JPH0393056A - Magneto-optical recording medium - Google Patents

Abstract

PURPOSE:To allow overwriting and to improve a linear recording density and track density by constituting a recording layer of a perpendicularly magnetized layer and a magneto-optical recording layer and forming the magneto-optical recording layer into multilayered structure having a transfer layer and a reproducing layer. CONSTITUTION:The perpendicularly magnetized film 20 and the transfer layer 13 of the magneto-optical recording layer 10 are so set as to attain Tc2<Tc1, Hc1<Hc2 when the coercive force of the magnetized film 20 is designated as Hc1, the Curie point as Tc1, the coercive force of the transfer layer 13 as Hc2, and the Curie point as Tc2. Only the magnetized film 20 is subjected to recording by a magnetic head at the time of recording. The magnetically recorded signals are transferred to the transfer layer 13 by irradiation with a laser beam. Simultaneously an intermediate layer 12 and a reproducing layer 11 are formed with magnetic domain patterns as well. The reproducing layer 11 is shut off from the magnetic coupling with the transfer layer 13 by the irradiation with the laser beam and the magnetic domain patterns are formed at the time of reproducing. As a result, the magnetic domain patterns in the diameter of the laser beam are read out in a partly masked form. Even the signals formed to the pitch below the diameter of the laser beam are reproduced with high S/N.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a magneto-optical recording medium in which recorded signals are read out using magneto-optical characteristics, and in particular, the present invention relates to a magneto-optical recording medium in which a recorded signal is read out using magneto-optical characteristics. This invention relates to an improvement in a magneto-optical recording medium in which recorded signals are optically read out by transferring them onto a layer.

[Summary of the Invention] The present invention provides a recording layer of a magneto-optical recording medium composed of a perpendicularly magnetized film that can be recorded by a magnetic head, and a magneto-optically recorded layer to which recording signals recorded on the perpendicularly magnetized film are transferred. By doing so, it is possible to perform so-called overwriting, and at the same time,
The magneto-optical recording layer has a multilayer structure including at least a reproducing layer and a transfer layer, and the recording magnetic domain of the reproducing layer is reversed, reduced or expanded during reading, thereby improving the linear recording density.

[Conventional technology]

In the magneto-optical recording method, the temperature of a magnetic thin film is partially raised to exceed the Curie point or the temperature compensation point, thereby eliminating the coercive force in this area and reversing the direction of magnetization in the direction of the externally applied recording magnetic field. Therefore, the structure 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, for example, polycarbonate, which has an excellent magneto-optic effect. recording magnetic layer (e.g. rare earth-transition metal alloy non-quality thin film) or reflective layer,
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.

As a recording method for this magneto-optical recording medium, an optical modulation method or a magnetic field modulation method is adopted, and the magnetic field modulation method is particularly advantageous in that it is possible to overwrite. It is believed that.

However, when recording signals using the magnetic field modulation method, the boundary line of the magnetization direction is determined by the isothermal line during cooling of the recording magnetic layer, so the shape of the recorded magnetic domain usually becomes feather-shaped (or crescent-shaped). There is a tendency. When a magnetic domain is formed in this shape, carriers decrease and jitter increases as the signal period becomes shorter.
Linear recording density is naturally limited.

In terms of linear recording density and overwrite characteristics, the perpendicular magnetic recording method, which records by direct magnetic field, is advantageous, and the shape of the recording magnetic domain can be made close to a rectangle, but in this case, the magnetic head There is a limit to track density due to constraints.

Under these circumstances, a new recording method that combines the magneto-optical recording method and the perpendicular magnetic recording method was reported by Otani et al. at the 36th Applied Physics Conference (Ia-2B-6). In this recording method, recording is performed magnetically on a perpendicularly magnetized film, and immediately after that, the signal of the perpendicularly magnetized film is transferred to the magneto-optical recording layer using a tracking-controlled light beam.The essence of recording is Since it is magnetic recording, overwriting is possible, and it is expected that it will be able to combine the advantages of magnetic recording and optical recording in terms of track density and linear recording density.

[Problem to be solved by the invention]

In this way, the above-mentioned recording method is excellent in terms of recording density and overwriting characteristics during recording, but because the written signal is reproduced optically, it is difficult to ensure the S/N ratio. There are major restrictions on the laser wavelength and numerical aperture of the lens during playback.

In other words, when trying to reproduce a signal smaller than the optical beam diameter, the S/N usually drops significantly, and for example, when an information bit (magnetic domain) with a diameter of 0.2 μm is
At present, it is impossible to read out using a laser beam of m.

Therefore, the actual situation is that the actual recording density decreases and the advantages of the above-mentioned recording methods cannot be fully utilized. Therefore, the present invention has been proposed in view of the above-mentioned conventional situation, and is intended to improve the limitations of recording density especially during reproduction, and to achieve excellent track density and linear recording density during both recording and reproduction. The purpose of the present invention is to provide a magneto-optical recording medium that can achieve the above-mentioned functions and can be overwritten by a magnetic head. [Means for Solving the Problems] In order to achieve the above-mentioned object, the magneto-optical recording medium of the present invention includes a magneto-optical recording layer and a perpendicularly magnetized film formed on a substrate, and the magneto-optical recording The layer has at least a transfer layer to which the recording signal of the perpendicularly magnetized film is transferred, and a reproduction layer that is magnetically coupled to the transfer layer at room temperature and converts the recording signal into an optical signal by a magneto-optic effect, The coercive force of the perpendicularly magnetized film is Hc, and the Curie point is Tc. , when the coercive force of the transfer layer is He2 and the Curie point is Tct, Tcz<Tc
,,Hc<Hcz, and the reproducing layer is characterized in that magnetic coupling with the transfer layer is cut off by temperature rise due to laser beam irradiation during reproduction, and the recording magnetic domain is deformed. [Function] In the magneto-optical recording medium of the present invention, writing a signal is performed by:
It is performed on a perpendicularly magnetized film using a magnetic hesode. At this time, the coercive force Hc+ of the perpendicularly magnetized film<the coercive force He of the transfer layer
．． Therefore, the transfer layer is not affected in any way. The signals recorded on this perpendicularly magnetized film can be generated by irradiating a laser beam onto the transfer layer provided in contact with this perpendicularly magnetized film and raising its temperature, thereby creating an exchange coupling between the perpendicularly magnetized film and the transfer layer or magnetostatic coupling. It is transferred to the transfer layer using bonding. Here, the linear recording density of the magnetic domain pattern to be transferred is determined by the linear recording density during writing by the magnetic head, whereas the trunk width is determined by the spot diameter of the irradiated laser beam. Furthermore, the direction of magnetization of the reproducing layer that is magnetically coupled to the transfer layer changes accordingly, and a magnetic domain pattern corresponding to the transfer layer is formed. On the other hand, during reproduction, by irradiating the reproduction layer with laser light, the magnetization signal recorded in the reproduction layer is converted into an optical signal by the magneto-optic effect (magnetic Kerr effect, Faraday effect) and read out.

At this time, the magnetic coupling between the reproducing layer and the transfer layer is partially severed due to the temperature distribution within the laser beam diameter, and for example, by applying an external magnetic field, the magnetic domain in the portion where the magnetic coupling is severed is deformed (external Depending on the direction of the magnetic field, wJ is enlarged, wJ is small, or reversed).

As a result, the magnetic domain backoon within the laser beam diameter is read out in a partially masked form, and even signals formed at a pitch smaller than the laser beam diameter are reproduced with a high S/N.

〔Example〕

Hereinafter, the present invention will be explained based on specific embodiments with reference to the drawings.

As shown in FIG. 1, the magneto-optical recording medium of this example includes a magneto-optical recording layer (10) consisting of a reproduction layer (11), an intermediate layer (12), and a transfer layer (13) on a transparent substrate (1). and a perpendicularly magnetized film (20) are laminated.

Here, the perpendicularly magnetized film (20) may be of any material as long as it has perpendicular magnetic anisotropy.
An alloy Fs film such as Cr, PtCo, PdCo, TbFeCo, TbFe, etc. is suitable. The film thickness of this perpendicular magnetization film (20) may be 100 or more, and usually 100 or more.
It is set to be around 3,000 pieces.

On the other hand, the reproducing layer (11) comprising the magneto-optical recording layer (10)
, the intermediate layer (12) and the transfer layer (13) are also perpendicularly magnetized films, but the reproducing layer (11) in particular requires a magneto-optic effect to convert the magnetization signal into an optical signal. Alternatively, a magnetic film with a large Faraday rotation angle is used, and usually a rare earth-transition metal alloy film is used. In addition, a reproduction layer (11), an intermediate J! (12) and the transfer layer <13> are preferably magnetically coupled at room temperature by exchange coupling. Therefore, the intermediate layer (12) and the transfer layer (13) also contain a rare earth transition like the reproduction layer (11). It is considered to be a metal alloy film. The rare earth-transition metal alloy films used for the reproduction layer (11), intermediate layer (12), and transfer layer (13) include TbFeCo, TbFe, and Gd.
FeCo, etc. are exemplified, and types. &tl戊 etc. should be selected.

In the magneto-optical recording layer (IO) having the above structure, the reproducing layer (l
The film thickness of 1) is preferably 250 or more in consideration of the S/N during reproduction. Further, the thickness of the intermediate layer (I2) is preferably 50 mm or more from the viewpoint of reliably blocking the magnetic coupling between the reproducing layer (11) and the transfer layer (13) during temperature rise. The film thickness of the transfer layer (l3) is 100 or more because it is necessary to make the equivalent magnetic field or floating M magnetic field due to exchange coupling with the perpendicular magnetization film (20) smaller than the coercive force of the transfer layer (1l). It is preferable to set it as above, and it is more preferable to set it as 200-500 pieces.

The perpendicular magnetization film (20) and the transfer layer (13) must be exchange-coupled or magnetostatically coupled, and the coercive force of the perpendicular magnetization film must be set to Hc. The Curie point is Tc, the coercive force of the transfer layer is Hcz, and the Curie point is Tc. When Tc
, <Tc+, HC, <HC! It is set so that

On the other hand, a reproducing layer (11), an intermediate layer (12) . The coercive force and Curie point of the transfer layer (l3) are T?
■Tcb, Tcc (='rag), and the coercive forces are Hc2, Hcb. When Hc( (=Hcz), T Cb > Tllt (one room temperature), and'
l”a.<Tc■Tcc, the coercive force He of the reproducing layer (11) is sufficiently small near the Curie point Tc6 of the intermediate layer (l2), and the coercive force He of the transfer layer (13) is at room temperature TII.
It is set to be sufficiently larger than the required magnetic field (the magnetic field applied during reading) in the temperature range from T to a predetermined temperature Trm (temperature raised during reading) higher than the Chierie point Tab of the intermediate layer (l2). ．． Note that a dielectric layer may be formed between the magneto-optical recording layer (10) and the transparent substrate (1), and a protective film or the like may be formed on the surface of the perpendicular magnetization film (20), if necessary. Okay, but illustration is omitted here. Next, the principle of recording and reproducing in the magneto-optical recording medium having the above-mentioned #I strength will be explained. First, during recording, the magnetic head M records only on the perpendicularly magnetized film (20), similar to the normal perpendicular magnetic recording method. At this time, the transfer layer (l3) provided in contact with the perpendicular magnetization film (20)
) are not affected in any way. For example, the coercive force Hc of the transfer H(,13) at room temperature! The magnetic head M
It is sufficient to set Hc2>>Hm compared to the magnetic field Hm generated by the magnetic field. Next, the signal (magnetic domain pattern) magnetically recorded on the perpendicular magnetization film (20) is transferred with a focused laser beam 7! (13) is transferred to the transfer layer <l3). The above is schematically illustrated in FIGS. 2 and 3. In addition, in FIG. 3, the perpendicular magnetization film (20
) and the transfer layer (13) are shown, and the intermediate II (12
) and the reproduction layer (11) are not shown. In addition, in FIG. 3, the perpendicular magnetization film (2o) and the transfer! The arrow in (13) represents the direction of magnetization or atomic magnetic moment. If the traveling direction of the magneto-optical recording medium is the direction of arrow X, the preceding magnetic head M causes perpendicular magnetization 1t! (20), a magnetic domain pattern P1 as shown by diagonal lines in FIG. 2 is formed. This magnetic domain pattern P has a wide track width W1 corresponding to the magnetic gap width of the magnetic head M, but has a high linear recording density and is rectangular in shape. Next, the focused laser beam LB is applied to the transfer layer (13).
When the transfer layer (13) is irradiated with light, the temperature of the transfer layer (13) increases, the coercive force rapidly decreases, and the magnetization finally disappears. Then, since the transfer N (13) is magnetostatically or exchange-coupled with the perpendicular magnetization film (20), the signal magnetically recorded on the perpendicular magnetization III (20) is transferred during the cooling process. For example, when the transfer layer (13) and the perpendicular magnetization film (20) are magnetostatically coupled, a stray magnetic field is generated by the magnetization of the perpendicular magnetization Ill (20>. In this state, the transfer layer (
13) with a laser beam, the transfer 111 (1
3) Curie point Tc. When the vicinity is heated, the magnetization direction of the transfer layer (l3) is oriented in the direction of the floating magnetic field during the cooling process,
The signal is transcribed. At this time, the Curie point Tc of the perpendicularly magnetized film (20). Since Tcz<Tc+, the magnetization of the perpendicularly magnetized film (20) will not disappear unless the temperature rise due to the laser beam irradiation exceeds the Curie point Tc+. Similarly, when the perpendicular magnetization film (20) and the transfer layer (l3) are exchange-coupled, the signal of the perpendicular magnetization film (20) is transferred to the transfer layer (l3) for the following reasons.
13). In other words, in stacked two-layer films with the same or similar origin of ferromagnetism, a magnetic force acts when the atomic magnetic moments of each layer (different from i6i) are aligned in a certain direction. ．． For example, if the atomic magnetic moment of the perpendicularly magnetized film (20) points upward in FIG. 3, the transfer 7! directly above it! The atomic magnetic moment in (13) is also a force that points upward. This force (exchange force) acts equivalently to the magnetic field, and similarly to the case of magnetostatic coupling, the transfer Ji (13) is irradiated with a laser beam and the Curie point Tc of the transfer layer (13) is reached. When the vicinity is heated, the direction of the atomic magnetic moment of the transfer layer (l3) is oriented in the direction of the atomic magnetic moment of the perpendicularly magnetized film (20) during the cooling process, and the signal is transferred. In both cases of magnetostatic coupling and exchange coupling, the signal is transferred only within the beam diameter range of the laser beam, so the transfer N (13) has a second
A magnetic domain pattern P2 as shown in black in the figure is formed. Note that such a magnetic domain pattern is transferred to the transfer 71 (1
3), but also on the intermediate N (12>) and the reproducing layer (11) which are magnetically coupled (exchange coupled) thereto.

This pattern P2 has a track width Wt determined by the beam diameter of the laser beam LB, and is formed into a fine rectangular magnetic domain pattern. This is in contrast to the feather-shaped magnetic domain pattern recorded in conventional magnetic field modulation methods in terms of pattern shape. As described above, regarding the recording density of the magneto-optical recording medium of this embodiment, the linear recording density during writing is determined by the magnetic head, and the track density is determined by the temperature profile during focused beam irradiation. On the other hand, from the viewpoint of reproduction, the linear recording density is determined mostly by the wavelength of the laser beam and the numerical aperture of the lens, and the track density is determined by crosstalk. Here, when comparing the linear recording density that can be written with a magnetic head and the linear recording density that can be reproduced with a laser beam, the reproduction performance of the latter is currently rate-limiting, and in order to improve the actual recording density,
This needs to be improved.

Therefore, during reproduction, high-density recording is handled by changing the magnetic domain pattern recorded in the reproduction layer (11) and lowering the apparent spatial frequency of the recorded bits. The principle of reproduction will be explained below.

Now, as shown in FIGS. 4 and 5, the reproduction layer (11)
, a predetermined magnetic signal is transferred to the intermediate layer (12) and the transfer layer (l3) to form a magnetic domain pattern. Note that in FIG. 4, the perpendicular magnetization film <2o) is not shown, and in FIG. 5, the region where the direction of magnetization is upward is shown blacked out. Here, when trying to read a signal using the laser beam LB, if the beam diameter of the laser beam LB is larger than the pitch of the recording pits (magnetic domain pattern), multiple recording pits will exist within the beam diameter. , it is impossible to read them individually with conventional magneto-optical recording media.

However, in the optical recording medium of this embodiment, magneto-optical recording J! Since J (10) is a multilayer film composed of a reproduction layer (11), an intermediate layer (12) and a transfer layer (13), the magnetic domain pattern of the reproduction layer (1l) can be deformed, and the beam Even if the diameter is larger than the pitch of the recording pits, each recorded bit can be read with a high S/N ratio.
When i(10) is irradiated with laser beam LB,
A temperature distribution occurs within the beam diameter, and as shown in FIG.
fii ) temperature increases. At this time, the temperature TP in the shaded area. is the Curie point Tch of the intermediate layer (l2)
If this is the case, the magnetization of the intermediate layer (12) will disappear, and the magnetic coupling (exchange coupling) between the reproduction layer (11) and the transfer layer (13) will be interrupted. In this state, playback layer (11
) coercive force He. Larger external magnetic field H. When the reproducing layer (11) in the shaded area is applied, as shown in FIG.
The direction of magnetization of is reversed and aligned with the direction of the external magnetic field H (here, downward). On the other hand, the area other than the shaded area. That is, in the region where the temperature is low, the reproducing layer (l1)
The magnetic coupling between and transfer Fl (13) is maintained,
The magnetic domain pattern transferred to transfer Fl (13) is maintained as it is. Therefore, as shown in Fig. 8, the shaded area within the beam diameter appears as if it were masked, and the magnetic domain pattern recorded in this area disappears due to the depth, and there is only a single magnetic domain within the beam diameter. It becomes as if only patterns exist. In other words, the spatial frequency of the recording pit seen from the reproduction light during reproduction appears lower than it actually is, and the optical transfer function (o”r
F：oρticat Transfer Function
on ) absolute value (MTF: ModulaLion T
transfer function) becomes larger, and the reproduction resolution improves. In this method, the spatial frequency ν>2
N. A. /λ (where N.^. is the numerical aperture of the objective lens, and λ is the wavelength of the laser beam.) It is possible to set MTF>O during reproduction, and the signal Detection becomes possible. Here, if the coercive force Hcc of the transfer layer (l3) is set larger than the strength of the external magnetic field HPI, the external magnetic field
{,． The magnetization (magnetic domain pattern) of the transfer layer (13) does not change due to the application of .

Also, during cooling, the temperature T=Tc. When , the exchange force comes into play again, and if the following conditions are satisfied, the transfer layer (l
3) The magnetization pattern is between the intermediate N (12) and the reproduction layer (11).
, and the state returns to the same state as before playback.

That is, T≦Tc1, ~ΔT, and 2MsaHcaha +2Ms. HBhi <aw2M
scHcchc+2MscHrshc>σ. however,
The thickness of the intermediate layer (12) is the same as that of the transfer layer (13) and the reproduction layer (l1).
) is negligible compared to the domain wall width, and Ms& is the saturation magnetization of the reproduction layer (11). h. is the playback layer (
1l) film thickness, M sc is transfer! (13) is the saturation magnetization, hC is the thickness of the transfer layer (l3), and σ1 is the interfacial domain wall density (erg/cd).

The above formula is Regeneration I'! This is an example of a condition where the same sublattice magnetization is dominant even if the transfer layer (11) and the transfer layer (l3) are made of a ferrimagnetic material or a ferrimagnetic material.

In the above, we have taken as an example the case where the same sublattice magnetization is dominant even if the reproduction layer (l1) and the transfer N (13) are made of Huet magnetic material or ferrimagnetic material, and reproduction i(
11) The reading method of reproducing while reversing the recorded magnetic domain has been described. When a rare earth-transition metal magnetic film has ferrimagnetism in which the sublattice magnetization of the transition metal and the sublattice magnetization of the rare earth element are opposite to each other, each layer is a transition metal sublattice magnetization dominant film or a rare earth By selecting the direction of the external magnetic field HPI+ applied during reproduction depending on whether the film is an elemental sublattice magnetization dominant film, the magnetic domain pattern can be reversed, reduced or expanded, and the apparent spatial frequency can be changed as in the previous embodiment. can be suppressed and the reproduction resolution can be improved.

Next, we actually tested a sample disc and evaluated its recording and playback characteristics, and the results are shown below.

Z Masu Team Track Pinch A dielectric film made of silicon nitride is formed on a glass substrate having a track groove of 1.6 μm, and on this a reproduction layer made of GdFeCo, an intermediate layer made of TbFe, and a transfer layer made of TbFeCo are formed. A perpendicularly magnetized film made of TbFeCO was formed on top of the TbFeCO film. A film of each layer was formed by continuous sputtering using a high frequency magnetron battering device. Thickness of each layer, Curie point. The magnetic properties (coercive force) are as shown in Table 1.

A single-layer magneto-optical recording layer and a perpendicular magnetization film were laminated on a glass substrate similar to the example in Table 1. The film thickness, Curie point, and magnetic properties (coercive force) of each layer are shown in Table 2.

Table 2 Signals were recorded on the sample disks of these Examples and Comparative Examples using a magnetic head, transferred using an optical pickup, and then reproduced to determine the signal-to-noise level ratio (S/N).
We investigated the recording frequency dependence of The results are shown in Figure 9.

Note that the numerical aperture N. of the objective lens of the pickup used during reproduction. A. is 0.50, laser wavelength is 780nm
The linear velocity was set to 7.5 m/sec, and an external magnetic field of 400 oersteds was applied to the sample disk of the example during reproduction.

As a result, a significant difference was observed between the example and the comparative example, especially in the high frequency band.

(Effects of the Invention) As is clear from the above explanation, in the magneto-optical recording medium of the present invention, signals are recorded using a magnetic head and transferred to the magneto-optical recording layer, so overwriting is possible. In addition, both linear recording density and trunk density can be significantly improved.Also, since the recorded magnetic domain is read out while deforming during playback, a high S/N can be achieved especially when performing high-density recording. It is possible to ensure that

[Brief explanation of drawings]

FIG. 1 is an enlarged schematic cross-sectional view of a main part of a configuration example of a magneto-optical recording medium to which the present invention is applied. FIG. 2 is a plan view schematically showing the magnetic domain pattern written in the perpendicular magnetization film and the transfer layer, and FIG. 3 is a schematic diagram showing the magnetization state at this time. FIG. 4 is a schematic diagram showing the initial magnetization state of the magneto-optical recording layer, and FIG. 5 is a schematic diagram showing the magnetic domain pattern of the reproducing layer at this time. Fig. 6 is a characteristic diagram showing the temperature profile due to laser irradiation during reproduction, Fig. 7 is a schematic diagram showing the magnetization state of the magneto-optical recording layer at this time, and Fig. 8 is a schematic diagram showing the magnetic domain pattern of the reproducing layer. It is a diagram. Figure 9 shows the S/N of an actually produced magneto-optical recording medium.
FIG. 4 is a characteristic diagram showing the recording frequency dependence of the sample in comparison with that of a comparative example.

Claims (1)

[Scope of Claims] A magneto-optical recording layer and a perpendicularly magnetized film for magnetic recording are laminated on a substrate, and the magneto-optically recorded layer includes a transfer layer to which a recording signal of the perpendicularly magnetized film is transferred, and a transfer layer to which a recording signal of the perpendicularly magnetized film is transferred. and a reproducing layer that is magnetically coupled to the perpendicularly magnetized film at room temperature and converts the recorded signal into an optical signal by a magneto-optic effect, and
c_1, the Curie point is Tc_1, and the coercive force of the transfer layer is Hc.
_2, when the Curie point is Tc_2, Tc_2<
A magneto-optical recording medium, wherein Tc_1, Hc_1<Hc_2, and the reproducing layer is magnetically coupled to the transfer layer by being heated by laser beam irradiation during reproduction, thereby deforming the recording magnetic domain.