Thermomagnetic recording method applying power modulated laser on a magnetically coupled double layer structure of perpendicular anisotropy film

Thermomagnetic recording method using a thermomagnetic recording medium having a superposed layer of magnetically coupled first and second magnetic thin films is disclosed. The magnetic thin film includes a portion where the respective magnetic moments of the first and second thin films are coupled in opposite directions to each other. The method comprises heating the superposed layer in a first heating state in which the superposed layer is heated at a temperature T.sub.1 which is higher than the Curie temperature T.sub.C1 of the first magnetic thin film and will not cause the inversion of the magnetic moment of the second magnetic thin film or in a second heating state in which the superimposed layer is heated at a temperature T.sub.2 which is higher than the Curie temperature T.sub.C1 and is high enough to cause the inversion of the magnetic moment of the second magnetic thin film by modulating heating condition according to an information signal to be recorded; and forming recording magnetization on the thermomagnetic recording medium by cooling the heated superimposed layer of the thermomagnetic recording medium. In the method, real-time overwriting information can be achieved without using a modulated external magnetic field.

BACKGROUND OF THE INVENTION 
The present invention relates to a thermomagnetic recording method using a 
laser beam to write data on a thermomagnetic recording medium, and more 
particularly relates to the method using a power modulated laser beam 
which can overwrite data without using external magnetic field modulation. 
In an optomagnetic recording method or a thermomagnetic recording method, a 
recording medium having a magnetic thin film having perpendicular 
anisotropy is initialized by magnetizing the thin film beforehand in one 
direction perpendicular to the surface of the recording medium, and then a 
bit perpendicularly magnetized in a direction opposite the initial 
direction of magnetization by local heating of the recording medium, for 
example, by laser beam irradiation to record binary information. 
In the optomagnetic recording method or the thermomagnetic recording 
method, the recorded information needs to be erased (initialization of the 
recording medium) prior to rewriting information, and hence it is 
impossible to carry out recording at a high transmission rate. Several 
overwrite systems, namely, recording systems not requiring such an 
independent erasing process preceding to rewriting, have been proposed. 
Among the thermomagnetic recording methods for overwriting system, 
prospective methods are, for example, a thermomagnetic recording method 
using an external magnetic field modulation in which the polarity of an 
external magnetic field acting on a recording medium is inverted according 
to an information signal, and a thermomagnetic recording method of a 
two-head system employing an erasing head initialize the medium prior to 
recording in addition to a recording head. In the thermomagnetic recording 
method of an external magnetic field modulation system, for example, as 
disclosed in Japanese Patent Provisional Publication (Kokai) No. 60-48806, 
to record information on an amorphous ferrimagnetic thin film recording 
medium having an easy direction of magnetization perpendicular to the 
surface of the thin film, a magnetic field of a polarity corresponding to 
an input digital signal is applied to a region on the recording medium to 
be irradiated by a heating beam. 
However, high-speed recording at a high information transmission rate by 
the thermomagnetic recording method of an external magnetic field 
modulation system requires an electromagnet capable of operating at a very 
high frequency on the order of megahertz (MHz). It is difficult to make 
such an electromagnet. Even if such an electromagnet is available, such an 
electromagnet is not capable of practical application due to its high 
power consumption rate and high heat generation. The thermomagnetic 
recording method of a two-head system has disadvantages that an additional 
head is necessary, the two heads must be spaced apart, load on the driving 
system increases to deteriorate the economic effect, and the apparatus is 
not suitable for mass production. 
OBJECT AND SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a 
thermomagnetic recording method capable of overwriting information. 
It is another object of the present invention to provide a thermomagnetic 
recording method using a power modulated laser beam without using a 
modulated external magnetic field. 
It is further object of the present invention to provide a thermomagnetic 
recording method in which, recorded information is stable upon 
preservation. 
It is still further object of the present invention to provide a 
thermomagnetic recording method in which signal reproduction can be 
achieved with high S/N ratio. 
According to the present invention, there is provided a thermomagnetic 
recording method using a thermomagnetic recording medium having a 
superposed film formed of magnetically coupled first and second magnetic 
thin films and including a portion in which the respective magnetic 
moments of the first and second magnetic thin films are coupled opposite 
to each other which comprises heating the thermomagnetic recording medium 
in a first heating state where the thermomagnetic recording medium is 
heated at a temperature T.sub.1 higher than the Curie temperature T.sub.C1 
of the first magnetic thin film and will not cause the inversion of the 
magnetic moment of the second magnetic thin film or in a second heating 
state in which the thermomagnetic recording medium is heated at a 
temperature T.sub.2 which is higher than the Curie temperature T.sub.C1 
and is high enough to cause the inversion of the magnetic moment of the 
second magnetic thin film by modulating heating condition according to an 
information signal to be recorded; and cooling the heated thermomagnetic 
recording medium to record binary information. By "modulating heating" is 
meant varying the applied heating condition in accordance with the 
information to be recorded. 
Overwriting information is achieved simply by modulating the intensity or 
duration of irradiation of a heating beam, such as a laser beam, according 
to an information signal to be recorded.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows the magnetization state of the superposed film structure 5, 
formed of a first magnetic thin film 3 and a second magnetic thin film 4 
magnetically coupled with each other. 
The superposed film structure 5 having a recording portion in which the 
respective directions of the respective magnetically coupled magnetic 
moments of a first magnetic thin film 3 and a second magnetic thin film 4 
are the same (state A) and a recording portion in which the respective 
directions of the magnetically coupled magnetic moments are opposite to 
each other (state C) is heated in a heating mode in which the laminated 
film is heated at a temperature T.sub.1 which is higher than the Curie 
temperature T.sub.C1 of the first magnetic thin film 3 and will not cause 
the inversion of the magnetic moment of the second magnetic thin film 4 
under the influence of an external magnetic field H.sub.1 or in a heating 
mode in which the laminated film is heated at a temperature T.sub.2 which 
is higher than the Curie temperature T.sub.C1 and is high enough to cause 
the inversion of the magnetic moment of the second magnetic thin film 4 
under the influence of the external magnetic field H.sub.1 by modulating 
heating condition according to an information signal to be recorded, and 
then the heated laminated film is cooled for recording magnetization of 
the thermomagnetic recording medium. 
G-1. Constitution of the Thermomagnetic Recording Medium 
The constitution of a thermomagnetic recording medium employed in the first 
embodiment of the present invention will be described briefly with 
reference to FIG. 2. A transparent dielectric film 2 serving as a 
protective film or an interference film is formed over one surface (lower 
surface, in FIG. 2) of a transparent substrate 1 such as a glass plate or 
an acrylic resin plate. A double-layer magnetic film 5 comprising a first 
magnetic thin film 3 and a second magnetic thin film 4 is formed over the 
transparent dielectric film 2. The surface (lower surface) of the 
double-layer magnetic film 5 is coated with a dielectric film 6 serving as 
a protective film. The dielectric films 2 and 6 may be omitted or the 
dielectric film 6 may be a metallic film. Further a metallic film (not 
shown) may be formed over the dielectric film 6. 
There are various possible magnetic materials for forming the first 
magnetic thin film 3 and second magnetic thin film of the double-layer 
magnetic film 5. In this embodiment, the magnetic material is assumed to 
be an amorphous alloy RE.sub.x TM.sub.1-x containing x=10 to 40 atm % of 
one or more than one of rare earth metals (RE) such as Nd, Sm, Gd, Tb, Dy 
and Ho, and 1-x=90 to 60 atm % of one or more than one of transition 
metals such as Cr, Mn, Fe, Co, Ni and Cu. The magnetic material may 
contain a small amount of elements other than those rare earth metals and 
transition metals. In such a magnetic amorphous RE-TM alloy, the magnetic 
moment of RE and that of TM are coupled antiparallel with each other 
except when RE is Nd or Sm. Accordingly, the amorphous magnetic RE-TM 
alloy is so-called ferrimagnetic material, and the total magnetization is 
the difference between the respective sublattice magnetization of RE and 
TM (addition of the respective sublattice magnetization when the direction 
of magnetic moment is taken into consideration). When RE is either Nd or 
Sm, or a mixture of Nd and Sm, the respective magnetic moments of RE and 
TM are coupled in parallel, and hence the amorphous magnetic RE-TM alloy 
is so-called ferromagnetic material. In this case, the total magnetization 
is the addition of the respective sublattice magnetization of RE and TM. 
In this embodiment, RE is Gd, Tb, Dy or Ho. 
Laser light R for recording or reproducing is projected, for example, as 
shown in FIG. 3, on the thermomagnetic recording medium 10 from the side 
of the transparent substrate 1 (FIG. 2), while the respective magnetic 
field H.sub.1 and H.sub.2 of magnets 11 and 12 are applied to the 
thermomagnetic recording medium 10 from the side of the protective 
dielectric film 6 as shown in FIG. 3 or from the side of the transparent 
substrate 1. In FIG. 3, the magnets 11 and 12 are spaced apart, however, 
as will be described hereinafter, the magnets 11 and 12 may be disposed 
adjacent to each other or the magnets 11 and 12 may be the same magnets. 
In the case of FIG. 3, the thermomagnetic recording medium 10 is a disk, 
which is driven for rotation by the spindle 16 of a driving motor 15, and 
the magnets 11 and 12 are different from each other in polarity; the 
magnets 11 and 12 may be the same in polarity, which will be described 
hereinafter. 
Possible modes of the double-layer magnetic film 5 formed by superposing 
the magnetic thin films 3 and 4 of a RE-TM alloy in a temperature range 
below the respective Curie temperatures T.sub.C1 and T.sub.C2 of the 
magnetic thin films 3 and 4 are manifested by four modes A to D as shown 
in FIG. 4. The respective easy directions of magnetization of the magnetic 
thin films 3 and 4 are supposed to be perpendicular to the surface of the 
films, namely, the magnetic thin films 3 and 4 are supposed to be 
so-called perpendicular anisotropy magnetic thin films, however, only 
either one of the magnetic thin films 3 and 4 may be a perpendicular 
anisotropy magnetic thin film. 
Referring to FIG. 4, in the modes A and B, the respective directions of the 
respective magnetic moments of the respective TMs of the first magnetic 
thin film 3 and the second magnetic thin film 4 are the same as indicated 
by arrows shown by solid lines, and those of the REs of the magnetic thin 
film 3 and the magnetic thin film 4 are the same as indicated by arrows 
shown by broken lines. In the modes C and D, the respective directions of 
the respective magnetic moments of the respective TMs of the first 
magnetic thin film 3 and the second magnetic thin film 4 are opposite to 
each other as indicated by arrows shown by solid lines in FIG. 4, and 
those of the REs of the magnetic thin film 3 and the magnetic thin film 4 
are opposite to each other as indicated by arrows shown by broken lines in 
FIG. 4, so that a region where the directions of the magnetic moment of 
the TM and the magnetic moment of RE change through an angle of 
180.degree., namely, an interface magnetic wall, is formed at the 
interface of the first magnetic thin film 3 and the second magnetic thin 
film 4. This interface magnetic wall is designated as a interface wall 7. 
An interface magnetic wall energy per unit area (.sigma.w erg/cm.sup.2) is 
stored in the interface wall 7. 
G-2. Transition of Mode of Magnetization 
Magnetic energies E.sub.A, E.sub.B, E.sub.C and E.sub.D (erg/cm.sup.2) of 
the modes A to D shown in FIG. 4 when an external magnetic field H (0e), 
is applied to the double-layer magnetic film 5 are expressed approximately 
by Zeeman energy and the interface wall energy density .sigma..sub.w in 
erg/cm.sup.2 unit as follows 
##EQU1## 
where M.sub.s1 and M.sub.s2 are the saturation magnetizations M.sub.s 
(emu/cm.sup.3) of the magnetic thin films 3 and 4, respectively, and 
h.sub.1 and h.sub.2 are the respective thicknesses (cm) of the magnetic 
thin films 3 and 4. The saturation magnetization M.sub.s is obtained by 
subtracting the sublattice magnetization M.sub.TM of TM (transition metal) 
from the sublattice magnetization M.sub.RE of RE (rare earth metal). The 
saturation magnetization M.sub.s is defined generally by 
EQU M.sub.s .tbd..vertline.M.sub.RE -M.sub.TM .vertline. 
However, in the present invention, the saturation magnetization M.sub.s is 
defined by 
EQU M.sub.s .tbd.M.sub.RE -M.sub.TM 
Accordingly, when M.sub.RE .gtoreq.M.sub.TM or M.sub.RE &lt;M.sub.TM, M.sub.s 
.gtoreq.0, or M.sub.s &lt;0, respectively. Further, the respective 
rectangular ratios, i.e., the ratio of M.sub.s to H.sub.c, of the magnetic 
thin films 3 and 4 are 1 (one), and the direction of the external magnetic 
field H.sub.1 indicated by an arrow in FIG. 4 is supposed to be a positive 
direction. However, in practice, the rectangular ratio need not 
necessarily be 1 (one). Although the magnetic energies of the modes A to D 
are determined approximately on the basis of Zeeman energy and the 
magnetic domain wall energy density ow, more strictly, the stray magnetic 
field of the adjacent bits must be taken into consideration, however, the 
stray magnetic field is not taken into consideration herein. 
When the coercive force of the first magnetic thin film or magnetic field 
to invert the magnetization of the first magnetic thin film 3 is H.sub.c1 
(0e), and that of the second magnetic thin film 4 is H.sub.c2, the energy 
necessary for inverting the magnetization of the first magnetic thin film 
3, namely, the coercive force energy E.sub.1 (erg/cm.sup.2), and the 
energy necessary for inverting the magnetization of the second magnetic 
thin film 4, namely, the coercive force energy E.sub.2 (erg/cm.sup.2), are 
expressed by 
EQU E.sub.1 =2.vertline.M.sub.s1 .vertline.h.sub.1 H.sub.c1 
EQU E.sub.2 =2.vertline.M.sub.s2 .vertline.h.sub.2 H.sub.c2 
To change the mode of magnetization from a mode i (i=A to D) to a mode j 
(j=A to D, j.noteq.i) E.sub.i -E.sub.j =E.sub.ij must be greater than the 
coercive force energy (E.sub.1, E.sub.2 or E.sub.1 +E.sub.2). For example, 
to change the mode of magnetization from the mode A to the mode B, an 
inequality: 
EQU E.sub.AB =E.sub.A -E.sub.B &gt;E.sub.1 +E.sub.2 
must be satisfied. Accordingly, 
EQU -2M.sub.s1 h.sub.1 H-2M.sub.s2 h.sub.2 H&gt;2.vertline.M.sub.s1 
.vertline.h.sub.1 H.sub.c1 +2.vertline.M.sub.s2 .vertline.h.sub.2 H.sub.c2 
G-3. Change of mode of Magnetization According to Temperature Variation-(1) 
Change of the mode of magnetization of the magnetic thin films of the 
double-layer magnetic film 5 with temperature variation caused by laser 
irradiation or the like will be described with reference to FIG. 1. 
Suppose that a recording bit of the double-layer magnetic film 5 of the 
thermomagnetic recording medium lo is in the mode A (FIG. 1) at room 
temperature T.sub.R, and the bit of the double-layer magnetic film 5 in 
the mode A is irradiated by laser light for recording. The intensity of 
the laser light or the duration of laser irradiation is controlled 
according to a recording signal to heat the double-layer magnetic film 5 
selectively to a first temperature T.sub.1 or to a second temperature 
T.sub.2. The first temperature T.sub.1 is higher than the Curie 
temperature T.sub.c1 of the first magnetic thin film 3 and is a 
temperature at which the inversion of magnetization of the second magnetic 
thin film 4 will not occur when the second magnetic thin film 4 is 
subjected to the influence of an external magnetic field H.sub.1, while 
the second temperature T.sub.2 is higher than the first temperature 
T.sub.1 and is a temperature high enough to cause the inversion of 
magnetization of the second magnetic thin film 4 when the second magnetic 
thin film 4 is subjected to the influence of the external magnetic field 
H.sub.1. That is, an area around the bit heated by the laser light is 
under the influence of the external magnetic field H.sub.1 of the magnet 
11, which is strong enough to invert the magnetization of the second 
magnetic thin film 4 at the temperature T.sub.2. 
Upon cooling the double-layer magnetic film 5 to the temperature T.sub.c1 
after the same has been heated to such a temperature, a spontaneous 
magnetization appears in the first magnetic thin film 3. The external 
magnetic field Hi, and the saturation magnetization M.sub.s1 and thickness 
h.sub.1 of the first magnetic thin film 3 are determined selectively to 
meet an inequality: 
EQU .sigma..sub.w &gt;2.vertline.M.sub.s1 .vertline.h.sub.1 .vertline.H.sub.1 
.vertline. 
at the temperature T (near the temperature T.sub.c1) where a spontaneous 
magnetism appears in the first magnetic thin film 3 to make the exchange 
energy acting between the two magnetic thin layers dominant rather than 
the Zeeman energy in determining the direction of the magnetism of the 
first magnetic thin film 3. Accordingly, when the temperature T of the 
double-layer magnetic film 5 coincides with the temperature T.sub.c1, the 
double-layer magnetic film 5 is in the mode A or the mode B where the 
respective directions of magnetization of the first and second magnetic 
thin films are the same. When the heating temperature is T.sub.1, the mode 
A is established upon cooling. And, when the heating temperature is 
T.sub.2, the mode B is established, upon cooling. 
As shown in FIG. 3, the magnet 12 applies an external magnetic field 
H.sub.2 meeting conditions which will be described in G-4 to the 
thermomagnetic recording medium 10 at a temperature near the room 
temperature T.sub.R to invert the magnetism of the second magnetic thin 
film 4, so that the mode C (FIG. 4) is established in the double-layer 
magnetic film 5. 
When a recording bit in the mode C of the double-layer magnetic film 5 of 
the thermomagnetic recording medium 10 is heated to a temperature above 
the temperature T.sub.c1, the magnetization of the first magnetic thin 
film 3 disappears and the same state of magnetization as that of a 
recording bit in the mode A in the initial state heated to the temperature 
above the temperature T.sub.c1 is established in the recording bit. 
Accordingly, the mode A is established as the recording bit is cooled 
after being heated to the temperature T.sub.1, while the mode B is 
established in the recording bit cooled after being heated to the 
temperature T.sub.2. Thus, a mode of recording magnetization according to 
the temperature T.sub.1 or T.sub.2 is obtained. As described hereinafter, 
a recording bit in the mode B is changed into a recording bit in the mode 
C at least prior to the successive overwriting operation to turn the 
direction of magnetization of the second magnetic thin film 4 in the 
direction of magnetization of the second magnetic thin film in the mode A. 
A recording bit in which the respective directions of the magnetically 
coupled magnetic moments of the first magnetic thin film 3 and second 
magnetic thin film 4 of the double-layer magnetic film 5, namely, a 
recording bit in the mode A, and a recording bit in which the respective 
directions of magnetically coupled magnetic moments of the same are 
opposite to each other, namely, a recording bit in the mode C, are heated 
to the temperature T.sub.1 or T.sub.2 according to information signals by 
regulating the heating condition to establish new mode of magnetization 
for recording magnetization regardless of the initial mode of 
magnetization, and thereby overwrite is achieved. 
G-4. Conditions for Overwrite-(1) 
Conditions for overwrite will be described hereinafter. 
In changing the temperature T of the double-layer magnetic film 5 under the 
influence of the magnetic field H.sub.1 as shown in FIG. 1, conditions for 
restraining the mode A from changing to the other modes in the temperature 
range from the room temperature T.sub.R to a temperature below the Curie 
temperature T.sub.c1 of the first magnetic thin film 3 (T.sub.R 
.ltoreq.T&lt;T.sub.c1) are expressed by inequalities: 
##EQU2## 
and conditions for restraining the mode C from changing to the other modes 
are expressed by inequalities: 
EQU 2M.sub.s1 h.sub.1 H.sub.1 +.sigma..sub.w &lt;2.vertline.M.sub.s1 
.vertline.h.sub.1 H.sub.c1 
EQU -2M.sub.s2 h.sub.2 H.sub.1 +.sigma..sub.w &lt;2.vertline.M.sub.s2 
.vertline.h.sub.2 H.sub.c2 
A condition for restraining the inversion of sublattice magnetization of 
the second magnetic thin film 4 while the temperature T of the 
double-layer magnetic film 5 is in the range from a temperature above the 
temperature T.sub.c1 to a temperature below the second temperature T.sub.2 
(T.sub.c1 &lt;T&lt;T.sub.2) is expressed by an inequality: 
EQU .vertline.H.sub.1 .vertline.&lt;H.sub.c2 
and a condition for causing the inversion of the sublattice magnetization 
of the second magnetic thin film 4 when the temperature T of the 
double-layer magnetic film 5 is above the temperature T.sub.2 is expressed 
by an inequality: 
EQU .vertline.H.sub.1 .vertline.&gt;H.sub.c2 
In cooling the double-layer magnetic film 5 after heating the same to such 
a temperature, a condition for allowing the direction of the magnetization 
of the first magnetic thin film 3 is determined by the exchange coupling 
with the direction of the magnetization of the second magnetic thin film 
4, upon the reduction of the temperature of the recording bit of the 
double-layer magnetic film 5 of the thermomagnetic recording medium 10 
approximately to the Curie temperature T.sub.c1 of the first magnetic thin 
film 3 (T is approximately equal to T.sub.c1) so that 
EQU .sigma..sub.w &gt;2.vertline.M.sub.s1 .vertline.h.sub.1 .vertline.H.sub.1 
.vertline. 
and conditions for restraining the mode A from changing to the other modes 
in the temperature range of T.sub.R .ltoreq.T&lt;T.sub.c1 are the same as 
those in the heating process, while conditions for restraining the mode B 
from changing to the other modes are 
##EQU3## 
Conditions for restraining the mode A from changing to the other modes when 
the external magnetic field is H.sub.2 (down as shown in FIG. 1) at room 
temperature are 
##EQU4## 
and conditions for causing the mode B to change into the mode C (by 
applying the upwardly directed H.sub.2 field as shown in FIG. 1) are 
##EQU5## 
Thus the use of a two-layer magnetic film 5 which meets all the foregoing 
conditions enables overwrite. 
In the foregoing description, the variation of mode of magnetization has 
been explained with reference to the modes A, B and C, however, overwrite 
is possible when the modes A, B and D are employed when the direction of 
magnetization of the second magnetic thin film 4 is to be turned in a 
direction opposite to that described hereinbefore. That is, in such a 
case, the modes A, B, C and D correspond to the modes B, A, D and C of the 
foregoing embodiment, respectively, and the saturation magnetization Ms is 
defined by 
EQU Ms.tbd.M.sub.TM -M.sub.RE 
to apply the foregoing conditions for overwrite without change. 
G-5. State of Magnetization for Reproduction and Preservation-(1) 
To enable overwrite, it is necessary, as mentioned hereinbefore, that the 
directions of magnetization in the second magnetic thin film 4 are the 
same regardless of the signal recording condition (the direction of 
magnetization of the first magnetic thin film 3), and there are recording 
bits in which the respective directions of the sublattice magnetization of 
the first magnetic thin film 3 and second magnetic thin film 4 of the 
double-layer magnetic film 5 are the same and recording bits in which the 
respective directions of the sublattice magnetization of the first 
magnetic thin film 3 and the second magnetic thin film 4 are opposite to 
each other. However, problems occur in reproducing recorded information or 
in preserving the thermomagnetic recording medium, when there are 
recording bits in which the directions of sublattice, magnetization are 
antiparallel. 
When the respective directions of sublattice magnetization of the magnetic 
thin films 3 and 4 are antiparallel, the thickness h.sub.1 of the first 
magnetic thin film 3 needs to be thick to improve the reproducing 
characteristics, and it is preferable, as apparent from the calculation of 
the magnetizing process, that the thickness h.sub.2 Of the second magnetic 
thin film 4 also is thick, and hence the thickness of the double-layer 
magnetic film 5 is considerably large, which requires a laser having a 
large output capacity. However, under existing circumstances, it is 
difficult to acquire such a laser having a large output capacity of small 
size, for example, a semiconductor laser. Furthermore, even if the 
thermomagnetic recording medium is preserved at room temperature, the 
double-layer magnetic film having the magnetic thin films in which the 
respective directions of sublattice magnetization are antiparallel is 
unstable to heat and magnetic field. Still further, in a recording bit in 
which the directions of secondary lattice magnetization are antiparallel, 
only the magnetic thin film 3 stores information. Therefore, in 
reproducing the recorded information by the use of Kerr effect, the 
information can be read only from the side of the magnetic thin film 3. 
Particularly, when the Curie temperature T.sub.c2 Of the second magnetic 
thin film 4 is higher than the Curie temperature T.sub.c1 of the first 
magnetic thin film 3 (T.sub.c1 &lt;T.sub.c2), the Kerr rotational angle 
.THETA.k.sub.2 Of the second magnetic thin film 4, in general, is greater 
than the Kerr rotational angle .THETA.k.sub.1 of the first magnetic thin 
film 3 (.THETA.k.sub.1 &lt;.THETA.k.sub.2). Accordingly, the SN ratio is 
improved when the signal is read from the side of the second magnetic thin 
film 4, however, such a way of reading signals is impossible when the 
directions of sublattice magnetization of the magnetic thin films are 
antiparallel. Accordingly, it is desirable that the direction of 
sublattice magnetization of the second magnetic thin film 4 and the 
direction of sublattice magnetization of the first magnetic thin film are 
the same in reproducing recorded information or in preserving the 
thermomagnetic recording medium. 
Conditions for making the respective directions of sublattice magnetization 
of the first and second magnetic thin films will be described hereinafter. 
Conditions for changing the mode C into the mode B by applying a third 
external magnetic field H.sub.3 (the positive direction of the magnetic 
field is indicated by the arrow shown in FIG. 1) to a magnetic recording 
medium having recording bits in the modes A and C are 
EQU 2M.sub.s1 h.sub.1 H.sub.3 +.sigma..sub.w &lt;2.vertline.M.sub.s1 
.vertline.h.sub.1 H.sub.c1 
EQU -2M.sub.s2 h.sub.1 H.sub.3 +.sigma..sub.w &gt;2.vertline.M.sub.s2 
.vertline.h.sub.2 H.sub.c2 
At the same time, necessary conditions for inhibiting the transition of the 
mode A to the mode B, C or D are 
##EQU6## 
The operating temperature and the temperature of the magnetic recording 
medium during reproducing need to meet those conditions. When a further 
external magnetic field H.sub.ex, for example, as shown in FIG. 1, is 
applied, in addition to the third external magnetic field H.sub.3, to the 
thermomagnetic recording medium, further conditions to align the 
magnetization direction during preservation, which will be described 
hereinafter, in addition to the foregoing conditions need to be satisfied. 
The positive direction of the external magnetic field H.sub.ex is the same 
as that of the external magnetic field H.sub.3, and hence, in some cases, 
H.sub.ex &lt;0. First, conditions for restraining the mode A from changing 
are obtained by substituting H.sub.3 of the foregoing expressions by 
H.sub.ex. Conditions for restraining the mode B from changing to mode C 
are 
##EQU7## 
Although the conditions for reproducing and preservation have been 
explained with reference to the variation of the modes A, B and C, the 
same transition of the modes is possible when the modes A, B and D are 
used when the direction of magnetization of the second magnetic thin film 
4 in the initial state is opposite to that explained hereinbefore. In such 
a case, the modes A, B, C and D correspond to the modes B, A, D, and C in 
the foregoing description, respectively. Thus, the mode D at the 
completion ot overwrite is changed to the mode A which is suitable for 
reproducing and preservation. 
G-6. Example of the Thermomagnetic Recording Medium-(1) 
Concrete examples of magnetic materials for forming the magnetic thin films 
3 and 4 of the double-layer magnetic film 5 of the thermomagnetic 
recording medium 10 will be described hereinafter. 
RE-TM ferrimagnetic thin films serving as the first magnetic thin film 3 
and the second magnetic thin film 4 were formed in that order over a glass 
plate serving as the transparent substrate 1 (FIG. 2) by a DC magnetron 
sputtering apparatus to form a double-layer magnetic film 5. In this case, 
the dielectric film 2 was omitted. The RE-TM ferrimagnetic thin films 3 
and 4 were formed by alternately superposing RE (rare earth metal) and TM 
(transition metal) layers. To prevent the oxidation of the double-layer 
magnetic film 5 consisting of the magnetic thin films 3 and 4, the 
double-layer magnetic film 5 was coated with a protective film 6 having a 
thickness of 800 angstroms (the lower surface in FIG. 2). The individual 
films were formed under the same conditions as those for forming the 
double-layer magnetic film 5. The magnetic characteristics of the films 
and the interface wall energy density .sigma.w were evaluated. The 
materials, film thickness and characteristics at the room temperature of 
the magnetic thin films 3 and 4 are tabulated in Table 1. 
TABLE 1 
______________________________________ 
First magnetic 
Second magnetic 
thin film thin film 
______________________________________ 
Material TbFe GdTbFeCo 
Thickness (.ANG.) 
310 1500 
Curie point (.degree.C.) 
137 192 
Saturation 95 145 
magnetization 
(emu/cm.sup.3) 
Coercive force 
9.8 2.6 
(kOe) 
______________________________________ 
In both the first magnetic thin film 3 and the second magnetic thin film 4, 
the sublattice magnetization of the RE is greater than that of the TM (RE 
rich) at the room temperature. The interface magnetic wall energy density 
at the room temperature was 2.0 erg/cm.sup.2 which was calculated by 
comparing the MH Loops of the respective single layer films and the 
double-layer film. 
An external magnetic field of 20 kOe was applied to the thermomagnetic 
recording medium having the double-layer magnetic film 5 thus formed to 
initialize the double-layer magnetic film 5 in the mode A shown in FIG. 4. 
Then, the thermomagnetic recording medium was heated to a temperature T 
with the external magnetic field H.sub.1 (FIG. 1) of 0.3 kOe applied 
thereto, and then the thermomagnetic recording medium was cooled to the 
room temperature. During the heating and cooling process, the 
thermomagnetic recording medium was irradiated from the side of the 
transparent substrate 1 (the glass plate) by a linearly polarized light of 
830 nm in wavelength to observe the condition of magnetization by Kerr 
effect. When the temperature T was 150.degree. C., the mode A of 
magnetization remained unchanged after heating and cooling. When the 
temperature T was 200.degree. C., the mode of magnetization changed from 
the mode A to the mode B, in which the respective directions of 
magnetization of the magnetic thin films 3 and 4 were inverted after 
heating and cooling. 
Then, the external magnetic field H.sub.2 (FIG. 1) of 5 kOe was applied to 
the thermomagnetic recording medium which was in the mode B at the room 
temperature, whereby the mode of magnetization of the double-layer 
magnetic film 5 was caused to changed from the mode B to the mode C. When 
the external magnetic field H.sub.2 was applied to the thermomagnetic 
recording medium in the mode A, the mode of magnetization remained 
unchanged. 
The same thermomagnetic recording medium in the mode C at the room 
temperature was heated to the temperature T with the external magnetic 
field H.sub.1 applied thereto, and then the thermomagnetic recording 
medium was cooled to the room temperature. When the temperature T was 
150.degree. C., the mode of magnetization was the mode A after heating and 
cooling, while the mode of magnetization was the mode B after heating and 
cooling, when the temperature T was 200.degree. C. 
The sample double-layer magnetic film subjected to the measurement meets 
the conditions for overwriting described in article G-4. at temperatures: 
the room temperature, 50.degree. C., 75.degree. C., 100.degree. C., 
125.degree. C. and 150.degree. C. At temperatures other than those 
temperatures, since M.sub.s ' 1/H.sub.c and .sigma..sub.w vary 
continuously, it is considered that, substantially, the foregoing 
conditions are satisfied. The value of the external magnetic field 
necessary for changing the mode of magnetization at each temperature 
agreed well with the value calculated on the basis of the expressions 
representing the conditions, from which it is apparent that the foregoing 
expressions representing the conditions for overwriting provides 
appropriate conditions for the practical application of the thermomagnetic 
recording method of the present invention. 
FIG. 5 shows the transition of the mode of magnetization of the 
double-layer magnetic film 5 consisting of the magnetic thin films 3 and 4 
having properties shown in Table 1 with the variation of the external 
magnetic field H at the room temperature. In FIG. 5, H.sub.01 is intensity 
of the external magnetic field where the transition of the mode D to the 
mode A (or the mode C to the mode B) occurs; H.sub.02 is intensity of the 
same where the transition of the mode B to the mode C (or the mode A to 
the mode D) occurs; H.sub.03 is intensity of the same where the transition 
of the mode C to the mode A (or the mode D to the mode B) occurs. The 
intensity of the external magnetic field causing the transition of the 
mode of magnetization was measured on the basis of Kerr effect. From the 
intensity of the external magnetic field H.sub.03 and the data shown in 
Table 1, calculated interface wall energy density .sigma..sub.w equals to 
2.0 erg/cm.sup.2. The calculated values of H.sub.01 and H.sub.02 
calculated by using the calculated interface wall energy density .sigma.w 
agreed well with the measured values, respectively. 
Referring to FIG. 5, when the mode of magnetization during recording is 
either the mode A or the mode C (or either the mode B or the mode D), the 
reproduction of recording bits in the modes A or B is possible when an 
external magnetic field of a magnetic intensity of H.sub.exA (or 
H.sub.exB) meeting 
EQU -H.sub.01 &lt;H.sub.exA &lt;-H.sub.02 (or H.sub.01 &lt;H.sub.exB &lt;H.sub.02) 
is applied to the double-layer magnetic film. 
When the mode of magnetization of the double-layer magnetic film 5 is the 
mode A or the mode B in reproducing recorded signals, both the magnetic 
thin films 3 and 4 of the double-layer magnetic film 5 are available for 
reading the signals, which improves the SN ratio of the reproduced signals 
as mentioned above as compared with the SN ratio of the reproduced signals 
reproduced from the double-layer magnetic film 5 in the mode C (or the 
mode D). 
When the mode of magnetization at the end of overwrite is either the mode A 
or the mode C (or either the mode B or the mode D), preservation and 
reproduction in the mode A or the mode B are possible by applying an 
external magnetic field of H.sub.exA (or H.sub.exB) meeting the foregoing 
condition to the double-layer magnetic film 5. If another magnetic field 
of H.sub.exC is applied to the double-layer magnetic film 5 in applying 
the external magnetic field of H.sub.exA (or H.sub.exB) is applied, in 
preservation or in reproduction, an inequality: 
EQU -H.sub.02 &lt;H.sub.exC &lt;H.sub.02 (or -H.sub.02 &lt;H.sub.exD &lt;H.sub.02) 
must be satisfied. 
G-7. Another Embodiment 
In the embodiment shown in FIG. 1, when the magnetic compensating point of 
the second magnetic thin film 4 is between the room temperature and the 
temperature T.sub.2, the external magnetic fields of H.sub.1 and H.sub.2 
are in the same direction as shown in FIG. 6. 
When the external magnetic fields of H.sub.1 and H.sub.2 are the same in 
the direction of magnetic field as shown in FIG. 6, both the external 
magnetic fields of H.sub.1 and H.sub.2 can be formed by a single magnet 13 
as shown in FIG. 7. A thermomagnetic recording medium 10 is irradiated by 
a laser light R at a position near the magnet 13 where the intensity of 
the magnetic field of the magnet 13 is not the maximum, so that the 
intensity H.sub.1 of the external magnetic field at the irradiated 
position is smaller than the intensity H.sub.2 (H.sub.1 &lt;H.sub.2). 
Further embodiment will be described hereinafter, in which the external 
magnetic field H.sub.2 is not required to change the mode B into the mode 
C, when the second magnetic thin film has a magnetic compensation 
temperature between room temperature and recording temperature T.sub.2. In 
this embodiment a similar double-layer magnetic film 5, as shown in FIG. 
2, is employed and the magnetization condition changes as shown in FIG. 8. 
Referring to FIG. 8, a double-layer magnetic film 5 has a recording portion 
in which the respective directions of the respective magnetic moments of a 
first magnetic thin film 3 and a second magnetic thin film 4, which are 
coupled magnetically, are the same (mode A) and a recording portion in 
which the respective directions of the respective magnetic moments of the 
magnetically coupled first magnetic thin film 3 and the second magnetic 
thin film 4 are opposite to each other (mode C). When the film (in mode A) 
is heated in a heating mode in which the superposed magnetic film is 
heated at a temperature T.sub.1 which is higher than the Curie temperature 
T.sub.C1 of the first magnetic thin film 3, there is no inversion of the 
sublattice magnetization of the second magnetic thin film 4. When the film 
is heated in a heating mode in which the superposed magnetic film 5 is 
heated at a temperature T.sub.2 which is higher than the Curie temperature 
T.sub.C1 there is an inversion of the sublattice magnetization of the 
second magnetic thin film 4. Thus, by modulating the heating condition, an 
information signal is recorded, and then the heated superposed magnetic 
film 5 is cooled for permanently recording the magnetic moment in the 
thermomagnetic recording medium. The second magnetic thin film 4 has a 
compensation temperature between the room temperature and the temperature 
T.sub.2 at which the superposed magnetic film 5 is heated in the first 
heating mode. The direction of the sublattice magnetization of the second 
magnetic thin film 4 is inverted during the course of cooling. 
Possible modes of the double-layer laminated magnetic film 5 formed by 
laminating the magnetic thin films 3 and 4 of a RE-TM alloy (in a 
temperature range below the respective Curie temperatures T.sub.C1 and 
T.sub.C2 of the magnetic thin films 3 and 4), are manifested by four modes 
A to D as shown in FIG. 10. The respective easy directions of 
magnetization of the magnetic thin films 3 and 4 are perpendicular to the 
surface of the thin films, namely, the magnetic thin films 3 and 4 are 
so-called perpendicular anisotropy magnetic thin films. 
Referring to FIG. 10, in the modes A and B, the respective directions of 
the respective magnetic moments of the respective TMs of the first 
magnetic thin film 3 and the second magnetic thin film 4 are the same as 
indicated by arrows shown by solid lines, and those of the REs of the 
magnetic thin film 3 and the magnetic thin film 4 are the same as 
indicated by arrows shown by broken lines. In the modes C and D, the 
respective directions of the respective magnetic moments of the respective 
TMs of the first magnetic thin film 3 and the second magnetic thin film 4 
are opposite to each other as indicated by arrows shown by solid lines in 
FIG. 10, and those of the REs of the first magnetic thin film 3 and the 
second magnetic thin film 4 are opposite to each other as indicated by 
arrows shown by broken lines in FIG. 10, so that a region where the 
directions of the magnetic moment of TM and the magnetic moment of RE 
change through an angle of 180.degree. C., namely, an interface magnetic 
wall, is formed in the interface of the first magnetic thin film 3 and the 
second magnetic thin film 4. This interface magnetic wall is designated as 
an interface wall 7. An interface magnetic wall energy per unit area 
(.sigma.w erg/cm.sup.2) is stored in the interface wall 7. 
G-8. Change of Mode of Magnetization According to Temperature Variation-(2) 
Change of the mode of magnetization of the magnetic thin films of the 
superposed magnetic film 5 with temperature variation caused by laser 
irradiation or the like will be described with reference to FIG. 8, in 
which arrows of solid lines shown in the magnetic thin films 3 and 4 
indicate the magnetic moment (magnetization) of TM (transition metal) and 
arrows of broken lines indicate the magnetic moment (magnetization) of Re 
(rare earth metal). 
Suppose that a recording bit of the double-layer magnetic film 5 of the 
thermomagnetic recording medium 10 is in the mode A (FIG. 8) at the room 
temperature T.sub.R, and the bit of the superposed magnetic film 5 in the 
mode A is irradiated by laser light for recording. The intensity of the 
laser light or the duration of laser irradiation is controlled according 
to a recording signal to heat the laminated magnetic film 5 selectively to 
a first temperature T.sub.1 or to a second temperature T.sub.2. The first 
temperature T.sub.1 is higher than the Curie temperature T.sub.C1 of the 
first magnetic thin film 3 and is a temperature at which the inversion of 
the sublattice magnetization of the second magnetic thin film 4 will not 
occur when the second magnetic thin film 4 is subjected to the influence 
of an external magnetic field H.sub.ex, which will be described 
hereinafter, while the second temperature T.sub.2 is higher than the first 
temperature T.sub.1 and the compensation temperature of the second 
magnetic thin film 4, and is a temperature high enough to cause the 
inversion of the sublattice magnetization of the second magnetic thin film 
4 when the second magnetic thin film 4 is subjected to the influence of 
the external magnetic field H.sub.ex. FIG. 11 shows the temperature 
characteristics of the magnetization and the coercive force of the second 
magnetic thin film 4. The second magnetic thin film 4 has a compensation 
temperature T.sub.comp between the room temperature T.sub.R and the second 
temperature T.sub.2. Qualitatively, the intensity of the external magnetic 
field H.sub.ex is greater than the coercive force H.sub.c (H.sub.ex 
&gt;H.sub.c) at the room temperature T.sub.R and at the second temperature 
T.sub.2, and the coercive force H.sub.c is greater than the intensity of 
the external magnetic field H.sub.ex (H.sub.ex &lt; H.sub.c) at the 
temperature T.sub.1 and at the compensation temperature T.sub.comp. 
Accordingly, the direction of sublattice magnetization of the second 
magnetic thin film 4 is inverted at the room temperature T.sub.R and at 
the temperature T.sub.2 by the external magnetic field H.sub.ex Of the 
magnet 11 which is shown in FIG. 9. More strict quantitative conditions 
will be described in article G-9. 
In the initial condition in the mode A, the total magnetization of the 
second magnetic thin film is pointing upward in FIG. 8 because the 
sublattice magnetization of RE (shown as a dashed arrow) is larger than 
that of TM (shown as a solid arrow). When the temperature of the 
superposed thin film 5 is raised over the compensation temperature 
T.sub.comp (FIG. 11), the total magnetization of the second magnetic thin 
film 4 is pointing downward because over the compensation temperature the 
sublattice magnetization of TM is larger than that of RE, as shown in FIG. 
11. When the temperature is further raised to T.sub.2, the total 
magnetization M.sub.s2 again inverted to follow the direction of the 
external magnetic field H.sub.ex. In other words, the sublattice 
magnetizations are inverted at the temperature T.sub.2 under the influence 
of the external magnetic field H.sub.ex. Upon cooling the superposed 
magnetic film 5 to the temperature T.sub.C1 after the same has been heated 
to the temperature T.sub.1 or T.sub.2, spontaneous magnetization appears 
again in the first magnetic thin film 3, in which the exchange energy 
between the first magnetic thin film 3 and the second magnetic thin film 4 
is more dominant than the Zeeman energy of the external magnetic field in 
determining the direction of the magnetic moment of the first magnetic 
thin film 3. That is, the saturation magnetization M.sub.s1 and the film 
thickness h.sub.1 of the first magnetic thin film 3 in relation to the 
external magnetic field H.sub.ex and the interface wall energy density 
.sigma.w are decided selectively so as to meet an inequality 
EQU .sigma..sub.w &gt;2.vertline.M.sub.s1 .vertline.h.sub.1 .vertline.H.sub.ex 
at a temperature near the temperature T.sub.c1 at which a spontaneous 
magnetization appears in the first magnetic thin film 3. Accordingly, when 
the temperature T of the thermomagnetic recording medium coincides with 
the temperature T.sub.C1, mode A or B is established, in which the 
respective directions of the respective magnetization of the magnetic thin 
films 3 and 4 of the superposed magnetic film 5 are the same. The mode A 
is established when the heating temperature is T.sub.1, and the mode B is 
established when the heating temperature is T.sub.2. 
When the recording bit of the thermomagnetic recording medium is cooled 
further, for example, to a temperature near the room temperature T.sub.R, 
the initial mode A is established in the bit, or the mode B, in which the 
direction of the magnetic moment is opposite to that of the initial state, 
is established. However, to meet the conditions for the transition of the 
mode of magnetization from the mode B to the mode C, the magnetic moment 
of the second magnetic thin film 4 is inverted, and thereby the mode of 
magnetization of the laminated magnetic film 5 assumes the mode C as shown 
in FIG. 10. 
During the course of cooling from the temperature T.sub.2, the direction of 
the total magnetization M.sub.s2 is changed from the M.sub.s2 at T.sub.2, 
just cooled to lower temperature than the compensation temperature. Though 
the superposed thin film 5 is still under influence of the external 
magnetic field H.sub.ex, the total magnetization of the magnetic thin film 
4 again is changed the direction to point upward as shown in FIG. 8, the 
mode C, since RE&gt;TM. 
When a recording bit in the mode C of the superposed magnetic film 5 of the 
thermomagnetic recording medium 10 is heated to a temperature above the 
temperature T.sub.C1, the magnetization of the first magnetic thin film 
disappears and the same mode of magnetization as that of a recording bit 
heated from the initial mode A is established in the recording bit. 
Accordingly, the mode of a recording bit heated to the temperature T.sub.1 
and then cooled becomes the mode A, while the mode of a recording bit 
heated to the temperature T.sub.2 and then cooled becomes the mode C. 
Therefore, the mode of recording magnetization is dependent on the heating 
temperatures T.sub.1 and T.sub.2. 
Thus, a recording bit in which the respective directions of the 
magnetically coupled magnetic moments of the first magnetic thin film 3 
and second magnetic thin film 4 of the superposed magnetic film 5 are the 
same, namely, a recording bit in the mode A, and a recording bit in which 
the respective directions of magnetically coupled magnetic moments of the 
first magnetic thin film 3 and the second magnetic thin film 4 are 
opposite to each other, namely, a bit in the mode C, are heated to the 
temperature T.sub.1 or T.sub.2 by regulating the heating condition 
according to information signals to establish a new mode of magnetization 
in the recording bit to enable overwrite irrespective of the initial mode 
of magnetization of the recording bit. 
G-9. Conditions for Overwrite-(2) 
Conditions for overwrite will be described hereinafter. 
In changing the temperature T of the superposed magnetic film 5 under the 
influence of the external magnetic field H.sub.ex as shown in FIG. 8, 
conditions for restraining the mode A from changing to the other modes in 
the temperature range from the room temperature T.sub.R to a temperature 
below the Curie temperature T.sub.C1 of the first magnetic thin film 3 
(T.sub.R .ltoreq.Y&lt;T.sub.c1) are expressed by inequalities: 
##EQU8## 
and conditions for restraining the mode C from changing are expressed by 
inequalities: 
EQU 2M.sub.s1 h.sub.1 H.sub.ex +.sigma..sub.w &lt;2.vertline.M.sub.s1 
.vertline.h.sub.1 H.sub.c1 
EQU -2M.sub.s2 h.sub.2 H.sub.ex +.sigma..sub.w &lt;2.vertline.M.sub.s2 
.vertline.h.sub.2 H.sub.c2 
A condition for restraining the inversion of the sublattice magnetization 
of the second magnetic thin film 4 while the temperature T of the 
superposed magnetic film 5 is in the range from a temperature above the 
temperature T.sub.C1 to a temperature below the second temperature T.sub.2 
(T.sub.C1 &lt;T&lt;T.sub.2) is expressed by an inequality: 
EQU .vertline.H.sub.ex .vertline.&lt;H.sub.c2 
and a condition for causing the inversion of the sublattice magnetization 
of the second magnetic thin film 4 when the temperature T of the 
superposed magnetic film 5 is above the temperature T.sub.2 (T&gt;T.sub.2) is 
expressed by an inequality: 
EQU .vertline.H.sub.ex .vertline.&gt;H.sub.c2 
In cooling the superposed magnetic film 5 after heating the same to such a 
temperature, a condition for allowing the magnetization of the first 
magnetic thin film 3 is determined by the exchange energy with the 
magnetization of the second magnetic thin film 4 upon the fall of the 
temperature T of the recording bit of the superposed magnetic film 5 of 
the thermomagnetic recording medium 10 near to the Curie temperature 
T.sub.C1 of the first magnetic thin film 3 (T is approximately equal to 
T.sub.c1) is: 
EQU .sigma..sub.w &gt;2.vertline.M.sub.s1 .vertline.h.sub.1 .vertline.H.sub.ex 
.vertline. 
and conditions for restraining the mode A from changing to the other modes 
in the temperature range of T.sub.R .ltoreq.T&lt;T.sub.C1 are the same as 
those for the heating process, while conditions for allowing the 
transition of the mode B to the mode C are: 
##EQU9## 
When all the foregoing conditions for overwrite are satisfied by the 
superposed magnetic film 5, overwrite is feasible. 
While the thermomagnetic recording medium is preserved at the room 
temperature, naturally, the mode A is maintained, however, the mode C may 
be maintained or the transition of the mode C to the mode B may occur. 
Conditions for maintaining the mode C are: 
EQU .sigma..sub.w &lt;2.vertline.M.sub.s1 .vertline.h.sub.1 H.sub.c1 
EQU .sigma..sub.w &lt;2.vertline.M.sub.s2 .vertline.h.sub.2 H.sub.c2 
and conditions for allowing the transition of the mode C to the mode B are: 
EQU .sigma..sub.w &lt;2.vertline.M.sub.s1 .vertline.h.sub.1 H.sub.c1 
EQU .sigma..sub.w &gt;2.vertline.M.sub.s2 .vertline.h.sub.2 H.sub.c2 
Thus, the least necessary condition for the mode C is to satisfy either one 
of the sets of conditions. 
When the conditions for restraining the transition of the mode C to other 
mode among those conditions for overwrite are not satisfied partly, 
namely, when the mode of magnetization changes from the mode C to the mode 
A while the thermomagnetic recording medium is heated from the room 
temperature T.sub.R to the Curie temperature T.sub.C1 of the first 
magnetic thin film 3 (T.sub.R &lt;T&lt;T.sub.c1), there must be a temperature 
TCA to provide 
EQU 2M.sub.s1 h.sub.1 H.sub.ex -.sigma..sub.w 2.vertline.M.sub.s1 
.vertline.h.sub.1 H.sub.c1 =2.vertline.M.sub.s1 .vertline.h.sub.1 H.sub.c1 
in that temperature range. In such a case, a necessary condition for 
overwrite is: 
EQU T.sub.BC &lt;T.sub.CA 
where T.sub.BC is a temperature at which the mode of magnetization changes 
from the mode B to the mode C while the thermomagnetic recording medium is 
cooled after being heated to the temperature T.sub.2. However, even if 
this condition is not satisfied, overwrite is possible when the inversion 
of the magnetic moment of the first magnetic thin film 3 does not occur 
while the temperature drops from the temperature T.sub.BC to the 
temperature T.sub.CA. Naturally, the temperature T of the thermomagnetic 
recording medium must be below the temperature T.sub.CA during 
reproduction, when there is such a temperature T.sub.CA. 
Although the variation of mode of magnetization has been explained in the 
foregoing description with reference to the modes A, B and C, overwrite is 
possible when the modes A, B and D are employed when the initial direction 
of magnetic moment of the second magnetic thin film 4 is turned in the 
opposite direction. That is, in the latter case, the modes A, B, C and D 
correspond to the modes B, A, D and C of the foregoing embodiment, 
respectively, and the saturation magnetic flux density M.sub.s is defined 
by: 
EQU M.sub.s .tbd.M.sub.TM -M.sub.RE 
to apply the foregoing conditions for overwrite without change. 
G-10. Mode of Magnetization for Reproduction and Preservation-(2) 
As explained previously, problems occur in reproducing recorded information 
or in preserving the thermomagnetic recording medium, when there are 
recording bits in which the directions of the sublattice magnetic moments 
are antiparallel. Accordingly, it is desirable that the direction of 
sublattice magnetization of the second magnetic thin film 4 and the 
direction of sublattice magnetization of the first magnetic thin film 3 
are the same in reproducing recorded information or in preserving the 
thermomagnetic recording medium. 
Conditions for making the respective directions of sublattice magnetization 
of the first and second magnetic thin films parallel will be described 
hereinafter. Conditions for changing the mode of magnetization from the 
mode C to the mode B by applying another external magnetic field H.sub.ex1 
(the positive direction of the magnetic field is indicated by the arrow 
indicating the direction of the external magnetic field H.sub.ex) to a 
magnetic recording medium having recording bits in the modes A and C are: 
EQU 2M.sub.s1 h.sub.1 H.sub.ex1 .sigma..sub.w &lt;2.vertline.M.sub.s1 
.vertline.h.sub.1 H.sub.c1 
EQU -2M.sub.s2 h.sub.2 H.sub.ex1 .sigma..sub.w &gt;2.vertline.M.sub.s2 
.vertline.h.sub.2 H.sub.c2 
Naturally, the following conditions for inhibiting the transition of the 
mode A to the other mode, in addition to the foregoing conditions, must be 
satisfied. 
##EQU10## 
The operating temperature and the temperature of the thermomagnetic 
recording medium during reproducing need to meet those conditions. When a 
further external magnetic field H.sub.ex2 is applied to the thermomagnetic 
recording medium, further conditions, which will be described hereinafter, 
in addition to the foregoing conditions need to be satisfied. The positive 
direction of the external magnetic field H.sub.ex2 is the same as that of 
the external magnetic field H.sub.ex1 and hence, in some cases, H.sub.ex2 
&lt;0. First, conditions for restraining the mode A from changing are 
obtained by substituting the H.sub.ex1 Of the foregoing inequalities by 
H.sub.ex2. Conditions for restraining the mode B established by changing 
the mode C from changing are: 
##EQU11## 
Although the conditions for reproducing and preservation have been 
explained with reference to the variation of the modes A, B and C, the 
same transition of the modes is possible when the modes A, B and D are 
used when the direction of magnetization of the second magnetic thin film 
4 in the initial state is opposite to that explained hereinbefore. In such 
a case, the modes A, B, C and D correspond to the modes B, A, D and C in 
the foregoing description, respectively. Thus, the mode D at the 
completion of overwrite is changed to the mode A which is suitable for 
reproducing and preservation. 
G-11. Example of the Thermomagnetic Recording Medium-(2) 
Concrete examples of magnetic materials for forming the magnetic thin films 
3 and 4 of the superposed magnetic film 5 of the thermomagnetic recording 
medium 10 will be described hereinafter. 
RE-TM ferrimagnetic thin films serving as the first magnetic thin film 3 
and the second magnetic thin film 4 were formed in that order over a glass 
plate serving as the transparent substrate 1 by a DC magnetron sputtering 
apparatus to form a superposed magnetic film 5. In this case, the 
dielectric film 2 was omitted. The RE-TM ferrimagnetic thin films 3 and 4 
were formed by alternately laminating RE (rare earth metal) and TM 
(transition metal) layers. To prevent the oxidation of the superposed 
magnetic film 5 consisting of the magnetic thin films 3 and 4, the 
superposed magnetic film 5 was coated with a protective film 6 having a 
thickness of 800 angstroms (the lower surface in FIG. 2). The individual 
films were formed under the same conditions as those for forming the 
laminated magnetic film 5. The magnetic characteristics of the films and 
the magnetic domain wall energy density .sigma.w were evaluated. The 
materials, film thickness and characteristics at the room temperature of 
the magnetic thin films 3 and 4 are tabulated in Table 2. 
TABLE 2 
______________________________________ 
First magnetic 
Second magnetic 
thin film thin film 
______________________________________ 
Material TbFe GdTbFeCo 
Thickness (.ANG.) 
310 2000 
Curie point (.degree.C.) 
135 202 
Saturation 115 134 
magnetization 
(emu/cm.sup.2) 
Coercive force 
9.2 1.8 
(kOe) 
______________________________________ 
In both the first magnetic thin film 3 and the second magnetic thin film 4, 
the sublattice magnetization of RE is greater than that of TM (RE rich) at 
the room temperature. The respective magnetic compensation points of the 
first magnetic thin film 3 and the second magnetic thin film 4 are 
temperatures around 130.degree. C. and 155.degree. C., respectively. The 
magnetic domain wall energy density .sigma..sub.w at the room temperature 
is 1.8 erg/cm.sup.2. 
An external magnetic field of 20 kOe was applied to the thermomagnetic 
recording medium having the superposed magnetic film 5 thus formed to 
initialize the superposed magnetic film 5 in the mode A as shown in FIG. 
10. Then, the thermomagnetic recording medium was heated to a temperature 
T with the external magnetic field H.sub.ex (FIG. 1) of 2.3 kOe applied 
thereto, and then the thermomagnetic recording medium was cooled to the 
room temperature. During the heating and cooling process, the 
thermomagnetic recording medium was irradiated from the side of the 
transparent substrate 1 (the glass plate) by a linearly polarized light of 
830 nm in wavelength to observe the condition of magnetization by Kerr 
effect. When the temperature T was 150.degree. C., the mode A of 
magnetization remained unchanged after heating and cooling. When the 
temperature T was 200.degree. C., the direction of the magnetization of 
the second magnetic thin film 3 occurred upon the arrival of the 
temperature of the thermomagnetic recording medium at 178.degree. C. 
During the cooling process, the mode B was established when the 
thermomagnetic recording medium was cooled to 130.degree. C. When the 
thermomagnetic recording medium was cooled further down to 47.degree. C., 
the mode of magnetization changed from the mode B to the mode C, which was 
maintained when the thermomagnetic recording medium was cooled to the room 
temperature. The mode C remained unchanged when H.sub.ex =0. 
The same thermomagnetic recording medium in the mode C at the room 
temperature was heated and cooled with the external magnetic field 
H.sub.ex Of 2.3 kOe applied thereto. When the heating temperature T was 
150.degree. C., the mode of magnetization after heating and cooling was 
Mode A, while the mode of magnetization after heating and cooling was the 
mode B when the heating temperature T was 200.degree. C. 
The present invention is not limited to the foregoing embodiment. For 
example, the ferrimagnetic thin films of the superposed magnetic film 
employed in the foregoing embodiment may be substituted by ferromagnetic 
thin films. 
Although the present invention has been described in the preferred 
embodiment thereof, in which the magnetic coupling energy between the two 
magnetic thin films is produced by the exchange coupling of the magnetic 
thin films, the magnetic coupling energy may be produced by the 
magnetostatic coupling or by the exchange coupling and magnetostatic 
coupling of the two magnetic thin films. 
According to the thermomagnetic recording method of the present invention, 
the intensity of a heating beam such as, for example, a laser beam, or the 
duration of irradiation by the heating beam is modulated according to 
information signals to regulate the heating temperature at which the 
thermomagnetic recording medium is heated between first and second heating 
temperatures for the effective recording of information in the 
thermomagnetic recording medium.