Magnetic recording medium

A magnetic recording medium is formed by disposing on a substrate a magnetic recording layer comprising a vertically magnetizable film of Co alone or a Co alloy, or Fe alone or a ferromagnetic Fe alloy. The lubricity, wear resistance and corrosion resistance of the magnetic recording medium are improved by forming an upper layer of Co oxide on the magnetic recording layer. The upper layer per se can be a magnetizable film, particularly a vertically magnetizable film.

FIELD OF THE INVENTION AND RELATED ART 
The metal film-type magnetic recording medium obtained by forming a 
ferromagnetic metal film by the film deposition technique on a 
non-magnetic substrate ordinarily of a plastic film or sheet has recently 
called much attention as a high recording density medium, because a thin 
magnetic recording layer having a higher magnetic flux density and a 
higher coercivity can be easily formed compared with the so-called 
"coating-type" magnetic recording medium having a coating layer wherein 
ferromagnetic powder is dispersed in a polymer binder. 
As a magnetic recording layer of the metal film type medium, a film of a 
Co-based alloy such as Co-Ni alloy has frequently been used because of 
large crystalline anisotropy and coercive force and also of relatively 
large corrosion resistance. Relatively inexpensive Fe alone or a 
ferromagnetic alloy containing Fe (hereinafter inclusively referred to as 
"Fe-based magnetic metal") is extremely susceptible to oxidation when 
formed in a film and has involved a problem in respect of corrosion 
resistance of a magnetic layer, while it is superior to the above 
mentioned Co-based alloy in respect of saturation flux density 
(hereinafter simply referred to as "Bs") which is an important magnetic 
property as well as a coercive force (hereinafter simply referred to as 
"He"). 
By the way, when the recording system is considered, the magnetic recording 
media may be classified into those adapted for the vertical or 
perpendicular magnetization recording system and the longitudinal or 
parallel magnetization recording system. The vertical magnetization 
system, when compared with the conventional longitudinal magnetization 
system, is capable of providing an extremely increased recording density 
and therefore the practical use thereof is extremely important for 
development of the magnetic recording. As recording media for the vertical 
magnetization system, Co-based metals represented by Co and Co-Cr alloy 
and Ba-ferrite have been developed. 
The Ba-ferrite medium comprises a coating layer of Ba-ferrite powder 
dispersed in a binder formed on a substrate and has an advantage that it 
can be produced through the methods for producing conventional recording 
media. The Ba-ferrite medium, however, involves a defect that it has a 
small Bs (saturation flux density). 
On the other hand, a vertically magnetizable film of Co or Co-alloy formed 
by the film deposition process including the vacuum evaporation process, 
the sputtering process, the plating process, etc., has a larger Bs than 
the Ba-ferrite layer and is therefore capable of realizing a higher 
recording density by that much. While the Co or Co-alloy film has 
excellent magnetic properties, it involves an obstacle to the 
commercialization thereof that it is poor in wear or abrasion resistance. 
In the meantime, as a measure for improving the corrosion resistance of a 
magnetic recording layer, it has been considered to provide a 
corrosion-resistant protective layer of a corrosion-resistant metal such 
as Cr, V or Ni, or an oxide such as Al.sub.2 O.sub.3 or SiO.sub.2. Such a 
protective layer of corrosion resistant metal or oxide as described above 
however requires a certain thickness in order to exhibit a sufficient 
corrosion resistance so that the reproduction efficiency is lowered due to 
spacing loss. This is particularly pronounced in a shorter wavelength 
side. Further, the above described class of protective film does not 
provide a good lubricity or wear resistance so that it has not been 
commercially used. 
In order to improve the wear resistance of a magnetic recording layer, it 
has been considered to deposit an organic lubricating material such as a 
metal soap, an aliphatic acid ester or perchloropolyether on the magnetic 
recording layer, whereas a protective lubricating material showing a 
sufficient durability has not been found. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a magnetic recording 
medium having excellent durability inclusive of corrosion resistance, 
lubricity and wear resistance. 
Another object of the present invention is to provide a magnetic recording 
medium adapted to high density recording. 
According to the present invention, there is provided a magnetic recording 
medium, comprising: a substrate, a magnetic recording layer and an upper 
layer of Co (cobalt) oxide disposed in laminated form in the order named; 
the magnetic recording layer comprising a vertically magnetizable film of 
Co alone or a Co alloy, or Fe (iron) alone or a ferromagnetic alloy 
containing Fe. 
The above mentioned and other objects and features of the invention will be 
better understood upon consideration of the following detailed description 
concluding with specific examples of practice and taken in conjunction 
with the accompanying drawings. In the description appearing hereinafter, 
"%" referring to a composition is by weight unless otherwise noted 
specifically.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a magnetic recording medium comprising a substrate 1, a 
vertically magnetizable magnetic recording layer 2 on the substrate, and 
an upper layer 3 of Co oxide formed on the magnetic recording layer 2. The 
magnetic recording layer 2 comprises a vertically magnetizable film of Co 
or a Co alloy formed by vacuum evaporation, sputtering, ion plating, or 
wet plating on the substrate 1. As ferromagnetic metal films forming 
vertically magnetizable films, i.e., ferromagnetic metal films having an 
easily magnetizable direction substantially perpendicular to the film 
surface, there have been known those of Co, Co-Cr, Co-V, Co-Mo, Co-W, 
Co-Cr-Pd, Co-Cr-Mo, Co-Cr-Rb, etc. Among these, a Co-Cr film has a 
particularly large vertical magnetic anisotropy and is relatively easily 
formed as a vertically magnetizable film. For this reason, the magnetic 
recording layer 2 is desirably formed of Co-Cr. The magnetic recording 
layer 2 may be formed in a thickness of, e.g., 0.1 to 2 .mu.m directly on 
the substrate 1 or alternatively on the substrate 1 through an 
intermediate layer of a metal film of Ti, Bi, Ge, etc. or an amorphous 
film of SiO.sub.2, Co-Zr, Co-Si-Br, etc. Such an intermediate layer can 
show a function of, e.g., improving the orientation of the magnetic 
recording layer 2. Further, there may be formed a high permeability 
magnetic layer between the substrate 1 and the magnetic recording layer 2 
or between the substrate 1 and the above mentioned intermediate layer for 
the purpose of increasing the recording efficiency or increasing the 
reproduction output. 
The upper layer 3 of a Co oxide may be formed on the vertically 
magnetizable 2 by sputtering of a Co target in an inert gas atmosphere 
containing a prescribed pressure of oxygen or by effecting vacuum 
evaporation or ion plating of Co as the evaporation source in a dilute 
oxygen atmosphere. The upper layer 3 is very excellent in lubricity and 
wear resistance so that the head touch and running or feeding 
characteristic of the magnetic recording medium can be remarkably 
improved. Further, as shown in FIG. 1, the magnetic recording layer 2 has 
a columnar microstructure grown in the film thickness direction and the 
upper layer 3 formed thereon has the same columnar structure. As a result, 
the upper layer 3 is firmly combined with the lower magnetic recording 
layer 2 in structure and provides a magnetic recording medium having an 
extremely good durability. 
The upper layer 3 comprising Co oxide may be formed by sputtering or vacuum 
evaporation and changes its magnetic property from ferromagnetism to 
non-magnetism and also changes its coercivity Hc depending on the amount 
of oxygen contained in the atmosphere for formation thereof. The amount of 
oxygen to be contained in the atmosphere for formation of the upper layer 
3 varies depending on a particular apparatus to be used. Thus, the upper 
layer 3 may be produced in an inert gas atmosphere containing 5 to 50% of 
oxygen with respect to the partial pressure of the inert gas in the case 
of sputtering or a dilute oxygen atmosphere at a pressure of the order of 
10.sup.-3 Torr in the case of vacuum evaporation. In either method, the 
oxygen content in the resultant upper layer naturally increases as the 
oxygen content in the formation atmosphere increases. The oxygen content 
in the thus formed upper layer is much smaller than that contained in CoO 
detectable by X ray diffraction and cannot be readily quantitatively 
measured. Generally speaking, however, as the oxygen content in the upper 
layer increases, the saturation flux density Bs tends to decrease. 
Further, as the incident angle of the evaporated particles becomes close 
to a vertical against the substrate, a vertically magnetizable layer can 
be readily formed. On the other hand, as the incident angle increases, a 
longitudinally magnetizable layer is readily formed. The magnetic 
properties of the upper layer affect the lower magnetic recording layer 2. 
For this reason, the thickness of the upper layer 3 should be determined 
depending on the oxygen content in the upper layer 3 so that the layer 3 
does not hinder the recording and reproduction characteristic of the 
magnetic recording layer 2. More specifically, a longitudinally 
magnetizable upper layer 3 containing little oxygen has a large saturation 
flux density Bs and a small Hc so that it functions as a magnetic shield 
layer against the lower magnetic recording layer 2. Therefore, where an 
upper layer 3 having an Hc of the order of 300 Oe or below and a large Bs 
is disposed on or above the magnetic recording layer 2, the thickness of 
the upper layer 3 should be decreased. Further, the upper layer 3 should 
preferably have a Bs of 10,000 gauss or below since the improvement in 
lubricity and wear resistance is little if the oxygen content is too 
small. 
More specifically, when the saturation flux density is represented by 
Bs.sub.1, the coercivity by Hc.sub.1 and the thickness by .delta..sub.1 
for the lower magnetic recording layer 2, and the saturation flux density 
is represented by Bs.sub.2, the coercivity by Hc.sub.2 and the thickness 
by .delta..sub.2 for the upper layer 3, the .delta..sub.2 should desirably 
be selected so as to satisfy the following relationship: 
EQU .delta..sub.1 Bs.sub.1 /Hc.sub.1 &gt;.delta..sub.2 Bs.sub.2 /10/Hc.sub.2. 
If the upper layer 3 contains too much oxygen and is non-magnetic or has a 
very small Bs, the upper layer 3 functions as a spacing between the lower 
magnetic recording layer 2 and a magnetic head. This influence is 
pronounced particularly when a high density recording is aimed at as in 
the magnetic recording medium according to the present invention. For this 
reason, the thickness of the upper layer 3 should preferably be 1/10 or 
less, further preferably 1/30 or less, of the minimum wavelength of a 
recording signal range. If the upper layer 3 is too thin, it cannot show 
sufficient effects in respect of lubricity, wear resistance and 
durability. The upper layer 3 should preferably have a thickness of at 
least 50 .ANG.. The upper limit for the thickness is not very significant 
but may generally be of the order of 0.5 .mu.m. 
If the upper layer 3 is formed as a vertically magnetizable film, the upper 
layer 3 functions as a magnetic recording layer like the magnetic 
recording layer 2 so that the thickness of the upper layer 3 can be thick 
and free of the restriction as described above. A vertically magnetizable 
upper layer 3 can be obtained by effecting evaporation or sputtering of Co 
in a certain reduced pressure range of oxygen atmosphere or in an inert 
gas atmosphere containing oxygen in a certain range of proportion in such 
a manner that the evaporated particles are incident on the substrate 1 at 
a substantially perpendicular initial incident angle with respect to the 
substrate 1. The oxygen partial pressure providing a vertically 
magnetizable upper layer 3 depends on a certain production process or 
apparatus used. In general, however, a vertically magnetizable film may be 
relatively easily formed in an oxygen atmosphere at a pressure of the 
order of 10.sup.-3 Torr for the evaporation process or in an inert gas 
atmosphere at a total pressure of, e.g., 10.sup.-4 to 10.sup.-1 Torr 
containing oxygen in a proportion of 10-20% with respect to the partial 
pressure of the inert gas for the sputtering process. The thus formed 
vertically magnetizable upper layer 3 has a Bs of generally 1000-6000 
gauss and an Hc of generally 150 to 1200 Oe. It is preferred that the 
upper layer 3 has magnetic properties not remarkably different from those 
of the magnetic recording layer 2 and, more specifically, it is preferred 
that both Bs and Hc of the upper layer 3 are of the same order as those of 
the magnetic recording layer 2 in view of recording and reproduction 
characteristics. 
The lubricating effect of the upper layer 3 depends on the surface 
unevenness thereof. If the surface unevenness is 0.005 .mu.m or larger in 
terms of an average of ten measured values of surface roughness Rz 
(JIS-B0601), a kinematic coefficient of friction of 0.3 or less is 
obtained. If the maximum roughness difference in height Rmax between 
concavity and convexity locally exceeds about 0.05 .mu.m, there occurs a 
drop-off of a signal at that place. More specifically, Rz is obtained by 
measuring roughnesses (or local differences in height) for a prescribed 
length (1 mm) of magnetic recording medium or tape by a roughness meter 
(Talystep mfd. by Taylor Hobson) and averaging ten measured values 
selected from all the measured values in order of magnitude. The maximum 
roughness Rmax is obtained as the maximum value among the thus selected 
ten measured values. As the surface roughness of the upper layer 3 depends 
on the surface roughness of the substrate 1, the surface roughness of a 
substrate for the magnetic recording medium according to the present 
invention should preferably be 0.005 .mu.m or larger in terms of Rz and 
0.1 .mu.m or smaller, particularly 0.05 .mu.m or smaller, in terms of 
Rmax. 
The substrate 1 for the magnetic recording medium according to the present 
invention may generally and preferably be a film having a thickness of the 
order of 5 to 100 .mu. comprising polyester, polyimide, polyamide, 
polysulfone, polyacetate, etc. In addition, those of glass, aluminum, 
surface-oxidized aluminum, etc., may also be used for the substrate 1 as 
desired. Basically, the substrate 1 may comprise any non-magnetic solid 
material providing a desired surface for forming a magnetic recording 
layer thereon. 
It has been further found that the lamination of the magnetic recording 
layer 2 and the upper layer 3 on a polymer film as a substrate 1 provides 
a magnetic recording medium having remarkably small tendency of curling. 
This may be attributable to the facts that the formation of a Co or Co 
alloy film on a film of a polymer such as polyester, polyimide or 
polyamide provides a laminated film generally curling with the metal film 
inside, whereas the formation of a Co oxide film on a polymer film 
provides a laminated film curling with the Co oxide film outside. 
While the above described embodiment of the magnetic recording medium of 
the present invention comprises a vertically magnetizable film of Co alone 
or a Co alloy as the magnetic recording layer, an Fe-based magnetic metal 
layer may also be used as a magnetic recording layer. The Fe-based 
magnetic metal provides large Bs and He, particularly a large Bs. 
In the case where an Fe-based metal layer is used as a magnetic recording 
layer 2, it is preferred that an upper layer 3 as shown in FIG. 1 also 
functions as a magnetic recording layer in order to prevent a spacing loss 
and to provide improved recording and reproduction characteristics. If the 
upper layer functions as a magnetic recording layer, the upper layer can 
be formed in a reliably large thickness so that a magnetic recording 
medium having further improved corrosion resistance and wear resistance 
can be provided. 
Hereinbelow, a case where an upper layer of a Co oxide also functions as a 
magnetic recording layer will be explained. 
A magnetic recording layer according to the present invention shown in FIG. 
2 comprises a substrate 1, and a magnetic recording layer 6 of an Fe-based 
magnetic metal and an upper layer 3 of a ferromagnetic Co oxide. As the 
magnetic recording layer 6 magnetically interacts with a magnetic head 
through the upper layer 3, the decrease in recording and reproduction 
efficiency thereof is marked on the shorter wavelength side. For this 
reason, it is important for the magnetic recording layer 6 to show good 
recording and reproduction characteristics on the longer wavelength side. 
The Fe-based magnetic metal may be Fe alone or a Fe-containing alloy such 
as Fe-Co, Fe-Ni, Fe-Mn, Fe-Cr, Fe-V, Fe-Cu, Fe-Ti, Fe-Co-Ni, Fe-Co-B, 
Fe-Co-Cr or Fe-Co-V. 
An Fe-alloy containing a smaller amount of Fe has a lower Bs and is 
disadvantageous also in respect of a material cost, the Fe-based magnetic 
metal should preferably be Fe alone or a ferromagnetic alloy containing 
25% or more of Fe. More preferably, the Fe-based metal may have a 
composition around an alloy of Fe 60 atomic %-Co 40 atomic % which 
provides the maximum Bs or, more specifically, may be an alloy comprising 
20 to 60 atomic % of Co and the remainder of Fe in view of the magnetic 
properties. It is not excluded that a further minor quantity of additive 
is contained as desired. 
The magnetic recording layer 6 may be formed as a film of about 0.05 to 2 
.mu.m in thickness on the substrate 1 by a method similar to those used 
for the formation of the magnetic recording layer 2 shown in FIG. 1 
inclusive of vacuum evaporation, ion plating, sputtering, etc. 
On the other hand, the upper layer 3 is formed as a Co oxide film on the 
magnetic recording layer 2. As described before, the upper layer 3 also 
functions as a magnetic recording layer for itself, so that the thickness 
thereof is not limited to a small value as is the case with a non-magnetic 
protective layer. A larger thickness should rather be preferred in order 
to improve the reliability with respect to corrosion resistance. Thus, a 
thickness of the order of 0.01 to 0.5 .mu.m, particularly 0.05 to 0.2 
.mu.m, is preferred. 
Similarly as explained with reference to an embodiment shown in FIG. 1, the 
upper layer 3 may be formed by vacuum evaporation or ion plating of Co in 
the presence of diluted oxygen or by sputtering of Co in an inert gas 
atmosphere containing oxygen. The magnetic properties of the upper layer 3 
comprising Co oxide depends on the oxygen partial pressure in the 
atmosphere for formation thereof. Further, the conditions for the film 
formation depend on the capacity of a film formation apparatus, a gas 
withdrawal speed, a film formation speed, etc., and may not be determined 
in a single way. Ordinarily, however, a Co oxide film having excellent 
magnetic properties may be obtained by vacuum evaporation containing 
oxygen at a partial pressure of about 10.sup.-3 to 10.sup.-2 Torr or by 
sputtering in an inert gas atmosphere containing 5 to 16% partial pressure 
of oxygen. 
The upper layer 3 is placed in a position to face a magnetic head and the 
recording and reproduction characteristics on the shorter wavelength side 
thereof are important. Thus, the upper layer 3 should preferably show a Bs 
of 2000 gauss or larger and an Hc of 300 Oe or larger, particularly an Hc 
of 500 0e or larger. 
An upper layer 3 of Co oxide showing a Bs of about 5000 gauss or below may 
be formed as a vertically magnetizable layer if the film formation is 
carried out under a condition that the evaporated particles are incident 
on the substrate 1 almost perpendicularly. The thus formed vertically 
magnetizable film shows excellent recording and reproduction 
characteristics on the shorter wavelength side and, in the present 
invention, is combined with magnetic recording layer 6 having a large Bs 
to provide a large reproduction output for a wide wavelength range, i.e., 
for a wide frequency range, as described hereinafter. 
Further, in the embodiment of the magnetic recording medium shown in FIG. 
2, a thin intermediate layer of, e.g., SiO.sub.2, or another layer may be 
interposed in a thickness of, e.g., 0.1 .mu.m or below between the 
magnetic recording layer 2 and the upper layer 3. 
The magneric recording medium according to the present invention may be 
preferably produced in a good productivity when a continuous vapor 
deposition apparatus having two vapor deposition rooms is used and the 
magnetic recording layer 2 or 6 and the upper layer 3 are produced 
successively. 
The magnetic recording medium according to the present invention can assume 
an arbitrary form inclusive of a disk, sheet, tape or card and may be 
suitably adapted to a use wherein a corrosion-resistant and wear-resistant 
protective layer is desired on the magnetic recording layer. 
As has been described hereinabove, by providing an upper layer of a Co 
oxide on a magnetic recording medium having a magnetic recording layer 
comprising a vertically magnetizable film of Co or a Co alloy or a 
Fe-based magnetic metal film, the wear resistance and the corrosion 
resistance of the magnetic recording medium are remarkably improved. 
Further, when the upper layer of Co oxide per se is made a magnetic 
recording layer, a spacing loss in avoided to provide an excellent 
magnetic recording medium for high density recording. Furthermore, if the 
upper layer of Co oxide is formed as a vertically magnetizable film, it 
increases a reproduction output on the shorter wavelength (high frequency) 
side, so that a magnetic recording medium further adapted for high density 
recording and broad range of recording is provided. 
Hereinbelow, the present invention will be explained with reference to 
experimental examples. 
EXAMPLE 1 
A 40 .mu.m-thick polyimide film was used as a substrate, a film having a 
composition of 80 wt. % Ni-20 wt. % Fe was formed thereon by sputtering in 
a thickness of 0.5 .mu.m, and a vertically magnetizable film having a 
composition of 80 wt. % Co-20 wt. % Cr was formed further thereon in a 
thickness of 0.3 .mu.m. 0n the Co-Cr film was further formed a Co oxide 
film by sputtering of Co in an Ar gas atmosphere containing 18% of oxygen. 
The Co oxide film showed no spontaneous magnetization, thus being 
nonmagnetic, as a result of measurement by means of a vibrating sample 
type magnetometer with respect to a Co oxide film formed under the same 
conditions directly on a polyimide film. 
In the manner as described above, floppy or flexible disk samples No. 1 to 
No. 5 respectively having Co oxide films of 0, 0.005, 0.01, 0.03 and 0.1 
.mu.m, respectively, in thickness. The recording and reproduction 
characteristics and the durability of the thus obtained floppy disks were 
measured by using a one-side access type vertical head. 
Table 1 shown below summarizes the results of the measurement of D.sub.50 
and durability. The D.sub.50 value was measured as a recording density (or 
frequency) at which the reproduction output reached 50% of the maximum 
output level in the reproduction output frequency characteristic curve. 
The durability of a floppy disk was evaluated in terms of a number of 
passes until 3dB or more of decrease in output or flaw occurs. 
TABLE 1 
______________________________________ 
Disk Thickness of 
Durability 
No. Co oxide film 
(passes) D.sub.50 
______________________________________ 
1 0 10,000 Unmeasurable 
2 0.005 .mu.m 120,000 90 KBPI 
3 0.01 .mu.m 750,000 70 KBPI 
4 0.03 .mu.m &gt;1,000,000 
48 KBPI 
5 0.1 .mu.m &gt;1,000,000 
31 KBPI 
______________________________________ 
As shown in the above table, the disk No. 1 having no Co oxide film caused 
wearing of the magnetic recording layer immediately after the head contact 
and became unusable after about 10,000 passes. The disks No. 4 and No. 5 
each having a sufficient thickness of Co oxide layer did not cause output 
change even after a million passes and were found to be very excellent in 
durability. The disks Nos. 2 and 3 caused output change after 120,000 
passes and 750,000 passes, respectively, and were rather inferior in 
durability to the disks Nos. 4 and 5. Neither of these disks, however, 
caused such a serious damage that the magnetic layer was scraped or 
scratched as was observed for the disk No. 1. On the other hand, as the 
thickness of the Co oxide film increased, the short wavelength recording 
capability represented by D.sub.50 was decreased. Accordingly, the 
thickness of the Co oxide layer should be determined while taking both 
recording wavelength and the durability into consideration. 
REFERENCE EXAMPLE 
Films of 80 wt. % Co-20 wt. % Cr were formed in different thicknesses 
respectively by sputtering on a 50 .mu.m-thick polyethylene terephthalate 
(PET) film. Separately, Co oxide films were formed in different 
thicknesses respectively by sputtering on the PET film. The degrees of 
curl or warp of the thus formed laminate films are plotted versus the 
thicknesses of the deposited films in FIG. 3. The curl is indicated by the 
reciprocal of curvature radius (.gamma.), while a plus (+) value indicates 
that a laminate curls with the deposited film inside and a minus (-) value 
indicates that a laminate curls with the deposited film outside. 
EXAMPLE 2 
On a 50 .mu.m-thick PET film was formed a vertically magnetizable film of 
80 wt. % Co-20 wt. % Cr in a thickness of 0.5 .mu.m, on which was further 
formed a Co oxide film by sputtering of Co in an Ar gas atmosphere 
containing 16% of oxygen. The Co oxide film was formed in thicknesses of 
0.01, 0.03, 0.05, 0.07 and 0.1 .mu.m to provide totally five floppy disk 
samples. The degrees of curl for these floppy disks are plotted versus the 
thickness of the Co oxide film in FIG. 4. As shown in FIG. 4, with respect 
to the disks of this example wherein a 0.5 .mu.-thick Co-Cr film was used, 
the laminate disk having a 0.5 .mu.-thick Co oxide film showed a least 
degree of curl and was found to be sufficiently flat for a practical use. 
A recording and reproduction experiment was conducted by using a floppy 
disk having no Co oxide film (referred to as "disk No. 6", one before 
formation of the Co oxide film as described above) and a floppy disk 
having a 0.05 .mu.m-thick Co oxide film (referred to as "disk No. 7") and 
by using a ring-type magnetic head. As a result, the floppy disk No. 7 
according to the present invention showed a durability of over one million 
passes and a uniform reproduction output containing little fluctuation in 
output within one track since the head uniformly contacted the disk. On 
the other hand, with respect to the disk No. 6 having no Co oxide film, 
portions of the Co-Cr film showing a large pressure of contact with the 
head were scraped off in short time due to influence of the curl. 
EXAMPLE 3 
Floppy disks having the structure which gave least curl in the above 
Example 2 were produced by using Ar atmospheres containing varying oxygen 
contents of 18%, 16%, 14% and 12% during the Co oxide film formation. 
Thus, each disk was prepared by forming a 0.5 .mu.m-thick Co-Cr vertically 
magnetizable film and a 0.05 .mu.m-thick Co oxide film successively on a 
50 .mu.m-thick PET film. The Co-Cr film showed a Bs of 5200 gauss, and an 
Hc of 580 Oe. The Co oxide films formed in the Ar gas atmospheres 
containing 18%, 16%, 14% and 12% showed Bs of 0, 1100, 4600 and 7200 
gauss, respectively. Judging from results obtained with respect to Co 
oxide films formed under the identical conditions directly on the PET 
film, the Co oxide film formed with 14% of oxygen was a vertically 
magnetizable film, while the other Co oxide films were nonmagnetic or 
longitudinally magnetizable films. 
These floppy disks were subjected to a recording and reproduction test by 
using a ring head. The results are shown in Table 2 below. 
TABLE 2 
__________________________________________________________________________ 
Oxygen Magnetizable 
Output at 
Disk No. 
content (%) 
Bs (Gauss) 
Hc (Oe) 
direction 
50 KBPI Durability 
__________________________________________________________________________ 
7 16 1100 250 longitudinal 
-73 dBVp - p 
&gt;1,000,000 
8 18 0 -- -- -72 dBVp - p 
&gt;1,000,000 
9 14 4600 870 vertical 
-68 dBVp - p 
&gt;1,000,000 
10 12 7200 1100 longitudinal 
-70 dBVp - p 
&gt;1,000,000 
__________________________________________________________________________ 
These disks did not show substantial difference in reproduction output at 
longer wavelength sides. At 50 KBPI, however, the disk No. 9 showed a 
highest output. The disk No. 9 gave highest outputs at further shorter 
wavelengths. With respect to durability, no output change or flaw was 
observed even after one million passes for any of the disks. Thus, when 
the Co oxide film is a vertically magnetizable film, the short wavelength 
recording characteristic which is the most important feature of the 
vertical recording system is not impaired, because the Co oxide film also 
functions as a magnetic recording layer. 
EXAMPLE 4 
Five polyimide films No. 1 to No. 5 of 12 .mu.m in thickness and different 
surface roughnesses were respectively used as a substrate. No. 1 polyimide 
film showed an average roughness Rz (average of ten measured values) of 
below measurement limit and a maximum roughness Rmax of 0.02 .mu.m; No. 2 
film, Rz of below measurement limit and Rmax of 0.18 .mu.m; No. 3 film, Rz 
of 0.015 .mu.m and Rmax of 0.039 .mu.m; No. 4 film, Rz of 0.04 .mu.m and 
Rmax of 0.096 .mu.m; and No. 5 film, Rz of 0.12 .mu.m and Rmax of 0.19 
.mu.m, all according to the measurement by Talystep mfd. by Taylor Hobson 
Co. 0n each of the polyimide films was formed a 0.42 .mu.m-thick 
vertically magnetizable film of Co 79 wt. %-Cr 21 wr. % by continuous 
vacuum evaporation with electron beam heating. Further, a 0.01 .mu.m-thick 
Co oxide film was formed on the Co-Cr film by electron beam heating of Co 
in an oxygen atmosphere of 6 milli-Torr to produce totally 5 magnetic 
recording tapes. The substrate temperature during the evaporation was 
200.degree. C.; the Co-Cr film showed a Bs of 4400 gauss and an Hc of 950 
Oe; and the Co oxide film formed was nonmagnetic, for all of the magnetic 
tapes thus obtained. 
Table 3 shows the kinetic friction coefficient, and the running property 
and the number of signal dropout when subjected to recording and 
reproduction by means of a VHS-type video tape deck. The dropout level 
D.sub.1 indicates 100 or less dropouts/min.; D.sub.2, 101 to 1000 
dropouts/min.; and D.sub.3, more than 1000 dropouts/min. 
TABLE 3 
__________________________________________________________________________ 
Kinematic 
friction 
Film No. 
Rz (.mu.m) 
Rmax (.mu.m) 
coefficient 
Running property 
Dropout 
__________________________________________________________________________ 
Tape 1 Nil 0.02 0.37 *1 D1 
according 
2 Nil 0.18 0.33 *1 D3 
to the 3 0.015 
0.039 
0.17 Good D1 
invention 
4 0.04 0.096 
0.18 Good D2 
5 0.12 0.19 0.16 Good D3 
Comparative 
3 0.015 
0.039 
0.34 *2 
Example 
__________________________________________________________________________ 
*1 Having a tendency to stick to a drum or a fixed head. 
*2 The Co--Cr film was scraped off to provide no output. 
As shown in Table 3, a magnetic tape obtained by using a very smooth film 
having a very small Rz gives a large kinematic friction coefficient and is 
liable to cause sticking to a head. On the other hand, a tape obtained by 
using a film having a larger Rz or Rmax has a good running property. Too 
large a Rz or Rmax causes many dropouts. Thus, the magnetic tapes 
according to the present invention did not cause degradation of image 
quality even in 30 minutes of still mode reproduction and were found to 
have excellent durability, whereas a magnetic tape prepared for comparison 
without providing a Co oxide film could not continue reproduction because 
the Co-Cr film was scraped off. Further, the magnetic tape obtained 
without a Co oxide film showed an extensive curl with the Co-Cr film 
inside, whereas the tapes according to the invention coated with an upper 
Co oxide film showed practically sufficient flatness. 
EXAMPLE 5 
A 0.1 .mu.m-thick metal layer of Fe alone as a magnetic recording layer was 
formed on one side of a 12 .mu.m-thick PET film as a substrate by a 
continuous vacuum evaporation apparatus with an electron beam heating 
system. In this instance, the incident angle of the Fe evaporated 
particles against the substrate film was restricted to at least 
65.degree.. 
Further, by causing evaporation of Co in an oxygen atmosphere of 
4.times.10.sup.-3 Torr in the same evaporation apparatus and with the 
incident angle restricted to the range of 0.degree.-45.degree., a 0.08 
.mu.m-thick Co oxide film layer was formed. The thus produced laminate was 
slit into a 12.7 mm width to obtain a magnetic tape (No. 6) according to 
the present invention. 
On the other hand, for the sake of comparison, two magnetic tapes were 
produced, one by forming only the Fe layer and the other by forming only 
the Co oxide layer, respectively, on the PET film followed by slitting 
into 12.7 mm. 
The thus obtained three types of magnetic tapes were subjected to 
measurement of magnetic properties and corrosion resistance. Further, the 
tape No. 6 according to the invention was further subjected to measurement 
of a kinematic friction coefficient, a still mode durability and a 
frequency characteristic. 
The magnetic properties were measured by using a vibrating sample-type 
magnetometer. The corrosion test was conducted by placing a magnetic tape 
sample under constant temperature and constant humidity conditions of 
60.degree. C. and a relative humidity of 90%, and by observing the tape 
surface through an optical microscope to measure a time in which rust has 
appeared. The still mode durability was tested by using a still mode of a 
commercially available VTR deck and evaluated in terms of a time in which 
the reproduction output has decreased to one half of the initial value. 
The results are shown in Table 4 appearing hereinafter and the frequency 
characteristic is shown in FIG. 5. 
EXAMPLE 6 
By using the same continuous evaporation apparatus as in Example 5, a 0.1 
.mu.m-thick metal film of Fe 50%-Co 50% was formed with a minimum incident 
angle of 60.degree. and then a 0.03 .mu.m-thick intermediate layer of 
SiO.sub.2 on one side of a 9 .mu.m-thick aramide film. 
Further, by causing evaporation of Co in an oxygen atmosphere of 
5.times.10.sup.-3 Torr in the same evaporation apparatus and with the 
incident angle restricted to the range of 0.degree.-45.degree., a 0.2 
.mu.m-thick Co oxide film (upper layer) was formed. The Co oxide film was 
found to be a vertically magnetizable film which has a larger residual 
magnetization in the direction vertical or perpendicular to the film than 
in the longitudinal or parallel direction. 
The thus produced laminate was slit into 12.7 mm to obtain a magnetic tape 
(No. 7) according to the invention. 
On the other hand, for the sake of comparison, two magnetic tapes were 
prepared, one by forming only the Fe-Co magnetic recording layer and the 
other by forming only the Co oxide film (upper layer), respectively, in 
the same manner as described above on a 9 .mu.m-thick aramide film 
followed by slitting into 12.7 mm. 
The thus obtained three types of magnetic tapes were subjected to 
measurement of magnetic properties and corrosion test as in Example 5. 
Further, the magnetic tape No. 7 according to the invention was similarly 
further subjected to measurement of a kinematic friction coefficient, a 
still mode durability, and a frequency characteristic. 
The results are also shown in Table 4 and FIG. 5. 
TABLE 4 
__________________________________________________________________________ 
Still 
Direction 
Corrosion 
Kinematic 
mode 
Bs Hc of easy resistance 
friction 
durability 
Material 
(Gauss) 
(Oe) 
magnetization 
(hrs.) 
coefficient 
(min.) 
__________________________________________________________________________ 
Ex. 5 
Magnetic 
Fe 11000 680 
longitudinal 
&lt;2 -- -- 
layer only 
Upper Co oxide 
7200 1050 
" &gt;1000 -- -- 
layer only 
Tape Co oxide/Fe 
9300 730 
" &gt;1000 0.28 &gt;60 
No. 6 
Ex. 6 
Magnetic 
Fe--Co 15000 700 
" &lt;5 -- -- 
layer only 
Upper Co oxide 
4600 820 
vertical 
&gt;1000 -- -- 
layer only 
Tape Co oxide/Fe 
8100 820 
longitudinal + 
&gt;1000 0.26 &gt;60 
No. 7 vertical 
Comp. Ex. 
Co--Ni 8200 930 
longitudinal 
&lt;500 0.30 25 
__________________________________________________________________________ 
As shown in Table 4, the tapes Nos. 6 and 7 according to the present 
invention showed clearly superior corrosion resistance to the tape having 
only the magnetic recording layer of Fe or Fe alloy and also showed a 
remarkable improvement over the tape of the comparative example. Further, 
the tapes Nos. 6 and 7 showed a rather smaller kinematic friction 
coefficient to be better in running property and much improvement in still 
mode durability, respectively, compared with the tape of the comparative 
example. 
Further, as shown in FIG. 5, the magnetic tapes Nos. 6 and 7 according to 
the invention showed a better reproduction output characteristic over a 
whole frequency range. Particularly, the tape No. 7 having a vertically 
magnetizable Co oxide film showed a remarkable improvement in the high 
frequency region over the tape of the comparative example.