Magnetooptical recording medium and method of producing the same

To obtain a magnetooptical recording medium with a magnetooptic effect suitable for ultraviolet radiation, x-layers and y-layers are sequentially laminated on a glass substrate, where x is a TbFeCo layer and y is a Pt or NdCo layer. This multilayer thin film structure can provide a hysteresis loop of a rectangular ratio 1 because a vertical magnetic anisotropic constant Ku is larger than a demagnetizing energy of 2.pi.Ms.sup.2 (where M is a saturated magnetization). FOM (Figure of Merit)=R.sqroot. [.theta..sub.k.sup.2 +.eta..sub.k.sup.2 ] where .theta..sub.k is a Kerr rotation angle and .eta..sub.k is a Kerr ellipticity) is 0.05 or more in the range of ultraviolet rays (of a wavelength of 400 nm or less).

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a magnetooptical recording medium which 
records information utilizing a magnetooptic effect to ultraviolet 
radiation with a wavelength of less than 400 nm of a metal thin film or 
multilayer thin film and to a method of producing the same. 
2. Description of the Related Art 
Conventionally, metal films including a TbFeCo (Terbium/Iron/Cobalt) 
amorphous film, a Bi (bismuth)-Substitutional garnet thin film, and CoPt 
(Cobalt/platinum) multilayer thin film have been widely used as 
magnetooptical recording mediums. These thin films are significant to a 
polar rotation angle (hereinafter merely referred to as Kerr rotation 
angle) or Kerr ellipticity (e.g. more than 0.1.degree.) of a magnetooptic 
effect to visible rays ranging 400 to 860 nm. Those thin films have a 
large vertical magnetic anisotropy which provides the magnetization 
direction perpendicular to the thin film. Typically, the TbFeCo amorphous 
thin film family is practically used. 
An amorphous film of TbFeCo is disclosed in a paper written by T. Suzuki, 
J. Appl. Phys. 69(8), pp 4756-4760 (1991) and both Bi-substitution garnet 
thin film and CoPt multilayer thin film are disclosed in a paper written 
by W. B. Zeper, F. Greidanus and P. F. Carcia; IEEE Trans. MAG25, pp 
3764-3766 (1989). 
There has been a strong demand to increase the recording density of the 
recording medium. Although a magnetooptical recording medium can record 
data of a higher density as compared to a conventional magnetic recording 
medium, further increases in the recording density of the magnetooptical 
recording medium are desired. 
Techniques including the magnetic super-resolution method and the magnetic 
domain expanding method have been proposed to increase recording density. 
Basically, the spot diameter of a recording beam must be decreased by 
shortening the wavelength of light. 
The TbFeCo amorphous thin film, currently and popularly used as a 
magnetooptical recording medium, has a Kerr rotation angle of more than 
0.3.degree. adjacent to a wavelength of 760 nm. However, it is well-known 
that the Kerr rotation angle and Kerr ellipticity decrease as the 
wavelength is shortened, so that the reproduced signal strength remarkably 
decreases. It is predicted that shortening the wavelength of light will 
lead to enable optical recording in high density by ultraviolet rays. For 
that reason, the TbFeCo series amorphous thin film currently used cannot 
be used for the magnetooptical recording medium which uses a 
short-wavelength beam such as ultraviolet ray. 
It is known that the Co-Pt multilayer thin film, or Bi-substitution garnet 
thin film has a large polar Kerr effect near to 400 nm or more. However, 
since these materials have a crystalline structure and produce large 
reproduction noises due to the crystalline grains, they are unsuitable in 
practical use. 
Since the R-TM alloy thin film (where R is a rare earth element such as Tb 
or Dy, and TM is a transition metal such as Fe, Co, or Ni) has an 
amorphous structure, the grain noise is very small (by 5 to 10 dB (30 kHz 
band width)) and the S/N ratio is large, compared with the crystalline 
film. This alloy thin film is put to practical use. Much study and 
research has been devoted to the alloy thin film to improve the Kerr 
rotation angle for short wavelengths. For example, the enhancement effect 
of Pt is disclosed in Japanese Patent Laid-Open Publication No. Hei 
5-128600 (JP-A-05-12860), while the enhancement effect of a light rare 
earth element is disclosed in Japanese Patent Nos. 1949740(JP-B-1949740) 
and 2026003(JP-B-2026003), and in Japanese Patent Laid-Open Publication 
Nos. 6-103621(JP-A-06-103621) and 6-60452(JP-A-06-60452). 
However, with the exception of Japanese Patents No. 1949740 and 2026003, 
these references only discuss a wavelength region .lambda..gtoreq.400 nm 
(a photon energy of less than about 3 eV). Although Japanese Patents No. 
1949740 and 2026003 disclose experimental data for the wavelengths 
reaching 200 nm, they do not disclose accurate measurements of wavelengths 
less than 400 nm because of light absorption. Therefore, wavelengths 
shorter than 400 nm have not been studied. 
A magnetooptical recording medium requires a large vertical magnetization 
anisotropy to obtain an orientation of magnetization perpendicular to the 
film surface. Specifically, it is necessary to set the magnetic 
anisotropic constant Ku to a value larger than the demagnetizing field 
energy 2.pi.Ms.sup.2. 
A magnetooptical recording medium which has a sufficient magnetooptic 
effect and a large vertical magnetic anisotropy in the ultraviolet-ray 
region is therefore greatly desired. 
SUMMARY OF THE INVENTION 
An objective of the present invention is to provide a magnetooptical 
recording medium which has a magnetooptic effect suitable for ultraviolet 
radiation. 
Another objective of the invention is to provide a method of producing a 
magnetooptical recording medium which has a magnetooptic effect relative 
to ultraviolet radiation. 
A magnetooptical recording medium according to the present invention may 
comprise a (R-TM) thin film, where R is a rare earth element of 15 to 50 
by atomic % and TM is an iron group transition metal; at least 70% of R 
being Tb and/or Dy and the remainder of R being at least one element 
selected from the group consisting of Y, La, Ce, Sm, Gd, Tb, Dy, Ho, Er, 
and Yb; the TM being Fe and/or Co; the content of the Fe ranging 0 to 
100%; the magnetooptical recording medium having a vertical magnetic 
anisotropic constant Ku larger than a demagnetizing field energy of 
2.pi.Ms.sup.2 (where Ms is a saturation magnetization); and FOM (Figure of 
Merit)=R.sqroot. [.theta..sub.k.sup.2 +.eta..sub.k.sup.2 ] is 0.05 or more 
in a range of ultraviolet radiation with a wavelength of 400 nm or less, 
where .theta..sub.k is a Kerr rotation angle and .eta..sub.k is a Kerr 
ellipticity. 
Another magnetooptical recording medium according to the present invention 
comprises a (R-TM-M) thin film, where R is a rare earth element of 15 to 
50 by atomic %, TM is an iron group transition metal and M is a metal; at 
least 70% of R is Tb and/or Dy, and the remainder of R being at least one 
element selected from the group consisting of Y, La, Ce, Sm, Gd, Tb, Dy, 
Ho, Er, and Yb; the TM being Fe and/or Co; the content of the Fe ranging 0 
to 100%; M is at least one element selected from the group consisting of 
Pt, Ta, Pd, Zr, Nb, Mo, Ru, Rh, Cu, Ag, Au, and W; the magnetooptical 
recording medium has a vertical magnetic anisotropic constant Ku larger 
than a demagnetizing field energy of 2.pi.Ms.sup.2 (were Ms is a saturated 
magnetization); and FOM (Figure of Merit)=R.sqroot. [.theta..sub.k.sup.2 
+.eta..sub.k.sup.2 ] is 0.05 or more in a range of ultraviolet radiation 
with a wavelength of 400 nm or less, where .theta..sub.k is a Kerr 
rotation angle and .eta..sub.k is a Kerr ellipticity. 
According to the present invention, a multilayer thin film may be formed of 
either a (R-TM) thin film and a M' thin film, or a (R-TM-N) thin film and 
a M' thin film. 
The M' thin film is formed of at least one element selected from the group 
consisting of Pt, Ta, Pd, Zr, Nb, Mo, Ru, Rh, Cu, Ag, Au, and W. The 
magnetooptical recording medium has a vertical magnetic anisotropic 
constant Ku larger than a demagnetizing field energy of 2.pi.Ms.sup.2 
(where Ms is a saturated magnetization), and FOM (Figure of 
Merit)=R.sqroot. [.theta..sub.k.sup.2 +.eta..sub.k.sup.2 ] is 0.05 or more 
in a range of ultraviolet radiation with a wavelength of 400 nm or less, 
where .theta..sub.k is a Kerr rotation angle and .eta..sub.k is a Kerr 
ellipticity. 
Moreover, according to the present invention, the M' thin film may 
preferably have a thickness of 15 .ANG. or less. 
According to the present invention, a multilayer thin film is formed of 
either a (R-TM) thin film and a (R+TM') thin film or a (R-TM-M) thin film 
and a (R-TM') thin film. R' is a rare earth element of Nd and/or Pr of 20 
to 80 by atomic % and the remaining R' contains at least one element 
selected from the group consisting of Y, La, Ce, Sm, Gd, Tb, Dy, Ho, Er 
and Yb, while TM' contains Fe and/or Co. The magnetooptical recording 
medium has a vertical magnetic anisotropic constant Ku larger than a 
demagnetizing field energy of 2.pi.Ms.sup.2 (where Ms is a saturated 
magnetization), and FOM (Figure of Merit)=R.sqroot. [.theta..sub.k.sup.2 
+.eta..sub.k.sup.2 ] is 0.05 or more in a range of ultraviolet radiation 
with a wavelength of 400 nm or less, where .theta..sub.k is a Kerr 
rotation angle and .eta..sub.k is a Kerr ellipticity. 
The above-mentioned magnetooptical recording medium can preferably be 
produced by forming a thin film on a substrate using the metal gal 
condensing method. 
In the magnetooptical medium, FOM (Figure of Merit)=R.sqroot. 
[.theta..sub.k.sup.2 +.eta..sub.k.sup.2 ], where .theta..sub.k is a Kerr 
rotation angle and .eta..sub.k is a Kerr ellipticity) is 0.05 or more in a 
range of ultraviolet radiation with a wavelength of 400 nm or less. This 
feature enables high density recording and reproduction to be performed 
using ultraviolet radiation with a small spot diameter. 
Such a metal film according to the present invention has a vertical 
magnetic anisotropic constant Ku larger than a demagnetizing field energy 
of 2.pi.Ms.sup.2 (where Ms is a saturated magnetization). Hence, the metal 
thin film has a vertical magnetic anisotropy that is radial to the 
magnetooptical recording medium. High-density magnetooptical recording can 
be achieved by utilizing this vertical magnetic anisotropy. The metal thin 
film has a negative Kerr rotation angle to visible rays. However, as the 
wavelength of the irradiated light is shortened, the absolute value of the 
Kerr rotation angle increases and then decreases. The metal thin film may 
have a positive Kerr rotation angle is a short wavelength range. A 
sufficiently large FOM can be obtained in the range of ultraviolet rays of 
less than 400 nm. 
A multilayer thin film wherein the (R-TM) or (R-TM-M) film and the M' film 
are alternately laminated can provide a large FOM value. Hence a suitable 
magnetooptical recording medium with a large FOM can be obtained by 
utilizing the multilayer thin film structure. 
Because the M' film is not a magnetic thin film, an excessively thick M' 
film causes the coercive force to be decreased, so that recording and 
reproduction can not be sufficiently accomplished. A sufficient 
magnetooptic effect can be obtained by setting the M' film to a thickness 
of less than 15 .ANG., while the coercive force can be maintained at more 
than a predetermined value. 
A multilayer thin film where the (R-TMN) or (R-TM-M) film and the (R'-TM') 
film are alternately laminated can provide a sufficiently large 
magnetooptic effect as well as a sufficient vertical magnetic anisotropy. 
Preferably, a heavy rare earth element is utilized as R, while a light 
rare earth element such as Nd or Pr is utilized as R'. 
For combinations of a rare earth metal and a transition metal, combinations 
of a heavy rare earth metal and Fe are the most common for vertical 
magnetic films. TbFe, GdFe, DyFe, GdTbFe, TbDyFe, and the like are typical 
of such combinations. For example, TbFe is characterized by a Curie point 
Tc of 140 to 250.degree. C., a Kerr rotation angle .theta..sub.k of 
approximately 0.3.degree., a saturated magnetization Ms of 50 to 100 
emu/cc, and a vertical magnetic anisotropic constant Ku of 10.sup.5 to 
10.sup.6 erg/cc. 
Heavy rare earth elements such as Tb, Dy, or Gd are rare resources in the 
Earth's crust and require a very complicated separation process. As a 
result, these elements are very expensive. 
Because the atomic magnetic moment of the heavy rare earth element and the 
atomic magnetic moment of Fe are coupled to each other reversely and in 
parallel, the saturated magnetization Ms and the Currie point Tc depend 
largely on the composition, so that it is difficult to mass-produce 
uniform, quality products. Unlike the heavy rare earth elements, light 
rare earth elements such as Nd and Pr are broadly distributed in the 
Earth's crust. 
Although it has been previously reported that, while the light rare earth 
metal-iron group transition metal amorphous thin film typified by NdFe 
increases its magnetooptic effect as the wavelength of light is shortened, 
it sharply decreases around a wavelength of 400 nm, an experiment by the 
present inventors found that the absolute value of the Kerr rotation angle 
increases even for wavelengths less than 400 nm (International Application 
No. PCT/JP97/02415). It is considered that the enhancement effect results 
from an effect of the light rare earth element. 
It has been reportedly considered that the amorphous thin film made of a 
light rare earth element and iron has a high saturated magnetization but 
cannot provide a vertical magnetic anisotropic energy which can cancel the 
demagnetizing field effect of the thin film. Moreover, since the 
hysteresis loop of a magnetic curve does not indicate a sufficiently large 
vertical magnetic anisotropy, but has the so-called "Snake" curve, it is 
impossible to have a hysteresis loop with a large rectangular ratio. This 
poor rectangular ratio makes a recording bit unstable and deteriorates the 
S/N ratio. 
However, the light rare earth element-iron group transition metal amorphous 
thin film according to the present invention can provide a large magnetic 
magnetooptic effect in the region of ultraviolet radiation. Moreover, the 
multilayer thin film structure according to the present invention can 
provide a hysteresis loop with a high rectangular characteristic. 
Moreover, the use of the light rare earth element-iron group transition 
metal amorphous thin film according to the present invention allows a 
sufficient vertical magnetic anisotropy and a sufficient rectangular ratio 
to be obtained. In addition, the consumption of heavy rare earth elements 
can be reduced.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Next, preferred embodiments of the present invention will be described 
below with reference to the attached drawings. 
A thin film is first deposited on a substrate through a sputtering and 
vapor-depositing process. An RF magnetron sputtering process or DC 
magnetron sputtering process may be employed as the sputtering process. 
The magnetooptical recording medium according to the present invention can 
also be produced with various metal gas condensing methods, and is not 
limited to the above-mentioned sputtering process. For example, ion beam 
sputtering, MBE (molecular beam epitaxy), CVD (chemical vapor deposition), 
or vacuum deposition may also be used with this invention. 
In an experiment to be described later, Tb.sub.28 Fe.sub.64.8 Co.sub.7.2 
and Pt were used as the target material. 
FIG. 1 shows the dependency on argon pressure and deposition rate of the 
hysteresis loop of a TB.sub.23 (FeCo).sub.77 single layer. Referring to 
FIG. 1, the rectangular ratio is 1 to all film growth rates under a 
pressure of 5 mTorr. The slower the film growth rate, the larger the 
coercive force (Hc). 
The conditions for production of a TbFeCo layer, Pt, and NdCo are decided 
based on the experimental results. Particularly, the film growth rate is 
set to a slower value to obtain a higher coercive force. That is, the film 
growth rate at the atmosphere or argon under a pressure of 5 mTorr is 
about 0.5 .ANG./sec. 
The substrate is a glass substrate and the substrate temperature is 
maintained at a room temperature. The entire multilayer thin film 
structure is formed of a X.ANG.TbFeCo/Y.ANG.Pt film, where X=10 to 50, and 
Y=0.5 to 12. 
FIG. 2 shows a schematic structure of a metal multilayer thin film for a 
magnetooptical recording medium. A y-layer and a x-layer are sequentially 
laminated on the glass substrate, where x is a TbFeCo layer and y is a Pt 
or NbCo layer. The thickness of the entire thin film is about 500 .ANG.. 
As the x-layer is used a (R-TM) film formed of a rare earth element R by 15 
to 50 atomic % (where 70% of R or more is Tb and/or Dy and the remaining 
of R is at least one selected from the group consisting of Y, La, Ce, Sm, 
Gd, Tb, Dy, Ho, Er, and Yb) and an iron group transition metal TM (where 
TM is formed of Fe of 0 to 100% and Co). The x-layer may preferably 
contain M of several % or less (where M is at least about one element 
selected from the group consisting of Pt, Ta, Pd, Zr, Nb, Mo, Ru, Rh, Cu, 
Ag, Au, and W). The x-layer may formed of a single layer of Pt, Ta, Pd, 
Zr, Nb, Mo, Ru, Rh, Cu, Ag, Au, or W. 
The y-layer may be preferably formed of M' of at least one selected from 
the group consisting of Pt, Ta, Pd, Zr, Nb, Mo, Ru, Rh, Cu, Ag, Au, and 
W). The y-layer may be preferably formed of a (R'-TM') thin film, where 
R', representing a rare earth element, that is, Nd and/or Pr of 20 to 80% 
by atomic %, and the remaining of R' is at least one element selected from 
the group consisting of Y, La, Ce, Sm, Gd, Tb, Dy, Ho, Er, and Yb, and TM' 
is of Fe and/or Co. 
Those metal films are characterized in that the vertical magnetic 
anisotropic constant Ku is larger than a demagnetizing field energy of 
2.pi.Ms.sup.2 (where Ms is a saturated magnetization) and that FOM (Figure 
of Merit)=R.sqroot. [.theta..sub.k.sup.2 +.eta..sub.k.sup.2 ] is 0.05 or 
more within a range of ultraviolet rays of a wavelength of 400 nm or 
less), where .theta..sub.k is a Kerr rotation angle and .eta..sub.k is a 
Kerr ellipticity ratio. 
Embodiment 1 
A metal multilayer thin film of TbFeCo/Pt is formed using the sputtering 
and vaporizing process. A Pt layer is used as the y-layer shown in FIG. 2. 
The total thickness is about 500 .ANG.. 
FIG. 3 shows the dependency on photon energy of Kerr rotation angle (in 
degree) and Kerr ellipticity (in degree) of a 50 .ANG. TbFeCo(Pt)/X.ANG.pt 
film (where X=0, 1, 2, 3, 6, and 12 .ANG.). The wavelength (.ANG.) is 
represented by 12400/photon energy. The content of Pt in TbFeCo(Pt) is 
0.5% or less. 
It is understood that the absolute value of a Kerr rotation angle increases 
over all photon energy ranges as Pt increases. The absolute value of a 
Kerr ellipticity .eta..sub.k decreases at less than 4.5 eV (2,756 .ANG.), 
but tends to increase at 4.5 or more eV. 
FIG. 4 shows a dependency on photon energy of the FOM value of a 50 
.ANG.TbFeCo(Pt)/X.ANG.Pt film (where X=1, 3, 6, and 12 .ANG.). When X=3, 
6, and 12 .ANG., the FOM increases. 
FIG. 5 shows a dependency on photon energy of the Kerr rotation angle and 
Kerr ellipticity of a 30 .ANG.TbFeCo(Pt)/X.ANG. film (where X=0.5, 1, 2, 
3, and 6 .ANG.). The absolute value of a Kerr rotation angle increases 
with an increase in Pt. The absolute value of a Kerr ellipticity increases 
at 5 eV or more with an increase in Pt. 
FIG. 6 shows a dependency on photon energy of the FOM value of a 30 
.ANG.TbFeCo(Pt)/X.ANG.Pt film. The FOM value increases over all X values 
including 6 .ANG., compared with that of the TbFeCo single layer. 
FIG. 7 shows a dependency on photon energy of the Kerr rotation angle and 
Kerr ellipticity of a 10 .ANG.TbFeCo(Pt)/X.ANG.Pt film (where X=0.5, 1, 
and 2 .ANG.). The absolute value of Kerr rotation angle increases with an 
increase in Pt while the absolute value of Kerr ellipticity increase at 5 
eV or more with an increases in Pt. 
FIG. 8 shows a dependency on photon energy of the FOM value of a 10 
.ANG.FeCo(Pt)/X.ANG.Pt film (where X=0.5, 1, and 2 .ANG.). Particularly, 
in the case of X=2 .ANG., this film structure can provide a larger FOM 
value than that of the single layer film over all wavelength regions. 
FIG. 9 shows a composition between a 50 .ANG.TbFeCo(Pt)/X.ANG.Pt film and a 
single layer film of TbFeCo in (a) Kerr rotation angle .theta..sub.k, (b) 
Kerr ellipticity .eta..sub.k and (c) FOM value at a photon energy of 5 eV. 
The Kerr rotation angle and Kerr ellipticity tend to be saturated with the 
Pt film of about 3 .ANG.. Under the above-mentioned condition, a 
sufficient effect can be obtained with the Pt layer of about 3 .ANG.. 
FIG. 10 shows the hysteresis loop of a Kerr effect (X=3, 6, and 12 .ANG.). 
The applied magnetic direction is perpendicular to the film surface. Any 
one of the films has a large vertical magnetic anisotropy Ku and the 
magnetization direction is perpendicular to the film surface with no 
magnetic direction applied. As the thickness of the Pt film increases, the 
coercive force Hc decreases. However, it is preferable to set the 
thickness of the Pt film to less than 12 .ANG.. 
FIG. 11 shows a dependency on a Pt layer of the coercive force Hc (10 
.ANG., 30 .ANG., 50 .ANG. TbFeCo(Pt)/X.ANG.Pt). In the TbFeCo(Pt) film has 
a thickness of 30 or 50 .ANG., when the Pt layer is very thin, Hc 
increases and then sharply decreases. When the Pt layer is 6 .ANG., Hc is 
relatively small. In the 10 .ANG.TbFeCo film, Hc is small even when the Pt 
layer is 1 .ANG.. Hence, in such a multilayer thin film, it is preferable 
to set the thickness of the Pt layer to 15 .ANG. or less. 
FIG. 12 shows a dependency on the thickness Pt of the vertical magnetic 
anisotropy coefficient Ku. In the 50 .ANG.TbFeCo film, Ku tends to be 
nearly saturated with the Pt layer of the thickness of about 3 .ANG. or 
more. 
Embodiment 2 
A metal multilayer thin film TbFeCo/NdCo is laminated using, for example, 
the sputtering process. In FIG. 2, a NdCo layer is used as the y-layer. 
The total thickness of the film is about 500 .ANG.. 
FIG. 13 shows a dependency on photon energy of the Kerr rotation angle and 
Kerr ellipticity of a 50 .ANG.TbFeCo/X.ANG.NdCo multilayer thin film 
(where X=1, 3, and 6 .ANG.). The absolute layer of the Kerr rotational 
angle decreases when the Pt film has a thickness of 1 .ANG. but decreases 
when the Pt thickness has a thickness of 3 .ANG. or more. When the Pt film 
has a thickness of 6 .ANG. or more, the absolute value of the Kerr 
rotation angle increases sharply. The Kerr ellipticity increases at more 
than about 5 eV with an increase in Pt. 
FIG. 14 shows a dependency on photon energy of FOM value (=R.sqroot. 
[.theta..sub.k.sup.2 +.eta..sub.k.sup.2 ]) of a 50 .ANG.TbFeCo/X.ANG.NdCo 
multilayer thin film. The FOM value of the multilayer thin film 
corresponding to X=6 increases over all photon energy ranges, compared 
with that of the single-layer film. 
FIG. 15 shows a dependency on photon energy of the Kerr rotation angle and 
Kerr ellipticity of a 30 .ANG.TbFeCo/X.ANG.NdCo film (where X=0.5, 1, 3, 
and 6 .ANG.). Particularly, when X=0.5 .ANG., both the absolute value of a 
Kerr rotation angle and the absolute value of a Kerr ellipticity are 
larger than that of the single layer film in the range of ultraviolet ray 
with a photon energy of 4 eV or more. 
FIG. 16 shows a dependency on photon energy of the FOM value of a 30 
.ANG.TbFeCo/X.ANG.NdCo film (where X=0.5, 1, 3, and 6 .ANG.). When X=6, 
the FOM value of a multilayer thin film increases over a photon energy of 
more than 4 eV, as compared with that of a single layer film. 
FIG. 17 shows a dependency on photon energy of the Kerr rotation angle and 
Kerr ellipticity of a 10 .ANG.TbFeCo/X.ANG.NdCo film (where X=1, 3, and 10 
.ANG.). Particularly, in the case of X=3 and 10 .ANG., the absolute value 
of a Kerr rotation angle is comparatively larger than that of the single 
film layer. 
FIG. 18 shows a dependency on photon energy of the FOM value of a 10 
.ANG.TbFeCo/X.ANG.NdCo film (where X=1, 3, and 10 .ANG.). When X=10, the 
FOM value of the multilayer thin film largely increases over all photon 
energy ranges, as compared with that of the single layer film. 
Particularly, the tendency is noticeable over a photon energy of 3 eV or 
more. When X=1, the FOM value is sufficiently larger than that of the 
single film layer over a photon energy of 4 eV or more. 
FIG. 19 shows a Kerr rotation angle hysteresis loop of 10 
.ANG.TbFeCo/X.ANG.NdCo multilayer thin film (where X=0, 1, 3, and 10). The 
multilayer thin film of (TbFeCo/NdCo) shows a high coercive force having a 
rectangular ratio of 1. When X=1 or 3, the multilayer thin film has a 
sufficient coercive force. 
FIG. 20 shows the dependency on temperature of a hysteresis loop of the 
Kerr rotation angle of a ((10 .ANG.TbFeCo/10 .ANG.NdCo).times.25) layer. 
This multilayer thin film indicates a rectangular ratio of 1 over the 
temperature ranges. 
FIG. 21 shows the dependency on X of the coercive force of a (10, 30, 
50).ANG.TbFeCo/X.ANG.NdCo film at a temperature of 80.degree. C. As a 
hole, as the thickness of NdCo increases, the coercive force tends to 
decrease but is relatively large. 
Embodiment 3 
A metal multilayer thin film TbFeCo/Pt is formed using, for example, the 
sputtering process. In the third embodiment, a Pt reflective layer of 75 
nm is formed on a glass substrate, as shown in FIG. 22. y-layer (Pt layer) 
and x-layer (TbFeCo layer) are sequentially laminated on the reflective 
layer. The thickness of each of the layers x and y is 25 nm. In this 
embodiment, A Pt protective layer is not formed on the uppermost x-layer. 
Hence, it is considered that the quality of the multilayer thin film can 
be accurately checked, with the effect of the Pt protective layer 
precluded. 
FIG. 23 shows a dependency on photon energy of the Kerr rotation angle and 
Kerr ellipticity of a 50 .ANG.TbFeCo(Pt)/Y.ANG.Pt multilayer thin film 
(where Y=1, 3, 6 .ANG.). The absolute value of the Kerr rotational angle 
of the multilayer thin film is larger than the single layer film at a 
photon energy of 4 eV or more. The thicker the thickness of the Pt layer, 
the larger the absolute value. 
FIG. 24 shows the relationships between the thickness Y of a Pt layer, Kerr 
rotation angle ratio (the absolute value of a ratio to the Kerr rotation 
angle of a single layer), and FOM value ratio (a FOM value ratio of a 
single film) at a photon energy of 5 eV of a 50 .ANG.TbFeCo(Pt)/Y.ANG.Pt 
multilayer thin film. The thicker the thickness of the Pt layer, the 
larger both the Kerr rotation angle ratio and FOM value ratio. The effect 
of the Pt layer is nearly saturated at the thickness Y of about 10 .ANG.. 
FIG. 25 shows a dependency on photon energy of both Kerr rotation angle and 
Kerr ellipticity of a X.ANG.TbFeCo(Pt)/6.ANG.Pt film. When the thickness X 
of a TbFeCo(Pt) is 10 .ANG., the absolute value of the Kerr rotation angle 
has a relatively large value (e.g. about -0.35) at a photon energy of 
about 5 eV. As the thickness X increase, the Kerr rotation angle becomes 
smaller at a photon energy of 5 eV. With the Pt layer disposed, the 
absolute value of the Kerr ellipticity becomes large at a photon energy of 
5 eV or more. The thicker the Pt layer, the larger the absolute value of 
the Kerr ellipticity. 
FIG. 26 shows a dependency on photon energy of the FOM value of a 
X.ANG.TbFeCo(Pt)/6 .ANG.Pt film. A multilayer thin film including a Pt 
layer has a larger FOM value than that of the single layer film. When the 
thickness X is 10 .ANG., the Pt layer has the highest FOM value. When X=30 
.ANG., the Pt layer has a high FOM value. When X=50 and 80 .ANG., the Pt 
layer has nearly the same FOM value. 
FIG. 27 shows a dependency on photon energy of both Kerr rotation angle and 
Kerr ellipticity of a Z(50 .ANG.TbFeCo(Pt)/6 .ANG.Pt) film, where Z 
represents the ratio of the thickness of each layer to the entire 
thickness in (50 .ANG.TbFeCo(Pt)/6 .ANG.Pt). Z=1/10 means 5 
.ANG.TbFeCo(Pt)/0.6 .ANG.Pt. Z=1 means 50 .ANG.TbFeCo(Pt)/6 .ANG.Pt. In 
the multilayer thin film, the absolute value of the Kerr rotation angle is 
larger than that of a single layer film. Particularly, the multilayer thin 
films has a much larger absolute value than a single layer film at a 
photon energy of 4 to 6 eV. At a photon energy of 3 to 5 eV, the absolute 
value of the Kerr ellipticity of the single layer film is larger than that 
of the multilayer thin film. At a photon energy of 5 eV or more, the 
absolute value of the Kerr ellipticity is larger than that of the single 
layer film. 
FIG. 28 shows a dependency on photon energy of the FOM value of a Z(50 
.ANG.TbFeCo(Pt)/6 .ANG.Pt) film. The multilayer thin film has a larger 
value than the single layer film at a photon energy of 4 eV or more. 
As described above, according to the present invention, FOM (Figure of 
Merit) (=R.sqroot. [.theta..sub.k.sup.2 +.eta..sub.k.sup.2 ] where 
.theta..sub.k is a Kerr rotation angle and .eta..sub.k is a Kerr 
ellipticity) is more than 0.05 in the range of ultraviolet rays (of a 
wavelength of 400 nm or less). This enables high density recording and 
reproduction using ultraviolet radiation in a small spot diameter. 
Moreover, the metal thin film according to the present invention has a 
vertical magnetic anisotropic constant Ku larger than a magnetic energy of 
2 .pi.Ms.sup.2 (where M is a saturated magnetization). Hence, the metal 
thin film has a vertical magnetic anisotropy being a radial magnetic 
anisotropy of a magnetooptical recording medium. High-density 
magnetooptical recording can be accomplished by using the metal thin film 
according to the present invention. 
A multilayer thin film structure in which a (R-TM) or (R-TM-M) film and a 
M' film are alternately laminated can provide a sufficiently large FOM 
value, so that a more suitable magnetooptical recording medium with a 
large FOM value can be realized. The M' film having a thickness of 15 
.ANG. or less can provide a sufficient magnetooptic effect and maintain a 
coercive force equal to or greater than a predetermined value. 
A multilayer thin film structure in which a (R-TM) or (R-TM-M) thin film 
and a M' thin film are laminated can provide a sufficiently large 
magnetooptic effect, a sufficient vertical magnetic anisotropy, and a 
hysteresis loop of a rectangular ratio of 1.