JPH0370854B2 - - Google Patents

Description

[Detailed description of the invention] [Technical field to which the invention pertains] The present invention relates to a method for manufacturing a magnetic recording medium, and in particular a method for more effectively performing magnetic field orientation treatment without impairing the surface smoothness of a coating film. It is.

[Prior art and its problems]

In the production of magnetic recording media, it is common to apply a magnetic paint to a support such as a non-magnetic film, followed by a magnetic field orientation treatment. That is,
A paint dispersed in a solvent together with magnetic particles and a binder is applied onto a support such as a polyester film.
The recording ability of the magnetic recording medium in this direction is increased by passing it through a magnetic field while it is still undried to orient the magnetic particles of the coating in the direction of the magnetic field.

Conventionally, this orientation process has been performed in a direction parallel to the surface of the recording ash body, and information has been recorded by magnetizing in this direction. However, when performing high-density recording, the self-demagnetizing field increases, so the magnetization vector rotates and residual magnetization decreases, putting a limit on high-density recording. Recently, therefore, a method has been proposed in which information is recorded by magnetizing the magnetic recording medium in a direction perpendicular to the surface thereof. In this method, the magnetization is perpendicular to the medium surface, so the higher the recording density, the lower the self-demagnetizing field.
Suitable for high-density recording.

By the way, in the perpendicular magnetization recording method, as shown in Fig. 1, magnetization is performed in a direction perpendicular to the surface of the medium coated on the substrate 3, so the axis of easy magnetization of the recording medium must be perpendicular to the medium surface. It is. Therefore, the magnetic field orientation treatment of the magnetic coating must also be performed in a direction perpendicular to the medium surface. Therefore, an orientation process is performed by applying a magnetic field by magnets 5 and 6 in a direction perpendicular to the surface of the coating film to the undried coating film 4 on which the magnetic coating material 1 has been applied by the coating device 2. It has been found that the smoothness of the coating film surface deteriorates significantly. This is undesirable as a recording medium, and was not observed when the alignment treatment was performed parallel to the medium surface. This is because the paint film is magnetized by a magnetic field perpendicular to the paint film surface, and magnetic poles of the same sign are generated on the paint film surface, which increases the static magnetic energy on the surface and makes it unstable, causing unevenness on the surface. Therefore, this is what is described as stabilizing. Therefore, it is found that it is extremely difficult to perform vertical alignment processing using such a magnetic field application method and to obtain a storage medium with a smooth surface.

[Purpose of the invention]

The present invention was made as a result of various studies to solve these difficulties, and it is possible to improve the coating film by applying a rotating magnetic field that rotates within the coating film plane perpendicular to the orientation direction to the wet coating film. The present invention provides a method for vertically aligning magnetic particles without impairing surface smoothness.

[Summary of the invention]

That is, in the present invention, as the principle is shown in FIG. 2, as an orienting magnetic field for the undried coating film 4,
By using a rotating magnetic field within the surface of the coating instead of using a magnetic field perpendicular to the surface of the coating, the magnetic particles within the coating can be vertically aligned without magnetizing the coating. In this case, the strength of the magnetic field is less than or equal to the magnetic field Hc required for reversing the magnetization of the magnetic particles. As a method of applying the orienting magnetic field, for example, an alternating magnetic field H ₁ produced by the coil pairs 7 and 8 in the same figure and an alternating magnetic field H ₂ produced by the coil pairs 9 and 10 are superimposed on the inner surface of the coating film at an angle θ, and then H ₁ , H ₂ is given as a temporal phase angle. It is desirable that both θ and θ are close to 90°, but they do not necessarily have to be 90°. Furthermore, the second
Although the principle is schematically shown in the figure, in practice it is necessary to adjust the size, arrangement, etc. of the magnetic field generating means so that the magnetic field is uniformly applied to the effective portion of the coating film. Further, it is desirable that the amplitudes of H ₁ and H ₂ be approximately the same, but they do not necessarily have to be equal. In Fig. 2, the coil serving as the magnetic field generating means is fixed, and a rotating magnetic field is generated by giving a temporal phase to the alternating magnetic fields H ₁ and H ₂ .
The coil itself may be rotated. Furthermore, if the medium itself is disk-shaped, it is also possible to rotate the medium in a fixed magnetic field.

[Embodiments of the invention]

Next, examples according to the present invention will be described.

Example A magnetic paint having the following composition was prepared.

Magnetic particles, Ba ferrite CoTi substituted product (σ _g =
62emu/g, Hc=1000Oe, particle size=0.1μm)
100 parts by weight (weight) Vinyl chloride vinyl acetate copolymer (VAGH)
15 parts Polyurethane (N-3022) 15 parts Methyl isobutyl ketone 100 parts Toluene 100 parts Microhexanone 100 parts This paint was coated on the polyethylene terephthalate film 3, and while it was still wet, it was rotated for orientation as shown in Figure 2. passed through a magnetic field. The frequency of the AC magnetic field used was 100Hz, the aforementioned θ≒≒90°,
The strengths of H ₁ and H ₂ were set to be equal. The coating film 4a subjected to such orientation treatment was dried and calendered to produce a magnetic tape. Magnetic measurements and surface roughness measurements were performed on this tape, and the vertical squareness ratio and surface roughness obtained by demagnetizing field correction were plotted against the orientation magnetic field H (=H ₁ = H ₂ ). is shown in Figure 3. In the figure, curve A indicates the squareness ratio in the vertical direction, and curve B indicates the surface roughness. As seen in this figure, it can be seen that this alignment method allows vertical alignment treatment without increasing the surface roughness of the coating surface.

Comparative Example As a comparative example to the above example, FIG. 4 shows the results of aligning the same paint using a perpendicular magnetic field as shown in FIG. 1. In addition, curves A and B in the figure
shows the same characteristics as in FIG. In this case, as a result of the coating being magnetized in the direction perpendicular to the film surface, a magnetic pole is generated on the film surface, and due to the demagnetizing field, the orientation magnetic field is approximately
4KOe or more is required, and when such an orientation magnetic field is used, the surface roughness increases, and even after calendering, significant surface roughness remains.
It can be seen that this is not desirable as a recording medium.

〔Effect of the invention〕

Note that the orientation method according to the present invention is not limited to the case where the above-mentioned Ba ferrite substituted product is used as the magnetic particles.
It is also effective when using γ-Fe ₂ O ₃ , its Co adherend, and CrO ₂ , and especially when using Co metal fine powder, by using this method, it is possible to obtain a surface with a squareness ratio of 0.8. A roughness of 0.1 μm was obtained.

The reason why vertical orientation can be obtained by applying a rotating magnetic field in this way is not necessarily clear, but it is thought to be due to the following reasons.
In other words, the magnetic particles in the wet coating film cannot follow the rotation angle of the rotating magnetic field, and the most stable state is when the magnetic particles are oriented perpendicular to the plane of the rotating magnetic field. This is thought to be because.

Also. This means that the energy ( ^per unit volume) of the magnetic particles in the rotating magnetic field H, Angle formed with H, Ms: The time average of the direction showing the minimum value of saturation magnetization is considered to be explained by the fact that in the magnetic field strength range where magnetization reversal does not occur, the direction is θ=90°.

As described above, according to the method of the present invention, a vertically oriented magnetic recording medium can be obtained without deteriorating the surface smoothness, and its practicality is great.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of the orientation treatment by the conventional method, FIG. 2 is an explanatory diagram of the orientation treatment by the method of the present invention, and FIG. 3 is the orientation magnetic field dependence of the orientation degree and surface roughness of the recording medium according to the present invention. The figure shown in FIG. 4 is a diagram showing the degree of orientation and surface roughness of a recording medium according to a conventional method. 1...Magnetic paint, 3...Base film, 4...
...Magnetic coating film, 7, 8, 9, 10...Rotating magnetic field generating coil.

Claims (1)

[Scope of Claims] 1. A method for manufacturing a perpendicular magnetic recording medium, characterized in that orientation treatment is performed by applying a rotating magnetic field that rotates in a plane perpendicular to the orientation direction of magnetic particles. 2. The method of manufacturing a perpendicular magnetic recording medium according to claim 1, wherein the rotating magnetic field is within the surface of the coating film, and the orientation direction of the magnetic particles is perpendicular to the coating surface.