JPH07320324A - Magneto-optical recording medium and reproducing method thereof - Google Patents

Abstract

(57) [Summary] [Object] It is an object of the present invention to obtain a large Kerr rotation angle while ensuring a sufficient reflectance in a magneto-optical recording medium. An object of the present invention is to provide a magneto-optical recording medium that can obtain a sufficient reflectance even with the thickness of the interference layer. A magnetic layer and a semi-reflective layer that reflects a part of light from the light source on the side closer to the light source than the magnetic layer and transmits the light on the other side on the substrate, and a semi-reflective layer on the side farther from the light source than the magnetic layer. In a magneto-optical recording medium each having a reflective layer that reflects light, the semi-reflective layer is a thin film mainly composed of one element or two or more elements selected from Au, Ag and Cu. recoding media.

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 such as magneto-optical disk and magneto-optical tape for recording signals by thermo-magnetic recording and reproducing signals utilizing magneto-optical effect, and a signal reproducing method using the same. Is.

[0002]

2. Description of the Related Art A conventional magneto-optical recording medium has a structure having a reflective layer, a magnetic layer, and one or two transparent optical interference layers on a substrate. The signal quality of a magneto-optical recording medium increases as the light reflectance and the magnetic Kerr rotation angle increase.Therefore, in a conventional medium, the magnetic layer is sandwiched between optical interference layers and reflective layers to cause multiple reflection of light in the medium. The car rotation angle is increased.

At this time, the reflectance decreases due to the interference effect of light, but since the Kerr rotation angle contributes more to the signal quality than the reflectance, the signal quality is improved as a whole. Noise during signal reproduction is usually limited by shot noise determined by the reflectance of the medium.

Therefore, when the reflectance decreases, the shot noise also decreases. However, when the shot noise becomes extremely small, the noise of the entire device called system noise dominates the noise during signal reproduction, and the shot noise The noise limit signal cannot be detected. Further, if the reflectance becomes extremely small, it also interferes with the focus servo and tracking servo of the light spot.

The reflectivity of the medium greatly changes depending on the film thickness of the optical interference layer, and the Kerr rotation angle is around the film thickness of the optical interference layer where the reflectivity is minimal (hereinafter referred to as an interference point). It becomes maximum. By selecting a film thickness of the optical interference layer that minimizes the reflectance, the fluctuation of the reflectance with respect to the fluctuation of the thickness of the optical interference layer is suppressed to the minimum, so that the manufacturing margin of the medium can be increased.

However, in the conventional medium, when the thickness of the optical interference layer is selected as the interference point, the reflectance becomes too small. Therefore, in the conventional medium, the film thickness of the optical interference layer is designed so that the Kerr rotation angle is not extremely increased and the reflectance is about 20%, and it can be said that the performance of the magnetic layer is sufficiently brought out. Absent.

[0007]

It is desired that the recording density of the magneto-optical recording medium be further improved from the current state, and shortening the wavelength of the recording / reproducing laser beam is the most effective method. Therefore, it is considered that the laser used for the magneto-optical recording medium will continue to have a shorter wavelength in the future.

When the wavelength of light becomes shorter, the reflectance of the magnetic layer itself lowers and the refractive index of the light interference layer generally increases, so that the reflectance at the interference point lowers more remarkably than at present. Become. Therefore, as described above, in order to perform reproduction at the shot noise limit or to obtain a sufficient servo signal, the Kerr rotation angle is sacrificed and the reflectance is sacrificed by significantly shifting the film thickness of the optical interference layer from the interference point. It is necessary to secure it.

When the signal of the magneto-optical recording medium is reproduced through the transparent substrate, the birefringence of the substrate deteriorates the signal quality.
When the birefringence of the substrate is large, the layer structure must be reversed to reproduce the signal from the opposite side of the substrate without passing through the substrate. In this case, light is directly incident on the light interference layer from the air, and the reflectance at the interference point is significantly reduced.

Therefore, even when the signal is reproduced without passing through the substrate, it is necessary to largely shift the film thickness of the optical interference layer from the interference point to secure the reflectance. The problem to be solved by the present invention is to obtain a large Kerr rotation angle while ensuring a sufficient reflectance in a magneto-optical recording medium, and even with an optical interference layer film thickness that maximizes the Kerr rotation angle. It is to obtain sufficient reflectance.

[0011]

As a result of intensive studies to solve the above problems, the inventors of the present invention have reflected a part of the light from the light source on the side closer to the light source than the magnetic layer of the magneto-optical recording medium. A semi-reflective layer that transmits the other light is provided, and the semi-reflective film is made of Au, Ag,
It has been found that it is possible to provide a magneto-optical recording medium having a large Kerr rotation angle while ensuring a relatively large reflectance by forming a thin film mainly containing one element or two or more elements selected from Cu.

The gist of the present invention is to provide at least a magnetic layer on a substrate, a semi-reflective layer that reflects a part of the light from the light source and transmits the other to the side closer to the light source than the magnetic layer, and from the magnetic layer to the light source. In a magneto-optical recording medium each having a reflective layer for reflecting light from a light source on the far side, the semi-reflective layer is Au, Ag,
A magneto-optical recording medium comprising a thin film mainly composed of one element or two or more elements selected from Cu, and homodyne detection of light from a magnetic layer by light from a semi-reflective layer using the medium. The method for reproducing a magneto-optical recording signal is characterized by the following.

The present invention will be described in detail below. Examples of the substrate used in the present invention include a glass substrate, a plastic substrate such as polycarbonate, and a plastic film such as PET. When using a substrate with a small birefringence such as polycarbonate, it is desirable to have a layer structure in which light is incident on the magnetic layer through the substrate. Therefore, a layer structure in which light is incident on the magnetic layer is desirable.

The magnetic layer used in the present invention is TbF.
Rare earth-transition metal amorphous alloy thin film such as eCo, Co / P
An artificial lattice thin film such as t can be used. As the characteristics of the magnetic layer, the perpendicular magnetic anisotropy is large and the film becomes a perpendicular magnetization film, the magneto-optical effect is large, and the Curie temperature is 170 to 30.
It is desired to be suitable for thermomagnetic recording with a laser at about 0 ° C.

If the film thickness of the magnetic layer is too thin, a sufficient magneto-optical effect cannot be obtained, and if it is too thick, the recording sensitivity decreases.
It is preferably about 15 nm to 50 nm. The present invention is characterized in that a semi-reflective layer that reflects a part of the light from the light source and transmits the other is provided on the side closer to the light source than the magnetic layer. This semi-reflective layer ensures a certain degree of reflectance by reflecting the light from the light source.

At the same time, a part of the light is transmitted, but this transmitted light causes multiple reflection in the magnetic layer, the reflection layer, and the light interference layer to increase the Kerr rotation angle. The semi-reflective layer transmits the light having the increased Kerr rotation angle in the opposite direction again and reaches the light receiving element. As a semi-reflective layer, it is important to obtain a relatively large reflectance even with a thin film thickness that sufficiently transmits light,
A thin film mainly containing one element or two or more elements selected from Au, Ag and Cu satisfies such a condition.

If the thickness of the reflective layer is too thick, light cannot be transmitted, and if it is too thin, the reflectance becomes insufficient, which causes a problem. The film thickness is preferably in the range of 5 to 40 nm, and 1
0-30 nm is more preferable. The wavelength of the light source is 600 nm
In the case of about 800 nm, there is no particular problem even if the main constituent of the thin film forming the semi-reflective layer is Au, Ag, or Cu, or any combination of two or more elements therein.

However, when the wavelength becomes a short wavelength of less than 600 nm, the effect as a semi-reflective layer becomes insufficient even if the thin film mainly composed of Au, Cu or an alloy thereof is used as the semi-reflective layer. Therefore, when manufacturing a magneto-optical recording medium compatible with a short wavelength light source of less than 600 nm, the semi-reflective layer is
It is preferable to use a thin film mainly composed of g.

Since Au is excellent in corrosion resistance, it is possible to form the semi-reflective layer by itself, whereas Ag and Cu are inferior in corrosion resistance to Au, so it is difficult to form the semi-reflective layer by itself. Therefore, when the semi-reflective layer is composed of a thin film mainly composed of Ag, Cu, or an alloy thereof, in order to improve the corrosion resistance, one or more elements from the following element groups should be added to the semi-reflective layer. Is preferred.

As elements for improving the corrosion resistance, Au and P
t, Pd, Rh, Mo, Ta, and Ti are effective, and the addition concentration is preferably 0.5 atomic% or more and 10 atomic% or less. Further, since Au, Ag, and Cu all have high thermal conductivity, when these are used as the semi-reflective layer, the heat of the magnetic layer escapes to the semi-reflective layer, causing a decrease in recording sensitivity.

Therefore, when it is desired to improve the recording sensitivity, it is preferable to add the following elements to the semi-reflective layer in order to reduce the thermal conductivity. The elements that reduce the thermal conductivity are Ti, V, Cr, Mn, Fe, Co, Ni, Zr and N.
b, Mo, Ta, W, Hf and Bi are mentioned. In the present invention, the reflective layer is provided on the side opposite to the semi-reflective layer with respect to the magnetic layer.

That is, the magnetic layer is sandwiched between the semi-reflective layer and the reflective layer. An optical interference layer made of a dielectric or the like may be provided between the semi-reflective layer and the magnetic layer, between the magnetic layer and the reflective layer, or both. The reflective layer plays a role of reflecting light from the light source that has passed through the semi-reflective layer and the magnetic layer and returning it to the magnetic layer.

Therefore, the reflective layer is a layer in contact with it,
That is, it is desirable that the reflectance with respect to the magnetic layer or the optical interference layer is high, and it is preferable that the thin film is mainly composed of Al, Au, Ag, and Cu. If the wavelength is less than 600 nm, A
It is not suitable for use as a reflective layer because the reflectance of the magnetic layer or optical interference layer of u and Cu decreases.

When the wavelength is less than 600 nm, it is preferable to use a thin film mainly composed of Al or Ag as the reflective layer. Similar to the semi-reflective layer, the above-mentioned elements may be added to the reflective layer in order to improve the corrosion resistance or reduce the thermal conductivity. The thickness of the reflective layer is 5
Generally about 0 nm.

The optical interference layer plays a role of changing the phase difference between the light reflected at the interface and the light transmitted through the interface and reflected at another interface, and the reflectance and the like change depending on the phase difference. Become. Since the light interference layer transmits light, it is desired that the absorption coefficient is small at the wavelength of the light source used, and a transparent dielectric such as silicon nitride is generally used.

The refractive index of the light interference layer dominates the interference effect, and the larger the refractive index, the greater the interference effect. Generally, a material having a refractive index of about 2.2 is used as an optical interference layer. The method of reproducing a magneto-optical recording signal of the present invention is characterized in that homodyne detection is performed on light returning to the magnetic layer and reflected by the semi-reflective layer.

The phase difference between the light reaching the magnetic layer and returning to the magnetic layer and the light reflected by the semi-reflective layer must be kept constant at any position on the medium, but in the present invention, the magnetic layer and the semi-reflective layer are used. If the thickness of the layer between them, that is, the thickness of the optical interference layer does not change significantly, the phase difference is kept substantially constant.

[0028]

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to the following examples as long as the gist thereof is not exceeded. Example 1 First, an Au film having a thickness of 17 nm was provided as a semi-reflective layer on a polycarbonate disk substrate having a thickness of 1.2 mm by a sputtering method.

Further, a tantalum oxide film having a thickness of 120 nm is used as an optical interference layer, and TbFe having a thickness of 25 nm is used as a magnetic layer.
The Co amorphous alloy film is again used as an optical interference layer having a thickness of 40 nm.
The tantalum oxide film of was formed by the sputtering method. Finally, an Al film having a thickness of 50 nm was provided as a reflective layer by a sputtering method to obtain a magneto-optical recording medium.

The light interference layer provided immediately after the semi-reflective layer has a wavelength of 7
It was set to 120 nm so that the Kerr rotation angle became maximum at 80 nm. The reflectance was measured by a spectrophotometer and the Kerr rotation angle was measured by a polarization plane modulation method. The measurement wavelength was 780 nm. In the case of a magneto-optical recording medium having a structure in which light is transmitted through a substrate as in this embodiment, reflection also occurs on the surface of the substrate, but since this reflected light does not contribute to the actual signal, the reflectance and the curve The reflectance and Kerr rotation angle were obtained by removing the contribution of the reflected light on the substrate surface to the rotation angle by calculation. The results are shown in Table 1.

Comparative Example 1 A disk was prepared in the same manner as in Example 1 except that the semi-reflective layer was not provided, and used as a magneto-optical recording medium. However, the thickness of the optical interference layer provided directly on the substrate was set to 60 nm so that the Kerr rotation angle was maximized at a wavelength of 780 nm. The reflectance was measured by a spectrophotometer and the Kerr rotation angle was measured by a polarization plane modulation method. The measurement wavelength was 780 nm. In the case of a magneto-optical recording medium having a structure in which light is made incident through a substrate as in this comparative example, reflection also occurs at the substrate surface, but this reflected light does not contribute to the actual signal, so the reflectance and curve The reflectance and Kerr rotation angle were obtained by removing the contribution of the reflected light on the substrate surface to the rotation angle by calculation. The results are shown in Table 1.

Comparative Example 2 A disk was prepared in the same manner as in Example 1 except that the semi-reflective layer was an Al film having a thickness of 6 nm to obtain a magneto-optical recording medium. The light interference layer provided immediately after the semi-reflective layer was set to 120 nm so that the Kerr rotation angle was maximized at a wavelength of 780 nm. The reflectance was measured by a spectrophotometer and the Kerr rotation angle was measured by a polarization plane modulation method.

The measuring wavelength was 780 nm. In the case of a magneto-optical recording medium having a structure in which light is made incident through a substrate as in this comparative example, reflection also occurs at the substrate surface, but this reflected light does not contribute to the actual signal, so the reflectance and curve The reflectance and Kerr rotation angle were obtained by removing the contribution of the reflected light on the substrate surface to the rotation angle by calculation. The results are shown in Table 1.

Example 2 A disk was prepared in the same manner as in Example 1 except that the semi-reflective layer was an Ag film having a thickness of 19 nm, and was used as a magneto-optical recording medium. However, the light interference layer provided immediately after the semi-reflective layer has a wavelength of 450 nm.
Therefore, the Kerr rotation angle was set to 52 nm so as to be maximum. The reflectance was measured by a spectrophotometer and the Kerr rotation angle was measured by a polarization plane modulation method.

The measurement wavelength was 450 nm. In the case of a magneto-optical recording medium having a structure in which light is transmitted through a substrate as in this embodiment, reflection also occurs on the surface of the substrate, but since this reflected light does not contribute to the actual signal, the reflectance and the curve The reflectance and Kerr rotation angle were obtained by removing the contribution of the reflected light on the substrate surface to the rotation angle by calculation. The results are shown in Table 1.

Comparative Example 3 A disk was prepared in the same manner as in Comparative Example 1 except that the thickness of the optical interference layer provided directly on the substrate was set to 38 nm so that the Kerr rotation angle was maximized at a wavelength of 450 nm and used as a magneto-optical recording medium. did. The reflectance was measured by a spectrophotometer and the Kerr rotation angle was measured by a polarization plane modulation method.

The measurement wavelength was 450 nm. In the case of a magneto-optical recording medium having a structure in which light is made incident through a substrate as in this comparative example, reflection also occurs at the substrate surface, but this reflected light does not contribute to the actual signal, so the reflectance and curve The reflectance and Kerr rotation angle were obtained by removing the contribution of the reflected light on the substrate surface to the rotation angle by calculation. The results are shown in Table 1.

Example 3 An Al film having a thickness of 50 nm was first formed as a reflective layer on a PET film substrate having a thickness of 20 μm by a sputtering method.
Furthermore, a 40 nm thick tantalum oxide film is used as an optical interference layer.
A TbFeCo amorphous alloy film having a thickness of 25 nm was provided as a magnetic layer, and a tantalum oxide film having a thickness of 116 nm was provided again as an optical interference layer by a sputtering method.

Finally, as a semi-reflective layer, Au having a thickness of 14 nm is used.
The film was provided by a sputtering method to obtain a magneto-optical recording medium. The light interference layer provided immediately before the semi-reflective layer was set to 116 nm so that the Kerr rotation angle was maximized at a wavelength of 780 nm. The reflectance was measured by a spectrophotometer and the Kerr rotation angle was measured by a polarization plane modulation method. The measurement wavelength was 780 nm. The results are shown in Table 1.

Comparative Example 4 A magneto-optical recording medium was prepared in the same manner as in Example 3 except that the semi-reflective layer was not provided.

The final optical interference layer was set to 60 nm to maximize the Kerr rotation angle at a wavelength of 780 nm. In order to compare the signal quality, the reflectance was measured by a spectrophotometer and the Kerr rotation angle was measured by a polarization plane modulation method. The measurement wavelength was 780 nm. The results are shown in Table 1.

[0042]

[Table 1]

Example 1 shows a sufficiently high reflectance and a large Kerr rotation angle. On the other hand, in the case of Comparative Example 1, the Kerr rotation angle is large but the reflectance cannot be said to be sufficiently high. In Comparative Example 2, the reflectance is high but the Kerr rotation angle is half or less of that of the embodiment. From this, it is important to provide a semi-reflective layer in order to obtain high reflectance,
Further, in order to achieve both high reflectance and a large Kerr rotation angle, it is important that the semi-reflective film is composed of a thin film mainly containing one element or two or more elements selected from Au, Ag and Cu. Can understand

From Example 2 and Comparative Example 3, it was found that it is difficult to obtain a sufficient reflectance in the short wavelength region unless the semi-reflective layer is provided, and the semi-reflective layer is very effective in the short wavelength region. It turns out that From Example 3 and Comparative Example 4, it can be seen that it is difficult to obtain high reflectance even when light is directly incident on the light interference layer, and even in such a case, the semi-reflective layer is particularly effective.

[0045]

According to the present invention, it is possible to provide a magneto-optical recording medium exhibiting a reflectance and a large Kerr rotation angle sufficient for reproducing shot noise limit.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takumi Shimamori Sanboshi Kasei Co., Ltd. Research Institute, 1000, Kamoshida-cho, Midori-ku, Yokohama-shi, Kanagawa

Claims (5)

[Claims] 1. A magnetic layer at least on a substrate, a semi-reflective layer that reflects a part of light from a light source and transmits the other to a side closer to the light source than the magnetic layer, and a light source farther from the light source than the magnetic layer. In the magneto-optical recording medium, each of which has a reflective layer for reflecting the light from, the semi-reflective layer is composed of a thin film mainly containing one element or two or more elements selected from Au, Ag and Cu. Magneto-optical recording medium. 2. The magneto-optical recording medium according to claim 1, wherein the semi-reflective layer is a thin film containing Ag as a main component. 3. The magneto-optical recording medium according to claim 1, wherein the semi-reflective layer has a thickness of 5 nm or more and 40 nm or less. 4. The magneto-optical recording medium according to claim 1, claim 2 or claim 3, wherein the light returning to the magnetic layer is reflected by the semi-reflective layer and returned by homodyne detection. A method for reproducing a magneto-optical recording signal, comprising: 5. The method of reproducing a magneto-optical signal according to claim 4, having a layer structure suitable for allowing light for recording / reproducing information to enter the magnetic layer from the side opposite to the substrate.