Hard disk magnetic recording medium comprising magnetic powder and a binder and having a specified magnetic layer thickness and surface roughness

A method for manufacturing a magnetic paint by mixing a resin composition powder as a dispersing binder for a magnetic powder with the magnetic powder, or together with a filler, and grinding the mixture under a high shear stress is disclosed. A magnetic recording medium such as a magnetic disk, etc., prepared by using the magnetic paint, can have a magnetic coating film having a substantially uniform film thickness of not more than 0.9 .mu.m and a surface roughness of not more than 0.05 .mu.m Ra before surface finishing.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a magnetic recording medium, and particularly to 
a method for manufacturing a magnetic paint for forming a magnetic 
recording film on a magnetic recording medium, and a magnetic recording 
medium suitable for a high recording density magnetic disk having 
distinguished electrical characteristics and reliability, prepared by 
using the magnetic paint. 
2. Description of the Related Art 
Methods for preparing a magnetic recording medium by applying a magnetic 
paint comprising a magnetic powder dispersed in a polymer binder including 
epoxy resin, etc., have been proposed (e.g., Japanese Patent Publication 
No. 55-816), and methods for manufacturing a magnetic paint by grinding a 
magnetic powder together with epoxy resin, etc., are disclosed in Japanese 
Patent Publication No. 57-40566 and Japanese Patent Application Kokai 
(Laid-open) No. 56-100871, where the magnetic powder is ground together 
with a resin solution comprising epoxy resin, etc., dissolved in 
cyclohexanone under a high shear stress to prepare a magnetic paint, and 
then a substrate is coated with the thus obtained magnetic paint to 
prepare a magnetic recording medium. However, in the prior art, no 
consideration has been given to thorough mixing of the magnetic powder 
with epoxy resin as a binder for the dispersion of the magnetic powder 
before grinding. Thus, in the magnetic paints obtained according to the 
prior art methods, a small amount of the resin solution as added at the 
grinding is locally absorbed into a portion of the magnetic powder, and 
the grinding is carried out in the locally absorbed state. That is, it has 
been quite difficult due to the nature of grinding to obtain a magnetic 
paint of such a tactoid structure that the magnetic powder is uniformly 
dispersed throughout the mixture. Consequently, it has been impossible to 
form a thin film having a film thickness of not more than about 0.9 .mu.m 
as a magnetic recording film from the magnetic paint manufactured 
according to the prior art methods. Also, the film, even if formed, has a 
surface roughness of about 0.08 .mu.m Ra before surface finishing. 
With a keen demand for magnetic disks having higher recording density and 
consequent smaller film thickness, the surface finishing work is 
inevitably increased, resulting not only in an increased surface finishing 
time, but also in an increased occurrence of surface defects such as 
scratches on the film surface and a consequent increase in electrical 
defects. 
The magnetic paint manufactured according to the said prior art methods has 
an insufficient dispersion of magnetic powder throughout the magnetic 
paint, and it is substantially impossible to form a thin film having a 
film thickness of not more than 0.9 .mu.m from such a magnetic paint, and 
the film, even if formed, has a surface roughness of about 0.08 .mu.m Ra 
before the surface finishing. 
Furthermore, in high density recording magnetic disk drives which are now 
widely used, a thin film magnetic head is used as a magnetic head, and 
thus a magnetic disk medium having a large film thickness has another 
problem such as a failure to thoroughly write signals. Usually, a magnetic 
disk medium has a magnetic thin film so formed on a disk plate as a 
substrate as to have a smaller film thickness on the inner peripheral side 
and a larger film thickness on the outer peripheral side. Thus, the 
aforementioned problem concerning the thin film magnetic head usually 
occurs on the outer peripheral side of the magnetic disk medium. To solve 
the problem, it is necessary that the magnetic disk medium has a magnetic 
thin film having a smaller film thickness on the outer peripheral side. 
However up to now no method other than intensifying the film finishing 
work to the film surface on the outer peripheral side more than that to 
the film surface on the inner peripheral side has been taken. As a result, 
the surface finishing time and frequency of scratch occurrence, and 
consequently electrical defects are inevitably increased. The increase in 
the electrical defects may cause a maloperation in the magnetic recording 
--reproducing apparatus, and scratches developed on the film surface also 
damage the disk-facing side of the magnetic head and cause head crushes. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an improved method for 
manufacturing a magnetic paint containing a magnetic powder in a well 
dispersed state throughout the paint. 
Another object of the present invention is to provide a magnetic recording 
medium having a magnetic recording film as a thin layer, prepared from the 
magnetic paint manufactured according to the present method, where the 
thin film has such a small surface roughness before surface finishing as 
to improve the signal/noise (S/N) ratio of the magnetic recording medium 
and also has a film thickness of not more than 0.9 .mu.m on both the inner 
and outer peripheral sides. 
In order to make a magnetic recording film having a film thickness of not 
more than 0.9 .mu.m, it is necessary to use a magnetic paint diluted with 
a large amount of a solvent. Thus, the conventional magnetic paints are 
diluted with a large amount of a solvent and the magnetic powder in the 
paints are highly flocculated to form flocs of magnetic powder having 
sizes of 3 to 10 .mu.m. When a magnetic paint containing a large amount of 
flocs of magnetic powder having such particle sizes as 3 to 10 .mu.m is 
applied to a non-magnetic substrate, the large flocs of magnetic powder 
will exist spotwise on the film surface. Thereafter, even if a magnetic 
field is applied to the film to make magnetic alignment, the large flocs 
of magnetic powder will keep their shapes without crumbling. As a result, 
a large number of pin holes will be formed to such sizes that the 
nonmagnetic substrate as an underlayer can be seen through the pin holes, 
and electrical defects or errors will be increased. It is needless to say 
that the surface roughness of the resulting film will be increased and 
consequently disk noises will be increased. Even by surface finishing to 
make the film surface smooth, the disk noises will not be decreased, 
because the disk noises depend upon the surface roughness before surface 
finishing. 
The present invention provides a magnetic paint whose magnetic powder never 
floculates even if diluted with a large amount of a solvent. In the 
present magnetic paint, the magnetic powder forms loose flocs having 
smaller sizes. A magnetic powder having a BET specific surface area of not 
less than 40 m.sup.2 /g produces flocs of the magnetic powder having sizes 
of not more than 1 .mu.m, and a magnetic powder having a BET specific 
surface area of about 20 m.sup.2 /g produces flocs of the magnetic powder 
having sizes of 1 to 3 .mu.m. Thus, when such a magnetic paint as above is 
applied to a non-magnetic substrate and subjected to magnetic alignment, 
the small loose flocs of magnetic powder are very readily crumbled so that 
the magnetic powder can be uniformly dispersed in the film. As a result, 
the resulting film surface of a magnetic disk can be made smooth and the 
surface roughness of the film can be made smaller. That is, the disk noise 
will be lowered. 
Furthermore, since the magnetic powder is uniformly dispersed throughout 
the film, any pin holes, which allow the non-magnetic substrate as an 
underlayer to be seen, are not formed, resulting in less electrical 
defects. 
From a magnetic paint where a magnetic powder having a BET specific surface 
area of about 20 m.sup.2 /g is dispersed, a film having a film thickness 
of 0.4 to 0.5 .mu.m, as coated, can be formed all over the surface from 
the outer peripheral side to the inner peripheral side of a magnetic disk. 
From a magnetic paint where magnetic powder having a BET specific surface 
area of not less than 40 m.sup.2 /g, a film having a film thickness of 
0.02 to 0.9 .mu.m, as coated, can be formed all over the surface from the 
outer peripheral side to the inner peripheral side of a magnetic disk. By 
making the film thickness of the film on the magnetic disk smaller, the 
resolution of the magnetic disk can be increased and the line recording 
density D.sub.50 of the magnetic disk can be also remarkably increased, as 
will shown in Table which will be given later. 
It seems that the film thickness of the film on the magnetic disk may be 
made smaller by surface finishing, but it is now very difficult to make a 
film having a film thickness of, for example, 0.4.+-.0.04 .mu.m or 
0.2.+-.0.02 .mu.m from a film having a film thickness of more than 0.9 
.mu.m. Furthermore, more intensified surface finishing will increase the 
surface finishing time, which will give more chances for forming scratches 
leading to electrical defects. 
As a result of extensive studies, the present inventors have found that a 
magnetic paint of tactoid structure where magnetic powders are uniformly 
dispersed throughout the paint can be manufactured by using at least one 
of epoxy resin, phenol resin, etc., in a powdery form as a binder for 
dispersing a magnetic powder in place of the resin in solution which has 
so far been used, thoroughly mechanically mixing the resin powder with the 
magnetic powder in advance, then grinding the mixture under a high shear 
stress, thereby bringing the resin powders into a substantially molten 
state and obtaining a uniformly ground mixture with the magnetic powder, 
and then subjecting the magnetic paint of such a tactoid structure that 
the magnetic powder is uniformly dispersed throughout the mixture to the 
same ball milling as has so far been used. Furthermore, the present 
inventors have found that when a magnetic recording medium, for example, a 
14-inch magnetic disk, is prepared by applying the thus obtained magnetic 
paint to a non-magnetic disk plate as a substrate and subjecting the thus 
formed thin film on the substrate to a magnetic alignment, the magnetic 
recording medium can have a surface roughness of not more than 0.050 .mu.m 
Ra before surface finishing, and can also have a substantially uniform 
film thickness of not more than 0.9 .mu.m all over the medium surface from 
the inner peripheral side to the outer peripheral side.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Grinding of magnetic powder under a high shear stress is usually carried 
out in the presence of a small amount of a resin solution, but the resin 
solution is locally absorbed into a portion of the magnetic powder, and 
consequently the magnetic powder and the resin are not easily brought into 
a uniformly mixed state, whereas the solids are more easily mixed with one 
another to form a uniform mixture. In the present invention, the resin 
composition in a fine powdery state, particularly, epoxy resin, etc., 
finely pulverized to less than 1,000 .mu.m, preferably less than 200 
.mu.m, is thoroughly mixed with the magnetic powder in advance, and thus 
the magnetic powder and the resin composition can be readily mixed into a 
uniformly distributed mixture. The resin powder grains swell through 
addition of a solvent or a liquid dispersing binder thereto or melt 
through heating, and the molten resin powder grains as nucleii absorb the 
magnetic powder, resulting in a magnetic paint of complete tactoid 
structure. Thus, it seems that a magnetic thin film having a very low 
surface roughness can be formed from such a magnetic paint. 
The liquid dispersing binder includes, for example, epoxy resin, phenol 
resin, acrylic resin, etc., each having a molecular weight of not more 
than 700. A solid dispersing binder includes, for example, epoxy resin, 
phenol resin, etc., each having a molecular weight of not less than 700, 
mostly not less than 1,000. In the case of melting by heating, it is 
preferable to use a resin having a melting point of about 100.degree. C. 
As described above, powdery resin such as epoxy resin, phenol resin, etc., 
in a powdery state, is used for dispersing the magnetic powder in the 
present invention, and a thin film with a lower surface roughness can be 
obtained with the powdery resin of smaller grain size. The grain size of 
the powdery resin for use in the present invention is less than 1,000 
.mu.m, preferably less than 200 .mu.m. The smaller the grain size, the 
higher the effect. For example, a magnetic disk obtained from a magnetic 
paint containing a resin powder having grain sizes of about 200 .mu.m has 
a surface roughness of about 0.045 .mu.m Ra before surface finishing, 
whereas that obtained from a magnetic paint containing a resin powder 
having grain sizes of about 100 .mu.m has a surface roughness of about 
0.040 .mu.m Ra, as shown in FIG. 1. Thus, the magnetic disk can have a 
lower noise, as shown in FIG. 2. 
By increasing the amount of a solvent in a magnetic paint, flocculation of 
a magnetic powder proceeds, and it becomes impossible to obtain a thin 
film having a good surface roughness (flatness) by applying the magnetic 
paint. This tendency is particularly pronounced in a magnetic paint where 
magnetic powder takes a tactoid structure, for example, prior art magnetic 
paints where the magnetic powder is dispersed in an epoxy resin solution, 
as disclosed in Japanese Patent Publication No. 57-40566 and Japanese 
Patent Application Kokai (Laid-open) No. 56-100871. 
In the present magnetic paint, no flocculation of the magnetic powder 
proceeds even with increasing amount of a solvent in the magnetic paint, 
though the magnetic powder takes a tactoid structure in the paint, and 
thus it is possible to obtain a thin film having a low surface roughness, 
as mentioned above. A distinguishing feature of the present magnetic paint 
is that a thin film has a uniform thickness, for example, 0.5 .mu.m before 
surface finishing can be formed without any substantial difference in the 
film thickness from the inner peripheral side to the outer peripheral side 
all over the surface of a magnetic disk plate (8.8 inch or 14 inch in 
diameter) by high speed spin coating of the present magnetic. On the other 
hand, in case of high speed spin coating of a conventional magnetic paint, 
a thin film is formed with a film thickness gradient, that is, thinner on 
the inner peripheral side and thicker on the outer peripheral side, on a 
magnetic disk plate. A higher resolution can be produced on the magnetic 
disk thereby, and no writing problems with a magnetic thin film head 
occur. The thin film-finishing time can be considerably shortened. 
In grinding a magnetic powder with, for example, epoxy resin under a high 
shear stress in a kneader, a paint composition range obtained by grinding 
under a high shear stress is shown in FIG. 4, by way of the relationship 
between a weight ratio of a resin to a magnetic powder on the ordinate and 
a weight ratio of a solvent to all solids (magnetic powder +resin +filler, 
etc.) on the abscissa, while fixing the kinds of magnetic powders, resin. 
As can be seen from FIG. 4, the present paint composition range using 
epoxy resin in the powdery form is broader than the prior art composition 
range using epoxy resin in the solution form. That is, grinding in the 
presence of a smaller amount of a solvent, i.e. grinding under a higher 
shear stress, can be carried out, and thus the resulting magnetic disk can 
have a higher S/N ratio. Furthermore, grinding in the presence of a 
smaller amount of epoxy resin can be carried out, and thus the content of 
the magnetic powder can be increased. 
As described in detail above, a magnetic paint manufactured according to 
the present method has a tactoid structure where a magnetic powder is 
uniformly dispersed throughout the paint. When the thus manufactured 
magnetic paint is applied to, for example, an aluminum disk plate, 14 
inches in diameter, to prepare a magnetic disk, i.e., a hard disk, a 
magnetic thin film which has a uniform and very small thickness, for 
example, not more than 0.9 .mu.m all over the magnetic disk surface, that 
is, from the inner peripheral side to the outer peripheral side on the 
disk surface and which also has a surface roughness of not more than 0.050 
.mu.m Ra before surface finishing can be easily formed by a simple coating 
method such as spin coating. Thus, the surface finishing time can be 
reduced to substantially half of the time required in the prior art. This 
also leads to reduction of electrical defects due to scratches during 
surface finishing to substantially half. Furthermore, the noise of the 
thus obtained magnetic disk can be reduced by about 30%, and at the same 
time both output and resolution can be increased. Due to the small film 
thickness on the outer peripheral side of the present magnetic disk, no 
problems with a magnetic thin film head occur at all. 
Other object of the present invention is to provide a method for 
manufacturing a magnetic paint with an improved dispersion state of 
magnetic powder in the paint. By making the surface roughness of the film 
before surface finishing smaller than 0.020 .mu.m Ra, the S/N 
(signal/noise) ratio of a magnetic disk can be improved and the film 
thickness, as coated, can be made less than 0.6 .mu.m all over the surface 
from the inner peripheral side to the outer peripheral side of the 
magnetic disk. 
Further object of the present invention is to provide a magnetic recording 
- reproduction apparatus and a magnetic recording apparatus with a high 
S/N ratio and a high resolution. 
Still further object of the present invention is to provide a magnetic 
recording--reproduction apparatus with less maloperation and less 
occurrence of head crush. 
As a result of extensive studies, the present inventors have found that 
these objects can be attained by thoroughly mechanically mixing a magnetic 
powder having a BET specific surface area of not less than 40 m.sup.2 /g 
with pulverized epoxy or phenol resin having particle sizes of not more 
than 20 .mu.m as a binder for dispersing the magnetic powder in advance 
instead of the so far used solution of a solid resin such as epoxy or 
phenol resin in a block or plate shape, then adding an appropriate amount 
of a solvent or a liquid polymeric binder to the mixture, thereby swelling 
the pulverized resin or heating the mixture, thereby melting the powdery 
resin, then grinding the mixture under a high shear stress, thereby 
uniformly kneading the resin substantially in a molten state with the 
magnetic powder, and then subjecting the uniformly ground mixture to the 
same ball mill grinding as has so far been used, thereby obtaining a 
magnetic paint of such a tactoid structure that the magnetic powder is 
uniformly dispersed throughout the mixture. That is, the present inventors 
have found that, when the thus obtained magnetic paint was applied to a 
non-magnetic substrate and then subjected to magnetic alignment, the 
resulting magnetic disk, 5.25 inch in diameter, had a very small surface 
roughness such as not more than 0.020 .mu.m Ra before surface finishing 
and a substantially uniform film thickness of not more than 0.6 .mu.m. 
Thus, the objects of the present invention can be fully attained. The 
thickness of the magnetic film on the magnetic disk is measured with a 
X-ray micro film thickness meter and the measurements are calibrated in 
advance in the following manner. That is, part of the film on the magnetic 
disk is masked and the non-masked part of the film is fired with a plasma 
asher, and then a difference between the film surface and the aluminum 
substrate is optically measured. 
Furthermore, it is also possible in the present invention to form a very 
thin magnetic film having a film thickness of 0.02 .mu.m and a surface 
roughness of 0.008 .mu.m Ra before surface finishing without any visible 
defects. That is, a magnetic disk with a smooth film can be obtained 
without much surface finishing, particularly with no surface finishing in 
case of a film thickness of not more than 0.2 .mu.m, that is, only with 
vanishing (removal of projections from the film surface). 
Since the film thickness is as small as 0.6-0.02 .mu.m and the surface 
roughness is also as small as 0.02-0.008 .mu.m Ra before surface finishing 
in the present invention, surface finishing is not substantially required, 
and even if surface finishing is made, the amount of surface finishing is 
much smaller than that of the prior art and thus a magnetic 
recording-reproduction apparatus with less possibility for forming 
scratches and thus for maloperation and with the distinguished S/N ratio 
and resolution can be provided in the present invention. 
In order to grind the magnetic powder under a high shear stress, a small 
amount of a resin solution is usually added thereto before the grinding. 
However, the added resin solution is locally absorbed into a portion of 
the magnetic powder and the magnetic powder and the resin are hardly 
brought into a uniformly mixed state. On the other hand, a uniform mixture 
can be more easily obtained from solid-solid components. 
In the present invention, a resin composition of epoxy resin, etc. finely 
pulverized to sizes of less than 20 .mu.m and magnetic powder having a BET 
specific surface area of not more than 40 m.sup.2 /g are thoroughly mixed 
together in advance and thus a uniformly mixed state can be obtained 
between the magnetic powder and the resin composition. Then, a solvent is 
added thereto, whereby the resin particles are smaller and act as nuclei 
for adsorbing the magnetic powder. Thus, a magnetic paint of uniform 
tactoid structure can be obtained by grinding such a uniform mixture under 
a high shear stress. It seems that consequently a magnetic film with a 
very low surface roughness can be formed from the uniformly ground 
mixture. 
Solid thermo-setting resin such as epoxy resin, phenol resin, etc. is used, 
after pulverization, for dispersing the magnetic powder. The smaller the 
particle size of the resin powder, or the larger the specific surface area 
of the magnetic powder, the lower the surface roughness. That is, 
application of a film with a lower surface roughness is possible. The 
particle size of the resin powder is preferably not more than 20 .mu.m, 
more preferably not more than 10 .mu.m. The BET specific surface area of 
the magnetic powder is preferably not less than 40 m.sup.2 /g, more 
preferably not less than 50 m.sup.2 /g. 
By increasing the amount of the solvent in the magnetic paint, floculation 
of the magnetic powder usually proceeds in the magnetic paint, making the 
application of a film impossible and also lowering the surface roughness 
(smoothness) of the film. This tendency is particularly pronounced in a 
magnetic paint where the magnetic powder forms a tactoid structure, for 
example, a paint where the magnetic powder is dispersed in an epoxy resin 
solution, as disclosed in Japanese Patent Publication No. 57-40566 and 
Japanese Patent Application Kokai (Laid-open) no. 56-100871 as prior art. 
In the present magnetic paint, flocculation of the magnetic powder never 
proceeds even by increasing the amount of the solvent in the paint, though 
the magnetic powder forms a tactoid structure in the paint, and thus 
application of a film with a lower surface roughness as mentioned above is 
possible. The distinguishedness of the present magnetic paint is that, 
when a conventional magnetic paint is applied to a magnetic disk by spin 
coating, coating is made thinner on the inner peripheral side of the 
magnetic disk and thicker on the outer peripheral side, whereas in case of 
the present magnetic paint coating can be made with no substantial 
difference in the film thickness all over the surface from the inner 
peripheral side to the outer peripheral side of a magnetic disk, for 
example, 5.25 inch in diameter, by high speed spin coating. That is, a 
uniform film with a film thickness of, for example, 0.5 .mu.m before 
surface finishing, can be formed all over the surface from the inner 
peripheral side to the outer peripheral side of a magnetic disk, whereby 
the magnetic disk can have a higher resolution without any problem, which 
arises in writing with a thin film head, and the time of surface finishing 
can be also largely shortened. 
As already mentioned above, flocculation of magnetic powder proceeds by 
increasing the amount of the solvent in the magnetic paint, making the 
application of a film impossible and also deteriorating the surface 
roughness (smoothness). This is also true of the magnetic paints disclosed 
in Japanese Patent Application Kokai (Laid-open) No. 63-48612, where the 
individual particles of magnetic powder are dispersed at random in the 
paint to make a considerably stable magnetic paint, but the resulting film 
has a surface roughness of 0.30 .mu.m Ra and a film thickness of 0.2 .mu.m 
as limits before surface finishing and also has a large difference in the 
film thickness on the surface between the inner peripheral side and the 
outerperipheral side, and thus surface finishing is indispensable. A film 
with a surface roughness and a film thickness below the limit values has 
visually observable defects on the film surface. 
Even if a magnetic powder having a BET specific surface area of not less 
than 40 m.sup.2 /g used in the present invention is dispersed according to 
the process for preparing a magnetic paint disclosed in Japanese Patent 
Application Kokai (Laid-open) No. 63-48612, no magnetic disks with better 
characteristics than the above-mentioned are obtained. On the other hand, 
in magnetic paints where a magnetic powder is dispersed in a bundle form, 
for example, a form of several tens of bundles in the magnetic paint, that 
is, paints where the magnetic powder is dispersed in a bundle form in 
epoxy resin, as disclosed, for example, in Japanese Patent Publication No. 
57-40566 and Japanese Patent Application Kokai (Laid-open) No. 63-4422, 
flocculation of magnetic powder proceeds considerably by increasing the 
amount of the solvent in the magnetic paint. The limit to the film 
thickness of a film prepared from these magnetic paints is 0.9 .mu.m 
(Japanese Patent Publication No. 57-40566) and 0.45 .mu.m [Japanese Patent 
Application Kokai (Laid-open) No. 63-4422] and the limit to the surface 
roughness is 0.030 .mu.m Ra before surface finishing. 
In the present magnetic paint, flocculation of the magnetic powder never 
proceeds even by increasing the amount of the solvent in the paint, though 
the magnetic powder forms a tactoid structure in the paint, and thus 
application of a film with a lower surface roughness as mentioned above is 
possible. The distinguishedness of the present magnetic paint is that, 
when a conventional magnetic paint is applied to a magnetic disk by spin 
coating, coating is made thinner on the inner peripheral side of the 
magnetic disk and thicker on the outer peripheral side, whereas in case of 
the present magnetic paint coating can be made with no substantial 
difference in the film thickness on the surface between the inner and 
outer peripheral sides of a magnetic disk, for example, 5.25 inches in 
diameter, by high speed spin coating. That is, a uniform film with a film 
thickness of, for example, 0.05 .mu.m and a surface roughness of 0.01 
.mu.m Ra before surface finishing, can be formed all over the surface from 
the inner peripheral side to the outer peripheral side, whereby the 
magnetic disk can have a higher resolution and a lower noise without 
surface finishing. 
A most important problem of a disk with such an ultra-thin film is the 
reliability of the film. The present invention provides not a continuously 
deposited medium, but a coated medium and thus it is possible to add a 
reinforcing material to the film or make the film porous and impregnating 
the pores with a liquid lubricant. Various means are available for keeping 
the reliability of the film at a high level, and a particularly effective 
means is to add fine single crystal alumina particles or fine diamond 
particles having a smaller particle size than the thickness of the film 
before surface finishing as a reinforcing material. Another means is to 
make a film of double layer structure, for example, to apply a resin 
containing a small amount of a reinforcing material as dispersed in a 
polymeric binder as an underlayer and then provide a very thin magnetic 
layer of the present invention thereon as an upper layer. 
The reason why the magnetic powder undergo no flocculation even if the 
amount of the solvent is increased in the present magnetic paint is that 
thermosetting resin in a fine powdery state such as epoxy resin, phenol 
resin, etc., as uniformly dispersed among the magnetic powder, is strongly 
chemically adsorbed onto the surface of the magnetic powder through the 
grinding under a high shear stress. In case of epoxy resin, the terminal 
epoxy groups undergo ring opening through the grinding under a high shear 
stress and combine with the OH groups that exist on the surface of 
magnetic powder. These reactions are not expectable in case of 
thermoplastic resin used for dispersion of magnetic powder in a magnetic 
tape, such as polyvinyl chloride, vinyl chloride copolymer, 
nitrocellulose, etc. 
A kneader-grinder for use in the practice of the present invention has, for 
example, a grinding capacity of 1 l with a power of 2.2 kW and 4 Hp, where 
800 g of magnetic powder is charged and ground with rotating blades at 30 
rpm. In that case, the grinding is carried out under a shear stress of at 
least 105 dynes/cm.sup.2. In the present invention, it is necessary to 
conduct the grinding under a high shear stress of 10.sup.5 dynes/cm.sup.2 
or more. 
The present invention will be described in detail below, referring to 
Examples. 
EXAMPLE 1 
Epoxy resin in a plate form was pulverized in a mixer to obtain epoxy resin 
powder having grain sizes of about 200 .mu.m. 25 parts by weight of the 
thus obtained epoxy resin powder, 100 parts by weight of a magnetic powder 
and 10 parts by weight of single crystal alumina were thoroughly mixed, 
then admixed with 10 parts by weight of cyclohexanone, then further mixed 
in a kneader, and then admixed with 5 parts by weight of cyclohexanone. 
The mixture was then ground under a high shear stress for about 4 hours. 
The ground mixture was placed in a ball mill pot, and 200 parts by weight 
of a solvent mixture of cyclohexanone and isophorone was added thereto. 
The mixture was subjected to ball milling for 5 days to disperse the 
magnetic powder throughout the mixture. Then, a solution containing 25 
parts by weight of phenol resin and 6 parts by weight of vinyl resin in 
280 parts by weight of a solvent mixture of cyclohexanone, isophorone and 
dioxane was added to the ball-milled mixture to prepare a magnetic paint 
for magnetic disks. Then, an aluminum disk plate, 14 inches in diameter, 
whose surfaces were cleaned in advance, was spin coated with the magnetic 
paint at 1,000 rpm and then subjected to magnetic alignment according to a 
known procedure. The thus coated magnetic disk was cured at 210.degree. C. 
and its film thickness and surface roughness were measured. 
The thus obtained disk had a film thickness of 0.8 .mu.m at R 105 mm (an 
abbreviation for designating such a disk radius position, that is, a 
distance of 105 mm from the center of the disk--similar abbreviations will 
be hereinafter used for similar desigrations) and 0.9 .mu.m at R 170 mm 
before surface finishing and a surface roughness of 0.045 .mu.m Ra before 
surface finishing, whereby the disk noise could be reduced by 20%, as 
compared with that of the conventional magnetic disk having a surface 
roughness of 0.08 .mu.m Ra. 
EXAMPLE 2 
Epoxy resin powder having grain sizes of about 200 .mu.m was prepared, and 
a magnetic paint and a magnetic disk were prepared in the same mixing 
ratio and preparatory manner as in Example 1, except that the thus 
prepared epoxy resin powder was used in place of the epoxy resin powder of 
Example 1. The thus obtained magnetic disk had a film thickness of 0.8 
.mu.m at R 105 mm and 0.9 .mu.m at R 170 mm before the surface finishing 
and a surface roughness of 0.040 .mu.m Ra before the surface finishing, 
whereby the disk noise could be reduced by 25%, as compared with that of 
the conventional magnetic disk. 
EXAMPLE 3 
An aluminum disk plate, 14 inches in diameter, whose surfaces were cleaned 
in advance, was spin coated with the magnetic paint prepared in Example 3 
at 1,500 rpm, subjected to magnetic alignment according to a known 
procedure, and cured at 210.degree. C. Then, the film thickness and the 
surface roughness of the thus obtained magnetic disk were measured. The 
thus obtained magnetic disk had a film thickness of 0.6 .mu.m at R 105 mm 
and 0.6 .mu.m at R 170 mm before surface finishing, as shown in FIG. 3, 
and a surface roughness of 0.040 .mu.m Ra before surface finishing, and no 
visual defects were observed on the film surfaces. 
EXAMPLE 4 
100 parts by weight of magnetic powder, 10 parts by weight of single 
crystal alumina and 15 parts by weight of epoxy resin powder having grain 
sizes of about 100 .mu.m were thoroughly mixed, and then admixed with 20 
parts by weight of cyclohexanone. Then, the thus obtained mixture was 
further mixed in a kneader, then admixed with 5 parts by weight of 
cyclohexanone, and ground under a high shear stress for about 4 hours. 
The ground mixture was placed in a ball mill pot, admixed with 180 parts by 
weight of a solvent mixture of cyclohexanone and isophorone and subjected 
to ball milling for 5 days to disperse the magnetic powder throughout the 
mixture. Then, a solution containing 25 parts by weight of phenol resin 
and 6 parts by weight of vinyl resin in 350 parts by weight of a solvent 
mixture of cyclohexanone, isophorone and dioxane was added to the 
ball-milled mixture to prepare a magnetic paint for magnetic disks. Then, 
an aluminum disk plate, 14 inches in diameter, whose surfaces were cleaned 
in advance, was spin coated with the magnetic paint at 1,500 rpm, 
subjected to magnetic alignment according to a known procedure, and cured 
at 210.degree. C. Then, the film thickness and the surface roughness of 
the magnetic disk were measured. The magnetic disk had a thickness of 0.45 
.mu.m at R 105 mm and 0.45 .mu.m at R 170 mm before the surface finishing 
and a surface roughness of 0.045 .mu.m Ra before the surface finishing. No 
visual defects were observed on the film surface. 
EXAMPLE 5 
100 parts by weight of magnetic powder and 12 parts by weight of epoxy 
resin powder having grain sizes of 100 .mu.m were thoroughly mixed, then 
admixed with 15 parts by weight of cyclohexanone, and further mixed in a 
kneader. Then, the mixture was admixed with 5 parts by weight of 
cyclohexanone and ground under a high shear stress for about 4 hours. 
Then, the ground mixture was placed in a ball mill pot, admixed with 165 
parts by weight of a solvent mixture of cyclohexanone and isophorone, and 
subjected to ball milling for 5 days to disperse the magnetic powder 
throughout the mixture. Then, a solution containing 12 parts by weight of 
phenol resin and 4 parts by weight of vinyl resin in 200 parts by weight 
of a solvent mixture of cyclohexanone, isophorone and dioxane was added to 
the ball-milled mixture to prepare a magnetic paint for magnetic disks. 
Then, an aluminum disk plate, 14 inches in diameter, whose surfaces were 
cleaned in advance, was spin coated with the magnetic paint at 1,500 rpm, 
subjected to magnetic alignment according to a known procedure, and cured 
at 210.degree. C. The thus obtained magnetic disk had a film thickness of 
0.75 .mu.m at both R 105 mm and R 170 mm and a surface roughness of 0.05 
.mu.m Ra before the surface finishing. 
EXAMPLE 6 
Epoxy resin powder having grain sizes of about 20 .mu.m was prepared. 25 
parts by weight of the epoxy resin powder, 100 parts by weight of magnetic 
powder and 10 parts by weight of single crystal alumina were thoroughly 
mixed, then admixed with 6 parts by weight of cyclohexanone, and further 
mixed in a kneader. Then, the mixture was admixed with 3 parts by weight 
of cyclohexanone, and ground under a high shear stress for about 4 hours. 
Then, the ground mixture was placed in a ball mill pot, admixed with 200 
parts by weight of a solvent mixture of cyclohexanone and isophorone, and 
subjected to ball milling for 5 days to disperse the magnetic powder 
throughout the mixture. Then, a solution containing 25 parts by weight of 
phenol resin and 6 parts by weight of vinyl resin in 280 parts by weight 
of a solvent mixture of cyclohexanone, isophorone and dioxane was added to 
the ball-milled mixture to prepare a magnetic paint for magnetic disks. 
Then, an aluminum disk plate, 14 inches in diameter, whose surfaces were 
cleaned in advance, was spin coated with the magnetic paint at 1,000 rpm, 
subjected to magnetic alignment according to a known procedure, and cured 
at 210.degree. C. 
The film thickness and the surface roughness of the thus obtained magnetic 
disk were determined. The magnetic disk had a film thickness of 0.8 .mu.m 
at R 105 mm and 0.9 mm at R 170 mm before surface finishing, and a surface 
roughness of 0.030 .mu.m Ra before surface finishing, whereby the disk 
noise could be reduced by 30%, as compared with that of the conventional 
disk. 
COMATIVE EXAMPLE 1 
100 parts by weight of magnetic powder and 10 parts by weight of single 
crystal alumina were mixed in a kneader, then admixed with a solution 
containing 14 parts by weight of epoxy resin in 21 parts by weight of 
cyclohexanone, and further mixed. Furthermore, a solution containing 6 
parts by weight of epoxy resin in 9 parts by weight of cyclohexanone was 
added to the mixture, and then the mixture was ground under a high shear 
stress for about 4 hours. 
The ground mixture was placed in a ball mill pot, and 5 parts by weight of 
epoxy resin and 180 parts by weight of a solvent mixture of cyclohexanone 
and isophorone were added thereto, and then the mixture was subjected to 
ball milling for 5 days to disperse the magnetic powder throughout the 
mixture. Then, a solution containing 25 parts by weight of phenol resin 
and 6 parts by weight of vinyl resin in 290 parts by weight of a solvent 
mixture of cyclohexanone, isophorone and dioxane was added thereto to 
prepare a magnetic paint for magnetic disks. 
Then, an aluminum disk plate, 14 inches in diameter, whose surfaces were 
cleaned in advance, was spin coated with the magnetic paint at 1,000 rpm, 
subjected to magnetic alignment according to a known procedure and cured 
at 210.degree. C. The film thickness and the surface roughness of the 
magnetic disk were measured. The magnetic disk had a film thickness of 0.9 
.mu.m at R 105 mm and 1.2 .mu.m at R 170 mm before surface finishing, as 
shown in FIG. 3, and a surface roughness of 0.08 .mu.m Ra before surface 
finishing. 
COMATIVE EXAMPLE 2 
100 parts by weight of magnetic powder and 10 parts by weight of single 
crystal alumina were placed into a kneader, and mixed. Then, a solution 
containing 12 parts by weight of epoxy resin in 18 parts by weight of 
cyclohexanone was added thereto, and the mixture was thoroughly mixed. 
Furthermore, a solution containing 3 parts by weight of epoxy resin in 4.5 
parts by weight of cyclohexanone was added thereto, and the mixture was 
ground under a high shear stress for about 4 hours. 
Then, a magnetic paint and a magnetic disk were prepared in the same mixing 
ratio and in the same manner as in Example 4. The thus obtained magnetic 
disk had a film thickness of 0.5 .mu.m at R 105 mm and 0.8 .mu.m at R 170 
mm before surface finishing and a surface roughness of 0.28 .mu.m Ra 
before surface finishing. Flocs of the magnetic powder were observable all 
over the film surfaces of the magnetic disk. 
COMATIVE EXAMPLE 3 
Epoxy resin was pulverized to coarse powder having grain sizes of about 3 
mm, and a magnetic paint was manufactured, using the thus obtained epoxy 
resin powder in the same mixing ratio and the same manner as in Example 4. 
Then, an aluminum disk plate, 14 inches in diameter, whose surfaces were 
cleaned in advance, was spin coated with the magnetic paint at 1,000 rpm, 
subjected to magnetic alignment according to a known procedure, and cured 
at 210.degree. C. The thus obtained magnetic disk had a film thickness of 
0.6 .mu.m at R 105 mm and 0.8 .mu.m at R 170 mm before surface finishing, 
and a surface roughness of 0.12 .mu.m Ra before surface finishing. 
In the foregoing embodiments according to the present invention, epoxy 
resin, phenol resin and vinyl resin were used as a polymer binder, but 
other ordinary organic polymer compounds having a good bonding to the 
magnetic powder, such as vinyl-based resins, for example, vinyl 
chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-vinyl 
alcohol copolymer, acrylonitrite-acrylic acid-2-hydroxyethylmethacrylate 
copolymer, etc.; rubber-based resins, for example, acrylonitrile-butadiene 
copolymer, etc.; cellulosic resins, for example, nitrocellulose, 
acetylcellulose, etc.; epoxy resins, for example, phenoxy resin, etc.; 
urethane-based resins, for example, urethane, urethane prepolymer, etc. 
can be also used. 
The vinyl resin for use in the present invention as a polymer binder 
includes, for example, polyvinylbutyral, polyvinylformal, polyvinyl 
acetate, etc., among which polyvinylbutyral is particularly preferable. 
EXAMPLE 7 
Epoxy resin in a plate form was pulverized in a mixer to obtain epoxy resin 
powder having grain sizes of about 200 .mu.m. 25 parts by weight of the 
thus obtained epoxy resin powder, 100 parts by weight of magnetic powder 
and 10 parts by weight of single crystal alumina were thoroughly mixed, 
and then admixed with 10 parts by weight of phenol resin in a liquid 
state, then further mixed in a kneader, and then admixed with 15 parts by 
weight of phenol resin in a liquid state, and the mixture was then ground 
under a high shear stress for about 4 hours. 
The ground mixture was placed in a ball mill pot, admixed with 160 parts by 
weight of a solvent mixture of cyclohexanone and isophorone and subjected 
to ball milling for 5 days to disperse the magnetic powder throughout the 
mixture. Then, a solution containing 6 parts by weight of vinyl resin in 
340 parts by weight of a solvent mixture of cyclohexanone, isophorone and 
dioxane was added thereto to prepare a magnetic paint for magnetic disks. 
Then, an aluminum disk plate, 8.8 inches in diameter, whose surfaces were 
cleaned in advance, was spin coated with the magnetic paint at 1,300 rpm, 
subjected to magnetic alignment according to a known procedure, and cured 
at 210.degree. C. Then, the film thickness of the magnetic disk was 
measured. The magnetic disk had a film thickness of 0.5 .mu.m at R 65 mm 
and 0.5 .mu.m at R 105 mm before the surface finishing. Then, the magnetic 
disk was lapped to a film thickness of 0.4 .mu.m and a surface roughness 
of 0.013 .mu.m Ra. The thus obtained magnetic disk was coated with a 
lubricant according to a known procedure, and the electrical 
characteristics of the magnetic disk were measured. It was found that the 
S/N ratio was 3.4. 
EXAMPLE 8 
25 parts by weight of phenol resin powder having grain sizes of about 1,000 
.mu.m, 100 parts by weight of magnetic powder and 10 parts by weight of 
single crystal alumina were thoroughly mixed, then admixed with 10 parts 
by weight of epoxy resin in a liquid state, and further mixed in a 
kneader. Then, the mixture was admixed with 15 parts by weight of epoxy 
resin in a liquid state and ground under a high shear stress for about 4 
hours. 
The ground mixture was placed in a ball mill pot, admixed with 180 parts by 
weight of a solvent mixture of cyclohexanone, isophorone and dioxane, and 
subjected to ball milling for 5 days to disperse the magnetic powder 
throughout the mixture. Then, a solution containing 6 parts by weight of 
vinyl resin in 280 parts by weight of a solvent mixture of cyclohexanone, 
isophorone and dioxane was added thereto to prepare a magnetic paint for 
magnetic disks. 
Then, an aluminum disk plate, 8.8 inches in diameter, whose surfaces were 
cleaned in advance, was spin coated with the magnetic paint at 1,000 rpm, 
subjected to magnetic alignment according to a known procedure, and cured 
at 210.degree. C. The film thickness of the magnetic disk was measured. 
The magnetic disk had a film thickness of 0.7 .mu.m at R 65 mm and 0.8 
.mu.m at R 105 mm before surface finishing. The magnetic disk was lapped 
to a film thickness of 0.45 .mu.m and a surface roughness of 0.012 .mu.m 
Ra. After the thus obtained magnetic disk was coated with a lubricant 
according to a known procedure, the electrical characteristics of the 
magnetic disk were measured. It was found that the S/N ratio was 3.3. 
COMATIVE EXAMPLE 4 
100 parts by weight of magnetic powder and 10 parts by weight of single 
crystal alumina were placed in a kneader, mixed, then admixed with a 
solution containing 14 parts by weight of epoxy resin in 21 parts by 
weight of cyclohexanone, and further mixed. Furthermore, a solution 
containing 6 parts by weight of epoxy resin in 9 parts by weight of 
cyclohexanone was added thereto, and the mixture was ground under a high 
shear stress for about 4 hours. 
The ground mixture was placed in a ball mill pot, admixed with 5 parts by 
weight of epoxy resin and 180 parts by weight of a solvent mixture of 
cyclohexanone and isophorone and subjected to ball milling for 5 days to 
disperse the magnetic powder throughout the mixture. Then, a solution 
containing 25 parts by weight of phenol resin and 6 parts by weight of 
vinyl resin in 290 parts by weight of a solvent mixture of cyclohexanone, 
isophorone and dioxane was added thereto to prepare a magnetic paint for 
magnetic disks. 
Then, an aluminum disk plate, 8.8 inches in diameter, whose surfaces were 
cleaned in advance, was spin coated with the magnetic paint at 1,000 rpm, 
subjected to magnetic alignment according to a known procedure and cured 
at 210.degree. C., and the film thickness of the thus obtained magnetic 
disk was measured. The magnetic disk had a film thickness of 0.9 .mu.m at 
R 65 mm and 1.2 .mu.m at R 105 mm before the surface finishing. The 
magnetic disk was lapped to a film thickness of 0.45 .mu.m and a surface 
roughness of 0.013 .mu.m Ra. After the magnetic disk was coated with a 
lubricant, the electrical characteristics of the magnetic disk were 
measured. It was found that the S/N ratio was 2.6. 
EXAMPLE 9 
Epoxy resin in a plate form was pulverized in a mixer to obtain epoxy resin 
powder having grain sizes of about 1,000 .mu.m. 35 parts by weight of the 
epoxy resin powder and 100 parts by weight of magnetic powder were placed 
into a kneader, and thoroughly mixed. Then, the epoxy resin was melted by 
heating the kneader, and the mixture was ground under a high shear stress 
for about 4 hours. 
The ground mixture was placed into a ball mill pot, admixed with 200 parts 
by weight of a solvent mixture of cyclohexanone and isophorone, and 
subjected to ball milling for 5 days to disperse the magnetic powder 
throughout the mixture. Then, a solution containing 35 parts by weight of 
phenol resin and 10 parts by weight of vinyl resin in 340 parts by weight 
of a solvent mixture of cyclohexanone, isophorone and dioxane was added 
thereto to prepare a magnetic paint for magnetic disks. 
Then, an aluminum disk plate, 8.8 inches in diameter, whose surfaces were 
cleaned in advance, was spin coated with the magnetic paint at 1,000 rpm, 
subjected to magnetic alignment according to a known procedure, and cured 
at 210.degree. C., and the film thickness and the surface roughness of the 
thus obtained magnetic disk were measured. The magnetic disk had a film 
thickness of 0.5 .mu.m at both R 65 mm and R 105 mm before surface 
finishing, and a surface roughness of 0.045 .mu.m Ra before surface 
finishing, whereby the disk noise could be reduced by 20%, as compared 
with that of the conventional disk (surface roughness: 0.075 .mu.m Ra 
before surface finishing). 
COMATIVE EXAMPLE 5 
100 parts by weight of magnetic powder was placed into a kneader, admixed 
with a solution containing 14 parts by weight of epoxy resin in 21 parts 
by weight of cyclohexanone, and thoroughly mixed. Then, a solution 
containing 6 parts by weight of epoxy resin in 9 parts by weight of 
cyclohexanone was added thereto, and the mixture was ground under a high 
shear stress for about 4 hours. 
The ground mixture was placed into a ball mill pot, admixed with 15 parts 
by weight of epoxy resin and 160 parts by weight of a solvent mixture of 
cyclohexanone and isophorone, and subjected to ball milling for 5 days to 
disperse the magnetic powder throughout the mixture. Then, a solution 
containing 35 parts by weight of phenol resin and 10 parts by weight of 
vinyl resin in 310 parts by weight of a solvent mixture was added thereto 
to prepare a magnetic paint for magnetic disks. 
Then, an aluminum disk plate, 8.8 inches in diameter, whose surfaces were 
cleaned in advance, was spin coated with the magnetic paint at 1,000 rpm, 
subjected to magnetic alignment according to a known method, and cured at 
210.degree. C. Then, the film thickness and the surface roughness of the 
thus obtained magnetic disk were measured. The magnetic disk had a 
thickness of 0.9 .mu.m at R 65 mm and 1.2 .mu.m at R 105 mm before surface 
finishing, and a surface roughness of 0.075 .mu.m Ra before surface 
finishing. 
EXAMPLE 10 
Epoxy resin powder having grain sizes of about 10 .mu.m was prepared by jet 
milling. 
25 parts by weight of the thus obtained epoxy resin powder, 100 parts by 
weight of a magnetic powder and 10 parts by weight of single crystal 
alumina were thoroughly mixed, then admixed with 10 parts by weight of 
cyclohexanone, then further mixed in a kneader, and then admixed with 5 
parts by weight of cyclohexanone. The mixture was then ground under a high 
shear stress for about 4 hours. 
The ground mixture was placed in a ball mill pot, and 200 parts by weight 
of a solvent mixture of cyclohexanone and isophorone was added thereto. 
The mixture was subjected to ball milling for 5 days to disperse the 
magnetic powder throughout the mixture. Then, a solution containing 25 
parts by weight of phenol resin and 6 parts by weight of vinyl resin in 
350 parts by weight of a solvent mixture of cyclohexanone, isophorone and 
dioxane was added to the ball-milled mixture to prepare a magnetic paint 
for magnetic disks. The magnetic paint was preserved for 10 days with 
stirring. Then, an aluminum disk plate, 8.8 inches in diameter, whose 
surfaces were cleaned in advance, was spin coated with the magnetic paint 
at 1,500 rpm and then subjected to magnetic alignment according to a known 
procedure. The thus coated magnetic disk was cured at 210.degree. C. and 
its film thickness and surface roughness were measured. 
The thus obtained disk had a film thickness of 0.38 .mu.m at R 65 mm and 
0.40 .mu.m at R 105 mm before surface finishing and a surface roughness of 
0.030 .mu.m Ra before surface finishing, and no visual defects were 
observed on the film surfaces. In this magnetic paint, no reflocculation 
of the magnetic powder occurred even when the magnetic paint was preserved 
for a long time. Thus it was possible to obtain a thin film having a low 
surface roughness, as mentioned above. 
EXAMPLE 11 
Epoxy resin powder having grain sizes of about 3 .mu.m was prepared by jet 
milling, and a magnetic paint and a magnetic disk were prepared in the 
same mixing ratio and preparatory manner as in Example 10, except that the 
thus prepared epoxy resin powder was used in place of the epoxy resin 
powder of Example 10. The thus obtained magnetic disk had a film thickness 
of 0.38 .mu.m at R 65 mm and 0.38 .mu.m at R 105 mm before surface 
finishing and a surface roughness of 0.027 .mu.m Ra before surface 
finishing, and no visible defects were observed on the film surfaces. 
As described Examples 10 and 11, the powdery resin having particle sizes of 
not more than 10 .mu.m is particularly effective in the present invention. 
For formation of a thin film coating disk, a magnetic paint containing a 
large amount of the solvent is usually used. In such a magnetic paint, 
flocculation of the magnetic powder occurs easily. Thus, it is impossible 
to obtain a thin film having a low surface roughness and preserve a 
magnetic paint for a long time. 
As shown in the above examples, the present inventors have found that there 
is no trouble in the thin film coating by using fine powdery resins having 
particle sizes of not more than 10 .mu.m. 
COMATIVE EXAMPLE 6 
100 parts by weight of magnetic powder, 100 parts by weight of single 
crystal alumina and 15 parts by weight of epoxy resin powder having grain 
sizes of about 100 .mu.m were thoroughly mixed, and then admixed with 20 
parts by weight of cyclohexanone. Then, the thus obtained mixture was 
further mixed in a kneader, then admixed with 5 parts by weight of 
cyclohexanone, and ground under a high shear stress for about 4 hours. 
The ground mixture was placed in a ball mill pot, admixed with 180 parts by 
weight of a solvent mixture of cyclohexanone and isophorone and subjected 
to ball milling for 5 days to disperse the magnetic powder throughout the 
mixture. Then, a solution containing 25 parts by weight of phenol resin 
and 6 parts by weight of vinyl resin in 350 parts by weight of a solvent 
mixture of cyclohexanone, isophorone and dioxane was added to the 
ball-milled mixture to prepare a magnetic paint for magnetic disks. The 
magnetic paint could be preserved for 10 days with stirring. Then, an 
aluminum disk plate, 8.8 inches in diameter, whose surfaces were cleaned 
in advance, was spin coated with the magnetic paint at 1,500 rpm and then 
subjected to magnetic alignment according to a known procedure. The thus 
coated magnetic disk was cured at 210.degree. C and its film thickness and 
surface roughness were measured. 
The thus obtained disk had a film thickness of 0.42 .mu.m at R 65 mm and 
0.43 .mu.m at R 105 mm before surface finishing and a surface roughness of 
0.120 .mu.m Ra before surface finishing. Flocs of the magnetic powder were 
observable on the film surfaces of the magnetic disk. 
The magnetic power used in the foregoing Examples and Comparative Examples 
had a BET surface area of 30 m.sup.2 /g or less, whereas the following 
Examples show cases of using a magnetic powder having a BET surface area 
of 40 m.sup.2 /g or more. 
EXAMPLE 12 
Epoxy resin in a plate form was pulverized in a jet mill to obtain an epoxy 
resin powder having a particle size of about 8.0 .mu.m. Then, 25 parts by 
weight of the thus obtained powdery epoxy resin, 100 parts by weight of a 
magnetic powder, Co-coated iron oxide powder (Hc 770 Oe) with a BET 
surface area of 40 m.sup.2 /g and 5 parts by weight of single crystal 
alumina were thoroughly mixed, and then admixed with 10 parts by weight of 
cyclohexanone. The mixture was further mixed in a kneader-grinder. Then, 
the mixture was admixed with 5 parts by weight of cyclohexanone and ground 
under a high shear stress for about 4 hours. 
Then, the ground mixture was placed in a ball mill pot, admixed with 140 
parts by weight of a solvent mixture of cyclohexanone and isophorone and 
subjected to ball mill grinding for 3 days to disperse the magnetic powder 
throughout the mixture. Then, the ball milled mixture was admixed with a 
solution of 25 parts by weight of phenol resin and 6 parts by weight of 
vinyl resin in 490 parts by weight of a solvent mixture of cyclohexanone, 
isophorone and dioxane to prepare a magnetic paint for a magnetic disk. 
Then, the thus prepared paint was applied to an aluminum substrate disk, 
5.25 inches in diameter, whose surface was cleaned in advance, by spin 
coating and subjected to magnetic alignment according to a known 
procedure. The thus obtained magnetic disk was cured at 210.degree. C. and 
then its film thickness and surface roughness were measured. 
The thus obtained disk had a film thickness of 0.50 .mu.m at R38 mm and 
also at R60 mm before surface finishing and a surface roughness of 0.020 
.mu.m Ra before surface finishing. Then, the disk was subjected to surface 
finishing according to a known procedure to make the film thickness 0.30 
.mu.m both at R38 mm and R60 mm after surface finishing. 
Then, the disk was subjected to electrical measurement with a metal-in-gap 
type head having a gap length of 0.3 .mu.m under measurement conditions 
such as a maximum recording frequency of 26.25.KFCI and a minimum 
recording frequency of 6.56 KFCI, whereby reproduction output and noise 
were measured. From these measurements, a ratio of the signal at the 
maximum recording frequency to the noise, i.e. S/N and the signal at the 
maximum recording frequence, divided by the signal at the minimum 
recording frequency and represented in percentage, i.e. resolution, were 
obtained and shown in FIG. 5, where the ordinate shows the S/N and the 
abscissa show the resolution. Same measurement was carried out in the 
following Examples and Comparative Examples and the results are also shown 
in FIG. 6. 
EXAMPLE 13 
100 parts by weight of magnetic powder, co-coated iron oxide powder (Hc 770 
Oe) with a BET specific surface area of 45 m.sup.2 /g, 5 parts by weight 
of single crystal alumina and 25 parts by weight of epoxy resin powder 
having particle sizes of about 3.0 .mu.m were thoroughly mixed and then 
admixed with 10 parts by weight of cyclohexanone. The mixture was further 
mixed in a kneader-grinder and then admixed with 5 parts by weight of 
cyclohexanone and ground under a high shear stress for about 4 hours. 
Then, the ground mixture was placed in a ball mill pot, admixed with 140 
parts by weight of a solvent mixture of cyclohexanone and isophorone and 
subjected to ball mill grinding for 3 days to disperse the magnetic powder 
throughout the mixture. Then, the ball milled mixture was admixed with a 
solution of 25 parts by weight of phenol resin and 6 parts by weight of 
vinyl resin in 550 parts by weight of a solvent mixture of cyclohexanone, 
isophorone and dioxane to prepare a magnetic paint for a magnetic disk. 
Then, the thus prepared paint was applied to an aluminum substrate disk, 
5.25 inches in diameter, whose surface was cleaned in advance, by spin 
coating and subjected to magnetic alignment according to a known 
procedure. After curing, the film thickness and the surface roughness were 
measured. The disk had a film thickness of 0.40 .mu.m both at R38 mm and 
R60 mm before surface finishing and a surface roughness of 0.018 .mu.m Ra 
before surface finishing. No visible defects were found on the film 
surface. Then, the disk was subjected to surface finishing according to a 
known procedure to make the film thickness 0.22 .mu.m both at R38 mm and 
R60 mm after surface finishing. 
EXAMPLE 14 
100 parts by weight of magnetic powder, Co-coated iron oxide powder (Hc 770 
Oe) with a BET surface area of 50 m.sup.2 /g and 25 parts by weight of 
epoxy resin powder having a particle size of about 2.0 .mu.m were 
thoroughly mixed and then admixed with 10 parts by weight of cyclohexanone 
and further mixed in a kneader-grinder. Then, the mixture was admixed with 
5 parts by weight of cyclohexanone and ground under a high shear stress 
for about 4 hours. 
Then, the ground mixture was placed in a ball mill pot and admixed with 165 
parts by weight of a solvent mixture of cyclohexanone and isophorone and 
subjected to ball mill grinding for 3 days to disperse the magnetic powder 
throughout the mixture. Then, the ball milled mixture was admixed with a 
solution of 25 parts by weight of phenol resin and 6 parts by weight of 
vinyl resin in 600 parts by weight of a solvent mixture of cyclohexanone, 
isophorone and dioxane to prepare a magnetic paint for a magnetic disk. 
Then, the thus prepared paint was applied to an aluminum substrate disk, 
5.25 inches in diameter, whose surface was cleaned in advance, by spin 
coating, and subjected to magnetic alignment according to a known 
procedure. After curing the film thickness and the surface roughness were 
measured. The disk had a film thickness of 0.30 .mu.m both at R38 mm and 
R60 mm before surface finishing and a surface roughness of 0.015 .mu.m Ra 
before surface finishing. Then, the disk was subjected to surface 
finishing according to a known procedure to make the film thickness 0.20 
.mu.m both at R38 mm and R60 mm after surface finishing. 
COMATIVE EXAMPLE 7 
100 parts by weight of magnetic powder, iron oxide powder (Hc 350 Oe) with 
a BET specific surface area of 30 m.sup.2 /g and 5 parts by weight of 
single crystal alumina were charged into a kneader-grinder. Then, the 
mixture was admixed with a solution of 14 parts by weight of epoxy resin 
in 21 parts by weight of cyclohexanone and subjected to further mixing. 
Then, the mixture was admixed with a solution of 6 parts by weight of 
epoxy resin in 9 parts by weight of cyclohexanone and ground under a high 
shear stress for about 4 hours. 
The ground mixture was placed in a ball mill pot, admixed with 5 parts by 
weight of epoxy resin and 180 parts by weight of a solvent mixture of 
cyclohexanone and isophrone and subjected to ball mill grinding for 5 days 
to disperse the magnetic powder throughout the mixture. Then, the ball 
milled mixture was admixed with a solution of 25 parts by weight of phenol 
resin and 6 parts by weight of vinyl resin in 350 parts by weight of a 
solvent mixture of cyclohexanone, isophorone and dioxane to prepare a 
magnetic paint for a magnetic disk. 
Then, the thus prepared paint was applied to an aluminum substrate disk, 
5.25 inches in diameter, whose surface was cleaned in advance, by spin 
coating and subjected to magnetic alignment according to a known 
procedure. After curing, the film thickness and surface roughness of the 
disk were measured. The thus obtained disk had a film thickness of 0.9 
.mu.m at R38 mm and 1.0 .mu.m at R60 mm before surface finishing and a 
surface roughness of 0.08 .mu.m Ra before surface finishing. Then, the 
disk was subjected to surface finishing according to a known procedure to 
make the film thickness 0.3 .mu.m at R38 mm and 0.35 .mu.m at R60 mm after 
surface finishing. 
COMATIVE EXAMPLE 8 
100 parts by weight of magnetic powder, Co-coated iron oxide powder (Hc 770 
Oe) with a BET specific surface area of 40 m.sup.2 /g and 5 parts by 
weight of single crystal alumina were charged into a kneader-grinder and 
mixed. Then, the mixture was admixed with a solution of 14 parts by weight 
of epoxy resin in 21 parts by weight of cyclohexanone and subjected to 
further mixing. Then, the mixture was admixed with a solution of 6 parts 
by weight of epoxy resin in 9 parts by weight of cyclohexanone and 
subjected to grinding under a high shear stress for about 4 hours. 
Then, the ground mixture was placed in a ball mill pot, admixed with 5 
parts by weight of epoxy resin and 180 parts by weight of a solvent 
mixture of cyclohexanone and isophorone and subjected to ball mill 
grinding for 5 days to disperse the magnetic powder throughout the 
mixture. Then, the ball milled mixture was admixed with a solution of 25 
parts of phenol resin and 6 parts by weight of vinyl resin in 450 parts by 
weight of a solvent mixture of cyclohexanone, isophorone and ioxane to 
prepare a magnetic paint for a magnetic disk. 
The thus prepared paint was applied to an aluminum substrate disk, 5.25 
inches in diameter, whose surface was cleaned in advance, by spin coating, 
and subjected to magnetic alignment according to a known procedure. After 
curing, the film thickness and the surface roughness were measured. The 
disk had a film thickness of 0.4 .mu.m at R38 mm and 0.7 .mu.m at R60 mm 
before surface finishing and a surface roughness of 0.23 .mu.m R before 
surface finishing. COMATIVE EXAMPLE 9 
Epoxy resin in a plate form was pulverized a mixer to obtain an epoxy resin 
powder having particle sizes of about 100 .mu.m. 25 parts by weight of the 
thus prepared powdery epoxy resin, 100 parts by weight of magnetic powder, 
iron oxide powder (Hc 340 Oe) with a BET specific surface area of 22 
m.sup.2 /g and 5 parts by weight of single crystal alumina were thoroughly 
mixed. Then the mixture was admixed with 10 parts by weight of 
cyclohexanone and subjected to further mixing in a kneader-grinder. Then, 
the mixture was admixed with 5 parts by weight of cyclohexanone and ground 
under a high shear stress for about 4 hours. 
Then, the ground mixture was placed in a ball mill pot, admixed with 200 
parts by weight of a solvent mixture of cyclohexanone and isophorone and 
subjected to ball mill grinding for 5 days to disperse the magnetic powder 
throughout the mixture. Then, the ball milled mixture was admixed with a 
solution of 25 parts by weight of phenol resin and 6 parts by weight of 
vinyl resin in 280 parts by weight of a solvent mixture of cyclohexanone, 
isophorone and dioxane to prepare a magnetic paint for a magnetic disk. 
Then, the thus prepared paint was applied to an aluminum substrate disk, 
5.25 inches in diameter, whose surface was cleaned in advance, by spin 
coating, and subjected to magnetic alignment according to a known 
procedure. After curing at 210.degree. C., the film thickness and the 
surface roughness of the disk were measured. The disk had a film thickness 
of 0.8 .mu.m at R38 mm and 0.9 .mu.m at R60 mm before surface finishing 
and a surface roughness of 0.045 .mu.m Ra before surface finishing. 
EXAMPLE 15 
Epoxy resin in a plate form was pulverized in a jet mill to obtain an epoxy 
resin powder having particle sizes of about 8.0 .mu.m. 27.5 parts by 
weight of the thus prepared powdery epoxy resin, 100 parts by weight of 
magnetic powder, Co-coated iron oxide powder (Hc 770 Oe) with a BET 
specific surface area of 40 m.sup.2 /g and 5 parts by weight of single 
crystal alumina were thoroughly mixed. Then, the mixture was admixed with 
100 parts by weight of cyclohexanone and subjected to further mixing in a 
kneader-grinder. Then, the mixture was further admixed with 5 parts by 
weight of cyclohexanone and ground under a high shear stress. 
The ground mixture was placed in a ball mill pot, admixed with 160 parts by 
weight of a solvent mixture of cyclohexanone and isophorone and subjected 
to ball mill grinding for 3 days to disperse the magnetic powder 
throughout the mixture. Then, the ball-milled mixture was admixed with a 
solution of 27.5 parts by weight of phenol resin and 6 parts by weight of 
vinyl resin in 800 parts by weight of a solvent mixture of cyclohexanone, 
isophorone and dioxane to prepare a magnetic paint for a magnetic disk. 
Then, the thus prepared paint was applied to an aluminum substrate disk, 
5.25 inches in diameter, whose surface was cleaned in advance, by spin 
coating, and subjected to magnetic alignment according to a known 
procedure. After curing at 210.degree. C., the film thickness and the 
surface roughness of the disk were measured. The disk had a film thickness 
of 0.20 .mu.m both at R38 mm and R60 mm, a surface roughness of 0.014 
.mu.m Ra and a Br/Bm ratio in the circumferential direction of 0.85 before 
surface finishing. No visible defects were detected at all on the film 
surface. Then, the magnetic disk was subjected to tape vanishing to remove 
the projections on the film surface. The film thickness of the magnetic 
disk was not changed after vanishing and was 0.20 .mu.m all over the 
surface from the inner peripheral side to the outer peripheral side of the 
disk. Then, the magnetic disk could perform recording and reproduction by 
making a magnetic head run over the magnetic disk at a distance of 0.20 
.mu. m from the magnetic disk. 
EXAMPLE 16 
100 parts by weight of magnetic powder, Co-coated iron oxide powder (Hc 770 
Oe) with a BET specific surface of 45 m.sup.2 /g, 3 parts by weight of 
single crystal alumina and 27.5 parts by weight of fine epoxy resin powder 
having particle sizes of about 3.0 .mu.m were thoroughly mixed. Then, the 
mixture was admixed with 10 parts by weight of cyclohexanone and subjected 
to further mixing in a kneader-grinder. Then, the mixture was admixed with 
5 parts by weight of cyclohexanone and ground under a high shear stress. 
Then, the ground mixture was placed in a ball mill pot, admixed with 160 
parts by weight of a solvent mixture of cyclohexanone and isophorone and 
subjected to ball mill grinding for 3 days to disperse the magnetic powder 
throughout the mixture. Then, the ball milled mixture was admixed with a 
solution of 12.5 parts by weight of phenol resin and 6 parts by weight of 
vinyl resin in 990 parts by weight of a solvent mixture of cyclohexanone, 
isophorone and dioxane to prepare a magnetic paint for a magnetic disk. 
Then, the thus prepared paint was applied to an aluminum substrate disk, 
5.25 inches in diameter, whose surface was cleaned in advance, by spin 
coating, and subjected is magnetic alignment according to a known 
procedure. After curing, the film thickness and the surface roughness were 
measured. The disk had a film thickness of 0.10 .mu.m both at R38 mm and 
R60 mm, a surface roughness of 0.012 .mu.m Ra and a Br/Bm ratio in the 
circumferential direction of 0.84 before surface finishing. No visible 
defects were detected at all on the film surface. 
Then, the magnetic disk was subjected to tape vanishing to remove 
projections on the film surface. The film thickness of the magnetic disk 
was not changed after the vanishing and was 0.10 .mu.m all over the 
surface from the inner peripheral side to the outer peripheral side. The 
magnetic disk could perform recording and reproduction by making a 
magnetic head run over the magnetic disk at a distance of 0.20 .mu.m from 
the magnetic disk. 
EXAMPLE 17 
100 parts by weight of magnetic powder, Co-coated iron oxide powder (Hc 770 
Oe) with a BET specific surface area of 50 m.sup.2 /g, 3 parts by weight 
of single crystal alumina and 35 parts by weight of fine epoxy resin 
powder having particle sizes of 2.0 .mu.m were thoroughly mixed. Then, the 
mixture was admixed with 10 parts by weight of cyclohexanone and ground 
under a high shear stress in a kneader-grinder. 
The ground mixture was placed in a ball mill pot, admixed with 130 parts by 
weight of a solvent mixture of cyclohexanone and isophorone and subjected 
to ball mill grinding for 3 days to disperse the magnetic powder 
throughout the mixture. Then, the ball milled mixture was admixed with a 
solution of 15 parts by weight of phenol resin and 3 parts by weight of 
vinyl resin in 1,400 parts by weight of a solvent mixture of 
cyclohexanone, isophorone and dioxane to prepare a magnetic paint for a 
magnetic disk. 
Then, the thus prepared paint was applied to an aluminum substrate disk, 
5.25 inches in diameter, whose surface was cleaned in advance, by spin 
coating, and subjected to magnetic alignment according to a known 
procedure. After curing, the film thickness and the surface roughness were 
measured. The disk had a film thickness of 0.07 .mu.m both at R38 mm and 
R60 mm, a surface roughness of 0.011 .mu.m Ra and a Br/Bm ratio in the 
circumferential direction of 0.80 before surface finishing. No visible 
defects were detected at all on the film surface. 
Then, the magnetic disk was subjected to tape vanishing to remove 
projections on the film surface. The film thickness of the magnetic disk 
was not changed after the vanishing and was 0.07 .mu.m all over the 
surface from the inner peripheral side to the outer peripheral side. The 
magnetic disk could perform recording and reproduction by making a 
magnetic head run over the magnetic head at a distance of 0.20 .mu.m from 
the magnetic disk. 
EXAMPLE 18 
100 parts by weight of magnetic powder, Co-coated iron oxide powder (Hc 770 
Oe) with a BET specific surface area of 55 m.sup.2 /g, a very small amount 
of fine diamond particles and 40 parts by weight of epoxy resin powder 
having particle sizes of about 2.0 .mu.m were thoroughly mixed. Then, the 
mixture was admixed with 10 parts by weight of cyclohexanone and ground 
under a high shear stress. 
Then, the ground mixture was placed in a ball mill pot, admixed with 130 
parts by weight of a solvent mixture of cyclohexanone and isophorone and 
subjected to ball mill grinding for 3 days to disperse the magnetic powder 
throughout the mixture. 
Then, the mixture was admixed with a solution of 20 parts by weight of 
phenol resin and 6 parts by weight of vinyl resin in 1,850 parts by weight 
of a solvent mixture of cyclohexanone, isophorone and dioxane to prepare a 
magnetic paint for a magnetic disk. 
Then, the thus prepared paint was applied to an aluminum substrate disk, 
5.25 inches in diameter, whose surface was cleaned in advance, by spin 
coating and subjected to magnetic alignment according to a known 
procedure. After curing, the film thickness and surface roughness were 
measured. The disk had a film thickness of 0.05 .mu.m both at R38 mm and 
R60 mm, a surface roughness of 0.010 .mu.m Ra and a Br/Bm ratio in the 
circumferential direction of 0.75 before surface finishing. No visible 
defects were detected at all on the film surface. 
Then, the magnetic disk was subjected to tape vanishing to remove 
projections on the film surface. The film thickness of the magnetic disk 
was not changed at all after the vanishing and was 0.05 .mu.m all over the 
surface from the inner peripheral side to the outer peripheral side. The 
magnetic disk could perform recording and reproduction by making a 
magnetic head run over the magnetic disk at a distance of 0.20 .mu.m from 
the magnetic disk. 
EXAMPLE 19 
100 parts by weight of magnetic powder, Co-coated iron oxide powder (Hc 770 
Oe) with a BET specific surface area of 50 m.sup.2 /g and 40 parts by 
weight of epoxy resin powder having particle sizes of about 2.0 .mu.m were 
thoroughly mixed. Then, the mixture was admixed with 10 parts by weight of 
cyclohexanone and ground in a high shear stress. 
The ground mixture was placed in a ball mill pot, admixed with 130 parts by 
weight of a solvent mixture of cyclohexanone and isophrorone and subjected 
to ball mill grinding for 3 days to disperse the magnetic powder 
throughout the mixture. Then, the ball milled mixture was admixed with a 
solution of 20 parts by weight of phenol resin and 6 parts by weight of 
vinyl resin in 2,350 parts by weight of a solvent mixture of 
cyclohexanone, isophorone and dioxane to prepare a magnetic paint for a 
magnetic disk. Then, the thus prepared paint was applied to an aluminum 
substrate disk, 5.25 inches in diameter, whose surface was cleaned in 
advance, by spin coating, and subjected to magnetic alignment. After 
curing, the film thickness and the surface roughness were measured. The 
disk had a film thickness of 0.02 .mu.m both at R38 mm and R60 mm and a 
surface roughness of 0.008 .mu.m Ra. No visible defects were detected at 
all on the film surface. 
Then, the magnetic disk was subjected to tape vanishing to remove 
projections on the film surface. The film thickness of the magnetic disk 
was not changed after the vanishing and was 0.02 .mu.m all over the 
surface from the inner peripheral side to the outer peripheral side. The 
magnetic disk could perform recording and reproduction by making a 
magnetic head run over the magnetic disk at a distance of 0.20 .mu.m from 
the magnetic disk. 
Line recording density D.sub.50 for giving a reproduction output which 
corresponds to 50% of the output of isolated pulse wave was obtained for 
the magnetic disks of Examples 12 to 18. The results are given in the 
following Table. 
TABLE 
______________________________________ 
Thickness of 
Example magnetic D.sub.50 
No. Magnetic powder 
film (.mu.m) (k FCI) 
______________________________________ 
12 Co-coated iron oxide 
0.30 26.9 
Hc: 770 Oe 
BET: 40 m.sup.2 /g 
13 Co-coated iron oxide 
0.22 30.0 
Hc: 770 Oe 
BET: 45 m.sup.2 /g 
14 Co-coated iron oxide 
0.20 31.2 
Hc: 770 Oe 
BET: 50 m.sup.2 /g 
15 Co-coated iron oxide 
0.20 31.5 
Hc: 770 Oe 
BET: 40 m.sup.2 /g 
16 Co-coated iron oxide 
0.10 37.5 
Hc: 770 Oe 
BET: 45 m.sup.2 /g 
17 Co-coated iron oxide 
0.07 39.0 
Hc: 770 Oe 
BET: 50 m.sup.2 /g 
18 Co-coated iron oxide 
0.05 40.4 
Hc: 770 Oe 
BET: 55 m.sup.2 /g 
______________________________________ 
As is obvious from Table 1, the line recording density D.sub.50 of magnetic 
disks can be made 30 kFCI or higher, which is considerably higher than the 
present maximum level of D.sub.50 =18 kFCI, at a distance of 0.2 .mu.m 
between the magnetic head and the magnetic disk by making the magnetic 
film of a magnetic disk thinner. By making the distance between the 
magnetic head and the magnetic disk 0.15 .mu.m, D.sub.50 =40-50 kFCI can 
be obtained. 
In Examples 12 to 18, Co-coated iron oxide powder is used as a magnetic 
material, and D.sub.50 can be increased by using barium ferrite powder, Fe 
metal powder, etc. as the magnetic material. 
As described in detail above, the magnetic paints prepared according to 
Examples 12 to 19 had a tactoid structure where the magnetic powder was 
uniformly dispersed in the paints. By using the magnetic paints in a 
magnetic recording medium, for example, a magnetic disk based on a 
5.25-inch aluminum substrate disk, a thin magnetic film having a uniform 
film thickness, for example, 0.6 to 0.02 .mu.m, all over the surface from 
the inner peripheral side to the outer peripheral side of the magnetic 
disk and a very small surface roughness of 0.020 to 0.008 .mu.m Ra can be 
readily obtained by coating. Thus, the surface finishing time can be 
considerably reduced, as compared with the prior art and the electrical 
defects due to scratches formed during the surface finishing can be 
effectively reduced. Furthermore, the noise of the present magnetic disk 
can be reduced by about 30%, and also an increase in the output resolution 
can be expected. Furthermore, problems with a magnetic head can be solved 
because of the small thickness on the outer peripheral side of the present 
magnetic head. 
The recording-reproduction apparatus according composed of a magnetic 
material having a saturation magnetic film density of 4 kG or more, 
preferably 9 kG or more and a magnetic disk and a magnetic recording disk 
comprising a non-magnetic substrate and a magnetic coating film having a 
film thickness of 0.02 .mu.m to 0.6 .mu.m and a surface roughness of not 
more than 0.02 .mu.m Ra before surface finishing, provided on the 
non-magnetic substrate, where the magnetic material of the magnetic head 
is in a thin film having a film thickness of 5 .mu.m or more. 
According to the present invention, it is possible for the first time to 
form a very thin, smooth magnetic film with a very small surface 
roughness. The present invention is applicable not only to the formation 
of a single layer of a magnetic film on a non-magnetic substrate, but also 
to the formation of at least two layers of the magnetic film on the 
non-magnetic substrate. 
The continuously deposited medium formed by sputtering, vapor deposition, 
plating, etc. has distinguished electromagnetic conversion characteristics 
but is not better in the reliability than the coated medium. The coated 
medium can contain a reinforcing material, a lubricant, etc. in the film, 
whereas the continuously deposited medium cannot contain such additives as 
above. This is the main reason for the poor reliability of the 
continuously deposited medium. Thus, it is possible to increase the 
reliability of the continuously deposited medium up to the level of the 
coated medium by forming a very thin magnetic layer of the present 
invention containing a reinforcing material such as a very small amount of 
fine diamond particles, etc. on the surface of the continuously deposited 
medium, where the holding power of the very thin magnetic layer of the 
present invention is made equal to that of the continuously deposited 
medium. 
In the foregoing Examples of the present invention, epoxy resin, phenol 
resin and vinyl resin are used as a polymeric brinder. Other ordinary 
organic polymer compounds having a good binding to the magnetic powder 
such as usually used vinylic resins such as vinyl chloride-vinyl acetate 
copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, 
acrylonitrite-acrylic acid-2-hydroxyethylmethacrylate copolymer, etc., 
rubber-based resins such as acrylonitrite-butadiene copolymer, etc., 
cellulose-based resins such as nitrocellulose, acetylcellulose, etc., 
epoxy resins such as phenoxy resin, etc., urethane-based resins such as 
polyurethane, urethane prepolymer, etc. can be also used. 
The vinyl resin for use in the present invention as the polymeric brinder 
includes, for example, polyvinylbutyral, polyvinylformal, polyvinyl 
acetate, etc., among which polyvinylbutyral is particularly preferable. 
The magnetic powder for use in the present invention is preferably 
Co-coated iron oxide powder with a BET specific surface area of 40 m.sup.2 
/g or more as used in Examples 12 to 19. Iron oxide powder, metal powder, 
barium ferrite powder, iron carbide powder, etc. with a BET specific 
surface area of 40 m.sup.2 /g or more can be used as the magnetic powder.