JPH06124500A - Magneto-optical recording medium and playback method of this medium - Google Patents

Abstract

PURPOSE:To reproduce a magnetic domain which is smaller than the diameter of a beam spot and to eliminate a cross talk by a method wherein, when a signal is replayed, the magnetization of a recording layer is transferred to a playback layer, it is converted into an optical signal by a magneto-optical effect and it is read out. CONSTITUTION:The intensity of a laser beam in a playback operation is set at a temperature Tr at which the effective vertical magnetic anisotropy constant of the temperature rise region of an optical spot is reversed. Then, in a playback layer, a high-temperature region becomes a vertical magnetization film and others are set to the state of an inside-the-face magnetization film. The playback layer which has become the vertical magnetization film is exchange-coupled to a recording layer, and the signal of the recording layer is transferred to the playback layer. The magnetic signal which has been transferred is converted into an optical signal by the magneto-optical effect of the playback layer, and it is detected. In addition, in the playback operation, the boundary temperature between a playback track and an adjacent track is set to be smaller than the temperature Tr. Then, the playback layer in the adjacent track does not become a vertical magnetization film, a recorded signal in the adjacent track is not transferred to the playback layer, and a cross talk can be eliminated.

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 by utilizing a magneto-optical effect and a reproducing method of the medium, and more particularly to improving linear recording density and track density. The present invention relates to a magneto-optical recording medium capable of increasing the density of a medium and a reproducing method of 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.

By the way, the linear recording density of an optical disk such as a magneto-optical recording medium is mainly determined by the S / N of the reproducing layer, and the signal amount of the reproducing layer is the period of the bit string of the recorded signal and the reproducing optics. It largely depends on the laser wavelength of the system and 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 f which becomes the detection limit is determined as follows.

F = λ / 2NA On the other hand, the track density is limited mainly by crosstalk. This crosstalk is mainly determined by the distribution (profile) of the laser beam on the medium surface, and is represented by a function of λ / 2NA as with the bit period.

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 before the reproduction of a signal, the magnetization direction of the reproducing layer is aligned in one direction, and then the recording layer is held. We are trying to improve the linear recording density and track density by transferring the signal to the reproducing layer to reduce the inter-symbol interference at the time of reproducing and making it possible to reproduce the signal having the period less than the diffraction limit of light.

[0010]

However, in the magneto-optical reproducing method described in Japanese Patent Application Laid-Open No. 3-93058, the magnetization of the reproducing layer must be aligned in one direction before being irradiated with laser light. Therefore, it is necessary to add a reproducing layer initialization magnet to the conventional device. Therefore, the reproducing method has problems that the magneto-optical recording device is complicated, the cost is high, and it is difficult to reduce the size.

[0011]

In view of the above problems, the present invention eliminates the need for initialization of the reproducing layer, so that even in a conventional magneto-optical recording / reproducing apparatus, a signal with a period less than the diffraction limit of light is used. It is an object of the present invention to provide a magneto-optical recording medium and a reproducing method of the medium, which can reproduce the data and improve the linear density and the track density.

Further, the above-mentioned object is an in-plane magnetized film at room temperature, and a first magnetic layer (hereinafter referred to as a reproduction layer) which becomes a perpendicular magnetized film when heated, and a second magnetic layer (which is called a reproducing layer) ( Hereinafter, a signal is recorded on the recording layer by using a magneto-optical recording medium having a recording layer), and when the signal is reproduced, the magnetization of the recording layer is transferred to the reproducing layer to achieve an optical effect. It is achieved by converting into a signal and reading.

This is achieved by a magneto-optical recording medium having an in-plane magnetized film at room temperature and a recording layer composed of a perpendicular magnetized film and a reproducing layer which becomes a perpendicular magnetized film when heated.

[0014]

The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a schematic view showing an example of a film structure of a magneto-optical medium of the present invention, FIG. 2 is an explanatory view showing a reproducing method of a magneto-optical signal of the present invention, and FIG. FIG. 4 is a schematic diagram showing the temperature distribution of the medium around the spot, FIG. 4 is a schematic diagram showing a transferred magnetization appearance state, and FIG. 5 is an example of the temperature dependence of the magnetization Ms of the reproducing layer and the performance index (Rθ _K ). Figures and 6
It shows an example of the temperature dependence of 2πMs ^{2 of the} reproducing layer and the perpendicular magnetic anisotropy constant Ku.

The magneto-optical recording medium of the present invention is an in-plane magnetized film at room temperature, and is exchange-coupled with the reproducing layer and the reproducing layer, which changes to a perpendicularly magnetized film when heated, and at any one of room temperature and temperature rising. It has at least two recording layers which are perpendicular magnetization films.

The recording layer is, for example, a rare earth-iron group amorphous alloy such as TbFeCo, DyFeCo, TbD.
A material having a large perpendicular magnetic anisotropy, such as yFeCo, capable of stably storing minute recording pits is most desirable.
Further, it may be a perpendicular magnetic film such as garnet, Pt / Co, Pd / Co, etc., which can magnetically transfer information to the reproducing layer.

As the reproducing layer, a rare earth-iron group amorphous alloy such as GdCo, GdFeCo, TbFeCo, DyFeCo, GdTbFeCo, GdD is used.
yFeCo and TbDyFeCo are preferable. For these materials, Nd, for the reason of increasing the Kerr rotation angle at short wavelength,
A light rare earth metal such as Pr or Sm may be added.

Alternatively, a platinum group-iron group periodic structure film such as Pt / Co, Pd / Co platinum group-iron group alloy such as PtCo, PdCo
And so on.

In the reproducing layer, in the high temperature portion within the laser spot for data reproduction, the magnetization direction needs to shift from the in-plane direction to the direction perpendicular to the film surface. The magnetic layer for the reproducing layer originally has, for example, perpendicular magnetic anisotropy (Ku>
0) At room temperature, since the saturation magnetization Ms is large, it is possible to use a magnetic material that is affected by its own demagnetizing field to become an in-plane magnetized film, and Ms becomes smaller as the temperature rises during reproduction to become a perpendicular magnetized film. .

As such a material, a compensation temperature is not always required, but it is desirable to use a material having a compensation temperature between room temperature and Curie temperature. Specifically, the compensation temperature of the reproducing layer is preferably 100 ° C. or higher,
C. or higher is more desirable, and 240.degree. C. or higher is most desirable.

The saturation magnetization of the reproducing layer is 150 at room temperature.
It is preferably emu / cc or more, and more preferably 200 emu / cc or more at room temperature.

Further, it is preferable that the Curie temperature of the reproducing layer is higher because the Kerr rotation angle does not decrease during reproduction and a sufficiently large reproduced signal can be obtained, and that of which the Curie temperature is higher than that of the recording layer is preferable. . More specifically, the Curie temperature of the reproducing layer is preferably 250 ° C. or higher, more preferably 300 ° C. or higher.

When a magnetic material containing GdFeCo as a main component is used in the reproducing layer, the Curie temperature can be increased by increasing the Co content. However, if too much Co is added, Ku becomes small and eventually becomes a negative value (in-plane anisotropy). Therefore, even if the temperature rises and the saturation magnetization becomes sufficiently small, a perfect perpendicular magnetization film is not formed. Therefore, a good signal cannot be obtained. (If reverse sputtering in which a negative potential bias is applied to the substrate side during film formation is performed or annealing treatment is performed after film formation, the GdCo film may have perpendicular magnetic anisotropy.
Membranes are not unusable, but this can be a slight disadvantage in terms of productivity. Therefore, when the composition of the main elements of the reproducing layer is Gd _x (Fe _100-y Co _y ) _100-x , y is more preferably 10 to 60 at% and more preferably 20 to 50 at%.
% Is more desirable.

The thickness of the reproducing layer is such that the information recorded in the recording layer is reproduced by allowing incident light to pass through to the recording layer or by causing a domain wall formed between the recording layer and the recording layer to penetrate into the reproducing layer side. Since the mask part of the light spot becomes incomplete, it cannot be made too thin. On the other hand, if the thickness of the reproducing layer is too large, there is a limit because the laser power required for reproducing becomes large. Therefore, the thickness of the reproducing layer is preferably 150 to 1000 Å,
More preferably, 300 to 900Å is desirable.

Elements such as Cr, Al, Ti, Pt and Nb for improving corrosion resistance may be added to the reproducing layer and the recording layer. Further, another magnetic layer such as an intermediate layer for adjusting the exchange coupling force may be provided.

In addition to the reproducing layer and the recording layer, SiN, AlOx, Ta are added to enhance the reproduced signal due to the interference effect.
Dielectrics such as Ox, Al, AlTi, AlCr, Al
A metal layer such as Ta or Cu may be provided for controlling thermal conductivity. Further, a protective coat made of the dielectric layer or polymer resin may be provided as a protective film.

The present inventor diligently studied the magnetization transfer process from the recording layer to the reproducing layer. As a result, if a magnetic layer that is an in-plane magnetized film at room temperature but becomes a perpendicular magnetized film at high temperature is used as the reproducing layer, a laser is generated. It was found that only the hot part of the spot transfers the magnetization of the recording layer. According to this method, it is possible to reproduce a signal having a period equal to or shorter than the light detection limit without requiring an extra operation such as pre-aligning the magnetization of the reproduction layer in one direction.

The principle of the regeneration process of the present invention will be described below.

A data signal is recorded on the recording layer of the magneto-optical recording medium shown in FIG. Recording is performed by irradiating the external magnetic field while irradiating a laser beam with a power such that the recording layer has a Curie temperature or higher, or after initializing the magnetization in the same direction, the laser is applied while applying the magnetic field in the recording direction. Modulates the power.

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 is near the Curie temperature of the recording layer, a recording magnetic domain smaller than the diameter of the light spot is obtained. As a result, a signal having a period below the diffraction limit of light can be recorded.

At the time of data reproduction, the medium is continuously irradiated with a reproduction laser beam to detect reflected light or transmitted light from the recording medium. At this time, the temperature of the laser irradiation portion rises, and the temperature distribution on the medium has a shape extending in the moving direction of the medium, as shown in FIG.

When the saturation magnetization of the magnetic thin film 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 the main orientation of is determined. Here, 2πM _s ² is the demagnetizing energy.

As shown in FIG. 5, the Ms of the reproduction layer becomes small because the temperature rises during reproduction. Therefore, as shown in FIG. 6, 2πMs ² is rapidly reduced, and the magnitude relationship with the perpendicular magnetic anisotropy constant Ku is reversed (the temperature at which the temperature is reversed is Tr
Then, K⊥> 0 and the film becomes a perpendicular magnetization film (Ku also decreases slightly as the temperature rises, but its reduction rate is generally small compared to 2πMs ² ).

Further, since Ms is large in the portion where the temperature is equal to or lower than Tr, the in-plane magnetized film remains as it satisfies the equation of K⊥ <0.

When this magnetic thin film is laminated directly on the perpendicular magnetic film or via an intermediate layer or the like, since the exchange coupling force and the magnetostatic coupling force from the perpendicular magnetic film act, the temperature region of the perpendicular magnetic film is high. Although it shifts to the side, if the transition temperature from the in-plane magnetized film to the perpendicular magnetized film in the single layer film is set to be slightly lower, even when laminated with the perpendicular magnetized film, the perpendicular magnetized film at room temperature becomes The condition that the film becomes a perpendicular magnetic film at a high temperature during reproduction is established.

That is, the intensity of the laser beam during reproduction is shown in FIG.
If it is set so that only the high temperature part of the light spot indicated by is above the temperature Tr, as shown in FIG. 2, only the high temperature part which is a part of the light spot becomes a perpendicular magnetization film in the reproducing layer, and most of the other part. The state in which the in-plane magnetized film remains is realized. The reproducing layer, which has become a perpendicularly magnetized film, is magnetically coupled to the recording layer by exchange coupling, so that the signal of the recording layer is transferred to the reproducing layer. The transferred magnetic signal is converted into an optical signal by the magneto-optical effect (Kerr rotation angle or Faraday rotation angle) of the reproducing layer and detected.

In this way, considering that the area of the high temperature portion of the light spot shown in FIG. 3 can be determined by the set intensity of the laser light, each of the signals having a period less than the diffraction limit of the light recorded in the recording layer is obtained. It can be transferred to the reproducing layer in units of recording bits, and as a result, a signal having a period less than the diffraction limit of light can be reproduced without intersymbol interference.

Further, in reproducing, if the temperature distribution is such that the temperature Tt at the boundary between the reproducing track and the adjacent track is Tt <Tr, the reproducing layer of the adjacent track does not become a perpendicular magnetization film, and the recording of the adjacent track occurs. The crosstalk is completely eliminated without the signal recorded on the layer being transferred to the reproduction layer. This state is shown in FIG.

In the above description, the case where the reproducing layer and the recording layer are magnetically coupled by the exchange interaction has been described, but the recording layer and the reproducing layer may be magnetically coupled by magnetostatic coupling during reproduction.

The present invention will be described in more 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 SiN, Tb, Gd, Fe, Co, and Al was attached to a DC magnetron sputtering apparatus, and a glass substrate was fixed to a substrate holder, and then 1 × 10 ^-5 Pa or less. The interior of the chamber was evacuated by a cryopump until the high vacuum was reached.

Ar gas was supplied at 0.3 Pa while evacuation was performed.
Gd, Tb, C after being introduced into the chamber until
Direct current power is applied to each of the target o to form a GdTbCo layer 800Å on the glass substrate, and then alternating current is applied to the SiN target to form a SiN layer as a protective film.
0Å Film was formed. The composition of the GdTbFe layer was adjusted by changing the power of each of the Gd, Tb, and Fe targets during sputtering film formation, and was set so that the compensation temperature was 180 ° C. and the Curie temperature was 350 ° C. or higher.

Also, the Kerr rotation angle was within 10% of the attenuation from the value at room temperature near 150 ° C., reflecting the high Curie temperature. When the average rotation rate of the magnetic moment with respect to the external magnetic field magnetized perpendicularly to the film surface of the reproducing layer was measured, as shown in FIG. 8, the reproducing layer was affected by the exchange coupling force from the recording layer at about 120 ° C. It was confirmed that the film having the magnetization direction in the film plane was changed to the perpendicular magnetization film even under the condition of receiving the effective external magnetic field.

Here, the average rotation rate of the magnetization is M⊥ / (Ms
・ Hex), M ⊥ is VSM by heating the sample with a resistance heating type heater in a vacuum of 1 × 10 ⁻⁴ Pa or less.
Using a (vibrating sample magnetometer), 600 in the direction perpendicular to the film surface.
An external magnetic field of Oe (denoted by Hex) was applied to measure the magnetization of the sample, and the saturation magnetization Ms was similarly determined by applying an external magnetic field of 15 KOe.

Next, after the inside of the chamber was evacuated to a high vacuum by a cryopump in the same manner as described above except that a polycarbonate substrate having a pre-groove of φ130 mm was mounted, A
r gas was introduced into the chamber until it reached 0.3 Pa,
A SiN layer is formed on the substrate to prevent oxidation and interference.
After depositing a film of 00Å, a GdTbCo layer is used as a reproducing layer.
0 Å and a TbFeCo layer of 400 Å was formed as a recording layer. After that, SiN is added to prevent oxidation and enhance interference effect.
A 300 Å layer and a 400 Å Al layer were formed as a heat conduction control layer. The film formation was carried out successively without breaking the vacuum to prepare a magneto-optical recording medium having a five-layer structure as shown in FIG. 7, which is an example of the present invention.

The refractive index n of the SiN layer is about 2.1 in both cases.
The composition of the TbFeCo layer is 2 for Tb, Fe, and Co, respectively.
The ratio was 1, 72, 7 at%.

The composition of the GdTbFe layer has a compensation temperature of about 26.
The temperature was set to 0 ° C and the Curie temperature was set to 350 ° C.

Next, the recording / 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 7
.About.13 mW, and the reproducing power was set to be the highest in the range of 2.5 to 3.5 mW. Linear speed 9m /
The carrier signal of 5 to 15 MHz is recorded on the recording layer as 1 second.
The magnetic field modulation method was used to write data at every MHz, and the recording frequency dependence of the C / N ratio (ratio of carrier level to noise level) was examined. The results are shown in Fig. 9.

(Experimental Examples 2 to 16) In the same manner, a medium having a composition, film thickness, compensation temperature (Tcp) and Curie temperature (Tc) as shown in Table 1 was prepared for the reproducing layer and the recording layer, and evaluated. I went. The film structure was the same as in Experimental Example 1 except that the heat conductive layer was not provided.

(Comparative Experimental Example) A magneto-optical recording medium having the same structure as in Experimental Example 1 was prepared except that the reproducing layer and the heat conduction control layer were deleted and the recording layer was changed to 800 Å. The recording frequency dependence was investigated. The results are shown in Fig. 9 (2. conventional method).

It can be seen from FIG. 9 and Table 1 that when the method of the present invention is adopted, a good C / N ratio can be obtained even in recording with a short mark length recorded at a high recording frequency.

[0054]

[Table 1]

[0055]

According to the magneto-optical recording medium and the reproducing method of the present invention, it is possible to reproduce a magnetic domain smaller than the beam spot diameter by using a simple device (conventional device) which does not require an initialization magnet. Crosstalk can be eliminated, and track density and linear recording density can be improved to achieve high-density recording.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an example of a film structure of a magneto-optical recording medium of the present invention.

FIG. 2 is an explanatory diagram showing a method of reproducing a magneto-optical signal according to the present invention.

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

FIG. 4 is a schematic diagram showing a transfer magnetization appearance state.

FIG. 5 is a diagram showing an example of the temperature dependence of the magnetization Ms of the reproducing layer and the figure of merit (Rθ _K ).

FIG. 6 is 2πMs ^{2 of the} reproducing layer and the perpendicular magnetic anisotropy constant Ku.
It is a figure showing the temperature dependence of.

FIG. 7 is a film configuration diagram of an embodiment of a magneto-optical recording medium of the present invention.

FIG. 8 is a table showing the average rotation rate of the magnetic moment with respect to an external magnetic field applied perpendicularly to the film surface.

FIG. 9 is a diagram showing recording frequency and mark length dependence of C / N ratio.

Claims (2)

[Claims] 1. A magneto-optical recording medium comprising an in-plane magnetized film at room temperature, which has a first magnetic layer that changes to a perpendicular magnetized film when heated, and a second magnetic layer made of the perpendicular magnetized film. The first magnetic layer is heated by laser light irradiation to form a perpendicular magnetization film by using a magnetic field, and the magnetic signal recorded in the second magnetic layer is transferred to the first magnetic layer while A signal reproducing method in a magneto-optical recording medium, which is characterized in that the signal is converted into an optical signal and read by the effect. 2. An in-plane magnetized film at room temperature, comprising a first magnetic layer that changes to a perpendicular magnetized film when heated, and a second magnetic layer composed of a perpendicular magnetized thin film. Magneto-optical recording medium.