Magneto-optical recording medium and the recording method therefor

Laser light is irradiated on a multilayered magnetic thin film having a spin-glass characteristic consisting of a rare-earth metal and a conductive material, the film being heated to a predetermined temperature below the spin-glass transition temperature when recording information and heated to a temperature above the spin-glass transition temperature when erasing the information.

FIELD OF THE INVENTION 
The invention relates to a recordable and erasable magneto-optical medium 
and the recording method therefor. 
BACKGROUND OF THE INVENTION 
A well known method of magnetic recording on a magnetic medium is carried 
out by irradiating light locally onto a region of the magnetic recording 
medium in a magnetic field so that the heat energy generated by the 
irradiation changes the magnetic state of the region. This method has been 
known to provide a high-density recording promoted by the laser light. 
However, since most conventional magnetic media of this kind are 
ferromagnetic, the above mentioned method has a drawback that the magnetic 
spins are oriented in parallel with the surface of the medium due to the 
internal magnetic field, preventing high density recording. On the other 
hand, TbFeCo alloy, which is an only commercially available 
perpendicular-magnetic recording medium for such magneto-optical 
recording, has disadvantages that it is not only expensive but also easily 
oxidized. 
BRIEF SUMMARY OF THE INVENTION 
The invention is directed to solve the above mentioned problems. Namely an 
object of the invention is to provide a new magnetic recording medium and 
the recording method therefor. 
Firstly, the invention is characterized in that it utilizes as a 
magneto-optical recording medium a magnetic thin film consisting of 
alternate layers of a rare-earth metal and a conductive material, and 
having a spin-glass characteristic. Secondly, the invention is 
characterized in that the thin film alloy or the material having said 
multilayer structure and spin-glass characteristic is heated during 
recording information to a temperature below the peak temperature and then 
cooled down to a room temperature, while it is heated to a temperature 
above the peak temperature during erasing the information and then cooled 
down to a room temperature. 
This makes it possible to provide an economical yet high-density 
magneto-optical recording medium which has a perpendicular-magnetization 
characteristic and can be recorded/erased by laser light.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
By depositing on a substrate alternate layers of a rare-earth metal 11 and 
a conductive material 12 each having thickness of 3-5 atoms as shown in 
FIG. 1 to form a multilayer structure, a magnetic material 20 may be 
obtained having a spin-glass characteristic due to interlayer interactions 
between the magnetic spins of the rare-earth metal 11 layers mediated by 
the conduction electrons. 
The above mentioned spin-glass characteristic refers to a 
magnetization-temperature relationship "a" as shown in FIG. 2, which is a 
fairly new magnetic phenomenon found in 1972 (See Physical Review B6, 4220 
(1972)). Since then, quite a few experiment have been made mainly from a 
fundamental physical point of interest, and presently about 20 kinds of 
magnetic materials having such spin-glass characteristics are known. All 
of these magnetic materials are alloys. This spin-glass characteristic, 
like other magnetic ordering, has not been completely understood, leaving 
some of the essential problems unsolved. For example, a question whether 
or not the spin-glass characteristic arises from a phase transition has 
not been answered. 
No example of a recording medium has been reported utilizing a magnetic 
material having the spin-glass characteristic. It is because the 
characteristic has been believed to be reversible. The inventor of this 
application has found from a detailed study of such magnetic material 
having the spin-glass characteristic that, as shown in FIG. 2, the 
material heated to a certain temperature T.sub.B below temperature T.sub.C 
at which the magnetization reaches its peak value, will retain its 
magnetization M.sub.B even after it is brought back to a room temperature 
T.sub.A, and that the material restores magnetization M.sub.A if it is 
brought back to a temperature T.sub.A after it is heated to a temperature 
T.sub.D above the peak temperature T.sub.C. The invention takes advantage 
of this phenomenon in utilizing the magnetic material having this 
spin-glass characteristic as a recording medium. Since such magnetic 
material having the spin-glass characteristic serves generally as a 
perpendicular-recording medium, and is recordable and erasable by laser 
light, the material may provide a high-density, large-capacity recording 
medium. 
EXAMPLE 
By repeating molecular beam epitaxial growth of a Mn (manganese) layer 2 
and a subsequent Eu (europium) layer 3, on a Si (silicon) substratum 1, a 
multilayer thin film of Mn layers 2 and Eu-layers 3 is formed. Small angle 
X-ray diffraction observation of this multilayer thin film gives the 
results shown in FIG. 4. In FIG. 4, the ordinate is the intensity of the 
diffracted X ray and the abscissa is the incident angle .theta. times 2 of 
the X-ray irradiated upon said thin film. Values in the parentheses 
indicate the thickness of one Mn-layer 2 plus one Eu-layers 3 in one cycle 
on the multilayers. Small angle X-ray diffraction is performed by 
irradiating X-ray beam onto the surface of the multilayer thin film at a 
given incident angle, and by measuring the intensities of the reflected 
X-rays into various diffraction angles. As the incident angle is varied, a 
diffraction peak appears. The existence of this diffraction peak implies 
the existence of the interfaces between the multilayers. It has been known 
that the diffraction peak becomes sharper as the interfaces becomes more 
distinctive. The following equation holds for the diffraction peak. 
EQU 2d sin .theta.=n.lambda. . . . (1) 
where .lambda. is the wave length of the X-ray, and n is an integer. Given 
the incident angle .theta. of the X-ray at the time of the peak 
diffraction, one-cycle thickness d may be found from Eq (1). 
Auger electron spectroscopy of the above magnetic thin film of stacked 
layers yields the depth profile as shown in FIG. 5. The result shows that 
the crests (maxima) and the trough (minima) of the Eu and Mn signal curves 
coincide, leading us to the conclusion that the interfaces are well 
defined. 
Thus, it is confirmed from FIGS. 4 and 5 that the interfaces of the stacked 
layers of said thin film are spaced apart by several atomic orders and 
free from perturbations. 
Next, the investigation of the above thin multilayer film by energy 
dispersion X-ray spectroscopy gives the result as shown in FIG. 6. As seen 
from this figure, only those signals for the silicon of the substratum, 
Eu, and Mn were detected, indicating the absence of impurities. 
The elementary analysis of said multilayer thin film by means of the Auger 
electron spectroscopy is shown in FIG. 7, in which the responses observed 
are only those for Ar gas used in the analysis, Eu, and Mn, also 
indicating the absence of impurities. 
Series of data depicting the spin-glass characteristic of said multilayer 
thin film may be obtained as shown in FIG. 8 from the measurement of the 
magnetization of the film as it is heated under a constant magnetic field 
after it was cooled under zero magnetic field. It has been found that this 
multilayer thin film has a perpendicular-magnetization property. Further, 
it has been found that, as the magnetic multilayer thin film is first 
cooled under zero magnetic field and then heated by laser light under a 
constant magnetic field to an arbitrary temperature below the peak 
temperature and brought back to the initial temperature, the magnetic thin 
film retains the magnetization induced at the time of stopping the laser 
light heating, which depends on the temperature at that time, as shown in 
by the data b1-b3 in the figure. It has been also confirmed that the 
magnetization of the magnetic thin film restores its initial magnetization 
if the film is heated up above the peak temperature by raising the power 
of the laser light and then stopping laser heating to bring the thin film 
back to its initial temperature, as shown by the curve "a" in the same 
figure. Incidentally, the ordinate represent the magnetization parallel to 
the magnetic thin film, and abscissa the absolute temperature. 
The spin-glass characteristic of said magnetic thin film varies with the 
magnetization as shown in FIG. 9. The ordinate represents the 
magnetization in unit of emu per unit gram, and the abscissa, the absolute 
temperature, the parameter being the intensity of the magnetic field (in 
Gauss) to be applied onto said magnetic thin film. It is seen from this 
result that the spin-glass characteristic of said magnetic thin film is 
intensified as the magnetizing field becomes weaker. 
Next, the spin-glass characteristic is investigated for the different 
thickness of the Eu layers and the Mn layers, with the result shown in 
FIG. 10. In this figure the ordinate is the Mn layer thickness and the 
abscissa ia the Eu layer thickness, and x's indicate the points where no 
spin-glass character is observed, while o's indicate the points where 
spin-glass characteristic is observed. It may be seen from this result, 
for at least Eu-Mn magnetic thin film, that the spin-glass characteristic 
may be obtained for the Eu layer thickness equal to or less than 40 .ANG. 
and Mn layer thickness equal to or less than 70 .ANG., and that the 
spin-glass characteristic appears more intensely for less Mn layer 
thickness as shown in FIG. 11, which is advantageous for magnetic 
recording. To note, the ordinate is the magnetization in unit of emu per 
unit gram, and the abscissa, the absolute temperature, the parameter being 
Mn layer thickness (in .ANG.). 
The reason why the Eu and Mn layer thickness may be less than 40 .ANG. and 
less than 70 .ANG., respectively, and why thinner Mn layers are more 
advantageous is due to the fact that the spin-glass characteristic arises 
from the interaction between the Eu layers mediated by the conduction 
electrons which are free from the influence of the magnetic spins in the 
Eu layers (indicating that the interlayer material (between the Eu layers) 
should be a conductive material), so that the spin-glass characteristic is 
weakened as the Mn layers become thick loosing the interlayer 
interactions. It is also anticipated that this view applies not only to 
Eu-Mn magnetic thin film but also to aforementioned magnetic thin film of 
a rare-earth and a conductive material. Further, rare-earth metals are 
known to possess conduction electrons which exist independently of the 
magnetic spins. Therefore, although the invention has been shown above 
with reference to a magnetic thin film comprising Mn layers 2 and Eu 
layers 3, it should be understood that the invention is not limited to 
this magneto-optical recording medium and may be carried out by 
alternative multilayer film of a rare-earth metal and a conductive 
material layers stacked alternately on each other. 
In this manner it is possible to manufacture low-cost magnetic films having 
spin-glass characteristics using rather inexpensive material such as Eu 
and without using expensive Tb (terbium). Such magnetic films have a 
perpendicular magnetization property which permits recording and erasing 
information by means of laser light. The magnetic films may be thus 
utilized as high-density recording media. 
It is inferred that conventional magnetic materials having spin-glass 
characteristic such as (1) Au.sub.x Fe.sub.1-x, (2) Eu.sub.x Sr.sub.1-x S, 
(3) Fe.sub.x Al.sub.1-x, (4) Fe.sub.x Cr.sub.1-x, (5) Fe.sub.x Pd.sub.1-x, 
(6) Mn.sub.x Pd.sub.1-x, (7) Pd.sub.x Fe.sub.y Mn.sub.1-x-y, (8) Ni.sub.x 
Mn.sub.1-x, (9) Fe.sub.x Ni.sub.1-x, (10) Fe.sub.x Mn.sub.1-x, (11) 
Zn.sub.x Co.sub.1-x, and (12) Co.sub.x Ti.sub.1-x O.sub.4 may be also 
utilized as a magnetic material to obtain similar functions described 
above.