Magneto-optical recording medium with exchange-coupled reproducing layer containing platinum

In a magnetooptical computer disk having one or more optically transparent dimensionally stable substrates and one or more double layer systems consisting of two exchange-coupled magnetic layers which consist of alloys of rare earth metals with transition metals and have vertical magnetic anisotropy, the first magnetic layer adjacent to the substrate has a composition corresponding to the formula I EQU RE.sub.x TM.sub.100-x-y-a Pt.sub.y A.sub.a I and the second magnetic layer, which is arranged on that side of the first magnetic layer which faces away from the substrate, has a composition corresponding to the formula II EQU RE.sub.u TM.sub.100-u-b B.sub.b II where PA0 RE is one or more elements from the group consisting of Gd, Tb, Dy and Ho, PA0 TM is one or more elements from the group consisting of Fe and Co, PA0 A and B independently of one another are elements from the group consisting of Cr, Nb, Ta, Al, Si, Ni and Mo, and PA0 5.ltoreq.x.ltoreq.30, PA0 0.5.ltoreq.y.ltoreq.20, PA0 15.ltoreq.u.ltoreq.30, PA0 0.ltoreq.a.ltoreq.25 and PA0 0.5.ltoreq.b.ltoreq.25.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a magnetooptical computer disk having one 
or more optically transparent dimensionally stable substrates and one or 
more double layer systems consisting of two exchange-coupled magnetic 
layers which consist of alloys of rare earth metals with transition metals 
and have vertical magnetic anisotropy. The present invention relates in 
particular to magnetooptical computer disks having a Pt-containing 
reproducing layer. 
2. Description of the Related Art 
Known magnetooptical recording layers for recording and reading information 
are, for example, monocrystalline garnet layers (e.g. yttrium iron 
garnet), polycrystalline layers of MnBi or amorphous layers of alloys of 
lanthanides (RE) and transition metals (TM), abbreviated below to (RE-TM). 
Recently, the amorphous (RE-TM) layers have been preferred since these 
recording layers can be produced over large areas by sputtering methods or 
vapor deposition methods, and the recorded signals can be read with a high 
signal-to-noise ratio. Many amorphous (RE-TM). alloys, for example Tb-Fe, 
Tb-Fe-Co, Gd-Tb-Fe-Co, Dy-Fe-Co, Nd-Tb-Fe-Co or Nd-Dy-Fe-Co, additionally 
have the advantage that the ferrimagnetic coupling of the RE and TM atoms 
results in a high coercive force in a direction at right angles to the 
plane of the layer. 
These known magnetooptical computer disks are used for recording or writing 
data with the aid of laser beams (for example pulse-modulated), which are 
focused on the magnetooptical recording layers and strike them at right 
angles. 
During recording or writing of data, an external auxiliary magnetic field 
is applied to the magnetooptical computer disks, the field lines of which 
field are oriented at right angles to the surface of the magnetooptical 
recording layers. The direction of the external magnetic field is opposite 
to the direction of magnetization of the magnetooptical recording layer. 
In addition, the magnetooptical recording layers may have a 
correspondingly oriented immanent (intrinsic) magnetic field. In a known 
alternative recording method, the external magnetic field is 
time-modulated. 
It is known that the magnetooptical recording layers which consist of 
amorphous ferrimagnetic (RE-TM) alloys, are magnetized at right angles to 
their surface and may have a plurality of layers are heated at the point 
of contact during recording of data by the write laser beam. As a result 
of the heating, the coercive force H.sub.c of the alloys decreases. If the 
coercive force H.sub.c falls below the sum of the field strengths of the 
applied (external) auxiliary magnetic field and of the intrinsic field at 
a critical temperature dependent on the particular alloy used, a region 
which has a direction of magnetization opposite to the original direction 
is formed at the point of contact. Such a region is also referred to as a 
magnetic domain. 
The diameter and the shape of the domains formed depend both on the size of 
the laser spot, the laser power, the laser pulse time and the strength of 
the external magnetic field and on the magnetization M.sub.s and the 
coercive force H.sub.c of the recording layer. Round smooth-edged domains 
are desirable since they give a high signal and a high signal-to-noise 
ratio during reading. 
In the write process, smooth-edged domains are obtained in particular when 
the magnetization and the coercive force of the magnetooptical storage 
layer have a suitable temperature dependence and the Curie temperature 
T.sub.c of the storage layer is at least approximately reached during 
heating by the laser beam. 
At temperatures substantially above T.sub.c, large, overlapping domains 
having poor signal-to-noise ratios are obtained. On the other hand, at 
temperatures substantially below T.sub.c, the nucleation of domains having 
the opposite magnetization is possible only with very high external 
magnetic fields which are therefore unsuitable for use. 
It is known that, during recording of the data, the write laser beam is 
moved relative to the magnetooptical computer disk or its magnetooptical 
recording layer and above the surface of said disk or layer. In general, 
the laser beam is focused on the recording layer by a displaceable optical 
apparatus, and the relevant magnetooptical computer disk is rotated at 
constant angular velocity (CAV). 
It is known that the data recorded in the magnetooptical computer disks 
can, if required, be deleted by controlled local heating of their 
magnetooptical recording layer, for example by means of an unmodulated 
continuous laser beam with the simultaneous action of an external or an 
intrinsic magnetic field whose field lines are oriented at right angles to 
the surface of the recording layer, after which further data can be 
recorded, i.e. the write process is reversible. 
The data are usually read using the linearly polarized light of a 
continuous-wave laser whose power is not sufficient to heat the material 
above the critical temperature. This laser beam is reflected either by the 
recording layer itself or by a reflector layer arranged behind it, the 
result being an interaction between the magnetic moments in the recording 
layer and the electromagnetic field of the laser light. Because of this 
interaction, the plane of polarization of the reflected laser light is 
rotated through a small angle relative to the original plane. If this 
rotation of the plane of polarization occurs when the light is reflected 
by the recording layer itself, the term Kerr effect is used and the angle 
of rotation is accordingly referred to as the Kerr angle; if, on the other 
hand, the plane is rotated during passage of the light twice or a greater 
number of times through the recording layer, the terms Faraday effect and 
Faraday angle are used. The direction of rotation of the plane of 
polarization depends on the magnetization direction at the relevant point 
of the storage layer. This rotation of the plane of polarization of the 
laser light reflected by the magnetooptical computer disk can be measured 
with the aid of suitable optical and electronic apparatuses and converted 
into signals, as described in, for example, U.S. Pat. No. 4 466 035. The 
magnetooptical read signal is proportional to the product of the Kerr 
angle and the reflectivity of the magnetooptical layer system. A high Kerr 
angle accordingly results in a high read signal and a correspondingly 
improved signal-to-noise ratio. 
For the laser wavelengths currently used (from 780 nm to 830 nm), the Kerr 
angle of the stated (RE-TM) alloys is, as a rule, from 0.2.degree. to 
0.3.degree.. The Kerr angle of alloys based on the heavy RE elements Gd, 
Tb and Dy generally decreases with decreasing wavelength. Since it is 
expected that short-wavelength lasers will be used for future 
magnetooptical computer disks, the stated (RE-TM) alloys have the 
disadvantage of a reduced magnetooptical read signal. 
As described in the stated U.S. Pat. No. 4,466,035, the Kerr angle can be 
increased by using suitable dielectric layers on the front and back of the 
magnetooptical recording layer and by employing a metallic reflector on 
the back of the magnetooptical recording layer. When the Kerr angle is 
increased with the aid of dielectric layers, a decrease in the 
reflectivity must, as a rule, be accepted. Since a minimum reflectivity is 
required for operating a magnetooptical storage medium in a prior art 
drive, the increase in the magnetooptical read signal in the manner 
described is subject to limits. In particular, the stated disadvantageous 
reduction of the Kerr angle with decreasing wavelengths of the recording 
laser cannot be avoided by optical matching with dielectric layers. 
It is known that the Kerr angle of an amorphous (RE-TM) layer can be 
increased by alloying with Pt. At the same time, however, considerable 
reduction in the coercive force in the direction at right angles to the 
film surface is observed. In the known magnetooptical recording layers, 
the advantage of the increased Kerr angle can therefore often be utilized 
to a limited extent since the Pt concentration must be kept low because of 
the coercive force 
Another great disadvantage of the stated (RE-TM) alloys is their poor 
corrosion resistance. Direct contact of the layers with air or water vapor 
results in progressive oxidation of the magnetooptical layer over a large 
area, said oxidation initially causing a reduction in the Kerr angle and 
in the reflectivity and hence a decrease in the signal-to-noise ratio and 
finally leading to completely oxidized layers which are useless for 
magnetooptical purposes. 
A possible method for improving the corrosion resistance of magnetooptical 
computer disks based on (RE-TM) alloys is the application of a transparent 
protective layer on the front and back for avoiding direct contact of the 
recording layer with air and for inhibiting the entry of oxygen or water 
molecules by diffusion. This is possible only by means of very dense, 
crack-free and pore-free layers, for example of Si.sub.3 N.sub.4 or AlN. 
However, the additional deposition of the transparent protective layers 
before and after application of the recording layer makes the production 
process for magnetooptical computer disks substantially longer and more 
expensive. Furthermore, defects in the protective layer, for example 
pinholes or cracks, can lead to corrosion of the lower-lying 
magnetooptical recording layer. An expensive quality control of the 
deposited protective layers is therefore necessary in order to ensure 
their protective effect. 
Owing to the high sensitivity of (RE-TM) alloys to oxidation, reactions of 
said alloys with reactive gases in the residual gas may also occur in the 
coating chamber. Particularly during the period after application of an 
(RE-TM)-containing layer and before application of the subsequent layer 
(for example, due to the changing of the sputtering target or transport of 
the coated disk), a superficial oxide layer may form as a result of 
reaction with the O.sub.2 or H.sub.2 O molecules of the residual gas. 
In the known exchange-coupled double layer systems described further below, 
there is a weakening of the exchange interaction at room temperature 
between the reproducing layer and the storage layer owing to nonmagnetic 
intermediate layers which form after production of the first magnetic 
layer and before application of the second magnetic layer by a reaction of 
the (RE-TM) alloy with reactive gases in the residual gas of the coating 
unit. This exchange interaction is even decisively reduced by monolayers 
of a nonmagnetic intermediate layer, for example of a metal oxide. 
Although the superficial oxide layer can be substantially removed by 
etching processes, for example a plasma etching process in an Ar 
atmosphere, the production process for the magnetooptical computer disk is 
however made longer and more expensive as a result. 
An alternative method for improving the corrosion behavior of amorphous 
(RE-TM) alloys is the alloy of corrosion inhibitors, i.e. elements which 
delay the corrosion of the recording layer. A number of (RE-TM) alloys 
with homogeneously alloyed corrosion inhibitors are known. 
For example, EP-A 229 292 describes a magnetooptical recording medium which 
consists of a (RE-TM) alloy containing an additional element, for example 
Ti, Cr, Al, Pt, Zr, V, Ta, Mo, W, Cu, Ru, Rh, Pd, Nb, Ir or Hf. The 
addition of said element delays the decrease in the coercive force and in 
the Kerr angle during storage of the magnetooptical layer in direct 
contact with humid air. 
U.S. Pat. No. 4,693,943 describes a magneto-optical recording medium having 
an amorphous (RE-TM) alloy with the composition [(dTb).sub.1-y 
(FeCo).sub.y ].sub.1-p Cr.sub.p, where 0.5.ltoreq.y.ltoreq.0.9 and 
0.001.ltoreq.p.ltoreq.0.3. The addition of Cr substantially improves the 
corrosion stability of the magnetooptical recording medium. We have found 
that the corrosion stability increases monotonically with increasing Cr 
content. 
It is also known that the corrosion stability of (RE-TM) layers can be 
further improved by simultaneously alloying a plurality of elements with 
said layers. EP-A 302 393 describes a magnetooptical recording medium 
containing a (RE-TM) alloy with which from 1 to 10 atom % of one or more 
elements from the group consisting of Nb, Ti, Ta, Cr and Al and from 2 to 
10 atom % of one or more elements from the group consisting of Pt, Au, Pd 
and Rh are also alloyed. 
A substantial disadvantage of the use of corrosion inhibitors is that the 
magnetic and magnetooptical properties of the recording medium are as a 
rule adversely affected by alloying with a corrosion inhibitor. In many 
cases, the Kerr angle is reduced and the temperature dependence of the 
magnetization and of the coercive force are unfavorably changed, having 
adverse effects on the write and read behavior of the recording medium. 
Exchange-coupled double layer systems which contain a first magnetic layer 
having a low coercive force and a second magnetic layer having a high 
coercive force are described in, for example, EP-A 51 296, U.S. Pat. No. 
4,628,485, U.S. Pat. No. 4,753,853, EP-A 305 185, EP-A 330 394 and EP-A 
333 467. 
EP-A 51 296 describes a thermomagnetic recording medium having a first and 
a second magnetically anisotropic layer, the second magnetic layer having 
a higher coercive force and a lower Curie temperature than the first 
magnetic layer. The first magnetic layer essentially contains a Gd alloy 
U.S. Pat. No. 4,628,485 describes a magnetooptical recording medium having 
a first thin magnetic layer of low Curie temperature and high coercive 
force (recording layer), an adjacent second magnetic layer (reproducing 
layer) having a high Kerr angle and further dielectric and metallic layers 
for optically increasing the Kerr angle. 
U.S. Pat. No. 4,645,722 describes a magnetooptical recording medium having 
a first magnetic layer which possesses a high coercive force and a second, 
multi-stratum magnetic layer system which has a higher Kerr angle and/or a 
higher reflectivity than the first magnetic layer. 
U.S. Pat. No. 4,753,853 describes an exchange-coupled magnetooptical double 
layer system in which a first layer has a lower Curie temperature and a 
high coercive force and consists of a TM-rich (Gd-Fe-Co) alloy. The second 
layer has a high Curie temperature and a low coercive force and consists 
of a TM-rich (Tb-Fe) alloy. 
EP-A 330 394 discloses a magneto-optical recording medium of the double 
layer type whose magnetic layer having a low coercive force and high Curie 
temperature contains Gd and one or more of the two elements Tb and Dy. The 
coercive force of the two magnetic layers and the ratio of the domain wall 
energy between these two layers to the product of saturation magnetization 
and thickness of the layer having a low coercive force must satisfy 
specific conditions. 
The magnetic layer having a low coercive force may be doped with one or 
more elements from the group consisting of Ni, Cr, Ti, Al, Si, Pt, In and 
Cu. However, no data at all is given with regard to the concentration of 
these elements Moreover, EP-A 330 394 does not disclose the purpose for 
which the layer having a low coercive force is to be doped with these 
elements. 
EP-A 364 212 describes a magnetooptical recording medium having a first 
magnetic layer (reproducing layer) of an amorphous R.sub.1 -Fe-Co-Cr 
alloy, where R.sub.1 is one or more elements from the group consisting of 
Tb and Dy, and a second magnetic layer (recording layer) of an amorphous 
R.sub.2 -Fe-Co-Cr alloy, where R.sub.2 is one or more elements from the 
group consisting of Tb, Dy and Gd. The proportion of Co in the first 
magnetic layer is smaller than the proportion of Co in the second magnetic 
layer. 
EP-A 364 196 describes a magnetooptical recording medium which is very 
similar to that in EP-A 364 212, except that the proportion of Cr in the 
first magnetic layer is greater than the proportion of Cr in the second 
magnetic layer. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a magnetooptical 
computer disk having a high read signal, in particular at short 
wavelengths, high data stability and good corrosion stability. 
We have found that this object is achieved, surprisingly, by magnetooptical 
computer disks having one or more optically transparent dimensionally 
stable substrates and one or more double layer systems consisting of two 
exchange-coupled magnetic layers which consist of alloys of rare earth 
metals with transition metals and have vertical magnetic anisotropy, 
wherein the first magnetic layer adjacent to the substrate has a 
composition corresponding to the formula I 
EQU RE.sub.x TM.sub.100-x-y-a Pt.sub.y A.sub.a I 
and the second magnetic layer, which is arranged on that side of the first 
magnetic layer which faces away from the substrate, has a composition 
corresponding to the formula II 
EQU RE.sub.u TM.sub.100-u-b B.sub.b II. 
RE is one or more elements from the group consisting of Gd, Tb, Dy and Ho, 
TM is one or more elements from the group consisting of Fe and Co, 
A and B independently of one another are one or more elements from the 
group consisting of Cr, Nb, Ta, Al, Si, Ni and Mo, and 
5.ltoreq.x.ltoreq.30, 
0.5.ltoreq.y.ltoreq.20, 
15.ltoreq.u.ltoreq.30, 
0.ltoreq.a.ltoreq.25 and 
0.5.ltoreq.b.ltoreq.25. 
The component, essential according to the invention, of the novel 
magnetooptical computer disk is the novel double layer system having two 
magnetic layers, of which the first layer (also referred to below as 
reproducing layer) is platinum and at least the second magnetic layer 
(also referred to below as storage layer) contains one or more elements 
from the group consisting of Cr, Nb, Ta, Al, Si, Ni and Mo. 
In prior art magnetooptical single layers, the Pt concentration must be 
kept low and hence no decisive increase in the Kerr angle and in the 
magnetooptical read signal is achieved. 
In the novel magnetooptical computer disks, the decrease in coercive force 
due to the addition of Pt to the reproducing layer is compensated by the 
use of an exchange-coupled storage layer having a high coercive force and 
vertical magnetic anisotropy. 
Since this permits substantially higher Pt concentrations to be established 
in the reproducing layer, it is possible, in contrast to known 
magnetooptical computer disks, to achieve substantially improved 
magnetooptical read signals and a considerably improved data stability. 
This applies in particular when using laser light wavelengths lower than 
800 nm. 
The Pt concentration and the thickness of the reproducing layer are 
determined by the following factors. As the Pt concentration increases, 
the vertical anisotropy of the reproducing layer and hence the exchange 
interaction of the two layers are reduced. The thickness of the 
reproducing layer should exceed the depth of penetration of the laser 
light used, in order to ensure an optimum increase in the Kerr angle. With 
a reduced exchange interaction, an interruption in the exchange coupling, 
i.e. separate switching of storage layer and reproducing layer, can occur 
even at reproducing layer thicknesses which are less than the depth of 
penetration of the laser light Because of the complicated hysteresis 
behavior, this case should be avoided in practice. 
The Pt concentration to be established in the reproducing layer also 
depends on the type and concentration of the RE and TM components stated 
in the formulae I and II and of the one or more elements of the group 
consisting of Cr, Nb, Ta, Al, Si, Ni and Mo. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred novel recording media contain platinum and chromium in the 
reproducing layer and chromium in the storage layer 
In this preferred embodiment of the novel magnetooptical computer disks, it 
is particularly advantageous that the properties of Pt or of other 
elements from the group consisting of Cr, Nb, Ta, Al, Si, Ni and Mo can be 
combined in a suitable manner. This is useful in particular where there 
are undesirable side effects with the addition of individual elements. 
For example, it is known that, in magnetooptical recording layers of 
(RE/TM) alloys, the Kerr angle can be increased by adding Pt and reduced 
by adding Cr. With the simultaneous presence of Cr and Pt in the (RE-TM) 
alloy of the reproducing layer of the novel magnetooptical computer disks, 
it is therefore possible, by adding Pt, to compensate the reduction in the 
Kerr angle due to the Cr content and hence to obtain a high read signal in 
conjunction with good corrosion stability. 
We have also found that the corrosion stability of (RE-TM) alloys achieved 
with Cr can be substantially improved for Cr contents up to 10 atom % by 
adding Pt. 
We have furthermore found that the Curie temperature T.sub.c of 
(RE-TM)-containing magnetooptical layers is increased by Pt and reduced by 
Cr (cf. Table 1). Table 1 shows the Curie temperature T.sub.c and the 
coercive force H.sub.c as a function of the content of D for 
magnetooptical layers of (Tb.sub.24 Fe.sub.68 Co.sub.8).sub.100-x D.sub.x, 
where D is Pt or Cr. 
TABLE l 
______________________________________ 
T.sub.c (.degree.C.) H.sub.c (kA/m) 
X D = Pt D = Cr D = Pt 
______________________________________ 
0 205 205 -- 
5 220 165 1015 
7 225 150 570 
9 231 128 305 
10 235 118 215 
12 248 100 90 
15 266 60 20 
18 302 18 0 
______________________________________ 
As is evident from Table 1, with the knowledge of the quantitative effects 
of Cr and/or Pt on the Curie temperature or the coercive force, it is 
possible to find compositions which have an optimum property profile. 
It should be borne in mind that T.sub.c and H.sub.c are also dependent on 
the type and amount of the RE and TM components stated in formulae I and 
II. 
Additional variations of T.sub.c and H.sub.c arise through the exchange 
coupling of the two magnetic layers in the double layer systems. 
In the novel double layer system, the reproducing layer contains Pt and at 
least the storage layer contains one or more additional elements from the 
group consisting of Cr, Nb, Ta, Al, Si, Ni and Mo. 
By the addition of one or more of these elements, intrinsic corrosion 
inhibition of the reproducing layer is achieved and hence the formation of 
nonmagnetic intermediate layers is substantially restricted. While the 
corrosion-inhibiting elements from the group consisting of Cr, Nb, Ta, Al, 
Si, Ni and Mo prevent oxidation of the (RE-TM) layer over a large area, 
the addition of Pt results in substantially increased stability to local 
corrosion phenomena (e.g. pitting) The double layer system therefore has 
an excellent life. In a particularly preferred embodiment of the 
invention, both the reproducing layer and the storage layer therefore 
contain one or more elements from the group consisting of Cr, Nb, Ta, Al, 
Si, Ni and Mo. Due to the inhibition of corrosion, a higher exchange 
coupling between the storage layer and the reproducing layer is achieved, 
in particular with low vertical orientation of the reproducing layer. The 
Pt concentration can thus be substantially increased without there being 
any adverse effects on the coercive force of the double layer system. With 
the simultaneous use of an element which increases the Kerr angle and of a 
corrosion-inhibiting element in the reproducing layer, there is thus a 
synergistic effect with regard to both the Kerr angle and the corrosion 
stability. 
Novel computer disks which contain platinum in the reproducing layer and 
chromium in the storage layer are also preferred because of the corrosion 
protection effect. Particularly preferred computer disks are those which 
contain platinum and chromium in the reproducing layer. 
The Pt in the reproducing layer and the one or more elements from the group 
consisting of Cr, Nb, Ta, Ta, Al, Si, Ni and Mo in the storage layer and 
possibly in the reproducing layer are uniformly or nonuniformly 
distributed in the vertical direction, i.e. across the layer thickness and 
in the horizontal direction, i.e. in the plane of the layer. 
It is advantageous if the distribution of one or more elements from the 
group consisting of Cr, Nb, Ta, Al, Si, Ni and Mo varies in the horizontal 
direction, i.e. if the concentration of these elements changes 
continuously or discontinuously in the plane of the layer. In a 
particularly preferred embodiment, the Cr concentration in the 
chromium-containing layer or layers increases continuously or 
discontinuously from the inner edge to its or their outer edge. 
The amorphous (RE-TM) alloys of the reproducing layer and of the storage 
layer may contain, in a known manner, one or more elements from the group 
consisting of Gd, Tb, Dy and Ho as RE elements and one or more elements 
from the group consisting of Fe and Co as TM elements. The ferrimagnetic 
coupling between these stated RE and TM atoms leads to a high coercive 
force in a direction at right angles to the plane of the layer. 
Consequently, a maximum Kerr angle on exposure of the medium at right 
angles to the surface and high data security are ensured. However, the 
magnitude of the vertical anisotropy and hence the maximum value of the 
coercive force in a direction at right angles to the plane of the layer 
differ in the two layers. Known compounds having a high coercive force are 
Tb- and/or Dy-containing alloys, for example Tb-Fe, Tb-Fe-Co, Dy-Fe or 
Dy-Fe-Co, but the lastmentioned alloys have only a relatively small Kerr 
angle. Alloys such as Gd-Co, Gd-Fe and Gd-Fe-Co are also known, said 
alloys having a high Kerr angle but a reduced coercive force at right 
angles to the plane of the layer. 
Known exchange-coupled layer systems therefore use an alloy based on Gd-Co, 
Gd-Fe or Gd-Fe-Co as the reproducing layer and an alloy based on Tb-Fe, 
Tb-Fe-Co, Dy-Fe or Dy-Fe-Co as the storage layer, the greater Kerr angle 
of the reproducing layer leading to a high magnetooptical read signal and 
the higher coercive force of the storage layer resulting in improved 
stability of the magnetically reversed region and hence of the stored 
information. 
Finally, the Curie temperature of the reproducing layer can be increased in 
a known manner above the Curie temperature of the storage layer by 
increasing the proportion of Co, resulting in a further increase in the 
magnetooptical read signal. While in a single-layer storage medium the 
increase in the Curie temperature is accompanied by a reduction in the 
sensitivity, the increase in the Curie temperature of the reproducing 
layer in an exchange-coupled double layer medium is not associated with a 
loss of sensitivity if the exchange coupling of the two layers is high at 
room temperature but small in the region of the Curie temperature of the 
storage layer. If the last-mentioned condition is not fulfilled, a change 
in the direction of magnetization of the storage layer with continuing low 
recording powers is not possible when the Curie temperature of the 
reproducing layer is too high, and the magnetooptical storage medium thus 
cannot be recorded on at small laser recording powers. 
In order to achieve an improved temperature dependence of the exchange 
coupling, the novel double layer systems may possess, between the two 
magnetic layers, one or more further magnetic intermediate layers which 
have a Curie temperature which is lower than the Curie temperature of the 
storage layer but higher than room temperature. Examples of these further 
magnetic layers are described in EP-A 258 978. 
The stated intermediate layer promotes a high exchange coupling between the 
reproducing layer and the storage layer at room temperature. However, in 
the region of the recording temperature, i.e. the Curie temperature of the 
storage layer, the exchange coupling between the storage layer and the 
reproducing layer is greatly reduced owing to the lack of long-range 
magnetic order in the intermediate layer. This permits magnetic reversal 
of the storage layer despite the existing orientation of the reproducing 
layer. On cooling, the sublattice magnetization of the storage layer 
increases more rapidly than the exchange coupling between the storage 
layer and the reproducing layer. At a critical temperature, the 
reproducing layer is therefore oriented according to the orientation of 
the storage layer, while the undesirable reversed process, i.e. 
orientation of the storage layer according to the orientation of the 
reproducing layer, is prevented. 
The reproducing layer and storage layer of the novel computer disks have a 
thickness in the conventional range. In general, this range is from 10 to 
500 nm. The thickness of the reproducing layer is preferably from 20 to 40 
mm and that of the storage layer is preferably from 40 to 60 mm. 
The novel magnetooptical double layer is produced during the production of 
the novel magnetooptical computer disk. 
The novel magnetooptical computer disk contains the optically transparent 
dimensionally stable substrate as a further essential component in 
addition to the novel magnetooptical double layer. 
Examples of advantageous dimensionally stable substrates are the 
conventional, disk-shaped, optically transparent, dimensionally stable 
substrates. In general, these consist of optically transparent ceramic 
materials or of plastics. Usually, they have a diameter of from 50 to 200 
mm, advantageously from 80 to 150 mm, in particular from 90 to 130 mm. In 
general, they are from 0.5 to 1.5 mm, preferably from 0.8 to 1.3 mm, 
thick. 
An example of a suitable optically transparent dimensionally stable ceramic 
material is glass. Examples of suitable optically transparent 
dimensionally stable plastics are polycarbonate, polymethyl methacrylate, 
polymethylpentene, cellulose acetobutyrate, mixtures of polyvinylidene 
chloride and polymethyl methacrylate and mixtures of polystyrene and 
poly-(2,6-dimethylphen-1,4-ylene ether). Among these, the dimensionally 
stable substrates consisting of plastics are particularly advantageous. 
That surface of the dimensionally stable substrate which faces the novel 
magneto-optical double layer may have structures. 
The structures in the surface of said substrate are in the micrometer 
and/or submicrometer range. They are used for exact guidance of the read 
laser beam and ensure a rapid and precise response of the tracking and 
autofocusing means in the laser-optical write and read heads of the disk 
drives, i.e. they permit or improve the tracking. Moreover, these 
structures may themselves be data, as is the case, for example, in the 
known audio or video compact disks, or they may be used for coding the 
recorded data. The structures consist of raised parts and/or indentations. 
These are in the form of continuous concentric or spiral tracks or 
isolated hills and/or holes. Moreover, the structure may have a more or 
less smooth wave shape. The tracks are preferred. In their cross-section, 
these have a rectangular, sawtooth-like, V-shaped or trapezoidal contour. 
Their indentations are generally referred to as grooves and their raised 
parts as land. Tracks having from 50 to 200 nm deep and from 0.4 to 1.0 
.mu.m wide grooves separated in each case by a 1-3 .mu.m wide land are 
particularly advantageous. 
The particularly preferably used dimensionally stable substrate is produced 
in a conventional manner by shaping the plastic or plastic blend forming 
the substrate by the injection molding method, if necessary under clean 
room conditions, as described in, for example, DE-A-37 27 093. 
The novel magnetooptical computer disk may contain one or more further 
layers in addition to the dimensionally stable substrate and the novel 
magnetooptical double layer. 
The arrangement of the various layers depends on whether, during recording 
or reading, the magnetooptical storage layer is exposed to laser light 
from the substrate side or the air side. Exposure from the substrate side 
is preferred. 
In the latter case, a conventional antireflection layer of an optically 
transparent dielectric material having a high refractive index may be 
present between the dimensionally stable substrate and the novel 
magnetooptical double layer system. The refractive index is greater than 
that of the substrate but smaller than that of the reproducing layer. This 
material usually contains oxides and/or nitrides or consists of these 
compounds. 
Furthermore, a further optically transparent dielectric layer containing or 
consisting of oxides, carbides and/or nitrides may be present on that side 
of the novel magnetooptical double layer system which faces away from the 
substrate. 
In addition, a conventional reflector layer, which usually consists of 
metals, may be present on that side of the novel magnetooptical recording 
layer which faces away from the dimensionally stable substrate, either 
directly thereon or on a transparent layer of oxides, carbides and/or 
nitrides is arranged thereon. 
Moreover, the novel magnetooptical computer disk may have a conventional 
dielectric protective or anticorrosion layer on that side of the reflector 
layer which faces away from the novel magnetooptical recording layer, 
and/or on one or more sides of the novel magnetooptical double layer 
system, said protective or anticorrosion layer likewise containing or 
consisting of carbides, oxides and/or nitrides. 
The additional layers (reflector layer or protective/anticorrosion layers) 
may also consist of a plurality of several strata. These layers may be 
X-ray amorphous or polycrystalline. 
The thickness of these additional layers is in general known and is 
described in, for example, the prior art cited at the outset. 
However, it is also possible to expose the magnetooptical recording layer 
from the air side, directly or through any transparent antireflection 
and/or protective layers present. In this case, the antireflection layer 
is arranged on that side of the magnetooptical double layer system which 
faces away from the substrate. Accordingly, the reflector layer is present 
between the substrate and the magnetooptical recording layer. 
Further examples of suitable possibilities for arranging the various layers 
in the novel magnetooptical computer disk are disclosed in U.S. Pat. No. 
4,710,418. 
The arrangement of the two magnetic layers of the novel double layer system 
is chosen in each case in such a way that the reproducing layer is 
arranged between the laser light source and the storage layer. 
The reflector layer, the anticorrosion layer and the further layers are 
produced during production of the novel magnetooptical computer disk, the 
order of the individual production or process steps depending on the 
particular composition of said disk. 
Two of the novel magnetooptical computer disks described above may 
furthermore be combined with one another in the form of a sandwich, in 
such a way that their recording layers face one another and there is a 
certain distance between them. The conventional techniques for joining two 
magnetooptical computer disks, as disclosed in, for example, U.S. Pat. No. 
4,751,124 or DE-A-37 18 302, are used for this purpose. 
The production of the novel magnetooptical computer disk starts from the 
dimensionally stable substrate described above, on one surface of which 
the novel magnetooptical double layer of the desired thickness and having 
the composition required according to the invention and, if required, the 
further dielectric and/or metal layers are applied in the required order, 
number and thickness and with the particular composition required, after 
which a defined magnetization oriented at right angles to the surface of 
the novel magnetooptical double layer is induced in a conventional manner 
in said double layer. 
The two magnetic layers of the novel magnetooptical double layer and any 
further layers present are applied by the conventional methods for the 
production of thin layers by vapor deposition, reactive vapor deposition, 
ion plating, ion cluster beam deposition (ICB), sputtering, reactive 
sputtering, magnetron sputtering or reactive magnetron sputtering, the 
sputtering methods being preferably used. 
In sputtering, the corresponding metals, carbides, oxides, nitrides and/or 
the other compounds which may be used are sputtered in the desired order 
and amount from a sputtering target placed on the cathode, under reduced 
pressure in a process gas atmosphere and are deposited on the 
dimensionally stable substrate or on a layer already present thereon. 
Usually, the process gas contains a noble gas, such as argon. 
In reactive sputtering, further reactive gases, such as hydrogen, 
hydrocarbons, oxygen, nitrogen, etc., in the desired amount, are mixed 
with the process gas at a suitable time. Thus, by sputtering a metal, for 
example, in the presence of hydrocarbons, oxygen and/or nitrogen in the 
process gas, it is possible directly to deposit the relevant metal oxide, 
nitride, carbide, carbide/oxide, carbide/nitride, oxide/nitride or 
carbide/oxide/nitride layers. The thickness, the structure and the 
composition of the relevant layers can be adjusted in a conventional 
manner via the sputtering rate, the deposition rate, the process gas 
pressure and the process gas composition. 
In reactive magnetron sputtering, the target is known to be present in a 
magnetic field. 
Examples of suitable sputtering processes are disclosed in U.S. Pat. No. 
4,670,353, U.S. Pat. No. 4,670,316 or DE-A-37 35385. 
The two magnetic layers of the novel magneto-optical double layer of the 
computer disk according to the invention can be produced, for example, by 
sputtering or magnetron sputtering of two (RE-TM) alloys of suitable 
external shape, in the form of a sputtering target, at separate times, 
under reduced pressure in a process gas atmosphere, and by depositing the 
lanthanide/transition metal alloys from the gas phase onto the surface of 
the dimensionally stable substrate or onto a layer already present 
thereon, according to the invention a platinum-containing (RE-TM) alloy 
being used for depositing the first magnetic layer. In addition, one or 
both of the two (RE-TM) alloys contain one or more elements from the group 
consisting of Cr, Nb, Ta, Al, Si, Ni and Mo. 
In the sputtering targets, the concentration of platinum and, where 
relevant, of Cr, Nb, Ta, Al, Si, Ni and Mo may be spatially constant or 
may change as a function of the radius or in the horizontal direction. 
This change in the concentrations may be continuous or discontinuous. 
Data in the form of magnetically reversed spots can be recorded on the 
novel magnetooptical computer disks in a conventional manner from the side 
of the optically transparent dimensionally stable substrate with the aid 
of a pulse-modulated write laser beam which is focused on the novel 
magnetooptical double layers, strikes them at right angles and has a 
wavelength .lambda. of less than 1,000 nm. Thereafter, the data can be 
read with the aid of a continuous-wave laser beam focused on said double 
layers and striking them at right angles, the light reflected by the 
reproducing layers themselves or by reflector layers being measured, 
analyzed and converted into signals. In the case of the novel 
magnetooptical computer disks, the conventional laser-optical disk drives 
having laser-optical heads which contain semiconductor lasers can be used 
for this purpose. 
The novel magnetooptical computer disks have particular advantages. 
The novel computer disks have substantially improved magnetooptical read 
signals and considerably improved data stability. 
In addition, the novel magnetooptical double layers can be adapted in an 
excellent but simple manner to the property profile of the other layers 
which may be present in the novel magnetooptical computer disks, resulting 
in possibilities for optimizing magnetooptical computer disks which were 
unknown to date and/or previously thought to be impossible to realize. 
The novel magnetooptical computer disks have high corrosion stability. 
Compared with the known magnetooptical computer disks, there is also good 
corrosion protection at the points particularly at risk.

EXAMPLES 1 TO 5 
The composition of the magnetic layers was chemically analyzed in each case 
by ICP (induced coupled plasma) spectroscopy. The Kerr angle was measured 
using an He-Ne laser of wavelength 633 nm, from the air side, at right 
angles to the plane of the layer, as a function of a variable external 
magnetic field. The deposited magnetic layers have an axis of easy 
magnetization in a direction at right angles to the surface of the thin 
layer. 
To investigate the corrosion stability, the layers were stored in a 
conditioning cabinet at a constant temperature and humidity (80.degree. C, 
80% relative humidity). The measurement of the Kerr angle was repeated at 
certain time intervals. It was found that the residual Kerr angle 
decreases with increasing storage time. The decrease depends on the 
chemical composition of the magnetic layer and can be characterized by the 
parameter t.sub.0.5, which is the time after which the residual Kerr angle 
has decreased to 50% of its initial value. 
EXAMPLE 1 
A target consisting of Tb, Fe, Co and Pt was used in a d.c. voltage 
sputtering unit. A glass sheet (diameter 130 mm) was arranged parallel to 
the target, at a distance of 65 mm. The vacuum chamber of the sputtering 
unit was evacuated to a reduced pressure of 1.times.10.sup.-7 mbar. 
Thereafter, Ar gas was introduced to a pressure of 6.times.10.sup.-3 mbar. 
By applying a d.c. voltage at a sputtering power of 500 W, a thin layer 
having a thickness of 80 nm and the composition (Tb.sub.21 Fe.sub.58 
Co.sub.21).sub.91 Pt.sub.9 was deposited. 
It was found experimentally that the residual Kerr angle of the (Tb.sub.21 
Fe.sub.58 Co.sub.21).sub.91 Pt.sub.9 layer is 20% greater than the 
residual Kerr angle of a comparative layer without Pt. It was also found 
that the coercive force of the layer is less than 100 kA/m and the layer 
is therefore unsuitable for magnetooptical data storage. The t.sub.0.5 
value of the layer was determined as 40 h. 
EXAMPLE 2 
A target consisting of Tb, Fe, Co, Cr and Pt was used in a d.c. voltage 
sputtering unit. A glass sheet (diameter 130 mm) was arranged parallel to 
the target, at a distance of 65 mm. The vacuum chamber of the sputtering 
unit was evacuated to a reduced pressure of 1.times.10.sup.-7 mbar. 
Thereafter, Ar gas was introduced to a pressure of 6.times.10.sup.-3 mbar. 
By applying a d.c. voltage at a sputtering power of 500 W, a thin layer 
having a thickness of 80 nm and the composition (Tb.sub.21 Fe.sub.58 
Co.sub.21).sub.87 Pt.sub.9 Cr.sub.4 was deposited. 
It was found that the residual Kerr angle of the layer is 20% greater than 
the residual Kerr angle of a comparative layer without Pt. It was also 
found that the coercive force of the layer is less than 100 kA/m, and the 
layer is therefore unsuitable for magnetooptical data storage. The 
t.sub.0.5 value of the layer was determined as 700 h. The corrosion 
stability of the deposited layer is thus substantially increased compared 
to with the layer from Example 1. 
EXAMPLE 3 
A first target, consisting of Tb, Fe, Co, Cr and Pt, and a second target, 
consisting of Tb, Fe, Co and Cr, were used in a d.c. voltage sputtering 
unit. A glass sheet (diameter 130 mm) was arranged parallel to the 
targets, at a distance of 65 mm. The vacuum chamber of the sputtering unit 
was evacuated to a reduced pressure of 1.times.10.sup.-7 ; mbar. 
Thereafter, Ar gas was introduced to a pressure of 6.times.10.sup.-3 mbar. 
By applying a d.c. voltage at a sputtering power of 500 W to the first 
target, a thin layer having a thickness of 30 nm and the composition 
(Tb.sub.21 Fe.sub.58 Co.sub.21).sub.87 Pt.sub.9 Cr.sub.4 was initially 
deposited. By applying a d.c. voltage at a sputtering power of 500 W to 
the second target, a thin layer having a thickness of 50 nm and the 
composition Tb.sub.20 Fe.sub.48 Co.sub.22 Cr.sub.10 was then deposited. 
The Kerr angle of the double layer was measured from the substrate side as 
a function of a variable external magnetic field, at right angles to the 
plane of the layer. The residual Kerr angle measured was 20% greater than 
the Kerr angle of a comparative layer without Pt. It was also found that 
the coercive force of the layer system is greater than 200 kA/m. The layer 
system is thus suitable for magnetooptical data storage. 
In the investigation of the corrosion stability, the measurement of the 
Kerr angle was repeated from the air side and substrate side at certain 
time intervals. It was found that the t.sub.0.5 value of the stated layer 
exceeds 700 h. The novel layer system thus has better corrosion stability 
than the layer from Example 2. 
EXAMPLE 4 
A first target consisting of Tb, Fe, Co and Pt, and a second target, 
consisting of Tb, Fe, Co and Cr, were used in a d.c. voltage sputtering 
unit. Six glass sheets (diameter 130 mm) were each arranged parallel to 
the targets, at a distance of 65 mm. The vacuum chamber of the sputtering 
unit was evacuated to a reduced pressure of 1.times.10.sup.-7 mbar. 
Thereafter, Ar gas was introduced to a pressure of 6.times.10.sup.-3 mbar. 
By applying a d.c. voltage at a sputtering power of 500 W to the first 
target, a layer having the composition (Tb.sub.21 Fe.sub.58 
Co.sub.21).sub.96 Pt.sub.4 was deposited on each of the six glass sheets. 
The layer thicknesses were 10, 20, 30, 40, 50 and 60 nm. By applying a 
d.c. voltage at a sputtering power of 500 W to the second target, a thin 
layer having a thickness of 50 mm and the composition Tb.sub.20 Fe.sub.48 
Co.sub.22 Cr.sub.10 was then deposited. 
EXAMPLE 5 
The production of a total of six double layer systems, each on a glass 
sheet, was carried out as in Example 4, except that the reproducing layer 
had the composition (Tb.sub.21 Fe.sub.58 Co.sub.21).sub.91 Pt.sub.9. 
The data measured on the double layer systems of Examples 4 and 5 are shown 
in Table 2. In this Table, H.sub.c is the coercive force, .theta..sub.k 
/.theta..sub.k (0) is the increase in the Kerr angle of the double layer 
system compared with the Kerr angle of a single-layer system having the 
composition of the storage layer, and d is the thickness of the 
reproducing layer. 
TABLE 2 
______________________________________ 
H.sub.c [kA/m] .theta..sub.k /.theta..sub.k (0) 
d [nm] Example 4 Example 5 Example 4 
Example 5 
______________________________________ 
0 780 780 1.0 1.0 
10 570 380 1.25 1.25 
20 430 215 1.56 1.58 
30 420 205 1.76 1.76 
40 402 165 1.73 1.82 
50 370 132 1.72 1.80 
60 360 123 1.63 1.76 
______________________________________ 
When Examples 4 and 5 (cf. Table 2) are compared by way of example, it is 
found that the desired property profile can be established via the 
composition of the reproducing layer and storage layer and their 
thicknesses.