JP2790685B2 - Magneto-optical recording method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for recording a signal by applying a bias magnetic field while irradiating a magneto-optical recording medium with a light beam such as a laser beam.

[Conventional technology]

2. Description of the Related Art In recent years, as a high-density and large-capacity memory, an optical memory that performs recording or reproduction using a light beam has been actively developed. Above all, a magneto-optical recording medium has received much attention as a rewritable optical memory. In recording on such a magneto-optical recording medium, the magnetization direction of the magnetic layer of the medium is usually aligned in a predetermined direction, and the intensity is adjusted according to the recording signal while applying a bias magnetic field in a direction opposite to the predetermined direction to the medium. This is performed by irradiating a modulated light beam. Reproduction is performed by irradiating a medium on which a signal is recorded as described above with a polarized light beam and changing its polarization direction by a magneto-optical effect.

However, in the above-described recording method, it is necessary to temporarily align the magnetization directions of the magnetic layer when rewriting the signal, that is, to erase the previously recorded signal. On the other hand, Japanese Patent Application Laid-Open No. 51-107121 and the like apply a bias magnetic field whose polarity is inverted in accordance with a signal while irradiating a medium with a light beam having a constant intensity, so that the above-described erasing process is unnecessary. Overwriteable recording methods have been proposed. Also, JP-A-63-153752 or JP-A-62-1759.
No. 48 proposes a method of performing overwriting by irradiating an intensity-modulated light beam to a magneto-optical recording medium having two magnetic layers exchange-coupled to each other.

On the other hand, in order to perform higher-density and higher-speed recording, there is a limit to the information represented by one recording bit only by two values corresponding to the magnetization direction (upward and downward) of the magneto-optical recording film. Means that the recording density of the current magneto-optical recording medium is the wavelength of the semiconductor laser used for recording (currently about 800 nm
m) is almost decided. For example, if you want to double the recording density, Semiconductor lasers with different wavelengths are needed, but will be available in the distant future.

However, for example, if the recording bits recorded on the magnetic layer can indicate four values, the recording density is doubled. In the case of this four-level recording, assuming that recording and reproduction can be performed at the same speed as conventional binary recording, the recording and reproduction speed is also two.
Double. A method for performing such quaternary recording is proposed in U.S. Pat. No. 4,612,587. In this method, recording is performed by irradiating two light beams having different wavelengths and independently modulated intensity to a magneto-optical recording medium having two magnetic layers having different absorption wavelengths.

[Problems to be solved by the invention]

However, the recording method of the above-mentioned US patent has a problem that the magnetization directions of the two magnetic layers must be aligned before recording, and overwriting cannot be performed. Also,
Since two light beams are used, two light sources are required, and the configuration of the recording apparatus is complicated and expensive.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a magneto-optical recording method capable of overwriting a four-level signal by using a recording apparatus having a simple and inexpensive configuration to solve the above-mentioned problems of the prior art.

[Means for solving the problem]

The object of the present invention is to provide a first magnetic layer exhibiting perpendicular magnetic anisotropy, having a higher Curie temperature and a lower coercive force _HL at room temperature than the first magnetic layer, A second magnetic layer, which is exchange-coupled, wherein the exchange force acting on each of the magnetic layers is lower than the coercive force of the second magnetic layer, and the medium is irradiated with a light beam by using a medium. In the magneto-optical recording method of recording a signal by applying a bias magnetic field while applying a bias magnetic field, the polarity of the bias magnetic field having a constant power of the light beam and having an intensity equal to or higher than the _HL is determined according to the signal as follows.
This is achieved by modulating the data into two states and performing quaternary recording.

(A) a first state in which the light beam has power to heat the medium to near the Curie temperature of the first magnetic layer, and a bias magnetic field has a polarity in a predetermined direction; A second state in which the bias magnetic field has a polarity in a predetermined direction, having a power to heat the magnetic layer to a temperature near the Curie temperature of the second magnetic layer, and (c) a light beam heats the medium to a temperature near the Curie temperature of the first magnetic layer. A third state in which the bias magnetic field has a polarity opposite to the predetermined direction, and (d) the light beam has a power to heat the medium to near the Curie temperature of the second magnetic layer. A fourth state in which the magnetic field has a polarity opposite to the predetermined direction.

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

FIG. 1A is a schematic sectional view showing an example of a magneto-optical recording medium used for recording according to the present invention. In FIG. 1, a first magnetic layer 2 and a second magnetic layer 3 are laminated on a light-transmitting substrate 1 provided with a pre-group. The first magnetic layer 2 has a low Curie point (T _L ) and a high coercive force (H _H ), and the second magnetic layer 3 has a high Curie point (T _H ) and a low coercive force (H _L ). Here, “high” and “low” indicate a relative relationship when the two magnetic layers are compared (coercive force is compared at room temperature). However, usually T _L of the first magnetic layer 2 70 to 180
° C., H _H is, 3~20KOe, the T _H of the second magnetic layer 3 100-400
C and _HL are preferably in the range of about 0.1 to 2 KOe.

As the material of each magnetic layer, a material exhibiting perpendicular magnetic anisotropy and exhibiting a magneto-optical effect can be used, but GdCo, GdFe, TbF
e, DyFe, GdTbFe, TbDyFe, GdFeCo, TbFeCo, GdTbCo, GdDyFeC
An amorphous magnetic alloy of a rare earth element such as o, TbDyFeCo, HoGdFeCo and a transition metal element is preferable.

In the magneto-optical recording medium used in the present invention, the first magnetic layer and the second magnetic layer are coupled by exchange coupling.
In these magnetic layers, H _H is the coercive force of the first magnetic layer at room temperature, _HL is the coercive force of the second magnetic layer at room temperature,
When M _S2 is the saturation magnetization of the second magnetic layer, h ₂ is the thickness of the second magnetic layer, and σ _W is the domain wall energy between the two layers, the following relational expression is satisfied.

here It shows an exchange force for arranging the second magnetic layer 3 in a stable direction with respect to the first magnetic layer 2. This relational expression is for allowing the magnetization state of the bit finally completed by recording to be stably present. That is, satisfying the above relationship means that the exchange force acting on each of the first magnetic layer and the second magnetic layer is smaller than the coercive force of each magnetic layer at room temperature.

Both magnetic layer to the characteristics as described above, for example, be between the first magnetic layer and the second magnetic layer providing the magnetic layer (small ie coercivity) large in M _S it is valid.

In the two exchange-coupled magnetic layers, the exchange force is 2
There are cases where the layers act to direct the magnetization directions of the layers in the same direction (parallel), and cases where the layers act in the opposite direction (antiparallel).
Which one is determined by the composition of each magnetic layer, but the recording method of the present invention can be similarly used regardless of the type of medium.

FIG. 1B is a schematic sectional view showing another configuration example of the magneto-optical recording medium used in the method of the present invention. 1B, the same members as those in FIG. 1A are denoted by the same reference numerals, and detailed description thereof will be omitted. In this example, the third magnetic layer 4 for adjusting the exchange force between the first magnetic layer 2 and the second magnetic layer 3 is provided as described above. Also, the first
Adjacent to the magnetic layer 2, a fourth magnetic layer 5 having a larger magneto-optical effect (that is, having a higher Curie temperature) than the first magnetic layer is provided. And the first and fourth
Are bonded with a strong exchange force such that their magnetization directions are always in a stable direction. Such a fourth
By providing the magnetic layer 5, a larger C / N ratio can be obtained during signal reproduction.

Protective layers 6, 7 made of a dielectric material such as Si ₃ N ₄ are provided above and below the four magnetic layers to prevent corrosion of these magnetic layers. In addition, the protective layer 7 has an adhesive layer 8
A bonding substrate 9 made of the same material as the substrate 1 is bonded. If two to seven layers are also laminated on the bonding substrate 9 and these are adhered, recording and reproduction can be performed on both sides.

When the magnetic layer as described above is formed of a rare earth-transition metal amorphous alloy, sputtering, electron beam evaporation, or the like is usually used as a film forming method. Adjustment of the saturation magnetization, coercive force, and the like of the magnetic film is performed by changing the composition ratio between the rare earth element and the transition metal element in the film. For example, by using a sputtering apparatus, a target mainly containing a rare earth element (evaporation source) is simultaneously sputtered with a target mainly containing a transition metal element, and the composition of a film to be formed can be adjusted by adjusting the respective sputtering rates. Is being done. Also, sputtering and electron beam evaporation are often used to form a protective layer.

The magneto-optical recording medium described above is also described in detail in the above-mentioned Japanese Patent Application Laid-Open No. 63-153752. That is, the present invention can use the same medium as that used in the conventional overwriting of binary signals.

FIG. 2 is a schematic diagram showing an example of a magneto-optical recording / reproducing apparatus for implementing the recording method of the present invention. In FIG. 2, reference numeral 35 denotes a disk-shaped magneto-optical recording medium having the above-described configuration. The medium 35 is rotated by a motor (not shown), and information is recorded and reproduced by the recording and reproducing head 31. The recording / reproducing head 31 has a light source such as a semiconductor laser that emits a light beam for irradiating the medium 35, and a bias magnetic field applying unit including, for example, an electromagnet. The recording / reproducing head 31 is driven by a recording signal input from a recording signal source 32. The recording / reproducing head 31
Has a built-in analyzer and photodetector. Then, the reflected light of the light beam irradiated by the medium 35 at the time of reproduction is received by the photodetector through the analyzer, and the output is sent to the reproduction circuit 33 to reproduce the recording signal. The principle of this signal reproduction is similar to that of the aforementioned US Pat. No. 4,612,587.

FIG. 3 is a schematic diagram for explaining one embodiment of the recording method of the present invention. In FIG. 3, square frames indicate the first magnetic layer in the upper row and the second magnetic layer in the lower row. Arrows in the frames indicate the directions of magnetization of the magnetic layer in each recording state. The lower frame arrow indicates the polarity of the bias magnetic field H _B to be applied to the medium. The arrow above the frame indicates the power of the light beam applied to the medium. This embodiment shows a case where a medium whose exchange force acts to direct magnetization in an anti-parallel manner is used.

In the present embodiment, the recording / reproducing head 31 irradiates the disk surface with laser beams modulated into two types (first type and second type) according to the signal from the signal source 32 shown in FIG. I do. The first type of laser power P _L is a power sufficient to raise the temperature of the disk to near the Curie temperature of the first magnetic layer 2, and the second type of laser power P _H is the power of the type The power can be raised to near the temperature. That is, in FIG. 4 schematically showing the relationship between the coercive force and the temperature of the two magnetic layers 2 and 3, the first type laser power P _L is around _TL , and the second type laser power P _H is T _H The temperature of the disk can be raised to the vicinity.

At the same time, a bias magnetic field capable of orienting only the second magnetic layer 3 to the laser beam irradiation position at room temperature while switching its polarity according to a signal from the recording signal source 32.
Add H _B. Bias magnetic field H _B is usually about 50Oe～1000Oe. The direction of the combination of laser power value and the bias magnetic field H _B becomes possible to record the next four values.

In FIG. 3, the state of magnetization before recording may be either I or I '. Here, while applying the upward bias magnetic field H _B as shown in a (this direction is positive), by adding the first type of the laser power P _L, the first magnetic layer 2 is near the Curie temperature The second magnetic layer 3 has a coercive force at which the bit is stably present at this temperature, and the magnetization is directed upward by the bias magnetic field. In the first type of recording, a force (exchange force) in which the magnetization of the first magnetic layer 2 is arranged in a stable direction (here, antiparallel) with respect to the direction of the magnetization of the second magnetic layer 3 is applied. In this way, a bit as indicated by a is formed from either the state I or I '.

Next, in FIG. 3, a bias magnetic field H _B as b,
By applying the second type of laser power P _H while applying the voltage upward, the temperature of the disk is raised to near the Curie temperature of the second magnetic layer 3.

Magnetization of the second magnetic layer 3 at this time, when the temperature rises above the compensation temperature T _C in Figure 4, the orientation is changed in the opposite direction. (That is, it changes from upward (↑) to downward (↓) in FIG. 3).
When the temperature is raised to near the Curie temperature T _{H of} the second magnetic layer 3
Magnetization of the direction of the bias magnetic field H _B, i.e. upward (↑)
Becomes Subsequently, when the recording bit leaves the focal point of the laser beam and drops in temperature, and when the temperature falls below the compensation temperature T _{C in} FIG. 4, the magnetization of the second magnetic third layer 3 changes from upward (↑) to downward (↓). Changes to Subsequently, when the bit becomes equal to or lower than the Curie temperature _{TL of} the first magnetic layer, the second magnetic layer 3 is added to the first magnetic layer.
Magnetization is generated in a stable direction, that is, upward (↑). Furthermore, in the process of being formed bits are cooled to room temperature, the coercive force of the second magnetic layer is smaller than the bias magnetic field H _B, the magnetization reversal of the second magnetic layer occurs. (That is, the state changes from downward (↓) to upward (↑).) In this manner, a bit as shown by b is formed from both the states I and I '.

Next, in FIG. 3 is irradiated with the first type of the laser power P _L while applying downward bias magnetic field H _B to (this direction is negative), such as c from Izure of I, I ' Bits are formed.

Next, in FIG. 3, when the second type of laser power P _H is irradiated while applying the bias magnetic field H _B downward, a bit like d is formed from both I and I ′.

Thus the recording bit states a, b, c, d are controlled in the direction and laser power of the bias magnetic field H _B at the time of recording, since the state before recording is independent, overwriting (overwrite) is It is possible.

Example 1 A protective layer Si ₃ N ₄ , a magnetic layer Tb ₁₈ Fe ₇₈ Co ₄ , and a magnetic layer Gd ₁₈ Dy ₆ F were sputtered on a polycarbonate disk substrate on which a pre-group and a preformat signal were engraved.
Each layer of e ₆₀ Co ₁₆ and the protective layer Si ₃ N ₄ was formed in this order. Tables 1 and 2 show film forming conditions, film thicknesses, magnetic properties, and the like of each layer.

Next, the substrate on which the film formation was completed was bonded to a polycarbonate bonding substrate using a hot melt adhesive to produce a magneto-optical disk.

The magneto-optical disk was set in a recording / reproducing apparatus, and 830 nm focused at about 1 μm while rotating at a linear velocity of about 8 m / sec.
The laser beam and the bias magnetic field H _B wavelengths, while modulated at a frequency of 1 MHz, and irradiated. At this time, the following table-
Respectively by combining the bias magnetic field H _B and the laser power as 4 were successively four kinds recorded.

Thereafter, when a reproduction was performed by irradiating a laser beam of 1 mW, each recording signal could be reproduced.

Next, the same recording was performed on the area where the above recording was performed at 0.75 MHz.
The measurement was performed at the same wave number of z.

As a result, the previously recorded 1 MHz signal component was not detected, and only the 0.75 MHz signal component was detected, confirming that overwriting was possible.

When the reproduced signals of the four recordings were observed with an oscilloscope, the first and third recorded / reproduced signals had opposite polarities and a signal amplitude of about 400 mV.

The polarities of the second and fourth recording / reproducing signals were opposite to each other, and the signal amplitude was about 250 mV.

From these results, it was found that each of the four types of recording can be reproduced as four types of independent signals (that is, four-level recording is possible).

Next, the Kerr rotation angle of the disk of this example was measured by a measuring device shown in FIG. In FIG. 5, light from a semiconductor laser 51 passes through an analyzer 54, is incident on a disk 57 sandwiched between electromagnets, and is substantially perpendicularly (inclined by about 5 degrees) and reflected. The reflected light passes through the analyzer 55 and the optical power meter
Reach 52. Rotational position of analyzer 54 and optical power meter 52
Reach The rotational position of the analyzer 54 and the output of the optical power meter 52 are recorded in the XY recorder 56.

The Kerr rotation angles corresponding to the first to fourth recording states are measured as follows. To the first recording state, the electromagnet 53
Thereby, the first and second magnetic layers of the recording film of the disk are magnetized in one direction. Next, a magnetic field having a magnitude (for example, 2KOe) at which the magnetization of only the second magnetic layer is reversed is applied in the opposite direction.

Next, while rotating the analyzer 54, the optical power meter
Find the position where the output of 52 becomes minimum.

For the second recording state, both magnetic layers may be magnetized in one direction in the same direction as the first recording. The first to the third recording state
After magnetizing both magnetic layers in one direction in the opposite direction to the recording of
Only the magnetization of the second magnetic layer is reversed.

For the fourth recording state, both magnetic layers may be magnetized in one direction in the opposite direction to the second recording.

When the minimum position (extinction position) of the optical power meter output corresponding to each recording state was measured, the first and second positions were measured.
Is symmetric with the third and fourth recording states,
The first, second, third, and fourth rotation angles are +0.32.
Degrees, +0.20 degrees, -0.32 degrees, and -0.2 degrees (plus,
The minus sign is attached for convenience. The extinction position on the aluminum reflective film is 0 degree).

In the above embodiment, between the first and second magnetic layers,
Although the case where the exchange force for making the magnetization antiparallel acts is shown, the quaternary recording can be similarly performed in the case where the exchange force for making the magnetization parallel occurs.

FIG. 6 is a schematic diagram for explaining a recording process in the case where an exchange force acts to make the magnetization parallel between the magnetic layers and these magnetic layers have the characteristics as shown in FIG. Here power P _H, P _L, the magnitude of the magnetic field H _B is the same as that of FIG. 3. The magnetization before recording may be in either J or J 'state. This medium, when upward adding a bias magnetic field H _B, the magnetization of the second magnetic layer is directed upward. Then, the magnetization of the first magnetic layer disappears by applying a light beam of power P _L. When the temperature decreases due to the passage of the light beam, an exchange force from the second magnetic layer acts on the first magnetic layer, and an upward magnetization appears. In this way, a bit as shown at a 'is formed.

Next, when a light beam of power P _H in said medium,
The magnetization of both magnetic layers disappears. Here idea to upward applying a bias magnetic field H _B, after passing through the light beam, upward magnetization in the second magnetic layer first appears. The temperature of the medium is the compensation temperature
When the _temperature becomes equal to or lower than T _C, the magnetization of the second magnetic layer is inverted and becomes downward. When the temperature of the medium becomes equal to or lower than _TL , a downward magnetization appears in the first magnetic layer due to the exchange force from the second magnetic layer. Further, when the temperature of the medium drops to room temperature, the magnetization of the second magnetic layer is oriented upward by the bias magnetic field. In this way, a bit like b 'is formed from either the state of J or J'.

When a light beam of power P _L while applying a bias magnetic field H _B downward, a 'a process similar to the case of, occurs become opposite only polarity, c' bits as shown in it is formed. When a light beam of power P _H is applied while applying a bias magnetic field H _B downward, a process similar to that in the case of b ′ occurs with the opposite polarity, and a bit as shown in d ′ is formed. .

As described above, even when a stable medium is used while the magnetizations of both magnetic layers are parallel, quaternary recording can be performed by the method of the present invention. In the process shown in FIG. 3 or FIG.
When the state before recording is the state shown in b, d, b ', d',
Obviously, quaternary recording is performed in exactly the same process. That is, the method of the present invention enables overwriting of a quaternary signal.

〔The invention's effect〕

As described in detail above, for a magneto-optical recording medium having predetermined characteristics, a bias magnetic field capable of reversing the polarity and a laser power whose magnitude is changed are set according to the recording signal in the following four sets. Matching , It is possible to perform overwriteable quaternary recording using the same recording / reproducing apparatus as conventional binary recording.

[Brief description of the drawings]

1A and 1B are schematic sectional views each showing an example of the configuration of a magneto-optical recording medium used in the method of the present invention, and FIG. 2 is an example of a magneto-optical recording / reproducing apparatus used in the method of the present invention. FIG. 3 is a schematic diagram for explaining one embodiment of the recording method of the present invention, and FIG. 4 is a diagram showing the temperature characteristics of the coercive force of the magnetic layer used in the method described in FIG. FIG. 5 is a schematic diagram showing the configuration of a Kerr rotation angle measuring device, and FIG. 6 is a schematic diagram showing a recording process in the case of using a medium in which an exchange force acts in parallel to the recording method of the present invention. 1: translucent substrate with pre-groove, 2, 3, 4, 5: magnetic layer, 6, 7: protective layer, 8: adhesive layer, 9: bonding substrate, 31: recording / reproducing head 32: recording signal Source 33: playback circuit 35: magneto-optical recording medium.

Claims (1)

(57) [Claims] A first magnetic layer exhibiting perpendicular magnetic anisotropy, having a higher Curie temperature than the first magnetic layer and a lower coercive force _HL at room temperature, and exchange-coupled with the first magnetic layer. And using a magneto-optical recording medium in which the exchange force acting on each of the magnetic layers is lower than the coercive force of the second magnetic layer. In a magneto-optical recording method for recording a signal by applying a magnetic field, the power of the light beam and the polarity of a bias magnetic field having a strength equal to or higher than the _HL are modulated into the following four states according to the signal. A magneto-optical recording method for performing quaternary recording. (A) a first state in which the light beam has power to heat the medium to near the Curie temperature of the first magnetic layer, and a bias magnetic field has a polarity in a predetermined direction; A second state in which the bias magnetic field has a polarity in a predetermined direction, having a power to heat the magnetic layer to a temperature near the Curie temperature of the second magnetic layer, and (c) a light beam heats the medium to a temperature near the Curie temperature of the first magnetic layer. A third state in which the bias magnetic field has a polarity opposite to the predetermined direction, and (d) a light beam has a power to heat the medium to near the Curie temperature of the second magnetic layer. And a fourth state in which the bias magnetic field has a polarity opposite to the predetermined direction.