JPH06251445A - Magneto-optical recording medium and information reproducing method using this medium - Google Patents

Abstract

PURPOSE:To make possible reproduction of magnetic domains smaller than a beam spot diameter and to attain high-density recording by having a first magnetic layer which turns to an intra-surface magnetized film on heating up and a second magnetic layer consisting of a perpendicularly magnetized film. CONSTITUTION:This magneto-optical medium has two layers; the reproducing layer which is the perpendicularly magnetized film at room temp. and turns to the intra- surface magnetized film on heating up and the recording layer which is the perpendicularly magnetized film even at room temp. and on heating up. Information signals are first recorded on the recording layer. The recording is executed by modulating external magnetic fields while irradiating the recording layer with a laser beam of such power at which the recording layer is heated up to its Curie temp. or above or by modulating the laser power while impressing the magnetic fields in a recording direction after once erasing the signals. A need for the initializing magnetic fields is, therefore, eliminated and inter-code interference is suppressed even if bit synchronization is smaller than the beam diameter of the laser beam. Information is thus reproduced with high C/N.

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 for recording / reproducing information with a laser beam utilizing a magneto-optical effect and an information reproducing method using the medium.

[0002]

2. Description of the Related Art As a rewritable high density recording system,
Using the thermal energy of a semiconductor laser to write magnetic domains in a magnetic thin film to record information, using the magneto-optical effect,
Attention has been paid to a magneto-optical recording medium for reading this information.

In recent years, there is an increasing demand for increasing the recording density of this magneto-optical recording medium to make it a recording medium having a larger capacity.

The linear recording density of an optical disk such as a magneto-optical recording medium is mainly determined by the laser wavelength of the reproducing optical system,
It depends largely on the numerical aperture of the objective lens.

That is, when the laser wavelength λ of the reproducing optical system and the numerical aperture NA of the objective lens are determined, the bit period which is the detection limit is determined to be λ / 2NA.

Therefore, in order to realize high density in the conventional optical disk, it is necessary to shorten the laser wavelength of the reproducing optical system and increase the numerical aperture NA of the objective lens.

However, there is a limit to the improvement of the laser wavelength and the numerical aperture of the objective lens. Therefore, a technique for improving the recording density by devising the configuration of the recording medium and the reading method has been developed.

For example, in Japanese Patent Laid-Open No. 3-93058, a medium composed of a reproducing layer and a recording layer is used, and an external magnetic field is applied in one direction while a signal held in the recording layer is transferred to the reproducing layer for reproduction. We are trying to improve the linear recording density by reducing the intersymbol interference and making it possible to reproduce signals with a period less than the light diffraction limit.

[0009]

[Patent Document 1] Japanese Unexamined Patent Application Publication No.
In the magneto-optical reproduction method described in -93058, the magnetization of the reproduction layer must be aligned in one direction by an external magnetic field during reproduction. Therefore, it is necessary to apply a magnetic field during reproduction. Therefore, the reproducing method has problems that the magneto-optical recording apparatus becomes complicated and the cost becomes high.

[0010]

In view of the above problems, the present invention uses a high density magneto-optical recording medium capable of reproducing a signal having a period equal to or shorter than the diffraction limit of light without applying a magnetic field during reproduction, and a medium using the medium. The purpose is to provide a method of reproducing information that has been used.

The above object can be achieved by a magneto-optical recording medium having a perpendicular magnetic film at room temperature and a recording layer composed of a perpendicular magnetic film and a reproducing layer which becomes an in-plane magnetic film in a high temperature region. .

A magneto-optical recording medium having a first magnetic layer, which is a perpendicular magnetic film at room temperature and becomes an in-plane magnetic film when the temperature is raised, and a second magnetic layer made of a perpendicular magnetic film, is used to apply light to the medium. A beam spot is irradiated, the first magnetic layer serves as an in-plane magnetized film in a high temperature portion of the spot, and the first magnetic layer serves as a perpendicular magnetized film in a low temperature portion and the second magnetic layer is formed. Due to exchange coupling, the magnetization of the first magnetic layer is aligned in a stable direction with respect to the magnetization direction based on the information of the second magnetic layer, and the influence of the perpendicular magnetic film portion of the first magnetic layer in the spot is exerted. This is achieved by reproducing the information by detecting the magneto-optical change of the light beam.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The magneto-optical recording medium of the present invention and the information reproducing method using the medium will be described below in detail with reference to the drawings.

The magneto-optical recording medium of the present invention is a perpendicularly magnetized film at room temperature, a reproducing layer which changes into an in-plane magnetized film when heated, and a recording layer which is a vertically magnetized film even at room temperature and at elevated temperature.
Having at least a layer. The reproducing layer and the recording layer are exchange-coupled to each other when they are perpendicularly magnetized films.

The reproducing layer is, for example, a rare earth-iron group amorphous alloy such as GdCo, GdFeCo, GdTbF.
eCo, GdDyFeCo, NdGdFeCo, etc. are desirable. It is preferable that the magnetic anisotropy is small and the compensation temperature is between room temperature and the Curie temperature.

The recording layer has a large perpendicular magnetic anisotropy, for example, a rare earth-iron group amorphous alloy such as T.
bFeCo, DyFeCo, TbDyFeCo, etc., or garnet, or a platinum group-iron group periodic structure film, for example, Pt / Co, Pd / Co platinum group-iron group alloy, for example Pt
Co, PdCo, etc. are desirable.

The reproducing layer and the recording layer are made of Cr, Al, Ti, Pt, Nb.
It is also possible to add elements for improving corrosion resistance such as.

Further, in addition to the reproducing layer and the recording layer, a dielectric such as SiN, AlOx, TaOx, SiOx may be provided in order to enhance the interference effect. Also, to improve the thermal conductivity, Al,
AlNx, AlTa, AlTi, AlCr, Cu or the like may be provided.

Further, an intermediate layer for adjusting exchange coupling force,
An auxiliary layer for recording assistance and reproduction assistance may be provided. Further, a protective coat made of the dielectric layer or polymer resin may be provided as a protective film.

The information reproducing method of the present invention will be described below.

As shown in FIG. 1A, first, an information signal is recorded on the recording layer of the magneto-optical recording medium of the present invention. Recording is performed by irradiating the external magnetic field while irradiating the laser beam with the power such that the recording layer becomes the Curie temperature or higher, or after erasing once, the laser power is modulated while applying the magnetic field in the recording direction. To do. Alternatively, the laser power is modulated while applying an external magnetic field.

At this time, if the intensity of the laser light is determined in consideration of the linear velocity of the recording medium so that only a predetermined area within the light spot will be near the Curie temperature of the recording layer, the recording magnetic domain not larger than the diameter of the light spot will be recorded. Can be formed, and as a result, a signal having a period equal to or shorter than the diffraction limit of light can be recorded.

At the time of reproducing information, the medium is irradiated with a reproducing laser beam, and at this time, the temperature of the irradiated portion rises. Since the medium moves at a constant speed, the temperature distribution on the medium is shown in Fig. 3.
As shown in the figure, it becomes a shape that extends in the moving direction of the medium, and part of the inside of the light spot (the trailing edge with respect to the moving direction of the light spot)
Becomes a high temperature distribution.

With respect to a single-layer magnetic thin film, when the saturation magnetization is M _S and the perpendicular magnetic anisotropy constant is Ku, the effective perpendicular magnetic anisotropy constant K defined by K⊥ = Ku-2πM _S ² It is known that ⊥ determines the main direction of magnetization. Here, 2πM _s ² is the demagnetizing energy. For example, if the reproducing layer has the temperature dependence of M _s and Ku shown in FIG.
The Ms of the reproduction layer increases because the temperature rises during reproduction. Therefore, as shown in FIG. 6, 2πMs ² rapidly increases and the magnitude relationship with the perpendicular magnetic anisotropy constant Ku is reversed, so that K⊥ <0 and the in-plane magnetized film is obtained.

That is, as shown in FIG. 2, the intensity and the linear velocity of the laser light at the time of reproduction are set so that the magnetization of the reproduction layer becomes an in-plane magnetized film in the high temperature portion which is a part of the light spot. If the saturation magnetization M _S and the perpendicular magnetic anisotropy constant Ku of the reproducing layer are set in consideration, only the high temperature portion in the light spot becomes the in-plane magnetic film, and the other portions become the perpendicular magnetic film. To do.
Since the reproducing layer, which is a perpendicularly magnetized film, is magnetically coupled to the recording layer by exchange coupling, the magnetization direction of the reproducing layer is stable with respect to the magnetization direction based on the information of the recording layer. That is, the information recorded in the recording layer is transferred to the reproducing layer. Then, the transferred information is converted into an optical signal by the magneto-optical effect of the reproducing layer (specifically, the magneto-optical effect of laser light reflected from the reproducing layer) and detected. In this case, the magneto-optical effect does not occur in the portion where the reproducing layer in the light spot is the in-plane magnetized film.

Although the reproducing layer and the recording layer are magnetically coupled by the exchange interaction in the above description, the recording layer and the reproducing layer may be magnetically coupled by magnetostatic coupling during reproduction. Also, when this magnetic thin film is laminated directly on the perpendicular magnetization film or through an intermediate layer, etc., since the exchange coupling force and the magnetostatic coupling force from the perpendicular magnetization film work, the temperature region where the in-plane magnetization occurs is the high temperature side. However, if the in-plane magnetization transition temperature of the single-layer film is set slightly lower, it becomes a perpendicular magnetization film at room temperature and becomes an in-plane magnetization film at high temperature even when laminated with a perpendicular magnetization film. The situation holds.

As described above, in the information reproducing method using the magneto-optical recording medium of the present invention, even if the bit period is smaller than the beam diameter of the laser beam without requiring an initializing magnetic field, the intersymbol Interference is suppressed, and information can be reproduced with high C / N.

Next, the above magneto-optical recording medium is improved,
A case where an intermediate layer is provided between the reproducing layer and the recording layer shown in FIG. 1B will be described.

The intermediate layer is located between the reproducing layer and the recording layer,
The Curie temperature is higher than room temperature and lower than the Curie temperatures of the reproducing layer and the recording layer. Examples of the material of the intermediate layer include rare earth-iron group amorphous alloys such as TbFe and GdF.
e, TbFeCo, GdFeCo or A to them
Examples thereof include those to which a non-magnetic element such as 1, Cu and Cr is added.

As shown in FIG. 4A, this intermediate layer plays a role of mediating the exchange imaging force from the recording layer to the reproducing layer until the Curie temperature is reached, and the information of the recording layer is transferred to the reproducing layer. To be done.

However, in the high temperature portion of the intermediate layer which has reached the Curie temperature, the exchange coupling between the reproducing layer and the recording layer is broken.
For this reason, the exchange coupling force from the recording layer is lost in the reproducing layer, so that the perpendicular magnetization anisotropy constant is apparently smaller than in other portions. Therefore, the magnitude of the apparent perpendicular magnetic anisotropy and the demagnetizing field energy are reversed, and the magnetization direction of the reproducing layer is oriented in the in-plane direction.

When the intermediate layer having a low Curie temperature is provided as described above, even if the reproducing layer does not exhibit the characteristic of changing from the perpendicularly magnetized film to the in-plane magnetized film when the temperature is raised in a single layer state (in other words, , Even if it is an in-plane magnetized film from room temperature to the Curie temperature), in the laminated state of the intermediate layer and the recording layer, it is possible to realize a state in which when the temperature of the reproducing layer is increased by the perpendicular magnetized film at room temperature, it becomes the in-plane magnetized film. Will also be possible. Therefore, there is an advantage that the range of material selection is widened.

Thus, as shown in FIG. 4B, part of the light spot of the reproducing layer (the portion where the reproducing layer is not the in-plane magnetized film, that is, the low temperature portion) is the perpendicular magnetized film.
In this portion, the reproducing layer and the recording layer are exchange-coupled, and the information of the recording layer is transferred to the reproducing layer. Then, the transferred information is converted into an optical signal by the magneto-optical effect of the reproducing layer (specifically, the magneto-optical effect of laser light reflected from the reproducing layer) and detected. Since the other portions become in-plane magnetized films and the magneto-optical effect does not occur, it is possible to reproduce with high resolution satisfactorily even if higher density recording is performed.

The present invention will be described in detail below with reference to experimental examples, but the present invention is not limited to the following experimental examples as long as the gist thereof is not exceeded.

(Experimental Example 1) Each target of Si, Tb, Gd, Fe, and Co was attached to a DC magnetron sputtering apparatus, and a glass substrate was fixed to a substrate holder, and then a height of 1 × 10 ^-5 Pa or less was obtained. Chamber until vacuum
The inside was evacuated with a cryopump.

Ar gas is supplied to 0.4 Pa while evacuation is performed.
The SiN layer to 80%
0Å film is formed, and then a GdFeCo layer 400 is formed as a reproducing layer.
Å Film formation, then 400 TbFeCo layer as recording layer
Å was formed, and then a SiN layer was formed as a protective film at 700Å.

At the time of forming the SiN layer, in addition to Ar gas, N ₂
A gas was introduced, and a film was formed by DC reactive sputtering.

The GdFeCo layer and the TbFeCo layer are G
Direct-current power was applied to each of the d, Fe, Co, and Tb targets to form a film by co-sputtering, and the composition thereof was adjusted by changing the power of each target during sputtering film formation.

The composition of the GdFeCo layer has a compensation temperature of 12
The Curie temperature was set to 400 ° C or higher at 0 ° C.

The composition of the TbFeCo layer was set so that the compensation temperature was room temperature or lower and the Curie temperature was 200 ° C.

When the residual θ _K when the magnetic field was 0 was measured while raising the temperature of this laminated film, it was 15 as shown in FIG.
It was confirmed that the residual Kerr rotation angle at around 0 ° C disappeared and an in-plane magnetized film was formed.

Example 2 Next, a magneto-optical recording medium having the same film structure as in Experimental Example 1 was prepared except that a polycarbonate substrate having a 130 mm diameter pregroove was mounted. next,
Recording and reproducing characteristics were measured using this magneto-optical recording medium.

N.V. of the objective lens of the measuring device. A. Is 0.5
5. The laser wavelength was 780 nm. Recording power is 8
5.8 to 13 M on the recording layer at mW and linear velocity of 9 m / s
A carrier signal of Hz was written by a magnetic field modulation method (recording magnetic field ± 300 Oe), and the recording frequency dependence of the C / N ratio was examined. The reproduction power was set so that the C / N ratio was maximized.
The results are shown in Table 1.

(Embodiment 3) Each target of Si, Tb, Gd, Fe and Co was attached to a DC magnetron sputtering apparatus, and a polycarbonate resin substrate provided with a pre-groove was fixed to a substrate holder and then 1 ×. 10 ^-5 P
The interior of the chamber was evacuated by a cryopump until a high vacuum of a or less was obtained.

Ar gas is supplied at 0.4 Pa while evacuation is performed.
And then the SiN layer 82.
0Å film is formed, and then a GdFeCo layer 400 is formed as a reproducing layer.
Å Film formation, then 5 TbFeCoAl layer as an intermediate layer
0 Å film is formed, and then a TbFeCo layer is formed as a recording layer 30
A 0 Å film was formed, and then a 700 Å SiN layer was formed as a protective film to prepare a magneto-optical recording medium of the present invention.

At the time of forming the SiN layer, N ₂ is added in addition to Ar gas.
A gas was introduced, and a film was formed by DC reactive sputtering.

The GdFeCo layer and the TbFeCo layer are G
Direct-current power was applied to each of the d, Fe, Co, and Tb targets to form a film by co-sputtering, and the composition thereof was adjusted by changing the power of each target during sputtering film formation.

The GdFeCo reproducing layer has a compensation temperature of 130.
The Curie temperature was set to 350 ° C. or higher in ° C.

The TbFeCoAl intermediate layer was set so that the compensation temperature was not higher than room temperature and the Curie temperature was 150 ° C.

The TbFeCo recording layer was set so that the compensation temperature was room temperature or lower and the Curie temperature was 200 ° C.

N.V. of the objective lens of the measuring device. A. Is 0.5
5. The laser wavelength was 780 nm. The recording power is 7.8 mW and the linear velocity is 9 m / s.
A 12 MHz carrier signal was written every 2 MHz by a magnetic field modulation method (recording magnetic field ± 300 Oe), and the recording frequency dependence of the C / N ratio was examined. The reproduction power was set so that the C / N ratio was maximized. The results are shown in Table 1.

(Comparative Example) TbFe was removed by removing the reproducing layer.
A magneto-optical recording medium having the same structure as in Experimental Example 2 was prepared except that the Co recording layer was set to 800 Å, and the recording frequency dependence of the C / N ratio was similarly examined. The results are shown in Table 1. From Table 1,
It can be seen that when the method of the present invention is adopted, a good C / N ratio can be obtained even at a high recording frequency.

(Experimental Example 4) By the same method as in Example 3, a magneto-optical recording medium having the following film thickness and composition conditions was prepared.

A SiN layer was formed in a thickness of 780 Å, a GdFeCo layer was formed in a thickness of 400 Å as a reproducing layer, TbFeCoSi was formed in an amount of 100 Å as an intermediate layer, a TbFeCo layer was formed in an amount of 300 Å as a recording layer, and then a SiN layer was formed as a protective film. Was deposited to 700Å.

The composition of the GdFeCo layer has a compensation temperature of 11
The Curie temperature was set to 320 ° C. at 0 ° C.

The composition of the TbFeCoSi layer was set so that the compensation temperature was below room temperature and the Curie temperature was 110 ° C.

The composition of the TbFeCo layer was set so that the compensation temperature was not higher than room temperature and the Curie temperature was 200 ° C.

After production, the recording / reproducing characteristics were evaluated in the same manner as in Examples 2 and 3. The results are shown in Table 1.

[0059]

[Table 1]

[0060]

According to the magneto-optical recording medium of the present invention and the information reproducing method using the medium, a simple device (conventional device) that does not require an initializing magnet can be used to generate a magnetic domain smaller than the beam spot diameter. Playback became possible and high density recording was achieved.

[Brief description of drawings]

FIG. 1A is a schematic view showing a film structure of a magneto-optical recording medium of the present invention. B is a schematic view showing an improved form of the film structure of a magneto-optical recording medium of the present invention.

FIG. 2 is a diagram for explaining an information reproducing method of the present invention using the magneto-optical recording medium shown in FIG. 1A.

FIG. 3 is a schematic diagram showing a temperature distribution of a medium around a light spot when the medium is moved.

FIG. 4 is a diagram illustrating an information reproducing method of the present invention using the magneto-optical recording medium shown in FIG. 1B.

FIG. 5: 2πMs ^{2 of} reproducing layer and perpendicular magnetic anisotropy constant Ku
Diagram showing an example of the temperature dependence of

FIG. 6 is a graph showing the temperature dependence of the Kerr rotation angle of the magneto-optical recording medium of the present invention.

Claims (3)

[Claims] 1. A magneto-optical recording medium comprising a first magnetic layer which is a perpendicular magnetization film at room temperature and which becomes an in-plane magnetization film when heated, and a second magnetic layer which is a perpendicular magnetization film. 2. The magneto-optical recording medium according to claim 1, comprising a perpendicular magnetic film having a Curie temperature lower than that of the first magnetic layer and the second magnetic layer between the first magnetic layer and the second magnetic layer. A magneto-optical recording medium having three magnetic layers. 3. A magneto-optical recording medium having a perpendicular magnetic film at room temperature, which has a first magnetic layer which becomes an in-plane magnetic film when heated, and a second magnetic layer which is a perpendicular magnetic film. A beam spot is irradiated, the first magnetic layer serves as an in-plane magnetized film in a high temperature portion of the spot, and the first magnetic layer serves as a perpendicular magnetized film in a low temperature portion and the second magnetic layer is formed. Due to exchange coupling, the magnetization of the first magnetic layer is aligned in a stable direction with respect to the magnetization direction based on the information of the second magnetic layer, and the influence of the perpendicular magnetic film portion of the first magnetic layer in the spot is exerted. An information reproducing method for a magneto-optical recording medium, characterized in that information is reproduced by detecting a magneto-optical change of the optical beam.