U.S. patent application number 09/732092 was filed with the patent office on 2001-04-26 for magneto-optical recording medium and reproducing method for information recorded on the medium.
This patent application is currently assigned to Fujitsu Limited. Invention is credited to Kuroda, Sumio, Matsumoto, Koji, Mihara, Motonobu, Shono, Keiji, Tamanoi, Ken.
Application Number | 20010000483 09/732092 |
Document ID | / |
Family ID | 27275349 |
Filed Date | 2001-04-26 |
United States Patent
Application |
20010000483 |
Kind Code |
A1 |
Tamanoi, Ken ; et
al. |
April 26, 2001 |
Magneto-optical recording medium and reproducing method for
information recorded on the medium
Abstract
A magneto-optical recording medium capable of perfectly masking
a mark adjacent to a mark to be reproduced to thereby improve a
reproduction output. The magneto-optical recording medium includes
a transparent substrate, a magnetic reproducing layer laminated on
the transparent substrate, a nonmagnetic intermediate layer
laminated on the magnetic reproducing layer, and a magnetic
recording layer laminated on the nonmagnetic intermediate layer.
The reproducing layer has an easy direction of magnetization in a
plane at room temperature, and has an easy direction of
magnetization perpendicular to a film surface at a given
temperature or higher. The nonmagnetic intermediate layer is thin
enough to allow magnetostatic bond between the recording layer and
the reproducing layer at the given temperature or higher. Instead
of the nonmagnetic intermediate layer, a magnetic intermediate
layer having an easy direction of magnetization in a plane from
room temperature to its Curie temperature may be interposed between
the reproducing layer and the recording layer.
Inventors: |
Tamanoi, Ken; (Kawasaki-shi,
JP) ; Shono, Keiji; (Kawasaki-shi, JP) ;
Kuroda, Sumio; (Kawasaki-shi, JP) ; Mihara,
Motonobu; (Kawasaki-shi, JP) ; Matsumoto, Koji;
(Kawasaki-shi, JP) |
Correspondence
Address: |
Patrick G. Burns
GREER, BURNS & CRAIN, LTD.
300 S. Wacker Drive
25th Floor
Chicago
IL
60606-6501
US
|
Assignee: |
Fujitsu Limited
1015 Kamikodanaka, Nakahara-ku Kanagawa 211
Kawasaki-shi
JP
|
Family ID: |
27275349 |
Appl. No.: |
09/732092 |
Filed: |
December 7, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09732092 |
Dec 7, 2000 |
|
|
|
08368607 |
Jan 4, 1995 |
|
|
|
Current U.S.
Class: |
428/821 ;
G9B/11.016; G9B/11.048; G9B/11.052 |
Current CPC
Class: |
G11B 11/10584 20130101;
Y10S 428/90 20130101; Y10T 428/265 20150115; Y10T 428/12861
20150115; G11B 11/10515 20130101; Y10T 428/26 20150115; G11B
11/10593 20130101 |
Class at
Publication: |
428/694.0ML |
International
Class: |
G11B 005/66 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 14, 1994 |
JP |
6-002449 |
Mar 1, 1994 |
JP |
6-031661 |
Sep 5, 1994 |
JP |
6-211102 |
Claims
What is claimed is:
1. A magneto-optical recording medium comprising: a transparent
substrate; a magnetic reproducing layer laminated on said
transparent substrate, said reproducing layer having an easy
direction of magnetization in a plane at room temperature and
having an easy direction of magnetization perpendicular to a film
surface at a given temperature or higher; a nonmagnetic
intermediate layer laminated on said reproducing layer; and a
magnetic recording layer laminated on said nonmagnetic intermediate
layer, said recording layer having an easy direction of
magnetization perpendicular to a film surface; wherein said
nonmagnetic intermediate layer is thin enough to allow
magnetostatic bond between said recording layer and said
reproducing layer.
2. A magneto-optical recording medium according to claim 1, wherein
said nonmagnetic intermediate layer has a thickness ranging from 1
nm to 10 nm.
3. A magneto-optical recording medium according to claim 1, wherein
said reproducing layer and said recording layer are formed from a
rare earth-transition metal amorphous alloy film.
4. A magneto-optical recording medium according to claim 1, wherein
said nonmagnetic intermediate layer is formed from a substance
selected from the group consisting of Al, Si, Ti, oxides, and
nitrides thereof.
5. A magneto-optical recording medium comprising: a transparent
substrate; a magnetic reproducing layer laminated on said
transparent substrate, said reproducing layer having an easy
direction of magnetization in a plane at room temperature and
having an easy direction of magnetization perpendicular to a film
surface at a given temperature or higher; a magnetic intermediate
layer laminated on said reproducing layer, said magnetic
intermediate layer having an easy direction of magnetization in a
plane; and a magnetic recording layer laminated on said magnetic
intermediate layer, said recording layer having an easy direction
of magnetization perpendicular to a film surface.
6. A magneto-optical recording medium according to claim 5, wherein
a Curie temperature Tc of said magnetic intermediate layer and a
temperature Tread of said reproducing layer in applying a
reproducing power thereto are related to satisfy Tread<Tc.
7. A magneto-optical recording medium according to claim 6, wherein
said magnetic intermediate layer is magnetized in a perpendicular
direction by exchange bond to said recording layer at said given
temperature or higher, and said reproducing layer and said
recording layer are exchange-bonded together through said magnetic
intermediate layer.
8. A magneto-optical recording medium according to claim 7, wherein
said magnetic intermediate layer is formed from a light rare
earth-transition metal amorphous alloy film represented by
R.sub.XFe.sub.YCo.sub.1-X-Y (R=Nd, Sm), where 0<X<0.5 and
0.ltoreq.Y<0.5.
9. A magneto-optical recording medium comprising: a transparent
substrate; a magnetic opening portion control layer laminated on
said transparent substrate, said control layer having an easy
direction of magnetization in a plane and having a transmittance of
60% or more; a magnetic reproducing layer laminated on said control
layer, said reproducing layer having an easy direction of
magnetization in a plane at room temperature and having an easy
direction of magnetization perpendicular to a film surface at a
given temperature or higher; and a magnetic recording layer
laminated on said reproducing layer, said recording layer having an
easy direction of magnetization perpendicular to a film
surface.
10. A magneto-optical recording medium according to claim 9,
wherein a Curie temperature Tc1 of said control layer, a Curie
temperature Tc2 of said reproducing layer, a Curie temperature Tc3
of said recording layer, a room temperature Troom, and a
temperature Tread of said reproducing layer in applying a
reproducing power thereto are related to satisfy
Tc2>Tc3>Tc1>Troom, and Tread>Tc1 at an opening portion
of said control layer.
11. A magneto-optical recording medium according to claim 10,
wherein said control layer, said reproducing layer, and said
recording layer are formed from a rare earth-transition metal
amorphous alloy film.
12. A magneto-optical recording medium according to claim 10,
wherein said control layer has a transmittance of 75% or more.
13. A magneto-optical recording medium according to claim 10,
wherein said control layer has a thickness ranging from 1 nm to 10
nm.
14. A magneto-optical recording medium comprising: a transparent
substrate; a magnetic reproduction assisting layer laminated on
said transparent substrate, said assisting layer having an easy
direction of magnetization perpendicular to a film surface; a
magnetic reproducing layer laminated on said assisting layer, said
reproducing layer having an easy direction of magnetization in a
plane at room temperature; and a magnetic recording layer laminated
on said reproducing layer, said recording layer having an easy
direction of magnetization perpendicular to a film surface; wherein
a Curie temperature Tc1 of said assisting layer, a Curie
temperature Tc2 of said reproducing layer, and a Curie temperature
Tc3 of said recording layer are related to satisfy Tc3<Tc1 and
Tc3<Tc2; and a coercive force Hc1 of said assisting layer and a
coercive force Hc3 of said recording layer are related to satisfy
Hc3>Hc1.
15. A magneto-optical recording medium according to claim 14,
wherein said assisting layer, said reproducing layer, and said
recording layer are formed from a rare earth-transition metal
amorphous alloy film.
16. A magneto-optical recording medium according to claim 14,
wherein said assisting layer has a thickness ranging from 25 nm to
60 nm and a coercive force of 600 Oe or less at room temperature,
and said assisting layer is composed of Gd, Fe, and Co, the content
of Gd being set in the range of 20 at % to 27 at%.
17. A magneto-optical recording medium according to claim 14,
wherein said reproducing layer has a thickness ranging from 1 nm to
40 nm, and said reproducing layer is composed of Gd, Fe, and Co,
the content of Gd being set in the range of 29 at % to 40 at%.
18. A magneto-optical recording medium according to claim 14,
wherein said recording layer has a thickness of 60 nm or less and a
Curie temperature of 250.degree. C. or less.
19. A magneto-optical recording medium according to claim 14,
further comprising a nonmagnetic intermediate layer interposed
between said reproducing layer and said recording layer, said
nonmagnetic intermediate layer having a thickness ranging from 0.5
nm to 20 nm.
20. A magneto-optical recording medium according to claim 14,
further comprising a paramagnetic intermediate layer interposed
between said reproducing layer and said recording layer, said
paramagnetic intermediate layer having a thickness ranging from 0.5
nm to 20 nm.
21. A magneto-optical recording medium according to claim 19,
wherein said nonmagnetic intermediate layer is formed from a
substance selected from the group consisting of Al, Si, Ti, Cu, Cr,
and nitrides thereof.
22. A magneto-optical recording medium according to claim 19,
wherein said assisting layer and said reproducing layer are formed
of materials having the same composition, and said assisting layer
has a thickness larger than that of said reproducing layer.
23. A magneto-optical recording medium according to claim 19,
wherein when said assisting layer is a single layer having a
thickness to be used, said single layer has an easy direction of
magnetization perpendicular to a film surface.
24. A magneto-optical recording medium according to claim 23,
wherein when said reproducing layer is a single layer having a
thickness to be used, said single layer has an easy direction of
magnetization in a plane.
25. A reproducing method for information recorded on a
magneto-optical recording medium comprising a transparent
substrate; a magnetic reproducing layer laminated on said
transparent substrate, said reproducing layer having an easy
direction of magnetization in a plane at room temperature and
having an easy direction of magnetization perpendicular to a film
surface at a given temperature or higher; a nonmagnetic
intermediate layer laminated on said reproducing layer; and a
magnetic recording layer laminated on said nonmagnetic intermediate
layer, said recording layer having an easy direction of
magnetization perpendicular to a film surface; wherein said
nonmagnetic intermediate layer is thin enough to allow
magnetostatic bond between said recording layer and said
reproducing layer; said reproducing method comprising the steps of:
directing a laser beam onto said recording medium as applying a
bias magnetic field to heat said recording medium to temperatures
lower than a Curie temperature of said recording layer; and forming
a temperature distribution in a beam spot, said temperature
distribution comprising a low-temperature area where the direction
of magnetization of said reproducing layer is an in-plane
direction, an intermediate-temperature area where Hr.ltoreq.Hs+Hc
is satisfied and magnetization of said recording layer is
transferred to said reproducing layer by magnetostatic bond, and a
high-temperature area where Hr>Hs+Hc is satisfied and the
direction of magnetization of said reproducing layer is identical
with the direction of said bias magnetic field; where Hr represents
a strength of said bias magnetic field, Hs represents a
magnetostatic bonding force between said reproducing layer and said
recording layer, and Hc represents a coercive force of said
reproducing layer.
26. A reproducing method according to claim 25, wherein said
recording layer is formed from a transition metal rich rare
earth-transition metal amorphous alloy film.
27. A reproducing method for information recorded on a
magneto-optical recording medium comprising a transparent
substrate; a magnetic reproducing layer laminated on said
transparent substrate, said reproducing layer having an easy
direction of magnetization in a plane at room temperature and
having an easy direction of magnetization perpendicular to a film
surface at a given temperature or higher; a nonmagnetic
intermediate layer laminated on said reproducing layer; and a
magnetic recording layer laminated on said nonmagnetic intermediate
layer, said recording layer having an easy direction of
magnetization perpendicular to a film surface; wherein said
nonmagnetic intermediate layer is thin enough to allow
magnetostatic bond between said recording layer and said
reproducing layer; said reproducing method comprising the steps of:
directing a laser beam onto said recording medium to heat said
recording medium to temperatures lower than a Curie temperature of
said recording layer; and forming a temperature distribution in a
beam spot, said temperature distribution comprising a
low-temperature area where the direction of magnetization of said
reproducing layer is an in-plane direction, an
intermediate-temperature area where magnetization of said recording
layer is transferred to said reproducing layer by magnetostatic
bond, and a high-temperature area where said magnetization
transferred to said reproducing layer is spontaneously extinguished
by high temperatures to make the direction of magnetization of said
reproducing layer identical with the direction of magnetization to
be erased.
28. A reproducing method for information recorded on a
magneto-optical recording medium comprising a transparent
substrate; a magnetic reproducing layer laminated on said
transparent substrate, said reproducing layer having an easy
direction of magnetization in a plane at room temperature and
having an easy direction of magnetization perpendicular to a film
surface at a given temperature or higher; a nonmagnetic
intermediate layer laminated on said reproducing layer; and a
magnetic recording layer laminated on said nonmagnetic intermediate
layer, said recording layer having an easy direction of
magnetization perpendicular to a film surface; wherein said
nonmagnetic intermediate layer is thin enough to allow
magnetostatic bond between said recording layer and said
reproducing layer; said reproducing method comprising the steps of:
directing a laser beam onto said recording medium as applying a
bias magnetic field to heat said recording medium to temperatures
lower than a Curie temperature of said recording layer; and forming
a temperature distribution in a beam spot, said temperature
distribution comprising a low-temperature area where Hr>Hs+He is
satisfied and the direction of magnetization of said reproducing
layer is identical with the direction of said bias magnetic field,
an intermediate-temperature area where Hr.ltoreq.Hs+Hc is satisfied
and magnetization of said recording layer is transferred to said
reproducing layer by magnetostatic bond, and a high-temperature
area where Hr>Hs+Hc is satisfied and the direction of
magnetization of said reproducing layer is identical with the
direction of said bias magnetic field; where Hr represents a
strength of said bias magnetic field, Hs represents a magnetostatic
bonding force between said reproducing layer and said recording
layer, and Hc represents a coercive force of said reproducing
layer.
29. A reproducing method according to claim 28, wherein said
recording layer is formed from a rare earth rich rare
earth-transition metal amorphous alloy film.
30. A reproducing method according to claim 25, wherein a magnetic
field leaked from a permanent magnet of an objective lens actuator
provided on an optical head is used as said bias magnetic field for
reproduction.
31. A reproducing method according to claim 28, wherein a magnetic
field leaked from a permanent magnet of an objective lens actuator
provided on an optical head is used as said bias magnetic field for
reproduction.
32. A reproducing method for information recorded on a
magneto-optical recording medium comprising a transparent
substrate; a magnetic reproduction assisting layer laminated on
said transparent substrate, said assisting layer having an easy
direction of magnetization perpendicular to a film surface; a
magnetic reproducing layer laminated on said assisting layer, said
reproducing layer having an easy direction of magnetization in a
plane at room temperature; and a magnetic recording layer laminated
on said reproducing layer, said recording layer having an easy
direction of magnetization perpendicular to a film surface; wherein
a Curie temperature Tc1 of said assisting layer, a Curie
temperature Tc2 of said reproducing layer, and a Curie temperature
Tc3 of said recording layer are related to satisfy Tc3<Tc1 and
Tc3<Tc2; and a coercive force Hc1 of said assisting layer and a
coercive force Hc3 of said recording layer are related to satisfy
Hc3>Hc1; said reproducing method comprising the steps of:
directing a laser beam onto said recording medium as applying a
bias magnetic field to heat said recording medium to temperatures
lower than a Curie temperature of said recording layer; and forming
a temperature distribution in a beam spot, said temperature
distribution comprising a low-temperature area where the direction
of magnetization of said assisting layer is identical with the
direction of said bias magnetic field, and a high-temperature area
where magnetization of said recording layer is transferred to said
assisting layer by exchange bond.
Description
BACKGROUND OF THE INVENTION
1. 1. Field of the Invention
2. The present invention relates to a high-density magneto-optical
recording medium and a reproducing method for information recorded
on the medium.
3. 2. Description of the Related Art
4. A magneto-optical disk is known as a high-density recording
medium, and an increase in information quantity gives rise to a
desire for higher densities of the medium. While the higher
densities may be realized by reducing the space of recorded marks,
the recording and reproducing of the marks are limited by the size
of a light beam (beam spot) on the medium. When the presence of
only one recorded mark in the beam spot is set, an output waveform
corresponding to "1" or "0" may be observed as a reproduction
signal according to whether the recorded mark is present or absent
in the beam spot.
5. However, when the presence of plural recorded marks in the beam
spot is set by reducing the space of the recorded marks, no change
in reproduction output occurs regardless of movement of the beam
spot on the medium. Accordingly, the output waveform becomes linear
and the presence or absence of the recorded mark in the beam spot
cannot be identified. To reproduce such small recorded marks having
a period smaller than the size of the beam spot, it is sufficient
to reduce the beam spot to a small size. However, since the size of
the beam spot is limited by the wavelength .lambda. of a light
source and the numerical aperture NA of an objective lens, the beam
spot cannot be sufficiently reduced to a small size.
6. There has recently been proposed a reproducing method using
magnetically induced super resolution such that a recorded mark
smaller in size than the beam spot be reproduced by the use of an
existing optical system. According to this method, the resolution
of reproduction is improved by masking other marks during
reproduction of one mark in the beam spot. Accordingly, a super
resolution disk medium is required to have at least a mask layer or
a reproducing layer for masking other marks so that only one mark
may be reproduced during signal reproduction, in addition to a
recording layer for recording marks. A magneto-optical recording
medium using a perpendicular magnetization film as the reproducing
layer is proposed in Japanese Patent Laid-open No. 3-88156. In the
prior art described in this publication, however, an initializing
magnetic field of about several kOe is required to initialize the
reproducing layer. Accordingly, a recording apparatus cannot be
made compact.
7. On the other hand, a magneto-optical recording medium using a
magnetic film as the recording layer is proposed in Japanese Patent
Laid-open Nos. 5-81717 and 5-342670. This magnetic film has an easy
direction of magnetization in a plane at room temperature and has
an easy direction of magnetization perpendicular to a film surface
at a given temperature or higher. The principle of reproduction in
this prior art will now be described in brief with reference to
FIGS. 31A, 31B, and 31C. As shown in FIG. 31C, a magneto-optical
disk 2 is formed by laminating a magnetic reproducing layer 6 and a
magnetic recording layer 8 on a transparent substrate 4. The
magnetic reproducing layer 6 has an easy direction of magnetization
in a plane at room temperature. However, when the medium is heated
to a given temperature or higher by applying a reproducing power,
the easy direction of magnetization is changed to a perpendicular
direction. The magnetic recording layer 8 is a perpendicular
magnetization film. Reference numeral 10 denotes a light beam.
8. The intensity distribution of the light beam is a Gaussian
distribution as shown in FIG. 31A. Accordingly, when the disk is at
rest, the temperature distribution on the disk is also a similar
distribution such that the central portion is higher in temperature
than the peripheral portion. In actual, however, the disk 2 is
rotated in the direction of arrow R shown in FIG. 31C during
reproduction. Accordingly, the temperature distribution on the disk
in rotation becomes a distribution as shown in FIG. 31B so that a
high-temperature area in the beam spot is shifted to the forward
direction of rotation of the disk. Owing to such a temperature
distribution during reproduction, the easy direction of
magnetization of the magnetic reproducing layer 6 becomes an
in-plane direction in a low-temperature area in the beam spot.
Therefore, the Kerr rotation angle of reflected light becomes
almost zero in the low-temperature area. In the high-temperature
area, the easy direction of magnetization of the magnetic
reproducing layer 6 is changed from an in-plane direction to an
perpendicular direction.
9. The perpendicular magnetization of the magnetic reproducing
layer 6 at this time is bonded to the magnetization of the magnetic
recording layer 8 by an exchange force, and the direction of
magnetization of the reproducing layer 6 is made identical with the
direction of magnetization of the recording layer 8, thereby
allowing the magnetization recorded in the recording layer 8 to be
transferred to the reproducing layer 6. The area size of such
transfer can be changed by a reproducing layer beam power. In this
manner, the size of the masking reproducing layer is controlled so
as to allow the reproduction of only one recorded mark, thereby
obtaining the same effect as that in the case of substantially
reducing the area of the beam spot.
10. As mentioned above, the intensity distribution of the laser
beam 10 directed onto the disk 2 is a Gaussian distribution, and
the disk 2 is rotated in the direction of arrow R. As a result, a
low-temperature area 10a and a high-temperature area 10b are formed
on the reproducing layer 6 (see FIG. 32). The high-temperature area
10b is shifted to the forward direction of rotation of the disk 2
with respect to the laser beam 10. In the prior art disclosed in
Japanese Patent Laid-open No. 5-81717, however, the in-plane
magnetization of the reproducing layer 6 in the low-temperature
area 10a in the beam spot is bonded to the perpendicular
magnetization of the recording layer 8, causing inclination of the
in-plane magnetization to generate a perpendicular component as
shown in FIG. 32.
11. As a result, the masking effect is reduced and a mark recorded
on the recording layer adjacent to a mark to be reproduced cannot
be perfectly masked. Accordingly, the magnetization of the
recording layer in the low-temperature area is also transferred to
the reproducing layer, so that individual marks cannot be
identified because of interference to cause a reduction in
reproduction output.
12. Japanese Patent Laid-open No. 5-342670 mentioned above
discloses a magneto-optical recording medium having a magnetic
intermediate layer interposed between a magnetic reproducing layer
and a magnetic recording layer. The magnetic intermediate layer is
provided to prevent the possibility that when the exchange bonding
force between the recording layer and the reproducing layer is too
strong, the magnetization of the reproducing layer becomes
perpendicular magnetization also in an area where the laser beam is
not directed, reducing the masking effect of the reproducing layer.
The magnetic intermediate layer described in this publication is
considered from its composition to have a Curie point lower than a
temperature of the medium to be heated by the reproducing laser
beam. While the operation of the magnetic intermediate layer is not
described in detail in this publication, it may be considered as
follows:
13. When the magnetic intermediate layer is heated to temperatures
higher than its Curie temperature, the magnetization of the
intermediate layer disappears. At this time, in the low-temperature
area in the beam spot, a stable in-plane mask is formed in the
reproducing layer, while in the high-temperature area, the
magnetization of the recording layer is transferred to the
reproducing layer by magnetostatic bond. Accordingly, information
recorded on the medium in the high-temperature area can be read
out. However, the transfer of the magnetization by a magnetostatic
bonding force is weaker than the transfer of the magnetization by
an exchange bonding force. Thus, the medium having the magnetic
intermediate layer described in this publication is not
satisfactory in transfer characteristic of the magnetization in the
high-temperature area in the beam spot. In addition, the
magnetostatic bonding force between the recording layer and the
reproducing layer is absorbed by the magnetic intermediate layer,
thus further hindering the transfer characteristic of the
magnetization.
SUMMARY OF THE INVENTION
14. It is therefore an object of the present invention to provide a
magneto-optical recording medium which can perfectly mask a mark
adjacent to a mark to be reproduced to thereby improve a
reproduction output.
15. It is another object of the present invention to provide a
magneto-optical recording medium which can prevent the crosstalk
between a track to be reproduced and a track adjacent to this track
to thereby realize the improvement in the reproduction output.
16. According to a first aspect of the present invention, there is
provided a magneto-optical recording medium comprising a
transparent substrate; a magnetic reproducing layer laminated on
said transparent substrate, said reproducing layer having an easy
direction of magnetization in a plane at room temperature and
having an easy direction of magnetization perpendicular to a film
surface at a given temperature or higher; a nonmagnetic
intermediate layer laminated on said reproducing layer; and a
magnetic recording layer laminated on said nonmagnetic intermediate
layer, said recording layer having an easy direction of
magnetization perpendicular to a film surface; wherein said
nonmagnetic intermediate layer is thin enough to allow
magnetostatic bond between said recording layer and said
reproducing layer.
17. Preferably, the nonmagnetic intermediate layer has a thickness
ranging from 1 nm to 10 nm. The nonmagnetic intermediate layer is
formed from a substance selected from the group consisting of Al,
Si, Ti, oxides, and nitrides thereof.
18. According to the first aspect, the nonmagnetic intermediate
layer is interposed between the magnetic recording layer and the
magnetic reproducing layer, so that the exchange bonding force
between the two magnetic layers can be perfectly cut off. Owing to
the enough small thickness of the nonmagnetic intermediate layer,
when the reproducing power is applied to the medium to heat the
reproducing layer to the given temperature or higher, the
magnetization of the recording layer is transferred to the
reproducing layer by the magnetostatic interaction, thereby
reproducing the information recorded on the recording layer. The
exchange interaction between the recording layer and the
reproducing layer is cut off by the nonmagnetic intermediate layer,
thereby preventing the inclination of magnetization of the
reproducing layer from the in-plane direction due to the exchange
interaction in a low-temperature area in a beam spot, with the
result that the reproduction output can be improved.
19. According to a second aspect of the present invention, there is
provided a magneto-optical recording medium comprising a
transparent substrate; a magnetic reproducing layer laminated on
said transparent substrate, said reproducing layer having an easy
direction of magnetization in a plane at room temperature and
having an easy direction of magnetization perpendicular to a film
surface at a given temperature or higher; a magnetic intermediate
layer laminated on said reproducing layer, said magnetic
intermediate layer having an easy direction of magnetization in a
plane; and a magnetic recording layer laminated on said magnetic
intermediate layer, said recording layer having an easy direction
of magnetization perpendicular to a film surface.
20. Preferably, the magnetic intermediate layer is formed from a
light rare earth-transition metal amorphous alloy film represented
by R.sub.XFe.sub.YCo.sub.1-X-Y (R=Nd, Sm), where 0<X<0.5 and
0.ltoreq.Y<0.5.
21. According to the second aspect, the magnetic intermediate layer
having an easy direction of magnetization always in a plane is
interposed between the magnetic recording layer and the magnetic
reproducing layer. Accordingly, the in-plane magnetization of the
magnetic intermediate layer is stable in a low-temperature area in
a beam spot in applying a reproducing power to the medium, so that
there does not occur the inclination of magnetization of the
magnetic intermediate layer due to the perpendicular magnetization
of the recording layer. In a high-temperature area in the beam
spot, the easy direction of magnetization of the reproducing layer
is perpendicular. At this time, the perpendicular direction of
magnetization of the magnetic intermediate layer becomes an easy
direction of magnetization because of the perpendicular
magnetization of both the recording layer and the reproducing
layer, and is made identical with the direction of magnetization of
the recording layer by the exchange bond to the magnetization of
the recording layer. Furthermore, the magnetization of the
reproducing layer is exchange-bonded to the magnetization of the
magnetic intermediate layer, and the direction of magnetization of
the reproducing layer therefore becomes identical with the
direction of magnetization of the intermediate layer. As a result,
the direction of magnetization of the recording layer is
transferred to the reproducing layer.
22. According to a third aspect of the present invention, there is
provided a magneto-optical recording medium comprising a
transparent substrate; a magnetic opening portion control layer
laminated on said transparent substrate, said control layer having
an easy direction of magnetization in a plane and having a
transmittance of 60% or more; a magnetic reproducing layer
laminated on said control layer, said reproducing layer having an
easy direction of magnetization in a plane at room temperature and
having an easy direction of magnetization perpendicular to a film
surface at a given temperature or higher; and a magnetic recording
layer laminated on said reproducing layer, said recording layer
having an easy direction of magnetization perpendicular to a film
surface.
23. Preferably, a Curie temperature Tc1 of said control layer, a
Curie temperature Tc2 of said reproducing layer, a Curie
temperature Tc3 of said recording layer, a room temperature Troom,
and a temperature Tread of said reproducing layer in applying a
reproducing power thereto are related to satisfy
Tc2>Tc3>Tc1>Troom, and Tread>Tc1 at an opening portion
of said control layer.
24. According to the third aspect, the control layer having a
transmittance of 60% or more is provided. As the low-temperature
area in the reproducing beam spot shows temperatures lower than the
Curie temperature of the control layer, the in-plane magnetization
remains, in the control layer. As a result, the magnetization of
the reproducing layer follows the magnetization of the control
layer owing to the exchange bonding force to become the in-plane
magnetization. Accordingly, the control layer serves as a mask in
the low-temperature area, thereby making impossible the
reproduction of a mark in the reproducing layer. In the
high-temperature area in the reproducing beam spot, the control
layer is heated to temperatures higher than the Curie temperature,
causing disappearance of the magnetization of the control layer. As
a result, the bond in magnetization between the reproducing layer
and the control layer is cut to bring the magnetization of the
reproducing layer into perfectly perpendicular magnetization,
thereby allowing the reproduction of the mark in the reproducing
layer through the control layer.
25. According to a fourth aspect of the present invention, there is
provided a magneto-optical recording medium comprising a
transparent substrate; a magnetic reproduction assisting layer
laminated on said transparent substrate, said assisting layer
having an easy direction of magnetization perpendicular to a film
surface; a magnetic reproducing layer laminated on said assisting
layer, said reproducing layer having an easy direction of
magnetization in a plane at room temperature; and a magnetic
recording layer laminated on said reproducing layer, said recording
layer having an easy direction of magnetization perpendicular to a
film surface; wherein a Curie temperature Tc1 of said assisting
layer, a Curie temperature Tc2 of said reproducing layer, and a
Curie temperature Tc3 of said recording layer are related to
satisfy Tc3<Tc1 and Tc3<Tc2; and a coercive force Hc1 of said
assisting layer and a coercive force Hc3 of said recording layer
are related to satisfy Hc3>Hc1.
26. According to the fourth aspect, the direction of magnetization
of the reproducing layer is an in-plane direction in the
low-temperature area in the beam spot. Accordingly, the direction
of magnetization of the assisting layer is made identical with the
direction of a bias magnetic field to form a perpendicular mask. In
the high-temperature area in the beam spot, the magnetization of
the reproducing layer is exchange-bonded to the magnetization of
the recording layer, and the magnetization of the reproducing layer
is also exchange-bonded bonded to the magnetization of the
assisting layer. Accordingly, the direction of magnetization of the
recording layer is transferred to the assisting layer, thereby
allowing reading of information recorded in the recording
layer.
27. Accordingly, when a magneto-optical output is differentially
detected, the low-temperature area in the beam spot acts as a mask,
so that a magneto-optical signal in the low-temperature area is not
read out, but a magneto-optical signal only in the high-temperature
area is read out. Therefore, a mark having a size less than the
diffraction limit of a reproducing laser wavelength can be read
out.
28. According to another aspect of the present invention, there is
provided a reproducing method for information recorded on a
magneto-optical recording medium comprising a transparent
substrate; a magnetic reproducing layer laminated on said
transparent substrate, said reproducing layer having an easy
direction of magnetization in a plane at room temperature and
having an easy direction of magnetization perpendicular to a film
surface at a given temperature or higher; a nonmagnetic
intermediate layer laminated on said reproducing layer; and a
magnetic recording layer laminated on said nonmagnetic intermediate
layer, said recording layer having an easy direction of
magnetization perpendicular to a film surface; wherein said
nonmagnetic intermediate layer is thin enough to allow
magnetostatic bond between said recording layer and said
reproducing layer; said reproducing method comprising the steps of
directing a laser beam onto said recording medium as applying a
bias magnetic field to heat said recording medium to temperatures
lower than a Curie temperature of said recording layer; and forming
a temperature distribution in a beam spot, said temperature
distribution comprising a low-temperature area where the direction
of magnetization of said reproducing layer is an in-plane
direction, an intermediate-temperature area where Hr.ltoreq.Hs+Hc
is satisfied and magnetization of said recording layer is
transferred to said reproducing layer by magnetostatic bond, and a
high-temperature area where Hr>Hs+He is satisfied and the
direction of magnetization of said reproducing layer is identical
with the direction of said bias magnetic field; where Hr represents
a strength of said bias magnetic field, Hs represents a
magnetostatic bonding force between said reproducing layer and said
recording layer, and Hc represents a coercive force of said
reproducing layer.
29. Preferably, the reproducing layer and the recording layer are
formed from a rare earth-transition metal amorphous alloy film.
When the recording layer is formed from a rare earth rich rare
earth-transition metal amorphous alloy film, there is formed in the
beam spot a temperature distribution comprising a low-temperature
area where the direction of magnetization of the reproducing layer
is made identical with the direction of the bias magnetic field, an
intermediate-temperatur- e area where the magnetization of the
recording layer is transferred to the reproducing layer, and a
high-temperature area where the direction of magnetization of the
reproducing layer is made identical with the direction of the bias
magnetic field.
30. According to still another aspect of the present invention,
there is provided a reproducing method for information recorded on
a magneto-optical recording medium comprising a transparent
substrate; a magnetic reproducing layer laminated on said
transparent substrate, said reproducing layer having an easy
direction of magnetization in a plane at room temperature and
having an easy direction of magnetization perpendicular to a film
surface at a given temperature or higher; a nonmagnetic
intermediate layer laminated on said reproducing layer; and a
magnetic recording layer laminated on said nonmagnetic intermediate
layer, said recording layer having an easy direction of
magnetization perpendicular to a film surface; wherein said
nonmagnetic intermediate layer is thin enough to allow
magnetostatic bond between said recording layer and said
reproducing layer; said reproducing method comprising the steps of
directing a laser beam onto said recording medium to heat said
recording medium to temperatures lower than a Curie temperature of
said recording layer; and forming a temperature distribution in a
beam spot, said temperature distribution comprising a
low-temperature area where the direction of magnetization of said
reproducing layer is an in-plane direction, an
intermediate-temperature area where magnetization of said recording
layer is transferred to said reproducing layer by magnetostatic
bond, and a high-temperature area where said magnetization
transferred to said reproducing layer is spontaneously extinguished
by high temperatures to make the direction of magnetization of said
reproducing layer identical with the direction of magnetization to
be erased.
31. The above and other objects, features and advantages of the
present invention and the manner of realizing them will become more
apparent, and the invention itself will best be understood from a
study of the following description and appended claims with
reference to the attached drawings showing some preferred
embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
32. FIG. 1 is a sectional view of a magneto-optical recording
medium according to a first preferred embodiment of the present
invention;
33. FIG. 2 is a view illustrating a reproducing method for the
recording medium according to the first preferred embodiment;
34. FIG. 3 is a graph showing the mark length dependency of C/N
with the thickness of a nonmagnetic intermediate layer changed;
35. FIG. 4 is a sectional view of a magneto-optical recording
medium according to a second preferred embodiment of the present
invention;
36. FIG. 5 is a view illustrating a reproducing method for the
recording medium according to the second preferred embodiment;
37. FIG. 6 is a sectional view of a magneto-optical recording
medium according to a third preferred embodiment of the present
invention;
38. FIG. 7 is a graph showing the temperature characteristics of
coercive forces of a magnetic reproducing layer and a magnetic
recording layer;
39. FIG. 8 is a view illustrating a reproducing method for the
recording medium according to the third preferred embodiment;
40. FIG. 9 is a view illustrating the relation between a
reproducing laser beam irradiation area and a reproducible
area;
41. FIG. 10 is a sectional view of a magneto-optical recording
medium according to a fourth preferred embodiment of the present
invention;
42. FIG. 11 is a view illustrating an erasing condition of data on
the recording medium according to the fourth preferred
embodiment;
43. FIG. 12 is a view illustrating a recording condition of data on
the recording medium according to the fourth preferred
embodiment;
44. FIG. 13 is a view illustrating a reproducing method for the
recording medium according to the fourth preferred embodiment;
45. FIG. 14 is a sectional view of a magneto-optical recording
medium according to a fifth preferred embodiment of the present
invention;
46. FIG. 15 is a graph showing the temperature dependency of
magnetization M of a recording layer;
47. FIG. 16 is a graph showing the temperature dependencies of
magnetization of a reproducing layer and a TM rich recording
layer;
48. FIG. 17A is a view showing a magnetization condition of a
medium having a TM rich recording layer during reproduction;
49. FIG. 17B is a view showing a temperature distribution in a beam
spot during reproduction;
50. FIG. 18 is a graph showing the temperature dependencies of
magnetization of a reproducing layer and an RE rich recording
layer;
51. FIG. 19A is a view showing a magnetization condition of a
medium having an RE rich recording layer during reproduction;
52. FIG. 19B is a view showing a temperature distribution in a beam
spot during reproduction;
53. FIG. 20 is a view illustrating a leaked magnetic field in an
objective lens actuator;
54. FIG. 21 is a graph showing the mark length dependency of C/N in
Example 1 in comparison with the prior art;
55. FIG. 22 is a graph showing the mark length dependency of C/N in
Example 2 in comparison with the prior art;
56. FIG. 23 is a graph showing the mark length dependency of C/N in
Example 3 in comparison with the prior art;
57. FIGS. 24A and 24B are views illustrating the relation between a
reproducing power and a reproducing condition;
58. FIG. 25 is a graph showing the reproducing power dependency of
C/N in Example 4 in comparison with the prior art;
59. FIG. 26 is a graph showing the reproducing power dependency of
crosstalk in Example 4 in comparison with the prior art;
60. FIG. 27 is a graph showing the reproducing power dependency of
C/N in Example 5;
61. FIG. 28 is a graph showing the reproducing power dependency of
crosstalk in Example 5;
62. FIG. 29 is a graph showing the reproducing power dependency of
C/N in Example 9 in comparison with Example 1;
63. FIG. 30 is a graph showing the reproducing power dependency of
C/N in Example 10 in comparison with Example 1;
64. FIGS. 31A, 31B, and 31C are views illustrating the principle of
reproduction in the prior art; and
65. FIG. 32 is a view illustrating the problem in the prior
art.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
66. The structure of a magneto-optical recording medium 12
according to a first preferred embodiment of the present invention
will be described with reference to FIG. 1. The magneto-optical
recording medium 12 is usually in the form of disk. A dielectric
layer 16 formed of SiN or the like by sputtering, for example, is
laminated on a transparent substrate 14 formed of glass or the
like. The dielectric layer 16 prevents oxidation and corrosion of a
magnetic layer laminated thereon. Resins such as polycarbonate,
polymethyl methacrylate, and amorphous olefin may be adopted as the
transparent substrate 14. Metal nitrides such as AlN, metal oxides
such as SiO.sub.2 and Al.sub.2O.sub.3, and metal sulfides such as
ZnS may be adopted as the dielectric layer 16.
67. A magnetic reproducing layer 18 formed from a rare
earth-transition metal amorphous alloy film such as GdFeCo is
laminated on the dielectric layer 16. The magnetic reproducing
layer 18 has an easy direction of magnetization in a plane at room
temperature, and has an easy direction of magnetization
perpendicular to its film surface at temperatures higher than a
given temperature to which the layer 18 is heated by a reproducing
beam power. A nonmagnetic intermediate layer 20 formed of SiN or
the like is laminated on the magnetic reproducing layer 18. Metal
nitrides such as AlN, metal oxides such as SiO.sub.2 and
Al.sub.2O.sub.3, and metal sulfides such as ZnS may be adopted as
the nonmagnetic intermediate layer 20.
68. A magnetic recording layer 22 formed from a rare
earth-transition metal amorphous alloy film such as TbFeCo is
laminated on the nonmagnetic intermediate layer 20. The magnetic
recording layer 22 has an easy direction of magnetization
perpendicular to its film surface. As the nonmagnetic intermediate
layer 20 is interposed between the magnetic reproducing layer 18
and the magnetic recording layer 22, the exchange bond between the
magnetic reproducing layer 18 and the magnetic recording layer 22
is perfectly cut off. The nonmagnetic intermediate layer 20 must be
thin enough to permit the magnetostatic bond between the magnetic
recording layer 22 and the magnetic reproducing layer 18 when the
magnetic reproducing layer 18 is heated to the given temperature.
Specifically, the thickness of the nonmagnetic intermediate layer
20 is preferably in the range of 1 nm to 10 nm, which will be
hereinafter described in detail.
69. There exists an optimum thickness of the dielectric layer 16.
This may be obtained as follows:
70. When the reflectivity R of the medium is minimum, the Kerr
rotation angle becomes maximum, and the Kerr enhancement is
amplified. The interference conditions are given by the following
equation.
2nd=.lambda.(2 m+1)/2
71. where n is the refractive index; .lambda. is the laser
wavelength; d is the thickness of the dielectric layer; and m is
the order. Substituting n=2.15, .lambda.=780 nm, and m.apprxeq.0
for the above equation, d=780/(4.times.2.15)=90.7.congruent.90 is
obtained.
72. Accordingly, the optimum thickness of the dielectric layer 16
is 90 nm.
73. A protective film 24 is laminated on the magnetic recording
layer 22 to complete the magneto-optical recording medium 12. The
protective film 24 prevents entry of water, oxygen, or other
substances such as halogen from the air to protect the magnetic
recording layer 22. Metal nitrides such as SiN and AlN, metal
oxides such as SiO.sub.2 and Al.sub.2O.sub.3, and metal sulfides
such as ZnS may be adopted as the protective film 24.
74. A reproducing method for information recorded on the
magneto-optical recording medium 12 according to the first
preferred embodiment will be described with reference to FIG. 2.
The directions of magnetization of the magnetic reproducing layer
and the magnetic recording layer are shown by arrows, and the
recording medium 12 is rotated in the direction of arrow R. Let Ts
denotes a temperature at which the direction of magnetization of
the magnetic reproducing layer 18 changes from an in-plane
direction to a perpendicular direction. In a low-temperature area
28a where the temperature T of the medium irradiated with a
reproducing beam 26 is lower than the temperature Ts, tile
exclaiming bond between tile magnetic reproducing layer 18 and the
magnetic recording layer 22 is perfectly cut off because the
nonmagnetic intermediate layer 20 is interposed between the two
magnetic layers 18 and 22. Accordingly, the magnetic reproducing
layer 18 perfectly shows in-plane magnetization without being
affected by the perpendicular magnetization of the magnetic
recording layer 22.
75. In a high-temperature area 28b where the temperature T of the
medium is higher than the temperature Ts, the magnetic reproducing
layer 18 shows perpendicular magnetization. The direction of
magnetization of the magnetic recording layer 18 in the
high-temperature area 28b is made identical with the direction of
magnetization of the magnetic recording layer 22 because the layer
18 is magnetostatically bonded to the layer 22 by the floating
magnetic field from the perpendicular magnetization of the layer
22. Accordingly, the magnetization of the magnetic recording layer
22 is transferred to the magnetic reproducing layer 18, and a
reproduction output can be improved by reading only the
high-temperature area 28b. As shown, a part of the high-temperature
area 28b where the magnetization of the magnetic reproducing layer
becomes perpendicular magnetization is present outside the beam
spot; however, the magnetization area outside the beam spot is not
reproduced, so that only the perpendicular magnetization part of
the high-temperature area 28b inside the beam spot can be read.
76. In this preferred embodiment, the magnetic reproducing layer 18
in the low-temperature area shows stable in-plane magnetization,
which therefore acts as a perfect mask to thereby obtain a high
reproduction output without interference with adjacent marks during
reproduction. As mentioned above, the nonmagnetic intermediate
layer 20 is formed of metals such as Al, Si, and Ti, their oxides
or nitrides. The nonmagnetic intermediate layer 20 must be thin
enough to permit the magnetostatic bond between the magnetic
recording layer 22 and the magnetic reproducing layer 18 at
temperatures higher than the given temperature. The preferable
range of the thickness of the nonmagnetic intermediate layer 20
will now be described.
77. Referring to FIG. 3, there is shown a mark length dependency of
a carrier-to-noise ratio, i.e., C/N in the case of changing the
thickness of the nonmagnetic intermediate layer 20. As apparent
from this graph, when the thickness of the nonmagnetic intermediate
layer 20 is in the range of 1 nm to 10 nm, the C/N is remarkably
improved. If the thickness of the nonmagnetic intermediate layer 20
is larger than 10 nm, e.g., it is 15 nm, a sufficient magnetostatic
bond between the magnetic reproducing layer 18 and the magnetic
recording layer 22 cannot be obtained. Accordingly, the thickness
of the layer 20 larger than 10 nm is not preferable. Further, if
the thickness of the layer 20 is smaller than 1 nm, the exchange
bond between the reproducing layer 18 and the recording layer 22
cannot be cut off. Accordingly, the thickness of the layer 20
smaller than 1 nm is not preferable. Thus, the preferable thickness
of the nonmagnetic intermediate layer 20 ranges from 1 nm to 10
nm.
78. The structure of a magneto-optical recording medium 12'
according to a second preferred embodiment of the present invention
will be described with reference to FIG. 4. In the following
description of this preferred embodiment, the same parts as those
in the first preferred embodiment shown in FIG. 1 are denoted by
the same reference numerals, and the explanation thereof will be
omitted to avoid repetition. This preferred embodiment is different
from the first preferred embodiment in the point that a magnetic
intermediate layer 30 is interposed between the magnetic
reproducing layer 18 and the magnetic recording layer 22. The other
layers are similar in structure to those in the first preferred
embodiment.
79. The magnetic intermediate layer 30 is a magnetic layer having
an easy direction of magnetization in a plane, and has a large
saturation magnetization Ms. Therefore, the in-plane magnetization
direction is stable over the range of room temperature to its Curie
temperature. The magnetic intermediate layer 30 is formed from a
light rare earth-transition metal amorphous alloy film represented
by R.sub.XFe.sub.YCo.sub.1-X-Y (R=Nd, Sm) where 0<X<0.5 and
0.ltoreq.Y<0.5.
80. A reproducing method for information recorded on the
magneto-optical recording medium 12' according to the second
preferred embodiment will be described with reference to FIG. 5.
The magneto-optical recording medium 12' is rotated in the
direction of arrow R. In the low-temperature area 28a in the laser
spot formed on the medium irradiated with the reproducing laser
beam 26, the temperature T of the medium is lower than the
temperature Ts. Accordingly, the magnetic reproducing layer 18
shows in-plane magnetization. In the low-temperature area 28a, the
saturation magnetization Ms of the magnetic intermediate layer 30
is large, and the in-plane magnetization direction of the layer 30
is therefore stable. Accordingly, there does not occur the
inclination of magnetization of the layer 30 due to the
perpendicular magnetization of the magnetic recording layer 22. As
a result, the in-plane magnetization direction of the magnetic
reproducing layer 18 is also stable in the low-temperature area
28a.
81. In the high-temperature area 28b where the temperature T of the
medium is higher than the temperature Ts, the easy direction of
magnetization of the magnetic reproducing layer 18 is
perpendicular. Accordingly, the easy direction of magnetization of
the magnetic intermediate layer 30 becomes perpendicular because of
the perpendicular magnetization of the recording layer 22 and the
reproducing layer 18, and the direction of magnetization of the
layer 30 is made identical with that of the layer 22 by the
exchange bond between the two layers 30 and 22. Further, the
direction of magnetization of the reproducing layer 18 is also made
identical with that of the intermediate layer 30 by the exchange
bond between the two layers 18 and 30. As a result, the
magnetization of the recording layer 22 is transferred to the
reproducing layer 18, and the magnetization thus transferred to the
reproducing layer 18 can be read by directing the reproducing laser
beam 26 to the medium. Further, when the thickness of the
reproducing layer 18 is small enough to permit the laser beam to
reach the intermediate layer 30, a large Kerr rotation angle is
obtained in reproduced light because the Kerr rotation angle of the
intermediate layer 30 formed from a light rare earth (e.g.,
Nd)-transition metal amorphous alloy film is larger than the Kerr
rotation angle of the reproducing layer 18, thereby obtaining a
high reproduction output.
82. The magnetic intermediate layer 30 in this preferred embodiment
is different from the magnetic intermediate layer mentioned in
Japanese Patent Laid-open No. 5-342670 in the following point. In
the intermediate layer mentioned in this publication, the
magnetization of the intermediate layer disappears at a temperature
of heating by the reproducing laser beam. To the contrary, the
magnetization of the intermediate layer 30 in this preferred
embodiment does not disappear at a temperature of heating by the
reproducing laser beam because of a sufficiently high Curie
temperature. Accordingly, the magnetization of the recording layer
22 is transferred to the intermediate layer 30 by the exchange bond
therebetween, and the magnetization of the intermediate layer 30 is
transferred to the reproducing layer 18 by the exchange bond
therebetween. Thus, the magnetization transfer characteristic in
this preferred embodiment is greatly improved.
83. The structure of a magneto-optical recording medium 12"
according to a third preferred embodiment of the present invention
will be described with reference to FIG. 6. In the following
description of this preferred embodiment, the same parts as those
in the first preferred embodiment shown in FIG. 1 are denoted by
the same reference numerals, and the explanation thereof will be
omitted to avoid repetition. In this preferred embodiment, a
magnetic opening portion control layer 32 is laminated on the
dielectric layer 16 by DC magnetron sputtering, for example. The
magnetic reproducing layer 18 is laminated on the control layer 32.
The control layer 32 always has an easy direction of magnetization
in a plane over the range of room temperature to its Curie
temperature. The transmittance of the control layer 32 must be 60%
or more, preferably, 75% or more. The thickness of the control
layer 32 is in the range of 1 nm to 10 nm, preferably, 1 nm to 5
nm.
84. Supposing that Tc1, Tc2, and Tc3 denotes the Curie temperatures
of the control layer 32, the reproducing layer 18, and the
recording layer 22, respectively, Troom denotes room temperature,
and Tread denotes a surface temperature of the medium in applying a
reproducing power thereto, it is necessary to satisfy
Tc2>Tc3>Tc1>Troom and also satisfy Tread>Tc1 at an
opening portion of the control layer 32. For example, the Curie
temperature of the control layer 32 is about 90.degree. C.; the
Curie temperature of the reproducing layer 18 is about
300.degree.C.; and the Curie temperature of the recording layer 22
is about 200.degree. C.
85. Referring to FIG. 7, there is shown the coercive
force-temperature characteristics of the reproducing layer 18 and
the recording layer 22. The temperature characteristic of the
recording layer 22 is shown by a curve 34, and the temperature
characteristic of the reproducing layer 18 is shown by a curve 36.
The point P on the curve 36 is a transition point where the
direction of magnetization of the reproducing layer 18 changes from
in-plane magnetization to perpendicular magnetization. The control
layer 32 is an in-plane magnetization film, and has the Curie
temperature of about 90.degree. C. The coercive force of the
control layer 32 is similar to that of the reproducing layer 18 in
the range of room temperature to about 90.degree. C.
86. A reproducing method for the magneto-optical recording medium
12" according to the third preferred embodiment will be described
with reference to FIGS. 8 and 9. The arrows shown represent the
directions of magnetization of the magnetic layers 18, 22, and 32.
The medium 12" is rotated in the direction of arrow R. In FIG. 9,
40 denotes a track to be reproduced, and 40a and 40 b denote tracks
adjacent to the track 40.
87. In the high-temperature area 28b in the beam spot 28 formed on
the medium 12" irradiated with the reproducing laser beam 26, the
temperature of the medium becomes higher than the Curie temperature
of the control layer 32, so that the magnetization of the control
layer 32 disappears to form an opening at this area. In the
high-temperature area 28b, the direction of magnetization of the
reproducing layer 18 changes from in-plane magnetization to
perpendicular magnetization, and the magnetization of the recording
layer 22 is therefore transferred to the reproducing layer 18.
Accordingly, a mark transferred to the reproducing layer 18 can be
read through the opening of the control layer 32. In the
low-temperature area 28a in the beam spot 28, if the control layer
32 is absent, the in-plane magnetization of the reproducing layer
18 is inclined to the perpendicular direction to some extent by the
exchange bond to the recording layer 22.
88. When the control layer 32 is present, the in-plane
magnetization of the control layer 32 remains because the
temperature of the low-temperature area 28a is lower than the Curie
temperature of the control layer 32. Accordingly, the reproducing
layer 18 shows in-plane magnetization due to the exchange bond to
the control layer 32, and the perpendicular component of the
magnetization of the reproducing layer 18 is almost absent. Even
when a fine perpendicular component of the magnetization of the
layer 18 is present, it cannot be read owing to the control layer
32 functioning as a mask. In this manner, a microscopic mark can be
reproduced by utilizing the presence or absence of the
magnetization of the control layer 32. Further, as shown in FIG. 9,
the reproduction of adjacent marks on the same track or marks
recorded on the adjacent tracks 40a and 40b is avoided to thereby
suppress crosstalk.
89. The structure of a magneto-optical recording medium 12a
according to a fourth preferred embodiment of the present invention
will be described with reference to FIG. 10. In the following
description of this preferred embodiment, the same parts as those
in the first preferred embodiment shown in FIG. 1 are denoted by
the same reference numerals, and the explanation thereof will be
omitted to avoid repetition. In this preferred embodiment, a
magnetic reproduction assisting layer 42 is laminated on the
dielectric layer 16 by DC magnetron sputtering, for example. The
magnetic reproducing layer 18 is laminated on the assisting layer
42.
90. The assisting layer 42 has an easy direction of magnetization
perpendicular to its film surface. Letting Tc1, Tc2, and Tc3 denote
the Curie temperatures of the assisting layer 42, the reproducing
layer 18, and the recording layer 22, respectively, the following
relations are satisfied.
Tc1>Tc3
Tc2>Tc3
91. Letting Hc1, Hc2, and Hc3 denote coercive forces of the
assisting layer 42, the reproducing layer 18, and the recording
layer 22, respectively, at room temperature, the following
relations are satisfied.
Hc3>Hc1
Hc3>Hc2
92. The assisting layer 42, the reproducing layer 18, and the
recording layer 22 are preferably formed from a rare
earth-transition metal amorphous alloy film. More specifically, the
assisting layer 42 and the reproducing layer 18 are preferably
formed of GdFeCo alloy or GdFe alloy, and the recording layer 22 is
preferably formed of TbFeCo alloy or DyFeCo alloy. Preferably, the
assisting layer 42 has a thickness of 25 nm to 60 nm and a coercive
force of 600 Oe or less at room temperature. Further, it is
desirable that the assisting layer 42 is composed of Gd, Fe, and
Co, and that the content of Gd is set in the range of 20 at % to 27
at%.
93. It is known that the depth of penetration of light into metal
is about 25 nm. Accordingly, if the thickness of the assisting
layer 42 is smaller than 25 nm, the magnetization of the
reproducing layer 18 is also reproduced, causing the possibility of
generation of noise. Therefore, the thickness of the assisting
layer 42 must be larger than or equal to 25 nm. On the other hand,
an excess thickness of the assisting layer 42 is not preferable in
respect of improvement in sensitivity or the medium. The relation
between the thickness of the assisting layer 42, the sensitivity of
the medium, and a magneto-optical signal output was examined. As
the result, it was found that the thickness of the assisting layer
42 is preferably set to 60 nm or less in order to obtain an enough
magneto-optical signal output.
94. It is preferable that the assisting layer 42 has a coercive
force of 600 Oe or less because the coercive force at room
temperature must be smaller than a bias magnetic field for
recording. Further, it is required that the assisting layer 42 is
formed of a material showing a large Kerr rotation angle. To meet
this requirement, the Curie temperature is also preferably set
high. As the result of examination of the composition of the
assisting layer 42, it was found that GdFeCo is preferable as
mentioned above. Further, it was found that a material containing
20 at % to 27 at % of Gd and showing perpendicular magnetization is
especially preferable in order to reduce the coercive force at room
temperature.
95. Preferably, the reproducing layer 18 has a thickness of 1 nm to
40 nm, and is composed of Gd, Fe, and Co. Further, it is desirable
that the content of Gd is in the range of 29 at % to 40 at%. The
reproducing layer 18 is a layer for controlling the exchange
bonding force between the assisting layer 42 and the recording
layer 22. Since the exchange bonding force between the assisting
layer 42 and the recording layer 22 must be cut off at room
temperature, the thickness of the reproducing layer 18 is
preferably large at room temperature. However, since the
magnetization of the recording layer 22 must be transferred to the
reproducing layer 18 when heated by a reproducing power, the
thickness of the reproducing layer is preferably small at a raised
temperature in the heated condition. That is, the thickness is
preferably large at room temperature, whereas the thickness is
preferably small at the raised temperature. As the result of
testing to meet these requirements contrary to each other, it was
found that when the thickness of the reproducing layer 18 is in the
range of 1 nm to 40 nm, a high magneto-optical signal output is
obtained.
96. As mentioned above, the preferable composition of the
reproducing layer 18 is GdFeCo. In particular, it is required that
the layer 18 shows in-plane magnetization at room temperature and
shows the magnetic characteristic that the layer 18 is
exchange-bonded to the recording layer 22 at the raised
temperature. It is further required that the layer 18 has a large
saturation magnetization in order to reduce the thickness of the
whole magnetic film (three-layer film). As the result of testing to
examine the composition that meets this requirement, it was found
that the content of Gd in the reproducing layer 18 is preferably in
the range of 29 at% to 40 at%.
97. Preferably, the recording layer 22 has a thickness of 60 nm or
less, and has a Curie temperature of 250.degree. C. or less. Since
the recording layer 22 is a layer for recording information, the
preferable composition of the recording layer 22 is TbFeCo or
DyFeCo each having a large magnetic anisotropy as mentioned above.
As the result of examination on an optimum thickness of the
recording layer 22, it was found that the thickness of the
recording layer 22 is made preferably less than or equal to 60 nm
in order to obtain a high magneto-optical recording signal by the
exchange bond between the recording layer 22 and the reproducing
layer 18 at the raised temperature. Further, the Curie temperature
of the recording layer 22 is made preferably less than or equal to
250.degree. C. in order to realize a highly sensitive
magneto-optical recording medium.
98. An erasing method for information recorded on the recording
medium 12a according to the fourth preferred embodiment will be
described with reference to FIG. 11. A laser beam is directed onto
the recording medium 12a to thereby heat the medium to a
temperature near the Curie temperature of the recording layer 22,
and a bias magnetic field is downward applied to thereby erase the
information. In FIG. 11, the arrows show the direction of
magnetization of each magnetic layer after erasing the information.
Although the arrows in the reproducing layer 18 are directed
rightward, this means that the direction of magnetization of the
reproducing layer 18 in an in-plane direction, but is not limited
to a specific direction in a plane.
99. Referring to FIG. 12, there is shown a condition after
recording information on the recording medium 12a according to the
fourth preferred embodiment. In recording information, a bias
magnetic field is upward applied and the medium is heated to a
temperature near the Curie temperature of the recording layer 22,
thereby making the direction of magnetization of the recording
layer 22 upward. At this time, the directions of magnetization of
the reproducing layer 18 and the assisting layer 42 are also made
the same (upward) as that of the recording layer 22. When the
temperature lowers, the saturation magnetization of the reproducing
layer 18 increases to bring the direction of magnetization of the
reproducing layer into an in-plane direction. Accordingly, the
magnetization of the recording layer 22 and the assisting layer 42
is upward directed in an area irradiated with a recording power. In
an area where no recorded data is present (i.e., in an area where
the power corresponding to a reproducing power is applied), the
reproducing layer 18 is heated to temperatures higher than the
temperature of exchange bond to the recording layer 22, while in a
cooling stage the magnetization of the reproducing layer 18 is
directed in a plane and the magnetization of the assisting layer 42
is directed upward.
100. A reproducing method for information recorded on the
magneto-optical recording medium 12a according to the fourth
preferred embodiment will be described with reference to FIG. 13.
The magneto-optical recording medium 12a is rotated in the
direction of arrow R. During reproduction of information in this
preferred embodiment, a bias magnetic field having the same
direction as that during recording is applied, that is, an upward
bias magnetic field Hb is applied. A beam spot is formed on the
medium irradiated with the reproducing laser beam 26, and a
temperature distribution is accordingly formed. The low-temperature
area 28a in the beam spot shows a temperature lower than the
temperature of exchange bond between the recording layer 22 and the
reproducing layer 18, so that the magnetization of the reproducing
layer 18 is directed in a plane and the magnetization of the
assisting layer 42 is directed upward.
101. In the high-temperature area 28b in the beam spot, the
recording layer 22 and the reproducing layer 18 are exchange-bonded
together, and the reproducing layer 18 and the assisting layer 42
are exchange-bonded together, so that the magnetization of the
recording layer 22 is transferred to the assisting layer 42.
Accordingly, the information recorded in the recording layer 22 in
the high-temperature area 28b is reproduced. When the
high-temperature area 28b comes outside the beam spot to cause a
decrease in temperature, the magnetization of the reproducing layer
18 is directed in a plane and the magnetization of the recording
layer 22 keeps its recorded condition. Thus, the low-temperature
area 28a and the high-temperature area 28b are formed in the beam
spot by adjusting the reproducing laser power, and an opening is
formed only in the high-temperature area 28b, while a mask whose
magnetization is directed upward is formed, that is, an up spin
mask is formed. Accordingly, a magneto-optical signal can be read
from only the high-temperature area, thus allowing super resolution
reproduction.
102. Referring to FIG. 14, there is shown the structure of a
magneto-optical recording medium 12b according to a fifth preferred
embodiment of the present invention. In the following description
of this preferred embodiment, the same parts as those in the fourth
preferred embodiment shown in FIG. 10 are denoted by the same
reference numerals, and the explanation thereof will be omitted to
avoid repetition. In the magneto-optical recording medium 12a
according to the fourth preferred embodiment, the assisting layer
42 and the reproducing layer 18 tend to be strongly exchange-bonded
together at room temperature, and it is sometimes difficult to
invert the direction of magnetization of the assisting layer 42 by
a bias magnetic field.
103. In the fifth preferred embodiment, a nonmagnetic or
paramagnetic intermediate layer 43 is interposed between the
assisting layer 42 and the reproducing layer 18 in order to weaken
the exchange bonding force between the assisting layer 42 and the
reproducing layer 18. The insertion of the intermediate layer 43
brings about magnetically weak bond between the assisting layer 42
and the reproducing layer 18, thereby allowing easy inversion of
the direction of magnetization of the assisting layer 42 by a bias
magnetic field. The intermediate layer 43 is formed from Si, Al,
Ti, Cu, or nitrides thereof. The thickness of the intermediate
layer 43 is preferably in the range of 0.5 nm to 20 nm. The
structure of the other parts are similar to that in the fourth
preferred embodiment shown in FIG. 10.
104. According to the fifth preferred embodiment mentioned above,
the insertion of the nonmagnetic or paramagnetic intermediate layer
43 between the assisting layer 42 and the reproducing layer 18
provides a high-performance magneto-optical recording medium.
However, as the compositions of the magnetic layers 42, 18, and 22
are different from each other, three kinds of sputter targets for
the magnetic layers and one kind of sputter target for the
intermediate layer are required, that is, four kinds of sputter
targets are totally required. As a modification of the fifth
preferred embodiment, the assisting layer 42 and the reproducing
layer 18 may be formed from magnetic films having the same
composition. For example, a GdFeCo amorphous alloy film having a
large thickness becomes a perpendicular magnetization film because
a demagnetizing field acting on the magnetic film is small, whereas
the alloy film having a small thickness becomes an in-plane
magnetization film at room temperature because the demagnetizing
field is large.
105. In transferring the magnetization of the recording layer 22 to
the reproducing layer 18 in the magneto-optical recording medium 12
according to the first preferred embodiment, a mask area for
in-plane magnetization is narrowed more with an increase in the
reproducing power, and a transfer area is therefore widened to
cause the interference of the adjacent mark and reduce a
reproduction output. This problem may be solved by defining a given
relation in magnetic characteristic between the reproducing layer
18 and the recording layer 22 and controlling the magnitude of a
bias magnetic field during reproduction. A reproducing method for
information recorded on the magneto-optical recording medium 12
using such a solution for this problem will now be described.
106. FIG. 15 shows the temperature dependency of magnetization M of
the recording layer 22. The direction of magnetization of the
reproducing layer 18 upon transition of the easy direction of
magnetization from an in-plane direction to a perpendicular
direction depends on a magnetostatic bonding force Hs by a magnetic
field leaked from the magnetization of the recording layer 22. Let
M1 denote the magnitude of magnetization of the recording layer 22
at least required for making the direction of magnetization of the
reproducing layer 18 identical with that of the recording layer 22
by the magnetostatic bonding force. In a temperature area where the
magnitude of magnetization of the recording layer 22 is less than
M1, the direction of magnetization of the reproducing layer 18 is
not identical with that of the recording layer 22.
107. Let Hc denote the coercive force of the reproducing layer 18
and Hs denote the magnetostatic bonding force between the recording
layer 22 and the reproducing layer 18. When a bias magnetic field
Hr for reproduction is applied in the above temperature area, the
direction of magnetization of the reproducing layer 18 is made
identical with the direction of the bias magnetic field for
reproduction under the condition of Hr>He+Hs, thereby forming a
mask whose magnetization direction is perpendicular. Thus, the
above-mentioned problem can be solved by forming the-mask (up spin
mask) whose magnetization direction is perpendicular in the beam
spot.
108. A reproducing method according to this preferred embodiment
will be described in detail with reference to FIGS. 16 to 20. As
described above, the reproducing layer 18 and the recording layer
22 of the magneto-optical recording medium 12 according to the
first preferred embodiment are preferably formed from a rare
earth-transition metal amorphous alloy film. FIG. 16 shows the
temperature dependencies of magnetization of the recording layer 22
and the reproducing layer 18 in the case where the recording layer
22 is formed from a transition metal (TM) rich rare
earth-transition metal amorphous alloy film. FIGS. 17A and 17B show
the magnetization condition of the medium during reproduction and
the temperature distribution in the beam spot, respectively.
109. As shown in FIG. 16, the temperature Tt at which the
magnetization of the reproducing layer 18 changes from in-plane
magnetization to perpendicular magnetization is set between the
compensation temperature Tcomp and the Curie temperature Tc of the
recording layer 22. The temperatures at which the magnetization of
the recording layer 22 becomes less than M1 exist in the vicinity
of the compensation temperature (T1 to T2) and in the vicinity of
the Curie temperature (T3 to Tc). In FIG. 17A, the medium is
rotated in the direction of arrow R. In FIG. 17B, reference numeral
28 denotes the beam spot, and reference numeral 29 denotes a mark
recorded on the medium.
110. As shown in FIG. 17B, in a low-temperature area A in the beam
spot 28, the reproducing layer 18 still shows in-plane
magnetization, which masks the magnetization of the recording layer
22. In an intermediate-temperature area B in the beam spot 28, the
condition of Hr.ltoreq.Hc+Hs is satisfied, and the magnetization of
the recording layer 22 is therefore transferred to the reproducing
layer 18 by the magnetostatic bonding. In a high-temperature area C
in the beam spot 28, the condition of Hr>He+Hs is satisfied, and
the direction of magnetization of the reproducing layer 18 is made
identical with the direction of the bias magnetic field Hr, thus
forming an up spin mask. In this manner, the in-plane magnetization
mask area A is reduced with an increase in the reproducing power;
however, at the same time, the up spin mask area C where the
direction of magnetization is made identical with that of the bias
magnetic field for reproduction is formed. Accordingly, the
recorded mark transfer area B is substantially almost unchanged to
avoid a reduction in the reproduction output.
111. FIG. 18 shows the temperature dependencies of magnetization of
the recording layer 22 and the reproducing layer 18 in the case
where the recording layer 22 is formed from a rare earth (RE) rich
rare earth-transition metal amorphous alloy film. FIG. 19A shows
the magnetization condition of the medium during reproduction, and
FIG. 19B shows four magnetization areas caused by the temperature
distribution in the beam spot. The temperatures at which the
magnetization of the recording layer 22 becomes less than M1 exist
in the vicinity of the compensation temperature (T1 to T2) and in
the vicinity of the Curie temperature (T3 to Tc). In this preferred
embodiment, the use of the RE rich rare earth-transition metal
amorphous alloy film as the recording layer 22 allows the
compensation temperature Tcomp of the recording layer 22 to be
increased near the Curie temperature Tc.
112. Accordingly, the temperature Tt at which the magnetization of
the reproducing layer 18 changes from in-plane magnetization to
perpendicular magnetization can be set near the compensation
temperature of the recording layer 22, and two up spin masks are
formed in the beam spot so that the direction of magnetization of
the reproducing layer 18 is made identical with that of the bias
magnetic field for reproduction (the areas B and D in FIG. 19B).
That is, when Tt>T, an in-plane magnetization mask area A is
formed in the beam spot, and when T2>T>Tt and Tc>T>T3,
the condition of Hr>Hc+Hs is satisfied to form two up spin mask
areas B and D in the beam spot so that the direction of
magnetization of the reproducing layer 18 is made identical with
that of the bias magnetic field for reproduction.
113. When T3>T>T2, the condition of Hr.ltoreq.Hc+Hs is
satisfied to form a recorded mark transfer area C where the
magnetization of the recording layer 22 is transferred to the
reproducing layer 18 by the magnetostatic bonding. In this manner,
the in-plane magnetization mask area A is greatly reduced with an
increase in the reproducing power; however, at the same time, the
two up spin mask areas B and D where the direction of magnetization
is made identical with that of the bias magnetic field for
reproduction. Accordingly, the recorded mark transfer area C is
substantially almost unchanged to avoid a reduction in the
reproduction output.
114. In the case where the coercive force Hc of the reproducing
layer 18 is greatly small, a magnetic field leaked from a permanent
magnet used in an objective lens actuator provided on an optical
head may be used as the bias magnetic field for reproduction. This
will be described with reference to FIG. 20. An objective lens
actuator 44 includes two yokes 48 and 50 provided on a yoke base 46
and two permanent magnets 52 and 54 provided on the two yokes 48
and 50, respectively. The objective lens actuator 44 further
includes a focusing coil (not shown) for performing the focusing of
an objective lens 56 and a tracking coil (not shown) for performing
the tracking. The permanent magnets 52 and 54 are located greatly
near a magneto-optical disk 12a, so that lines of magnetic force 58
leaked from the permanent magnets 52 and 54 have an influence upon
the magneto-optical disk 12a. Thus, when the coercive force Hc of
the reproducing layer 18 is greatly small, the magnetic fields
leaked from the permanent magnets 52 and 54 can be used as the bias
magnetic field for reproduction.
EXAMPLE 1
115. SiN dielectric layer 16, GdFeCo reproducing layer 18, SiN
intermediate layer 20, TbFeCo recording layer 22, and SiN
protective layer 24 were sequentially formed on glass substrate 14
by RF sputtering. The formation of each layer by the sputtering was
performed in a vacuum chamber under an ultimate vacuum of
5.times.10.sup.-5 Pa or less. Specifically, the SiN layers 16, 20,
and 24 were formed by the sputtering employing an Ar gas pressure
of 0.2 Pa and an applied power of 0.8 kW, while the magnetic layers
18 and 22 were formed by the sputtering employing an Ar gas
pressure of 0.5 Pa and an applied power of 1.0 kW.
116. The composition of the reproducing layer 18 is
Gd.sub.29Fe.sub.55Co.sub.16, and the composition of the recording
layer 22 is Tb.sub.20Fe.sub.72Co.sub.8. The thicknesses of the
layers 16, 18, 20, 22, and 24 are 90 nm, 40 nm, 5 nm, 40 nm, and 45
nm, respectively. The Curie temperature and the compensation
temperature of the reproducing layer 18 is 330.degree. C. and
210.degree. C., respectively, and the Curie temperature and the
compensation temperature of the recording layer 22 is 220.degree.
C. and room temperature, respectively.
117. When only the GdFeCo reproducing layer 18 was used, the easy
direction of magnetization changed from an in-plane direction to
perpendicular direction at about 150.degree. C. In the conventional
structure having four layers laminated together on the substrate 14
and excluding the intermediate layer, the easy direction of
magnetization of the reproducing layer 18 changed from an in-plane
direction to a perpendicular direction at about 90.degree. C. In
this example having five layers laminated together on the substrate
14 and including the nonmagnetic intermediate layer 20, the easy
direction of magnetization of the reproducing layer 18 changed from
an in-plane direction to a perpendicular direction at about
120.degree. C. This indicates that the exchange bonding force
between the recording layer 22 and the reproducing layer 18 was
weakened.
118. FIG. 21 shows the mark length dependency of C/N in this
example and the conventional medium having four layers excluding
the intermediate layer. The conditions of measurement are a linear
velocity of 3 m/s, a reproducing power of 1.2 mW, and a recording
power of 2.3 to 2.5 mW. In the conventional medium having four
layers excluding the intermediate layer, the C/N even for a long
mark length is low. To the contrary, in the medium of this example
having the nonmagnetic intermediate layer 20, it was confirmed that
the C/N was largely improved.
EXAMPLE 2
119. SiN dielectric layer 16, GdFeCo reproducing layer 18, Al
intermediate layer 20, TbFeCo recording layer 22, and SiN
protective layer 24 are sequentially formed on glass substrate 14
by sputtering. The formation of each layer was performed in a
vacuum chamber under an ultimate vacuum of b 5.times.10.sup.-5 Pa
or less. All the layers except the intermediate layer 20 were
formed under the same conditions as those in Example 1. The Al
intermediate layer 20 was formed by DC sputtering employing an Ar
gas pressure of 0.2 Pa and an applied power of DC 1.0 kW. The
compositions of the reproducing layer 18 and the recording layer 22
are the same as those in Example 1. The thicknesses of the layers
16, 18, 20, 22, and 24 are 90 nm, 25 nm, 3 nm, 40 nm, and 45 nm,
respectively.
120. FIG. 22 shows the mark length dependency of C/N in the
conventional medium excluding the intermediate layer and in the
medium of this example including the nonmagnetic metal intermediate
layer. The conditions of measurement are a linear velocity of 9
m/s, a reproducing power of 2.2 mW, and a recording power of 4.8 to
5.2 mW. It was confirmed that the C/N was largely improved by
interposing the nonmagnetic metal intermediate layer 20 between the
reproducing layer 18 and the recording layer 22.
EXAMPLE 3
121. SiN dielectric layer 16, GdFeCo reproducing layer 18, NdCo
intermediate layer 30, TbFeCo recording layer 22, and SiN
protective layer 24 were sequentially formed on glass substrate 14
by sputtering. The formation of each layer was performed in a
vacuum chamber under an ultimate vacuum of 5.times.10.sup.-5 Pa or
less. All the layers except the magnetic intermediate layer 30 were
formed under the same conditions as those in Example 1. The NdCo
intermediate layer 30 was formed by DC sputtering employing an Ar
gas pressure of 0.2 Pa and an applied power of DC 1.0 kW.
122. The compositions of the reproducing layer 18 and the recording
layer 22 are the same as those in Example 1. The composition of the
magnetic intermediate layer 30 is Nd.sub.27Co.sub.73. The
thicknesses of the layers 16, 18, 30, 22, and 24 are 90 nm, 40 nm,
10 nm, 40 nm, and 45 nm, respectively. FIG. 23 shows the mark
length dependency of C/N in the conventional medium excluding the
intermediate layer and in the medium of this example including the
magnetic intermediate layer. The conditions of measurement are a
linear velocity of 9 m/s, a reproducing power of 2.2 mW, and a
recording power of 4.8 to 5.2 mW. As apparent from FIG. 23, it was
confirmed that the C/N was largely improved by interposing the
magnetic intermediate layer 30 between the reproducing layer 18 and
the recording layer 22.
EXAMPLE 4
123. Tb-SiO.sub.2 dielectric layer 16, DyFe control layer 32,
GdFeCo reproducing layer 18, TbFeCo recording layer 22, and
Tb-SiO.sub.2 protective layer 24 were sequentially formed on glass
substrate 14 by sputtering. The dielectric layer 16 and the
protective layer 24 were formed by RF magnetron sputtering, and the
control layer 32, the reproducing layer 18, and the recording layer
22 were formed by DC magnetron sputtering.
124. The composition of the control layer 32 is Dy.sub.12Fe.sub.88;
the composition of the reproducing layer 18 is
Gd.sub.30Fe.sub.50Co.sub.20; and the composition of the recording
layer 22 is Tb.sub.20Fe.sub.72Co.sub- .8. The thicknesses of the
layers 16, 32, 18, 22, and 24 are 90 nm, 10 nm, 50 nm, 50 nm, and
90 nm, respectively. The sputtering of the dielectric layer 16 and
the protective layer 24 was performed by employing an Ar gas
pressure of 0.5 Pa and an applied power of RF 2 kW, and the
sputtering of the control layer 32, the reproducing layer 18, and
the recording layer 22 was performed by employing an Ar gas
pressure of 0.5 Pa and an applied power of DC 1 kW. The Curie
temperature of the control layer 32, the reproducing layer 18, and
the recording layer 22 are 90.degree. C. 300.degree. C., and
200.degree. C. respectively.
125. The relation between a reproducing power and a reproducing
condition in this example will be described with reference to FIGS.
24A and 24B. FIG. 24A shows the reproducing condition when the
reproducing power is less than 2 mW, and FIG. 24B shows the
reproducing condition when the reproducing power is greater than or
equal to 2 mW. In the condition of FIG. 24A where the medium is
irradiated with a reproducing laser beam 26 having power less than
2 mW, the temperature of the medium does not sufficiently rise.
Accordingly, the reproducing layer 18 shows in-plane magnetization,
but has a perpendicular component of the magnetization to some
extent due to the exchange bond to the recording layer 22. However,
the control layer 32 is an in-plane magnetization film at this
temperature, and the perpendicular component of the magnetization
transferred to the reproducing layer 18 is therefore almost
ignorably small to read.
126. In the condition of FIG. 24B where the medium is irradiated
with a reproducing laser beam 26 having power greater than or equal
to 2 mW, a low-temperature area 28a and a high-temperature area 28b
are formed in a beam spot. In the high-temperature area 28b, the
control layer 32 is heated to temperatures higher than the Curie
temperature, and the magnetization of the control layer 32
therefore disappears to form an opening. Further, in the
high-temperature area 28b, the easy direction of magnetization of
the reproducing layer 18 changes from an in-plane direction to a
perpendicular direction, and the magnetization of the recording
layer 22 is transferred to the reproducing layer 18 by the exchange
bonding force therebetween. As the thickness of the control layer
32 is 10 nm to allow a high transmittance of 60%, a mark
transferred to the reproducing layer 18 can be read through the
opening of the control layer 32. In the low-temperature area 28a,
the temperature of the control layer 32 is lower than the Curie
temperature, and the control layer 32 masks the reproducing layer
18 to prevent reproduction of any portions other than the recorded
mark transferred to the reproducing layer 18.
127. FIG. 25 shows the reproducing power dependency of C/N in this
example in comparison with the conventional medium excluding the
control layer 32. This example is shown by a solid line, and the
conventional medium is shown by a dashed line. As apparent from
FIG. 25, the C/N in this example is remarkably improved over the
C/N in the conventional medium. FIG. 26 shows the reproducing power
dependency of crosstalk in this example in comparison with the
conventional medium excluding the control layer. This example is
shown by a solid line, and the conventional medium is shown by a
dashed line. The crosstalk herein referred to is the dB value of a
difference between a present signal and a previous signal. As
apparent from FIG. 26, the crosstalk in this example is remarkably
improved over the crosstalk in the conventional medium.
EXAMPLE 5
128. Tb-SiO.sub.2 dielectric layer 16, DyFeCo control layer 32,
GdFeCo reproducing layer 18, TbFeCo recording layer 22, and
Tb-SiO.sub.2 protective layer 24 were sequentially formed on glass
substrate 14 by sputtering. The composition of the control layer 32
is Dy.sub.35Fe.sub.62Co.sub.3; the composition of the reproducing
layer 18 is Gd.sub.30fe.sub.50Co.sub.20; and the composition of the
recording layer 22 is Tb.sub.20Fe.sub.72Co.sub.8. The thicknesses
of the layers 16, 32, 18, 22, and 24 are 90 nm, 5 nm, 70 nm, 30 nm,
and 90 nm, respectively. The Curie temperature of the control layer
32 is 100.degree. C., and the Curie temperatures of the reproducing
layer 18 and the recording layer 22 are the same as those in
Example 4. In this example, the thickness of the control layer 32
is smaller than that in Example 4, so that the transmittance of the
control layer 32 is 85%, which is higher than the transmittance in
Example 4. The mechanism of magnetically induced super resolution
is similar to that in Example 4.
129. FIG. 27 shows the reproducing power dependency of C/N in this
example, and FIG. 28 shows the reproducing power dependency of
crosstalk in this example. As apparent from FIG. 27 in comparison
with FIG. 25, the C/N in this example is higher than the C/N in
Example 4 because the transmittance of the control layer 32 in this
example is higher than that in Example 4. the crosstalk in this
example is similar to that in Example 4.
EXAMPLE 6
130. Targets of TbFeCo, first GdFeCo, second GdFeCo, and Si and a
polycarbonate substrate having a track pitch of 1.2 .mu.m were set
in a sputtering device, and a chamber of the sputtering device was
evacuated to 10.sup.-5 Pa. Then, SiN dielectric layer 16, GdFeCo
reproduction assisting layer 42, GdFeCo reproducing layer 18,
TbFeCo recording layer 22, and SiN protective layer 24 were
sequentially formed on the polycarbonate substrate 14 by DC
sputtering.
131. The SiN layers 16 and 24 were formed in the chamber under an
ultimate vacuum of 5.times.10.sup.-5 Pa by sputtering employing an
Ar gas pressure of 0.2 Pa and an applied power of 0.8 kW, and the
magnetic layers 42, 18, and 22 were formed in the chamber under the
same ultimate vacuum as above by sputtering employing an Ar gas
pressure of 0.5 Pa and an applied power of 1.0 kW. The composition
of the assisting layer 42 is Gd.sub.20Fe.sub.54Co.sub.26; the
composition of the reproducing layer 18 is
Gd.sub.39Fe.sub.37Co.sub.24; and the composition of the recording
layer 22 is Tb.sub.19Fe.sub.73Co.sub.8. The thicknesses of the
layers 16, 42, 18, 22, and 24 are 70 nm, 40 nm, 12 nm, 40 nm, and
100 nm, respectively.
132. As apparent from these compositions, the assisting layer 42
and the recording layer 22 are TM rich, and the reproducing layer
18 is RE rich. The Curie temperatures of the assisting layer 42,
the reproducing layer 18, and the recording layer 22 are
360.degree. C., 330.degree. C., and 220.degree. C., respectively.
The coercive forces Hc1, Hc2, and Hc3 of the assisting layer 42,
the reproducing layer 18, and the recording layer 22 at room
temperature are related to satisfy Hc3>Hc1 and Hc3>Hc2.
133. The recording characteristic of the magneto-optical recording
medium thus formed was examined. The wavelength of laser used is
780 nm. A laser power of 9 mW was directed to the recording medium
with a bias magnetic field being applied downward, thereby erasing
data recorded on the recording medium. The recording of data was
performed by employing a recording power of 4 mW, a frequency of
7.5 MHz, and a duty ratio of 26% as rotating the medium at a linear
velocity of 3 m/sec. In recording, a bias magnetic field is applied
upward. Under these conditions, a mark having a length of about 0.4
.mu.m was recorded on the medium.
134. The reproducing characteristic of the recording medium was
next examined. The reproduction of data was performed as upward
applying a bias magnetic field. With a reproducing power of 1.5 mW,
no magneto-optical signal output for a previously recorded signal
was obtained. This is considered to be due to the fact that the
whole area of the reproduction assisting layer 42 in the beam spot
formed an up spin mask. With a reproducing power of 1.6 mW, the
magnetization of the recording layer 22 was transferred through the
reproducing layer 18 to the assisting layer 42 to obtain a
magneto-optical signal output. This is considered to be due to the
fact that an area having temperatures higher than the temperature
at which the magnetization of the recording layer 22 is transferred
to the reproducing layer 18 was formed to form an up spin mask and
an opening. The ratio of carrier to noise (C/N) at this time was 42
dB. With a reproducing power of 1.7 mW, the direction of
magnetization of the assisting layer 42 was made identical with
that of the bias magnetic field, that is, the upward direction, and
the diameter of an area (opening) where the reproducing layer 18
was exchange-bonded to the recording layer 22 was about 0.4 .mu.m,
with the result that a C/N value of 48 dB was obtained.
EXAMPLE 7
135. A magneto-optical recording medium was prepared under the
conditions similar to those in Example 6 with the exception that
the composition and the Curie temperature of the assisting layer 42
were changed. The composition of the assisting layer 42 is
Gd.sub.23Fe.sub.58Co.sub.19, and the Curie temperature of the
assisting layer 42 is 300.degree. C. The recording and reproducing
characteristics were measured similarly to Example 6 to obtain a
C/N value of 46 dB with a reproducing power of 1.8 mW.
EXAMPLE 8
136. A magneto-optical recording medium was prepared by interposing
an SiN film having a thickness of 5 nm between the reproducing
layer 18 and the recording layer 22 of the recording medium
prepared in Example 6. The recording and reproducing
characteristics were measured similarly to Example 6 to obtain a
C/N value of 49 dB with a reproducing power of 1.7 mW. Prior to
this measurement, the thickness of the SiN film was examined to
find that the thickness range of 3 nm to 10 nm is suitable for
super resolution reproduction.
EXAMPLE 9
137. Targets of TbFeCo, first GdFeCo, second GdFeCo, and Si and a
polycarbonate substrate having a track pitch of 1.2 .mu.m were set
in a sputtering device, and a chamber of the sputtering device was
evacuated to 10.sup.-5 Pa. Then, an SiN film having a thickness of
70 nm was formed on the substrate by DC sputtering under the
following conditions. This film serves not only to protect the
magnetic film from oxidation, but also to exhibit an enhance effect
such that a magneto-optical signal is enhanced.
138. gas pressure: 0.3 Pa
139. sputter gas: Ar, N.sub.2
140. pressure ratio: Ar : N.sub.2=6:4
141. applied power: 0.8 kW
142. Then, the chamber was evacuated to 10.sup.-5 Pa again, and the
films of the first GdFeCo, second GdFeCo, and TbFeCo were
continuously formed in this order on the SiN film by DC sputtering
under the following conditions.
143. gas pressure: 0.5 Pa
144. sputter gas: Ar
145. applied power: 1 kW
146. The composition of the assisting layer 42 is
Gd.sub.20Fe.sub.54Co.sub- .26; the composition of the reproducing
layer 18 is Gd.sub.39Fe.sub.37 Co.sub.24; and the composition of
the recording layer 22 is Tb.sub.19Fe.sub.73Co.sub.8. The
thicknesses of the layers 42, 18, and 22 are 40 nm, 12 nm, and 50
nm, respectively.
147. As apparent from these compositions, the assisting layer 42
and the recording layer 22 are TM rich, and the reproducing layer
18 is RE rich. The Curie temperatures of the assisting layer 42,
the reproducing layer 18, and the recording layer 22 are
360.degree. C., 330.degree. C., and 220.degree. C. respectively.
The coercive forces Hc1, Hc2, and Hc3 of the assisting layer 42,
the reproducing layer 18, and the recording layer 22 at room
temperature are related to satisfy Hc3>Hc1 and Hc3>Hc2.
148. The recording characteristic of the magneto-optical recording
medium thus formed was examined. The wavelength of laser used is
780 nm. A laser power of 9 mW was directed to the recording medium
with a bias magnetic field being applied downward, thereby erasing
data recorded on the recording medium. The recording of data was
performed by employing a recording power of 4 mW, a frequency of
7.5 MHz, and a duty ratio of 26% as rotating the medium at a linear
velocity of 3m/sec. In recording, a bias magnetic field is applied
upward. Under these conditions, a mark having a length of about 0.4
.mu.m was recorded on the medium.
149. The reproducing characteristic of the recording medium was
next examined. The reproduction of data was performed as upward
applying a bias magnetic field. With a reproducing power of 1.5 mW,
no magneto-optical signal output for a previously recorded signal
was obtained. This is considered to be due to the fact that the
whole area of the reproduction assisting layer 42 in the beam spot
formed an up spin mask. With a reproducing power of 1.6 mW, the
magnetization of the recording layer 22 was transferred through the
reproducing layer 18 to the assisting layer 42 to obtain a
magneto-optical signal output. This is considered to be due to the
fact that an area having temperatures higher than the temperature
at which the magnetization of the recording layer 22 is transferred
to the reproducing layer 18 was formed to form an up spin mask and
an opening. A C/N value at this time was 42 dB. With a reproducing
power of 1.7 mW, the direction of magnetization of the assisting
layer 42 was made identical with that of the bias magnetic field,
that is, the upward direction, and the diameter of an area
(opening) where the reproducing layer 18 was exchange-bonded to the
recording layer 22 was about 0.4 .mu.m, with the result that a C/N
value of 48 dB was obtained.
150. Then, similar recording and reproducing characteristics were
measured by employing the same conditions as those mentioned above
for the reproducing layer 18 and the recording layer 22 but
changing the composition of the assisting layer 42. The results of
measurement are shown in Table 1. In changing the composition, the
content of Gd was changed with the ratio of Fe and Co fixed.
1TABLE 1 Gd content (at %) 19 20 21 23 24 26 27 28 C/N (dB) 20 48
48 49 48 46 45 32 Coercive force 0 106 232 159 93 70 60 5 (Oe)
151. It is understood from Table 1 that when the content of Gd is
in the range of 20 at % to 27 at%, high C/N values are
obtained.
EXAMPLE 10
152. To examine the thickness of the assisting layer 42, a test was
performed by changing the thickness of the assisting layer 42 in a
recording medium similar in structure to that in Example 9. The
recording and reproducing system is similar to that in Example 9.
The results are shown in Table 2.
2TABLE 2 Thickness (nm) 20 25 30 40 50 55 60 70 C/N (dB) 30 45 46
48 48 48 47 42
153. As apparent from Table 2, when the thickness is in the range
of 25 nm to 60 nm, high C/N values are obtained.
EXAMPLE 11
154. Plural magneto-optical disks were prepared by changing the
composition of the reproducing layer 18 in a magneto-optical
recording medium having the same structure as that of the medium in
Example 9 to examine magneto-optical signal outputs. The results
are shown in Table 3. In changing the composition, the content of
Gd was changed with the ratio of Fe and Co fixed. The thickness of
the reproducing layer 18 is 12 nm.
3TABLE 3 Gd content (at %) 28 29 30 34 39 40 42 C/N (dB) 40 44 46
48 48 45 39
155. As apparent from Table 3, it is understood that when the
content of Gd is in the range of 29 at % to 40 at%, high C/N values
are obtained.
EXAMPLE 12
156. Plural magneto-optical disks were prepared by changing the
thickness of the reproducing layer 18 in a magneto-optical
recording medium having the same structure as that of the medium in
Example 9 to examine magneto-optical signal outputs. The results
are shown in Table 4.
4TABLE 4 Thickness (nm) 0 1 5 10 20 30 40 45 C/N (dB) x 44 46 48 47
45 44 36
157. As apparent from Table 4, it was found that when the thickness
of the reproducing layer 18 is in the range of 1 nm to 20 nm, high
C/N values are obtained. In Table 4, the mark x indicates that
magnetically induced super resolution reproduction was
impossible.
EXAMPLE 13
158. The thickness and the Curie temperature of the recording layer
22 were examined by using a magneto-optical recording medium having
the same structure as that of the medium in Example 9. When the
recording layer 22 has a Curie temperature of 300.degree. C., a
recording power of about 6 mW was required in recording a mark on
the medium under the same conditions as those in Example 9.
Although the linear velocity of the medium was set to 3 m/sec in
Example 9, an actual magneto-optical disk is rotated at a velocity
three to five times the above linear velocity. Accordingly, when
the linear velocity is 15 m/sec, a semiconductor laser capable of
outputting a recording power of about 30 mW is required. However,
the maximum output of a semiconductor laser mounted on an existing
magneto-optical recording device is about 12 aW. Therefore, when
the Curie temperature of the recording layer 22 is 300.degree. C.,
the magneto-optical recording medium cannot be put to practical
use. In these circumstances, the Curie temperature of the recording
layer 22 was examined to find out that the Curie temperature of
about 250.degree. C. allows a mark to be sufficiently recorded on
the medium even at the linear velocity of 15 m/sec.
EXAMPLE 14
159. The thickness of the recording layer 22 was examined. The
thicknesses of the assisting layer 42 and the reproducing layer 18
were set to 55 nm and 15 nm, respectively, and the recording layer
22 having a Curie temperature of 250.degree. C. was adopted. When
the thickness of the recording layer 22 was changed to 70 nm, a
mark could not be sufficiently recorded by a laser power of 12 mW
at the linear velocity of 15 m/sec. However, when the thickness of
the recording layer 22 was changed to 60 nm, a mark could be
sufficiently recorded even by the laser power of 12 mW.
Accordingly, it was found that the thickness of the recording layer
22 is preferably 60 nm, or less.
EXAMPLE 15
160. The crosstalk of a magneto-optical signal was examined by
using a magneto-optical recording medium having the following
structure. The crosstalk herein referred to means reading of a
magneto-optical signal recorded on a track adjacent to a track to
be reproduced. The composition of the assisting layer 42 is
Gd.sub.20Fe.sub.54Co.sub.26; the composition of the reproducing
layer 18 is Gd.sub.39Fe.sub.37Co.sub.24; and the composition of the
recording layer 22 is Tb.sub.19Fe.sub.73Co.sub.8. The thicknesses
of the layers 42, 18, and 22 are 50 nm, 15 nm, and 55 nm,
respectively. The Curie temperatures of the layers 42, 18, and 22
are 360.degree. C., 330.degree. C., and 250.degree. C.,
respectively.
161. The track pitch of the substrate is 1 .mu.m; the land width of
the substrate is 0.8 .mu.m; and the groove width of the substrate
is 0.2 .mu.m. The magneto-optical recording medium having the above
structure was formed on this substrate. A signal with a mark length
of 0.4 .mu.m and a mark space of 0.4 .mu.m was recorded on the
magneto-optical recording medium, and the signal recorded was
reproduced by a laser having a wavelength of 780 nm. The crosstalk
from the adjacent track was -40 dB or less. Thus, it was found that
the crosstalk to be solved in case of a narrow track pitch can be
improved.
EXAMPLE 16
162. Targets of TbFeCo, first GdFeCo, second GdFeCo, and Si and a
polycarbonate substrate having a track pitch of 1.2 .mu.m were set
in a sputtering device, and a chamber of the sputtering device was
evacuated to 10.sup.-5 Pa. Then, an SiN film having a thickness of
70 nm was formed on the substrate by DC sputtering under the
following conditions. This film serves not only to protect the
magnetic film from oxidation, but also to exhibit an enhance effect
such that a magneto-optical signal is enhanced.
163. gas pressure: 0.3 Pa
164. sputter gas: Ar, N.sub.2
165. pressure ratio: Ar : N.sub.2=6:4
166. applied power: 0.8 kW
167. Then, the chamber was evacuated to 10.sup.-5 Pa again, and the
films of the first GdFeCo, Si, second GdFeCo, and TbFeCo were
continuously formed in this order on the SiN film by DC sputtering
under the following conditions.
168. gas pressure: 0.5 Pa
169. sputter gas: Ar
170. applied power: 1 kW
171. The composition of the assisting layer 42 is
Gd.sub.20Fe.sub.54Co.sub- .26; the composition of the reproducing
layer 18 is Gd.sub.39Fe.sub.37Co.sub.24; and the composition of the
recording layer 22 is Tb.sub.19Fe.sub.73Co.sub.8. Si intermediate
layer 43 was interposed between the assisting layer 42 and the
reproducing layer 18. The thicknesses of the layers 42, 43, 18, and
22 are 40 nm, 5 nm, 12 nm, and 50 nm, respectively. The Curie
temperatures of the assisting layer 42, the reproducing layer 18,
and the recording layer 22 are 360.degree. C. 330.degree. C., and
220.degree. C., respectively. The coercive forces Hc1, Hc2, and Hc3
of the assisting layer 42, the reproducing layer 18, and the
recording layer 22 at room temperature are related to satisfy
Hc3>Hc1 and Hc3>Hc2.
172. The recording characteristic of the magneto-optical recording
medium thus formed was examined. The wavelength of laser used is
780 nm. A laser power of 9 mW was directed to the recording medium
with a bias magnetic field being applied downward, thereby erasing
data recorded on the recording medium. The recording of data was
performed by employing a recording power of 4 mW, a frequency of
7.5 MHz, and a duty ratio of 26% as rotating the medium at a linear
velocity of 3 m/sec. In recording, a bias magnetic field is applied
upward. Under these conditions, a mark having a length of about 0.4
.mu.m was recorded on the medium.
173. The reproducing characteristic of the recording medium was
next examined. The reproduction of data was performed as upward
applying a bias magnetic field. With a reproducing power of 1.5 mW,
no magneto-optical signal output for a previously recorded signal
was obtained. This is considered to be due to the fact that the
whole area of the reproduction assisting layer 42 in the beam spot
formed an up spin mask. With a reproducing power of 1.6 mW, the
magnetization of the recording layer 22 was transferred through the
reproducing layer 18 to the assisting layer 42 to obtain a
magneto-optical signal output. This is considered to be due to the
fact that an area having temperatures higher than the temperature
at which the magnetization of the recording layer 22 is transferred
to the reproducing layer 18 was formed to form an up spin mask and
an opening. A C/N value at this time was 42 dB. With a reproducing
power of 1.7 mW, the direction of magnetization of the assisting
layer 42 was made identical with that of the bias magnetic field,
that is, the upward direction, and the diameter of an area
(opening) where the reproducing layer 18 was exchange-bonded to the
recording layer 22 was about 0.4 .mu.m, with the result that a C/N
value of 48 dB was obtained.
EXAMPLE 17
174. A magneto-optical recording medium similar to that in Example
16 was formed with the exception that the intermediate layer 43 was
formed from SiN. The SiN intermediate layer 43 has a thickness of 5
nm. The reproducing characteristic of this magneto-optical
recording medium was examined to find that a high signal output
similar to that in Example 16 could be obtained. Further, a similar
test by changing the SiN intermediate layer 43 to an Al or AlN
intermediate layer was performed to find that a high signal output
could similarly be obtained.
EXAMPLE 18
175. Plural magneto-optical recording media each having a structure
similar to that in Example 16 were prepared by changing the
thickness of the intermediate layer 43 to examine magneto-optical
signal outputs. The results are shown in Table 5.
5TABLE 5 Thickness (nm) 0 0.3 0.5 3 5 10 20 23 C/N (dB) 35 35 43 45
48 47 43 25
176. As apparent from Table 5, when the thickness of the
intermediate layer 43 is in the range of 0.5 nm to 20 nm, high C/N
values can be obtained. When the thickness of the intermediate
layer 43 is excessively small, it is considered that the magnetic
bond between the assisting layer 42 and the reproducing layer 18 is
excessively strong to cause a reduction in C/N. When the thickness
of the intermediate layer 43 is excessively large, it is considered
that the magnetic bond between the assisting layer 42 and the
reproducing layer 18 is excessively weak to cause a reduction in
C/N.
EXAMPLE 19
177. The multilayer film having the structure in Example 16 was
formed on a substrate having a track pitch of 1 .mu.m. The
substrate has a land width of 0.8 .mu.m and a groove width of 0.2
.mu.m. A signal with a mark length of 0.4 .mu.m and a mark space of
0.4 .mu.m was recorded on the magneto-optical recording medium
obtained above, and the signal recorded was reproduced by a laser
having a wavelength of 780 nm. The crosstalk from the adjacent
track was -40 dB or less. Thus, it was confirmed that the crosstalk
to be solved in case of a narrow track pitch can be improved.
EXAMPLE 20
178. Targets of TbFeCo, GdFeCo, and Si and a polycarbonate
substrate having a track pitch of 1.2 .mu.m were set in a
sputtering device, and a chamber of the sputtering device was
evacuated to 10.sup.-5 Pa. Then, an SiN film having a thickness of
70 nm was formed on the substrate by DC sputtering under the
following conditions.
179. gas pressure: 0.3 Pa
180. sputter gas: Ar, N.sub.2
181. pressure ratio: Ar : N.sub.2=6:4
182. applied power: 0.8 kW
183. Then, the chamber was evacuated to 10.sup.-5 Pa again, and the
films of the GdFeCo, Si, GdFeCo, and TbFeCo were continuously
formed in this order on the SiN film by DC sputtering under the
following conditions.
184. gas pressure: 0.5 Pa
185. sputter gas: Ar
186. applied power: 1 kW
187. The composition of the assisting layer 42 is
Gd.sub.20Fe.sub.54Co.sub- .26; the composition of the reproducing
layer 18 is the same as that of the assisting layer 42; and the
composition of the recording layer 22 is
Tb.sub.19Fe.sub.73Co.sub.8. Si intermediate layer 43 was interposed
between the assisting layer 42 and the reproducing layer 18. The
thicknesses of the layers 42, 43, 18, and 22 are 40 nm, 5 nm, 12
nm, and 50 nm, respectively. The Curie temperatures of the
assisting layer 42, the reproducing layer 18, and the recording
layer 22 are 360.degree. C., 360.degree. C., and 220.degree. C.,
respectively. Further, an SiN film having a thickness of 100 nm was
formed on the recording layer 22 by a similar method. This SiN film
serves to prevent oxidation of the magnetic film.
188. The recording characteristic of the magneto-optical recording
medium thus formed was examined. The wavelength of laser used is
780 nm. A laser power of 9 mW was directed to the recording medium
with a bias magnetic field being applied downward, thereby erasing
data recorded on the recording medium. The recording of data was
performed by employing a recording power of 4 mW, a frequency of
7.5 MHz, and a duty ratio of 26% as rotating the medium at a linear
velocity of 3 m/sec. In recording, a bias magnetic field is applied
upward. Under these conditions, a mark having a length of about 0.4
.mu.m was recorded on the medium.
189. The reproducing characteristic of the recording medium was
next examined. The reproduction of data was performed as upward
applying a bias magnetic field. With a reproducing power of 1.5 mW,
no magneto-optical signal output for a previously recorded signal
was obtained. This is considered to be due to the fact that the
whole area of the reproduction assisting layer 42 in the beam spot
formed an up spin mask. With a reproducing power of 1.6 mW, the
magnetization of the recording layer 22 was transferred through the
reproducing layer 18 to the assisting layer 42 to obtain a
magneto-optical signal output. This is considered to be due to the
fact that an area having temperatures higher than the temperature
at which the magnetization of the recording layer 22 is transferred
to the reproducing layer 18 was formed to form an up spin mask and
an opening. A C/N value at this time was 42 dB. With a reproducing
power of 1.7 mW, the direction of magnetization of the assisting
layer 42 was made identical with that of the bias magnetic field,
that is, the upward direction, and the diameter of an area
(opening) where the reproducing layer 18 was exchange-bonded to the
recording layer 22 was about 0.4 .mu.m, with the result that a C/N
value of 48 dB was obtained.
EXAMPLE 21
190. A magneto-optical recording medium similar to that in Example
20 was formed with the exception that the intermediate layer 43 was
formed from SiN. A test similar to that in Example 20 was performed
to find that a high signal output similar to that in Example 20
could be obtained. Further, a similar test by changing the SiN
intermediate layer 43 to an Al or AlN intermediate layer was
performed to find that a high signal output could similarly be
obtained.
EXAMPLE 22
191. Targets of TbFeCo, GdFeCo, and Si and a polycarbonate
substrate having a track pitch of 1.2 .mu.m were set in a
sputtering device, and a chamber of the sputtering device was
evacuated to 10.sup.-5 Pa. Then, an SiN film having a thickness of
70 nm was formed on the substrate by DC sputtering under the
following conditions.
192. gas pressure: 0.3 Pa
193. sputter gas: Ar, N.sub.2
194. pressure ratio: Ar : N.sub.2=6:4
195. applied power: 0.8 kW
196. Then, the chamber was evacuated to 10.sup.-5 Pa again, and the
films of the GdFeCo, Si, GdFeCo, and TbFeCo were continuously
formed in this order on the SiN film by DC sputtering under the
following conditions.
197. gas pressure: 0.5 Pa
198. sputter gas: Ar
199. applied power: 1 kW
200. The composition of the assisting layer 42 is
Gd.sub.23Fe.sub.51Co.sub- .26; the composition of the reproducing
layer 18 is the same as that of the assisting layer 42; and the
composition of the recording layer 22 is
Tb.sub.19Fe.sub.73Co.sub.8. Si intermediate layer 43 was interposed
between the assisting layer 42 and the reproducing layer 18. The
thicknesses of the layers 42, 43, 18, and 22 are 30 nm, 5 nm, 10
nm, and 50 nm, respectively. The Curie temperatures of the
assisting layer 42, the reproducing layer 18, and the recording
layer 22 are 360.degree. C., 360.degree. C., and 220.degree. C.,
respectively. Further, an SiN film having a thickness of 100 nm was
formed on the recording layer 22 by a similar method. The recording
and reproducing characteristics of this magneto-optical recording
medium were examined. Measurement similar to that in Example 20 was
made to find that a C/N value of 48 dB was obtained by a
reproducing power of 2 mW.
EXAMPLE 23
201. The common composition of the assisting layer 42 and the
reproducing layer 18 was replaced by TbFeCo or DyFeCo amorphous
alloy to perform a test similar to that in Example 20. As the
result, even when the thickness was small, an in-plane
magnetization film could not be realized. Accordingly, it was found
that the optimum common composition of the assisting layer 42 and
the reproducing layer 18 is GdFeCo.
EXAMPLE 24
202. The multilayer film having the structure in Example 20 was
formed on a substrate having a track pitch of 1 .mu.m. The
substrate has a land width of 0:8 .mu.m and a groove width of 0.2
.mu.m. A signal with a mark length of 0.4 .mu.m and a mark space of
0.4 .mu.m was recorded on the magneto-optical recording medium
obtained above, and the signal recorded was reproduced by a laser
having a wavelength of 780 nm. The crosstalk from the adjacent
track was -40 dB or less. Thus, it was confirmed that the crosstalk
to be solved in case of a narrow track pitch can be improved.
EXAMPLE 25
203. SiN dielectric layer 16, GdFeCo reproducing layer 18, SiN
intermediate layer 20, TbFeCo recording layer 22, and SiN
protective layer 24 were sequentially formed on glass substrate 14
by RF sputtering. The formation of each layer by the sputtering was
performed in a vacuum chamber under an ultimate vacuum of
5.times.10.sup.-5 Pa or less. Specifically, the SiN layers 16, 20,
and 24 were formed by the sputtering employing an Ar gas pressure
of 0.2 Pa and an applied power of 0.8 kW, while the magnetic layers
18 and 22 were formed by the sputtering employing an Ar gas
pressure of 0.5 Pa and an applied power of 1.0 kW.
204. The composition of the reproducing layer 18 is
Gd.sub.29Fe.sub.55Co.sub.16, and the composition of the recording
layer 22 is Tb.sub.18Fe.sub.68Co.sub.14. The thicknesses of the
layers 16, 18, 20, 22, and 24 are 90 nm, 40 nm, 5 nm, 40 nm, and 45
nm, respectively. The Curie temperature and the compensation
temperature of the reproducing layer 18 is 330.degree. C. and
220.degree. C., respectively, and the Curie temperature and the
compensation temperature of the recording layer 22 is 260.degree.
C. and 80.degree. C., respectively.
205. A bit having a mark length of 0.4 .mu.m was recorded on the
magneto-optical recording medium prepared above, and the
reproducing power dependency of C/N was measured. The result of
measurement is shown by a solid line in FIG. 29. A broken line in
FIG. 29 shows the reproducing power dependency of C/N in the
recording medium prepared in Example 1. The recording and
reproducing conditions are a linear velocity of 5 m/sec, a
recording power of 5.4 mW, and a duty ratio of 25%. A magnetic
field (about 40 Oe) leaked from an objective lens actuator was used
for the bias magnetic field for reproduction. It was confirmed that
even when the reproducing power was increased, the reproduction
output was not decreased in the recording medium prepared in this
example as compared with the recording medium prepared in Example
1.
EXAMPLE 26
206. SiN dielectric layer 16, GdFeCo reproducing layer 18, SiN
intermediate layer 20, TbFeCo recording layer 22, and SiN
protective layer 24 were sequentially formed on glass substrate 14
by RF sputtering. The composition of the recording layer 22 is
Tb.sub.26Fe.sub.61Co.sub.13, and the composition of the reproducing
layer 18 is the same as that in Example 25. The Curie temperature
and the compensation temperature of the recording layer 22 are
220.degree. C. and 140.degree. C., respectively. The thicknesses of
the layers 16, 18, 20, 22, and 24 are 90 nm, 40 nm, 5 nm, 40 nm,
and 45 nm, respectively.
207. A bit having a mark length of 0.4 .mu.m was recorded on the
magneto-optical recording medium prepared above, and the
reproducing power dependency of C/N was measured. The result of
measurement is shown by a solid line in FIG. 30. A broken line in
FIG. 30 shows the reproducing power dependency of C/N in the
recording medium prepared in Example 1. The recording and
reproducing conditions are a linear velocity of 5 m/sec, a
recording power of 5.4 mW, and a duty ratio of 25%. A magnetic
field (about 40 Oe) leaked from an objective lens actuator was used
for the bias magnetic field for reproduction. It was confirmed that
even when the reproducing power was increased, the reproduction
output was not decreased in the recording medium prepared in this
example employing the RE rich rare earth-transition metal amorphous
alloy film for the recording layer 22.
208. The present invention as described above has an effect that a
reproduction output can be improved by perfectly masking a mark
adjacent to a mark to be reproduced. In addition, the crosstalk can
also be improved.
* * * * *