U.S. patent application number 10/669038 was filed with the patent office on 2004-10-07 for magneto-optical recording medium, manufacturing method thereof and magneto-optical data recording and playback device.
Invention is credited to Bouet, Laurence, Despax-Bonningue, Corine, Furuya, Akinori, Ohkubo, Toshifumi, Rousset, Abel, Tailhades, Philippe, Tanabe, Takaya, Yamamoto, Manabu, Yoshikawa, Hiroshi.
Application Number | 20040197604 10/669038 |
Document ID | / |
Family ID | 26436739 |
Filed Date | 2004-10-07 |
United States Patent
Application |
20040197604 |
Kind Code |
A1 |
Furuya, Akinori ; et
al. |
October 7, 2004 |
Magneto-optical recording medium, manufacturing method thereof and
magneto-optical data recording and playback device
Abstract
This invention relates to magneto-optical recording media such
as magneto-optical disks and cards, manufacturing methods of the
medium and a magneto-optical recording and playback device to
record and play back data using the magnetic-optical recording
media. The magneto-optical recording medium has a recording layer
and a reflective layer on a substrate, and the recording layer has
a layered structure in which at least on spinel ferrite (or
rutile-type oxide or hematite) layer and at least one garnet
ferrite layer are piled together. The manufacturing method
comprises the steps of heat treatment after the formation of the
recording layer. In the magneto-optical recording and playback
device to record and playback data, the wavelength of light for
recording data is different from that for reading data, which is
preferable for a magneto-optical recording medium comprising a
garnet ferrite layer.
Inventors: |
Furuya, Akinori; (Tokyo,
JP) ; Yoshikawa, Hiroshi; (Tokyo, JP) ;
Tanabe, Takaya; (Tokyo, JP) ; Yamamoto, Manabu;
(Tokyo, JP) ; Ohkubo, Toshifumi; (Tokyo, JP)
; Bouet, Laurence; (Toulouse, FR) ; Tailhades,
Philippe; (Toulouse, FR) ; Despax-Bonningue,
Corine; (Toulouse, FR) ; Rousset, Abel;
(Toulouse, FR) |
Correspondence
Address: |
THELEN REID & PRIEST LLP
P.O. BOX 640640
SAN JOSE
CA
95164-0640
US
|
Family ID: |
26436739 |
Appl. No.: |
10/669038 |
Filed: |
September 22, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10669038 |
Sep 22, 2003 |
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09529919 |
Apr 21, 2000 |
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6759137 |
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09529919 |
Apr 21, 2000 |
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PCT/JP99/04674 |
Aug 30, 1999 |
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Current U.S.
Class: |
428/822.2 ;
369/13.01; 428/820.3; G9B/11.048; G9B/11.049 |
Current CPC
Class: |
G11B 11/10543 20130101;
Y10T 428/12931 20150115; G11B 11/10532 20130101; G11B 7/1275
20130101; G11B 11/10536 20130101; Y10T 428/263 20150115; Y10T
428/24967 20150115; G11B 11/10582 20130101; G11B 11/10586 20130101;
Y10T 428/12465 20150115; G11B 11/10584 20130101; Y10T 428/12667
20150115; Y10T 428/2457 20150115; Y10T 428/24479 20150115; Y10T
428/12646 20150115; Y10T 428/31 20150115 |
Class at
Publication: |
428/694.0GT ;
369/013.01 |
International
Class: |
G11B 011/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 28, 1998 |
JP |
10-244156 |
Apr 1, 1999 |
JP |
11-095518 |
Claims
1-18. (canceled).
19. A magneto-optical recording medium having a recording layer and
a reflective layer on a substrate, the recording layer comprising:
a garnet ferrite recording layer; and at least one underlayer for
the garnet ferrite recording layer selected from the group
consisting of a spinel ferrite layer, rutile-type oxide layer and a
hematite layer, wherein the underlayer is formed on the substrate
or the reflective layer; the garnet ferrite recording layer is
formed adjacent to the underlayer after the formation of the
underlayer; and the recording layer is heat-treated after the
formation of the garnet ferrite layer at a temperature of 500 to
700.degree. C., thereby reducing the internal compressive stress of
the garnet ferrite layer by the tensile stress provided from the
underlayer.
20. A magneto-optical recording medium according to claim 19,
wherein said recording layer has tracks on which data are recorded,
and said recording layer is formed at least on the tracks.
21. A magneto-optical recording medium according to claim 19,
wherein said recording layer is located between said substrate and
said reflective layer.
22. A magneto-optical recording medium according to claim 19,
wherein said reflective layer is located between said substrate and
said recording layer.
23. A magneto-optical recording medium according to claim 19,
wherein the thickness of said garnet ferrite layer is 40 to 400
nm.
24. A magneto-optical recording medium according to claim 19,
wherein the thickness of said underlayer is 10 to 100 nm.
25. A magneto-optical recording medium according to claim 19,
wherein said recording layer has a multi-layered structure in which
a plurality of garnet ferrite layers and a plurality of spinel
ferrite layers, rutile-type oxide layers or hematite layers are
layered.
26. A magneto-optical recording medium according to claim 25,
wherein the thickness of said recording layer is 40 to 1000 nm.
27. A magneto-optical recording medium according to claim 19,
wherein grooves are formed on the surface of at least one of said
substrate, said reflective layer, and said recording layer.
28. A magneto-optical recording medium according to claim 19,
wherein loads are attached to the surface of at least one of said
substrate, said reflective layer, and said recording layer.
29. A magneto-optical recording medium according to claim 19,
wherein a transparent layer is formed on the surface of said
recording layer or said reflective layer.
30. A magneto-optical recording medium according to claim 19,
wherein grooves are formed on the surface of said transparent
layer.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a magneto-optical recording medium
such as magneto-optical disk and card, a manufacturing method for
the medium and a magneto-optical recording and playback device to
record and play back data using the magneto-optical recording
medium. This invention specially relates to a magneto-optical
recording medium having a recording layer comprising a garnet
ferrite layer and being ideal for high density and proximity
recording, a manufacturing method for the medium and a
magneto-optical recording and playback device for the medium.
[0003] 2. Background Art
[0004] In conventional magneto-optical recording media on the
market, the recording layer is mainly made from a thin metal layer.
Data are recorded onto the recording layer as recorded bits by
changing the optical properties such as the transmission or
reflection rate of minute spots on the recording layer by a light
beam for recording data. An amorphous alloy of rare earth metals
such as TbFeCo is a typical material for the thin metal layer, and
the alloy is favorable for recording data because it generally has
a high absorption coefficient (>10.sup.5cm.sup.-1). However, the
thin metal layer is prone to deterioration due to oxidation and
other factors. For this reason, it has to be sealed and protected
by a plastic layer, for example.
[0005] On the other hand, a magneto-optical recording medium having
a recording layer that consists of an oxide such as garnet ferrite,
which is a kind of ferrite having a garnet type structure of
crystals and has a large Faraday rotation angle, has been
developed. In such a recording medium, the degree of deterioration
of properties due to oxidation is smaller than that of the case in
which a metal material is used for the recording layer, because the
material itself of the recording layer is already an oxide.
Therefore, it has the distinctive feature that the above mentioned
special protection is not necessary.
[0006] In the case in which garnet ferrite is used as the material
for magneto-optical recording, however, internal stress occurs in a
garnet ferrite layer when spattering for formation of the layer on
a substrate. This sometimes results in cracks in the garnet ferrite
layer, rough morphology of the surface of the layer and very large
crystal particles, which are not preferable because they cause the
problem of medium noise when recording and playing back. In order
to overcome the above-mentioned problems, a method for improvement
of the morphology of the layer by adjusting the thermal expansion
coefficient of the substrate and by performing inverse spattering,
for example, after annealing, is disclosed in Japanese Patent
Publication No. Hei 8-249740 (1996).
[0007] Meanwhile, a new type of recording layer having a plurality
of layers made from several kinds of materials for magneto-optical
recording has recently been developed for the purpose of
improvement of the S/N and C/N ratio. However, it is said that
metal materials such as the above amorphous alloy are not suitable
for multi-layered structures since they have a relatively small
Kerr Effect and a high absorption coefficient. As a result, a
recording layer having multiple-layers made from garnet ferrite,
which is known as an oxide-type material for optical recording and
which has large Faraday Effect, has attracted much attention. For
instance, a multi-layered recording medium having piled layers made
from bismuth-substituted type garnet ferrite having an excellent
Faraday rotation angle in the wavelength range of visible light has
been proposed (see Itoh, Koike, Numata, Inoue and Kawanishi
"Multi-Layered Magnetic Garnet Ferrite Film for Magneto-Optical
Recording", Abstract of 10th Academic Lectures for Application of
Magnetics in Japan, p.31, November 1986).
[0008] However, a multi-layered recording medium having a recording
layer made from bismuth-substituted garnet ferrite requires a high
power light beam when writing data because the absorption
coefficient of the recording layer is small. Taking this problem
into consideration, Japanese Patent Publication No. Hei 6-282868
discloses a multi-layered type magneto-optical recording medium
having a light absorption layer which accelerates the recording
process by efficient transfer of the applied by a light beam to a
recording layer which is near the light absorption layer. However,
the above mentioned magneto-optical recording medium uses direct
energy gap semiconductors such as GaAs, InP, CdS, CdSe, ZnSe and
ZnS, which are easily oxidized during layer formation, as the light
absorption layer. Therefor , a protective layer is required on the
surface of the light absorption layer. The protectiv layer is
formed by deposition of SiO.sub.2, A1.sub.2O.sub.3, TiO.sub.2 or
the like in the range of 2.about.100 .mu.m by a CVD method or
spattering to create a film. Accordingly, the manufacturing process
shown in Japanese Patent Publication No. 6-282868 becomes complex
because formation Of the light absorption layer and the protective
layer is necessary in addition to formation of the recording layer,
which leads to increased production costs. Besides, properties such
as the S/N ratio of the magneto-optical recording medium
manufactured by the above method have not yet reached a
satisfactory level.
[0009] Japanese Patent Publication No. Hei 6-290497 (1994)
discloses a manufacturing method for a magneto-optical recording
medium having a recording layer that has a double-layered garnet
ferrite structure, in which a non-magnetic garnet ferrite
underlayer is used, and discloses that the multi-recording layer
keeps the garnet ferrite crystal particle diameter at 1 .mu.m or
below to reduce the disparities in bit shapes and medium noise.
However, the above method is impractical because the manufacturing
process is complex. Further, in the case in which the multi-layered
structure is formed by two types of garnet ferrite having different
compositions, the elements in each garnet ferrite layer disperse at
the vicinity of layer boundaries after heat treatment. Therefore,
compositional deviations occur in the direction perpendicular to
the layer surface in the multi-layered structure, which causes the
problem of deterioration of properties such as the S/N ratio and
repeatability. Furthermore, high density recording is hindered if
the above mentioned method is employed because it is impossible for
the above mentioned garnet ferrite layer formation method to
provide minute crystals on the order of nanometers.
[0010] On the other hand, various approaches to the improvement of
the S/N and C/N ratios have been examined from not only the aspect
of the magneto-optical recording medium itself but also from the
aspect of devices for recording and playing back data on the
magneto-optical recording medium.
[0011] The method to realize high resolving power by raising the
optical refraction rate by fulfilling a liquid between a sample and
an object lens is known. An application of this method using a
small solid lens has been proposed under the name of SIL lens
(Appl. Phys. Lett., 57(24), 1990; U.S. Pat. No. 5,004,307). And, a
data recording and playback system using the SIL lens for
magneto-optical recording medium has been also proposed (U.S. Pat.
No. 5,125,750). This system is characterized in that the distance
between the SIL lens and the recording layer of the recording
medium is kept within the wavelength of the light, namely on the
sub-micron order, so as to gain a small focus spot diameter which
is a feature of the SIL lens, and so as to gain an increase in the
recording density. However, even in this system, the S/N and C/N
ratios have not yet reached a satisfactory level. Besides, this
system lacks generality because it is not suitabl for a
magneto-optical recording medium that has a relatively thick
protective layer, which is, for example, disclosed in Japanese
Patent Publication No. Hei 6-282868.
SUMMARY OF THE INVENTION
[0012] The present invention was made referring to the state of the
prior art mentioned above. The present invention aims to provide
data recording and playback with excellent S/N and C/N ratios, from
the both aspects of a magneto-optical recording medium and a
magneto-optical recording and playback device. Namely, it is an
object of the present invention to produce a new magneto-optical
recording medium that has a recording layer containing a garnet
ferrite layer, which has high resolution, high recording density,
high S/N and C/N ratios and the magnet properties of which can be
easily controlled.
[0013] It is another object of the present invention to produce a
new magneto-optical recording and playback device that is suitable
for data recording and playback on such a magneto-optical recording
medium and is able to increase the S/N and C/N ratios.
[0014] The above mentioned object is achieved by a magneto-optical
recording medium having a recording layer and a reflective layer on
a substrate characterized in that the recording layer has a layered
structure in which a garnet ferrite layer and any one of a spinel
ferrite layer, a rutile-type oxide layer and a hematite layer are
layered. The layered structure in the recording layer is preferably
present at least on tracks in which data are recorded. If the
layered structure is not present between the tracks, it is
preferable that only a garnet ferrite layer be present between the
tracks.
[0015] The recording layer may be located between the substrate and
the reflective layer. On the other hand, the reflective layer may
be located between the substrate and the recording layer. The
preferable thickness of the garnet ferrite layer is 40 to 400 nm,
and that of the spinel ferrite layer, the rutile-type oxid layer
and the hematite layer is 10 to 100 nm. The recording layer may
have a multi-layer structure in which a plurality of garnet ferrite
layers and a plurality of spinel ferrite layers, rutile-type oxide
layers and hematite layers are layered. If so, the preferable
thickness of the recording layer is 40 to 1000 nm.
[0016] Further, grooves may be formed on, or loads may be attached
to the surface of at least one of the substrate, the reflective
layer or the recording layer. Here, a "load" is a member to change
the effective reflection index on the surface of the layers, which
has a rectangular section in general and forms convexities on the
surface of the layers to which it is attached. The material of the
load is not-restricted.
[0017] In the magneto-optical recording medium of the present
invention, a transparent layer may be formed on the surface of the
recording layer or the reflective layer. If so, grooves may be
formed on the surface of the transparent layer.
[0018] The magneto-optical recording medium of the present
invention can be produced by a manufacturing method characterized
by comprising a step of heat treatment at a temperature of 500 to
700.degree. C., preferably 600 to 630.degree. C., after the
formation of the recording layer.
[0019] The other object of the present invention mentioned above is
achieved by a magneto-optical recording and playback device to
record and play back data by use of a magneto-optical recording
medium, characterized in that the wavelength of light is different
for recording data into the magneto-optical recording medium and
for reading data from the magneto-optical recording medium. This
magneto-optical recording and playback device can be used for a
magneto-optical recording medium having a recording layer
comprising a garnet ferrite layer, preferably for a magneto-optical
recording medium having a recording layer and a reflective layer on
a substrate, and the recording layer having a layered structure in
which a garnet ferrite layer and any one of a spinel ferrite layer,
a rutile-type oxide layer and a hematite layer are layered. It is
preferable that the light for recording and reading are provided by
one light source.
[0020] This invention has the effect that a garnet ferrite-type
recording medium having excellent magnetic properties and suitable
as a magneto-optical recording medium can be produced without a
complicated process. And, passivation does not occur since no
metal-type material is used as the magneto-optical recording
material.
[0021] Besides, a magneto-optical recording medium with high
anisotropy, high resolution, high recording density and low noise
can be produced since a garnet ferrite layer having minute
morphology suitable for high density recording can be obtained, and
internal stress in a recording layer can be cancelled by a
combination of a spinel ferrite layer, a rutile-type oxide layer or
a hematite layer-with the garnet ferrite layer. Further, the
control of the magnetic properties of the garnet ferrite layer is
easily accomplished. Further, the S/N ratio of the recording medium
is remarkably improved because of the synergistic effect of high
output effect derived from the enormous Faraday effect originally
provided by the garnet ferrite and the low noise effect.
[0022] In the case in which the reflective layer is located between
the substrate and the recording layer, the production process of
the magneto-optical recording medium can be simplified and the
production costs can be reduced since a protective film is not
necessary even if the reflective layer is made of metal-type
material. Further, in this case, an optical pickup mechanism such
as a read head can be set substantially closer to the medium
surface during playback since there is no protective film. This
makes it possible to obtain a higher S/N ratio than before.
[0023] In the case in which the recording layer comprises a
plurality of garnet ferrite layers and a plurality of spinel
ferrite layers, rutile-type layers or hematite layers, the number
of heat treatments can be reduced. And, it is easy to obtain a
recording layer having excellent magnetic properties since precise
control of the stress in the recording medium is possible.
[0024] Further in the case in which grooves are formed, or loads
are attached to the surface of at least one of the substrate, the
reflective layer or the recording layer, servo control of the
recording position on the recording medium can be carried out.
[0025] Further, if a transparent layer is layered on the surface of
the recording layer or the reflective layer, the light irradiated
from a playback head and focused on the recording layer will not be
subject to the effects of dust and scratches on the surface of the
recording medium. Incidentally, if grooves are formed on the
transparent layer, the grooves can be utilized as guides for the
servo control.
[0026] According to the manufacturing method of the magneto-optical
recording medium of the present invention, it is possible to endow
with magnetic properties only to the garnet ferrite layer present
on the track parts, and to make the garnet ferrite layer except for
the track parts non-magnetic by control of the temperature of the
heat treatment. Therefore, it is possible to reduce the noise
derived from parts other than the tracks, and the SIN ratio
increases considerably because of the synergistic effect of the
high output derived from the enormous Faraday effect originally
provided by the garnet ferrite and the noise reduction effect.
Further, it is also possible to reduce the magnetic interference
with the data recorded in the track parts from the parts other than
the track parts.
[0027] The manufacturing method of the present invention can
produce a magneto-optical recording medium having a layered
structure in which a garnet ferrite layer and any one of a spinel
ferrite layer, a rutile-type oxide layer and a hematite layer are
layered, at least on tracks in which data are recorded, as well as
the other magneto-optical recording media of the present invention.
And, in the case in which a reflective layer is located between a
substrate and a recording layer, no protective means such as a
coating layer for the reflective layer is necessary even if the
reflective layer is made of metal type materials. Therefore, it is
possible to simplify the manufacturing process and to reduce the
production cost. Furthermore, it is possible to locate a light
pick-up mechanism such as a reading head substantially closer to
the recording layer, which results in a increase of the S/N
ratio.
[0028] If the magneto-optical recording medium of the present
invention has a transparent layer other than the substrate,
compatibility with a conventional medium can be obtained, and a
light beam focused on the recording layer will be less affected by
dust that may be adhering to the surface of the recording medium or
by scratches that may be present thereon. If grooves are formed on
the surface of the transparent layer, servo control of the
recording position becomes possible by detecting the change of the
refraction and reflection rate on the surface of the transparent
layer, which is caused by the grooves.
[0029] Taking the light absorption properties of a magneto-optical
recording medium to be used into consideration, in the
magneto-optical recording and playback device of the present
invention, the wavelength of light is set to be different for
recording data and reading data. Therefore, the S/N ratio and C/N
ratio can be increased by optimizing the recording and playback,
referring to the properties of the medium to be used.
[0030] For example, if the light absorption rate of the
magneto-optical recording medium is high for light with a short
wavelength, efficient data recording and reduction of the power of
the light beam for data recording are possible by shortening the
wavelength of the light beam for data recording. On the other hand,
if the light absorption rate of the magneto-optical recording
medium is low for light with a long wavelength, reducing
undesirable heating of a recording layer is possible by lengthening
the wavelength of the light beam for data reading, and increasing
of C/N ratio is possible because a reflected light beam with high
power can be obtained.
[0031] The magneto-optical recording and playback device of the
present invention can be preferably used for the magneto-optical
recording medium of the present invention that has a garnet ferrite
layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 is a figure showing two types of section structure of
a magneto-optical recording medium having a single layer of spinel
ferrite, rutile-type oxide or hematite.
[0033] FIG. 2 is a figure showing two types of section structure of
a magneto-optical recording medium having a single garnet ferrite
layer.
[0034] FIG. 3 is a figure showing two types of section structure of
a magneto-optical recording medium having, two layers of garnet
ferrite/spinel ferrite (or rutile-type oxide, or hematite).
[0035] FIG. 4 is a figure showing magnetization curves of
magneto-optical recording media that have a single layer of spinel
ferrite, a single layer of garnet ferrite and two layers of garnet
ferrite/spinel ferrite (or rutile-type oxide).
[0036] FIG. 5 is a figure showing magnetization curves of
magneto-optical recording media that have a single layer of
hematite, a single layer of garnet ferrite and two layers of garnet
ferrite/hematite.
[0037] FIG. 6 is a figure showing two types of section structure of
a magneto-optical recording medium having a recording layer which
consists of multiple layers of garnet ferrite/spinel ferrite(or
rutile-type oxide, or hematite).
[0038] FIG. 7 is a figure showing two types of section structur of
a magneto-optical recording medium which has two layers of garnet
ferrite/spinel ferrite (or rutile-type oxide), and on which grooves
for servo control are formed.
[0039] FIG. 8 is a figure showing two types of section structure of
a magneto-optical recording medium having a recording layer which
consists of multiple layers of garnet ferrite/spinel ferrite (or
rutile-type oxide, or hematite), and on which grooves for servo
control are formed.
[0040] FIG. 9 is a figure showing two types of section structure of
a magneto-optical recording medium having a recording layer which
consists of two layers of garnet ferrite/spinel ferrite (or
rutile-type oxide), and on which loads for servo control are
attached.
[0041] FIG. 10 is a figure showing two types of section structure
of a magneto-optical recording medium having a recording layer
which consists of multiple layers of, garnet ferrite/spinel ferrite
(or rutile-type oxide, or hematite), and on which grooves for servo
control are formed.
[0042] FIG. 11 is a figure showing two types of section structures
of magneto-optical recording media having a recording layer which
consists of two or multiple layers of garnet ferrite/spinel ferrite
(or rutile-type oxide), and which lacks a metal reflective layer,
and on which loads for servo control are attached.
[0043] FIG. 12 is a figure showing two types of section structure
corresponding to FIG. 3(b) and FIG. 6(b), and on which transparent
layers are formed.
[0044] FIG. 13 is a figure showing two types of section structure
corresponding to FIG. 7(b) and FIG. 8(b), and on which transparent
layers are formed.
[0045] FIG. 14 is a figure showing two types of section structure
corresponding to FIG. 13, and flattening is done for the surface
the transparent layers.
[0046] FIG. 15 is a figure showing two types of section structure
corresponding to FIG. 9(b) and FIG. 10(b), and on which transparent
layers are formed.
[0047] FIG. 16 is a figure showing two types of section structure
corresponding to FIG. 12(a) and (b), and on which grooves for servo
control are attached.
[0048] FIG. 17 is a figure showing a section structure of the
magneto-optical recording medium having a recording layer in which
spinel ferrite layers are formed on tracks.
[0049] FIG. 18 is a figure showing a section structure of the
magneto-optical recording medium of FIG. 17, a garnet ferrite layer
of which is directly or indirectly coated with a metal reflective
layer.
[0050] FIG. 19 is a graph showing X-ray diffraction intensity of
magneto-optical recording media, one of which has a recording layer
that consists of a single layer of garnet ferrite, and the other of
which has a recording layer that consists of two layers of spinel
and garnet ferrite.
[0051] FIG. 20 is a graph showing the dependence of the Faraday
rotation angle of magneto-optical recording media having various
recording layers on the wavelength of light.
[0052] FIG. 21 is a graph showing the dependence of the absorption
coefficient of a BiDyGalG thin layer on the wavelength.
[0053] FIG. 22 is a figure showing the optical system outline of an
embodiment of the magneto-optical data recording and playback
device of the present invention.
[0054] FIG. 23 is a figure showing the optical system outline of
another embodiment of the magneto-optical data recording and
playback device of the present invention.
[0055] FIG. 24 is a figure showing the optical system outline of
another embodiment of the magneto-optical data recording and
playback device of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0056] Spinel ferrite, which is a kind of ferrite having a spinel
type crystal structure, which is known as a material for
magneto-optical recording, has a high Faraday rotation effect
(Brevet Francais No. 933315258 (1993)). It does not crack and can
provide small crystal particles on the order of nanometers, which
makes it suitable for high density recording. Nevertheless, there
is the problem that sufficient signal output cannot be obtained
since the absorption coefficient of the spinel ferrite layer itself
is large. However, it is quite easy to form a layer having tensile
stress if spin 1 ferrite is used, since it has a different crystal
structure from garnet ferrite.
[0057] With regard to rutile-type oxide and hematite which are the
same as ferrite in terms of being inorganic oxides, it also easy to
form a layer having tensile stress, and minute crystal particles on
the order of nanometers can be obtained.
[0058] In the recording medium of the present invention, a garnet
ferrite layer and any one of a spinel ferrite layer, a rutile-type
oxide layer or a hematite layer are layered together so that
undesirable compressive stress applied to the garnet ferrite layer
can be canceled by the tensile stress of the spinel ferrite,
rutile-type oxide and hematite layer.
[0059] Accordingly, a recording layer having excellent magnetic
properties such as a large square ratio (residual
magnetization/saturation magnetization), increased magnetic
coercive force and high vertical anisotropy, and having improved
morphology can be obtained. Because of this, a high density
recording medium suitable for magneto-optical recording can be
produced.
[0060] In the medium in which a plurality of garnet ferrite layers
and a plurality of spinel ferrite layers, rutile-type oxide layers
or hematite layers are layered alternately or at random, it is easy
to obtain a recording layer having excellent magnetic properties
because the internal stress of the recording layer can be
controlled over a wide range from compressive stress to tensile
stress. Furthermore, the number of heat treatments can be
reduced.
[0061] As a substrate for the medium, a heat-resistant glass such
as quartz glass or Pyrex glass is generally used. In the present
invention, ferrite represented by the general formula
R.sub.x-yCo.sub.yFe.sub.3-xO.s- ub.4 (0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.x, and R is at least one kind of rare earth
element including Dy) including Fe.sub.3O.sub.4, and .gamma.-
Fe.sub.2O.sub.3, for example, is used as the spinel ferrite. On the
other hand, ferrite represented by the general formula
Bi.sub.xR.sub.3-x+uM.sub.yFe.sub.5-y+vO.sub.12
(0.ltoreq.x.ltoreq.3, 0.ltoreq.y.ltoreq.5, -3.ltoreq.u.ltoreq.3,
-3.ltoreq.v.ltoreq.=3, -5.ltoreq.x.ltoreq.5, R is at least-one kind
of rare earth element including Dy, and M is a tervalent metal
being interchangeable with iron) including iron garnet, for
instance, is used as the garnet ferrite. Rutile-type oxide is
represented by RO.sub.2 (R is a transition metal such as Ti and
Cr), and TiO.sub.2 is generally used. As the hematite,
.alpha.-M.sub.xFe.sub.2-xO.sub.3 (0.ltoreq.x.ltoreq.1, M is Ti, V,
Cr, Mn, Zr, Nb, Mo and the like) can be used in the present
invention. As the material for the reflective layer, a metal such
as aluminum, gold, chrome, platinum or alloy thereof can be
employed.
[0062] If a reflective layer made of a metal material with a
thermal expansion coefficient larger than that of glass is formed
on a glass substrate in advance by spattering, the reflective layer
is subject to tensile stress after the spattering. Therefore, if a
recording layer is further layered on the reflective layer, the
compressive stress in a garnet ferrite layer can be canceled
effectively by producing tensile stress in the reflective layer and
in a spinel ferrite, rutile-type oxide or hematite layer.
[0063] Furthermore, a garnet ferrite layer formed on a spinel
ferrite, rutile-type oxide or hematite layer inherits the
morphology afforded by the minute crystal particles, which is the
feature of spinel ferrite, rutile-type oxide or hematite, and
produces minute garnet ferrite crystals. Therefore, a garnet
ferrite layer having minute morphology suitable for high-density
recording can be obtained.
[0064] Accordingly, a magneto-optical recording medium with high
resolution, high recording density and low noise can be produced.
Further, it is also possible to produce a magneto-optical recording
medium having an excellent S/N ratio because of the synergistic
effect of high output effect derived from very large Faraday effect
originally provided by the garnet ferrite and the low noise
effect.
[0065] If a reflective layer is located between a substrate and a
recording layer, it is unnecessary to consider passivation and to
provide a protective film on the reflective layer, even if the
reflective layer is made of metal, since an oxide-type recording
layer which is extremely stable over a long time covers the
reflective layer. This allows the simplification of the production
process of a magneto-optical recording medium and the reduction of
production costs. Also, this means that an optical pickup mechanism
such as a read head can be set substantially closer to the medium
surface during playback since no protectiv film is present. This
makes it possible to obtain a higher S/N ratio.
[0066] If grooves are formed or loads are attached to the surface
of at least one of the substrate, the reflective layer or the
recording layer, the effective refraction index of the surface can
be changed from place to place. Therefore, servo control of the
recording position on the recording medium is possible by detecting
the change in strength of the reflected light from the surface
caused by the change of the refraction index thereof.
[0067] If a transparent layer is further formed on the surface of
the recording layer or the reflective layer, irradiated light tends
to be less affected by any scratches or dust on the recording
medium. Further, if grooves are formed on the surface of the
transparent layer, the grooves can be utilized as guides for the
servo control.
[0068] It is preferable that the thickness of the garnet ferrite
layer in the present invention is from 40 to 400 nm becaus
sufficient magnetic properties cannot be obtained if it is less
than 40 nm, and cracks may occur if it is more than 400 nm. On the
other hand, it is preferable that the thickness of the spinel
ferrite, rutile-type oxide or hematite layer is from 10 to 100 nm
because sufficient magnetic properties cannot be obtained if it is
less than 10 nm, and coloration of the layer causes deterioration
of the S/N ratio if it is more than 100 nm.
[0069] In the case in which the recording layer comprises a
plurality of garnet ferrite layers and a plurality of spinel
ferrite, rutile-type oxide or hematite layers, it is preferable
that the thickness of the recording layer is from 100 to 1000 nm
because sufficient magnetic properties cannot be obtained if it is
less than 100 nm, and the transparency of the recording layer
becomes worse if it is more than 1000 nm.
[0070] In the present invention, the recording layer can comprise
another layer made of material for magnetic recording other than
garnet ferrite, spinel ferrite, rutile-type oxide and hematite if
necessary. However, it is desirable for the garnet ferrite layer to
adjoin the spinel ferrite, rutile-type oxide or hematite layer.
[0071] Hereafter, the present inventions will be explained in more
detail, referring to the figures.
COMPARATIVE EXAMPLES
[0072] For comparison with the present invention, recording media
each of which has a recording layer consisting of a single layer
made of spinel ferrite, rutile-type oxide, hematite or garnet
ferrite, were manufactured.
[0073] FIG. 1 shows two types of section structure of a
magneto-optical recording medium having a recording layer which
consists of a single layer of spinel ferrite or rutile (TiO.sub.2)
(hereunder, referred to as "Comparative Example 1") or of hematite
(hereunder, referred to as "Comparative Example 2"). In the case
shown in FIG. 1(a), the spinel ferrite (or rutile) layer 2 on the
quartz glass substrate 1 is covered with th metal reflective layer
4. On the other hand, in the case shown in FIG. 1(b), the layer
order of the metal reflective layer 4 and the spinel ferrite (or
rutile) layer 2 on the quartz glass substrate 1 is different from
that of FIG. 1(a).
[0074] The manufacturing process of the Comparative Example 1 in
which spinel ferrite was employed was as follows. First, in the
case of FIG. 1(a), the spinel ferrite layer 2 made from
Mn.sub.0.013Co.sub.0..sub.73Fe- .sub.2.14O.sub.4 and having a
thickness of 100 nm was formed on the quartz glass substrate 1 by
RF spattering. After that, it was heat-treated in a normal-pressure
atmosphere of 20% oxygen and 80% nitrogen for 10 minutes at
400.degree. C., and finally, the spinel ferrite layer 2 was coated
with the metal reflective layer 4. According to AFM (Atomic Force
Microscope) surface observation, the surface roughness and the
crystal particle diameter of the spinel ferrite layer 2 after the
heat-treatment were 2 nm and 30 nm at most, respectively, which
indicated that the surface of the spinel ferrite layer 2 was very
flat.
[0075] In the case of FIG. 1(b), the manufacturing process of the
Comparative Example 1 was conducted under the same conditions as
the case of FIG. 1(a) except that the metal reflective layer 4 was
formed in advance on the quartz glass substrate 1. According to AFM
surface observation, the surface roughness and the crystal particle
diameter of the spinel ferrite layer 2 were 2 nm, and 30 nm at
most, respectively, which indicated that the surface of the spinel
ferrite layer 2 was very flat.
[0076] On the other hand, the Comparative Example 1 in which rutile
(TiO.sub.2) was employed instead of spinel ferrite was manufactured
under the same conditions as mentioned above. According to AFM
surface observation, the surface roughness and the crystal particle
diameter of rutile layers 2 of both FIG. 1(a) and (b) were 2 nm and
30 nm at most, respectively, which indicated that the surface of
the rutile layer 2 was very flat. Comparative Example 2 was
manufactured under the same conditions as Comparative Example 1. In
Comparative Example 2, .alpha.-Fe.sub.2O.sub.3 was employed as
hematite. According to AFM surface observation, the surface
roughness and the crystal particle diameter of the hematite layers
2 of the both of FIG. 1(a) and (b) were 2 nm and 30 nm at most,
respectively, which indicated that th surface of the hematite layer
2 was very flat.
[0077] FIG. 2 shows two types of section structure of a
magneto-optical recording medium having a recording layer which
consists of a single layer of garnet ferrite (hereunder, referred
to as, "Comparative Example 3"). In the case shown in FIG. 2(a),
the garnet ferrite layer 3 on the quartz glass substrate 1 is
covered with the metal reflective layer 4. On the other hand, in
the other case shown in FIG. 2(b), the layer order of the metal
reflective layer 4 and the garnet ferrite layer 3 on the quartz
glass substrate 1 is different from that of FIG. 2(a).
[0078] The manufacturing process of Comparative Example 3 was as
follows. First, in the case of FIG. 2(a), the garnet ferrite layer
3 made from Bi.sub.2DyFe.sub.4GaO.sub.12 and having a thickness of
350 nm was formed on the quartz glass substrate 1 by a RF
spattering method. After that, it was heat-treated in a
normal-pressure atmosphere of 100% oxygen for 10 minutes at
650.degree. C., and finally, the garnet ferrite layer 3 was coated
with the metal reflective layer 4. According to AFM surface
observation, the surface roughness and the crystal particle
diameter of the garnet layer 3 after the heat-treatment were 4 nm
and 70 nm, respectively, and cracks of 1 to 3 .mu.m were found on
the surface.
[0079] For the case of FIG. 2(b), the manufacturing process of
Comparative Example 3 was conducted under the same conditions as
the case of FIG. 2(a) except that the metal reflective layer 4 was
formed in advance on the quartz glass substrate 1. According to AFM
surface observation, the surface roughness and the crystal particle
diameter of the garnet layer 3 were 4 nm, and 70 nm, respectively,
and cracks of 1 to 3 .mu.m were found on the surface.
[0080] [EMBODIMENTS 1 and 2]
[0081] FIG. 3 shows two types of section structure of a
magneto-optical recording medium having a recording layer which
consists of two layers of garnet ferrite/spinel ferrite (or
rutile)(hereunder, referred to as "Embodiment 1") or of two layers
of garnet ferrite/hematite (hereunder, referred to as "Embodiment
2"). In the case shown in FIG. 3(a), the garnet ferrite layer 3 is
layered on the spinel ferrite (or rutile or hematite) layer 2
formed on the quartz glass substrate 1, and the garnet ferrite
layer 3 is covered with the metal reflective layer 4. On the other
hand, the case shown in FIG. 3(b) is different from the case of
FIG. 3(a) in a point that the spinel ferrite(or rutile or hematite)
layer 2 and the garnet ferrite layer 3 are layered on-the metal
reflective layer 4 formed on the quartz glass substrate 1.
Incidentally, recording and playback are executed by irradiation of
a laser from the bottom and through the quartz glass substrate 1 in
the case of FIG. 3(a), while they are executed by direct
irradiation of laser onto the recording layer from the top in the
case of FIG. 3(b).
[0082] The manufacturing process of the Embodiments 1 and 2 was as
follows. First, for the case of FIG. 3(a), the spinel ferrite (or
rutile or hematite) layer 2 as an underlayer was formed on the
quartz glass substrate 1 by RF spattering, and it was heat-treated
under the same conditions as Comparative Examples 1 and 2. After
that, the garnet ferrite layer 3 having a large Faraday effect was
formed on the spinel ferrite (or rutile or hematite) layer 2 by RF
spattering, and it was heat-treated under the same conditions as
Comparative Example 3. Finally, the garnet ferrite layer 3 was
coated with the metal reflective layer 4. For the case of FIG.
3(b), the manufacturing process for Embodiment 1 was conducted
under the same conditions as mentioned above except that the metal
reflective layer 4 was formed in advance on the quartz glass
substrate 1.
[0083] Mn.sub.0.13Co.sub.0..sub.73Fe.sub.2.14O.sub.4, TiO.sub.2,
.alpha.-Fe.sub.2O.sub.3 and Bi.sub.2DyFe.sub.4GaO.sub.12were used
as spinel ferrite, rutile, hematite and garnet ferrite,
respectively. The thickness of the spinel ferrite (or rutile or
hematite) layer 2 and the garnet ferrite layer 3 were 100 nm and
350 nm, respectively.
[0084] According to AFM surface observation, whichever material was
selected from spinel ferrite, rutile or hematite as the underlayer
in both structures of FIG. 3(a) and (b), there were no cracks on
the surface of the garnet ferrite layer 3 after heat treatment and
the surface roughness, and the crystal particle diameter in this
layer were 3 nm and 40 nm at most, respectively, which indicated
that the surface of the garnet ferrite layer 3 was very flat. It is
apparent from these results that the morphology of the garnet
ferrite layer 3 was remarkably improved and minute garnet ferrite
crystals could be obtained by use of the spinel ferrite (or rutile
or hematite) layer 2 as the underlayer for the garnet ferrite layer
3. The reason for this seems to be inheritance of the morphology
produced by the minute crystal particles by the spinel ferrite(or
rutile or hematite) layer 2 to the garnet ferrite layer 3.
Therefore, a magneto-optical recording medium with high resolution,
high recording density and low noise can be produced. Further, it
is also possible to produce a magneto-optical recording medium
having an excellent S/N ratio because of the synergistic effect of
high output effect derived from very large Faraday effect
originally produced by the garnet ferrite and the low noise
effect.
[0085] Further, in the case shown in FIG. 3(b) where the metal
reflective layer 4 is located between the quartz glass substrate 1
and spinel ferrite (or rutile or hematite) 2, no protective means
such as a protective film is necessary for the metal reflective
layer 4 because passivation does not occur. This allows the
simplification of the production process and a reduction of
production costs. Also, this means that an optical pickup mechanism
such as a read head can be set substantially closer to the medium
surface during playback since there is no protective means. This
makes it possible to obtain a higher S/N ratio than before.
[0086] It is preferable that the thickness of the garnet ferrite
layer 3 in Embodiments 1 and 2 is from 40 to 400 nm because
sufficient magnetic properties cannot be obtained if it is less
than 40 nm, and cracks may occur if it is more than 400 nm. On the
other hand, it is preferable that the thickness of the spinel
ferrite (or rutile or hematite) layer 2 is from 10 to 100 nm
because improvements in the morphology of the surface of the garnet
ferrite layer 3 adjacent to the layer 2 may not be obtained if it
is less than 10 nm, and coloration of the layer causes
deterioration of the S/N ratio if it is more than 100 nm.
[0087] FIG. 4 shows magnetization curves of Comparative Examples 1
and 3, and Embodiment 1. FIGS. 4(a), (b) and (c) correspond to the
magnetization properties of Comparative Example 1 having a single
layer of spinel ferrite, Comparative Example 2 having a single
layer of garnet ferrite and Embodiment 1 having both layers,
respectively.
[0088] Comparative Example 1 having a single layer structure of
spinel ferrite had the hystetesis shown in FIG. 4(a), and the
magnetic coercive force (a), the saturation magnetization (a) and
the residual magnetization (a) were 50000e ; 250 emu/cc and 150
emu/cc, respectively. Accordingly, Comparative Example 1 has
sufficient property for a practical use in terms of the magnetic
coercive force (more than 20000e ). However, practical problems
such as deterioration of the S/N ratio can be expected since the
square ratio, which was 0.6 (the residual magnetization
(a)/saturation magnetization (a)=150/250=0.6), is small. It is said
that a square ratio equal to or more than about 0.8 is ideal for
practical use.
[0089] On the other hand, Comparative Example 3 having a single
layer structure of garnet ferrite had the hysteresis shown in. FIG.
4 (b), and the magnetic coercive force (b), the saturation
magnetization (b) and the residual magnetization (b) were 12000, 13
emu/cc and 10 emu/cc, respectively. Accordingly, Comparativ Example
3 has sufficient square ratio for a practical use since it was
about 0.8(10/13). However, practical problems such as an increase
in noise during high density recording can be expected since the
magnetic coercive force 12000e was small (for practical use, a
magnetic coercive force of at least 20000e is said to be
ideal).
[0090] Contrary to Comparative Examples 1 and 3, Embodiment 1,
having a two-ply structure of garnet ferrite/spinel ferrite (or
rutile), had the hysteresis shown in FIG. 4(c), and the magnetic
coercive force (c), saturation magnetization (c) and the residual
magnetization (c) were 20000e , 13 emu/cc and 10 emu/cc,
respectively. Accordingly, Embodiment 1 had sufficient magnetic
properties for a practical use in terms of both the magnetic
coercive force, and a square ratio of about 0.8 (10/13). In
addition, regardless of whether spinel ferrite or rutile was
selected as the material of the underlayer for the garnet ferrite
layer 3, the values of the above magnetic properties were
definitely the same.
[0091] As is obvious from the above, the magnetic properties, which
were poor in the recording media having a single layer of either
garnet ferrite or spinel ferrite (or rutile), were remarkably
improved in the recording media having a two-ply structure of
garnet ferrite/spinel ferrite (or rutile). Whichever was selected
from spinel ferrite or rutile as the material of the underlayer,
the S/N ratio of Embodiment 1 actually improved by 20 dB over
Comparative Example 3.
[0092] FIG. 5 shows the magnetization curves of Comparative
Examples 2 and 3, and Embodiment 2. FIGS. 5(a), (b) and (c)
correspond to the magnetization curve of Comparative Example 2,
having a single layer of hematite, Comparative Example 3, having a
single layer of garnet ferrite, and Embodiment 2 having both
layers, respectively.
[0093] As is obvious from FIG. 5(a), Comparative Example 2, which
has a recording layer consisting of a single layer of hematite, is
non-magnetic. On the other hand, Comparative Example 3, which has a
recording layer consisting of a single layer of garnet ferrite, has
the magnetization hysteresis properties shown in FIG. 5(b). Namely,
the magnetic coercive force, the saturation magnetization and the
residual magnetization were 12000e , 13 emu/cc and 10 emu/cc,
respectively, as mentioned above. On the other hand, Embodiment 2,
having a recording layer which consists of a garnet ferrite
layer/hematite layer had the same magnetic coercive force,
saturation magnetization and residual magnetization as Comparative
Example 3, which is shown in FIG. 5(c). It was also found that S/N
ratio was improved by 5 dB over Comparative Example 3.
[0094] According to the magneto-optical recording medium of
Embodiment 1, the morphology of the garnet ferrite layer 3 is
improved by the effect derived from the spinel ferrite layer 2 as
an underlayer. However, the magnetic properties, such as the
magnetic coercive force of the medium, change due to the effect of
the spinel ferrite (or rutile) layer 2, which is ferromagnetic.
According to the magneto-optical recording medium of Embodiment 2,
however, the magnetic properties of the garnet ferrite layer 3 do
not change because the hematite layer 2 is non-magnetic. Therefore,
stabler reading and recording of data is possible in Embodiment 2.
Besides, output signal for playback from the magneto-optical
recording medium of the embodiment 1 where the spinet ferrite layer
2 is used as an underlayer may decrease because the absorption
coefficient of the spinel ferrite layer 2 itself is large. In
Embodiment 2, however, no such inconvenience occurs because no
spinel ferrite underlayer is used.
[0095] [EMBODIMENT 3]
[0096] FIG. 6 shows two types of section structure of a
magneto-optical recording medium having a recording layer which
consists of multiple layers of garnet ferrite/spinel ferrite (or
rutile or hematite) (hereunder, referred to as "Embodiment 3").
[0097] In the case shown in FIG. 6(a), the multiple recording layer
5 consisting of a plurality of garnet ferrite layers and a
plurality of spinel ferrite (or rutile or hematite) layers is
layered on the quartz glass substrate 1, and it is covered with the
metal reflective layer 4. On the other hand, in the case shown in
FIG. 6(b), the layer order of the metal reflective layer 4 and the
multiple recording layer 5 is different from that of FIG. 6(a).
[0098] Embodiment 3 was manufactured by the same method used for
Embodiments 1 and 2, except that heat treatment for the multipl
recording layer 5, which was previously formed by layering a
plurality of spinel ferrite (or rutile or hematite) layers and
garnet ferrite layers, was performed only once. The layer structure
type shown in FIG. 6(a) and that shown in FIG. 6(b) correspond to
the case shown in FIG. 3(a) of Embodiment 1 or 2 and the case shown
in FIG. 3(b) of Embodiment 1 or 2, respectively.
[0099] According to observation of the magnetic properties of
Embodiment 3, when either spinel ferrite or rutile, but not garnet
ferrite, was selected as the material of the multiple recording
layer 5, it had 20000e of magnet coercive force, 13 emu/cc of
saturation magnetization and 10 emu/cc of residual magnetization,
and the square ratio was about 0.8, and these values were the same
as those of Embodiment 1. Besides, the surface roughness and the
crystal particle diameter of the multiple recording layer 5 after
heat-treatment were 3 nm and 40 nm at most, respectively. In other
words, Embodiment 3 having a multiple recording layer made from
garnet ferrite/spinel ferrite or rutile could provide the same
magnetic properties and morphology as Embodiment 1. On the other
hand, Embodiment 3, where hematite was used as the material for the
multiple recording layer 5 with garnet ferrite, had the same magnet
properties and morphology as Embodiment 2.
[0100] According to Embodiment 3, the number of heat treatments can
be reduced because it is not necessary to heat treat each layer
included in the multiple recording layer 5 in order to obtain the
same magnetic properties and morphology as Embodiments 1 and 2.
Further, a recording layer having excellent magnetic properties can
be more easily obtained because internal stress of the recording
layer can be precisely controlled.
[0101] The thickness of the multi-layered recording layer 5 is
preferably between 40 and 1000 nm because sufficient magnetic
properties cannot be obtained if it is less than 40 nm and the
transparency of the recording layer is degraded if it is more than
1000 nm.
[0102] The following embodiments can also be prepared in accordance
with substantially the same manufacturing methods employed in
Embodiments 1 to 3.
[0103] [EMBODIMENT 4]
[0104] FIG. 7 shows two types of section structure of a
magneto-optical recording medium having a recording layer which
consists of two layers of garnet ferrite/spinel ferrite (or rutile
or hematite), and grooves for servo control on its surface
(hereunder, referred to as "Embodiment 4"). In the case shown in
FIG. 7(a), the garnet ferrite layer 3 is layered on the spinel
ferrite (or rutile or hematite) layer 2 formed on the quartz glass
layer 1, and the garnet ferrite layer 3 is covered with the metal
reflective layer 4. On the other hand, the case shown in FIG. 7(b)
is different from that of FIG. 7(a) in that the spinel ferrite(or
rutile or hematite) layer 2 is layered on the metal reflective
layer 4 formed on the quartz glass substrate 1.
[0105] As shown in the Figure, in Embodiment 4, the grooves 6 are
formed on the surface of the spinel ferrite (or rutile or hematite)
layer 2, the garnet ferrite layer 3 and the metal reflective layer
4 by employing a quartz glass substrate 1 having grooves with a
predetermined width and depth on its surface, thereby changing the
effective refraction index on the recording medium from place to
place. Then, the change of the refraction index causes a change of
the reflected light strength from the recording medium. Therefore,
servo control of the recording position on the recording medium is
possible by detection of the change. Namely, the grooves 6 on the
recording medium function as guides for the servo control of the
recording position.
[0106] If the recording medium is disk shaped, and is mounted and
rotated on a rotating means such as an electric or ultrasonic
motor, for example, the grooves 6 are formed along the
circumferential direction of the medium. On the other hand, if the
recording medium is not rotated but is mounted on an linear moving
mechanism or cyclic vibrating mechanism such as a linear ultrasonic
motor or a laminated piezo device, the grooves 6 are formed along
the linear direction or the cyclic vibrating direction thereof.
Incidentally, the grooves 6 need not be consecutive and can be a
set of non-consecutive pit-shaped patterns.
[0107] [EMBODIMENT 5]
[0108] FIG. 8 shows two types of section structure of a
magneto-optical recording medium having a recording layer which
consists of multiple layers of garnet ferrite/spinel ferrite (or
rutile or hematite) and grooves for servo control on the surface
(hereunder, referred to as "Embodiment 5"). In the case shown in
FIG. 8(a), a multiple recording layer 5 consisting of a plurality
of garnet ferrite layers and a plurality of spinel ferrite (or
rutile or hematite) layers is layered on the quartz glass substrate
1, and it is covered with the metal reflective layer 4. On the
other hand, in the case shown in FIG. 8(b), the layer order of the
metal reflective layer 4 and the multipl recording layer 5 is
different from that of FIG. 8(a).
[0109] In the case shown in FIG. 8(a), a quartz glass substrat 1
having pre-formed grooves is employed. However, the grooves 6 can
be formed on the surface of the recording medium by employing a
metal reflective layer 4 having grooves with a predetermined width
and depth on its surface. In the same way as in Embodiment 4, the
grooves 6 can be formed along the circumferential, linear moving or
vibrating direction of the recording medium, and they need not be
consecutive.
[0110] [EMBODIMENT 6]
[0111] FIG. 9 shows two types of section structure of a
magneto-optical recording medium having a recording layer which
consists of two layers of garnet ferrite/spinel ferrite (or rutile
or hematite) and loads for servo control on its surface (hereunder,
referred to as "Embodiment 6"). In the case shown in FIG. 9(a), the
garnet ferrite layer 3 is layered on the spinel ferrite (or rutile
or hematite) layer 2 formed on the quartz glass substrate 1, and
the garnet ferrite layer 3 is covered with the metal reflective
layer 4. The loads 7 made of aluminum are attached to the surface
of the metal reflective layer 4. On the other hand, the case shown
in FIG. 9(b) is different from the case of FIG. 9(a) in that the
metal reflective layer 4 is formed on the quartz glass substrate 1
and loads 7 made from silicon oxide are attached directly to the
recording layer consisting of spinel ferrite (or rutile or
hematite) layer 2 and garnet ferrite layer 3. The material of the
loads 7 is not limited, therefore, a variety of metal, oxide and
dielectric materials can be employed.
[0112] As shown in the figure, in Embodiment 6, concavities and
convexities are formed on the surface of the recording medium by
attaching loads 7 having a predetermined size onto the surface, and
the effective refraction index on the surface can be changed. The
change of the refraction index causes a change of the reflection
rate of light from the recording medium. Therefore, servo control
of the recording position on the recording medium is possible by
detection of the change. Namely, the loads 7 on the recording
medium function as guides for the servo control of the recording
position.
[0113] If the magneto-optical recording medium is disk shaped, and
is mounted and rotated on a rotating means like an electric or
ultrasonic motor, for example, the loads 7 are attached along the
circumferential direction of the recording medium. On the other
hand, if the recording medium is not rotated but is mounted on a
linear moving mechanism or cyclic vibrating mechanism such as a
linear ultrasonic motor or a laminated piezo device, the loads 7
are attached along the linear direction or the cyclic vibrating
direction thereof. Incidentally, the loads 7 need not be
consecutive.
[0114] [EMBODIMENT 7]
[0115] FIG. 10 shows two types of section structure of a
magneto-optical recording medium having a recording layer which
consists of multiple layers of garnet ferrite/spinel ferrite (or
rutile or hematite) and loads for servo control on its surface
(hereunder, referred to as "Embodiment 7"). In the case shown in
FIG. 10(a), the metal reflective layer 4 is layered on the multiple
recording layer 5 which consists of a plurality of spinel ferrite
(or rutile or hematite) layers and a plurality of garnet ferrite
layers, and is formed on the quartz glass layer 1. The loads 7 made
from aluminum are attached onto the surface of the metal reflective
layer 4. On the other hand, the case shown in FIG. 10(b) is
different from that of FIG. 10(a) in that the multiple recording
layer 5 is layered on the metal reflective layer 4 formed on the
quartz glass substrate 1, and the loads 7 made from silicon oxide
are attached directly to the multiple recording layer 5.
[0116] In Embodiment 7, the material of the loads 7 is not limited,
and a variety of metal, oxide and dielectric materials can be
employed. The concavities and convexities formed by the loads 7 can
be used as guides for servo control. And, in the same way as in
Embodiment 6, the loads 7 may be attached along the
circumferential, linear moving or vibrating direction of the
recording medium, and they need not be consecutive.
[0117] [EMBODIMENT 8]
[0118] FIG. 11 shows two types of section structure B6 of
magneto-optical recording media (hereunder, they are referred to as
"Embodiment 8") which lack the metal reflective layer, and in which
loads 7 are directly attached to the surface of the recording layer
consisting of the garnet ferrite layer 3 and the spilel ferrite (or
rutile or hematite) layer 2, or of the multi-layered recording
layer 5 consisting of a plurality of garnet ferrite layers and
spinel ferrite(or rutile or hematite) layers.
[0119] It is apparent from the figure that the loads 7 of
appropriate size are directly attached onto the recording layer or
the multi-layered recording layer 5 with a predetermined space
between each other. The loads 7 in Embodiment 8 are made of a
ferromagnetic and anisotropic material such as CoCr alloy (50000e
of magnetic coercive force and 300 emu/cc of saturation
magnetization) which can be useful as a magnetic recording layer.
And, they also have the function of a metal reflective layer
because they are made from a metal material. Therefore, in
Embodiment 8, the formation of a metal reflective layer can be
omitted, maintaining the servo controlability of the recording
medium. Further, data recording by using not only a light beam but
also a magnetic field is possible. Furthermore, the loads 7 have no
substantial effect on the magnetic properties of the garnet ferrite
layer in the recording layer or the multi-layered recording layer 5
because the loads 7 are disposed on the surfaces of the recording
layer with a predetermined space.
[0120] [EMBODIMENT 9]
[0121] FIGS. 12(a) and (b) show section structures of
magneto-optical recording media (hereunder, they are referred to as
"Embodiment 9") which correspond to the types shown in the FIGS.
3(b) and 6(b), respectively, covered with a transparent layer. In
Embodiment 9, polycarbonate is used as the material of the
transparent layer 8, however, it can be made of other transparent
materials if necessary. A proper thickness of the transparent layer
8 is in the range of 100 nm to 2 mm.
[0122] As is apparent from the figure, compatibility with
conventional media surfaces can be obtained, and the distance
between the surface of the recording medium and a recording layer,
on which recording is done, in the medium can be extended by
providing the transparent layer 8 on the recording layer.
[0123] Therefore, the laser irradiated by the playback head and
focused on the recording layer is less likely to be affected by
dust that may be adhering to the surface of the recording medium or
by scratches that may be present thereon.
[0124] If the optical thickness of the transparent layer 8 is
sufficiently shorter than the wavelength of the light used for
magneto-optical recording, for example 700 nm or less, Embodiment 9
can be used as a magneto-optical recording medium for near-field
recording. This recording method utilizes evanescent light that
exists in the proximity of the surface of recording medium surface,
typically in the region of one wavelength distance from the surface
of the recording medium. On the other hand, if the optical
thickness of the transparent layer 8 is equivalent to or more than
the wavelength of the light, Embodiment 9 can be used as a
magneto-optical recording medium for conventional far field
magneto-optical systems such as those including a general
condensing lens.
[0125] [EMBODIMENT 10]
[0126] FIG. 13(a) and (b) show section structures of
magneto-optical recording media (hereunder, they are referred to as
"Embodiment 10") which correspond to the types shown in FIGS. 7(b)
and 8(b), respectively, further covered with a transparent layer.
According to Embodiment 10, it is possible to cause a change of the
effective refraction and reflection rates internally in the
recording medium because the recording layer having grooves for
servo control is covered with the transparent layer 8 as shown in
the figure. Accordingly, more accurate control can be performed
because external influence is excluded when detecting the change of
the reflection rate at an internal part of the recording
medium.
[0127] Besides, in Embodiment 10, the laser irradiated by the
playback head is less likely to be affected by dust that may be
adhering to the surface of the recording medium or by scratches
that may be present thereupon because the laser is focused on the
recording layer inside of the recording medium. The grooves on the
recording layer are formed along the circumferential, linear moving
or vibrating direction of the recording medium. Incidentally, the
grooves need not be consecutive.
[0128] [EMBODIMENT 11]
[0129] FIG. 14 shows section structures of magneto-optical
recording media (hereunder, they are referred to as "Embodiment
11") which correspond to Embodiment 10, but flattening is carried
out for transparent layer 8. In the case of FIG. 14(b), a quartz
glass substrate 1 having grooves on its surface is employed.
Flattening makes it possible, in addition to the effects of
Embodiment 10, for an optical pickup mechanism to fly in the
proximity of the surface of the recording medium rotating at high
speed in order to record and reproduce data. For example, access to
the proximity of the recording medium is performed by mounting the
optical pickup mechanism on an air-bearing slider head that is
generally used in conventional magnetic recording disk drives.
[0130] [EMBODIMENT 12]
[0131] FIGS. 15(a) and (b) show section structures of
magneto-optical recording media (hereunder, they are referred to as
"Embodiment 12") which correspond to the types shown in FIGS. 9(b)
and 10(b), respectively, further covered with a transparent layer.
Embodiment 12 has the same effects as Embodiment 11. Further, it is
the same as Embodiment 6 in that the loads 7 are attached along the
circumferential, linear moving or vibrating direction of the
recording medium, and they need not be consecutive.
[0132] [EMBODIMENT 13]
[0133] FIGS. 16(a) and (b) show section structures of
magneto-optical recording media (hereunder, they are referred to as
"Embodiment 13") which correspond to the types shown in FIGS. 12(a)
and (b), respectively, and the grooves 6 are formed on the surface
of the transparent layer 8. Thereby, servo control of recording
position is possible by detecting the change of the effective
refraction index on the transparent layer 8. And, compatibility
with conventional media surfaces can be obtained, and the distance
between the surface of the recording medium and the recording
layer, on which recording is done, can be extended by providing the
transparent layer 8 on the recording layer. This makes it less
likely for the laser irradiated by the playback head and focused on
the recording layer to be affected by dust that may be adhering to
the surface of the recording medium or by the scratches that may be
present thereon. The grooves 6 on the surface of the recording
layer 8 are formed along the circumferential, linear moving or
vibrating direction of the recording medium. Incidentally, the
grooves 6 need not be consecutive.
[0134] Hereafter, other magneto-optical recording media which have
different layer structures from those mentioned above will be
explained.
[0135] With regard to the magneto-optical recording media of
Embodiments 1-13, there is the concern that sufficient S/N ratio
may not be obtained because of increased noise that is derived from
the magnetization of a part of the recording layer other than track
parts, on which data are recorded, by annealing in the
manufacturing process.
[0136] Therefore, in the magneto-optical recording medium of the
present invention which is described hereafter, the recording layer
has a structure in which a garnet ferrite layer is present on the
surface of a spinel ferrite layer, a rutile type oxide layer or a
hematite layer, which are formed only along track parts on which
data are recorded.
[0137] FIG. 17 shows a cross section of one embodiment (hereunder,
referred to as "Embodiment 14") of a magneto-optical recording
medium of the present invention that has a recording layer in which
the spinel ferrite layers 2 are formed only along the track parts.
In Embodiment 14, the recording layer is constructed by forming the
garnet ferrite layer 3 so as to cover the spinel ferrite layers 2
formed along track parts which are parallel with a predetermined
space on the substrate 1 made of quartz glass. The garnet ferrite
layers 3a having magneto-optical property are present on the spinel
ferrite layers 2. On the other hand, the non-magnetic garnet
ferrite layers 3b are present between the spinel ferrite layers
2.
[0138] The manufacturing process of Embodiment 14 was as follows.
First, a spinel ferrite layer was formed as an underlayer on the
substrate 1 by a spattering method and heat treatment was carried
out. As the material of the spinel ferrite layer,
R.sub.x-yCo.sub.yFe.sub.3-zO.sub.4 (0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.x, R is at least one element of the rare earth
elements including Dy) can be used in the present invention. In
Embodiment 14, Mn.sub.0..sub.13Co.sub.0..sub.73Fe.sub.2..su-
b.14O.sub.4 was employed. In more detail, the spinel ferrite layer
was formed by RF spattering until the thickness of the layer
reached 100 nm in Embodiment 14. After that, it was heat-treated at
atmospheric pressure in an atmosphere of 20% oxygen and 80% of
nitrogen for 10 minutes at 400.degree. C.
[0139] Next, photoresist was applied onto the spinel ferrite layer
and the part of the spinel ferrite layer which did not form track
parts was removed by inverse spattering after exposing the
photoresist to light. After removing the photoresist layer from the
surface of the spinel ferrite layer 2 on the tracks, the garnet
ferrite layer 3, which has a large Faraday Effect, was formed so as
to cover the spinel ferrite layer 2 by RF spattering, and heat
treatment was carried out. As the material of the garnet
ferrite-layer 3, Bi.sub.xR.sub.3-x+uM.sub.yFe.sub.5-y+vO.su- b.12
(0.ltoreq.x.ltoreq.3, 0.ltoreq.y.ltoreq.5, -3.ltoreq.u.ltoreq.3,
-3.ltoreq.v.ltoreq.3, R is at least one rare earth element
including Dy, and M is a tervalent metal being interchangeable with
iron) can be used in the present invention. In Embodiment 14,
Bi.sub.2DyFe.sub.4GaO.sub.12 was employed. In more detail, the
garnet ferrite layer 3 was formed by RF spattering until the
thickness of the layer reached 350 nm in Embodiment 14. After that,
it was heat-treated at atmospheric pressure in an atmosphere of
100% oxygen for 10 minutes at 630.degree. C.
[0140] The formation of the garnet ferrite layer 3 by spattering or
the like so as to cover the spinel ferrite layer 2 formed only
along the track parts on the substrate 1 and the heat treatment
gave magneto-optical properties only to the garnet ferrite layers
3a on the spinel ferrite layers 2, and made the garnet ferrite
layers 3b between the spinel ferrite layers 2 non-magnetic, for the
reason described below.
[0141] According to the result of AFM surface observation, there
were no cracks on the surface of the garnet ferrite layer 3 of
Embodiment 14. Further, the surface roughness and the crystal
particle diameter of the garnet ferrite layer 3 were 3nm and 40 nm,
respectively, which indicated that the surface of the layer was
very flat. Namely, the morphology of the garnet ferrite layer 3 was
remarkably improved by the use of the spinel ferrite layers 2 as
underlayers of the garnet ferrite layer 3.
[0142] Incidentally, rutile type oxide layers or hematite layers
can also be employed instead of the spinel ferrite layers 2, and
the magneto-optical recording medium corresponding to the
embodiment 14 can be manufactured by using those layers and
conditions mentioned above.
[0143] By the way, as explained before, garnet ferrite-layers
formed by spattering and heat treatment are generally subject to
compression stress. However, spinel ferrite layers, rutile type
oxide layers and hematite layers, after the same manufacturing
process, are usually subject to tensile stress.
[0144] In Embodiment 14, therefore, the compressive stress applied
to the garnet ferrite layer 3 can be canceled by the tensile stress
applied to the spinel ferrite layer 2. Accordingly, a recording
layer having large square ratio (residual magnetization/saturation.
magnetization) can be easily obtained, and a magneto-optical
recording medium suitable for high density recording can be
produced.
[0145] In Embodiment 14, only the garnet ferrite layers 3a exhibit
a magnet-optical effect, but the garnet ferrite layers 3b between
the spinel ferrite layers 2, namely between the tracks, do not
exhibit a magnet-optical effect. Therefore, the noise of the
magneto-optical recording medium can be reduced considerably. For
example, it was found that noise was reduced by 3 dB in the case in
which the width of track was set to be half the pitch of the
tracks. Further, recording marks on the recording layer remain
stable if fluctuation of the power of the radiated light beam is
large, because the width of the recording mark on which data is
recorded is physically limited by the width of the track.
[0146] Incidentally, the same effects as mentioned above can be
obtained if rutile type oxide layers or hematite layers are
employed instead of the spinel ferrite layers 2.
[0147] It is preferable that the thickness of the spinel ferrite
layer 2 (or rutile type oxide layer, or hematite layer) and the
garnet ferrite layer 3 in Embodiment 14 are from 10-100 nm and from
40-400 nm, respectively. If the thickness of the spinel ferrite
layer 2 (or rutile type oxide layer, or hematite layer) is less
than 10 nm, it may be difficult to improve the morphology of the
garnet ferrite layer 3 adjacent to the layer. And, if the thickness
of the spinel ferrite layer 2 (or rutile type oxide layer, or
hematite layer) is more than 100 nm, coloration of the layer, which
will cause deterioration of the S/N ratio, may occur. On the other
hand, if the thickness of the garnet ferrite layer 3 is less than
40 nm, sufficient magnetic properties may not be obtained. And, if
the thickness of the garnet ferrite layer 3 is more than 400 nm,
cracks may occur on the surface.
[0148] In order to further improve the efficiency of recording and
reading, the garnet ferrite layer 3 may be coated preferably with a
metal reflective layer 4, which is made from a metal such as
aluminum, gold, chrome and alloy thereof, directly or via a
dielectric layer 10 as shown in FIGS. 18(a) and (b).
[0149] FIG. 19 shows the X-ray diffraction intensity of the
magneto-optical recording medium having a recording layer which
consists of a single layer of garnet ferrite and one having a
recording layer which consists of two layers of spinel ferrit and
garnet ferrite. This figure also shows the relation between the
X-ray diffraction intensity of the magneto-optical recording media
and the temperature of the heat treatment applied to the garnet
ferrite layer in the manufacturing process. The garnet ferrite
layer and/or the spinel ferrite layer in each magneto-optical
recording medium were formed under the same conditions as
Embodiment 14 except that the temperature of the heat treatment was
variously changed.
[0150] The X-ray diffraction intensity shown in FIG. 19 was
obtained by measuring the peak strength of the diffraction angle
(2.theta.=32.degree.) during the radiation of X-rays (K.alpha.)
onto the above magneto-optical recording media. This diffraction
angle can act as an indicator showing the crystal state of the
garnet ferrite layer.
[0151] It is apparent from FIG. 19 that a practical magneto-optical
effect appears if the temperature of the heat treatment is over
about 650.degree. C. in the case in which the recording layer
consists of a single layer of garnet ferrite. From the viewpoint of
crystallization of the above case, crystallization of the garnet
ferrite layer began at about 600.degree. C., and the
crystallization conditions that make magneto-optical recording
possible are obtained at about 650.degree. C. On the other hand, in
the case in which the recording layer consists of two layers of
garnet ferrite and spinel ferrite, a practical magneto-optical
effect already appears when the temperature of the heat treatment
exceeds about 570.degree. C. From the viewpoint of crystallization
of this case, crystallization of the garnet ferrite layer already
begins at about 500.degree. C., and the crystallization conditions
that make magneto-optical recording possible are obtained at about
570.degree. C.
[0152] It is obvious from these facts that the crystallization
conditions which exhibit sufficient magneto-optical properties for
a recording layer of a magneto-optical recording medium can be
obtained at a lower heat treatment temperature by a combination of
a garnet ferrite layer and a spinel ferrite layer. In the
magneto-optical recording media shown in FIGS. 17 and 18,
therefore, it is possible to provide magneto-optical properties
only to the garnet ferrite layers 3a on the spinel ferrite layers
2, and is possible to make the garnet ferrite layers 3b which are
not formed on the spinel ferrite layers 2 non-magnetic. In fact, in
the manufacturing process of Embodiment 14, the temperature of heat
treatment was set at 630.degree. C., whereby magneto-optical
properties were provided only to the garnet ferrite layers 3a on
the spinel ferrite layers 2.
[0153] Incidentally, it was found that temperatures over
700.degree. C. were not appropriate for heat treatment because the
peak strength did not increase even if the temperature was raised
over 700.degree. C, and deterioration of the magnetic properties
was sometimes observed at such temperatures. Therefore, heat
treatment for the track parts should be preferably executed at
temperatures of 500 to 700.degree. C. Further, it is more
preferable to execute heat treatment at temperatures of 600 to
630.degree. C. in order to provide a strong contrast in
magneto-optical properties between the garnet ferrite layer on the
track parts and that on other parts. Namely, if heat treatment is
carried out in, the range of 600 to 630.degree. C. in the
manufacturing process of Embodiment 14, only the track parts on
which the garnet ferrite layer 3 and the spinel ferrite layer 2 are
present can have practical magneto-optical properties, which is
apparent from FIG. 19. Incidentally, magneto-optical recording
media of the present invention which have the same structure of the
recording layer as Embodiment 14 provide more options for the
material of the substrate 1 because the temperature of heat
treatment can be lower than the case in which the recording layer
consists of a single layer of garnet ferrite.
[0154] The measurement of the magnet properties of the two
magneto-optical recording media used in the analysis in FIG. 19
revealed that the magneto-optical recording medium which had a
recording layer consisting of a single layer of garnet ferrite had
12000e of magnetic coercive force, 13 emu/cc of saturation
magnetization and 10 emu/cc of residual magnetization. Accordingly,
this medium had sufficient square ratio for a practical use since
it was about 0.8 (10/13). However, problems such as increased noise
during high density recording can be expected since the magnetic
coercive force was small, i.e., 12000e (for a practical use, a
magnetic-coercive force of at least 20000e is said to be
ideal).
[0155] Unlike to the above medium, the medium which had a recording
layer in which a spinel ferrite layer and a garnet ferrite layer
were piled together had 20000e of magnetic coercive force, 13
emu/cc of saturation magnetization and 10 emu/cc of residual
magnetization. Accordingly, this medium had sufficient magnetic
properties for a practical use in terms of both the magnetic
coercive force and the square ratio which was about 0.8 (10/13). In
addition, the S/N ratio was improved by 3 dB over the
magneto-optical recording medium having a recording layer
consisting of a single layer of garnet ferrite. From these data, it
was found that the, magneto-optical properties of the
magneto-optical recording medium was remarkably improved by
constructing a multi-layered structure in which a spinel ferrite
layer and a garnet ferrite layer were piled together only on the
track parts of the recording layer.
[0156] In Embodiment 14, the garnet ferrite layer 3 was formed
after the track parts were formed by the spinel ferrite layer 2 on
the substrate 1. However, it is possible in the present invention
to first form the garnet ferrite layer 3 directly on the substrate
1, and to further form the spinel ferrite layer 2 along the track
parts on the garnet ferrite layer 3. Incidentally, in the present
invention, unrewritable data may be recorded in advance by
formation of recording marks in the track parts by patterning in
accordance with predetermined data.
[0157] Further, the recording layer of the present invention can be
constituted by a plurality of garnet ferrite layers and a plurality
of spinel ferrite layers (or rutile-type oxide layers or hematite
layers), while the recording layer in Embodiment 11 consists of one
spinel ferrite layer 2 and one garnet ferrite layer 3. In such a
case, it is preferable that the thickness of the recording layer is
from 40 to 1000 nm because sufficient magnetic properties cannot be
obtained if it is less than 40 nm, and the transparency of the
recording layer becomes worse if it is more than 1000 nm.
[0158] Furthermore, a transparent layer may be formed on the garnet
ferrite layer 3 of Embodiment 14. In the same way as in Embodiment
9, polycarbonate can be used as the material of the transparent
layer, the thickness of the transparent layer is preferably in the
range of 100 nm to 2 mm, compatibility with a conventional medium
surface can be obtained and a light beam focused on the recording
layer is less likely to be affected by dust that may be adhering to
the surface of the recording medium or by scratches that may be
present thereon. Incidentally, grooves for servo control of the
recording position may be formed on the surface of the transparent
layer.
[0159] Incidentally, the recording layer of the magneto-optical
recording medium of the present invention can contain other layers
made from materials other than garnet, ferrit, spinel ferrite,
rutile type oxide and hematite for adaptation to various purposes,
while the recording layers of Embodiments 1 to 14 consist of garnet
ferrite, spinel ferrite, rutile type oxide and/or hematite.
[0160] Next, the magneto-optical data recording and playback device
of the present invention, which is preferable for recording and
playing back data for the magneto-optical recording media of the
present invention which were mentioned above, will be
described.
[0161] FIG. 20 is a graph showing the dependence of the Faraday
rotation angle of magneto-optical recording media having various
recording layers on the wavelength (.lambda.) of light. The solid
line (1) in FIG. 20 shows the dependence of the Faraday rotation
angle of a magneto-optical recording medium having a recording
layer consisting of a single layer of BiDyGalG, which is a kind of
garnet ferrite, on the wavelength of light under a magnetic field
of 10 kOe. The alternating long and short dash of line (2) of the
same Figure shows the dependence of the Faraday rotation angle of a
magneto-optical recording medium having a recording layer
consisting of a single layer of
Mn.sub.0.13Co.sub.0.73Fe.sub.2.14O.sub.4, which is a kind of spinel
ferrite, on the wavelength of light under a magnetic field of 10
kOe. And, the dotted line (3) shows the dependence of the Faraday
rotation angle of the magnet-recording medium shown in FIG. 3(a),
which is "Embodiment 1" having a recording layer of a garnet
ferrite layer and a spinel ferrite layer, on the wavelength of
light under the same magnetic field. It was found that the
magneto-optical properties of the magneto-optical recording medium
of Embodiment 1 seemed to be dependent on the garnet ferrite
constituting the recording layer, and that a very large playback
signal would be gained from the magneto-optical recording medium of
Embodiment 1 in the range of visible light.
[0162] Next, the dependence of the absorption coefficient (.alpha.)
of a BiDyGalG thin layer on the wavelength (.lambda.) of light is
shown in FIG. 21. The amorphous metal used in a conventional
magnetic-optical recording medium such as TbFeCo does not show any
change of absorption character over the range of visible right.
However, it was found that the shorter the wavelength of light, the
bigger the light absorption coefficient of the BiDyGalG thin layer
became, from FIG. 21. Namely, light absorption increased for
wavelengths shorter than near 500 nm but decreased for wavelength
longer than near 500 nm. For example, it is obvious from FIG. 21
that the absorption coefficient for light with a wavelength of 410
nm is ten times as high as that for light with a wavelength of 630
nm. The magneto-optical recording medium of Embodiment 1 has the
same dependence of absorption coefficient on wavelength as shown in
FIG. 21, because the magneto-optical property of the medium of
Embodiment 1 is mainly dependent on the garnet ferrite thin layer 3
in the recording layer.
[0163] In the magneto-optical recording and playback device of the
present invention, the wavelength of light is different for
recording and for reading in order to make maximum use of the light
absorption properties of the magneto-optical recording medium to be
used for recording and reading. If a magneto-optical recording
medium having a recording layer containing a layer which has the
property shown in FIG. 21 is used for recording and reading data,
for example, a light beam with a short wavelength, which is easily
absorbed, will be radiated on the medium when recording. On the
other hand, in the above case, a light beam with a long wavelength,
which is less strongly absorbed than the above light, will be
radiated onto the medium when reading. Thereby, it will be possible
to heat the recording layer efficiently when recording, and to
restrain undesirable heating of the recording layer during
playback.
[0164] Further, in the above case, the power of the light beam with
short wavelength for recording can be reduced compared to a
conventional device, because the light beam for recording is
efficiently absorbed by the recording layer. On the other hand, the
power of the light beam for reading will be increased compared to a
conventional device, because a light beam with a long wavelength is
scarcely absorbed and well reflected. Therefore, it will be
possible to increase the C(career)/N(noise) ratio during
playback.
[0165] FIG. 22 shows an optical system outline of one embodiment
(hereunder, referred to as "Embodiment 15") of a magneto-optical
recording and playback device of the present invention. Hereunder,
the recording and reading method of Embodiment 15 will be explained
referring to FIG. 22.
[0166] In Embodiment 15, the disk-shaped magneto-optical recording
medium 11 is supported on a rotator, not shown in the figure, to be
rotated. The light beam emitted from the light source for recording
21 is focused on the magneto-optical recording medium 11 through
the first collimator lens 22, the first beam splitter 41 and the
objective lens 44 in sequence when data are recorded on the
magneto-optical recording medium 11. The micro area on the
recording layer of the magneto-optical recording medium 11, to
which the light beam is radiated, is heated by absorption of the
light beam, whereby, data are recorded.
[0167] On the other hand, when data are read from the
magneto-optical recording medium 11, the light beam emitted from
the light source for reading 23 is focused on the magneto-optical
recording medium 11 through the second collimator lens 22, second
beam splitter 42 and the objective lens 44 in sequence. After that,
most part of the light beam is reflected off the magneto-optical
recording medium 11. The reflected light beam is focused on the
light detector 31 through the-objective lens 44, the .lambda./4
plate 61 and the focusing lens 32 in sequence. At the light
detector 31, recording signals are detected. In Embodiment 15, two
semiconductor lasers are used as the light source for recording 21
and for reading 23. However, other kinds of laser can be used as
the light source as long as they do not interfere with the action
of the recording and playback device.
[0168] As the magneto-optical recording medium 11 in embodiment 15,
any kind of medium can be used. Preferably, a magneto-optical
recording medium having a garnet ferrite layer is used. More
preferably, the magneto-optical recording media of Embodiments
1-14, in which one or more garnet ferrite layers and one or more
spinel ferrite layers (or rutile-type oxide layers or hematite
layers) are layered together by spattering or CVD method are
used.
[0169] When data are recorded or read from the magneto-optical
recording medium 11 by use of the magneto-optical recording and
playback device of Embodiment 15, the wavelength of the light beam
emitted from the light source for recording 21 is set to be shorter
than a certain standard value and the wavelength of the light beam
emitted from the light source for reading 23 is set to be longer
than the standard value. The standard value is appropriately
determined according to the properties of the recording layer of
the magneto-optical recording medium. If the magneto-optical
recording medium having the properties shown in FIG. 21 is used,
for example, 500 nm will be employed as the standard value.
[0170] Therefore, if the medium having the light absorption
property shown in FIG. 21 is used as the magnetic-optical recording
medium 11 in Embodiment 15, it is preferable to adopt an argon
laser which can emit a light beam with a wavelength of around 480
nm as the light source for recording 21 and to adopt an ordinary
red semiconductor laser which can emit a light beam with a
wavelength of 500 to 700 nm as the light source for reading 23.
Data recording using a light beam with a wavelength of around 480
nm makes efficient recording possible because the absorption rate
of the recording layer increases. On the other hand, data reading
using a light beam with a wavelength of 630 nm, for example, makes
efficient receiving of reflections possible because the absorption
rate of the recording layer for the wavelength becomes relatively
small. Accordingly, sufficient reflected light can be received
without radiation by an excessively powerful light beam.
[0171] FIG. 23 shows an optical system outline of another
embodiment (hereunder, referred to as "Embodiment 16") of the
magneto-optical recording and playback device of the present
invention. Embodiment 16 is different from Embodiment 15 with
regard to the point of the wavelength conversion element 62
disposed on the pass between the second collimator lens 24 and the
second beam splitter 42. In Embodiment 16, the same type of light
sources can be used as the light source for recording 21 and for
reading 23 because the wavelength conversion element 62 is set
across the pass of light beam for playback.
[0172] In other words, if light sources which emit the light with
the same wavelength are used as the light sources for recording 21
and reading 23, the wavelength of the light beam for reading can be
converted to a longer one which is scarcely absorbed by the
magneto-optical recording medium 11 by use of the wavelength
conversion element 62. Therefore, consumption of energy for
playback can be reduced because it is not necessary to radiate a
high power light beam onto the magneto-optical recording medium 11,
which also enables to extend the life of the medium. Incidentally,
the wavelength of the light beam for recording may be converted
into a shorter one by disposing the wavelength conversion element
62 on the pass of the light beam for recording.
[0173] FIG. 24 shows an optical system outline of another
embodiment (hereunder, referred to as "Embodiment 17") of the
magneto-optical recording and playback device of the present
invention. In Embodiment 17, the light beams for recording and for
reading, which have different wavelengths, are provided only by the
common light source 25.
[0174] In Embodiment 17, a titanium sapphire laser is employed as
the common light source 25 for two light beams for recording and
reading data. The light beam emitted from the common light source
25 is guided into the beam splitter 43 via the first collimator
lens 22 and is split into the two beams of transmitted and
reflected lights, according to the direction of its polarization.
The beam of transmitted light is focused on the recording layer of
the magneto-optical recording medium 11 through the first half
mirror 46 and the objective lens 44 in sequence, and is used for
recording data.
[0175] On the other hand, the beam of reflected light is guided
into the non-linear optical element 63 and its wavelength is
doubled. After that, the beam of reflected light is focused on the
recording layer of the magneto-optical recording medium 11 via the
same pass of the beam of transmitted light after passing through
the reflection mirror 45, band pass filter 64 and second half
mirror 47. The light reflected on the recording layer is guided
into the light detector 31 through the same pass of Embodiment 15.
Accordingly, the production cost can be reduced as compared to the
case in which two separate light sources are used for recording and
reading each.
[0176] In Embodiment 17, any other conversion means instead of the
non-linear optical element 63 can be used if it is capable of
converting the wavelength of a part of light beam emitted from the
common light source 25. Incidentally, it is possible to shift the
wavelength of a part of light beam emitted from the common light
source 25 to a shorter wavelength by another non-linear optical
element or the like for recording, and to use the other part of the
light beam for reading.
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