JPH07176089A - Magneto-optical recording medium and method for reproducing magneto-optical recorded information using the same - Google Patents

Abstract

(57) [Summary] The magneto-optical disk comprises a magnetic double layer consisting of a read layer 3 and a recording layer 4 exhibiting perpendicular magnetization in the region from room temperature to the Curie temperature.
Has its magnetization oriented in accordance with this external magnetic field when a relatively small external magnetic field is applied at room temperature. It is configured to face the direction according to the exchange coupling force. [Effect] Utilizing the temperature distribution of the light emitted during playback,
While adjusting the area of the readout layer 3 that is in a high temperature state,
By applying an appropriate external magnetic field Hr, it is possible to improve the recording density without using a short wavelength laser and without increasing the crosstalk amount even if the track pitch is narrowed. Moreover, the initialization magnetic field is unnecessary.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is applied to a magneto-optical recording / reproducing apparatus, for example, a magneto-optical recording medium such as a magneto-optical disk, a magneto-optical tape, a magneto-optical card, etc., and reproducing magneto-optical recorded information using the same. It is about the method.

[0002]

2. Description of the Related Art A magneto-optical disk is being researched and developed as a rewritable optical disk, and a part thereof has already been put to practical use as an external memory for a computer.

In a magneto-optical disk using a perpendicular magnetic film as a recording medium, recording and reproduction are performed by using light, so that a large recording capacity can be realized as compared with a floppy disk or a hard disk using an in-plane magnetic film.

However, the recording density of a magneto-optical disk is
The size of the light beam spot on the magneto-optical disk is restricted. That is, when the recording bit diameter and the recording bit interval are smaller than the size of the light beam spot,
Since a plurality of recording bits enter the light beam spot, it becomes impossible to reproduce each recording bit separately. In order to reduce the size of the beam spot in order to improve the recording density, it is effective to shorten the wavelength of the laser beam, but the semiconductor laser currently on the market is 680 nm.
The semiconductor laser with the shortest wavelength and the shorter wavelength is still under development. Therefore, it is difficult to further increase the recording density of the magneto-optical disk by using the long-wavelength laser currently on the market.

On the other hand, for example, JJAP, vol.31 (199
2), pp.568-575, a magneto-optical disk having two magnetic layers of a recording layer and a reading layer is used, and two methods of reading bits smaller than the light beam spot [FAD (front
aperture detection) and RAD (rear aperture detecti
on)] has been proposed, and it has been shown that the recording density in the beam traveling direction can be improved.

[0006]

However, of these methods, in the FAD method, the shape of the part of the beam spot that is involved in readout becomes a crescent shape (Fig. 6 of the above document), and the track When reproduction is performed with a narrow pitch, a phenomenon (crosstalk) in which signals from adjacent tracks are mixed easily occurs, which causes a problem that it is difficult to improve the track density. Further, in the RAD method, a magnetic field (magnet) for initialization is required, and there is a problem that the device becomes large.

The present invention has been made in view of the above-mentioned conventional problems, and an object thereof is not to require an initializing magnet and to increase crosstalk even if the track pitch is narrowed. It is another object of the present invention to provide a magneto-optical recording medium capable of improving the recording density in the beam traveling direction and a reproducing method of magneto-optical recording information using the magneto-optical recording medium.

[0008]

In order to solve the above-mentioned problems, a magneto-optical recording medium according to the invention of claim 1 has perpendicular magnetic anisotropy for reading out magneto-optically recorded information.
It has a magnetic double layer consisting of a read layer which is a magnetic layer having a thickness of h ₁ and a recording layer which is a magnetic layer having a thickness of h ₂ and which has perpendicular magnetic anisotropy for magneto-optical recording of information. t
In a), the coercive force of the read layer is Hc ₁ (t
a), the saturation magnetization is Ms ₁ (ta), the coercive force of the recording layer is Hc ₂ (ta), the saturation magnetization is Ms ₂ (ta), the domain wall energy at the interface is σw (ta), and the temperature is equal to or higher than room temperature. (T
m), the coercive force of the read layer is Hc ₁ (t), the saturation magnetization is Ms ₁ (t), the coercive force of the recording layer is Hc ₂ (t), and the saturation magnetization is Ms _{2 at} a temperature (t) equal to or higher than m). (t), where the domain wall energy at the interface is σw (t), Hw ₁ (ta) = σw (ta) / 2Ms ₁ (ta) h ₁ Hw ₁ (t) = σw (t) / 2Ms ₁ (t) Let h ₁ be Hc ₁ (ta) + Hw ₁ (ta) <− Hc ₁ (t) + Hw
It is characterized by satisfying the condition of ₁ (t).

In order to solve the above-mentioned problems, a reproducing method of magneto-optical recording information according to a second aspect of the present invention uses the magneto-optical recording medium according to the first aspect, wherein Hw ₂ (ta) = σw ( If ta) / 2Ms ₂ (ta) h ₂ Hw ₂ (t) = σw (t) / 2Ms ₂ (t) h ₂ , then Hc ₁ (ta) + Hw ₁ (ta) <Hr <−Hc ₁ (t) + H
w ₁ (t) Hr <Hc ₂ (ta) -Hw ₂ (ta) Hr <Hc ₂ (t) -Hw ₂ (t) While applying an external magnetic field Hr satisfying the relationship, light is irradiated to read information. It is characterized by that.

[0010]

According to the structure of claim 1, when the information is read from the magneto-optical recording medium, when the light is irradiated, the temperature distribution in the light irradiation region becomes almost Gaussian distribution. Therefore, only the area in the vicinity of the center of the light irradiation area becomes an area having a temperature higher than the predetermined temperature (tm) and higher than the surrounding area (t). In the magnetization direction of the read layer in the region heated to this high temperature (t), the applied external magnetic field strength is -Hc.
If it is smaller than ₁ (t) + Hw ₁ (t), it operates in the direction according to the exchange coupling force from the recording layer.

On the other hand, the high temperature (t) in the light irradiation region
The temperature of the region other than the region heated up to, that is, the region around the high temperature region does not rise so much even when irradiated with light and remains at about room temperature (ta). In such a peripheral region, the applied external magnetic field strength is Hc ₁ (ta) + Hw ₁ (t
If it is larger than a), the magnetization direction of the read layer is oriented according to the external magnetic field.

Therefore, when reproducing information as described in claim 2, Hc ₁ (ta) + Hw ₁ (ta) <Hr <-H
When the magneto-optical recording medium is irradiated with light in consideration of the temperature distribution while applying an external magnetic field Hr satisfying c ₁ (t) + Hw ₁ (t), the temperature rises in the area around the light irradiation area, that is, the temperature rises. The magnetization of a region around room temperature (ta), which is not kept, is directed by the external magnetic field Hr in a direction according to the external magnetic field Hr. That is, in a region of the read layer having a temperature of about room temperature (ta), the magnetizations are oriented in the same perpendicular direction. On the other hand, the magnetization of the region corresponding to the vicinity of the central portion of the light irradiation region in the high temperature (t) state does not follow the external magnetic field Hr even when the external magnetic field Hr in the above range is applied, and the magnetization of the recording layer Due to the exchange coupling force of, the magnetization direction of the recording layer is followed.

The external magnetic field H applied during the above reproduction
As described in claim 2, r is a magnetic field Hc ₂ (ta) −Hw ₂ (t when the magnetization of the recording layer is reversed at room temperature (ta).
a) and magnetic field Hc when reversing near high temperature (t)
Since it is set to be smaller than ₂ (t) -Hw ₂ (t), the external magnetic field Hr does not cause the magnetization reversal of the recording layer.

Therefore, only the region near the center of the light irradiation region in the read layer operates according to the information recorded in the recording layer, and the magnetization of the read layer in the periphery thereof always faces the same direction by the external magnetic field Hr. As a result, the information in a region other than the vicinity of the center of the light irradiation region is masked.

As a result, the temperature is raised to a temperature equal to or higher than the predetermined temperature (tm), and only the central region smaller than the diameter of the light beam can be involved in the reproduction. Therefore, as in the conventional FAD method described above, the reproduction is performed. Unlike the case where the region related to the disk has a crescent shape, crosstalk from adjacent tracks is reduced and the resolution during reproduction is improved. Further, the information in the recording layer is not affected by the external magnetic field Hr applied during reproduction. As a result, it is possible to improve the recording density in the beam traveling direction without increasing crosstalk even if the track pitch is narrowed. Moreover, these things can be realized without requiring a magnetic field for initialization.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention is shown in FIGS.
The explanation is based on the following.

As shown in FIG. 1, the magneto-optical disk (magneto-optical recording medium) of this embodiment has a substrate 1, a transparent dielectric layer 2,
The reading layer 3, the recording layer 4, the protective layer 5, and the overcoat layer 6 are laminated in this order.

For the read layer 3, a GdFeCo rare earth transition metal alloy, which is a ferrimagnetic material, is used.
GdFeCo rare earth transition metal alloy single layer (thickness 50 nm)
2 shows the temperature dependence of the coercive force (Hc) of the rare earth metal sublattice magnetization at room temperature is larger than the transition metal sublattice magnetization, the compensation temperature is around 130 ° C., and the Curie temperature is 330. ℃. Since the perpendicular magnetic anisotropy of this film is relatively small, the value of Hc at room temperature is relatively small.

The figure also shows a Kerr hysteresis loop at a typical temperature. Gd used here
In the case of a single layer, the FeCo film has a magnetization that is oriented in a substantially in-plane direction when the external magnetic field is 0 at 200 ° C. or higher, but at a low temperature, the magnetization is oriented in the perpendicular direction. The sample prepared to obtain such data was a glass substrate 1 as shown in FIG.
0 as a magnetic layer 11 with a GdFeCo film of 50 nm, A
The 1N dielectric layer 12 is laminated in a thickness of 50 nm, and measurement is performed by irradiating light of 633 nm from the glass substrate 10 side.

The recording layer 4 has a ferrimagnetic material D.
A yFeCo rare earth transition metal alloy has been used. Dy
The temperature dependence of the coercive force (Hc) of the FeCo rare earth transition metal alloy single layer (thickness 50 nm) is as shown in FIG.
The compensation temperature is room temperature and the Curie temperature is 230 ° C. The figure also shows the Kerr hysteresis loop at typical temperatures. In the case of a single layer, the DyFeCo film used here has perpendicular magnetization in the temperature region from room temperature to the Curie temperature.

The sample prepared for obtaining such data is, as shown in FIG. 3, a DyFeCo film having a thickness of 50 nm and an AlN dielectric layer 12 having a thickness of 50 nm laminated on a glass substrate 10 as a magnetic layer 11. Yes, glass substrate 10
The measurement is performed by irradiating light of 633 nm from the side.

FIGS. 5 to 6 show Kerr hysteresis loops at a typical temperature viewed from the GdFeCo side of a sample having a magnetic double layer in which the above-mentioned GdFeCo film and DyFeCo film are laminated by 50 nm each. As shown in FIG. 7, the prepared sample is a glass substrate 1
3, a readout layer 14 made of a GdFeCo film, DyF
Recording layer 15 made of eCo film and AlN dielectric layer 1
6 is laminated in order. The measurement was performed by irradiating light of 633 nm from the glass substrate 13 side.

FIG. 5 shows a loop at room temperature, and FIG. 6 shows a loop at 120 ° C. Each figure shows the relationship between the external magnetic field H applied in the direction perpendicular to the film surface of the sample shown in FIG. 7 and the polar Kerr rotation angle (θ _K ) when light is incident from the direction perpendicular to the film surface. ing. Also, the Kerr effect of the glass substrate is cancelled. The solid line arrows in the figure represent the direction in which the locus of the loop is drawn. Further, in each drawing, the magnetization direction of the transition metal sublattice magnetization in the magnetic double layer under a typical magnetic field is schematically shown. The white arrows in the figure represent the directions of the transition metal sub-lattice magnetization, with the upper one representing the transition metal sub-lattice magnetization and the lower one representing the transition metal sub-lattice magnetization. Shows. The arrow shown by the broken line is the direction of the external magnetic field H. When the external magnetic field H is positive, an upward magnetic field perpendicular to the film surface is applied, while when the external magnetic field H is negative, a downward magnetic field is applied. The white arrow indicating the direction of the transition metal sublattice magnetization is
The direction is shown corresponding to the direction adopted by the external magnetic field H.

As is apparent from FIG. 5, when an external magnetic field H having a magnitude of H ₁ or more shown in the figure is vertically applied at room temperature, the transition metal sublattice magnetization of the read layer (GdFeCo film) 14 is Face the direction according to the external magnetic field H. In the figure, in the magnetic field of H ₁ or higher, the magnetic field is directed downward according to the external magnetic field H
In a magnetic field of H ₂ or less, the magnetic field goes up according to the exchange coupling force with the recording layer (DyFeCo film) 15. In the figure, the recording layer 1
The direction of the transition metal sublattice magnetization of No. 5 is the same in all the external magnetic fields H (upward in the figure).
Is because it reflects that it has a compensation temperature at room temperature.

Further, FIG. 6 shows that -1.2≤H≤1.2 (kO
It is a Kerr loop when the external magnetic field H is changed within the range of e). At 120 ° C., the transition metal sublattice magnetization of the readout layer 14 faces downward according to the external magnetic field H in the external magnetic field H of H ₃ or more shown in the figure, and exchange coupling from the recording layer 15 in the external magnetic field H of H ₄ or less. Turn up according to the force. In the drawing, all the directions of the transition metal sublattice magnetization of the recording layer 15 are always drawn upwards because the recording layer 15 is -1.2≤
This is because it reflects that no magnetization reversal occurs in the range of H ≦ 1.2 (kOe).

Next, the sample measured here was irradiated with light of 633 nm from the side of the AlN dielectric layer 16 to measure the Kerr hysteresis loop of the recording layer 15, and the results are shown in FIGS. 8 and 9.
Shown in. The measurement was performed in the range of −10 ≦ H ≦ 10 (kOe). FIG. 8 shows the results at room temperature and FIG. 9 shows the results at 120 ° C. At room temperature, the recording layer 1 is in the range of −10 ≦ H ≦ 10 (kOe).
The magnetization of 5 did not reverse. At 120 ° C, H =
The magnetization of the recording layer 15 was reversed at 2.2 kOe.

Using a magnetic double layer with the above characteristics,
The recording density of the magneto-optical disk can be improved by adopting a method of reproducing information by irradiating a laser under an appropriate external magnetic field. That is, it is possible to reproduce recorded bits smaller than the size of the light beam.

This will be described below with reference to FIGS. 5 and 6. During the reproducing operation, the reproducing light beam 7 shown in FIG. 1 is applied to the readout layer 3 from the substrate 1 side through the condenser lens 8. In the area of the readout layer 3 irradiated with the reproduction light beam 7, the temperature in the vicinity of the central portion of the readout layer 3 rises most and becomes higher than that of the peripheral portion. This is the reproduction light beam 7
However, since it is narrowed down to the diffraction limit by the condenser lens 8, its light intensity distribution becomes a Gaussian distribution, and the temperature distribution of the irradiation site on the magneto-optical disk also becomes a Gaussian distribution.

When the intensity of the reproduction light beam 7 is set so that the temperature in the vicinity of the center rises and the temperature in the peripheral region becomes close to room temperature, only the area in the vicinity of the center smaller than the light beam diameter is involved in reproduction. To do. That is, the temperature-increased region near the center of the readout layer 3 has the characteristics as shown in FIG. 6, while the temperature in the peripheral region of the readout layer 3 other than the region corresponding to the center of the reproduction light beam 7 is high. It is close to room temperature and has the characteristics shown in FIG. Therefore, the magnet 9 is used to generate an external magnetic field H between H ₁ and H _4.
By applying r, the transition metal sub-lattice magnetization of the readout layer 3 in the region near the center has a strength smaller than H ₄ of the external magnetic field Hr, and therefore, in accordance with the exchange coupling force from the recording layer 4, Since the intensity of the external magnetic field Hr is larger than H ₁ , the transition metal sublattice magnetization of the readout layer 3 in the peripheral portion follows the external magnetic field Hr applied from the magnet 9.

As described above, the magnetic double layer used in this embodiment must satisfy the condition of H ₁ <H ₄ ... (1), and further, H ₁ <Hr <H ₄ ... ( If an external magnetic field Hr satisfying 2) is applied at the time of reproduction, the information of the recording layer 4 is transferred to the read layer 3 only at the temperature rising portion corresponding to the region near the center of the reproduction light beam 7. Only the vicinity of the center of 7 will be involved in information reproduction. That is, the transition metal sublattice magnetization of the read layer 3 other than the temperature rising portion functions to mask the information of the recording layer 4.

Then, when the reproducing light beam 7 moves (actually, the magneto-optical disk rotates) to reproduce the next recording bit, the temperature of the previous reproducing portion drops to around room temperature and the reading layer The transition metal sub-lattice magnetization of No. 3 no longer follows the transition metal sub-lattice magnetization of the recording layer 4, and is oriented in the same perpendicular direction as the transition metal sub-lattice magnetization of the region other than the temperature rising portion due to the external magnetic field Hr. Therefore, the previously reproduced portion is masked.

The conditions for realizing this phenomenon will be generally described as follows. The thickness of the read layer 3 made of the GdFeCo film is h ₁ , the thickness of the recording layer 4 made of the DyFeCo film is h ₂ , the coercive force of the read layer 3 at room temperature is Hc ₁ (ta), and the saturation magnetization is Ms ₁ (ta), The coercive force of the recording layer 4 is Hc ₂ (ta), the saturation magnetization is Ms ₂ (ta), the domain wall energy at the interface is σw (ta), and 120
The coercive force of the read layer 3 at C is Hc ₁ (t), the saturation magnetization is Ms ₁ (t), the coercive force of the recording layer 4 is Hc ₂ (t), the saturation magnetization is Ms ₂ (t), and the domain wall at the interface is Energy is σw
(T), H ₁ and H ₄ are H ₁ = Hc ₁ (ta) + Hw ₁ (ta) (3) H ₄ = −Hc ₁ (t) + Hw ₁ (t)・ It is (4). The above Hw ₁ (ta) and Hw ₁ (t) are respectively Hw ₁ (ta) = σw (ta) / 2Ms ₁ (ta) h ₁ Hw ₁ (t) = σw (t) / 2Ms ₁ (t ) H ₁ . Substituting the equations (3) and (4) into the equation (2), Hc ₁ (ta) + Hw ₁ (ta) <Hr <−Hc ₁ (t) + Hw ₁ (t) (5) , An external magnetic field H according to the magnetic characteristics of the magneto-optical disk
The range of r is set.

From the above (1), the magnetic double layer provided on the magneto-optical disk is Hc ₁ (ta) + Hw ₁ (ta) <− Hc ₁ (t) + Hw ₁ (t) ( 6) must be satisfied.

When a reproduction is performed using a combination of a magneto-optical disk having a magnetic double layer satisfying the condition of the above expression (6) and an external magnetic field Hr satisfying the condition of the expression (5), the state is as follows: It is shown in FIG. In the figure, the arrows shown in the read layer 3 and the recording layer 4 represent the direction of the transition metal sublattice magnetization of each layer, and the information written in the read layer 4 is the information written in the read layer only at the temperature-increased portion. It is transcribed to 3. On the other hand, in a region at room temperature other than near the beam center, the transition metal sublattice magnetization of the readout layer 3 is oriented in the same perpendicular direction (downward in the figure) by the external magnetic field Hr applied upward, and The information written in 4 is masked.

Therefore, the information is not reproduced from the portion where the temperature is lowered, and the signal mixing from the adjacent bits, which is a cause of noise, is eliminated. However, at this time, the external magnetic field H
r must be large enough not to destroy the information in the recording layer 4.

By the way, when Hw ₂ (ta) = σw (ta) / 2Ms ₁ (ta) h ₂ Hw ₂ (t) = σw (t) / 2Ms ₁ (t) h ₂ is set, the recording layer 4 near room temperature is obtained. The external magnetic field for reversing the magnetization is Hc ₂ (ta) −Hw ₂ (ta), and when reversing the magnetization of the recording layer 4 in a heated state by being irradiated with the reproduction light beam 7. External magnetic field is Hc
Since it is represented by ₂ (t) -Hw ₂ (t), it is necessary to set the external magnetic field Hr applied at the time of reproduction to a range smaller than these values.

Therefore, the external magnetic field Hr is Hr <Hc ₂ (ta) -Hw ₂ (ta) (7) Hr <Hc ₂ (t) -Hw ₂ (t) (8) You need to be satisfied.

As a result of the above, (5), (6),
The equations (7) and (8) are the conditions to be satisfied by the magneto-optical disk and the external magnetic field Hr during reproduction. As can be seen from FIGS. 5 and 6, the magnetic double layer used in this example has Hc ₁ (ta) + Hw ₁ (ta) = 450 (Oe) −Hc ₁ (120 ° C.) + Hw ₁ (120 ° C.) = 800 (O
e), which satisfies the above formula (6).

As can be seen from FIGS. 8 and 9, Hc ₂ (ta) -Hw ₂ (ta)> 10 (kOe) Hc ₂ (120 ° C.)-Hw ₂ (120 ° C.) = 2.2 (kOe) ) Met.

Therefore, if Hr that satisfies 450 <Hr <800 (Oe) is selected, the equations (5), (7) and (8) are satisfied.

As described above, the magneto-optical disk of this embodiment having the magnetic double layer satisfying the above conditions is used.
When information is reproduced with an appropriate laser power and an appropriate external magnetic field Hr, it becomes possible to reliably reproduce a recording bit smaller than the diameter of the reproduction light beam 7. In addition, since it is not affected by the adjacent recording bits, the recording density can be remarkably increased. Further, since a magnetic field for initialization is not required, the device using such a magneto-optical disk can be downsized.

Next, a specific example of the magneto-optical disk will be described in more detail. The substrate 1 has a diameter of 86 mm and an inner diameter of 15
mm, 1.2 mm thick disk-shaped glass substrate (not shown), but on one surface there is an uneven guide track for guiding the light beam with a pitch of 1.6 μm and a groove width (recess). The width is 0.8μm, and the width of the land (projection) is 0.8μ
It is formed by m.

On the surface of the substrate 1 on which the guide track is formed, AlN is formed as a transparent dielectric layer 2 with a thickness of 80 nm. Further, a GdFeCo film, which is a rare earth-transition metal alloy thin film, having a thickness of 50 nm is formed as the readout layer 3 on the transparent dielectric layer 2. The composition of GdFeCo is, Gd _0.22 (Fe _{0. 82}
Co _0.18 ) _0.78 , the rare earth metal sublattice magnetization is more dominant than the transition metal sublattice magnetization at room temperature, the compensation temperature is around 130 ° C., and the Curie temperature is 330 ° C.

A DyFeCo film, which is a rare earth-transition metal alloy thin film, having a thickness of 5 is formed as a recording layer 4 on the readout layer 3.
It is formed with 0 nm. The composition of DyFeCo is Dy
_0.25 (Fe _0.83 Co _0.17 ) _0.75 , room temperature is the compensation temperature,
The Curie temperature is 230 ° C.

Due to the combination of the read layer 3 and the recording layer 4, the direction of the transition metal sublattice magnetization of the read layer 3 is, as described above, at the room temperature under an external magnetic field of 450 (Oe) or more. 8 at 120 ° C according to the direction of the magnetic field
It follows the exchange coupling force from the recording layer 4 under an external magnetic field of 00 (Oe) or less.

An AlN film having a thickness of 20 nm is formed as a protective layer 5 on the recording layer 4. Further, on the protective layer 5, a polyurethane acrylate-based ultraviolet curable resin is formed as the overcoat layer 6 with a thickness of 50 nm.

Next, description will be given of an experiment conducted for recognizing a motion using the magneto-optical disk having the above-mentioned structure and a result obtained by the experiment. The wavelength of the semiconductor laser of the optical pickup used in the experiment is 780 nm, and the objective lens numerical aperture (NA) is 0.55. First, the rotation speed is set to 18 at the land portion in the radius of 26.5 mm of the magneto-optical disc
Under 00 rpm (5 m / sec linear velocity), a single frequency recording bit with a length of 0.765 μm was pre-recorded. The record is
First, the magnetization direction of the recording layer 4 is aligned in one direction (erased state), and then the direction of the external recording magnetic field is fixed in the opposite direction to the erasing direction, which corresponds to a length of 0.765 μm. This was done by modulating the laser at the recording frequency (in this case about 3.3 MHz). The recording laser power was about 8 mW.

The recording bit string thus recorded is reproduced by changing the reproduction laser power and the applied magnetic field during reproduction,
The result of examining the amplitude of the reproduced signal waveform is shown in FIG. In the figure, the horizontal axis represents the reproduction laser power, which was measured in the range of 0.5 mW to 3.0 mW. The vertical axis represents the reproduction signal amplitude, which is normalized by the amplitude when the reproduction laser power is 0.5 mW. The curve indicated by A in the figure is the measurement result of the magneto-optical disk of the present embodiment, and is the external magnetic field H applied during reproduction
r was set to 650 (Oe). The curve indicated by B in the figure is the measurement result of a conventional magneto-optical disk manufactured for comparison.

A conventional magneto-optical disk has a substrate similar to the above with AlN 80 nm / DyFeCo 20 nm / AlN.
25 nm / AlNi 30 nm are laminated in this order, and AlN
The overcoat layer similar to the above is provided on i. That is, the conventional magneto-optical disk has only one layer of DyFeCo magnetic layer which is a rare earth transition metal alloy, both sides of which are transparent dielectric layers, sandwiched by AlN which is a protective layer, and finally a reflection film. This is a structure in which AlNi is provided.

This structure is called a reflection film structure, and is a typical structure of a 3.5-inch size single-plate type magneto-optical disk which is already on the market. Further, as is well known, the DyFeCo magnetic layer in the conventional magneto-optical disk has perpendicular magnetization from room temperature to high temperature. It should be noted that no external magnetic field is applied when the reproducing operation is performed on the conventional magneto-optical disk.

The broken line shown in the figure is 0.5 mW from the origin.
Is a line connecting the amplitude standard values in the above equation, and represents a proportional straight line showing the relationship between the signal amplitude and the reproduction laser power in the reproduction of the magneto-optical signal represented by the following equation.

Reproduction signal amplitude ∝ Medium reflected light quantity × polar Kerr rotation angle In this equation, the medium reflected light quantity increases in proportion to the reproduction laser power, and therefore can be replaced with the reproduction laser power.

In the figure, the measurement result curve B of the conventional magneto-optical disk is below the proportional straight line for the following reason. That is, when the reproducing laser power is increased, the amount of light reflected from the medium increases accordingly, but the temperature of the recording medium rises. The magnetization of a magnetic material generally has the property of decreasing as the temperature rises and becoming zero at the Curie temperature. Therefore, in the conventional magneto-optical disk, the polar Kerr rotation angle becomes smaller as the temperature rises, so that the measurement result curve B is located below the proportional straight line.

On the other hand, in the measurement result curve A of the magneto-optical disk of the present embodiment, the signal amplitude sharply increases as the reproducing laser power increases, and is above the proportional straight line, and is larger than the increase in the reproducing laser power. It can be seen that an increase in amplitude is obtained. This result indicates that when the temperature is low, the information written in the recording layer 4 is masked by the effect of the external magnetic field Hr and the reading layer 3, so that the reading is not performed and the transition metal sublattice magnetization of the reading layer 3 increases as the temperature rises. This reflects the characteristic of the magnetic double layer at the time of reproducing under the appropriate external magnetic field Hr that information is read by following the transition metal sublattice magnetization of the recording layer 4, and its operation is reflected. It is a proof.

Next, the result of examining the reproduced signal quality when the recording bit is made smaller will be described. Here, reproduction of smaller recording bits means improvement of recording density.

The graph of FIG. 11 shows the relationship between the recording bit length and the reproduction signal quality (C / N). In this experiment, the linear velocity of the disk was set to 5 m / se as in the previous experiment.
Set to c, record at different recording frequencies, and
N was measured. Other than that, the optical pickup and the recording method are the same as in the previous experiment. The curve indicated by A in the figure is
According to the measurement results of the magneto-optical disk of this example, the reproducing laser power was 3.0 mW and the external magnetic field Hr applied during reproduction was 650 (Oe). The curve B in the figure shows the measurement result of a conventional magneto-optical disk similar to that used in the previous experiment. The reproduction laser power was 1 mW and no magnetic field was applied during reproduction.

In the case of a long bit whose recording bit length is 0.7 μm or more, there is almost no difference in C / N between the two,
When the thickness is 0.7 μm or less, the difference between the measurement result curve A of the magneto-optical disk of this embodiment and the measurement result curve B of the conventional magneto-optical disk appears remarkably.

In the conventional magneto-optical disk, the C / N is low when the recording bit length is 0.7 μm or less.
This is because as the bit length becomes smaller, the number (area) of bits existing in the irradiation diameter of the light beam increases, making it impossible to identify individual bits.

The cutoff spatial frequency is one of the indices representing the optical resolution of the optical pickup.
It is determined by the wavelength of the laser as the light source and the numerical aperture of the objective lens. Using the laser wavelength (780 nm) of the optical pickup used in this experiment and the numerical aperture value (0.55) of the objective lens, the cutoff frequency was calculated and converted to the recording bit length, which was 780 nm / ( 2 * 0.55) /2=0.355 μm. In other words, the optical resolution limit of the optical pickup used in this experiment is 0.355 μm in bit length. Reflecting this, the measurement result of the conventional magneto-optical disk described above shows that the C / N at 0.35 μm is almost zero.

On the other hand, in the magneto-optical disk of this embodiment, the C / N slightly decreased as the bit length decreased, but the C / N was large even in the bit shorter than the optical resolution of 0.355 μm. The value has been obtained.

From the above results, it was confirmed that by using the magneto-optical disk of this example, it is possible to reproduce a bit smaller than the optical diffraction limit. As a result, the recording density can be greatly improved as compared with the conventional magneto-optical disk.

Next, in addition to the effect of improving the recording density confirmed in the above experiment, another important effect, that is, the result of an experiment for reducing the amount of crosstalk will be described.

Generally, in a magneto-optical disk, for example, if the land specification is used, the width of the land is set as wide as possible and a guide track is formed in which the width of the groove is narrowed, and only the land portion is used for recording and reproduction. Therefore, for example, the crosstalk in a land specification disc means that when an arbitrary land is reproduced, it leaks from the bits written in the lands on both sides. The reverse is true for a disc with a groove specification.

For example, the IS10089 standard (ISO 5.2
5 "rewritable optical disc), the crosstalk amount with respect to the shortest recording bit (0.765 μm) is -26 dB or less in a guide track with a pitch of 1.6 μm. In the experiment, the magneto-optical disk using the above-mentioned glass substrate having the same land width and groove width of 0.8 μm was used, and the amount of crosstalk from both adjacent grooves during reproduction of the land portion was measured.

The graph of FIG. 12 shows the results of measurement of this experiment for the magneto-optical disk of this embodiment and the conventional magneto-optical disk. In this graph, the horizontal axis represents the reproduction laser power and the vertical axis represents the crosstalk amount. In the figure, A is the measurement result of the magneto-optical disk of this example,
The laser power during reproduction was 3.0 mW, and the external magnetic field Hr applied during reproduction was 650 (Oe). B in the figure is a measurement result of a conventional magneto-optical disk for comparison, as in the experiment conducted previously.

As is clear from this graph, in the conventional magneto-optical disk (B), the crosstalk amount is -15 dB.
On the other hand, in the magneto-optical disk (A) of the present embodiment, a result of clearing −30 dB and −26 dB defined by the ISO standard was obtained.

The reason why such a result is obtained will be described with reference to FIG. FIG. 13 shows the state of the magneto-optical disk as seen from directly above, and the recording bits indicated by circles (broken lines) are recorded in the middle land portion and the groove portions on both sides thereof. The larger circle (solid line) in the drawing is the condensed light beam, and in this case, the servo is applied to the land portion in the middle. In the figure, the land width and groove width are 0.8 μm, the light beam diameter is 1.73 μm (Airy disk diameter: 1.22 * 780 nm / 0.55), and the recording bit diameter is 0.355 μm for convenience of explanation. It is shown by

In the figure, there are seven recording bits below the light beam. With a conventional magneto-optical disk, it is impossible to separate the signals in the light beam. This is the reason why the conventional magneto-optical disk could hardly obtain C / N for a recording bit of 0.35 μm and that the crosstalk from the adjacent track was large.

On the other hand, in the magneto-optical disk of the present embodiment, the transition metal sublattice magnetization of the read layer 3 is the same as that of the recording layer 4 only in the region near the center of the light beam (that is, the region where the temperature is higher than the surroundings). While operating according to the direction of the transition metal sublattice magnetization (ie, the recorded information),
The transition metal sublattice magnetization of the readout layer 3 in the portion other than the vicinity of the center of the light beam is uniformly oriented in the same vertical direction according to the external magnetic field Hr applied during reproduction.

Therefore, as described above, even if there are 7 bits in the light beam, only one bit located at the center of the light beam contributes to the reproduction.
Even with a very small bit of μm, a large C /
N is obtained. Further, in the above-mentioned conventional FAD method, since the region involved in reproduction has a crescent shape, crosstalk from adjacent tracks has been a problem, but in the present embodiment, near the center of the light beam involved in reproduction. As shown in the figure, the area (1) is substantially circular, so that the crosstalk from both adjacent tracks is very small.

As described above, the magneto-optical disk having the magnetic double layer in which the reading layer 3 and the recording layer 4 are laminated is used, and the laser beam is applied while applying the external magnetic field Hr having the intensity in the above range during reproduction. By irradiating, it is possible to realize the improvement of the recording density in the beam traveling direction without requiring the initialization magnet and without increasing the crosstalk even if the track pitch is narrowed. I was confirmed.

The composition of the read layer 3 and the recording layer 4,
The film thickness, the type of alloy, etc. are not limited to those in the above embodiments. According to the gist of the present invention, the thickness of the magnetic double layer,
It is clear that the magnetic characteristics and the external magnetic field Hr applied at the time of reproduction should satisfy the above equations (5), (6), (7) and (8). The rare earth-transition metal alloy is a material system in which the coercive force, the magnitude of the magnetization, and the domain wall energy at the interface change greatly if the ratio of the rare earth to the transition metal is changed. Therefore, if the ratio of GdFeCo or DyFeCo is changed, Along with this, the above-mentioned H ₁ and H ₄ also change. The same applies when the film thickness is changed. Furthermore, the types of rare earth metal transition metal alloys are, for example, TbFeCo, GdTbFe,
GdTbFeCo, GdDyFeCo, NdGdFeC
It is also possible to change to o or the like.

[0073]

As described above, the magneto-optical recording medium according to the invention of claim 1 is a read layer which is a magnetic layer of thickness h ₁ having perpendicular magnetic anisotropy for reading magneto-optically recorded information. And a coercive force of the read layer at room temperature (ta), which has a magnetic double layer consisting of a recording layer which is a magnetic layer having a thickness h _{2 and} having perpendicular magnetic anisotropy for magneto-optical recording of information. Is Hc ₁ (ta), the saturation magnetization is Ms ₁ (ta), the coercive force of the recording layer is Hc ₂ (ta), and the saturation magnetization is Ms ₂ (t).
a), the domain wall energy at the interface is σw (ta) at room temperature or higher and at a predetermined temperature (tm) or higher (t),
The coercive force of the readout layer is Hc ₁ (t) and the saturation magnetization is Ms.
₁ (t), the coercive force of the recording layer is Hc ₂ (t), the saturation magnetization is Ms ₂ (t), and the domain wall energy at the interface is σw (t), and Hw ₁ (ta) = σw (ta) / If 2Ms ₁ (ta) h ₁ Hw ₁ (t) = σw (t) / 2Ms ₁ (t) h ₁ , then Hc ₁ (ta) + Hw ₁ (ta) <− Hc ₁ (t) + Hw
_The configuration satisfies the condition of ₁ (t).

Further, the reproducing method of the magneto-optical recording information according to the invention of claim 2 uses the magneto-optical recording medium according to claim 1 as described above, and Hw ₂ (ta) = σw (ta) / 2Ms _{If 2} (ta) h ₂ Hw ₂ (t) = σw (t) / 2Ms ₂ (t) h ₂ , then Hc ₁ (ta) + Hw ₁ (ta) <Hr <−Hc ₁ (t) + H
w ₁ (t) Hr <Hc ₂ (ta) -Hw ₂ (ta) Hr <Hc ₂ (t) -Hw ₂ (t) While applying an external magnetic field Hr satisfying the relationship, light is irradiated to read information. It is a thing.

Therefore, in the magneto-optical recording medium according to the first aspect, by the method according to the second aspect, the reproduction is performed by adjusting the temperature distribution when the light is irradiated and the external magnetic field Hr to be applied to perform the reading. In the layer, only the high temperature (t) region equal to or higher than the predetermined temperature (tm) corresponding to the vicinity of the center of the light irradiation region can be involved in the reproduction, and crosstalk does not increase even if the track pitch is narrowed. , Because it becomes possible to reproduce the recording bit smaller than before,
The recording density can be significantly improved.

Furthermore, this method does not require a magnetic field for initialization, and therefore, by using such a magneto-optical recording medium, there is an effect that the recording / reproducing apparatus and the like can be downsized.

[Brief description of drawings]

FIG. 1 is a schematic diagram for explaining a schematic configuration and a reproducing operation of a magneto-optical disk according to an embodiment of the present invention.

FIG. 2 is a graph showing temperature dependence of coercive force and a Kerr hysteresis loop at a typical temperature when the read layer used in the magneto-optical disk shown in FIG. 1 is a single layer.

FIG. 3 is a cross-sectional view of a sample manufactured to obtain the measurement result shown in the graph of FIG.

FIG. 4 is a graph showing temperature dependence of coercive force and Kerr hysteresis loop at a typical temperature when the recording layer used in the magneto-optical disk shown in FIG. 1 is a single layer.

5 is a graph showing a Kerr hysteresis loop at room temperature as seen from the read layer side of the magnetic double layer in the magneto-optical disk shown in FIG.

6 is a graph showing a Kerr hysteresis loop at 120 ° C. seen from the read layer side of the magnetic double layer in the magneto-optical disk shown in FIG. 1.

7 is a cross-sectional view of a sample manufactured to measure the Kerr hysteresis loop of FIGS. 5 and 6. FIG.

8 is a graph showing a Kerr hysteresis loop at room temperature viewed from the recording layer side of the magnetic double layer in the magneto-optical disc shown in FIG.

9 is a graph showing a Kerr hysteresis loop at 120 ° C. viewed from the recording layer side of the magnetic double layer in the magneto-optical disk shown in FIG. 1.

FIG. 10 is a graph showing the relationship between the reproduction laser power and the reproduction signal amplitude in the magneto-optical disk of this example and the conventional magneto-optical disk.

FIG. 11 is a graph showing the relationship between the recording bit length and the reproduction signal quality in the magneto-optical disk of this example and the conventional magneto-optical disk.

FIG. 12 is a graph showing the amount of crosstalk in the magneto-optical disk of the present example and the conventional magneto-optical disk.

FIG. 13 is a schematic diagram showing a state during a reproducing operation of the magneto-optical disk in the present embodiment.

[Explanation of symbols]

 3 reading layer 4 recording layer 7 reproducing light beam 9 magnet 11 magnetic layer 14 reading layer 15 recording layer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Akira Takahashi 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Within Sharp Corporation (72) Kenji Ota 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Within the corporation

Claims (2)

[Claims] _1. A reading layer, which is a magnetic layer having a thickness h _{1 and} having perpendicular magnetic anisotropy for reading magneto-optically recorded information, and a perpendicular magnetic anisotropy for magneto-optically recording information. The recording layer has a magnetic double layer having a thickness of h _{2 and is} a magnetic layer, and the coercive force of the readout layer at room temperature (ta) is Hc ₁ (ta) and the saturation magnetization is Ms ₁ (ta). The coercive force of the layer is Hc ₂ (ta) and the saturation magnetization is Ms ₂ (t).
a), the domain wall energy at the interface is σw (ta), the coercive force of the readout layer is Hc ₁ (t), and the saturation magnetization is Ms at a temperature (t) equal to or higher than room temperature and a predetermined temperature (tm).
₁ (t), the coercive force of the recording layer is Hc ₂ (t), the saturation magnetization is Ms ₂ (t), and the domain wall energy at the interface is σw (t), where Hw ₁ (ta) = σw (ta) / If 2Ms ₁ (ta) h ₁ Hw ₁ (t) = σw (t) / 2Ms ₁ (t) h ₁ , then Hc ₁ (ta) + Hw ₁ (ta) <− Hc ₁ (t) + Hw
_A magneto-optical recording medium which satisfies the condition of ₁ (t). 2. The magneto-optical recording medium according to claim 1, wherein Hw ₂ (ta) = σw (ta) / 2Ms ₂ (ta) h ₂ Hw ₂ (t) = σw (t) / 2Ms ₂ (t ) When _{h 2, Hc 1 (ta)} + Hw 1 (ta) <Hr <-Hc 1 (t) + H
w ₁ (t) Hr <Hc ₂ (ta) -Hw ₂ (ta) Hr <Hc ₂ (t) -Hw ₂ (t) While applying an external magnetic field Hr satisfying the relationship, light is irradiated to read information. A method for reproducing magneto-optical recording information, which is characterized in that