Magnetic orientation system

A two-part arrangement of permanent magnets with low retentivity conducting elements for the flow of magnetic force is used in order to orientate magnetically anisotropic pigments in a suspension in the direction of tape travel during the production of a magnetic tape by means of magnetic fields. The first part of the arrangement acts on the suspension with an inhomogeneous strong magnetic field of about 5000 A/cm which is narrowly restricted physically, without anticipating the orientation in the direction of tape travel. Orientation in the direction of tape travel takes place in the second part of the arrangement which has a physically extended but weaker field.

The invention relates to a system which orientates highly coercive magnetic 
particles during the production of magnetic store layers. 
The method of producing magnetic store layers is known and is explained 
with reference to FIG. 1. A non-magnetic film 1 composed, for example, of 
polyethylene terephthalate is guided from a supply spool 12 via guiding 
elements 13 to a winding spool 14. The magnetic suspension 8 is applied to 
the film 1 by the casting device 15. The coated film, still in the liquid 
state, passes through the orientation system 17 and then through a drying 
device 18. The suspension consists of a binder-containing lacquer with 
solvent and magnetic particles such as iron oxides, CrO.sub.2 or metal 
powder. The solvent evaporates in the drying device so that the magnetic 
layer 11 is formed from the magnetic suspension. Nowadays, the particles 
of the suspension almost exclusively have a pronounced magnetic primary 
axis which depends on the anisotropy of the shape and/or the anisotropy of 
the crystals. Elongate particles are preferably used. The orientation 
system generates a primary axis of magnetisation in the direction of tape 
travel. 
A magnetising coil through which the cast film is guided can be used as 
orientation system as described in French Pat. No. 100 8218 and in the 
Journal Hochfrequenztechnik und Elektroakustik, April 1963, Vol 72, 
Edition 2, pages 54-63. Such a coil has the advantage of an extensive 
homogeneous field in the direction of tape travel, which can be adapted to 
the conditions predetermined by the suspension by altering the coil 
current. The main disadvantages of using a coil is that it has to be 
operated in a highly explosive atmosphere. This disadvantage has meant 
that it is preferable nowadays to use orientation systems made up of 
permanent magnets. 
The simplest forms of permanent magnets, horseshoe magnets and bar magnets, 
for the orientation of magnetic pigment particles are described in U.S. 
Pat. No. 2,796,359. 
A frequently used and long known system (U.S. Pat. No. 2,711,901) makes use 
of magnetic strips which oppose each other with like poles. The film with 
the suspension cast on it is guided along the line of symmetry. The 
magnetic field acting on a pigment particle alters the bearing by 
180.degree. during the passage of the particle. Owing to this field 
configuration, a good orientation is achieved only if the maximum value of 
the directional field is substantially larger than the coercive field of 
the pigment particles. FIG. 2 of the Patent specification shows the 
configuration of the longitudinal component of the directional field 
acting on the particles. The resultant transverse field is zero. With such 
a orientation system composed of barium ferrite magnets, it is possible to 
generate maximum field intensities of approximately 800 A/cm. This is 
sufficient for the orientation of pigments having coercive fields between 
100 and 400 A/cm. With metal pigments having coercive field intensities of 
approximately 800 A/cm, the orientation is unsatisfactory with such a 
system. 
German Offenlegungsschrift No. 24 44 971 describes a directional 
arrangement which allows several such systems to act in succession. A 
perpendicular field component, which the control magnet should possess, 
should thus be corrected. This is preferably achieved by building up the 
consecutive pairs of geometrically different control magnet strips of like 
polarity. 
The split field of a magnetic circuit has also been proposed for generating 
a magnetic primary direction in magnetic store layers (British Pat. No. 
902,838). An important detail in this Patent specification is the fact 
that, owing to the action of the split field of the magnetic circuit on 
the moving magnetisable particles, these particles should be rotated by 
the split field in the primary direction in which the magnetisation of the 
signal to be recorded is to lie. It is immaterial whether the split field 
is generated by a current or by permanent magnets. Details about the 
strength of the split field are not given. 
The split field of a magnetic circuit is also used for other purposes in 
the production of magnetic tape. For example, it is used for distributing 
the suspension uniformly over the cast web (German Offenlegungsschrift No. 
23 46 390) or to draw out the swollen elevations at the edges (German 
Offenglegungsschrift No. 29 36 035). Both applications make use of the 
force of attraction, not the torque, of the magnetic field toward the 
pigment particles, which is unimportant in the directional process. It is 
explicitly stated in the last paragraph on page 4 of German 
Offenlegungsschrift No. 23 46 390, that orientation is not the object. In 
German Offenlegungsschrift No. 29 36 035, the pigment is held by the split 
field and pressed outwards by the movement of the film. On the other hand, 
the torque exerted by the field is important in the directing process. 
Bate and Dunn describe, in the IEEE Trans. on Magnetics Vol Mag 16 (1980) 
pages 1124-25, orientation systems in which two magnetic circuits with 
their split scatter fields are combined. With this arrangement, the 
magnetic field is substantially weakened by low-retentivity conducting 
elements, in the fading portion of the control magnet system possessing a 
direction opposed to that of the main field. FIG. 4 of the articles shows 
a double circuit system of this type. The cast film passes through the two 
magnetic circuits along the line of symmetry. The low-retentivity 
conducting elements serve to weaken the fading field. The field 
configuration of the longitudinal component on the plane of symmetry is 
also shown in FIG. 4 of the same article. In order to evaluate the quality 
of orientation of the pigment particles, the following characteristic 
values are adopted industrially: velour effect, the ratio of residual 
magnetism to saturation in the longitudinal direction of the tape M.sub.R 
/M.sub.S and the ratio of the residual magnetisation in the longitudinal 
and transverse direction M.sub.R// /M.sub.R .angle.. 
The velour effect is due to the fact that it is not always possible to 
orientate the pigment particles in the direction of tape travel and 
parallel to the surface of the layer. The magnetic primary axis is at an 
inclination to this direction in this case. Owing to the oblique position 
of the primary axis, different residual magnetisation is achieved by the 
recording field with its approximately circular configuration of field 
lines, depending in which direction the magnetic layer passes through the 
recording field. This is due to the fact that the angle between the field 
direction on the play-back edge and the primary axis for the two 
directions of tape travel is different. This results in a different level 
of the velour effect for opposing recording directions particularly when 
scanning short wave lengths (approximately 5 .mu.m). The velour effect is 
characterised by the logarithm of the ratio of the two levels: 
EQU V(dB)=20 log (P1/P2) 
With a wave length of 4 .mu.m, a value of V=2 dB implies that the velour 
effect is strong. The values M.sub.R /M.sub.S and M.sub.R// 
/M.sub.R.angle. are used as a gauge of the scatter of the primary axes of 
the particles round the direction of tape travel. In order to determine 
M.sub.R /M.sub.S, the residual magnetisation M.sub.R and the saturation 
magnetisation M.sub.S are measured in the direction of the primary axis. 
The ratio M.sub.R /M.sub.s indicates the value of the anisotropy. The 
closer the ratio is to the value 1, the better the particles are 
orientated. As absolute saturation is never achieved completely, it is 
necessary to indicate the field modulation when comparing various samples. 
The M.sub.R /M.sub.S value has the disadvantage that nowadays it varies 
practically only between 0.7 and 0.96. It must be measured very accurately 
in order to indicate differences. In order to determine M.sub.R// 
/M.sub.R.angle. (orientation ratio) the residual magnetisation M.sub.R// 
is measured in the direction of the primary axis and the residual 
magnetisation M.sub.R.angle. in the direction lying perpendicular thereto 
in the plane of the layer. This value has the advantage that the apparatus 
required for measuring it is less expensive than that required for 
determining M.sub.R /M.sub.S. The saturation residual magnetisation is 
virtually never achieved in the perpendicular direction owing to the high 
demagnetisation factor of transversely magnetised elongate particles. 
M.sub.R// /M.sub.R.angle. is even more dependent on the modulation field 
intensity than M.sub.R /M.sub.S. The former orientation systems have a 
number of defects which are found to be disturbing, particularly nowadays, 
in the production of store materials with coercive fields exceeding 500 
A/cm. Powdered metal pigments having coercive fields exceeding 800 A/cm 
are being brought onto the market at present. These values will increase 
significantly in future (1600-4000 A/cm). As the total coercive field 
H.sub.C represents only a mean value of the coercive fields of the 
particles, higher fields than the mean coercive field H.sub.C are required 
for orientating the particles. The production of such strong fields in a 
hazardous environment necessitates considerable production costs if 
current-carrying magnetising coils are to be used. When using permanent 
magnets, it is very difficult to generate the strong fields needed for the 
orientation of highly coercive pigments. The object of the invention is 
therefore to provide a directional system with which highly coercive store 
material having a high quality of anisotropy in the direction of tape 
travel can be produced. 
The object has been achieved according to the invention by magnet system 
for the orientation of the magnetic primary axes of pigment particles of a 
magnetic, binder-containing dispersion, which is applied to a substrate by 
casting machines, comprising permanent magnets and low-retentivity 
elements for conducting the magnetic flux, the orientation system 
comprising two partial systems wherein the field of the first directional 
system has an intensity between 1600 and 16,000 A/cm and is concentrated 
over a maximum cross-section of 5.times.5 mm.sup.2, and wherein the field 
lines have a markedly curved configuration and a strong field gradient, 
and the field produces, as the film lies near the slit, a main direction 
of the magnetising vectors of the particles which is orientated obliquely 
to the direction of tape travel, and the field of the following second 
partial system has an intensity between 400 and 2400 A/cm and a dimension 
in the longitudinal direction of at least 0.5 cm and essentially a 
longitudinal component which is orientated parallel to the direction of 
film travel, and the magnets which are at a distance from the film are 
arranged in mirror image fashion to the direction of film travel. 
The pigment particles are magnetised by the first field in such a way that 
their magnetising vectors do not deviate by more than 90.degree. from the 
principal direction. Such a condition corresponds to the remanent 
condition of a previously saturated material. According to the invention, 
the main direction of magnetisation does not lie parallel to the surface 
of the layer nor parallel to the direction of tape travel owing to the 
type of directional field, but has a component in the direction of film 
travel. Moreover, the magnetising vectors of these particles may be 
focussed in the principal direction by rotation of the particles. To 
achieve these conditions, it is necessary, according to the invention, for 
the maximum magnetic field of the first orientation system to be at least 
twice as large as the coercive field of the pigment particles. 
The second part of this orientation system has the object, according to the 
invention, of converting the magnetisation vectors of the beam into a 
position parallel to the direction of tape travel by mechanical rotation 
of the particles. This is achieved by the magnetic field which acts in 
this second part of this orientation system and has only one longitudinal 
component in the direction of film travel in the field region effecting 
the rotation. The system is made up mirror symmetrically to the tape of 
film. 
The period for which the particles are subject to the field effect depends 
on the size of the field and the casting speed during the casting process. 
This period must be adapted to the period required to rotate the particles 
in the direction of film travel. This rotation period lies between 0.01 
and 0.1 seconds depending on the type of pigment, type of lacquer 
(viscosity) and field intensity. The type of orientation is measured as a 
function of the film speed in order to estimate the rotation times for a 
specific casting solution and specific arrangement of the second part of 
the orientation system in accordance with one of the methods of 
measurement indicated above. The second part of the orientation system can 
be made up of parts according to the sub-claims, and the size of the field 
of one such part should not fall below 0.5 cm in the longitudinal 
direction. Furthermore, the maximum field intensity of the individual 
partial magnetic systems should decrease in an even manner as the distance 
from the casting device increases. In particular, the maximum field 
intensity of a partial magnet system should only amount to 20% of the 
maximum field intensity of the preceding system in each case. 
A few magnetic systems are described by way of example below with reference 
to the following figures.

FIG. 2 shows a cross-section through an example of the first part of the 
orientation system for the generation of a high inhomogeneous magnetic 
field. According to the invention, it consists of a strip-shaped permanent 
magnet 2, with a magnetic primary axis 3, which is comprised of two 
strip-shaped magnetic yokes 4/5. These yokes are composed of a material 
having a saturation magnetisation exceeding 1.8 Tesla. They form a slit 5 
which may be from 30 .mu.m to 2 mm wide. Inhomogeneous fields 19 having an 
intensity of .about.5000 A/cm are regulated across the slit. While the 
film 1 with the layer 8 moves in direct contact with the slit in the 
direction of the arrow 9, according to the invention, the slit field 19 
penetrates through the film onto the suspension. 
The layer thus obtains a magnetic primary axis 10 which, in the centre, 
lies obliquely to the direction of tape travel 9, as indicated by the 
arrow. 
The edge of a strip-shaped transversely magnetised permanent magnet, for 
example, can also be used for generating the strong inhomogeneous field of 
the first part of the orientation system. 
The effect of such an edge is shown in FIG. 3. 
Permanent magnet systems, which are known also to be inventive, are used 
for the second part of the orientation system, which again can be made up 
of one or more partial systems. 
FIG. 4 shows one system. It consists of two magnetic circuits which are 
arranged symmetrically about the coated film (1,8) and the scatter fields 
19 of which act in the same direction. An individual circuit is made up 
fundamentally like the first part of the orientation system (FIG. 2). In 
contrast to the first part of the orientation system, the slits 6/7 in the 
second part of the orientation system are at a certain distance 20 from 
the film and the cast suspension. The distance 20 should be at least 3 mm. 
This distance produces a maximum longitudinal field of at least 800 A/cm 
in the centre of the arrangement. The necessary size of the field 19 of 
2-3 cm in length is obtained by the curved surface 16 of the low 
retentivity yokes 4/5. Barium ferrites or rare earth magnets, for example, 
can be used as permanent magnetic materials. FIG. 5 shows the field 
configuration on the ordinate and abscissa of the longitudinal components 
over the line of symmetry of such a magnetic system running parallel to 
the plane of the film along the abscissa. The longitudinal component has a 
marked peak in the centre of the system. Small subsidiary peaks of 
opposing direction are located to the right and left of this peak. The 
direction of the main peak of the longitudinal component of the second 
part of the orientation system coincides with the field direction over the 
line of symmetry of the slit in the first part of the orientation system. 
The direction of the magnetic field of the second system is identical to 
the direction of the first field beyond the center of each slit. The 
values of the subsidiary peaks are substantially lower than the value of 
the main peak, so they can be neglected. 
Another arrangement according to the invention for the second part of the 
orientation system is shown in FIG. 6. A permanent magnetic plate 22 is 
transversely magnetised in direction 3. It is connected of magnetically 
anisotropic material with a high crystal anisotropy, allowing 
magnetisation perpendicularly to the surface. These surfaces then bear the 
magnetic charge with positive and negative poles. The plate has a slit 23 
of approximately 6 mm through the centre of which the film 1 with the 
layer 6 travels. The configuration of the longitudinal components is 
qualitatively the same as in FIG. 5. The field lines 21 belong to the 
region which yields the main peak of the longitudinal component, while the 
field lines 24 show the formation of the subsidiary peaks in the opposite 
direction. 
According to the invention, the systems shown in FIGS. 4 and 6 can also 
follow each other repeatedly to produce the second partial system, the 
field direction in the main peak remaining unchanged. 
The system shown in FIG. 7 in which two like poles face each other can also 
be used as the second part of the orientation system. It has on its line 
of symmetry parallel to the plane of the film two extreme values of the 
longitudinal component which are equal in value with opposite polarity 
values. The advantage of this system lies in the fact that the field is 
pressed out of the intermediate space by the like poles facing each other 
and an extended field region is thus produced along the film. The second 
partial system can also be made up of several individual systems with 
opposing like poles when producing the second partial system by the 
arrangement of the like poles facing each other. 
EXEMPLARY EMBODIMENT 
An industrial casting machine of the type shown in principle in FIG. 1 was 
used for experimental investigations into the invention. The film speed 
was approximately 15 m/minute in all tests. A slit caster containing no 
magnetic elements was used as an applicator system 15 for the suspension 8 
so that no external magnetic forces could act on the particles before the 
suspension entered the magnetic orientation system 17. 
The individual systems mentioned in the following list were available for 
the orientation system consisting of two parts. The corresponding numbers 
serve for identification in the following statements. Either barium 
ferrite of the 300K type or the Co-Sm alloy Secolite made by 
Thyssen-Edelstahlweke Dortmund was used as permanent magnet material. Low 
retentivity Fe alloys were used as magnetic flux conducting material. The 
characteristic data of the individual systems are given below. 
Individual system 1 (single circuit system) 
This individual system is used exclusively for the first part of the total 
system. It is shown in cross-section in FIG. 2. The cross-section of the 
permanent magnet 2 was 3.times.24 mm and was magnetised over the short 
edge. The external dimensions of the yokes 4/5 were 14.times.40 mm.sup.2. 
Two individual magnetic systems of this type with various slit widths 5 
were used. 
1.1 Slit width 0.8 mm 
1.2 Slit width 0.4 mm 
The following individual systems (2 and 3) are used exclusively for the 
second part of the total system. They are all made up in mirror image 
fashion to the film and layer travel so that only one longitudinal 
component of the directional field acts therein whereas the transverse 
components balance each other. 
Individual system 2 (two circuit system) 
The two circuit system is shown in principle in FIG. 4. Ferrite rods having 
a cross-section of 12.times.19 mm.sup.2, which were magnetised over the 
short axis, were used as permanent magnets. The total width of the yokes 
was 24 mm. The slit width was 0.8 mm. Two systems with differing field 
intensities were available. 
______________________________________ 
Total height of the 
Maximum field 
permanent magnet 
intensity 
Designation (mm) A/cm 
______________________________________ 
2.1 76 1 655 
2.2 38 920 
______________________________________ 
The field configuration shown in FIG. 5, with a strong field peak in the 
centre and the two weak subsidiary peaks of opposing field direction at 
the edge, is characteristic of this system. 
Individual system 3 (opposing like poles) 
Consisting of magnetic strips 25, 26 which are arranged in mirror image 
fashion and face each other with like poles. This system is shown in 
principle in FIG. 7. Two arrangements which differ substantially in their 
magnetic material and cross-section are tested in this case. 
______________________________________ 
Maximum 
Cross-section field in- 
Magnetic width .times. height 
Distance 
tensity 
Designation 
material mm mm A/cm 
______________________________________ 
3.1 Ferrite 25 .times. 40 
8 920 
3.2 Co--Sm 18 .times. 6 
8 2,210 
______________________________________ 
The orientation depends on the suspension used as well as the orientation 
system. Both the pigment and the lacquer have a decisive influence. 
Distance is the distance left between the two poles of the mirror image 
fashioned permanent magnets. The following two different suspensions were 
tested: 
______________________________________ 
Particle 
Pigment 
Pigment Coercivity 
Viscosity 
Dis- length 
type material field A/cm 
cpi persion 
.mu.m 
______________________________________ 
A Fe.sub.3 O.sub.4 
751 difficult 
0.7 
B Fe 844 2,500 good 0.3 
______________________________________ 
The quality of the orientation is described by the test values M.sub.R 
/M.sub.S ; M.sub.R// /M.sub.R.angle. described above. The field modulation 
for measuring the first two values was 3,200 A/cm. 
The following Tables 1 and 2 each show the orientation through one of the 
individual systems in the top part and the results achieved with the 
combination according to the invention in the second part. 
TABLE 1 
______________________________________ 
Pigment type: B 
Orientation system 1st part2nd partmmDistance 
M.sub.R.parallel. /M.sub.S 
##STR1## 
##STR2## 
______________________________________ 
Individual systems 
without 
without 0.665 0.583 1.255 
1.1 0 0.664 0.594 1.216 
3.1 8 0.729 0.535 1.538 
3.1 12 0.716 0.547 1.511 
3.1 20 0.707 0.557 1.364 
2.1 7 0.764 0.506 1.419 
3.2 8 0.795 0.480 1.878 
Combinations according to the invention 
.rarw.1.1 
.rarw.3.1 
8 0.779 0.480 1.903 
.fwdarw.1.1 
.rarw.3.1 
8 0.779 0.486 1.915 
.rarw.1.1 
.rarw.2.1 
7 0.799 0.471 1.950 
.fwdarw.1.1 
.rarw.2.1 
7 0.797 0.471 1.925 
.rarw. 1.1 
.rarw.3.2 
8 0.798 0.477 1.967 
.rarw.1.1 
.rarw.2.1 + 3.2 
7 0.802 0.488 1.852 
______________________________________ 
TABLE 2 
______________________________________ 
Pigment type A 
Orientation system 
Distance 
1st Part 2nd Part mm M.sub.R.parallel. /M.sub.S 
______________________________________ 
Individual system 
without without 0.64 
1.1 -- 0 0.68 
1.2 -- 0 0.68 
3.1 8 0.645 
2.1 7 0.70 
1.1 3.1 8 0.72 
1.2 3.1 8 0.715 
1.2 2.1 7 0.73 
1.2 2.1 + 2.2 7 0.74 
______________________________________ 
The results in the Tables show that better orientations can be achieved 
with the systems according to the invention consisting of two parts than 
with a system according to the prior art. M.sub.R// /M.sub.S and 
M.sub.R.angle. /M.sub.S values are invariably higher, resulting in better 
anisotropy of the pigment particles in a magnetic layer.