Magnetic recording medium having a magnetic layer comprising hexagonal ferrite particles

A magnetic recording medium is described, which comprises a nonmagnetic support having thereon at least one magnetic layer containing ferromagnetic particles dispersed in a binder, wherein the magnetic recording medium has at least one magnetic layer containing at least hexagonal ferrite magnetic particles and the magnetic layer containing the hexagonal ferrite magnetic particles has a coercive force, Hc, of from 1,300 to 5,000 Oe, a ratio of the Hc to an anisotropic magnetic field HK thereof, Hc/HK, of from 0.30 to 1.0, and a squareness ratio of in-plane direction, SQ, of from 0.65 to 1.00.

FIELD OF THE INVENTION 
The present invention relates to a magnetic recording medium for 
high-density recording which has one or more magnetic layers or has one or 
more magnetic layers and one or more nonmagnetic layers, in which the 
uppermost layer contains hexagonal ferrite magnetic particles. 
BACKGROUND OF THE INVENTION 
Widely used conventional magnetic recording media such as video tapes, 
audio tapes, and magnetic disks comprise a nonmagnetic support having 
thereon a magnetic layer comprising particles of ferromagnetic iron oxide, 
Co-modified ferromagnetic iron oxide, CrO.sub.2, ferromagnetic metal, or 
hexagonal ferrite dispersed in a binder. Of these ferromagnetic materials, 
hexagonal ferrite is known as a material having excellent suitability for 
high-density recording. The following are examples of magnetic recording 
media employing magnetic particles of hexagonal ferrite. 
JP-A-60-157719, for example, discloses a magnetic recording medium having a 
magnetic layer which contains magnetic particles having a particle 
diameter of from 0.1 to 0.3 .mu.m and has a vertical-direction squareness 
ratio of 0.7 or more and a surface roughness of 0.05 .mu.m or less. (The 
term "JP-A" as used herein means an "unexamined published Japanese patent 
application.") This prior art technique is intended to provide a magnetic 
recording medium for high-density recording which has a sufficiently high 
vertical-direction squareness ratio and excellent surface smoothness. 
JP-A-62-109226 discloses a magnetic recording medium comprising a support 
having thereon a magnetic layer which has a thickness of 1.8 .mu.m or 
less, contains platy magnetic particles having an average particle 
diameter of 0.2 .mu.m or less and an average aspect ratio of flatness of 6 
or more, and has a specific vertical-direction squareness ratio and a 
specific vertical-direction coercive force. This prior art technique is 
intended to provide a magnetic recording medium which has excellent 
running durability during use and satisfactory suitability for overwriting 
and attains high recording density and high output. 
JP-A-64-89022 discloses a magnetic recording medium which employs a binder 
having a saturation magnetization of 60 emu/g or more, a specific surface 
area by BET method of from 25 to 70 m.sup.2 /g, an average particle 
diameter of from 0.01 to 0.2 .mu.m, and a coercive force of from 400 to 
2,000 Oe, and containing a polar group in an amount of 1.times.10.sup.-5 
eq/g or more. This prior art technique is intended to improve reproduced 
output and attain a high C/N ratio and improved running durability. 
JP-A-3-280215 discloses a magnetic recording medium in which the 
longitudinal-direction coercive force is from 1,000 to 4,000 Oe and the 
residual magnetization in the longitudinal direction is higher than that 
in the vertical direction, which in turn is higher than that in the 
in-plane width direction. This prior art technique is intended to provide 
a magnetic recording medium for high-density recording which has a 
satisfactory balance between long-wavelength output and short-wavelength 
output. 
JP-A-5-40370 discloses a magnetic recording medium in which 100 parts by 
weight of magnetic particles having a specific surface area of from 23 to 
45 m.sup.2 /g and a coercive force of from 400 to 2,000 Oe are dispersed 
in from 10 to 40 parts by weight of a resin binder. This prior art 
technique is intended to provide a magnetic recording medium for 
high-density recording which is reduced in noise and has excellent 
orientation. 
JP-A-5-12650 discloses a magnetic recording medium which comprises a 
support, a magnetic layer containing hexagonal ferrite and having a 
thickness of from 0.1 to 0.6 .mu.m, and a nonmagnetic layer provided 
between the magnetic layer and the support and having a larger thickness 
than the magnetic layer. This prior art technique is intended to improve 
surface properties, short-wavelength output, erasion characteristics, and 
durability. 
JP-A-5-225547 discloses a magnetic recording medium comprising a 
nonmagnetic support, a nonmagnetic layer provided thereon, and a magnetic 
layer provided on the nonmagnetic layer and containing 0.1 .mu.m or less 
magnetic particles. This prior art technique is intended to provide a 
magnetic recording medium having excellent high-frequency electromagnetic 
characteristics, satisfactory suitability for signal overwriting, and good 
durability. 
In JP-A-3-286420, IEEE. Trans. Mag., Vol. 24, No. 6, Nov. 1988, p. 2850, 
there is a description to the effect that the electromagnetic 
characteristics of a magnetic recording medium containing hexagonal 
ferrite are influenced by the anisotropic magnetic field HK of the 
ferrite. The former reference discloses a magnetic recording medium which 
has two magnetic layers provided on a nonmagnetic layer and in which the 
lower magnetic layer has an axis of easy magnetization in the longitudinal 
direction and the upper magnetic layer contains magnetic particles having 
an anisotropic magnetic field of 3,000 Oe or less; this prior art 
technique is intended to provide a magnetic recording medium which attains 
high output over a wide range from a long-wavelength region to a 
short-wavelength region. 
Furthermore, a large number of inventions concerning a squareness ratio SQ 
in magnetic recording media are disclosed in JP-A-60-164925 and 
JP-A-3-49025. 
However, the invented prior art recording media employing hexagonal ferrite 
described above have failed to fully exhibit their performances although 
effective in some degree. There has been much room for an improvement in 
output, especially in the ultrashort-wavelength region (usually, recording 
wavelengths of 0.5 .mu.m and less), but conditions under which such 
improvement is attained have been unable to be found. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a magnetic recording 
medium remarkably improved in electromagnetic characteristics, in 
particular ultrashort-wavelength output necessary to high-density 
recording, which has long been required of magnetic recording media. 
The present inventors made intensive studies in order to obtain a magnetic 
recording medium having satisfactory electromagnetic characteristics. 
As a result, this and other objects of the present invention have been 
attained by a magnetic recording medium comprising a nonmagnetic support 
having thereon at least one magnetic layer containing ferromagnetic 
particles dispersed in a binder, wherein the magnetic recording medium has 
at least one magnetic layer containing at least hexagonal ferrite magnetic 
particles and the magnetic layer containing the hexagonal ferrite magnetic 
particles has a coercive force, Hc, of from 1,300 to 5,000 Oe, a ratio of 
the Hc to an anisotropic magnetic field HK thereof, Hc/HK, of from 0.30 to 
1.0, and a squareness ratio of in-plane direction, SQ, of from 0.65 to 
1.00. 
DETAILED DESCRIPTION OF THE INVENTION 
The present invention is characterized in that the magnetic layer 
containing hexagonal ferrite magnetic particles has specific values of Hc, 
Hc/HK, and SQ of in-plane direction. As a result, the magnetic recording 
medium of the present invention retains remarkably improved 
ultrashort-wavelength output necessary to high-density recording. 
Although the reason why the magnetic recording medium of the present 
invention shows excellent electromagnetic characteristics has not been 
elucidated, the following explanations are possible. The Hc/HK ratio, 
i.e., the ratio of coercive force Hc to anisotropic magnetic field HK, is 
a factor which influences the mechanism of the reversal of magnetization; 
a magnetic layer having a higher Hc/HK ratio has a greater possibility 
that the reversal of magnetization occurs as a result of simultaneous 
rotation. That is, magnetic materials which attain a higher Hc value are 
more apt to undergo such phenomenon than magnetic materials showing the 
same HK value. It is presumed that a magnetic layer in which the reversal 
of magnetization occurs as a result of simultaneous rotation shows more 
rapid switching with changing recording magnetic field and, hence, the 
regions of the reversal of magnetization which are recorded on the 
magnetic recording medium account for a reduced proportion. Namely, it is 
thought that a magnetic recording medium having a high Hc/HK ratio 
basically has the ability to attain high output. On the other hand, rapid 
switching tends to disadvantageously cause demagnetization, i.e., the 
phenomenon in which recorded signals are erased when the recording head 
separates from the recording medium. Consequently, a high Hc/HK ratio 
alone is incapable of enabling the magnetic recording medium to fully 
exhibit the basic high-output performance. It is thought that the Hc 
should be increased in order to inhibit demagnetization as much as 
possible. On the other hand, SQ values less than 0.65 weaken the effect of 
Hc/HK ratio, probably because such a nearly randomly oriented state 
results in an increased proportion of the regions of the reversal of 
magnetization and this counteracts the effect of a high Hc/HK ratio. To 
sum up, a magnetic recording medium having a high Hc/HK ratio basically 
has the ability to attain high output, but it should further has a high Hc 
value and an SQ value in a specific range so as to fully exhibit that 
performance. The magnetic recording medium of the present invention 
satisfies the above requirement, which fact is thought to be the reason 
why the magnetic recording medium of the present invention has high 
output. 
In the present invention, the magnetic layer has an Hc/HK ratio of from 
0.30 to 1.0, preferably from 0.40 to 1.0, and more preferably from 0.60 to 
1.0. The Hc of the magnetic layer is from 1,300 to 5,000 Oe, preferably 
from 1,300 to 3,000 Oe, and more preferably from 1,700 to 2,500 Oe. 
Further, the magnetic layer has a squareness ratio of in-plane direction, 
SQ of from 0.65 to 1.00, preferably from 0.65 to 0.95, and more preferably 
from 0.80 to 0.95. The HK of the magnetic layer is desirably 3,000 Oe or 
more. 
The performance of the head to be used for recording or reproducing is 
preferably taken in account when an Hc value is decided. Specifically, Hc 
values of 1,300 Oe or more are suitable for the currently used heads 
having a Bs of about 1 T (tesla), while Hc values of 1,700 Oe or more are 
suitable for heads employing a high-Bs material, e.g., Fe-Ta-N, and having 
a Bs of about from 1.2 to 1.8 T. The upper limit of Hc varies depending on 
head materials, and is hence unable to be specified unconditionally. 
However, it may be about 4,000 Oe. 
In producing the magnetic recording medium of the present invention, the 
magnetic layer supported on the nonmagnetic support which layer contains 
hexagonal ferrite magnetic particles and satisfies the property 
requirements specified above (hereinafter this magnetic layer being 
referred to also as "upper layer" or "upper magnetic layer") may be 
provided on a nonmagnetic layer comprising inorganic nonmagnetic particles 
dispersed in a binder, or on a lower magnetic layer comprising 
ferromagnetic particles dispersed in a binder, or on a multilayer 
structure comprising the nonmagnetic layer and the lower magnetic layer. 
In this invention, the term "lower layer" is often used when the 
nonmagnetic layer and/or the lower magnetic layer provided under the upper 
layer is referred to without being distinguished from each other, although 
the term "lower nonmagnetic layer" or "lower magnetic layer" is used when 
one of the two underlying layers is referred to. In the case of forming 
both lower nonmagnetic layer and lower magnetic layer as the lower layer, 
either layer may be formed first, and the effect of the invention is 
basically obtainable regardless of the sequence of layer formation. If 
desired and needed, the upper magnetic layer, the lower nonmagnetic layer, 
and the lower magnetic layer each may have a multilayer structure. 
The upper layer may contain another kind of ferromagnetic particles in 
combination with the ferrite particles if desired and needed. However, the 
proportion of the hexagonal ferrite magnetic particles is usually from 50 
to 100% by weight, preferably from 80 to 100% by weight, based on all 
ferromagnetic particles in the upper layer. The ferromagnetic particles 
for use in the lower layer are not particularly limited, and the same 
hexagonal ferrite magnetic particles as in the upper layer are usable. The 
lower layer is free from the requirements concerning Hc, Hc/HK, and 
in-plane direction SQ which the upper layer is required to satisfy. The 
term "ferromagnetic particles" used hereinafter means any kind of 
ferromagnetic particles including hexagonal ferrite magnetic particles, 
unless otherwise indicated. 
In the magnetic recording medium of the present invention, the magnetic 
layer containing hexagonal ferrite particles may be the only layer. 
However, a lower nonmagnetic layer is preferably provided between the 
magnetic layer and the support, because the formation of a lower 
nonmagnetic layer not only contributes to an improvement in surface 
properties but also facilitates a thickness reduction for the upper layer. 
A lower magnetic layer containing acicular ferromagnetic particles or 
other magnetic particles is also preferably provided as another lower 
layer between the upper magnetic layer and the support, because the lower 
magnetic layer contributes to an improvement in long-wavelength 
electromagnetic characteristics. 
The residual magnetic flux density (Br) of the magnetic layer containing 
hexagonal ferrite particles is preferably 1,000 G or more. If the Br 
thereof is less than 1,000 G, output decreases over the whole wavelength 
region. There is no particular upper limit to the Br thereof. The SFD of 
the magnetic layer is 0.5 or less, preferably 0.3 or less. 
The thickness of the magnetic layer containing hexagonal ferrite particles 
is preferably 3 .mu.m or less, and may be varied according to purposes. 
For example, if the magnetic layer Containing hexagonal ferrite particles 
is the only magnetic layer, the thickness thereof is preferably from 0.5 
to 3 .mu.m. If a lower layer is provided, the thickness of the upper layer 
is preferably from 0.01 to 1 .mu.m. 
If a lower magnetic layer is provided, the ferromagnetic particles 
contained therein are preferably fine ferromagnetic particles of metal 
comprising iron as the main component or particles of either 
cobalt-modified iron oxide or iron oxide. If a lower nonmagnetic layer is 
provided, the inorganic nonmagnetic particles contained therein are 
preferably particles of at least one of titanium dioxide, barium sulfate, 
zinc oxide, and s-iron oxide. 
The lower layer and the upper layer are preferably coated by a wet-on-wet 
coating method according to U.S. Pat. No. 4,844,946. 
Examples of methods that can be used for practicing the present invention 
include the following. However, usable methods are, of course, not limited 
thereto, and methods other than those can be used to attain the object of 
the invention as long as the requirements specified hereinabove are 
satisfied. 
A magnetic coating fluid containing hexagonal ferrite particles dispersed 
therein and a magnetic coating fluid containing nonmagnetic particles or 
ferromagnetic particles dispersed therein are applied to a nonmagnetic 
support in such amounts as to result in an upper-layer thickness of 2.0 
.mu.m or less. Before the coating dries, the coated support is passed 
through a magnetic field for longitudinal orientation. Calendering is then 
conducted with metal rolls arranged in a multi-stage stack. Thus, the 
magnetic recording medium of the present invention can be produced. 
Although the reasons for those treatments have not been elucidated, the 
following explanations are possible. The orientation treatment in which 
the coated support having a wet coating is passed through a magnetic field 
for longitudinal orientation is intended to longitudinally orient a 
certain proportion of the hexagonal ferrite particles contained in the 
magnetic recording medium. Longitudinal orientation herein means to align 
individual hexagonal ferrite particles so that the axis of easy 
magnetization for each particle is directed to the direction of the length 
of the magnetic recording medium. 
An important point in this longitudinal orientation is that this treatment 
is performed so that the finished magnetic recording medium has a 
regulated SQ value within the range specified hereinabove. For attaining 
this, the wet coating which is being passed through a longitudinal 
magnetic field should be dried to some degree during the orientation. 
For this purpose, it is preferred to use an apparatus capable of feeding 
heated dry air to the longitudinal magnetic-field zone and capable of 
evacuating the air. It is also possible to utilize a difference in boiling 
point between the organic solvents used in the magnetic coating fluid and 
the nonmagnetic coating fluid; namely, a low-boiling organic solvent and a 
high-boiling organic solvent may be used in combination. Although the 
degree of drying of the coating formed on the support is difficult to 
determine, it can be estimated from a measurement of the concentration of 
the organic solvents in the gas discharged from the magnetic-field zone 
for longitudinal orientation. Specifically, the orientation zone is 
preferably constructed so that about from 70 to 90% of the organic 
solvents contained in the applied magnetic and nonmagnetic coating fluids 
can be evaporated. 
The hexagonal ferrite magnetic particles for use in the upper layer are 
then explained below. 
Examples of the hexagonal ferrite contained in the upper layer in the 
present invention include substitutional ferrites such as barium ferrite, 
strontium ferrite, lead ferrite, and calcium ferrite, and Co-substituted 
ferrites. Specific examples thereof include barium ferrite and strontium 
ferrite both of the magnetoplumbite type and barium ferrite and strontium 
ferrite both of the magnetoplumbite type containing a spinel phase as a 
part thereof. Besides the constituent atoms, these ferrites may contain 
other atoms such as, e.g., Al, Si, S, Sc, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, 
Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg, Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, 
Zn, Ni, Sr, B, Ge, and Nb. In general, hexagonal ferrites containing 
elements such as Co--Ti, Co--Ti--Zr, Co--Ti--Zn, Ni--TiZn, and Nb--Zn, can 
be used. The longitudinal-direction SFD of the upper layer is preferably 
0.3 or less, because such an SFD value advantageously results in a 
narrower coercive-force distribution. The coercive force can be 
controlled, for example, by regulating the particle diameter or particle 
thickness of the hexagonal ferrite, by regulating the thickness of the 
spinal phase of the hexagonal ferrite, by regulating the amount of a 
substituent element in the spinel phase, or by changing the substitution 
sites in the spinel phase. The hexagonal ferrite for use in the present 
invention is usually in the form of hexagonal platy particles; the 
diameter of these particles, which means the width of the hexagonal 
plates, is determined with an electron microscope. 
In the present invention, the particle diameter (plate diameter) of the 
hexagonal ferrite is usually from 0.01 to 0.2 .mu.m, preferably from 0.02 
to 0.1 .mu.m. The average thickness (plate thickness) of the fine 
particles is usually from 0.001 to 0.2 .mu.m, preferably from 0.003 to 
0.05 .mu.m. The aspect ratio thereof regarding degree of flatness 
(particle diameter/plate thickness) is from 1 to 15, preferably from 3 to 
7. The crystallite size thereof is from 50 to 450 .ANG., preferably from 
100 to 350 .ANG.. Further, the specific surface area of these fine 
hexagonal-ferrite particles as measured by the BET method (S.sub.BET) is 
from 25 to 100 m.sup.2 /g, preferably from 40 to 70 m.sup.2 /g. Specific 
surface areas thereof less than 25 m.sup.2 /g are undesirable in that an 
increased noise results, while specific surface areas thereof exceeding 
100 m.sup.2 /g are undesirable in that satisfactory surface properties are 
difficult to obtain. The magnetic particles preferably have a water 
content of from 0.01 to 2%; the water content thereof is preferably 
optimized according to the kind of the binder used. The pH of the magnetic 
particles, which is preferably optimized according to the kind of the 
binder used, is from 4 to 12, preferably from 6 to 10. If desired and 
needed, the surface of the magnetic particles may be treated, for example, 
with Al, Si, P, or oxide thereof. Preferred is a surface treatment with 
Al.sub.2 O.sub.3 or SiO.sub.2. The amount or proportion of the 
surface-treating agent, which is desirably varied according to the kind of 
the binder used, is from 0.1 to 10% based on the amount of the magnetic 
particles. This surface treatment is advantageous in that it reduces the 
adsorption of a lubricant, e.g., a fatty acid, to 100 mg/m.sup.2 or less. 
Although there are cases where the magnetic particles contain soluble 
inorganic ions of, e.g., Na, Ca, Fe, Ni, and Sr, these ions do not 
particularly influence the properties of the upper layer as long as the 
concentration thereof is 500 ppm or less. The magnetic particles have a 
.sigma.s of 50 emu/g or more, preferably 60 emu/g or more, and a tap 
density of preferably 0.5 g/cc or more, more preferably 0.8 g/cc or more. 
For producing the hexagonal ferrite for use in the present invention, any 
of various methods may be used such as a glass crystallization method, a 
coprecipitation method, and a hydrothermal reaction method. 
Known ferromagnetic particles may be employed as the ferromagnetic 
particles used in the lower magnetic layer in the present invention. 
Examples thereof include .gamma.-FeO.sub.x (.sub.x =1.33-1.5), Co-modified 
.gamma.-FeO.sub.x (.sub.x =1.33-1.5), ferromagnetic alloy fine particles 
containing Fe, Ni, or Co as the main component (75% or more), and acicular 
barium ferrite. Preferred of these are particles of a ferromagnetic alloy 
containing .alpha.-Fe as the main component and the Co-modified 
.gamma.-FeOx. Besides the atoms specified above, the ferromagnetic 
particles may contain other atoms such as Al, Si, S, Sc, Ti, V, Cr, Cu, Y, 
Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg, Pb, Bi, La, Ce, Pr, Nd, 
P, Co, Mn, Zn, Ni, Sr, B, Ge, and Nb. 
Before being dispersed, the fine ferromagnetic particles may be treated 
with, for example, a dispersant, a lubricant, a surfactant, or an 
antistatic agent. These treatments are described in, for example, 
JP-B-44-14090, JP-B-45-18372, JP-B-47-22062, JP-B-47-22513, JP-B-46-28466, 
JP-B-46-38755, JP-B-47-4286, JP-B-47-12422, JP-B-47-17284, JP-B-47-18509, 
JP-B-47-18573, JP-B-39-10307, JP-B-48-39639, and U.S. Pat. Nos. 3,026,215, 
3,031,341, 3,100,194, 3,242,005, and 3,389,014. (The term "JP-B" as used 
herein means an "examined Japanese patent publication.") 
The fine ferromagnetic alloy particles among the ferromagnetic particles of 
the above-enumerated kinds may contain a small amount of hydroxide or 
oxide. For obtaining the fine ferromagnetic alloy particles for use in the 
present invention, a known method may be used. Examples thereof include: a 
method in which reduction is conducted with an organic acid double salt 
(consisting mainly of oxalate) and a reducing gas such as hydrogen; a 
method comprising reducing iron oxide with a reducing gas such as hydrogen 
to obtain Fe or Fe-Co particles; a method comprising pyrolyzing a metal 
carbonyl compound; a method in which reduction is conducted by adding a 
reducing agent such as sodium boron hydride, hypophosphite, or hydrazine 
to an aqueous solution of ferromagnetic metal; and a method comprising 
vaporizing metal in a low-pressure inert gas to obtain fine particles. The 
thus-obtained ferromagnetic alloy particles may be used after undergoing a 
known gradual oxidation treatment. This treatment can be conducted by any 
of the following: a method comprising immersing the particles in an 
organic solvent, followed by drying; a method comprising immersing the 
particles in an organic solvent and feeding an oxygen-containing gas to 
form an oxide film on the surfaces, followed by drying; and a method in 
which an oxide film is formed on the surfaces by controlling the partial 
pressures of oxygen gas and an inert gas, without using an organic 
solvent. The ferromagnetic particles have a specific surface area as 
determined by the BET method of from 25 to 80 m.sup.2 /g, preferably from 
40 to 70 m.sup.2 /g. Specific surface areas thereof less than 25 m.sup.2 
/g are undesirable in that an increased noise results, while specific 
surface areas thereof more than 80 m.sup.2 /g are undesirable in that 
satisfactory surface properties are difficult to obtain. The us of the 
magnetic iron oxide particles is 50 emu/g or more, preferably 70 emu/g or 
more, while the .sigma.s of the fine ferromagnetic metal particles is 
preferably 100 emu/g or more, more preferably from 110 to 170 emu/g. The 
coercive force thereof is preferably from 500 to 2,500 Oe, more preferably 
from 800 to 2,000 Oe. 
The tap density of y-iron oxide is preferably 0.5 g/cc or more, more 
preferably 0.8 g/cc or more. In alloy particles, the tap density thereof 
is preferably from 0.2 to 0.8 g/cc. Tap densities of alloy particles more 
than 0.8 g/cc tend to result in acceleration of the oxidation of the 
ferromagnetic particles during compaction, so that a sufficient so is 
difficult to obtain. If the tap density of alloy particles is less than 
0.2 g/cc, insufficient dispersion tends to result. In using .gamma.-iron 
oxide, the proportion of divalent iron to trivalent iron is preferably 
from 0 to 20%, more preferably from 5 to 10%. Further, the amount of 
cobalt atoms is from 0 to 15%, preferably from 2 to 8%, based on the 
amount of iron atoms. 
The lower magnetic layer formed under the upper magnetic layer containing a 
hexagonal ferrite in the magnetic recording medium of the present 
invention preferably has a higher degree of orientation in the 
longitudinal direction than in the vertical direction. The lower magnetic 
layer preferably has a coercive force of from 500 to 2,500 Oe, a 
squareness ratio of from 0.6 to 0.95, a Br of from 1,000 to 4,000 G, and 
an SFD of 0.6 or lower. 
The center-line average surface roughness of each of the lower layer and 
the upper layer in the present invention is preferably 0.006 .mu.m or 
less, if the individual layers (which each may have a multilayer 
structure) are separately coated. 
The lower nonmagnetic layer is explained below. 
The inorganic nonmagnetic particles for use in the lower nonmagnetic layer 
of the magnetic recording medium of the present invention can be selected 
from inorganic compounds such as metal oxides, metal carbonates, metal 
sulfates, metal nitrides, metal carbides, and metal sulfides. Specific 
examples of such inorganic compounds include m-alumina having an 
.alpha.-alumina structure content of 90% or more, .beta.-alumina, 
.gamma.-alumina, silicon carbide, chromium oxide, cerium oxide, 
.alpha.-iron oxide, corundum, silicon nitride, titanium carbide, titanium 
oxide, silicon dioxide, tin oxide, magnesium oxide, tungsten oxide, 
zirconium oxide, boron nitride, zinc oxide, calcium carbonate, calcium 
sulfate, barium sulfate, and molybdenum disulfide. These may be used alone 
or in combination. Particularly preferred of these are titanium dioxide, 
zinc oxide, iron oxide, and barium sulfate. The particle sizes of these 
nonmagnetic particles are preferably from 0.005 to 2 .mu.m. It is, 
however, possible to use a combination of two or more kinds of nonmagnetic 
particles having different particle sizes if desired and needed. 
Alternatively, the same effect can be produced by using one kind of 
nonmagnetic particles having a wide particle diameter distribution. The 
especially preferred range of the particle size is from 0.01 to 0.2 .mu.m. 
The tap density thereof is from 0.05 to 2 g/cc, preferably from 0.2 to 1.5 
g/cc. The water content thereof is from 0.1 to 5%, preferably from 0.2 to 
3%. The pH thereof is from 2 to 11, especially preferably from 6 to 9. The 
specific surface area thereof is from 1 to 100 m.sup.2 /g, preferably from 
5 to 50 m.sup.2 /g, and more preferably from 7 to 40 m.sup.2 /g. The 
crystallite size thereof is preferably from 0.01 to 2 .mu.m. The oil 
absorption thereof as measured with DBP is from 5 to 100 ml/100g, 
preferably from 10 to 80 ml/100g, and more preferably from 20 to 60 
ml/100g. The specific gravity thereof is from 1 to 12, preferably from 3 
to 6. The particle shape thereof may be any of the acicular, spherical, 
polyhedral, platy, or hexagonal platy shapes. In acicular particles, the 
aspect ratio thereof is preferably from 2 to 15. The ignition loss thereof 
is preferably 20% or lower. The inorganic particles for use in the present 
invention preferably have a Mohs' hardness of from 4 to 10. The roughness 
factors of the surfaces of these kinds of particles are desirably from 0.8 
to 1.5, preferably from 0.9 to 1.2. The SA adsorption thereof is from 1 to 
20 .mu.mol/m.sup.2, preferably from 2 to 15 .mu.mol/m.sup.2. The 
nonmagnetic particles for use in the lower layer preferably have a heat of 
wetting by water in the range of from 200 to 600 erg/cm.sup.2 at 
25.degree. C. A solvent which gives a heat of wetting in the above range 
can be used. The appropriate number of water molecules present on the 
surfaces thereof at 100.degree.to 400.degree. C. is from 1 to 10 per 100 
.ANG..sup.2. The isoelectric-point pH thereof in water is preferably from 
3 to 6. The surfaces of these particles are preferably treated with 
Al.sub.2 O.sub.3, SiO.sub.2, TiO.sub.2, ZrO.sub.2, SnO.sub.2, Sb.sub.2 
O.sub.3, or ZnO. Of these, Al.sub.2 O.sub.3, SiO.sub.2, TiO.sub.2, and 
ZrO.sub.2 are preferred from the standpoint of dispersibility, with 
Al.sub.2 O.sub.3, SiO.sub.2, and ZrO.sub.2 being particularly preferred. 
These may be used in combination or alone. A treated surface layer formed 
by coprecipitation may be used according to purpose. It is also possible 
to use a treated surface layer having a structure formed by first treating 
with alumina and then treating the resulting surface layer with silica, or 
to use a treated surface layer having a structure which is the reverse of 
the above structure. Although the treated surface layer may be made porous 
if desired and needed, a homogeneous and dense surface layer is generally 
preferred. 
Specific examples of inorganic nonmagnetic particles for use in the present 
invention include UA5600, UA5605, and Nanotite manufactured by Showa Denko 
K.K.; AKP-20, AKP-30, AKP-50, HIT-55, HIT-100, and ZA-G1 manufactured by 
Sumitomo Chemical Co., Ltd.; G5, G7, and S-1 manufactured by Nippon 
Chemical Industrial Co., Ltd.; TF-100, TF-120, TF-140, R516, DPN250, and 
DPN250BX manufactured by Toda Kogyo Co., Ltd.; TTO-51B, TTO-55A, TTO-55B, 
TTO-55C, TTO-55S, TTO-55D, FT-1000, FT-2000, FTL-100, FTL-200, M-1, S-1, 
SN-100, R-820, R-830, R-930, R-550, CR-50, CR-80, R-680, and TY-50 
manufactured by Ishihara Sangyo Kaisha, Ltd.; ECT-52, STT-4D, STT-30D, 
STT-30, and STT-65C manufactured by Titan Kogyo K.K.; T-1 manufactured by 
Mitsubishi Material Co., Ltd.; NS-0, NS-3Y, and NS-8Y manufactured by 
Nippon Shokubai Kagaku Kogyo Co., Ltd.; MT-100S, MT-100T, MT-150W, 
MT-500B, MT-600B, and MT-100F manufactured by Teika Co., Ltd.; FINEX-25, 
BF-1, BF-10, BF-20, BF-1L, and BF-10P manufactured by Sakai Chemical 
Industry Co., Ltd.; DEFIC-Y and DEFIC-R manufactured by Dowa Mining Co., 
Ltd.; and Y-LOP manufactured by Titan Kogyo K.K. and nonmagnetic particles 
obtained by calcining them. 
Especially preferred inorganic nonmagnetic particles are titanium dioxide 
particles. Hence, titanium dioxide is described in detail with respect to 
production processes thereof. For producing titanium dioxide products, the 
sulfuric acid process and the chlorine process are mainly used. In the 
sulfuric acid process, a raw ore of ilmenite is leached with sulfuric acid 
to extract, e.g., Ti, Fe as sulfates. The iron sulfate is removed by 
crystallization, and the remaining titanyl sulfate solution is purified by 
filtration and then subjected to hydrolysis with heating to thereby 
precipitate hydrous titanium oxide. This precipitate is separated by 
filtration and then washed to remove impurities. Calcination of the 
resulting precipitate at 80 to 1,000.degree. C after addition of a 
particle size regulator or the like gives crude titanium oxide. The 
titanium oxide is of rutile form or anatase form according to the 
nucleating agent added in hydrolysis. This crude titanium oxide is ground, 
sieved, and subjected to, e.g., surface treatment, thereby to produce a 
titanium dioxide product. In the chlorine process, natural or synthetic 
rutile is used as the raw ore. The ore is chlorinated under 
high-temperature reducing conditions to convert the Ti to TIC14 and the Fe 
to FeCl.sub.2, and the iron chloride is solidified by cooling and 
separated from the liquid TICl.sub.4. The crude TICl.sub.4 obtained is 
purified by rectification and a nucleating agent is added thereto. This 
crude TICl.sub.4 is instantaneously reacted with oxygen at a temperature 
of 1,000.degree. C. or more to obtain crude titanium oxide. For imparting 
pigmenting properties to the crude titanium oxide yielded in the above 
oxidative decomposition step, the same finishing technique as in the 
sulfuric acid process is employed. The surface treatment of the titanium 
oxide material may be conducted as follows. The material is dry-ground, 
and water and a dispersant are then added thereto. The resulting slurry is 
subjected to wet grinding, followed by centrifugal separation to separate 
coarse particles. The resulting slurry of fine particles is then 
transferred to a surface treatment tank, where surface covering with a 
metal hydroxide is performed. First, an aqueous solution of a 
predetermined amount of a salt of, e.g., Al, Si, Ti, Zr, Sb, Sn, Zn is 
added to the slurry and an acid or alkali is added to neutralize the 
resulting slurry to thereby form a hydrous oxide and cover the surfaces of 
the titanium oxide particles with the oxide. The water-soluble salts 
formed as by-products are removed by decantation, filtration, and washing. 
The slurry is subjected to final pH adjustment, filtration, and washing 
with pure water. The resulting cake is dried with a spray dryer or band 
dryer. Finally, the dry particles are ground with a jet mill to give a 
product. In place of such a wet process, the surface treatment can be 
conducted by passing vapors of AlCl.sub.3 and SiCl.sub.4 through titanium 
oxide particles and then passing water vapor to treat the particle 
surfaces with Al and Si. With respect to processes for the production of 
other pigments, reference may be made to Characterization of Powder 
Surfaces, published by Academic Press. 
Carbon black may be incorporated into the lower layer, whereby the known 
effect of reducing Rs can be produced. For this purpose, carbon black such 
as furnace black for rubbers, thermal black for rubbers, coloring black, 
and acetylene black can be used. The specific surface area of the carbon 
black is from 100 to 500 m.sup.2 /g, preferably from 150 to 400 m.sup.2 
/g, and the DBP absorption thereof is from 20 to 400 ml/100g, preferably 
from 30 to 200 ml/100g. The particle diameter thereof is from 5 to 80 
m.mu., preferably from 10 to 50 m.mu., and more preferably from 10 to 40 
m.mu.. The carbon black preferably has a pH of from 2 to 10, a water 
content of from 0.1 to 10%, and a tap density of from 0.1 to 1 g/cc. 
Specific examples of carbon black for use in the present invention include 
BLACKPEARLS 2000, 1300, 1000, 900, 800, 880, 700, and VULCAN XC-72 
manufactured by Cabot Corporation; #3050B, #3150B, 3250B, #3750B, #3950B, 
#950, #650B, #970B, #850B, and MA-600 manufactured by Mitsubishi Kasei 
Corporation; CONDUCTEX SC manufactured by Columbia Carbon Co.; RAVEN 8800, 
8000, 7000, 5750, 5250, 3500, 2100, 2000, 1800, 1500, 1255, and 1250 
manufactured by RAVEN; and Ketjen Black EC manufactured by Lion Akzo Co., 
Ltd. These carbon blacks may be surface-treated with a dispersant or 
another agent or grafted with a resin before use. Carbon black whose 
surfaces have been partly graphitized may also be used. Further, before 
being added to a coating fluid, the carbon black may be dispersed into a 
binder. These carbon blacks can be used in an amount of 50% or less by 
weight based on the inorganic particles and 40% or less based on the total 
weight of the nonmagnetic layer. These carbon blacks can be used alone or 
in combination. With respect to carbon blacks usable in the present 
invention, reference may be made to, for example, Carbon Black Binran 
(Carbon Black Handbook), edited by Carbon Black Association. 
Organic particles for use in the present invention include acrylic-styrene 
resin particles, benzoguanamine resin particles, melamine resin particles, 
and phthalocyanine pigments. Other usable examples thereof include 
polyolefin resin particles, polyester resin particles, polyamide resin 
particles, polyimide resin particles, and poly(ethylene fluoride) resins. 
For producing these organic particles, techniques such as those described 
in JP-A-62-18564 and JP-A-60-255827 can be used. 
It should be noted that although an undercoat layer is provided in ordinary 
magnetic recording media, this undercoat layer, which has a thickness of 
0.5 .mu.m or less, is intended to improve adhesion between the support and 
the magnetic or another layer and is different from the lower layer in the 
present invention. In the present invention also, an undercoat layer is 
preferably provided to improve adhesion between the lower layer and the 
support. 
The same binders, lubricants, dispersants, additives, solvents, and 
dispersing techniques as those for the upper magnetic layer can be used 
for the lower nonmagnetic layer. In particular, with respect to the 
amounts and kinds of binders and the amounts and kinds of additives and 
dispersants, known techniques usable for magnetic layers can be applied. 
The thickness of the lower nonmagnetic layer is from 0.2 to 5 .mu.m, 
preferably from 1 to 3 .mu.m. 
The binder for use in the upper layer and the lower layer in the present 
invention may be a conventionally known thermoplastic resin, thermosetting 
resin, or reactive resin, or a mixture thereof. 
The thermoplastic resin may be one having a glass transition temperature of 
from -100.degree.to 150.degree. C., a number-average molecular weight of 
from 1,000 to 200,000, preferably from 10,000 to 100,000, and a degree of 
polymerization of about from 50 to 1,000. Examples of the thermoplastic 
resins include polymers or. copolymers containing a structural unit 
derived from vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, 
acrylic acid, acrylic ester, vinylidene chloride, acrylonitrile, 
methacrylic acid, methacrylate, styrene, butadiene, ethylene, vinyl 
butyral, vinyl acetal, or vinyl ether, polyurethane resins, and various 
rubber-type resins. 
Examples of the thermosetting or reactive resin include phenolic resins, 
epoxy resins, thermosetting polyurethane resins, urea resins, melamine 
resins, alkyd resins, reactive acrylic resins, formaldehyde resins, 
silicone resins, epoxypolyamide resins, mixtures of polyester resin and 
isocyanate prepolymer, mixtures of polyester polyol and polyisocyanate, 
and mixtures of polyurethane and polyisocyanate. These resins are 
described in detail in Plastic Handbook published by Asakura Shoten. It is 
also possible to use a known resin of the electron beam-curing type for 
each of the layers. Examples of the resins and production processes 
therefor are described in detail in JP-A-62-256219. 
The resins enumerated above can be used alone or in combination. Preferred 
examples of those include combinations of a polyurethane resin with at 
least one member selected from vinyl chloride resins, vinyl chloride-vinyl 
acetate resins, vinyl chloride-vinyl acetate-vinyl alcohol resins, and 
vinyl chloride-vinyl acetate-maleic anhydride copolymers, and further 
include combinations of these with polyisocyanate. 
The polyurethane resins may have a known structure such as polyester 
polyurethane, polyether polyurethane, polyether polyester polyurethane, 
polycarbonate polyurethane, polyester polycarbonate polyurethane, or 
polycaprolactone polyurethane. For obtaining further improved 
dispersibility and durability, it is preferred to use, according to need, 
one or more of the above-enumerated binders which have, incorporated 
therein through copolymerization or addition reaction, at least one polar 
group selected from --COOM, --SO.sub.3 M, --OSO.sub.3 M, 
--P.dbd.O(OM).sub.2, --O--P.dbd.O(OM).sub.2 (where M is a hydrogen atom or 
an alkali metal salt group), --OH, --NR.sub.2, --N.sup.+ r.sub.3 (R 
represents a hydrocarbon group), an epoxy group, --SH, and --CN. The 
amount of the polar group(s) is from 10.sup.-1 to 10.sup.-8 mol/g, 
preferably from 10.sup.-2 to 10.sup.-6 mol/g. 
Specific examples of those binders for use in the present invention include 
VAGH, VYHH, VMCH, VAGF, VAGD, VROH, VYES, VYNC, VMCC, XYHL, XYSG, PKHH, 
PKHJ, PKHC, and PKFE manufactured by Union Carbide Corp.; MPR-TA, MPR-TA5, 
MPR-TAL, MPR-TSN, MPR-TMF, MPR-TS, MPR-TM, and MPR-TAO manufactured by 
Nisshin Chemical Industry Co., Ltd.; 1000W, DX80, DX81, DX82, DX83, and 
100FD manufactured by Denki Kagaku Kogyo K.K.; MR-105, MR110, MR100, and 
400X-110A manufactured by Nippon Zeon Co., Ltd.; Nippolan N2301, N2302, 
and N2304 manufactured by Nippon Polyurethane Industry Co. Ltd.; Pandex 
T-5105, T-R3080, T-5201, Burnock D-400, D-210-80, Crisvon 6109, and 7209 
manufactured by Dainippon Ink & Chemicals, Inc.; Vylon UR8200, UR8300, 
UR8600, UR5500, UR4300, RV530, and RV280 manufactured by Toyobo Co., Ltd.; 
Daipheramin 4020, 5020, 5100, 5300, 9020, 9022, and 7020 manufactured by 
Dainichiseika Color & Chemicals Mfg., Co., Ltd.; MX5004 manufactured by 
Mitsubishi Kasei Corporation; Sunprene SP-150, TIM-3003, and TIM-3005 
manufactured by Sanyo Chemical Industries, Co., Ltd.; and Saran F310 and 
F210 manufactured by Asahi Chemical Industry Co., Ltd. 
The amount of the binder used in the lower nonmagnetic layer or in the 
lower or upper magnetic layer in the present invention is from 5 to 50% by 
weight, preferably from 10 to 30% by weight, based on the amount of the 
nonmagnetic particles or the ferromagnetic particles, respectively. In 
employing a vinyl chloride resin, it is preferred to use the same in an 
amount of from 5 to 30% by weight in combination with from 2 to 20% by 
weight polyurethane resin and from 2 to 20% by weight polyisocyanate. In 
using polyurethane in the present invention, this resin preferably has a 
glass transition temperature of from -50.degree.to 100.degree. C., an 
elongation at break of from 100 to 2,000%, a stress at break of from 0.05 
to 10 kg/cm.sup.2, and a yield point of from 0.05 to 10 kg/cm.sup.2. 
The magnetic recording medium of the present invention has one or more 
layers. It is, of course, possible to form the nonmagnetic layer and the 
magnetic layers so that these layers differ from each other in binder 
amount, the proportion of a vinyl chloride resin, polyurethane resin, 
polyisocyanate, or another resin in the binder, the molecular weight of 
each resin contained in each magnetic layer, polar group amount, the 
aforementioned physical properties of resin according to need. For 
attaining this, known techniques concerning multilayered magnetic layers 
are applicable. For example, in the case of forming layers having 
different binder amounts, an increase in binder amount in the upper 
magnetic layer is effective in diminishing the marring of the upper 
magnetic layer surface, while an increase in binder amount in either the 
upper magnetic layer or the lower nonmagnetic layer to impart flexibility 
is effective in improving head touching. 
Examples of the polyisocyanate for use in the constituent layers of the 
magnetic recording medium of the present invention include isocyanates 
such as tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, 
hexamethylene diisocyanate, xylylene diisocyanate, naphthylene 
1,5-diisocyanate, o-toluidine diisocyanate, isophorone diisocyanate, and 
triphenylmethane triisocyanate, products of the reactions of these 
isocyanates with polyalcohols, and polyisocyanates formed through 
condensation of isocyanates. These isocyanates are commercially available 
under the trade names of: Coronate L, Coronate HL, Coronate 2030, Coronate 
2031, Millionate MR, and Millionate MTL manufactured by Nippon 
Polyurethane Co., Ltd.; Takenate D-102, Takenate D-110N, Takenate D-200, 
and Takenate D-202 manufactured by Takeda Chemical Industries, Ltd.; and 
Desmodule L, Desmodule IL, Desmodule N, and Desmodule HL manufactured by 
Sumitomo Bayer Co., Ltd. For each of the layers, these polyisocyanates may 
be used alone, or used in combination of two or more thereof, taking 
advantage of a difference in curing reactivity. 
The carbon black for use in the magnetic layer in the present invention 
includes furnace black for rubbers, thermal black for rubbers, coloring 
black, and acetylene black. The carbon black preferably has a specific 
surface area of from 5 to 500 m.sup.2 /g, a DBP absorption of from 10 to 
400 ml/100g, a particle diameter of from 5 to 300 m.mu., a pH of from 2 to 
10, a water content of from 0.1 to 10%, and a tap density of from 0.1 to 1 
g/cc. Specific examples of carbon blacks usable in the present invention 
include BLACKPEARLS 2000, 1300, 1000, 900, 800, 700, and VULCAN XC-72 
manufactured by Cabot Corporation; #80, #60, #55, #50, and #35 
manufactured by Asahi Carbon Co., Ltd.; #2400B, #2300, #900, #1000, #30, 
#40, and #10B manufactured by Mitsubishi Kasei Corporation; and CONDUCTEX 
SC manufactured by Columbia Carbon Co.; and RAVEN 150, 50, 40, and 15 
manufactured by RAVEN. These carbon blacks may be surface-treated with a 
dispersant or another agent or grafted with a resin before use. A carbon 
black whose surfaces have been partly graphitized may also be used. 
Further, before being added to a magnetic coating fluid, the carbon black 
may be dispersed into a binder. These carbon blacks can be used alone or 
in combination. The carbon black is preferably used in an amount of from 
0.1 to 30% by weight based on the amount of the ferromagnetic particles. 
The carbon black incorporated in the magnetic layer functions to prevent 
static buildup in the layer, to reduce the coefficient of friction of the 
layer, as a light screen for the layer, and to improve the strength of the 
layer. Such effects are produced to different degrees depending on the 
kind of carbon black used. Therefore it is, of course, possible in the 
present invention to properly use carbon blacks according to the purpose 
so as to give an upper magnetic layer, a lower nonmagnetic layer, and a 
lower magnetic layer which differ in the kind, amount, and combination of 
carbon blacks, on the basis of the above-described properties including 
particle size, oil absorption, electrical conductivity, and pH. With 
respect to carbon blacks usable in the magnetic layer in the present 
invention, reference may be made to, for example, Carbon Black Binran 
(Carbon Black Handbook) edited by Carbon Black Association. 
In the present invention, an abrasive material is used in the upper 
magnetic layer and may also be used in the lower magnetic layer. Known 
abrasive materials mostly having a Mohs' hardness of 6 or more can be used 
alone or in combination. Examples thereof include .alpha.-alumina having 
an .alpha.-alumina structure content of 90% or more, .beta.-alumina, 
silicon carbide, chromium oxide, cerium oxide, .alpha.-iron oxide, 
corundum, artificial diamond, silicon nitride, silicon carbide, titanium 
carbide, titanium oxide, silicon dioxide, and boron nitride. A composite 
made up of two or more of these abrasive materials (e.g., one obtained by 
surface-treating one abrasive material with another) may also be used. 
Although in some cases these abrasive materials contain compounds or 
elements other than the main component, the same effect is obtained with 
such abrasive materials as long as the content of the main component is 
90% or more. These abrasive materials preferably have a particle size of 
from 0.01 to 2 .mu.m. If desired and needed, abrasive materials having 
different particle sizes may be used in combination, or a single abrasive 
material having a widened particle diameter distribution may be used so as 
to produce the same effect. The abrasive material preferably has a tap 
density of from 0.3 to 2 g/cc, a water content of from 0.1 to 5%, a pH of 
from 2 to 11, and a specific surface area of from 1 to 30 m.sup.2 /g. 
Although abrasive materials that can be used in the present invention may 
have any particle shape selected from the acicular, particulate, 
spherical, and cubical forms, a particle shape having a sharp corner as 
part of the contour is preferred because abrasive materials of this shape 
have high abrasive properties. 
Part or all of the additives to be used in the present invention may be 
added at any step in a process for producing a magnetic or nonmagnetic 
coating fluid. For example, it is possible: to mix the additives with 
ferromagnetic particles prior to a kneading step; to add the additives 
during the kneading of ferromagnetic particles, a binder, and a solvent; 
to add the additives at a dispersing step; to add the additives after 
dispersion; or to add the additives immediately before coating. There are 
cases where the purpose is achieved by applying part or all of the 
additives, according to the purpose, by simultaneous or successive coating 
after magnetic layer application. Further, it is possible, according to 
purpose, to apply a lubricant on the magnetic layer surface after 
calendering or slitting. 
Examples of marketed lubricant products for use in the present invention 
include NAA-102, NAA-415, NAA-312, NAA-160, NAA-180, NAA-174, NAA-175, 
NAA-222, NAA-34, NAA-35, NAA-171, NAA-122, NAA-142, NAA-160, NAA-173K, 
hardened castor oil fatty acid, NAA-42, NAA-44, Cation SA, Cation MA, 
Cation AB, Cation BB, Naymeen L-201, Naymeen L-202, Naymeen S-202, Nonion 
E-208, Nonion P-208, Nonion S-207, Nonion K-204, Nonion NS-202, Nonion 
NS-210, Nonion HS-206, Nonion L-2, Nonion S-2, Nonion S-4, Nonion 0-2, 
Nonion LP-20R, Nonion PP-40R, Nonion SP-60R, Nonion OP-80R, Nonion OP-85R, 
Nonion LT-221, Nonion ST-221, Nonion OT-221, Monoguri MB, Nonion DS-60, 
Anon BF, Anon LG, butyl stearate, butyl laurate, and erucic acid 
manufactured by NOF Corporation; oleic acid manufactured by Kanto Chemical 
Co., Ltd.; FAL-205 and FAL-123 manufactured by Takemoro Yushi Co., Ltd.; 
Enujerub LO, Enujerub IPM, and Sansosyzer E4043 manufactured by Shin Nihon 
Rika Co., Ltd.; TA-3, KF-96, KF-96L, KF-96H, KF410, KF420, KF965, KF54, 
KF50, KF56, KF-907, KF851, X-22-819, X-22-822, KF905, KF700, KF393, 
KF-857, KF-860, KF-865, X-22-980, KF-101, KF-102, KF-103, X-22-3710, 
X-22-3715, KF-910, and KF-3935 manufactured by Shin-Etsu Chemical Co., 
Ltd.; Armide P, Armide C, and Armoslip CP manufactured by Lion Ahmer Co., 
Ltd.; Duomin TDO manufactured by Lion Fat and Oil Co., Ltd.; BA-41G 
manufactured by Nisshin Oil Mills Co., Ltd.,; and Profan 2021E, Newpole 
PE61, Ionet MS-400, Ionet MO-200, Ionet DL-200, Ionet DS-300, Ionet 
DS-1000, and Ionet DO-200 manufactured by Sanyo Chemical Co., Ltd. 
Examples of organic solvents for use in the present invention include 
ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, 
diisobutyl ketone, cyclohexanone, isophorone, and tetrahydrofuran; 
alcohols such as methanol, ethanol, propanol, butanol, isobutyl alcohol, 
isopropyl alcohol, and methylcyclohexanol; esters such as methyl acetate, 
butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate, and 
glycol acetate; glycol ethers such as glycol dimethyl ethers, glycol 
monoethyl ethers, and dioxane; aromatic hydrocarbons such as benzene, 
toluene, xylene, cresol, and chlorobenzene; chlorinated hydrocarbons such 
as methylene chloride, ethylene chloride, carbon tetrachloride, 
chloroform, ethylene chlorohydrin, and dichlorobenzene; and other 
compounds such as N,N-dimethylformamide and hexane. These solvents may be 
used in arbitrary proportions. These organic solvents need not be 100% 
pure, and may contain impurities, such as isomers, unreacted raw 
materials, by-products, decomposition products, oxidation products, and 
water, besides the main components. The content of these impurities is 
preferably 30% or less, more preferably 10% or less. It is preferred in 
the present invention that the organic solvent used for the upper layer be 
of the same kind as that used for the lower layer. Different solvent 
amounts may be used for the two layers. It is important that solvents 
having higher surface tensions (e.g., cyclohexanone, dioxane) should be 
used for the lower layer to enhance the stability of coating. 
Specifically, the arithmetic mean thereof for the upper layer solvents 
should be not lower than that for the lower layer solvents. From the 
standpoint of improving dispersibility, solvents which are polar to some 
degree are preferred, and a preferred solvent composition is one at least 
50% of which is accounted for by one or more solvents having a dielectric 
constant of 15 or more. The preferred range of solubility parameter is 
from 8 to 11. 
The thickness of each constituent layer of the magnetic recording medium 
according to the present invention is as follows. The thickness of the 
nonmagnetic support is from 1 to 100 .mu.m, preferably from 4 to 20 .mu.m. 
The total thickness of the upper layer and the lower layer is from 1/100 
to 2 times the thickness of the nonmagnetic support. An undercoat layer 
may be provided between the nonmagnetic support and the lower layer in 
order to improve adhesion. The thickness of this undercoat layer may be 
from 0.01 to 2 .mu.m, preferably from 0.02 to 0.5 .mu.m. Further, a back 
coat layer may be provided on the nonmagnetic support on the side opposite 
to the magnetic layer. The thickness of this back coat layer may be from 
0.1 to 2 .mu.m, preferably from 0.3 to 1.0 .mu.m. These undercoat layer 
and back coat layer may be the same as known ones. The nonmagnetic support 
for use in the present invention may be a known film. Examples thereof 
include films of polyesters such as poly(ethylene terephthalate) and 
poly(ethylene naphthalate), polyolefins, cellulose triacetate, 
polycarbonates, polyamides, polyimides, poly(amide-imide)s, polysulfone, 
aramids, aromatic polyamides, and polybenzoxazole. In using a thin support 
having a thickness of 7 .mu.m or less, the support is preferably made of a 
high-strength material such as poly(ethylene naphthalate) or polyamide. If 
desired and needed, a laminate support such as that described in 
JP-A-3-224127 may be used in order that the magnetic layer surface and the 
base surface have different surface roughnesses. These supports may be 
subjected beforehand to, e.g., corona discharge treatment, plasma 
treatment, adhesion-promoting treatment, heat treatment, dust-removing 
treatment. In order to attain the objects of the present invention, it is 
preferred to employ a nonmagnetic support having a center-line average 
surface roughness of 0.03 .mu.m or less, preferably 0.01 .mu.m or less, 
and more preferably 0.005 .mu.m or less, as measured at a cut-off of 0.08 
mm. In addition to the requirement of low center-line average surface 
roughness, the nonmagnetic supports are required to be preferably free 
from projections as large as 1 .mu.m or more. The state of the surface 
roughness of the support can be freely controlled by changing the size and 
amount of a filler which is incorporated into the support if desired and 
needed. Examples of the filler include oxides or carbonates of Ca, Si, and 
Ti and fine organic powders such as acrylic powder. The support preferably 
has a maximum height SR.sub.max of 1 .mu.m or less, a ten-point average 
roughness SR.sub.z of 0.5 .mu.m or less, a center-plane peak height SRp of 
0.5 .mu.m or less, a center-plane valley depth SRv of 0.5 .mu.m or less, a 
center-plane areal ratio SSr of from 10% to 90%, and an average wavelength 
S.lambda.a of from 5 .mu.m to 300 .mu.m. The number of surface projections 
having a size of from 0.01 to 1 .mu.m present on these supports can be 
controlled with a filler of from 0 to 2,000 per 0.1 mm.sup.2. 
The nonmagnetic support for use in the present invention preferably has an 
F-5 value in the tape running direction of from 5 to 50 kg/mm.sup.2 and an 
F-5 value in the tape width direction of from 3 to 30 kg/mm.sup.2. 
Although the F-5 value in the tape length direction is generally higher 
than that in the tape width direction, this does not apply in the case 
where the width-direction strength, in particular, should be enhanced. The 
degrees of thermal shrinkage of the support in the tape running direction 
and in the tape width direction are preferably 3% or less, more preferably 
1.5% or less, under conditions of 100.degree. C. and 30 minutes, and are 
preferably 1% or less, more preferably 0.5% or less, under conditions of 
80.degree. C. and 30 minutes. The strength at break thereof in each of 
both directions is preferably from 5 to 100 kg/mm.sup.2, and the modulus 
thereof is preferably from 100 to 2,000 kg/mm.sup.2. 
A process for preparing a magnetic coating fluid to be used for producing 
the magnetic recording medium of the present invention comprises at least 
a kneading step and a dispersing step, and may further comprise a mixing 
step that may be conducted, if needed, before and after the two steps. 
Each step may include two or more stages. Each of the materials for use in 
the present invention, including ferromagnetic particles, inorganic 
nonmagnetic particles, binder, carbon black, abrasive material, antistatic 
agent, lubricant, and solvent, may be added in any step either at the 
beginning of or during the step. Further, the individual raw materials may 
be added portion-wise in two or more steps. For example, a polyurethane 
may be added portion-wise in each of the kneading step, the dispersing 
step, and the mixing step for viscosity adjustment after the dispersion. 
Conventionally known manufacturing techniques can, of course, be used as 
part of the process to attain the object of the present invention. Use of 
a kneading machine having high kneading power, such as a continuous 
kneader or pressure kneader, in the kneading step is advantageous in that 
improved gloss is obtained. In using a continuous kneader or pressure 
kneader, the ferromagnetic or nonmagnetic particles are kneaded together 
with all or part (preferably at least 30%) of the binder, the binder 
amount being in the range of from 15 to 500 parts by weight per 100 parts 
by weight of the ferromagnetic particles. Details of this kneading 
treatment are given in JP-A-l-166338 and JP-A-64-79274. For preparing a 
coating fluid for the nonmagnetic layer, use of a dispersing medium having 
a high specific gravity is desirable. A preferred example thereof is 
zirconia beads. 
The following constitutions can be proposed as exemplary coating 
apparatuses and methods for producing multilayered magnetic recording 
media such as that of the present invention. 
1. A lower layer is first applied with a coating apparatus commonly used 
for magnetic coating fluid application, e.g., a gravure coating, roll 
coating, blade coating, or extrusion coating apparatus, and an upper layer 
is then applied, while the lower layer is in a wet state, by means of a 
support-pressing extrusion coater such as those disclosed in JP-B-1-46186, 
JP-A-60-238179, and JP-A-2-265672. 
2. An upper layer and a lower layer are applied almost simultaneously using 
a single coating head having therein two slits for passing coating fluids, 
such as those disclosed in JP-A-63-88080, JP-A-2-17971, and JP-A-2-265672. 
3. An upper layer and a lower layer are applied almost simultaneously with 
an extrusion coater equipped with a back-up roll, such as that disclosed 
in JP-A-2-174965. 
In order to prevent the electromagnetic characteristics and other 
properties of the magnetic recording medium from being impaired by 
aggregation of ferromagnetic particles, shearing is preferably applied to 
the coating fluid present in the coating head by a method such as those 
disclosed in JP-A-62-95174 and JP-A-1-236968. The viscosity of each 
coating fluid should be in the range as specified in JP-A-3-8471. 
In the present invention, the methods described above are preferably used 
for producing a multilayered magnetic recording medium. Also in the case 
of forming two magnetic layers and one nonmagnetic layer, each of the 
above-described methods is easily applicable to the formation of these 
three layers. It is, however, possible to use a method in which a 
nonmagnetic layer is applied and dried before a lower magnetic layer and 
an upper magnetic layer are simultaneously formed thereon, or a method in 
which a nonmagnetic layer and a lower magnetic layer are simultaneously 
formed and dried before an upper magnetic layer is formed thereon. 
A known orientation apparatus may be used for producing the magnetic 
recording medium of the present invention. However, like-pole-facing 
cobalt magnets, unlike-pole-facing cobalt/solenoid magnets, and 
superconducting magnets are preferred. During the application of a 
magnetic field, the amount of the organic solvent contained in the coating 
is preferably regulated to a value within the range specified hereinabove 
by controlling the temperature and amount of the air fed for drying or by 
controlling the rate of coating. In other words, it is preferred that the 
place in which the coating is dried be made controllable. The rate of 
coating is from 20 to 1,000 m/min, preferably from 100 to 800 m/min, and 
more preferably from 200 to 600 m/min, and the temperature of the drying 
air is usually from 40.degree.to 100.degree. C., preferably from 
60.degree.to 100.degree. C., and more preferably from 80.degree.to 
100.degree. C. As stated above, predrying may be performed to an 
appropriate degree before the coated support enters the magnet zone. 
Examples of calendering rolls that can be used for producing the magnetic 
recording medium of the present invention include rolls of a 
heat-resistant plastic, e.g., epoxy, polyimide, polyamide, or 
poly(imide-amide), and metal rolls. Preferred is calendering with metal 
rolls. The calendering temperature is usually from 20.degree.to 
150.degree. C., preferably from 70.degree.to 120.degree. C., and more 
preferably from 100.degree.to 110.degree. C. The linear pressure is 
usually from 50 to 500 kg/cm, preferably from 200 to 400 kg/cm, and more 
preferably from 300 to 400 kg/cm. 
The magnetic recording medium of the present invention has the following 
properties. The coefficients of friction of the upper magnetic layer 
surface and the opposite side surface with SUS420J are 0.5 or less, 
preferably 0.3 or less, throughout the temperature range of from 
-10.degree. C. to 40.degree. C. and the humidity range of from 0% to 95%. 
The surface resistivity on both sides is preferably from 10.sup.4 to 
10.sup.12 .OMEGA./sq, and the electrification potential thereof is 
preferably from -500 V to 500 V. The modulus at 0.5% elongation of the 
upper magnetic layer is preferably from 100 to 2,000 kg/mm.sup.2 in both 
the running and width directions, and the strength at break thereof is 
preferably from 1 to 30 kg/cm.sup.2. The modulus of the magnetic recording 
medium is preferably from 100 to 1,500 kg/mm.sup.2 in both running and 
width directions, the residual elongation thereof is preferably 0.5% or 
less, and the thermal shrinkage thereof at temperature of 100.degree. C. 
or less is preferably 1% or less, more preferably 0.5% or less, and 
especially preferably 0.1% or less. The glass transition temperature (the 
temperature at which the loss modulus in a dynamic viscoelasticity 
measurement at 110 Hz becomes maximum) of the upper magnetic layer is 
preferably from 50.degree.to 120.degree. C., while that of the lower 
nonmagnetic or lower magnetic layer is preferably from 0.degree. to 
100.degree. C. The loss modulus is preferably from 1.times.10.sup.8 to 
8.times.10.sup.9 dyne/cm.sup.2, and the loss tangent is preferably 0.2 or 
less. Too large loss tangents tend to result in troubles due to sticking. 
The residual solvent content in the upper layer is preferably 100 
mg/m.sup.2 or less, more preferably 10 mg/m.sup.2 or less. It is preferred 
that the residual solvent content in the upper layer be lower than that in 
the lower layer. The void content in each of the upper layer and the lower 
layer is preferably 30% by volume or less, more preferably 20% by volume 
or less. Although a lower void content is desirable for attaining higher 
output, there are cases where a certain degree of void content is 
preferred according to purpose. For example, in the case of a magnetic 
recording medium for data recording use where suitability for repeated 
running operations is important, higher void contents in most cases bring 
about better running durability. 
The upper layer has a center-line surface roughness Ra of 0.008 .mu.m or 
less, preferably 0.003 .mu.m or less, and an RMS surface roughness 
R.sub.RMS as determined with an AFM is preferably from 2 nm to 15 nm. The 
upper layer has preferably a maximum height SRmax of 0.5 .mu.m or less, a 
ten-point average roughness SRz of 0.3 .mu.m or less, a center-plane peak 
height SRp of 0.3 .mu.m or less, a center-plane valley depth SRv of 0.3 
.mu.m or less, a center-plane areal ratio SSr of from 20% to 80%, and an 
average wavelength S.lambda.a of from 5 .mu.m to 300 .mu.m. The upper 
layer surface may have from 0 to 2,000 projections having a size of from 
0.01 .mu.m to 1 .mu.m. The number of these projections can be easily 
controlled, for example, by regulating the surface irregularities of the 
support with a filler or by the surface irregularities of calendering 
rolls. 
The magnetic recording medium of the present invention, which preferably 
has a lower layer and an upper layer, can be made to have a difference in 
physical property between the lower layer and the upper layer according to 
purpose, as can be easily presumed. For example, the upper layer is made 
to have a heightened modulus to improve running durability and, at the 
same time, the lower layer is made to have a lower modulus than the upper 
layer to improve the head touching of the magnetic recording medium.

The present invention is explained below by the following examples, but the 
invention is not construed as being limited thereto. In the examples, all 
parts, percents and ratios are by weight unless otherwise indicated. 
EXAMPLES &lt;Production of Hexagonal-Ferrite Magnetic Particles, 1&gt; 
Various compounds as raw materials for hexagonal-ferrite production were 
weighed out in the following amounts in terms of oxide amounts. 
______________________________________ 
B.sub.2 O.sub.3 
7.1 mol 
BaO 10.0 mol 
Fe.sub.2 O.sub.3 
X1 mol 
M1O Y1 mol 
M2O.sub.2 Z1 mol 
______________________________________ 
The weighed compounds were sufficiently mixed by a powder mixer. The 
resulting mixture was placed in a zirconia crucible equipped with a 
stirrer, and melted by heating at from 1,300 to 1,350.degree. C. The melt 
was jetted into the nip between a pair of revolving cooling rolls made of 
stainless steel to obtain an amorphous substance. This amorphous substance 
was placed in an electric furnace, where the substance was heated to 
500.degree. C. at a rate of 150.degree. C./hr, maintained at that 
temperature for 6 hours, subsequently heated to 800.degree. C., maintained 
at this temperature for 5 hours, and then cooled to room temperature at a 
rate of 120.degree. C./hr to obtain crystal powders. This crystal powders 
were ground with a planetary mill, and the ground powders were immersed in 
a 6 N aqueous acetic acid solution at 80.degree. C. for 5 hours. 
Subsequently, the powders were washed with a large amount of water, 
dehydrated, dried at 100.degree. C., and then deaerated with a muller to 
finally obtain ferromagnetic powders. 
X-Ray analysis revealed that the ferromagnetic particles thus produced 
mainly had the M-form magnetoplumbite structure. The compositions of the 
thus-produced barium ferrites and the powder and magnetic properties 
thereof are shown in Table 1. 
TABLE 1 
______________________________________ 
Spe- 
cific 
Magnetic surface 
powder X1 M1 M2 Y1 Z1 Hc .sigma.s 
area 
unit mol -- -- mol mol Oe emu/g m.sup.2 /g 
______________________________________ 
A 8.8 Co Ti 1.2 1.2 1050 55 30 
B 8.8 Zn Ti 1.2 1.2 1120 54 31 
C 9.0 Co Ti 1.0 1.0 1260 55 28 
D 9.0 Zn Ti 1.0 1.0 1420 57 30 
E 9.6 Ni Ti 0.7 0.7 1680 60 30 
F 11.2 Co Nb 0.4 0.4 1800 61 34 
G 11.2 Zn Nb 0.4 0.4 1960 60 32 
______________________________________ 
&lt;Production of Hexagonal-Ferrite Magnetic Powders, 2&gt; 
Various compounds as raw materials for hexagonal-ferrite production were 
weighed out in the following amounts in terms of element amount. 
______________________________________ 
Fe.sup.3+ X2 mol 
M3.sup.2+ Y2 mol 
M4.sup.4+ Z2 mol 
______________________________________ 
The above compounds were dissolved in 4 liters of distilled water. 
______________________________________ 
Ba.sup.2+ 1.57 mol 
______________________________________ 
The Ba compound was dissolved in 3 liters of distilled water. 
______________________________________ 
NaOH 164 mol 
______________________________________ 
The NaOH was dissolved in 4 liters of distilled water. The three aqueous 
solutions were mixed in a 20-l stainless-steel tank, while nitrogen gas 
was continuously bubbled into the mixture from the tank bottom with 
stirring. The slurry thus obtained was introduced into an autoclave and 
heated at 280.degree. C for 4 hours with stirring. After being cooled to 
room temperature, the reaction mixture was taken out and subjected to 
solid-liquid separation. The solid obtained was sufficiently washed with 
water and dried at 100.degree. C. The dry solid was placed in an electric 
furnace and maintained at 850.degree. C. for 10 hours. Thereafter, the 
solid was cooled to room temperature, taken out of the furnace, and then 
deaerated with a muller to finally obtain hexagonal-ferrite magnetic 
particles (hereinafter also referred to simply as "magnetic particles"). 
X-Ray analysis revealed that the magnetic particles thus produced mainly 
had the M-form magnetoplumbite structure. The particle and magnetic 
properties of the thus-produced barium ferrites are shown in Table 2. An 
examination of the barium ferrites with an electron microscope revealed 
that the plate diameter thereof was 0.03 .mu.m and the aspect ratio 
thereof regarding degree of flatness was 3. 
TABLE 2 
______________________________________ 
Spe- 
cific 
Magnetic surface 
powder X2 M3 M4 Y2 Z2 Hc .sigma.s 
area 
unit mol -- -- mol mol Oe emu/g m.sup.2 /g 
______________________________________ 
H 16.0 Co Ti 0.8 0.8 1150 55 49 
I 16.0 Co Ti 0.6 0.6 1380 54 50 
J 15.0 Zn Ti 0.56 
0.56 
1450 55 48 
K 18.0 Ni Ti 0.4 0.4 1820 59 53 
L 18.0 Zn Nb 0.3 0.3 2090 60 47 
M 16.0 Zn Nb 0.25 
0.25 
2130 55 52 
N 16.0 Ni Nb 0.15 
0.15 
2460 56 46 
O 16.0 Co Nb 0.4 0.6 1290 53 47 
______________________________________ 
&lt;Production of Coating 
______________________________________ 
Upper Magnetic Coating Fluid X: 
Barium ferrite (magnetic particles A to O) 
100 parts 
Vinyl chloride copolymer 
12 parts 
Containing 1 .times. 10.sup.-4 eq/g --PO.sub.3 Na 
Degree of polymerization 
300 
Polyester polyurethane resin 
3 parts 
Neopentyl glycol/caprolactonepolyol/ 
MDI = 0.9/2.6/1 
Containing 1 .times. 10.sup.-4 eq/g --SO.sub.3 Na 
group 
.alpha.-Alumina (particle size, 0.3 .mu.m) 
2 parts 
Carbon black (particle size, 0.015 .mu.m) 
5 parts 
Butyl stearate 1 part 
Stearic acid 2 parts 
Methyl ethyl ketone 125 parts 
Cyclohexanone 125 parts 
Lower Magnetic Coating Fluid Y: 
Fine ferromagnetic iron oxide particles 
100 parts 
Composition, Co-adsorbed iron oxide 
Hc 800 Oe 
BET specific surface area 
45 m.sup.2 /g 
Crystallite size 200 .ANG. 
Surface-treating agent 
5 wt % Al.sub.2 O.sub.3 
2 wt % SiO.sub.2 
Particle size 0.12 .mu.m 
(major axis length) 
Aspect ratio 8 
.sigma.s 76 emu/g 
Vinyl chloride copolymer 
12 parts 
Containing 1 .times. 10.sup.-4 eq/g --SO.sub.3 Na 
Degree of polymerization 
300 
Polyester polyurethane resin 
3 parts 
Neopentyl glycol/caprolactonepolyol/ 
MDI = 0.9/2.6/1 
Containing 1 .times. 10.sup.-4 eq/g --SO.sub.3 Na 
group 
.alpha.-Alumina (particle size, 0.3 .mu.m) 
2 parts 
Carbon black (particle size, 0.10 .mu.m) 
0.5 parts 
Butyl stearate 1 part 
Stearic acid 5 parts 
Methyl ethyl ketone 100 parts 
Cyclohexanone 20 parts 
Toluene 60 parts 
Lower Nonmagnetic Coating Fluid Z: 
Inorganic nonmagnetic particles, TiO.sub.2 
80 parts 
Crystal system rutile 
Average primary-particle diameter 
0.035 .mu.m 
BET specific surface area 
40 m.sup.2 /g 
pH 7 
TiO.sub.2 content 90% or more 
DBP oil absorption 27-38 ml/100 g 
Surface-treating agent 
8 wt % Al.sub.2 O.sub.3 
Carbon black 20 parts 
Average primary-particle diameter 
16 m.mu. 
DBP oil absorption 80 ml/100 g 
pH 8.0 
BET specific surface area 
250 m.sup.2 /g 
Volatile content 1.5% 
Vinyl chloride copolymer 
12 parts 
Containing 1 .times. 10.sup.-4 eq/g --SO.sub.3 Na 
Degree of polymerization 
300 
Polyester polyurethane resin 
5 parts 
Neopentyl glycol/caprolactonepolyol/ 
MDI = 0.9/2.6/1 
Containing 1 .times. 10.sup.-4 eq/g --SO.sub.3 Na 
group 
Butyl stearate 1 part 
Stearic acid 1 part 
Methyl ethyl ketone/cyclohexanone 
250 parts 
(8/2 mixed solvent) 
______________________________________ 
With respect to each of the above three coating fluids, the ingredients 
were kneaded with a continuous kneader and then dispersed with a sand 
mill. To the resulting dispersions was added a polyisocyanate in an amount 
of 3 parts for nonmagnetic coating fluid Z and in an amount of 5 parts for 
each of upper magnetic coating fluid X and lower magnetic coating fluid Y. 
The dispersions were filtered through a filter having an average opening 
diameter of 1 .mu.m. Thus, lower nonmagnetic coating fluid Z, upper 
magnetic coating fluid X, and lower magnetic coating fluid Y were 
prepared. 
&lt;Production of Magnetic Recording Media&gt; 
EXAMPLE 1 
A poly(ethylene naphthalate) support having a thickness of 7 .mu.m and a 
center-line surface roughness of 0.002 .mu.m was coated by simultaneous 
double coating with lower nonmagnetic coating fluid Z at a dry thickness 
of 3 .mu.m and with upper magnetic coating fluid X at a dry thickness of 
0.8 .mu.m. The coated support was passed through an orientation zone which 
had a 1 m-long solenoid magnet having a magnetic force of 3,000 G and to 
which 100.degree. C. dry air was continuously fed. Thus, longitudinal 
orientation was performed together with drying. Thereafter, the web was 
calendered with a 7-roll calender in which all the rolls were metal rolls, 
at a linear pressure of 300 kg/cm and a temperature of 100.degree. C. The 
calendered web was slit into a 8-mm width to produce a 8-mm video tape. 
Thus, magnetic recording media (hereinafter abbreviated as "media") 1 to 
15 (excluding 5) were obtained which corresponded to the magnetic powders 
used in upper magnetic coating fluid X. 
EXAMPLE 2 
Media 5 and 16 were produced in the same manner as in Example 1, except 
that a poly(ethylene naphthalate) support having a thickness of 7 .mu.m 
and a center-line surface roughness of 0.002 .mu.m was coated by 
simultaneous double coating with lower nonmagnetic coating fluid Z at a 
dry thickness of 2 .mu.m and with upper magnetic coating fluid X 
(containing magnetic powder E) at a dry thickness of 1.5 .mu.m, and that 
the power supply to the solenoid was switched off to dry the coating 
without orientation. 
EXAMPLE 3 
Medium 20 was produced in the same manner as in Example 1 (the medium 
employing magnetic powder G), except that lower magnetic coating fluid Y 
was used in place of lower nonmagnetic coating fluid Z. The magnetic 
properties and HK of this medium were regarded as the same as those of 
medium 8, which had a lower nonmagnetic layer, because those properties 
were influenced by the ferromagnetic particles contained in the lower 
layer. 
EXAMPLE 4 
A poly(ethylene naphthalate) support having a thickness of 7 .mu.m and a 
center-line surface roughness of 0.002 .mu.m was coated only with upper 
magnetic coating fluid X (containing magnetic powder F or G) at a dry 
thickness of 3.0 .mu.m. The subsequent procedure was carried out in the 
same manner as in Example 1 to produce media 17, 18, and 19. 
Media 1 to 20 thus produced were evaluated by the methods described below. 
The results obtained are shown in Table 3. 
Evaluation Methods 
(Magnetic Properties) 
Measurements were made in an applied magnetic field of 10 kOe with VSM-5, 
manufactured by Toei Kogyo Co., Ltd. 
(HK) 
Using torquemeter TRT-2, manufactured by Toei Kogyo K.K., a demagnetized 
sample was examined for rotational hysteresis loss Wr from a low intensity 
of magnetic field to 10 kOe. The values of Wr were plotted against the 
reciprocal of intensity of applied magnetic field, 1/H, and the intensity 
of applied magnetic field at which Wr became 0 on the higher 
magnetic-intensity side was determined by-extrapolating a straight portion 
of the Wr curve; this intensity was taken as HK. The values of Rh and Hp 
(the intensity of magnetic field at the peak of the curve of r against 
1/H) which were obtained from the same measurement are given in Table 3. 
Rh is integrated rotational hysteresis. 
(Electromagnetic Characteristics) 
A 1.0-T head (output 1) or a 1.5-T head (output 2) was mounted on 8-mm 
video deck FUJIX8, manufactured by Fuji Photo Film Co., Ltd., to record 10 
MHz signals. The recorded signals were reproduced and the output thereof 
was measured with an oscilloscope. Medium 1 was used as the reference, 
with the output value therefor with respect to each head being taken as 0 
dB. 
TABLE 3 
__________________________________________________________________________ 
Magentic 
Bm Hc SQ Hc/HK 
HK Rh Hp Output 1 
Output 2 
Medium 
particles 
G Oe -- -- Oe -- Oe dB dB Remarks 
__________________________________________________________________________ 
1 A 1850 
1110 
0.85 
0.18 6167 
1.70 
1420 
0.0 0.0 Comp. 
2 B 1850 
1220 
0.85 
0.33 3697 
1.05 
1550 
0.8 0.2 Comp. 
3 C 1850 
1380 
0.80 
0.28 4929 
1.40 
1600 
0.4 -0.6 Comp. 
4 D 1870 
1560 
0.72 
0.35 2166 
1.10 
1820 
1.9 1.5 Inv. 
5 D 1870 
1510 
0.56 
0.34 4441 
0.80 
1830 
0.5 0.4 Comp. 
6 E 1850 
1730 
0.85 
0.43 4023 
1.20 
1960 
1.0 2.5 Inv. 
7 F 1900 
1890 
0.86 
0.28 6750 
1.40 
2130 
0.2 0.8 Comp. 
8 G 1870 
2040 
0.88 
0.48 4250 
0.80 
2200 
0.5 2.7 Inv. 
9 H 1800 
1180 
0.84 
0.33 3576 
1.00 
1350 
0.3 0.2 Inv. 
10 I 1780 
1590 
0.87 
0.36 4417 
0.95 
1540 
2.0 1.4 Inv. 
11 J 1760 
1640 
0.79 
0.51 3216 
0.70 
1790 
2.4 1.8 Inv. 
12 K 1850 
2010 
0.82 
0.23 8739 
1.70 
2250 
0.3 0.4 Comp. 
13 L 1840 
2250 
0.80 
0.22 10227 
1.80 
2460 
-0.2 0.3 Comp. 
14 M 1750 
2350 
0.81 
0.76 3092 
1.10 
2560 
-0.5 3.8 Inv. 
15 N 1740 
2640 
0.83 
0.68 3882 
0.55 
2910 
-1.5 3.5 Inv. 
16 G 1860 
1960 
0.55 
0.50 3920 
0.50 
2260 
1.4 0.9 Comp. 
17 G 1870 
2040 
0.88 
0.53 3849 
0.90 
2150 
1.2 2.2 Inv. 
18 F 1910 
1880 
0.86 
0.28 6714 
1.60 
2100 
0.4 1.0 Inv. 
19 G 1870 
2020 
0.88 
0.46 4391 
1.00 
2210 
1.6 2.6 Inv. 
20 G 1870 
2040 
0.88 
0.48 4250 
0.80 
2200 
0.7 2.6 Inv. 
21 O 1860 
1350 
0.88 
0.45 3000 
1.05 
1550 
0.7 -0.1 Inv. 
__________________________________________________________________________ 
All the media satisfying the constitutional requirements of the present 
invention, including the media having a multilayer coating consisting of a 
lower nonmagnetic layer and an upper magnetic layer, the medium having a 
multilayer coating consisting of a lower magnetic layer and an upper 
magnetic layer, and the media having a single-layer coating consisting of 
an upper layer, showed increased outputs. In contrast, the media not 
satisfying the constitutional requirements of the invention showed lower 
outputs. 
The magnetic recording medium of the present invention, which has a 
magnetic layer containing hexagonal-ferrite magnetic particles, can have 
remarkably improved ultrashort-wavelength output necessary to high-density 
recording, because the magnetic layer has specific values of Hc, Hc/HK, 
and SQ of in-plane direction. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.