JPH1064132A - Method for reproducing magneto-optical recording medium in which light intensity modulated overwriting is possible, and reproducing device therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to obtain a high C/N value, to increase the speed of rewriting information by combining magnetic super-high resolution reproduction and to surely reproduce the information recorded with a high density by impressing a magnetic field on a recording medium at the time of reproducing the information recorded on this medium. SOLUTION: By impressing the magnetic field Hr on the recording medium at the time of reproduction, the range where the intensity of a reproducing beam may be set, is made to the range of prescribed mW. This range is wider than the range in the case where the magnetic field is not impressed on the medium at the time of reproduction. Further, the setting of the intensity of the reproducing beam at a large value is made possible and, therefore, the effect of the magnetic super-high resolution is sufficiently obtd. and the high C/N value is obtd. Namely, the temp. at which the magnetization of a memory layer inverts (the destruction of the information) is raised by reproducing the information while impressing the magnetic field Hr on the medium. The combination of the overwriting and the magnetic super-high resolution is thus made practicable.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reproducing method for a magneto-optical recording medium capable of optical intensity modulation overwriting.

[0002]

2. Description of the Related Art Information can be recorded at high density.
An optical recording medium capable of high-speed reproduction has attracted attention as a recording medium for audio and images, and further for a computer. Read-only CDs have become widespread for audio use, and in recent years have also rapidly spread for computers. Also, a write-once type in which information can be recorded only once and a rewritable type (re-writable type) in which information once recorded can be rewritten many times have become widespread. In particular, one type of rewritable type magneto-optical recording medium is 100
It is possible to rewrite information more than ten thousand times, and it is widely used mainly as a computer recording medium.

Conventionally, in order to rewrite information once recorded on a magneto-optical recording medium with new information, it is necessary in principle to erase the recorded information and then record new information. Was. That is, as in the case of rewriting information on a magnetic recording medium, overwriting in which information once recorded is not directly erased but is directly rewritten with new information (overwriting)
Was impossible.

[0004] Therefore, it is a disadvantage that the time required for rewriting information is lengthened by the amount required for erasing the information, that is, the information rewriting speed is low. In order to overcome this disadvantage, a light intensity modulation overwrite recording method capable of overwriting only by modulating the recording beam intensity, and an apparatus and a medium used therein have been invented. The invention has been filed in several countries,
Of these, patents have already been registered in the United States (Japanese Unexamined Patent Publication
75948 = DE3,619,618A1 = USP 5,239,524). Hereinafter, this invention will be referred to as "basic invention". Hereinafter, the present invention will be briefly described.

The overwritable magneto-optical recording medium used in this magneto-optical recording / reproducing method has a perpendicular magnetic anisotropy (perpendicular magnetic layer) as a layer for recording information.
orlayers). This magnetic layer is made of, for example, amorphous TbFe, TbFeCo, GdFe, GdFeCo, DyF
e, a heavy rare earth metal-transition metal amorphous alloy such as DyFeCo, etc., in which the magnetization of the heavy rare earth metal (RE sublattice magnetization) and the magnetization of the transition metal (TM sublattice magnetization) They are facing each other. The difference between these two sublattice magnetizations indicates the magnitude and direction of the magnetization as the magnetic layer.

As the magnetic layer, a layer (hereinafter, referred to as a memory layer or an M layer) basically serving as a recording and reproducing layer composed of a magnetic thin film capable of perpendicular magnetization, and a recording auxiliary layer composed of a magnetic thin film capable of perpendicular magnetization are also provided. (Hereinafter referred to as recording layer or W layer), and both layers are exchange-coupled (ex
This is an overwritable multilayer magneto-optical recording medium that is change-coupled and can keep only the magnetization of the W layer in a predetermined direction at room temperature without changing the magnetization direction of the M layer.

The W layer has a lower coercive force Hc and a higher Curie point Tc at room temperature than the M layer. Then, the information has a mark having a magnetization in the direction perpendicular to the substrate (referred to as “A direction”) in the M layer (and possibly also the W layer) and a magnetization in the opposite direction (referred to as “reverse A direction”). Record by mark. In this medium, the direction of magnetization of the W layer can be aligned in one direction by magnetic field means (for example, initialization magnetic field Hini.). In addition, at that time, the magnetization direction of the M layer does not reverse, and the magnetization direction of the W layer once aligned does not reverse even when subjected to the exchange coupling force from the M layer. The direction of magnetization of the M layer does not reverse even when subjected to exchange coupling force from the W layer aligned in one direction.

In the recording method according to the basic invention, the recording medium is arranged such that only the direction of magnetization of the W layer is aligned in one direction by magnetic field means before recording. Then, the medium is irradiated with a laser beam pulse-modulated according to the binary information. The intensity of the laser beam is controlled to two values, a high level PH and a low level PL, which correspond to the high and low levels of the pulse. This low level is higher than the reproduction level PR for irradiating the medium during reproduction. As is already known, it is common to turn on the laser at a "very low level" even when not recording, for example to access a predetermined recording location on the medium. This very low level,
This level is the same as or approximate to the reproduction level PR.

At the temperature that the medium reaches when the medium is irradiated with a low-level laser beam, the magnetization direction of the W layer does not change, and the magnetization direction of the M layer changes between the M layer and the W layer. Is in the direction of non-existence. This is called a low temperature process, and the temperature region where this process occurs is called a low temperature process temperature TL. On the other hand, at a higher temperature reached by the medium when the medium is irradiated with a high-level laser beam, the direction of magnetization of the W layer follows the direction of the recording magnetic field, and M
The direction of magnetization of the layer is such that no domain wall exists between the M layer and the W layer. This is called a high temperature process, and the temperature region where this process occurs is called a high temperature process temperature TH.

After the laser beam irradiation, the magnetization of the W layer imitated by the magnetic field means in the direction of the recording magnetic field by the high-level laser beam irradiation again imitates the direction of the magnetic field means. Therefore, if the direction of the magnetization of the magnetic field means and the direction of the recording magnetic field are reversed, new recording can be repeatedly recorded (that is, overwritten) on the already recorded M layer. This is the principle of light modulation overwrite magneto-optical recording.

[0011] In a slightly different expression, the contents described above form recording marks by irradiating a high-level laser beam and erase recording marks by irradiating a low-level laser beam, so that new information can be written over old information. It can also be said to overwrite (overwrite). Further, a type in which the initialization of the W layer is realized by providing a magnetic layer called an initialization layer instead of the initialization magnetic field (Japanese Patent Laid-Open No. 63-268103 = USP 4,878,132) was invented. , Practical use has begun.

By the way, recently, opportunities for recording or reproducing a large amount of information such as image information have been increasing. Under such circumstances, there is an increasing demand for recording more information on an optical recording medium or reproducing the information accurately. Various approaches have been taken to meet that demand. First, in a disk, there is a method of increasing the recording density in the radial direction by reducing the track pitch. For this purpose, it is necessary to reduce the recording mark width. Until now, the standard track pitch was 1.6 μm, but recently, attempts have been made to reduce the track pitch to 1.4 μm or 1.2 μm.
m, and also 1.0 μm. Next, in order to increase the recording density in the circumferential direction, it is necessary to reduce the recording mark length. In order to reduce the width and length of the recording mark, the sensitivity of the recording medium is set so that the recording mark is formed only in the high energy area near the center of the recording beam. Can be formed. That is,
When recording, if the composition design of the magnetic layer is performed so that the recording mark is formed only in the high temperature part near the center of the recording beam,
High-density recording is possible.

However, the problem lies in reproduction. Reproduction is performed by optically detecting a mark in the beam spot. It is therefore necessary to always have basically only one mark in the beam. If a plurality of marks exist in the beam, the information will be mixed and cannot be correctly reproduced. Such a state in which information is mixed is called optical crosstalk. That is, it is impossible to retrieve and reproduce only the information represented by the mark in a part of the beam spot.

For example, in the case of an infrared laser, the beam spot diameter can be reduced to only about 1.0 μm even with an ideal optical system. Therefore, in the radial direction, the track pitch is 1.4
If it is smaller than about μm, marks recorded on adjacent tracks will be reproduced simultaneously, and if the mark length or mark interval is considerably less than 1.0 μm, the preceding and succeeding marks will be reproduced simultaneously.

Therefore, it is conceivable to shorten the wavelength of the reproduction beam. It is known that how small a beam can be narrowed is proportional to the wavelength of the beam. By this means, the reproduction beam spot size is made smaller, thereby enabling the reproduction of information recorded at high density. If this is realized, the problem of optical crosstalk can be avoided. However, the wavelength of a semiconductor laser that can be used as a light source of an optical pickup has a limit in terms of its output and stability. For example, semiconductor lasers with a wavelength of 830 nm have been generally used in the past, but despite the fact that the wavelength was shortened by taking a considerable period of time, the wavelength of 680 nm only became mainstream recently. is there. That is, the reduction rate of the beam size due to the shortening of the wavelength is only 18%. Therefore, it is difficult to drastically reduce the beam spot size to 1/2 or 1/3 of the current size by this means.

However, there has been invented a recording / reproducing method and a medium which can accurately reproduce information recorded at a considerably high density even if the reproducing beam spot size is the same as before. This is called super-resolution reproduction and medium. The basic idea of this method is as follows. Although the temperature of the recording medium rises due to the irradiation of the reproduction beam, since the medium is moving, the temperature on the rear side in the reproduction beam spot becomes relatively high due to the heat storage effect. If a part of the beam spot is masked using this characteristic of the temperature distribution and only the unmasked part is reproduced, only the information of a small part in the spot can be reproduced. That is, the reproducing beam spot size is substantially reduced. That is, information recorded at a considerably high density can be accurately reproduced.

Specifically, (1) a type in which a low-temperature portion in a reproduction beam spot becomes an opening (a high-temperature portion becomes a mask); and (2) an opening in a high-temperature portion (a low-temperature portion becomes a mask). There are also two types: (3) a type in which the high-temperature portion is an opening, and a type that realizes a smaller opening by masking a higher-temperature portion in the opening portion. ing. As the principle of mask generation, a method based on a change in transmittance due to a phase change in addition to a method using a change in magnetization direction due to a change in magnetic coupling force has been disclosed.

Here, the principle of utilizing the change in the magnetization direction due to the change in the magnetic coupling force will be briefly described with reference to FIG. FIG. 4B shows a cross section of a main portion of the magneto-optical recording medium and a temperature distribution of a portion irradiated with the reproducing beam. This magneto-optical recording medium has two magnetic layers,
Information is recorded by the direction of magnetization in the perpendicular direction of the recording layer. A mask layer is provided on the recording layer, and the magnetization of the mask layer is in the in-plane direction in a region below a predetermined temperature (low temperature region), but is in a region above a predetermined temperature (high temperature region).
In this case, the direction is the same as the magnetization of the recording layer due to the exchange coupling force with the magnetization of the recording layer. The predetermined temperature is set to be a small area near the center of the temperature distribution due to the irradiation of the reproduction beam. As a result, as shown in FIG. 4A, only the mark in a high-temperature area of a part of the reproduction beam spot is read. Then, even if the mark is within the reproduction beam spot, the mark that is out of the high temperature region is not read.

[0019]

In the light intensity modulation overwrite magneto-optical recording and reproduction, a high level PH and a low level PL which are binary recording beam intensities at the time of recording and a reproduction beam intensity Pr at the time of reproduction are used. Modulate the laser beam intensity to three levels. This means that the modulation level number of the laser beam intensity in the light intensity modulation overwrite magneto-optical recording / reproduction is one more than the modulation level number of the laser beam intensity in the conventional non-overwrite magneto-optical recording / reproduction.

On the other hand, the beam intensity that can be emitted from a general semiconductor laser as a light source naturally has an upper limit. For this reason, the intensity range (power margin) that can be set as each modulation level is narrower in the light intensity modulation overwrite magneto-optical recording / reproduction than in the conventional non-overwrite magneto-optical recording / reproduction. In other words, in the case of the optical intensity modulation overwrite magneto-optical recording / reproduction, the intensity range that can be set as the reproduction level is narrower to the lower side as the low level exists between the high level and the reproduction level.

However, in order to increase the reproduction signal C / N, the higher the reproduction beam intensity, the more advantageous it is. Therefore, there is a problem in that light intensity modulation overwrite magneto-optical reproduction is disadvantageous in this respect. Further, in magnetic super-resolution reproduction, as described above, since the temperature distribution in the reproduction beam spot is used, it is necessary in principle to make this temperature distribution larger than a predetermined size. That is, it is preferable that the reproducing beam intensity is higher than that of the conventional non-overwrite magneto-optical recording and reproducing. However, in light intensity modulated overwrite magneto-optical recording / reproduction, the upper limit of the reproduction beam intensity must be kept low in principle due to the existence of a low level. There is a problem that it is difficult to combine.

The present invention solves the above-mentioned problems, and achieves a high C / C in optical intensity modulation overwrite magneto-optical recording and reproduction.
N value is obtained, and combined with magnetic super-resolution reproduction,
Provided are a reproducing method and a reproducing apparatus for a magneto-optical recording medium capable of overwriting with light intensity modulation, which can rewrite information at a high speed and can reliably reproduce information recorded at high density.

[0023]

According to the present invention, a magnetic field Hr is applied when information recorded on a recording medium is reproduced. By applying a magnetic field at the time of reproduction, it acts in a direction to prevent information from being erased (change in magnetization direction) due to a temperature rise due to reproduction beam irradiation, so that the reproduction beam intensity is lower than when there is no reproduction magnetic field Hr. The level can be set higher.

[0024]

Next, the principle of the present invention will be described briefly. In a magneto-optical recording medium capable of optical intensity modulation overwriting, the recorded information is held in the memory layer as a region having an upward and a downward magnetization perpendicular to the medium surface until it is rewritten with new information. However,
The magnetization of the recording layer is aligned (initialized) in one upward or downward direction after the information is recorded and before the next information is rewritten. Therefore, if the recording layer is adjacent to the memory layer, the memory layer faces upward, or
In any one of the downward magnetization regions, the RE sublattice magnetizations or the TM sublattice magnetizations of the memory layer and the recording layer face in opposite directions. That is, an interface domain wall is formed between these two layers. Then, in that portion, a force acts to eliminate the interface domain wall.

This force has the following effect on the magnetization of the memory layer:
It works so as to reverse the magnetization so that it is directed in the same direction as the magnetization direction of the recording layer. Its size is σw / (2 · M
s1 · t1). However, the coercive force, saturation magnetization, film thickness, and Curie temperature of the memory layer are each Hc
1, Ms1, t1, and Tc1, and the exchange coupling force acting between the memory layer and the recording layer is σw.

The above can be said to be the same even when the recording layer and the memory layer are not adjacent to each other, provided that the magnetization directions of the magnetic layers adjacent to the memory layer are aligned in one direction. When there is no external magnetic field acting on the memory layer, the temperature of the magnetic layer rises due to the irradiation of the reproducing beam, whereby the coercive force Hc1 of the memory layer decreases, and Hc1 <σw / (2 · Ms1 · t1) Then, in the interface domain wall region in the memory layer, magnetization reversal starts to occur in a direction in which the interface domain wall is eliminated. That is, information is erased (destructed).

On the other hand, when an external magnetic field of magnitude Hext is applied from the outside in the same direction as the magnetization direction of the region where the interface domain wall of the memory layer exists, Hc1 <−Hext + σw / (2 · Ms1 At the temperature region t1), magnetization reversal starts to occur in the region where the interface domain wall of the memory layer exists for the first time. That is, it is possible to prevent the erasure of information up to a temperature region where Hc1 becomes smaller.

Considering the actual reproduction, as long as the magnitude of the magnetic field Hr is within the temperature range from Tamb to Tr, and Hc1> −Hr + σw / (2 · Ms1 · t1), the interface domain wall region of the memory layer is required. Does not occur. Therefore, this equation is modified to obtain Hr> −Hc1 + σw / (2 · Ms1 · t1) (1) Next, considering a region of the memory layer where there is no interface magnetic wall, Hr> Hc1 + σw / (2 · Ms1 · t
When the condition (1) is satisfied, the magnetization reversal occurs. Therefore, the condition under which the magnetization reversal does not occur in this region must satisfy Hr <Hc1 + σw / (2 · Ms1 · t1) (2).

Therefore, from the formulas (1) and (2), the range of Hr at which the magnetization reversal does not occur in either region of the memory layer is as follows: -Hc1 + σw / (2 · Ms1 · t1) <Hr <Hc1
+ Σw / (2 · Ms1 · t1) FIG. 3 shows the relationship of this equation. Note that both Hc1 and the term σw / (2 · Ms1 · t1) become 0 when the Curie point Tc1 of the memory layer is exceeded.
If it exceeds 1, there is no Hr that satisfies the above equation. Therefore, the above equation automatically includes that Tr <Tc1.

Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited thereto.

[0031]

Embodiment 1 A disc substrate made of polycarbonate having a guide groove for tracking and having a thickness of 1.2 mm, a diameter of 130 mm and a track pitch of 1.2 μm is prepared. A silicon nitride (dielectric) layer, TbFeCo
A (memory) layer, a TbDyFeCo (recording) layer, and a silicon nitride (dielectric) layer are formed in this order by sputtering.
The memory layer has a TM-rich composition in which the sublattice magnetization of the transition metal is larger than the sublattice magnetization of heavy rare earth, and the recording layer has a sublattice magnetization of the heavy rare earth which is larger than that of the transition metal. It has a large RE-rich composition and has a compensation temperature between room temperature and Curie temperature. In this way, a magneto-optical disk that can be overwritten by light intensity modulation is created.

Next, a magneto-optical recording / reproducing apparatus is prepared. The light source wavelength of this device is 780 nm, and the NA of the objective lens of the optical pickup is 0.55. In addition, this device is equipped with a 4 KOe initialization magnetic field Hini.
The magnetic field passes through each rotation. next,
Set the above magneto-optical disk in the recording / reproducing device, and
The maximum allowable value of the reproducing beam intensity is measured by the following procedure while rotating at rpm.

First, a signal having a frequency of 2 MHz and a duty ratio of 50% is recorded at a position of a radius of 45 mm on the disk while modulating a recording beam into two values of PH = 9.0 mW and PL = 4.0 mW. At this time, a magnetic field having a magnitude of 300 Oe is applied in the same direction as the initialization magnetic field Hini. Next, the recorded signal is reproduced at a reproduction beam intensity of 1.0 mW without applying a magnetic field. The C / N value of the reproduced signal is 57.3 dB.

Then, after overwriting and recording a signal having a frequency of 5 MHz and a duty ratio of 50% without changing the recording beam intensity, the signal is reproduced at a reproduction beam intensity of 1.0 mW without applying a magnetic field. The C / N value of the reproduced signal at this time is 51.2
Get dB. That is, the previously recorded 2 MHz signal is not observed, and the overwrite recording is indeed performed. next,
Reproduction is performed while applying a magnetic field of 1.5 Koe in the same direction as the magnetic field applied during recording. First, the reproduction beam intensity
The C / N value at 1.0 mW is 51.2 dB, the same as when no magnetic field is applied.

Next, the reproducing beam intensity was set to 1.1 mW and C
After measuring the / N value, the reproducing beam intensity is returned to 1.0 mW, and the C / N value is measured again. Furthermore, the reproduction beam intensity is increased to 1.
After measuring the C / N value with 2 mW, the reproducing beam intensity was
After returning to 1.0 mW, the C / N value is measured again. As described above, the C / N value is measured while increasing the reproduction beam intensity in steps of 0.1 mW, and then the reproduction beam intensity is returned to 1.0 mW and the C / N value is again measured.
The procedure of measuring the / N value is repeated.

Until the reproducing beam intensity becomes 2.5 mW,
The C / N value increases as the reproduction beam intensity increases, and a C / N value of 54.5 dB is obtained at a reproduction beam intensity of 2.5 mW. When the reproducing beam intensity is returned to 1.0 mW, the same C / N value as before increasing the reproducing beam intensity is obtained. That is, it is understood that the magnetization reversal does not occur up to the reproduction beam intensity of 2.5 mW, that is, the information is not erased.

When the reproducing beam intensity exceeds 2.6 mW, C
/ N drops sharply. Then, the reproduction beam intensity is
Even if it returns to 1.0 mW, the original C / N value cannot be obtained. This is because at a beam intensity of 2.6 mW or more, the coercive force Hc1 of the memory layer decreases due to the heat, and the relationship of Hr> −Hc1 + σw / (2 · Ms1 · t1) (1) cannot be maintained, and the interface of the memory layer cannot be maintained. This indicates that the magnetization of the domain wall region is inverted, that is, the information in the memory layer is erased (destructed).

That is, it can be seen that when a magnetic field of 1.5 Koe is applied during reproduction, information in the memory layer is not destroyed up to a reproduction beam intensity of 2.5 mW.

[0039]

[Comparative Example 1] After recording a signal of frequency 2 MHz, the same procedure as in Example 1 is performed until the signal of 5 MHz is overwritten. Next, without applying a magnetic field, reproduction was performed in the same procedure as in Example 1 while increasing the reproduction beam intensity in steps of 0.1 mW, the C / N value of the reproduction signal was measured, and the reproduction beam intensity was further increased by 1.0 mW. And reproduce the C / C
The procedure of measuring the N value is repeated.

Until the reproducing beam intensity reaches 2.0 mW,
The C / N value increases as the reproduction beam intensity increases, and a C / N value of 53.5 dB is obtained at a reproduction beam intensity of 2.0 mW. When the reproducing beam intensity is returned to 1.0 mW, the same C / N value as before increasing the reproducing beam intensity is obtained. That is, it is understood that the magnetization reversal does not occur up to the reproduction beam intensity of 2.0 mW, that is, the information is not erased.

When the reproducing beam intensity exceeds 2.1 mW, C
/ N drops sharply. Then, the reproduction beam intensity is
Even if it returns to 1.0 mW, the original C / N value cannot be obtained. In other words, it indicates that the information in the memory layer is erased (destructed) at a beam intensity of 2.1 mW or more. As described above, it is shown that a higher reproduction beam intensity can be set by applying a magnetic field during reproduction. That is, it is shown that the reproduction beam intensity can be improved by about 30% by applying a magnetic field of 1.5 KOe.

[0042]

Embodiment 2 The same substrate as in Embodiment 1 is prepared. A silicon nitride (dielectric) layer, a TbFeCo (memory) layer, a TbDyFeCo (recording) layer,
TbFe (switching layer), TbCo (initialization layer), and silicon nitride (dielectric) layer are formed by sputtering in this order. In this way, a magneto-optical disk that can be overwritten by light intensity modulation is created. Here, the memory layer and the recording layer have the same composition as in Example 1. Further, the switching layer has a function of transmitting the magnetization of the initialization layer only when necessary by the exchange coupling force. The magnetization direction of the initialization layer is previously aligned in one direction. In this way, a magneto-optical disk that can be overwritten by light intensity modulation is created. The magneto-optical recording / reproducing apparatus used in the first embodiment is prepared, and after removing the initialization magnetic field Hini. From this apparatus, the above-described magneto-optical disk is set.

Next, the magneto-optical disk is set in a recording / reproducing apparatus, and the maximum allowable value of the reproducing beam intensity is measured by the following procedure while rotating at 2400 rpm. First, a signal having a frequency of 2 MHz and a duty ratio of 50% is recorded while modulating a recording beam into two values of PH = 10.0 mW and PL = 4.4 mW at a position of a radius of 45 mm on the disk. At this time, the size
A magnetic field of 300 Oe is applied in the same direction as the magnetization direction of the initialization layer.

Next, the recorded signal is converted into a reproduction beam intensity.
Reproduction is performed at 1.0 mW without applying a magnetic field. C /
N value obtains 57.3dB. Subsequently, a signal having a frequency of 5 MHz and a duty ratio of 50% is overwritten and recorded without changing the recording beam intensity, and then reproduced at a reproduction beam intensity of 1.0 mW without applying a magnetic field. C / N of the reproduced signal at this time
The value gets 51.0dB. That is, the previously recorded 2 MHz signal is not observed, and the overwrite recording is indeed performed.

Next, reproduction is performed while applying a magnetic field of 1.5 Koe in the same direction as the magnetic field applied during recording. First, the C / N value when the reproducing beam intensity is 1.0 mW is 51.0 dB, which is the same as when no magnetic field is applied. Next, after measuring the C / N value with the reproduction beam intensity set to 1.1 mW, the reproduction beam intensity is returned to 1.0 mW and the C / N value is measured again.
Further, after measuring the C / N value by setting the reproducing beam intensity to 1.2 mW, the reproducing beam intensity was returned to 1.0 mW, and the C / N ratio was again measured.
Measure the value. Thus, the reproducing beam intensity is set to 0.1 mW
After measuring the C / N value in increments, the reproduction beam intensity
The procedure of returning to 1.0 mW and measuring the C / N value again is repeated.

Until the reproducing beam intensity reaches 2.8 mW,
The C / N value increases as the reproduction beam intensity increases, and a C / N value of 55.1 dB is obtained at a reproduction beam intensity of 2.8 mW. When the reproducing beam intensity is returned to 1.0 mW, the same C / N value as before increasing the reproducing beam intensity is obtained. That is, it is understood that the magnetization reversal does not occur up to the reproduction beam intensity of 2.8 mW, that is, the information is not erased.

When the reproducing beam intensity exceeds 2.9 mW, C
/ N drops sharply. Then, the reproduction beam intensity is
Even if it returns to 1.0 mW, the original C / N value cannot be obtained. This is because at a beam intensity of 2.9 mW or more, the coercive force Hc1 of the memory layer decreases due to the heat, and the relationship of Hr> −Hc1 + σw / (2 · Ms1 · t1) (1) cannot be maintained, and the interface of the memory layer cannot be maintained. This indicates that the magnetization of the domain wall region is inverted, that is, the information in the memory layer is erased (destructed).

That is, it can be seen that when a magnetic field of 1.5 Koe is applied during reproduction, information in the memory layer is not destroyed up to a reproduction beam intensity of 2.8 mW.

[0049]

Comparative Example 2 After recording a signal of 2 MHz in frequency, the signal is recorded in exactly the same procedure as in Example 2 until the signal of 5 MHz is overwritten. Next, without applying a magnetic field, reproduction was performed in the same procedure as in Example 1 while increasing the reproduction beam intensity in steps of 0.1 mW, and the C / N value of the reproduction signal was measured.
The reproduction is repeated by returning the reproduction beam intensity to 1.0 mW, and the procedure of measuring the C / N value of the reproduction signal is repeated.

Until the reproducing beam intensity reaches 2.2 mW,
The C / N value increases as the reproduction beam intensity increases, and a C / N value of 53.1 dB is obtained at a reproduction beam intensity of 2.2 mW. When the reproducing beam intensity is returned to 1.0 mW, the same C / N value as before increasing the reproducing beam intensity is obtained. That is, it is understood that the magnetization reversal does not occur up to the reproduction beam intensity of 2.2 mW, that is, the information is not erased.

When the reproducing beam intensity exceeds 2.3 mW, C
/ N drops sharply. Then, the reproduction beam intensity is
Even if it returns to 1.0 mW, the original C / N value cannot be obtained. That is, it indicates that the information in the memory layer is erased (destructed) at a beam intensity of 2.3 mW or more. As described above, it is shown that a higher reproduction beam intensity can be set by applying a magnetic field during reproduction. That is, it is shown that the reproduction beam intensity can be improved by about 30% by applying a magnetic field of 1.5 KOe.

[0052]

Embodiment 3 The same substrate as in Embodiment 1 is prepared. A silicon nitride (dielectric) layer, a GdFeCo (supermagnetic resolution reproducing) layer, a TbFeCo (memory) layer, a TbDyFeCo (recording) layer, and a silicon nitride (dielectric) It is formed by sputtering in the order of layers. In this way, a magneto-optical disk capable of supermagnetic resolution reproduction and overwritable by light intensity modulation is created. Here, the memory layer and the recording layer have the same composition as in Example 1.
Further, the supermagnetic resolution reproducing layer is magnetized in the in-plane direction at room temperature, but has a property of being perpendicularly magnetized when the temperature exceeds a predetermined temperature.

The magneto-optical disk described above is set in the magneto-optical recording and reproducing apparatus used in Example 1, and the maximum allowable value of the reproducing beam intensity is measured by the following procedure while rotating at 2400 rpm. First, a signal having a frequency of 14 MHz and a duty ratio of 50% is recorded at a position of a radius of 45 mm on the disk while modulating a recording beam into two values of PH = 9.0 mW and PL = 4.4 mW. At this time, a magnetic field of 300 Oe is applied in the same direction as the initialization magnetic field.

Next, reproduction is performed while applying a magnetic field of 1.5 KOe in the same direction as the magnetic field applied during recording. First, at a reproduction beam intensity of 1.0 mW, a signal recorded previously is hardly observed. That is, super-resolution does not occur with a reproduction beam having an intensity of 1.0 mW, and no reproduction signal is observed. When the reproduction beam intensity is increased from 1.0 mW to 0.1 mW steps, a reproduction signal starts to be observed at 2.1 mW,
Further, as the reproducing beam intensity increases, the C /
The N value increases. Then, when the reproduction beam intensity is 2.6 mW, the C / N value of the reproduction signal reaches 44.0 dB. This value does not change during repetitive reproduction at a reproduction beam intensity of 2.6 mW. In other words, this indicates that the magnetization of the memory layer is not reversed at the read beam intensity of 2.6 mW, that is, information is not erased (destructed). However, the reproducing beam intensity was 2.7 mW
, The C / N value drops sharply, returns to 2.6 mW and does not return to the C / N value. That is, the reproduction beam intensity is 2.7m
It indicates that the information in the memory layer is destroyed when the value is W or more. That is, it is understood that when a magnetic field is applied during reproduction, the reproduction beam intensity can be set in the range of 2.1 to 2.6 mW.

Subsequently, a signal having a frequency of 5 MHz and a duty ratio of 50% is overwritten and recorded at a reproduction beam intensity of 2.6 mW without changing the recording beam intensity. At this time, the C / N value of the reproduced signal is 51.0 dB. That is, the previously recorded 14 MHz signal is not observed, and the overwrite recording is indeed performed.

[0056]

Comparative Example 3 The procedure is the same as that of Example 3 until a signal having a frequency of 14 MHz is recorded. Next, without applying a magnetic field,
When the reproduction beam intensity is increased from 1.0 mW to 0.1 mW, a reproduction signal starts to be observed at 2.1 mW, and when the reproduction beam intensity is 2.2 mW, the C / N value is 28.0 dB. But,
When the reproducing beam intensity is set to 2.3 mW, the C / N value drops sharply, and does not return to the original C / N value even when it is returned to 2.2 mW.
That is, this indicates that the information in the memory layer is destroyed when the reproduction beam intensity is 2.3 mW or more.

As described above, the range in which the reproduction beam intensity can be set by applying a magnetic field during reproduction is 2.1 to 2.6 mW.
Which is the range when no magnetic field is applied during reproduction.
It can be seen that it is wider than the range of 2.1 to 2.2 mW. Further, since the reproducing beam intensity can be set to a large value, the effect of the magnetic super-resolution can be sufficiently obtained, and a C / N value as high as 16 dB can be obtained. That is, by performing reproduction while applying a magnetic field, the temperature at which the magnetization of the memory layer is reversed (destruction of information) can be increased, and the combination of overwriting and magnetic super-resolution can be made practical. Become.

[0058]

As described above, according to the present invention, a high C / N value can be obtained even in light intensity modulation overwrite magneto-optical recording and reproduction, and further, information rewriting can be performed by combining magnetic super-resolution reproduction. It is possible to provide a reproducing method and a reproducing apparatus for a magneto-optical recording medium capable of overwriting with light intensity modulation capable of reliably reproducing information recorded at high speed and with high density.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram illustrating a reproduction principle according to the present invention.

FIG. 2 is a block diagram of a playback device according to the present invention.

FIG. 3 is a conceptual diagram showing a relationship between a magnitude of a reproducing magnetic field and a temperature characteristic of a memory layer according to the present invention.

FIG. 4 is a conceptual diagram illustrating the principle of super-resolution reproduction using a change in magnetization direction due to a change in magnetic coupling force.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Rotating means 2 ... Light source 3 ... Optical pickup 4 ... Modulator 5 ... Input part 6 ... Magnetic field setting means 7 ... Magnetic field applying means 8 ... Detector 9 ... Demodulator D ... Magneto-optical recording medium

Claims (7)

[Claims] 1. A method for reproducing a magneto-optical recording medium having at least two magnetic layers and capable of light intensity modulation and capable of overwriting, wherein a magnetic field H is used when reproducing information recorded on the recording medium.
A method for reproducing a magneto-optical recording medium capable of overwriting light intensity modulated by applying r. 2. The reproducing method for a magneto-optical recording medium capable of overwriting with light intensity modulation according to claim 1, wherein the magnitude of the magnetic field Hr is −Hc1 + σw / (2 · Ms1 · t1) in a temperature range from Tamb to Tr. <Hr <Hc1
2. The method according to claim 1, wherein the following condition is satisfied: + σw / (2 · Ms1 · t1). Here, the coercive force, the saturation magnetization, the film thickness, and the Curie temperature of the memory layer are set to Hc1, Ms1, t1, and Tc, respectively.
1. The exchange coupling force acting between the memory layer and the recording layer is defined as σw. 3. The reproducing method for a magneto-optical recording medium capable of optical intensity modulation overwriting according to claim 2, wherein the magnetic field Hr is −Hc1 + σw / (2 · Ms1 · t1) <Hr <in a temperature range from Tamb to Tr. Hc1
+ Σw / (2 · Ms1 · t1), and the direction of reproduction Hr at a temperature near Tr is one of the magnetization directions of the interface domain wall region generated between the memory layer and the adjacent magnetic layer. A method for reproducing a light intensity modulated overwritable magneto-optical recording medium, wherein the magnetization direction is the same as the magnetization direction on the memory layer side. However, if the room temperature is Tam
b, let Tr be the maximum temperature reached by the magnetic layer by the irradiation of the reproduction beam. Further, the coercive force, saturation magnetization, film thickness, and Curie temperature of the memory layer are set to Hc1, Ms, respectively.
1, t1, and Tc1, and the exchange coupling force acting between the memory layer and the recording layer are defined as σw. 4. The reproducing method for a magneto-optical recording medium capable of overwriting light intensity modulated according to claim 1, wherein the magneto-optical recording medium capable of overwriting light intensity modulated has a magnetization direction of at least one magnetic layer. A method for reproducing a light intensity modulated overwritable magneto-optical recording medium, comprising an initialization layer for aligning in a predetermined direction. 5. A reproducing method for a magneto-optical recording medium capable of overwriting light intensity modulated light according to claim 1, wherein the magneto-optical recording medium capable of overwriting light intensity modulated light utilizes a temperature distribution in a reproducing beam spot. A method for reproducing a light intensity modulated overwritable magneto-optical recording medium, comprising: 6. A reproducing apparatus for reproducing information recorded on a light intensity modulation overwritable magneto-optical recording medium having at least two magnetic layers, the reproducing apparatus comprising: a rotating unit for rotating the medium; An optical pickup for irradiating a predetermined portion of the medium with a beam emitted from a light source and causing a reflected beam from the medium to enter a detector, a magnetic field applying means for applying a magnetic field to the medium during reproduction, and a magnetic field applying means A reproducing apparatus for a magneto-optical recording medium capable of overwriting light intensity modulated, comprising a magnetic field setting means for setting. 7. The reproducing apparatus for a magneto-optical recording medium capable of optical intensity modulation overwriting according to claim 6, wherein Hr applied to the medium at the time of reproduction by the magnetic field applying means is:
In a temperature range from Tamb to Tr, −Hc1 + σw / (2 · Ms1 · t1) <Hr <Hc1
+ Σw / (2 · Ms1 · t1). A reproducing apparatus for a magneto-optical recording medium capable of overwriting with light intensity modulation. However, if the room temperature is Tam
b, let Tr be the maximum temperature reached by the magnetic layer by the irradiation of the reproduction beam. Further, the coercive force, saturation magnetization, film thickness, and Curie temperature of the memory layer are set to Hc1, Ms, respectively.
1, t1, and Tc1, and the exchange coupling force acting between the memory layer and the recording layer are defined as σw.