JPH0393058A - Signal reproduction method in magneto-optical recording medium - Google Patents

Abstract

PURPOSE:To improve linear recording density and track density by arranging the direction of magnetization in a reproduction layer, and reading while transferring a magnetic signal from a recording holding layer only in an area whose temperature is raised by means of the irradiation of laser light. CONSTITUTION:The direction of magnetization in the reproduction layer 21 is arranged and it is set to a deletion state. Then, the reproduction layer 21 is raised to a prescribed temperature by irradiating the reproduction layer 21 with laser light, while the magnetic signal recorded in the recording holding layer 22 is transferred only in the area heated to the prescribed temperature of the reproduction layer 21 the magnetic signal is converted into an optical signal by a magneto-optical effect and reading is executed. Consequently, there is no influence of the adjacent tracks, crosstalk is cancelled and a signal can be reproduced at high C/N even if track density and linear recording density are high.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for reproducing signals in a magneto-optical recording medium in which recorded signals are read out using magneto-optical characteristics, and in particular to a method for improving linear recording density and track density. It is related to the technology to do so. [Summary of the Invention] The present invention comprises a recording layer of a magneto-optical recording medium with a multilayer film including a magnetically coupled reproduction layer and a recording layer, and erases data by aligning the direction of magnetization of the reproduction layer in advance. At the same time, during reproduction, the reproduction layer is heated to a predetermined temperature or higher by irradiation with laser light, and the magnetic signals written in the recording layer are read while being transferred to the reproduction layer only in this raised area. By doing so, the aim is to eliminate crosstalk and improve linear recording density and track density. [Conventional technology] In the magneto-optical recording method, a magnetic thin film is partially heated to exceed the Curie point or temperature compensation point, and the coercive force in this area is extinguished to magnetize it in the direction of an externally applied recording magnetic field. The basic principle is to reverse the direction of the magneto-optical recording medium. Therefore, the principle of a magneto-optical recording medium is to have an axis of easy magnetization perpendicular to the film surface on the entire surface of a transparent substrate made of polycarbonate or the like. A recording magnetic layer (e.g. rare earth-transition metal alloy amorphous thin film) and a reflective layer with excellent magneto-optical effects,
It is known that a recording section is provided by laminating dielectric layers, and signals are read by irradiating laser light from the transparent substrate side. Incidentally, the linear recording density of not only magneto-optical recording media but also optical discs such as digital audio discs (so-called compact discs) and video discs is determined mainly by the S/N during reproduction, and also by the signal-to-noise ratio of the reproduced signal. The amount depends largely on the period of the pit train of the recorded signal, the laser wavelength of the reproduction optical system, and the numerical aperture of the objective lens.

At present, the laser wavelength λ of the reproduction optical system and the numerical aperture N. A. Once determined, the pit circumference ltl which becomes the detection limit
If is determined. That is, r=λ/2·N. A. It is. On the other hand, track density is primarily limited by crosstalk. This crosstalk is mainly determined by the distribution (profile) of the laser beam on the medium surface, and like the bit period, it is also λ/2·N. A. is schematically represented by the function of

Therefore, in order to achieve high density with conventional optical discs, it is necessary to shorten the laser beam length λ of the reproducing optical system and to reduce the numerical aperture N of the objective lens. Eight. The basic stance is to increase the .

However, there are limits to the improvement of the laser wavelength and the numerical aperture of the objective lens, and on the other hand, techniques have been developed to improve the recording density by devising the structure of the recording medium and the reading method.

For example, the applicant of the present application has previously disclosed in Japanese Patent Application Laid-Open Nos. 1-143041 and 1-143042 that recording pits (magnetic domains) are enlarged during reproduction. We have proposed a method that improves the reproduction resolution by reproducing the data while eliminating it. This method consists of a reproduction layer and an intermediate layer. By using an exchange-coupled polyN film consisting of a recording layer as a recording medium, and expanding or erasing the magnetic domain of the reproduction layer heated by a reproduction light beam during reproduction,
This reduces intersymbol interference during reproduction, making it possible to reproduce signals with a period below the optical diffraction limit. [Problems to be Solved by the Invention] However, in the above-mentioned method, although the linear recording density is improved, the crosstalk is similar to that of a normal optical disc, and it is difficult to improve the track density.

The present invention has been proposed in view of the conventional situation, and an object of the present invention is to provide a signal reproducing method that can eliminate crosstalk and improve not only linear recording density but also track density. With the goal.

? Means for Solving the Problems] In order to achieve the above object, the present invention provides a magneto-optical recording method for recording on a multilayer film having at least a magnetically coupled reproducing layer and a recording layer. After recording a signal on the recording layer of the medium and aligning the magnetization direction of the reproducing layer, the reproducing layer is heated by irradiating the reproducing layer with a laser beam, and the recording layer is heated. It is characterized by converting the recorded magnetic signal into an optical signal by the magneto-optic effect while transferring it to the reproducing layer and reading it.

That is, in the present invention, a multilayer film having at least a reproducing layer and a recording layer is used as a recording medium, and the reproducing layer is kept in the same state (erased state) over the entire surface before reproducing a signal, and is heated to a certain temperature by being irradiated with re-living light. It is set so that the signal previously recorded on the recording layer is transferred to the reproduction layer only in the above areas,
The area below the temperature does not have any influence on the signal and is optically equivalent to being masked, thereby improving both the linear recording density and the track density.

In the present invention, the recording layer of the magneto-optical recording medium used may be composed of at least two perpendicularly magnetized films (a reproducing layer and a recording holding layer), such as an exchange-coupled film made of a rare earth-transition metal alloy thin film. A multilayer film (at least a 2N film, a three-layer film, preferably a four-layer film or more) is suitable.

Of course, it is not limited to this, but garnet film, CoCr, PLC
It may be a perpendicularly magnetized film such as O, PdCo, etc., or it may be a coating film of a magnetic paint in which hexagonal ferrite I powder such as barium ferrite is dispersed. However, the reproducing layer and the recording layer must be magnetically coupled by magnetostatic coupling or exchange coupling. Furthermore, the reproducing layer must have a large Kerr rotation angle and Faraday rotation angle. Signals may be recorded on the record-holding layer using an optical modulation method or a magnetic field modulation method, similar to ordinary magneto-optical recording media. A magnetic signal may be recorded on the perpendicularly magnetized film using a magnetic head in the same manner as in a perpendicular magnetic recording medium, and then the magnetic signal recorded on the perpendicularly magnetized film may be transferred to the recording layer by irradiation with laser light. Then, as shown in FIG. 1, a signal is recorded on the recording layer (1) of the magneto-optical recording medium having the above structure, while the reproduction layer (2) aligns the direction of magnetization and erases the signal. Leave it as it is.

In this example, the direction of magnetization of the reproducing layer (2) is aligned upward in the figure.

Here, the reproduction JiJ (2) is erased using an external magnetic field H. You can do it with That is, if H■>Hc+ (where Hc+ is the magnetization reversal magnetic field of the reproducing layer (2)), then the reproducing layer (2)
> of the external magnetic field H! It can be aligned in the l direction. Further, at this time, if Htm<<Hcz [where Hc2 is the magnetization switching magnetic field of the recording layer (1)], the signal recorded on the recording layer (1) will not be affected.

During reproduction, the reproduction layer (2) is irradiated with laser light LB,
The area irradiated with the laser beam LB is heated and its temperature rises. At this time, as shown in FIG. 2, when the reproducing layer (2) reaches a certain temperature T■ or higher, the signal recorded in the recording holding N(1) is transferred to the reproducing N(2). For example, if the recording layer (1) and the reproducing layer (2) are magnetically coupled by magnetostatic coupling, the stray magnetic field H sL from the recording layer (1) is the reaction of the reproducing layer (2). Magnetic field Hd, magnetic domain generation in reproduction layer (2). Magnetic field■1? l+ The externally applied magnetic field H■ applied during reproduction is set to the predetermined temperature T. When the following, Hs2+Hd+±Hps<Hn+・H H H (1) is satisfied, and when the temperature is above the predetermined temperature T■, Hsz+Hd+fHrm>Hr++ ・ ・ ・
If the magnetization, coercive force, film thickness, etc. of each layer are set so as to satisfy the equation (2), then the temperature T. Signals are transferred only in the heated region according to the floating am field generated from the recording layer (1).

Similarly, if the recording layer (1) and the reproducing layer (2) are magnetically coupled by exchange coupling, the equivalent magnetic field HW+ (-
6 H / 2 Ms+ h + : However, σ1 is the interfacial domain wall energy density that occurs between the eyebrows of the reproduction layer (2) and the magnetic layer in contact with the reproduction layer (2>, and Ms+ is the energy density of the interfacial domain wall between the reproduction layer (2)
The saturation magnetization h in 2) is the thickness of the reproducing layer (2). ]
is the magnetic domain generated magnetic field Hn. of the reproduction layer (2). On the other hand, when the temperature is below the predetermined temperature TPI, HwI±Hrw<Hn+'' (3)? exists, and when the temperature is above the predetermined temperature TPI, Hw+±Hr*>l
{n+ + ” (4), then the temperature T
(2) Signals are transferred only in the areas heated above due to the exchange force with the recording layer (1). The transferred magnetic signal is generated by the magneto-optic effect (
It is converted into an optical signal by the Rikiichi effect or Faraday effect) and is reproduced by detecting the Kerr rotation angle of the previous laser beam LB. During playback, as shown in FIG.
．． The temperature T8 at the boundary between and the adjacent track t, is TN<
If the temperature distribution is set to be TPI, the signal recorded in the recording storage N (1) below the adjacent track L will not be transferred to the reproduction layer (2), and crosstalk will be completely eliminated. be done.

[Effect]

In the signal reproducing method of the present invention, the reproducing layer for reading the signal has the magnetization direction aligned and is in the same state (erased state) over the entire surface.
The signal is transferred and read only in the area irradiated with the laser beam.

Therefore, there is no influence from adjacent tracks, and crosstalk is eliminated.

In addition, due to the temperature distribution when the laser beam is irradiated, the front end of the laser beam in the traveling direction maintains the erased state and appears as if it were masked, and the vertical period is smaller than the beam diameter of the laser beam. In this case, the apparent spatial frequency is suppressed and reproduction is performed with a high C/N.

〔Example〕

Hereinafter, specific embodiments to which the present invention is applied will be described with reference to the drawings.

m This example is an example in which magnetostatic coupling is used for magnetic coupling between the recording layer and the reproducing layer.

As shown in FIG. 4, the magneto-optical recording medium of this embodiment consists of a transparent substrate (11) made of polycarbonate, glass, etc., and a first reproducing layer (12). The second reproducing layer (13) and the recording layer (14) are covered with dielectric films (15) and (16).
), and furthermore, the outermost layer is also coated with a lecture electrical body film (1
7). Dielectric film (15),
Materials for (16) and (17) include silicon nitride,
Transparent dielectric materials such as silicon oxide and aluminum nitride can be used. The first reproducing layer (l2) has a high Curie point (for example, 2
00°C or higher), the perpendicularly magnetized film has a large force/rotation angle, and has a coercive force of several hundred oersted or less. Second rebirth 11! (13) is a perpendicularly magnetized film with large perpendicular magnetic anisotropy, a low Curie point (for example, below 200°C), and a coercive force of around 2 kilo Oersteds (k(le)) at room temperature. 1 reproduction layer (12) and a second reproduction layer (1
3) is exchange coupled.

When the reproduction layer has a two-layer structure in this way, the temperature dependence of the magnetization reversal magnetic field of the reproduction layer changes sharply after a certain temperature, and moreover, the reversal magnetic field changes above the Curie point of the second reproduction layer (13). The value can be as low as about 100 (Oe), making it possible to stably transfer minute bits.

On the other hand, the recording/retention layer (14) is made of a material that has a large perpendicular magnetic anisotropy and a Curie point higher than that of the second reproducing layer (13). The Curie point of the recording layer (14) serves as a guideline for setting the output margin of the laser beam used for reading and writing, and must be set at least 50°C higher than the Curie point of the second reproduction layer (I3). There is. Actually, the first reproducing layer (12) is made of CdFeCo, the second reproducing layer (l3) is made of TbFe, and the record-holding layer (l4) is made of TbFeCo.
The Curie point of 14) is 280℃. The coercive force is l O (k
Oe) or higher. When such a magneto-optical recording medium was used to reproduce a signal recorded on the recording layer (I4) while being transferred to the reproduction layer (I2) using reproduction light, an extremely high C/N was achieved. Real 1 u rectangle i This example is an example in which exchange coupling is used for magnetic coupling between the recording layer and the reproducing layer.

The fifth precept for magneto-optical recording media using exchange coupling is
As shown in the figure, there is an exchange-coupled 2N film (hereinafter referred to as medium A) consisting of a reproduction layer (21) and a recording layer (22), and as shown in FIG. An intermediate layer (25) is interposed between the retention layer (24) (
Hereinafter, it will be referred to as medium B. ), as shown in FIG.
It has a two-layer structure of a reproducing layer (27) and a recording layer (29) is laminated via an intermediate layer (28) (hereinafter referred to as medium C).
It is called. ) is possible. Here, in the medium A, in order to satisfy the conditions of the above-mentioned equations (3) and (4), it is necessary to increase the thickness of the reproducing layer (21) and the recording layer (22). Due to limitations on laser beams, there is a possibility that recording and erasing cannot be performed when the linear velocity is high.

Therefore, it may be possible to reduce the interfacial domain wall energy density by intervening an intermediate layer so that the film thickness of each layer can be made thinner.However, in medium B, the equivalent magnetic field Hw+ due to the exchange force is applied to the reproducing layer. The temperature at which the magnetic domain becomes equal to the magnetic field Hn+ generated by the magnetic domain becomes extremely sensitive to M&L formation, film thickness, etc., and unevenness on the disk circumference has a significant effect, tending to increase noise and jitter during reproduction. On the other hand, in the medium C, the first reproducing layer (26) is made of a material with a high Curie point, a large force rotation angle, and a small coercive force, and the second reproducing layer (27) is made of a material with a high Curie point, a large force rotation angle, and a small coercive force. By using a material with a low coercivity and a coercive force of about 2 (koe), a magneto-optical recording medium with a good reproduction S/N can be obtained.

The present inventors have developed StsN-
Karanal v::t4 bodies N (MW 8 0 0 A)
, the first regenerated N consisting of GdFeCo (thickness: 300)
, a second reproduction layer made of TbFe6 (It!! thickness 150 layers), an intermediate layer made of GdFeCo (thickness 100 layers), T
Recording layer made of bFeCo (thickness: 350 mm), Si
．． N. A dielectric layer (lI!! 800 mm thick) consisting of A sample disk was created by sequentially stacking the layers. Then, a 5 MHz carrier signal was written in the recording layer, and the dependence of the carrier and noise reproduction output on the laser output was investigated at a linear velocity of 5 m/sec and a reproduction external magnetic field of 500 (Oe). At the same time, we also measured crosstalk. For crosstalk, set #M0.8am group on the board to 0. are formed at intervals of 8 tim (therefore, land portions with a width of 0.8 μm are formed between the group portions),
Measurements were made by recording a 4.8 MHz signal on each of the land and group parts. The results are shown in FIG. In the figure, line i
represents the reproduced output of the carrier, lii represents the reproduced output of the noise, and line 111 represents the crosstalk.

The reason why carriers are not observed at a laser output of 1.6 mW or less is because the medium temperature does not reach the temperature required for transfer at this power.

On the other hand, the laser output 2. Above 0 mW,
Carriers are observed rapidly. Crosstalk is almost undetectable at laser outputs below 2.5mW, so in this sample disk, the laser output was set to 2.5mW. 0
By setting the power to be at least mW and at most 2.5mW, it is possible to reproduce signals at a high C/N without crosstalk. Therefore, in order to confirm the advantage of reproducing while transferring, we investigated the pit period dependence of C/N. The linear velocity during measurement was 5 m/sec. The reproduction magnetic field is 50
0 (Oe), the laser wavelength is 780 nm, and the numerical aperture of the objective lens is N. A. is 0.53. The results are shown in Figure 9. In FIG. 9, curve a indicates the reproduction layer 3. It shows the characteristics when initialized with an external magnetic field of 5 (koe) and then reproduced while transferring the magnetic domain with a laser output of 2.8 mW.Curve b shows the characteristics when initialized after initialization with an external magnetic field of 5 (koe) after reproducing once under the same conditions. This shows the characteristics when reproduced as is with a laser output of 1.4 mW. Therefore, curve b is C
Corresponds to /H. Looking at FIG. 9, it can be seen that the C/N is significantly improved by adopting the method of the present invention.
[Effect of the invention] As is clear from the above explanation, in the present invention,
Since the magnetic signals recorded in the recording layer are read out while being transferred to the reproducing layer, crosstalk can be eliminated and signals can be read with high C/N even when recording density and linear recording density are high. It is possible to play it. Therefore, it is very useful and significant in achieving high density recording in magneto-optical recording media.

[Brief explanation of drawings]

FIGS. 1 and 2 are schematic diagrams showing the reproduction principle of the present invention, with FIG. 1 showing the initial state and FIG. 2 showing the reproducing state. Figure 3 is a schematic diagram showing the temperature distribution in the track width direction when laser light is irradiated. FIG. 4 is a schematic cross-sectional view showing an example of the configuration of a magneto-optical recording medium using magnetostatic coupling. FIG. 5 is a schematic sectional view showing an example of the structure of a magneto-optical recording medium using exchange coupling, and FIG. 6 is a schematic sectional view showing another example of the structure of a magneto-optical recording medium using exchange coupling. FIG. 7 is a schematic cross-sectional view showing still another example of the configuration of a magneto-optical recording medium using exchange coupling. Figure 8 is a characteristic diagram showing the dependence of carrier output and crosstalk on laser output when reproduction is performed while transferring magnetic domains to the reproduction layer, and Figure 9 is a characteristic diagram of the C/N when reproduction is performed while transferring magnetic domains. FIG. 3 is a characteristic diagram showing bit period dependence in comparison with that in normal reproduction.

Claims (1)

[Claims] A magneto-optical recording medium whose recording layer is a multilayer film comprising at least a magnetically coupled reproducing layer and a recording layer,
After recording a signal on the recording layer and aligning the magnetization direction of the reproducing layer, the reproducing layer is heated by irradiating the reproducing layer with a laser beam, and the signal recorded on the recording layer is heated. A method for reproducing signals in a magneto-optical recording medium, characterized in that a magnetic signal is transferred to a reproducing layer, converted into an optical signal by a magneto-optic effect, and then read.