JPH076426A - Magneto-optical recording medium - Google Patents

Abstract

PURPOSE:To increase recording density by regulating the Curie temp. of a magneto-optical recording layer to a specified temp. or above and using a specified alloy contg. at least one of specified additive elements as the material of a reflecting layer. CONSTITUTION:In the magneto-optical recording medium with a magneto-optical recording layer and a reflecting layer, the Curie temp. of the recording layer is regulated to >=220 deg.C and an Ag alloy contg. an element selected among at least Rh, Pt and Au is used as the material of the reflecting layer. The objective magneto-optical recording medium having excellent reproducing characteristics at <=600nm wavelength and excellent corrosion resistance is obtd. and a high density optical information record can be reproduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magneto-optical recording medium for recording / reproducing information by light using thermo-magnetic recording and magneto-optical effect, and an optical information recording / reproducing method using the same.

[0002]

2. Description of the Related Art In recent years, along with a dramatic increase in the amount of information, there is an increasing demand for improving the recording density of information recording media. The optical disc has a high recording density and is excellent in random accessibility and portability. In particular, a magneto-optical disk can be repeatedly recorded and has excellent reliability, so that it has already been commercialized as a recording medium for external storage devices for computers and recording devices.

[0003]

The storage capacity of magneto-optical disks currently commercialized is about 300 megabytes (MByte) per side of a disk having a diameter of 5.25 inches. In order to increase this to about 1 gigabyte (GB Byte), it has been proposed to shorten the track pitch or change the modulation method of the recording signal, and the prospect of its practical application is emerging.

However, it cannot be said that it is impossible to further improve the recording density within the range of the current technology because the recording density is already close to the theoretical limit. The theoretical limit of the recording density on the optical disk is determined by the size of the focused spot of the laser light used for recording and reproduction. Therefore, in order to further increase the recording density, it is indispensable to narrow down the spot of the laser light.

The spot diameter d of the laser beam is given by the following equation (1) according to the wavelength λ of the laser beam used and the numerical aperture NA of the objective lens.
It is represented by. However, k is a constant determined by the aperture shape of the lens and the intensity distribution of the incident light beam.

[0006]

## EQU1 ## d = k.λ / NA (1) In order to reduce the spot diameter d of the laser light, the wavelength λ
It is necessary to use a light source having a short aperture and an objective lens having a large numerical aperture NA. If the numerical aperture of the lens is increased, the depth of focus becomes shallower, and the tolerance to the tilt of the disk and the unevenness of the thickness of the substrate sharply decreases, so that the servo capability of the optical head deteriorates.

Therefore, the numerical aperture of the lens is currently 0.55.
It cannot be much larger than the degree. Therefore, in order to reduce the spot diameter of the laser light to improve the recording density, the current light source of the optical head is 830 nm, 7
It is essential to use a light source with a wavelength shorter than 80 nm. It is the reflectance and the magnetic Kerr rotation angle that dominate the quality of the reproduction signal of the magneto-optical disk, and more specifically, it is represented by the product of the square root of the reflectance and the magnetic Kerr rotation angle. This is called a performance coefficient.

A heavy rare earth-transition metal amorphous alloy typified by TbFeCo is used as the recording layer of the magneto-optical disk currently commercialized. The coefficient of performance of these alloys shows a relatively large value when the wavelength of the laser light of the existing magneto-optical disk drive is about 800 nm, but sharply decreases when the wavelength of the light becomes shorter than 600 nm.

Further, the detection sensitivity of the photodiode of the photodetection element used to detect the drive signal is 800 nm.
Although it is high in the vicinity, it drops sharply at 600 nm or less. These facts mean that with the current technology, the intensity of the reproduction signal is extremely reduced on the short wavelength side, and stable reproduction of the recording signal becomes impossible. As described above, in order to realize a magneto-optical recording / reproducing system using a short wavelength light source aiming at an improvement in recording density, a reduction in reproduction signal intensity on the short wavelength side becomes a problem.

As described above, in order to obtain a large reproduction signal, it is important to increase the performance coefficient of the recording medium and improve the sensitivity of the photodetector. Light is detected by a photodetector used in a magneto-optical disk drive and a photodiode. Light excites an electron near a pn junction into a conduction band, the electron in the conduction band moves through the pn junction, and a current flows through the photodiode. Is done by flowing.

Since the light absorption coefficient of the Si semiconductor constituting the photodiode becomes large on the short wavelength side, light having a short wavelength is absorbed near the surface of the photodiode and it is difficult to reach the vicinity of the pn junction. As a result, the number of electrons excited near the pn junction decreases, and the light detection sensitivity decreases. Therefore, it is theoretically difficult to improve the detection sensitivity of the photodetector on the short wavelength side, and it is important to increase the performance coefficient of the recording medium in order to obtain a large reproduced signal on the short wavelength side.

A magneto-optical recording medium is generally constructed by providing a plurality of layers such as a magneto-optical recording layer, a light interference layer, a light reflecting layer, and a protective layer on a transparent substrate. The figure of merit that governs the reproduced signal strength must be considered in all of these layers. The present invention is a material constituting all layers,
Optimizing the layer thickness and layer structure, the magneto-optical disk 600
It is an object of the present invention to provide a magneto-optical recording medium having a high performance coefficient at a wavelength of nm or less and an optical information recording / reproducing system using the medium and a short wavelength light of 600 nm or less.

[0013]

DISCLOSURE OF THE INVENTION As a result of intensive investigations by the present inventors to provide a high recording density magneto-optical recording / reproducing system, in a magneto-optical recording medium having at least a magneto-optical recording layer and a reflective layer on a substrate. The magneto-optical recording layer has a Curie temperature of 220 ° C. or higher, and the reflective layer is made of an Ag alloy containing at least one element selected from the group of elements Rh, Pd, Pt and Au.
It has been revealed that it becomes possible to provide a magneto-optical recording medium exhibiting excellent reproducing characteristics and corrosion resistance at the following wavelengths, and to realize high-density optical information recording / reproducing.

According to the present invention, in a magneto-optical recording medium having at least a magneto-optical recording layer and a reflective layer on a substrate, the Curie temperature of the magnetic layer is 220 ° C. or higher and at least the element groups Rh, Pd and Pt are used as the reflective layer. , An Au alloy containing one element selected from Au, and a light characterized in that information is recorded and reproduced by using a laser beam having a wavelength of 600 nm or less for the medium. This is an information recording / reproducing method.

The present invention will be described in more detail below. Examples of the substrate of the magneto-optical recording medium used in the present invention include glass, plastics such as polycarbonate, and a substrate in which a grooved resin for guiding an optical head is formed on glass. It is preferable to optimize the depth and spacing of the grooves according to the wavelength to be used, that is, the shorter the wavelength, the shallower the grooves and the finer the spacing.

It is desirable that the birefringence of such a substrate is as small as possible so as not to impair the quality of the magneto-optical signal. The substrate generally has a thickness of about 1 to 2 mm. The magneto-optical recording layer of the present invention is a layer for condensing laser light having a wavelength of 600 nm or less and heating it to the Curie point, and performing thermomagnetic recording by using an external magnetic head. In order to record, the magnetization axis must be perpendicular to the film surface.
It is desirable that the magneto-optical recording layer has a large perpendicular magnetic anisotropy.

Reproduction is performed by the Kerr effect using laser light having a wavelength of 600 nm or less (light weaker than laser light at the time of recording). When the wavelength of the reproduction light is shortened and the focused spot diameter is reduced, Since the energy density of light per unit area becomes high, the temperature of the magneto-optical recording layer when irradiated with reproducing light is likely to be higher than the conventional temperature of about 800 nm.

When the temperature of the magneto-optical recording layer increases, the magnetic Kerr rotation angle decreases and the output of the reproduction signal decreases, but the rate of decrease is more remarkable as the Curie temperature of the magneto-optical recording layer is lower.
Therefore, it is necessary to properly adjust the physical properties of the magneto-optical recording layer. The Curie temperature of the conventional magneto-optical recording layer for a wavelength of about 800 nm is about 170 ° C.
When recording / reproducing is performed with a light source having a wavelength of 88 nm, recording sensitivity is extremely improved and C / N is lowered.

C of Tb-Fe-Co which is the composition of the recording layer
o Curie temperature rises with increasing concentration,
It was found that when the Curie temperature is around 220 ° C., the recording sensitivity is lowered and the C / N is improved. Therefore 600n
It is essential that the Curie temperature of the material of the magneto-optical recording layer for recording / reproducing using light having a wavelength of m or less is 220 ° C. or higher.

As a magneto-optical recording layer satisfying such conditions, the Co concentration in the 3d transition metal in the magneto-optical recording layer is 1
5 atomic% or more Tb-Fe-Co amorphous alloy or 3d transition metal Co concentration 35 atomic% or more Nd-T
Examples include b-Fe-Co amorphous alloys. The upper limit of the Co concentration in the 3d transition metal is preferably about 60 atom%. The film thickness of the magneto-optical recording layer must be determined according to the wavelength used in consideration of the recording sensitivity to the power of the laser beam, the performance coefficient, and the optical constant of the reflective layer.
The thickness of the magneto-optical recording layer is preferably 30 nm or less when combined with a wavelength of m or less and a reflective layer mainly composed of Ag.

The reflection layer plays a role of reflecting the light transmitted through the magneto-optical recording layer and returning it to the magneto-optical recording layer again. As a result, the efficiency of light utilization is increased, the reflectance and the magnetic Kerr rotation angle are increased, and the coefficient of performance is increased. Further, the composition of the reflective film affects the thermal conductivity of the magneto-optical recording medium. While aluminum or an aluminum alloy is used as the reflective layer of a magneto-optical disk corresponding to a wavelength of about 800 nm which is currently commercialized, in the present invention, a reflective layer made of an Ag alloy containing a specific element is used. It is characteristic that it is done.

If conventional aluminum, aluminum alloy, or gold is used as the reflective layer, a sufficient coefficient of performance cannot be obtained at a wavelength of 600 nm or less, but a reflective layer mainly containing Ag is used and the above-mentioned 220 is used. It has been found that a good performance coefficient can be obtained only by combining with a magneto-optical recording layer having a Curie temperature of ℃ or higher.

Focusing on metals having a high reflectance,
A patent has been filed for using Au, Ag, Cu, Al for the reflective layer of the magneto-optical recording medium. (JP-A-58-83
364, JP-A-59-132434, JP-A-59-
No. 8150, JP-A-59-38781, etc.) The reflectance of the reflective layer, which is noted in these patents, is
It is the reflectance when light is directly incident on the reflective layer from the air.

However, it is clear from the following points that such a reflectance is not essential for the coefficient of performance.
That is, the reflection of light in the reflective layer of the magneto-optical recording medium is caused by the interface between the magneto-optical recording layer and the reflective layer, or the interface between the heat insulating layer (interference layer) and the reflective layer provided between the magneto-optical recording layer and the reflective layer. This is because the difference occurs between the optical constants of the two and the reflection at the interface between the air and the reflective layer is completely different.

Further, as a result of detailed examination of the role played by the Ag alloy reflective layer, the effect provided by the Ag alloy reflective layer is to increase the Kerr rotation angle of the magneto-optical recording layer rather than to obtain a high reflectance. Was found to be large. For example, the reflectance when using a conventional reflective layer made of Al alloy and when using the Ag alloy of the present invention,
Table 1 below shows the comparison of Kerr rotation angle and performance coefficient. The advantage of using Ag alloy is not reflectance but
Rather, it can be seen that it is the car rotation angle. Measurement wavelength is 50
It is 0 nm.

[0026]

[Table 2] The reason why the Kerr rotation angle is remarkably increased when a Ag alloy is used for the reflective layer and a good coefficient of performance is obtained as a result is
Even at a wavelength where the refractive index n of Ag is 600 nm or less,
It was found by simulation using optical constants that the reason is to show a small value of 5 or less.

Other high reflectance metals such as Au and C
In the case of u, the refractive index n is small near the wavelength of 800 nm and is equal to Ag, but in the wavelength region of wavelength 600 nm or less, it is considerably larger than Ag and exceeds 0.5. found. Furthermore, when the optical constants of various metals and semiconductors were measured, it was 600 n.
It was found that Ag is the only element that exhibits a small refractive index of 0.5 or less in the short wavelength region of m or less.

That is, at a wavelength of about 800 nm, good characteristics can be obtained by using a reflecting layer mainly composed of Au, Cu, Al, etc. other than Ag, but at a wavelength of 600 nm or less, Ag can be obtained.
It means that good characteristics cannot be obtained unless a reflective layer mainly composed of is used. Further, the reflective layer mainly composed of Ag exhibits good characteristics at a wavelength of 600 nm or less for the first time when combined with a magneto-optical recording layer having a Curie temperature of 220 ° C. or higher. A magneto-optical medium corresponding to a wavelength of about 800 nm has a temperature of 200 ° C. or less in order to improve recording sensitivity.
Generally, a magneto-optical recording layer having a Curie temperature of about 170 ° C. is used. Even if such a magneto-optical recording layer and a reflective layer mainly containing Ag are combined, an excellent performance coefficient cannot be obtained at a wavelength of 600 nm or less because the magneto-optical effect originally possessed by the magneto-optical recording layer is small. .

By the way, Ag alone has a problem in corrosion resistance, and it is difficult to directly apply it to the reflective layer. Then, as a result of earnestly studying the improvement of the corrosion resistance of Ag, it was found that the Ag alloy containing at least one element selected from the group of elements Rh, Pd, Pt, and Au had a sufficiently small refractive index of 0.5 or less at a wavelength of 600 nm or less. It has been found that excellent corrosion resistance can be realized at the same time.

It was also found that the added concentration of each element has an optimum range. That is, it was found that when the concentration of the additive element is too high, the refractive index increases and exceeds 0.5. These elements have the effect of improving the corrosion resistance of the Ag alloy even when added in a very small amount. The addition range (concentration range) of each element in the Ag alloy is shown below.

[0031]

[Table 3] Concentration range of additive element Rh: 0.1 atom% or more, 4.7
Atom% or less Pd: 0.1 atom% or more, 5.1 atom% or less Pt: 0.1 atom% or more, 7.0 atom% or less Au: 0.1 atom% or more, 28.8 atom% or less Reflective layer Two or more elements may be selected from the above element group and added, or an element other than the above element group may be added, but in order to keep the refractive index at 0.5 or less, The Ag concentration in the alloy must be 70 atomic% or more.
The thickness of the reflective layer is preferably 30 nm to 100 nm.

The interference layer is a layer made of a dielectric material and causes light to interfere. Generally provided between the substrate and the magneto-optical recording layer, multiple reflection of light between the substrate and the magneto-optical recording layer,
It plays a role of increasing the apparent magnetic Kerr rotation angle and an interference effect. A sufficient interference effect cannot be obtained unless its thickness is changed according to the wavelength of light used. 400 nm
Considering a wavelength of ~ 600 nm, the thickness of the interference layer is 20 nm.
It is preferably set to 55 nm. This is the case of using the primary interference point, but the thickness of the interference layer can be set to 120 nm to 200 nm by utilizing the secondary interference point.

The interference layer provided between the substrate and the magneto-optical recording layer serves to enhance the adhesion between the substrate and the magneto-optical recording layer, serves to insulate the magneto-optical recording layer from the substrate, and penetrates through the plastic substrate. It also has the role of protecting the magneto-optical recording layer from moisture coming in. Materials used as the interference layer include silicon nitride, tantalum oxide, silicon oxide,
Amorphous thin films made of aluminum oxide, titanium oxide, zinc sulfide, etc., and mixtures thereof are common.

The interference layer made of a dielectric material can be further provided between the magneto-optical recording layer and the reflective layer. This makes it possible to further enhance the light interference effect and increase the magnetic Kerr rotation angle. Each layer described above is
As a physically protective protective layer, it is preferable to use a hard material such as an acrylic UV-curable resin. After coating the reflective layer with a thickness of about 2 to 20 μm by spin coating, ultraviolet irradiation is performed. It is generally formed by curing with.

The above-mentioned dielectric layer may be provided inside the protective layer made of an organic material such as an ultraviolet curable resin (between the protective layer made of an organic material and the reflective layer) to form a protective layer. .

[0036]

EXAMPLES The present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples unless it exceeds the gist. Example 1 A 1.2 mm thick glass substrate was used as the substrate. A tantalum oxide thin film was formed as a dielectric layer on the substrate by the reactive sputtering method. After that, the surface was etched with high frequency plasma for 5 minutes to smooth the surface.

The initial film thickness was set so that the thickness of the tantalum oxide layer after etching was about 35 nm. Next, on this tantalum oxide layer, an amorphous alloy of Tb ₂₀ Fe ₆₄ Co ₁₆ {numerical value indicates composition ratio in atomic% and Co concentration in 3d transition metal (Fe) is 20%} is used, and direct current magnetron sputtering method is used. To a thickness of about 20 nm as a magneto-optical recording layer.

A Rh tip was placed on an Ag target and DC magnetron sputtering was performed to provide a reflective layer having a thickness of about 50 nm. As a result of analysis, the composition of the reflective layer was Rh.
It was 0.6 atom% and Ag was 99.4 atom%. Approximately 4 tantalum oxide layer was sputtered on the reflective layer as a protective layer.
After being taken out from the sputtering device having a thickness of 0 nm, an ultraviolet curable resin was spin-coated to provide an organic protective layer having a thickness of about 3 μm to obtain a magneto-optical recording medium.

When the magnetic Kerr loop was measured while increasing the temperature of the sample prepared by the same method as above except that the protective layer for ultraviolet curable resin was not provided,
A loop was observed even at 220 ° C., and it was found that the Curie temperature of the magneto-optical recording layer was 220 ° C. or higher. The recording characteristics and reproducing characteristics of the magneto-optical recording medium were evaluated by a magneto-optical recording dynamic characteristics evaluator using an Ar laser having a wavelength of 488 nm as a light source. Recording conditions and reproduction conditions are as follows.

Recording conditions: linear velocity 10 m / s, recording frequency 2 MHz, duty 50%, recording applied magnetic field 24 kA / m, and recording laser power varied. Reproduction conditions: linear velocity 10 m / s reproduction laser power 1 mW. Table-2 shows the maximum value of / N
It was shown to.

Example 2 The magneto-optical recording layer was formed of Nd ₁₀ Tb ₁₅ Fe ₃₈ Co ₃₇ {numerical values indicate atomic percentage of component ratio, Co concentration in 3d transition metal (Fe) is 49%} amorphous alloy, thickness about A magneto-optical recording medium was obtained by the same method as in Example 1 except that the thickness was 20 nm. The Curie temperature was 220 ° C. or higher. The recording characteristics and reproducing characteristics of the magneto-optical recording medium were evaluated in the same manner as in Example 1. C / N when recording power is changed
The maximum value of is shown in Table-2.

Comparative Example 1 The magneto-optical recording layer was formed of Tb ₂₀ Fe ₇₄ Co ₆ {numerical value indicates atomic percentage of component ratio, Co concentration in 3d transition metal (Fe) is 7.5%} amorphous alloy, thickness about A magneto-optical recording medium was obtained by the same method as in Example 1 except that the thickness was 20 nm. The Curie temperature was about 170 ° C. Example 1
The recording characteristics and reproducing characteristics of the magneto-optical recording medium were evaluated in the same manner as in. Table 2 shows the maximum C / N values when the recording power was changed.

Comparative Example 2 A magneto-optical recording medium was produced in the same manner as in Example 1 except that the reflective layer was made of Ag alone and had a thickness of about 50 nm. The recording characteristics and reproducing characteristics of the magneto-optical recording medium were evaluated in the same manner as in Example 1. Table 2 shows the maximum C / N values when the recording power was changed.

Comparative Example 3 A magneto-optical recording medium was prepared in the same manner as in Example 1 except that the reflective layer was made of AlTa alloy and had a thickness of about 50 nm. The recording characteristics and reproducing characteristics of the magneto-optical recording medium were evaluated in the same manner as in Example 1. Table 2 shows the maximum C / N values when the recording power was changed.

[0045]

[Table 4] In Examples and Comparative Example 2, good C / N was obtained, and it is understood that the combination of the magnetic layer having a Curie temperature of 220 ° C. or higher and the reflective layer mainly containing Ag is important.

In order to investigate the corrosion resistance of the reflective layer itself used in the above Examples and Comparative Examples, samples prepared by the same method as in Examples and Comparative Examples except that no UV-curable resin protective layer was provided were prepared at 85 ° C. The sample was stored in a high temperature and high humidity tank having a humidity of 85% for 500 hours. When visually observing the occurrence of corrosion,
Corrosion occurred only in the sample corresponding to Comparative Example 2. It can be seen that there is a problem in the corrosion resistance of Ag alone, and the addition of Rh to Ag is effective in improving the corrosion resistance.

[0047]

As described in detail above, according to the present invention, it is possible to provide a high-performance magneto-optical recording medium compatible with wavelengths shorter than 600 nm, which is shorter than the existing one. A high recording density magneto-optical recording system is realized.

Claims (6)

[Claims] 1. In a magneto-optical recording medium having at least a magneto-optical recording layer and a reflective layer on a substrate, the magneto-optical recording layer has a Curie temperature of 220 ° C. or higher, and the reflective layer is one of the following element groups. A magneto-optical recording medium comprising an Ag alloy containing at least one of the above. Element group: Rh, Pd, Pt, Au 2. The optical recording medium according to claim 1, wherein the content of the elements contained in the Ag alloy is within the following range and the Ag concentration in the Ag alloy is 70 atomic% or more. . [Table 1] Concentration range of additional element Rh: 0.1 atom% or more, 4.7 atom% or less Pd: 0.1 atom% or more, 5.1 atom% or less Pt: 0.1 atom% or more, 7. 0 atomic% or less Au: 0.1 atomic% or more and 28.8 atomic% or less 3. The magneto-optical recording medium according to claim 2, wherein the magneto-optical recording layer has a thickness of 30 nm or less. 4. An optical interference layer made of a dielectric material having a thickness of 20 nm or more and 55 nm or less is provided on the side of the magneto-optical recording layer opposite to the side on which the reflective layer is provided. 3. The magneto-optical recording medium described in 3. 5. The magneto-optical recording layer comprises Co in 3d transition metal.
A Tb-Fe-Co amorphous alloy having a concentration of 15 atomic% or more,
Alternatively, the magneto-optical recording medium according to claim 4, wherein the 3d transition metal is an amorphous Nd-Tb-Fe-Co alloy having a Co concentration of 35 atomic% or more. 6. An optical information recording / reproducing method, wherein recording / reproducing is performed on the magneto-optical recording medium according to claim 1 by using laser light having a wavelength of 600 nm or less.