Signal reproducing apparatus using movement of magnetic wall

In a signal reproducing apparatus, information is reproduced by moving a magnetic wall on a magnetic recording medium while effecting relative movement between a light spot and the medium. A light spot is irradiated for forming a temperature distribution for moving the magnetic wall on the medium. A differential detecting unit takes a positive or negative value in accordance with a direction of magnetization in the light spot. An edge detecting unit takes a positive or negative value in accordance with directions of magnetization, before and after the magnetic wall as a boundary, in the relative moving direction in the light spot. A reproduction signal is generated on the basis of the detection result from the differential detecting unit and the detection result from the edge detecting unit.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a signal reproducing apparatus for 
reproducing recorded information by irradiating a light spot on a 
magneto-optical medium and, more particularly, to a signal reproducing 
apparatus using a magnetic wall movement reproducing method. 
2. Related Background Art 
Recently, a high-density magneto-optical medium by which information is 
recorded and reproduced by using a fine light spot has attracted 
attention. FIG. 1 shows an optical system of a magneto-optical 
recording/reproducing apparatus for performing recording and reproduction 
of information with respect to a magneto-optical medium. Referring to FIG. 
1, this apparatus includes a semiconductor laser 28 as a light source. 
Divergent light emitted from the semiconductor laser 28 is collimated into 
a parallel light beam by a collimator lens 29. This parallel light beam 
from the collimator lens 29 is fed into an objective lens 32 via a beam 
shaping prism 30 and a polarizing beam splitter 31 and focused into a fine 
light spot on a magnetic layer of a magneto-optical medium 33 by the 
objective lens 32. Meanwhile, an external magnetic field is applied from a 
magnetic head 34 to the magneto-optical medium 33. 
The reflected light from the magneto-optical medium 33 returns to the 
polarizing beam splitter 31 via the objective lens 32. A portion of the 
reflected light is separated by the polarizing beam splitter 31 and 
supplied to a control optical system. The control optical system further 
separates the separated light beam and supplies one separated light beam 
to a reproducing optical system 36 to generate an information signal. The 
control optical system supplies the other separated light beam to a 
photodetector 44 via a condenser lens 42 and a half prism 43 and to a 
photodetector 46 via a knife edge 45 to generate a control signal for 
automatic focusing or automatic tracking. The reproducing optical system 
36 includes a halfwave plate 37 for rotating the polarizing direction of a 
light beam through 45.degree., a condenser lens 38 for focusing the light 
beam, a polarizing beam splitter 39 for separating the light beam, and 
photodetectors 40 and 41 for detecting the two light beams separated by 
the polarizing beam splitter 39. A magneto-optical signal is obtained by 
differentially detecting signals from the photodetectors 40 and 41. 
A method of obtaining the magneto-optical signal will be described below 
with reference to FIG. 2. First, in the magneto-optical medium 33, 
information is recorded as a pit (magnetic domain) as a difference between 
the magnetizing directions. Therefore, when linearly polarized light is 
given, the polarizing direction of the linearly polarized light rotates 
clockwise or counterclockwise in accordance with the difference between 
the magnetizing directions. Assume, for example, that the polarizing 
direction of linearly polarized light incident on the magneto-optical 
medium 33 is the direction of a coordinate axis P shown in FIG. 2, 
reflected light for downward magnetization is R+ which is rotated 
+.theta.k, and reflected light for upward magnetization is R- which is 
rotated -.theta.k. When an analyzer is placed in a direction as shown in 
FIG. 2, light transmitting through the analyzer is A with respect to R+ 
and B with respect to R-. By detecting these light beams by 
photodetectors, information can be obtained as the difference between the 
light intensities. In FIG. 1, the polarizing beam splitter 39 functions as 
an analyzer; i.e., the polarizing beam splitter 39 is an analyzer in a 
direction of +45.degree. from the P axis with respect to one separated 
light beam and an analyzer in a direction of -45.degree. from the P axis 
with respect to the other separated light beam. That is, the signal 
components obtained by the photodetectors 40 and 41 have opposite phases. 
Accordingly, by differentially detecting these signals, a reproduction 
signal with reduced noise can be obtained. 
As described above, in a magneto-optical medium a pit (magnetic domain) as 
information is recorded as perpendicular magnetization in a thin magnetic 
film by using thermal energy of a semiconductor laser. This information is 
read by using a magneto-optical effect. Recently, demands on a higher 
recording density of this magneto-optical medium have increased. 
Generally, it can be said that the linear recording density of an optical 
disk as one magneto-optical medium depends upon the laser wavelength of a 
reproducing optical system and the NA (Numerical Aperture) of an objective 
lens. That is, the diameter of a light spot is determined when the laser 
wavelength .lambda. of a reproducing optical system and the NA of an 
objective lens are determined. Consequently, the size of a reproducible 
pit (magnetic domain) has a limitation of about .lambda./(2NA). Therefore, 
to realize a high density in a conventional optical disk, it is necessary 
to shorten the laser wavelength of a reproducing optical system or 
increase the NA of an objective lens. However, improvements of the laser 
wavelength and the NA of an objective lens also have their limits. 
Accordingly, development of a technique is being attempted which increases 
the recording density by improving the construction of a recording medium 
or a method of reading a recording medium. 
For example, Japanese Patent Application Laid-Open No. 3-93058 has proposed 
a reproducing method in which a signal is recorded in a recording holding 
layer of a multilayered film having a reproduction layer and the recording 
holding layer which are magnetically coupled with each other and, after 
the directions of magnetization are aligned, laser light is irradiated on 
the reproduction layer to heat the reproduction layer, thereby reading the 
signal while transferring the signal recorded in the recording holding 
layer to the heated area in the reproduction layer. Also, Japanese Patent 
Application Laid-Open No. 6-290496 has proposed a magnetic wall movement 
reproducing method in which a light spot is irradiated on a 
magneto-optical medium, which is formed by stacking a plurality of 
magnetic layers, to transfer a pit (magnetic domain) recorded as 
perpendicular magnetization in a recording layer to a reproduction layer, 
and magnetic walls of the pit (magnetic domain) transferred to the 
reproduction layer are moved to make this pit (magnetic domain) larger 
than the pit (magnetic domain) in the recording layer, thereby reproducing 
the pit. 
This magnetic wall movement reproducing method will be described below. 
FIGS. 3A to 3D are schematic views for explaining a magneto-optical medium 
used in the magnetic wall movement reproducing method and the action of 
the magneto-optical medium. FIG. 3A is a schematic view showing the 
surface of the magneto-optical medium. FIG. 3B is a schematic view showing 
the section of the magneto-optical medium. Referring to FIGS. 3A and 3B, a 
reproducing light spot 48 and an information track 47 on the 
magneto-optical medium are shown. The magneto-optical medium is 
constituted by three magnetic layers, i.e., first, second, and third 
magnetic layers 50, 51, and 52. Arrows in each layer indicate the 
directions of atomic spins. Magnetic walls 49 are formed in regions where 
the directions of spins are opposite to each other. 
FIG. 3C is a graph showing a temperature distribution formed in this 
magneto-optical medium. Assume that in a position X.sub.s the medium 
temperature is a temperature T.sub.s near the Curie temperature of the 
second magnetic layer 51. FIG. 3D shows the distribution, which 
corresponds to the temperature distribution in FIG. 3C, of a magnetic 
energy density .sigma.1 in the first magnetic layer 50. As shown in FIG. 
3D, when a gradient of the magnetic wall energy density .sigma.1 exists in 
an X direction, a force F1 is produced with respect to magnetic walls 
present in a position X in the individual layers. This force F1 so acts as 
to move the magnetic walls to a portion where the magnetic wall energy is 
low. In the first magnetic layer 50, magnetic wall coercivity is small, 
and the magnetic wall mobility is large. Therefore, the magnetic walls in 
the first magnetic layer 50 alone are easily moved by the force F1. 
However, the medium temperature is still lower than T.sub.s in a region 
before the position X.sub.s (on the right-hand side in FIG. 3D). 
Accordingly, by exchange coupling with the third magnetic layer 52 having 
a large magnetic wall coercivity, magnetic walls in the first magnetic 
layer 50 are fixed to positions corresponding to the positions of magnetic 
walls in the third magnetic layer 52. 
If one of the magnetic walls 49 exists in the position X.sub.s of the 
medium as shown in FIG. 3B, the medium temperature rises to the 
temperature T.sub.s near the Curie temperature of the second magnetic 
layer 51, and this breaks the exchange coupling between the first and 
third magnetic layers 50 and 52. As a consequence, the magnetic wall 49 in 
the first magnetic layer 40 instantaneously moves, as indicated by an 
arrow, to a region where the temperature is higher and the magnetic wall 
energy density is lower. That is, when the reproducing light spot 48 
passes by, the magnetic wall moves as described above, and atomic spins in 
the first magnetic layer 50 in the spot are pointed in the same direction. 
The magnetic wall instantaneously moves as the medium moves, and all 
atomic spins in the light spot are reversed and pointed in the same 
direction. Consequently, a signal reproduced by the light spot always has 
a fixed amplitude regardless of the size of a pit (magnetic domain) 
recorded in the third magnetic layer 52; i.e., the signal is free from the 
problem of waveform interference resulting from optical diffraction 
limits. Accordingly, it is possible to reproduce a pit (magnetic domain) 
smaller than about .lambda./(2NA) which is the resolution limit determined 
by the laser wavelength .lambda. and the NA of an objective lens. 
Consequently, the recording density can be increased. 
FIG. 4 is a schematic view showing the arrangement of an optical system 
used in magnetic wall movement reproduction. Referring to FIG. 4, a 
recording/reproducing semiconductor laser 53 has a wavelength of, e.g., 
780 nm. A heating semiconductor laser 55 has a wavelength of, e.g., 1.3 
.mu.m. These semiconductor lasers 53 and 55 are so arranged that their 
laser beams are incident as P-polarized light on a recording medium. The 
laser beams emitted from the semiconductor lasers 53 and 55 are shaped 
into substantially circular beams by beam shaping means (not shown) and 
converted into parallel light beams by collimator lenses 54 and 56. This 
optical system further comprises a dichroic mirror 57 and a polarizing 
beam splitter 58. The dichroic mirror 57 is so designed as to transmit 
100% of light with a wavelength of 780 nm and reflect 100% of light with a 
wavelength of 1.3 .mu.m. The polarizing beam splitter 58 transmits 70 to 
80% of P-polarized light and reflects almost 100% of S-polarized light as 
a vertical component. 
The parallel light beams converted by the collimator lenses 54 and 56 enter 
an objective lens 59 via the dichroic mirror 57 and the polarizing beam 
splitter 58. This part of the optical system is so designed that a light 
beam of 780 nm becomes large with respect to the aperture of the objective 
lens 59 and a light beam of 1.3 .mu.m becomes small with respect to the 
aperture of the objective lens 59. Accordingly, even when the same 
objective lens 59 is used, the action of the NA of the lens is small to 
the 1.3-.mu.m light beam, so the size of a light spot on a recording 
medium 60 becomes larger than that formed by the 780-nm light beam. The 
reflected light from the recording medium 60 is formed into a parallel 
light beam through the objective lens 59 and reflected by the polarizing 
beam splitter 58 to form a light beam 61. This light beam 61 is incident 
on an optical system (not shown) and subjected to, e.g., wavelength 
separation, thereby generating a servo error signal or an information 
reproduction signal. 
The relationship between the recording/reproducing light spot and the 
heating light spot on the recording medium shown in FIG. 4 will be 
described below with reference to FIGS. 5A and 5B. Referring to FIG. 5A, a 
recording/reproducing light spot 62 has a wavelength of 780 nm, and a 
heating light spot 63 has a wavelength of 1.3 .mu.m. Pits (magnetic 
domains) recorded in a land 65 have magnetic walls 64, and grooves 66 are 
also formed. A region 67 is a heated region whose temperature is raised by 
the heating light spot 63. In this manner, by coupling the 
recording/reproducing light spot 62 and the heating light spot 63 on the 
land 65 between the grooves 66, a temperature gradient as shown in FIG. 5B 
can be formed on the moving medium. The relationship between the 
temperature gradient and the recording/reproducing light spot 62 is as 
explained in FIGS. 3A to 3D. Consequently, magnetic wall movement 
reproduction as described above can be performed. 
In the above conventional magnetic wall movement reproducing method, 
reproduction is performed by using a reproducing light spot and a heating 
light spot. This increases the number of parts such as semiconductor 
lasers and complicates the structure. Therefore, it is possible to 
simplify the structure by performing magnetic wall movement reproduction 
by using a reproducing light spot alone without using any heating light 
spot. However, when reproduction is thus performed only with a reproducing 
light spot, the peak of a high-temperature portion on a magneto-optical 
medium comes inside the reproducing light spot. Accordingly, magnetic 
walls move in a direction opposite to the moving direction of the 
magneto-optical medium and in the same direction as the moving direction 
of the magneto-optical medium. Consequently, the influences of the two 
signals mix in the reproduction signal, and this makes the information 
difficult to reproduce. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a signal reproducing 
apparatus capable of image reproduction with a single beam and performing 
magnetic wall movement reproduction with a simple arrangement. 
The above object is achieved by a signal reproducing apparatus for 
reproducing information by moving a magnetic wall on a magnetic recording 
medium, comprising: 
means for irradiating a light spot for forming a temperature distribution 
for moving the magnetic wall on the medium; 
means for moving the light spot and the medium relative to each other; 
differential detecting means for taking a positive or negative value in 
accordance with a direction of magnetization in the light spot; 
edge detecting means for taking a positive or negative value in accordance 
with directions of magnetization, before and after the magnetic wall as a 
boundary, in the relative moving direction in the light spot; and 
reproducing means for generating a reproduction signal on the basis of the 
detection result from the differential detecting means and the detection 
result from the edge detecting means. 
Other features and advantages of the invention will become apparent from 
the following detailed description taken in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An embodiment of the present invention will be described in detail below 
with reference to the accompanying drawings. FIG. 6 is a block diagram 
showing the arrangement of an embodiment of the present invention. 
Referring to FIG. 6, a disk-like magneto-optical medium 100 for recording 
and reproducing information is rotated at a predetermined speed by a 
spindle motor 101. As this magneto-optical medium 100, a magneto-optical 
medium having three magnetic layers as explained in FIGS. 3A to 3D is 
used. As an information reproducing method, a reproducing method using 
magnetic wall movement is used. Also, the magneto-optical medium 100 is a 
cartridge type medium and can be exchanged. A control circuit 102 for 
controlling individual components is connected to an external information 
processing apparatus such as a computer via an interface controller 103. 
The control circuit 102 controls exchange of information with the 
information processing apparatus and controls recording and reproduction 
of information with respect to the magneto-optical medium 100 by 
controlling individual parts. 
An optical head 104 is a recording/reproducing head for recording and 
reproducing information by irradiating a light beam to the magneto-optical 
medium 100. The optical head 104 includes various optical elements such as 
a semiconductor laser, an objective lens for narrowing a laser beam from 
the semiconductor laser into a fine light spot, and a photodetector for 
detecting the reflected light from the magneto-optical medium 100. The 
arrangement of the optical head 104 will be described in detail later. A 
magnetic head 105 is arranged to oppose the optical head 104 with the 
magneto-optical medium 100 interposed between them. This magnetic head 105 
applies a magnetic field to the magneto-optical medium 100 in information 
recording. An optical head control circuit 106 controls the position of 
the optical head 104 and the position of the light beam irradiated from 
the optical head 104 to the magneto-optical medium 100. That is, the 
optical head control circuit 106 accesses a desired track by moving the 
optical head 104 in the radial direction of the magneto-optical medium 
100, performs focus control by which the light beam from the optical head 
104 is focused on the surface of the rotating magneto-optical medium 100, 
and performs tracking control by which the light beam from the optical 
head 104 follows an information track. Note that a spindle motor 
controller 109 controls rotation of the magneto-optical medium 100 in 
accordance with an instruction from the control circuit 102. 
An information recording circuit 107 records information in the 
magneto-optical medium 100 under the control of the control circuit 102. 
An information reproducing circuit 108 reproduces recorded information 
from the magneto-optical medium 100 on the basis of a read signal from the 
optical head 104. When the information processing apparatus issues a 
recording instruction, the control circuit 102 controls the individual 
components to move the optical head 104 to a target information track and 
transfers an information signal, transmitted from the information 
processing apparatus, to the information recording circuit 107. The 
information recording circuit 107 modulates the information signal and 
drives the magnetic head 105 with this modulated signal. Also, the 
information recording circuit 107 supplies a fixed driving current for 
recording to the internal semiconductor laser of the optical head 104. In 
this manner, while a light beam with a fixed intensity is irradiated on 
the magneto-optical medium 100, a magnetic field modulated in accordance 
with the information signal is applied to record the information on the 
information track of the magneto-optical medium 100. As another 
information recording method, it is also possible to perform recording by 
irradiating a light beam, whose intensity is modulated in accordance with 
an information signal, from the optical head 104, while a magnetic field 
in a fixed direction is applied from the magnetic head 105. 
When the information processing apparatus issues a reproduction 
instruction, on the other hand, the control circuit 102 controls the 
individual parts to move the optical head 104 to a target information 
track. The control circuit 102 also controls the information recording 
circuit 107 to supply a fixed driving current for reproduction to the 
internal semiconductor laser of the optical head 104. Consequently, the 
optical head 104 scans a fixed light beam on the information track of the 
magneto-optical medium 100. The optical head 104 detects the reflected 
light from the magneto-optical medium 100, and the information reproducing 
circuit 108 reproduces recorded information on the basis of the read 
signal. The reproduction signal is transferred to the external information 
processing apparatus via the interface controller 103 under the control of 
the control circuit 102. Although magnetic wall movement reproduction as 
described above is performed in this embodiment, this embodiment realizes 
magnetic wall movement reproduction using a single reproducing light spot 
by combining an optical edge reproducing method with this magnetic wall 
movement reproduction. This optical edge reproduction will be described 
below prior to explaining a reproducing operation by the embodiment. 
First, a pit (magnetic domain) is formed as a difference between the 
directions of perpendicular magnetization in a magneto-optical medium. The 
formation methods are classified into two methods: one is a pit position 
recording method in which the center of a pit is given the meaning of 
information; and the other is a pit edge recording method in which the 
edge of a pit is given information. The recording density can be increased 
more by the pit edge recording than the pit position recording. Therefore, 
pit edge recording and reproducing methods are being extensively 
researched and developed recently. 
The inventor of the present application has proposed a method of optically 
detecting the edge of a magneto-optical pit (magnetic domain) in Japanese 
Patent Application Laid-Open No. 4-279710. This optical edge reproducing 
method will be described next. FIG. 7 shows the way a light spot moves on 
a pit (magnetic domain) recorded in a magneto-optical medium. Referring to 
FIG. 7, the light spot moves in the order of (a), (b), and (c). That is, 
the light spot moves from a pit (magnetic domain) with downward 
magnetization to a boundary region (including a magnetic wall) and further 
moves to a pit (magnetic domain) with upward magnetization. Consider the 
distribution of light on a photodetector when a light spot moves this way. 
Assume that, as shown in FIG. 2, reflected light from downward 
magnetization is R+, reflected light from upward magnetization is R-, and 
their P- and S-axis components are (P+, S+) and (P+, S-), respectively. 
When the light spot moves in the order of (a), (b), and (c) in FIG. 7, the 
P-axis component is P+, i.e., remains almost unchanged. Accordingly, the 
amplitude distribution of light (ignore the size and see only the shape) 
on a reproducing photodetector is as shown in (a) of FIG. 8, and the 
intensity distribution of light (ignore the size and see only the shape) 
is as shown in (b) of FIG. 8. There is almost no difference between them. 
On the other hand, the S-axis component forms a uniform distribution of S+ 
or S- in the light spot in (a) or (c) of FIG. 7. Consequently, the 
amplitude of light (ignore the size and see only the shape) on a 
reproducing photodetector is as shown in (c) of FIG. 8, and the intensity 
of light (ignore the size and see only the shape) is as shown in (d) of 
FIG. 8. 
FIG. 9 shows a reproducing optical system used in optical edge 
reproduction. Although FIG. 9 shows only the reproducing optical system, 
the same parts as in FIG. 1 can be used as the rest. Note that the same 
reference numerals as in FIG. 1 denote the same parts in FIG. 9. This 
system includes a halfwave plate 37, a condenser lens 38, and a polarizing 
beam splitter 39. Two-division photodetectors 68 and 69 have division 
lines in a direction perpendicular to information tracks on a 
magneto-optical medium. That is, when an information track is projected 
onto the two-division photodetectors by using the optical system shown in 
FIG. 1 and the reproducing optical system shown in FIG. 9, the image 
perpendicularly crosses the division lines. The system further comprises 
amplifiers 70, 71, and 72 for differential detection, and an edge 
detection reproduction signal 73 is output from the differential amplifier 
72. 
FIG. 10 shows changes in the light intensity on the two-division 
photodetectors 68 and 69 when a light spot moves from a pit (magnetic 
domain) with downward magnetization to a pit (magnetic domain) with upward 
magnetization via a boundary where magnetizations are reversed as shown in 
FIG. 7. (a), (b), and (c) in FIG. 10 indicate changes in the light 
intensity on the two-division photodetector 68, and (d), (e), and (f) in 
FIG. 10 indicate changes in the light intensity on the two-division 
photodetector 69. In FIG. 10, the X axis indicates the position on the 
two-division photodetector shown below, and the Y axis indicates the 
magnitude of the intensity. The Y axis corresponds to the division line of 
the two-division photodetector. 
When the light spot is on a pit (magnetic domain) with downward 
magnetization as shown in (a) of FIG. 7, the distribution of the light 
intensity on the two-division photodetector 68 is as shown in (a) of FIG. 
10, and the distribution of the light intensity on the two-division 
photodetector 69 is as shown in (d) of FIG. 10. The shape of each 
distribution is symmetrical about the Y axis, and the intensity peak is on 
the Y axis. The peak size in (a) of FIG. 10 is larger than that in (d) of 
FIG. 10. In this case, detection signals obtained by individual 
photodetectors 68-1 and 68-2 of the two-division photodetector 68 are the 
same, and detection signals obtained by individual photodetectors 69-1 and 
69-2 of the two-division photodetector 69 are the same. Therefore, both 
signals obtained by differential detection by the differential amplifiers 
70 and 71 are 0. 
In contrast, when the light spot is on a pit (magnetic domain) with upward 
magnetization as shown in (c) of FIG. 7, the distribution of the light 
intensity on the two-division photodetector 68 is as shown in (c) of FIG. 
10, and the light intensity distribution on the two-division photodetector 
69 is as shown in (f) of FIG. 10; i.e., (a) and (d) in FIG. 10 are 
replaced with each other. Similar to the above case, detection signals 
obtained by the individual photodetectors 68-1 and 68-2 are the same, and 
detection signals obtained by the individual photodetectors 69-1 and 69-2 
are the same. Accordingly, both signals obtained by the differential 
amplifiers 70 and 71 are 0. 
On the other hand, when the light spot is in a position where downward 
magnetization is reversed to upward magnetization as shown in (b) of FIG. 
7, the distributions of the light intensity on the two-division 
photodetectors 68 and 69 are as shown in (b) and (e), respectively, of 
FIG. 10. Each distribution has two peaks on the positive side and the 
negative side, centering around the Y axis, of the X axis. The peak on the 
negative side is larger in (b) of FIG. 10, and the peak on the positive 
side is larger in (e) of FIG. 10. If this is the case, a signal of 
(68-2)-(68-1) is obtained from the differential amplifier 70, i.e., a 
signal of a negative value is obtained. On the other hand, a signal of 
(69-2)-(69-2) is obtained from the differential amplifier 71, i.e., a 
signal of a positive value is obtained. In addition, a signal of 
(differential amplifier 71)-(differential amplifier 70) is obtained when 
the differential amplifier 72 differentially detects the output signals 
from the differential amplifiers 70 and 71. Consequently, a signal of a 
positive value is obtained as the edge detection reproduction signal 73. 
When the light spot moves from downward magnetization to upward 
magnetization in this manner, a peak signal of a positive value is 
obtained as the edge detection reproduction signal 73 in a position where 
the magnetizations are reversed. In contrast, when a light spot moves from 
upward magnetization to downward magnetization, a peak signal of a 
negative value is obtained by the same principle in a position where the 
magnetizations are reversed. In this method, the edge detection 
reproduction signal 73 is constantly 0 when magnetization in a light spot 
is uniform. When a magnetization boundary (magnetic wall) enters a light 
spot, a positive or negative peak signal is obtained. 
A practical information reproducing operation of this embodiment will be 
described below. FIG. 11A is a plan view when a light spot is irradiated 
on the magneto-optical medium 100. FIG. 11B shows the states of the 
individual magnetic layers of the magneto-optical medium 100 at that time. 
The magneto-optical medium 100 has an information track 1, a land 2, and 
grooves 3. Each groove 3 has a function of blocking the influence of 
magnetic wall movement from an adjacent information track. For this 
purpose, these grooves are made deeper than conventional grooves, or only 
these grooves are annealed to erase magnetization. A light spot 4 is used 
in reproduction (recording). In reproducing information, the optical head 
104 irradiates the light spot 4 with an optical power of a fixed intensity 
by which no recording is performed. 
When the light spot 4 is thus irradiated, a temperature distribution 
indicated by oval contour lines is formed on the magneto-optical medium as 
shown in FIG. 11A. The moving direction of the magneto-optical medium 100 
is the direction of an arrow A (to the left in FIG. 11A). The 
magneto-optical medium 100 is the one shown in FIGS. 3A to 3D as described 
earlier and includes first, second, and third magnetic layers 8, 9, and 
10. The first magnetic layer 8 is a reproduction layer, the second 
magnetic layer 9 is an adjusting layer, and the third magnetic layer 10 is 
a recording layer. Arrows in each layer indicate the directions of atomic 
spins, and magnetic walls 11 and the like are formed in regions where the 
spin directions are opposite to each other. 
In a high-temperature portion 5, the second magnetic layer 9 is at the 
Curie temperature or higher. For this reason, magnetization disappears in 
the second magnetic layer 9 as shown in FIG. 11B. When magnetic walls 12 
and 13 of a magnetic domain recorded in the third magnetic layer 10 comes 
to the boundary between a low-temperature portion and the high-temperature 
portion 5, the magnetic wall 12 moves (in the direction of an arrow B) 
toward the high-temperature portion in a direction opposite to the moving 
direction of the medium, and the magnetic wall 13 moves (in the direction 
of an arrow C) toward the high-temperature portion in the same direction 
as the moving direction of the medium. That is, the magnetic wall 12 moves 
in a region 6, and the magnetic wall 13 moves in a region 7. In this 
state, however, the information of the magnetic wall 12 and the 
information of the magnetic wall 13 mix in the light spot 4, so no desired 
information can be reproduced. Therefore, this embodiment realizes 
accurate magnetic wall movement reproduction with a single beam, even if 
two magnetic wall movements exist in a light spot, by using the optical 
edge reproducing method as described previously in magnetic wall movement 
reproduction. 
FIG. 12 shows a reproducing optical system and a signal processing circuit 
used in this embodiment. The reproducing optical system is provided in the 
optical head 104, and the signal processing circuit is provided in the 
information reproducing circuit 108. Note that although the optical head 
104 incorporates an optical system including a semiconductor laser and an 
objective lens as explained in FIG. 1 in addition to the reproducing 
optical system, these components are omitted from FIG. 12. Referring to 
FIG. 12, elements 15 to 19 of this reproducing optical system 14 are the 
same as explained in FIG. 9. That is, the elements 15, 16, and 17 are a 
halfwave plate, a condenser lens, and a polarizing beam splitter, 
respectively. 
The elements 18 and 19 are two-division photodetectors having division 
lines in a direction perpendicular to information tracks on the 
magneto-optical medium. Amplifiers 20 to 22 for differential detection are 
also the same as explained in FIG. 9. An edge detection reproduction 
signal 23 is obtained from the output of the differential amplifier 22. 
This edge detection reproduction signal is explained in FIGS. 7 to 10, so 
a detailed description thereof will be omitted. Addition amplifiers 24 and 
25 add signals from the two-division photodetectors 18 and 19, 
respectively. That is, a signal of (18-1)+(18-2) is obtained by the 
addition amplifier 24, and a signal of (19-1)+(19-2) is obtained by the 
addition amplifier 25. A differential amplifier 26 obtains a signal of 
(addition amplifier 25)-(addition amplifier 24). The differential 
amplifier 26 outputs a differential detection reproduction signal 27 as in 
the related art shown in FIG. 1. On the basis of the edge detection 
reproduction signal 23 and the differential detection reproduction signal 
27, a magnetic wall movement information reproducing circuit 110 
reproduces recorded information and outputs the information as a magnetic 
wall movement information reproduction signal 111. The operation of the 
magnetic wall movement information reproducing circuit 110 will be 
described in detail later. 
Assuming that magnetic walls instantaneously move in the high-temperature 
regions 6 and 7 in FIGS. 11A and 11B, four types of combinations of 
magnetizing directions, (a), (b), (c), and (d) in FIG. 13, are obtained in 
the high-temperature regions 6 and 7. That is, in (a) of FIG. 13, the 
directions of magnetization are upward in both the regions 6 and 7. In (b) 
of FIG. 13, the direction of magnetization in the region 6 is upward and 
the direction of magnetization in the region 7 is downward. In (c) of FIG. 
13, the direction of magnetization in the region 6 is downward and the 
direction of magnetization in the region 7 is upward. In (d) of FIG. 13, 
the directions of magnetization are downward in both the regions 6 and 7. 
Reference numerals 6 and 7 in FIG. 13 correspond to 6 and 7 in FIGS. 11A 
and 11B, i.e., denote the magnetic wall moving regions in the 
high-temperature portion 5. Assume that the regions 6 and 7 have nearly 
equal influences on the light spot 4. 
It is ideally desirable that the regions 6 and 7 have the same size and 
their boundary be in the center of the light spot 4. Actually, however, 
the boundary between the regions 6 and 7 slightly moves in the moving 
direction of the magneto-optical medium 100 because the medium is moving. 
However, as shown in FIG. 11B, the size of the region 7 in which the 
magnetic wall moves is larger than that of the region 6 on the opposite 
side. Accordingly, by controlling the temperature rise caused by the light 
spot 4, i.e., by controlling the semiconductor laser driving current by an 
internal laser driving circuit (not shown) of the information recording 
circuit 107 in FIG. 6 and thereby controlling the light intensity of the 
light spot, the influences of the light spot 4 on the reproduction signals 
in the regions 6 and 7 can be made nearly equal to each other. 
For the sake of simplicity, it is assumed that the sizes of the regions 6 
and 7 are the same and their boundary is in the center of the light spot 4 
in FIG. 13. Also, distributions indicated by (a) to (d) in FIG. 13 
indicate the distributions of light on the two-division photodetector 18. 
Although the distributions of light on the two-division photodetector 19 
are omitted, the distributions are (d), (c), (b), and (a) in FIG. 13 when 
the magnetization patterns are (a), (b), (c), and (d), respectively, in 
FIG. 13. In (a) of FIG. 13, the directions of magnetization are uniformly 
upward in the light spot 4, so the distribution on the two-division 
photodetector 18 is symmetrical about the Y axis as in (a) of FIG. 10. In 
(b) of FIG. 13, the boundary between downward magnetization and upward 
magnetization exists in the light spot 4, so the distribution on the 
two-division photodetector 18 has two peaks on the positive side and 
negative side, centering around the Y axis, of the X axis as in (b) of 
FIG. 10. In (c) of FIG. 13, the combination of the directions of 
magnetization is the reverse of that shown in (b) of FIG. 13, so the 
relationship between the left and right shapes of the distribution on the 
two-division photodetector 18 is opposite to that in (b) of FIG. 13. In 
(d) of FIG. 13, the directions of magnetization are uniformly downward in 
the light spot 4, so the distribution on the two-division photodetector 18 
is symmetrical about the Y axis as in (c) of FIG. 10. 
A differential detection reproduction signal and an edge detection 
reproduction signal obtained when the light spot 4 is scanned on an 
information track of the magneto-optical medium 100 will be described 
below with reference to FIG. 14. (a) of FIG. 14 shows magnetization 
patterns in the regions 6 and 7 in which the magnetic walls shown in FIGS. 
11A and 11B move. Assume that, as shown in (a) of FIG. 14, the 
magnetization pattern changes in the order of (I), (II), (III), and (IV) 
when the light spot 4 is scanned on an information track. (b) of FIG. 14 
shows the differential detection reproduction signal 27 obtained for the 
magnetization patterns in (a) of FIG. 14. The differential detection 
reproduction signal 27 is an output signal from the differential amplifier 
26 in FIG. 12. The differential detection reproduction signal 27 is a 
negative signal when the directions of magnetization are uniformly upward 
as in magnetization pattern (I), and a positive signal when the directions 
of magnetization are uniformly downward as in magnetization pattern (III). 
When the directions of left magnetization and right magnetization are 
different as in magnetization pattern (II) or (IV), the value of the 
differential detection reproduction signal 27 is 0. 
(c) of FIG. 14 shows the edge detection reproduction signal 23 obtained for 
the magnetization patterns in (a) of FIG. 14. The edge detection 
reproduction signal is an output signal from the differential amplifier 
22. The edge detection reproduction signal is a positive signal when the 
direction of magnetization is upward in the region 6 and downward in the 
region 7 as in magnetization pattern (II), and a negative signal when the 
direction of magnetization is downward in the region 6 and upward in the 
region 7 as in magnetization pattern (IV). When the directions of 
magnetization are uniform as in magnetization pattern (I) or (III), the 
value of the edge detection reproduction signal is 0. In FIGS. 9 and 10, 
it is explained that a positive or negative peak signal is obtained at an 
instant the magnetic wall passes by. In FIG. 14, however, one 
magnetization pattern is held until it changes to the next magnetization 
pattern. Therefore, the edge detection reproduction signal is not a peak 
signal but takes a positive or negative fixed value as shown in (c) of 
FIG. 14. 
As is apparent from the above description, the differential detection 
reproduction signal and the edge detection reproduction signal are 
independent signals with respect to the four magnetization patterns. That 
is, the magnetizing directions in the regions 6 and 7 cannot be specified 
only by the respective values of the differential detection reproduction 
signal and the edge detection reproduction signal. However, the 
magnetizing directions in the regions 6 and 7 can be identified by 
combining the differential detection reproduction signal and the edge 
detection reproduction signal. In the region 6, for example, the direction 
of magnetization is upward when the values of the differential detection 
reproduction signal and the edge detection reproduction signal are 
(negative, 0) or (0, positive), and downward when the values are 
(positive, 0) or (0, negative). Analogously, in the region 7, the 
direction of magnetization is upward when the values of the differential 
detection reproduction signal and the edge detection reproduction signal 
are (negative, 0) or (0, negative), and downward when the values are (0, 
positive) or (positive, 0). 
The edge detection reproduction signal 23 and the differential detection 
reproduction signal 27 are output to the magnetic wall movement 
information reproducing circuit 110 where a reproduction signal is 
generated on the basis of the two signals. More specifically, the magnetic 
wall movement information reproducing circuit 110 samples the edge 
detection reproduction signal 23 and the differential detection 
reproduction signal 27 and outputs +1, -1, and 0 when each signal is a 
positive signal, a negative signal, and a 0 signal, respectively. The edge 
detection reproduction signal 23 and the differential detection 
reproduction signal 27 are represented by the combination of +1, -1, and 0 
as follows. When both the directions of left magnetization and right 
magnetization are upward as in magnetization pattern (I) of (a) of FIG. 
14, (edge detection reproduction signal 23, differential detection 
reproduction signal 27)=(-1, 0). When the direction of left magnetization 
is downward and the direction of right magnetization is upward as in 
magnetization pattern (II), the combination is (0, +1). When both the 
directions of left magnetization and right magnetization are downward as 
in magnetization pattern (III), the combination is (+1, 0). When the 
direction of left magnetization is upward and the direction of right 
magnetization is downward as in magnetization pattern (IV), the 
combination is (0, -1). 
The combinations are thus four types of (-1, 0), (0, +1), (+1, 0), and (0, 
-1), and they are different combinations. That is, the pattern of the left 
magnetization and right magnetization can be unconditionally known if the 
combination is found. Focusing attention only on the right side of each 
magnetization pattern in (a) of FIG. 14, the magnetic wall movement 
information reproducing circuit 110 outputs a reproduction signal 
indicating that upward magnetization is detected as the magnetic wall 
movement information reproduction signal 111, when the combination of 
(edge detection reproduction signal 23, differential detection 
reproduction signal 27) is (-1, 0) or (0, +1). In contrast, when the 
combination is (+1, 0) or (0, -1), the magnetic wall movement information 
reproducing circuit 110 outputs a reproduction signal indicating that 
downward magnetization is detected as the magnetic wall movement 
information reproduction signal 111. 
Focusing attention only on the left side of each magnetization pattern in 
(a) of FIG. 14, the magnetic wall movement information reproducing circuit 
110 outputs a reproduction signal indicating that upward magnetization is 
detected as the magnetic wall movement information reproduction signal 
111, when the combination of (edge detection reproduction signal 23, 
differential detection reproduction signal 27) is (-1, 0) or (0, -1). In 
contrast, when the combination is (0, +1) or (+1, 0), the magnetic wall 
movement information reproducing circuit 110 outputs a reproduction signal 
indicating that downward magnetization is detected as the magnetic wall 
movement information reproduction signal 111. The side on which attention 
is to be focused is previously determined in accordance with the 
apparatus. In this way, the magnetic wall movement information reproducing 
circuit 110 reproduces recorded information from the magneto-optical 
medium 100 and outputs the information to the control circuit 102. 
In the present invention as has been described above, the differential 
detection reproduction signal and the edge detection reproduction signal 
are simultaneously detected, and recorded information in a magneto-optical 
medium is reproduced on the basis of these two signals. Therefore, even if 
magnetic walls move in two directions in a reproducing light spot, two 
pieces of information are not mixed, so the recorded information can be 
accurately reproduced. Accordingly, it is possible to accurately perform 
magnetic wall movement reproduction with a single beam and greatly 
simplify an optical system compared to a system using two beams.