Magneto-optical recording medium and method of manufacturing same

A magneto-optical recording medium operable with a low external magnetic field intensity during recording and erasing and attenuating a variation in the external magnetic field intensity comprises a magneto-optical recording layer and protective and reflective layer provided adjacent to the magneto-optical recording layers, each of which has a size of microstructures (which are associated with the non-uniformity in composition, density, crystallizability, and the like and visible under a transmission electron microscope) less than the width of a magnetic domain wall in the magneto-optical recording layer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a magneto-optical recording medium for 
recording data by heating with the use of a laser beam and reproducing it 
by means of the magneto-optical effect and a method of making the same. 
2. Description of the Prior Art 
A prior art magneto-optical recording medium is known having such a 
substantial arrangement as shown in FIG.7 or 8. In the arrangement, a 
magneto-optical recording layer 3 of a perpendicular magnetic anisotropy 
film, e.g. of amorphous rare-earth-transition-metal alloy film, which 
offers a relatively higher magneto-optical effect, is provided on a 
substrate 1 through a protective layer 2. The magneto-optical recording 
layer 3 is then covered with another protective layer 4 directly or 
through a reflective layer 5. In operation, the recording and erasing 
procedures are carried out, in a thermomagnetic manner, by locally heating 
the magneto-optical recording layer 3 to over a compensation temperature 
or about a curie temperature with the irradiation of a laser beam 6 onto 
the magneto-optical recording medium and simultaneously, applying an 
external magnetic field 7 in a direction at right angle to the layer 3. 
Also, the reproduction is made by irradiating linearly polarized weak 
power laser beam 6 onto the magneto-optical recording layer 3 carrying 
recorded data and detecting the direction of magnetization in the incident 
area by means of the Kerr magneto-optical effect or the Faraday 
(magneto-optical) effect. 
Specifically, there are two methods for magneto-optical recording-an 
optical modulation recording method for recording by modulating the 
irradiating laser beam power according to a signal to be recorded under a 
specific recording magnetic field strength, and a magnetic-field 
modulation recording method for recording by modulating the direction of 
the recording magnetic field according to a signal to be recorded under a 
specific constant laser power irradiation. 
The disadvantages of the foregoing prior art magneto-optical recording 
medium are that, particularly in recording and erasing as follows: 
1) A relatively greater rate of the external magnetic field intensity (e.g. 
about 300 Oe for the magneto-optical recording layer of TbFeCo) is needed 
for satisfactory saturation of the S/N (signal-to-noise) ratio of a 
reproduced signal, thus causing the magnetic field supplying device of a 
recording apparatus to be bulky in size or making it difficult to use a 
magnetic-field modulation recording method which requires a frequency 
range of some megahertz in the magnetic field. 
2) The required intensity of the external magnetic field varies depending 
on the area of the magneto-optical recording layer. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a magneto-optical 
recording medium operable with a low external magnetic field intensity 
during recording and erasing and a small variation in the required 
external magnetic field intensity, and a method of manufacturing the same. 
For achievement of the foregoing object, according to the present 
invention, the size of each microstructure (which is formed due to the 
poor uniformity in composition, density, crystallinity, and the like and 
visible with a transmission electron microscope as shown in FIG.9) in a 
magneto-optical recording layer and protective and reflective layers is 
made, smaller than the width of a magnetic domain wall in the 
magneto-optical recording layer. 
With the aforementioned arrangement, the intensity of external magnetic 
field needed for recording or erasing can be reduced and also, minimized 
in variation. 
For undertaking a satisfactorily saturated reproduction S/N ratio by the 
recording and erasing with the use of a low external magnetic field 
intensity, it is essential that: 1) each bit (or a magnetic domain) 
recorded in the magneto-optical recording layer under the low external 
magnetic field intensity is clearly defined as a region of unidirectional 
magnetization without boundary obscurity and has an appropriate size of 
area relative to the diameter of a laser beam spot; and 2) no recorded bit 
is present after the erasing procedure with the low external magnetic 
field intensity. To satisfy the foregoing requirements during the 
recording and erasing by locally heating the magneto-optical recording 
layer to as high as over a compensation temperature or about a curie 
temperature and applying an external magnetic field vertical to the layer, 
each of the magneto-optical layer and the protective and reflective layers 
needs to have high uniformity in both the optical and thermal properties 
ensuring no disorder in the thermal distribution of a locally heated area 
and also, the development and erasure of bits in the magneto-optical 
recording layer at a high temperature should be carried out without 
difficulty by lower levels of the external magnetic field intensity. 
The existence of microstructures in the magneto-optical recording layer, 
which are oriented due to the poor uniformity of composition, density, 
crystallinity, and the like and visible through a transmission electron 
microscope, involves a microscopic inequality in the magnetic domain wall 
energy which causes a magnetic domain wall defining a bit (the magnetic 
domain), which is to be developed during the recording (or erased during 
the erasing), to be hooked up (known as pinning phenomenon of magnetic 
domain wall). Accordingly, if the external magnetic field strength is too 
small, the development or complete erasure of a bit which is clearly 
defined without boundary fault and has an appropriate size of area will 
fail to be accomplished. 
The magnetic domain wall is an area where the direction of electron spin 
determining the magnetization is gradually shifted. Hence, when the size 
of the microstructure is smaller than the width of the magnetic domain 
wall, the interaction between the magnetic domain wall and the microscopic 
magnetic domain wall energy inequality resulting from the microstructure 
will be decreased, allowing the magnetic domain wall to be easily 
displaced. As the result, the recording and erasing with a low level of 
external magnetic field intensity becomes feasible. 
Also, to more fully accomplish the recording or erasing with a low external 
magnetic field intensity, the protective and reflective layers adjacent to 
the magneto-optical recording layer may also have a size of microstructure 
smaller than the width of a magnetic domain wall. This is for the reason 
that: 1) the microstructural characteristics of the protective layer on 
which the magneto-optical recording layer is disposed affect the 
properties of the magneto-optical recording layer; and 2) the existence of 
microstructures in the protective or reflective layer adjacent to the 
magneto-optical recording layer causes a disorder in the thermal 
distribution of a locally heated area during the recording or erasing. 
As described above, the magneto-optical recording medium and the method of 
making the same according to the present invention allow the external 
magnetic field strength needed for recording and erasing operation to be 
reduced and minimized in variation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a schematic cross sectional view of a magneto-optical recording 
medium showing a first embodiment of the present invention, in which a 
magneto-optical recording layer 13 is interposed between two protective 
layers 12 and 14. As shown in FIG. 1, there are provided a substrate 11 
made of glass or plastics, two protective layers 12 and 14 made of 
ZnSeSiO.sub.2, and a magneto-optical recording layer 13 made of an 
amorphous rare-earth-transition-metal alloy, e.g. TbFeCo, DyFeCo, 
NdTbFeCo, or YTbFeCo. The layers are formed on the substrate 11 
respectively by sputtering procedure such that the thickness of the 
protective layer 12, the magneto-optical recording layer 13, and the 
protective layer 14 are 83 nm, 100 nm, and 83 nm respectively. To have 
smaller size of microstructures in each layer, the sputtering procedure is 
carried out in an intermittent manner (preferably, the ratio of film 
depositing time to intermission time is 1:4 to 4:1) under the conditions 
of 40.degree. C. to 100.degree. C. of the substrate temperature, at least 
2 W/cm.sup.2 of the target application power, 1 to 20 mTorr of the Ar gas 
pressure, and 10.sup.-6 Torr or lower of the residual gas pressure. 
FIG. 2 shows transmission electron microscope photographs showing the 
magneto-optical recording layer 13 and the protective layer 12 produced by 
the foregoing procedure, in which the average size of microstructure is 
less than 5 nm, which is much smaller than that, about 20 to 30 nm, in a 
prior art magneto-optical recording layer or protective layer shown in 
FIG. 9. The width of a magnetic domain wall in the amorphous 
rare-earth-transition-metal alloy, e.g. TbFeCo, is about and virtually, 
greater than the size of the microstructure in the magneto-optical 
recording or protective layer according to the present invention. 
The comparison of the magneto-optical recording medium of the first 
embodiment with a prior art magneto-optical recording medium (FIG. 7) is 
shown in FIG. 3, in respect of the dependence of the reproduction S/N 
ratio on the external magnetic field strength in recording and erasing. 
Both the recording and erasing operations are carried out using the same 
intensity of external magnetic field. The reproduction S/N ratio is 
saturated when the external magnetic field intensity is 150 to 200 Oe as 
expressed by the curves 31 to 34 representing the magneto-optical 
recording mediums of the first embodiment, while compared with about 350 
Oe in the prior art magneto-optical recording medium represented by the 
curve 35. Thus, when the size of the microstructure in the magneto-optical 
and protective layers is smaller than the width of the magnetic domain 
wall, the recording and erasing magnetic field strength needed for 
saturation of the reproduction S/N ratio can be reduced to as low as 
possible. 
The magneto-optical recording medium of the present invention has 
uniformity in the properties of each layer, thus lowering the variations 
in the external magnetic field intensity and providing good uniformity in 
the characteristics during the recording or erasing. 
Although the first embodiment employs an amorphous 
rare-earth-transition-metal alloy film as the magneto-optical recording 
layer and a ZnSeSiO.sub.2 film as the protective layer, a variety of other 
materials can be used with equal success; for example, a Heusler alloy 
film, a spinel ferrite film, or a hexagonal system ferrite film as the 
magneto-optical layer and a ZnS film, a ZnSe film, an oxide film, a 
nitride film, or a mixture film thereof as the protective layer, according 
to the principles of the present invention. 
The present invention also resides in a modified magneto-optical recording 
medium having an arrangement shown in FIG. 4, in which a reflective layer 
45 is added to the magneto-optical recording medium of FIG. 1, of which 
magneto-optical recording layer 13 and protective layer 14 are decreased 
in thickness. 
FIG. 5 is a schematic cross sectional view of a magneto-optical recording 
medium showing a second embodiment of the present invention, in which a 
magneto-optical recording layer 53 is interposed between a protective 
layer 52 and a reflective layer 54. As shown in FIG. 5, there are provided 
a substrate 51 made of glass or plastics, two protective layers 52 and 55 
made of ZnSeSiO.sub.2, a magneto-optical recording layer 53 made of an 
amorphous rare-earth-transition-metal alloy, e.g. TbFeCo, and a reflective 
layer 54 made of Al. The layers are formed on the substrate 51 
respectively by sputtering procedure such that the thickness of the 
protective layer 52, the magneto-optical recording layer 53, the 
reflective layer 54, and the protective layer 55 are 83 nm, 40 nm, 40 nm, 
and 83 nm respectively. The same sputtering procedure as of the first 
embodiment is used for producing the magneto-optical recording medium of 
the second embodiment. As the result, the size of microstructure in each 
layer is smaller than the width of a magnetic domain wall in the 
magneto-optical recording layer 53, which can be seen via a transmission 
electron microscope. 
FIG. 6 is a diagram showing the comparison of the magneto-optical recording 
medium of the second embodiment with a prior art magneto-optical recording 
medium (FIG. 8) in respect of the dependence of the reproduction S/N ratio 
on the external magnetic field strength in recording and erasing. Also, 
both the recording and erasing operations are carried out using the same 
intensity of external magnetic field. The reproduction S/N ratio is 
saturated when the external magnetic field intensity is about 200 Oe as 
expressed by the curve 61 representing the magneto-optical recording 
medium of the second embodiment, while compared with around 350 Oe in the 
prior art magneto-optical recording medium represented by the curve.65. 
Thus, like the first embodiment, the size of the microstructures in the 
magneto-optical and protective and reflective layers is smaller than the 
width of the magnetic domain wall of the magneto-optical recording layer, 
so that the recording and erasing magnetic field strength needed for 
saturation of the reproduction S/N ratio can be reduced and the 
magneto-optical recording medium capable of lowering the variations in the 
external magnetic field intensity and providing good uniformity in the 
characteristics during the recording or erasing can be obtained. 
Although the second embodiment employs an amorphous 
rare-earth-transition-metal alloy film as the magneto-optical recording 
layer, a ZnSeSiO.sub.2 film as the protective layer, an aluminum film as 
the reflective layer, various other materials can be used with equal 
success: for example, a Heusler alloy film, a spinel ferrite film, or a 
hexagonal system ferrite film as the magneto-optical layer; a ZnS film, a 
ZnSe film, an oxide film, a nitride film, or a mixture film thereof as the 
protective layer; and a copper film, a gold film, or a platinum film as 
the reflective layer, according to the principles of the present 
invention.