Magneto-optical recording element

In a magneto-optical recording element comprising a substrate, a magnetic layer and a dielectric layer, the dielectric layer is formed by deposition of a composition comprising Si.sub.3 N.sub.4 and a refractive index-improving agent such as Al.sub.2 O.sub.3 or Y.sub.2 O.sub.3. This dielectric layer has a high refractive index and the enhancement effect is improved. Moreover, this dielectric layer is excellent in the adhesion and resistance characteristics.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a magneto-optical recording element, 
especially an erasable magneto-optical recording element, in which the 
figure of merit is improved and the adhesion to a substrate and various 
resistance characteristics are improved in a magneto-optical recording 
medium. Furthermore, the present invention relates to a method for the 
fabrication of this magneto-optical recording medium. 
2. Description of the Prior Art 
Recently, research has been vigorously made on high-density recording using 
a magneto-optical recording medium. According to this recording method, 
laser beams are projected on a recording medium to locally heat the 
recording medium and write bits into the recording medium, and the 
recorded information is read out by utilizing the magneto-optical effect. 
According to this method, large quantities of informations are recorded at 
a high density. This magneto-optical recording medium is obtained by 
forming an amorphous metal vertical magnetization film composed of a rare 
earth element and a transition metal mainly by sputtering. 
In this magneto-optical recording system, improvement of the recording 
medium per se and formation of a dielectric layer between the substrate 
and recording medium have been proposed as means for improving the 
magneto-optical characteristics. 
More specifically, in a photomagnetic recording element comprising a 
transparent substrate, a transparent dielectric layer and a magnetic layer 
of a magneto-optical recording medium formed on the transparent substrate 
through the transparent dielectric layer, if the thickness t of the 
dielectric layer is set so that multiple reflection is caused when laser 
beams are projected from the substrate side for reproduction, that is, the 
thickness t of the dielectric layer is set so that the following condition 
is satisfied: 
EQU t=.lambda./4n.multidot.(2m+1) 
wherein 
.lambda. stands for the reproduction wavelength of laser beams, 
n stands for the refractive index of the dielectric layer, and 
m is a number of 0, 1, 2, 3, . . . , 
an enhancement of a Polar Kerr effect can be obtained and figures of merit 
can be prominently improved. 
As the dielectric material, there can be mentioned oxides such as 
CeO.sub.2, ZrO.sub.2, TiO.sub.2, Bi.sub.2 O.sub.3 and SiO and non-oxides 
such as Si.sub.3 N.sub.4, AlN, CdS, SiC and ZnS. Non-oxides are 
advantageous in that oxygen originating from the dielectric material is 
not present in the interface of the amorphous metal vertical magnetization 
film and deterioration of the magnetic layer owing to diffusion of oxygen 
is obviated, and if a non-oxide dielectric material excellent in the 
impermeability to water and oxygen in air is selected and used, a 
dielectric layer which is stable for a long time and has a high 
reliability can be obtained. 
A plastic material has been used for a substrate of an optical disc for 
high-density recording because the plastic material is light in the 
weight, cheap in the cost and excellent in the durability and safety and a 
large quantity of substrates provided with guide tracks can be reproduced 
by injection molding. Thus, substrates for magneto-optical recording have 
been prepared by using polymeric material having an excellent light 
transmittance, such as a polycarbonate resin and a polymethyl methacrylate 
resin. 
Under this background, it is considered that in a magneto-optical recording 
element comprising a magnetic layer formed on a substrate of this plastic 
material through a non-oxide type dielectric layer, if reproduction is 
performed by projecting laser beams from the substrate side, the higher 
the refractive index of the dielectric layer than that of the substrate, 
the larger is the enhancement effect. 
As the non-oxide type dielectric material having a high refractive index, 
there can be mentioned ZnS (n=2.35), CdS (n=2.6) and SiC (n&gt;3). However, 
these dielectric materials are relatively poor in resistance 
characteristics and if a layer of such a dielectric material is placed in 
a high-temperature high-humidity environment for a long time, oxygen and 
water in air are supplied into the magnetic layer through many pinholes 
formed at the step of preparing this dielectric layer and deterioration 
phenomena such as oxidation are caused in the magnetic layer. Although 
Si.sub.3 N.sub.4 has a refractive index of from 1.9 to 2.1, it provides a 
dense film free of pinholes, which is excellent in various resistance 
characteristics. Accordingly, it is desired that the refractive index of 
the Si.sub.3 N.sub.4 dielectric layer will be improved while excellent 
resistance characteristics are effectively utilized. However, no proposal 
has been made in the art to realize this desire. 
SUMMARY OF THE INVENTION 
Under this background we made research on the Si.sub.3 N.sub.4 dielectric 
material, and as a result, it was found that if a predetermined amount of 
a specific additive is incorporated in Si.sub.3 N.sub.4, all of the 
above-mentioned problems can be solved. We have now completed the present 
invention based on this finding. 
It is therefore a primary object of the present invention to provide a 
magneto-optical recording element comprising a dielectric layer of 
Si.sub.3 N.sub.4 excellent in the oxidation resistance, the long-time 
stability and various resistance characteristics such as the resistance to 
exposure to a high-temperature high-humidity environment, in which the 
refractive index of the Si.sub.3 N.sub.4 dielectric layer is increased and 
the figures of merit are improved. 
Another object of the present invention is to provide a method in which 
formation of a dielectric layer of Si.sub.3 N.sub.4 is accomplished with a 
good productivity by sputtering and a dielectric layer excellent in 
various characteristics is obtained. 
More specifically, in accordance with one fundamental aspect of the present 
invention, there is provided a magneto-optical recording element 
comprising a substrate, a dielectric layer containing silicon nitride and 
a magnetic layer for magneto-optical recording having an axis of easy 
magnetization perpendicular to the film surface thereof, wherein the 
dielectric layer consists of a deposition film formed of a composition 
comprising silicon nitride and an agent for improving the refractive index 
of silicon nitride and the dielectric layer has a refractive index of at 
least 2.15. 
In accordance with another fundamental aspect of the present invention, 
there is provided a method for the fabrication of a magneto-optical 
recording element, which comprises maintaining a substrate on which a film 
is to be formed, a first target composed of a magnetic layer-forming metal 
and a second target composed of a composition comprising (a) silicon 
nitride and (b) at least one additive selected from single elements, 
oxides, nitrides, sulfides and silicides of elements of the groups IIIa, 
IVa, IIb, IIIb, IVb and VIb of the Periodic Table in an inert gas 
atmosphere maintained at 1.times.10.sup.-3 to 50.times.10.sup.-3 Torr and 
forming a magnetic layer and a dielectric layer alternately on the 
substrate by sputtering.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the invention, the shape of the substrate on which the magnetic layer is 
to be formed is not particularly critical. However, the description will 
now be made with reference to a substrate for a recording disc 
hereinafter. 
FIG. 1 is a sectional view showing a typical layer structure of the 
magneto-optical recording element according to the present invention. 
Referring to FIG. 1, a magnetic layer 3 is maintained on a substrate 1 for 
a recording disc through a first Si.sub.3 N.sub.4 dielectric layer 2, and 
a second Si.sub.3 N.sub.4 layer 4 is formed on the magnetic layer 3 and a 
protecting layer 5 is formed on the dielectric layer 4. 
For formation of the Si.sub.3 N.sub.4 dielectric layer on the surface of 
the substrate 1 for a recording disc, PVD (physical vacuum deposition) or 
CVD (chemical vacuum deposition) is preferably adopted so as to comply 
with mass production because the magnetic layer 3 is formed by the 
film-forming technique such as sputtering. For example, as means for 
forming the Si.sub.3 N.sub.4 dielectric layer 4 by sputtering, there may 
be adopted a method conducted by using an additive-containing Si.sub.3 
N.sub.4 sintered body described hereinafter as the target, a method 
conducted by a composite target comprising an Si.sub.3 N.sub.4 target and 
a target of an additive or a reactive sputtering method conducted in a 
nitrogen atmosphere by using an alloy target in which an Si additive is 
incorporated. 
In the present invention, a glass or a plastic material such as a 
polycarbonate resin (hereinafter referred to as "PC resin") or a 
polymethyl methacrylate resin (hereinafter referred to as "PMMA resin") 
may be used as the material of the substrate 1. It is preferred that the 
surface portion to which the Si.sub.3 N.sub.4 dielectric layer 2 is bonded 
be formed of a plastic material. For example, a substrate 1 composed 
entirely of a plastic material is advantageous in that the weight is 
light, the cost is cheap and the durability and safety are good and that 
large quantities of substrates provided with guide tracks can be 
reproduced by injection molding. Moreover, the figures of merit can be 
improved by interposition of the Si.sub.3 N.sub.4 dielectric layer 2. 
It is important that the additive for improving the refractive index of 
Si.sub.3 N.sub.4 per se, that is, the refractive index improver, should be 
incorporated in the Si.sub.3 N.sub.4 dielectric layer of the present 
invention so that the refractive index is at least 2.15. 
As the additive, there can be mentioned single elements, oxides, nitrides, 
sulfides and silicides of elements of the group IIIa of the Periodic Table 
such as Y, La and Ce, elements of the group IVa of the Periodic Table such 
as Ti and Zr, elements of the group VIa of the Periodic Table such as Cr 
and Mo, elements of the group Vb of the Periodic Table such as Sb and Bi, 
elements of the group IVb of the Periodic Table such as Si, Ge, Sn and Pb, 
elements of the group IIIb of the Periodic Table such as Al and elements 
of the group IIb of the Periodic Table such as Zn and Cd. For example, 
there can be mentioned single elements such as Al, Ti, Si and Ge and 
compounds such as Al.sub.2 O.sub.3, Y.sub.2 O.sub.3, La.sub.2 O.sub.3, 
CeO.sub.2, Bi.sub.2 O.sub.3, GeO.sub.2, ZrO.sub.2, CdO, Cr.sub.2 O.sub.3, 
SnO.sub.2, PbO, AlN, TiN, YN, ZnS, Sb.sub.2 S.sub.3, TiSi.sub.2 and 
YSi.sub.2. These additives may be used singly or in the form of a mixture 
of two or more of them. 
Some of the refractive index improvers to be incorporated in the Si.sub.3 
N.sub.4 dielectric layer in the present invention have heretofore been 
used as sintering aids for Si.sub.3 N.sub.4. However, in the present 
invention, Si.sub.3 N.sub.4 is contained in the dielectric layer in the 
state quite different from the sintered state. Namely, Si.sub.3 N.sub.4 is 
contained in the amorphous state in the dielectric layer of the present 
invention. Accordingly, it is quite surprising that by incorporation of an 
additive such as mentioned above, the refractive index of the dielectric 
layer is improved. 
Of course, the present invention includes an embodiment in which prominent 
additional advantages described hereinafater are attained by using a 
sintered body of Si.sub.3 N.sub.4 as the target for formation of a 
dielectric layer by sputtering. 
It sometimes happens that the composition of the target for formation of 
the dielectric layer is different from the composition of the actually 
formed dielectric layer. Generally, the Si.sub.3 N.sub.4 content in the 
dielectric layer tends to be higher than the Si.sub.3 N.sub.4 content in 
the target. However, if the above-mentioned additive is contained in the 
dielectric layer in an amount of at least 0.1 mole%, especially 0.5 to 8.0 
mole%, the refractive index is satisfactorily improved. It is preferred 
that the additive be contained in the target in an amount of 4 to 20 
mole%. 
As the dielectric layer suitable for attaining the objects of the present 
invention, there can be mentioned a deposition film formed of a 
composition comprising 0.1 to 5.0 mole% of alumina or aluminum nitride and 
0.1 to 3.0 mole% of an oxide or nitride of a rare earth element with the 
balance being silicon nitride. 
As examples of the dielectric layer-forming target composed of a 
composition comprising Si.sub.3 N.sub.4 and a sintering aid, which is 
preferably used in the present invention, there can be mentioned Si.sub.3 
N.sub.4 (90 mole%)-Al.sub.2 O.sub.3 (6 mole%)-Y.sub.2 O.sub.3 (4 mole%), 
Si.sub.3 N.sub.4 (90 mole%)-Al.sub.2 O.sub.3 (6 mole%)-La.sub.2 O.sub.3 (4 
mole%), Si.sub.3 N.sub.4 (90 mole%)-Al.sub.2 O.sub.3 (6 mole%)-CeO.sub.2 
(4 mole%) and Si.sub.3 N.sub.4 (90 mole%)-AlN(5 mole%)-La.sub.2 O.sub.3 (5 
mole%). The refractive index can be increased with increase of the 
proportion of the Si atom in Si.sub.3 N.sub.4, and we have found that if a 
target for forming of a dielectric layer, which is formed by adding 5 to 
20 mole% of Si to Si.sub.3 N.sub.4, is used, the refractive index can be 
prominently increased. 
The Si.sub.3 N.sub.4 dielectric layer of the present invention contains the 
above-mentioned additive as the indispensable component, but it may 
further contain other component so far as the effect of the additive is 
not lost. For example, the dielectric layer may contain a minor amount of 
SiO.sub.2 or WC. 
From the results of experiments conducted repeatedly by us, it has been 
confirmed that if the content of the additive in the target is at least 4 
mole%, prominent effects can be attained. It is preferred that the maximum 
refractive index of the Si.sub.3 N.sub.4 dielectric layer be determined 
within such a range that the reflectance is not greatly reduced and a 
sufficient quantity of light can be assured for focussing at the time of 
rotation of the disc, though this maximum refractive index differs 
according to the optical constant of the magnetic layer and the material 
of the substrate. When a PC resin, a PMMA resin or a glass is used as the 
substrate, since the refractive index is 1.59, 1.5 or 1.5, respectively, 
it is practically preferred that the refractive index of the Si.sub.3 
N.sub.4 dielectric layer be increased within a range not exceeding 3.5. 
If the refractive index of the Si.sub.3 N.sub.4 dielectric layer is 
increased, the enhancement effect is improved and simultaneously, the 
thickness t of the dielectric layer can be reduced based on the formula 
t=.lambda./4n.multidot. (2m+1) for obtaining the enhancement effect, with 
the result that the film-forming time required for formation of the 
dielectric layer can be shortened by scores of % and the deviation of the 
enhancement effect owing to the unevenness of the layer thickness 
distribution on the substrate can be reduced. 
Moreover, according to the present invention, if Si.sub.3 N.sub.4 is 
contained as the main component in the target for formation of the 
Si.sub.3 N.sub.4 dielectric layer in an amount of at least 60 mole%, 
preferably at least 80 mole%, a film free of pinholes can be formed, 
though this lower limit of the Si.sub.3 N.sub.4 content differs to some 
extent according to the kind of the additive. Therefore, deterioration 
such as oxidation is not caused in the dielectric layer even if it is 
exposed to a high-temperature high-humidity environment for a long time, 
and inherent excellent resistance characteristics of Si.sub.3 N.sub.4 can 
be retained. 
Furthermore, in the Si.sub.3 N.sub.4 dielectric layer of the present 
invention, the linear expansion coefficient can be increased to 
4.times.10.sup.-6 to 10.times.10.sup.-6 /.degree.C., and this can be 
brought close to those of the glass substrate, the PC resin and the PMMA 
resin that are 9.times.10.sup.-6 to 10.times.10.sup.-6 /.degree.C., 
6.6.times.10.sup.-5 /.degree.C. and 5.times.10.sup.-5 to 9.times.10.sup.-5 
/.degree.C., respectively. Therefore, occurrence of peeling of the 
magnetic layer or cracking is hardly caused and the adhesion to each 
substrate is improved, with the result that a high reliability is ensured 
for a long time and inherent excellent magneto-optical characteristics can 
be maintained. Incidentally, the linear expansion coefficient of Si.sub.3 
N.sub.4 per se is 1.9.times.10.sup.-6 /.degree.C. 
In the magneto-optical recording element of the present invention, a 
magnetic layer 3 composed of an amorphous metal easy magnetization film 
such as TbFe, GdCo, TbFeCo, DyFeCo, GdTbFeCo or GdDyFeCo is formed on the 
glass or plastic substrate 1 through the above-mentioned Si.sub.3 N.sub.4 
dielectric layer 2, and in order to prevent oxidation or other 
deterioration of the magnetic layer 3, a second Si.sub.3 N.sub.4 
dielectric protecting layer 4 is preferably formed on the magnetic layer. 
It is preferred that also this protecting layer be the above-mentioned 
Si.sub.3 N.sub.4 dielectric layer of the present invention, and in this 
case, one target can be commonly used. 
In the magneto-optical recording element of the present invention, a 
certain interposing layer may be formed between the substrate 1 and the 
Si.sub.3 N.sub.4 dielectric layer 2 or between this dielectric layer 2 and 
the magnetic layer 3 so as to effectively improve the magneto-optical 
characteristics. 
In the magneto-optical recording element of the present invention, a 
protecting resin layer 5 may be formed on the second dielectric layer 4. A 
known ultraviolet ray-curable acrylic resin, polyester resin or acrylic 
urethane resin may be used for the protecting resin layer 5. 
As shown in FIG. 2, the protecting layer 5 of an ultraviolet ray-curable 
resin may be formed directly on the surface of the magnetic layer 3 while 
omitting the second dielectric layer 4. 
Furthermore, as shown in FIG. 3, there may be adopted a layer structure 
comprising a magnetic layer 3 formed on a substrate 1, a dielectric layer 
4 of the present invention formed on the magnetic layer 3 and a protecting 
resin layer 5 formed on the dielectric layer 4. In this case, an 
enhancement effect can be obtained by applying reproducing laser beams 
from the side opposite to the substrate 1. Moreover, there may be adopted 
an embodiment in which a metal vacuum deposition layer 6 is formed instead 
of the protecting resin layer 5 and an enhancement effect is obtained by 
applying reproducing laser beams from the side of the substrate 1. 
The method for the fabrication of the magneto-optical recording element of 
the present invention will now be described. 
Sputtering, vacuum deposition or ion plating may be adopted for the 
fabrication of the magneto-optical recording element of the present 
invention. The fabrication method using a magnetron sputtering apparatus 
shown in FIG. 7 will now be described in detail by way of an example. 
Referring to FIG. 7, a first target 12 composed of a sintered body of 
Si.sub.3 N.sub.4, a second target 13 composed of a magnetic alloy and a 
disc-shaped substrate 14 which is rotated and driven to form a combination 
of films thereon are arranged within a vacuum tank 11. 
High-frequency sputtering is effected between the first target 12 and the 
substrate 14 and sputtering is effected by application of a high-frequency 
voltage or direct current voltage between the second target 13 and the 
substrate 14. 
A planar magnetron type cathode is arranged below the first and second 
targets 12 and 13, whereby the efficiency of ionization of the discharge 
gas molecules is increased by utilizing the Penning discharge effect by 
crossed electric and magnetic fields and a high film-forming speed 
suitable for mass production can be attained. 
According to the present invention, in the above-mentioned apparatus, 
formation of the Si.sub.3 N.sub.4 dielectric layer and formation of the 
magnetic layer are carried out in an optional lamination order. At first, 
the apparatus is evacuated to a high vacuum degree less than 
1.times.10.sup.-5 Torr, and a sputtering inert gas such as argon or 
nitrogen is introduced so that a predetermined pressure is maintained. If 
the pressure of the atmosphere gas is lower than 1.times.10.sup.-3 Torr, a 
stable discharge state cannot be obtained and formation of a film becomes 
difficult. If the pressure of the atmosphere gas exceeds 
50.times.10.sup.-3 Torr, the amount of argon (Ar) or oxygen (O) contained 
in the magnetic film is increased and attainment of the objects of the 
present invention becomes difficult, and no good uniformity or stability 
can be obtained. Therefore, the pressure of the atmosphere gas is set at 
1.times.10.sup.-3 to 50.times.10.sup.-3 Torr, preferably 3.times.10.sup.-3 
to 20.times.10.sup.-3 Torr. 
In case of the recording element shown in FIG. 1, a high-frequency electric 
power is applied between the first target 12 and the substrate 14 to form 
an Si.sub.3 N.sub.4 dielectric layer, and when a predetermined thickness 
is obtained, sputtering is stopped. Then, a high-frequency electric power 
is applied between the second target 13 and the substrate 14 to form a 
magnetic layer, and when a predetermined thickness is obtained, this 
sputtering is stopped. Then, sputtering for formation of an Si.sub.3 
N.sub.4 dielectric layer is carried out again. 
According to the present invention, by using as the first target a 
composite Si.sub.3 N.sub.4 sintered body in which an additive as described 
hereinbefore is incorporated, the refractive index of the dielectric layer 
is improved and additionally, the following advantages can be attained. 
The vacuum degree in the sputtering apparatus for formation of a film 
before introduction of a non-oxidizing gas (such as Ar or N.sub.2) may be 
at a level of 1.times.10.sup.-5 Torr (1.3.times.10.sup.-3 Pa) or less. 
According to the conventional technique, the required vacuum degree is 
1.times.10.sup.-6 Torr or less. Namely, according to the present 
invention, the required vacuum degree can be moderated about 10 times. 
Therefore, the time required for evacuation can be shortened and the 
production efficiency can be increased. The reason why the vacuum degree 
attained before introduction of the non-oxidizing gas is thus controlled 
is that if the vacuum degree is larger than the critical value, the 
residual gas in the apparatus (such as water, oxygen or nitrogen) is 
included in the dielectric layer to form silicon oxynitride (Si--N--O), 
with the result that the refractive index is reduced and becomes close to 
that of SiO.sub.2. 
The conventional Si.sub.3 N.sub.4 target is porous and the porosity is 
higher than about 30%. In contrast, in the present invention, since a 
sintering aid for silicon nitride is used as the additive, an Si.sub.3 
N.sub.4 target having a porosity lower than 5% can be formed and used, and 
the following disadvantages involved in the conventional technique can be 
eliminated. 
Since the conventional target has pores therein, the discharge stability at 
the sputtering step is poor. Moreover, the effect of cooling the target 
for controlling the temperature of the target is insufficient because of 
the presence of pores. Furthermore, it is considered that an impurity gas 
is contained in closed pores present in the interior of the conventional 
Si.sub.3 N.sub.4 sintered body target, and a uniform and stable quality 
cannot be obtained because of this impurity gas. 
The present invention will now be described in detail with reference to the 
following examples that by no means limit the scope of the invention. 
EXAMPLE 1 
Starting Si.sub.3 N.sub.4 having a purity of 99.9% was mixed with Al.sub.2 
O.sub.3 and Y.sub.2 O.sub.3, and the mixture was molded, sintered and 
processed into a disc-like shape having a diameter of 6 inches and a 
thickness of 5 mm. The so-obtained composite Si.sub.3 N.sub.4 was set in a 
high-frequency binary magnetron sputtering apparatus. One of a glass 
substrate, a PC resin substrate and a PMMA resin substrate was arranged in 
the sputtering apparatus as the substrate 1, and the sputtering apparatus 
was evacuated to 5.times.10.sup.-7 Torr. Then, Ar gas having a purity of 
99.999% was introduced until the pressure was elevated to 
5.times.10.sup.-3 Torr. Then, an electric power of 50 W was applied to the 
substrate 1 to effect etching and pre-sputtering was conducted for 5 
minutes on the substrate at a high-frequency power of 1 KW to form a 
composite Si.sub.3 N.sub.4 dielectric layer 2. The film-forming conditions 
were set so that the thickness of the formed composite Si.sub.3 N.sub.4 
dielectric layer 2 was .lambda./4n (in which .lambda. stands for the 
wavelength of reproducing laser beams, which was adjusted to 8000 .ANG. in 
this example, and n stands for the refractive index of the composite 
Si.sub.3 N.sub.4 dielectric layer). Then, pre-sputtering was carried out 
for 60 minutes at a high-frequency power of 200 W to form a DyFeCo layer 
having a thickness of about 1500 .ANG. in each element. 
A composite Si.sub.3 N.sub.4 protecting layer 4 was formed on the 
so-obtained magnetic layer 3 under the same conditions as adopted for 
formation of the above-mentioned composite Si.sub.3 N.sub.4 dielectric 
layer 2. 
Incidentally, from the results of the fluorimetric X-ray analysis, it was 
found that the target used for formation of the composite Si.sub.3 N.sub.4 
dielectric layer 2 and composite Si.sub.3 N.sub.4 protecting layer 4 
comprised 90 mole% of Si.sub.3 N.sub.4, 6 mole% of Al.sub.2 O.sub.3 and 4 
mole% of Y.sub.2 O.sub.3. 
With respect to the so-obtained magneto-optical recording elements of the 
present invention, the figure of merit, the resistance characteristics and 
the adhesion were determined according to the following methods. 
(1) Figure of Merit 
With respect to the magneto-optical recording element formed by using the 
glass substrate (having a refractive index of 1.5), reproducing laser 
beams (having a wavelength of 8000 .ANG.) were projected from the 
substrate side and the Kerr rotation angle .theta.k and reflectance R were 
measured, and the figure of merit .eta.(=.sqroot.R.times..theta.k) was 
determined. The obtained results are shown in Table 1. 
For comparison, data of the comparative photomagnetic recording element, 
which was prepared in the same manner as described above except that 
Si.sub.3 N.sub.4 dielectric and protecting layers were formed without 
incorporation of any additive, are shown in Table 1. 
In order to express the enhancement effect by a numerical value, a 
magneto-optical recording element having the same composite dielectric 
layer 2 and magnetic layer 3 as described above but being not provided 
with the composite Si.sub.3 N.sub.4 protecting layer 4 as shown in FIG. 2 
was prepared, reproducing laser beams (having a wavelength of 8000 .ANG.) 
were projected to the element from the magnetic layer side and the 
inherent figure of merit .eta.' of the magnetic layer was determined. 
Similarly, the enhancement effect of the comparative element was 
determined. 
TABLE 1 
______________________________________ 
Comparison, 
Pesent Invention 
Si.sub.3 N.sub.4 (99.9%) 
______________________________________ 
Laser Beams Applied 
from the Substrate Side 
.theta.k (.degree.) 
0.731 0.600 
R (%) 16.5 19.5 
##STR1## 0.297 0.265 
Laser Beams Applied from 
0.190 0.192 
the Magnetic Layer Side 
##STR2## 
Enhancement Effect, 
3.9 2.8 
20 log .eta./.eta.' (dB) 
Refractive Index of 
2.32 2.15 
Dielectric Layer 
Thickness (.ANG.) of 
862 930 
Dielectric Layer 
______________________________________ 
From the data shown in Table 1, it is seen that in the element of the 
present invention, the refractive index of the dielectric layer is 
increased and the enhancement becomes higher than in the comparative 
element and the figure of merit is larger by about 12%. 
(2) Resistance Characteristics 
With respect to the magneto-optical recording element formed by using the 
glass laminate, changes of the Kerr rotation angle and coersive force were 
traced from the point just after the fabrication of the element placed in 
a high-temperature high-humidity atmosphere maintained at a temperature of 
65.degree. C. and a relative humidity of 90 to 95%. The obtained results 
are shown in FIGS. 4 and 5. Incidentally, the data obtained with respect 
to comparative elements comprising an SiC or CdS dielectric layer are 
shown in FIG. 6. Incidentally, these data were obtained from the Kerr 
hysteresis loops by using a Kerr effect-measuring apparatus (supplied by 
Nippon Bunko K.K.). The wavelength of reproducing laser beams used for the 
measurement was 6328 .ANG.. 
In FIG. 4, the ratio of the Kerr rotation angle .theta.kr(t) after the 
lapse of time t to the Kerr rotation angle .theta.kr(o) just after the 
fabrication is plotted. Incidentally, .theta.kr indicates the residual 
Kerr rotation angle. Marks indicate the values obtained with respect to 
the element of the present invention and curve (a) is a time dependency 
curve obtained by connecting these values. Marks indicate the values 
obtained with respect to the comparative element prepared in the same 
manner as described above except that no additive was incorporated in the 
Si.sub.3 N.sub.4 dielectric layer and Si.sub.3 N.sub.4 protecting layer, 
and curve (b) is a time dependency curve obtained by connecting these 
values. 
From FIG. 4, it is seen that even after the lapse of 500 hours, the 
characteristic of the element of the present invention is hardly changed 
as compared with the characteristic of the comparative element and the 
composite Si.sub.3 N.sub.4 dielectric layer of the present invention is 
comparable or superior to the known Si.sub.3 N.sub.4 protecting layer as 
the layer for protecting a magneto-optical recording magnetic film. 
In FIG. 5, the ratio of the coercive force Hc(t) after the lapse of time t 
to the coercive force Hc(o) just after the fabrication is plotted. Marks 
indicates the values obtained with respect to the element of the present 
invention and curve (c) is a time dependency curve obtained by connecting 
these values. Marks indicate the values obtained with respect to the 
above-mentioned comparative element and curve (d) is a time dependency 
curve obtained by connecting these values. 
From FIG. 5, it is seen that the element of the present invention retains 
an excellent characteristic even after the lapse of 500 hours as in the 
case of the known Si.sub.3 N.sub.4 layer. 
In FIG. 6, the change of the ratio .theta.kr(t)/.theta.kr(o) with the lapse 
of time t is plotted. Curve (a) shown in FIG. 4 is similarly shown in FIG. 
6, and the date obtained with comparative elements formed by using an SiC 
dielectric material or CdS dielectric material are shown in FIG. 6. 
Namely, marks indicate the values obtained with respect to the element 
of the present invention, marks indicate the values obtained with 
respect to the comparative element obtained by laminating the SiC 
dielectric layer, the DyFeCo layer and the SiC dielectric protecting layer 
in this order on the glass substrate and marks .quadrature. indicate the 
values obtained with respect to the comparative element obtained by 
laminating the CdS dielectric layer, the DyFeCo layer and the CdS 
dielectric protecting layer in this order on the glass substrate, and 
curves (a), (e) and (f) are time dependency curves obtained by connecting 
these values , and .quadrature., respectively. Incidentally, the DyFeCo 
layer was formed in the same manner as described above. 
Incidentally, each of SiC and CdS shown in FIG. 6 is a medium having a 
refractive index larger than 2. As is seen from FIG. 6, in the comparative 
element formed by using the SiC dielectric material, the Kerr rotation 
angle was reduced after the lapse of 200 hours and the element could not 
be practically used. In the comparative element, the Kerr rotation angle 
was degraded after the lapse of about 10 hours and the degradation degree 
was extremely high. Thus, it was confirmed that a non-oxide type 
dielectric material having a refractive index larger than 2 is not 
suitable as an oxidation-resistant protecting layer. 
(3) Adhesion Test 
With respect of the magneto-optical recording elements obtained in this 
example, an adhesive tape (transparent tape) was sufficiently bonded to 
the surface of the composite Si.sub.3 N.sub.4 protecting layer and the 
tape was then pulled and peeled, and this operation was conducted 5 times 
on the same area. This bonding-peeling test was conducted on other areas 
of the protecting layer. Thus, the adhesion between the substrate and the 
dielectric layer was evaluated. 
The results of this adhesion test are shown in Table 2. 
TABLE 2 
__________________________________________________________________________ 
Present Invention 
Comparison 
__________________________________________________________________________ 
Composition of Target for 
Si.sub.3 N.sub.4 (90 mole %)-Al.sub.2 O.sub.3 
Si.sub.3 N.sub.4 (99.9 mole %) 
Si.sub.3 N.sub.4 Dielectric Layer 
(6 mole %)-Y.sub.2 O.sub.3 (4 mole %) 
Film-Forming Speed 
(.ANG./sec) of Si.sub.3 N.sub.4 
Dielectric Layer 
1-KW power for 
2.32 2.11 
formation of film 
2-KW power for 
4.29 3.68 
formation of film 
Substrate Temperature (.degree.C.) 
1-KW power for 
35 40 
formation of film 
2-KW power for 
43 46 
formation of film 
Adhesion to Substrate 
PC resin .circle. .DELTA. 
PMMA resin .circle. .DELTA. 
glass .circle. .circle. 
__________________________________________________________________________ 
Note 
.circle. : excellent adhesion with no peeling of the dielectric layer 
.DELTA.: fair adhesion with peeling of the dielectric layer during 
repetition of the bondingpeeling test 
From the results shown in Table 2, it is seen that in the element of the 
present invention, the dielectric layer has a good adhesion to all of the 
substrates, but when the comparative Si.sub.3 N.sub.4 (99.9 mole%) 
dielectric layer-forming target is used, the adhesion to plastic 
substrates is relatively poor and peeling of the dielectric layer is 
caused when the operation of bonding the transparent tape and peeling it 
is repeated. 
It also is seen that according to the present invention, the film-forming 
speed is higher than in the comparison at the same applied electric power 
irrespectively of the material of the substrate and the substrate 
temperature can be lowered at the film-forming step in the present 
invention. The fact that the substrate temperature can be reduced at the 
film-forming step is especially important when a plastic substrate having 
a low heat distortion temperature is used, and this can be attained by 
preventing the secondary incidence of electrons on the substrate and 
reducing the heat radiation to the target. In the above example, a high 
electric power efficiency (film-forming speed/applied electric power) is 
attained and this high electric power efficiency is effective for carrying 
out the film-forming operation at a lower temperature. Namely, according 
to the present invention, there is provided an Si.sub.3 N.sub.4 dielectric 
layer which can be formed at a high speed at such as low substrate 
temperature as not giving any thermal influence to a plastic substrate. 
Incidentally, in the example, the substrate temperatures were compared 
while the thickness was maintained at a level providing an enhancement 
effect. 
EXAMPLE 2 
The procedures of Example 1 were repeated in the same manner except that 
composite Si.sub.3 N.sub.4 dielectric layer-forming targets shown in Table 
3 were used. With respect to each of the obtained dielectric layers, the 
refractive index and enhancement effect .eta./.eta.' were determined. The 
obtained results are shown in Table 3. 
TABLE 3 
__________________________________________________________________________ 
Refractive 
Enhancement 
Index of 
Effect, 20 
Thickness (.ANG.) 
Composition of Dielectric Layer-Forming 
Dielectric 
log .eta./.eta.' 
of Dielectric 
Target Layer (dB) Layer 
__________________________________________________________________________ 
Si.sub.3 N.sub.4 (95 mole %)-Si (5 mole %) 
2.45 5.5 820 
Si.sub.3 N.sub.4 (80 mole %)-Si (20 mole %) 
2.61 5.7 770 
Si.sub.3 N.sub.4 (90 mole %)-Al.sub.2 O.sub.3 (6 mole %)- 
2.41 5.4 830 
La.sub.2 O.sub.3 (4 mole %) 
Si.sub.3 N.sub.4 (90 mole %)-AlN (10 mole %) 
2.26 5.1 885 
__________________________________________________________________________ 
From the results shown in Table 3, it is seen that in each element, the 
refractive index and enhancement effect were improved. When these elements 
were subjected to the resistance and adhesion tests in the same manner as 
described in Example 1, it was found that excellent results similar to 
those obtained in Example 1 were obtained. Moreover, with respect to each 
element, it was confirmed that the dielectric layer could be formed at a 
high film-forming speed at a low temperature. 
In the same manner as described in Example 1, magneto-optical recording 
elements were prepared by using Si.sub.3 N.sub.4 (90 mole%)-Al.sub.2 
O.sub.3 (6 mole%)-Y.sub.2 O.sub.3 (4 mole%), Si.sub.3 N.sub.4 (90 
mole%)-Al.sub.2 O.sub.3 (6 mole%)-CeO.sub.2 (4 mole%) and Si.sub.3 N.sub.4 
(90 mole%-AlN(5 mole%)-La.sub.2 O.sub.3 (5 mole%) as the dielectric 
layer-forming target. It was confirmed target. It was confirmed that the 
objects of the present invention can be attained by using these targets. 
EXAMPLE 3 
Magneto-optical recording elements were prepared in the same manner as 
described in Example 1 except that sintered bodies having a composition 
shown below are used as the target for forming a dielectric layer. 
Run 1: Si.sub.3 N.sub.4 (89 mole%)-Al.sub.2 O.sub.3 (9 mole%)-Y.sub.2 
O.sub.3 (2 mole %) 
Run 2: Si.sub.3 N.sub.4 (93 mole%)-Al.sub.2 O.sub.3 (5 mole%)-Y.sub.2 
O.sub.3 (2 mole%) 
Run 3: Si.sub.3 N.sub.4 (97 mole%)-Al.sub.2 O.sub.3 (2 mole%)-Y.sub.2 
O.sub.3 (1 mole%) 
The obtained results are shown in Table 4. 
TABLE 4 
__________________________________________________________________________ 
No.Run 
##STR3## 
##STR4## 20 log .eta./.eta.' (dB)Enhancement 
Effect, Dielectric LayerRefractive 
Index of Dielectric 
LayerThickness 
(.ANG.) 
__________________________________________________________________________ 
of 
1 0.666 
18.3 
0.285 0.191 3.5 2.26 885 
2 0.651 
18.5 
0.280 0.190 3.4 2.23 900 
3 0.613 
19.3 
0.269 0.190 3.0 2.18 920 
__________________________________________________________________________ 
EXAMPLE 4 
A magneto-optical recording element was prepared in the same manner as 
described in Example 1 except that the evacuation was carried out so that 
the vacuum degree attained before sputtering was 5.times.10.sup.-6 Torr. 
The obtained recording element had characteristics similar to those of the 
recording element obtained in Example 1. 
EXAMPLE 5 
A target of composite silicon nitride comprising Si.sub.3 N.sub.4, 8.7 
mole% of Al.sub.2 O.sub.3 and 2.1 mole% of Y.sub.2 O.sub.3 and having a 
porosity of 2%, which had a diameter of 6 inches and a thickness of 5 mm, 
was set in a high-frequency magnetron sputtering apparatus, and sputtering 
was carried out at a supplied power of 1 KW to form a composite silicon 
nitride film on a glass substrate. 
In order to examine the influences of the preparation conditions on the 
refractive index, the vacuum degree attained before the starting of 
sputtering was changed in the range of from 5.times.10.sup.-7 Torr to 
1.5.times.10.sup.-5 Torr. 
The discharge gas used at the sputtering step was Ar gas having a purity of 
99.999% and the Ar gas pressure at the sputtering step was adjusted to 
5.times.10.sup.-3 Torr. 
For comparison, the above procedures were repeated in the same manner 
except that a target of Si.sub.3 N.sub.4 (having a purity of 99.9%) having 
a porosity of 28% was used. 
The obtained results are shown in FIG. 8. 
From the results shown in FIG. 8, it is seen that according to the present 
invention, a film having a constant refractive index can be obtained if 
the attained vacuum degree is less than 1.times.10.sup.-5 Torr. 
From the results of the measurement using a quadrupole type mass analyzer, 
it was found that when the attained vacuum degree was low, the main 
component of the residual gas was water (H.sub.2 O). 
As is apparent from the foregoing description, in accordance with the 
present invention, there is provided a magneto-optical recording element 
in which the refractive index of an Si.sub.3 N.sub.4 dielectric material 
showing an excellent oxidation resistance to a magnetic layer can be 
increased while maintaining excellent resistance characteristics such as a 
high resistance to a high-temperature high-humidity environment, and 
therefore, the figure of merit is improved and the adhesion is increased.