JPH0344388B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a novel magneto-optical recording medium with improved reproduction signal strength.

[Background of the invention]

Magneto-optical recording media include, for example, GdCo, GdTbCo,
A magnetic thin film that can be perpendicularly magnetized, such as GdTbFe or DyTbFe, is laminated on a substrate.The magnetization direction of this magnetic thin film is aligned either upward or downward with respect to the film direction, and then the area to be recorded is placed. By irradiating the part with a laser beam and heating the part to a temperature above the Curie point of the magnetic material, the original magnetization direction is freely released, and at the same time, by applying a weak magnetic field in the opposite direction to the part, the part can be is magnetized in the opposite direction to that of the film, and then the laser irradiation is stopped to fix the magnetization in the opposite direction. As a result, if the magnetization direction of the film is set to 0 and the opposite direction is set to 1, the minute spot area irradiated with the laser beam remains recorded as one of the digital signals of 0 and 1.

The difference in the magnetization direction (that is, upward or downward) of the magnetic thin film recorded in this way can be determined by irradiating it with linearly polarized light, and the rotation state of the polarization plane of the reflected or transmitted light differs depending on the direction of magnetization. It is read using a phenomenon (magnetic Kerr effect or magnetic Faraday effect). In other words, if the plane of polarization rotates by +θrad when the direction of magnetization is upward with respect to the incident light, then
When the direction of magnetization is downward with respect to the incident light, the plane of polarization rotates by -θ rad. All angles below are expressed in radians (rad.).

Therefore, if a polarizer (also called an analyzer, which allows only the polarized light components that match the principal axis to pass through) is placed at the end of the reflected or transmitted light, if the polarized light incident on the analyzer is linearly polarized, then the The intensity of light passing through the analyzer increases or decreases depending on the rotation of the plane of polarization. In other words, one rotation angle −
When an analyzer is placed on the optical path with its principal axis aligned in a direction almost perpendicular to θ, if polarized light with a rotation angle of +θ is incident, it will pass through the analyzer,
When polarized light with a rotation angle of -θ is incident, almost no light is transmitted through the analyzer.

Therefore, if a detector such as a photoelectric conversion element is placed in front of the analyzer, the strength of light intensity will be converted into the strength of current. In other words, the digital recording signal due to the difference in the magnetization direction of the magnetic thin film is converted into the rotation angle ±θ of the plane of polarization, which is then converted into the intensity of light through an analyzer, and finally into the intensity of current by the detector. Therefore, the strength of the reproduced signal (that is, the difference in strength of the current caused by the detector) depends on 2θ.

However, in general, reflected light or transmitted light from a recording medium is not linearly polarized light in a strict sense, but becomes elliptically polarized light having an ellipticity χ. Usually χ compared to θ
is very small, so only θ (θ≠0) is used. However, depending on the magnetic material and the wavelength of the incident light, θ and χ may become almost equal.
In that case, ignoring χ results in a large loss in the reproduced signal strength.

[Purpose of the invention]

Therefore, an object of the present invention is to make it possible to effectively utilize the ellipticity χ of reflected light or transmitted light of a recording medium even in a conventional magneto-optical reproducing device, thereby improving the reproduced signal strength and the S/N ratio. An object of the present invention is to provide a magneto-optical recording medium.

[Summary of the invention]

Therefore, the present inventor came up with the idea of making χ contribute to improving the reproduced signal strength S by providing a phase shifter, and conducted theoretical calculations to find out what kind of phase shifter should be provided to improve the reproduced signal strength S. heading,
The present invention has now been accomplished.

Now, consider a magneto-optical recording medium in which a retarder 2 having a phase difference of Δ is laminated on a magnetic thin film 1, as shown in FIG. 1 (cross-sectional view). In this case, the light is the first
As shown by arrows in the figure, in the case of a transmission type, the magnetic thin film 1 is irradiated from the substrate side, and in the case of a reflective type, the magnetic thin film 1 is irradiated from the retarder 2 side. Such a magneto-optical recording medium K is mainly reproduced by a reproducing apparatus having a conceptual configuration as shown in FIG. 2 (reflection type) or FIG. 3 (transmission type). In FIGS. 2 and 3, K is a recording medium, L is a laser light source (polarized light source), BS is a beam splitter, A is an analyzer, and D is a detector.

Here, (a) the Kerr rotation angle or Faraday rotation angle of the magnetic thin film is θ, the Kerr ellipticity or Faraday ellipticity is χ, the amplitude reflectance or amplitude transmittance is γ, and (b) the phase difference of the retarder. is Δ, and the principal axis azimuth is φ(φ
is substantially zero), (c) the azimuth angle of the analyzer is α, and (d) the azimuth angle of the polarized light emitted from the laser light source is zero, then the light intensity I〓 after passing through analyzer A is , the calculation is complicated, so if you omit the middle part, I〓∝｜γ｜ ² ×｜θ(sin2α・cosΔ)＋χ(sin2α・s
inΔ)+1/2(1+cos2α)+φsin2αsin ² Δ｜…(
Equation 1) is derived. In this case, the mode shown in Figure 2 (reflection type)
In this case, it passes through the retarder twice, once when it enters and once when it is reflected, but since the plane of polarization of the linearly polarized light almost coincides with the main axis of the retarder when it enters, there is no effect of the retarder, and the reflection type (Figure 2) , the transmission type (FIG. 3) both have the same formula 1.

Now, the light intensity when the direction of magnetization of the magnetic thin film is the same as the direction of incidence of the incident light is Ia ↑, the complex Kerr or Faraday rotation angle by the magnetic thin film is θ+iχ, and the light intensity when the direction of magnetization is opposite to the direction of incidence is If Ia ↑ and the complex Kerr or Faraday rotation angle is −θ−iχ, then
The optical intensity difference, that is, the reproduced signal strength S is S=|Ia ↑ −Ia ↑ |, so S∝|γ| ² × | θ(sin2α・cosΔ)+χ(sin2α・sinΔ
)｜=｜γ｜ ² ×｜sin2α(θcosΔ+χsinΔ)｜…(Equation 2
) becomes.

Alternatively, instead of the direct method using an analyzer, a Wallaston prism, a Rossillon prism or a polarizing beam splitter (PBS) can be used.
In the so-called differential method, which has polarization directions orthogonal to each other and is divided into two parts with approximately equal intensities and guided to each detector, and the difference in output from both detectors is taken, the reproduced signal strength S is 1/4πrad. If ^{the light intensity I} at the ^{time of} Therefore, by substituting Equation 1 into the right-hand side, S∝|θcosΔ+χsinΔ| is derived.

On the other hand, in the case of the direct method, the optimal value of the azimuth angle α of the analyzer in Equation 2 varies depending on the detector used, the laser as the light source, the analyzer, etc., but in any case, α is the extinction position for the incident polarized light. (α=1/2πrad.).

Therefore, it can be expressed as α=(1/2π+α′) rad. Since α′ is a sufficiently small angle, sin2α=sin(π+2α′)=−sin2α′≒−2α′ cos2α=cos(π+2α ′)≒cosπ=−1.

Therefore, if the reflectance or transmittance γ is constant,
The reproduced signal strength S is S∝|θcosΔ+χsinΔ|.

Therefore, whether the analysis method is a direct method or a differential method, S∝｜θcosΔ+χsinΔ｜… (Formula 3) holds true.

Therefore, no retarder is provided (that is, Δ=0,
sinΔ=0), it can be seen that χ cannot be reflected or used in the reproduced signal strength S. However, depending on the value of Δ, θcosΔ becomes small, so if Δ is selected that always satisfies |θcosΔ+χsinΔ|>|θ| (Equation 4), a retarder with such a phase difference Δ can be obtained. By providing this, the reproduced signal strength S of the medium will be improved. This is the principle of the invention.

Next, we will specifically find some values of Δ that satisfy Equation 4.

Now, considering the vectors 〓=θ χ, 〓=cosΔ sinΔ, inner product C of 〓 and 〓 (C=
〓・〓) becomes C=|θcosΔ+χsinΔ|. on the other hand,
If C is the inner product of 〓 and 〓, C means the mapping of a to the unit vector 〓.

Therefore, if the relationship between the vectors 〓, 〓 and the projection C is illustrated, it will be as shown in FIG. 4 when both θ and χ are positive. Then, the length of C is always greater than the length of θ when Δ is greater than zero and smaller than twice the angle formed by the vector 〓 and the θ axis. The angle formed by the vector 〓 and the θ axis is determined by tan ⁻¹ χ/θ=Δ, and at that time C becomes maximum and equal to the length of the vector 〓: √ ² + ² .

In other words, the value of |θcosΔ+χsinΔ| is always larger than |θ| when 2tan ^-1 χ/θ>Δ>0rad., and the maximum value is √ ² + ² .

If a phase shifter with tanΔ=χ/θ is provided, the reproduced signal strength S will be lower than that without a phase shifter.
is improved by a factor of √θ ² +χ ² /θ (Formula 5).

Also, when χ is positive and θ is negative, it becomes as shown in Figure 5, and the value of |θcosΔ+χsinΔ| is always larger than |θ| when πrad.>Δ>2tan ^-1 χ/θ−πrad. It is.

Similarly, when χ and θ are both angles, the value of |θcosΔ+sinΔ| is always larger than |θ| when 2tan ^-1 χ/θ+πrad.＞Δ＞πrad. Since we are considering the phase difference, it has the same meaning as 2tan ^-1 χ／θ＞Δ＞0rad.

Therefore, to summarize, |θcosΔ+χsinθ| is always larger than |θ| because when χ/θ>0, 2tan ^-1 χ/θ>Δ>0rad. When χ/θ<0, πrad.>Δ >2tan ^-1 χ/θ−πrad.

However, all solutions are within the range of 0 to πrad. Therefore, since it is a trigonometric function, the solution will be obtained even if you add or subtract an integer multiple of πrad. to each side of these inequalities. In that case, a retarder having a phase difference of Δ derived from the above formula plus or minus an integral multiple of π rad. will be used. This is also within the scope of the present invention.

Therefore, a basic embodiment of the present invention is a magneto-optical recording medium mainly composed of a magnetic thin film having a Kerr rotation angle or a Faraday rotation angle θ and a Kerr ellipticity or a Faraday ellipticity χ, in which χ/θ> When 0, 2tan ^-1 χ/θ>Δ>0゜When χ/θ<0, πrad.>Δ>2tan ^-1 χ/θ−πrad. is set to zero, and the main axis azimuth angle φ of the retarder is set to be substantially zero when used as a reference.

Here, in order to further aid understanding of the present invention, I would like to give a qualitative explanation by citing FIGS. 6 1 to 4.

As shown in FIG. 6, linearly polarized light incident on a magnetic thin film is incident on a minute spot region having upward magnetization with respect to the film surface, and when reflected there, for example, the plane of polarization of the reflected light is +θ due to Kerr effect
At the same time, a phase lag (Kerr ellipticity) occurs between the P component and the S component, and as a result, the reflected light becomes elliptically polarized light with a major axis tilt of +θ, and on the other hand,
The corresponding downward reflected light becomes elliptically polarized light whose major axis has an inclination of −θ. In FIG. 6, linearly polarized light in the absence of the Kerr effect is shown by a double-headed vector on the x-axis.

However, when the retarder of the present invention is provided,
As shown in FIG. 6 and 2, as the inclination of the long axis of the elliptically polarized light of the reflected light increases from θ to θ', the ellipse becomes flatter. The magnitude of θ′ is expressed as θ′=|θcosΔ+χsinΔ|.

Providing a retarder that provides the best embodiment of the present invention, that is, a retarder having a retarder Δ that satisfies Δ=tan ⁻¹ χ/θ,
As shown in FIG. 6, the reflected light becomes linearly polarized light, and its slope increases to θ''. The magnitude of θ'' is expressed as θ''=√ ² + ² .

FIG. 6 4 summarizes the relationship between FIGS. 1 and 3 in order to show the relationship between the two.

The reflected light from the medium is finally converted into an electric signal by a detector to provide a reproduced signal strength S.

Therefore, let us calculate what the reproduced signal strength S will be in the case of the best embodiment.

In the case of the direct method In this case, the reproduction light from the magnetic thin film is directed in the direction α
After passing through an analyzer (α=1/2π+α′・|α′|<<1), it is put into a detector.

If you omit the calculations in the middle, the answer will be as follows.

S∝−4√ ² + ²・α′ The dark condition in which one of the light beams, −θ″ or +θ″, does not pass through the analyzer at all is α′=√ ² + ² , but in the usual method, one of the lights passes through the analyzer to some extent. Since we have adopted the condition for α′ to be greater than √ ² + ² , α′=√ ² + ²・A (A>1).

Therefore, S∝-4(θ ² +χ ² )A.

In the case of the 45° differential method, S∝−4√ ² + ² .

Therefore, compared to the conventional method that does not use χ, this embodiment allows S to It will be twice as expensive.

Next, for comparison, let us consider an example in which the 1/4λ plate disclosed in Japanese Patent Application Laid-open No. 59-63041 filed by Philips Corporation, which is the earlier application of the present invention, is used instead of the retarder of the present invention. . In this case, although there is no specific disclosure in the earlier application, it is stated in lines 9 to 7 from the lower left column on page 4 of the publication of the earlier application that ``Thereby, the portion of the layer that is magnetized in the opposite direction in the initial direction is Only the light reflected by the photoelectric detector 12 is incident on the photoelectric detector 12.'' This leads to the following points.

The light reflected by the layer magnetized in the initial direction becomes linearly polarized light.

The analyzer must have an orthogonal orientation α to this linearly polarized light.

In order to convert the light reflected by the layer magnetized in the initial direction, that is, elliptically polarized light, into linearly polarized light, as shown in , the 1/4λ plate has to change its principal axis to the length of the ellipse of the reflected light (elliptically polarized light). Place it in line with the axis (or short axis).

From these matters, it can be understood as follows. Compared to the case where the magnetic thin film has no magnetization, the plane of polarization of the reflected light (polarized light) of the reflected light from the magnetic thin film is rotated by +θ or −θ and becomes elliptically polarized light.
Among these elliptically polarized lights, when the +θ light passes through the 1/4λ plate, the inclination is θ, as shown in Figure 7.1.
It becomes linearly polarized light of +χ, and -θ light becomes thick elliptically polarized light with an inclination of θ-χ and an ellipticity of 2θ, as shown in FIG.

Let us calculate the reproduced signal strength S in this case. In the prior invention, linearly polarized light of θ+χ is passed through an analyzer having an orthogonal orientation α. in this case,
α=1/2π+θ+χ. Note that, as mentioned above, in the conventional method, α is set to be larger than this value, so the invention of the prior application is different from the conventional method. As a result, when S is calculated, it is as follows.

S∝-4(θ ² +χ ² ) Comparing the S value of this prior invention with the S value of the direct method of the above-mentioned best embodiment of the present invention, the latter S value is A
It is understood that this is twice as high.

The present invention will be explained below with reference to Examples.

〔Example〕

A film with a thickness of approximately 1000 Å is placed on a transparent glass substrate B.
A Gd-Co magnetic thin film 1 is formed by sputtering. This magnetic thin film 1 has a Kerr rotation angle θ of approximately
At 0.0087 rad. (30 minutes), Kerr ellipticity χ is approximately 0.0058 rad.
(20 minutes) (when the wavelength of incident light λ = 633 nm).

Here, assuming Δ=tan ^-1 0.0058/0.0087 and calculating Δ, Δ is 0.59 rad. (33.69 degrees)
is required. Therefore, a retarder with a phase difference Δ=0.59 rad. is laminated on top of the magnetic material. Note that the retarder is made of a thin film of quartz, mica, or the like, and a retarder having an arbitrary phase difference is easily available.

In the recording medium of this example, the reproduced signal strength S is improved by about 20% compared to the recording medium without a phase shifter.

Note that when a plastic material is used as the substrate B of the recording medium, since some plastic materials have various retarders Δ, the retarder and the substrate may also be used, and this is also within the scope of the present invention. contained within.

In addition, when using a reproducing device with a beam splitter BS as shown in Figure 2 A and B,
This beam splitter may also have a phase difference δ, in which case the overall phase difference will be Δ+δ and the reproduced signal strength will decrease.
In order to cancel this δ, the phase difference of the retarder used in the present invention may be set to Δ−δ.

〔Effect of the invention〕

As described above, according to the invention, by providing a specific phase shifter, the reproduced signal strength can be increased to a maximum of √ ² + ² /θ
It can be improved by up to twice as much.

[Brief explanation of drawings]

FIG. 1 is a sectional view conceptually showing a cross section of a magneto-optical recording medium according to an embodiment of the present invention. FIGS. 2 and 3 are explanatory diagrams illustrating the basic configuration of a magneto-optical reproducing apparatus used for reproducing a magneto-optical recording medium. FIG. 4 and FIG. 5 are explanatory diagrams of the inner product of vectors. FIGS. 6 and 7 are conceptual diagrams explaining the polarization state of polarized light. Explanation of symbols of main parts, B...Substrate, 1...Magnetic thin film, 2...Retarder.

Claims (1)

[Claims] 1 Kerr rotation angle or Faraday rotation angle θ (θ≠0)
In a magneto-optical recording medium mainly composed of a magnetic thin film having Kerr ellipticity or Faraday ellipticity χ, a retarder having a phase difference Δ that satisfies the formula: |θcosΔ+χsinΔ|>|θ| 1. A magneto-optical recording medium, characterized in that the main axis azimuth angle φ of the phase shifter is set to zero, and the principal axis azimuth angle φ of the phase shifter is substantially zero.