Method of manufacturing a magnetic thin film

An organic metal compound containing elements constituting a magnetic material and an oxygen gas are introduced into plasmas at a low pressure and a thin film of magnetic oxide is prepared on an organic film or aluminum at low temperature.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention concerns a magnetic thin film capable of recording data at 
high density for use in storage media such as video tapes, magnetic discs 
and optomagnetic discs, and a method of manufacturing the same. 
2. Description of the Prior Art 
Magnetic recording has been directed to higher density and digital 
recording in recent years. Magnetic recording method mainly employed so 
far has been the so-called interplaner magnetization method in which 
directions of easy magnetization exist on a plane of a magnetic recording 
medium. However, since the directions of magnetization in the magnetic 
recording medium are oriented so as to repell with each other as the 
recording density is increased in this system, it has been difficult to 
increase the recording density. In view of the above, a magnetic recording 
method called perpendicular magnetization, in which directions of easy 
magnetization are in the direction perpendicular to the plane of the 
magnetic recording medium, has been recently developed as a new magnetic 
recording method, which has enabled the art to outstandingly increase the 
recording density as disclosed in "An Analysis for the Magnetization Mode 
for High Density Magnetic Recording", by S. Iwasaki and Y. Nakamura, IEEE 
Transaction, Magn. MAG-13, No. 5, p 1272 (1977). A cobalt-chromium (Co-Cr) 
alloy film has been developed as the perpendicular magnetic recording 
recording medium mainly by a sputtering process as disclosed in "Co-Cr 
Recording Film with Perpendicular Magnetic Anisotropy", by S. Iwasaki and 
K. Ouchi, IEEE Transactions, Magn. MAG-14, 5, 849 (1978). In addition to 
the Co-Cr alloy, barium ferrite (BaO.6Fe.sub.2 O.sub.3) has been obtained 
by the sputtering process as disclosed in "Structure and Magnetic 
Properties of C-Axis Well Oriented Ba-Ferrite Films Deposited by 
Targets-Facing Type of Sputtering", by Hoshi, Matsuoka, Naoe and Yamanaka, 
The Transactions of the Institute of Electronics and Communication 
Engineers of Japan (C), J. 66-C, 1, p 9-16 (January, 1983). 
In these perpendicular magnetic recording media, although the Co-Cr alloy 
film can be prepared at a low temperature, the perpendicular magnetic 
anisotropy thereof as the measure for the magnitude of the perpendicular 
magnetization is smaller than that of barium ferrite and strontium 
ferrite. This causes a problem that no complete perpendicular 
magnetization film can be obtained, but some inplanar magnetized 
components remain. Further, since the Co-Cr alloy is a metal material, it 
is readily oxidized as other magnetic materials such as Fe and Co-Ni. 
While on the other hand, a substantially complete perpendicular 
magnetization film can be prepared with barium ferrite and strontium 
ferrite since a substantially complete C-axis oriented film can be 
obtained therewith. However, since the substrate temperature has to be 
500.degree. C. or higher for preparing a barium ferrite or strontium 
ferrite film, it is difficult to prepare strontium ferrite or barium 
ferrite on a polyimide or aluminum substrate. 
While on the other hand, the perpendicular magnetization is necessary also 
in the optothermal magnetic recording for attaining high density 
recording. 
However, the optothermal magnetic recording method is different from the 
magnetic recording method in that the change in the magnetic property due 
to heat application is utilized for recording and the optical effect 
relevant to the magnetic property is utilized for reproducing. That is, 
heat of a laser beam is utilized for recording and Kerr effect or 
Faraday's effect of the optomagnetic recording medium is utilized for 
reproduction as disclosed by Osamu Imamura, The Journal of the Institute 
of Television Engineers of Japan, Vol. 39, No. 4 (1985), p 365-368. 
Further, if an optomagnetic disc is used, for example, as the medium, a 
large Kerr effect (large Kerr rotation angle) is required in order to 
improve the SN ratio (signal to noise ratio) of the disc as disclosed in 
"A Guide To Getting Strong Magneto Optical Effect", Masanori Abe, Journal 
of Magnetics Society of Japan, Vol. 8, No. 5 (1984), p 366-372. 
In view of the above, optomagnetic recording media with large Kerr rotation 
angle such as manganese-bismuth (MnBi), gadolinium-cobalt (GdCo) and 
gadolinium-terbium-iron (GdTbFe) have been developed by the vacuum 
deposition or sputtering process. 
However, since these recording media utilize metal thin films such as Gd, 
Tb and Fe which are readily oxidized, they are considered not adaptable to 
external memory devices or the like for computers that require high 
reliability. 
While on the other hand, it has been attempted to use ferromagnetic oxides, 
which are highly stable chemically, as the optomagnetic or magnetic 
recording (medium as disclosed by) J. W. D. Martens and A. B. Voermans, 
IEEE Transactions on Magnetics, vol. MAG-20 No. 5, September, 1984, and 
mainly cobalt ferrite films or oxide iron films are prepared by a heat 
treatment at 400.degree. C.-800.degree. C. using a sputtering or gas phase 
heat decomposing process (by H. Schmid in Austrian Pat. No. 162,382 
(1949), and R. H. Sawyer in U.S. Pat. No. 2,642,339 (1953). In these 
optothermo magnetic recording media, although MnBi, GdCo and GdTbFe alloys 
can produce perpendicular magnetization films through synthesis at lower 
temperature, there has been a problem that the reliability is reduced due 
to the oxidation of the film. Particularly, in the case of using 
inexpensive substrates such as of polycarbonate or polyimide, these 
substrates are liable to adsorb water and, accordingly, the alloys may be 
oxidized due to the absorbed water. 
While on the other hand, ferrites such as cobalt ferrite are free from 
oxidation of the film and, stable and inexpensive. However, since a heat 
treatment at 700.degree. C.-800.degree. C. (crystallization) is necessary 
upon preparing the film through sputtering or chemical vapor deposition 
process (CVD process) for obtaining a film of a large Kerr effect, it is 
difficult to use those substrates, for example, made of low melting glass, 
aluminum, polycarbonate and polyimide. In addition, since the ferrites 
such as cobalt ferrites have a spinel type isotropic crystal structure, 
there is a problem that they cannot form a perpendicular magnetization 
film by the crystal magnetic anisotropy such as in barium ferrite or 
Co-Cr. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a reliable magnetic oxide film. 
Another object of this invention is to provide a method of manufacturing a 
reliable magnetic oxide film by use of a plasma chemical vapor deposition 
process on a substrate with a low heat resistivity such as an organic film 
or aluminum at a low temperature or without heating the substrate. 
According to this invention, a magnetic oxide film is prepared at a low 
temperature or without heating the substrate not by way of conventional 
sputtering or chemical vapor deposition process, but by flowing a gas of 
an organic metal compound containing constituent elements for the magnetic 
material into high frequency plasmas, magnetron discharge plasmas or 
electron cyclotron resonance plasmas (ECR plasma) under a reduced pressure 
(10-10.sup.-4 Torr), and by way of a plasma CVD process utilizing the 
activity of these plasmas. 
Specifically, organic metal compounds containing elements constituting the 
magnetic material, for example, alkoxide compounds such as triethoxy iron 
(Fe(OC.sub.2 H.sub.5).sub.3), triethoxy cobalt (Co(OC.sub.2 
H.sub.5).sub.3), diethoxy barium (Ba(OC.sub.2 H.sub.5).sub.2), 
.beta.-diketone complexes such as dipivaloyl methane barium 
(Ba(DPM).sub.2) (DPM=C.sub.11 H.sub.19 O.sub.2), iron acetyl acetonate 
(Fe(C.sub.5 H.sub.7 O.sub.2).sub.3), cobalt acetyl acetonate (Co(C.sub.5 
H.sub.7 O.sub.2).sub.3) and barium acetyl acetonate (Ba(C.sub.5 H.sub.7 
O.sub.2).sub.2) and ferrocene-like compounds such as 
bis(cyclopentadienyl)iron (Fe(C.sub.5 H.sub.5).sub.2) and 
bis(cyclopentadienyl)cobalt (Co(C.sub.5 H.sub.5).sub.2) in the form of 
vapors are introduced together with oxygen (O.sub.2) as the reaction gas 
into a reaction vessel at a reduced pressure, and plasmas are generated in 
the reaction vessel to deposit magnetic oxide material on a substrate at a 
low temperature below 350.degree. C. 
For instance, when cobalt ferrite (CoFe.sub.2 O.sub.4) is prepared through 
the reaction of iron acetyl acetonate, cobalt acetyl acetonate and oxygen, 
the reaction proceeds by the following reaction scheme: 
EQU 8Fe(C.sub.5 H.sub.7 O.sub.2).sub.3 +4Co(C.sub.5 H.sub.7 O.sub.2).sub.3 
+233O.sub.2 .fwdarw.4CoFe.sub.2 O.sub.3 +180CO.sub.2 +126H.sub.2 O 
If the heat decomposing reaction is carried out under the normal pressure 
(760 Torr), a temperature at 500.degree. C. or higher is required for the 
substrate, but the reaction can occur in plasmas at a temperature from 
100.degree. C. to 350.degree. C. under a reduced pressure (10 
Torr-10.sup.-4 Torr), and, further, the reaction occurs at a temperature 
below 100.degree. C. in high density plasmas such as magnetron discharge 
plasmas and electron cyclotron resonance plasmas. 
The oxide magnetic film can thus be deposited under a low temperature, 
because there are present in plasmas many chemical species such as active 
radicals or ions that cause chemical reactions at low temperature, and the 
reaction that cannot occur in view of the energy in the usual heat CVD 
process is possible within the plasmas. 
In addition, the plasma CVD process can synthesize high melting materials 
such as oxides, carbides and nitrides at low temperature as compared with 
the usual CVD process, and as well as can produce film of columnar 
structure at high purity and with good crystallinity even at a low 
temperature since this accompanies heat decomposing deposition. Thus, this 
is a most suitable method to render the magnetization film of isotropic 
crystal structure such as ferrites (horizontal magnetization film) into a 
perpendicular magnetization film on an organic film (since ferrites are 
isotropic crystals, perpendicular magnetization film is obtained by 
utilizing the perpendicular anisotropy caused by the configurational 
anisotropy due to the columnar structure of the film). 
With the constitution as has been described above, a thin magnetic oxide 
film (perpendicular magnetic recording medium) can be obtained at a low 
temperature below 350.degree. C. by skillfully utilizing the plasma 
activity in the method according to this invention.