Magnetic recording apparatus with magnetic recording medium having high recording density

A longitudinally oriented magnetic recording medium is described which has advantageous properties for digital recording, ie. a very small pulse width and high output level, and wherein the quotient l.sub.r /(d.multidot.L) (l.sub.r =residual polarization in mT, d=layer thickness in .mu.m, L=mean particle length in nm) is greater than 18 and the pulse width stated as PW.sub.50 is smaller than 500 nm.

FIELD AND BACKGROUND OF THE INVENTION 
The present invention relates to an essentially longitudinally oriented 
magnetic recording medium which has a very small layer thickness and is 
suitable for high-density recording. 
Recently, the recording wavelength has been steadily decreased in order to 
satisfy the need for increased recording density for magnetic recording 
media. For example, the recording wavelength for the 8 mm video system is 
0.58 .mu.m. This gives rise to the problem of a loss of thickness in 
signal playback, ie. the playback level does not increase linearly as a 
function of increasing layer thickness but exhibits a saturation effect. 
Thus, only a very thin layer is required for short wavelengths. 
In order to meet this requirement, magnetic recording media in which a 
binder-free ferromagnetic metal layer was applied in a very small 
thickness by means of a vacuum method were developed in the past 10 years. 
Although these metal evaporated recording media have a small thickness 
loss and achieve a very high playback level, the mass production of such 
tapes still gives rise to considerable difficulties in comparison with 
magnetic recording media in which the magnetic pigments are dispersed in 
binders. Moreover, these ME tapes change under the influence of 
atmospheric oxygen. 
However, it has recently been possible to meet the requirement for a small 
layer thickness also by means of a thin magnetic layer in which the finely 
divided magnetic particles are dispersed in polymeric binder and this 
layer is cast on a non-magnetic substrate. 
Such application methods are described, for example, in U.S. Pat. No. 
2,819,186, German Laid-Open Application DE-A 4,302,516, EP 0 520 155, EP 0 
566 100 and the German Applications P 44 43 896 and P 195 04 930. 
Magnetic recording with high recording density is now predominantly carried 
out by a digital method. This means that, in contrast to the analog video 
recording, no sinusoidal signals are recorded and instead the information 
is recorded on the recording medium by switching over the direction of the 
current of the recording head. The magnetization pattern produced in such 
a switching process is referred to as the magnetization transition. 
However, this transition does not occur abruptly but more or less 
gradually, for example in the form of a Gauss curve. The playback signal 
of such a magnetization transition is pulse-like because inductive 
reading, which is typically used in the video system, is based on 
differentiation. Since the magnetization transition as described above 
does not occur abruptly, the read pulses have a certain width, which is 
usually defined by the PW.sub.50 value. This value indicates the distance 
on the recording medium between the two points at which the actual signal 
assumes precisely 50% of the maximum value, as shown in FIG. 1. It is 
clear that highdensity recording requires very small pulse widths. At the 
same time, a very large pulse magnitude must be ensured in order to 
achieve an adequate output level.

DESCRIPTION OF THE INVENTION 
It is an object of the present invention to provide a magnetic recording 
medium of the generic type stated at the outset which meets the 
abovementioned requirements. 
We have found that this object is achieved, according to the invention, by 
a recording medium in which the quotient l.sub.r /(d.multidot.L) is 
greater than 18. 
Where 
l.sub.r =residual polarization in mT 
d=thickness of the magnetic layer (in .mu.m) and 
L=mean length of the magnetic particles. 
In addition, the pulse width PW.sub.50 should be less than 500 nm. The 
invention is illustrated below. 
The measured values were obtained using an experimental setup under the 
following conditions: 
Relative head/tape speed: v=3.17 m/s Read/write head: V8 type, measured gap 
zero point g.sub.w =236 nm 
The recording current was adjusted so that the level of the fundamental 
wave of the square-wave signal is a maximum in square wave recording at a 
wavelength .lambda.=3 g.sub.w =708 nm, ie. 3 times the measured head gap 
width. The measurement was carried out using a spectrum analyzer 
(resolution bandwidth 30 kHz). If this condition is varied, for example by 
reducing the recording current by 20-30%, the differences between novel 
recording medium and comparison remain or become even larger. 
A square-wave signal having a frequency of 96 kHz was recorded on the tape. 
The signal was then read by means of the read head and was scanned with a 
digitizer. The scanning rate was 5 ns. A single pulse was calculated from 
the scan signal, said pulse being obtained by averaging the centered read 
pulses. A total of 126 pulses were averaged. The pulse width is calculated 
from the single pulse measurement as above by determining the distance on 
the tape between the two points at which the read amplitude is precisely 
50% of the maximum read signal, once again reference being made to the 
figure. 
The residual polarization l.sub.r is defined as the magnetic moment divided 
by the sample volume. It is measured using a commercial magnetometer. 
There are in principle no restrictions to the composition of the magnetic 
recording medium, preferably consisting of the layer containing the 
magnetic pigments and a nonmagnetic substrate. 
The prior art magnetic pigments, such as iron oxide, Co-doped iron oxides, 
metal pigments and metal alloys, chromium dioxide and others, may be used, 
as may the conventional polymeric binders or binder mixtures and the other 
additives, such as dispersants, nonmagnetic pigments, lubricants, curing 
agents, wetting agents and solvents. 
Suitable components of the magnetic layer and of the nonmagnetic layer are 
described, for example, in DE-A 43 02 516. 
Known films of polyesters, such as polyethylene terephthalate or 
polyethylene naphthalate, and polyolefins, cellulose triacetate, 
polycarbonates, polyamides, polyimides, polyamidoimides, polysulfones, 
aramids or aromatic polyamides, serve as substrates. The substrate may be 
subjected beforehand to a corona discharge treatment, a plasma treatment, 
a slight adhesion treatment, a heat treatment, a dust removal treatment or 
the like. In order to achieve the object of the invention, the nonmagnetic 
substrate is one having a center line average surface roughness of in 
general 0.03 .mu.m or less, preferably of 0.02 .mu.m or less, in 
particular of 0.01 .mu.m or less. It is also desirable for the substrate 
not only to have such slight center line average surface roughness but 
also to have no large protuberances of 1 .mu.m or more. The roughness 
profile of the surface of the substrate can, if desired, be freely 
controlled according to the size and amount of the filler to be added to 
the substrate. Examples of suitable fillers are oxides and carbonates of 
Ca, Si and Ti and fine organic powders of acrylic substances. 
The process for the preparation of the magnetic dispersion comprises at 
least one kneading stage, one dispersing stage and, if required, one 
mixing stage, which may be provided before and after the preceding stages. 
Particular stages may each comprise two or more stages. In the preparation 
of the composition, all starting materials, ie. the ferromagnetic powder, 
the binder, the carbon black, the abrasive, the antistatic agent, the 
lubricant and the solvent, may be added to the reactor immediately at the 
beginning of the process or subsequently in the course of the process. The 
individual starting materials may be divided into a plurality of portions, 
which are added to the process in two or more stages. For example, the 
polyurethane is divided into a plurality of portions and is added in the 
kneading stage and in the dispersing stage and also in the mixing stage 
for adjusting the viscosity after dispersing. 
In order to achieve the object of the present invention, a known 
conventional technology may also be used as part of the process for the 
production of the novel magnetic recording medium. For example, a kneading 
apparatus having a high kneading force, for example a continuous kneader 
or a pressure kneader, may be used in the kneading stage in order to 
obtain a novel magnetic recording medium having a high Br value. If such a 
continuous kneader or pressure kneader is used, the ferromagnetic powder 
is kneaded with the total binder, preferably 30% by weight or more. For 
example, 100 parts by weight of a ferromagnetic powder are mixed with from 
15 to 500 parts by weight of a binder. 
After fine filtration through a narrow-mesh filter having a mesh size of 
not more than 5 .mu.m, the dispersions are applied by means of a 
conventional coating apparatus at speeds in the usual range of 100-500 
m/min, oriented in the recording direction in a magnetic field and then 
dried and subjected to calendering and, if required, a further 
surface-smoothing treatment. 
Essentially longitudinally oriented is intended to mean the magnetic 
particles are present oriented essentially in the plane of the layer in 
the recording direction, but may also be oriented inclined at an angle up 
to 25.degree. to the plane of the layer. 
The coating may be effected by means of a doctor blade coater, a knife 
coater, a doctor, an extrusion coater, a reverse roll coater or a 
combination. The two layers can preferably be applied simultaneously by 
the wet-on-wet method. 
The magnetic recording medium thus obtained is then slit longitudinally or 
punched into the usual width for use and subjected to the conventional 
electroacoustic tests and the mechanical tests. 
Particularly advantageous results are obtained when a very thin magnetic 
upper layer whose thickness is less than 1 .mu.m is cast on a nonmagnetic 
lower layer whose layer thickness is 1-8 .mu.m. 
The invention is illustrated with reference to practical examples and 
comparative examples, but without restricting the invention to the 
specific formulation examples and the apparatus for the production of such 
a magnetic recording medium. A magnetic recording medium consisting of a 
thin magnetic upper layer which was cast on a nonmagnetic lower layer was 
produced by means of an apparatus as described in more detail in German 
Application P 195 04 930. The two layers are based on the following 
formulation: 
______________________________________ 
a) Composition of the lower layer 
Parts by weight 
______________________________________ 
Vinyl polymer having polar groups 
85 
Polyurethane having polar groups 
85 
TiO.sub.2 (55 m.sup.2 /g BET) 
1000 
Lubricant 25 
Polyisocyanate 30 
Solvents (tetrahydrofuran, dioxane) 
2209 
______________________________________ 
The viscosity of this lower layer is 50 mPa.s and the flow limit is 18 Pa. 
______________________________________ 
b) Composition of the upper layer 
Parts by weight 
______________________________________ 
Magnetizable metal pigment 
1000 
.alpha.-Al.sub.2 O.sub.3 (particle size = 0.2 .mu.m) 
70 
Vinyl polymer having polar groups 
77 
Polyurethane having polar groups 
77 
Phosphoric ester 10 
Lubricant 25 
Polyisocyanate 22.5 
Solvents (tetrahydrofuran, dioxane) 
6170 
______________________________________ 
The viscosity of this upper layer is 8 mPa.s and the flow limit is 2.5 Pa. 
The measurement of the viscosity and of the flow limit was carried out 
using a Carri-Med CSL Rheometer in the plate-and-cone measuring system at 
25.degree. C., and the evaluation was effected according to Bingham 
(descending curve). 
TABLE 1 
______________________________________ 
Mean 
particle 
S = length 
Example 
H.sub.c [kA/m] 
M.sub.s [kA/m] 
M.sub.r /M.sub.s ] 
d [.mu.m] 
SFD [nm] 
______________________________________ 
1 (Com- 
144 326 0.89 1.5 0.27 105 
parison) 
2 (Com- 
148 326 0.83 0.2 0.28 
105 
parison) 
3 (Com- 
181 363 0.9 1.2 0.3 80 
parison) 
4 187 334 0.87 0.17 0.31 
80 
5 183 346 0.84 0.14 0.3 80 
6 182 336 0.83 0.13 0.31 
80 
Fuji SDC 
133 280 0.88 0.4 0.3 150 
(Comp.) 
______________________________________ 
Table 1 shows the magnetic and mechanical data of the magnetic recording 
media obtained using different magnetic pigments, which are shown in the 
Table. d in .mu.m is the dry layer thickness of the upper layer, and the 
mean particle length (in nanometer) of the magnetic particles after volume 
averaging is shown in the last column. The particle size was measured 
under the electron microscope at 100,000 times magnification. A magnetic 
recording medium which has the name Fuji SDC and is available on the 
market for Hi-8 recording is also included as a comparison. 
Table 2 below shows the results which were obtained with the various 
magnetic recording media with respect to 
pulse width PW.sub.50 in nm 
the ratio of l.sub.r /(d.multidot.L) 
where 
l.sub.r is the residual polarization in mT, 
d is the thickness of the magnetic layer in .mu.m and 
L is the mean particle length of the magnetic pigments in nm. 
TABLE 2 
______________________________________ 
PW.sub.50 
l.sub.r /(d .multidot. L) 
Example (nm) 
(nT/.mu.m/nm) 
______________________________________ 
1 (Comparison) 682 2.3 
2 (Comparison) 586 16.2 
3 (Comparison) 625 4.3 
4 460 28.8 
5 401 32.6 
6 389 33.7 
Fuji SDC (Comp.) 625 5.2 
______________________________________ 
It is evident that the novel recording media have a very small pulse width 
PW.sub.50, ie. smaller than 500 nm, and at the same time a high quotient 
l.sub.r /(d.multidot.L), ie. greater than 18.