C) may be satisfied. That is, when a magnetic material having a very small coercive force such as GdFeCo is used for the reproducing layer, Hr is set to 200 to 300 (Oe), and then the expression (C) is obtained in most temperature ranges other than the vicinity of the compensation temperature. Can be satisfied.

Therefore, the mask in the high temperature portion can be generated without considering the magnetic characteristics of the reproducing layer and the intermediate layer. The temperature in the interval between Tt1 and Tt2 is preferably 50 ° C. or higher and 150 ° C. or lower. If this temperature interval is too narrow, a sufficient reproduction signal cannot be obtained, and if this temperature interval is too wide, the effect of super-resolution becomes small. Materials used for the reproducing layer include GdFeCo, GdCo, GdFe, and GdDyF.
e, GdDyCo, GdDyFeCo and the like are preferably used.

Among them, an alloy mainly composed of GdFeCo having a small coercive force and a high Curie temperature is preferably used. When expressed as Gdx (FeyCo _100-y ) _100-x (atomic%), it is preferable that 19 ≦ x ≦ 27 and 0 ≦ y ≦ 90. More preferably, 21 ≦ x ≦ 27 and 5 ≦ y ≦ 85. More preferably, 22 ≦ x ≦ 26 and 5 ≦ y ≦ 85.
However, 5 atomic% of additives such as Ti, Cr, Pt, and Mo
You may add about the following.

The volume magnetic susceptibility Ms1 of the reproducing layer at room temperature (the temperature of the medium use environment, which is 25 ° C. in the present invention) is preferably 250 emu / cc or less, more preferably 200 emu / cc or less. If Ms1 is too large, the magnetization does not sufficiently rise in the aperture, and the reproduction signal becomes small. However, if Ms1 had a certain size H
Since w1 decreases, it is preferably 50 emu / cc or more, more preferably 80 emu / cc or more. Since the intermediate layer is predominantly rare earth metal, when the magnetization of the reproducing layer is predominantly transition metal, the force exerted by the external magnetic field is in the opposite direction.Therefore, the magnetization of the reproducing layer is predominantly rare earth metal at room temperature. Is preferable for forming

The coercive force Hc1 of the reproducing layer must be sufficiently small to satisfy the expression (A) at low temperatures. The coercive force Hc1 of the reproducing layer at room temperature is preferably 500 (Oe) or less, more preferably 300 (Oe) or less. When the thickness of the reproducing layer is too thin, reproducing light is transmitted, so that the effect of the mask is weakened and the super-resolution capability is lowered.

On the other hand, if the thickness is too large, Hw is lowered to make it difficult to form an aperture and the sensitivity is deteriorated. The thickness of the reproducing layer is preferably 12 nm or more and 50 nm or less. More preferably, it is 15 nm or more and 40 nm or less. The intermediate layer preferably has a rare earth metal-dominant magnetization at room temperature, and the magnetization decreases as the temperature rises, and the perpendicular magnetic anisotropy improves.

In the state of the reproducing layer alone which is not exchange-coupled with other layers at room temperature, the intermediate layer is preferably an in-plane magnetized film in order to reduce the exchange coupling between the reproducing layer and the recording layer. In order to sufficiently weaken the exchange coupling force between the reproducing layer and the recording layer, it is preferable to have a volume magnetic susceptibility of 200 emu / cc or more at room temperature.

Further, in order to sufficiently improve the perpendicular magnetic anisotropy when the temperature rises, it is preferable that the temperature at which the volume magnetic susceptibility is 150 emu / cc or less is 50 ° C. or more and 160 ° C. or less. GdFe, GdFeC as the material of the intermediate layer
O, GdDyFe, GdDyFeCo and the like are preferably used.

Above all, an alloy mainly composed of GdFeCo having a small coercive force and a high Curie temperature is preferably used. When expressed as Gdx (FeyCo _100-y ) _100-x (atomic%), it is preferable that 28 ≦ x ≦ 36 and 80 ≦ y ≦ 100. More preferably 30 ≦ x ≦ 34 and 85 ≦ y ≦ 100
It is. However, additives such as Ti, Cr, Pt, and Mo may be added in an amount of about 5 atomic% or less.

The cutting layer must have a Curie temperature lower than that of any of the reproducing layer, the intermediate layer and the recording layer. The Curie temperature of the cutting layer is preferably about 100 to 250 ° C. More preferably, it is 120 to 230 ° C.
It is preferable that the cutting layer has a high perpendicular magnetic anisotropy and generates a strong force in the magnetization of the reproducing layer. It is necessary that the film thickness of the cutting layer is such that the exchange coupling force is almost completely cut when the cutting layer exceeds the Curie temperature.

If the film thickness is too thick, the recording sensitivity deteriorates. The film thickness is preferably 2 nm or more and 30 nm or less.
Materials used for the cutting layer include TbFe and TbFe
Co, DyFeCo, DyFe, TbDyFeCo and the like are preferable. Above all, TbFeCo with high perpendicular magnetic anisotropy
An alloy mainly composed of is preferably used.

When expressed as Tbx (FeyCo _100-y ) _100-x (atomic%), it is preferable that 21 ≦ x ≦ 30 and 80 ≦ y ≦ 100. More preferably, 23 ≦ x ≦ 28 and 85 ≦ y ≦ 95. However, if an additive such as Ti, Cr, Pt, or Mo is added to 5
You may add about atomic% or less.

Since the recording layer is a layer in which recording is stably stored, it is necessary that the recording layer has a Curie temperature of such a level that it is not deteriorated by the reproducing beam. The Curie temperature is preferably about 250 to 350 ° C. If the Curie temperature is too high, the laser power required for recording will be too high.

The recording layer also needs to have a high perpendicular magnetic anisotropy in order to give a strong force to the magnetization of the reproducing layer.
The material of the recording layer is TbFeCo, TbCo, Dy
FeCo, TbDyFeCo, GdTbFe, GdTb
FeCo and the like are preferably used.

For the recording layer, an alloy mainly composed of TbFeCo, which has a large perpendicular magnetic anisotropy, is preferably used. Above all, when expressed as Tbx (FeyCo _100-y ) _100-x (both are atomic%), it is preferable that 18 ≦ x ≦ 26 and 70 ≦ y ≦ 85. However, additives such as Ti, Cr, Pt, and Mo may be added in an amount of about 5 atomic% or less.

The thickness of the recording layer is preferably 10 nm or more and 50 nm or less. In order to stably store the recording in the recording layer, the coercive force of the recording layer is preferably larger than the coercive force of the reproducing layer. The coercive force of the recording layer is preferably 3 kOe or more. Since an alloy of a rare earth metal and a transition metal is very likely to be oxidized, it is preferable to adopt a mode in which protective films are provided on both sides of the magnetic layer. As the protective film, simple substances such as Si oxide, Al oxide, Ta oxide, Ti oxide, Si nitride, Al nitride, and Si carbide, or a mixture thereof are preferably used. The thickness of the protective film is preferably about 50 nm to 150 nm.

The magnetic anisotropy of the magnetic layer can be improved by plasma etching the surface after forming the substrate-side protective film. Providing a high heat-conducting substance made of a simple substance such as Al, Cu, Au, or Ag or an alloy mainly containing it as a heat dissipation layer on the recording layer side of the magnetic layer directly or through a protective layer is This is a desirable structure for stabilizing the distribution. The thickness of the heat radiation layer is 10 nm to 100
About nm is preferable.

When reproducing, the reproducing magnetic field for forming the mask is applied. The magnitude of the reproducing magnetic field is 200
It is preferably (Oe) or less. Particularly preferably, 1
It is 50 (Oe) or less. Since the method of the present invention can suppress the crosstalk to a very low level, it is possible to perform recording on both the land and the groove in addition to the normal land recording or the groove recording.

[0042]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, which should not be construed as limiting the scope of the invention. Examples 1 to 5 A polycarbonate substrate having a guide groove with a track pitch of 1.0 μm was introduced into a sputtering device, and 5 × 10 5
Evacuation was performed to a vacuum degree of ^-5 Pa or less.

Thereafter, 80 nm of oxidized Ta was formed as a protective layer on the substrate by reactive sputtering. Next, on the oxidized Ta, 30 including Gdx (Fe ₈₀ Co ₂₀ ) _1-x is formed.
a reproducing layer of 10 nm, a 10 nm intermediate layer of Gd ₃₁ Fe ₆₉ , a 10 nm cutting layer of Tb ₂₃ (Fe ₉₀ Co ₁₀ ), and a 40 nm recording layer of Tb ₂₃ (Fe ₇₅ Co ₂₅ ) ₇₇ .

Finally, a 50 nm protective layer made of SiN was provided. The value of x was changed from 0.19 to 0.27 as shown in Table 1. The Curie temperatures of the reproducing layer, the intermediate layer, the cutting layer, and the recording layer were measured and found to be 300 ° C or higher and 250 ° C, respectively.
The temperature was 200 ° C, 300 ° C or higher. At room temperature (25 ° C.), the magnetization of the rare earth metal was predominant in the intermediate layer, and the volume magnetic susceptibility was 260 emu / cc.

The compensation temperature was 190 ° C. The temperature at which the volume susceptibility reached 150 emu / cc was 120 ° C. When the intermediate layer was alone, it was an in-plane magnetized film at room temperature. The cutting layer and the recording layer had almost a compensating composition at room temperature and had a coercive force of 20 kOe or more.

The disc manufactured in this manner was used at a wavelength of 6
The CNR (narrow band signal-to-noise ratio) was evaluated by optical modulation recording using an evaluator having a numerical aperture of 80 nm and a numerical aperture of 0.55. Recording conditions are a linear velocity of 7 m / s, a frequency of 9 MHz, a recording power of 9 mW, and a recording duty of 30%. Reproduction magnetic field Hr is 3
It was set to 00 (Oe).

When the reproduction power was changed and the measurement was performed,
No signal was observed at a reproducing power of less than 1.6 mW in any of the discs, but at 1.6 mW or more, magnetic domains were transferred to the reproducing layer at a high temperature portion, and a super-resolution signal was generated. Example 3 also has a reproducing power of 2.2 for the disc.
The CNR at mW was 46.5 dB.

When the reproducing power is 2.4 mW or more, a mask in a high temperature portion is formed, and when the reproducing power is 2.6 mW, the CNR is 49.
It became 2 dB. CNR at playback power of 2.6 mW
Are summarized in Table 1. Table 1 also shows the crosstalk measured on the adjacent tracks when recording was performed at 2 MHz. Crosstalk is defined as 20log ₁₀ (I2 / I1) using the signal intensity I1 at the recording track and the signal intensity I2 at the adjacent track.

Comparative Examples 1 to 5 Discs were prepared in the same manner as in Example 1 except that the cutting layer was not provided. Table 1 shows the results of the measurement performed on the disk thus manufactured in the same manner as in the examples.

Comparative Example 6 Using the same substrate, Ta oxide having a thickness of 90 nm was used as a protective layer.
Tb ₂₁ (Fe ₉₃ Co ₇ ) ₇₉ is 28 nm as a recording layer, SiN is 30 nm as an intermediate layer, and Al is 40 n as a reflective layer.
A conventionally used disk provided with m was prepared.
Then, the CN ratio was measured under the same conditions as in Example 1 with no reproducing magnetic field applied and the reproducing power Pr = 2.0 mW.
B. Further, when the crosstalk was measured in the same manner as in the example, it was -18 dB.

[0051]

[Table 1]

[0052]

By using the magneto-optical recording medium of the present invention, the CNR can be improved in a double mask super-resolution medium having both high super-resolution capability and low crosstalk, and can be obtained over a wide composition range. As a result, productivity is significantly improved.

[Brief description of drawings]

FIG. 1 is an explanatory view of a longitudinal section of a RAD magneto-optical recording medium of the present invention.

FIG. 2 is a vertical cross-sectional explanatory view of a conventional RAD magneto-optical recording medium.

Claims (3)

[Claims] 1. A reproducing layer, an intermediate layer, a cutting layer, and a recording layer, which are made of an alloy magnetic material containing a rare earth metal and a transition metal, are provided in this order on a substrate, and the coercive force of the recording layer at room temperature is that of the reproducing layer. A magneto-optical recording medium, which has a coercive force larger than that of the magnetic layer, an intermediate layer having a rare earth metal-dominant magnetization, and a Curie temperature of a cutting layer lower than a Curie temperature of any other magnetic layer. 2. The magneto-optical recording medium according to claim 1, wherein the intermediate layer is an in-plane magnetized film at room temperature when it is not exchange-coupled with other layers. 3. The reproducing method for a magneto-optical recording medium according to claim 1, wherein the reproducing light is a reproducing light having such a power that the maximum attainable temperature of the medium exceeds the Curie temperature of the cutting layer, and the reproducing layer is magnetized at room temperature. A reproducing method for a magneto-optical recording medium, characterized in that reproducing is performed while applying an external magnetic field having a sufficient intensity to align in one direction.