Perpendicular magnetization film and the preparation thereof

A perpendicular magnetization film of iron-chromium alloy having an easy magnetization axis perpendicular to a plane of substrate, and the preparation thereof. The perpendicular magnetization film provides a dominant peak corresponding to a lattice constant of 2.07 to 2.08 .ANG. in X-ray diffraction spectrum, and possesses a large saturation magnetization of more than 100 emu/cm.sup.3. The perpendicular thin film can be prepared by depositing iron and chromium on the substrate under vacuum or an atmosphere of argon gas at a low pressure.

BACKGROUND OF THE INVENTION 
The present invention relates to a perpendicular magnetization film used as 
a perpendicular magnetic storage medium which is suitable for high-density 
recording, and the preparation thereof. 
For a high-density magnetic storage, a perpendicular magnetic storage 
medium is effectively used. A magnetic storage medium used for such a 
purpose is prepared by the use of magnetic thin film which has an easy 
magnetization axis perpendicular to a plane of film. For perpendicular 
magnetic storage, there have been researched a magnetic storage of 
cobalt-chromium alloy prepared by means of sputtering or 
vacuum-evaporation method, and also a magnetic storage of barium-ferrite 
alloy prepared by means of coating or sputtering method. For applying 
cobalt-chromium thin film to a perpendicular magnetic storage, a structure 
of single crystal or a structure close to single crystal must be provided. 
In a process of preparing an alloy having the above structure, a substrate 
needs to be heated to a temperature of more than 100.degree. C. or in some 
cases to a temperature of more than 200.degree. C. On the other hand, for 
applying barium-ferrite thin film to a perpendicular magnetic storage by 
means of coating method, a barium and ferrite powder consisting of uniform 
particles of about 0.1 .mu.m in diameter must be provided. It costs much 
to produce such barium and ferrite powder. Moreover, it is necessary to 
mix a binder to form a film. In a film containing such a binder, a 
saturation magnetization becomes smaller depending on a content of a 
binder, so that capacity of magnetic storage is decreased. In a process of 
preparing a barium-ferrite film by means of sputtering method, a substrate 
needs to be heated to a temperature of about 500.degree. C. Thus, a 
substrate of cheap plastic materials cannot be used. 
An object of the present invention is to solve the problem that an ordinary 
plastic material cannot be used for a substrate of a perpendicular 
magnetic storage of cobalt and chromium, and that a material of substrate 
is restricted within expensive polyimides, aluminum, glass, or the like 
because a substrate must be heated to a temperature of more than 
100.degree. C. in a process of preparation thereof. Another object of the 
invention is to solve the problem that cobalt is rare and expensive 
resources. Still another object of the invention is to solve the problem 
that a perpendicular magnetic storage of barium-ferrite alloy has low 
saturation magnetization compared with cobalt-chromium alloy. 
The above and other objects and the advantages of the present invention 
will become apparent from the following description. 
SUMMARY OF THE INVENTION 
These objects of the invention are solved by providing a perpendicular 
magnetization film comprising a magnetic thin film of iron-chromium alloy, 
and also providing the preparation thereof. The film is supported on a 
substrate and has an easy magnetization axis perpendicular to a plane of 
substrate. The alloy contains chromium in an amount of 20 to 60 atomic %. 
The process for preparing the perpendicular magnetization film comprises 
depositing iron and chromium on the substrate under vacuum or atmosphere 
of argon gas at a low pressure.

DETAILED DESCRIPTION OF THE INVENTION 
In the present invention, a magnetic thin film supported on a substrate is 
composed of chromium and iron wherein a content of chromium is 20 to 60 
atomic %, preferably 30 to 50 atomic %, and a magnetic thin film possesses 
an easy magnetization axis perpendicular to a plane of substrate. 
When a content of chromium is less than 20% in the alloy, a crystal phase 
rich in iron is deposited on a substrate, and a film does not represent a 
perpendicular uniaxial anisotropy, and accordingly, a perpendicular 
anisotropy of a magnetic thin film is not present because of an existence 
of a strong demagnetizing field which is induced by a large saturation 
magnetization of the film. 
A thickness of a magnetic thin film is preferably 100 to 10,000 .ANG., and 
most preferably 500 to 5000 .ANG.. When the thickness of the film is below 
100 .ANG., a leakage flux is so small that a sensitivity for reading out 
the written informations is low. On the other hand, when the thickness is 
more than 10,000 .ANG., it will be difficult to write down the 
informations throughout a thickness of film. Thus, using a film of more 
than 10,000 .ANG. results in a waste of material and cost. 
The magnetic thin film of the invention is corrosion resistant because it 
contains much chromium. 
To achieve the object of the invention, it is preferable that a magnetic 
film of iron-chromium alloy has present only a peak corresponding to a 
lattice constant at 2.07 to 2.08 .ANG. in a X-ray diffraction spectrum. 
Such a magnetic film having the above-mentioned peak represents a large 
perpendicular magnetic anisotropy. At present, the crystal structure from 
which this diffraction peak originates is unknown. In a magnetic film 
which represents a peak at about 2.04 .ANG. in a diffraction spectrum 
corresponding to a 110-plane of iron in a body-centered cubic structure, a 
perpendicular magnetic anisotropy of the film contrarily tends to be 
small. In the invention, a magnetic thin film having a dominant peak 
corresponding to a range of 2.07 to 2.08 .ANG. of lattice spacing is 
preferably used. 
When a magnetization in a direction perpendicular to a plane of substrate 
is more intense than a magnetization of any other direction of 
magnetization, a magnetic thin film is hereinafter referred to that the 
film has an easy magnetization axis perpendicular to a plane of substrate. 
That is to say, a residual magnetization determined from a 
magnetization-hysterisis curve along a direction perpendicular to a plane 
of substrate is large compared with residual magnetization in any other 
direction within a surface of substrate. 
Examples of substrates in the invention are a metal plate of aluminum or 
stainless steel, a sheet or film of polyesters, polyimides or 
polymethacrylates, and the like. A material of a substrate is not 
restricted within the above-mentioned, but the material must have a 
softening point of more than about 50.degree. C. and a thickness in a 
range of about 10 .mu.m to about 10 mm. 
A perpendicular magnetization film of the invention can be deposited by 
means of evaporation method such as sputtering or vacuum-evaporation. 
A process for preparing a perpendicular magnetization film by means of 
sputtering is disclosed in the following description. For allowing a 
magnetic thin film of iron and chromium to have perpendicular magnetic 
anisotropy, necessary conditions to be suitably decided in a sputtering 
process include a composition of alloy or content of chromium, a 
temperature of substrate, a rate of deposition, and a pressure of 
atmospheric argon gas. A content of chromium in the alloy is 20 to 60 
atomic %, and preferably 30 to 50 atomic %. A perpendicular magnetization 
film of the invention is obtained by setting up a temperature of 
substrate, rate of sputtering and pressure of argon gas. Preferably, a 
structure of deposited film should be determined so that a diffraction 
peak corresponding to a lattice constant of 2.07 to 2.08 .ANG. becomes 
dominant in a X-ray diffraction spectrum. An upper limit of the range of a 
permitted temperature of substrate is decided according to a softening 
point of substrate. Preferably, a temperature of substrate is low so as to 
obtain a crystal structure having a lattice constant of 2.07 to 2.08 .ANG. 
of a perpendicular magnetic film. In this regard, a substrate should be 
cooled below room temperature with coolant such as water in a sputtering 
process. Usually, a temperature of substrate is selected in a range of 
about -50.degree. to about 150.degree. C., preferably 0.degree. to 
80.degree. C., most preferably 0.degree. to 50.degree. C. A rate of 
sputtering is changed by adjusting a supplied power to a sputtering 
apparatus. When a power for sputtering is high, a deposition rate of 
magnetic film becomes high, and also a temperature of surface of substrate 
rises. Thus, a substrate should be cooled when a power of sputtering is 
large. A pressure of argon gas is usually set in a range of 
1.times.10.sup.-3 to 1.times.10.sup.-2 Torr. 
A mangetic thin film produced in the aforementioned conditions shows a 
crystal structure having a lattice constant of 2.07 to 2.08 .ANG.. The 
fact is judged from X-ray diffraction analysis. The surface of the 
obtained magnetic thin film might be equal to a 110-plane of a 
body-centered cubic structure of chromium-rich iron, but the above fact 
cannot be concluded at present. 
Examples of sputtering apparatuses used to prepare a magnetic thin film of 
the invention are DC (direct current)-sputtering apparatus, RF (radio 
frequency)-sputtering apparatus or ion-beam sputtering apparatus, and the 
like. In a magnetic thin film prepared by the aforementioned process, a 
perpendicular mangetic anisotropy constant Ku is positive and more than 
10.sup.5 erg/cm.sup.3, a coercive force is more than 300 oersted (Oe), so 
that the obtained magnetic thin film is a perpendicular magnetization 
film. 
Another process for preparing a perpendicular magnetization film of the 
invention by means of vacuum-evaporation is disclosed in the following 
description. Examples of vacuum-evaporation processes which can be applied 
to the invention are resistive-heating-evaporation, 
electron-beam-heating-evaporation, and the like. For preparing a 
perpendicular magnetization film of iron-chromium alloy, conditions to be 
controlled in the process of vacuum-evaporation method essentially include 
a component of film and a temperature of substrate. Concentration of 
chromium in the alloy is 20 to 60 atomic % and preferably, 30 to 50 atomic 
% to obtain a perpendicular magnetization film. An evaporation can be 
performed either by evaporating iron and chromium, respectively, or by 
evaporating an alloy of iron and chromium including an amount of 20 to 50 
atomic % of chromium. Evaporation of an alloy of iron an chromium can be 
suitably accomplished by reason that a vapor pressure of iron and a vapor 
pressure of chromium are not so different from each other. In a 
vacuum-evaporation process, a temperature of substrate is unavoidably 
increased by a radiation of heat from an evaporator, and a substrate 
should be cooled to a temperature in a range of about 0.degree. to about 
100.degree. C., preferably 0.degree. to 80.degree. C., and most preferably 
0.degree. to 50.degree. C. 
In the above conditions of vacuum-evaporation, the obtained magnetic film 
possesses a perpendicular magnetic anisotropy Ku of more than 10.sup.5 
erg/cm.sup.3. This value of Ku is nearly the same as in the case of 
sputtering-evaporation. 
A magnetic thin film prepared by the above-described processes has a large 
saturation magnetization, a large vertical magnetic anisotropy, and a 
large coersive force. Those magnetic properties are suitable as a 
perpendcular magnetic storge for recording high-density information. 
A suitably prepared example of magnetic thin film which is deposited by 
means of sputtering-evaporation method under the following conditions, 
i.e. concentration of chromium of 35 atomic %, deposition rate of 150 
.ANG./min, and pressure of argon gas of 1.5.times.10.sup.-3 Torr, realizes 
a perpendicular magnetization film of iron-chromium alloy in a thickness 
of 4,000 .ANG. having a perpendicular magnetic anisotropy constant Ku of 
more than 5.times.10.sup.5 erg/cm.sup.3, and coersive force Hc.sub..perp. 
of more than 500 Oe. A perpendicular magnetic anisotropy constant Ku is 
derived from the relationship: Ku=K.sub..perp. +2.pi.Ms.sup.2, where 
K.sub..perp. is an apparent uniaxial anisotropy constant measured by a 
torque meter, and Ms is a saturation magnetization. Hc.sub..perp. is 
defined as a coersive force decided from a magnetization curve measured by 
applying a magnetic field perpendicular to a plane of substrate. 
At present, it is not known why the perpendicular magnetic thin film of 
iron-chromium alloy of the invention possesses a large perpendicular 
magnetic anisotropy and a large coercive force. 
It is reported, for example, by Shunichi Iwasaki in Nikkei Electronics, 
Oct., infra p. 141 (1982) that a magnetic thin film having a large 
perpendicular anisotropy can be suitably used for high-density recording. 
It is a matter of course that the perpendicular magnetization film of 
iron-chromium alloy of the invention can be used for recording 
high-density information. 
A perpendicular magnetization film in the present invention and its 
preparation are experimentally explained by the following Examples. It is 
to be understood that the present invention is not limited to Examples, 
and various changes and modifications may be made in the invention without 
departing from the spirit and scope thereof. 
EXAMPLES 1 TO 3 
Magnetic thin films of iron-chromium alloy were deposited on the 
glass-substrate of 1 mm in thickness by means of rf-magnetron sputtering 
apparatus. The target was an iron plate of a diameter of 3 inches and a 
thickness of 0.5 mm. Chromium chips of 10 mm square were put on the iron 
plate. The concentration of chromium in the film was controlled by 
changing the number of chromium chips. The distance between the substrate 
and the target was 5 cm. The pressure of argon gas was 1.5.times.10.sup.-3 
Torr. The temperature of the substrate was room temperature. The supplied 
power for sputtering was 50 W. Prior to the deposition, a sputtering was 
performed to clean the surface of target. After the above process was 
completed, the substrate was exposed to the target by opening the shutter. 
Thereafter the magnetic film was deposited on the substrate for 30 
minutes. 
The thickness of the obtained magnetic thin film was measured by means of 
stylus step monitor. The composition of the film was measured by means of 
X-ray analyzer. The saturation magnetization Ms was measured by means of 
vibrating-sample-magnetometer. The apparent uniaxial magnetic anisotropy 
constant K.sub..perp. pependicular to the substrate was measured by a 
torque meter. The perpendicular magnetic anisotropy constant Ku was 
derived from the relationship: Ku=K.sub..perp. +2.pi.Ms.sup.2. 
The results are shown in Table 1 and FIG. 1, wherein Ku changes in 
accordance with the variation of content of chromium in the film. The 
X-ray diffraction spectrum of the magnetic thin film obtained in the 
process of Example 2 is plotted in the graph of FIG. 2A. FIG. 3 is a graph 
illustrating a variance of intensity of diffracted X-rays versus a content 
of chromium where the diffraction angles 2.theta. were 44.4.degree. and 
43.6.degree., respectively. In the above measurement, the copper anode was 
used under an accelerating voltage of 30 kV and emission current of 50 m 
A. The power for sputtering was 50 W. 
COMATIVE EXAMPLES 1 AND 2 
Magnetic thin films containing a smaller amount of chromium were prepared 
under the same condition as in Example 1. Magnetic properties of the 
obtained magnetic thin films were measured. The results are shown in Table 
1 and FIG. 1. The X-ray diffraction spectrum of the magnetic thin film 
prepared in the process of Comparative Example 1 is plotted in the graph 
of FIG. 2B. A variance of intensity of diffracted X-rays versus a content 
of chromium are plotted in the graph of FIG. 3, wherein the diffraction 
angles 2.theta. were 44.4.degree. and 43.6.degree., respectively. From the 
results of Table 1 and FIG. 1, it is seen that a perpendicular 
magnetization film cannot be obtained where a content of chromium is 
small. From the results of FIG. 3, it is seen that the peak of X-ray 
diffraction spectrum at an angle of 2.theta.=44.4.degree. (corresponding 
to a 110-plane of a body-centered cubic structure of Fe) is not 
recognized, but the peak at an angle of 2.theta.=43.6.degree. appears in 
the magnetic thin films which have a large amount of perpendicular 
magnetic anisotropy. 
COMATIVE EXAMPLES 3 TO 6 
Magnetic thin films were prepared in the same condition as in Example 2 
except that the temperatures of the substrate were different from the 
temperatures in Example 2. The magnetic properties of the obtained 
magnetic films were measured in the same manner as in Example 1. The 
results are shown in Table 1. The X-ray diffraction spectrum of the 
obtained magnetic thin film in Comparative Example 6 is plotted in the 
graph of FIG. 2C. From the results of Table 1 and the graph of FIG. 2C, it 
is seen that the oriented 110-plane of the body-centered cubic structure 
of Fe was deposited on the substrate at a high temperature, and the 
perpendicular magnetic anisotropy is vanishing. 
EXAMPLES 4 AND 5 
Magnetic thin films were prepared under the same condition as in Example 2 
except that the supplied powers for sputtering or deposition rates were 
different from Examples 1 to 3. The measured magnetic properties of the 
obtained magnetic thin films are shown in Table 1. The powers for 
sputtering were 150 W in Example 4 and 300 W in Example 5, respectively. 
EXAMPLE 6 
A magnetic thin film was deposited on the substrate under the same 
condition as in Example 2 except that the substrate was a film of 
polyethylene terephthalate of 76 .mu.m thick, and the substrate was cooled 
to a temperature of 20.degree. C. and the supplied power for sputtering 
was 200 W. In the obtained magnetic thin film, the concentration of 
chromium was 34.2 atomic %, saturation magnetization was 250 emu/cm.sup.3, 
perpendicular magnetic anisotropy constant was 5.2.times.10.sup.5 
erg/cm.sup.3 and coersive force was 560 Oe. The curl of the plane of 
substrate was not actually recognized. In X-ray diffraction analysis, only 
a diffraction peak corresponding to a lattice constant of 2.074 .ANG. was 
recognized. 
TABLE 1 
__________________________________________________________________________ 
Concen- Temperature 
Pressure 
Deposi- 
Saturation 
Coersive 
Perpendicular 
tration of 
of of argon 
tion magnetiza- 
force 
anisotropy 
chromium substrate 
gas rate tion Ms 
Hc.sub..perp. 
constant Ku 
atomic % .degree.C. 
Torr. .ANG./min 
emu/cm.sup.3 
Oe erg/cm.sup.3 
__________________________________________________________________________ 
Ex. 1 
33.0 room temp. 
1.5 .times. 10.sup.-3 
150 260 620 +0.5 .times. 10.sup.6 
Ex. 2 
34.5 " 1.5 .times. 10.sup.-3 
130 280 600 +0.6 .times. 10.sup.6 
Ex. 3 
38.3 " 1.5 .times. 10.sup.-3 
130 100 680 +0.22 .times. 10.sup.6 
Com. 
21.0 " 1.5 .times. 10.sup.-3 
130 1470 400 &lt;-2 .times. 10.sup.6 
Ex. 1 
Com. 
27.0 " 1.5 .times. 10.sup.-3 
140 780 500 -1.3 .times. 10.sup.6 
Ex. 2 
Com. 
35.5 100 1.5 .times. 10.sup.-3 
130 300 380 -0.43 .times. 10.sup.6 
Ex. 3 
Com. 
32.8 150 1.5 .times. 10.sup.-3 
140 910 320 -0.07 .times. 10.sup.6 
Ex. 4 
Com. 
33.3 200 1.5 .times. 10.sup.-3 
140 840 280 -0.78 .times. 10.sup.6 
Ex. 5 
Com. 
35.2 250 1.5 .times. 10.sup.-3 
160 610 270 -0.49 .times. 10.sup.6 
Ex. 6 
Ex. 4 
34.7 room temp. 
1.5 .times. 10.sup.-3 
470 270 370 +0.29 .times. 10.sup.6 
Ex. 5 
33.5 " 1.5 .times. 10.sup.-3 
1,000 
160 370 +0.11 .times. 10.sup.6 
__________________________________________________________________________ 
According to the present invention, there is provided a perpendicular 
magnetization film of iron-chromium alloy. The perpendicular magnetization 
film makes it possible to realize a corossive resistant and high-density 
magnetic storage at a low cost. In the process for preparing the 
perpendicular magnetization film, a deposition of the film is carried out 
at a low temperature of substrate, thereby a cheap material of low 
heat-resistance can be used for a substrate.