Magneto-optical recording medium having magnetization variable between in-plane magnetization and perpendicular magnetization

A magneto-optical recording medium comprising a readout layer, an intermediate layer, and a recording layer. Two light beams having high and low light intensities respectively are applied while applying an external magnetic field. The magnetization direction of the recording layer is changed according to information, by reversing the sub-lattice magnetic moment of the readout layer in the case of the high light intensity. The intermediate layer exhibits a perpendicular magnetization during the irradiation, and also exhibits an in-plane magnetization at room temperature. Since it is not necessary to provide a device for generating a magnetic field for initializing every recording, it is possible to avoid increase in the size of the recording and reproducing apparatus. Moreover, the magnetization direction of the readout layer can be easily controlled with an external magnetic field, and the optical modulation overwriting property can be stabilized.

FIELD OF THE INVENTION 
The present invention relates to a magneto-optical recording medium to be 
adopted in magneto-optical recording devices, such as a magneto-optical 
disk, a magneto-optical tape, a magneto-optical card, etc., and also 
concerns a method of recording and reproducing magneto-optical information 
using these devices. 
BACKGROUND OF THE INVENTION 
A magneto-optical disk memory has been put into practical use as a 
rewritable optical disk memory. However, in order to record information 
again in the already-recorded portion, it is necessary to record it after 
erasing previously recorded information. Therefore, it has a disadvantage 
in that it takes a long time to re-record. 
In order to solve this problem, magnetic modulation overwriting is used as 
a method to perform overwriting, that is, recording without erasing 
already-recorded information before re-recording. 
However, this method performs overwriting by modulating the size of the 
magnetic field to be applied. Therefore, it has a disadvantage in that a 
device for generating a magnetic field has to be arranged in the vicinity 
of the magneto-optical disk in order to obtain a magnetic field of a 
sufficient size. Moreover, it also has a disadvantage in that recording 
takes much time because it is not possible to modulate the magnetic field 
at a high speed. 
Accordingly, in order to solve the above-described problems, Japanese 
Laid-Open Patent Application 175948/1987 (Tokukaisho 62-175948) discloses 
an optical modulation overwriting method. This method uses a 
magneto-optical recording medium having a double-layered structure 
composed of a recording layer and a recording auxiliary layer each of 
which is a film having a perpendicular magnetization, and modulates only 
the laser power of the light beam, thereby enabling overwriting. 
This method, disclosed in Japanese Laid-Open Patent Application 
175948/1987, enables optical modulation overwriting. However, since the 
magnetization direction of the recording auxiliary layer is changed when 
overwriting, it is necessary for each recording process to initialize the 
magnetization direction of the recording auxiliary layer before 
overwriting. Therefore, besides a device for generating a magnetic field 
for recording, a device for generating an initializing magnetic field is 
necessary. As a result, increased size and manufacturing costs of the 
apparatus are problems. 
Moreover, another problem of conventional magneto-optical disk memories is 
as follows: when the diameter and the interval of the recorded bit is 
small compared with the diameter of the light beam of the focused 
semiconductor laser, the reproducing property deteriorates. This is 
because the focused laser beam covers adjacent recorded bits therein, 
thereby making it impossible to reproduce each recorded bit separately. 
In order to solve such a problem, Japanese Laid-Open Patent Application 
81717/1993 (Tokukaihei 5-81717) discloses a technique using a 
magneto-optical recording medium of a double-layered structure. Namely, 
this medium has both a readout layer, which is a film showing an in-plane 
magnetization at room temperature and which becomes a film showing a 
perpendicular magnetization according to rises in temperature, and a 
recording layer, which is a film showing a perpendicular magnetization. 
This disclosure proposes an optical modulation overwriting method which 
reproduces each recorded bit separately even if the recorded bit is 
smaller than the diameter of the laser beam, and which does not need an 
initializing magnetic field. 
However, in the technique proposed in Japanese Laid-Open Patent Application 
81717/1993, since the recording layer and the readout layer are directly 
in contact with each other, a magnetic exchange-coupling force 
therebetween is strongly exerted. Therefore, since the technique cannot 
easily control the magnetization direction of the readout layer by using 
the recording magnetic field, it has a problem in that the optical 
modulation overwriting property is unstable. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a magneto-optical 
recording medium and a magneto-optical recording and reproducing method 
which do not need a device for generating an initializing magnetic field 
which arranges the magnetization direction before performing optical 
modulation overwriting, and which can prevent increases of both the size 
and the manufacturing cost of the apparatus. 
Another object of the present invention is to provide a magneto-optical 
recording medium and a magneto-optical recording and reproducing method 
which can easily control the magnetization direction of the readout layer 
with an external magnetic field, thereby stabilizing the optical 
modulation overwriting property. 
Still another object of the present invention is to provide a 
magneto-optical recording medium and a magneto-optical recording and 
reproducing method which can prevent signal contamination from adjacent 
bits causing noise, and can positively reproduce a recorded bit recorded 
with a pitch smaller than the spot diameter of the light beam, thereby 
remarkably increasing recording density. 
In order to achieve the above-described object, a first magneto-optical 
recording medium in accordance with the present invention is characterized 
in that the magneto-optical recording medium comprises: 
a recording layer which is constituted of a magnetic substance wherein 
information is magneto-optically recorded and which shows a perpendicular 
magnetization from room temperature T1 to a Curie temperature Tc3; 
a readout layer which is provided on the recording layer, and which is 
constituted of a magnetic substance of a rare-earth transition metal alloy 
from which the information recorded in the recording layer is reproduced 
by receiving irradiation of a light beam, the readout layer having a 
compensation temperature Tcomp1 and showing a perpendicular magnetization 
from room temperature to a temperature Tr2; and 
an intermediate layer which is provided between the recording layer and the 
readout layer, which is constituted of a magnetic substance, the 
intermediate layer having a compensation temperature Tcomp2 and shows a 
perpendicular magnetization only in a temperature range from a temperature 
Ti1 to a temperature Ti2 including the Tc3 and an in-plane magnetization 
from room temperature to the Ti1. 
In this arrangement, supposing that T3 indicates a temperature not lower 
than Ti1, not higher than Tcomp1, and not higher than Tcomp2; that T4 
indicates a temperature not lower than Tc3, not lower than Tcomp1, not 
lower than Tcomp2, and not higher than Tr2; that and Hc1 and Hc3 
respectively indicate a coercive force of the readout layer and a coercive 
force of the recording layer, for a predetermined magnetic field Hw, the 
following conditions are satisfied: 
when a temperature is T1, Hc1&lt;Hw and Hw&lt;Hc3; 
when a temperature is T3, Hc3&lt;Hw and Hw&lt;Hc1; and 
when a temperature is T4, Hc1&lt;Hw. 
A second magneto-optical recording medium in accordance with the present 
invention is as described in the first magneto-optical recording medium 
and is characterized in that when compositions of the readout layer, the 
intermediate layer, and the recording layer are set respectively to 
Gd.sub.0.28 (Fe.sub.x Co.sub.1-x).sub.0.72, Gd.sub.0.28 (Fe.sub.y 
Co.sub.1-y) .sub.0.72, and Dy.sub.0.20 (Fe.sub.0.78 Co.sub.0.22).sub.0.80, 
Y satisfies a condition 45%&lt;Y&lt;65%. 
A first magneto-optical recording and reproducing method in accordance with 
the present invention is characterized in that the method comprises a 
recording step of recording information as a magnetization onto a 
recording layer and a reproducing step of reproducing the information as 
the magnetization recorded on the recording layer, wherein the recording 
step includes the steps of: 
applying a magnetic field Hw onto the first or second magneto-optical 
recording medium in accordance with the present invention; and 
irradiating a light beam I having a low light intensity or a light beam II 
having a high light intensity alternatively onto the magneto-optical 
recording medium according to a recording signal, namely, (a) when 
irradiating a light beam I, making a sub-lattice magnetic moment of a 
rare-earth metal of the readout layer point in a direction of the Hw and 
making a magnetization direction of the recording layer point in a 
predetermined direction due to an exchange-coupling force from the readout 
layer; and (b) when irradiating a light beam II, making a sub-lattice 
magnetic moment of a rare-earth metal of the readout layer point in a 
direction reverse to the direction of the Hw and making a magnetization 
direction of the recording layer point in a predetermined direction 
reverse to the direction which the recording layer points when the light 
beam I is irradiated due to an exchange-coupling force from the readout 
layer, and 
the reproducing step includes the steps of: 
irradiating a light beam III having a light intensity lower than the light 
intensity of the light beam I onto the first or second magneto-optical 
recording medium; and 
arranging a sub-lattice magnetization of the readout layer to a direction 
stable for a sub-lattice magnetization of the recording layer after 
irradiation of the light beam III. 
According to the above arrangement, during recording, the magnetic field Hw 
is applied to the portion irradiated by the light beam in the 
magneto-optical recording medium in the same manner as the first 
magneto-optical recording and reproducing method. Note that the 
composition of each layer can be respectively arranged like that of the 
second magneto-optical medium, for example. The magnetization of the 
readout layer, referred to as M1 hereinafter, points in a direction that 
is the same as that of Hw since Hc1 is smaller than Hw at room 
temperature. The magnetization of the recording layer, referred to as M3 
hereinafter, points in a predetermined direction, upward or downward, 
according to the recorded information. However, since the intermediate 
layer is a film showing an in-plane magnetization, M3 is not copied to the 
readout layer. 
Here, like the first magneto-optical recording and reproducing method, (a) 
a light beam I having a low light intensity is irradiated so that the 
temperature of each magnetic layer is raised to T3, while the intermediate 
layer shows a perpendicular magnetization. Accordingly, an 
exchange-coupling force is exerted between the readout layer and the 
recording layer. In contrast, Hc1 is larger than Hw and Hc3 is smaller 
than Hw. Therefore, the exchange-coupling force copies the magnetization 
from the readout layer to the recording layer through the intermediate 
layer, and M3 points in a predetermined direction .alpha., upward or 
downward. 
When the temperature of each magnetic layer falls to T1 after irradiation 
of the light beam I, since Hc3 is larger than Hw, M3 still points in the 
direction .alpha. in spite of Hw. In contrast, the intermediate layer 
shows an in-plane magnetization. Therefore, since the exchange-coupling 
force is not exerted between the readout layer and the recording layer, M1 
still points in a direction that is the same as that of Hw, that is, an 
initializing direction, in spite of M3. This means that the direction 
.alpha. is recorded in M3. 
(b) Alternatively, a light beam II having a high light intensity is 
irradiated instead of the light beam I having a low light intensity so 
that the temperature of each magnetic layer is raised to T4, and Hc1 
becomes smaller than Hw. Moreover, since the temperature is higher than 
Tcomp1, the sub-lattice magnetic moment of the transition metal of the 
readout layer grows large, so the readout layer becomes TM rich. 
Therefore, the sub-lattice magnetic moment of the rare-earth metal of the 
readout layer, referred to as R1 hereinafter, points reversely to the 
direction of Hw, and M1 points in the direction of Hw. 
The magnetization of the intermediate layer shows an in-plane magnetization 
if T4 is between Ti2 and Tr2, and it comes into the same condition as that 
of the readout layer if T4 is between Tcomp2 and Ti2. In contrast, the 
magnetization of the recording layer disappears because its temperature is 
not lower than its Curie temperature Tc3. 
In the temperature-falling process to T3 after irradiation of the light 
beam II, since the temperature is not higher than Tcomp1, the sub-lattice 
magnetic moment of the transition metal of the readout layer grows small 
so that the readout layer becomes RE rich again. On the other hand, the 
magnetization of the intermediate layer is either in-plane as described 
above, or is of the same condition as that of the readout layer. Moreover, 
the magnetization of the recording layer does not exist because its 
temperature is not lower than its Curie temperature Tc3. Therefore, R1 
keeps pointing reversely to the direction of Hw, and M1 points in the 
direction of R1, that is, reversely to that of Hw. 
When the temperature falls to T3, since Hc1 is larger than Hw, directions 
of R1 and M1 are fixed to the above-described directions respectively. On 
the other hand, since the intermediate layer shows a perpendicular 
magnetization, an exchange-coupling force is exerted between the readout 
layer and the recording layer. Therefore, the magnetization direction is 
copied from the readout layer to the recording layer through the 
intermediate layer, so the magnetization of the recording layer points in 
a predetermined direction .beta. reverse to .alpha., that is, downward if 
.alpha. is upward and is upward if .alpha. is downward. 
When the temperature falls to T1, since Hc3 is larger than Hw, M3 stably 
keeps pointing in the above-described direction .beta. in spite of Hw. On 
the other hand, since the intermediate layer shows an in-plane 
magnetization, the exchange-coupling force between the readout layer and 
the recording layer is no longer exerted or becomes weak, and meanwhile, 
since Hc1 is smaller than Hw, M1 is initialized to the direction of Hw. 
This means that the direction .beta. is recorded in M3. 
By providing the intermediate layer as described above, an 
exchange-coupling force is strengthened so as to help copying from the 
readout layer to the recording layer when the magnetization is copied, 
while the exchange-coupling force is weakend so as to prevent copying from 
the recording layer to the readout layer when the magnetization direction 
of the readout layer is initialized. 
Therefore, enabling to reduce the intensity of the magnetic field necessary 
to initialization, it becomes possible to make initialization using the 
magnetic field Hw, which is used for recording. Consequently, it becomes 
unnecessary to provide a device for generating an initializing magnetic 
field arranging the magnetization direction before performing optical 
modulation overwriting, thereby making it possible to prevent increase in 
the size and increase in the manufacturing costs of the apparatus. 
Moreover, since the intermediate layer controls the magnetic 
exchange-coupling force between the recording layer and the readout layer 
as described above, the magnetization direction of the readout layer can 
be easily controlled with an external magnetic field, and the optical 
modulation overwriting property can be stabilized. 
During reproduction, the light beam III, whose light intensity is lower 
than that of the light beam I, is irradiated to the above-described 
magneto-optical recording medium, thereby raising the temperature of the 
layers to a predetermined temperature T2 lower than T3. Since an external 
magnetic field such as the magnetic field Hw is not applied at that time, 
the direction of M1 is determined according to the magnetic 
exchange-coupling force exerted from the recording layer. Though the 
intermediate layer shows an in-plane magnetization, it has a certain 
amount of the perpendicular magnetization component because it has been 
put between the readout layer and the recording layer which show 
perpendicular magnetizations. Therefore, the magnetic exchange-coupling 
force is exerted from the recording layer to the readout layer through the 
intermediate layer. Consequently, the direction of M1 is positively 
determined. 
In this way, the magnetization recorded in the recording layer is copied to 
the readout layer and reproduced as information. 
A third magneto-optical recording medium in accordance with the present 
invention is characterized in that the magneto-optical recording medium 
comprises: 
a recording layer which is constituted of a magnetic substance where 
information is magneto-optically recorded and which shows a perpendicular 
magnetization from room temperature T1 to its Curie temperature Tc3; 
a readout layer which is provided on the recording layer and which is 
constituted of a magnetic substance of a rare-earth transition metal alloy 
from which the information recorded in the recording layer is reproduced 
by receiving irradiation of a light beam, the readout layer having a 
compensation temperature Tcomp1, and showing a perpendicular magnetization 
only from a temperature Tr1 to a temperature Tr2 and an in-plane 
magnetization from room temperature to the Tr1; and 
an intermediate layer, which is provided between the recording layer and 
the readout layer and which is constituted of a magnetic substance, the 
intermediate layer having a compensation temperature Tcomp2 and which 
shows a perpendicular magnetization only in a temperature range from a 
temperature Ti1 to a temperature Ti2 including the Tc3 and an in-plane 
magnetization at temperatures not higher than the Ti1. In this 
arrangement, supposing that T3 indicates a temperature not lower than Ti1, 
not higher than Tcomp1, and not higher than Tcomp2; that T4 indicates a 
temperature not lower than Tc3, not lower than Tcomp1, not lower than 
Tcomp2, and not higher than Tr2; and that Hc1 and Hc3 respectively 
indicate a coercive force of the readout layer and a coercive force of the 
recording layer, for a predetermined magnetic field Hw, the following 
conditions are satisfied: 
when a temperature is T1, Hc1&lt;Hw and Hw&lt;Hc3; 
when a temperature is T3, Hc3&lt;Hw and Hw&lt;Hc1; and 
when a temperature is T4, Hc1&lt;Hw. 
A second magneto-optical recording and reproducing method in accordance 
with the present invention is characterized in that the method comprises a 
recording step of recording information as a magnetization onto a 
recording layer and a reproducing step of reproducing the information as 
the magnetization recorded on the recording layer, 
wherein the recording step includes the steps of: 
applying a magnetic field Hw onto the third magneto-optical recording 
medium in accordance with the present invention; 
irradiating a light beam I having a low light intensity or a light beam II 
having a high light intensity alternatively onto the magneto-optical 
recording medium according to a recorded signal, namely, (a) when 
irradiating a light beam I, making a sub-lattice magnetic moment of a 
rare-earth metal of the readout layer point in a direction of the Hw and 
making a magnetization direction of the recording layer point in a 
predetermined direction due to an exchange-coupling force from the readout 
layer; and (b) when irradiating a light beam II, making a sub-lattice 
magnetic moment of a rare-earth metal of the readout layer point in a 
direction reverse to a direction of the Hw and making a magnetization 
direction of the recording layer point in a predetermined direction 
reverse to the direction which the recording layer points when the light 
beam I is irradiated, due to an exchange-coupling force from the readout 
layer, and 
the reproducing step includes the steps of: 
irradiating a light beam III having a light intensity lower than the light 
intensity of the light beam I and having a distribution of light intensity 
where light intensity is higher in a center of the light beam than in a 
surrounding area surrounding the center, onto the magneto-optical 
recording medium; and 
arranging a sub-lattice magnetization of the readout layer to a direction 
stable for a sub-lattice magnetization of the recording layer after 
irradiation of the light beam III. 
According to the above arrangement, during recording, the above-described 
magnetic field Hw is applied to the magneto-optical recording medium like 
the second magneto-optical recording and reproducing method. At this time, 
the magnetization of the readout layer, referred to as M1 hereinafter, 
shows an in-plane magnetization at room temperature. Moreover, the 
magnetization of the recording layer, referred to as M3, points in a 
predetermined direction, that is, upward or downward, according to the 
recorded information. However, since the intermediate layer shows an 
in-plane magnetization, M3 is not copied to the readout layer. 
Here, a light beam I having a low light intensity and a light beam II 
having a high light intensity are alternatively irradiated as follows in 
the same manner as the second magneto-optical recording and reproducing 
method. 
When the temperatures of the layers are raised to the temperature T2 in the 
vicinity of Tr1 due to irradiation of the light beams, since Hc1 is 
smaller than Hw, M1 points in the direction of Hw. Moreover, M3 points in 
a predetermined direction, and is not copied to the readout layer because 
the intermediate layer shows an in-plane magnetization. 
Afterward, information is recorded similiarly to the first magneto-optical 
recording and reproducing method, provided that the magnetization of the 
readout layer becomes in-plane again when the temperature falls to T1, 
which makes this method different from the first magneto-optical recording 
and reproducing method. 
Consequently, it becomes not necessary to provide a device for generating 
an initializing magnetic field arranging the magnetization direction 
before performing optical modulation overwriting, thereby making it 
possible to prevent increase in the size and increase in the manufacturing 
cost of the apparatus. 
Moreover, similiarly to the first magneto-optical recording and reproducing 
method, by providing the intermediate layer, since the intermediate layer 
controls the magnetic exchange-coupling force between the recording layer 
and the readout layer, the magnetization direction of the readout layer 
can be easily controlled with an external magnetic field, and the optical 
modulation overwriting property can be stabilized. 
Reproduction is performed as follows. The light beam III whose light 
intensity is lower than that of the light beam I is irradiated to the 
above-described magneto-optical recording medium, thereby raising the 
temperature of the layers to a temperature T2 lower than T3. 
Here, the information recorded in the recording layer is reproduced 
substantially similiarly to the first magneto-optical recording and 
reproducing method. However, since the light intensity distribution of the 
light beam III shows a distribution where the intensity is higher in the 
center of the light beam than in the surrounding area, when the light beam 
III is irradiated, the temperature in the vicinity of the center of the 
irradiated portion is higher than that in the surrounding area in the 
readout layer. Therefore, in the readout layer, the temperature becomes 
not lower than T2, so the magnetization changes from the in-plane 
magnetization to the perpendicular magnetization only at the area in the 
vicinity of the center in the irradiated portion. Accordingly, the 
magnetization is copied from the recording layer to the readout layer only 
at the above-described area in the vicinity of the center. 
Therefore, it is made possible to reproduce only the information from the 
area in the vicinity of the center of the readout layer irradiated with 
the light beam III. Consequently, it becomes possible to prevent signal 
contamination from adjacent bits causing a noise and to positively 
reproduce a recorded bit recorded with a pitch smaller than the spot 
diameter of the light beam, thereby remarkably increasing recording 
density.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
[First Embodiment] 
Referring to FIGS. 1 through 4, the following description will discuss one 
embodiment of the present invention. 
As shown in FIG. 2, a magneto-optical disk 10 (magneto-optical recording 
medium) in the present embodiment is arranged by stacking a substrate 1 , 
a transparent dielectric layer 2, a readout layer 3, an intermediate layer 
4, a recording layer 5, a protective layer 6, and a backcoating layer 7 in 
this order. Recording and reproduction are performed by focusing a laser 
beam 8 (light beam) onto the readout layer 3 through a focusing lens 9. 
The substrate 1 is a disk-shaped glass substrate with an outer diameter of 
86 mm, an inner diameter of 15 mm, and a thickness of 1.2 mm. Although not 
shown in FIG. 2, in the surface of one side of the substrate 1, there are 
provided guide tracks with lands and grooves for guiding a light beam. The 
guide tracks have a pitch of 1.6 .mu.m, a width of grooves of 0.8 .mu.m, 
and a width of lands of 0.8 .mu.m. The transparent dielectric layer 2 is 
provided by forming an AlN layer of a thickness of 80 nm on the side where 
the guide tracks are provided in the substrate 1 to improve the 
reproducing property with a light interfering function. 
Each of the readout layer 3, the intermediate layer 4, and the recording 
layer 5, which is an amorphous film composed of a rare-earth and 
transition metal alloy, is made of a ferri-magnetic material where the 
sub-lattice magnetic moment of the rare-earth metal and that of the 
transition metal are anti-parallel to each other. 
The readout layer 3 is prepared by forming GdFeCo as a thin rare-earth and 
transition metal alloy film with a thickness of 40 nm. The readout layer 3 
has a composition of Gd.sub.0.28 (Fe.sub.0.65 Co.sub.0.35).sub.0.72 and a 
compensation temperature of 250.degree. C., and its magnetization 
condition is perpendicular at room temperature and in-plane at not lower 
than about 385.degree. C. 
The intermediate layer 4 is prepared by forming GdFeCo which is a thin 
rare-earth and transition metal alloy film with a thickness of 20 nm. The 
intermediate layer 4 has a composition of Gd.sub.0.28 (Fe.sub.0.50 
Co.sub.0.50).sub.0.72 and a compensation temperature of 240.degree. C., 
and its magnetization condition is in-plane from room temperature to 
200.degree. C., perpendicular at not lower than 200.degree. C., and 
in-plane at not lower than 300.degree. C. 
The recording layer 5 is prepared by forming DyFeCo as a thin rare-earth 
and transition metal alloy film with a thickness of 40 nm. The recording 
layer 5 has a composition of Dy.sub.0.20 (Fe.sub.0.78 
Co.sub.0.22).sub.0.80 and a Curie temperature of about 250.degree. C. 
The protective layer 6 is provided by forming an AlN layer of a thickness 
of 20 nm to prevent the magnetic layers from oxidation and erosion. 
Moreover, the backcoating layer 7 is provided by forming a polyurethane 
acrylate series ultraviolet-rays hardening resin of a thickness of 5 .mu.m 
to prevent the magnetic layers from oxidation, erosion, and damage. 
The guide tracks on the surface of the substrate 1 was formed directly onto 
the surface of the glass by reactive ion etching. Moreover, the 
transparent dielectric layer 2, the readout layer 3, the intermediate 
layer 4, the recording layer 5, and the protective layer 6 were together 
formed by the sputtering method by keeping a vacuous condition in the same 
sputtering apparatus. 
Each AlN layer of the transparent dielectric layer 2 and the protective 
layer 6 was formed by a reactive sputtering method which sputters an A1 
target in an N.sub.2 gas atmosphere. 
The readout layer 3, the intermediate layer 4 and the recording layer 5 
were formed by sputtering a so-called complex target where a Gd tip or a 
Dy tip is arranged on an FeCo alloy target, or by sputtering three-element 
alloy targets, that is, GdFeCo and DyFeCo targets, by using Ar gas. 
The overcoating layer 7 was formed by spreading a resin with a spin coater 
and afterward hardening the resin by irradiating an ultra-violet ray by 
the use of an ultraviolet ray irradiating apparatus. 
Next, FIG. 1 shows conceptually a magnetic property of the magneto-optical 
disk 10 in the present embodiment that is provided with the 
above-described layers. 
The recording layer 5 has its compensation temperature virtually at room 
temperature and its Curie temperature at a temperature Tc3 . The readout 
layer 3 shows a perpendicular magnetization at room temperature T1, and 
has its compensation temperature Tcomp1 in the vicinity of the Curie 
temperature Tc3 of the recording layer 5. Moreover, the readout layer 3 
has its Curie temperature at a temperature Tr2, or alternatively, shows an 
in-plane magnetization at temperatures not lower than the temperature Tr2. 
The intermediate layer 4 shows an in-plane magnetization at room 
temperature and a perpendicular magnetization at not lower than a 
temperature Ti1. Moreover, the intermediate layer 4 has its compensation 
temperature Tcomp2 in the vicinity of the Curie temperature Tc3 of the 
recording layer 5 and shows an in-plane magnetization at not lower than a 
temperature Ti2. 
Moreover, the following definitions are made: T3 indicates a temperature 
not lower than Ti1, not higher than Tcomp1, and not higher than Tcomp2. T4 
indicates a temperature not lower than Tc3, not lower than Tcomp1, not 
lower than Tcomp2, and not higher than Tr2. Hw indicates a recording 
magnetic field which is a magnetic field externally applied during 
recording. Hc1 and Hc3 respectively indicate the coercive force of the 
readout layer 3 and that of the recording layer 5. Moreover, the coercive 
forces Hc1 and Hc3 and the temperatures T1, T3, and T4 satisfy the 
conditions: 
when the temperature is T1, Hc1&lt;Hw and Hw&lt;Hc3; 
when the temperature is T3, Hc3&lt;Hw and Hw&lt;Hc1; and 
when the temperature is T4, Hc1&lt;Hw. 
Referring to FIG. 3, the next description will discuss a recording process 
that is carried out by optical modulation overwriting on the 
magneto-optical disk 10. In this figure, T1, T3, and T4 are the 
temperatures as described above. The temperature T2 is an arbitrary 
temperature lower than Ti1, and the .temperature becomes higher in the 
order of T1, T2, T3, and T4. A1, A2, . . . , B1, B2, . . . indicate 
respective conditions shown in the figure. Each condition shows states of 
the readout layer 3, the intermediate layer 4, and the recording layer 5 
at the highest box, the middle box, and the lowest box, respectively. In 
each layer, a black arrow indicates the direction of the sub-lattice 
magnetic moment of the rare-earth metal, and a white arrow indicates the 
direction of the total magnetization. 
"RE rich" represents a condition wherein the sub-lattice magnetic moment of 
the rare-earth metal is larger than the sub-lattice magnetic moment of the 
transition metal. Meanwhile, "TM rich" represents a condition wherein the 
sub-lattice magnetic moment of the transition metal is larger than the 
sub-lattice magnetic moment of the rare-earth metal. When the layer is RE 
rich, the direction of the sub-lattice magnetic moment of the rare-earth 
metal and the direction of the total magnetization are parallel, while 
when the layer is TM rich, the direction of the sub-lattice magnetic 
moment of the rare-earth metal and the direction of the total 
magnetization are anti-parallel. In the magneto-optical 10, the readout 
layer 3 and the intermediate layer 4 are RE rich and the recording layer 5 
is TM rich at room temperature. 
Two conditions A1 and A2 exist dependently on the recorded information at 
room temperature. Namely, according to the recorded information, the 
magnetization of the recording layer 5, referred to as M3 hereinafter, 
points downward in the condition A1 and upward in the condition A2. 
In the conditions A1 and A2, the recording magnetic field Hw pointing 
downward in the figure is applied from a magnetic field generating 
apparatus (not shown) onto a portion on the magneto-optical disk 10, which 
receives the light beam. Here, since Hc1 is smaller than Hw at room 
temperature, the magnetization of the readout layer 3, referred to as M1, 
points in the direction of Hw (downward in the figure). Moreover, as 
described above, M3 points in a predetermined direction, upward or 
downward, according to the recorded information. However, since the 
intermediate layer 4 is a film showing an in-plane magnetization, M3 is 
not copied to the readout layer 3. 
Here, a light beam I having a low light intensity or a light beam II having 
a high light intensity are alternatively irradiated. The light beam I and 
the light beam II have a power which can raise the temperature to T3 and 
T4, respectively. In the temperature-raising process, when the temperature 
of each layer is raised to T2 so that each layer becomes the condition B1 
or B2, the intermediate layer 4 still shows an in-plane magnetization 
similiarly to the conditions A1 and A2. Moreover, since Hc3 is still large 
enough, the recording layer 5 still shows the magnetization direction of 
the conditions A1 or A2. Furthermore, the exchange-coupling force between 
the readout layer 3 and the recording layer 5 is weak since the 
intermediate layer 4, which shows an in-plane magnetization, exists 
therebetween, so the magnetization of the readout layer 3 points in the 
direction same as that of Hw. 
Next, (a) the light beam I is continuously irradiated so that the 
temperature of each magnetic layer is raised to T3 (the condition C1). 
Therefore, since the temperature of the recording layer 5 is not lower 
than Tc3, M3 is extremely small. Moreover, the intermediate layer 4 shows 
a perpendicular magnetization. Accordingly, an extremely strong 
exchange-coupling force is exerted between the readout layer 3 and the 
recording layer 5. Here, Hc1 is larger than Hw and Hc3 is smaller than Hw. 
Therefore, the exchange-coupling force copies the magnetization from the 
readout layer 3 to the recording layer 5 through the intermediate layer 4. 
Thus, the sub-lattice magnetic moment of the recording layer 5 becomes 
parallel to the sub-lattice magnetic moment of the readout layer 3, and M3 
points in a predetermined direction .alpha. (upward in the figure). 
Namely, the sub-lattice magnetic moment of the rare-earth metal of the 
recording layer 5, referred to as R3, is parallel, that is, downward in 
the figure, to the sub-lattice magnetic moment of the rare-earth metal of 
the readout layer 3, referred to as R1. Moreover, M3 points upward in the 
figure since the recording layer 5 shows TM rich. 
The irradiation of the light beam I does not raise the temperature of the 
layers to temperatures higher than the above-described temperature. When 
the temperature of each layer falls to T1 (the condition A2) through T2 
(the condition B2) after irradiation of the light beam I, M3 still points 
in the direction .alpha. in spite of Hw since Hc3 is larger than Hw. On 
the other hand, the intermediate layer 4 shows an in-plane magnetization. 
Therefore, since the exchange-coupling force is not exerted between the 
readout layer 3 and the recording layer 5, M1 still points in the 
direction same as that of Hw, that is, an initializing direction, in spite 
of M3. This means that the direction .alpha. is recorded in M3. 
(b) In the condition C1, The light beam II having a high light intensity is 
irradiated instead of the light beam I having a low light intensity so 
that the temperature of each magnetic layer is raised to T4 (the condition 
D1). Accordingly, Hc1 becomes smaller than Hw. Moreover, since the 
temperature is not lower than Tcomp1, the sub-lattice magnetic moment of 
the transition metal of the readout layer 3 grows large; thus, the readout 
layer 3 becomes TM rich. Therefore, the sub-lattice magnetic moment R1 of 
the rare-earth metal of the readout layer 3 points reversely to the 
direction of Hw, that is, upward in the figure, and M1 points in the 
direction of Hw. 
The magnetization of the intermediate layer 4 shows an in-plane 
magnetization as shown in the figure if T4 is higher than Ti2 and not 
higher than Tr2 in this embodiment. 
On the otherhand, the magnetization of the intermediate layer 4 comes into 
the same condition as that of the readout layer 3 if T4 is not lower than 
Tcomp2 and not higher than Ti2. Namely, since it is not lower that Tcomp2, 
the sub-lattice magnetic moment of the transition metal of the 
intermediate layer 4 grows large so that the layer shows TM rich. 
Therefore, the sub-lattice magnetic moment of the rare-earth metal of the 
intermediate layer 4 points reversely to the direction of Hw, that is, 
upward in the figure, and the magnetization of the intermediate layer 4 
points in the direction of Hw. 
On the other hand, the magnetization of the recording layer 5 disappears 
because its temperature is not lower than its Curie temperature Tc3. 
In the temperature-falling process to T3 which takes place after 
irradiation of the light beam II, since the temperature becomes not higher 
than Tcomp1, the sub-lattice magnetic moment of the transition metal of 
the readout layer 3 becomes small so that the readout layer 3 becomes RE 
rich again. On the other hand, the magnetization of the intermediate layer 
4 is either in the in-plane condition as described above or in the same 
condition as that of the readout layer 3. Moreover, the magnetization of 
the recording layer 5 does not exist because its temperature is not lower 
than its Curie temperature Tc3. Therefore, R1 remains pointing reversely 
to the direction of Hw, and M1 points in the direction of R1, that is, 
reversely to that of Hw. 
In the case when the temperature falls to T3 (the condition C2), since Hc1 
is larger than Hw, directions of R1 and M1 are fixed to the 
above-described directions respectively. Therefore, the condition does not 
change from C2 to C1. On the other hand, since the intermediate layer 4 
shows a perpendicular magnetization, an exchange-coupling force is exerted 
between the readout layer 3 and the recording layer 5. Therefore, the 
magnetization direction is copied from the readout layer 3 to the 
recording layer 5 through the intermediate layer 4, and the sub-lattice 
magnetic moments of the recording layer 5 become parallel to the 
sub-lattice magnetic moments of the readout layer 3. Thus, the 
magnetization M3 of the recording layer 5 points in a predetermined 
direction .beta., (that is, downward in the figure,) reverse to .alpha.. 
Namely, the sub-lattice magnetic moment R3 of the rare-earth metal of the 
recording layer 5 becomes parallel to the sub-lattice magnetic moment R1 
of the rare-earth metal of the readout layer 3, that is, upward in the 
figure, and since the recording layer 5 is TM rich here, M3 points 
downward in the figure. 
In the case when the temperature falls to T1 (the condition A1) through T2 
(the condition B1), since Hc3 is larger than Hw, M3 stably keeps pointing 
in the above-described direction .beta. in spite of Hw. On the other hand, 
since the intermediate layer 4 shows an in-plane magnetization, the 
exchange-coupling force between the readout layer 3 and the recording 
layer 5 is no longer exerted or becomes weak. Here, since Hc1 is smaller 
than Hw, M1 is initialized to the direction of Hw. This means that the 
direction .beta. is recorded in M3. 
As described above, whether the initial condition is A1 or A2, by the light 
beam I having a comparatively low light intensity, the temperatures of the 
layers are raised according to the paths a1 and a2 and fall according to 
the paths b1 and b2 as shown in the figure to reach the condition A2, 
thereby allowing information to be recorded. On the other hand, whether 
the initial condition is A1 or A2, by the light beam II having a 
comparatively high light intensity, the temperatures of the layers are 
raised according to the paths a1, a2, and a3 and fall according to the 
paths c1, c2 and c3 as shown in the figure to reach the condition A1, 
thereby allowing information to be recorded. 
In other words, the intermediate layer 4, which is provided as described 
above, makes it possible to strengthen an exchange-coupling force so as to 
help copying when the magnetization is copied from the readout layer 3 to 
the recording layer 5, and also to weaken the exchange-coupling force so 
as to prevent copying from the recording layer 5 to the readout layer 3 
when the magnetization direction of the readout layer 3 is initialized. 
This means that the magnetization direction of the readout layer 3 is not 
changed by the recording layer 5 after recording. For this reason, as 
described above, by performing the initialization of the readout layer 3 
at the same time when recording is finished, it is not necessary to 
perform initialization again prior to recording at the beginning of the 
next recording. Consequently, it becomes unnecessary to provide a device 
for generating an initializing magnetic field that arranges the 
magnetization direction before performing optical modulation overwriting, 
and it becomes possible to prevent increases of the size and the 
manufacturing costs of the apparatus. 
Moreover, since the intermediate layer 4 controls the magnetic 
exchange-coupling force between the recording layer 5 and the readout 
layer 3 as described above, the magnetization direction of the readout 
layer 3 can be easily controlled by using an external magnetic field Hw, 
and the optical modulation overwriting property can be stabilized. 
The following description will discuss an information reproducing process 
with reference to FIG. 4. In the figure, legends such as arrows are the 
same as those in FIG. 3. 
During reproduction, different from recording, an external magnetic field 
like the recording magnetic field Hw is not applied. The light beam III 
whose light intensity is lower than that of the light beam I is irradiated 
to the magneto-optical disk 10, thereby raising the temperature of the 
layers to T2. 
Since an external magnetic field, such as the magnetic field Hw, is not 
applied at this time, the direction of M1 is determined according to the 
magnetic exchange-coupling force exerted from the recording layer 5. 
Meanwhile, although the intermediate layer 4 shows an in-plane 
magnetization, it has a predetermined amount of the perpendicular 
magnetization component because it is put between the readout layer 3 and 
the recording layer 5 which show perpendicular magnetizations. Therefore, 
the exchange-coupling force is exerted from the recording layer 5 to the 
readout layer 3 through the intermediate layer 4. Consequently, the 
sub-lattice magnetic moments of the readout layer 3 are positively 
arranged to the direction parallel to the sub-lattice magnetic moments of 
the recording layer 5 respectively; thus, the direction of M1 is 
positively determined. In other words, the sub-lattice magnetic moment R1 
of the rare-earth metal of the readout layer 3 becomes parallel, that is, 
upward in the condition A1 and downward in the condition A2, to the 
sub-lattice magnetic moment R3 of the rare-earth metal of the recording 
layer 5. Moreover, since the readout layer 3 shows RE rich, M1 points in 
the same direction as that of R1, that is, upward in the condition A1 and 
downward in the condition A2. In this way, the magnetization recorded in 
the recording layer 5 is copied to the readout layer 3 and reproduced as 
information. 
The next description will discuss various measured results in the actual 
recording and reproduction in accordance with the above-described 
processes. 
The magneto-optical disk 10 was used for recording and reproduction and 
rotated at the linear speed of 10 m/s at the position of laser beam 
irradiation. The recording magnetic field Hw of 20 kA/m was applied. The 
first laser power of the light beam I and the second laser power of the 
light beam II were set to 6 mW and 10 mW respectively, and recording was 
performed by modulating the laser powers with a frequency of 2.5 MHz. As a 
result, reversed magnetic domains were formed with a cycle of 4 .mu.m and 
a length of 2 .mu.m on the recording layer 5. 
Moreover, recorded information was reproduced at a laser power of 2 mW. The 
magneto-optical signal of 2.5 MHz was obtained from the readout layer 3 in 
accordance with the reversed magnetic domains which were formed on the 
recording layer 5, and therefore a good reproducing signal with a C/N 
ratio of 48.5 dB was obtained. 
Next, overwriting was performed by modulating the laser powers with a 
frequency of 5 MHz onto the reversed magnetic domains which were formed at 
2.5 MHz. As a result, the reversed magnetic domains, which had been formed 
at 2.5 MHz, vanished and new reversed magnetic domains were formed with a 
cycle of 2 .mu.m and a length of 1 .mu.m on the recording layer 5. 
Moreover, recorded information was reproduced at a laser power of 2 mW. The 
magneto-optical signal of 5 MHz and a C/N ratio of 47.2 dB was obtained 
from the readout layer 3 in accordance with the reversed magnetic domains 
which were formed on the recording layer 5. Namely, the signal component 
of 2.5 MHz, which was previously recorded, was lowered to the level of 8.5 
dB; thus it was confirmed that good optical modulation overwriting with an 
erasing ratio of -40 dB was performed. 
Next, the above-described measurements were performed by using the readout 
layer 3 and the intermediate layer 4, whose compositions had been changed, 
and TABLE 1 shows the results thereof. Here, the compositions of the 
readout layer 3, the intermediate layer 4 and the recording layer 5 were 
set respectively to Gd.sub.0.28 (Fe.sub.x Co.sub.1-x).sub.0.72, 
Gd.sub.0.28 (Fe .sub.y Co.sub.1-y).sub.0.72 and Dy.sub.0.20 (Fe.sub.0.78 
Co.sub.0.22).sub.0.80, and TABLE 1 shows numeric values of Tcomp1, Tr2, 
Tcomp2, Ti1 and Ti2, a C/N ratio at 2.5 MHz, and an erasing ratio in 
overwriting a newly-recorded pattern of 5 MHz onto the already-recorded 
pattern of 2.5 MHz by optical modulation. Here, the unit of X and Y are 
both %, and that of Tcomp1, Tr2, Tcomp2, Ti1, Ti2 is .degree. C., and that 
of the C/N ratio and the erasing ratio, which is referred to as ER, is dB. 
TABLE 1 
______________________________________ 
X Y Tcomp1 Tr2 Tcomp2 Ti1 Ti2 C/N ER 
______________________________________ 
65 45 250 385 -- -- -- 46.1 -10 
65 50 250 385 240 200 300 47.1 -40 
65 53 255 380 245 175 330 47.5 -41 
65 56 250 375 240 140 360 47.1 -40 
65 60 245 380 250 80 370 47.3 -35 
65 65 250 380 240 20 380 47.2 -10 
70 45 250 385 -- -- -- 46.1 -10 
70 50 250 360 260 200 300 46.5 -42 
70 53 245 365 255 175 355 45.8 -40 
70 56 245 360 255 135 360 46.2 -39 
70 60 240 355 250 85 365 46.5 -35 
70 60 240 360 260 20 385 46.4 -10 
______________________________________ 
As clearly shown in the table , if Y=65% , the intermediate layer 4 shows a 
perpendicular magnetization at room temperature; thus good optical 
modulation overwriting property was not obtained, and the erasing ratio is 
as bad as -10 dB. Moreover, if Y=45%, there is no temperature range 
wherein the intermediate layer 4 shows a perpendicular magnetization; thus 
the exchange-coupling between the recording layer 5 and the readout layer 
3 was not sufficient; the result is that good optical modulation 
overwriting property was not obtained, and that the erasing ratio is as 
bad as -10 dB. In contrast, since an intermediate layer 4, which satisfies 
the condition 45%&lt;Y&lt;65%, is provided, a good erasing ratio between -35 dB 
and -42 dB was obtained, and a stable and good optical modulation 
overwriting property was obtained. Therefore, the above-described Y needs 
to satisfy the condition 45%&lt;Y&lt;65%. 
[Second Embodiment] 
Referring to FIGS. 5 through 8, the following description will discuss 
another embodiment of the present invention. Here, for convenience of 
explanation, those members that have the same functions described in the 
aforementioned embodiment are indicated by the same reference numerals and 
the description thereof is omitted. 
The magneto-optical disk 20 (the magneto-optical recording medium) in the 
present embodiment has a similar arrangement to that of the first 
embodiment. However, a readout layer 23, which will be described below, is 
provided instead of the readout layer 3. The readout layer 23, which is an 
amorphous film composed of a rare-earth and transition metal alloy, is a 
ferri-magnetic material where the sub-lattice magnetic moment of the 
rare-earth metal and that of the transition metal are antiparallel to each 
other. The readout layer 23 is prepared by forming GdFeCo, which is a thin 
film composed of a rare-earth and transition metal alloy with a thickness 
of 40 nm. 
The readout layer 23 has a composition of Gd.sub.0.28 (Fe.sub.0.6 0 
Co.sub.0.40).sub.0.72 and a compensation temperature of 250.degree. C., 
and its magnetization condition is in-plane at room temperature, 
perpendicular at not lower than about 80.degree. C., and in-plane at not 
lower than about 350.degree. C. 
The magneto-optical disk 20 was made by a similar method to that of the 
first embodiment 1. 
Next, FIG. 5 shows conceptually a magnetic property of the magneto-optical 
disk 20 in the present embodiment that is provided with the 
above-described layers. 
The readout layer 23 shows an in-plane magnetization at room temperature T1 
and a perpendicular magnetization at not lower than a temperature Tr1, and 
has its compensation temperature Tcomp1 in the vicinity of the Curie 
temperature Tc3 of the recording layer 5. Moreover, the readout layer 23 
has its Curie temperature at a temperature Tr2, or alternatively, shows an 
in-plane magnetization at temperatures not lower than the temperature Tr2. 
The intermediate layer 4 and the recording layer 5 are respectively the 
same as those in the first embodiment. 
Moreover, the following definitions are made: T3 indicates a temperature 
not lower than Ti1, not higher than Tcomp1, and not higher than Tcomp2. T4 
indicates a temperature not lower than Tc3, not lower than Tcomp1, not 
lower than Tcomp2, and not higher than Tr2. Hw indicates a recording 
magnetic field which is a magnetic field externally applied during 
recording. Hc1 and Hc3 respectively indicate the coercive force of the 
readout layer 23 and that of the recording layer 5. Tr1 is lower than Ti1. 
Moreover, the coercive forces Hc1 and Hc3 and the temperatures T1, T3, and 
T4 satisfy the conditions: 
when the temperature is T1, Hc1&lt;Hw and Hw&lt;Hc3; 
when the temperature is T3, Hc3&lt;Hw and Hw&lt;Hc1; and 
when the temperature is T4, Hc1&lt;Hw. 
Referring to FIG. 6, the next description will discuss a recording process 
by which optical modulation overwriting onto the magneto-optical disk 20 
is carried out. In the figure, T1, T3, and T4 are the temperatures as 
described above. Temperature T2 is an arbitrary temperature between Tr1 
and Ti1. The temperature becomes higher in the order of T1, T2, T3, and 
T4. A1, A2, . . . , B1, B2, . . . indicate respective conditions shown in 
the figure. Each condition shows the states of the readout layer 23, the 
intermediate layer 4, and the recording layer 5 at the highest box, the 
middle box, and the lowest box, respectively. In each layer, a black arrow 
indicates the direction of the sub-lattice magnetic moment of the 
rare-earth metal, and a white arrow indicates the direction of the total 
magnetization. In the magneto-optical 20, at room temperature, the readout 
layer 23 and the intermediate layer 4 are RE rich and the recording layer 
5 is TM rich. 
Two conditions A1 and A2 exist dependently on the recorded information at 
room temperature. Namely, according to the recorded information, the 
magnetization of the recording layer 5, referred to as M3 hereinafter, 
points downward in the condition A1 and upward in the condition A2. 
In the conditions A1 and A2, the recording magnetic field Hw pointing 
downward in the figure is applied from a magnetic field generating 
apparatus (not shown) onto the portion which receives the light beam on 
the magneto-optical disk 20. Here, the magnetization referred to as M1 of 
the readout layer 23 is in-plane. Moreover, the magnetization of the 
recording layer 5, referred to as M3, points in a predetermined direction, 
upward or downward, according to the recorded information. However, since 
the intermediate layer 4 shows an in-plane magnetization, M3 is not copied 
to the readout layer 23. 
Here, a light beam I having a low light intensity or a light beam II having 
a high light intensity are alternatively irradiated. The light beam I and 
the light beam II have a power which can raise the temperature of the 
layers to T3 and T4, respectively. In the temperature-raising process, 
when the temperature of each layer is raised to T2 so that each layer 
becomes the condition B1 or B2, M1 points in the direction of Hw since Hc1 
is smaller than Hw. Moreover, M3 still points in the same direction as 
above-described one. Since the intermediate layer 4 shows an in-plane 
magnetization, M3 is not copied to the readout layer 23. 
Next, information is recorded similarly to the first embodiment. However, 
different from the first embodiment, when the temperature falls to T1 (the 
conditions A1 or A2), the readout layer 23 shows an in-plane magnetization 
again. 
By providing the above-described intermediate layer 4, since the 
intermediate layer 4 controls the magnetic exchange-coupling force between 
the recording layer 5 and the readout layer 23 similarly to the first 
embodiment, the magnetization direction of the readout layer 23 can be 
easily controlled with an external magnetic field Hw, and the optical 
modulation overwriting property can be stabilized. 
The next description will discuss an information reproducing process with 
reference to FIGS. 7 and 8. In the figure, legends such as arrows are the 
same as those in FIG. 6. 
During reproduction, the light beam III, whose light intensity is lower 
than that of the light beam I, is irradiated to the magneto-optical disk 
20, thereby raising the temperature of the layers to T2. 
Here, the information recorded in the recording layer 5 is reproduced in a 
similar way to that of the first embodiment. However, since the laser beam 
8 is focused by the focusing lens 9, its light intensity distribution 
shows a Gaussian distribution so that the light intensity of the center of 
the light beam III is higher than that of the surrounding area. Therefore, 
when the light beam III is irradiated, the temperature in the vicinity of 
the center of the irradiated portion of the readout layer 23 becomes 
higher than the temperature of the surrounding area. Consequently, 
different from the first embodiment, in the readout layer 23, the 
temperature is raised to not lower than T2 so that the magnetization state 
changes from in-plane to perpendicular only at the area in the vicinity of 
the above-described center. Subsequently, the magnetization is copied from 
the recording layer 5 to the readout layer 23 only at the area in the 
vicinity of the above-described center. When the irradiation of the light 
beam III is finished, the readout layer 23 goes back to an in-plane 
magnetization. 
Therefore, it is possible to reproduce only the information from the area 
in the vicinity of the center of the readout layer 23 irradiated with the 
light beam III. Consequently, it becomes possible to prevent signal 
contamination from adjacent bits that cause noise and to positively 
reproduce a recorded bit recorded with a pitch smaller than the spot 
diameter of the light beam, thereby remarkably increasing recording 
density. 
The next description will discuss various measured results in the real 
recording and reproduction according to the above-described processes. 
The magneto-optical disk 20 was used for recording and reproducing and 
rotated at the linear speed of 10 m/s at the position of laser beam 
irradiation. The recording magnetic field Hw of 20 kA/m was applied. The 
first laser power of the light beam I and the second laser power of the 
light beam II were set to 6 mW and 10 mW respectively, and recording was 
performed by modulating the laser powers with a frequency of 5 MHz. As a 
result, the reversed magnetic domains were formed with a cycle of 2 .mu.m 
and a length of 1 .mu.m on the recording layer 5. 
Moreover, recorded information was reproduced at a laser power of 2.5 mW. 
The magneto-optical signal of 5 MHz was obtained from the readout layer 23 
according to the reversed magnetic domains which were formed on the 
recording layer 5, and a good reproducing signal with a C/N ratio of 48.5 
dB was obtained. 
Next, overwriting was performed by modulating the laser powers with a 
frequency of 10 MHz onto the reversed magnetic domains which were formed 
at 5 MHz. As a result, the reversed magnetic domains which were formed at 
5 MHz vanished and new reversed magnetic domains were formed with a cycle 
of 1 .mu.m and a length of 0.5 .mu.m on the recording layer 5. 
Moreover, recorded information was reproduced at a laser power of 2.5 mW. 
The magneto-optical signal of 10 MHz and a C/N ratio of 47.8 dB were 
obtained from the readout layer 23 according to the reversed magnetic 
domains which were formed on the recording layer 5. In short, the signal 
component of 5 MHz, which was previously recorded, was lowered to the 
level of 8.5 dB, so it was confirmed that good optical modulation 
overwriting with an erasing ratio of -40 dB was performed. 
Next, the above-described measurement was performed by changing the 
compositions of the readout layer 23 and the intermediate layer 4, and 
TABLE 2 shows the result. That is, the compositions of the readout layer 
23, the intermediate layer 4, and the recording layer 5 were set 
respectively to Gd.sub.0.28 (Fe.sub.x Co.sub.1-x).sub.0.72, Gd.sub.0.28 
(Fe.sub.y Co.sub.1-y).sub.0.72, and Dy.sub.0.20 (Fe.sub.0.78 
Co.sub.0.22).sub.0.80, and TABLE 2 shows numeric values of Tcomp1, Tr2, 
Tcomp2, Ti1 and Ti2, a C/N ratio at 10 MHz, and an erasing ratio in 
overwriting in which optical modulation makes a newly-recorded pattern of 
10 MHz onto the already-recorded pattern of 5 MHz. Here, the unit of X and 
Y are both %, that of Tcomp1, Tr2, Tcomp2, Ti1 and Ti2 is .degree. C., and 
that of the C/N ratio and the erasing ratio, referred to as ER, is dB. 
TABLE 2 
______________________________________ 
T- T- 
X Y comp1 Tr1 Tr2 comp2 Ti1 Ti2 C/N ER 
______________________________________ 
60 45 250 80 365 -- -- -- 47.3 -10 
60 50 250 85 365 240 200 300 47.8 -40 
60 53 255 70 360 240 175 330 47.2 -41 
60 56 250 75 365 240 140 345 48.0 -40 
60 60 245 80 360 245 80 360 48.5 -9 
57 45 250 100 345 -- -- -- 47.5 -10 
57 50 255 105 355 260 200 300 47.3 -42 
57 53 250 100 350 255 175 355 47.8 -40 
57 56 245 100 350 245 135 360 46.8 -39 
57 60 240 95 360 250 85 365 46.9 -9 
______________________________________ 
As clearly shown in the table, if Y=60%, the lowest temperature Ti1, where 
the intermediate layer 4 shows a perpendicular magnetization, is not 
higher than the lowest temperature Tr1, where the readout layer 23 shows a 
perpendicular magnetization. Therefore, a good optical modulation 
overwriting property is not obtained, and the erasing ratio is as bad as 
-9 dB. Moreover, if Y=45%, there is no temperature range wherein the 
intermediate layer 4 shows a perpendicular magnetization so that the 
exchange-coupling between the recording layer 5 and the readout layer 23 
was not sufficient; the result was that a good optical modulation 
overwriting property was not obtained, and that the erasing ratio is as 
bad as -10 dB. On the other hand, by providing the intermediate layer 4 
which satisfies the condition 45%&lt;Y&lt;60%, a good erasing ratio from -35 dB 
to 42 dB was obtained, and a stable and good optical modulation 
overwriting property was obtained. Therefore, the above-described Y needs 
to satisfy the condition 45%&lt;Y&lt;60%. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included 
within the scope of the following claims.