Magnetic recording medium

A magnetic recording medium wherein a ferromagnetic substance consisting of Fe, Co, Ni or alloy thereof is vacuum evaporated and deposited on a substrate made of a plastic film or a sheet of non-magnetic metal. The thin ferromagnetic film has the columnar crystal structure, and the columnar crystals are coated with a layer of oxide of the ferromagnetic substance.

BACKGROUND OF THE INVENTION: 
The present invention relates generally to an improved a magnetic recording 
medium of a thin ferromagnetic film formed by vacuum deposition and a 
method for manufacture thereof. 
In the conventional process for manufacture of so-called coated type 
magnetic tapes, magnetic powder such as gamma-Fe.sub.2 O.sub.3 or 
CrO.sub.2 is mixed with a binder, and the mixture is applied over a 
substrate and cured. In this process, the binder cannot be eliminated 
because it is used to disperse the magnetic powder. As a result, the 
information recording density of the coated type magnetic tapes cannot be 
improved beyond a certain limit. In order to solve this problem, there has 
been invented and demonstrated a thin ferromagnetic type magnetic tape 
which may be manufactured without the use of a binder. The thin metal or 
ferromagnetic film may be prepared by several methods such as chemical 
plating, vacuum deposition, sputtering, ion plating and so on. Of these 
methods, vacuum deposition is most advantageous in that the evaporation 
rate is fast and this method is very simple. However, the thin 
ferromagnetic film type tape has the problem of satisfactorily increasing 
its coercive force. For instance, with the chemical plating method, 
studies and experiments of increasing the coercive force of a thin cobalt 
film by adding a suitable impurity such as phosphorus have been long 
conducted. With the ion plating method, a thin film is formed under a high 
pressure of the order of 10.sup.-2 to 10.sup.-3 torr so as to control the 
grain size, thereby improving the coercive force. The thin magnetic film 
type tape prepared by the chemical plating method is however not 
satisfactory in practice because the strength of joint between the thin 
metal film and the substrate is not sufficient and the film forming rate 
is too slow. The thin magnetic film prepared by the ion plating method is 
also not satisfactory because its thin film forming rate is too slow. 
The film forming rate of the vacuum deposition method is faster than any 
other method, but the coercive force of thin metal film tapes fabricated 
by the vacuum deposition method is in general less than 100 Oe which is 
considerably less than 200 to 500 Oe of the conventional coated type 
magnetic tapes so that they are unsatisfactory in practice. The volume or 
bulk coercive force of Fe, Co or Ni is tens Oe at the highest. The 
coercive force of a thin film formed by the vacuum deposition of these 
metals is 100 Oe at the highest, because the ferromagnetic metal is 
evaporated in a nearly ideal vacuum (10.sup.-4 to 10.sup.-6 torr) in the 
form of atoms and is recrystalized on a substrate. 
A method is known in which the beam of evaporating metal is made incident 
at an angle greater than 45.degree. on a substrate in order to increase 
the coercive force of a thin magnetic film. However, the evaporation or 
thin film forming rate is too low to be used in practice. 
SUMMARY OF THE INVENTION 
One of the objects of the present invention is to provide a magnetic 
recording medium having a high coercive force and capable of high density 
recording. 
Another object of the present invention is to provide an extremely thin 
magnetic recording medium whose output variation is minimum. 
To the above and other ends, the present invention provides a thin metal 
film type magnetic recording medium wherein a thin ferromagnetic film 
continuously formed by vacuum deposition on a substrate of a plastic film 
or a sheet of nonmagnetic metal has a columnar crystal structure, and the 
columnar crystals are coated with a layer of oxide of a ferromagnetic 
substance. The present invention also provides a method for manufacture of 
the magnetic recording media of the type described.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Prior to the description of the preferred embodiments of the present 
invention, the underlying principle thereof will be described. The 
inventors made extensive studies and experiments on the crystal structures 
of thin ferromagnetic films prepared by vacuum deposition or evaporation, 
and found that in practice satisfactory coercive force depends upon 
somewhat limited crystal structures and the chemical oxidation states of 
the crystal grain boundaries. A method is known wherein a thin magnetic 
film of cobalt formed on an aluminum substrate is coated with an oxide 
layer. The oxide film is formed by burning. A lubricating layer is further 
formed on the oxide film. In general, the thin metal films prepared by the 
vacuum deposition process exhibit various crysal structures depending upon 
materials used. When the evaporation rate is higher (in excess of 1,000 
A/min), amorphous and uniform films tend to be formed. 
The investigation of Fe, Ni, Co and alloys thereof which are ferromagnetic 
showed that the same results are obtained, as with other materials. The 
coercive force of thin films of these metals is in the order of 100 Oe at 
the highest, and there is a tendency that the thicker the thin film, the 
lower the coercive force becomes. 
The inventors used a wide variety of substrates and vacuum evaporation and 
deposition conditions which had never been employed before to prepare the 
thin ferromagnetic films with various crystal structures and found that 
the thin magnetic films thus prepared have a coercive force of 200 to 
1,000 Oe which is equal to or higher than the coercive force of 
conventional magnetic tapes. 
Referring to FIG. 1, a substrate 1 is made of a plastic film of 
polyethylene terephthalate, polycarbonate, polypropylene or the like. If 
required, the substrate is coated with a layer 2 for facilitating uniform 
deposition and strong adhesion of a ferromagnetic film. Most desirable 
effects can be attained when the layer 2 is made of a metal such as Al, 
Ti, Mo, Si and the like as will be described in detail hereinafter. The 
thin ferromagnetic film 3 has columnar crystals coated with an oxide 4. 
The ferromagnetic materials used in deposition of thin films are Fe, Co, 
Ni, alloys thereof and alloys of Fe, Co and Ni with suitable nonmagnetic 
materials. 
Next the vacuum evaporation process for growing such special or columnar 
crystals 3 will be described. In general, according to the vacuum 
evaporation process, a evaporation metal is heated in an atmosphere at 
10.sup.-4 to 10.sup.-6 torr and an evaporant is deposited on a substrate 
to recrystallize. Gases in a vacuum evaporation apparatus contain water, 
those gases contained in the air and those liberated from the substrate 1. 
It is difficult to operate the vacuum evaporation process under the same 
and stable conditions. One solution to this problem is to introduce a 
limited volume of gases so as to improve the reproducibility of both 
composition and volume of gases in the apparatus. 
The composition and volume of gases in the apparatus effect the crystal 
growth of a thin metal film. The inventors found out that when oxygen gas 
is introduced into the vacuum evaporation apparatus, the quality 
uniformity as well as reproducibility may be considerably improved. For 
instance, when Fe is vacuum deposited at an evaporation rate of 3,000 
A/min, at an oxygen partial pressure of 1.times.10.sup.-4 torr, upon a 
substrate of polyethylene terephthalate, a magnetic tape having the 
coercive force ranging from 300 to 600 Oe can be obtained. 
Factors affecting the growth of such columnar crystals are the evaporation 
rate, physical properties of a substrate and the atmosphere. Furthermore 
the method for inclining a substrate to the incident beam of evaporating 
metal or the method for effecting the vacuum deposition in magnetic or 
electrical fields will also influence the growth of columnar crystals. It 
is preferable to use an electron microscope, although not limited, to 
observe the crystal structures of thin magnetic films. The ratio of height 
to width of a columnar crystal grown by the process in accordance with the 
present invention is in general higher than three, and the columnar 
crystal is inclined at an angle relative to the major surface of a 
substrate or undercoating. Thus it can be seen that for a ratio of three 
the thin film of ferromagnetic material is from about 300 A to 3000 A 
thick. The inventors found that the columnar crystals incline in general 
at angles less than 60.degree. relative to the vertical, and they are 
similar in size to the thickness of a film. Furthermore the inventors 
found that columnar crystals having a width (cross sectional) of less than 
500 A exhibit very satisfactory results. The inventors found that the 
combination especially with the process for inclining the substrate 
relative to the incidence beam can attain very satisfactory results. 
Preferably the thickness of the oxide coating 4 of the columnar 
ferromagnetic crystal 3 is more than 20 A. The thicker oxide coating 4 
will not adversely affect the coercive force, but is not preferable in the 
case of a ferromagnetic material which is de-magnetized by oxidation 
because the remenant magnetization decreases. 
For ordinary or general purpose magnetic tapes, the cross section of the 
column of the columnar crystals 3 is preferably between 100 A and 1,000 A, 
though not limited. 
It has not been explained why a thin ferromagnetic film consisting of 
columnar crystals 3 coated with the oxide film 4 has high coercive force 
and excellent properties required for magnetic tapes. A possible 
explanation is that opposed to the conventional vacuum deposition 
processes wherein crystals are grown in an air atmosphere or an atmosphere 
substantially similar to air, the crystal growth is effected in a highly 
pure atmosphere so that the uniform grain size results. Furthermore oxygen 
gas may serve to produce finely divided crystal grains of uniform size. 
This is assumed by the fact that the coercive force is highly stable at 
various pressures and temperatures and that anisotrophy is produced by 
form anisotropy of controlled grain size. 
The control of grain size alone can be attained by ion plating under a 
controlled gas pressure, but the columnar crystals cannot be grown. The 
inventors assume that the columnar crystals grown by the vacuum deposition 
process have properties substantially similar to those of needle crystals 
of gamma--Fe.sub.2 O.sub.3 grown by the coating process. Even when the 
columnar crystals 3 are partly coated with the oxide film 4, the same 
effects may be attained. 
The thin magnetic films prepared by a magnetic substance mainly consisting 
of Fe, Co, Ni and alloys thereof have the problems of low output, higher 
loss in a high frequency range and higher noise, but these problems may be 
substantially solved by the present invention so that thin magnetic films 
having excellent recording properties may be provided. The inventors found 
that of thin magnetic films prepared by vacuum evaporation and deposition 
of Fe, Co, Ni and alloys thereof, a thin magnetic film having a volume 
resistivity in excess of 1.2 times as high as that of the composition 
metal exhibits considerably improved output, negligible loss at a high 
frequency range and very small noise. 
FIG. 2 shows the relationship between the volume resistivity and relative 
output and noise of a thin magnetic film prepared by vacuum evaporation of 
Co of 3,000 A in thickness on a polyester film. The ratio 
(.rho./.rho..sub.B) of a volume resistivity .rho. to that .rho..sub.B of 
bulk of Co is plotted along the abscissa while the output relative to a 
reference input and noise are plotted along the ordinate, the output and 
noise levels being 0 dB when .rho./.rho..sub.B is 1.0. Recording biase 
frequency of 200 KHz was used in the measurement of output. The solid 
curve indicates the output of 1 KHz; the broken line curve, the output of 
20 KHz; and the one-dot chain curve, the noise of 1 KHz. The open circle 
symbol O indicates a thin magnetic film prepared by the simple vacuum 
deposition; the triangle symbol .DELTA., a thin magnetic film prepared by 
inclining a substrate to the incident beam of evaporating metal; the cross 
symbol X, a thin magnetic film prepared by vacuum evaporation in an 
atmosphere containing oxygen; the closed circle symbol o, a thin magnetic 
film heat treated at 100.degree. C. after vacuum deposition; and the 
square symbol , a thin magnetic film heat treated at 70.degree. C. after 
vacuum evaporation. 
It is seen that the resistivity of a thin magnetic film may be increased by 
the vacuum evaporation process wherein a substrate is inclined to the 
incident beam of evaporating metal, the introduction of oxygen gas or by 
the after-heat-treatment. Furthermore, the higher the ratio 
.rho./.rho..sub.B, the higher the output becomes, and when the ratio 
.rho./.rho..sub.B is in excess of 1.2, the output of 1 KHz increases by 
about 10 dB while the output of 20 KHz increases by about 15 dB. On the 
other hand, noise of 1 KHz decreases by about 7 dB. Thus the 
characteristics and properties of thin magnetic films can be considerably 
improved by the increase in resistivity. The difference between the 
methods for increasing the resistivity will not result in any appreciable 
difference in characteristics and properties. 
In summary, the present invention provides magnetic films prepared by 
vacuum evaporation of a magnetic substance mainly consisting of Fe, Co, Ni 
and alloys thereof. The thin magnetic films have a volume resistivity in 
excess of 1.2 times as high as that of bulk of the composition metal or 
alloy. The output may be considerably improved, especially in a high 
frequency range, and noise is remarkably reduced so that the magnetic 
tapes having excellent characteristics and properties hitherto 
unattainable by the conventional magnetic tapes may be provided. 
Next referring to FIG. 3, the undercoating 2 will be described in detail. A 
magnetic tape shown in FIG. 3 consists of a polyethylene terephthalate 
film, an undercoating 2 made of aluminum, a thin magnetic film 5 in 
accordance with the present invention and an overcoating 6 made of a metal 
or organic compound for improving the transportability of the tape. 
A magnetic tape with the above construction was compared with a 
conventional magnetic tape provided with no aluminum undercoating 2, in 
terms of the varation in output voltage in the longitudinal direction. The 
variation of the conventional tape was 10 to 20%/m, while the variation of 
the magnetic tape in accordance with the present invention was less than 
7.5%/m. The latter was prepared by the vacuum deposition of a cobalt alloy 
on a polyester film. 
The magnetic substances are Fe and Ni, exept the above cobalt Co and alloys 
of Fe, Ni, Co. When the thickness of the aluminum undercoating 2 is 
thicker than 100 A, the desired effects may be obtained. The thickness of 
the magnetic tape is preferably less than one micron. 
In addition to aluminum, Ti, Cr, Mo, Ta and so on may be used, but in view 
of the characteristic forming a film and cost, aluminum is most 
preferable. The same effects may be attained by an aluminum oxide 
undercoating. 
Next referring to FIGS. 4, 5 and 6, the process for manufacture of magnetic 
tapes in accordance with the present invention will be described. First 
referring to FIG. 4, within a vacuum chamber 8, the substrate 7 wound on a 
roll 9 is unwound and taken up by a takeup reel 11. The substrate 7 being 
made of a high molecular weight compound or non-magnetic material and 
being guided by guide rollers 10. An evaporation source 12 is connected to 
a power source 14 through insulated terminals 13 so that upon energization 
of the evaporation source 12, a metal or alloy (not shown) is heated and 
evaporated. The vacuum chamber 8 is provided with a partition wall 15 
having an aperture, and the evaporant is deposited upon the substrate 7 in 
the vicinity of the aperture. The vacuum chamber 8 is evacuated by a 
vacuum pump such as an ion pump, and gases 19 and 20 which are introduced 
into the vacuum chamber 8 are controlled by variable leak valves 17 and 
18, respectively. 
In this embodiment, the substrate 7 is transported in opposed relationship 
with the evaporation source 12, but the substrate 7 may be suitably 
inclined to the incident beam of evaporating metal in order to produce a 
desired anisotropy. The angle of inclination may be determined depending 
upon a selected ferromagnetic substance. The evaporation source 12 may be 
any of the conventional evaporation sources such as an electron beam 
evaporator, a high frequency induction heating device and the line. 
One of the essential features of the present invention resides in the fact 
that the oxygen gas is introduced into the vacuum chamber 8. In addition 
to oxygen gas, inert gases and other reactive gases may be introduced, but 
it is essential that the partial pressure of oxygen in the vacuum 
deposition atmosphere must be higher than the partial pressures of other 
gases. In other words, the oxygen gas in the atmosphere must have the 
highest partial pressure. The purification of gases to be introduced into 
the vacuum chamber 8 is effected in any suitable conventional manner, and 
a suitable arrangement for introducing gases into the vacuum chamber in a 
stable manner is of course provided. 
In operation, a liquefied nitrogen trap is used to evacuate the vacuum 
chamber 8, and then the ion pump 16 is driven to evacuate the chamber 8 to 
3.times.10.sup.-6 torr. Thereafter oxygen with a purity higher than 99.99% 
is introduced into the chamber 8 until the pressure within the chamber 8 
rises to 10.sup.-6 to 10.sup.-3 torr, and then cobalt is heated and 
evaporated by the high frequency induction heating device to deposite 
cobalt 2,500 to 3,000 A in thickness on a substrate made of polyimide 100 
microns in thickness. 
FIG. 5 shows the relationship between the vacuum pressure and the coercive 
force of thus obtained magnetic tape (curve A), and for the sake of 
comparison shown is the coercive (curve B) of a conventional type magnetic 
tape prepared by keeping the degree of vacuum in the vacuum chamber 
between 10.sup.-6 and 10.sup.-3 torr by opening and closing a valve (not 
shown) interconnecting between the vacuum chamber and the vacuum pump or 
exhaust system. 
From FIG. 5 it is readily seen that the presence of oxygen gas in the 
vacuum deposition atmosphere contributes to the considerable increase in 
coercive force. This effect may be equally obtained when Fe, Ni, Co and 
alloys thereof are used. 
It is preferable that oxygen gas is introduced into the vacuum chamber 
until the degree of vacuum thereof drops to 10.sup.-3 to 10.sup.-5 torr, 
but the degree of vacuum may be controlled so as to be 10.sup.-6 to 
10.sup.-7 torr when the initial degree of vacuum is increased. 
In FIG. 6 there are shown the characteristic curves A and B of secular 
variation of magnetic tapes fabricated in accordance with the present 
invention and those C and D of conventional magnetic tapes, these test 
tapes being left at 45.degree. C. and at humidity of 80% and their 
coercive force as well as rectangular hysteresis ratios being measured. It 
is apparent that the magnetic tapes in accordance with the present 
invention exhibit by far excellent magnetic characteristics than the 
conventional ones in coercive force and triangular ratio. 
In summary, according to the present invention oxygen gas is introduced 
into the vacuum chamber wherein vacuum deposition such as electron beam 
deposition, ion plating and so is effected, so that the magnetic tapes 
having stable and improved magnetic characteristics such as coercive 
force, rectangular hysteresis ratio and so on may be provided. In 
addition, the present invention may be applied to the manufacture of 
laminations consisting of alternate magnetic and non-magnetic layers and 
of magnetic tapes with a substrate consisting of a film made of a high 
molecular compound and coated with a non-magnetic metal layer. In addition 
to oxygen, argon or nitrogen whose partial pressure is lower than the 
partial pressure of oxygen may be introduced in a quantity of 1/10 to 1/5 
by volume of oxygen, but the use of the atmosphere consisting of 100% of 
oxygen can attain the most desirable effects.