JPH08249737A - Optical recording medium and reproducing method - Google Patents

Abstract

PURPOSE: To attain satisfactory reproduction output even when recording bits are made fine by making the heat conductivity of a thermal diffusion layer formed on a magnetic layer optionally with a dielectric layer in-between higher than that of the magnetic layer and the heat conductivity of the dielectric layer lower than that of the magnetic layer. CONSTITUTION: A dielectric film 12 is formed as a transparent protective film or an interference film on a substrate 11, perpendicularly magnetized films are formed as magnetic films 13, 14, 15 on the film 12 and a thermal diffusion layer 16 is disposed on the film 15. The film 12 has lower heat conductivity than the films 13-15 and the layer 16 has higher heat conductivity than the films 13-15. The film 13 is made of a perpendicularly magnetized film having relatively low coercive force of magnetic domain walls and high mobility of magnetic domain walls as compared with the film 15 and the film 14 has a lower Curie temp. than the films 13, 15. By this structure, heat energy absorbed in a magnetic layer consisting of the films 13-15 is rapidly radiated from the film 15 through the layer 16 and recording density is increased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical recording medium for recording / reproducing information with a laser beam by utilizing a magneto-optical effect, and more specifically, an optical recording medium capable of increasing the density of the medium. And a method of reproducing recorded information.

[0002]

2. Description of the Related Art As a rewritable high density recording method,
Using the thermal energy of the semiconductor laser, write the magnetic domain in the magnetic thin film, record the information, use the magneto-optical effect,
Attention has been paid to a magneto-optical recording medium for reading this information.
Further, 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 greatly depends on the laser wavelength of the reproducing optical system and the numerical aperture of the objective lens. That is, since the diameter of the beam waist is determined when the laser wavelength λ of the reproduction optical system and the numerical aperture N _{A of the} objective lens are determined, the spatial frequency at the time of signal reproduction becomes about 2N _A / λ as a detectable limit. 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 N _A 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 Unexamined Patent Publication No. 3-93058, signal recording is performed on a recording holding layer of a multilayer film having a magnetically coupled renormalization layer and a recording holding layer, and magnetization of a reproducing layer is performed. A signal reproducing method has been proposed in which the signals recorded in the recording holding layer are read while being transferred to the temperature rising region of the reproducing layer after the signals are aligned and aligned with each other and heated.

According to this method, the area heated by the laser to reach the transfer temperature and the signal is detected with respect to the laser spot diameter for reproduction can be limited to a smaller area. It is possible to reduce inter-interference and reproduce a signal having a period less than the diffraction limit of light.

[0005]

However, in the magneto-optical reproducing method described in Japanese Patent Application Laid-Open No. 3-93058, the signal detection area that is effectively used becomes smaller than the laser spot diameter for reproduction, and therefore the reproduced signal is reduced. The amplitude is significantly reduced,
It has a drawback that sufficient reproduction and output cannot be obtained. Further, 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.

The present invention has been made to solve the above-mentioned problems of the prior art, solves the problem of the restriction of the recording density defined by the conditions at the time of reproduction, and aims to miniaturize the recording information bits. Even if the
/ N).

[0007]

The present invention is an optical recording medium in which at least a first magnetic layer, a second magnetic layer and a third magnetic layer are sequentially laminated on a substrate, and the first magnetic layer is provided. Comprises a perpendicularly magnetized film having a relatively small domain wall coercive force and a large domain wall mobility as compared with the third magnetic layer, and the second magnetic layer has a lower Curie temperature than the first and third magnetic layers. In the optical recording medium, which is composed of magnetic layers, and the second and third magnetic layers are perpendicularly magnetized films, in the optical recording medium, a direct or dielectric layer is formed on the side opposite to the side in contact with the second magnetic layer on the third magnetic layer. A thermal diffusion layer is formed via the dielectric layer, the thermal conductivity of the dielectric layer is smaller than that of the magnetic layers 1 to 3, and the thermal diffusion layer is larger than the magnetic layers of 1 to 3; It is a characteristic optical recording medium.

In the above optical recording medium, the thermal conductivity of the thermal diffusion layer is 50 W / m · K or more, the specific heat is 0.2 J / (g · K) or more, and the layer thickness is 100 n.
It is a more preferable embodiment that it is m or less.

The present invention is also a method of reproducing recorded information from the above-mentioned optical recording medium, wherein the light beam is moved relative to the optical recording medium from the side of the first magnetic layer. The recorded information is reproduced by moving the domain wall formed in the first magnetic layer by irradiating and making a temperature distribution having at least a temperature region higher than the Curie temperature of the second magnetic layer. And the reproduction method. In the above reproducing method, it is a more preferable aspect that pulse-modulated light is used as the reproducing light and that the modulation cycle of the pulse-modulating reproducing light is set shorter than the cycle of the information unit of the reproducing signal. Further, it is a more preferable embodiment that the Curie temperature of the second magnetic layer is 100 ° C. or lower.

According to the present invention, as shown in FIG. 1, a dielectric film 12 as a light-transmitting protective film or an interference film is formed on a substrate 11, and the dielectric film 12 is magnetized at room temperature. First to third magnetic films 13, 14 and 1 that are magnetically coupled
Each perpendicular magnetization film 5 is formed.

Here, the Curie temperature of each magnetic film is set to T_c1,
T_c2, T_c3And when _C2Is high and
T_C1, T_c3> T_c2And Where the coercive force of the first magnetic layer
H_C1Is the Curie temperature T of the second magnetic layer_C2Small enough in the vicinity
Coercive force H of the third magnetic layer_C3From room temperature to T_C2taller than
Magneto-optical at a desired temperature and well above the required magnetic field
Use as a recording medium.

Further, the first magnetic layer is a perpendicular magnetization film having a large domain wall mobility. Further, the heat diffusion layer 16 is provided so that the heat energy absorbed in the magnetic layer is quickly radiated.

The thermal diffusion layer has a thermal conductivity of 50 W / m.
A sufficient heat diffusion effect can be obtained by using a material having a specific heat of 0.2 J / g · K or more and K or more.

In the present invention, as a material for forming the thermal diffusion layer to satisfy the above, it is preferable to use Al, Ag, Au or a material obtained by adding Ti, Cr or the like to Al. It is also effective to stack a material exhibiting large heat conduction through a dielectric film having a thickness of 10 to 20 nm.

The thickness of the thermal diffusion layer 16 generally requires that the recording power is 3 to 4 times the reproducing power, and that the recording medium input power is 15 mW at maximum, and it is preferably 100 nm or less. If the thermal diffusion layer is made thicker than this, it is necessary to increase the reproduction power accordingly.

FIG. 2 shows a reproduced signal waveform in the optical recording / reproducing method of the present invention.

As described above, in order to make the temperature profile at the time of reproduction on the medium steep, reproduction may be performed with a pulse laser beam having a narrow width at the frequency interval corresponding to the minimum bit length. At the time of reproduction, the laser beam is driven according to the reproduction laser drive signal (FIG. 2-B) to irradiate the optical recording medium.

The reflected light of the laser light thus radiated is
After passing through the lens, it is detected by a detector such as a PIN photodiode. This signal is amplified by the detection circuit, and the detection signal (FIG. 2-C) is obtained. Since this detection signal has a pulse shape synchronized with the pulse of the reproduction laser light, it is used as a reproduction signal (FIG. 2-D) through a low pass filter.

Next, FIG. 3A is a schematic sectional view of the optical recording medium of the present invention.

The magnetic layer of this medium is the first magnetic layer 13,
The second magnetic layer 14 and the third magnetic layer 15 are sequentially stacked. The arrow in each layer represents the direction of atomic spin. At the boundary of the regions where the spin directions are opposite to each other,
The domain wall 19 is formed. The recording signal of this recording layer is also shown as a graph on the lower side.

FIG. 3B is a graph showing the temperature distribution formed on the optical recording medium of the present invention. This temperature distribution is
It may be induced on the medium by the light beam itself for reproduction, but it is desirable to use another heating means together to raise the temperature from before the spot of the reproduction light beam to increase the temperature. Forms a temperature distribution with a temperature peak behind. Here, at the position X _S ,
The medium temperature is a temperature T _S near the Curie temperature of the second magnetic layer.
It has become.

FIG. 3C is a graph showing the distribution of the domain wall energy density σ ₁ of the first magnetic layer corresponding to the temperature distribution of FIG. 3B. Thus, the domain wall energy density σ ₁
When there is a gradient of, the force F ₁ obtained from the following formula acts on the domain wall of each layer existing at the position X.

[0023]

[Equation 1] This force F ₁ acts so as to move the magnetic domain wall to the lower domain wall energy. Since the first magnetic layer has a small domain wall co-magnetism and a large domain wall mobility, by itself, the domain wall is easily moved by this force F ₁ .

However, position X_S To the front (right in the figure)
Side), the medium temperature is still T _S Lower, domain wall
Because it is exchange-coupled with the third magnetic layer having a large coercive force,
At the position corresponding to the position of the domain wall in the third magnetic layer,
The domain wall in the magnetic layer 1 is also fixed.

In the present invention, as shown in FIG. 3A, when the domain wall 19 is at the position X _S of the medium, the medium temperature rises to a temperature T _S near the Curie temperature of the second magnetic layer,
The exchange coupling between the first magnetic layer and the third magnetic layer is broken. As a result, the domain wall 19 in the first magnetic layer is indicated by the arrow 20.
As indicated by, the temperature instantaneously moves to the region where the temperature is high and the domain wall energy density is small.

Below the spot of the reproducing light beam, the domain wall 1 is formed.
When 9 passes, all atomic spins of the first magnetic layer in the spot are aligned in one direction. Then, each time the domain wall 19 comes to the position X _S as the medium moves, the domain wall 19 is moved below the spot.
Move momentarily and the directions of atomic spins in the spot are reversed and aligned in one direction. As a result, as shown in FIG. 3 (a), the reproduction signal amplitude does not depend on the recorded domain wall spacing (that is, the recording mark length), but becomes a constant and maximum amplitude, which is caused by the optical diffraction limit. It is completely free from problems such as waveform interference. Therefore, even if the recording density is improved and the magnetic domains are made finer, a sufficient reproduction output can be obtained and the recording density can be increased.

Further, when the power of the laser beam during reproduction is constant, the temperature profile spreads due to thermal diffusion in the medium, but in order to make this temperature profile steep,
For example, the frequency interval corresponding to the minimum bit length (10 to 1
Reproduction may be performed with a pulsed laser beam having a narrow width (5 MHz). Further, a thermal diffusion layer having good thermal conductivity may be provided in the third magnetic layer so that the thermal energy absorbed in the magnetic layer can be quickly released. As the transparent substrate, for example, polycarbonate, glass or the like can be used. Examples of the dielectric layer include Si ₃ N ₄ , AlN, SiO ₂ , and S.
Transparent dielectric materials such as iO, ZnS, MgF ₂ can be used. The same thing can be used for the dielectric layer that is finally formed again as a protective film. Each of these layers can be formed by, for example, continuous sputtering using a magnetron sputtering apparatus, continuous filling vapor deposition, or the like. In particular, each magnetic layer is formed continuously without breaking the vacuum,
They are exchange-coupled to each other. Further, a protective coat made of a polymer resin may be applied. Alternatively, the substrates after the film formation may be attached.

In the above-mentioned medium, it is considered that each magnetic layer is made of various magnetic materials. For example, Pr, Nd, Sm, Gd, Tb, Dy, Ho, etc. are added to the magnetic layer. Mn, Cu, Ti, Al, Si, P
You may add elements, such as t and In.

In the case of a heavy rare earth-iron group amorphous alloy, the saturation magnetization can be controlled by the composition ratio of the rare earth element and the iron group element. The Curie temperature can also be controlled by the composition ratio. Further, although it is possible to control it by the Curie temperature composition ratio, in order to control it independently of the saturation magnetization, a part of Fe as the iron group element is C
A method of controlling the substitution amount using a material substituted with o can be more preferably used. That is, Fe, 1 wt% is Co
By substituting with, it is expected that the Curie temperature will rise by about 6 ° C. Therefore, using this relationship, the addition amount of Co is adjusted so as to obtain the desired Curie temperature. Also, Cr, Ti
Conversely, the Curie temperature can be lowered by adding a small amount of a non-magnetic element such as. Alternatively, the Curie temperature can be controlled by using two or more kinds of rare earth elements and adjusting the composition ratio thereof.

Besides, materials such as garnet, a platinum group-iron group periodic structure film, or a platinum group-iron group alloy can also be used.

The first magnetic layer is, for example, GdC.
It is desirable to employ a rare earth-iron group amorphous alloy having a small perpendicular magnetic anisotropy such as o, GdFeCo, GdFe, NdGdFeCo, or a bubble memory material such as garnet.

As the third magnetic layer, for example, TbFeC
o, DyFeCo, TbDyFeCo, and other rare earth-iron group amorphous alloys, Pt / Co, Pd / Co, and other platinum group-iron group periodic structure films, which have large perpendicular magnetic anisotropy and maintain a stable magnetization state. What you can do is desirable.

When the medium temperature reaches a temperature T _S near the Curie temperature of the second magnetic layer, the magnetic coupling between the recording layer (third magnetic layer) and the moving layer (first magnetic layer) is broken, The domain wall of the first magnetic layer having a small domain wall coercive force and a large domain wall mobility easily moves alone.

When the Curie temperature of the second magnetic layer is set to 100 ° C. or lower, even if the reproducing power is relatively small,
The amount of movement of the domain wall in the magnetic layer is large. On the other hand, when the Curie temperature of the second magnetic layer is set to 140 ° C. or higher, the domain wall does not move unless the reproducing power is set high, and the amount of domain wall movement is large. In the case of a heavy rare earth-iron group amorphous alloy, the Curie temperature of the second magnetic layer can be controlled by the composition ratio. Also,
It can be controlled by the Curie temperature composition ratio, but in order to control it independently of the saturation magnetization, a method in which a material in which a part of Fe is replaced with Co is used as the iron group element and the replacement amount is controlled is known. It can be used more preferably.

[0035]

EXAMPLE A substrate is made of a light transmissive resin substrate such as a glass plate or polycarbonate, and a track groove for tracking servo is formed on one surface of, for example, 1.6 μm.
It is formed with an m pitch, on which, for example, Si ₃ N _{4 is formed.}
A dielectric film made of a film, first to third magnetic layers, and a protective film thereon are successively formed by, for example, continuous sputtering or vapor deposition using a magnetron sputtering apparatus.

As the first magnetic layer, for example, GdCo,
It can be composed of GdFeCo and GdFe. The second magnetic layer is, for example, DyFe, DyFeCo,
The third magnetic layer can be made of TbFe, TbFeCo, D, or the like.
It can be formed of yFeCo or the like. Example 1 A dielectric film made of Si ₃ N ₄ on a polycarbonate substrate having a track groove with a track pitch of 1.6 μm, and G
A first magnetic layer made of a dFeCo film, a second magnetic layer made of a TbFeCo film, and a third magnetic layer made of a DyFeCo film.
The magnetic layer, Al as a heat diffusion layer, and a protective film made of a Si ₃ N ₄ film were sequentially formed by continuous sputtering using a magnetron sputtering apparatus to prepare an optical disk. The thickness and magnetic characteristics of each layer in this case are shown below.

[0037]

[Table 1] FeCo-rich means a FeCo sub-lattice magnetization dominant film at room temperature, and Tb-rich means a Tb sub-lattice magnetization dominant film at room temperature.

FIG. 4 shows the measurement results of the recording frequency dependency of C / N using the reproducing pulsed light of the present invention using the magneto-optical recording medium according to this example.

The solid line in the figure indicates the numerical aperture N.V. of the objective lens. A
= 0.50, laser wavelength 780 nm, linear velocity 8 m / se
c, the recording power is 7 mW, the recording external magnetic field is 500 (Oe), there is no externally applied magnetic field during reproduction, and the reproduction power is 4 mW. The dotted line in the figure shows the case where the reproduction power is 1.5 mW. When the reproducing power was set to 1.5 mW, the same result as the frequency dependency of C / N when the magnetic layer was a TbFeCo single layer was shown. It is considered that the recording power does not change at such a reproducing power because the heating temperature does not reach the Curie temperature of the second magnetic layer. On the other hand, when the reproducing power is 4 mW, the C / N is greatly increased when the reproducing power is 1.5 mW and the magnetic domain length is 0.6 μm or less.
A signal component was obtained although the C / N was low even at m or less. It was also confirmed that the C / N was reproduced when the place reproduced with the reproduction power of 4 mW was reproduced again.

Next, among the film thicknesses and compositions of the first magnetic layer, the second magnetic layer, the third magnetic layer, and the thermal diffusion layer, only the Curie temperature of the second magnetic layer, A sample of the optical recording medium which was changed was prepared, and the relationship between the reproducing power of the medium and the amount of domain wall movement was measured.

[0041]

[Table 2]

[0042]

[Table 3] The following method was used to measure the amount of domain wall movement when the reproducing power was changed to the medium.

A reference synchronizing signal is taken out from the medium, a signal is recorded for one pulse based on this synchronizing signal, and a change in the position of the reproducing pulse when the reproducing power is changed is measured by a time interval analyzer. It is a thing. The sync signal is created from the sector mark. The measurement accuracy increases as the signal recording position is closer to this synchronizing signal, so recording was performed at the 14T portion of the sector mark.

The 14T portion of the 3.5 inch 1st ISO medium has a sufficient length of 7 μm or more at the innermost circumference.

FIG. 5 shows the relationship among the sector mark, recording pulse, reproducing pulse and time interval.

FIG. 6 shows the relationship between the reproducing power and the domain wall movement amount.

When the Curie temperature of the second magnetic layer is 100 ° C. or lower, the domain wall moves from the reproducing power of 2 to 3 mW.
When the Curie temperature is 140 ° C and 180 ° C, the reproducing power is 5
Unless it is set to mW, the domain wall does not start to move, and the amount of movement is small.

Next, by using the medium in which the Curie temperature of the second magnetic layer is 80 ° C. and the medium in which the Curie temperature of the layer is 140 ° C., C / N is obtained by using the reproducing pulsed light in the present invention. FIG. 7 shows the measurement result of the recording frequency dependence of the.

The result of the measurement is the objective lens N. A = 0.5,
Laser wavelength 780 nm, linear velocity 7 mW, recording external magnetic field 50
0 (Oe), no external magnetic field was applied during reproduction, and reproduction power was 2 mW.

In the medium (1), the recording magnetic domain changed even when the reproducing power was 2 mW, the magnetic domain length was 0.6 μm or less, and C
/ N significantly increased, and a signal component was obtained although the C / N was low even at 0.3 μm or less. On the other hand, in the medium, the increase in C / N below 0.6 μm is small,
A signal component could not be obtained below μm. It was also confirmed that the C / N was reproduced when the place reproduced at 2 mW was reproduced again.

[0051]

As described above, according to the present invention,
Since it is possible to obtain a sufficient reproduction output, the recorded information magnetic domain can be sufficiently reduced. By this fact, the recording density can be improved, and the recording magnetic domains can be formed not only on the runt portion but also on the groove portion. Therefore, the recording magnetic domains can be formed on both sides. Further, the recording density can be further improved.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a configuration diagram of an optical recording medium used in the present invention.

FIG. 2 is a diagram showing various signal waveforms in the optical recording / reproducing apparatus.

FIG. 3 is an example of a schematic cross-sectional view of an optical recording medium.

FIG. 4 is a diagram showing the recording frequency dependence of C / N in the experiment of Example 1.

FIG. 5 shows the relationship between sector marks, recording pulses, etc. in the experiment of Example 1.

FIG. 6 shows the relationship between the reproduction power and the domain wall movement amount in the experiment of Example 1.

FIG. 7 is a diagram showing recording frequency dependence in the experiment of Example 1.

[Explanation of symbols]

 Reference Signs List 11 substrate 12 dielectric film 13 first magnetic layer 14 second magnetic layer 15 third magnetic layer 16 thermal diffusion layer 17 dielectric film 18 surface protective layer 19 domain wall 20 light beam spot for reproduction 21 domain moving direction 22 Medium movement direction

Claims (7)

[Claims] 1. At least a first, a second, and a third on a substrate.
The magnetic recording medium in which the magnetic layers are sequentially laminated, the first magnetic layer has a relatively small domain wall coercive force and a large domain wall mobility as compared with the third magnetic layer. The second magnetic layer is a magnetic layer having a Curie temperature lower than that of the first and third magnetic layers, and the second and third magnetic layers are perpendicular magnetization films. A thermal diffusion layer is formed on the magnetic layer on the side opposite to the side in contact with the second magnetic layer directly or via a dielectric layer, and the dielectric layer has a smaller thermal conductivity than the first to third magnetic layers. An optical recording medium, wherein the heat diffusion layer has a thermal conductivity higher than that of the magnetic layers 1 to 3 above. 2. The thermal conductivity of the heat diffusion layer according to claim 1 is 50 W / m · K or more, and the specific heat is 0.2 J / (g · g).
K) or above, which is an optical recording medium. 3. The thickness of the heat diffusion layer according to claim 1 is 10
An optical recording medium having a thickness of 0 nm or less. 4. An optical recording medium according to claim 1, wherein the Curie temperature of the second magnetic layer is 100 ° C. or lower. 5. A method for reproducing recorded information from the optical recording medium according to claim 1, wherein the first magnetic layer side is provided while moving a light beam relative to the optical recording medium. From the second magnetic layer to obtain a temperature distribution having a temperature region higher than the Curie temperature of the second magnetic layer to move the domain wall formed in the first magnetic layer and reproduce the recorded information. And how to play. 6. A reproducing method, wherein pulse-modulated light is used as the reproducing light according to claim 5. 7. The reproducing method according to claim 2, wherein the modulation cycle of the pulse-modulated reproduction light is set shorter than the cycle of the information unit of the reproduction signal.