GB2184618A

GB2184618A - Magneto-optical memory medium

Info

Publication number: GB2184618A
Application number: GB08629107A
Authority: GB
Inventors: Tadashi Kobayashi
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 1985-12-11
Filing date: 1986-12-05
Publication date: 1987-06-24
Also published as: GB8629107D0

Abstract

In a magneto-optical memory medium having a transparent substrate (1), at least one recording layer (3) comprising a magnetic film, and at least one dielectric layer (2), and capable of reading out information magnetically recorded on the recording layer by a read-out light beam (5) being applied thereto and by the use of the magneto-optical effect, the film thickness of the dielectric layer is smaller than the film thickness for which the read-out performance index is maximum when the wavelength of the read-out light beam is 800 nm. Embodiments are described in which read- out is effected by a photodetector having self- multiplication, such as an avalanche diode, (Figs. 1-8 not shown) and by a photodetector having no self-multiplication, such as a pin photodiode, (Figs. 9-11 not shown). <IMAGE>

Description

SPECIFICATION Magneto-optical Memory Medium Background of the Invention Field of the Invention This invention relates to a magneto-optical memory medium capable of reading out magnetically recorded information by a read-out light beam being applied thereto and by the use of the magneto-optical effect.

Related Background Art In recent years, studies of magneto-optical memory mediums capable of recording a large volume of information highly densely and moreover capable of erasing the information have been actively made.

Such a magneto-optical memory medium uses a magnetic film as a recording layer, and records information by partially reversing the direction of magnetization of the magnetic film as by applying a light beam condensed into the form of a spot. Also, the thus magnetically recorded information is read out by applying a linearly polarized light beam to the magnetic film and by the use of the magneto-optical effect.

That is, the linearly polarized light beam has its direction of polarization rotated in a direction conforming to said direction of magnetization by the magnetic film (magneto-optical Kerr rotation) and is reflected. This reflected light is then received by a photoelectric converting element through an analyzer, whereby the rotation in said direction of polarization is converted into a variation in intensity of light and is detected thereby. However, in such a magneto-optical memory medium, the angle of rotation of said magnetooptical Kerr rotation is small, and this has led to a problem that the read-out signal detected is weak.

So, for example, in Japanese Laid-Open Patent Application No. 25036/1985, etc., a method is proposed which uses, in addition to the magnetic film, a dielectric layer or a reflecting layer in the construction of the magneto-optical memory medium to improve the read-out characteristic. Therein, an optimum layer construction for a particular wavelength is examined.

On the other hand, a light source of a wavelength of the order of 800 nm is now chiefly used as the light source of the read-out light beam applied to the magneto-optical memory medium. Light sources of shorter wavelengths are desired from the viewpoint of recording density and are actively studied and developed.

Accordingly, in view of the future tendency of semiconductor lasers toward shorter wavelengths, it is desired that in a medium, the read-out characteristic be constant within a considerably wide wavelength range.

However, in the magneto-optical memory medium shown in the aforementioned Japanese laid-open patent application, only the read-out light of a particular wavelength is taken into consideration, and where the read-out light is made into a shorter wavelength, it has been impossible to obtain a sufficient read-out performance.

Summary of the Invention It is an object of the present invention to overcome the above-noted disadvantage peculiar to the prior art and to provide a magneto-optical memory medium in which a good read-out performance can be obtained even when the wavelength of the read-out light is varied over a wide range.

The above object of the present invention is achieved by making, in a magneto-optical memory medium having a transparent substrate, at least one recording layer comprising a magnetic film, and at least one dielectric layer, and capable of reading out information magnetically recorded on said recording layer by a read-out light beam being applied thereto and by the use of the magneto-optical effect, the film thickness of said dielectric layer smaller than the film thickness for which the read-out performance index is maximum when the wavelength of the read-out light beam is 800 nm.

By such a construction, in the magneto-optical memory medium of the present invention, it is difficult for the read-out performance index to be affected by any fluctuation of the wavelength of the reproducing light and the read-out performance index maintains a practically warrantable value in a wide wavelength range thereof (as one block of this wavelength range, mention may be made, for example, of 750-840 nm and 550-640 nm).

Brief Description of the Drawings Figure 1 is a graph showing the relation of the thickness of the dielectric layer VS. the read-out performance index F and the intensity reflectivity R when information is read out from a magneto-optical memory medium provided with a dielectric layer and a reflecting layer by the use of a photodetector having a self-multiplication characteristic.

Figure 2 is a graph showing the wavelength dependencies of R and F when in Figure 1, the film thickness of the dielectric layer is fixed at 750 A.

Figure 3 is a graph showing the wavelength dependencies of R and F when in Figure 1, the film thickness of the dielectric layer is fixed at 1800 A.

Figures 4 to 8 are schematic cross-sectional views showing examples of the construction of the magneto-optical memory medium of the present invention.

Figure 9 is a graph showing the relation of the thickness of the dielectric layer VS. the read-out performance index F and the intensity reflectivity R when information is read out from a magneto-optical memory medium provided with a dielectric layer and a reflecting layer by the use of a photodetector having no self-multiplication characteristic.

Figure 10 is a graph showing the wavelength dependencies of R and F when in Figure 9, the film thickness of the dielectric layer is fixed at 750 A.

Figure 11 is a graph showing the wavelength dependencies of R and F when in Figure 9, the film thickness of the dielectric layer is fixed at 1800 A.

Figure 12 is a schematic view showing an example of the construction of a magneto-optical information reproducing apparatus using the magneto-optical memory medium of the present invention.

Description of the Preferred Embodiments The invention will hereinafter be described in detail with reference to the drawings.

When reading out information from a magneto-optical memory medium, between a case where a photo-detector having a self-multiplication characteristic such as an avalanche photodiode (APD) is used and a case where a photodetector having no self-multiplication characteristic such as a PIN photodiode is used, the cause of dominant noise differs and correspondingly, the manner in which the read-out performance index F is calculated differs. That is, when the intensity reflectivity of the medium is R and the magneto-optical Kerr rotation angle is 0k, in the former case, F= < - 6k and in the latter case, F=R .6k.

Accordingly, the desirable construction of the magneto-optical memory medium must be considered discretely in the respective cases. So, the magneto-optical memory medium in the case where a photodetector having a self-multiplication characteristic will first be described.

Figure 1 is a graph in which variations in the read-out performance index F and the intensity reflectivity R in a case where use is made of a magneto-optical memory medium having a layer construction of glass/Tb-Fe(1 00 )/SiO2/AI are plotted relative to the film thickness of a layer formed of SiO2 which is an example of a dielectric layer. Here, the read-out performance index F is the square root of the intensity reflectivity R multiplied by the magneto-optical Kerr rotation angle Ok, and is an amount representative of the quality of the read-out characteristic.

Figure 1 shows that when the film thickness of SiO2 is about 750 A and about 1800 A, the optimum interference action of the reproducing light is caused and the maximum value of F is provided. The wavelength dependencies of F and R when the film thickness of SiO2 is fixed at 750 A are shown in Figure 2.

As the wavelength becomes shorter, both F and R become considerably smaller. Particularly R is almost null for the short wavelength. Also, the wavelength dependencies of F and R when the film thickness of SiO2 is fixed at 1800 A are shown in Figure 3, and it is seen that at such time, F is 0 for the light of a wavelength in the vicinity of 600 nm and the recorded information cannot be read out at all. Considering the future tendency of semiconductor lasers toward shorter wavelengths, it is desired that in a medium, F be constant in a considerably wide wavelength range, but this cannot be realized by such a construction.

With reference to Table 1 below, the present invention will now be described with respect to representative constructions of the magneto-optical memory medium, i.e., items (1) to (4) below. In Table 1, the upper stage in each column indicates a comparative example when the film thickness of each layer is set so as to substantially coincide with the optimum value of F, and the lower stage in each column indicates the layer construction of the present invention in which the film thickness of the dielectric layer is made small so that the wavelength dependencies become better.

TABLE 1

F(800 nm) F(600 nm)/ Construction (min) F(800 nm) Variation in R Glass substrate/GdTbFeCo [Comp. Ex. ] 17.0 0.79 0.421-0.464 (1-1) Glass substrate/ZnS(700 A)/GdTbFeCo 26.5 0.73 0.190-0.275 [Comp. Ex. ] Glass substrate/ZnS(600 A)/GdTbFeCo 25.7 0.82 0.176-0.206 [Invention] (1-2) Glass substrate/SiC(500 A)/GdTbFeCo 30.3 0.67 0.077-0.217 [Comp. Ex.] Glass substrate/SiC(400 A)/GdTbFeCo 26.8 0.90 0.070-0.112 [Invention] (2-1) Glass substrate/GdTbFeCo(400 )/SiO 21.2 0.41 0.374-0.432 (2500 )/AI [ Comp. Ex.] Glass substrate/GdTbFeCo(400 )SiO 19.3 0.60 0.373#0.484 (2300 A)/AI [Invention] (2-2) Glass substrate/GdTbFeCo (170 )/SiO 31.3 0.64 0.13#0.195 (650 A)/Al [Comp. Ex.] Glass substrate/GdTbFeCo (170 )/SiO 30.4 0.77 0.184~0.241 (500 A)/Al [Invention] (3-1) Glass substrate/SiC(500 A)/GdTbFeCo 35.7 0.66 0.05#0.140 (400 A)/Si0(2500 A)/Al [ Comp. Ex.] Glass substrate/SiC(400 A)/GdTbFeCo 33.0 0.82 0.041-0.125 (400 A)/SiO(2300 A)/Al [Invention] (3-2) Glasssubstrate/SiO(1050 )/GdTbFeCo 35.1 0.73 0.107-0.125 (170AwSi0(650A)/Al [Comp. Ex.] Glass substrate/Si0(800 A)1GdTbFeCo 33.6 0.81 0.1250.164 (170 A)/Si0(500 )/Al [Invention] (3-3) Glass substrate/MgF2(1450 A)/SiO 37.1 0.71 0.084-0.136 (1050 A)/GdTbFeCo(1 70 A)/SiO(650 A)/ Al [Comp. Ex.] Glass substrate/MgF2( 1100 )/SiO 34.7 0.84 0.08#0.126 (800 A)/GdTbFeCo(170 )/SiO(500 A)/ Al [Invention] (4-1) Glass substrate/GdTbFeCo(170 )/SiO 25.0 0.54 0.16#0.222 (550 A)/GdTbFeCo [ Comp. Ex.] Glass substrate/GdTbFeCo (170 )/SiO 22.3 0.76 0.27#0.327 (250 A)/GdTbFeCo [Invention] (1) As shown in Figure 4, a dielectric layer 2 and a magnetic layer 3 were successively provided on a glass substrate 1 to form a magneto-optical memory medium. In all of the following examples, a reading light beam 5 enters from the glass substrate 1 side, and the reflected light by the medium is detected, whereby information is read out.

(1-1) In the glass substrate/ZnS/GdTbFeCo layer construction, if the film thickness of ZnS is made smaller from 700 A to 600 A, F at 800 nm is hardly varied (a reduction of about 3%), but F at 600 nm is improved to a reduction of 0.73 times to 0.82 times for F at 800 nm.

(1-2) In the glass substrate/SiC/GdTbFeCo layer construction, if the film thickness of SiC is made smaller from 500 A to 400 , F (600)/F (800) is further improved to a reduction of 0.90 times. However, the reflectivity is considerably reduced.

(2) As shown in Figure 5, a magnetic layer 3, a dielectric layer 2 and a reflecting layer 4 were successively provided on a glass substrate 1 to form a magneto-optical memory medium.

(2-1) In the glass substrate/GdTbFeCo (400 )/SiO/AI layer construction, the film thickness of the GdTbFeCo layer is great and therefore, it is not so effective to vary the film thickness of the dielectric layer, but if combined with (1-2), there is an effect as shown in item (3-1) below.

(2-2) In the glass substrate/GdTbFeCo (170 A)/SiO/AI construction, the film thickness of the magnetic layer is small and therefore, if the film thickness of the dielectric layer is made small, F at 800 nm is not much varied, but the wavelength dependency of F is improved.

(3) As shown in Figure 6, a first dielectric layer 21, a magnetic layer 3, a second dielectric layer 22 and a reflecting layer 4 were successively provided on a glass substrate 1 to form a magneto-optical memory medium. A third dielectric layer 23 was further provided between the glass substrate 1 and the first dielectric layer 21. The magneto-optical memory medium shown in Figure 7 was also made. The third dielectric layer 23 and the first dielectric layer 21 were formed of materials different in refractive index.

(3-1) The glass substrate/SiC/GdTbFeCo (400 )/SiO/AI layer construction is a combinations of the constructions of (1-2) and (2-1), and in this construction, the wavelength dependency of F is improved.

Again in this construction, the reflectivity is low, but this is because the refractive index of SiC is great, and if a dielectric material smaller in refractive index than SiC is employed, the reflectivity will become higher.

(3-2) The glass substrate/SiO/GdTbFeCo (170 A)/SiO/Al construction comprises the layer construction of (2-2) and a dielectric layer interposed between the glass substrate and the magnetic layer, and since in the layer construction of (2-2), the reflectivity is already considerably reduced, use is made of SiO which is small in refractive index.

(3-3) The glass substrate/MgF2/SiO/GdTbFeCo (170 )/SiO/AI construction comprises the layer construction of (3-2) and a third dielectric layer interposed between the glass substrate and the dielectric layer, and in this construction, the film thickness which provides h14 (A=wavelength of light) when MgF2 is 800 nm is 1450 A, and the film thickness which provides A/4 when MgF2 is 600 nm is 1100 A.

(4) As shown in Figure 8, a first magnetic layer 31, a dielectric layer 2 and a second magnetic layer 32 were successively provided on a glass substrate 1 to form a magneto-optical memory medium.

(4-1) The glass substrate/GdTbFeCo (170 A)/SiO/GdTbFeCo layer construction has a dielectric layer interposed between two magnetic layers, and the second magnetic layer also has the function as a reflecting layer.

In both of the cases (3) and (4), the wavelength dependency of F is improved.

In the above-described embodiments it is desirable that the film thickness of the dielectric layer be made small so that the value of F when the wavelength of the read-out light is 600 nm is 75% or more of the value of F when the wavelength of the read-out light is 800 nm. However, too small a thickness would reduce the absolute value of F itself. With such a point taken into account, the optimum value of the film thickness of the dielectric layer will hereinafter be considered.

Table 2 below shows the values of F at wavelength 800 nm in the magneto-optical memory medium of the aforementioned type (1 ) wherein the refractive index and film thickness of the dielectric layer were changed variously. In the leftmost column of Table 2, there is shown the refractive index n of the dielectric layer. In the uppermost stage of Table 2, there is shown the film thickness h of the dielectric layer, and 100% means a film thickness for which F is maximum at wavelength 800 nm, and 95% means a film thickness which is 95% of the thickness for which F is maximum at wavelength 800 nm. 90%, 85% and 80% are also similar in significance. Table 3 below corresponds to Table 2 and shows the ratio of F of each medium at 600 nm to F of each medium at 800 nm. The numerical values in the leftmost column and the uppermost stage of Table 3 are similar in significance to those in Table 2.

The greater is the value of F in Table 2, the greater is the interference effect of the dielectric layer and the better is the read-out characteristic, and the greater is the value of F (600 nm)/F (800 nm) in Table 3, the better is the wavelength dependency of the read-out characteristic of the medium. That is, the fluctuation of the value of the read-out characteristic is small even if the wavelength is changed.

In Tables 2 and 3, values are shown with respect only to the constructions in which F (600 nm)/F (800 nm) 50.75.

TABLE 2

h=100% 95% 90% 85% 80% n=1.75 * 20.7 min 2 23.8 23.8 2.25 26.5 26.4 2.5 28.6 28.4 27.9 2.75 30.0 29.4 28.5 3 *3 31.2 30.8 29.6 *5 28.1 F(800 nm)=V#0k TABLE 3

h=100% 95% 90% 85% 80% n=1.75 q 0.76 2 0.74 0.76 2.25 0.73 0.76 2.5 0.70 0.74 0.78 2.75 0.69 0.74 0.79 3 *4 0.63 0.67 0.72 *6 0.80 F(600 nm)/F(800 nm) The following is seen from Tables 2 and 3. As is apparent from the comparison of the values in the column of h=100%, if the value of n is small, the interference effect of the dielectric layer is also small and the effect of having provided the dielectric layer is not so great (see *1 in Table 2). Accordingly, when the value of n is small, the wavelength dependency of F is good (see *2 in Table 3) even if the film thickness h of the dielectric layer is 100%, but it is not suitable for practical use.

On the other hand, when the value of n is great, the value of F itself is great (*3) if h is 100%, but the wavelength dependency of F is bad (*4). If, at n=3, h is 85%, the value of F itself is not much reduced (*5) whereas the wavelength dependency of F is considerably improved (*6). A similar effect is also found when his 95% or 90%.

Also, when n assumes the mean value between 1.75 and 3, h is 100% and the value of F is considerably high and the interference effect is found, and if h is 8595%, the value of F is not much reduced, whereas the wavelength dependency of F is improved. Accordingly, by using a dielectric material of great refractive index (preferably 2 or higher, and more preferably 2.25 or higher) and making the film thickness thereof into a thickness of 8595% of the optimum film thickness at 800 nm, the value of F itself can be maintained practically warrantable and moreover, the wavelength dependency of F can be improved.

A magneto-optical memory medium of the aforedescribed type (2) having the construction of glass substrate/magnetic layer (GdTbFeCo, 170 A)/dielectric layer (n=1.23~3)/reflecting layer layer (Al) was made, and the read-out performance index was found in the same manner as the above-described examples.

As can be seen from Figure 1, in the construction of this type, considering in the range O-A/2n (A=wavelength of light) of h, there are two h's for which F exhibits a maximum value. In the film thickness which assumes the second maximum value, F is O in the vicinity of wavelength 600 nm as shown in Figure 3 and therefore, the wavelength dependency of F is especially bad. So, film thicknesses in the vicinity of the first maximum value will be considered in the following.

TABLE 4

h=100% 95% 90% 85% 80% n=1.25 32.0 min 31.9 31.8 31.6 31.4 1.5 31.6 31.6 31.4 31.2 30.9 1.75 31.2 31.1 30.9 30.6 2 30.8 30.7 30.6 30.3 29.9 2.25 30.4 30.3 30.2 29.9 29.4 2.5 29.9 29.9 29.7 29.4 28.9 2.75 29A 29.3 29.1 28.7 28.2 3 28.9 28.7 28.4 27.9 F(800 nm)=vw0k TABLE 5

h=100% h=100% | 95% | 90% 1 t| 85% 80% n=1.25 0.69 0.71 0.72 0.74 0.75 1.5 0.67 0.70 0.72 0.74 0.76 1.75 0.67 0.70 0.73 0.75 2 0.63 0.67 0.71 0.74 0.77 2.25 0.58 0.64 0.69 0.73 0.76 2.5 0.54 0.62 0.67 0.72 0.76 2.75 0.54 0.61 0.68 0.73 0.76 3 0.57 0.65 0.71 0.76 F(600 nm)/F(800 nm) Table 4 shows the value of F at 800 nm of a magneto-optical memory medium of type (2) in which the refractive index and film thickness of the dielectric material were changed variously, and this table is similar to Table 2. Table 5 shows the ratio of F of said plural mediums at 600 nm to F of said plural mediums at 800 nm, and is similar to Table 3.

In both of Tables 4 and 5, values are shown with respect only to the constructions in which F (600 nm)/F (800 nm)#0.75.

It is seen from Tables 4 and 5 that in the construction type (2), unlike the type (1), a smaller refractive index n of the dielectric layer for h = 100% results in a better value of F itself and better wavelength dependency of F. It is also seen that the use of a dielectric material of small n, e.g. n= 1.25 or 1.5, for the dielectric layer also results in even better wavelength dependency of F if h is 9580%, and that even in a case where use must be made of a dielectric material of great n from the viewpoints of corrosion resistance and making of the film, the adoption of h of 9580% can improve the wavelength dependency of F without much reducing the value of F.

Tables 4 and 5 refer to a case where Al is used for the reflecting layer, but the same thing can be said even when other metals are used.

A similar experiment was carried out with respect to the magneto-optical memory of the aforedescribed type (3) having the construction of glass substrate/dielectric layer (A)/magnetic layer/ dielectric layer (B)/reflecting layer.

This construction is a combination of the construction of the type (1) and the construction of the type (2), and again it was ascertained that by making the film thickness of the dielectric layer into 9580% of the optimum film thickness at 800 nm, the wavelength dependency of F could be improved without the value of F itself being much reduced. For this purpose, the film thickness of one of the dielectric layers (A) and (B) may be made somewhat smaller, but making the film thicknesses of both layers is more effective.

In any of the magneto-optical memory mediums of the types (1), (2) and (3), the wavelength dependency of F is improved by making the film thickness of the dielectric layer small, and this results from the wavelength for which the interference effect occurs intensely being made shorter from the vicinity of 800 nm by making the film thickness of the dielectric layer, and thereby reducing the reduction in F.

Some specific embodiments will be shown below.

Embodiment 1 In a magneto-optical memory medium of the glass substrate/SiC/GeTbFeCo construction, the optimum film thickness of SiC at wavelength 800 nm was about 500 A, and the then F was as great as 30.3 min, but F(600 nm)/F (800 nm) was as small as 0.67. When the film thickness of SiC was made into 90% of 500 , i.e., 450 A, F (800 nm) was not much varied, that is, was 29.2 min, whereas F (600 nm)/F (800 nm) could be improved to 0.76.

Embodiment 2 In a magneto-optical memory medium of the glass substrate/GdTbFeCo (170 A)/SiO/Al construction, the optimum film thickness of SiO at wavelength 800 nm was about 650 A, and then F was as great as 31.3 min, but F (600 nm)/F (800 nm) was as small as 0.64. When the film thickness of SiO was made into 80% of 650 A, i.e., 520 A, F (800 nm) was not much varied, that is, was 30.6 min, but F (600 nm)/F (800 nm) could be improved to 0.75.

Embodiment 3 In a magneto-optical memory medium of the glass substrate/SiO(A)/GdTbFeCo (170 A)/SiO(B)/AI construction, the optimum film thickness of SiO (A) at wavelength 800 nm was about 1050 A and the optimum film thickness of SiO(B) at wavelength 800 nm was about 650 A, and the then F was as great as 31.1 min, but F (600 nm)/F (800 nm) was 0.73. When the film thickness of SiO(A) was made into 80% of 1050 A, i.e. 840 A and the film thickness of SiO(B) was made into 80% of 650 A, i.e., 520 A, F (800 nm) was not much varied, that is, was 33.9 min, but F (600 nm)/F (800 nm) could be improved to 0.80.

The magneto-optical memory medium in a case where a photodetector having no self-multiplication characteristic will now be described.

Figure 9 is a graph in which variations in F and R in a magneto-optical memory medium having the glass substratetmagnetic layer: Tb-Fe (100 A)/SiO2/Al layer construction are plotted relative to the film thickness of SiO2 (wavelength of light=800 nm). Here, F is the read-out performance index in a case where a photodetector having no self-multiplication characteristic such as a PIN photodiode is used, and it is the intensity reflectivity R multiplied by the magneto-optical Kerr rotation angle 0k and is an amount representative of the quality of the read-out characteristic.

Figure 9 shows that when the film thickness of the dielectric layer (SiO2) is about 450 A and 2000 A, the optimum interference action of the reproducing light is caused and the maximum value of F is provided.

The wavelength dependencies of F and R when the film thickness of SiO2 is fixed at 750 A are shown in Figure 10. As the wavelength becomes shorter, both F and R become considerably smaller. Also, the wavelength dependencies of F and R when the film thickness of SiO2 is fixed at 2000 A are shown in Figure 11. It is seen that at this time, F becomes 0 for a light of a wavelength in the vicinity of 650 nm and recorded information cannot be read out at all. Considering the future tendency of semiconductor lasers toward shorter wavelengths, it is desired that in a medium, F be constant in a considerably wide wavelength range, but it cannot be realized by a magneto-optical memory medium of such layer construction.

The magneto-optical memory medium of the present invention will now be described by way of example with reference to Table 6 below. in Table 6, the upper stage of each column indicates the result of the measurement of the conventional magneto-optical memory medium in which the film thickness of each layer is set so as to substantially coincide with the optimum value of F at 800 nm, and the lower stage indicates the result of the measurement of a magneto-optical memory medium having the layer construction of the present invention in which the film thickness of the dielectric layer is made small so that the wavelength dependency becomes better.

TABLE 6

F (800 nm) F (600 nm)/ Construction (min) F (800 nm) Variation in R (2-3) Glass substrate/GdTbFeCo (170A)/SiO 15.2 0.74 0.227#O.282 (400 A)/Al [ Comp. Ex.] Glass substrate/GdTbFeCo (170 A)/SiO 14.8 0.82 0.281-0.334 (300 A)/AI [ Invention ] (3-4) Glass substrate/SiO (2200 A)/GdTbFeCo 15.2 0.68 0.165--0.291 (170 A)/SiO(400 A)/AI [Comp. Ex.] Glass substrate/SiO (2000 )/GdTbFeCo 14.6 0.81 0.245--0.346 (170 A)/SiO (300 A)/Al [Invention] In the glass su bstrate/GdTbFeCo (170 A)/SiO/Al layer construction of the aforedescribed type (2), if the film thickness of the dielectric layer (SiO) is made smaller from 400 A to 300 , F at 800 nm is not much varied, i.e., from 15.2 min to 14.8 min, but F at 600 nm is 0.74 times to 0.82 times as great as that at 800 nm and the wavelength dependency of F is improved.

In the glass substrate/SiO(A)/GdTbFeCo (170 A)/SiO(B)/AI layer construction of the aforedescribed type (3), if the film thickness of the SiO(A) layer is made smaller from 2200 A to 2000 A and the film thickness of the SiO(B) layer is made smaller from 400 A to 300 A, F (600 nm)/F (800 nm) is improved from 0.68 to 0.81 and moreover, the value of F (800 nm) is hardly reduced.

In a magneto-optical memory medium of such construction, depending on the manner in which the materials of the dielectric and magnetic layers are chosen, F (600 nm)/F (800 nm) can be made into 0.80 or more simply by reducing the film thickness of only one of the two dielectric layers. However, if the film thickness of one of the two dielectric layers is made too small, the absolute value of F itself will also be reduced. With such a point taken into account, the optimum value of the film thickness of the dielectric layer will hereinafter be considered.

Table 7 below shows the values of the read-out performance index F when the refractive index and film thickness of the dielectric layer are changed variously in a magneto-optical memory medium of the aforedescribed type (2) having the glass substrate/magnetic layer (GdTbFeCo, 170 A)/dielectric layer (n=1.25--3)/reflecting layer (Al) construction.

As seen from Figure 9, in the magneto-optical memory medium of the layer construction of type (2), considering the film thickness h of the dielectric layer in the range of O-A/2n (A: wavelength of light, n: refractive index of the dielectric layer), there are two h's for which F exhibits the maximum value. In the film thickness which assumes the second maximum value, F becomes 0 in the vicinity of wavelength 650 nm, as shown in Figure 11, and therefore, the wavelength dependency is especially bad. So, a film thickness in the vicinity of the first maximum value will be considered in the following.

TABLE 7

n=100% 95% 90% 85% 80% 75% n=1.25 15.3 min 15.3 15.3 15.3 15.2 15.1 1.5 15.2 15.2 15.2 15.2 15.1 15.0 1.75 15.1 15.1 15.1 15.0 14.9 2 15.0 15.0 14.9 14.8 2.25 14.9 14.9 14.9 14.8 14.7 2.5 14.8 14.7 14.7 14.6 2.75 14.7 14.7 14.6 14.6 14.5 3 14.5 14.5 14.4 F(800 nm)=R.#k TABLE 8

h=100% h=100% 95% | 90% | 85% | 80% | 75% n=1.25 0.74 0.75 0.76 0.77 0.78 0.80 1.5 0.74 0.75 0.76 0.78 0.79 0.80 1.75 0.74 0.75 0.77 0.79 0.80 2 0.75 I 0.77 0.78 I 0.80 2.25 0.71 0.73 0.75 0.77 0.79 2.5 0.74 0.76 0.78 0.80 2.75 0.68 0.71 0.74 0.77 0.79 3 0.74 0.76 0.79 F (600 nm)/F (800 nm) In the leftmost column of Table 7, there is shown the refractive index n of the dielectric layer.In the uppermost stage of Table 7, there is shown the film thickness h of the dielectric layer, and 100% means the film thickness for which F is maximum at wavelength 800 nm, and 95% means the film thickness of 95% of the film thickness for which F is maximum at wavelength 800 nm. 90%, 85% and 80% are also similar in significance. Table 8 corresponds to Table 7, and shows the ratio of F of each medium of Table 7 at 600 nm.

The numerical values in the leftmost column and the uppermost stage of Table 8 are similar in significance to those of Table 7.

The greater is the value of F in Table 7, the greater is the interference effect in the dielectric layer, and the greater is the value of F (600 nm)/F (800 nm) in Table 8, the better is the wavelength dependency of the medium, that is, the smaller is the fluctuation of the value of the read-out characteristic even if the wavelength is changed. In Table 7, there are shown only the values of F (800 nm) in which the reduction is within 0.2 min relative to the value of F (800 nm) at h=100%, and in Table 8, there are shown those which correspond thereto. When h < 75%, F (800 nm) is too much reduced (1 min or more), and this is not practical.

The following is seen from Tables 7 and 8. In the medium of type (2), for n = 100%, a smaller value of the refractive index n of the dielectric layer results in a greater value of F. The film thickness of the dielectric layer which can reduce the reduction in F (800 nm) (within 1 min) and improve the value of F (600 nm)/F (800 nm) is in the range of h=95% to 75% whether n is small or great, and when n is great, h#80%.

Tables 7 and 8 refer to a case where Al is used for the reflecting layer, but the same thing can be said also when other metals are used.

The medium of type (3) having the glass su bstrateldielectric layer (A)/magnetic layer/dielectric layer (B)/reflecting layer construction will now be considered.

The medium of type (3) comprises the medium of type (2) and a dielectric layer interposed between the glass substrate and the magnetic layer, and by the film thickness of the dielectric layer being made into 95~75% of the film thickness for which F (800 nm) is maximum, the wavelength dependency of F can be improved (F (600 nm)/F (800 nm) can be improved to the order of 0.8) without the value of F being not much reduced (within 1 min). For this purpose, the film thickness of one of the dielectric layers (A) and (B) may be made small, but making the film thicknesses of both dielectric layers small would be more effective.

In any of types (2) and (3), the wavelength dependency of F is improved by making the film thickness of the dielectric layer smaller than the film thickness for which the read-out performance index F at the wavelength 800 nm of light is maximum, and this results from making the wavelength for which the interference effect occurs intensely shorter than 800 nm by making the film thickness of the dielectric layer small, and thereby reducing the reduction in F.

Some specific embodiments will be shown below.

Embodiment 4.

In the glass substrate/GdTbFeCo (170 A)/SiO/Al construction, the optimum film thickness of SiO at wavelength 800 nm was about 400 A and the then F was as great as 15.2 min, but F (600 nm)/F (800 nm) was 0.74. When the film thickness of SiO was made into 75% of 400 , i.e., 300 , F (800 nm) was not much varied, that is, was 14.8 min, but F (600 nm)/F (800 nm) could be improved to 0.82.

Embodiment 5 In the glass substrate/SiO(A)/GdTbFeCo (170 A)/SiO(B)/Al construction, the optimum film thickness of SiO(A) at wavelength 800 nm was about 2200 , the optimum film thickness of SiO(B) at the same wavelength was about 400 , and the then F was as great as 15.2 min, but F (600 nm)/F (800 nm) was as small as 0.68. When the film thickness of SiO(A) was made into 90% of 2200 A, i.e., 2000 A, and the film thickness of SiO(B) was made into 75% of 400 A, i.e., 300 A, F (800 nm) was not much varied, that is, was 14.6 min, but F (600 nm)/F (800 nm) could be improved to 0.81.

As is apparent from the foregoing description, even when a photodetector having no self-multiplication characteristic is used, the read-out characteristic becomes good, that is, suffers from little fluctuation over a wide range of the wavelength of reproducing light, by using the magneto-optical memory medium of the present invention in which the film thickness of the dielectric layer is 9575% of the film thickness for which the F when the wavelength of light is 800 nm is maximum.

Finally, the construction of a magneto-optical information reproducing apparatus using the magnetooptical memory medium of the present invention is schematically shown in Figure 12. In Figure 12, a light beam 7 emitted from a semiconductor laser 6 and polarized in a predetermined direction is collimated by a collimator lens 8 and enters a polarizing beam splitter 9. The light beam passed through this polarizing beam splitter 9 is then condensed on the magneto-optical memory medium 11 of the present invention by an objective 10. The medium 11 comprises a dielectric layer and a magnetic layer provided on a disc-like transparent substrate, and is placed on a turn table 12 and rotated thereon as indicated by an arrow.The reflected light beam 13 of said light beam 7 having its plane of polarization rotated in accordance with the information magnetically recorded on the medium 11 again passes through the objective 10, is reflected by the polarizing beam splitter 9 and is directed to an analyzer 14. The transmission axis azimuth of the analyzer 14 is inclined (e.g. at 45") with respect to said predetermined direction, and the aforementioned reflected light beam 13 transmitted through the analyzer 14 becomes intensity-modulated in conformity with the recorded information on the medium 11. This modulated light is condensed by a sensor lens 15 and photoelectrically converted by a photodetector 16, and is taken out as a reproduction signal S. The photodetector 16 may be one having a self-multiplication characteristic such as APD, or one having no self-multiplication characteristic such as a PIN photodiode, and depending on which of them is used, the read-out performance index when the reproduction signal S is obtained differs as previously described.

The present invention is not restricted to the above-described embodiments, but other various applications thereof are possible. For example, the shape of the medium is not limited to a disc-like shape, but may also be a card-like shape or a tape-like shape. The present invention covers all these applications.

Claims

1. A magneto-optical memory medium having a transparent substrate, at least one recording layer comprising a magnetic film, and at least one dielectric layer, and capable of reading out information magnetically recorded on said recording layer by a read-out light beam being applied thereto and by the use of the magneto-optical effect, wherein the film thickness of said dielectric layer is smaller than the film thickness for which the read-out performance index F is maximum when the wavelength of said read-out light beam is 800 nm.

2. A magneto-optical memory medium according to Claim 1, wherein said read-out performance index F is expressed by the following equation: F=# 6k, where R is the intensity reflectivity of the read-out light by said medium, and 6k is the magneto-optical Kerr rotation angle of said medium.

3. A magneto-optical memory medium according to Claim 2, wherein the value of the read-out performance index F at the wavelength 600 nm of said read-out light beam is equal to or greater than 75% of the value of F at wavelength 800 nm.

4. A magneto-optical memory medium according to Claim 3, wherein said medium comprises the dielectric layer and the recording layer disposed in succession from said transparent substrate side.

5. A magneto-optical memory medium according to Claim 3, wherein said medium further has a reflecting layer, and comprises the recording layer, the dielectric layer and the reflecting layer disposed in succession from said transparent substrate side.

6. A magneto-optical memory medium according to Claim 5, wherein said medium further has one or more dielectric layer between said transparent substrate and said recording layer.

7. A magneto-optical memory medium according to Claim 3, wherein said medium has two recording layers and comprises the first recording layer, the dielectric layer and the second recording layer disposed in succession from said transparent substrate side.

8. A magneto-optical memory medium according to Claim 2, wherein said medium comprises the dielectric layer and the recording layer disposed in succession from said transparent substrate side, and the film thickness of said dielectric layer is 85%~95% of the film thickness for which F is maximum at wavelength 800 nm.

9. A magneto-optical memory medium according to Claim 2, wherein said medium further has a reflecting layer and comprises the recording layer, the dielectric layer and the reflecting layer disposed in succession from said transparent substrate side, and the film thickness of said dielectric layer is 80%~95% of the film thickness for which F is maximum at wavelength 800 nm.

10. A magneto-optical memory medium according to Claim 9, wherein said medium further has one or more dielectric layers between said transparent substrate and said recording layer.

11. A magneto-optical memory medium according to Claim 1, wherein said read-out performance index F is expressed by the following equation: F=R ~ Ok, where R is the intensity reflectivity of the read-out light by said medium, and 6k is the magneto-optical Kerr rotation angle of said medium.

12. A magneto-optical memory medium according to Claim 11,wherein the value of the read-out performance index F at the wavelength 600 nm of said read-out light beam is equal to or greater than 80% of the value of F at wavelength 800 nm.

13. A magneto-optical memory medium according to Claim 12, wherein said medium comprises the dielectric layer and the recording layer disposed in succession from said transparent substrate side.

14. A magneto-optical memory medium according to Claim 12, wherein said medium further has a reflecting layer, and comprises the recording layer, the dielectric layer and the reflecting layer disposed in succession from said transparent substrate side.

15. A magneto-optical memory medium according to Claim 14, wherein said medium further has one or more dielectric layers between said transparent substrate and said recording layer.

16. A magneto-optical memory medium according to Claim 11, wherein the film thickness of said dielectric layer is 75%~95% of the film thickness for which F is maximum at wavelength 800 nm.

17. A magneto-optical memory medium according to Claim 16, wherein said medium further has a reflecting layer, and comprises the recording layer, the dielectric layer and the reflecting layer disposed in succession from said transparent substrate side.

18. A magneto-optical memory medium according to Claim 17, wherein said medium further has one or more dielectric layers between said transparent substrate and said recording layer.

19. A magneto-optical memory medium having a magnetic recording layer and a dielectric layer having a thickness smaller than that giving maximum read-out performance for a reading optical beam having a wavelength of 800 nm.

20. A magneto-optical memory medium substantially as described in the description with reference to the drawings.

21. A magneto-optical memory medium substantially as described in any of the examples in the description.