JPH056587A - Magneto-optical memory element - Google Patents

Abstract

(57) [Summary] [Object] An object of the present invention is to provide a magneto-optical storage element capable of obtaining a sufficiently large Kerr rotation angle. A transparent dielectric film is formed by forming a rare earth-transition metal amorphous magnetic thin film having an easy axis of magnetization in a direction perpendicular to the film surface, a transparent dielectric film and a reflective film on a substrate. Thickness 30
nm to 60 nm, the real part A of the virtual refractive index of the transparent dielectric film and the reflective film is 0 <A ≦ 0.5, and the imaginary part B is 0 ≧.
B ≧ −1.5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magneto-optical storage element capable of recording / reproducing / erasing information.

[0002]

2. Description of the Related Art Conventionally, one of the difficulties in using a magneto-optical storage element as a memory element has been a low reproduction signal level. In particular, in a so-called Kerr effect reproducing method in which a magneto-optical storage element is irradiated with laser light and information is reproduced by the reflected light, the Kerr rotation angle is generally small, so that the signal noise ratio (S / N) may be increased. It was difficult. Therefore, conventionally, the magnetic material that is the recording medium has been improved, and a dielectric film of SiO or SiO ₂ has been formed on the recording medium to increase the Kerr rotation angle.

In addition to this method, the applicant has proposed a method for improving the apparent Kerr rotation angle by forming a reflective film behind the recording medium in the Kerr effect reproducing type magneto-optical storage element (Japanese Patent Application No. Sho. 55-85695). This structure is called a reflective film structure.

The characteristic of this structure is that the laser beam reflected on the surface of the recording medium and the laser beam transmitted through the recording medium and then reflected by the reflecting film are combined, so that the above structure is not provided with the reflecting film. That is, the apparent car rotation angle is greatly improved.

[0005]

However, in the above-described reflective film structure, the increase rate of the Kerr rotation angle varies depending on the wavelength of the laser light used, the type and thickness of the magnetic film, the film thickness of the reflective film, and the like. In addition, even with this structure, there is a problem that a sufficiently large Kerr rotation angle cannot be obtained.

Therefore, an object of the present invention is to provide a magneto-optical storage element capable of obtaining a sufficient Kerr rotation angle.

[0007]

In order to achieve the above object, the present invention provides a substrate and a rare earth-transition metal system having a film thickness of about 30 nm or less and having an easy axis of magnetization in a direction perpendicular to the film surface. An amorphous magnetic thin film, a transparent dielectric film, and a reflective film are provided, and the thickness of the transparent dielectric film is 30 nm to 60 n.
m, the real part A of the virtual refractive index of the transparent dielectric film and the reflective film is 0 <A ≦ 0.5, and the imaginary part B is 0 ≧ B ≧ −.
It is a magneto-optical storage element characterized by being set to 0.5.

[0008]

FIG. 1 shows an embodiment of a magneto-optical storage element having a reflective film structure according to the present invention. 1 is a substrate such as glass, 2 is Gd
TbFe amorphous thin film, 3 is a SiO ₂ transparent film, and 4 is a Cu metal film. It has been confirmed that when the thickness of the SiO ₂ transparent film 3 is changed in this structure, the Kerr rotation angle is greatly changed. FIG. 2 is a graph showing how the Kerr rotation angle changes when the wavelength of the laser light is 6328 nm and the thickness of the SiO ₂ transparent film 3 is changed. Comparing the Kerr rotation angle in the figure with the Kerr rotation angle of 0.27 ° when the SiO ₂ transparent film 3 and the Cu metal film 4 are not present, the importance of the existence of the Cu metal film 4 and the SiO ₂ transparent film 3 can be seen. .. Also, SiO
₂ The Kerr rotation angle without the transparent film 3 is 0.5 ° at maximum even if the other conditions (magnetic film thickness, reflective film thickness, etc.) are changed, so the SiO ₂ transparent film 3 should have a suitable film thickness. It is also well understood from the figure that the car rotation angle can be greatly increased by adjusting to.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 3 is a structural explanatory view of a magneto-optical storage element according to the present invention. In the figure, 5 is air, and a glass substrate may be arranged in this portion. 6 is GdTbFe, GdTbD
It is a rare earth-transition metal type amorphous thin film such as yFe, TbDyFe, TdFe, DyFeCoSn, and the thin film 6 is 3
It has a film thickness of 0 nm or less, for example, a film thickness of 15 nm. With such a film thickness, incident laser light can pass through the thin film 6, and the above-described effect of increasing the Kerr rotation angle can be obtained. 7 is a reflection film made of a metal film or SiO, SiO
₂ , MgF ₂ , Si ₃ N ₄ , Ta ₂ O ₅ , TiO ₂ , CeO
₂ , a transparent film of ZrO ₂ , Al ₂ O _3, etc. and Cu,
A reflective film in which a metal film such as Ag, Au, Al or the like is layered, and the refractive index of the reflective film 7 (if the transparent film and the metal film are layered as described above, a virtual refractive index ) Real part A
Has a value of 0 <A ≦ 0.5, and the imaginary part B has a value of 0 ≧ B ≧ −1.5.

Next, it will be theoretically clarified how appropriate the refractive index of the reflection film 7 is. The refractive index of the air 5 is n ₀ , and the refractive index of the reflective film 7 is n ₂ . Here, the refractive index n ₁ of the rare earth-transition metal thin film 6 differs between clockwise circularly polarized light and counterclockwise circularly polarized light depending on the magnetization state of the film. It is assumed that the _one refractive index is n ₁ ⁺ and the other refractive index is n ₁ ⁻ .
At this time, the reflectances r ₁ ⁺ , r ₁ ⁻ of both left and right circularly polarized light at the interface A between the air 5 and the rare earth-transition metal thin film 6 are expressed by the formula 1.

[0011]

[Equation 1]

Further, the reflectances r ₂ ⁺ and r ₂ ⁻ of both left and right circularly polarized light at the interface B between the rare earth-transition metal thin film 6 and the reflective film 7 are given by
It is represented by.

[0013]

[Equation 2]

From this, the left and right circular deflections R ⁺ and R which interfere with each other inside the rare earth-transition metal thin film 6 and then appear from the interface A.
^-Is the number 3.

[0015]

[Equation 3]

However, δ ⁺ = 4πn ₁ ⁺ d / λ, δ ⁻ = 4πn
₁ ^- d / λ, d: film thickness of magnetic film, λ: wavelength of light.

When linearly polarized light is incident perpendicularly to the film surface, if the traveling direction of light is the Z axis and the vibrating surface is the XZ plane, the x and y direction components Rx of the left and right circularly polarized light that emerges from the interface A. , Ry
Is the number 4.

[0018]

[Equation 4]

Here, θx = arg (Rx), θy = ar
If g (Ry) is set, the Kerr rotation angle α can be expressed by Equation 5.

[0020]

[Equation 5]

The state of change of the Kerr rotation angle with respect to the refractive index of the reflective film was investigated by substituting the following numerical values into the expression of the Kerr rotation angle α. The rare earth-transition metal film 6 is GdT
The refractive indices n ₁ ⁺ and n ₁ ⁻ of the bFe magnetic film were set to 6 when the wavelength of the incident laser light was 6328Å.

[0022]

[Equation 6]

The above n ₁ is the average value of the refractive index of the magnetic film for the left and right circularly polarized light, and δn is the deviation from the average value of the refractive index for the left and right circularly polarized light. Then, the film thickness d of the rare earth-transition metal film 6 was set to 30 nm and 15 nm, and the film thickness d was examined. A graph shown in FIG. 4 was obtained from the mathematical expression of the Kerr rotation angle α using the above numerical values. FIG. 7A is a graph when the film thickness d of the rare earth-transition metal film 6 is 30 nm, and FIG. 8B is a graph when the film thickness d of the rare earth-transition metal film 6 is 1.
It is a graph at the time of 5 nm. What is shown in the figure is, so to speak, a contour line of the Kerr rotation angle relating to the refractive index of the reflection film. The horizontal axis is the real part of the refractive index, and the vertical axis is the even part of the refractive index. From the figure, when the film thickness d of the rare earth-transition metal film 6 is 30 nm, the real part A of the refractive index n ₂ of the reflective film 7 is 0 <Al ≦ 0.5, and the imaginary part B is 0 ≧ B ≧ −3. Sometimes the Kerr rotation angle is the largest, and when the film thickness d of the rare earth-transition metal film 6 is 15 nm, the reflection film 7
It can be seen that the Kerr rotation angle is the largest when the real part A of the refractive index n ₂ is 0 <A ≦ 0.3 and the imaginary part B is 0.2 ≧ B ≧ −8. From these results shown in FIG. 4 and other results not shown, when the film thickness of the rare earth-transition metal film 6 is about 30 nm or less, the refractive index of the reflective film is 0 <A in the real part A.
It was found that a Kerr rotation angle which is practically satisfactory can be obtained when ≦ 0.5 and the imaginary part B is 0 ≧ B ≧ −1.5. A general metal film, for example, a Cu film formed by spattering has a refractive index of about 0.25-3.1i,
In many cases, the value does not exist within the preferable refractive index range of the reflective film obtained as the above conclusion. In such a case, the refractive index of the reflective film can be controlled by the following method.
That is, the refractive index of the metal film can be apparently greatly changed by using a two-layer film formed by laminating a transparent dielectric film such as SiO ₂ and a metal film instead of the metal film as a reflection film. This apparent refractive index is called a virtual refractive index here. For example, on the back surface of the rare earth-transition metal film 6, SiO ₂
In a device having a structure in which a transparent dielectric film and a Cu film are layered in this order, the change in refractive index (change in virtual refractive index) when the film thickness of the SiO ₂ transparent dielectric film is changed is shown. 5 shows. FIG. 5A is a partially enlarged view thereof. As shown in FIG. 5, the thickness of the dielectric film at S ₂ is 0 nm, 50 nm, 100
The value of the refractive index draws a circle on the complex plane as it increases to nm, 150 nm, and 200 nm. Further, according to FIG. 5A, the SiO ₂ transparent dielectric film has a film thickness of about 30 to 60 nm and a refractive index value of 0.1-1.2i to 0.05-0.3.
The value of the refractive index is 0.2 when the SiO ₂ transparent dielectric film does not exist, that is, when the film thickness is 0 nm.
This is a significant change from 5-3.1i. Then, it can be seen that the refractive index of the reflective film shifts to the range of the preferable refractive index described above.

In principle, when a transparent film having a refractive index n ″ is layered on a film having a refractive index n ′, the imaginary refractive index of the two-layer film is a radius of 8 with a center of the formula 7 on the complex plane. Is a circle, where | r |

[0025]

[Equation 7]

[0026]

[Equation 8]

[0027]

[Equation 9]

Therefore, the above-mentioned SiO ₂ transparent dielectric film and C
The transparent dielectric film for controlling the virtual refractive index of the reflective film is not limited to the two-layer film including the u film, but MgF ₂ film, Si ₃ N ₄
Film, Ta ₂ O ₅ film, TiO ₂ film, CeO ₂ film, ZrO ₂
A film, an Al ₂ O ₂ film or the like can be used.

As described above, it has been found that the refractive index can be controlled by changing the structure of the reflective film, and that the Kerr rotation angle which is practically satisfactory can be obtained by the control.

When the magneto-optical storage element is used, the reference of the magnitude of the read signal is the reflectance R and the Kerr rotation angle α.
Is Rα ² . Therefore, the magnitude of the read signal is almost dependent on the magnitude of the Kerr rotation angle α 1. According to an experiment, the value of the reflectance R decreases in the region where the Kerr rotation angle α increases, but the value of the Kerr rotation angle α has a great influence on the magnitude of the read signal. The large range of is approximately equal to the large range of the Kerr rotation angle.

For example, the film thickness d of the rare earth-transition metal film is 15
nm, and the refractive index n ₂ of the reflecting film is 0.2-0.4i, the reflectance R is 0.22, and the Kerr rotation angle α is 0.62 °.
α ² is about 0.085. On the other hand, when the film thickness d of the rare earth-transition metal film is relatively thick, 100 nm, and there is no reflection film effect, the reflectance R is 0.54, the Kerr rotation angle α is 0.21 °, and Rα ² is about 0.024. Is. As described above, it can be seen that the read signal is very large when the effect of the reflective film is exerted. And as a result, it is understood that the larger the car rotation angle, the better.

When a magneto-optical storage element is used, the recording sensitivity depends on the absorption rate of the rare earth-transition metal film.
From the viewpoint of the absorptance, the absorptance is higher when the film thickness of the rare earth-transition metal film is thinner and the reflective film structure is used. For example, when the film thickness d of the rare earth-transition metal film is 15 nm and the refractive index n ₂ of the reflective film is 0.2-0.4i, 72% of incident light is absorbed by the rare earth-transition metal film. On the other hand, when the film thickness d of the rare earth-transition metal film is 100 nm and is relatively large and there is no reflection film effect, the incident light of 46 is incident on the rare earth-transition metal film.
Only% is absorbed. As described above, it is understood that the element having the reflective film structure is also excellent in terms of recording sensitivity.

[0033]

As described in detail above, the magneto-optical storage element of the present invention can increase the Kerr rotation angle, which can increase the read signal. Further, since the magneto-optical storage element of the present invention has a large absorptance, it can have excellent recording sensitivity.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a magneto-optical storage element having a reflective film structure of the present invention.

FIG. 2 is a diagram showing characteristics of a Kerr rotation angle of a magneto-optical storage element.

FIG. 3 is a diagram illustrating the configuration of a magneto-optical storage element.

FIG. 4 is a diagram showing a characteristic of a Kerr rotation angle with respect to a refractive index of a reflective film.

FIG. 5 is a diagram showing changes in virtual refractive index.

[Explanation of symbols]

1 Substrate 2 GaTbFe Amorphous Thin Film 3 SiO ₂ Transparent Film 4 Cu Metal Film 5 Air 6 Rare Earth-Transition Metal Amorphous Thin Film 7 Reflective Film

Front page continued (72) Inventor Kenji Ota 22-22 Nagaike-cho, Abeno-ku, Osaka

Claims (1)

1. A substrate, a rare earth-transition metal amorphous magnetic thin film having a film thickness of about 30 nm or less and having an easy axis of magnetization in a direction perpendicular to the film surface, and a transparent dielectric. The transparent dielectric film has a body film and a reflection film, and the thickness of the transparent dielectric film is 30 nm to 60 nm.
and the real part A of the virtual refractive index of the transparent dielectric film and the reflective film is 0 <A ≦ 0.5, and the imaginary part B is 0 ≧ B ≧.
A magneto-optical storage element having a value of −1.5.