WO1996035210A1 - Method and device for producing magnetic recording media - Google Patents

Abstract

The invention concerns a method and device for producing magnetic recording media with a disoriented particulate magnetic layer applied to a non-magnetic layer carrier. For the purpose of disorientation, the magnetic layer containing magnetically anisotropic pigments, after being applied to the layer carrier, is first oriented perpendicular to the plane of the magnetic layer by means of a unipolar magnetic field constant in time. The still-liquid magnetic layer is then passed into a field-free space where the layer is disoriented before it starts to dry. The device according to the invention comprises a magnetic arrangement which is mounted in a mirror-symmetrical manner either side of the layer carrier and generates a unipolar magnetic field oriented perpendicular to the plane of the magnetic layer. According to the invention, disc-shaped magnetic recording media can be produced from particulate magnetic layers without any portion of the reading voltage amplitude being dependent on orientation.

Description

Method and device for producing magnetic recording media

The invention relates to a method for producing a magnetic recording medium with a non-magnetic layer support and an oriented magnetic layer applied thereon, a magnetic dispersion which contains magnetically anisotropic pigments being coated on the layer support in the liquid state, and thereafter the still liquid dispersion layer is subjected to an orientation treatment by means of a magnetic field, is dried and then solidified.

The invention also relates to a magnet arrangement with an inlet part and an outlet part for the orientation treatment of a magnetic recording medium and a magnetic recording medium in disk form for storing information on individual circular tracks, consisting of a non-magnetic substrate and one or more disoriented magnetic layers.

In the current particulate magnetic recording media for recording video, audio and data signals, shape-anisotropic, mostly needle-shaped, ferromagnetic pigment particles are used, which are embedded in a binder matrix. The shape anisotropy, that is, the length-to-thickness ratio of the particles, causes a pronounced magnetic anisotropy in the sense that the magnetically easy axis of the particles is oriented parallel to the longitudinal direction. When coating a continuous material web, a distance-dependent shear force arises in the liquid magnetic dispersion between the application element and the moving layer support, with the effect that the needle-shaped pigment particles are at least partially rotated in the coating direction (see, for example, M. Pahl in: "Practical rheology of plastic melts and solutions ", VDI-Verlag, Düsseldorf, 1983, p. 38

- 39). In the following drying process, this preferred direction is "frozen" by fixing the orientation of the longitudinal axis of the particle and thus nevertheless its magnetically easy axis. In some applications, for example in disk-shaped magnetic recording media with a flexible layer support, this longitudinal magnetic orientation is undesirable because it is written and read in a ring-shaped and concentric manner with a magnetic head, and the reading signal is thus periodically modulated. There has been no shortage of efforts to solve this problem by disorienting the liquid

Eliminate magnetic dispersion.

A method of the generic type mentioned and a device suitable for carrying it out * are known from patent specification EP-B 0 121 093. According to EP-B 0 121 093, the magnetic orientation treatment is carried out in such a way that the rectilinearly moving layer support with the liquid magnetic layer is guided through a plurality of temporally unchangeable magnetic fields with alternating polarity which are arranged one behind the other in the direction of movement and are alternately polarized by groups of permanent magnets be generated. The direction of the magnetic fields is essentially perpendicular to the magnetic layer plane and the magnitude of the magnetic fields gradually decreases in the direction of movement.

This treatment section is preceded by a pretreatment section for magnetizing the liquid magnetic layer, which in an advantageous embodiment consists of a known annular gap magnetic head which touches the back of the layer support. From DE-A-35 23 396 a wedge-shaped arrangement with alternating magnetic poles is known, in which the magnetic layer carrier is guided asymmetrically. The following disadvantages occur in the production of spatially or areally isotropic magnetic layers according to EP-B-0 121 093 and DE-A-35 23 396:

The vertical orientation of the magnetically anisotropic pigment particles with magnetic fields of alternating polarity requires, in some cases, considerably large magnetic fields, which are far larger in magnitude than the coercive field strength of the pigment particles. So that the partial orientation of the magnetic particles already achieved in a magnetic treatment zone by the magnetic field in the subsequent treatment zone is further strengthened by the field directed there in an antiparallel manner to the previous one, a change in the direction of the magnetic field is also a change - Magnetization of the particles required. As is known, this is only achieved with high magnetic fields, the amount of which is at least equal to 1.5 to 2 times the coercive field strength H _{c of} the magnetic particles. In modern particulate recording media, the H _{c is} usually 25 to 65 kA / m, but in some products also more than 100 kA / m. In addition to the use of expensive special alloys as magnetic materials, for example SmCo, this also means that in practice very small pole distances to the layer carrier of less than 1 mm. In the pretreatment section, the surface of the substrate is even touched by the magnetic head. However, it is known that any contact of the moving layer support with fixed internals in the casting machine inevitably leads to abrasion, which greatly increases the number of errors in the recording medium. Extremely high demands are also placed on the precision of the magnetic pole production and the geometrical arrangement of the magnet groups, because the two magnet groups above and below the plane of the substrate have to be made exactly mirror-symmetrical, which is intentionally not done in the case of DE-A-35 23 396 . Magnet arrangements of this type are therefore not suitable for disorienting a magnetic layer.

DE-A 40 32 129 also deals with the problem of producing isotropic, that is to say statistically oriented, recording media, for example flexible magnetic disks with annular, concentrically arranged information tracks. In the final state, these data products must be completely magnetically isotropic, so that the read voltage signals are free of any orientation-dependent modulation. The device according to the teaching of DE-A 40 32 129 essentially consists of a rotatably arranged drum which extends over the entire coating width, the axis of rotation of which is arranged at right angles to the coating direction and which is equipped as completely as possible with magnetic elements of alternating polarity on the jacket surface , so that the local magnetic fields emerge vertically from the lateral surface. According to this teaching, even the slight orientation of the magnetically anisotropic particles in the liquid phase of the magnetic layer, which is caused by the coating process, can be largely eliminated by optimizing the controllable process parameters - distance of the jacket surface to the magnetic layer, rotational speed of the drum.

The basic process idea of DE-A 40 32 129 thus differs from the European patent discussed at the outset in that the random orientation of the magnetically anisotropic particles is brought about by swirling the particles by superimposing a large number of time-varying magnetic fields, the The amount and direction change in rapid succession as you pass through the treatment zone. However, the use of special magnets with the largest possible energy product is also important in this technical teaching, because at a distance of the layer support from the drum which is acceptable in practice, the magnetic effect of the drum attached to one side of the layer support is otherwise is too low. The large number of these magnets represents a considerable cost factor and also a manufacturing expense. In addition, the speed of the rotating drum is not known a priori and must be optimized for the respective magnetic layers by tests. Even with the optimal embodiment of the invention, no complete isotropy is achieved after the treatment; there is still a residual modulation of the reading signal up to 3% of the track-average reading voltage amplitude.

Further publications in which several, in some cases very many, magnetic fields act on the magnetically anisotropic pigments are described in JP-A 63-052 329, 01-169 726, 01-169 728, 60-125 931, 59-229 746, 01 -169,725. These disorient the magnetic pigments according to the same principles as the already discussed EP-B-0 121 093, DE-A-35 23 396 or DE-A-40 32 129.

The object of the present invention was to provide a method and the simplest possible device for producing magnetic recording media which enable the production of isotropic magnetic layers which contain magnetically anisotropic pigments. The method should work without contact, that is, all parts of the device should be arranged at a sufficient distance from the layer support and from the liquid magnetic layer applied thereon. In addition, the method and device should deliver good results even with small magnetic fields.

It has been found that this object can be achieved by a method in which the layer support with the magnetic layer still liquid after the application is subjected to an orientation treatment by means of a magnetic field, the

Orientation treatment takes place in the following treatment zones arranged one after the other: a) a directional zone which contains a unipolar, temporally constant magnetic field, the direction of which is oriented perpendicular to the plane of the layer support

b) a transition zone following the directional zone a), in which the magnetic field of the directional zone a) is continued and in the direction of movement except for at most

10% of the field strength in the directional zone a) decreases, and

c) a field-free removal zone following the transition zone b), the length of which is dimensioned such that the magnetically anisotropic pigments spontaneously disorient in the liquid dispersion layer.

The invention also includes a magnet arrangement for magnetic field treatment of a magnetic recording medium from:

- Magnets (7, 7 ') which are arranged mirror-symmetrically on both sides of a plane of symmetry which corresponds to the plane of the dispersion layer (4), wherein

- Magnetic poles of opposite names face each other

- With neighboring magnetic poles of the same name in each half of the room on both sides of the

Plane of symmetry, so that

- The magnetization vectors (9, 9 ') are rectified, and wherein - the distance of the pole faces from the plane of symmetry in the inlet part (a) of the arrangement is constant and

- In the outlet part (b) increases evenly.

The invention is explained in more detail below with the aid of the description, the figures and the examples.

FIG. 1 shows a cross section through the magnet arrangement according to the invention with the field-generating magnets (7, 7 ').

Figure 2 shows an advantageous embodiment of the. Magnet arrangement, field-generating magnets (7, 7 ') and field-forming parts (8, 8', 10, 10 ') being attached. Figures show typical circumferential distributions (circumferential angle φ) of reading

3a and 3b voltage amplitudes U (3a) and U '(3b) of a magnetic head, measured with clamped and rotating disk-shaped magnetic recording media, in the case of FIG. 3b) according to the invention and in the case of FIG. 3a) not according to the invention disk-shaped recording medium.

The magnet arrangement according to the invention consists of two halves, each containing magnets which are oriented identically to one another. The magnets of the two halves face each other with respect to a plane of symmetry (4) with magnetic poles of the same name.

The distance between the opposing pole faces is constant in an inlet part a) and increases in an outlet part b).

In the inlet part a), the distance of the pole faces from the plane of symmetry can be set and should be at least about 2 mm, so that contact of the layer support and the dispersion layer is avoided when using the magnet arrangement according to the invention. On the other hand, with regard to the effectiveness of the arrangement, the distance to the plane (4) should at most be so large that the magnitude of the magnetic field in the plane of symmetry is at least 20 kA / m. In each half, a plurality of magnetic elements can be arranged one behind the other in the x direction, the distances between the individual magnetic elements being such that the amount of the magnetic field is greater than 20 kA at every point in the plane of symmetry (4) in the inlet part a) / m.

In the outlet part b), the distances of the pole faces from the plane of symmetry (4) and the distances of the adjacent magnets in the x direction are chosen so that the amount of

Magnetic field decreases in the plane of symmetry and that the gradient of the magnetic field in the plane of symmetry in the x direction is always less than 1600 kA m ² , preferably less than 600 kA / m ² . At the outlet end of part b) of the arrangement, the distance of the pole faces from the plane of symmetry (4) is such that the amount of the magnetic field in plane (4) between the terminal magnetic poles is a maximum of 10% of the

Field in the inlet part a) in the plane of symmetry.

In an expedient embodiment, the individual magnetic elements (7, 7 ') in both halves of the arrangement consist of cuboid permanent magnets made of ferritic Material with an induction flux density of at least 250 mT. The dimensions of the ferrite blocks (7, 7 ') in the practical example are 130 mm x 50 mm x 18 mm, the magnetization vectors (9, 9 ") are perpendicular to the rectangular base area 130 mm x 50 mm. Transversely to the plane of symmetry (4) , i.e. in the y-direction, there are so many ferrite cuboids placed side by side that the total length of each ferrite line in y-

The direction is at least as large as the width of the substrate to be treated.

Other embodiments of the magnetic elements (7, 7) are also suitable within the scope of the invention, for example direct current electromagnets with or without an iron core, or permanent magnets made from alloys of elements from the group

Iron, cobalt, nickel, aluminum, silicon or rare earths, especially samarium, neodymium.

FIG. 2 shows an advantageous embodiment of the magnet arrangement with the field-generating magnets (7, 7) in the inlet part a) and soft magnetic metal sheets (8, 8 ',

10, 10 ') in the inlet part a) and outlet part b). In the specific case, the magnets (7, 7 ') consist of ferrite blocks of the type described. The soft magnetic metal sheets are arranged symmetrically with respect to the plane (4) so that the lower sheets (8, 8') and the upper sheets (10, 10 ') touch the pole faces of the magnets. To accommodate the magnets, the metal sheets are milled in their full width, that is to say in the y direction, advantageously approximately 1 mm. The top and bottom plates are made of soft magnetic steel with a high relative permeability greater than 10, preferably greater than 100. According to the invention, however, they can also consist of other highly permeable, soft magnetic Fe alloys, for example Fe-Si, Fe-Ni, Fe-Ni. Co.

The lower plates (8, 8 ') are adjacent to the plane of symmetry (4) and are at a distance of 2 to 8 mm, preferably 5 mm. The upper limit of 8 mm corresponds to the limit of the effectiveness according to the invention for disorienting the dispersion layers described in the examples. On both sides of the plane (4), the lower plates (8, 8 ') are angled at an angle α to the direction of movement x, so that they widen in the entire area of the transition zone b) with a constant opening angle α in the direction of movement x. The high relative permeability of the soft magnetic sheets (8, 8 ') causes the magnetic field lines to end perpendicular to the surface of these sheets in the entire field-filled area. At the same time, the soft magnetic property of the metal sheets (8, 8 ') that the magnetic field between the two arrangement halves is independent of the manufacturing tolerances of the magnets, in particular their individual induction flux density, and that the magnetic field in the inlet part a) is homogeneous. Furthermore, the lower plates (8, 8 ') also shield the interior of the transition zone b) from harmful magnetic stray fields which, for example, originate from the opposite rear side of the magnets. According to the above explanations for FIG. 1, the geometric dimensions of the lower plates in the specific embodiment were selected as follows: plate thickness in the z direction 3 mm, width in the y direction 700 mm and length in the x direction in the directional zone a) 90 mm , in the transition zone b) along the angled section 100 mm, opening angle α to the x direction was 20 °.

The distance between the lower plates (8, 8 ') from the plane of symmetry in the inlet zone was 5 mm. The distance between the lower plates (8, 8 ') increases evenly in the outlet part (b) and is wedge-shaped in the example. However, it can be any other steady _; smooth curve shape of the magnetic field generating arrangement in the outlet part (b) can be used, which also realize this uniform increase in distance, for example

Quarter circle, parabolic, hyperbolic, elliptical, etc.

The top plates (10, 10 ') serve to reduce the stray field of the magnets (7, 7') facing away from the plane of symmetry (4). The exact dimensions of the top plates are not essential to the invention. However, for their effectiveness it is advantageous to dimension their length in the x and y directions so that the facing pole faces of all magnets (7, 7 ') are at least covered.

In the concrete embodiment of the magnet arrangement described, the magnetic field was measured by means of a commercially available Hall measuring probe. The measurement was carried out only in the plane of symmetry (4), which corresponds to the layer plane when the magnet arrangement is used to disorient liquid dispersion layers.

The vertical magnetic field component H _z in the z direction of the arrangement was constant in the inlet part a) and was 30 kA / m with a steady decrease in the measured value in the outlet part b). In this area, the gradient dH _z / dx of H _z in the x direction and in the plane of symmetry (4) was approximately 0 to a maximum of 600 kA / m ² . At the end of part b), H _z = 3 kA / m, that is 1/10 of the measured value in part a). The measured horizontal components H _x , Hy, were negligible in the entire magnetic field range and were also at most 3 kA / m.

A comparative measurement with the exceptionally set maximum distance value of 8 mm on both sides of the layer support plane to the lower plates (8, 8 ') gave the measured value 20 kA / m for H ₂ in the inlet part a). This value provides the actually required limit for the effectiveness of the disorientation in the device according to the invention, regardless of the special magnetic properties of the magnets and soft magnetic sheets used.

In front of the inlet part a), the measurement of the H _z field in the plane of symmetry initially gives an opposite polarity to the H ₂ value in the inlet part a). This is due to the stray field which results from the pole faces of the magnets or the top plates (10, 10 ') facing away from the plane of symmetry. When approaching the inlet part a), the polarity is reversed and the value of H ₂ rises steeply, the gradient of H _z in the x direction is 1600 kA / m ² , until finally the constant value of the magnetic field H _z in the inlet part a) of 30 kA / m is reached.

The magnet arrangement is used in the following way:

Immediately after coating with the still liquid magnetic dispersion layer, the layer support is guided through the magnet arrangement perpendicularly to the magnetic field, that is, along the plane of symmetry (4). It is essential that the inlet part a) of the arrangement is first crossed. This corresponds to the directional zone a), in which the magnetic pigments are initially placed perpendicular to the layer plane (4) by the action of the largest field in the arrangement. The layer support is then passed through the outlet part b), which corresponds to the transition zone b). In this, the perpendicularly orienting magnetic field influence decreases so slowly that the magnetic pigments initially maintain their orientation and at the outlet end with the subsequent field-free discharge zone c) due to their mutual magnetic

Repulsive forces disorient themselves spontaneously.

Examples 1 to 3 according to the invention always showed the great effectiveness of the method according to the invention in the specific magnet arrangement described. Due to the mirror symmetry of the magnet halves, no special requirements were placed on the guiding accuracy of the layer carrier, the guiding tolerance perpendicular to the layer plane was Δ z = ± 0.5 mm.

A very special advantage is that with the method according to the invention, magnetic layers with coercive field strength H _{c of} up to 56 kA / m were effectively disoriented in the examples with a maximum value of the magnetic field in the directional zone a) of only 30 kA / m. Thus, liquid dispersion layers can be effectively disoriented with a magnetic field that is smaller than the H _{c of} the magnetic pigments. This property allows an effective disorientation even of highly coercive, pigmented magnetic layers, which for example also contain metal pigment with H _c greater than 100 kA / m, with the arrangement according to the invention using the method according to the invention.

For comparison, the direction of movement of the substrate with the liquid dispersion layer was also changed by the magnet arrangement in a preliminary test, so that the outlet part b) was first crossed. In this case, the directional zone a) is given by the outlet and inlet part and the transition zone b) corresponds to the field area with a steep field gradient in the x-direction of 1600 kA / m ² and reversal of the field polarity. No disorientation effect of the arrangement was found. In any case, this preliminary test provides an upper limit for the field gradient in the x-direction in the transition zone b).

With the device described, the known magnetic recording media can be easily disoriented, provided that after leaving the transition zone b) the magnetic layer is still fluid enough to allow a spontaneous disorientation of the magnetically anisotropic pigments in the removal zone c) lichen. In the field-free space, the pigment particles previously oriented in the z direction and polarized in the same direction experience torques which only become effective when the coating solution has low resistance to flow, i.e. low viscosity and yield point. The exact length of the removal zone c) is therefore not relevant to the present invention. In contrast, it is necessary to optimize the distance of the outlet end of the transition zone b) from the coating station. This distance should be as small as possible, which means that the entire device should be as close as possible to the coating station. should be brought so that the drying process begins as late as possible after passing the outlet end of the transition zone b). In the following examples, the distance from the entry point of the straightening zone a) was approximately 1.5 m from the coating point in the x direction at a transport speed of the support in the x direction of 90 m / min.

The magnetic recording media which can be produced according to the invention with the described magnet arrangement according to the method according to the invention consist of a non-magnetic layer support with at least one magnetic layer applied thereon, which in the finished state contains magnetically anisotropic ferromagnetic pigment particles embedded in a lacquer layer of known per se Contains binders and other auxiliaries. For example, all commonly used polymeric films can be used as layer supports. They can either be uncoated or wear one or more layers, which in turn can also contain magnetic pigments. The polymer films preferably consist of

Polyethylene terephthalate, polyethylene naphthalate, polypropylene sulfide. The film thickness is usually between 4 μm and 100 μm, preferably between 9 μm and 75 μm.

Magnetic anisotropic, ferromagnetic pigment particles are preferably considered: finely divided rod-shaped gamma-iron (III) oxide with an average particle size of 0.1 to 2 μm and in particular 0.1 to 0.9 μm or rod-shaped chromium dioxide same particle structure as given for iron oxide. Other suitable materials are gamma-iron (III) oxide with doping of heavy metals, in particular cobalt, and finely divided metal alloys containing iron, cobalt and / or nickel. Finely divided chromium dioxide is particularly suitable. Pigment mixtures are also suitable. With these particles, the magnetic anisotropy is predominantly determined by the rod-like shape. The ratio of length to thickness of the rods is greater than 2, mostly between 4 and 0. The average rod length is usually 0.1 to 0.4 μm.

The binders forming the magnetizable layer contain not less than 40% by weight of polyurethane. Solvent-containing polyurethane elastomers such as those described, for example, in DE-B 11 06 959 or in DE-B 28 53 694 are suitable for this purpose. Further suitable polyurethanes are in DE-A 32 26 995, 32 27 163 and 32 27 164. The polyurethanes can be used as sole binders or in blends with other polymers (such as polyvinyl formals, phenoxy resins, PVC copolymers). 10 to 40% of the second binder component is preferably added. With these binders, it is particularly advantageous that additional dispersants can be dispensed with in whole or in part.

A crosslinking of the magnetic recording media, which may be necessary, depending on the binder system and tape property profile, is the reaction of the polyurethanes or polyurethane binder mixtures with polyisocyanates. A large number of organic di-, tri- or polyisocyanates or isocyanate prepolymers up to a molecular weight of 10,000, preferably between 500 and 3000, can be used for the crosslinking. Polyisocyanates which carry more than 2 NCO groups per molecule are preferred. Polyisocyanates based on tolylene diisocyanate, hexamethylene diisocyanate or isophorone diisocyanate, which are formed by polyaddition to diols or triols or by biuret and isocyanurate formation, have been found to be particularly suitable. proven. An addition product of tolylene diisocyanate with trimethylolpropane and ethylene glycol is particularly favorable. The amount of polyisocyanate used is to be adapted to the respective binder system.

Depending on the binder used, water, cyclic ethers such as tetrahydrofuran and dioxane and cyclic ketones such as cyclohexanone are used as solvents. The polyurethanes are also soluble in other strongly polar solvents, such as dimethylformamide, N-methylpyrrolidone, dimethyl sulfoxide or ethyl glycol acetate. It is also possible to borrow the solvents mentioned with aromatics such as toluene or xylene and esters such as

Mix ethyl or butyl acetate.

In general, further additives for improving the magnetic layer are added to the dispersions of magnetic material and binder. Examples of such additives are fatty acids, polycarboxylic acids, mono-, di- or polysulfonic acids or polyphosphoric acids, their mixtures, esters or salts with metals of the first to fourth main groups in the periodic table of the elements, lecithins, fluorocarbons, and also fillers, such as carbon black, Graphite, quartz powder and / or non-magnetized Powder based on silicate or iron oxide. Such additives are usually less than 10% by weight, based on the solid magnetic layer.

The magnetic dispersions are produced in a known manner. For this purpose, the magnetic material with the binder used and sufficient solvent is dispersed in a dispersing machine, for example a pot ball mill or an agitator ball mill, with the addition of the further additives, if appropriate. To set the appropriate binder-pigment ratio, these additives can be added to the mixture either in the solid state or in the form of solutions or dispersions with 10 to 60 parts by weight, based on 100 parts of the

Liquid to be added. It has proven to be expedient to continue the dispersion until an extremely fine distribution of the magnetic material is achieved, which can take 1 to 5 days. Subsequent repeated filtering gives a completely homogeneous magnetic dispersion. Any necessary crosslinking agents are added to the dispersion before coating.

The magnetic dispersion is applied to the layer support with the aid of conventional coating machines, for example by means of a ruler, or by roller applicators. Before the still liquid coating mixture is dried on the carrier, which is expediently carried out at temperatures of 50 to 100 ° C. for 0.2 to 5

Minutes happens, the anisotropic magnetic particles are disoriented by the action of a magnetic field with the invention by ^'rect. The magnetic layer can then be smoothed and compacted on conventional machines by passing between heated and polished rollers, if appropriate using pressure and temperatures of 20 to 100 ° C., preferably 40 to 80 ° C. The thickness of the

Magnetic layer is generally 0.2 to 20 μm, preferably 0.7 to 10 μm.

Although the present invention can be used advantageously for all described magnetic media for disorientation, the following examples and comparative experiments relate to the production of disk-shaped magnetic recording media. It is known that with these products the areal isotropy of the magnetic layer, that is to say the degree of disorientation, can be measured particularly sensitively, for example with a certifier of the type ML 5000 Desk Top Disk Evaluator from Medialogic Inc., Plaineville, Mass., UNITED STATES. For this, the Magnetic record carrier, here a reference diskette from the Physikalischen-Technische Bundesantalt Braunschweig, Germany, clamped onto a read / write device with a rotation speed of 300 rpm and the circumferential distribution of the read voltage amplitude UL on a fixed radius and at the preferred 2F write frequency Read 250 kHz of a previously written square wave signal with a magnetic head. With the lowest available magnetic preferred direction, a characteristic sinusoidal double wave is obtained, as is known, as the envelope of the circumferential distribution U as an LF signal, as shown in FIG. 3a. In contrast, a completely isotropic, disoriented magnetic layer shows no double-wave component in the envelope of the circumferential distribution of the reading voltage amplitude U ', see FIG. 3b. (U _m ax - U _{L m} i _n) In theory, the known relationship between the modulation should (U _ma + U _min) and the directivity of M _x / M _y apply the directivity if small, and is close to 1:

- 1) (This relationship also applies to U ')

M _x and M _{y are} the remanent magnetizations of a suitable tape sample after exposure to magnetic fields in the x and y directions in the layer plane.

So that the modulation amplitude of the sine-wave component of the double U _L bezie¬ hung as U _L ', a more sensitive and accurate measure of the disorientation of the magnetic layer is shown Presenting ^ than the usually measured directivity.

The quantitative analysis and the comparison of the reading voltage distributions U or U 'according to FIGS. 3a and 3b over the circumferential angle φ additionally revealed the following surprising, but nevertheless very welcome, side effects in all the measured examples. In Figures 3a and 3b, the 100% information relates to the average level of the above. Reference disk.

The mean value of U _L 'over a full extent of the recording track in the case of the invention disoriented in FIG. 3b was equal to the average value of U _L in the non-oriented case FIG. 3a of the respective comparative example. The modulation noise of the reading voltage U, that is to say the small statistical RF signal fluctuations of the reading voltage amplitude on a recording track, was clear in the case of FIG. 3b, which was disoriented according to the invention, compared to case 3a, that is to say -0.5 dB compared to a modulation noise / useful voltage ratio of approximately 35-40 dB for floppy disks, which was calculated using Fourrier analysis from the measurement signal from the above-mentioned certifier. The fact that the modulation noise is reduced gives the person skilled in the art an unpredictable reduction in roughness (not shown in the drawing).

Both favorable side effects of using the method according to the invention are by no means self-evident, considering that the spontaneous disorientation of the magnetically anisotropic pigments in zone c) is preceded by a perpendicular orientation to the magnetic layer plane in zones a) and b). An increase in roughness of the magnetic layer was therefore foreseen for the person skilled in the art with the known deteriorations in the reading voltage level and the modulation noise.

The following examples as well as the comparative experiments show the effectiveness and advantages of the invention, applied to the selected magnetic media.

example 1

A 75 μm thick polyethylene terephthalate film was coated in two passes through a coating machine with a magnetic layer on both sides, with a dry layer thickness of 2.2 μm. The magnetic layers were liquid with a ruler

Mold coated, disoriented according to the invention, dried and calendered in the usual way. Disc-shaped magnetic recording media in the 5.25 inch format were punched out of the finished, wound coating web and finished in the usual way. The magnetic layer dispersion had the following composition:

The magnetic pigment consisted of needle-shaped γ-Fe ₂ 0 ₃ with the coercive force H _c about 23 kA / m, needle length on average 350 nm and length-thickness ratio about 6. This was combined with inorganic, non-magnetic pigment Al ₂ 0 ₃ and Soot in one Amount totaling 18% by weight, based on the finished dispersion. The binder solution consisted of a solution of 10 parts by weight of the combination polyurethane-phenoxy resin-nitrocellulose to 90 parts by weight of tetrahydrofuran. The proportion by weight of the binder solution in the finished dispersion was 31%. The rest of 51% consisted of tetrahydrofuran and dioxane solvents in a ratio of 1: 1 and less than 1% by weight of conventional auxiliaries.

The resulting magnetic layer was isotropic, that is to say without a preferred magnetic direction. The measured modulation stroke of the sine double wave from U _L was 0 to 1%. Guide factors 0.98 to 1.02 were measured on tape samples with a conventional vibration magnetometer, the mean value of the measurements was 1.0.

Comparative Example 1

The procedure was as in Example 1, but without the magnetic layer being disoriented according to the invention.

The resulting magnetic layer was partially oriented with the magnetically easy axis in the x direction. The orientation-dependent modulation stroke of U was 0 to 3%, on average 1%. Guide factor values 0.98 to 1.06 were measured, on average 1.02.

Example 2

The composition was analogous to Example 1 with the following changes: The magnetic pigment consisted of a pigment mixture of 80% by weight of a needle-shaped, co-adsorbed γ-Fe ₂ O ₃ with H _c 50 kA / m, length of the particles on average 230 nm and length-thickness ratio on average 5 and 20% by weight acicular CrO ₂ with H _c 48 kA / m, length on average 240 nm and length-thickness ratio on average about 10. The thickness of the dry magnetic layer was 1.0 μm. The disk-shaped magnetic recording media were punched out in the 3.5 inch format.

The resulting magnetic layer was isotropic, that is to say without a preferred magnetic direction. The measured modulation stroke of the sine double wave from U _L was 0 to 1%. Guide factors 0.98 to 1.02 on tape samples were measured with a conventional vibration magnetometer, the mean value of the measurements was 1.0.

Comparative Example 2

The procedure was as in Example 2, but without the disorientation according to the invention.

The resulting magnetic layer was partially oriented with the magnetically easy axis in the x direction. The orientation-dependent modulation stroke of U was 0 to 3%, im

Average 1.5%. Guide factor values of 0.98 to 1.08 were measured, on average 1.03.

Example 3

The composition was analogous to Example 2 with the following changes:

Portion of the magnetic pigment mixture consisted of co-adsorbed γ-Fe ₂ 0 ₃ with H _c * 56 kA / m, length of the particles on average 240 nm and length-thickness ratio on average 5. The thickness of the dry magnetic layer was 0. 7 μm. The magnetic layer was applied using a reverse roll roller applicator.

The resulting magnetic layer was isotropic, that is to say without a preferred magnetic direction. The measured modulation stroke of the sine double wave from U _L was 0 to 1%. Guide factors 0.98 to 1.02 on tape samples were measured with a conventional vibration magnetometer, the mean value of the measurements was 1.0.

Comparative Example 3

The procedure was as in Example 3, but without the disorientation according to the invention.

The resulting magnetic layer was partially oriented with the magnetically easy axis in the x direction. The orientation-dependent modulation stroke of U _L was 0 to 3%, on average 2.5%. Guide factor values 0.98 to 1.10 were measured, on average 1.05.

Claims

Claims: 1. A method for producing a magnetic recording medium with a non-magnetic substrate and a de-oriented magnetic layer applied thereon, wherein a magnetic dispersion which is magnetically anisotropic Contains pigments, is coated in the liquid state on the substrate and then the still liquid dispersion layer is subjected to an orientation treatment by means of a magnetic field - is dried and then solidified, characterized in that the orientation treatment takes place in the following treatment zones arranged in succession: a) a Directional zone, which contains a unipolar, time-constant magnetic field, the direction of which is oriented perpendicular to the plane of the dispersion layer b) a transition zone following the directional zone a), in which the magnetic field of the directional zone a) is continued and in the direction of movement up to a maximum of 10 % of the field strength in the directional zone a) decreases, and c) a field-free removal zone following the transition zone b), the length of which is dimensioned such that the magnetically anisotropic pigments spontaneously disorient in the liquid dispersion layer. 2. The method according to claim 1, characterized in that in the directional zone a) Amount of the unipolar magnetic field is not less than 20 kA / m. 3. The method according to claim 1 or 2, characterized in that the unipolar magnetic field in directional zone a) is a Gieich field. 4. The method according to claim 3, wherein the amount of the DC magnetic field is smaller than the coercive force of the magnetic pigments. 5. The method according to any one of claims 1 to 4, wherein at each point in the dispersion layer in the transition zone b) the gradient of the magnetic field perpendicular to the dispersion layer plane in the direction of movement of the layer support is less than 1600 kA / m ² . 6. The method according to claim 5, wherein the gradient of the magnetic field perpendicular to the layer plane is not greater than 600 kA / m ² . 7. The method according to any one of claims 1 to 6, wherein at no point in the dispersion layer in the directional zone a) and in the transition zone b) the amount of the magnetic field parallel to the dispersion layer is more than 3 kA / m. 8. Magnet arrangement with an inlet part a) and an outlet part b) for the orientation treatment of a magnetic recording medium with a non-magnetic layer support and a liquid dispersion layer applied thereon, characterized by magnets (7, 7 ') which are mirror-symmetrical on both sides of one Are arranged plane of symmetry, which corresponds to the level of the dispersion layer (4), with opposite magnetic poles facing each other and with adjacent magnetic poles of the same name in each half of the room on both sides of the plane of symmetry, so that the magnetization vectors (9, 9 ') are aligned, and the Distance of the pole faces from the plane of symmetry in the inlet part (a) Arrangement is constant and increases evenly in the outlet part (b). 9. Magnet arrangement according to claim 8, characterized by - Soft magnetic steel sheets (8, 8 ') and (10, 10') which are attached mirror-symmetrically on both sides of the plane of symmetry and which are bent at an angle in the outlet part (b) with respect to the inlet part (a) and - which the pole faces of the magnets (7, 7 ') touch. 10. Magnet arrangement according to claim 8, wherein the magnets (7, 7 ') are arranged only in the inlet part (a) - soft magnetic steel sheets (8, 8 ') and (10, 10') which are attached mirror-symmetrically on both sides of the plane of symmetry and which are bent at an angle in the outlet part (b) with respect to the inlet part (a) and touch the pole faces of the magnets (7, 7 '). 11. Magnet arrangement according to one of claims 8 to 10, wherein the magnets (7, 7) are permanent magnets. 12. A magnet arrangement according to claim 11, wherein the saturation magnetization of the magnets is not less than 250 mT. 13. Magnet arrangement according to one of claims 8 to 10, wherein the magnets (7, 7 ') are electromagnets. 14. Magnetic recording medium for storing information on individual circular tracks, consisting of a non-magnetic substrate with one or more deoriented magnetic layers, characterized in that the circumferential distribution of the reading voltage level (U _L ) on each circular track contains essentially no orientation-dependent portion. 15. Magnetic recording medium according to claim 14, characterized in that the circumferential distribution of the measured reading voltage level (U _L ') on each circular track contains a low-frequency modulation component of at most 1% of the target level. 16. Magnetic recording medium according to claim 14 or 15, characterized in that the circumferential distribution of the measured reading voltage level U _L 'on each circular track has a reduction in high-frequency modulation noise of at least - 0.5 dB. 0078/380/94