JPH0535498B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a Curie point writing type magneto-optical recording medium that can be read using the magnetic Kerr effect, and an overwritable magneto-optical recording medium using the same. Regarding the method.

[Conventional technology]

A magneto-optical disk is known as an erasable optical disk memory. Magneto-optical disks have advantages over magnetic recording media using conventional magnetic heads, such as high-density recording and non-contact recording and playback, but on the other hand, the recorded area must be erased before recording. It has the disadvantage that it cannot be magnetized in one direction (it must be magnetized in one direction). In order to compensate for this drawback, proposals have been made such as a method in which a recording/reproducing head and an erasing head are provided separately, or a method in which recording is performed while irradiating a continuous laser beam and simultaneously modulating the applied magnetic field.

[Problem that the invention seeks to solve]

However, these methods have drawbacks such as a large-scale apparatus, high cost, and the inability to perform high-speed modulation.

We have developed a magneto-optical recording method that eliminates the drawbacks of the above-mentioned known techniques and enables overwriting similar to that of magnetic recording media by simply adding a magnetic field generating means of a simple structure to the conventional device configuration. The present applicant filed Japanese Patent Application No. 61-158787 on July 8, 1985 (this application was filed as a domestic priority application filed on February 2, 1988).
This was proposed in the basic application No. 20384).

However, because this method is a completely new recording method, many research questions still remain. That is, the research involves the search for a magneto-optical recording medium that optimizes the magneto-optical effect during reproduction and provides good recording sensitivity.

As a result of further research, the present inventors obtained several results.

The present invention was completed in this way, and its purpose is not only to provide a recording method that allows overwriting, but also to provide a magneto-optical method that can optimize the magneto-optical effect during reproduction and obtain good recording sensitivity. The goal is to provide recording media.

[Means for Solving the Problems] The present invention capable of achieving the above-mentioned objects includes: a first magnetic layer having a Kyrie point T ₁ and a coercive force H ₁ ; and a second magnetic layer having a Kyrie point T ₂ and a coercive force H ₂ . A magneto-optical recording medium comprising a two-layer exchange-coupled perpendicularly magnetized film on a substrate, which satisfies the following conditions: _H1 ＞H ₂ ＞σw/2MsL ₂ T ₁ ＜T ₂ , 600Å＜L ₁ +L ₂ ＜1000Å 200Å＜L ₁ (Ms is the saturation magnetization of the second magnetic layer, L ₂ is its thickness,
This recording method is characterized in that L ₁ is the film thickness of the first magnetic layer, σw is the domain wall energy between the two magnetic layers), and that binary overwritable recording is performed using this.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIGS. 1a and 1b are schematic cross-sectional views each showing an embodiment of the magneto-optical recording medium of the present invention. The magneto-optical recording medium shown in FIG. 1a has a first magnetic layer 2 and a second magnetic layer 3 laminated on a transparent substrate 1 provided with pregrooves. The first magnetic layer 2 has a low Kyrie point T ₁ and a high coercive force H ₁ , and the second magnetic layer 3 has a high Kyrie point T ₂ and a low coercive force H ₂ . Here, "high" and "low" represent a relative relationship when comparing both magnetic layers (coercive force is compared at room temperature). Note that T ₁ ≈T ₂ may also be satisfied. Usually, T ₁ of the first magnetic layer 2 is 70 to 180°C, and H ₁ is 3 to 180°C.
10KOe, T 2 of second magnetic layer ₃ 100-400℃, H ₂ 0.5
It is recommended to select from within the range of ~2KOe.

The main component of each magnetic layer can be one that exhibits perpendicular magnetic anisotropy and exhibits a magneto-optical effect.
Amorphous magnetic alloys of rare earth elements and transition metal elements are preferred. For example, GdCo, GdFe, TbFe,
DyFe, GdTbFe, TbDyFe, GdTbFeCo,
Examples include TbFeCo and GdTbCo.

In the magneto-optical recording medium of the present invention, the first magnetic layer 2 is mainly involved in reproduction. That is, the magneto-optic effect exhibited by the first magnetic layer 2 is mainly used for reproduction, and the second magnetic layer 3 plays an important role in recording.

On the other hand, in the exchange-coupled bilayer film in the conventional magneto-optical recording method, conversely, the magnetic layer with a low Kyrie point and high coercive force is mainly involved in recording; The magnetic layer was mainly involved in reproduction.

In such a conventional exchange-coupled two-layer film, the saturation magnetization Ms of the magnetic layer mainly involved in reproduction, its film thickness L,
It is desirable that the following relationship exists between the domain wall energy σw between the two layers.

H ₁ > σw/2MsL > H ₂ However, in the exchange-coupled double-layer film of the recording medium used in the present invention, the saturation magnetization Ms of the second magnetic layer 3, its film thickness L ₂ , and the domain wall energy σw between the two magnetic layers The following relationship is required between:

H ₁ > H ₂ > σw/2MsL _2This is to stabilize the magnetization state of the bit (shown in Figure 2 f) that is finally completed by recording (the detailed reason will be explained later). .

Therefore, the film thickness, coercive force, magnitude of saturation magnetization, domain wall energy, etc. of each layer may be appropriately set so that both magnetic layers 2 and 3 (perpendicularly magnetized films) satisfy the above relational expression.

In addition to the above-mentioned requirements, the magneto-optical optical medium of the present invention has the following conditions: 600 Å<L ₁ +L ₂ <1000 Å 200 Å<L It also satisfies requirement ₁ . (The reason for this setting will be explained later.) Note that both magnetic layers 2 and 3 do not undergo exchange coupling, considering the magnitude of the effective bias magnetic field during recording or the stability of binary recorded bits. It is desirable that

In FIG. 1b, 4 and 5 are protective films for improving the durability of both magnetic layers or for improving the magneto-optical effect.

6 is an adhesive layer for bonding the bonding substrate 7 together. By laminating 2 to 5 layers on the bonding substrate 7 and gluing them together, recording and reproduction can be performed on both sides.

The recording process will be described below with reference to FIGS. 2 to 4. Before recording, the stable magnetization directions of both magnetic layers 2 and 3 may be parallel (same direction) or antiparallel (opposite directions). In FIG. 2, a case will be explained in which the stable directions of magnetization are parallel.

35 in FIG. 3 is a magneto-optical disk having the configuration described above. For example, suppose that the magnetization state of a certain part of this magnetic layer is initially as shown in FIG. 2a. The magneto-optical disk 35 is rotated by a spindle motor and passes through the magnetic field generating section 34 . At this time, if the magnitude of the magnetic field of the magnetic field generator 34 is set to a value between the coercive forces of both the magnetic layers 2 and 3 (the direction of the magnetic field is upward in this embodiment), the magnetic field will appear as shown in FIG. 2b. 2 The magnetic layer 3 is magnetized in a uniform direction,
On the other hand, the magnetization of the first magnetic layer 2 remains unchanged.

Next, when the magneto-optical disk 35 rotates and passes the recording/reproducing head 31, the recording signal generator 3
According to the signal from 2, 2 types (1st type and 2nd type)
The disk surface is irradiated with a laser beam with a laser power value of (seed). The first type of laser power is enough to raise the temperature of the disk to around the Curie point of the first magnetic layer 2, and the second type of laser power is enough to raise the temperature of the disk to around the Curie point of the second magnetic layer 3. It is a power that can be heated. That is, in FIG. 4, which schematically shows the relationship between the coercive force and temperature of both magnetic layers 2 and 3, the first type laser power is
Near T ₁ , the second type of laser power can raise the temperature of the disk to near T ₂ .

The first magnetic layer 2 is
The temperature rises to near the Curie point, but the second magnetic layer 3
has a coercive force that allows bits to exist stably at this temperature, so by setting the bias magnetic field appropriately during recording, bits like those in Figure 2 c can be obtained from any of Figure 2 b. Formed (preliminary record of type 1).

Here, setting the bias magnetic field appropriately means the following. That is, in the first type of preliminary recording, the magnetization of the first magnetic layer 2 receives a force (exchange force) that aligns it in a stable direction (here, in the same direction) as the direction of magnetization of the second magnetic layer 3. Therefore, conventionally, a bias magnetic field is not required. However, the bias magnetic field is set in a direction that assists the magnetization reversal of the second magnetic layer 3 in preliminary recording using the second type of laser power (described later) (that is, a direction that hinders the first type of preliminary recording). And this bias magnetic field is the first
For convenience, it is preferable to set the magnitude and direction to be the same in preliminary recording of both the seed and second type laser powers.

From this point of view, it is preferable to set the bias magnetic field to the minimum size necessary for the second type of laser power preliminary recording based on the principle shown below, and settings that take this into consideration are the appropriate settings mentioned above. It is.

Next, the second type of preliminary recording will be explained.

The second magnetic layer 3 is
When the temperature is raised to near the Curie point (second type of preliminary recording), the direction of magnetization of the second magnetic layer 3 is reversed by the bias magnetic field set as described above. Subsequently, the magnetization of the first magnetic layer 2 is also aligned in a stable direction (here, in the same direction) with respect to the second magnetic layer 3.
That is, a bit as shown in FIG. 2d is formed from any of the bits shown in FIG. 2b.

In this way, each location on the magneto-optical disk is preliminarily recorded in the state shown in FIG. Become.

Next, when the magneto-optical disk 35 is rotated and the pre-recorded bits c and d pass through the magnetic field generating section 34 again, the magnitude of the magnetic field of the magnetic field generating section 34 will be between the coercive forces of the magnetic layers 2 and 3 as described above. Therefore, the recording bit c remains in the state e (final recording state) without any change. On the other hand, the recording bit d undergoes magnetization reversal in the second magnetic layer 3 and enters the state f (another final recording state).

In order for the state of the recording bit f to exist stably, the magnitude of saturation magnetization Ms of the second magnetic layer 3, the film thickness
As described above, it is sufficient if there is the following relationship between L ₂ and the domain wall energy σw between the magnetic layers 2 and 3.

σw/2MsL ₂ <H ₂ <H ₁ Here, σw/2MsL ₂ represents the strength of the exchange force acting on the second magnetic layer. In other words, a magnetic field with a magnitude of σw/2MsL ₂ attempts to direct the magnetization direction of the second magnetic layer 3 in a stable direction (in this case, the same direction) as the magnetization direction of the first magnetic layer 2. . Therefore, in order for the second magnetic layer 3 to resist this magnetic field and not reverse its magnetization, assuming the coercive force of the second magnetic layer 3 to be H ₂ , H ₂ >σw/
2MsL ₂ is sufficient.

The states e and f of the recorded bits are controlled by the laser power during recording and do not depend on the state before recording, so overwriting is possible. Recorded bits e and f can be reproduced by irradiating a reproduction laser beam and processing the reproduction light by a recording signal regenerator 33.

Next, the reproduction of recorded bits e and f will be further described.

In the recording bits of the conventionally known exchange-coupled two-layer film, the directions of magnetization of the two types of bits are reversed in both magnetic layers depending on the respective recording state. In the present invention, the direction of magnetization of the first magnetic layer 2 (mainly involved in reproduction) is reversed depending on each recording, but the direction of magnetization of the second magnetic layer 3 (mainly involved in recording) is always changed by the magnetic field. This is the magnetization direction of the generating section 34 and does not change. As the thickness of the magnetic layer (which does not contribute to the magneto-optical effect) increases, the sensitivity of the magneto-optical recording medium decreases, so the thickness of the magnetic film L ₁ +
It is necessary to combine and optimize the film thicknesses of L ₁ and L ₂ so as to minimize L ₂ and increase the magneto-optical effect during reproduction. As will be explained with reference to data in Examples, the combination of film thicknesses that showed good sensitivity and a large magneto-optical effect was 600 Å L ₁ + L ₂ 1000 Å 200 Å L ₁ .

In the explanation of FIG. 2, the first magnetic layer 2 and the second magnetic layer 3 are
We showed an example of stability when the direction of magnetization is the same, but
The same can be said of a magnetic layer that is stable when the direction of magnetization is antiparallel. FIG. 5 shows the magnetization state during the recording process in this case, corresponding to FIG. 2.

〔Example〕

Example 1 A polycarbonate disk-shaped substrate with pregroove and preformat signals engraved thereon was set in a sputtering apparatus equipped with a three-dimensional target source at a distance of 10 cm from the target, and rotated.

In argon, from the first target at a sputtering speed of 100 Å/min and a sputtering pressure of 5×10 ^-3 Torr.
ZnS was provided as a protective layer with a thickness of 800 Å. Next, in argon, TbFe was sputtered from a second target at a sputtering speed of 100 Å/min and a sputtering pressure of 5×10 ^-3 Torr.
Sputter the alloy to a film thickness of 300 Å, T ₁ = approximately 140°C,
A first magnetic layer of Tb _19.5 Fe _80.5 with H ₁ =about 8 KOe was formed.

Next, under argon with a sputtering pressure of 5×10 ^-3 Torr.
Sputter TbFeCo alloy, film thickness 400 Å, T ₂ = approx.
A second magnetic layer of Tb _24.5 Fe ₆₈ Co _7.5 was formed at 190° C. and H ₂ =about 0.8 KOe.

Next, in argon, the first target was sputtered at a sputtering speed of 100 Å/min and a sputtering pressure of 5×10 ^-3 Torr.
ZnS was provided as a protective layer with a thickness of 3000 Å.

Next, the above substrate on which the film had been formed was bonded to a polycarbonate bonding substrate using a hot melt adhesive to create a magneto-optical disk. (600 Å<L ₁ +L ₂ = 700 Å<1000 Å 200 Å<L ₁ =300 Å) This magneto-optical disk was set in a recording/reproducing device, and the magnetic field generator of 2.5 KOe was applied at a linear velocity of approximately 8 m/sec.
A laser beam with a wavelength of 830 nm focused to approximately 1 μm is transmitted at 2 MHz with a duty of 50%.
Recording was carried out using binary laser powers of 4 mW and 8 mW while modulating the power. The bias magnetic field is
It was 100Oe.

After that, a 1.5 mW laser beam was irradiated to perform reproduction, and a binary signal could be reproduced.

Next, an experiment similar to that described above was conducted on a magneto-optical disk that had been completely recorded. As a result, previously recorded signal components were not detected, confirming that overwriting is possible.

Example 2 A magneto-optical disk sample was produced using the same method and the same materials as in Example 1, except that only the thickness of the first magnetic layer and the thickness of the second magnetic layer were changed.

As a guideline for evaluating the reproduced signal of each sample, the reflectance and Kerr rotation angle of the sample at a wavelength of 830 nm were measured. Furthermore, when the light receiving element is a photodiode, the product of the square value of the reflectance and the Kerr rotation angle, which is considered to be the reproduction performance index, was calculated. These results are shown in Table 1.

In the recording bit formed according to the present invention, the magnetization of the second magnetic layer does not change in direction from the recording state and does not contribute to the magneto-optical effect. When the thickness of the first magnetic layer 2 is small, when the second magnetic layer 3 is absent, or when the thickness is small, the reflected light intensity is small and the figure of merit is also small.

Sum of the film thicknesses of the first magnetic layer 2 and the second magnetic layer 3 L ₁ +
When L ₂ becomes larger than 400 Å, the reflected light intensity becomes almost constant.

Sum of the film thicknesses of the first magnetic layer 2 and the second magnetic layer 3 L ₁ +
When L ₂ is 600 Å or more, the figure of merit and Kerr rotation angle are saturated and become almost constant values, but the film thickness of the first magnetic layer 2
When L ₁ is less than 200 Å, the thickness of the second magnetic layer 3
It was found that even when L ₂ becomes large, the Kerr rotation angle is saturated at a value smaller than the value when the film thickness L ₁ of the first magnetic layer 2 is 200 Å or more. From this result, 600Å
It has become clear that <L ₁ +L ₂ and 200 Å<L ₁ are preferable.

Furthermore, recording and reproduction experiments were conducted using these samples in the same manner as in Example 1. L ₁ + L ₂ is 600Å
For the above samples, good reproduced signals similar to those of Example 1 were obtained.

In addition, if L ₁ + L ₂ is 1000 Å, the first type and the second type
Laser power required to record each species is L ₁
+L ₂ is approximately 2.5 times that of 600Å, and L ₁ +L ₂ is 1000
When exceeding Å, the laser power required for recording increased rapidly. That is, it has become clear that L ₁ +L ₂ <1000 Å is preferable.

As described above, the magnetization of the second magnetic layer 3 is
Even if the direction does not change during reproduction of both type 2 recorded signals (that is, does not contribute to the magneto-optical effect), the provision of the second magnetic layer 3 makes it appear that the first magnetic layer 2 is It was found that the same Kerr rotation angle and regeneration performance index could be obtained with a film thickness of L ₁ + L ₂ .

Next, for comparison, the first
A magnetic field larger than the coercive force of the magnetic layer 2 is applied, and the direction of the magnetization of the second magnetic layer 3 is changed between the first type and the second type of recording state similarly to the magnetization of the first magnetic layer 2 ( Measure the reflectance and Kerr rotation angle (same as in the case of a conventional exchange-coupled bilayer film), and measure the reflectance and Kerr rotation angle similarly.
A regeneration performance index was also calculated. The results are shown in Table 2.

In this case, the magneto-optical effect of the second magnetic layer 3 appears (because there is reversal of magnetization depending on the recording state),
The second magnetic layer 3 has a higher Curie temperature, and under the same film thickness conditions, the Kerr rotation angle of the second magnetic layer 3 is larger than that of the first magnetic layer 2. Even if the second magnetic layer 3 is smaller than 200 Å,
If the film thickness is about 300 to 400 Å, a large rotation angle,
Indicates the playback performance index. However, when the thickness of the first magnetic layer 2 is greater than 200 to 250 Å, the contribution of the magneto-optical effect of the second magnetic layer 3 is hardly seen.

This is because the second magnetic layer 3 shown in Table 1 is provided, which apparently shows the same Kerr rotation angle and figure of merit as if the first magnetic layer 2 had a film thickness of L ₁ + L ₂ . match the result.

In addition, in the sample with the same configuration as in Example 1, the material of the protective film was also Si ₃ , N ₄ SiC,
Even when SiO, Al ₂ O ₃ , etc. were used, the relationship between the rotation angle and the thickness of the first and second magnetic layers when the figure of merit was saturated was the same. In the case of SiC, which has a higher refractive index than Si ₃ N ₄ and ZnS, the figure of merit increased by about 10%, but the recording sensitivity decreased at the same film thickness L ₁ +L ₂ . In addition, it has a lower refractive index than Si ₃ N ₄ and ZnS.
For SiO and Al ₂ O ₃ , the figure of merit decreased by about 10%.

Furthermore, the material of the first magnetic layer 2 is changed from TbFe to
Similarly, when GdTbFe, TbFeCo, GdTbFeCo, DyTbFeCo, etc. were used instead, the relationship among the thicknesses of the first and second magnetic layers, the rotation angle, and the value at which the figure of merit reached saturation did not change.

〔Effect of the invention〕

As explained in detail above, as a magneto-optical recording medium, the first magnetic layer has a low Curie point _T1 and a high coercive force _H1 , and a relatively high Curie point _T2.
and a second magnetic layer having a low coercive force _H2 ,
The first magnetic layer has a thickness of 200 Å or more, and the sum of the thicknesses of the first and second magnetic layers is between 600 Å and 1000 Å. By providing a magnetic field generating section and recording with binary laser power, it has become possible to perform overwriting with good recording sensitivity and reproduced signals.

[Brief explanation of drawings]

FIGS. 1a and 1b are diagrams each showing an example configuration of a magneto-optical medium used in the present invention, and FIG. 2 is a diagram showing the magnetization directions of magnetic layers 2 and 3 during implementation of the recording method of the present invention. , FIG. 3 is a conceptual diagram of the recording/reproducing apparatus, and FIG. 4 is a schematic diagram showing the relationship between coercive force and temperature of both magnetic layers 2 and 3. FIG. 5 is a diagram showing the magnetization state of the magnetic layer in another embodiment of the present invention. 1: Transparent substrate with pregroove, 2, 3: Magnetic layer, 4, 5: Protective layer, 6: Adhesive layer, 7: Bonding substrate, 31: Recording/reproducing head, 32:
Recording signal generator, 33: Recording signal regenerator, 34:
Magnetic field generation section, 35: magneto-optical disk.

Claims (1)

[Claims] 1. Exchange coupling in a two-layer structure consisting of a first magnetic layer having a Kyrie point T ₁ and a coercive force H ₁ and a second magnetic layer having a Kyrie point T ₂ and a coercive force H ₂ 1. A magneto-optical recording medium comprising a perpendicularly magnetized film on a substrate, which satisfies the following conditions. H ₁ ＞H ₂ ＞σw／2MsL ₂ T ₁ ＜T ₂ , 600Å＜L ₁ +L ₂ ＜1000Å 200Å＜L ₁ (Ms is the saturation magnetization of the second magnetic layer, L ₂ is its thickness,
( _L1 is the thickness of the first magnetic layer, and σw is the domain wall energy between the two magnetic layers). 2 A two-layer structure consisting of a first magnetic layer having a Kyrie point (T ₁ ) and a coercive force (H ₁ ) and a second magnetic layer having a Kyrie point (T ₂ ) and a coercive force (H ₂ ). In a magneto-optical recording medium having an exchange-coupled perpendicularly magnetized film on a substrate, the following conditions H ₁ > H ₂ > σw/2MsL ₂ T ₁ < T ₂ , 600 Å < L ₁ + L ₂ < 1000 Å 200Å<L ₁ (Ms is the saturation magnetization of the second magnetic layer, L ₂ is the film thickness,
_L1 is the film thickness of the first magnetic layer, and σw is the domain wall energy between the two magnetic layers. (a) With respect to the medium, at a location different from the recording head, change the direction of magnetization of the first magnetic layer with coercive force H ₁ enough to magnetize the second magnetic layer with coercive force H ₂ in one direction. Magnetic field B of a magnitude that does not cause reversal
(b) Next, the recording head applies a bias magnetic field and at the same time irradiates laser power sufficient to raise the temperature of the medium to near the low Curie point _T1 , thereby changing the direction of magnetization of the second magnetic layer. Either the first type of preliminary recording, in which the direction of magnetization of the first magnetic layer is aligned in a stable direction with respect to the second magnetic layer without changing the magnetic field, or the medium is raised to near the high Curie point T ₂ while applying a bias magnetic field. A second type of preliminary recording in which the direction of magnetization of the second magnetic layer is reversed by irradiation with laser power sufficient to heat the layer, and at the same time the first magnetic layer is also magnetized in a stable direction relative to the second magnetic layer. (c) then moving the medium to cause the prerecorded bits to pass through the magnetic field B;
For the bits formed by the first type of preliminary recording, the direction of magnetization of both the first magnetic layer and the second magnetic layer remains unchanged, and for the bits formed by the second type of preliminary recording, the direction of magnetization is unchanged in the second magnetic layer. Binary recording in which the direction of magnetization of the first magnetic layer is reversed in the same direction as the magnetic field B, and the direction of magnetization of the first magnetic layer is left unchanged.