Magneto-optical recording medium having multiple magnetic layers

Magneto-optic recording medium has at least a 1st magnetic layer for recording information signals, a 2nd magnetic layer formed on the 1st magnetic layer and a 3rd magnetic layer formed on the 2nd magnetic layer. The 1st and 3rd magnetic layers are made of a first amorphous material of rare earth and 3d transition metal having perpendicular magnetic anisotropy in a direction perpendicular to a surface thereof, respectively. The 2nd magnetic layer is interposed between the 1st and 2nd magnetic layers as an intermediate layer. The 2nd magnetic layer is made of a second amorphous material of rare earth metal and 3d transition metal with rare earth metal rich (RE-rich) having a smaller exchange energy between the rare earth metal and a 3d transition metal than that of Gd--Fe or Gd--Fe--Co and having magnetic anisotropy in a direction parallel to a surface of the 2nd magnetic layer, wherein the 1st and the 3rd magnetic layers are magnetically coupled through the 2nd magnetic layer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an improvement of a magneto-optic 
recording medium capable of being written/read by irradiating a laser beam 
thereon. 
2. Description of the Related Art 
In a recording method for recording information on a recording medium where 
information marks (magnetic domain) are read out by utilizing a 
magneto-optic interaction, the information is recorded on the recording 
medium as binary information marks in such a manner that first, the 
recording medium having such a magnetic thin layer as a perpendicular 
magnetization thin layer is initialized so that a magnetization 
orientation of the magnetic thin layer is preliminarily aligned in one 
direction perpendicular to a surface of the magnetic thin layer. Then, 
magnetic domains corresponding to the information marks are formed to have 
an magnetization orientation in a reverse direction of the initial 
magnetization by spot-heating such as irradiation of a laser beam spot 
with applying the external magnetic field. 
In the recording method, before rewriting the information preliminarily 
recorded on the recording medium, it requires a considerable time to erase 
(or initialize) the information recorded. As a result, there is a problem 
that it is impossible to realize a recording of information at a high data 
transfer rate. 
As a countermeasure, in order to save the time of initialization, various 
kinds of overwriting methods without erasing information are proposed or 
put into practice. 
Among the overwriting methods, there is one so called the light intensity 
modulation direct overwriting technic, which seems promising as a high 
density recording technic combined with a magnetic super-resolution 
technic in future. 
As a basic technic of a recording medium to realize this light intensity 
modulation direct overwriting, there is one proposed in the Japanese 
Patent Laid-open Publication 62-175948/87. 
In the magneto-optic recording method disclosed therein, there is employed 
a recording medium having a laminated structure of first and second 
magnetic thin films of rare earth-transition metal. The outline of 
operation of the magnetic recording is as follows. 
A first elevated temperature state by heating the first magnetic thin film 
up to a temperature T1 higher than Curie temperature Tc of the first 
magnetic thin film under a first external magnetic field without causing 
inversion of the sublattice magnetization of the second magnetic thin 
film; and a second elevated temperature state by heating the second 
magnetic thin film up to a temperature T2 higher than the temperature T1 
and high enough to invert the sublattice magnetization of the second 
magnetic thin film under the first external magnetic field, which two 
temperature states are respectively conditioned corresponding to the 
digital information "0" or "1" to be recorded. 
In cooling process, recorded marks (magnetic domains) corresponding to the 
digital information "0" and "1" are formed on the first magnetic thin 
layer corresponding to the magnetization orientation of the sublattice of 
the second magnetic thin layer based on the exchange bonding force between 
the 1st and 2nd thin layers due to exchange interaction. Then, only the 
magnetization orientation of the sublattice of the second magnetic thin 
layer is inverted in one direction at room temperature by applying a 
second external magnetic field. Thereby, the direct overwrite can be 
realized. 
FIG. 1 is a schematic view for explaining the magnetization states of 1st 
and 2nd magnetic layers in response to a change of temperature from room 
temperature to recording temperature upon the recording operation or the 
rewriting operation. 
Next, the description is given of the detailed operation of the 
magneto-optic recording method in the prior art referred to FIG. 1. 
In FIG. 1, a magneto-optic disc (referred to as a "disc") comprises at 
least a 1st magnetic thin layer as a recording layer, which exhibits high 
coercivity at room temperature and has low reverse magnetization 
temperature, and a second magnetic thin layer as a reference layer, which 
exhibits low coercivity at room temperature and has higher reverse 
magnetization temperature. Both the layers have perpendicular 
magnetization. The disc is assumed to be rotated in a certain direction 
and a laser beam (not shown) is irradiated to provide heat to the 1st and 
2nd layers of the disc so as to raise the temperature T1 or T2 
corresponding to a low level of digital information or a high level 
thereof. Further, upon a recording or rewriting operation, an initial 
external magnetic field Hini and an external recording magnetic field Hex 
are always applied to the disc in directions as shown with arrows. Thus, 
the external recording magnetic field Hex is applied to the 1st and 2nd 
magnetic layers heated at the low temperature T1 or at the high 
temperature T2 by the laser beam during a rotation of the disc. 
Thereafter, the initial external magnetic field Hini is applied to the 1st 
and 2nd magnetic layer cooled down at room temperature during the rotation 
of the disc. 
As initial states, there are two cases, a state A and a state B. 
In the state A as the initial state, the 1st magnetic layers is assumed to 
be magnetized in a direction shown with an arrow "a" in response to a 
digital signal of a low level "0". The 2nd magnetic layer is magnetized in 
a direction shown with an arrow "a" as mentioned hereinafter. 
In the state B as another initial state, the 1st magnetic layer is 
magnetized in a direction shown with an arrow "b" in response to a digital 
signal of a high level "1", and the 2nd magnetic layer is magnetized in 
the direction "a". It should be noted that the 2nd magnetic layer is 
always magnetized in the direction shown with the arrow "a" after passing 
through the initial external magnetic field Hini in the direction of "a". 
As an elevated temperature state, there are two cases, a state C 
corresponding to the lower temperature T1 and a state D corresponding to 
the higher temperature T2. 
In the state C, the magnetization of the 1st magnetic layer vanishes at the 
lower temperature T1 higher than Curie temperature Tc1 of the 1st magnetic 
layer by being irradiated with the laser beam, but the 2nd magnetic layer 
remains in the initial magnetization even when the external recording 
magnetic field Hex shown with an arrow "b" is applied to the 2nd magnetic 
layer. 
In the state D, the magnetization of the 1st magnetic layer vanishes at the 
higher temperature T2 which is higher than the lower temperature T1 by 
being irradiated with the laser beam, and the magnetization orientation of 
the 2nd magnetic layer is changed to a direction shown with the arrow "b" 
due to the external recording magnetic field Hex in the direction "b". 
As interim states during the rotation of the disc, there are two cases, a 
state A1 which is changed from the state C and a state E which is changed 
from the state D. These states A1 and E are realized to be cooled down 
below Curie temperature Tc1 during the rotation of the disc. 
In the state E, the 1st magnetic layer is magnetized in the direction "b" 
subjected by a magnetic field ("b") of the 2nd magnetic layer in a 
temperature range lower than the Curie temperature Tc1 of the 1st magnetic 
layer as the disc is rotated to be cooled down. 
Then, as the disc further rotates, the state E is further changed into the 
state B under the initial magnetic field Hini applied to the 1st and 2nd 
magnetic layers in the direction "a" at the room temperature, wherein the 
magnetization orientation of the 2nd magnetic layer is changed 
corresponding to the direction "a" of the initial magnetic field Hini 
though the magnetization orientation "b" of the 1st magnetic layer remains 
as it is. 
In the state A1, the 1st magnetic layer is magnetized in the direction "a" 
subjected by a magnetic field ("a") of the 2nd magnetic layer in a 
temperature range lower than the Curie temperature Tc1 of the 1st magnetic 
layer as the disc is rotated to be cooled down. 
Then, as the disc further rotates, the initial magnetic field Hini is also 
applied thereto, however, the magnetization orientation thereof is the 
same as that of the 2nd magnetic layer. Thus, the state A is the same as 
the state A1. 
As well known, when the 1st and 2nd magnetic layers are facing to each 
other and they are magnetized in an opposite direction to each other as 
shown in the state B at the room temperature, an interface magnetic domain 
wall (referred to as interface wall hereinafter) is formed on an interface 
therebetween because of an exchange coupling force. 
The interface wall energy .sigma.w is represented as follows: 
EQU H.sub.w i =.sigma..sub.w /2M.sub.si h.sub.i ( 1) 
Wherein, 
H.sub.w i : the effective bias magnetic field received by an ith magnetic 
layer and caused by another magnetic layer adjacent to the ith magnetic 
layer, 
M.sub.si : the saturation magnetization of the ith magnetic layer, 
h.sub.i : the thickness of the ith magnetic layer. 
EQU .sigma..sub.w =2{(A.sub.1 .times.K.sub.1).sup.1/2 +(A.sub.2 
.times.K.sub.2).sup.1/2 } (2) 
Wherein, 
A.sub.1 : the exchange stiffness constant of the 1st magnetic layer, 
A.sub.2 : the exchange stiffness constant of the 2nd magnetic layer, 
K.sub.1 : the effective magnetic anisotropy constant of the 1st magnetic 
layer, 
K.sub.2 : the effective magnetic anisotropy constant of the 1st magnetic 
layer, 
As the condition of the overwriting, it is necessary to meet the following 
conditions so as to prevent the transition from the state A to the state B 
at room temperature. 
EQU H.sub.c1 &gt;H.sub.w 1 =.sigma..sub.w /2M.sub.s1 h.sub.1 ( 3) 
Wherein, 
H.sub.c1 : the coercivity of the 1st magnetic layer, 
H.sub.w 1 : the effective bias magnetic field received by the 1st magnetic 
layer and caused by the adjacent magnetic layer of the 1st magnetic layer, 
M.sub.s1 : the saturation magnetization of the 1st magnetic layer, 
h.sub.1 : the thickness of the 1st magnetic layer. 
Further, to prevent the transition from the state B to the state E at room 
temperature, it is necessary to meet the following condition. 
EQU H.sub.c2 &gt;H.sub.w 2 =.sigma..sub.w /2M.sub.s2 h.sub.2 ( 4) 
Wherein, 
H.sub.c2 : the coercivity of the 2nd magnetic layer, 
H.sub.w 2 : the effective bias magnetic field received by the 2nd magnetic 
layer and caused by an magnetic layer adjacent to the 2nd magnetic layer, 
M.sub.s2 : the saturation magnetization of the 2nd magnetic layer, 
h.sub.2 : the thickness of the 2nd magnetic layer. 
Further, in the state E, to prevent the polarity inversion of the 1st 
magnetic layer by the initial magnetic field Hini for initialization, it 
is necessary to meet the following condition. 
EQU H.sub.c1 .+-.H.sub.107 1 &gt;H.sub.ini ( 5) 
Here, as to the positive and negative signs ".+-." of the left side, the 
positive sign "+" is employed when the 1st magnetic layer is made of a 
rare earth metal rich (referred to as RE-rich) layer and the 2nd magnetic 
layer is made of a transition metal rich (referred to as TM-rich) layer, 
or an opposite case to the above, i.e., in the case of an antiparallel 
coupling. The negative sign "-" is employed when both the 1st and 2nd 
magnetic layers are made of the RE-rich layers, or the TM-rich layers, 
i.e., in the case of parallel coupling. 
On the other hand, it is necessary to meet the following condition so as to 
allow the transition from the state E to the state B. 
EQU H.sub.c2 +H.sub.w 2 &lt;H.sub.ini ( 6) 
Further, to allow the transition from the state C to the state A at about 
the Curie temperature Tc1 of the 1st magnetic layer, i.e., to allow the 
magnetization orientation of the 1st magnetic layer to be arranged in the 
magnetization orientation of the 2nd magnetic layer, it is necessary to 
meet the following condition. 
EQU H.sub.w 1 &gt;H.sub.c1 +H.sub.ex ( 7) 
Wherein, Hex: the external recording magnetic field. 
As seen from the above description, in order to satisfy the formula (3) and 
(4) at the room temperature, the interface wall energy .sigma..sub.w is 
desirable to be smaller. 
At the temperature where the magnetization orientation of the 1st magnetic 
layer is arranged in the magnetization direction of the 2nd magnetic 
layer, the interface wall energy .sigma..sub.w needed to be maintained at 
a state that the amount of the interface wall energy .sigma..sub.w is not 
changed or less changed if any, under a condition satisfying the formula 
(7). 
However, it was impossible to realize all the states mentioned above by 
only controlling the coercivity values, the saturation magnetization 
values and the thicknesses of the 1st and 2nd magnetic layers. 
In the magneto-optic readout method, the magnetic reversal portion 
corresponding to an information bit, which forms a magnetic domain, is 
read out based on magnetic Kerr effect. Thus, in order to improve the 
recording density, it is necessary to decrease a length of a recorded 
information bit, i.e., to diminish an area of the magnetic domain. 
However, as well known, a reproduction resolution of signal is practically 
determined by a wavelength .lambda. of a light source and a numerical 
aperture of an objective lens used in an optical pickup system, and its 
limitation of the readout is determined by 2NA/.lambda.. 
Thus, in order to increase the recording density, it is considered to 
decrease the wavelength .lambda. of the light source and/or to employ an 
objective lens having a high numerical aperture NA to allow a diameter of 
a laser spot to be diminished. However, a wavelength of the laser beam in 
a practical use is only 680 nm. Further, when the objective lens having 
the high numerical aperture NA is employed, a focal length thereof becomes 
short. This requires a high precision distance control between the lens 
and an optical disc, resulting in a severe precision of the optical disc 
in production. 
Accordingly, it is impossible to employ such an objective lens having the 
high numerical aperture NA. The numerical aperture NA of the objective 
lens to be practically used is at most 0.6. In other words, there is a 
limitation to improve the recording density by using the wavelength 
.lambda. of the light source and the numerical aperture NA of the 
objective lens. 
As a countermeasure for solving the problem in improving the recording 
density restricted by such readout conditions, there are proposed a method 
for readout signals and a detected medium used for the same, for instance, 
in Japanese Patent Laid-open Publication 6-150418/1994. 
In this readout method, the exchange interaction between a readout layer 
(first magnetic layer) and a recording layer (second magnetic layer)is 
utilized in such a manner that the information magnetic domain recorded on 
the recording layer is copied to the readout layer, and the information is 
detected from the readout layer. 
The structure of such a recording medium is as follows. 
The magneto-optic recording medium comprises a transparent substrate, a 
readout layer (a first magnetic layer) formed on the transparent substrate 
and a recording layer (a second magnetic layer) formed on the readout 
layer. The readout layer behaves the in-plane magnetization parallel to 
the surface thereof at room temperature. However, as the temperature 
rises, the in-plane magnetization of the readout layer is changed into the 
perpendicular magnetization. The recording layer has a function to 
magneto-optically record information thereon. The recording layer is made 
of a rare earth-transition metal alloy. 
The information is recorded on the magneto-optic recording medium as 
follows. 
Under a constant magnetic field applied to the medium to allow the 
recording layer to be magnetized, a magnetization orientation of the 
recording layer is reversed by being irradiated with a laser beam in such 
a manner that the power of the laser beam is selectively set to either a 
first laser power which is relatively lower laser power, or a second laser 
power which is relatively higher laser power responsive to the recording 
signals. 
The information recorded on the recording layer is reproduced as follows. 
When the laser beam is irradiated on the medium, there is generated a 
gradient of temperature in the diameter range of the laser beam spot on 
the readout layer. The readout layer which shows the in-plane 
magnetization at the room temperature, turns to the perpendicular 
magnetization in an area corresponding to a high temperature area in the 
gradient of temperature, and the orientation of the perpendicular 
magnetization is arranged in the direction parallel to the magnetization 
of the recording layer. Thereby, the information is read out. 
However, the material forming the readout layer in the magneto-optic 
recording medium mentioned in the foregoing has a wide temperature range 
for allowing the magnetization transition from the in-plane to the 
perpendicular magnetization. This wide temperature range causes a problem 
that it is inadequate to read out information with a high S/N (C/N) from 
the area information magnetic domain of the recording layer smaller than 
an area corresponding to the diameter of the laser beam spot irradiated on 
the readout layer. 
On the other hand, according to the present invention, as the material 
forming the readout layer (the first magnetic layer), there is employed an 
alloy having a small exchange energy between the rare earth metal and the 
3d transition metal. Thereby, it is possible to reduce the temperature 
range for allowing the magnetization transition from the in-plane to the 
perpendicular magnetization, resulting in realizing the magnetically 
induced super resolution readout for the high density recorded information 
recorded on the area information magnetic domain of the recording layer 
smaller than the area corresponding to the diameter of the laser beam spot 
with excellent S/N (C/N) compared with ordinary ones. 
SUMMARY OF THE INVENTION 
Accordingly, a general object of the present invention is to provide a 
magneto-optic recording medium, in which the above disadvantages have been 
eliminated. 
A specific object of the present invention is to provide a magneto-optic 
recording medium having at least a 1st magnetic layer for recording 
information signals, a 2nd magnetic layer formed on the 1st magnetic layer 
and a 3rd magnetic layer formed on the 2nd magnetic layer, the 
magneto-optic recording medium comprising: the 1st and 3rd magnetic layers 
each being made of a first amorphous material of rare earth and 3d 
transition metal having perpendicular magnetic anisotropy in a direction 
perpendicular to a surface thereof, respectively; and the 2nd magnetic 
layer interposed between the 1st and 3rd magnetic layers as an 
intermediate layer, the 2nd magnetic layer being made of a second 
amorphous material of a rare earth metal and a 3d transition metal with 
rare earth metal rich (RE-rich), the second amorphous material having a 
smaller exchange energy between the rare earth metal and the 3d transition 
metal than that of one selected from a group of Gd--Fe and Gd--Fe--Co and 
having in-plane magnetic anisotropy in a direction parallel to a surface 
of the 2nd magnetic layer, wherein the 1st and the 3rd magnetic layers are 
magnetically coupled through the 2nd magnetic layer. 
Another and more specific object of the present invention is to provide a 
magneto-optic recording medium comprising: a first magnetic layer formed 
on a transparent substrate for reading information recorded on a second 
magnetic layer, the first magnetic layer having an easy axis of 
magnetization in a direction parallel to a surface thereof at a room 
temperature, the easy axis of magnetization being changed into a 
perpendicular direction from the in-plane direction when heated up to a 
predetermined temperature higher than the room temperature, the first 
magnetic layer being made of a rare earth and a 3d transition metal in 
which an exchange energy between the rare earth metal and the 3d 
transition metal is made to be small, and a second magnetic layer provided 
on the first layer for magneto-optically recording information and 
transferring the information to the first magnetic layer for reading out 
the information, the second magnetic layer being made of a rare earth 3d 
transition metal alloy having an easy axis of magnetization in a direction 
perpendicular a surface thereof at a room temperature. 
Other objects and further features of the present invention will be 
apparent from the following detailed description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Description is now given of first to third embodiments of magneto-optic 
recording mediums according to the present invention referring to FIGS. 2 
to 16, wherein the like reference characters as shown in FIG. 1 denote 
like or corresponding parts throughout the drawings, and detailed 
descriptions of the like parts are omitted for simplicity except for new 
parts employed. 
First embodiment of a magneto-optic recording medium! 
FIG. 2 is a sectional view for explaining a structure of a magneto-optic 
recording medium of the present invention. 
Referring to FIG. 2, a magneto-optic recording medium 1 of the present 
invention comprises a transparent base 2 made of a glass plate or a 
polycarbonate plate, a dielectric layer 3 as a protection layer or a 
multiple interference layer provided on the transparent base 2, 1st, the 
thin films of 2nd and 3rd magnetic layers (referred to as magnetic layer) 
4, 5, 6, which are stacked in this order by using sputtering method, and a 
protection layer 7 made of a non-magnetic metal or a dielectric layer 
provided on the 3rd magnetic layer 6. 
The laser beam is irradiated from an upper surface of the transparent base 
2 as shown with an arrow A. 
Each of the 1st and 3rd magnetic layers 4, 6 is composed of an amorphous 
magnetic layer of a rare earth-3d transition metal having a perpendicular 
magnetic anisotropy, wherein the 1st magnetic layer 4 has a function as 
the recording layer and the 3rd layer 6 has a function of the reference 
layer as mentioned in the foregoing. 
The 2nd magnetic layer 5 as an intermediate layer interposed between the 
first and third magnetic layers 4, 6 is made of a rare earth-3d transition 
metal layer in which the rare earth metal is rich compared to the 3d 
transition metal (referred to as RE-rich layer hereinafter). The 2nd 
magnetic layer 5 of the RE-rich of the present invention is constructed to 
have a small exchange energy between the rare earth metal and the 
transition metal. The effective reduction of the exchange energy 
therebetween can be attained, for instance, by adding other metals such as 
Bi and Sn to the 2nd magnetic layer as mentioned hereinafter. Further, the 
2nd magnetic layer 5 has in-plane magnetic anisotropy in the direction 
parallel to the surface of the 2nd magnetic layer 5. 
Specifically, the 1st magnetic layer 4 is made of, for instance, Tb--Fe 
(terbium-iron alloy), or Tb--Fe--Co (terbiumiron-cobalt alloy). The 3rd 
magnetic layer 6 is made of, for instance, Dy--Fe--Co 
(dysprosium-iron-cobalt alloy). 
The 2nd magnetic layer 5 as the intermediate layer is made of the RE(rare 
earth metal)-rich layer, for instance, Gd--Fe (gadolinum-iron alloy) or 
Gd--Fe--Co (gadolinum-iron-cobalt alloy). 
Specifically, in a composition formula represented by Gd.sub.x (Fe.sub.1-y 
--Co.sub.y).sub.1-X, "x" and "y" denote an atom containing ratio, 
respectively, and are defined as follows: 
0.32 .ltoreq.x.ltoreq.0.50, 0.ltoreq.y.ltoreq.0.3 (x and y denote an atom 
containing ratio, respectively). 
Further, other metals such as Bi (bismuth) and Sn (tin) are added to the 
Gd--Fe--Co to reduce the exchange energy between the rare earth metal and 
3d transition metal. 
Next, a description is given of an important feature of the temperature 
dependence of the interface wall energy developed between the 1st and 3rd 
magnetic layers and effective control method of the interface wall energy. 
Here, the 2nd magnetic layer 5 of the present invention is provided as an 
intermediate layer interposed between the 1st and 3rd magnetic layers 4, 6 
so as to control the exchange coupling force between the 1st and 3rd 
magnetic layers 4, 6. 
FIG. 3 is a graph showing temperature dependence of interface wall energy 
(.sigma.w ) developed between 1st and 3rd magnetic layers with respect to 
either case where a RE-rich layer or a TM (3rd transition metal)-rich 
layer is employed as a 2nd magnetic layer in the magneto-optic recording 
medium, and temperature dependence of Bloch wall energy (.sigma.B) of both 
the 1st and 3nd magnetic layer. 
The values of interface wall energy and Bloch wall energy are calculated on 
the basis of a molecular field theory by using parameters determined from 
the experimental values of magnetization temperature characteristics and 
effective magnetic anisotropy energy (Ku-2.pi.Ms.sup.2) vs temperature 
characteristics with respect to each of the magnetic layers (1st to 3rd 
magnetic layers) used in the magneto-optic recording medium. The effective 
magnetic anisotropy energy (Ku-2.pi.Ms.sup.2) is represented as a 
difference between the intrinsic magnetic anisotropy energy Ku and the 
demagnetizing field energy 2.pi.Ms.sup.2 (Ms: saturation magnetization). 
In FIG. 3, an ordinate represents the interface wall energy (erg/cm.sup.2) 
or the Bloch wall energy (erg/cm.sup.2), and an abscissa represents 
absolute temperature (K) (referred to as temperature). Further, in FIG. 3, 
reference numerals 10, 11 denote the temperature dependence curves of 
Bloch wall energy .sigma.B developed in the 1st and 3rd magnetic layers 4, 
6, respectively, and 12, 13 temperature dependence curves of the interface 
wall energy developed between the 1st and 3rd magnetic layers 4, 6 when 
either of the RE-rich and TM-rich layers is employed as the 2nd magnetic 
layer 5. 
As seen from the formulas (3), (4), (5) and (6) mentioned in the foregoing, 
the interface wall energy developed between the 1st and 3rd magnetic layer 
is desirable to be smaller at the room temperature. Thus, as shown in the 
curves 12, 13 in FIG. 3, the interface wall energy between the 1st and 3rd 
magnetic layers 4, 6 is respectively made to be as small as 1.2 
erg/cm.sup.2 at the room temperature in either case employing the RE-rich 
or the TM-rich layer as the 2nd magnetic layer 5. 
As seen from FIG. 3, as the temperature rises, the interface wall energy 
therebetween is gradually decreased in either case, however, a decrement 
rate of the curve 12 is smaller than that of the curve 13. This fact means 
that the curve 12 is more preferable than the curve 13 as mentioned 
hereinafter. 
FIG. 4 is a graph showing temperature dependence of magnetization of the 
RE-rich layer of Gd--Fe and the TM-rich layer of Gd--Fe either of which is 
used as the 2nd magnetic layer in the magneto-optic recording medium. 
In FIG. 4, an ordinate represents magnetization (emu/cm.sup.3) and an 
abscissa represents temperature (K), and a reference character 14 denotes 
a temperature dependence curve of magnetization of the TM-rich layer of 
Gd--Fe and, 15 the magnetization vs temperature characteristic curve of 
the RE-rich of Gd--Fe. 
FIG. 5 is a graph showing temperature dependence of the intrinsic magnetic 
anisotropy energy (Ku) and of the effective magnetic anisotropy energy 
(Ku-2.pi.Ms.sup.2) with respect to the RE-rich layer of Gd--Fe and TM-rich 
layer of Gd--Fe either which is used as the 2nd magnetic layer in the 
magneto-optic recording medium. 
In FIG. 5, an ordinate represents the intrinsic magnetic anisotropy energy 
Ku (10.sup.5 erg/cm.sup.3) or the effective magnetic anisotropy energy 
Ku-2.pi.Ms.sup.2 (10.sup.5 erg/cm.sup.3) and an abscissa represents 
temperature (K), and reference characters 16, 17 respectively denote a 
temperature dependence curve of the intrinsic magnetic anisotropy energy 
(Ku) of the RE-rich layer of Gd--Fe and that of the TM-rich layer of 
Gd--Fe, and 18, 19 a temperature dependence curve of the effective 
magnetic anisotropy energy (Ku-2.pi.Ms.sup.2) of the RE-rich layer of 
Gd--Fe and that of the TM-rich layer of Gd--Fe. 
Next, a description is given of a reason why the curve 12 shown in FIG. 3 
is preferable compared with the curve 13 shown therein, referring to FIG. 
3. 
As mentioned in the foregoing, from a viewpoint of the magnetic copy 
performance from the 3rd magnetic layer 6 to the 1st magnetic layer 4, it 
is preferable that as the temperature rises, a decrement rate of the 
interface wall energy between the 1st and 3rd magnetic layer 4, 6 is 
lessened. This fact is understood from the formula (7) taking account of 
formula (1), i.e., a formula (7') derived therefrom as follows. 
EQU Hw1=.sigma.w /2M.sub.si h.sub.i &gt;Hc1+Hex (7') 
wherein a symbol .sigma.w denotes the interface wall energy between the 
1st and 3rd magnetic layers 4, 6. 
As seen from the formula (b 7'), it is desirable that the value of .sigma.w 
is larger than the room temperature. 
Referring to FIG. 3, when comparing the curves 12 with the curve 13, the 
curve 12 is more slowly decreased than the curve 13, as the temperature 
rises. 
Accordingly, it can be said that when employing the RE-rich layer as the 
2nd magnetic layer 5 it is possible to perform a better magnetic copy from 
the 3rd magnetic layer 6 to the 1st magnetic layer 4 than when employing 
the TM-rich layer as the 2nd magnetic layer 5. 
Further, as shown as the curves 18, 19 in FIG. 5, as the temperature rises, 
the effective magnetic anisotropy energy Ku-2.pi.Ms.sup.2 of the RE-rich 
of Gd--Fe (curve 18) employed as the 2nd layer 5 rapidly increases 
compared with that of the TM-rich layer of Gd--Fe (curve 19) (It should be 
noted that the curve 19 of the TM-rich layer of Gd--Fe droops downward). 
Taking account of a fact that an interface wall energy is generally 
expressed as a sum of the effective magnetic anisotropy energy and the 
exchange energy, it is said that a difference of the effective magnetic 
anisotropy energy between the RE-rich layer and the TM-rich layer 
responsive to a change of the temperature causes the difference of the 
interface wall energy between the two cases employing the RE-rich layer or 
the TM-rich layer as the 2nd magnetic layer 5, which is shown in FIG. 3. 
Further, referring to the curves 14, 15 shown in FIG. 4, as the temperature 
rises, the magnetization of the RE-rich layer of Ge--Fe is largely 
decreased compared with that of the TM-rich layer of Gd--Fe. (It should be 
noted that the curve 14 of the TM-rich layer swells upward). Thus, It can 
be said that as shown in FIG. 3, this difference of magnetization between 
the RE-rich layer and the TM-rich layer causes the difference of the 
interface wall energy between the two cases where the TM-rich layer or the 
RE-rich layer is employed as the 2nd magnetic layer 5, because, as seen 
from the temperature dependence curves 16, 17, 18 and 19 in FIG. 5, the 
intrinsic magnetic anisotropy energy (Ku) of the RE-rich layer (the curve 
16) is changed at the same rate as that (the curve 17) of the TM-rich 
layer responsive to the change of the temperature, but the effective 
magnetic anisotropy energy (Ku-2.pi.Ms.sup.2) of the RE-rich layer (the 
curve 18) is differently changed from that of the TM-rich layer (the curve 
19) responsive to the change of temperature. 
Taking account of the fact that an interface wall energy is expressed as a 
sum of effective magnetic anisotropy energy and exchange energy, and the 
effective magnetic anisotropy energy (Ku-2.pi.Ms.sup.2) is related to the 
demagnetizing field energy due to the saturation magnetization Ms as 
mentioned in the foregoing, the difference of magnetization between the 
RE-rich layer and the TM-rich layer responsive to the change of 
temperature causes the difference of the interface wall energy between the 
two cases where the RE-rich or TM-rich layers is employed as the 2nd 
magnetic layer 5. 
In other words, the temperature dependence curve 15 of the RE-rich layer 
shown in FIG. 4 is more preferable than the curve 14 of the TM-rich layer 
when they are employed as the 2nd magnetic layer 5, i.e., the 
magnetization of the 2nd magnetic layer 5 is more desirable to be 
decreased as the temperature rises. 
FIG. 6 is a graph showing coercivity Hc vs temperature characteristics of 
the 1st magnetic layer of Tb--Fe and the 3rd magnetic layer of Dy--Fe--Co 
either of which is employed in the magneto-optic recording medium. 
In FIG. 6, the ordinate represents coercivity Hc (kOe) and the abscissa 
represents temperature (K). Reference number 20 denotes the coercivity vs 
temperature characteristic curve of the 1st magnetic layer 4 of Tb--Fe and 
21 that of the 3rd magnetic layer 6 of Dy--Fe--Co layer. 
As seen from a comparison between the curves 20, 21 in FIG. 6, a 
temperature range capable of the magnetic copy from the 3rd magnetic layer 
6 of Dy--Fe--Co to the 1st magnetic layer 4 of Tb--Fe is from 370K to 
420K, wherein the coercivity Hc of the 3rd magnetic layer 6 is larger than 
that of the 1st magnetic layer 4 for the corresponding temperature. 
In order to discuss the stability of the interface wall within the 
temperature range mentioned above, the angle of the RE magnetic moment 
from the film plane as a function of the thickness direction in the 
magneto-optic recording medium 1 having the 1st, 2nd and 3rd magnetic 
layers 4, 5, 6 are shown in FIGS. 7 and 8. 
FIG. 7 is a graph showing the angle of the RE magnetic moment from the film 
plane as a function of the thickness direction in the magneto-optic 
recording medium with respect to Gd composition x in a composition 
notation of (Gd.sub.x --Fe.sub.1-x) when a TM-rich layer of Gd--Fe is used 
as the 2nd magnetic layer. 
In FIG. 7, the ordinate represents the angle of the rare earth moment from 
the film plane and the abscissa represents the thicknesses of the 1st, 2nd 
and 3rd magnetic layers 4, 5, 6. 
Specifically, the 1st magnetic layer 4 is made of Tb--Fe, the 2nd magnetic 
layer 5 is made of a TM-rich layer of Gd--Fe and the 3rd magnetic layer 6 
is made of Dy--Fe--Co. The thickness of the 2nd magnetic layer 5 of Gd--Fe 
is made to be 150 angstrom. 
Further, in each curve of X=0.15, X=0.18, X=0.19, X=0.203, X=0.21 and 
X=0.24, X denotes an atom containing ratio of Gd atom in the composition 
notation of (Gd.sub.x Fe.sub.1-x) in the TM-rich layer of Gd--Fe employed 
as the 2nd magnetic layer 5. 
FIG. 8 is a graph showing the angle of RE magnetic moment in the 
magneto-optic recording medium with respect to Gd composition in the 
composition notation of (Gd.sub.x Fe.sub.1-x) when an RE-rich layer of 
Gd--Fe is used as the 2nd magnetic layer. 
In FIG. 8, the ordinate represents the angle of the of rare earth moment 
from the film plane and the abscissa represents the thicknesses of the 
1st, 2nd and 3rd magnetic layers 4, 5, 6 as mentioned in the foregoing. 
In each curve of X=0.27, X=0.315, X=0.33, X=0.36, X=0.39 and X=0.41, X 
denotes an atom containing ratio of Gd atom in the composition notation of 
(Gd.sub.x Fe.sub.1-x) in the RE-rich layer of Gd--Fe employed as the 2nd 
magnetic layer 5. 
In FIGS. 7 and 8, the angle of the RE magnetic moment is defined such that 
the magnetic moment orientation parallel to the film plane is made to be 
zero, and a certain magnetization orientation perpendicular to the film 
plane is made to be +90.degree. and an opposite orientation is made to be 
-90.degree.. It is considered that most of the interface wall is present 
in the second magnetic layer 5 in a stable state because a main magnetic 
moment inversion is generated in the 2nd magnetic layer 5. 
Referring to FIG. 7, in the magneto-optic recording medium 1 employing the 
2nd magnetic layer 5 having the Gd atom containing ratio of 24 at % 
(X=0.24) of the 2nd magnetic layer 5, the center of the interface wall is 
considered to shifted to the 1st magnetic layer 4 from the 2nd magnetic 
layer 5. 
Similarly, as seen from FIG. 8, in the magneto-optic recording medium 1 
employing the 2nd magnetic layer 5 having the Gd atom containing ratio of 
27 at % (X=0.27) of the 2nd magnetic layer 5, the center of the interface 
wall is shifted to the 1st magnetic layer from the 2nd magnetic layer 5. 
FIG. 9 is a graph showing temperature dependence of the interface wall 
energy between the 1st magnetic layer of Tb--Fe layer and the 3rd magnetic 
layer of Dy--Fe--Co when the TM-rich layer of Gd--Fe is employed as the 
2nd magnetic layer in the magneto-optic recording medium, wherein an 
atomic containing ratio X of Gd atom in the composition notation of the 
(GdxFex-1) in the 2nd magnetic layer is varied. 
In the curve of Gd.sub.0.15 Fe.sub.085 shown in FIG. 9, (Gd.sub.0.15 
Fe.sub.0.25) denotes x=0.15 (atomic containing ratio of Gd) in the 
(GdxFe1-x) formula in the Gd--Fe layer (TM-rich) employed as the 2nd 
magnetic layer 5. 
Similarly, (Gd.sub.0.18 Fe.sub.0.82) denotes x=0.18, (Gd.sub.0.19 
Fe.sub.0.81) x=0.19, (Gd.sub.0.21 Fe.sub.0.79)x=0.21, and (Gd.sub.0.24 
Fe.sub.0.76)x=0.24. 
FIG. 10 is a graph showing the temperature dependence of the interface wall 
energy between the 1st magnetic layer of Tb--Fe layer and the 3rd magnetic 
layer of Dy--Fe--Co when the RE-rich layer of Gd--Fe is employed as the 
2nd magnetic layer in the magneto-optic recording medium, wherein an 
atomic containing ratio X of Gd atom in the composition notation of the 
(GdxFex-1) in the 2nd magnetic layer is varied. 
In the curve of Gd.sub.0.27 Fe.sub.0.73 shown in FIG. 10, (Gd.sub.0.27 
Fe.sub.0.73 ) denotes x=0.27 (atomic containing ratio of Gd) in the 
(GdxFe1-x) formula in the Gd--Fe layer (RE-rich) employed as the 2nd 
magnetic layer 5. 
Similarly, (Gd.sub.0.30 Fe.sub.0.70) denotes x=0.30, (Gd.sub.0.33 
Fe.sub.0.67) x=0.33, (Gd.sub.0.36 Fe.sub.0.64) x=0.36, (Gd.sub.0.39 
Fe.sub.0.61) x=0.39 and (Gd.sub.0.41 Fe.sub.0.59) x=0.41, respectively. 
As seen from FIGS. 9 and 10, in the magneto-optic recording medium 1, two 
kinds of mediums, each having the 2nd magnetic layer 5 of Gd atom 
containing ratio of x=0.24 or x=0.27, have an interface wall energy 
equivalent to an Bloch wall energy .sigma.B of the 1st magnetic layer 4 at 
the temperature of 395K. 
Specifically, as shown in FIG. 9, the interface wall energy of the 2nd 
magnetic layer 5 having a Gd composition of (Gd.sub.0.24 Fe.sub.0.76) 
coincides with the Bloch wall energy of the 1st magnetic layer 4 at the 
temperature of 395K. 
Further, as shown in FIG. 10, the interface wall energy of the 2nd magnetic 
layer 5 having a Gd composition of (Gd.sub.0.27 Fe.sub.0.73) coincides 
with the Bloch wall energy .sigma.B of the 1st magnetic layer 4 at the 
temperature of 395K. 
As a result, it is considered that in the magneto-optic recording medium 1 
employing the 2nd magnetic layer 5 having the composition of (Gd.sub.0.24 
Fe.sub.0.76) or (Gd.sub.0.27 Fe.sub.0.73) nearly equivalent to the 
compensation composition of the 2nd magnetic layer 5, the interface wall 
energy thereof is considered to permeate into the 1st magnetic layer 4, 
resulting in that the center of the wall is shifted to the 1st magnetic 
layer 4 from the second magnetic layer 5. 
As mentioned in the foregoing, the interface wall energy .sigma.w can be 
represented by the formula (2). 
EQU .sigma.w =2{(A.sub.1 K.sub.1)1/2+(A.sub.2 K.sub.2)}.sup.1/2(2) 
Wherein, 
A.sub.1 : the exchange stiffness constant of the 1st magnetic layer, 
A.sub.2 : the exchange stiffness constant of the 3rd magnetic layer, 
K.sub.1 : the effective magnetic anisotropy constant of the 1st magnetic 
layer, 
K.sub.2 : the effective magnetic anisotropy constant of the 3rd magnetic 
layer. 
Thus, the interface magnetic wall is considered to be shifted to a side 
where a product of the A and the K is smaller. 
Accordingly, it is possible to make the interface wall stable by reducing 
the perpendicular magnetic anisotropy energy of the 2nd magnetic layer 5. 
FIG. 11 is a graph showing the effective magnetic anisotropy energy vs 
temperature characteristics with respect to Gd--Fe components of the 
Gd--Fe layers (RE-rich) employed as the 2nd magnetic layer. 
In the curve of Gd.sub.0.27 Fe.sub.0.73 shown in FIG. 11, (Gd.sub.0.27 
Fe.sub.0.73) denotes x=0.27 (atomic containing ratio of Gd) in the 
composition notation of (Gd.sub.x Fe.sub.1-x) in the Gd--Fe layer 
(RE-rich) employed as the 2nd magnetic layer 5. 
Similarly, the curve of (Gd.sub.0.30 Fe.sub.0.70) denotes x=0.30, 
(Gd.sub.0.316 Fe.sub.0.684) x=0.316, (Gd.sub.0.3 Fe.sub.0.7) x=0.3, 
(Gd.sub.0.36 Fe.sub.0.64) x=0.36, (Gd.sub.0.39 Fe.sub.0.61) x=0.39 and 
(Gd.sub.0.41 Fe.sub.0.59) x=0.41, respectively. 
As seen from FIG. 11, a composition range where the effective anisotropy 
energy of the 2nd magnetic layer 5 becomes negative in the temperature 
range of 370K to 420K capable of magnetic copy, i.e., a composition range 
where the the 2nd magnetic layer 5 has the in-plane magnetic anisotropy, 
is more than 32 at % of Gd in the composition of the 2nd magnetic layer 5. 
In this composition range of Gd, most of the interface wall is present in 
a stable state in the 2nd magnetic layer 5. 
As mentioned above, in the magneto-optic recording medium 1 comprising the 
1st and 3rd magnetic layers of amorphous rare earth-3d transition metal 
having a perpendicular magnetic anisotropy and the 2nd magnetic layer 
interposed between the 1st and 3rd magnetic layers, the first and the 
third magnetic layers are magnetically coupled through the 2nd magnetic 
layer being laminated, and the 2nd magnetic layer provided for controlling 
the coupling of the 1st and 3rd magnetic layer is made of a amorphous rare 
earth-3d transition metal of the RE-rich in which an exchange energy 
between the rare earth metal and the 3d transition metal is smaller than 
that of the TR-rich, and the 2nd magnetic layer has an in-plane magnetic 
anisotropy in a direction parallel to the surface thereof at least at 
temperature of more than 100.degree. C. These features contributes to 
improve the read-write characteristics of the magneto-optic recording 
medium. 
Second embodiment! 
As mentioned in the first embodiment, the interface wall energy vs 
temperature is largely affected by the magnetization vs temperature 
characteristics of the intermediate layer as the 2nd magnetic layer 5. 
Therefore, the interface wall energy vs temperature characteristics can be 
improved by controlling the temperature dependence of the saturation 
magnetization of the intermediate layer. 
Specifically, it is effective to control the exchange energy between the 
rare earth metal and the 3d transition metal, i.e., the exchange energy 
between Gd atom and Fe atom in the intermediate layer of Gd--Fe. 
For instance, when such elements as Bi and Sn are added to the GdFe 
intermediate layer, the compensation temperature of the GdFe layer is 
reduced, resulting from a reduction of the exchange energy between Gd and 
Fe atoms. 
FIG. 12 is a graph showing magnetization vs temperature characteristic of 
the Gd--Fe layers with respect to the RE-rich and the RE-rich layer of 
which exchange energy between Gd atom and Fe atom is reduced employed as 
the 2nd magnetic layer. 
In FIG. 12, a reference numeral 22 designates a magnetization vs 
temperature characteristic curve of the Gd--Fe layer (RE-rich) having the 
exchange energy J.sub.GdFe of -1.7.times.10.sup.-15 erg between Gd and Fe 
atoms, and 23 a magnetization vs temperature characteristic curve of the 
Gd--Fe layer (RE-rich) having the exchange energy J.sub.GdFe of 
-1.2.times.10.sup.-15 erg between Gd and Fe atoms. 
FIG. 13 is a graph showing the intrinsic magnetic anisotropy energy (Ku) 
and effective magnetic anisotropy energy (Ku-2.pi.Ms.sup.2) vs temperature 
characteristics of the Gd--Fe layers with respect to the RE-rich layer and 
the RE-rich layer of which exchange energy between Gd atom and Fe atom is 
reduced, either of which Gd--Fe layers is used as the 2nd magnetic layer 
in the magneto-optic recording medium 1. 
In FIG. 13, the ordinate represents the intrinsic magnetic anisotropy 
energy Ku (10.sup.5 erg/cm.sup.3) or the effective magnetic anisotropy 
energy Ku-2.pi.Ms.sup.2 (10.sup.5 erg/cm.sup.3) and the abscissa 
represents the temperature (K). Reference character 24 denotes an 
intrinsic magnetic anisotropy energy (Ku) vs temperature characteristic 
curve of the Gd--Fe layers (RE-rich) having exchange energy J.sub.GdFe of 
-1.7.times.10.sup.-15 erg between Gd atom and Fe atom, 25 an intrinsic 
magnetic anisotropy energy (Ku) vs temperature characteristic curve of the 
Gd--Fe layers (RE-rich) having exchange energy J.sub.GdFe of 
-1.2.times.10.sup.-15 erg between Gd and Fe atoms, 26 an effective 
magnetic anisotropy energy (Ku-2.pi.Ms.sup.2) vs temperature 
characteristic curve of the Gd--Fe layers (RE-rich) having exchange energy 
J.sub.GdFe of -1.7.times.10.sup.-15 erg between Gd atom and Fe atom, and 
27 an effective magnetic anisotropy energy (Ku-2.pi.Ms.sup.2) vs 
temperature characteristic curve of the Gd--Fe layers (RE-rich) having 
exchange energy J.sub.GdFe of -1.2.times.10.sup.-15 erg between Gd and Fe 
atoms. 
As seen from comparison between the curve 22 (exchange energy J.sub.GdFe of 
-1.7.times.10.sup.-15 erg) and the curve 23 (exchange energy J.sub.GdFe of 
-1.2.times.10.sub.-15 erg) in FIG. 12, and comparison between the curve 23 
and the curves 14, 15 in FIG. 4, the magnetization of the curves 23 
rapidly decreases compared with those of curves 14, 15, 23, as the 
temperature rises. 
Further, as seen from comparison between the curve 27 (exchange energy 
J.sub.GdFe of -1.2.times.10.sup.-15 erg) and the curve 26 (exchange energy 
J.sub.GdFe of -1.7.times.10.sup.-15 erg) in FIG. 13, and comparison 
between the curve 27 and the curves 18, 19 in FIG. 5, the curves 27 of the 
effective magnetic anisotropy energy Ku-2.pi.Ms.sup.2 of rapidly decreases 
compared with those of curves 18, 19, 26, as the temperature rises. 
These features are effective to improve the read-write characteristics of 
the magneto-optic recording medium as mentioned in the embodiment 1. 
FIG. 14 is a graph showing interface wall energy vs temperature 
characteristics of the Gd--Fe layers with respect to the RE-rich layer and 
the RE-rich layer of which exchange energy between Gd atom and Fe atom is 
reduced, either of which layers is used as the 2nd magnetic layer in the 
magneto-optic recording medium 1. 
In FIG. 14, a reference character 28 denotes an interface wall energy vs 
temperature characteristic curve of the RE-rich layer employed as the 2nd 
magnetic layer where the exchange energy between Gd atom and Fe atom is 
made to be -1.7.times.10.sup.-15 erg. It should be noted that the curve 28 
is identical to the curve 12 shown in FIG. 3. Reference numeral 29 denotes 
the one where the exchange energy between Gd atom and Fe atom is made to 
be -1.2.times.10.sup.-15 erg. Further, the reference numerals 30, 31 
denote Bloch wall energy .sigma.B vs temperature characteristic curves of 
the 1st magnetic layer (Tb--Fe) 4 and the 3rd magnetic layer (Dy--Fe--Co) 
6, which are identical to ones shown in FIG. 3 
As seen from comparison between the curves 28, 29 in FIG. 14, the interface 
energy of the curve 29 of which exchange energy Is reduced to 
-1.2.times.10.sup.-15 erg, decreases more gradually degraded compared with 
the curve 28 of which exchange energy is -1.7.times.10.sup.-15 erg as the 
temperature rises. 
Thus, these features suggest that the magnetic copy performance of the 
magneto-optic recording medium is more improved, as seen from the 
description in the embodiment 1. 
Specifically, the condition of magnetic copy is determined by the formula 7 
as mentioned in the foregoing. 
From the formula 7, it is desirable for (Hc1-Hw1)&lt;0 and the absolute value 
of (Hc1-Hw1) is larger, wherein Hc1 represents the coercivity of the 1st 
magnetic layer 4 and Hw1 represents the effective bias magnetic field 
which the 1st magnetic layer is received from an adjacent layer. 
FIG. 15 is a graph showing (Hc1-Hw1) vs temperature characteristic curves 
of the 2nd magnetic layer with respect to the TM-rich layer and the 
RE-rich layer shown in FIG. 3 and the Re-rich layer of which exchange 
energy is reduced to -1.2.times.10.sup.-15 erg. 
In FIG. 15, the ordinate represents Hc1-Hw1 (kOe) and the abscissa 
represents the temperature (K). Reference numeral 26 denotes a (Hc1-Hw1) 
vs temperature characteristic curve when the 2nd magnetic layer is made of 
the TM-rich layer having the exchange energy J.sub.GdFe of 
-1.7.times.10.sup.-15 erg, 27 is the one when the 2nd magnetic layer is 
made of the RE-rich layer having the exchange energy J.sub.GdFe of 
-1.7.times.10.sup.-15 erg, and 28 is the one when the 2nd magnetic layer 
is made of a RE-rich layer having the exchange energy J.sub.GdFe of 
-1.2.times.10.sup.-15 erg. 
As seen from FIG. 15, the 2nd magnetic layer 5 shown with the reference 
numeral 28 shows the largest absolute value of Hc1-Hw1 within the negative 
range thereof among the three 2nd magnetic layers. Thus, it is considered 
that the recording medium having the 2nd magnetic layer shown with the 
reference character 28 is the best among the three recording mediums in 
magnetic copy performance. 
Similarly, the recording medium having the 2nd magnetic layer 5 shown with 
the reference character 27 is considered to be better than that shown with 
the reference numeral 26 in the magnetic copy performance. 
As a result, it is possible to improve the read-write characteristic of the 
triple-layered magneto-optic recording medium by employing the 2nd 
intermediate magnetic layer which is made of a RE-rich amorphous layer 
having Gd as the rare earth metal and Fe or Fe--Co as the 3rd transition 
metal and has the in-plane magnetic anisotropy at the room temperature and 
has a small exchange energy between the rare earth metal and the 3rd 
transition metal by adding such elements as Bi and Sn therein. 
According to the magneto-optic recording medium of the present invention, 
it is possible to suppress the interface magnetic wall energy to a small 
value at the room temperature and maintain it large at the temperature T 
by employing the 2nd magnetic layer having a moderate thickness interposed 
between the 1st and 2nd magnetic layers, wherein the 2nd magnetic layer is 
made of the rare earth-transition metal of RE-rich having the in-plane 
magnetic anisotropy at the room temperature and is added with other metals 
such as Bi, Sn. Thereby, it is possible to improve the stability of the 
magnetic wall at the room temperature and to improve the stability of the 
magnetic copy performance of the written magnetic domain at the 
temperature T1. 
Further, according to the present invention, it is possible to provide not 
only a magneto-optic recording medium capable of realizing the improvement 
of the light intensity modulation direct overwriting but also technics 
applicable to improvements and control of the exchange coupling force in 
the magnetic supper-resolution multilayer for the high density recording 
in future, and essential technics applicable to expansion and improvements 
of the magnetic read-write characteristics using the exchange coupling 
multilayer. 
Third embodiment! 
As mentioned in the first embodiment, the temperature characteristics of 
the interface wall energy between the 1st and the 3rd magnetic layers 4, 6 
is largely depended on the temperature characteristics of magnetization of 
the 2nd magnetic layer 5. 
In the third embodiment, the rare earth metal such as Er (erbium), Ho 
(holmium) is used as the 2nd intermediate magnetic layer (intermediate 
layer) 5 to improve the temperature dependence of the magnetization 
effectively. 
FIG. 16 is a graph showing magnetization vs temperature characteristic 
curves of five kinds of alloy thin layers, each composed of the 3d 
transition metal such as Fe (iron) and rare earth metal such as Gd 
(gadolinium), Tb (terbium), Dy (dysprosium), Ho (holmium) and Er (erbium). 
In order to improve the temperature characteristics of the interface wall 
energy, it is effective to reduce the magnetization thereof rapidly as the 
temperature rises. 
As seen from FIG. 16, in order to reduce the magnetization of the thin film 
rapidly as the temperature rises, it is effective to employ an Er--Fe thin 
layer or an Ho--Fe thin layer compared with those of Gd--Fe, Tb--Fe, and 
Dy--Fe thin layer as the intermediate layer 5. Thereby, a magneto-optic 
recording medium having an excellent read-write characteristic can be 
realized by employing the Er--Fe or Ho--Fe thin layer as the 2nd magnetic 
layer 5. 
Further, it is possible to add such elements as Bi, Sn and Lu to such 
Er--Fe or Ho--Fe thin layer so as to reduce the exchange energy between 
the rare earth metal and the 3d-transition metal. Thereby, further 
improvement of read-write characteristic of the magneto-optic recording 
medium will be expected as mentioned in the foregoing. 
According to the magneto-optic recording medium of the third embodiment in 
the present invention, it is possible to suppress the interface magnetic 
energy to a small value at the room temperature and maintain it large with 
increasing temperature by employing the 2nd magnetic layer having a 
moderate thickness interposed between the 1st and 2nd magnetic layers, 
wherein the 2nd magnetic layer is made of the RE-rich layer having the 
in-plane magnetic anisotropy at room temperature such as Ho--Fe, 
Ho--Fe--Co, Er--Fe, Er--Fe--Co and is added with other metals such as Bi, 
Sn. Thereby, it is possible to improve the stability of the interface 
magnetic wall at the room temperature and to improve the magnetic copy 
performance the written magnetic domain at the temperature T1. 
Next, the description is given of the magneto-optic recording mediums of 
the fourth to the seventh embodiments of the present invention referring 
to FIGS. 17 to 27. 
FIG. 17 is a schematic view for explaining a structure of a magneto-optic 
recording medium employed in the fourth to the seventh embodiments of the 
present invention. 
FIG. 18 is a graph showing magnetization vs temperature characteristic 
curves of materials used in the first magnetic layer for comparison; 
FIG. 19 is a graph showing an intrinsic magnetic anisotropy energy vs 
temperature characteristic curve and the demagnetizing field energy vs 
temperature curve of a material of Gd--Fe used in the first magnetic layer 
for comparison; 
FIG. 20 is a graph showing an intrinsic magnetic anisotropy energy vs 
temperature characteristic curve and a demagnetizing field energy vs 
temperature characteristic curve of a material of Gd--Fe--Co--Bi of the 
present invention; 
FIG. 21 is a graph showing intrinsic magnetic anisotropy energy vs 
temperature characteristic curves and demagnetizing field energy vs 
temperature curves of materials used as the first magnetic layer for 
comparison; 
FIG. 22 is a graph showing the remanent Kerr rotation angle of a 
magneto-optic recording medium when Gd--Fe is employed as the first 
magnetic layer and Dy--Fe--Co is employed as the second magnetic layer. 
FIG. 23 is a graph showing the remanent Kerr rotation angle of a 
magneto-optic recording medium when Gd--Fe--Co--Bi is employed as the 
first magnetic layer and Dy--Fe--Co is employed as the second magnetic 
layer; 
FIG. 24 is a graph showing magneto-optic effect vs temperature 
characteristics of magneto-optic recording mediums when a respective one 
of the Gd--Fe and Gd--Fe--Co--Bi is employed as the first magnetic layer 
(readout layer) and Dy--Fe--Co is employed as the second magnetic layer, 
wherein the magneto-optic effect is represented by the remanent Kerr 
rotation angle .theta.kr normalized by a saturation Kerr rotation angle 
.theta.k; 
FIG. 25 is a graph showing a magnetization vs temperature characteristic of 
materials used as the first magnetic layer; 
FIG. 26 is a graph showing the temperature dependence of the angle of 
magnetization direction on a final atomic plane of the first magnetic 
layer obtained from a calculation result, wherein a perpendicular 
direction to the film is made to be 0 degree, and an in-plane direction of 
the film is made to be 90 degrees; and 
FIG. 27 is a graph showing magnetization vs temperature characteristics of 
materials used as the first magnetic layer obtained by a calculation based 
on a molecular field theory. 
Next, the explanation is given of the fourth to the seventh embodiments. 
Fourth embodiment! 
Referring to FIG. 17, a magneto-optic recording medium 1a of the present 
invention comprises a transparent substrate 101, a dielectric layer 102, a 
first magnetic layer 103 (readout layer), a second magnetic layer 
(recording layer) 104, and a dielectric layer 105. They are stacked in 
this order. A protection layer 106 is provided on the dielectric layer 5, 
when necessary. The laser beam is irradiated on the surface of the 
magneto-optic recording medium 1a in a direction shown with an arrow B. 
The transparent substrate 101 is made of a glass plate or a polycarbonate 
plate. The dielectric layer 102 is made of a transparent dielectric film 
as a protection layer or multi-interference layer. The first and second 
magnetic layers 103, 104 are successively formed in vacuum by using a 
continuous sputtering method, resulting in a double layer magnetic thin 
film. 
The dielectric layer 105 is made of a non-magnetic metal layer or a 
dielectric film. The protection layer 106 is formed by using UV 
(ultraviolet-rays) curing resin on the dielectric layer 105 when 
necessary. 
The first magnetic layer 103 shows an in-plane magnetization with an easy 
axis of magnetization in a direction shown with an arrow X at the room 
temperature 300k, and as the temperature rises more than the room 
temperature, it shows a perpendicular magnetization having an easy axis of 
magnetization in a direction shown with an arrow Y. The first magnetic 
layer 103 is mainly made of a rare earth-3d transition metal alloy. 
Further, in the first magnetic layer 103, the exchange energy between the 
rare earth metal and the 3d transition metal is small. The first magnetic 
layer 103 which shows the perpendicular magnetization above the room 
temperature has reversibility to show the in-plane magnetization instantly 
when the temperature descends after the first magnetic layer 103 operates 
as a perpendicular magnetization layer. The information is read out from 
the first magnetic layer 103. Thus, it may be referred to as a readout 
layer. 
The second magnetic layer 104 is made of a rare earth-3d transition metal 
alloy having an easy axis of magnetization in a perpendicular direction 
shown with an arrow Y in the temperature range from the room temperature 
up to the Curie point. The information is recorded on the second magnetic 
layer 104. Thus, this layer may be referred to as a recording layer. 
As materials for forming the first magnetic layer 103, for instance, 
Gd--Fe--Co--Bi (gadolinium-iron-cobalt-bismuth alloy) is used. As 
materials for forming the second magnetic layer 104, for instance, 
Dy--Fe--Co (dysprosium-iron-cobalt) is used. The temperature dependence of 
the saturation magnetization for Gd--Fe, Gd--Fe--Co and Fe--Co--Bi films 
is shown in FIG. 18. 
In the FIG. 18, the abscissa represents the temperature (K), and the 
ordinate represents a magnitude of a saturation magnetization 
(emu/cm.sup.3). 
As shown in FIG. 18, in the temperature dependence of the saturation 
magnetization for Gd--Fe, Gd--Fe--Co and Gd--Fe--Co--Bi films, a magnitude 
of saturation magnetization reduces as the temperature rises. The degree 
of reduction is smaller in the order of Gd--Fe--Co--Bi, Gd--Fe and 
Gd--Fe--Co. 
It may be possible to change the temperature characteristics of Gd--Fe and 
Gd--Fe--Co by controlling an amount of Gd thereof, however, it is 
impossible to allow them to have such a rapidly descending curve as that 
of Gd--Fe--Co--Bi. 
Referring to FIGS. 19 and 20, the temperature dependence of the intrinsic 
magnetic anisotropy energy Ku and the demagnetizing field energy 
2.pi.Ms.sup.2 for Gd--Fe or Gd--Fe--Co--Bi film are shown, respectively. 
In FIGS. 19 and 20, the ordinate represents a magnitude of demagnetizing 
field energy 2.pi.Ms.sup.2 and a magnitude of an intrinsic anisotropy 
magnetic energy Ku. The unit of energy is 10.sup.5 erg/cm.sup.3. The 
abscissa represents the temperature (K). As seen from FIG. 18, the 
temperature characteristic curve of magnetization of Gd--Fe--Co--Bi 
indicates sharper drop than those of Gd--Fe and Gd--Fe--Co. 
Further, as seen from FIGS. 19 and 20, the temperature dependence of the 
demagnetizing field energy 2.pi.Ms.sup.2 of Gd--FeCo--Bi change more 
rapidly than those of Gd--Fe. 
From the above facts, it is said that the more rapidly the curve of 
saturation magnetization changes, the more rapidly the curve of 
demagnetizing field energy is changed. As an example of such a material, 
there is Gd--Fe--Co--Bi. 
Next, the description is given of examples of the magneto-optic recording 
medium 1a where Gd--Fe or Gd--Fe--Co--Bi is used as the first magnetic 
layer 103 thereof, for instance, both cases where the first magnetic layer 
103 is formed with Gd--Fe or Gd--Fe--Co--Bi. In both the cases, other 
constituting components (the transparent substrate 1, the dielectric 
layers 2 and 5, the second magnetic layer 4 and the protection layer 6) 
are formed with the same components as those shown in FIG. 17. 
The temperature dependence curves of the intrinsic magnetic anisotropy 
energy Ku and of the effective magnetic anisotropy energy Ku-2.pi.Ms.sup.2 
for the respective magneto-optic recording mediums 1a in which the first 
magnetic layer is formed by Gd--Fe or Gd--Fe--Co--Bi, are shown in FIG. 
21, respectively. 
In FIG. 21, the ordinate represents the effective magnetic anisotropy 
energy Ku-2.pi.Ms.sup.2 (10.sup.5 erg/cm.sup.3) and the abscissa 
represents the temperature (K). As shown in FIG. 21, as temperature rises, 
each intrinsic magnetic anisotropy energy Ku of Gd--Fe or Gd--Fe--Co--Bi 
changes similarly to each other and the effective magnetic anisotropy 
energy Ku-2.pi.Ms.sup.2 of Gd--Fe--Co--Bi increases rapidly compared with 
that of Gd--Fe. This fact can be explained by the temperature dependence 
of demagnetizing field energy 2.pi.Ms.sup.2 shown in FIGS. 19 and 20. 
It should be noted that such temperature dependence of the intrinsic 
magnetic anisotropy energy Ku and the effective magnetic anisotropy energy 
Ku-2.pi.Ms.sup.2 of Gd--Fe--Co--Bi is effective to improve the problem 
mentioned in the Description of the Related Arts. 
In other words, it contributes to solve such a problem that in the material 
forming the readout layer (the first layer), as the temperature range to 
allow the magnetization transition from the in-plane to the perpendicular 
magnetization is broad, it is insufficient to read out the information 
recorded on the area (information magnetic domain) much smaller than the 
diameter of the laser spot. 
Specifically, as the material for forming the readout layer (the first 
magnetic layer), it is desirable to employ an alloy of which exchange 
energy between the rare earth metal and the 3d transition metal is small. 
Thereby, the alloy has a narrow temperature range to allow the magnetic 
transition from the in-plane to the perpendicular magnetization, resulting 
in a magneto-optic recording medium capable of reading out the high 
density information recorded on the area (information magnetic domain) 
narrower than the diameter of the laser spot in the readout layer without 
degradation of S/N. This magneto-optic recording medium realizes a 
super-resolution readout. 
In FIG. 21, the temperature at which the effective magnetic anisotropy 
energy Ku-2.pi.Ms.sup.2 changes its sign from negative to positive, (i.e., 
the temperature at which the effective magnetic anisotropy energy 
Ku-2.pi.Ms.sup.2 becomes zero) is about 440 K in both cases of 
Gd--Fe--Co--Bi and Gd--Fe. 
Here, the negative sign of the effective magnetic anisotropy energy 
represents the in-plane magnetization state and the positive sign 
represents the perpendicular magnetization state. The super-resolution 
readout is realized around the temperature mentioned above. 
Thus, it is important to solve the problem mentioned in the Description of 
the Related Art that how the magnetization transition is rapidly changed 
near the temperature of 440K. 
In FIGS. 22 and 23, there are shown the temperature dependence of remanent 
Kerr rotation in the magneto-optic recording mediums 1a where Gd--Fe or 
Gd--Fe--Co--Bi is used as the first magnetic layer (readout layer), 
respectively and Dy--Fe--Co is used as the second magnetic layer. In the 
FIGS. 22 and 23, the ordinate represents remanent Kerr rotation angles 
.theta.kr (degree). 
Specifically, the first magnetic layer 103 of the magneto-optic recording 
medium 1a shown in FIG. 17 is made of Gd--Fe (FIG. 22) or Gd--Fe--Co--Bi 
(FIG. 23). 
The same components as those shown in FIG. 17 are used as other 
corresponding components (the transparent substrate 101, the dielectric 
layers 102 and 105, the second magnetic layer 104 made of Dy--Fe--Co, and 
the protection layer 106) except the first magnetic layer 103. Here, a 
thickness of the first magnetic layer 103 of Gd--Fe or Gd--Fe--Co--Bi is 
made to be 50 nm, and a thickness of the second magnetic layer 104 is made 
to be 30 nm. 
As shown in FIG. 22, in the temperature dependence curve of remanent Kerr 
rotation (degree) of the first magnetic layer 103 made of Gd--Fe in the 
magneto-optic recording medium 1a, the magneto-optic effect for reading 
out, i.e., a Kerr rotation angle, is slowly rising with respect to the 
temperature of 440K in a temperature range of 300 to 440K. 
On the other hand, as shown in FIG. 23, in the temperature dependence of 
remanent Kerr rotation angle of the first magnetic layer 103 made of 
Gd--Fe--Co--Bi in the magneto-optic recording medium 1a, the Kerr rotation 
thereof, is rapidly rising with respect to the temperature of 440K in a 
temperature range of 370 to 440K, which is narrower than the range of 300 
to 440K shown in FIG. 22. 
The difference between the characteristics shown in FIGS. 22 and 23 
suggests a further improvement of the super-resolution detection effect. 
FIG. 24 is a graph for clarifying the difference between the temperature 
dependence of the remanent Kerr rotation shown in FIGS. 22 and 23. 
In FIG. 24, the abscissa represents the temperature (K) and the ordinate 
represents .theta.kr/.theta.k which is the remanent Kerr rotation 
.theta.kr normalized by the saturation Kerr rotation .theta.k for 
comparison of readout characteristics, i.e., the temperature dependence of 
the magneto-optic effect. 
As shown in FIG. 24, a curve representing the first magnetic layer 103 
using Gd--Fe--Co--Bi shows a steep rise compared with a curve of the first 
magnetic layer 103 formed with Gd--Fe. The fact that the value of 
.theta.kr/.theta.k rapidly rises, is equivalent to that a magnetization 
transition occurs near 440K. Accordingly, it will be understood that it is 
effective to form the first magnetic layer with Gd--Fe--CoBi. 
This excellent result can be explained by an effectiveness of Bi, i.e., 
reduction of exchange integral between the rare earth metal (referred to 
as RE) and the 3d transition metal (referred to as TM). The exchange 
energy of Gd--Fe--Co is reduced by the addition of Bi, resulting in a 
rapid reduction of magnetization of Gd--Fe--Co--Bi by the rise of 
temperature. 
Generally, an amount of exchange energy between the RE and the TM in 
RE--Fe--Co alloy is represented by a formula (8) as follows. 
EQU J.sub.RE-Fe .multidot.S.sub.RE .multidot.S.sub.FE +J.sub.RE-Co 
.multidot.S.sub.RE .multidot.S.sub.Co (8) 
Wherein, 
J.sub.RE-Fe : exchange integral between RE (rare earth) and Fe 
J.sub.RE-Co : exchange integral between RE and Co 
S.sub.RE : RE spin 
S.sub.FE : Fe spin 
S.sub.Co : Co spin 
When the exchange energy between the RE and the TM is reduced, the 
sublattice magnetization of the RE supported by the sublattice 
magnetization of the TM is weakened, the RE sublattice magnetization is 
rapidly reduced as the temperature rises. As a result, it is considered 
that the change of the saturation magnetization due to the change of 
temperature is developed rapidly. 
The addition of Bi or Sn to Gd--Fe corresponds to the reduction of 
J.sub.RE-Fe in the formula (8). Accordingly, a similar result that the 
first magnetic layer 103 is made of Gd--Fe--Co--Bi, is also obtained in a 
case where the first magnetic layer 103 is made of Gd--Fe--Co--Sn. 
Fifth embodiment! 
The problem mentioned in the foregoing can be solved by reducing the 
exchange energy between the RE and the TM in the first magnetic layer 103 
of the magneto-optic recording medium 1a in the present invention. 
Furthermore, it will be understood from the formula (8) that it is possible 
to reduce the exchange energy by reducing the Fe spin of the first 
magnetic layer 103. 
The reduction of the Fe spin can be made by adding Ni and Co to the first 
magnetic layer 103. 
There is shown a result for verifying this effect in FIG. 25, wherein 
saturation magnetization vs temperature characteristics of three kinds of 
thin magnetic layers of Gd--Co--Ni, Gd--Fe (Gd-rich) and Gd--Fe (Fe-rich) 
are represented. 
With respect to the Gd--Co--Ni layer, the saturation magnetization rapidly 
reduces as the temperature rises. Accordingly, it is possible to solve the 
problem mentioned in the foregoing by employing Gd--Co--Ni as the first 
magnetic layer 103. 
Sixth embodiment! 
There is shown in FIG. 26 a calculation result for verifying the fourth and 
fifth embodiments of the present invention. 
In FIG. 26, the ordinate represents magnetization rotation angle (degree) 
on the readout surface of the first magnetic layer 103, and the abscissa 
represents the temperature (K), wherein the magnetization rotation angle 
in the perpendicular direction to the film surface is made to be zero. 
In the Gd--Fe--Co--Bi where the exchange integral between RE and TM is 
decreased, and the Gd--Co--Ni where the Fe spin is decreased, used as the 
first magnetic layer 103 in the magneto-optic recording medium 1a of the 
present invention, the magnetization transition of the first magnetic 
layer 103 is rapidly developed from the in-plane direction (90.degree.) to 
the perpendicular direction (0.degree.) compared with that of Gd--Fe--Co 
in the prior art. 
Further, these calculations of the fourth and fifth embodiments are 
performed under the temperature where the magnetization orientation of 
each of the first magnetic layers is completely aligned in the 
perpendicular direction so that differences of magnetization transition 
characteristics among the three kinds of the first magnetic layer 3 are 
made to be clear. 
Further, a result with respect to Gd--Fe--Co--Sn coincides with that of 
Gd--Fe--Co--Bi. 
Seventh embodiment! 
As well known, the larger an atomic number becomes, the smaller heavy RE 
spin becomes. 
Accordingly, it is considered that to replace Gd of Gd--Fe--Co with a heavy 
rare earth metal having an atomic number larger than that of Gd is 
effective to reduce a value of the exchange energy between RE and TM as 
well as a reduction of the exchange integral and a reduction the Fe spin. 
Changes of the saturation magnetization Ms due to a temperature change are 
calculated in cases where Gd of Gd--Fe--Co is replaced with heavy rare 
earth metal (Tb, Ho, Dy) based on the molecular field theory by using 
parameters of the Hund's rule. The results are shown in FIG. 27. 
As seen from FIG. 27, the saturation magnetization Ms of Tb--Fe--Co, 
Dy--Fe--Co and Ho--Fe--Co rapidly decreases as the temperature rises 
compared with that of Gd--Fe--Co. 
As a result, it will be understood that addition or replacement of heavy 
rare earth metal, i.e., Tb, Dy, Ho, Er, Tm, Yb, and Lu, made to the first 
magnetic layer 103 is effective to solve the problem mentioned in the 
foregoing. 
According to the magneto-optic recording medium in the present invention, 
as the materials for forming the readout layer (the first magnetic layer), 
an alloy which can realize the rapid transition from the in-plane to the 
perpendicular magnetization within a narrow temperature range due to a 
small exchange energy between the rare earth metal and the 3d transition 
metal, is employed, resulting in the magneto-optic recording medium 
capable of reading out the high density information recorded on the area 
smaller than the diameter of the laser spot, and eliminating a cross-talk 
problem when reading. Thus, it is possible to provide an magneto-optic 
recording medium as a super-resolution readout medium density with an 
improved high S/N (C/N) compared with that in the prior art.