Note: Descriptions are shown in the official language in which they were submitted.
~ 3 ~
BACKGRO~ND OF THE INVENTION
The present invention relates to a method of
reproducing a signal from a magneto-optical recording
medium by reading out data bits (magnetic domains)
through the magneto-optical interaction.
In a magneto--optical recording/reproducing method
which forms data bits or bubble magnetic domains with
irradiation of a laser beam to heat a recording medium
locally and reads out the signal by utillzing the
magneto-optical interaction, it is necessary, for
increasing the magneto-optical recording density, to
shorten the length of each bit length, i.e. to minimize
the data magnetic domain. However, in the ordinary
magneto-optical recording/reproducing system known
generallyt such attempt is limited by the wavelength of
thè laser beam in a reproducing mode, the numerical
aperture of the lens and so forth for ensuring a
satisfactory S/N in a xeproducing operation. Under the
current technical circumstances, for example, it is
impossible to read out a data bit (magnetic domain) of
0.2 ~um with a laser beam having a spot diameter of 1 ~m.
~k
~ 3 ~
OBJECT AND SU~ Y OF T~E INVENTION
~ ccordingly, it i5 an object of the present
invention to provide a method for reproducing signal
from a mayneto-optical recording medium in which closely
provided bits can be separately reproduced, to enable a
high density recording.
It is another object of the present invention to
provide a method for reproducing signal from a magneto~
optical recording medium to provide an improved signal
to noise ratio.
According to the present invention there is
provided a method of reproducing a signal from a
magneto-optical recording medium in which the medium i5
formed of a first magnetic film, a second magnetic film
and a third magnetic film magnetically coupled to one
another at room temperature TRT~ wherein the Curie
points Tcl, Tc2 and Tc3 of the first, second and third
magnetic films are in the relationship of TC2 > TRT~ TC2
Tcl, and Tc2 < Tc3, and the coercive force Hcl of the
first magnetic film is sufficiently small in the
vicinity of the Curie point Tc2 of the second magnetic
film, while the coercive force Hc3 of the third magnetic
film is sufficiently greater than a required magnetic
-- 2 --
1 3 ~ 3
field intensity within a temperature range between the
room temperature TRT and a predetermined temperature Tp~
higher than the Curie point Tc2 of the second magnetic
film, and in reproducing a signal from the magneto-
optical recording medium, the medium is heated to the
predetermined temperature TPB to interrupt the magnetic
coupling between the first and third magnetic film under
an application of magnetic field to cause change of
domain size in the first magnetic film.
BRIEF DESCRIPTION OP THE DRAWINGS
Fig. 1 schematically shows the structure of a
magneto-optical recording medium used in the method of
the present invention;
Figs. 2A through 2D illustrate the states of
magnetization obtained by the method of the invention;
FigsO 3A through 3C illustrate the states of
magnetization obtained by another method of the
invention;
Figs. 4A through 4D show the recorded bits on the
magneto-optical recording medium and output waveforms;
Figs. 5A and 5B show the waveform of a reproduction
output with the states of magnetization;
~3~g~
Fig. 6 graphically shows the reproduction
characteristic curves in relati.on to recording
frequencies;
Fig. 7 graphically shows a characteristic curve
representing the relationship between the temperature
and the inverse field intensity of the magnetic film;
and
Fig. 8 graphically shows the reproduction
characteristic curves in relation to recording
frequencies.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention uses a magneto~optical
recording medium S having, on its light transmitting
substrate 1 as shown in Fig. 1, a transparent dielectric
film 2 formed when necessary to serve as a protective
film or interference film, and further having thereon a
first magnetic film 11, a second magnetic film 12 and a
third magnetic film 13 which are magnetically coupled to
one another at room temperature TRT and have anisotropy
perpendicular to the film surface, wherein the
respective Curie points Tcl, Tc2 and Tc3 of the first,
second and third magnetic films 11, 12 and 13 are so
3~3~ ~$~
selected as to have relationship of TC2 ~ TRT an Tc2
Tcl, Tc2 ~ Tc3. The coerclve force Hcl o~ the first
magnetic film 11 is sufficiently small in the vicinity
of the Curie point Tc2 of the second magnetic film 12,
and the coercive force Hc3 of the third magnetic film 13
is sufficiently greater than a required magnetic field
intensity within a temperature range from the room
temperature TRT and a predetermined temperature TPB
higher than the Curie point Tc2 of the second magnetic
film 12.
And in reproduction of such recording medium~ the
data bits or the recorded magnetic domains of the first
magnetic film 11 are expanded or shrinked by the
combination of a demagnetizing field applied thereto and
an external magnetic field applied when necessary at the
aforesaid predetermined temperature TPB above the Curie
point Tc2 of the second magnetic film 12, and then the
signal is read out in such state.
If necessary, a surface protective film 4 is
further formed on the third magnetic film 13.
In recording the magneto-optical recording medium S
to form data magnetic domains, as generally known, a
laser beam is focused and irradiated while a bias
magnetic field is applied in the direction reverse to
_ 5 _
~3~3$~
the perpendicular magnetization of the first to third
magnetic films 11 to 13 .in the initial state, whereby
the first to third magnetic films 11 to 13 are heated
above the respective Curie points. And in the cooling
stage posterior to scanning with the laser beam, a
bubble magnetic domain inverted directionally by the
external magnetic field and the stray magnetic field is
formed to record, for example, data "1". That is,
binary data "1" or "O" is recorded in accordance with
the presence or absence of such data bubble magnetic
domain.
And particularly in the present invention to read
out or reproduce the recorded data from such magneto-
optical recording medium S, the medium portion to be
read out is heated, with irradiation of a laser beam or
the like, up to the predetermined temperature TPB above
the Curie point Tc2 of the second magnetic film 12, so
that the mutual magnetic coupling between the first and
third magnetic films 11 and 13 is interrupted when the
recorded data is read out in accordance with the Kerr
rotation angle or Faraday rotation angle resulting from
the magneto-optical interaction which depends on the
presence or absence of the magnetic domain. In this
state, therefore, the first magnetic film 11 is rendered
free from any magnetic restriction of the third magnetic
film 13, and the recorded-data magnetic domain is
expanded or shrinked by the xequired magnetic field
which corresponds to the sum of a demagnetizing field
and an external magnetic field appllied when necessary,
and also by the reduction of the coercive force of the
first magnetic film 11 caused at the temperature TPB.
As hereina~ter explained with reference to Figs. 4C
and 4D, the output changes during playback operation,
due to the expansion or shrinkage of the data bits, the
signal can be obtained by differentiation of the output
signal propositional to the Kerr rotation angle. In
this case two closely recorded data bits can be
separately reproduced to enable high density recording.
Further, if the first magnetic film 11 is composed
of a suitable material adapted to a~tain a large Kerr
rotation angle or Faraday rotation angle, the
substantial area of the data magnetic domain can be
increased due to the recorded data principally on the
first magnetic film 11 to consequently pxovide a greater
reproduction output, hence further improving the S/N
(C/N).
Since the reproduction is performed in a state
where the recorded-data magnetic domain has been
~ 3 ~ 3
expanded with substantial increase of the read magnetic
domain area, it becomes possible to increase the
reproduction output to eventually enhance the S/N.
And the read portion of the recording medium is
cooled after reproduction of the data with displacement
of the irradiation by the scanning laser beam, so that
the third magnetic film 13 of a high coercive force
functions as a magnetic recording retainer layer in the
process where the first to third magnetic films 11 to 13
are cooled to, e.g. the room temperature TRT, and the
resultant magnetization of the third magnetic film 13
acts to magnetize the second magnetic film 12 by
magnetic coupling, then to magnetize the first magnetic
film 11 coupled thereto magnetically, whereby the data-
bit magnetic domain in the initial recording state is
formed again to resume the recording state.
According to the present invention, as described
hereinabove, the second magnetic film 12 serving as an
intermediate layer of the magneto-optical recording
medium S is selectively placeable in either a
magnetically coupled state or a magnetically interrupted
state between the first and third magnetic films 11 and
13, so that in a reproducing operation, the second
magnetic film 12 as an intermédiate layer magnetically
separates the first and third magnetic films 11 and 13
from each other to enable expansion or shrinkage of the
data-recorded magnetic domain of the first magnetic film
11. Therefore, the third magnetic film 13 maintains its
function as a magnetic recording retainer layer to
retain the magnetization thereof, while the first
magnetic film 11 exhibits its function as a reproducing
layer to provide higher separation of signals to enable
high recorded density an enhanced reproduction output
when expansion of the magnetic domain occures.
Consequently, the recording density can be increased to
ensure a sufficient reproduction output despite
minimization of the magnetic domain for bit data,
whereby a higher density is rendered attainable in the
recording.
Now referring to Fig. 2, a description will be
given below with regard to the state of magnetization
obtained when each of the first to third magnetic films
11 to 13 is composed of ferromagnetic material. Suppose
now that the magnetic films 11 to 13 in an initial
nonrecorded state have unidirectional perpendicular
magnetizations as shown in Fig. 2A. And when data "1"
is recorded, a data bit or a data magnetic domain BM is
~3~8~
formed with the magnetization directionally reverse to
the initial state, as shown in Fig. 2B.
In reading out the data magnetic domain BM thus
formed, a laser beam LB is irradiated to the data
magnetic domain B~ as shown in Fig~ 2C in such a manner
that, as described previously, the aforesaid
predetermined temperature TPB is obtained in the center,
for example, of the irradiated portion. In this stage,
the second magnetic film 12 is heated beyond its Curie
point Tc2 so that the magnetism thereof is lost, whereby
the magnetic coupling between the first and third
magnetic films 11 and 13 is interrupted. When an
external magnetic field Hex is applied in such a state
in the same direction as the external bias magnetic
field applied in the recording operation, i.e~, in the
direction of the original magnetization of the magnetic
domain BN or that in the recording mode, then the
magnetic domain BM of the first magnetic film 11, whose
coercive force Hcl is rendered smaller at the
temperature TPB, is expanded by the sum of such external
magnetic field and the demagnetizing field.
In a state where the laser beam L~ is irradiated
outside the data magnetic domain ~M as shown in Fig. 2D,
the temperature rise caused in the data magnetic domain
-- 10 --
3. 3,~ 'J ~j
is relatively small, so that there occurs substantially
no expansion of the data bit or magnetic domain BM.
Thus, it becomes possible to expand merely the data-
recorded magnetic domain BM alone existing at the center
of the magnetic domains L~ in the central area of the
laser beam scanning in the reading mode.
Referring to Fig. 3, a description will be given
below with regard to the ~tate of magnetization obtained
when each of the first to third magnetic films 11 to 13
is composed of ferromagnetic material in another
example. Suppose now that the magnetic films 11 to 13
in an initial nonrecorded state have unidirectional
perpendicular magnetization as ~hown in Fig. 3A~ And
when data "1" is recorded, a data bit or a data magnetic
domain BM is formed with the magnetization directionally
reverse to the initial state, as shown in Fig. 3B. In
reading out the data magnetic domain BM thus formed, a
laser beam LB is irradiated to the dake magnetic domain
BM as shown in Fig. 3c in such a manner that, as
described previously, the aforesaid predetermined
temperature Tp~ i~ obtained in the center, for example,
of the irradiated portion. In this stage, the second
magnetic film 12 is heated beyond its Curie point Tc2 so
that the magnetism thereof is lost, whereby the magnetic
~ 3 ~ 3
coupling between the first and third magnetic films 11
and 13 is interrupted. When an external magnetic field
~ex is applied in such a state in the direct;on reverse
to the external bias magnetic field applied in the
recording operation, i.e. in the direction of the
original magnetization of the magnetric domain BM or
that in the recording mode, then the magnetic domain BM
of the first magnetic film ll/ whose coercive force ~cl
is rendered smaller at the temperature Tp~, is shrinked
to, e.g. a width W2 or is inverted by the combination of
such external magnetic field Hex and the demagnetization
field.
Accordingly, if the data reproduced from the
magnetic domain BM is outputted in the form of, for
example, a differential change in the Kerr rotation
angle, ao that a great output can be obtained. Since
the first magnetic film 11 has a function as a
reproducing layer to enhance the reproduction output by
shrinking or inverting the magnetic domain in the
reproducing operation, it becomes possible to obtain a
sufficiently great reproduction output despite
minimization of the magnetic domain as bit data, hence
realizing a higher recording density.
~3~g~
Fig. 4A shows a relationship between recorded bits
BM1~ BM2 ... formed in a guide truck T and laser beam
spot L~ which has a much larger diameter than the
recorded bits.
In Fig. 4A the allow A indicates a moving direction
o~ the magneto-optical recording medium.
In Fig. 4B, the solid line shows an output waveform
from the magneto optical recording medium having
recorded bits as shown in Fig. 4A with a conventional
method without causing expansion or shrinkage of the
recorded bits. In this conventional method the recorded
bits BM1 and BM2 can't be separely reproduced, because
signals obtained from each of recorded bits BM1, BM2
which are indicated by the dotted line, overlaps with
each other.
Fig. 4C shows an output waveform from the magneto-
optical recording medium of Fig. 4A, when the recorded
bits are expanded upon playback, where the closely
provided bits BM1~ B~2 can be separately reproduced.
Fig. 4D shows an output waveform when the recorded bits
are shrinked upon playback.
Consequently, when the laser beam scanning is
carried out on the magnetic recordin~ medium where data-
recorded magnetic domains BM are arrayed at equal
~L 3 ~ J r3 3
intervals as shown in Fig. 5A, its output obtained by
reading out the magnetic domain BM comes to have the
waveform of Fig. 5B in which a higher level is indicated
upward as compared with an ideal demagnetization level
in erasing the magnetic domain BM, in ca~e the domain is
expanded at playback.
Practically, when the first to third magnetic films
11 to 13 are composed of rare-earth and transition
metals and have such ferrimagnetism that the sublattice
magnetization of the transition metal and that of the
rare-earth metal are directionally opposite to each
other, it i5 necessary to selectively determine the
direction of an external magnetic field Hex, which is
applied in a reproducing operation, in accordance with
whether the sublattice magnetization of the transition
metal or that of the rare-earth metal is dominant in
each magnetic film.
Such determination will now be described below in
detail. Relative to the direction of an external
magnetic field Hex applied in a reproducing operation,
the direction of an external bias magnetic field applied
in a recording operation is regarded as a reference, and
separa~e considerations are made as to whether the
saturation magnetization of the third magnetic film 13,
- 14 -
~ '.J~J
which occurs immediately below the Curie point TC3
thereof and dominates the recording direction, is in a
transition metal sublattice dominant state or a rare~
earth sublattice dominant state. ~ere, the
demagnetizing field and the stray magnetic field appied
to the data magnetic domain BM in the first magnetic
film 11 are neglected from the consideration.
[1-1] In case the magnetization of the third
magnetic film 13 is in a transition metal sublattice
dominant state immediately below the Curie point Tc3;
(l-la) When the magnetization of the first magnetic
film 11 is in a transition metal sublattice dominant
state in the vicinity of the Curie point Tc2 of the
second magnetic film 12, the data-recorded magnetic
domain BM can be expanded by applying, in the
reproducing operation, an external magnetic field in the
same direction as the external magnetic field applied in
the recording operationO
(l-lb) When the magnetization of the first magnetic
~ilm 11 is close to zero in the vicinity of the Curie
point Tc2 of the second magnetic film 12, the
temperature in the reproducing operation is further
raised beyond the vicinity of the Curie point Tc2 of the
second magnetic film 12 so that the magnetization of the
- 15 -
~ 3 ~
first magnetic film 11 is placed in a transition metal
sublattice dominant state. In this case, the bubble
magnetic domain BM can be expanded by applying an
external magnetic field Hex in the same direction as in
the recording mode.
(l-lc) When the magnetization of the first magnetic
film 11 is in a rare-earth sublattice dominant state in
the vicinity of the Curie point Tc2 of the second
magnetic film 12, the magnetic domain BM can be expanded
by applying, in the reproducing operation, an external
magnetic field Hex in the direction reverse to that in
the recording mode.
[2-1] In case the magnetization of the third
magnetic film 13 is in a rare-earth sublattice dominant
state immediately below the Curie point Tc3 thereof:
~1-2a) When the magnetization of the first magnetic
film 11 is in a transition metal sublattice dominant
state in the vicinity of the Curie point Tc2 of the
second magnetic film 12, the bubble magnetic domain BM
can be expanded by applying, in the reproducing
operation, an external magnetic field Hex in the
direction reverse to that in the recording mode.
(1-2b) When the magnetization of the first magnetic
film 11 is close to zero in the vicinity of the Curie
- 16 -
~ 3~ 3
point Tc2 of the second magnetic film 12, the
temperature TPB in the reproducing operation is further
raised beyond the vicinity of the Curie point TC2 of the
second magnetic film 12 so that the magnetization o the
first magnetic film 11 is placed in a transition m~tal
sublattice dominant state. In this case~ the magnetic
domain BM can be expanded by applying an external
magnetic field Hex in the direction reverse to that in
the recording mode.
~ C) When the magnetization of the first magnetic
film 11 is ln a rare~earth sublattice dominant state in
the vicinity of the Curie point Tc2 of the second
magnetic film 12, the bubble magnetic domain BM can be
expanded by applying, in the reproducing operation, an
external bias magnetic field Hex in the same direction
as in the recording mode.
[1-2] In case the magnetization of the third
magnetic film 13 is in a tran~ition metal sublattice
dominant state immediately below the Curie point Tc3:
(2-la) When the magnetization of the first magnetic
film 11 is in a transition metal sublattice dominant
state in the vicinity of the Curie point Tc2 of the
second magnetic film 12, the data-recorded magnetic
domain BM can be contracted or inverted by applying, in
- 17 -
1 3 ~
the reproducing operation, an external magnetic field in
the direction reverse to that in the recording mode.
~2-lb) When ~he magnetization of the first magnetic
film ll is close to zero in the vicinity of the Curie
point TC2 Of the second magnetic film 12, the
temperature in the reproducing operation is further
raised beyond the vicinity of the Curie point Tc2 of the
second magnetic film 12 so that the magnetization of the
first magnetic film ll is placed in a transition metal
sublattice dominant state. In this case, the bubble
magnetic domain BM can be contracted or inverted by
applying an external magnetic field Hex in the direction
reverse to that in the recording mode.
(2-lC~ When the magnetization of the first magnetic
film ll is in a rare-earth subIattice dominant state in
the vicinity of the Curie point Tc2 of the second
magnetic film 12, the magnetic domain BM can be
contracted or reversed by applying, in the reproducing
operation, an external magnetic field Hex in the same
direction as in the recording mode.
[2-2] In case the magnetization of the third
magnetic film 13 is in a rare-earth sublattice dominant
state immediately below the Curie point Tc3 thereof:
~ 3 .~
(2-2a) When the magnetization of the first magnetic
film 11 is in a transition metal sublattice dominant
state in the vicinity of the Curie point Tc2 of the
second magnetic film 12, the bubble magnetic domain BM
can be contracted or inverted by applying, in the
reproducing operation, an external magnetic field Hex in
the same direction as in the recording mode.
(2-2b) When the magnetization of the first magnetic
film 11 is close to zero in the vicinity of the Curie
point ~c2 o~ the second magnetic film 12, the
temperature TpB in the reproducing operation is further
raised beyond the vicinity of the Curie point Tc2 of the
second magnetic film 12 so that the magnetization of the
first magnetic film 11 is placed in a transition metal
sublattice dominatnt ~tate. In this case, the magnetic
domain BM can be contracted or inverted by applying an
external magnetic field Hex in the same direction as in
the recording mode~
(2-2c) When the magnetization of the first magnetic
film 11 is in a rare-earth sublattice dominant state in
~he vicinity of the Curie point Tc2 of the second
magnetic film 12, the bubble magnetic domain BM can be
contracted or inverted by applying, in the reproducing
--19
operation, an external bias magnetic ield Hex in the
direction reverse to that in the recording mode.
EXAMPLE
The substrate 1 is composed of a transparent
material such as glass plate, plastic plate of acrylic
resin, polycarbonate resin or the like and, although not
shown, track grooves are formed for tracking servo on
one side of the substrate with a pitch of, for example,
1.6 ~m. And a dielectric film 2 composed of Si3N4,
first to third magnetic films 11 to 13, and further a
protective film 4 are formed sequentially on the
substrate 1 by the technique of continuous evaporation
or sputtering performed by, for example, a magnetron
sputtering apparatus.
The first magnetic film 11 may be composed of GdCo,
GdFeCo, GdFe or the like; the second magnetic film 12
may be composed of DyFe, DyFeCo, TbFe or the like; and
the third magnetic film 13 may be compose~ of TbFe, TbFe
Co, DyFeCo or the like and composition of each layer is
selected to give a suitable Tc and Hc characteristics.
In such third magnetic film 13, there are formable
magnetic domains B~ each having a diameter smaller than
0.1 ~m.
- 20 -
~ 3 ~ .3
A magneto-optical recording medium known a~ an
optical disc S was produced by sequentially depositing,
on a glass substrate having track grooves with a pitch
of 1.6 ~m, a dielectric film 2 of Si3N4, a first
magnetic film 11 of GdFeCo, a second magnetic film o~
DyFeCo, a third magnetic film 13 of DyFeCo, and a
protective film 4 of Si3N4 in the form of superposed
layers with continuous sputtering performed by a
magnetron sputter1ng apparatus. ~able 1 lists below the
respective thicknesses and magnetic characteristics of
such magnetic films 11 to 13 as individual layers.
TABLE 1
_ ~hickness Curie Holding force
Material (~) p(oiC)t (KOe)
.. .
MagneticGdFeCo 450 >330 0.2
film 11 (FeCo rich)
MagneticDyFeCo 100 93 10
film 12 (Dy rich)
MagneticDyFeCo 600 195 20
film 13 . (Dy rich~
In Table 1, "FeCo rich" implies a film where the
FeCo sublattice magnetization is dominant at room
temperature, and "Dy rich" implies a film where the Dy
sublattice magnetization is dominant at room
temperature.
Fig. 6 graphically shows the results of measuring
the dependency of the carrier level-to-noise level (C/N)
on the recording frequency in the magneto-optical
recording medium S of Embodiment 1. In Fig. 6, a solid-
line curve represents the characteristic obtained by the
use of an objective lens having a numerical aperture
N.A. of 0.50 and a pickup with a laser beam wavelength
of 780 nm, under the conditions including a pickup
linear velocity of 7.5 m/sec, a recording power of 7.0
mWr a recording external magnetic field intensity of 500
(Oe), a reproducing external magnetic field intensity
set to zero, and a reproducing power of 3.5 mW. Also in
Fig. 6, a dottedline curve represents the characteristic
obtained with a reproducing power of 1.5 mW. When the
reproducing power is set to 1.5 mW as in this example,
the frequency dependency of the C/N achieved is the same
as that in a conventional optical disc having merely a
single layer of TbFeCo as the entire magnetic film.
This seems to result from the phenomenon that, with the
reproducing power of such a small value, the heating
temperature fails to reach the Curie point Tc2 of the
~,! `r'' ''
second magnetic film 12 and therefore the recorded
magnetic domain is not deformed in the reproducing
operation. When the reproducing power is 3.5 mW, in
comparison with the above example of 1.5 mW, the C/N is
remarkably increased in a range where the magnetic
domain length or bit length e ls smaller than 0.7 ~m.
Even when the bit leng~h ~ is equal to 0.3 ~um, a desired
signal component is still obtained although the C/N is
low. In this case, the external magnetic field Hex was
zero at playback, there is a stray field from the area
surrounding the data bit. In a range where the bit
length 4 is greater than 0.7 um, the C/N is reduced to
the contrary due to increase of the noise N. It has
been confirmed that if the medium portion once
reproduced with a power of 3.5 mW is reproduced again,
the C/N is resumed regardless of whether the latter
reproducing power is 1.5 mW or 3.5 mW.
If the laser beam power is maintained constant
during the reproducing operation in Embodiment 1
mentioned above~ the temperature profile is spread due
to the thermal diffusion in the recording medium S, so
that the resolution in reproducing the micro data bit
~magnetic domain) is deteriorated. A steep temperature
profile i5 attainable by performing reproduction with a
~ 3 ~
pulse laser beam of a narrow width at an interval of the
frequency corresponding to the minimal bit length.
Furthermore, for the purpose of inducing pxompt
radiation of the thermal energy absorbed into the
magnetic film~ a radiation film of high thermal
conductivity such as an aluminum film may be deposited
on the therd magnetic film 13 (on the reverse side
thereof with respect to the side in contact with the
second magnetic film 12).
Fig. 7 graphically shows a characteristic curve
representing the relationship between the temperature
and the inverse field intensity of the irst magnetic
film 11 in the magneto-optical recording medium S of the
example. And Fig~ 8 graphically shows the results of
measuring the dependency of the carrier level-to-noise
level (C/N) on the recording frequency in the medium S.
In Fig. 8, a ~olid-line curve represents the
characteristic obtained by the use of an objective lens
having a numerical aperture N.A. of 0.50 and a pickup
with a laser beam wavelength of 780 nm, under the
conditions including a pickup linear velocity of 7.5
m/~ec, a recording power of 7.0 mW, a recording external
magnetic field having an intensity of 500 (Oe), a
reproducing external magnetic field having an
- 24 -
~ 3 ~ t~i
intensity of 600 (Oe) and applied in the same direction
as ~he recording external magnetic field, and a
reproducing power of 3.5 mW. Also in Fig. B, a broken-
line curve represents the characteristic obtained with a
reproducing power of 1.5 mW. When the reproducing power
is set to 1.5 mW as in this example, the frequency
dependency of the C/N achieved is the same as that in a
conventional optical disc having merely a single layer
of TbFeCo as the entire magnetic film~ This seems to
result from the phenomenon that, with the reproducing
power of such a small value, the heating temperature
ails to reach the Curie point Tc2 of the second
magnetic film 12 and therefore the recorded magnetic
domain is not deformed in the recording operation. When
the reproducing power is 3.5 mW, in comparison with the
above example of 1~5 mW, the C/N is remarkably increased
in a range where the magnetic domain length or bit
length ~ is smaller than 0.7 ~m. Even with the bit
length e is equal to 0.3 ~m, a desired signal component
is still obtained although the C/N is low. In a range
where the bit length e is greater than 0.7 ~m, the C/N
i5 reduced to the contrary duè to increase of the noise
N. Further in Fig. 8, a one-dot chain line represents
the characteri~tic measured with a reproducing power of
- 25 -
3.5 mW (~ < 0.5 ~m) and the external magnetic ~leld
intensity Hex set to 0 ~Oe). ~s shown, when the bit
length ~ is smaller than 0.5 ~um, the C/N obtained with
Hex = 600 (Oe) is higher than the ratio with Hex = 0
(Oe).
It has been confirmed that if the medium portion
once reproduced with a power of 3.5 mW is reproduced
againr the C/N is rPsumed regardless of whether the
latter reproducing power is 1.5 mW or 3.5 mW.
If the laser beam power is maintained constant
during the reproducing operation in Example mentioned
above, the temperature profile is spread due to the
thermal diffusion in the recording medium S, so that the
resolution in reproducing the micro data bit (magnetic
domain) is deteriorated. A steep temperature profile is
attainable by performing reproduction with a pulse laser
beam of a narrow width at an interval of the frequency
corresponding to the minimal bit length. Furthermore,
for the purpose of inducing prompt radiation of the
thermal energy absorbed into the magnetic film~ a
radiation film of high thermal conductivity such as an
aluminum film may be deposited on the third magnetic
film 13 (on the reverse Aide thereof with respect to the
side in contact with the second magnetic film 12).
- 26 -
J
As described hereinabove, according to the presen~
invention having a laminous structure of first, second
and third magnetic films 11, 12 and 13, such three
magnetic films are retained in a magnetically coupled
state with one another at normal temperature~ and when
heated in a reproducing operation, the second magnetic
film 12 functions to interrupt the magnetic coupling
between the first and third magnetic films ll and 13,
thereby expanding or shrinking the data-recorded
magnetic domain of the first magnetic film 11 to
consequently improve the resolution S/N (C/N) of the
reproduction output, and still the recording state can
be held with regard to the third magnetic film 13.
Therefore, after termination of the reprsduction, the
recording state can be restored to eventually ensure the
satisfactory reproduction characteristics without
impairing repeated reproduction.
Furthermore, since the present invention is capable
of providing a sufficiently great reproduction output as
mentioned above, it becomes possible to realize
dimensional reduction of the data magnetic domains B~ to
consequently increase the recording density. Be~ides
the above, even in another structure of the magneto-
optical recording medium where track grooves are formed
~ ~ 3~
in its base, the data magnetic domains BM can still be
reduced dimensionally. Therefore, the recording
magnetic domains are formable not merely in land
portions alone as in any ordinary medium but also in
both land portions and track grooves, hence further
enhancing the data recording density.
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