Magneto-optical recording medium

A magneto-optical recording medium, comprising a substrate, a magneto-optical recording layer on the substrate, a transparent thermally-insulating layer of one selected from the group consisting of tantalum oxynitride and tantalum oxide on the magneto-optical recording layer, and a metal reflecting layer on the transparent thermally-insulating layer, the metal reflecting layer having a thermal conduction represented by a product of a thermal conductivity multiplied by a layer thickness of not less than 1.3.times.10.sup.-6 WK.sup.-1. Heat dispersion is improved while maintaining a high recording sensitivity.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a magneto-optical recording medium in 
which information is recorded, reproduced and erased by a light such as a 
laser beam. More specifically, the present invention relates to a 
magneto-optical recording medium having a metal reflecting layer. 
2. Description of the Related Art 
Optical recording media are being investigated and developed because of 
their high density recording and large storage capacity. Particularly, a 
magneto-optical recording medium is desired and various materials and 
systems therefor have been published because of its wide applicability to 
various fields. 
A typical magneto-optical recording medium comprises a magneto-optical 
recording layer of an amorphous rare earth metal-transition metal alloy 
formed on a transparent substrate, the magneto-optical recording layer 
having an axis of easy magnetization perpendicular to the layer. 
Since the magneto-optical recording layer of the amorphous alloy magnetic 
layer is easily oxidized and has a small Kerr rotation angle, resulting in 
an unsatisfactory C/N (carrier/noise) ratio of a reproduced signal, and so 
various proposals have been made to solve these problems. 
For example, a four layer construction of a substrate/a transparent 
dielectric layer/a recording layer/a transparent dielectric layer/a metal 
reflecting layer in this order allows a utilization of Faraday and Ker 
effects together with a Kerr enhancement effect by the dielectric layer, 
thereby providing a high C/N ratio, and the four layer construction also 
provides a high durability by using the metal nitride as the dielectric 
layer. By providing a metal reflecting layer, the recording sensitivity is 
lowered in comparison with the case without the reflecting layer. To 
obtain an adequate recording sensitivity, there is a proposal to lower the 
thermal conduction of the metal reflecting layer by controlling the 
thickness of the metal layer or adding an additive to the metal layer. 
Nevertheless, the present inventors found that in the above four layer 
construction, if the thermal conduction of the metal reflecting layer is 
made lower, the recording sensitivity is improved but several problems 
occur. For example, when information is being recorded, a large shift or 
dislocation of a recording bit from a position to be recorded occurs by a 
thermal influence between the neighboring bits. This bit shift can be 
detected as a shift of the peak position of the reproduced signal. The 
larger the shift of the bit is, the larger the peak shift is and more an 
error in reproduction occurs. Further, during erasing of information in 
which a high power laser beam is irradiated, the recording layer may be 
deteriorated by an over-heat if the construction of the medium does not 
allow a disperse of heat. In fact, in an inventors' investigation, the C/N 
ratios of media were often lowered when a continuous wave erasure laser 
beam (high power) was irradiated. Namely, there is a problem of a medium 
in a long term stability against a laser beam. 
The object of the present invention is to solve the above problems and to 
provide a magneto-optical recording medium having the advantageous 
characteristics derived from the transparent dielectric layer and the 
metal reflecting layer while having a high C/N ratio and recording 
sensitivity as well as a low error rate of reproduced information and an 
excellent long term stability against a laser beam. 
SUMMARY OF THE INVENTION 
The above object of the present invention is attained by a magneto-optical 
recording medium comprising a substrate, a transparent dielectric layer, a 
magneto-optical recording layer and a metal reflecting layer in this 
order, in which at least a transparent heat insulating layer of a tantalum 
oxynitride or oxide is inserted between the magneto-optical recording 
layer and the metal reflecting layer, and the metal reflecting layer is 
selected so as to have a product of a thermal conductivity multiplied by a 
thickness thereof of not less than 1.3.times.10.sup.-6 WK.sup.-1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
One of the reasons for the above mentioned shift of bits during recording 
information, that is, the peak shift, is considered to be a thermal 
interference between the neighboring bits. Namely, it is considered that 
when the magneto-optical recording layer is heated above a temperature 
necessary for recording, the heat diffuses to the neighboring bits and 
affects its shape or position. Further, the reason for the lowering of the 
C/N ratio when a continueous wave erasure laser beam is irradiated with is 
also an over-heating of the magneto-optical recording layer above a 
temperature necessary for erasing. 
Therefore, to prevent the undesirable over-heat, a construction in which 
heat dispersion is improved is necessary, but only improving the thermal 
conductivity or only thickening of the metal reflecting layer result in a 
lowering of the recording sensitivity. Accordingly, considering the 
recording sensitivity together with a small peak shift and an excellent 
long term stability against an erasure laser beam, a construction in which 
the temperature of the recording layer is easily elevated to a certain 
temperature (a recording temperature) but is difficult to elevate more 
than that temperature so that an undesirable over-heat is prevented should 
be created. 
To obtain the above construction, the inventors made attention to a 
combined construction in which the over-heat is prevented by a metal 
reflecting layer having an excellent heat dispersion, and the lowering of 
the recording sensitivity is presented by a heat insulating layer having 
such an excellent heat insulation provided between the recording layer and 
the metal reflecting layer. Then, the inventors confirmed that by 
inserting a layer of tantalum oxynitride (hereinafter referred to as 
"TaON") or tantalum oxide (hereinafter referred to as "TaO") having an 
excellent heat insulation as the transparent heat insulating layer between 
the metal reflecting layer and the magneto-optical recording layer and by 
thickening the metal reflecting layer to improve the thermal conduction of 
the metal reflecting layer, a magneto-optical recording medium having a 
high C/N ratio, a high recording sensitivity, a small peak shift of 
reproducing signal, and an excellent long term stability against a high 
power laser beam could be obtained. 
In the above construction, the temperature of the magneto-optical recording 
layer can be easily elevated because of the heat insulation of the TaON or 
TaO having an extremely low thermal conductivity, thereby allowing a high 
recording sensitivity. This heat insulation or a high recording 
sensitivity is obtainable even if the metal reflecting layer used has a 
considerably high thermal conduction and by this metal reflecting layer 
having a considerably high thermal conduction, an excess heat is dispersed 
and a too much over-heat is prevented. (The thickness of the 
heat-insulating layer or transparent dielectric layer is selected to 
obtain an enhancement of the Kerr effect and in this thickness of the 
transparent dielectric layer, it is desirable that the transparent 
dielectric layer has as a low thermal conduction or a high heat insulation 
as possible, in combination with a high thermal conduction of the metal 
reflecting layer. In this respect, the TaON or TaO has a preferably low 
thermal conduction or a preferably high heat insulation among possible 
transparent dielectric materials. The low thermal conductivity of the TaON 
or TaO does not prevent a necessary heat dispersion by the metal 
reflecting layer.) Furthermore, a high transparency of TaON or TaO also 
attributes to a high C/N ratio of the recording medium. Thus, in 
accordance with the present invention, a high C/N ratio, a high 
sensitivity, a low peak shift, and a long term stability against a high 
power laser beam are obtained. 
In a preferred embodiment, another transparent dielectric layer of a metal 
nitride may be inserted between the magneto-optical recording layer and 
the transparent heatinsulating layer, in order to prevent an oxidation of 
a magneto-optical recording layer which is possible when a TaON or TaO 
layer is prepared in direct contact with the magneto-optical recording 
layer, depending on a method of depositing a TaON or TaO layer (for 
example, an effect of pre-sputtering prior to preparing a TaON or TaO 
layer). 
The thickness of the another transparent dielectric layer is preferably 
thinner as long as it prevents an oxidation of the magneto-optical 
recording layer, to allow as a larger thickness as possible of the heat 
insulating layer, and is generally 2 to 15 nm, preferably 2 to 10 nm, 
since the total thickness of this additional transparent dielectric layer 
of e.g., Si.sub.3 N.sub.4 or AlSiN and the transparent heat insulating 
layer of TaON or TaO is determined so as to obtain an enhancement of the 
Kerr effect. 
A preferred TaON, i.e., tantalum oxynitride in the present invention 
comprises 1 to 45 atomic %, more preferably 1 to 35 atomic % of nitrogen 
and 27 to 72 atomic %, more preferably 35 to 72 atomic % of oxygen, the 
remainder being tantalum, because of its transparency and heat insulation, 
although elements other than Ta, O and N may be contained in an mount of 
an order of impurity. The method for forming a TaON layer may be any thin 
film preparing method, including known PVD such as vacuum evaporation 
method, sputtering methods etc., or CVD. Among them, a reactive sputtering 
method using a Ta.sub.2 O.sub.5 target and a mixed gas of Ar and N.sub.2, 
or a reactive sputtering using a Ta target and a mixed gas of Ar, N.sub.2 
and O.sub.2 is preferred because of less extraordinary arc discharge and 
high deposition rate and thus stable operation and a high productivity. 
A preferred TaO in the present invention comprises 20 to 45 atomic % of Ta 
because of its transparency and heat insulation, the remainder being 
oxygen. The TaO layer may be prepared by various thin film preparing 
methods as mentioned above. In preparing a TaO film, a reactive sputtering 
using a Ta or Ta.sub.2 O.sub.5 target with a mixed gas of Ar and O.sub.2 
as well as a reactive sputtering using a Ta.sub.2 O.sub.5 target with Ar 
gas alone may be considered. However, a sputtering method using Ar gas 
only is not practically applicable because the transparency of the layer 
prepared is poor. 
The TaO prepared by sputtering method using a mixed gas of Ar and O.sub.2 
has a heat insulation and transparency almost equal to those of TaON and 
therefore may be applicable to the present invention in the same as TaON. 
Nevertheless, in preparing a TaO film, oxygen gas must be introduced in a 
vacuum chamber, while in preparing a TaON film, a mixed gas of Ar and N 
with a Ta.sub.2 O.sub.5 target can be used, to obtain a transparent layer. 
Therefore, TaON is superior to TaO when considering the deposition 
process, because the magneto-optical recording layer is not in contact 
with oxygen in the TaON formation process. 
The thickness of the transparent heat insulating layer is preferably 5 to 
50 nm, more preferably 10 to 40 nm considering the enhancement of the Kerr 
effect, although the thicker the thickness, the more the heat insulation. 
In the present invention, the metal reflecting layer has an important role 
of dispersing heat and it provides a larger effect of the invention if it 
has a higher thermal conduction. The thermal conduction of the metal 
reflecting layer is further improved when the thermal conductivity of the 
layer is higher and the thickness of the layer is thicker. In accordance 
with the investigation by the inventors, it was found that the effect of 
the present invention can be considered or compared by the value of a 
product of the thermal conductivity multiplied by the thickness of the 
layer. Thus, when the thermal conduction of the metal reflecting layer is 
defined by this product, in accordance with the present invention, the 
value of a product of the thermal conductivity multiplied by the thickness 
of the metal reflecting layer should be not less than 1.3.times.10.sup.-6 
WK.sup.-1 preferably not less than 2.0.times.10.sup.-6 WK.sup.-1, more 
preferably not less than 3.0.times.10.sup.-6 WK.sup.-1. 
Note that the thermal conductivity of the metal reflecting layer is 
determined by the following procedures. Namely, referring to FIG. 1, a 
metal layer 2 to be measured is prepared on a glass substrate 1 and has a 
width (l.sub.1) of 10 mm and a thickness (d) of 100 nm. Four Au electrodes 
3 to 6 are deposited with an equal space on the metal layer 2, the spacing 
(l.sub.2) between the electrodes being 4 mm. The electric resistance (R) 
of the metal layer 2 is measured by the four terminal method and the 
electric conductivity (.sigma.) and the thermal conductivity (k) are 
determined by the following formulae, where L represents the Lorenz number 
and T represents the measured temperature, and L and T are made 
2.45.times.10.sup.-8 W.OMEGA./K.sup.2 and 300K., respectively. 
EQU .sigma.=(1/R).times.(1.sub.2 /l.sub.1 d) 
EQU k=L.times.T.times..sigma. 
Examples of thus obtained electric and thermal conductivities of an Al 
layer and an AgAuTi and AlAuTi alloy layers described later in Examples 
are shown in Table 1. 
TABLE 1 
______________________________________ 
Electric and thermal conductivities 
of metal layers 
Electric Thermal 
conductivity 
conductivity 
(.OMEGA..sup.-1 m.sup.-1) 
(Wm.sup.-1 K.sup.-1) 
______________________________________ 
Al 1.6 .times. 10.sup.7 
1.2 .times. 10.sup.2 
Ag.sub.95.0 Au.sub.4.1 Ti.sub.0.9 
7.9 .times. 10.sup.6 
5.8 .times. 10.sup.1 
Al.sub.92.7 Au.sub.4.8 Ti.sub.2.5 
1.8 .times. 10.sup.6 
1.3 .times. 10.sup.1 
______________________________________ 
The material of the metal reflecting layer is preferably a metal having a 
high thermal conductivity, from the reasons described before, and Ag, Au, 
Al, Cu or an alloy containing any of these metals as a main component is 
preferably used. Since these metals have a high reflectivity of light, an 
advantage of a high C/N ratio is obtained. Among those, AgAu and AlAu 
alloys are superior in reflectivity of light, thermal conductivity and 
durability, and these alloys further containing Ti are further preferred 
for the durability. 
The thickness of the metal reflecting layer is selected so as to control 
the heat dispersion to an adequate range. If the thermal conductivity 
thereof is low, the thickness should be thick. If the thermal conductivity 
is high, the thickness may be thin. 
The metal reflecting layer may be prepared by a conventional method, 
including sputtering and vacuum evaporation. 
The magneto-optical recording layer of the present invention may be any one 
which allows recording, reproducing and erasing by the thermomagnetic 
effect and the magneto-optical effect, more specifically a magnetic metal 
layer in which the axis of easy magnetization is perpendicular to the 
layer and a reverse magnetic domain can be created optionally so that 
recording and erasing can be made by the therm omagnetic effect. For 
example, a layer of amorphous alloys of a rare earth element and 
transition metal element such as TbFe, TbFeCo, GdTbFe, GdFeCo, NdDyFeCo, 
NdDyTbFeCo, NdFe, PrFe and CeFe, a double-layer of these layers utilizing 
an exchang coupling interaction, an artificial superlattice such as Co/Pt, 
Co/Pd, etc. may be used. 
The substrate may be made of a polymer resin such as polycarbonate resin, 
acrylic resin, epoxy resin, 4-methylpentene resin or a copolymer thereof, 
or amorphous polyolefin or glass. The polycarbonate resin is preferred 
because of mechanical strength, environmental stability, thermal stability 
and humidity permeability. 
The above description concerns the basic construction of a magneto-optical 
recording medium of the present invention which is not limited thereto. 
Particularly, it is preferred that another transparent dielectric layer is 
inserted between the substrate and the magneto-optical recording layer to 
obtain an enhancement of the Kerr effect. The transparent dielectric 
layers are preferably made of silicon nitride or aluminum silicon nitride 
because they have a high durability under a high temperature and high 
humidity. 
Usually, on the metal reflecting layer, an organic protecting layer of an 
organic radiation-curing or thermo-setting resin is generally provided in 
order to provide a mechanical protection and an improvement of durability 
and an inorganic protection layer made of the same dielectric material may 
be provided between the metal reflecting layer and the organic protecting 
layer. 
The above medium is used as a single-sided medium, which is optionally 
provided with a protection plate or a protection film, or as a 
double-sided medium by bonding two such media on the side of the metal 10 
reflecting layer. 
Thus, in accordance with the present invention, by a combination of a 
transparent heatinsulating layer and a highly thermally conductive metal 
reflecting layer, a magneto-optical recording medium in which both 
characteristics of conflicting peak shift (bit error) and recording 
sensitivity are improved and a high C/N ratio and a long term stability 
against a high power laser beam are obtained is provided. 
The present invention is further described with reference to Examples. 
Magneto-optical recording discs of Examples 1 to 7 and Comparative 
examples 1 to 3 were manufactured and the optimum valve of a recording 
laser power, C/N ratio and peak shift thereof were measured. Also, the 
long term stability against a laser beam was determined by measuring a C/N 
ratio after the disc is irradiated with a continuous laser beam while 
rotating the disc at a lowered rpm for an adequate rotation number. A 
rotation number of the disc necessary in lowering of the C/N ratio by 2 dB 
from the original C/N ratio is used for estimation. 
EXAMPLE 1 
A magneto-optical recording medium having a laminate structure as shown in 
FIG. 2 was made. In FIG. 2, the reference numeral 11 denotes a transparent 
substrate, 12 a transparent dielectric layer, 13 a magneto-optical 
recording layer, 14 a transparent dielectric nitride layer, 15 a 
transparent heat insulating layer, and 16 a metal reflecting layer. 
A disc-like substrate 11 of polycarbonate (PC) resin having a diameter of 
130 mm and a thickness of 1.2 mm and having grooves at a pitch of 1.6 
.mu.m was mounted in a vacuum chamber of a magnetron sputtering equipment 
(ANELVA Corporation SPF-430H) capable of mounting three targets, and the 
chamber was evacuated to less than 4.times.10.sup.-7 Torr. 
A mixture gas of Ar and N.sub.2 (Ar:N.sub.2 =70:30 by volume) was 
introduced into the chamber and the flow rate of the gas mixture was 
regulated to a pressure of 10 mTorr. An AlSiN layer (112.5 nm thick) as 
the transparent dielectric layer 12 was deposited by RF sputtering, using 
a target of a sintered Al.sub.30 Si.sub.70 (a diameter of 100 mm and a 
thickness of 5 mm) and a glow discharge at an RF power of 500 W of 13.56 
MHz, while rotating the PC substrate. 
The thus deposited layer 12 had a composition of Al.sub.19 Si.sub.39 
N.sub.42 when analyzed by an Auger electron spectrometer (provided by 
Perkin-Elmer Co. as PHI-SAM 610). 
Then a Tb.sub.20.5 Fe70.9Co.sub.8.6 alloy layer (22.5 nm thick) as the 
magneto-optical recording layer 13 was deposited on the AlSiN layer 12 by 
DC sputtering a Tb.sub.19 Fe.sub.72.5 Co.sub.8.5 alloy target (a diameter 
of 100 mm and a thickness of 4.5 mm) at an Ar gas pressure of 4 mTorr and 
a glow discharge power of 150 W. 
The composition of the alloy layer was determined by an inductivity coupled 
plasma spectrometry (ICP). The compositions of the other alloys were also 
determined by the ICP. 
Then an AlSiN layer was deposited under the same conditions as above to 
form the transparent dielectric nitride layer 14 (5 nm thick). 
Then, a target was changed from Al.sub.30 Si.sub.70 to Ta.sub.2 O.sub.5, an 
evacuation of the chamber was effected to less than 4.times.10.sup.-7 Torr 
and a mixture gas of Ar and N.sub.2 (Ar:N.sub.2 =70:30 by volume) was 
introduced into the chamber and the flow rate of the gas mixture was 
regulated to a pressure of 10 mTorr. The target was a disc of sintered 
Ta.sub.2 O.sub.5 100 mm in diameter and 5 mm in thickness. A sputtering 
was carried out at an RF power of 400 W of 13.56 MHz, to deposit a TaON 
layer (30 nm thick) as the transparent heat insulating layer 15. The 
contents of the oxygen and nitrogen in the layer 15 measured by the Auger 
electron spectroscopy were 51 atomic % and 21 atomic %, respectively. 
A further DC sputtering was effected using an Ag.sub.94.5 Au.sub.4.4 
Ti.sub.1.1 alloy target (100 mm diameter and 3 mm thick) at an Ar gas 
pressure of 2 mTorr and a glow discharge power of 60 W, to deposit an 
Ag.sub.95.5 Au.sub.4.1 Ti.sub.0.9 alloy layer 70 nm thick as the metal 
reflecting layer 16. 
The PC substrate 11 was rotated at 20 rpm during the above depositions. 
Then, on the metal reflecting layer 16, an ultra-violet ray curable 
phenolic novolak epoxy acrylate resin was coated by a spin coater, which 
was then cured by irradiation with an ultra-violet ray and an organic 
protecting layer 17 (about 10 .mu.m thick) was formed. 
EXAMPLE 2 
Example 1 was repeated and a magneto-optical recording disc was made except 
that a thickness of the TaON layer was changed to 20 nm and the metal 
reflecting layer 16 was an Al.sub.92.7 Au.sub.4.8 Ti.sub.2.5 alloy layer 
(100 nm thick) perpared by DC sputtering using an Al.sub.92.5 Au.sub.4.5 
Ti.sub.3.0 target (100 mm diameter and 5 mm thick) at an Ar pressure of 
1.5 mTorr and a glow discharge power of 125 w. 
EXAMPLE 3 
A magneto-optical recording disc was made in the same manner as in Example 
2 except that a thickness of the TaON layer was changed to 30 nm and the 
metal reflecting layer was an Al.sub.92.7 Au.sub.4.8 Ti.sub.2.5 alloy 
layer having a thickness of 150 nm. 
EXAMPLE 4 
On the same substrate 11 as in Example 1, under similar conditions as in 
Example 1 or 2, an AlSiN layer (100 nm thick) as the transparent 
dielectric layer 12, a TbFeCo amorphous alloy layer (22.5 nm thick) as the 
magneto-optical recording layer 13, an AlSiN layer (5 nm thick) as the 
transparent dielectric nitride layer 14, an TaON layer (30 nm thick) as 
the transparent heat insulating layer 15, and an Al.sub.92.7 Au.sub.4.8 
Ti.sub.2.5 alloy layer (210 nm thick) as the metal reflecting layer 16 
were laminated in this order. An organic protecting layer 17 was formed on 
the metal reflecting layer 16 and a magneto-optical recording disc was 
made. 
EXAMPLE 5 
A magneto-optical recording disc as shown in FIG. 2 was made. 
On the same substrate 11 as in Example 1, under similar conditions as in 
Example 1, an AlSiN layer (112.5 nm thick) as the transparent dielectric 
layer 12, a TbFeCo amorphous alloy layer (22.5 nm thick) as the 
magneto-optical recording layer 13, an AlSiN layer (5 nm thick) as the 
transparent dielectric nitride layer 14, an TaO layer (30 nm thick) as the 
transparent heat insulating layer 15, and an Ag.sub.95.0 Au.sub.4.1 
Ti.sub.0.9 alloy layer (70 nm thick) as the metal reflecting layer 16 were 
laminated in this order. An organic protecting layer 17 was formed on the 
metal reflecting layer 16 and a magneto-optical recording disc was made. 
The TaO layer was deposited by RF sputtering a sintered Ta.sub.2 O.sub.5 
target (100 mm diameter and 5 mm thick) in a mixed gas of Ar and O.sub.2 
(Ar:O.sub.2 =70:30). The RF glow power was 400 W of 13.56 MHz, and the gas 
pressure was adjusted to 10 mTorr. The composition of the TaO layer was 
Ta.sub.32 O.sub.68 in accordance with the Auger electron spectrometer 
mentioned before. 
EXAMPLE 6 
A magneto-optical recording disc was made in the same manner as in Example 
1 except that the transparent dielectric layers 12 and 14 under and on the 
magneto-optical recording layer 13 were a silicon nitride (SIN) layer. 
The SiN layer was deposited by RF sputtering a metal Si target (100 mm 
diameter and 5 mm thick) in a mixed gas of Ar and N.sub.2 (Ar:N.sub.2 
=70:30). The RF glow power was 500 W of 13.56 MHz, and the gas pressure 
was adjusted to 15 mTorr. 
The thickness of the SiN layer as the transparent dielectric layer 12 on 
the substrate side of the recording layer was 112.5 nm and the thickness 
of the SiN layer as the transparent dielectric nitride layer 14 on the 
metal reflecting layer side was 5 nm. 
The SiN layer comprised 43% nitrogen in accordance with the Auger electron 
spectrometer mentioned before. 
EXAMPLE 7 
A magneto-optical recording disc as shown in FIG. 3 was made. In FIG. 3, 
the same reference numerals denote the same parts as in FIG. 2. The 
magneto-optical recording disc comprised a substrate 11, a transparent 
dielectric layer 12, a magneto-optical recording layer 13, a transparent 
heat insulating layer 15, a metal reflecting layer 16, and an organic 
protecting layer 17. 
On the same substrate 11 as in Example 1, under similar conditions as in 
Example 1, an AlSiN layer (112.5 nm thick) as the transparent dielectric 
layer 12, a TbFeCo amorphous alloy layer (22.5 nm thick) as the 
magneto-optical recording layer 13, an TaON layer (35 nm thick) as the 
transparent heat insulating layer 15, and an Ag.sub.95.0 Au.sub.4.1 
Ti.sub.0.9 alloy layer (70 nm thick) as the metal reflecting layer 16 were 
laminated in this order. An organic protecting layer 17 was prepared on 
the metal reflecting layer 16. However, a pre-sputtering of the Ta.sub.2 
O.sub.5 target for forming the TaON layer was effected before depositing 
the TbFeCo amorphous alloy layer, because if the pre-sputtering the 
Ta.sub.2 O.sub.5 target is effected after depositing the TbFeCo amorphous 
alloy layer, the TbFeCo amorphous alloy layer would be deteriorated. 
COMATIVE EXAMPLE 1 
A magneto-optical recording disc as shown in FIG. 4 was made. In FIG. 4, 
the same reference numerals denote the same parts as in FIG. 2. The 
magneto-optical recording disc comprised a substrate 11, a transparent 
dielectric layer 12, a magneto-optical recording layer 13, a transparent 
dielectric layer 14, a metal reflecting layer 16, and an organic 
protecting layer 17. 
On the same substrate 11 as in Example 1, under similar conditions as in 
Example 1 or 2, an AlSiN layer (112.5 nm thick) as the transparent 
dielectric layer 12, a TbFeCo amorphous alloy layer (22.5 nm thick) as the 
magneto-optical recording layer 13, an AlSiN layer (35 nm thick) as the 
transparent dielectric layer 14, and an Al.sub.92.7 Au.sub.4.8 Ti.sub.2.5 
alloy layer (60 nm thick) as the metal reflecting layer 16 were laminated 
in this order. An organic protecting layer 17 was formed on the metal 
reflecting layer 16. 
COMATIVE EXAMPLE 2 
Comparative example 1 was repeated except that the thickness of the metal 
reflecting layer 16 was changed to 100 nm. 
COMATIVE EXAMPLE 3 
Example 2 was repeated except that the thickness of the TaON layer as the 
transparent heat insulating layer 15 was changed to 10 nm and the 
thickness of the AlAuTi layer as the metal reflecting layer 16 was changed 
to 60 nm. 
(1) Then, the C/N ratio and the optimum valve of a recording laser power of 
the magneto-optical recording discs of Examples 1 to 7 and Comparative 
examples 1 to 3 were evaluated by a magneto-optical recording and 
reproducing unit (PULSTEC INDUSTRIAL CO., LTD., DDU-1000 type) under the 
following conditions. The optimum valve of a recording laser power was 
determined by varying the power of a semiconductor laser for recording and 
selecting the power when the second harmonics of the reproducing signal 
was minimum. 
______________________________________ 
Recording conditions: 
Disc rotation speed: 1800 rpm 
Position of recording on disc: 
30 mm radius 
Recording frequency: 3.7 MHz 
Applied magnetic field during recording: 
250 oersteds 
Recording pulse width: 90 nsec 
Reproducing conditions: 
Disc rotation speed: 1800 rpm 
Reproducing laser power: 1.5 mW 
______________________________________ 
(2) Further, the peak shift of the magneto-optical recording discs of 
Examples 1 to 7 and Comparative examples 1 to 3 were determined. 
The peak shift measured is an absolute value of a difference between the 
time T.sub.2 between pulses of signal for recording and an average value 
T'.sub.2 of the time T.sub.2 ' between peaks of reproduced signals, when 
the signal as shown in FIG. 5A (T.sub.1 =90 nsec, T.sub.2 =270 nsec, 
T.sub.3 =90 nsec) is recorded and the signal as shown in FIG. 5B is 
reproduced. Therefore, the peak shift is represented by the following 
formula. 
EQU Peak shift=.vertline.T.sub.2 -T.sub.2 '.vertline. 
The recording and reproducing were made by the magneto-optical recording 
and reproducing unit mentioned above. The conditions for recording and 
reproducing were the following. The time T.sub.2 ' between pulses of 
reproduced signals was measured by a frequency and time interval analyzer 
(type HP-5371A, produced by Hewlett Packard). 
______________________________________ 
Recording conditions: 
Disc rotation speed: 1800 rpm 
Position of recording on disc: 
30 mm radius 
Recording laser power: 6 mW 
Applied magnetic field during recording: 
250 oersteds 
Reproducing conditions: 
Disc rotation speed: 1800 rpm 
Reproducing laser power: 1.5 mW 
______________________________________ 
(3) Further, the long term stability against a laser beam of the 
magneto-optical recording discs of Examples 1 to 7 and Comparative 
examples 1 to 3 were evaluated. 
A laser beam was continuously irradiated to a certain track at a 30 mm 
radius of the rotating disc and the C/N ratio there was measured after the 
disc was rotated for a certain number. Thus, the number of the rotation of 
the disc was determined when the C/N ratio was decreased by 2 dB. If this 
disc rotation number is larger, the disc is considered to be more stable 
against a laser beam. The rotation speed of the disc while a continueous 
wave laser beam was irradiated with was set to be 300 rpm to accelerate 
the temperature elevation of the magneto-optical recording disc and the 
power of the continuously irradiated laser beam was 6 mW. The method of 
measuring the C/N ratio and conditions of recording and reproducing were 
the same as described before. 
The results, i.e., the optimum value of a recording laser power, C/N ratio, 
peak shift and disc rotation number when the C/N ratio decreased by 2 dB 
are shown in Table 2. 
TABLE 2 
__________________________________________________________________________ 
Thermal conduc- 
Optimum Rotation number 
tivity of metal 
recording 
C/N 
Peak 
when C/N is 
reflecting layer 
power 
max 
shift 
reduced by 2 dB 
Sample 
Construction (thickness, nm) 
(W/mk) (mW) (dB) 
(n sec) 
(cycles) 
__________________________________________________________________________ 
Ex. 1 PC/AlSiN/TbFeCo/AlSiN/TaON/AgAuTi 
5.8 .times. 10 
4.5 48.6 
9.6 3.4 .times. 10.sup.3 
112.522.553070 
Ex. 2 PC/AlSiN/TbFeCo/AlSiN/TaON/AlAuTi 
4.5 48.5 
10.2 
9.0 .times. 10.sup.2 
112.522.5520100 
Ex. 3 PC/AlSiN/TbFeCo/AlSiN/TaON/AlAuTi 
4.5 48.4 
10.0 
1.8 .times. 10.sup.3 
112.522.5530150 
Ex. 4 PC/AlSiN/TbFeCo/AlSiN/TaON/AlAuTi 
1.3 .times. 10 
4.5 48.4 
9.5 2.7 .times. 10.sup.3 
10022.5530210 
Ex. 5 PC/AlSiN/TbFeCo/AlSiN/TaO/AgAuTi 
5.8 .times. 10 
4.5 48.5 
9.6 3.3 .times. 10.sup.3 
112.522.553070 
Ex. 6 PC/SiN/TbFeCo/SiN/TaON/AgAuTi 
5.8 .times. 10 
4.5 48.4 
9.6 3.3 .times. 10.sup.3 
112.522.553070 
Ex. 7 PC/AlSiN/TbFeCo/TaON/AgAuTi 
5.8 .times. 10 
4.5 47.8 
9.7 3.3 .times. 10.sup.3 
112.522.53570 
Com. Ex. 1 
PC/AlSiN/TbFeCo/AlSiN/AlAuTi 4.5 48.3 
12.8 
2.0 .times. 10.sup.1 
112.522.53560 
Com. Ex. 2 
PC/AlSiN/TbFeCo/AlSiN/AlAuTi 5.5 48.4 
10.2 
7.8 .times. 10.sup.2 
112.522.535100 
Com. Ex. 3 
PC/AlSiN/TbFeCo/AlSiN/TaON/AlAuTi 
4.5 48.2 
12.6 
2.5 .times. 10.sup.1 
112.522.551060 
__________________________________________________________________________ 
From Table 2, the following can be seen. Namely, if a transparent heat 
insulating layer 15 is not used as in Comparative examples 1 and 2, even 
if the metal reflecting layer 16 is excellent in heat dispersion 
(Comparative example 2) or not (Comparative example 1), the 
magneto-optical recording disc cannot satisfy the all characteristics of a 
high recording sensitivity, and a small peak shift and a long term 
stability against a laser beam. In Comparative example 1 in which the heat 
dispersion of the metal reflecting layer 16 is poor, the optimum value of 
a recording laser power is advantageously low, 4.5 mW, but the peak shift 
is disadvantageously larger and the C/N ratio is decreased by 2 dB only 
after 20 rotations. In Comparative example 2 in which the heat dispersion 
of the metal reflecting layer 16 is excellent, the peak shift is 
relatively small and the stability against a laser beam is superior to 
that of Comparative example 1, but the optimum value of a recording laser 
power is disadvantageously high, 5.5 mW. 
It is also seen from Comparative example 3 that even if a transparent 
heatinsulating layer 15 is used, if a metal reflecting layer 16 has a poor 
heat dispersion, the advantageous effect of the present invention cannot 
be obtained. 
In sharp contrast, in all Examples 1 to 7 in which a transparent 
heatinsulating layer 15 and a metal reflecting layer 16 having an 
excellent heat dispersion are used, the optimum value of a recording laser 
power is kept to be low, 4.5 mW, while the peak shift and the long term 
stability against a laser beam are also excellent. 
Moreover, it is also seen in Examples 2 to 4 that the more advantageous 
effects of the present invention can be obtained if the metal reflecting 
layer 16 used has a more excellent heat dispersion. 
From the above, considering that the peak shift and the long term stability 
against a high power laser beam which are the same degree as those in 
Example 2 are enough, the effects of the present invention can be obtained 
by using a transparent heat insulting layer 15 of TaON or TaO and a metal 
reflecting layer 16 having a thermal conduction equal to or higher than 
that of the Al.sub.92.7 Au.sub.4.8 Ti.sub.2.5 alloy layer having a 
thickness of 100 nm. Namely, the value of a product of a thermal 
conductivity and a layer thickness of a metal reflecting layer is 
preferably not less than 1.3.times.10.sup.-6 WK.sup.-1. If higher effects 
of the present invention are desired, the thermal conduction should be 
more than that of the Al.sub.92.7 Au.sub.4.8 Ti.sub.2.5 alloy layer having 
a thickness of 150 nm and therefore the value of a product of a thermal 
conductivity and a layer thickness is preferably not less than 
2.0.times.10.sup.-6 WK.sup.-1. 
It is also seen from Examples 1 to 6 that TaON and TaO can be equally used 
as the transparent heat-insulating layer 15, AlSiN and SiN can be equally 
used as the transparent dielectric layers 12 and 14, and AgAuTi and AlAgTi 
alloys can be equally used as the metal reflecting layer 16. Further, the 
satisfactory effects of the present invention can be obtained even if the 
TaON layer is prepared in direct contact with the magneto-optical 
recording layer. 
Thus, in accordance with the present invention, the practically important 
characteristics of a magneto-optical recording medium are significantly 
improved.