JPH0479075B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Technical Field> The present invention relates to a magneto-optical memory element which is expected to be used as a memory element capable of recording, reproducing and erasing information.

<Prior Art> Conventionally, one of the difficulties when using a magneto-optical storage element as a memory element was that the reproduction signal level was low. In particular, in the so-called Kerr effect reproduction method, in which a magneto-optical memory element is irradiated with a laser beam and information is reproduced using the reflected light, the Kerr rotation angle is generally small, so it is difficult to increase the signal-to-noise ratio (S/N). It was difficult. For this reason, in the past, it was necessary to improve the magnetic material used as the recording medium, or to add SiO or other materials to the recording medium.
They tried to increase the Kerr rotation angle by forming a SiO ₂ dielectric film.

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 a magneto-optical storage element using the Kerr effect reproduction method (Patent Application No. 1983). −85695)
are doing. This structure is called a reflective film structure.

The feature 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 on the reflective film are combined, so the apparent appearance is lower than that of a structure without the reflective film. This means that the Kerr rotation angle is greatly improved. In this case, it has been confirmed that the increase rate of the Kerr rotation angle varies depending on the wavelength of the laser beam used, the type and thickness of the magnetic film, the thickness of the reflective film, etc.

What is shown in FIG. 1 is an example of a magneto-optical memory element having a reflective film structure that has already been proposed by the applicant. 1
is a substrate such as glass, 2 is a GdTbFe amorphous thin film, 3 is
4 is a SiO ₂ transparent film, and 4 is a Cu metal film. It has been confirmed that in this structure, when the thickness of the SiO ₂ transparent film 3 is changed, the Kerr rotation angle changes greatly. FIG. 2 is a graph showing how the Kerr rotation angle changes when the wavelength of the laser beam is 632.8 nm and the thickness of the SiO ₂ transparent film 3 is changed. Comparing the Kerr rotation angle shown in the figure with the Kerr rotation angle of 0.27° in the absence of the SiO ₂ transparent film 3 and the Cu metal film 4, the importance of the existence of the Cu metal film 4 and the SiO ₂ transparent film 3 can be seen.
In addition, the Kerr rotation angle without the SiO ₂ transparent film 3 is at most 0.5° even if other conditions (magnetic material film thickness, reflective film thickness, etc.) are changed, so the film thickness of the SiO ₂ transparent film 3 is It is also clear from the figure that the Kerr rotation angle can be greatly increased by making appropriate adjustments.

<Objective> An object of the present invention is to provide a structure of a magneto-optical memory element that can obtain an optimum Kerr rotation angle by a combination of a transparent dielectric film and a metal reflective film.

<Example> FIG. 3 is an explanatory diagram of the configuration of an example of the magneto-optical memory element according to the present invention. In the figure, 5 is air, and a glass substrate may be placed in this portion. 6 is
GdTbFe, GdTbDyFe, TbDyFe, TbFe,
It is a rare earth-transition metal based amorphous thin film such as DyFeCoSn, and this thin film 6 has a thickness of 30 nm or less, for example 15 nm.
It has a film thickness of With a film thickness of this level, the incident laser beam can pass through the thin film 6, and the above-mentioned effect of increasing the Kerr rotation angle can be obtained. 7 is a reflective film made of a metal film or SiO, SiO ₂ , MgF ₂ ,
Reflection made by layering a transparent film such as Si ₃ N ₄ , Ta ₂ O ₅ , TiO ₂ , CeO ₂ , ZrO ₂ , Al ₂ O ₃ etc. and a metal film such as Cu, Ag, Au, Al etc. on the lower surface of the transparent film. The real part A of the refractive index (virtual refractive index when a transparent film and a metal film are layered as described above) of this reflective film 7 is 0<
A≦0.5, and the imaginary part B has a value of 0≧B≧−1.5.

Next, the appropriate value of the refractive index of the reflective film 7 will be theoretically clarified. The refractive index of the air 5 is _n0 , the refractive index of the rare earth-transition metal thin film 6 is _n1 , and the refractive index of the reflective film 7 is _n2 . 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 state of magnetization of the film. Let the refractive index of one of the above be n ₁ ⁺ and the refractive index of the other be n ₁ ^- . At this time, the reflectance 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 is
If r ₁ ⁺ and r ₁ ^- , then r ₁ ⁺ = n ₀ − n ₁ ⁺ /n ₀ + n ₁ ⁺ , r ₁ ^- = n ₀ − n ₁ ^- /n ₀ + n ₁ ^- . Also, the reflectance 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 is
If r ₂ ⁺ and r ₂ ^- , then r ₂ ⁺ = n ₁ ⁺ −n ₂ /n ₁ ⁺ +n ₂ and r ₂ ^- = n ₁ ^- −n ₂ /n ₁ ^- +n ₂ . From this, if the left and right circularly polarized lights that interfere inside the rare earth-transition metal thin film 6 and then emerge from the interface A are R ⁺ and R ^- , then R ⁺ =
r ₁ ⁺ +r ₂ ⁺ e ^-i 〓 ⁺ /1+r ₁ ⁺ r ₂ ⁺ e ^-i 〓, R ^- = r ₁ ^- +r ₂ ⁺ e ^-i 〓
It is expressed as ^- /1+r ₁ ^- r ₂ ^- e ^-i 〓 ^- . However, δ ⁺ =4πn ₁ ⁺ d/λ, δ ^- =4πn ₁ ^- d/λ,
d: thickness of magnetic film, λ: wavelength of light.

When linearly polarized light is incident perpendicularly to the film surface, if the traveling direction of the light is the Z axis and the vibration plane is the X-Z plane, the interface A
x and y direction components of left and right circularly polarized light emerging from
Rx and Ry are Rx=1/2( ^R ++ ^R- ) and Ry=i/2(R ^-- R ⁺ ). Here θx=arg(Rx), θy=arg
(Ry), the Kerr rotation angle α is expressed by the following formula.

α=1/2tan ^-1 2｜Rx｜｜Ry｜ /｜Ry｜ ² −｜Rx｜ ² cos
(θx−θy) By substituting the following numerical values into the formula for the Kerr rotation angle α, the state of change in the Kerr rotation angle with respect to the refractive index of the reflective film was investigated. The rare earth-transition metal film 6 is
As a GdTbFe magnetic film, its refractive index n ₁ ^± is n ₁ ^± = n ₁ ±δn {n ₁ = when the wavelength of the incident laser beam is 6328 Å.
2.3−3.0i, δn=1/2(0.05−0.02i)}. The above n ₁ indicates the average value of the refractive index of the magnetic film for left and right circularly polarized light, and Δn indicates the deviation from the above average refractive index for 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 respective film thicknesses d were investigated. Using the above numerical values, the graph shown in FIG. 4 was obtained from the mathematical formula for the Kerr rotation angle α. In the figure a, the thickness d of the rare earth-transition metal film 6 is
This is a graph when the film thickness is 30 nm, and the graph b is a graph when the film thickness d of the rare earth-transition metal film 6 is 15 nm.
What is shown in the figure is, so to speak, contour lines of the Kerr rotation angle with respect to the refractive index of the reflective film. The horizontal axis is the real part of the refractive index, and the vertical axis is the imaginary part of the refractive index. From the same figure, when the thickness d of the rare earth-transition metal film 6 is 30 nm, the refractive index _n2 of the reflective film 7 has a curve when the real part A is 0<A≦0.5 and the imaginary part B is 0≧B≧−3. When the rotation angle is the largest and the thickness d of the rare earth-transition metal film 6 is 15 nm, the real part A of the refractive index _n2 of the reflective film 7 is 0<A≦0.3.
It can be seen that the Kerr rotation angle is the largest when the imaginary part B is 0.2≧B≧−0.8. From these results shown in FIG. 4 and other results not shown, when the thickness of the rare earth-transition metal film 6 is approximately 30 nm or less, the refractive index of the reflective film has a real part A of 0<A≦0.5. It has been found that a practically satisfactory Kerr rotation angle can be obtained when the imaginary part B is 0≧B≧−1.5.

Now, the refractive index of a general metal film, such as a Cu film formed by sputtering, is approximately 0.25-3.1i.
However, the value thereof is often not within the range of the preferable refractive index 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, by using a two-layer film consisting of a transparent dielectric film such as SiO ₂ and a metal film as a reflective film instead of a metal film, it is possible to change the refractive index of the metal film to a large extent in appearance. can. This apparent refractive index is referred to herein as a virtual refractive index. For example, in an element having a structure in which a SiO ₂ transparent dielectric film and a Cu film are layered in this order on the back surface of a rare earth-transition metal film 6, the refraction when the thickness of the SiO ₂ transparent dielectric film is changed is FIG. 5 shows the change in index (change in virtual refractive index). FIG. 5a is a partially enlarged view. As shown in Figure 5, the film thickness of the SiO ₂ transparent dielectric film is 0nm, 50nm, 100nm, 150nm,
As the refractive index increases to 200 nm, the value of the refractive index draws a circle on the complex plane. Also, according to FIG. 5a, when the thickness of the SiO ₂ transparent dielectric film is about 30 to 60 nm, the refractive index value is about 0.1-1.2i to 0.05-0.3i,
The refractive index value is significantly changed from 0.25-3.1i when the SiO ₂ transparent dielectric film is not present, that is, when the film thickness is 0 nm. It can be seen that the refractive index of the reflective film is within the above-mentioned preferred refractive index range.

In principle, when a transparent film with a refractive index n'' is layered on a film with a refractive index n', the virtual refractive index of the two-layer film is n''(1-r ² )/1+r ² on the complex plane. center radius n
″・｜2r｜/｜1−r ² |. However, |r｜ = n″−n′/n″+n′. Therefore, the above-mentioned SiO ₂ transparent dielectric film and Cu metal film Not only the two-layer film with MgF ₂ but also the transparent dielectric film for controlling the virtual refractive index of the reflective film.
membrane, Si ₃ N ₄ membrane, Ta ₂ O ₅ membrane, TiO ₂ membrane, CeO ₂ membrane,
ZrO ₂ film, Al ₂ O ₃ film, etc. can be used.

It has been found that by changing the structure of the reflective film as described above, it is possible to control its refractive index, and through this control, a practically satisfactory Kerr rotation angle can be obtained.

When a magneto-optical memory element is used, a measure of the magnitude of the read signal is Rα ² where R is the reflectance and α is the Kerr rotation angle. Therefore, the magnitude of the readout signal is mostly determined by the magnitude of the Kerr rotation angle α. According to experiments, the value of the reflectance R becomes smaller in the region where the Kerr rotation angle α increases, but since the magnitude of the readout signal is strongly influenced by the value of the Kerr rotation angle α, the readout signal The large range of is approximately the same as the large range of the Kerr rotation angle.

For example, if the thickness d of the rare earth-transition metal film is 15 nm,
When the refractive index _n2 of the reflective film is 0.2−0.4i, the reflectance R is
0.22, Kerr rotation angle α is 0.62°, and Rα ² is approximately 0.085. On the other hand, the thickness d of the rare earth-transition metal film is 100 nm.
The reflectance R when the film is relatively thick and has no effect is
0.54, Kerr rotation angle α is 0.21°, and Rα ² is approximately 0.024. As described above, it can be seen that when the reflective film has an effect, the readout signal is very large. The results show that the larger the Kerr rotation angle, the better.

Furthermore, when a magneto-optical memory element is used, the recording sensitivity is influenced by the absorption rate of the rare earth-transition metal film. And from the perspective of this absorption rate, rare earths -
The absorption rate is higher when the transition metal film is thinner and has a reflective film structure. For example, when the 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 the incident light is absorbed by the rare earth-transition metal film. On the other hand, the film thickness d of the rare earth-transition metal film is
When the rare earth-transition metal film is relatively thick at 100 nm and has no reflective film effect, only 46% of the incident light is absorbed by the rare earth-transition metal film. As described above, it can be seen that the element with the reflective film structure is superior in terms of recording sensitivity as well.

<Effect> As explained above, according to the present invention, the combination of the transparent dielectric film and the metal reflective film makes the real part A of the virtual refractive index 0<A≦0.5 and the imaginary part B 0≧
Since B≧−1.5, the absorption rate is large, so the recording sensitivity is excellent, and the Kerr rotation angle can be increased, and the read signal can be increased, so the magneto-optical storage element has excellent characteristics. can be provided.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of the structure of a magneto-optic memory element having a reflective film structure, FIG. 2 is a graph diagram of its Kerr rotation angle characteristics, FIG. 3 is an explanatory diagram of the structure of the magneto-optic memory element according to the present invention, and FIG. The figure is a graph showing the characteristics of the Kerr rotation angle with respect to the refractive index of the reflective film, and FIG. 5 is a graph showing the change in virtual refractive index when the film thickness of the SiO ₂ transparent dielectric film is changed. In the figure, 1: substrate, 2: GdTbFe 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.

Claims (1)

[Scope of Claims] 1. A substrate, a rare earth-transition metal based amorphous magnetic thin film having a film thickness of about 30 nm or less and having an axis of easy magnetization perpendicular to the film surface, a transparent dielectric film, and a metal. a reflective film, and the real part A of the virtual refractive index is set to 0<A≦ by the combination of the transparent dielectric film and the metal reflective film.
0.5, and the imaginary part B satisfies 0≧B≧−1.5.