Magnetic recording medium

An improved magnetic recording medium is disclosed, particularly a magnetic tape suitable for sound recording and video recording, which comprises as a recording element a mixture of cobalt-containing iron oxide magnetic particles consisting essentially of ferromagnetic iron oxide particles containing cobalt and optionally a divalent iron in the most outer layer thereof, and .gamma.-ferric oxide particles in the ratio of 10:90 to 50:50 by weight, said mixture of cobalt-containing iron oxide magnetic particles and .gamma.-ferric oxide particles being applied to a substrate (e.g. a polyester film) with a binding agent. The magnetic recording medium has excellent electromagnetic transducing properties and improved charging properties and is useful for conventional magnetic recording apparatus used at normal position with high sensitivity in wide range of frequency.

The present invention relates to an improvement of a magnetic recording 
medium, particularly a magnetic tape suitable for sound recording and 
video recording. 
There have widely been used magnetic tapes containing .gamma.-ferric oxide 
(.gamma.-Fe.sub.2 O.sub.3) particles as a recording element for sound 
recording or video recording, because the .gamma.-ferric oxide particles 
are chemically and magnetically stable and are not expensive. These 
conventional magnetic tapes have a comparatively low coercive force such 
as about 300 to 380 oersteds, and hence, show a good output power in the 
low frequency range but do not show a sufficient output power in the high 
frequency range. 
In order to improve the drawback of the .gamma.-ferric oxide particles, it 
is proposed to form a layer of iron oxide containing cobalt on the surface 
of the .gamma.-ferric oxide particles. The magnetic particles thus 
obtained have a high coercive force because of the cobalt containment, but 
on the other hand, the magnetic tape prepared from the magnetic particles 
tends to show an increased surface electrical resistance in comparison 
with the magnetic tape prepared from .gamma.-ferric oxide particles, and 
hence, the magnetic tape has drawbacks that it is easily electrostatically 
charged and is readily deposited with dust which induces drop out, and 
further, a noise occurs due to discharge. In order to eliminate these 
drawbacks, a conductive material such as carbon black is added to the 
magnetic composition, but increase of non-magnetic components unfavorably 
results in lowering of packed amount of magnetic particles which gives 
unfavorable effect on the characteristics of the magnetic recording 
medium. 
The present inventors have initiated intensive studies to improve the 
charging of the magnetic recording tape while keeping the excellent 
characteristics of the cobalt-containing iron oxide magnetic particles, 
and have found that a mixture of .gamma.-ferric oxide particles and 
cobalt-containing iron oxide magnetic particles is excellent as a 
recording element and can give the desired magnetic tape. 
An object of the present invention is to provide a magnetic recording 
medium having improved charging properties as well as excellent 
electromagnetic transducing properties. Another object of the invention is 
to provide a magnetic recording medium useful in wide range of frequency. 
A further object of the invention is to provide a magnetic tape useful for 
a conventional magnetic recording apparatus used at normal position with 
high sensitivity in a wide range of frequency. These and other objects of 
the invention will be apparent from the following description. 
The magnetic recording medium of the present invention comprises as the 
essential recording element a mixture of .gamma.-ferric oxide particles 
and cobalt-containing iron oxide magnetic particles consisting of 
ferromagnetic iron oxide particles containing cobalt in the most outer 
layer thereof, and the magnetic recording medium has a surface electrical 
resistance similar to that of the conventional magnetic recording medium 
comprising .gamma.-ferric oxide particles as the recording element and has 
excellent magnetic characteristics in the high frequency range similar to 
that of the known magnetic recording medium comprising the 
cobalt-containing iron oxide magnetic particles alone as the recording 
element. 
The cobalt-containing iron oxide magnetic particles used in the present 
invention comprise ferromagnetic iron oxide particles containing cobalt in 
the most outer layer thereof and are prepared by dispersing .gamma.-ferric 
oxide particles or ferromagnetic iron oxide particles which are obtained 
by partially reducing .gamma.-ferric oxide particles into an aqueous 
solution of a cobalt salt (e.g. cobalt sulfate, cobalt chloride, cobalt 
nitrate) which contains an alkali (e.g. sodium hydroxide, potassium 
hydroxide, lithium hydroxide) in an amount of one equivalent or more to 
the cobalt salt, and maintaining the mixture at a temperature of higher 
than room temperature up to the boiling point of the mixture, by which 
cobalt is distributed into the most outer layer of the .gamma.-ferric 
oxide particles or ferromagnetic iron oxide particles, wherein cobalt is 
contained in the most outer layer in an amount of 0.2% by weight or more, 
preferably 0.3 to 5% by weight, based on the weight of the particle. The 
detail of the preparation of the cobalt-containing iron oxide magnetic 
particles is disclosed in U.S. Ser. No. 898,127. 
The cobalt-containing iron oxide magnetic particles have a particle size in 
the longitudinal direction of not more than 1.mu., preferably 0.1 to 
1.mu., and an axis ratio (long axis/short axis) of not less than 2, 
preferably 5 to 15. 
The cobalt-containing iron oxide magnetic particles have a higher coercive 
force than that of .gamma.-ferric oxide particles, and further, since it 
has a core of ferromagnetic iron oxide, they are stable and can keep their 
excellent magnetic characteristics for a long period of time, and further, 
have also good magnetic characteristics in the low frequency range. The 
cobalt-containing iron oxide magnetic particles have improved magnetic 
characteristics in the high frequency range, but when the particles have 
too high of a coercive force than the coercive force of the .gamma.-ferric 
oxide particles with which they are mixed, the magnetic recording medium 
obtained therefrom shows inferior erasing properties. On the other hand, 
when the difference of the coercive force between the cobalt-containing 
iron oxide magnetic particles and the .gamma.-ferric oxide particles is 
too small, the desired improvement of the magnetic characteristics in the 
high frequency range can not be achieved. Accordingly, the difference of 
the coercive force between the cobalt-containing iron oxide magnetic 
particles and the .gamma.-ferric oxide particles is preferably in the 
range of 20 to 100 oersteds, more preferably 30 to 80 oersteds. 
It has further been found that it is preferable to incorporate a ferrous 
salt (e.g. ferrous sulfate, ferrous chloride, ferrous nitrate) into the 
surface layer of the cobalt-containing iron oxide in addition to a cobalt 
salt, by which the cobalt-containing iron oxide particles show a greater 
coercive force and also improved charging properties in comparison with 
the particles containing only a cobalt salt. That is, when the 
cobalt-containing iron oxide magnetic particles are prepared by dispersing 
.gamma.-ferric oxide particles or ferromagnetic iron oxide particles 
obtained by partial reduction of .gamma.-ferric oxide particles into an 
aqueous solution containing a ferrous salt as well as a cobalt salt and an 
alkali as mentioned hereinbefore, and maintaining the mixture at a 
temperature of higher than room temperature up to the boiling point of the 
mixture under an atmosphere that the divalent iron is substantially not 
oxidized to form the most outer layer containing divalent iron and cobalt, 
the resulting particles show a greater coercive force than the particles 
containing only cobalt in the most outer layer as is clear from the 
accompanying drawing. 
The drawing shows a relation between the coercive force of the 
cobalt-containing iron oxide magnetic particles and the cobalt content 
thereof, wherein a curve 1 shows the case of adding no ferrous salt into 
the aqueous solution and a curve 2 shows the case of adding a ferrous salt 
in an amount of three fold by mol of that of the cobalt salt. As is clear 
from this figure, when a divalent iron is incorporated into the 
cobalt-containing iron oxide particles, the particles show a largely 
increased coercive force in comparison with the particles containing only 
cobalt at the same cobalt level. This means that in order to obtain the 
particles having a prescribed coercive force, the content of cobalt can be 
decreased by incorporation of the divalent iron in addition to cobalt, and 
hence, the particles show an improved electrical conductivity. 
The cobalt-containing iron oxide magnetic particles used in the present 
invention have preferably a coercive force of 370 to 420 oersteds, a 
cobalt content of about 0.3 to 1.8% by weight and a divalent iron content 
of 0.1 to 8% by weight, more preferably 0.9 to 5.4% by weight, based upon 
the weight of the cobalt-containing iron oxide magnetic particle. 
The .gamma.-ferric oxide particles to be mixed with the cobalt-containing 
iron oxide magnetic particles include all conventional .gamma.-ferric 
oxide particles. The particles act to decrease the surface electrical 
resistance. The .gamma.-ferric oxide particles have preferably a coercive 
force of not more than 380 oersteds, and more preferably 320 to 370 
oersteds. 
The coercive force of the cobalt-containing iron oxide magnetic particles 
and the .gamma.-ferric oxide particles is measured as follows: 
The particles are packed into a capsule in an amount of about 1 g/cc, and 
then, the coercive force of the particles is measured at a magnetic field 
of 10,000 oersteds with a vibrating sample magneto meter (VSM III) (made 
by Toei Kogyo K.K.). 
The cobalt-containing iron oxide magnetic particles and .gamma.-ferric 
oxide particles are mixed in a ratio of 10:90 to 50:50 by weight, 
preferably 25:75 to 50:50 by weight. When the cobalt containing iron oxide 
particles are mixed with the .gamma.-ferric oxide particles in less than 
10% by weight based on the total weight of the mixture, the magnetic 
recording medium does not show the desired coercive force, but on the 
other hand, when the amount of the cobalt-containing iron oxide particles 
is over 50% by weight based on the total weight of the mixture, the 
magnetic recording medium shows an undesirably increased surface 
electrical resistance. By mixing both particles in the ratio 
above-mentioned, there can be obtained the desired magnetic recording 
medium having excellent electromagnetic transducing properties and less 
charging properties. 
The magnetic recording medium can be prepared from the mixture thus 
obtained in conventional methods. For example, a composition comprising 
the mixture of the particles as mentioned above is prepared by mixing the 
mixture with a binding agent, an organic solvent and other additives, and 
the magnetic composition thus prepared is applied to a substrate such as a 
polyester film by a conventional means such as a roll coater, a blade 
coater, an air knife coater, or the like, and thereafter is dried. 
The binding agent includes any conventional binding agents such as vinyl 
chloride-vinyl acetate copolymer, polyvinyl butyral, polyurethane resin, 
nitrocellulose, polyester resin, or the like. 
The organic solvent includes toluene, methyl isobutyl ketone, methyl ethyl 
ketone, cyclohexanol, tetrahydrofuran, ethyl acetate, or the like, which 
may be used alone or in combination of two or more thereof. 
The magnetic composition may also contain other additives such as 
dispersing agents (e.g. lecithin), lubricants (e.g. lauric acid, liquid 
paraffin), plasticizers (e.g. dioctyl phthalate, dibutyl phthalate), or 
the like. 
The magnetic recording medium thus prepared has a coercive force of not 
more than 380 oersteds, preferably 340 to 380 oersteds, and has excellent 
electromagnetic transducing properties suitable for a conventional 
magnetic recording apparatus used at normal position and further has 
improved charging properties.

The present invention is illustrated by the following Examples but is not 
limited thereto. 
EXAMPLE 1 
Acicular .gamma.-Fe.sub.2 O.sub.3 particles (particle size: about 0.3.mu., 
axis ratio (long axis/short axis): about 10, coercive force: 337 oersteds, 
maximum magnetization: 74 emu/g, 1 kg) were dispersed in water (6 liters), 
and thereto were added an aqueous solution (1 liter) of cobalt sulfate 
(CoSO.sub.4.7H.sub.2 O) (23.85 g) and an aqueous solution (1 liter) of 
sodium hydroxide (10.79 g). The mixture was reacted with agitation at 
100.degree. C. for 5 hours. After the reaction was finished, the resulting 
precipitates were separated by filtration, washed well with water, and 
then dehydrated and dried at 130.degree. C. for 2 hours. 
The cobalt-containing iron oxide magnetic particles thus obtained had a 
coercive force of 393 oersteds and a maximum magnetization of 75 emu/g. 
The cobalt-containing iron oxide magnetic particles and the same 
.gamma.-Fe.sub.2 O.sub.3 particles as used above were mixed in the ratio 
of 25:75 and 50:50 by weight. By using the resulting magnetic particles 
mixtures, two magnetic compositions were prepared in accordance with the 
following formulation. 
______________________________________ 
Magnetic particles mixture 
75 parts by weight 
Vinyl chloride-vinyl acetate 
25 parts by weight 
copolymer 
Dioctyl phthalate 5 parts by weight 
Toluene 100 parts by weight 
Methyl ethyl ketone 
100 parts by weight 
______________________________________ 
The magnetic compositions were applied onto a polyester film (thickness: 
12.mu.) in a thickness of about 6.mu. (in dry state). After drying the 
composition, the film was cut in a fixed width to give two magnetic tapes. 
EXAMPLE 2 
Acicular .gamma.-Fe.sub.2 O.sub.3 particles (particle size: about 0.3.mu., 
axis ratio: about 10, coercive force: 337 oersteds, maximum magnetization: 
74 emu/g, 1 kg) were dispersed in water (6 liters) and thereto were added 
an aqueous solution (1 liter) of cobalt sulfate (CoSO.sub.4.7H.sub.2 O) 
(42.93 g) and ferrous sulfate (FeSO.sub.4.7H.sub.2 O) (127.37 g) and an 
aqueous solution (1 liter) of sodium hydroxide (146.60 g). The mixture was 
reacted with agitation at room temperature for 3 hours. After the reaction 
was finished, the resulting precipitates were separated by filtration, 
washed well with water, and then dehydrated and dried at 130.degree. C. 
for 2 hours. 
The cobalt-containing iron oxide magnetic particles thus obtained had a 
coercive force of 395 oersteds and a maximum magnetization of 75 emu/g. 
The cobalt-containing iron oxide magnetic particles and the same 
.gamma.-Fe.sub.2 O.sub.3 particles as used above were mixed in the ratio 
of 30:70 and 50:50 by weight. By using the resulting magnetic particles 
mixtures, two magnetic compositions were prepared in accordance with the 
same formulation as used in Example 1, and further two magnetic tapes were 
prepared in the same manner as described in Example 1. 
REFERENCE EXAMPLE 1 
A magnetic tape was prepared in the same manner as described in Example 1 
except that the same .gamma.-Fe.sub.2 O.sub.3 particles as used in Example 
1 (75 parts by weight) were used alone instead of the magnetic particles 
mixture. 
REFERENCE EXAMPLE 2 
A magnetic tape was prepared in the same manner as described in Example 1 
except that the same cobalt-containing iron oxide magnetic particles as 
prepared in Example 1 (75 parts by weight) were used alone instead of the 
magnetic particles mixture. 
REFERENCE EXAMPLE 3 
A magnetic tape was prepared in the same manner as described in Example 2 
except that the same cobalt-containing iron oxide magnetic particles as 
prepared in Example 2 were used alone instead of the magnetic particles 
mixture. 
With respect to the magnetic tapes prepared in the above Examples 1 and 2 
and Reference Examples 1 to 3, there were measured various properties such 
as magnetic characteristics, electromagnetic transducing properties (i.e. 
sensitivity, frequency characteristics) and surface specific resistance. 
The results are shown in the following table. 
As is clear from the results, the magnetic tapes of the present invention 
show greater electromagnetic transducing properties and less surface 
electrical (specific) resistance in comparison with the magnetic tapes of 
Reference Examples wherein .gamma.-ferric oxide particles or 
cobalt-containing iron oxide magnetic particles are used alone. 
__________________________________________________________________________ 
Reference 
Reference 
Reference 
Example 1 
Example 2 
Example 1 
Example 2 
Example 3 
__________________________________________________________________________ 
Ratio by weight of 
cobalt-containing iron 
25/75 
50/50 
30/70 
50/50 
0/100 100/0 100/0 
oxide magnetic parti- 
cles/.gamma.-Fe.sub.2 O.sub.3 particles 
Coercive force 
(oersted) 356 367 357 370 347 401 415 
Sensitivity (dB) 
+2.3 
+2.2 
+2.3 
+2.2 
+2.3 +1.3 +1.2 
Frequency 
characteristics (dB) 
7 kHz +3.7 
+3.9 
+4.3 
+4.0 
+2.9 +4.4 +5.1 
12.5 kHz +6.8 
+7.0 
+7.7 
+7.1 
+5.9 +8.2 +9.3 
15 kHz +7.5 
+7.8 
+9.2 
+8.5 
+6.5 +10.0 +11.3 
Surface specific 
1.14 
3.14 
6.60 
2.13 
3.09 7.22 2.58 
resistance (.OMEGA./cm.sup.2) 
.times. 10.sup.9 
.times. 10.sup.9 
.times. 10.sup.8 
.times. 10.sup.9 
.times. 10.sup.8 
.times. 10.sup.10 
.times. 10.sup.10 
__________________________________________________________________________ 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the present invention, and all such 
modifications as would be obvious to one skilled in the art are intended 
to be included within the scope of the following claims.