JP2910084B2 - Signal reproducing method in magneto-optical recording medium - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a signal reproducing method for a magneto-optical recording medium for reading a recording signal by using magneto-optical characteristics, and particularly to improving a linear recording density and a track density. It is related to the technology for performing.

[Summary of the Invention]

According to the present invention, the recording layer of the magneto-optical recording medium is composed of a multilayer film including a reproducing layer and a recording holding layer that are magnetically coupled, and the magnetization direction of the reproducing layer is set in advance to be in the erased state, In some cases, the reproducing layer is heated to a predetermined temperature or higher by laser beam irradiation, and the magnetic signal written in the recording holding layer is read only while transferring the magnetic signal to the reproducing layer only in the region where the temperature has been raised. This is intended to eliminate the talk and improve the linear recording density and the track density.

[Conventional technology]

In the magneto-optical recording method, the temperature of the magnetic thin film is partially raised beyond the Curie point or the temperature compensation point, the coercive force in this part is eliminated, and the direction of magnetization is reversed to the direction of the externally applied recording magnetic field. Therefore, as a configuration of the magneto-optical recording medium, for example, one main surface of a transparent substrate made of polycarbonate or the like has an excellent magneto-optical effect having an easy axis of magnetization in a direction perpendicular to the film surface. A recording section is provided by laminating a recording magnetic layer (for example, a rare earth-transition metal alloy amorphous thin film), a reflective layer, and a dielectric layer, and a signal is read by irradiating a laser beam from the transparent substrate side. Is known.

Incidentally, the linear recording density of optical disks such as digital audio disks (so-called compact disks) and video disks is not limited to magneto-optical recording media, and is mainly determined by the S / N ratio during reproduction. The amount greatly depends on the period of the pit train of the recorded signal, the laser wavelength of the reproducing optical system, and the numerical aperture of the objective lens.

At present, when the laser wavelength λ of the reproducing optical system and the numerical aperture NA of the objective lens are determined, the pit period f serving as a detection limit is determined. That is, f = λ / 2 · NA.

On the other hand, track density is mainly limited by crosstalk. The crosstalk is mainly determined by the distribution (profile) of the laser beam on the medium surface, and is also roughly expressed by a function of λ / 2 · NA, similarly to the pit period.

Therefore, in order to realize a higher density in a conventional optical disc, the basic attitude is to shorten the laser wavelength λ of the reproducing optical system and increase the numerical aperture NA of the objective lens.

However, there is a limit to the improvement of the laser wavelength and the numerical aperture of the objective lens. On the other hand, techniques for improving the recording density by devising the configuration and reading method of the recording medium have been developed.

For example, the applicant of the present application has previously proposed in Japanese Patent Application Laid-Open Nos. 1-130441 and 1-143042 a method in which a recording pit (magnetic domain) is reproduced while being enlarged and disappeared during reproduction to improve the reproduction resolution. ing. This method uses an exchange-coupling multilayer film composed of a reproducing layer, an intermediate layer, and a recording layer as a recording medium, and expands or erases magnetic domains of the reproducing layer heated by a reproducing light beam during reproduction, thereby forming a code between codes during reproduction. The interference is reduced, and a signal having a period equal to or less than the diffraction limit of light can be reproduced.

[Problems to be solved by the invention]

However, in the above-described method, although the linear recording density is improved, the crosstalk is the same as that of an ordinary optical disk, and it is difficult to improve the track density.

The present invention has been proposed in view of such conventional circumstances, and provides a signal reproducing method capable of eliminating crosstalk and improving not only linear recording density but also track density. With the goal.

[Means for solving the problem]

In order to achieve the above object, the present invention uses a magneto-optical recording medium having a multilayer film in which a reproduction layer and a recording holding layer exchange-coupled to each other are formed, and performs signal recording on the recording holding layer. At the same time, the magnetization direction of the reproduction layer is aligned prior to reproduction, and at the time of reproduction, the reproduction layer is irradiated with a laser beam to raise the temperature of the reproduction layer, and the signal recorded on the recording holding layer is raised. It is characterized in that it is converted into an optical signal by the magneto-optical effect and read out while transferring only a part of the reproducing spot in accordance with the temperature distribution of the reproducing layer due to the temperature.

That is, in the present invention, a multilayer film having at least a reproducing layer and a recording holding layer is used as a recording medium, and before reproducing a signal, the reproducing layer is kept in the same state (erased state) over a certain temperature at which the reproducing light is irradiated. It is set so that the signal recorded in the recording holding layer in advance is transferred to the reproducing layer only in the area where the area has become, and in the area below the temperature, the part is optically masked without being involved in the signal at all. And improve 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 layers of a perpendicular magnetization film (reproducing layer and recording holding layer). For example, exchange coupling made of a rare earth-transition metal alloy thin film A multilayer film (at least a two-layer film, a three-layer film, preferably a four-layer film or more is preferable) is suitable. Of course, not limited to this, garnet film or CoC
It may be a perpendicular magnetization film of r, PtCo, PdCo or the like, or may be a coating film of a magnetic paint in which hexagonal ferrite powder such as barium ferrite is dispersed. However, it is necessary that the reproducing layer and the recording holding layer are magnetically coupled by magnetostatic coupling or exchange coupling. Also, the reproducing layer needs to have a large Kerr rotation angle and a large Faraday rotation angle.

In the recording holding layer, similar to a normal magneto-optical recording medium,
The signal may be recorded by an optical modulation method or a magnetic field modulation method.However, a perpendicular magnetization film is provided in contact with the recording holding layer, and a magnetic signal is applied to the perpendicular magnetization film by a magnetic head in the same manner as a perpendicular magnetic recording medium. After recording, the magnetic signal recorded on the perpendicular magnetization film by laser light irradiation may be transferred to the recording holding layer.

Then, as shown in FIG. 1, a signal is recorded on the recording holding layer (1) of the magneto-optical recording medium having the above-mentioned configuration, while the reproducing layer (2) is kept in the erased state with the magnetization directions aligned. In this example, the direction of magnetization of the reproducing layer (2) is aligned upward in the figure.

Here, the erasing of the reproducing layer (2) may be performed by the external magnetic field _HER . That is, _HER > Hc ₁ [where Hc ₁ is the reproducing layer (2)
Once you have the magnetization reversal field], it is possible to align the direction of magnetization of the reproducing layer (2) in the direction of the external magnetic field H _ER.
Also, at this time, if _HER << Hc ₂ [where Hc ₂ is the magnetization reversal magnetic field of the recording holding layer (1)], the signal recorded on the recording holding layer (1) is not affected.

During reproduction, the reproduction layer (2) is irradiated with laser light LB,
The area irradiated with the laser beam LB is heated and the temperature rises.

At this time, as shown in FIG. 2, when the reproducing layer (2) reaches a certain temperature _TPB or higher, a signal recorded in the recording holding layer (1) is transferred to the reproducing layer (2).

For example, if the recording holding layer (1) and the reproducing layer (2) are magnetically coupled by magnetostatic coupling, the floating magnetic field Hs ₂ from the recording holding layer (1), Demagnetizing field
Hd _1, the magnetic domain generated magnetic field Hn ₁ of the reproducing layer (2), the external applied magnetic field H _PB applied during reproduction, when: the predetermined temperature _{T PB, Hs 2 + Hd 1} ± H PB <Hn 1 ·· The magnetization, coercive force, and film of each layer satisfy the expression (1) and satisfy the expression Hs ₂ + Hd ₁ ± H _PB > Hn ₁ when the temperature is _{equal to} or higher than the predetermined temperature T _PB. If the thickness or the like is set, a signal is transferred according to the stray magnetic field generated from the recording holding layer (1) only in a region heated to the temperature _TPB or higher.

Similarly, assuming that the recording holding layer (1) and the reproducing layer (2) are magnetically coupled by exchange coupling, an equivalent magnetic field Hw ₁ [= σw / 2Ms
₁ h ₁ : where σw is an interface domain wall energy density generated between the reproducing layer (2) and the magnetic layer in contact with the reproducing layer (2), Ms ₁ is the saturation magnetization of the reproducing layer (2), and h ₁ is the reproducing Layer (2)
Is the film thickness. ] Is less than or equal to the predetermined temperature T _{PB with} respect to the magnetic domain generated magnetic field Hn ₁ of the reproducing layer (2), Hw ₁ ± H _PB <Hn ₁ (3), and the predetermined temperature T _PB In the above case, if Hw ₁ ± H _PB > Hn ₁ (4), the signal is transferred by the exchange force with the recording holding layer (1) only in the region heated to the temperature T _PB or higher. You.

The transferred magnetic signal is converted into an optical signal by the magneto-optical effect (Kerr effect or Faraday effect) of the reproducing layer (2), and is reproduced by detecting the Kerr rotation angle of the laser beam LB.

At the time of reproduction, as shown in FIG.
If the temperature T _N at the boundary between t _a and the adjacent track t _b is a temperature distribution such that T _N <T _PB , the signal recorded on the recording holding layer (1) below the adjacent track t _b Is not transferred to the reproducing layer (2), and the crosstalk is completely eliminated.

[Action]

According to the signal reproducing method of the present invention, the reproducing layer from which the signal is read has the same magnetization direction and is in the same state (erased state) over the entire surface, and the signal is transferred and read only in the region irradiated with the laser beam. It is.

Therefore, there is no influence of the adjacent track,
Crosstalk is eliminated.

Further, due to the temperature distribution at the time of irradiating the laser light, the front end in the traveling direction of the laser light has a shape as if the erased state is maintained and as if it were masked, and the pit period is smaller than the beam diameter of the laser light. Is also reduced in apparent spatial frequency, and is reproduced at high C / N.

〔Example〕

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

Example 1 This example is an example in which magnetostatic coupling is used for magnetic coupling between the recording holding layer and the reproducing layer.

As shown in FIG. 4, the magneto-optical recording medium of the present embodiment
A first reproducing layer (12), a second reproducing layer (13), and a recording holding layer (14) are formed on a transparent substrate (11) made of polycarbonate, glass, or the like by dielectric films (15), (16). And a dielectric film (17) is provided on the outermost layer as well. As a material for the dielectric films (15), (16), and (17), a transparent dielectric material such as silicon nitride, silicon oxide, or aluminum nitride can be used.

The first reproducing layer (12) has a high Curie point (for example, 20
(0 ° C. or higher), a perpendicular magnetization film having a large Kerr rotation angle and a coercive force of several hundred Oersteds or less.

The second reproducing layer (13) is a perpendicular magnetic film having a large perpendicular magnetic anisotropy, a low Curie point (for example, 200 ° C. or less), and a coercive force of about 2 kilooersted (kOe) at room temperature.

The first reproducing layer (12) and the second reproducing layer (1
3) is exchange coupled.

When the reproducing layer has a two-layer structure as described above, the reproducing layer changes sharply at a temperature where the temperature of the magnetization reversal magnetic field of the reproducing layer has a temperature dependency.
In addition, the switching field can be set to a low value of about 100 (Oe) above the Curie point of the second reproducing layer (13), and stable transfer of minute bits can be performed.

On the other hand, the recording holding layer (14) is made of a material having a large perpendicular magnetic anisotropy and a Curie point higher than the Curie point of the second reproducing layer (13). The Curie point of the record holding layer (14) serves as a guide for setting the output margin of the laser beam used for reading and writing.
Must be higher than the Curie point of the regeneration layer (13) by 50 ° C. or more.

Actually, the first reproducing layer (12) is made of GdFeCo, the second reproducing layer (13) is made of TbFe, and the recording holding layer (14) is made of TbFeCo. Coercivity is 10 (k
Oe) Set above.

Using such a magneto-optical recording medium, when a signal recorded on the recording holding layer (14) was reproduced while being transferred to the reproducing layer (12) by reproduction light, an extremely high C / N was realized.

Embodiment 2 This embodiment is an example in which exchange coupling is used for magnetic coupling between the recording holding layer and the reproducing layer.

The configuration of a magneto-optical recording medium using exchange coupling is a two-layer exchange coupling film including a reproducing layer (21) and a recording holding layer (22) as shown in FIG. 5 (hereinafter referred to as medium A). ), An intermediate layer (25) interposed between the reproducing layer (23) and the recording holding layer (24) as shown in FIG.
This is referred to as medium B. As shown in FIG. 7, the reproducing layer has a two-layer structure of a first reproducing layer (26) and a second reproducing layer (27) in the same manner as in the first embodiment, and an intermediate layer (28) is provided. (Hereinafter, referred to as a medium C) in which a recording holding layer (29) is laminated.

Here, in the medium A, in order to satisfy the above-mentioned expressions (3) and (4), the thickness of the reproducing layer (21) and the recording holding layer (22) must be increased. If the linear velocity is high due to the limitation of the laser power, there is a possibility that recording and erasing cannot be performed.

Therefore, it is considered that the interface magnetic wall energy density can be reduced by interposing an intermediate layer so that the thickness of each layer can be reduced. However, in the medium B, the equivalent magnetic field Hw ₁ due to the exchange force is reproduced. The temperature at which the magnetic domain generated magnetic field Hn ₁ becomes equal to the layer becomes very sensitive to the film composition, the film thickness, and the like, and the unevenness on the disk periphery is significantly affected, and noise and jitter tend to increase during reproduction.

On the other hand, in the medium C, the first reproducing layer (26) is made of a material having a high Curie point, a large Kerr rotation angle, and a small coercive force, and the second reproducing layer (27) has a Curie point. By using a material with low coercive force of about 2 (kOe),
A magneto-optical recording medium with good reproduction S / N can be obtained.

On the polycarbonate substrate, the present inventors have developed a dielectric layer (thickness: 800 膜厚) made of Si ₃ N _{4, a} first reproduction layer (thickness: 300 Å) made of GdFeCo, and a second reproduction layer (film) made of TbFe. 150 mm thick), an intermediate layer of GdFeCo (100 mm thick), TbFe
Recording holding layer made of Co (film thickness 350 Å), a dielectric layer composed of Si ₃ N ₄ (thickness 800 Å) are sequentially laminated by sputtering to prepare a sample disk.

Then, a carrier signal of 5 MHz was written into the recording holding layer, and the laser output dependence of the carrier and noise reproduction output was examined with a linear velocity of 5 m / sec and a reproduction external magnetic field of 500 (Oe). At the same time, crosstalk was also measured. Crosstalk is caused by forming 0.8 μm wide groups on the substrate at 0.8 μm intervals (therefore, 0.8 μm wide lands are formed between the groove portions), and 4.8 MHz signals are respectively applied to the land portions and the groove portions. Was recorded and measured.

The results are shown in FIG. In the figure, the line i represents the reproduced output of the carrier, the line ii represents the reproduced output of the noise, and the line iii represents the crosstalk.

The reason that no carrier is observed at a laser output of 1.6 mW or less is because the medium temperature has not reached the temperature required for transfer at this power.

On the other hand, at a laser output of 2.0 mW or more, carriers are sharply observed. Crosstalk is 2.5mW laser output
In the sample disk, the laser output is set to 2.0 mW or more and 2.5 mW or less, so that signal reproduction at high C / N without crosstalk is possible.

Therefore, in order to confirm the superiority of reproducing while transferring, the pit period dependency of C / N was examined. The linear velocity for measurement was 5 m / s, and the reproducing magnetic field was 500 (O
e), laser wavelength is 780nm, numerical aperture of objective lens N.
A. is 0.53. The results are shown in FIG.

In FIG. 9, a curve a shows the characteristics when the reproducing layer is initialized with an external magnetic field of 3.5 (kOe) and then reproduced while transferring magnetic domains with a laser output of 2.8 mW, and a curve b shows the same characteristics. The graph shows the characteristics in the case where reproduction was performed once under the conditions, and then reproduction was performed without initialization at a laser output of 1.4 mW. Therefore, the curve b corresponds to C / N in normal reproduction.

Referring to FIG. 9, the adoption of the method of the present invention allows C /
It can be seen that N is greatly improved.

〔The invention's effect〕

As is clear from the above description, in the present invention, since the magnetic signal recorded on the recording holding layer is read while being transferred to the reproducing layer, crosstalk can be eliminated, and the track density and the track density can be reduced. Even when the linear recording density is high, it is possible to reproduce a signal with a high C / N.

Therefore, it is very useful for achieving high density recording in a magneto-optical recording medium, and its significance is great.

[Brief description of the drawings]

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

Claims (1)

(57) [Claims] 1. A magneto-optical recording medium having a multilayer film in which a read-out layer and a record-holding layer exchange-coupled to each other are formed, and signal recording is performed on the record-holding layer, and the read-out is performed prior to the read-out. The magnetization direction of the layers is aligned, and at the time of reproduction, the reproduction layer is irradiated with laser light to raise the temperature of the reproduction layer, and the signal recorded on the recording holding layer is subjected to the temperature distribution of the reproduction layer due to the temperature increase. A signal reproducing method for a magneto-optical recording medium, wherein the signal is converted into an optical signal by a magneto-optical effect and read while transferring only a part of the reproducing spot in accordance with the signal.