Over-write capable magnetooptical recording medium with four magnetic layered structure dispensing with external initializing field

A medium comprises a memory layer, a writing layer, a switching layer, and an initializing layer, these four layers consisting of perpendicularly magnetizable magnetic films being stacked in order, wherein the memory layer and the writing layer are exchange-coupled, a direction of magnetization of only the writing layer can be aligned in a predetermined direction without changing a direction of magnetization of the memory layer, and the writing layer and the initializing layer are exchange-coupled via the switching layer at a temperature equal to or lower than the Curie temperature of the switching layer. The writing layer is RE-rich at room temperature and has compensation temperature between room temperature and its Curie temperature, and the initializing layer is RE-rich at room temperature and has no compensation temperature between room temperature and its Curie temperature.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a magnetooptical recording medium having a 
four-layered structure and capable of performing an over-write operation 
by modulating only an intensity of a light beam in accordance with 
information to be recorded without modulating a direction and strength of 
a bias field Hb. 
2. Related Background Art 
In recent years, many efforts have been made to develop an optical 
recording/reproduction method which can satisfy various requirements 
including high density, large capacity, high access speed, and high 
recording/reproduction speed, and a recording apparatus, a reproduction 
apparatus, and a recording medium used therefor. 
Of various optical recording/reproduction methods, the magnetooptical 
recording/reproduction method is most attractive due to its unique 
advantages in that information can be erased after it is recorded, and new 
information can be repetitively recorded. 
A recording medium used in the magnetooptical recording/reproduction method 
has a perpendicular magnetic layer or layers as a recording layer. The 
magnetic layer comprises, for example, amorphous GdFe or GdCo, GdFeCo, 
TbFe, TbCo, TbFeCo, and the like. Concentrical or spiral tracks are 
normally formed on the recording layer, and information is recorded on the 
tracks. In this specification, one of "upward" and "downward" directions 
of magnetization with respect to a film surface is defined as an "A 
direction", and the other one is defined as a "non-A direction". 
Information to be recorded is binary-coded in advance, and is recorded by 
two signals, i.e., a bit (B.sub.1) having an "A-directed" magnetization, 
and a bit (B.sub.0) having a "non-A-directed" magnetization These bits 
B.sub.1 and B.sub.0 correspond to "1" and "0" levels of a digital signal. 
However, in general, the direction of magnetization of the recording 
tracks can be aligned in the "non-A direction" by applying a strong 
external field before recording. This "aligning process" is called 
"initialize*" in a conventional sense. Thereafter, a bit (B.sub.1) having 
an "A-directed" magnetization is formed on the tracks. Information is 
expressed in accordance with the presence/absence and/or a bit length of 
the bit (B.sub.1). Note that a bit is often called a mark recently. 
PRINCIPLE OF BIT FORMATION 
In the bit formation, a characteristic feature of laser, i.e., excellent 
coherence in space and time, is effectively used to focus a beam into a 
spot as small as the diffraction limit determined by the wavelength of the 
laser light. The focused light is radiated onto the track surface to 
record information by producing bits less than 1 .mu.m in diameter on the 
recording layer. In the optical recording, a recording density up to 
10.sup.8 bits/cm.sup.2 can be theoretically attained, since a laser beam 
can be concentrated into a spot with a size as small as its wavelength. 
As shown in FIG. 2, in the magnetooptical recording, a laser beam (L) is 
focused onto a recording layer (1) to heat it, while a bias field (Hb) is 
externally applied to the heated portion in the direction opposite to the 
initialized* direction. A coercivity H.sub.C of the locally heated portion 
is decreased below the bias field (Hb). As a result, the direction of 
magnetization of that portion is aligned in the direction of the bias 
field (Hb). In this way, reversely magnetized bits are formed. 
Ferromagnetic and ferrimagnetic materials differ in the temperature 
dependencies of the magnetization and H.sub.C. Ferromagnetic materials 
have H.sub.C which decreases around the Curie temperature and allow 
information recording based on this phenomenon. Thus, information 
recording in ferromagnetic materials is referred to as T.sub.C recording 
(Curie temperature recording). 
On the other hand, ferrimagnetic materials have a compensation temperature 
T.sub.comp., below the Curie temperature, at which magnetization (M) 
becomes zero. The H.sub.C abruptly increases around this temperature and 
hence abruptly decreases outside this temperature. The decreased H.sub.C 
is canceled by a relatively weak bias field (Hb). Namely, recording is 
enabled. This process is called T.sub.comp. recording (compensation point 
recording). 
In this case, however, there is no need to adhere to the Curie point or 
temperatures therearound, and the compensation temperature. In other 
words, if a bias field (Hb) capable of canceling a decreased H.sub.C is 
applied to a magnetic material having the decreased H.sub.C at a 
predetermined temperature higher than a room temperature, recording is 
enabled. 
PRINCIPLE OF REPRODUCTION 
FIG. 3 shows the principle of information reproduction based on the 
magnetooptical effect. Light is an electromagnetic wave with an 
electromagnetic-field vector normally emanating in all directions in a 
plane perpendicular to the light path. When light is converted to linearly 
polarized beams (L.sub.P) and irradiated onto a recording layer (1), it is 
reflected by its surface or passes through the recording layer (1). At 
this time, the plane of polarization rotates according to the direction of 
magnetization (M). This phenomenon is called the magnetic Kerr effect or 
magnetic Faraday effect. 
For example, if the plane of polarization of the reflected light rotates 
thro .theta..sub.k degrees for the "A-directed" magnetization, it rotates 
through -.theta..sub.k degrees for the "non-A-directed" magnetization. 
Therefore, when the axis of an optical analyzer (polarizer) is set 
perpendicular to the plane inclined at -.theta..sub.k, the light reflected 
by a "non-A-direction" magnetized bit (B.sub.0) cannot pass through the 
analyzer. On the contrary, a component corresponding to a product of 
(sin2.theta..sub.k).sup.2 and the light reflected by a bit (B.sub.1) 
magnetized along the "A direction" passes through the analyzer and becomes 
incident on a detector (photoelectric conversion means). As a result, the 
bit (B.sub.1) magnetized along the "A direction" looks brighter than the 
bit (B.sub.0) magnetized along the "non-A direction", and causes the 
detector to produce a stronger electrical signal The electrical signal 
from the detector is modulated in accordance with the recorded 
information, thus reproducing the information. 
In order to re-use a recorded medium, (i) the medium must be 
re-initialized* by an initialize* device, or (ii) an erase head having the 
same arrangement as a recording head must be added to a recording 
apparatus, or (iii) as preliminary processing, recorded information must 
be erased using a recording apparatus or an erasing apparatus. 
Therefore, in the conventional magnetooptical recording method, it is 
impractical to perform an over-write operation, which can properly record 
new information regardless of the presence/absence of recorded 
information. 
If the direction of a bias field Hb can be desirably modulated between the 
"A-direction" and "non-A direction", an over-write operation is possible. 
However, it is impossible to modulate the direction of the bias field Hb 
at high speed. For example, if the bias field Hb comprises a permanent 
magnet, the direction of the magnet must be mechanically reversed. 
However, it is impossible to reverse the direction of the magnet at high 
speed. Even when the bias field Hb comprises an electromagnet, it is also 
impossible to modulate the direction of a large-capacity current at high 
speed. 
However, according to remarkable technical developments, a magnetooptical 
recording method capable of performing an over-write operation by 
modulating only an intensity of a light beam to be irradiated in 
accordance with binary coded information to be recorded without modulating 
a strength (including an ON/OFF state) or the direction of the bias field 
Hb, an over-write capable magnetooptical recording medium used in this 
method, and an over-write capable recording apparatus used in this method 
were invented, as disclosed in a patent application (Japanese Patent 
Laid-Open Application No. 62-175948 corresponding to DE 3,619,618 and to 
U.S. application Ser. No. 453,255). This invention will be referred to as 
the basic invention hereinafter. 
DESCRIPTION OF THE BASIC INVENTION 
The basic invention uses an "over-write capable multilayered magnetooptical 
recording medium which includes a recording layer (to be referred to as a 
memory layer or M layer hereinafter in this specification) which comprises 
a perpendicularly magnetizable magnetic thin film, and a reference layer 
(to be referred to as a "writing layer" or W layer hereinafter in this 
specification) which comprises a perpendicularly magnetizable magnetic 
thin film, and in which the two layers are exchange-coupled, and the 
direction of magnetization of only the W layer can be aligned in a 
predetermined direction without changing the direction of magnetization of 
the M layer at a room temperature. 
Information is expressed by a bit having an "A-directed" magnetization, and 
a bit having a "non-A-directed" magnetization in the M layer (in some 
cases, also in the W layer). 
In this medium, the direction of magnetization of the W layer can be 
aligned in an "A direction" by an external means (e.g., an initial field 
Hini.). At this time, the direction of magnetization of the M layer is not 
reversed. Furthermore, the direction of magnetization of the W layer which 
has been aligned in the "A direction" is not reversed upon application of 
an exchange coupling force from the M layer. In contrast to this, the 
direction of magnetization of the M layer is not reversed upon application 
of an exchange coupling force from the W layer aligned in the "A 
direction". 
The W layer has a lower coercivity H.sub.C and a higher Curie temperature 
T.sub.C than those of the M layer. 
According to a recording method of the basic invention, only the direction 
of magnetization of the W layer of the recording medium is aligned in the 
"A direction" by an external means until a time immediately before 
recording. This process will be specially referred to as "initialize" in 
this specification. The "initialize" process is unique to an over-write 
capable medium. 
Thereafter, a laser beam which is pulse-modulated in accordance with binary 
coded information is irradiated on the medium. The laser beam intensity 
has high level P.sub.H and low level P.sub.L. These high and low levels 
correspond to high and low levels of a pulse. Note that low level is 
higher than very low level* P.sub.R to be irradiated on the medium in a 
reproduction mode. Therefore, for example, an output waveform of a laser 
beam in the basic invention is as shown in FIG. 4A. 
Although not described in the specification of the basic invention, a 
recording beam need not always be a single beam but may be two proximity 
beams in the basic invention. More specifically, a leading beam may be 
used as a low-level laser beam (erasing beam) which is not modulated in 
principle, and a trailing beam may be used as a high-level laser beam 
(writing beam) which is modulated in accordance with information. In this 
case, the trailing beam is pulse-modulated between high level and base 
level (equal to or lower than low level, and its output may be zero). In 
this case, an output waveform is as shown in FIG. 4B. 
A bias field Hb whose direction and strength are not modulated is applied 
to a medium portion irradiated with the beam. The bias field Hb cannot be 
focused to a size as small as the portion irradiated with the beam (spot 
region), and a region where the bias field Hb is applied is considerably 
larger than the spot region. 
When a low-level beam is radiated, a bit in one of the "A direction" and 
the "non-A direction" is formed in the M layer regardless of the direction 
of magnetization of a previous bit. 
When a high-level beam is irradiated, a bit in the other direction is 
formed in the M layer regardless of the direction of magnetization of the 
previous bit. 
Thus, the over-write operation is completed. 
In the basic invention, a laser beam is pulse-modulated according to 
information to be recorded. However, this procedure itself has been 
performed in the conventional magnetooptical recording method, and a means 
for pulse-modulating the beam intensity on the basis of binary coded 
information to be recorded is a known means. For example, see "THE BELL 
SYSTEM TECHNICAL JOURNAL, Vol. 62 (1983), pp. 1923-1936 for further 
details. Therefore, the modulating means is available by partially 
modifying the conventional beam modulating means if required high and low 
levels of the beam intensity are given. Such a modification would be easy 
for those who are skilled in the art if high and low levels of the beam 
intensity are given. 
One characteristic feature of the basic invention lies in high and low 
levels of the beam intensity. More specifically, when the beam intensity 
is at high level, "A-directed" magnetization of the W layer is reversed to 
the "non-A direction" by an external means such as a bias field (Hb) and 
the like, and a bit having the "non-A-directed" [or "A-directed"] 
magnetization is thus formed in the M layer by means of the 
"non-A-directed" magnetization of the W layer. When the beam intensity is 
at low level, the direction of magnetization of the W layer is left 
unchanged from the initialized state, and a bit having the "A-directed" 
[or "non-A-directed"] magnetization is formed in the M layer under the 
influence of the W layer (this influence is exerted on the M layer through 
the exchange coupling force). 
In this specification, if expressions ooo [or .DELTA..DELTA..DELTA.] 
appear, ooo outside the parentheses in the first expression corresponds to 
ooo in the subsequent expressions ooo [or .DELTA..DELTA..DELTA.], and vice 
versa. 
A medium used in the basic invention is roughly classified into first and 
second categories. In either category, a recording medium has a 
multilayered structure including the M and W layers. 
The M layer is a magnetic layer, which exhibits a high coercivity at a room 
temperature, and has a low magnetization reversing temperature The W layer 
is a magnetic layer, which exhibits a relatively lower coercivity at a 
room temperature and has a higher magnetization reversing temperature than 
those of the M layer. Note that each of the M and W layers may comprise a 
multilayered structure. If necessary, a third layer (e.g., an adjusting 
layer for an exchange coupling force .sigma..sub.w) may be interposed 
between the M and W layers. In addition, a clear boundary between the M 
and W layers need not be formed, and one layer can be gradually converted 
into the other layer. 
In the first category, when the coercivity of the M layer is represented by 
H.sub.C1 ; that of the W layer, H.sub.C2 ; a Curie temperature of the M 
layer, T.sub.C1 ; that of the W layer, T.sub.C2 ; a room temperature, 
T.sub.R ; a temperature of the recording medium obtained when a laser beam 
at low level P.sub.L is irradiated, T.sub.L ; that obtained when a laser 
beam at high level P.sub.H is irradiated, T.sub.H ; a coupling field 
applied to the M layer H.sub.D1 ; and a coupling field applied to the W 
layer H.sub.D2, the recording medium satisfies Formula 1 below, and 
satisfies Formulas 2 to 5 at the room temperature: 
EQU T.sub.R &lt;T.sub.C1 .apprxeq.T.sub.L &lt;T.sub.C2 .apprxeq.T.sub.H Formula 1 
EQU H.sub.C1 &gt;H.sub.C2 +.vertline.H.sub.D1 .-+.H.sub.D2 .vertline.Formula 2 
EQU H.sub.C1 &gt;H.sub.D1 Formula 3 
EQU H.sub.C2 &gt;H.sub.D2 Formula 4 
EQU H.sub.C2 +H.sub.D2 &lt;.vertline.Hini..vertline.&lt;H.sub.C1 .+-.H.sub.D1 Formula 
5 
In the above formulas, symbol ".apprxeq." means "equal to" or 
"substantially equal to (.+-.20.degree. C.)". In addition, of double signs 
.+-. and .-+., the upper sign corresponds to an A (antiparallel) type 
medium, and the lower sign corresponds to a P (parallel) type medium 
(these media will be described later). Note that a ferromagnetic medium 
belongs to a P type. 
The relationship between a coercivity and a temperature is as shown in the 
graph of FIG. 6. In FIG. 6, a thin curve represents the characteristics of 
the M layer, and a bold curve represents those of the W layer. 
Therefore, when an external means, e.g., an initial field (Hini.) is 
applied to this recording medium at the room temperature, the direction of 
magnetization of only the W layer is reversed without reversing that of 
the M layer according to Formula 5. When the external means exerts an 
influence (e.g., the initial field (Hini.)) on the medium before 
recording, only the direction of magnetization of the W layer can be 
aligned in the "A direction". That is, the "initialize" process is 
performed In the following description, the "A direction" is indicated by 
an upward arrow in this specification, and the "non-A direction" is 
indicated by a downward arrow for the sake of simplicity. If the initial 
field Hini. becomes zero, the direction of magnetization of the W layer 
can be left unchanged without being re-reversed according to Formula 4. 
FIG. 7 schematically shows a state wherein only the W layer is magnetized 
by the external means in the "A direction" until a time immediately 
before recording. 
In FIG. 7, the direction of magnetization* in the M layer represents 
previously recorded information. In the following description, since the 
direction of magnetization of the M layer can be disregarded, it is simply 
indicated by X, as shown in CONDITION 1 in FIG. 7 or 8. 
In CONDITION 1, a high-level laser beam is radiated on the medium to 
increase a medium temperature to T.sub.H. Since T.sub.H is higher than the 
Curie temperature T.sub.C1, the magnetization of the M layer disappears. 
In addition, since T.sub.H is near the Curie temperature T.sub.C2, the 
magnetization of the W layer also disappears completely or almost 
completely. The bias field Hb in the "A direction" or "non-A direction" is 
applied to the medium in accordance with a type of medium. The bias field 
Hb may be a stray field from the medium itself. For the sake of 
simplicity, assume that the bias field Hb in the "non-A direction" is 
applied to the medium. Since the medium is moving, a given irradiated 
portion is immediately separated apart from the laser beam, and is cooled. 
When the medium temperature is decreased under the presence of Hb, the 
direction of magnetization of the W layer is reversed to the "non-A 
direction" to follow Hb (CONDITION 2 in FIG. 8). 
When the medium is further cooled and the medium temperature is decreased 
slightly below T.sub.C1, magnetization of the M layer appears again. In 
this case, the direction of magnetization of the M layer is influenced by 
that of the W layer through a magnetic coupling (exchange coupling) force, 
and is aligned in a predetermined direction. As a result, a 
"non-A-directed" bit (the P type medium) or an "A-directed" bit (the A 
type medium) is formed according to the type of medium. This state 
corresponds to CONDITION 3 (P type) or 4 (A type) in FIG. 8. 
A change in condition caused by the high-level laser beam will be called a 
high-temperature cycle herein. 
A laser beam at low level P.sub.L is irradiated on the medium to increase 
the medium temperature to T.sub.L. Since T.sub.L is near the Curie 
temperature T.sub.C1, the magnetization of the M layer disappears 
completely or almost completely. However, since T.sub.L is lower than the 
Curie temperature T.sub.C2, the magnetization of the W layer does not 
disappear. This state is represented by CONDITION 5 in FIG. 8. In this 
state, although the bias field Hb is unnecessary, it cannot be turned on 
or off at high speed (within a short period of time). Therefore, the bias 
field Hb in the high-temperature cycle is left applied inevitably. 
However, since the H.sub.C2 is kept high, the magnetization of the W 
layer will not be reversed by Hb. Since the medium is moving, a given 
irradiated portion is immediately separated apart from the laser beam, and 
is cooled. As cooling progresses, the magnetization of the M layer appears 
again. The direction of magnetization appearing in this case is influenced 
by the W layer through the magnetic coupling force, and is aligned in a 
predetermined direction. As a result, an "A-directed" bit (P type) or a 
"non-A-directed" bit (A type) is formed in the M layer according to the 
type of medium. This magnetization is left unchanged at the room 
temperature. This state corresponds to CONDITION 6 (P type) or 7 (A type) 
in FIG. 8. 
A change in condition caused by the low-level laser beam will be called a 
low-temperature cycle herein. 
As described above, "non-A-directed" and "A-directed" bits can be 
desirably formed by selecting the high- and low-temperature cycles 
independently of the direction of magnetization of the M layer before 
recording. More specifically, an over-write operation is enabled by 
pulse-modulating the laser beam between high level (high-temperature 
cycle) and low level (low-temperature cycle) in accordance with 
information. Refer to FIGS. 9A and 9B. FIGS. 9A and 9B illustrate 
directions of magnetization of P and A type media at the room temperature 
or formed when the medium temperature is returned to the room temperature. 
In the above description, both the first and W layers have no compensation 
temperature T.sub.comp. between the room temperature and the Curie 
temperature. However, when the compensation temperature T.sub.comp. is 
present, if the medium temperature exceeds it, 1 the direction of 
magnetization is reversed (in practice, although the directions of 
sublattice magnetization of RE and TM atoms are not changed, since the 
relationship between their strengths is reversed, the direction of 
magnetization of the alloy is reversed), and 2 A and P types are reversed. 
For these reasons, a description must be complicated accordingly. In this 
case, the direction of the bias field Hb is opposite to the direction 
.dwnarw. in the above description at the room temperature. That is, Hb in 
the same direction as the "initialized" direction .uparw. of magnetization 
of the W layer is applied. 
A recording medium normally has a disk shape, and is rotated during 
recording. For this reason, a recorded portion (bit) is influenced again 
by an external means, e.g., Hini. after recording. As a result, the 
direction of magnetization of the W layer is aligned in the original "A 
direction" . In other words, the W layer is "initialized". However, at 
the room temperature, the magnetization of the W layer can no longer 
influence that of the M layer, and the recorded information can be held. 
If linearly polarized light is irradiated on the M layer, since light 
reflected thereby includes information, the information can be reproduced 
as in the conventional magnetooptical recording medium. 
A perpendicular magnetic film constituting each of the M and W layers is 
selected from the group consisting of 1 amorphous or crystalline 
ferromagnetic and ferrimagnetic materials having no compensation 
temperature and having a Curie temperature, and 2 an amorphous or 
crystalline ferrimagnetic material having both the compensation 
temperature and the Curie temperature. 
The first category which utilizes the Curie temperature as the 
magnetization reversing temperature has been described. In contrast to 
this, the second category utilizes H.sub.C decreased at a temperature 
lower than the Curie temperature. In the second category, substantially 
the same description as the first category can be applied except that a 
temperature T.sub.S1 at which the M layer is magnetically coupled to the W 
layer is used in place of T.sub.C1 in the first category, and a 
temperature T.sub.S2 at which the direction of magnetization of the W 
layer is reversed by Hb is used in place of T.sub.C2. 
In the second category, when the coercivity of the M layer is represented 
by H.sub.C1 ; that of the W layer, H.sub.C2 ; a temperature at which the M 
layer is magnetically coupled to the W layer, T.sub.S1 ; a temperature at 
which the magnetization of the W layer is reversed by Hb, T.sub.S2 ; a 
room temperature, T.sub.R ; a medium temperature obtained when a laser 
beam at low level P.sub.L is irradiated, T.sub.L ; that obtained when a 
laser beam at high level P.sub.H is irradiated, T.sub.H ; a coupling field 
applied to the M layer, H.sub.D1 ; and a coupling field applied to the W 
layer H.sub.D2, the recording medium satisfies Formula 6 below, and 
satisfies Formulas 7 to 10 at the room temperature: 
EQU T.sub.R &lt;T.sub.S1 .apprxeq.T.sub.L &lt;T.sub.S2 .apprxeq.T.sub.H Formula 6 
EQU H.sub.C1 &gt;H.sub.C2 +.vertline.H.sub.D1 .-+.H.sub.D2 .vertline.Formula 7 
EQU H.sub.C1 &gt;H.sub.D1 Formula 8 
EQU H.sub.C2 &gt;H.sub.D2 Formula 9 
EQU H.sub.C2 +H.sub.D2 &lt;.vertline.Hini..vertline.&lt;H.sub.C1 .+-.H.sub.D1 Formula 
10 
In the above formulas, of double signs .+-. and .-+., the upper sign 
corresponds to an A (antiparallel) type medium, and the lower sign 
corresponds to a P (parallel) type medium (these media will be described 
later). 
In the second category, when the medium is at the high temperature T.sub.H, 
the magnetization of the W layer does not disappear, but is sufficiently 
weak. The magnetization of the M layer disappears, or is sufficiently 
weak. Even if sufficiently weak magnetization is left in both the M and W 
layers, the bias field Hb .uparw. is sufficiently large, and the Hb 
.dwnarw. forces the direction of magnetization of the W layer and that of 
the M layer in some cases to follow that of the Hb .dwnarw.. This state 
corresponds to CONDITION 2 in FIG. 10. 
Thereafter, the W layer influences the M layer via .sigma..sub.w 1 
immediately, or 2 when cooling progresses after irradiation of the laser 
beam is stopped and the medium temperature is decreased below T.sub.H, or 
3 when the irradiated portion is away from Hb, thereby aligning the 
direction of magnetization of the M layer in a stable direction. As a 
result, CONDITION 3 (P type) or 4 (A type) in FIG. 10 is established. 
On the other hand, when the medium is at the low temperature T.sub.L, both 
the W and M layers do not lose their magnetization. However, the 
magnetization of the M layer is relatively weak. In this case, there are 
two bit states, i.e., CONDITIONs 5 and 6 in FIG. 10 for P type, and there 
are also two bit states, i.e., CONDITIONs 7 and 8 in FIG. 10 for A type. 
In CONDITIONs 6 and 8, a magnetic wall (indicated by a bold line) is 
generated between the M and W layers, and the medium is in a relatively 
unstable (metastable) condition. The medium portion in this condition is 
applied with Hb .dwnarw. immediately before it reaches the irradiation 
position of the laser beam. Nevertheless, CONDITION 6 or 8 can be 
maintained. Because, since the W layer has sufficient magnetization at the 
room temperature, the direction of magnetization of the W layer will not 
be reversed by Hb .dwnarw.. The M layer in CONDITION 8, whose direction of 
magnetization is opposite to Hb .dwnarw., receives the influence of the 
exchange coupling force .sigma..sub.w larger than the influence of Hb 
.dwnarw., and the direction of magnetization of the M layer is held in the 
same direction as that of the W layer since the medium is of P type. 
Thereafter, the portion in CONDITION 6 or 8 is irradiated with a low-level 
laser beam. For this reason, the medium temperature is increased. Upon an 
increase in medium temperature, the coercivities of the two layers are 
decreased. However, since the W layer has a high Curie temperature, a 
decrease in coercivity H.sub.C2 is small, and the "A direction" 
corresponding to the "initialized" direction of magnetization is 
maintained without being overcome with Hb .dwnarw.. On the other hand, 
since the medium temperature is lower than the Curie temperature T.sub.C1 
of the M layer although the M layer has the low Curie temperature, the 
coercivity H.sub.C1 remains. However, since the coercivity H.sub.C1 is 
small, the M layer receives 1 the influence of Hb .dwnarw. and 2 the 
influence via the exchange coupling force .sigma..sub.w from the W layer 
(force for aligning the direction of magnetization of the M layer in the 
same direction as that of the W layer in P type). In this case, the latter 
influence is stronger than the former influence, and the following 
formulas are simultaneously satisfied: 
##EQU1## 
The lowest temperature at which these formulas are simultaneously 
satisfied will be called T.sub.LS. In other words, the lowest temperature 
at which the magnetic wall in CONDITION 6 or 8 disappears is T.sub.LS. 
As a result, CONDITION 6 transits to CONDITION 9, and CONDITION 8 transits 
to CONDITION 10. On the other hand, CONDITION 5 originally having no 
magnetic wall is the same as CONDITION 9, and CONDITION 7 is the same as 
CONDITION 10. Consequently, a bit in CONDITION 9 (P type) or 10 (A type) 
is formed upon irradiation of the low-level beam regardless of the 
previous state (CONDITION 5 or 6 for P type, or CONDITION 7 or 8 for A 
type). 
This state is maintained when the medium temperature is decreased to the 
room temperature after the laser beam irradiation is stopped or the bit 
falls outside the irradiation position. CONDITION 9 (P type) or 10 (A 
type) in FIG. 10 is the same as CONDITION 6 (P type) or 7 (A type) in FIG. 
8. 
As can be understood from the above description, the low-temperature cycle 
is executed without increasing the medium temperature up to the Curie 
temperature T.sub.C1 of the M layer. 
Even when the low-temperature cycle is executed at a temperature equal to 
or higher than T.sub.C1, since the medium temperature is increased from 
the room temperature to T.sub.C1 via T.sub.LS, CONDITION 6 transits to 
CONDITION 9 for P type, and CONDITION 8 transits to CONDITION 10 for A 
type at that time. Thereafter, the medium temperature reaches T.sub.C1, 
and CONDITION 5 shown in FIG. 8 is established. 
In the above description, both the M and W layers have no compensation 
temperature T.sub.comp. between the room temperature and the Curie 
temperature. However, when the compensation temperature T.sub.comp. is 
present, if the medium temperature exceeds it, 1 the direction of 
magnetization is reversed, and 2 A and P types are reversed. For these 
reasons, a description must be complicated accordingly. In this case, the 
direction of the bias field Hb is opposite to the direction in the above 
description at the room temperature. 
In both the first and second categories, the recording medium is preferably 
constituted by the M and W layers each of which comprises an amorphous 
ferrimagnetic material selected from transition metal (e.g., Fe, 
Co)--heavy rare earth metal (e.g., Gd, Tb, Dy, and the like) alloy 
compositions. 
When the materials of both the M and W layers are selected from the 
transition metal-heavy rare earth metal alloy compositions, the direction 
and level of magnetization appearing outside the alloys are determined by 
the relationship between the direction and level of sublattice 
magnetization of transition metal (TM) atoms, and those of heavy rare 
earth metal (RE) atoms inside the alloys. For example, the direction and 
level of TM sublattice magnetization are represented by a vector indicated 
by a dotted arrow , those of RE sublattice magnetization are represented 
by a vector indicated by a solid arrow .uparw., and the direction and 
level of magnetization of the entire alloy are represented by a vector 
indicated by a hollow arrow . In this case, the hollow arrow (vector) 
is expressed as a sum of the dotted and solid arrows (vectors). However, 
in the alloy, the dotted and solid arrows (vectors) are directed in the 
opposite directions due to the mutual effect of the TM sublattice 
magnetization and the RE sublattice magnetization. Therefore, when 
strengths of these magnetizations are equal to each other, the sum of the 
dotted and solid arrows (vectors), i.e., the vector of the alloy is zero 
(i.e., the level of magnetization appearing outside the alloy becomes 
zero). The alloy composition making the vector of the alloy zero is called 
a compensation composition. When the alloy has another composition, it has 
a strength equal to a difference between the strengths of the two 
sublattice magnetizations, and has a hollow arrow (vector or ) having a 
direction equal to that of the larger vector. Thus, a magnetization vector 
of the alloy is expressed by illustrating dotted and solid vectors 
adjacent to each other, as shown in, e.g., FIG. 11. The RE and TM 
sublattice magnetization states of the alloy can be roughly classified 
into four states, as shown in FIGS. 12(1A) to 12(4A). Magnetization 
vectors (hollow arrow or ) of the alloy in the respective states are 
shown in FIGS. 12(1B) to 12(4B). For example, the alloy in the sublattice 
magnetization state shown in FIG. 12(1A) has a magnetization vector shown 
in FIG. 12(1B). 
When one of the strengths of the RE and TM vectors is larger than the 
other, the alloy composition is referred to as "oo rich" named after the 
larger vector (e.g., RE rich). 
Both the M and W layers can be classified into TM rich and RE rich 
compositions. Therefore, when the composition of the M layer is plotted 
along the ordinate and that of the W layer is plotted along the abscissa, 
the types of medium as a whole of the basic invention can be classified 
into four quadrants, as shown in FIG. 5. In FIG. 5, the intersection of 
the abscissa and the ordinate represents the compensation composition of 
the two layers. 
The P type medium described above belongs to Quadrants I and III in FIG. 5, 
and the A type medium belongs to Quadrants II and IV. 
In view of a change in coercivity against a change in temperature, a given 
alloy composition has characteristics wherein the coercivity temporarily 
increases infinitely and then abruptly decreases before a temperature 
reaches the Curie temperature (at which the coercivity is zero). The 
temperature corresponding to the infinite coercivity is called a 
compensation temperature (T.sub.comp.). At a temperature lower than the 
compensation temperature, the RE vector (solid arrow) is larger than the 
TM vector (dotted arrow) (i.e., TM rich), and vice versa at a temperature 
higher than the compensation temperature. Therefore, the compensation 
temperature of the alloy having the compensation composition is assumed to 
be present at the room temperature. 
In contrast to this, no compensation temperature is present between the 
room temperature and the Curie temperature in the TM rich alloy 
composition. The compensation temperature below the room temperature is 
irrelevant in the magnetooptical recording, and hence, it is assumed in 
this specification that the compensation temperature is present between 
the room temperature and the Curie temperature. 
If the M and W layers are classified in view of the presence/absence of the 
compensation temperature, the medium can be classified into four types. A 
medium in Quadrant I includes all the four types of media. When both the M 
and W layers are classified in view of their RE or TM rich characteristics 
and in view of the presence/absence of the compensation temperature, 
recording media can be classified into the following nine classes. 
TABLE 1 
______________________________________ 
Class Type 
______________________________________ 
Quadrant I (P type) 
M layer: RE Rich 
W layer: RE Rich 
1 T.sub.comp. T.sub.comp. 1 
2 No T.sub.comp. T.sub.comp. 2 
3 T.sub.comp. No T.sub.comp. 
3 
4 No T.sub.comp. No T.sub.comp. 
4 
Quadrant II (A type) 
M layer: RE Rich 
W layer: TM Rich 
5 T.sub.comp. No T.sub.comp. 
3 
6 No T.sub.comp. No T.sub.comp. 
4 
Quadrant III (P type) 
M layer: TM Rich 
W layer: TM Rich 
7 No T.sub.comp. No T.sub.comp. 
4 
Quadrant IV (A type) 
M layer: TM Rich 
W layer: TM Rich 
8 No T.sub.comp. T.sub.comp. 2 
9 No T.sub.comp. No T.sub.comp. 
4 
______________________________________ 
In the above description, a two-layered film consisting of the M and W 
layers has been exemplified. An over-write operation is enabled even in a 
medium including a multi-layered film consisting of three or more layers 
as long as the medium has the above-mentioned two-layered film. In 
particular, in the above description, the initial field Hini. is used as 
the external means. However, in the basic invention, any other external 
means may be employed. That is, the direction of magnetization of the W 
layer need only be aligned in a predetermined direction before a time 
immediately before recording. 
For this reason, a structure using, as an external means, an exchange 
coupling force from an initializing layer in place of Hini. was invented 
(Japanese Journal "OPTRONICS", 1990, No. 4, pp. 227-231; International 
Application Laid-Open WO 90/02400 for further details). This invention 
will be referred to as an alternative invention hereinafter. The 
alternative invention will be described below. 
DESCRIPTION OF ALTERNATIVE INVENTION 
FIG. 13 shows a structure of a medium according to the alternative 
invention. This medium comprises a substrate and a magnetic film formed on 
the substrate. The magnetic film has a four-layered structure constituted 
by sequentially stacking an M layer 1 consisting of a perpendicularly 
magnetizable magnetic thin film, a W layer 2 consisting of a 
perpendicularly magnetizable magnetic thin film, a switching layer (to be 
referred to as an S layer hereinafter; also referred to as a control layer 
in the above-mentioned journal "OPTRONICS") 3 consisting of a 
perpendicularly magnetizable magnetic thin film, and an initializing layer 
(to be referred to as an I layer hereinafter) 4 consisting of a 
perpendicularly magnetizable magnetic thin film (in some cases, the S 
layer 3 may be omitted). The M and W layers are exchange-coupled to each 
other, and the direction of magnetization of only the W layer can be 
aligned in a predetermined direction without changing the direction of 
magnetization of the M layer at a room temperature In addition, the W and 
I layers are exchange-coupled to each other via the S layer at a 
temperature equal to or lower than a Curie temperature of the S layer. 
The I layer has a highest Curie temperature, and does not lose its 
magnetization upon radiation of a high-level laser beam. The I layer 
always holds magnetization in a predetermined direction, and serves as 
means for repetitively "initializing" the W layer to prepare for the next 
recording every time recording is performed. For this reason, the I layer 
is called the initializing layer. 
However, in a process of a high-temperature cycle (e.g., near T.sub.H), the 
magnetization of the W layer must be reversed). In this case, the 
influence from the I layer must become negligibly small. When the 
temperature is increased, an exchange coupling force .sigma..sub.w24 
between the W and I layers can be conveniently decreased. 
However, when sufficient .sigma..sub.w24 remains even at T.sub.H, the S 
layer is required between the W and I layers. If the S layer consists of a 
non-magnetic member, .sigma..sub.w24 can be reduced to zero or can become 
very small. However, .sigma..sub.w24 must be large enough to "initialize" 
the W layer at a certain temperature between T.sub.H and the room 
temperature. In this case, the S layer must apply an apparently sufficient 
exchange coupling force between the W and I layers. For this purpose, the 
S layer must consist of a magnetic member. Therefore, the S layer is 
converted to a magnetic member at a relatively low temperature to apply an 
apparently sufficient exchange coupling force .sigma..sub.w24 between the 
W and I layers, and is converted to a non-magnetic member at a relatively 
high temperature to apply a zero or very small exchange coupling force 
.sigma..sub.w24 between the W and I layers. For this reason, the S layer 
is called the switching layer. 
The principle of a four-layered film over-write operation will be described 
below with reference to FIG. 13. A typical example will be described 
below, but there are some examples in addition to this example. A hollow 
arrow indicates a direction of magnetization of each layer. 
A condition before recording corresponds to either CONDITION 1 or CONDITION 
2. Paying attention to an M layer, in CONDITION 1, an "A-directed" bit 
(B.sub.1) is formed, or in CONDITION 2, a "non-A-directed" bit (B.sub.0) 
is formed, a magnetic wall (indicated by a bold line) is present between 
the M layer and a W layer, and the medium is in a relatively unstable 
(metastable) state. 
LOW-TEMPERATURE CYCLE 
A laser beam is irradiated on the bit in CONDITION 1 or 2 to increase a 
temperature. First, magnetization of an S layer disappears. For this 
reason, CONDITION 1 transits to CONDITION 3, or CONDITION 2 transits to 
CONDITION 4. 
When the temperature is further increased, and reaches T.sub.LS, the 
magnetization of the M layer is weakened, and the influence from the W 
layer via an exchange coupling force is strengthened. As a result, the 
direction of magnetization of the M layer in CONDITION 4 is reversed, and 
at the same time, the magnetic wall between the two layers disappears. 
This condition corresponds to CONDITION 5. The bit in CONDITION 3 
originally has no magnetic wall between the two layers, and directly 
transits to CONDITION 5. 
When irradiation of the laser beam is stopped or an irradiated portion is 
separated from the irradiation position, the temperature of the bit in 
CONDITION 5 begins to fall, and CONDITION 1 is then established via 
CONDITION 3. 
This is the low-temperature cycle. 
When the temperature is further increased from that in CONDITION 5, and 
exceeds the Curie temperature of the M layer, magnetization disappears, 
and CONDITION 6 is established. When irradiation of the laser beam is 
stopped or an irradiated portion is separated from the irradiation 
position, the temperature of the bit in CONDITION 6 begins to fall, and 
then reaches a temperature slightly lower than the Curie temperature of 
the M layer. Thus, magnetization appears in the M layer. In this case, the 
direction of magnetization of the M layer is influenced by the W layer via 
the exchange coupling force, and is aligned in a stable direction with 
respect to the direction of magnetization of the W layer (i.e., in a 
direction not to form a magnetic wall between the layers). Since the 
medium is of P type, CONDITION 5 is reproduced. The temperature is further 
decreased, and CONDITION 3 is established accordingly. Thereafter, a bit 
in CONDITION 1 is formed. This process is another example of the 
low-temperature cycle. 
HIGH-TEMPERATURE CYCLE 
When a laser beam is irradiated on the bit in CONDITION 1 or 2 to increase 
a temperature, CONDITION 6 is established via CONDITION 5, as described 
above. 
When the temperature is further increased, the coercivity of the W layer is 
decreased considerably. For this reason, the direction of magnetization of 
the W layer is reversed by a bias field Hb .dwnarw.. This is CONDITION 8. 
When irradiation of the laser beam is stopped or an irradiated portion is 
separated from the irradiation position, the medium temperature begins to 
fall. The medium temperature then reaches a temperature slightly lower 
than the Curie temperature of the M layer. Thus, magnetization appears in 
the M layer. The direction of magnetization of the M layer is influenced 
by the W layer via the exchange coupling force, and is aligned in a stable 
direction with respect to the direction of magnetization of the W layer 
(i.e., in a direction not to form a magnetic wall between the layers). 
Since the medium is of P type, CONDITION 9 appears. 
When the temperature is further decreased, magnetization appears in the S 
layer. As a result, the W layer and an I layer are magnetically coupled 
(by the exchange coupling force). As a result, the direction of 
magnetization of the W layer is aligned in a stable direction with respect 
to the direction of magnetization of the I layer (i.e., in a direction not 
to form a magnetic wall between the layers). Since the medium is of P 
type, the direction of magnetization of the W layer is reversed to the "A 
direction", and as a result, an interface wall is formed between the M and 
W layers. This condition is maintained at the room temperature, and a bit 
in CONDITION 2 is formed. 
This is the high-temperature cycle. 
When the temperature is further increased after CONDITION 8 appears by the 
bias field Hb .dwnarw., the temperature then exceeds the Curie temperature 
of the W layer. As a result, CONDITION 7 appears. 
When irradiation of the laser beam is stopped or an irradiated portion is 
separated from the irradiation position, the medium temperature begins to 
fall. The medium temperature then reaches a temperature slightly lower 
than the Curie temperature of the W layer. Thus, magnetization appears in 
the W layer. The direction of magnetization of the W layer follows the 
direction of the bias field Hb .dwnarw.. As a result, CONDITION 8 appears. 
When the temperature is further decreased, a bit in CONDITION 2 is formed 
via CONDITION 9. This process is another example of the high-temperature 
cycle. 
OVER-WRITE OPERATION 
As described above, a bit (B.sub.1) in CONDITION 1 is formed in the 
low-temperature cycle, and a bit (B.sub.0) in CONDITION 2 is formed in the 
high-temperature cycle independently of a previous recording state. 
Therefore, an over-write operation is enabled. 
PROBLEMS TO BE SOLVED BY THE INVENTION 
The present inventor manufactured samples of an over-write capable 
magnetooptical recording medium which has a four-layered structure, and 
does not require an initial field Hini. according to the alternative 
invention described above, and examined their characteristics, especially, 
an allowable range (margin) of high level P.sub.H. 
Every time an over-write operation was performed while gradually changing 
the margin of P.sub.H, a bit error rate (BER) was measured by reproducing 
information. When the high-level intensity exceeded a predetermined 
P.sub.H value, the BER began to increase. The BER is preferably as low as 
possible, and a range in which the BER is not increased is determined as 
the P.sub.H margin. 
However, an additionally manufactured medium sample had a narrow P.sub.H 
margin, and this medium sample was unsatisfactory in a practical 
application. A problem to be solved by the present invention is the narrow 
P.sub.H margin. 
The narrow P.sub.H margin causes the following secondary problems. 1 The 
setting precision of the intensity level of a semiconductor laser normally 
used as a light source is about .+-.10%, individual lasers suffer from a 
variation of about .+-.10% upon manufacture, and the setting precision 
drifts by about .+-.10% as time elapses. 2 The setting precision drifts by 
about .+-.10% due to aging, or a change in environmental condition. 3 The 
intensity of a laser beam which reaches the medium via an optical system 
drifts by about .+-.10% as time elapses under the influence of dust, and 
the like. 4 Individual optical systems suffer from a variation of about 
.+-.10% upon manufacture, and the intensity of a laser beam which reaches 
the medium is varied by about .+-.10%. 5 Due to these variations or 
drifts, if the P.sub.H margin is narrow, the yield of recording 
apparatuses is decreased, and information is recorded at an intensity 
exceeding the P.sub.H margin, resulting in an increase in BER. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to enlarge a P.sub.H margin. 
The present inventor has made extensive studies, and found a cause of an 
increase in BER when P.sub.H was set beyond the P.sub.H margin. More 
specifically, the magnetization of the I layer, which must always be 
aligned in a predetermined direction, was found to be reversed in a state 
wherein the medium temperature is returned to room temperature after 
irradiation of the P.sub.H -level laser beam. It is presumed that when the 
medium temperature is increased to T.sub.H, the coercivity of the I layer 
is decreased, and for this reason, the magnetization of the I layer, which 
is opposite to the direction of the bias field Hb, is reversed by Hb. 
Therefore, the present inventor noticed that the magnetization of the I 
layer is not reversed if the bias field Hb is designed to have the same 
direction as that of the magnetization of the I layer, and manufactured 
and examined samples of an over-write capable magnetooptical recording 
medium having a four-layered structure, in which (1) the W layer is RE 
rich at room temperature, and has a compensation temperature between room 
temperature and its Curie temperature, and (2) the I layer is RE rich at 
room temperature, and has no compensation temperature between room 
temperature and its Curie temperature. Thus, the present inventor found 
that the P.sub.H margin can be enlarged, thus achieving the present 
invention. 
According to the present invention, there is provided an over-write capable 
magnetooptical recording medium comprising a memory layer consisting of 
perpendicularly magnetizable magnetic film, a writing layer consisting of 
perpendicularly magnetizable magnetic film, the writing layer being 
RE-rich at room temperature and having compensation temperature between 
room temperature and its Curie temperature, a switching layer consisting 
of perpendicularly magnetizable magnetic film, and an initializing layer 
consisting of perpendicularly magnetizable magnetic film, the initializing 
layer being RE-rich at room temperature and having no compensation 
temperature between room temperature and its Curie temperature, these four 
layers being stacked in order, wherein the memory layer and the writing 
layer are exchange-coupled, a direction of magnetization of only the 
writing layer can be aligned in a predetermined direction at room 
temperature without changing a direction of magnetization of the memory 
layer, and the writing layer and the initializing layer are 
exchange-coupled via the switching layer at a temperature not more than 
the Curie temperature of the switching layer. 
According to the finding of the present inventor, the I layer preferably 
consists of an alloy expressed by the following general formula: 
EQU Tb.sub.x (Fe.sub.100-y Co.sub.y).sub.100-x 
(for 25 atomic. %&lt;x&lt;40 atomic. %, and 10 atomic. %&lt;y&lt;100 atomic. %) 
The principle of the over-write operation of the four-layered film 
according to the present invention will be described below with reference 
to FIG. 1. The four-layered film comprises the M layer, the W layer, the S 
layer, and the I layer stacked in the order named. A large hollow arrow in 
each layer in FIG. 1 represents the direction of magnetization of the 
corresponding layer, and a small dotted arrow in the hollow arrow 
indicates the direction of transition-metal sublattice magnetization. An 
exchange coupling force acts to cause the directions of transition-metal 
sublattice magnetization of two layers to coincide with each other. 
Assume that the M layer is TM rich, and has no T.sub.comp. between the room 
temperature and its Curie temperature, the W layer is RE rich, and has 
T.sub.comp. between the room temperature and its Curie temperature 
according to the present invention, the S layer is TM rich, and has no 
T.sub.comp. between the room temperature and its Curie temperature, and 
the I layer is RE rich, and has no T.sub.comp. between the room 
temperature and its Curie temperature according to the present invention. 
However, the M and S layers are not limited to this example. 
A condition at the room temperature before recording corresponds to either 
CONDITION 1 or 2. Paying attention to the M layer, a "non-A directed" bit 
(B.sub.0) is formed in CONDITION 1, or an "A-directed" bit (B.sub.1) is 
formed in CONDITION 2. In condition 2 the M and W layers have the opposite 
directions of sublattice magnetization. For this reason, the medium is in 
a relatively unstable (metastable) state, and an interface wall (indicated 
by a bold line) is present between the M and W layers. 
LOW-TEMPERATURE CYCLE 
When a laser beam is irradiated on the bit in CONDITION 1 or 2 to increase 
a temperature, the medium temperature reaches a temperature T.sub.LS 
first. The magnetization of the M layer is weakened, and the influence 
from the W layer via the exchange coupling force is strengthened. As a 
result, sublattice magnetization of the M layer in CONDITION 2 is 
reversed, and at the same time, the magnetic wall between the M and W 
layer disappears. In this case, since the M layer is TM rich, and TM 
sublattice magnetization is larger than RE sublattice magnetization, the 
magnetization of the M layer is also reversed to follow the TM sublattice 
magnetization. This is CONDITION 3. Thus, a bit in the "non-A direction" 
is formed. 
Since a bit in CONDITION 1 originally has no magnetic wall between the M 
and W layers, it is left unchanged at T.sub.LS, and directly transits to 
CONDITION 3. 
In this manner, the low-temperature cycle is completed. More specifically, 
the "non-A-directed" bit (B.sub.0) is formed in the M layer. 
The lowest temperature (equal to the lowest temperature at which the 
interface wall between the M and W layers disappears) causing the 
low-temperature cycle is called T.sub.LS. If T.sub.LS is higher than the 
Curie temperature of the S layer, the low-temperature cycle occurs after 
the magnetization of the S layer disappears unlike in the process shown in 
FIG. 1. However, this process may also be adopted. 
Assume that the laser beam irradiation continues, the medium temperature is 
increased beyond T.sub.LS, and exceeds the Curie temperature of the S 
layer. Thus, the magnetization of the S layer disappears. 
Assume that the temperature is further increased, and the medium 
temperature exceeds T.sub.comp. of the W layer. Thus, the W layer transits 
from RE rich to TM rich. For this reason, the direction of magnetization 
of the W layer follows the direction of TM sublattice magnetization. In 
other words, the direction of magnetization of the W layer is reversed. 
This is CONDITION 4. 
When the beam irradiation further continues, the medium temperature then 
exceeds the Curie temperature of the M layer, and magnetization of the M 
layer disappears. However, sufficient magnetization remains in the W 
layer, and is not reversed by the bias field Hb. This is CONDITION 5. 
When the laser beam irradiation is stopped or the irradiated portion is 
separated from a radiation position, the temperature of the bit in 
CONDITION 5 begins to fall. When the medium temperature is decreased to a 
temperature slightly below the Curie temperature of the M layer, 
magnetization appears in the M layer. The direction of magnetization of 
the M layer is influenced by the W layer via .sigma..sub.w, and the 
direction of sublattice magnetization of the M layer follows that of the W 
layer. This state is stable in terms of energy. Since the W layer is TM 
rich at this temperature, the direction of magnetization of the W layer is 
controlled by the direction of TM sublattice magnetization. Therefore, 
CONDITION 4 is reproduced. 
When the temperature is further decreased, the medium temperature is then 
decreased below T.sub.comp. of the W layer. Thus, the W layer transits 
from TM rich to RE rich. For this reason, the direction of magnetization 
of the W layer follows the direction of RE sublattice magnetization 
(opposite to the direction of TM sublattice magnetization). In other 
words, the direction of magnetization of the W layer is reversed. 
When the temperature is further decreased, and is decreased to a 
temperature slightly below the Curie temperature of the S layer, 
magnetization appears in the S layer. This is CONDITION 3. 
This process is another example of the low-temperature cycle. 
HIGH-TEMPERATURE CYCLE 
When a laser beam is irradiated on the bit in CONDITION 1 or 2 to increase 
a temperature, CONDITION 5 is established via CONDITION 3 or 4, as 
described above. 
Assume that the beam irradiation further continues, and the temperature is 
increased. Thus, the coercivity of the W layer is considerably decreased. 
For this reason, the direction of magnetization of the W layer is reversed 
by the bias field Hb. This is CONDITION 7. 
Assume that the laser beam irradiation continues, the medium temperature is 
further increased, and exceeds the Curie temperature of the W layer. Thus, 
the magnetization of the W layer also disappears. This is CONDITION 6. 
When the laser beam radiation is stopped or the irradiated portion is 
separated from an irradiation position, the temperature of the bit in 
CONDITION 6 begins to fall. When the medium temperature is decreased to a 
temperature slightly below the Curie temperature of the W layer, 
magnetization appears in the W layer. The direction of magnetization of 
the W layer follows the direction of the bias field Hb. This is CONDITION 
7. As described above, there are two processes to CONDITION 7. 
Assume that the medium temperature is further decreased from a temperature 
in CONDITION 7, and reaches a temperature slightly below the Curie 
temperature of the M layer. Thus, magnetization appears in the M layer. At 
this time, the direction of magnetization of the M layer is influenced by 
the W layer via .sigma..sub.w, and the direction of sublattice 
magnetization of the M layer follows the direction of sublattice 
magnetization of the W layer. This state is stable in terms of energy. 
Since the W layer is TM rich at this temperature, the direction of 
magnetization of the W layer is controlled by the direction of TM 
sublattice magnetization. For this reason, CONDITION 8 appears. 
When the temperature is further decreased, the medium temperature is 
decreased below T.sub.comp. of the W layer. Thus, the W layer transits 
from TM rich to RE rich. For this reason, the direction of magnetization 
of W layer follows the direction of RE sublattice magnetization (opposite 
to the direction of TM sublattice magnetization). In other words, the 
direction of magnetization of the W layer is reversed. This is CONDITION 
9. More specifically, an "A-directed" bit (B.sub.1) is formed in the M 
layer. 
The above-mentioned process is the essential part of the high-temperature 
cycle. 
When the temperature is further decreased, and reaches a temperature 
slightly below the Curie temperature of the S layer, magnetization appears 
in the S layer. As a result, the W and I layers are magnetically coupled 
(by the exchange coupling force). More specifically, the direction of TM 
sublattice magnetization of the W layer follows that of the I layer. Since 
the W layer is RE rich at this temperature, the direction of magnetization 
of the W layer follows the direction of RE sublattice magnetization 
(opposite to the direction of TM sublattice magnetization). In other 
words, the direction of magnetization of the W layer is reversed. On the 
other hand, since the direction of TM sublattice magnetization of the W 
layer is opposite to that of the M layer, an interface wall (indicated by 
a bold line in CONDITION 2) is formed between the W and M layers. This is 
CONDITION 2. However, as one characteristic feature of this four-layered 
film medium, CONDITION 2 is maintained in a metastable state between the 
Curie temperature of the S layer and the room temperature. 
OVER-WRITE OPERATION 
As described above, a bit (B.sub.1) is CONDITION 1 is formed in the 
low-temperature cycle, and a bit (B.sub.0) in CONDITION 2 is formed in the 
high-temperature cycle independently of a previous recording state. 
Therefore, the over-write operation is enabled. 
RELATIONSHIP BETWEEN BIAS FIELD Hb AND I LAYER 
The bias field Hb is related to transition from CONDITION 5 to CONDITION 6 
or 7 in FIG. 1. In this case, the direction of Hb is the same as the 
direction of magnetization of the I layer. This is one characteristic 
feature of the present invention. For this reason, even when the I layer 
is exposed to the high temperature in the high-temperature cycle, and the 
coercivity of the I layer is decreased, the direction of magnetization of 
the I layer is never reversed under the protection of Hb. 
Therefore, even if high P.sub.H is set, since the direction of 
magnetization of the I layer is left unchanged, it is presumed that the 
BER is low. In other words, P.sub.H can be set to be high accordingly, and 
hence, the P.sub.H margin can be enlarged. 
In contrast to this, in the medium of the alternative invention as the 
prior art, since the direction of magnetization of the I layer is opposite 
to the direction of Hb, when the coercivity of the I layer is decreased at 
the high temperature, the direction of magnetization of the I layer is 
reversed. For this reason, when high P.sub.H is set, it is presumed that 
the BER is increased. In other words, P.sub.H cannot be set to be so high, 
and hence, the P.sub.H margin is narrowed. 
In the medium of the present invention, since the I layer does not have 
T.sub.comp. between the room temperature and the Curie temperature, the 
magnetization of the I layer will not be reversed in the middle of the 
process, and the direction of magnetization of the I layer is always the 
same as the direction of Hb. Thus, the magnetization of the I layer can be 
prevented from being accidentally reversed by Hb.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention will be described in detail below by way of its 
example. However, the present invention is not limited to this. 
Using an RF magnetron sputtering apparatus, a 500-.ANG. thick Tb.sub.22 
Fe.sub.73 Co.sub.5 M layer, a 1,000-.ANG. thick Tb.sub.5 Dy.sub.23 
Fe.sub.40 Co.sub.32 W layer, a 200-.ANG. thick Tb.sub.21 Fe.sub.79 S 
layer, and a 500-.ANG. thick Tb.sub.31 Fe.sub.20 Co.sub.49 I layer were 
sequentially stacked on a glass substrate having a diameter of 200 mm, 
thus manufacturing an over-write capable magnetooptical recording medium 
of this embodiment. 
EVALUATION TEST 1 
A 10-kOe external field was applied to the medium of this Example to align 
the direction of magnetization of the I layer in the "A direction". 
Then, the medium was rotated at a linear velocity of 10 m/sec, and a laser 
beam having a wavelength of 830 nm was irradiated on a given portion of 
the medium while applying the "A-directed" bias field Hb=300 Oe to the 
medium, thereby overwrite-recording reference information. In this case, 
the laser beam intensity was set to have high level P.sub.H =15.0 mW (on 
disk), and low level P.sub.L =5.0 mW (on disk), and the laser beam was 
pulse-modulated between these levels at a frequency of 4 MHz (reference 
information) (a duty ratio of pulse=50%). 
Then, the reference information was changed to a frequency of 3 MHz, and an 
over-write operation was similarly performed. 
Thereafter, reproduction was performed using a laser beam having a 
reproduction intensity P.sub.R =1.0 mW (on disk). When the obtained 
reproduced signal was analyzed by a spectrum analyzer, the reproduced 
signal did not include 4-MHz information at all, but included only 3-MHz 
information. Thus, it was demonstrated that a perfect over-write operation 
was performed. 
EVALUATION TEST 2 
Over-write and reproduction operations were repeated while changing P.sub.H 
between 8 to 20 mW at 1-mW intervals using the medium of this Example, and 
a medium of the prior art (alternative invention), thus obtaining a change 
in BER. 
When the P.sub.H margin was obtained from the measurement results, the 
medium of this Example had a margin wider than that of the medium of the 
prior art (alternative invention) by about 50%.