JP2908911B2 - Magneto-optical storage element - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magneto-optical storage element capable of recording, reproducing and erasing information. Conventionally, one of the drawbacks when using a magneto-optical storage element as a memory element is that the reproduction signal level is low. In particular, in a so-called Kerr effect reproducing system in which a magneto-optical storage element is irradiated with laser light and information is reproduced by reflected light, the signal-to-noise ratio (S / N) can be increased because the Kerr rotation angle is generally small. It was difficult. Therefore, conventionally, a magnetic material as a recording medium has been improved, or 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 of improving the apparent Kerr rotation angle by forming a reflective film behind a recording medium in a magneto-optical storage element of the Kerr effect reproducing method (Japanese Patent Application No. 55-85695). This structure is called a reflection film structure. The feature of this structure is that the laser light reflected on the recording medium surface and the laser light transmitted through the recording medium and then reflected on the reflection film are combined, so that the structure is not compared with the structure without the reflection film. That is, the apparent car rotation angle is greatly improved. However, in the above-mentioned reflecting film structure, the increasing rate of the Kerr rotation angle depends on the wavelength of the laser beam used, the type and thickness of the magnetic film, the thickness of the reflecting film, and the like. In addition to this, there is a problem that a car rotation angle of a sufficient size cannot be obtained even with this structure. SUMMARY OF THE INVENTION An object of the present invention is to provide a magneto-optical storage element capable of obtaining a sufficiently large Kerr rotation angle. [0007] To achieve the above object,
Therefore, the present invention has a substrate and a thickness of 15 nm or less,
Rare earth-transition gold with easy axis of magnetization perpendicular to film surface
Metal-based amorphous magnetic thin film and a transparent film having a thickness of about 60 nm or less.
Light dielectric film and less of gold, silver, copper and aluminum
And a metal reflective film including at least one of these in that order.
It is characterized by . FIG. 1 shows an embodiment of a magneto-optical memory device having a reflective film structure according to the present invention. 1 is a substrate such as glass, 2 is Gd
A TbFe amorphous thin film, 3 is a SiO ₂ transparent film, and 4 is a Cu metal film. It has been confirmed that changing the thickness of the SiO ₂ transparent film 3 in this structure greatly changes the Kerr rotation angle. FIG. 2 is a graph showing how the Kerr rotation angle changes when the wavelength of the laser beam is 6328 nm and the thickness of the SiO ₂ transparent film 3 changes. 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 understood. . Also, SiO
The _second transparent layer 3 is Kerr rotation angle when no other conditions (the magnetic film thickness, reflecting film thickness, etc.) appropriate to the thickness of the SiO ₂ transparent film 3 from a 0.5 ° at maximum be changed It can also be seen from the figure that the car rotation angle can be greatly increased by adjusting the angle. FIG. 3 is a diagram illustrating the configuration of a magneto-optical memory device according to the present invention. In the figure, reference numeral 5 denotes air, and a glass substrate may be provided in this portion. 6 is GdTbFe, GdTbD
A rare earth-transition metal based amorphous thin film such as yFe, TbDyFe, TdFe, DyFeCoSn, etc.
It has a thickness of 0 nm or less, for example, a thickness of 15 nm. With such a thickness, the 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, ZrO _2, Al ₂ transparent film of O _3, and Cu on the underside,
A reflective film on which a metal film of Ag, Au, Al, or the like is layered; and a refractive index of the reflective film 7 (a virtual refractive index when a transparent film and a metal film are layered as described above). ) 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, the appropriate value of the refractive index of the reflection film 7 is theoretically clarified. The refractive index of the air 5 is n ₀ and the refractive index of the reflection 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. One of the refractive indexes is n ₁ ⁺ , and the other is n ₁ ⁻ .
In this case the air 5 and the rare earth - transition metal reflectivity of both the circular polarization of the left and right at the interface A between the thin film 6 r ₁ ^+, r ₁ ^- is represented by the number 1. [0011] [0012] The rare earth - transition metal film 6 and the reflection film 7 reflectivity of both the circular polarization of the left and right at the interface B between r ₂ ^+, r ₂ ^- the number 2
It is represented by [0013] From this, the right and left circular deflections R ⁺ , R ⁺ , which interfere inside the rare earth-transition metal thin film 6 and then emerge from the interface A,
^-Becomes Equation 3. [Equation 3] [0016] However ^{_{δ + = 4πn 1 + d /}} λ, δ - = 4πn
₁ ^- d / λ, d: film thickness of the magnetic film, λ: wavelength of light. When linearly polarized light is perpendicular to the film surface, the traveling direction of the light is defined as the Z axis, and the vibration plane is defined as the XZ plane. , Ry
Is Equation 4. [Equation 4] Here, θx = arg (Rx), θy = ar
If g (Ry) is set, the Kerr rotation angle α is expressed by Expression 5. (Equation 5) By substituting the following numerical values into the equation of the Kerr rotation angle α, the state of the change of the Kerr rotation angle with respect to the refractive index of the reflection film was examined. The rare earth-transition metal film 6 is made of GdT
The refractive indexes n ₁ ⁺ and n ₁ ⁻ of the bFe magnetic film were set to 6 when the wavelength of the incident laser light was 6328 °. (Equation 6) The above-mentioned n ₁ represents the average value of the refractive index of left and right circularly polarized light of the magnetic film, and δn represents the deviation from the above-mentioned average value of the refractive index of right and left 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 each film thickness d was examined. Using the above numerical values, a graph shown in FIG. 4 was obtained from the mathematical expression of the Kerr rotation angle α. FIG. 3A is a graph when the film thickness d of the rare earth-transition metal film 6 is 30 nm, and FIG.
It is a graph at the time of 5 nm. What is shown in the figure are so-called contour lines of the Kerr rotation angle with respect 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 FIG earth - real part A is 0 refractive index n ₂ of the reflective film 7 when the film thickness d of 30nm transition metal film 6 <Al ≦ 0.5, 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 satisfies 0 <A ≦ 0.3 and the imaginary part B satisfies 0.2 ≧ B ≧ −8. From the results shown in FIG. 4 and other results (not shown), when the thickness of the rare earth-transition metal film 6 is about 30 nm or less, the refractive index of the reflection film is such that the real part A is 0 <A.
It was found that a practically satisfactory Kerr rotation angle was obtained when ≦ 0.5 and the imaginary part B was 0 ≧ B ≧ −1.5. Now, the refractive index of a general metal film, for example, a Cu film formed by sputtering, is approximately 0.25-3.1i,
In many cases, the value does not exist within the preferable range of the refractive index of the reflection 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 significantly 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 the reflection film. This apparent refractive index is referred to herein as a virtual refractive index. For example, the rear surface of the rare earth-transition metal film 6 is made of SiO ₂
In a device having a structure in which a transparent dielectric film and a Cu film are layered in this order, a change in refractive index (change in virtual refractive index) when the thickness of the SiO ₂ transparent dielectric film is changed is shown. It is shown in FIG. FIG. 5A is a partially enlarged view of FIG. As shown in FIG. 5, the thickness of the dielectric film at S ₂ is 0 nm, 50 nm, 100 nm.
The value of the refractive index draws a circle on the complex plane as it increases to nm, 150 nm, and 200 nm. According to FIG. 5 (a), the thickness of the SiO ₂ transparent dielectric film is about 30 to 60 nm, and the value of the refractive index is 0.1-1.2i to 0.05-0.3.
i, and when the SiO ₂ transparent dielectric film does not exist, that is, when the film thickness is 0 nm, the refractive index value is 0.2.
It changes greatly from 5-3.1i. Then, it can be seen that the refractive index of the reflective film has shifted within the above-mentioned preferred refractive index range. In principle, when a transparent film having a refractive index of n ″ is formed on a film having a refractive index of n ′, the virtual refractive index of the two-layered film is expressed by a radius of 8 with a center at Equation 7 on a complex plane. Where | r | is equation (9). [Equation 8] [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 reflection film is not limited to the two-layer film with the u film, but may be 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 reflection film, and a Kerr rotation angle that is practically satisfactory can be obtained by the control. Also, in FIG.
The results show that the rare earth-transition metal film has a thickness of 30 nm.
15 nm has the same refractive index of the reflective film
If there is, you can see that a large car rotation angle can be obtained . When a magneto-optical storage element is used, the standard of the magnitude of the read signal is that the reflectance is R and the Kerr rotation angle is α.
Is Rα ² . Therefore, the magnitude of the readout signal is substantially influenced by the magnitude of the Kerr rotation angle α. According to an experiment, the value of the reflectance R is conversely reduced in a region where the Kerr rotation angle α is large. However, since the magnitude of the read signal is greatly affected by the value of the Kerr rotation angle α 1, Is approximately equal to the range of the Kerr rotation angle. For example, the film thickness d of the rare earth-transition metal film is 15
nm, when the refractive index n ₂ of the reflective film is 0.2-0.4i, the reflectance R is 0.22, and the Kerr rotation angle α is 0.62 ° and R
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 at 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. It is. As described above, it can be seen that the readout signal is very large when the effect of the reflection film is provided. As a result, it is understood that the larger the car rotation angle is, the better. When a magneto-optical storage element is used, the recording sensitivity depends on the absorption of the rare earth-transition metal film.
From the viewpoint of the absorptance, the absorptivity is higher when the film thickness of the rare earth-transition metal film is thinner and the reflection film structure is used. Such as rare earth - film thickness d of the transition metal film is 15 nm, the refractive index n ₂ of the reflective film when 0.2-0.4i the rare earth - 72% of the incident light is absorbed in the transition metal film. On the other hand, when the film thickness d of the rare earth-transition metal film is relatively large, ie, 100 nm, and there is no reflective film effect, 46 of the incident light enters the rare earth-transition metal film.
Only% is absorbed. As described above, in terms of recording sensitivity, the film thickness of the rare earth-transition metal film in the reflective film structure is also small.
It turns out that the element of 15 nm or less is excellent. As described above in detail, the magneto-optical storage element of the present invention can increase the Kerr rotation angle, thereby increasing the read signal. Further, since the magneto-optical storage element of the present invention has a large absorptance, the recording sensitivity can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a configuration diagram of a magneto-optical storage element having a reflective film structure according to 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 a configuration of a magneto-optical storage element. FIG. 4 is a diagram showing characteristics of a Kerr rotation angle with respect to a refractive index of a reflection film. FIG. 5 is a diagram showing a change in a virtual refractive index. [Description of Signs] 1 substrate 2 GaTbFe amorphous thin film 3 SiO ₂ transparent film 4 Cu metal film 5 air 6 rare earth-transition metal based amorphous thin film 7 reflective film

(72) Inventor Kenji Ota 22-22 Nagaike-cho, Abeno-ku, Osaka City Inside Sharpe Co., Ltd. (72) Inventor Hideka Yamaoka 22-22 Nagaike-cho, Abeno-ku, Osaka City Sharpe Co., Ltd. References JP-A-59-54056 (JP, A) JP-A-59-52442 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G11B 11/10

Claims (1)

(57) [Claims] Magnetization and the substrate has a thickness of less 15 nm, in a direction perpendicular to the film plane
Rare earth-transition metal based amorphous magnetic thin film with easy axis
And a transparent dielectric film having a thickness of about 60 nm or less, and at least one of gold, silver, copper, and aluminum.
And a metal reflection film formed in this order.
A magneto-optical storage element .