Note: Descriptions are shown in the official language in which they were submitted.
12987~4
1 72932-6
FIELD OF THE INVENTION
This invention relates to magnetooptical recording
medium having excellent resistance to oxidation and excellent
magnetooptical recording characteristics and more partlcularly to
magnetooptical recording films in the magnetooptical recording
medium having an easy axis of magnetization perpendicular to the
film as well as excellent resistance to oxidation and excellent
magnetooptical recording characteristics.
BACKGROUND OF THE INVENTION
It is known that magnetooptical recording films
comprising transition metals such as iron, cobalt, etc. and rare
earth elements such as terbium (Tb), gadolinium (Gd), etc., have
and easy axis of magnetization perpendicular to the film and are
capable of forming a small inverse magnetic domain with
magnetization antiparallel to the magnetization of the film. By
corresponding the existence or nonexistence of this inverse
magnetic domain to "1" or "0", it becomes possible to record a
digital slgnal on such magnetooptical recording films as mentioned
above.
As magnetooptical recording films composed of such
.~
12987~
-- 2 --
transition metals and rare earth elements as mentioned
above, there are disclosed those of Tb-Fe system
containing 15-30 atom% of Tb, for example, in Japanese
Patent Publication No. 20691/1982. There are also used
magnetooptical recording films of Tb-Co system to which a
third component metal ha~ been added. Furthermore,
magnetooptical recording films of Tb-Fe system, Tb-Fe-Co
system and the like are known as well.
Though these magnetooptical recording films have
excellent recording reproducing characteristics, they
still involved such a serious problem from a practical
standpoint that they are subject to oxidation in the
course of ordinary use thereof and their characteristics
come to change with the lapse of time.
The mechanism of oxidation deterioration of
magnetooptical recording films containing such transition
metals and rare earth elements as mentioned above is
discussed, for example, in Journal of Applied Magnetism
Society of Japan, Vol.9, No.2, pp.93-96, and this paper
reports that this mechanism of oxidative deterioration may
be claqsified into three types as mentioned below.
a) Hole corrosion
By hole corrosion is meant the occurrence of
pinholes in the magnetooptical recording film. This
corrosion proceeds mainly under the circumstances of high
12,~1 37Q~
humidity, and it markedly proceeds, for example, in the
recording films of such system as Tb-Fe, Tb-Co or the
like.
b) Surface oxidation
Surface oxide layers are formed on magnetooptical
recording films, whereby Kerr-rotation angle~ k of the
films changes with time and eventually comes to decrease.
c) Selective oxidation of rare earth element
Rare earth elements present in magnetooptical
recording films are selectively oxidized, thereby coercive
force Hc of the films comes to largely change with time.
Various attempts have heretofore been made to
inhibit such oxidative deterioration of magnetooptical
recording films as mentioned above. For instance, there
is propo~ed a procedure in which a magnetooptical
recording film is designed to have a three-layer structure
wherein the film is sandwitched between anti-oxidizing
protective layers such as those of Si3N4, SiO, SiO2, AlN,
etc. The anti-oxidizing protective layers as proposed
above, however, involved such problems that they are
relatively expensive and, at the same time, they require
much time and labor to be formed on magnetooptical
recording films, and that a sufficient inhibition of
oxidative deterioration of the recording films is not
always expected even when such anti-oxidizing protective
ïZ91!3';~ 4
layers are formed on said recording films.
Furthermore, various attempts are being made to
improve resistance to oxidation of magnetooptical
recording films by incorporating a third component metal
into the recording films.
For instance, Journal of Applied Magnetism Society
of Japan cited above discloses an attempt to improve
resistance to oxidation of magnetooptical recording films
of Tb-Fe or Tb-Co system by incorporation into the films
of such third component metal as Co, Ni, Pt, Al, Cr, Ti
and Pd in an amount of up to 3.5 atom%. In connection
with the attempt, the said Journal reports that the
incorporation of small amounts of Co, Ni and Pt into Tb-Fe
or Tb-Co ls effective in inhibiting the surface oxidation
and hole corrosion of the resulting magnetooptical
recording film but has no effect on inhibition of the
selective oxidation of Tb contained as a rare earth
element in this magnetooptical recording film. This
disclo~ure means that when small amounts of Co, Ni and Pt
are added to Tb-Fe or Tb-Co, Tb present in the resulting
magnetooptical recording film is selectively oxidized, and
coercive force Hc of the film largely changes. Thus, even
when small amounts up to 3.5 atom% of Co, Ni and Pt are
added to Tb-Fe or Tb-Co, no sufficient improvement in
resistance to oxidation of the resulting magnetooptical
~Z~t~704
-- 5 --
recording film i5 made.
With the view of improving resistance to oxidation
of magnetooptical recording films, a teaching on the
magnetooptical recording films which are obtained by
adding Pt, Al, Cr and Ti in an amount up to 10 atom~ to Tb-
Fe or Tb-Fe-Co is disclosed in page 209 of the Proceedings
of The Nineth Conference of Applied Magnetism Society of
Japan (November 1985). Even when Pt, Al, Cr and Ti in an
amount up to 10 atom% are added to Tb-Fe or Tb-Fe-Co,
however, inhibition of selective oxidation of Tb present
in the resulting magnetooptical recording films is not
sufficient, though the surface oxidation and hole
corrosion can be inhibited to a fairly effective extent.
Thus, there was still left such a problem that coercive
force Hc of the resultant magnetooptical recording films
will largely change with time, and eventually the coercive
force Hc will largely decreases.
Japanese Patent L-0-P Publn. No. 255546/1986
di~close~ magnetooptical recording films which have been
improved in resistance to oxidation by adding such noble
metal elements as Pt, Au, Ag, Ru, Rh, Pd, Os and Ir within
such range that ~err-rotation angle necessary for
regeneration is to magnetooptical recording films
comprising rare earth elements and transition metals,
obtained.
12~8 7(:~4
Furthermore, Japanese Patent L-O-P Publn. No.
7806/1983 discloses magnetooptical recording films
comprising polycrystalline thin films having the
composition of PtCo in which Pt is contained in an amount
of 10-30 atom%.
However, the above-mentioned polycrystalline thin
films having this composition of PtCo involved such
problems that said polycrystalline thin films as formed
require heat treatment such as annealing because that are
polycrystalline, that particle-particle boundaries among
the crystals sometimes appear as noise signals, and that
these polycrystalline thin films are high in Curie point.
In the manner as described above, magnetooptical
recording films comprising Tb-Fe or Tb-Co and further
incorporated with such third metal components as Co, Ni,
Pt, Al, Cr, Ti and Pd have heretofore been known.
However, these known magnetooptical recording films
involve at least one of such problems that they are not
suf f icient in resistance to oxidation, small in C/N ratio
and high in noise level, and that no high C/N ratio can be
obtained unless a large bias magnetic f ield is applied
(i.e. they are poor in bias magnetic f ield dependability).
With the purpose of providing magnetooptical
recording films which are excellent in resistance to
oxidation, usable over a long period of time, high in C/N
lZ91~37~4
7 72932-6
ratio and low in noise level and, moreover, excellent in such bias
magnetic field dependability that a sufficiently high C/N ratio is
obtained even in a small bias field, the present inventors
prosecuted extensive researches and eventually have accomplished
the present invention on the basis of their finding that
magnetooptical recording films which contain specific amounts of
Pt and/or Pd, specific amounts of Fe and/or Co and also rare earth
elements such as Tb will exhibit excellent magnetooptical
recording characteristics.
DISCLOSURE OF THE INVENTION
The magnetooptical recording film of the magnetooptical
recording medium of the present invention contains:
(i) Pt and/or Pd,
(ii) rare earth element (RE), and
(iii) Fe and/or Co, the Pt and/or Pd being present in an
amount of more than 10 atom% but not more than 30 atom%, the Fe
and/or Co being present in an amount of at least 40 atom% but not
more than 70 atom%, and having a Co/(Fe + Co) ratio [atomic ratio]
of 0-0.3. The magnetic recording medium, which may be in a disc
form, includes a substrate on which the magnetooptical recording
film is formed.
The magnetooptical recording medium is excellent in
resistance to oxidation, usable over a long period of time, large
in C/N ratio and low in noise level and, moreover, excellent in
bias magnetic field
lZ9~371;?~
-- 8 --
dependability.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a graph showing the relationship between
the content (atom%) of Pt and/or Pd in a magnetooptical
recording film and resistance to oxidation (~C/N ratio).
Fig. 2 is a graph showing the relationship between bias
magnetic field (Oe) and C/N ratio of a magnetooptical
recording film. Fig. 3 is a graph showing the
relationship between the content (atom%) of Pt and/or Pd
and mlnimum bias magnetic field (Hsat. (Oe) ) in a
mage~ntooptical recording film. Fig. 4 is a graph showing
the relationship between the CO/(Fe + Co) ràtio tatomic
ratio~ and noise level (dBm) in a magnetooptical recording
film containing Pt. Fig. 5 is a graph showing the
relationship between the Co/(Fe + Co) ratio ~atomic ratio]
and noise level (dBm) in a magnetooptical recording film
containing Pd. Fig. 6 is a graph showing the relationship
between the Co/(Fe + Co) ratio [atomic ratio] and erasion
deterioration (~C/N ratio (dB) ) in a magnetooptical
recording film.
All of the magnetooptical discs in Figs. 1-6 is a
single layer of 1000 A.
BEST MODE OF P~ACTICING THE INVENTION
129~3'7C~4
g
The magnetooptical recording films of the present
invention are illustrated below in detail.
The present magnetooptical recording films contain
as essential components the following components
(i) Pt and/or Pd,
(ii) a rare earth element, and
(iii) Fe and/or Co.
Furthermore, it is preferable that the present
magnetooptical recording films are amorphous when measured
by the wide-angle X-ray diffraction method.
(i) Pt and/or Pd
The present magnetooptical recording films contain
Pt or Pd, or both, and the amount of Pt and/or Pd
contained in the magnetooptical recording films is more
than 10 atom% but not more than 30 atom%, preferably more
than 10 atom% but less than 20 atom%, and more preferably
at least 11 atom% but not more than 19 atom%.
The presence in the magnetooptical recording film
of Pt and/or Pd in an amount exceeding 10 atom% brings
about such advantages that resistance to oxidation of said
recording film becomes excellent, no hole corrosion occurs
and C/N ratio becomes low in the recording film.
Fig. 1 shows the relationship between the content
of Pt and/or Pd in the magnetooptical recording film and
C/N ratio when said recording film is retained for 1000
lZ9~3704
-- 10 --
hours under the circumstances of 85% RH and 80 C.
It is thus understood from Fig. 1 that when the
amount in the magnetooptical recording film of Pt and~or
Pd exceeds 10 atom%, resistance of said recording film
improves, no hole corrosion occurs even after a long-term
use and also C/N ratio will not deteriorate.
For instance, the magnetooptical recording film of
the present invention having the composition represented
13 28 50Cog or Pd12Tb28Fe53Co~ will not change in
C/N ratio at all even when it is retained under the
circumstance of 85% RH and 80C. In contrast thereto, a
magnetooptical recording film having the composition
represented by Tb25Fe68Co7 but containing no Pt or Pd will
greatly decrease in C/N ratio when it is retained for 1000
hours under the circumstances of 85% RH and 80C.
By incorporation into a magnetooptical recording
film of Pt and/or Pd in an amount within the range as
specified above, a sufficiently high C/N ratio can be
obtained even by a small bias magnetic field when an
information is recorded on the magnetooptical recording
film or when the information recorded is read out
therefrom. If a sufficiently high C/N ratio is obtained
by a small bias magnetic field, a magnet for producing a
bias magnetic field can be made small in size and,
moreover, heat generation from the magnet can be inhibited
1298~04
-- 1 1 --
and hence simplification of a driving devlce for an
optical disc bearing the magnetooptical recording film
thereon is made possible. Moreover, because a
sufficiently large C/N ratio is obtained by a small bias
magnetic field, it becomes easy to design a magnet for
magnetic field modulation recording capable of overwrite.
Fig. 2 shows the relationship between bias
magnetic field and C/N ratio (dB) of the present
magnetooptical recording film having the composition
represented by Ptl3Tb28Fe50Cog and of a magnetooptical
recording film having the composition represented by
b25 68 7-
It is understood from Fig. 2 that in theconventionally known magnetooptical recording film
represented by Tb25Fe68Cog, the C/N ratio is not saturated
unless a bias magnetic field of more than 250 Oe is
applied, whereas in the present magnetooptical recording
film represented by Ptl3Tb28Fe50Cog, recording can be
pçrformed even by a small bias magnetic field and the C/N
ratio is saturated at a level of 120 Oe or more. In the
following examples and comparative examples, a Hsat value
of the minimum bia~ magnetic field of each magnetooptical
recording film is shown in Table 1, at which the C/N ratio
is saturated. The smaller is this Hsat value, it follows
that the C/N ratio is saturated by a small bias magnetic
:lZ98704
- 12 -
field.
Fig. 3 shows the relationship between the content
of Pt and/or Pd and the minimum bias magnetic field (Hsat,
(Oe) ) of a magnetooptical recording film of PtTbFeCo
system and of a magnetooptical recording film of PdTbFeCo
system.
It is understood from Fig. 3 that the minimum bias
magnetic field, Hsat, becomes sufficiently small when the
content of Pt and/or Pd exceeds 10 atom%.
(ii) Rare earth element (RE)
In the present magnetooptical recording films,
rare earth element (RE) is contained, and usable as the
rare earth element is Nd, Sm, Pr, Ce, Eu, Gd, Tb, Dy or
Ho.
Of the rare earth elements illustrated above,
preferably usable are Nd, Pr, Gd, Tb and Dy, and
particularly preferred is Tb. The rare earth elements may
be used in combination of two or more, and in this case
the combination preferably contains at least 50 atom% of
Tb.
From the standpolnt of obtaining an optical
magneti~m having an easy axis of magnetization
perpendicular to the film, it is preferable that this rare
earth element present in a magnetooptical recording film
in such an amount as 0.25 < x ~ 0.45, preferably
~zg8704
- 13 -
0.3 < x < 0.4, wherein x represents RE/(RE + Fe + Co)
[atomic ratio].
(iii) Fe and/or Co
In the present magnetooptical recording films, Fe
or Co or both are contained, and Fe and/or Co is
r_-.
pre~e~Ably.present in the magnetooptical recording film in
an amount of at least 40 atom% but not more than
70 atom%, preferably at least 40 atom% but less than
60 atom%, and more preferably at least 40 atom% but not
more than 59 atom%.
Further, Fe and/or Co present in the
magnetooptical recording film is proforably in such an
amount that Co/(Fe + Co) ratio [atomic ratio~ is from 0 to
0.3, preferably from 0 to 0.2, and more preferably from
0.01 to 0.2.
When the amount of Fe and/or Co is in the range of
at least 40 atom% but not more than 70 atom%, there is
such an advantage that a magnetooptical recording film
which is excellent in resistance to oxidation and has an
easy axis of magnetization perpendicular to the film is
obtained.
In this connection, when Co is incorporated into a
magnetooptical recording film, there are observed such
phenomena as (a) Curie point of the magnetooptical
recording film increases and (b) Kerr-rotation angle ~ k)
lZ987(;~4
becomes large. As the result, recording sensitivity of a
magnetooptical recording film can be ad~usted by the
amount of Co to be incorporated and, moreover, a carrier
level of reproducing signal increases by incorporating
Co. According to the study conducted by the present
inventors, however, it has been found that when Co is
incorporated excessively into a magnetooptical recording
film, a noise level of reproducing signal rises and the
C/N ratio decreases. Accordingly, in the present
magnetooptical recording film, Co/(Fe + Co) ratio [atomic
ratio] is from O to 0.3, preferably from O to 0.2, and
more preferably from 0.01 to 0.2.
Fig. 4 shows the relationship between Co/(Co ~ Fe)
ratio ~atomic ratio] and noise level (dBm) in a
magnetooptical recording film of PtTbFeCo system, and Fig.
5 show~ the relationship between Co/(Co + Fe) ratio
[atomic ratio~ and noise level (dBm) in a magnetooptical
recording film of PdTbFeCo system.
Concretely, in the present magnetooptical
recording film (Co/(Fe + Co) ratio ~atomic ratio]: 0.15)
having the composition represented by Pt13Tb28Fe50Cog, the
noise level is -56 dBm, whereas in a magnetooptical
recording film (Co/(Fe + Co) ratio [atomic ratio]: 0.39)
having the composition represented by Pt13Tb28Fe36Co23,
the noise level is as large as -50 dBm. Furthermore, in
~zg8704
- 15 -
the present magnetooptical recording film (Co/(Fe + Co)
ratio [atomic ratio]: 0.12) having the composition
represented by Pdl4Tb2~Fe52Co~, the noise level is -56
dBm, whereas in a magnetooptical recording film (Co/(Fe
Co) ratio [atomic ratio]: 0.31) having the composition
y dl4Tb27Fe41Col8, the noise level is as
large as -51 dBm.
Furthermore, when the Co/(Fe + Co) ratio [atomic
ratio] exceeds 0.3, erasion deterioration is observed in
the resulting magnetooptical recording film. That is, the
Co/(Fe + Co) ratio [atomic ratio] in a magnetooptical
recording film which is larger than 0.3 or more strictly
in excess of 0.2 is not preferable since the
maynetooptical recording film sometimes subject to change
of property when an energy with which said recording film
is irradiated is made large in order to erase the
information once recorded in the magnetooptical recording
film.
Fig. 6 shows the relationship between Co/(Fe + Co)
ratio [atomic ratio] and erasion deterioration (~C/N ratio
(dB) ) in a magnetooptical recording film.
In the concrete, no change in film property occurs
at all even when the present magnetooptical recording film
having the composition of Ptl3Tb28Fe50Cog (Co/(Fe + Co)
ratio [atomic ratio] : 0.155) is irradiated with an
~29B'7~4
- 16 -
increased energy at the time of erasing the information
once recorded in the recording film, and even a new
information having the same value, as a value of C/N
ratio, as that prior to erasion can also be recorded on
the erased recording film. In contrast thereto, in a
magnetooptical recording film having the composition of
13 28 40 19 or Pt13Tb28Fe36Co23 and containing Co in
an amount of more than 0.3 in terms of Co/(Fe + Co) ratio
[atomic ratio], change in film property occurs and
magnetooptical characterisitcs decrease, and the C/N ratio
of a newly recorded information will also becomes smaller
than that of the information recorded prior to erasion
when the information once recorded is erased with an
energy larger than necessary.
Furthermore, no change in film property will occur
even when recording and erasing the information are
performed repeatedly. For instance, no decrease ln C/N
ratio is observed even when the recording and erasing
operations are performed 100,000 times in the present
magnetooptical recording film having the composition of
Ptl3Tb28Fesoco9
In the present invention, it is also possible to
improve Curie temperature, compensation temperature,
coercive force Hc or Kerr-rotation angle 9k, or cheapen
the cost of production by incorporating various elements
:12:~8'~
into the magnetooptical recording films. These elements
for the purpose intended may be used, for example, in the
proportion of less than 10 atom% based on the total number
of elements constituting the recording film.
Examples of useful elements for this purpose other
than those constituents of the magnetooptical recording
film include such elements as mentioned below.
(I) 3d transition elements other than Fe and Co
Concretely, such transition elements include Sc,
Ti, V, Cr, Mn, Ni, Cu and Zn.
Of these elements exemplified above, preferably
used are Ti, Ni, Cu and Zn.
(II) 4d transition elements other than Pd
Concretely, such transition elements include Y,
Zr, Nb, Mo, Tc, Ru, Rh, Ag and Cd.
Of these transition elements exemplified above,
preerably used are Zr and Nb.
(III) 5d transition elements other than Pt
Concretely, such transition elements include Hf,
Ta, W, Re, Os, Ir, Au and Hg.
Of these transition elements, preferably used is
Ta
(IV) Group III B elements
Concretely, B, Al, Ga, In, and Tl are used.
Of these elements, preferably used are B, Al and
t,
lZ987C~4
- 18 -
Ga.
(V) Group IV B elements
Concretely, C, Si, Ge, Sn and Pb are used.
Of these elements, preferably used are Si, Ge, Sn
and Pb.
(VI) Group V B elements
Concretely, N, P, As, Sb and Bi are used.
Of these elements, preferably used is Sb.
(VII) Group VI B elements
Concretely, S, Se, Te and Po are used.
Of these elements, preferably used is Te.
The composition of the recording film is
determined by ICP emission spectroscopic analysis.
A process for preparing the magnetooptical
recording films of the present invention is illustrated
hereinafter.
The present magnetooptical recording films may be
prepared by depositing a magnetooptical recording film
having the predetermined composition on a substrate,
wherein the substrate i5 maintained at about room
temperature, and a composite target with chips of elements
constituting the present magnetooptical recording film in
the predetermined proportions thereon or an alloy target
having the predetermined composition is deposited by the
sputtering method or electron beam evaporation method on
C
lZ987~14
_ 19 _
said substrate (this substrate may be fixed, or may
rotates on its axis or may rotates on its axis while
revolving).
The present magnetooptical recording films as
illustrated above may be formed at room temperature, and
the films as formed are not always in need of such heat
treatment as annealing or the like that is usually
required for allowing the films to have a magnetic easy
axis perpendicular to the film.
If necessary, in this connection, an amorphous
alloy thin film can also be formed on a substrate while
heating the substrate to 50-600C, or while cooling the
substrate to -50C.
At the time of sputtering, moreover, biasing a
substrate i5 also possible so that the substrate comes to
have a negative potential. By doing so, ions of an inert
gas such as argon accelerated in the electric field will
hit not only target substances but also a magnetooptical
recording film being formed and consequently a
magnetooptical recording film having excellent
characteristics is sometimes obtained.
The alloy targets which are used for the
preparation of the magnetooptical recording films of the
present invention contain (i) Pt and/or Pd, (ii) a rare
earth element, and (iii) Fe and/or Co, said Pt and/or Pd
lZ987~4
- 20 -
being present in an amount of more than 5 atom% but not
more than 40 atom~, and sald Fe and/or Co being present in
an amount of at least 30 atom% but not more than 80 atom%,
and have a Co/(Fe + Co) ratio [atomic ratio] of from O to
0.4.
Such alloy targets as illustrated above may be
prepared by either the melting method or the sintering
method by the use of the metals mentioned above.
The melting method is a process in which the
metals are melted with a low-frequency melting furnace and
then casted into targets, and the targets of better
quality may be obtained when impurities such as oxygen and
the like are removed from the molten metal by using a
calcia crucible.
On one hand, the sintering method is a process of
sintering metallic powder, in which the sintering of
powder of each component metal, or powder of alloy of
component metals, or of mixtures of powder of alloy of
several kinds of metals and metals is effected to give
desired alloy targets.
Such alloy targets as used in the present
invention, in comparison with TbFeCo alloy targets, are
excellent in resistance to oxidation and, moreover,
excellent in toughnesc so that after casting and
processing, the resulting alloy targets are prevented from
lZ987~
- 21 -
their cracking. Accordingly, the alloy targets used in
the present invention are not always in need of pre-
sputtering that is required for the conventionally used
TbFeCo alloy targets.
In the targets of the present invention, moreover,
permeability of target becomes small so that the targets
can be increased in thickness in comparison with the
conventional TbFeCo alloy targets and utility efficiency
of the present targets can also be enhanced.
In general, the composition of alloy target
deviates from that of a film obtained from said alloy
target to a maximum extent of 10 atom% according to the
sputtering conditions employed. On that account, the
composition of the alloy target is decided according to
the composition of the film and to the sputtering
conditions to be employed.
Since the magnetooptical recording films of the
present invention have a magnetic easy axis perpendicular
to the film, they are utilizable in such various fields as
films, magnetic recording materials such as vertical
magnetic recording magnetic bubble memories or
magnetooptical recording films, and photomodulaters that
utilize magnetooptical effects.
The present magnetooptical recording films having
the compostion mentioned above have a magnetic easy axis
perpendicular to the film and exhibit Kerr hysterisis loop
~z9~
- 22 -
of a favorable square-shaped form.
In the present specification, by "exhibition of
Kerr hysterisis loop of a favorable square-shaped form" is
2/~kl ratio of the Kerr-rotation
a saturation magnetization (~kl) in the maximum external
magnetic field to the Kerr-rotation angle at a remanent
magnetization (~k2) in the external magnetic field of zero
is at least than 0.8.
In the present invention, a thickness of
magnetooptical recording film is desirably 20 A - 5~ m,
preferably 50-5000 A, and more preferably 100-3000 A or
thereabout.
The case where the present magnetooptical
recording films have been utilized as recording films of
magnetooptlcal discs in illustrated hereinafter.
r~ The pre~ent magnetooptical recording films ~-
film ~ith a magnetization with easy axis perpendicular to
the film and, at the same time, in most of them, Kerr
hysterisis loop has a square-shaped form, ~ k under the
circum~tances where no external magnetic field exist~ is
practically the same as~ k at a saturation magnetization
in the maximum external magnetic field, and also coercive
force Hc is large, and hence they are suitable as
magnetooptical recording films. Furthermore, that ~k is
favorable means that ~f is also favorable, and accordingly
lZ98~
- 23 -
the present magnetooptical recording films are utilizable
in both of Kerr effect utilization system and Faraday
effect utilization system.
Furthermore, since the present magnetooptical
recording films are excellent in resistance to oxidation,
they are not always in need of use of such protective film
for prevention of oxidation as used in the conventional
magnetooptical recording films comprising heavy rare earth
elements and 3d transition metal alloys such as Tb-Fe, Tb-
Fe-Co, etc. .
Moreover, it is not always necessary to use anti-
oxidizing material~ in a substrate adjacent to the
recording film or other functional films (e.g. enhancing
film and reflective film), or adhesive layers for
lamination purposes.
Further, even when enhancing film and/or
reflection film is formed on the present magnetooptical
recording films, the film can be formed by the wet film
forming method such as the spin or spray coating procesq
that could not be employed in the conventional
magnetooptical recording films, in addition to the dry
film forming method quch as vacuum evaporation or
sputtering.
Accordingly, the structure of magnetooptical discs
bearing the present magnetooptical recording films as
129871~9L
recording films thereon may include such structures as
mentioned below.
(i) Substrate/recording film,
(ii) Substrate/enhancing film/recording film,
(iii) Substrate/recording film/reflection film,
(iv) Substrate/enhancing film/recording film/
reflection film, and
(v) Substrate/enhancing film/recording film/
enhancing film/reflection film.
The magnetooptical discs having such structures
illustrated above may also have on the outermost layer of
the recording film side a protective film or protective
lable for imparting scratch resistance or resistance to
oxidation to said outermost layer.
The enhancing films may be of organic or inorganic
materials so long as they have a refractive index larger
than that of a substrate.
As the substrate, there may be used inorganic
materials such as glass, aluminum, etc. and organic
materials such a~ polymethyl methacrylate, polycarbonate,
polymer alloy of polycarbonate and polystyrene, amorphous
polyolefins as di~closed in U.S. Patent 4,614,7~8, poly-4-
methyl-1-pentene, epoxy resin, polyether sulfone,
polysulfone, polyether imide, etc.
The reflective films may be of any materials so
12987~!4
- 25 -
long as they have a reflection index of at least 50%.
Further, the structure of the magnetooptical discs
is not limited only to the structures ~ v) mentioned
above, and the discs may be provided with a subbing layer,
anti-oxidizing film or highly permeable soft magnetic
film, if necessary, and the discs may be used either
singly or in the form of a laminated disc obtained by
bonding two discs each other.
It is also possible to prepare a magnetooptical
recording film by providing two layers of magnetooptical
recording films on a substrate. In that case, the
magnetooptical recording film having this structure is
preferably such that when coercive force and Curie
temperature of a first magnetooptical recording-film are
taken as Hc1 and Tc1, respectively, and when coercive
force and Curie temperature of a second magnetooptical
recording film are taken as Hc2 and Tc2, respectively, the
relationship of Hc1 > Hc2 and that of Tc1 < Tc2 are
established and that an outer layer (a layer on the side
opposite to a substrate, i.e., the second magnetooptical
recording film) comprises such magnetooptical recording
film as specified in the present invention.
~,t The magnetooptical recording film having such 2-
A' o~ ~r~ g
r~ layer structure may be sub~ect to o~or~irite.~ The
recording process wherein such two-layer magnetooptical
- 26 -
e~,,~/r/ ~
recording film is subjected to ovor~ritc is illustrated
below.
A magnetooptical recording apparatus has a
magnetic field (Ha) generating an upward magnetic field
(or a downward magnetic field) for writing, right below a
head through which a laser light is irradiated and in
front of said magnetic field, i.e. an upright side, a
magnetic field (Hb) generating a downward magnetic field
(or an upward magnetic field) having the relationship of
Hc1 > Hb > Hc2.
The magnetooptical recording film first passes
through above Hb, whereby magnetization direction of the
second magnetooptical recording film is arranged
downwardly (or upwardly). In that case, the magnetization
direction of the first magnetooptical recording film does
not change from the relationship of Hc1 > Hb.
Subsequently, the magnetooptical recording film, when it
passes through above Ha, is irradiated with a laser light
modulated either to a high level or low level in
correspondence to an information signal. The high level
laser light heats the magnetooptical recording film to a
level higher than the Curie temperature Tc2 of the ~econd
magnetooptical recording film, and hence both the
magnetization directions of the first and second
magnetooptical recording films are arranged by the weak
129~37~
- 27 -
magnetic field Ha to the magnetization direction of the
weak magentic field, that is, upwardly (or downwardly).
On one hand, the low level laser light heats the
magnetooptical recording film only to a temperature higher
than Curie temperature Tc1 of the first magnetooptical
recording film and less than Tc2, and hence only the first
magnetooptical recording film is magnetized by Ha, and the
magnetization direction of the second magnetooptical
recording film remains downwardly (or upwardly).
Therefore, the magnetization direction of the first
magnetooptical recording film, after passing through Ha
and in the course of back to room temperature, reverse to
the magnetization direction of the second magnetooptical
recording film and turns downward (or upward).
Accordingly, the magnetooptical recording film can be
sub~ected to overwrite by modulating an energy level of
laser light for writing.
Apart from the above-mentioned, in the case of a
magnetooptical recording film having a two-layer structure
wherein coercive force of a first magnetooptical layer (an
inner layer) is smaller than that of a second
magnetooptical recording film (an outer layer) and at
least the outer layer (a layer on the side opposite to a
substrate) comprises a magnetooptical recording film as
defined in the present invention, the magnetooptical
~Z9~7~
- 28 -
recording film so designed is found high in C/N ratio and
excellent in long-term reliability. That is, a recording
sensitivity depends on Curie point of the outer layer, the
information recorded in the outer layer i5 transferred to
the inner layer having a small coercive force and a large
Kerr-rotation angle, and when the recorded information is
read out, a high C/N ratio is obtained. In that case, to
obtain a high C/N ratio, it is desirable that Kerr-
rotation angle of the inner layer is at least 0.25 deg,
preferably at least 0.3 deg, coercive force of the inner
layer is not greater than 3 KOe, preferably not greater
than 2 KOe, and coercive force of the outer layer i9 at
least 3 KOe, preferably at least 4 KOe, and Curie point of
the outer layer is 100-300C.
In the magnetooptical recording film as mentioned
above, recording sensitivity depends on Curie point of the
second magnetooptical recording film, and because of a
small coercive force, the information recorded in the
second magnetooptical recording film is transferred to the
first magnetooptical recording layer having a large Kerr-
rotation angle, and a C/N ratio is obtained on reading-out
of the transferred information.
The present invention is illustrated below with
reference to examples, but it should be construed that the
invention in no way limited to those examples.
lZ9l37~4
- 28a - 72932-6
In the following examples, the composition of the
magnetooptical recording film is shown in atomic %. For instance,
Pt12Tb30Fe49Cog of Example 1 means 12 atom.% of Pt, 30 atom.% of
Tb, 49 atom.% of Fe and 9 atom.% of Co.
1~9~3704
- 29 -
Examples 1-25 and Comparative Examples 1-19
Using a composite target with chips of Pt and/or
Pd and a rare earth element arranged in a predetermined
proportion on Fe or Fe-Co target as a target, there was
deposited on a glass substrate at 20-50C by DC magnetron
sputtering a magnetooptical recording film having the
composition as denoted in Table 1. The condition under
which the film was formed included Ar pressure of 5 m
Torr., Ar flow rate of 3 sccm, ultimate degree of vacuum
of not more than 5 x 10 6 Torr, and a film thickness of
the alloy thin film of loO0 A.
As a result of determination by the wide angle X-
ray diffraction method, the magnetooptical films obtained
were all amorphous. The composition of the recording
films obtained was determined by ICP emission
~pectroscopic analysis.
The Kerr-rotation angle was measured by the
inclination incidence method ( ~ = 780 nm) at a remanent
magnetization in the external magnetic field of zero from
the side of the glass substrate. A concrete method of
measurement and apparatus therefor to be employed in the
inclination incidence method are described in "Measuring
Techniques of Magnetic Materials", compiled by Kazuo
Yamakawa (published by Torikepps K.K. on December 15,
1985), pp.261-263.
987~
- 30 -
Table 1 shows x value, Fe + Co amount (atomic %),
Co/(Fe + Co) ratio [atomic ratio], Kerr-rotation angle
(9k) and coercive force ~Hc) of the magnetooptical
recording films ohtained.
Furthermore, on a substrate comprising an
amorphous ethylene-tetracyclododecene copolymer was
deposited a magnetooptical recording film having a
predetermined composition by DC magnetic sputtering method
to prepare a magnetooptical disc bearing a single layer of
1000 A.
The magnetooptical disc obtained had a diameter of
130 mm, and using this magnetooptical disc, recording and
reproducing were carried out with a driving apparatus
.r~ (Nakamichi OMS-1000) under the conditions of recording
frequency number 1 MHZ (Duty ratio 50%), linear speed of
11.1 m/s, bias magnetic field o f 200 Oe at the time o f
writing, and readout laser power of 1.0 mW.
Table 1 also shows a carrier to noise ratio (C/N
ratio) and noise level when the recording was carried out
with a recording power (optimum recording power), of which
a level of second harmonic frequency as measured by
spectrum analyzer became minimum.
Then, the information recorded was erased with a
power larger by 3.0 mW than this optimum recording power,
and a new information was recorded on the erased recording
A~ r~ade~ fk
12987~4
- 31 -
film. This operation was repeated 10 times, and
thereafter the difference between C/N ratio before and
after erasion was measured and represented asa C/N ratio.
As regards bias magnetic field dependability, the
bias magnetic field dependability (H sat) was obtained by
a change in C/N ratio when the bias magnetic field under
the above-mentioned conditions was changed upto 50-500 Oe.
Further, with the purpose of long-term
reliability, the disc obtained was subjected to life test,
wherein the disc was allowed to stand in an oven under the
circumstances of high temperature and humidity of 80C and
85% ~H, and after the lapse of 1000 hours, C/N ratio was
measured to obtain the results as shown in Table 1.
lZ~1~'7(l~4
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-35-
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12987~4
- 36 -
Example 26
On an amorphous polyolefin substrate were
deposited a film comprising Pt13Tb28Fe55Co4 as a fir9t
magnetooptical recording film and a film comprising
Pt13Tb30Fe48Cog as a second magnetooptical recording film
so that the second magnetooptical recording film became an
outer layer. In this case, the magnetooptical recording
film thus prepared had such relation that coercive force
of the first magnetooptical recording film is larger than
that of the second magnetooptical recording film, and
Curie temperature of the first magnetooptical film is
lower than that of the second magnetooptical recording
film.
Then, using a disc drive having an upward magnetic
field (0.2 KOe) for writing and a downward magnetic field
(5 KOe) provided ad~acent to an upright side of said
upward magnetic field larger than coercive force of the
second magnetooptical recording film and smaller than
coercive force of the first magnetooptical recording film,
the abovementioned magnetooptical disc was sub~ected to
overlight under the conditions of linear speed of 11 m/s,
low laser power of 4 mW, and high level laser power of 6
mW by single beam modulation, whereupon the overwrite was
possible.
Furthermore, this magnetooptical recording disc
lZ98'7(;~L
was subjected to life test, wherein the disc was allowed
to stand under the circumstances of 80C and 85% RH for
1000 hours, whereupon no change in recording
characteristics was observed.
Example 2~
In the same procedure as described in Example 25,
a film comprising Pt5Tb20Fe60Col5 was deposited to a film
thickness of 300 A as a first magnetooptical recording
film and a film comprising Ptl3Tb28Fe51Co8 p
to a film thickness of 800 A as a second magnetooptical
recording film (an outer layer).
This first magnetooptical recording film thus
deposited had ~k of 0.40, Hc of 1.2 KOe, and Tc of 270C.
The second magnetooptical recording film thus deposited
had ~k of 0.24, Hc of 8.3 KOe, and Tc of 170C.
The magnetooptical recording film thus prepared
had a C/N ratio of 55 dB.
The magnetooptical disc bearing the above
magnetooptical recording film thereon was subjected to
life test, wherein the disc was allowed to stand under the
circumstances of 80C and 85% RH for 1000 hours, whereupon
no change in recording characteristics was observed.
