Magneto-optical medium and recording and/or reproducing apparatus thereof

An apparatus for recording and/or reproducing a magneto-optical recording medium having a recording layer, a reproducing layer and an intermediate layer for magnetically coupling the recording layer and the reproducing layer in a stationary state, in which the magnetic coupling between the recording layer and the reproducing layer is only able to take place in an area in which a temperature is increased to be higher than a predetermined temperature by the radiation of a light upon reproducing and in which a recorded information held in the recording layer is read out from the reproducing layer in the radiated area. This magneto-optical recording medium is characterized in that a Curie temperature of the intermediate layer is selected to be 150.degree. C. or more. Also, an apparatus which performs the above-mentioned functions and additionally which is capable of recording and/or reproducing conventional magneto-optical discs which reproduce and/or record over the entire area of irradiation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a magneto-optical recording medium and a 
magneto-optical medium recording and/or reproducing apparatus capable of 
recording and/or reproducing both from a magneto-optical medium in which a 
reproduced signal is read out from only one portion of a light radiated 
area (laser beam spot) upon reproducing to thereby record and/or reproduce 
an information at high density and from a magneto-optical recording medium 
of the conventional system in which a reproduced signal is read out from 
substantially the whole area of the laser beam spot. 
2. Description of the Prior Art 
An erasable magneto-optical disc has a magneto-optical recording layer. 
When this magneto-optical layer is irradiated with a laser beam and then 
heated, the magnetization direction (recording pit) of the heated portion 
is converted into a magnetization direction corresponding to the external 
magnetic field associated with a recording information. In this way, an 
information signal can be recorded. Upon playback, the recorded 
information signal is played back by utilizing a Kerr effect in which a 
laser beam is irradiated on the track of the recording pit and a polarized 
plane of a reflected light is rotated by the magnetization direction. In 
the case of multi-layer magneto-optical discs having a reflecting layer in 
addition to the magneto-optical layer, a Faraday effect also is utilized. 
A recording linear density of an information on the magneto-optical disc is 
determined by a carrier-to-noise (C/N) ratio of a reproduced signal. In 
the magneto-optical recording and/or reproduction of the conventional 
magneto-optical disc (hereinafter this conventional magneto-optical disc 
is referred to as an MO disc), as shown in FIG. 1, substantially the whole 
area of a beam spot 5, i.e., the laser beam light radiated area on the MO 
disc is employed as a reproduced signal detection area so that the linear 
recording density of the MO disc, which can be reproduced, is determined 
by the spot diameter of the laser beam. 
If a diameter d of the laser beam spot 5 is smaller than a pitch .tau. of a 
recording pit 4 as shown in FIG. 1A, then two recording pits 4 cannot 
enter the laser beam spot 5 and a reproduced output has a waveform shown 
in FIG. 1B, thus making it possible to read the reproduced signal. 
However, as shown in FIG. 1C, if the recording pits 4 are formed at high 
density and the diameter d of the laser beam spot 5 becomes larger than 
the pitch .tau. of the recording pit 4, then two recording pits 4, for 
example, enter the same laser beam spot 5 simultaneously and therefore the 
waveform of the reproduced output becomes substantially constant as shown 
in FIG. 1D. As a consequence, the two recording pits 4 cannot be 
reproduced separately and the reproduction becomes impossible. 
The spot diameter d depends upon a wavelength .lambda. of the laser beam 
and an numerical aperture NA of an objective lens. Accordingly, it has 
been proposed to make the recording high in density by utilizing a laser 
light of short wavelength .lambda. or by reducing the spot diameter d of 
the laser beam by increasing the numerical aperture NA of the objective 
lens. However, these proposals have unavoidable limits from a laser light 
source and optical system standpoint and these unavoidable limits hinder 
the magneto-optical disc from becoming high in recording density. 
Similarly, a track density is mainly restricted by a crosstalk component 
from adjacent tracks. In the prior art, the amount of the crosstalk 
component depends upon the laser beam spot diameter d, which also hinders 
the the magneto-optical disc from becoming high in recording density. 
The assignee of the present application has previously proposed a novel 
magneto-optical disc in which a readable linear recording density and the 
track density can be increased without varying the laser beam spot 
diameter and a method of reproducing such novel magneto-optical disc (see 
Japanese Laid-Open Patent Publication No. 3-88156 corresponding to U.S. 
Pat. No. 5,168,482). This novel magneto-optical disc will hereinafter be 
referred to as an MSR (magneto-optical super resolution) disc. 
In this MSR disc, by effectively utilizing a temperature distribution 
provided by the relative movement of the magneto-optical recording medium 
and the reproducing laser spot 5, the recording pits 4 of the 
magneto-optical recording medium will be read only from a predetermined 
temperature area upon playback, thereby a resolution and density can be 
increased. 
Two types of MSR disc systems are the rear aperture detection type and the 
front aperture detection type. 
First, the rear aperture detection type MSR disc reproducing system will be 
described with reference to FIGS. 2A and 2B. 
FIG. 2A is a schematic plan view illustrating a recording pattern of a 
magneto-optical recording medium 10 and FIG. 2B is a schematic 
cross-sectional view illustrating the magnetization state of the 
magneto-optical recording medium 10. In this case, as shown in FIG. 2A, 
the magneto-optical recording medium 10 is moved in the direction shown by 
an arrow D relative to the laser beam spot 5 formed by the laser beam. As 
shown in FIG. 2B, for example, the magneto-optical recording medium 10, 
has three layers including a reproducing layer 11 (formed of at least a 
vertical magnetization layer), and a recording layer 13 and, more 
preferably, an intermediate layer 12 interposed between the two layers 11 
and 13. In FIG. 2B, solid line arrows schematically indicate the 
directions of the magnetic moment and in the illustrated example, the 
downward arrows indicate initial state, e.g., "0" in binary value. 
Further, in FIG. 2B, the upward arrows, i.e., magnetic domains formed by 
the upward magnetization indicate "1" of binary value and in this state, 
an information recording pit 4 is formed at least on the recording layer 
13 in the form of "1". 
A reproducing mode in such magneto-optical recording medium 10 will be 
described below. 
Initially, by the application of an initialized magnetic field Hi from the 
outside, the reproducing layer 11 is magnetized in the downward direction 
in FIG. 2B and is thereby initialized. That is, magnetization of the 
reproducing layer 11 becomes uniform, i.e. uniformly "0," over pits and 
non-pit areas. After this initialization of the reproducing layer 11, the 
magnetization directions of the reproducing layer 11 and the recording 
layer 13 are held in the opposite direction by magnetic walls produced in 
the intermediate layer 12 in the area of the recording pits 4, where, the 
recording layer has a value of "1." These pits in which the recording and 
reproducing layers exhibit opposing magnetization directions are called 
latent image recording pits 81. 
On the other hand, the magneto-optical recording medium 10 is supplied at 
least at its reproducing portion with a reproducing magnetic field H.sub.r 
whose direction is opposite to that of the initialized magnetic field 
H.sub.i. In this state, when the area having a latent image recording pit 
81 comes under the laser beam spot 5, its temperature will increase due to 
the laser irradiation. Portions of the surface of the medium 10 which are 
irradiated for a longer time reach a higher temperature. The hatched high 
temperature area 14, shown in FIG. 2A represents a portion of the surface 
of the medium 10 which has been so heated by the laser beam spot 5. It 
will be noted that the high temperature area 14 makes up only a portion of 
the entire laser beam spot 5. When a latent image recording pit 81 reaches 
this high temperature area 14, the magnetic wall of the intermediate layer 
12 breaks down and the magnetization of the recording layer 13 is 
transferred to the reproducing layer 11 by an exchange force, whereby the 
latent image recording pit 81 existing in the recording layer 13 is 
embossed on the reproducing layer 11 as the reproducible recording pit 6. 
Accordingly, if the rotation of the polarizing plane of the laser beam spot 
5 by the Kerr effect due to the magnetization direction in the reproducing 
layer 11 or due to the Faraday effect is detected, then the recording pit 
4 can be read out. However, unless a latent image recording pit 81 has 
reached the high temperature area 14 of the laser beam spot 5, the latent 
image recording pit 81 is not embossed on the reproducing layer 11. 
Therefore the reproducible recording pits 6 exist only in the high 
temperature area 14 of narrow width. As a consequence, even when a 
plurality of recording pits 4 are entered into the laser beam spot 5, that 
is, even in the magneto-optical recording medium 10 of high density 
recording type, only the reproducible recording pits 6 can be read out, 
which can make it possible to perform the reproduction at high resolution. 
In order to carry out the above-mentioned playback of high resolution, the 
initialized magnetic field H.sub.i, the reproduced magnetic field H.sub.r, 
coercive force of each magnetic layer, thickness, magnetization, magnetic 
wall energy or the like are selected in response to temperatures of the 
high temperature area 14 and of the low temperature area 15 within the 
laser beam spot 5. More specifically, assuming that H.sub.C1 represents a 
coercive force of the reproducing layer 11, M.sub.S1 a saturated 
magnetization thereof and h.sub.1 a film thickness thereof, then a 
condition for initializing only the reproducing layer 11 is given by the 
following equation (1): 
EQU Hi&gt;H.sub.C1 +.rho..sub.W2 /2M.sub.S1 h.sub.1 ( 1) 
where .rho..sub.W2 is the magnetic wall energy between the reproducing 
layer 11 and the recording layer 13. 
Further, assuming that H.sub.C3 represents a coercive force of the 
recording layer 13, M.sub.S3 a saturated magnetization thereof and h.sub.3 
a film thickness thereof, then a condition such that the information of 
the recording layer 13 is maintained by the magnetic field is given by the 
following equation (2): 
EQU Hi&lt;H.sub.C3 -.rho..sub.W2 /2M.sub.S3 H.sub.3 ( 2) 
In order to maintain the magnetic wall provided by the intermediate layer 
12 between the reproducing layer 11 and the recording layer 13 even after 
the initialized magnetic field Hi, the condition expressed by the 
following equation (3) must be established: 
EQU H.sub.C1 &gt;.rho..sub.W2 /2M.sub.S1 H.sub.1 ( 3) 
Then, at a temperature T.sub.H selected within the high temperature area 
14, the condition expressed by the following equation (4) must be 
satisfied: 
EQU H.sub.C1 -.rho..sub.W2 /2M.sub.S1 H.sub.1 &lt;H.sub.r &lt;H.sub.C1 +.rho..sub.W2 
/2M.sub.S1 h.sub.1 ( 4) 
By the application of a reproducing magnetic field H.sub.r which satisfies 
the above-mentioned equation (4), the magnetization of the latent image 
recording pit 81 of the recording layer 13 can be transferred, i.e., 
embossed on the reproducing layer 11 only at its portion where the 
magnetic wall provided by the intermediate layer 12 exists. 
While the magneto-optical recording medium 10 of the MSR type is composed 
of the reproducing layer 11, the intermediate layer 12 and the recording 
layer 13 in a trilayer structure, the magneto-optical recording medium 10 
is not limited to the trilayer structure and may be applied to a 
four-layer structure in which a reproducing auxiliary layer 91 is provided 
on the intermediate layer 12 side of the reproducing layer 11 as shown in 
a schematic enlarged cross-sectional view forming FIG. 3. 
The reproducing auxiliary layer 91 assists the characteristics of the 
reproducing layer 11. By this reproducing auxiliary layer 91, the coercive 
force of the reproducing layer 11 can be compensated for at room 
temperature, and the magnetization of the reproducing layer 11 arranged by 
the initialized magnetic field H.sub.i can stably exist regardless of the 
existence of the magnetic wall. Further, the coercive force rapidly 
decreases near a reproducing temperature so that the magnetic wall 
confined within the intermediate layer 12 will spread to the reproducing 
auxiliary layer 91. Also, the magnetic wall will still break down 
satisfactorily, even with the auxiliary layer 91, and thereby the 
recording pit 4 can be embossed satisfactorily. 
When the magneto-optical recording medium 10 is formed in a four-layer 
structure fashion in which the reproducing auxiliary layer 91 is provided 
as described above, the coercive force H.sub.C1 of the reproducing layer 
11 is replaced with a coercive force H.sub.CA given by the following 
equation (5) and .rho..sub.W2 /M.sub.S1 h.sub.1 is replaced with 
.rho..sub.W2 /(M.sub.S1 h.sub.1 +M.sub.SS h.sub.S): 
EQU H.sub.CA =(M.sub.S1 h.sub.1 H.sub.C1 +M.sub.SS h.sub.S H.sub.CS)/(M.sub.S1 
h.sub.1 +M.sub.SS h.sub.S) (5) 
(inequality of H.sub.C1 &lt;H.sub.CA &lt;H.sub.CS is established in the 
above-mentioned rear aperture detection type MSR disc) where M.sub.SS, 
h.sub.S and H.sub.CS represent the saturated magnetization, the film 
thickness and the coercive force of the reproducing auxiliary layer 91, 
respectively. 
The MSR disc of the front aperture detection type will be described next 
with reference to FIGS. 4A and 4B. FIG. 4A is a schematic top view 
illustrative of the recording pattern of the magneto-optical recording 
medium 10 and FIG. 4B is a schematic cross-sectional view illustrative of 
the magnetization state. In FIGS. 4A and 4B, like parts corresponding to 
those of FIGS. 2A and 2B are marked with the same reference numerals and 
therefore need not be described in detail. In this case, the initialized 
magnetic field H.sub.i is not required. 
The reproducing mode of such magneto-optical recording medium 10 will be 
described. In this case, the following equation (6) must be established in 
the high temperature area 14 so that, even within the laser beam spot 5, 
the magnetizations of the reproducing layer 11 which reach the high 
temperature area 14 are converted to the downward direction in FIG. 4B by 
the reproducing magnetic field H.sub.r applied from the outside, thereby 
the recording pit 4 in the reproducing layer 11 is no longer reproducible. 
That is, in this MSR disc of the front aperture detection type, the 
resolution can be increased by reproducing recording pits 4 only within 
the low temperature area 15 of the beam spot 5. 
EQU H.sub.r &gt;H.sub.C1 +.rho..sub.W2 /2M.sub.S1 h.sub.1 ( 6) 
At that time, under the condition that the recording pit 4 is 
unreproducible, various conditions such as a coercive force or the like 
are set in such a fashion that the recording pit 4 is left as a latent 
image recording pit 81 in the recording layer 13. Thus, at room 
temperature, the magnetization of the recording layer 13, i.e., the 
recording pit 4 will be transferred to the reproducing layer 11 and 
returns to the reproducible condition. 
According to the above-mentioned MSR discs of the rear aperture detection 
type and the front aperture detection type, since the recording pit in the 
area of one portion of the reproducing laser beam spot is reproduced, the 
resolution in the playback mode can be improved. 
Further, it has been proposed that a magneto-optical recording medium be 
made in which the above-mentioned two MSR discs of the rear aperture 
detection type and the front aperture detection type are combined and the 
zones of varying temperature within a laser beam spot are utilized to 
further increase density and resolution. Specifically, the area of a 
magneto-optical recording medium under the laser beam spot 5 will have a 
high temperature area 14, an intermediate temperature area 16 and a low 
temperature area 15 (shown in FIG. 5). This allows the high temperature 
area 14 to function as the MSR disc of the front aperture detection type 
described in FIG. 4 and also to thereby allow the intermediate temperature 
area 16 and the low temperature area 15 to function as the two temperature 
areas necessary for a rear aperture MSR as described in connection with 
FIG. 2. 
According to the MSR disc provided by the combination of the rear aperture 
detection type MSR disc and the front aperture detection type MSR disc, 
since the reproducible recording pit 19 as shown by the hatched area in 
FIG. 5 is limited in the narrow intermediate temperature area 16 
sandwiched between the high temperature area 14 and the low temperature 
area 15, the resolution in the playback mode can be improved more. 
Incidentally, it is preferable that the MSR disc suitable for recording and 
reproducing of high resolution can be recorded and/or reproduced by an 
ordinary magneto-optical disc drive apparatus according to a recording 
and/or reproducing system which will be described below with reference to 
FIGS. 6 and 7. 
That is, for disc medium used in the data storage such as external storage 
of a computer, in order to facilitate the data processing and the data 
access, the track area on a disc medium D is divided at every sector S of 
a proper length so that data can be processed in units defined by the 
sector S as shown in FIG. 6. Then, sector control information such as a 
physical address on the disc D or the like are recorded on each sector S 
and the sector control information is written in advance in the disc as an 
emboss signal. 
FIG. 7 shows an ISO standard sector format of the WO (write once optical 
disc)/MO (erasable type optical disc). As shown in FIG. 7, one sector is 
composed of a header portion HD and a recording data portion DA, and the 
header portion HD is recorded (pre-formatted) on the optical disc medium 
in advance as the emboss signal as earlier noted. The header portion HD is 
composed of a sector synchronizing (sync.) portion and an address portion. 
The sector sync. portion is used to relatively identify the interval 
between the sectors and sector control information such as physical 
address on the disc or the like are recorded on the address portion. The 
physical address is composed of, for example, a track address and a sector 
address. In some cases, physical addresses might be sectors having serial 
numbers. The recording data is recorded only in the recording data portion 
DA in association with the sector control information of the header 
portion HD (associated information is stored in a directory area). 
However, since the magneto-optical recording medium, particularly, the 
magneto-optical recording medium of the front aperture detection type is 
arranged so as to read the recorded signal by changing the magnetization 
state of the reproducing layer in the playback mode, the magnetic 
characteristic thereof is changed at a relatively low temperature in such 
a manner that the magnetization is changed under a predetermined 
temperature condition. Accordingly, if the above magneto-optical recording 
medium is recorded and/or reproduced by the ordinary magneto-optical disc 
drive apparatus, the magnetic characteristic becomes unstable and a 
reproduced output fluctuates. There is then the risk that the recorded 
signal cannot be played back precisely. 
Furthermore, considering the recording and/or reproducing apparatus of the 
MSR disc, it is preferable that the MO disc, which is now widely and 
commercially available on the market, can be recorded and/or reproduced by 
this recording and/or reproducing apparatus. In that case, it is 
preferable that the recording and/or reproducing apparatus can use common 
hardware for recording and/or reproducing the above two discs, thereby 
simplifying the arrangement. 
OBJECTS AND SUMMARY OF THE INVENTION 
Therefore, it is an object of the present invention to provide an improved 
magneto-optical medium and apparatus for recording and/or reproducing a 
magneto-optical medium in which the aforesaid shortcomings and 
disadvantages encountered with the prior art can be eliminated. 
More specifically, it is an object of the present invention to provide a 
magneto-optical recording medium which can be recorded and/or reproduced 
at high density and which can be recorded and/or reproduced by a 
conventional magneto-optical recording and/or reproducing apparatus. 
It is another object of the present invention to provide a magneto-optical 
recording and/or reproducing apparatus of a simplified arrangement capable 
of recording and/or reproducing a magneto-optical medium at high density 
and which can also perform conventional magneto-optical recording and/or 
reproducing. 
As a first aspect, a magneto-optical recording medium of the present 
invention is comprised of at least a recording layer, a reproducing layer 
and an intermediate layer interposed between the recording layer and the 
reproducing layer, in which a recorded signal is read out by changing the 
magnetization state of the reproducing layer. In this magneto-optical 
recording medium, a Curie temperature of the intermediate layer is 
selected to be 150.degree. C. or more. 
As a second aspect, in a magneto-optical disc recording and/or reproducing 
apparatus of the present invention, a channel clock is switched when the 
MSR disc to which the front aperture detection type or rear aperture 
detection type reproducing method is applied is recorded and/or reproduced 
or when the MO disc is recorded and/or reproduced. 
The preceding and other objects, features, and advantages of the present 
invention will become apparent from the following detailed description of 
illustrative embodiments thereof to be read in conjunction with the 
accompanying drawings, in which like reference numerals are used to 
identify the same or similar parts in the several views.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings in detail and initially to FIG. 8, an embodiment 
of a magneto-optical recording medium 10 according to the present 
invention will be described in detail. In this case, the magneto-optical 
recording medium 10 is the MSR disc of the front aperture detection type. 
As shown in FIG. 8, a substrate 17 is made of a transparent material such 
as polycarbonate (PC) or the like. On one major surface of the substrate 
17, there are deposited a dielectric layer 18 made of, for example, an SiN 
layer having a thickness of 800 .ANG., a reproducing layer 11, an 
intermediate layer 12, a recording layer 13 and a surface protecting layer 
19 having a thickness of 800 .ANG. by some suitable method such as the 
sputtering-process or the like, in that order. 
In the ordinary magneto-optical drive apparatus, the revolution rate of the 
magneto-optical recording medium is 2400 r.p.m., a recording innermost 
periphery r is 30 mm, an upper limit of environmental temperature is 
50.degree. C. and an upper limit of the laser output is 1.5 mW according 
to the standards of ISO (International Organization for Standardization). 
When these standardized values are associated with the above-mentioned MSR 
disc, then the rise of temperature is about 100.degree. C. in the 
recording innermost periphery. Considering the environmental temperature, 
the temperature is assumed to rise up to about 150.degree. C. In the 
magneto-optical recording medium 10 according to the present invention, a 
Curie temperature T.sub.C2 of the intermediate layer 12 is selected to be 
150.degree. C. or more so that, when this magneto-optical recording medium 
10 is utilized in the ordinary magneto-optical disc drive apparatus, the 
temperature of the magneto-optical recording medium 10 will not exceed the 
Curie temperature T.sub.C2 of the intermediate layer 12. 
More specifically, even when the above-mentioned MSR disc 10 is utilized by 
the ordinary magneto-optical disc drive apparatus, it is possible to 
reliably avoid fluctuation of reproduced output due to the influence 
exerted on the recording pit by the erasure of magnetic characteristics of 
the intermediate temperature based on temperature dependence. Therefore, 
the magnetizations of the respective magnetic layers, particularly, the 
magnetization of the intermediate layer 12 can be held reliably so that 
this MSR disc can be accurately and positively recorded and/or reproduced 
by the ordinary magneto-optical disc drive apparatus. 
In this embodiment, the reproducing layer 11 is made of GdFeCo and 300 
.ANG. in thickness, and the recording layer 13 is made of TbFeCo or the 
like and 400 .ANG.. Then, the intermediate layer 12 is 200 .ANG. in 
thickness, for example, and the composition thereof is, for example, 
{Tb(Fe.sub.0.95 Co.sub.0.05)}.sub.0.35 Al.sub.0.05 of, the TbFeCoAl 
system. The Curie temperature T.sub.C2 thereof is selected to be 
150.degree. C. 
FIG. 9 shows a graph graphing the change of Curie temperature when z in 
(Tb.sub.0.25 (Fe.sub.0.95 Co.sub.0.05).sub.0.75).sub.1-z Al.sub.z was 
changed. From FIG. 9, it can be understood that this Curie temperature 
T.sub.C2 is proper because the Curie temperature is lowered in proportion 
to the increase of the containing amount of Al in the TbFeCoAl system. 
FIG. 10 is a graph graphing the change of Curie temperature when z in 
(Tb.sub.0.3 Fe.sub.0.7).sub.1-z Al.sub.z was changed. As shown in FIG. 10, 
it can be appreciated that the Curie temperature is lowered in accordance 
with the increase of z, i.e., the amount of Al in the TbFeAl system. On 
the other hand, FIG. 11 is a graph graphing the change of Curie 
temperature when z in Tb.sub.0.25 (Fe.sub.1-z Co.sub.z).sub.0.75 was 
changed. As shown in FIG. 10, it can be understood that the Curie 
temperature is lowered in proportion to the increase of the amount of Co. 
If the composition ratio of various materials is examined as described 
above, then it is possible to select the Curie temperature to be a 
predetermined one, i.e., 150.degree. C. or more. 
Since the Curie temperature Tc of the intermediate layer 12 is selected to 
be 150.degree. C. as described above, it is to be understood that this MSR 
disc can be normally recorded and/or reproduced by the ordinary 
magneto-optical disc drive apparatus. 
While a magneto-optical recording medium suitably applied to the MSR disc 
of the front aperture detection type is described in the above-mentioned 
embodiment of the present invention, the present invention is not limited 
to the magneto-optical recording medium thus arranged and various 
modifications may be possible. The present invention can be applied to the 
magneto-optical recording medium of the MSR type serving as both rear 
aperture detection type and front aperture detection type which can 
achieve the playback of high resolution. In this embodiment the magnetic 
layer of the magneto-optical recording medium is formed as a four-layer 
structure formed of the reproducing layer 11, the reproducing auxiliary 
layer 91, the intermediate layer 12, the recording layer 13 or the like 
as, for example, shown in FIG. 3. The recording pit is embossed on the 
reproducing layer 11 only at its narrow area of the predetermined 
temperature range as described in FIG. 5, by selecting the Curie 
temperature T.sub.C2 of the reproducing auxiliary layer 91 to be 
150.degree. C. or more. In this way, the magnetic characteristics can be 
prevented from being fluctuated at the relatively low temperature 
conventional systems and the magneto-optical disc drive apparatus can 
produce a stable reproduced output, which can therefore effect the 
accurate and reliable recording and/or reproduction in both conventional 
and high density systems. 
As described above, in the magneto-optical recording medium 10 of the 
present invention, the Curie temperature T.sub.C2 of the intermediate 
layer 12 is selected to be 150.degree. C. or more so that, even when this 
magneto-optical recording medium is utilized in the ordinary 
magneto-optical disc drive apparatus, the temperature of the 
magneto-optical recording medium 10 will not exceed the Curie temperature 
T.sub.C2 of the intermediate layer 12. Thus, the fluctuation of the 
reproduced output or the like due to the temperature dependence of the 
magnetic characteristics of the intermediate layer 12 can be avoided 
positively and the recording pit 4 of the recording layer 13 can be 
reliably held in the reproducing layer 11, which enables the 
magneto-optical recording medium of this invention to be recorded and/or 
reproduced even by the ordinary magneto-optical disc drive apparatus 
accurately and reliably. 
An apparatus for recording and/or reproducing the above-mentioned 
magneto-optical recording medium according to the present invention will 
hereinafter be described below. 
Referring to FIG. 12, there is a magneto-optical disc 20 which might be an 
MSR disc to which the above-mentioned front aperture detection type or 
rear aperture detection type reproducing method is applied or an MO disc. 
In the case of this example, the magneto-optical disc 20 is rotated at a 
constant revolution rate, i.e., in a constant angular velocity (CAV) 
fashion. The revolution rate of the MSR disc and the MO disc is selected 
to be the same value, e.g., 2400 r.p.m. Also, the MSR disc and the MO disc 
are the same in size and may be rotated at a constant linear velocity 
(CLV) fashion. 
In this embodiment, as earlier noted, the sector number of the disc per 
track and the sector format of the MSR disc and the MO disc are the same, 
that is, they are in accordance with the ISO standard sector format. The 
pre-formatted portions of the header portion HD of the MSR disc and the MO 
disc are equal to each other. 
In the case of this embodiment, the MSR disc or the MO disc have the same 
data format such as the recording modulation system of information or the 
like and the MSR disc has the linear recording density of recording data 
higher than that of the MO disc, for example, twice the density. However, 
the linear recording density of the MSR disc is selected to be equal to 
that of an MO disc in the pre-formatted portion recorded by the emboss 
signal because the emboss signal is recorded on the pre-formatted portion 
to which the reproducing method of the front aperture detection type or 
rear aperture detection type cannot be applied. Accordingly, in the case 
of MSR disc, the recording density is different at the header portion HD 
and the recording data portion DA. 
The MSR disc to which the reproducing method of the front aperture 
detection type is applied might be such one that the recording layer, for 
example, is made of TbFeCo, the intermediate layer is made of TbFeCoAl and 
the reproducing layer is made of GdFeCo. Further, the MSR disc to which 
the reproducing method of the rear aperture detection type is applied 
might be such one that the recording layer, for example, is made of 
TbFeCo, the intermediate layer is made of GdFeCo, the reproducing 
auxiliary layer is made of TbFeCoAl and the reproducing layer is made of 
GdFeCo. 
If the MSR disc is not recorded and/or reproduced by the MO disc recording 
and reproducing apparatus, then it is possible to use such an intermediate 
layer whose Curie temperature is lower than 150.degree. C., for example, 
about 120.degree. C. 
Further, in the case of this embodiment, pre-grooves for tracking are 
formed on the disc and these pre-grooves are formed on the MSR disc and 
the MO disc in accordance with the common specification. The tracking 
method is not limited to the method using the pre-grooves and various 
well-known methods can be applied. 
Furthermore, a disc cartridge has an identifying aperture to discriminate 
the MSR disc and the MO disc. 
The recording and/or reproducing apparatus of this invention will now be 
described with reference to a block diagram forming FIG. 12. In this case, 
FIG. 12 is formed of FIGS. 12A and 12B drawn on two sheets of drawings so 
as to permit the use of a suitably large scale. 
Referring to FIG. 12, a host computer 44 is connected to a system 
controller 40. The system controller 40 is controlled by an instruction 
from the host computer 44 to thereby record and/or reproduce the data as 
will be described later. Also, data is transmitted and received between 
the system controller 40 and the host computer 44. A servo circuit 43 is 
connected to and controlled by the system controller 40 to effect the 
focusing servo, tracking servo and so on. 
A disc type identifying or discriminating device 70 is adapted to detect 
the disc type identifying aperture to determine whether a disc installed 
on the recording and/or reproducing apparatus is the MSR disc or the MO 
disc. A disc identifying output signal from the device 70 is supplied to 
the system controller 40 and the system controller 40 carries out the 
control corresponding to the MSR disc and the MO disc in response to the 
disc identifying output signal. 
A laser light source 21 is provided to emit a laser beam and the laser beam 
from the laser light source 21 becomes incident on a magneto-optical disc 
20. Part of the laser beam from the laser light source 21 becomes incident 
on a photo-detector 22 which is used to monitor the laser power. A 
photo-electrically converted output from the photo-detector 22 is supplied 
to an auto power control circuit 23. The auto power control circuit 23 
compares the output of the photo-detector 22 and a laser power setting 
reference value REF from a laser power reference value generating circuit 
24. A difference output from the generator circuit 24 is supplied to a 
laser drive circuit 25 to control the output power of the laser light 
source 21. That is, under the control of the above-mentioned closed loop, 
the output power of the laser light source 21 becomes a value 
corresponding to the laser power setting reference value REF. 
The laser power reference value generating circuit 24 is supplied with a 
mode switching signal from the system controller 40 and changes the laser 
power reference value REF in the recording mode, reproducing mode or in 
the erasing mode in response to the mode switching signal. Also, the laser 
power reference value REF is changed in accordance with the MSR disc and 
the MO disc. In this case, the laser power set reference value REF in the 
playback mode of the MSR disc is set in advance such that the output laser 
power of the laser light source 21, that is, the area of the reproducing 
area 9 or 18 falls in a predetermined optimum value. That is, if the laser 
output power is changed, then the area of the area exceeding a threshold 
value temperature T.theta. is changed on the disc as S1 and S2 by the 
radiation of laser beam as shown in FIG. 13. This area is the high 
temperature area 14 so that, if the laser power is controlled as described 
above, then the area of the high temperature area 14 can be made to cover 
a predetermined area. 
If the MSR disc need not be recorded and/or reproduced by the MO disc 
recording and/or reproducing apparatus, the laser power reference value 
REF need not always be changed in accordance with the MSR disc and the MO 
disc. 
In the case of this embodiment, an external magnetic field H.sub.re is 
generated by supplying a drive current to a magnetic field generating coil 
51 from a driver 52. The magnetic field generating coil 51 is provided at 
the position opposing the laser light source 21 in the surface side 
opposite to the surface of the magneto-optical disc 20 radiated by the 
laser beam. The driver 52 is supplied with a reference value M.sub.ref 
from the reference value generating circuit 53 and drives the magnetic 
field generating coil 51 such that the magnitude of the external magnetic 
field H.sub.re from the magnetic field generating coil 51 falls in a 
predetermined value corresponding to the reference value. 
The reference value generating circuit 53 is supplied with the mode 
switching signal from the system controller 40 and controls the magnitude 
of the external magnetic field H.sub.re in response to the recording mode, 
the reproducing mode or the erasing mode or in response to the MSR disc or 
the MO disc. That is, in the recording mode and in the erasing mode, 
predetermined external magnetic fields suitable for the respective modes 
are generated regardless of the type of the disc. When the MO disc is 
reproduced, then the external magnetic field is inhibited from being 
generated. Also, when the MSR disc is reproduced, the aforementioned 
predetermined reproduced magnetic field H.sub.re is generated as this 
external magnetic field as earlier noted. 
A reflected light of the laser beam radiated on the magneto-optical disc 20 
from the laser light source 21 is introduced through an optical system 
(not shown) to a playback photodetector 31, in which it is 
photo-electrically converted. 
An output signal of this photo-detector 31 is supplied through a head 
amplifier 32 to an RF amplifier 33 provided as a signal processor circuit 
which then derives an RF signal. This RF signal from the RF amplifier 33 
is supplied to and then converted into a digital signal by a pulse shaping 
circuit 34. This digital signal is supplied to a phase-locked loop (PLL) 
circuit 35 which derives a clock signal synchronized with the reproduced 
signal. The PLL circuit 35 is supplied with a control signal from the 
system controller 40 and varies a synchronizing frequency in response to 
the difference between recording densities of the MSR disc and the MO 
disc. In this embodiment, since the channel clock frequencies of the MSR 
disc and the MO disc are selected in the ratio of 2:1, the PLL circuit 35 
may change only the frequency-dividing ratio. 
The digital signal from the pulse shaping circuit 34 and the clock signal 
from the PLL circuit 35 are supplied to a decoder/encoder 60. 
The decoder/encoder 60 is controlled in mode by the system controller 40 
and extracts the sector control information from the reproduced signal to 
thereby decode the physical address of each sector. Also, the 
decoder/encoder 60 decodes the recording data read out from the recording 
data portion DA. The data thus decoded is supplied through the system 
controller 40 to the host computer 44. Write data from the host computer 
44 is supplied through the system controller 40 to the decoder/encoder 60 
and is thereby modulated into data of a predetermined data format, that 
is, encoded by a (2, 7) modulation (run length limited code). 
The write data from the decoder/encoder 60 is supplied to a write pulse 
generating circuit 61. The system controller 40 supplies a channel clock 
generating circuit 62 with the switching signal corresponding to the 
identifying information illustrative of the MSR disc or MO disc. The 
channel clock generating circuit 62 supplies the write pulse generating 
circuit 61 with a channel clock which determines a timing at which the 
write pulse is generated from the generating circuit 61. In this 
embodiment, if the magneto-optical recording medium is the MSR disc, then 
a channel clock of frequency twice as high as the normal channel clock for 
the MO disc is supplied to the write pulse generating circuit 61 by the 
switching signal. 
The write pulse corresponding to the write data from the write pulse 
generating circuit 61 is supplied to the auto power control circuit 23 at 
the timing synchronized with the aforementioned channel clock. Upon 
recording, when the write pulse is added to the reference value REF from 
the laser power value generating circuit 24, the temperature of the disc 
is increased more than the Curie temperature of the recording layer, 
whereby the magnetization of the recording layer is inverted to the 
direction of the external magnetic field H.sub.re, thereby the binary 
value data being recorded. 
In this way, data is recorded on the MSR disc and the MO disc. In the case 
of the MSR disc, the frequency of the channel clock is twice that of the 
MO disc so that data can be recorded thereon at twice linear recording 
density. 
In the case of the MO disc, the recorded data is reproduced by the scanning 
of the laser beam spot in substantially the whole area of the spot 
diameter under the condition such that the external magnetic field 
H.sub.re is zero. At that time, the output clock of the PLL circuit 35 
will have a low clock frequency corresponding to the channel clock 
frequency of the MO disc. 
Further, in the case of the MSR disc, the recorded information of high 
density is read out from the high temperature area narrower than the laser 
beam spot diameter by the radiation of the laser beam spot on the disc 20 
under the condition such that the reproducing external magnetic field 
H.sub.re is generated from the magnetic field generating coil 51. At that 
time, the output clock of the PLL circuit 35 will have a high clock 
frequency corresponding to the channel clock frequency of the MSR disc. 
The focusing servo and the tracking servo are effected as follows: 
As shown in FIG. 12, the output of the head amplifier 32 is supplied to a 
matrix amplifier 41, and the matrix amplifier 41 generates a focusing 
servo signal and a tracking servo signal by using outputs of a plurality 
of divided sensing units of the photo-detector 31. The focusing servo 
signal and the tracking servo signal from the matrix amplifier 41 are 
supplied through a servo amplifier 42 to a servo system 43, whereby a 
focusing lens of an optical system (not shown) is positionally controlled 
by using, for example, an actuator to effect the focusing control and also 
positions of a tracking correction lens and an optical pickup are 
controlled to effect the tracking control. In this embodiment, the servo 
system can be made common to the MSR disc and the MO disc. 
While the MSR disc and the MO disc are discriminated by means of the disc 
identifying aperture formed through the disc cartridge as described in the 
above-mentioned embodiment, variations are also possible. For instance, a 
control track in which a disc identifying information is recorded is 
formed on the disc in advance so that, when the system is actuated, then 
the disc can be identified by reading the disc identifying information 
from this control track. In this case, the control track may be provided 
only in the MSR disc because the MO disc can be identified without the 
identifying information. 
Further, as the method of identifying the MSR and MO discs, the following 
method may be possible, in which an area in which a reference signal 
reproduced by the playback of the front aperture detection type or rear 
aperture detection type is formed on the MSR disc in advance. More 
specifically, upon actuation, if this area is placed in the playback mode 
under the condition such that the external magnetic field is applied 
thereto, the reference signal can be reproduced from the MSR disc but the 
reference signal cannot be reproduced from the MO disc. thereby the two 
MSR and MO discs can be discriminated from each other. 
While the output power of the laser light source 21 is controlled by 
setting the laser power set reference value REF to the proper value so 
that the area of the high temperature area 14 in the playback mode of the 
MSR disc may fall in the predetermined optimum value as described in the 
above embodiment, similar effects can be achieved by controlling the 
external magnetic field (reproduced magnetic field H.sub.re). 
Considering the reproducing method of the erase type, for example, 
precisely speaking, the temperature at which the high temperature area 14 
starts being formed is not the Curie temperature T.sub.C2 of the 
intermediate layer 12 in FIG. 4 but is a temperature associated with the 
reproduced magnetic field H.sub.re and which is expressed by the following 
equation (7); 
EQU H.sub.C1 +H.sub.w &lt;H.sub.re (7) 
where H.sub.C1 is the coercive force of the reproducing layer 11 and 
H.sub.w the exchange coupling force between the reproducing layer 11 and 
the recording layer 13. The exchange coupling force H.sub.w between the 
reproducing layer 11 and the recording layer 13 is reduced in accordance 
with the rise of temperature and becomes zero at the Curie temperature 
T.sub.C2 of the intermediate layer 12. 
A temperature characteristic of H.sub.C1 +H.sub.w is illustrated in FIG. 
14. In FIG. 14, T.sub.C1 represents the Curie temperature of the 
reproducing layer 11 and the coercive force of the reproducing layer 11 
becomes similar to that of the single layer at the temperature higher than 
the Curie temperature T.sub.C2 of the intermediate layer 12. 
In order to arrange the magnetization directions of the reproducing layer 
11 of the magneto-optical disc in the same direction, the application of a 
magnetic field larger than H.sub.C1 +H.sub.w is needed as shown in the 
above-mentioned equation (7). Accordingly, even in the same temperature 
distribution state, if the application of a magnetic field H.sub.r0 is 
effected as the application of the reproduced magnetic field H.sub.re, an 
area of the range higher than the Curie temperature T.sub.C2 becomes the 
high temperature area 14. However, if the magnitude of the reproduced 
magnetic field H.sub.re is equal to H.sub.r1, an area of the range of 
temperature Ta lower than the Curie temperature T.sub.C2 becomes the high 
temperature area 14 and the size of the high temperature area 14 is 
changed in response to the magnitude of the reproduced magnetic field 
H.sub.re. 
Therefore, the size of the high temperature area 14 can be controlled to 
fall in a predetermined value by controlling the external magnetic field 
H.sub.re. 
Also in the reproducing method of the rear aperture detection type, the 
size of the high temperature region 14 can be controlled to fall in the 
predetermined size by similarly controlling the external magnetic field. 
Further, the size of the high temperature area 14 in the playback mode of 
the MSR disc can be controlled to fall in a predetermined value by 
adjusting both the laser power and the external magnetic field. 
As described above, according to this embodiment, by switching the channel 
clock, the MO disc can be recorded and/or reproduced by the MSR disc 
recording and/or reproducing apparatus. Further, this MSR disc recording 
and/or reproducing apparatus has many common portions from a hardware 
standpoint and therefore can be made compact in size from a circuit scale 
and space factor standpoint. 
A second embodiment of the present invention will be described below. In 
this embodiment, while the rotational speed and the pre-format are similar 
to those of the MO disc system similarly to the aforementioned embodiment, 
the optimum channel clock and data format of the MSR disc are selected. 
For the MSR disc, for example, the channel clock is selected to be about 
2.5 times the channel clock of the conventional MO disc and the data 
format employs (1, 7) modulation and the edge recording system for the MSR 
disc while the data format of the MO disc system is the aforementioned (2, 
7) modulation. 
FIG. 15, which is formed of FIGS. 15A and 15B drawn on two sheets of 
drawings so as to permit the use of a suitably large scale, is a block 
diagram of the second embodiment of the recording and/or reproducing 
apparatus according to the present invention. In FIG. 15, like parts 
corresponding to those of FIG. 12 are marked with the same references and 
therefore need not be described. 
As shown in FIG. 15, the decoder/encoder 60 is comprised of an MO disc 
decoder/encoder unit 60A and an MSR disc decoder/encoder unit 60B in 
response to the difference between the data formats of the MO disc and the 
MSR disc. These decoder/encoder units 60A and 60B are switched in response 
to the identifying output of the disc type discriminating device 70 when 
the MO disc is recorded and/or reproduced or when the MSR disc is recorded 
and/or reproduced. A rest of the circuit arrangements of FIG. 15 is the 
same as that of the embodiment of FIG. 12. 
According to this embodiment, the linear recording density of the MSR disc 
can be increased. 
While a compatibility of the MO disc with the MSR disc is taken into 
consideration because the data format of the MO disc widely available on 
the market is not negligible as described above, it can be expected that 
an MO disc of the next generation will be developed in the future. 
Therefore, this embodiment utilizes the data format suitable for recording 
and/or reproducing the MSR disc as a data format of the MO disc of the 
next generation. For example, the (1, 7) modulation and the edge recording 
system are employed and this data format is applied to the recording 
and/or reproduction of the MSR disc and the MO disc. It is, however, 
needless to say that the data format most suited to the MSR disc is not 
limited to the above-mentioned system. 
With the above-mentioned arrangement, it is possible to realize a disc 
recording and/or reproducing apparatus of a small circuit scale which can 
record and/or reproduce the MO disc with ease while demonstrating the 
effect of the high recording density of the MSR disc. 
As described above, since the high density recording of the MSR disc can be 
realized without reducing the laser beam spot diameter, the laser beam 
spot diameter of the conventional MO disc can be utilized without 
modifications. Therefore, the optical system can be made common to the MSR 
disc and the MO disc. 
Furthermore, even when the MO disc and the recording density are varied in 
order to demonstrate the effect of the high density recording of the MSR 
disc at maximum, in the MSR disc recording and/or reproducing apparatus, 
the MO disc can be recorded and/or reproduced by switching the channel 
clock. Therefore, the MSR disc recording and/or reproducing apparatus 
having a compatibility with the MO disc can be realized without increasing 
the circuit scale too much. 
Having described the preferred embodiments of the invention with reference 
to the accompanying drawings, it is to be understood that the invention is 
not limited to those precise embodiments and that various changes and 
modifications thereof could be effected by one skilled in the art without 
departing from the spirit or scope of the invention as defined in the 
appended claims.