JPH0250531B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a magnetic recording medium, and more particularly to a method for effectively performing a magnetic field orientation treatment to obtain a perpendicularly magnetized recording medium.

Perpendicular magnetization recording has been rapidly researched in recent years as it exceeds the recording density limit of conventional longitudinal magnetic recording. As a characteristic necessary for this perpendicular magnetization recording medium, it is necessary to have magnetic anisotropy in a direction perpendicular to the magnetic coating surface. However, this technology of perpendicular magnetization recording is currently still in the early stages of research, and there are only a few reports regarding recording media. According to these reports, thin films produced by sputtering or vapor deposition of Co-Cr alloys are used in experiments in order to produce an axis of easy magnetization in the perpendicular direction. However, manufacturing perpendicular magnetization recording media by the sputtering method or the vapor deposition method has restrictions on the manufacturing process and inevitably increases the manufacturing cost, which is not preferred in practice.

Furthermore, even if an attempt is made to manufacture a perpendicularly magnetized recording medium using a coated medium such as a tape or sheet, it is impossible to create one having an axis of easy magnetization in the perpendicular direction, and thus no such example has been found to date. Even if an attempt is made to create a perpendicularly magnetized recording medium by coating a magnetic paint obtained by mixing acicular magnetic particles or amorphous magnetic particles with a binder on a non-magnetic support, it is impossible to create a perpendicularly magnetized recording medium by some method. An operation must be performed to make the axis of easy magnetization perpendicular to the surface of the magnetic coating. In other words, in a coated perpendicular recording medium, in order to align the axis of easy magnetization in the direction perpendicular to the magnetic coating surface, it is necessary to use the shape magnetic anisotropy of acicular magnetic particles such as γ-Fe ₂ O ₃ . In the case of particles that generate coercive force (Hc), the magnetic particles must be aligned perpendicular to the film surface, and, like barium ferrite, the magnetic crystalline anomaly rather than the shape must be aligned. In the case of magnetic particles that produce coercive force due to their anisotropy, the anisotropy must be oriented perpendicular to the surface of the magnetic coating. To do this, it is necessary to apply a magnetic field in the vertical direction, but even if magnetic particles are oriented perpendicular to the coating surface using the magnetic field orientation treatment used in conventional methods, the coating surface A phenomenon occurs in which the magnetic field rises in the direction of the applied magnetic field, and the surface becomes rough and its surface properties are significantly deteriorated. That is, according to the conventional method, the surface roughness of the vertically oriented magnetic tape obtained is as high as σ>5μ,
The surface is extremely rough. When such significant surface deterioration occurs, the distance between the magnetic head and the tape varies, which greatly affects the playback direction force.
In particular, it becomes unusable as a perpendicular magnetization recording medium used in a short wavelength region.

Therefore, an object of the present invention is to provide a manufacturing method for obtaining a perpendicular magnetization recording medium by applying an alternating magnetic field to a coated medium.

The method according to the present invention can be applied to any type of coated media, and is not limited to magnetic particles coated on a non-magnetic support such as a tape or a sheet. Any magnetic particles used in recording media can be used. Examples of particles that can be used include γ-
Fe ₂ O ₃ , Fe ₃ O ₄ , γ-Fe ₃ O ₃ and Fe ₃ O ₄ oxidation state intermediate iron oxide, containing Co γ-Fe ₂ O ₃ , containing Co
Fe ₃ O ₄ , Co-containing iron oxide in an oxidation state intermediate between γ-Fe ₂ O ₃ and Fe ₃ O ₄ , CrO ₂ , CoO ₂ with one or more metal elements such as Te, Sb, Fe , oxides containing Bi, barium ferrite, Fe,
Metals such as Co and Ni, Fe-Co, Fe-Ni, Fe-Co
-Ni, Fe-Co-B, Fe-Co-Cr-B, Mn-
Examples include alloys such as Bi, Mn-Al, Fe-Co-V, and iron nitride.

After coating a magnetic paint containing magnetic particles as described above on a non-magnetic support according to a conventional method, when the coating film is not yet dry and is in a flowable state, the coercive force of the magnetic powder is determined. Vertical alignment treatment is performed by applying an alternating current magnetic field with a strength within ±20% of the alignment direction from at least two directions perpendicular to the alignment direction. This alternating magnetic field is applied in a direction perpendicular to the orientation direction. Note that even if the direction of magnetic field application is said to be perpendicular to the alignment direction, a certain degree of deviation in direction is allowed in practice, and it is natural that such deviation in direction should be interpreted as being included in the perpendicular direction here. It is. Such an alternating magnetic field can be generated by an alternating magnetic field generator, such as a solenoidal alternating current magnet. The solenoid AC magnet used in the present invention has an AC magnetic field of 0 at a frequency of 50Hz.
It can be varied in the range from to about 5 KOe. Note that the frequency is preferably about 40 Hz or higher. In addition, the strength of the applied magnetic field is close to the coercive force (Hc) of the magnetic particles used, and ±
It is within the range of about 20%. Furthermore, the magnetic field application time is preferably about 100 msec or more. The magnetic field application time can be controlled by changing the length of the solenoid of the solenoid AC magnet used, or by appropriately adjusting the running speed of the tape through which the AC magnetic field is passed.

In addition, the AC magnetic field generator such as a solenoid AC magnet may be used in a single unit or in multiple units, but in either case, the AC magnetic field generator applies an AC magnetic field at right angles to the orientation direction. It needs to be placed so that it can be done. That is, unlike in the conventional method, the magnetic recording medium must be arranged so that the alternating magnetic field is not applied parallel to the vertical alignment direction, in other words, in a direction perpendicular to the longitudinal direction of the tape of the magnetic recording medium (i.e., in the thickness direction of the tape). Must be.

In the present invention, as described above, the reason why the tape is oriented in the thickness direction, that is, in the perpendicular direction, by applying an alternating magnetic field in a direction other than the tape thickness direction, that is, in a direction perpendicular to the orientation direction, is as follows. can be inferred. If the strength of the applied magnetic field is less than the coercive force (Hc) of the magnetic particles in the magnetic paint,
Switching of magnetization does not occur, but the particle simply repeats rotational motion due to the alternating magnetic field, and this becomes more intense as the magnitude of the magnetic field increases, but when the magnetic field exceeds the coercive force of the magnetic particle,
This time, magnetization switching occurs, so the magnetic particles no longer make large movements, and only the direction of the particles' spontaneous magnetization is reversed by the alternating magnetic field. When this magnetic field is increased, a high degree of orientation can be obtained. After applying an alternating magnetic field near the coercive force (Hc) of the magnetic particles, which determines whether switching of longitudinal magnetization occurs or not, one solenoid alternating current magnet is placed in the longitudinal direction of the tape, the tape is dried and the final It was found that the orientation direction of the magnetic particles in the tape obtained was within the plane of the width (Y)-thickness (Z) direction. This is due to the fact that the magnetic particles, which were repeatedly rotating in response to an external alternating magnetic field, pass through that magnetic field and as the magnetic coating dries, the movement of the particles is affected by the viscous resistance of the surrounding binder. It can be assumed that this is because as it is blocked by the alternating magnetic field, it becomes unable to rotate following the magnetic field, and remains in a stable position at approximately 90 degrees with respect to the alternating magnetic field. However, as the drying progresses further, the solvent in the magnetic paint evaporates and the paint film becomes thinner, and the particles that were distributed on the Y-Z plane are destroyed as the film thickness decreases, so eventually It is thought that the orientation will be aligned in the Y direction, and eventually the orientation in the Z direction will decrease by a considerable percentage even if an alternating current magnetic field near Hc is applied.

Therefore, if one more AC magnetic field device is arranged so that the direction can be changed by 90 degrees and an AC magnetic field can be applied in the width direction, the solenoid AC magnet placed in the longitudinal direction of the tape can be used to apply an AC magnetic field in the Y-Z plane. The magnetic particles directed towards the tape are moved in the tape longitudinal direction (X) -
It is possible to make particles oriented in the Z plane more reliably in the Z direction, ie in the vertical direction. Furthermore, one or a plurality of other solenoid AC magnets can be arranged in succession, in which case it is possible to more reliably keep the magnetic particles oriented in the Z direction oriented.
However, it is not practical to arrange too many alternating current magnetic field generators, and the arrangement may be appropriately determined in consideration of economic efficiency and the degree of perpendicular magnetization.

The method of the present invention eliminates the phenomenon that the magnetic paint stands up in the direction of the applied magnetic field, which was unavoidable when vertically aligning using conventional methods, and the surface of the tape coating deteriorates, resulting in deterioration of its surface properties. It is possible to obtain a practically extremely advantageous perpendicular magnetization recording medium in which no perpendicular magnetization occurs.

Hereinafter, the present invention will be explained in more detail with reference to Examples.

Example 1 A magnetic tape was prepared using a magnetic paint having the following composition.

Composition by weight γ-Fe ₂ O ₃ (Hc=380Oe) 100 Vinyl chloride-vinyl acetate copolymer (product name "VAGH") 15 Polyurethane resin (product name "Estane 5702")
15 Lecithin 1 Methyl ethyl ketone:anone (1:1) mixed solvent 300 After kneading the above composition, apply this magnetic paint on a polyethylene terephthalate film according to a conventional method, and while the coating film is not dry, apply the first As shown in the figure, 100 msec inside the solenoid AC magnet 1.
Tape 2 was passed at a speed of . This solenoid AC magnet was variable in the range of 50 Hz cm ² 0 to 5 KOe, and was arranged so that the direction of its magnetic field was parallel to the tape surface.

In addition, in the above-mentioned orientation process, magnetic tapes obtained by varying the strength of the applied alternating magnetic field are tested in the tape longitudinal direction (X direction), width direction (Y direction), and thickness (perpendicular) direction. Each squareness ratio Rs (=Br/
Bs) was measured and the results are shown in Figure 2.

According to Figure 2, in an orientation magnetic field around 400 Oe,
A phenomenon in which magnetic particles are arranged in the YZ plane is observed,
The component in the thickness direction becomes maximum, and the component in the Z direction
The value of Rs was approximately 40%. This value is about twice as high as the Rs of about 22% obtained when magnetic particles with normal uniaxial anisotropy are processed by conventional methods, and the residual magnetic flux density in the vertical direction is also high. It has been shown that it is only that high.

In addition, the surface roughness of the obtained magnetic tape was extremely small, with the standard deviation of the amplitude σ = 0.11 μ, even when measured in the as-applied state, and the surface roughness was not degraded at all like normal tape. Nakatsuta.

Example 2 The same magnetic paint as used in Example 1 was applied onto a base film in the same manner as in Example 1, and the paint was passed through a solenoid alternating current magnet before it dried.

In this example, as shown in FIG. 3, the tape 2 was passed through three solenoid AC magnets 1. The first magnet is in the tape longitudinal direction (X direction), the second magnet is in the tape width direction (Y direction), and the third magnet is in the same direction as the first magnet. It was arranged so that an alternating magnetic field could be generated in the X direction. Further, the total application time of the magnetic field in this case was 100 msec, which was applied by three solenoid AC magnets.

The Rs values in each direction were measured for the magnetic tapes obtained by performing the above-described orientation treatment by varying the strength of the applied alternating magnetic field, and the results are shown in FIG.

As is clear from FIG. 4, it was observed that Rs in the Z direction increased significantly near the coercive force (Hc = 380 Oe) of the magnetic particles. The residual magnetic flux density in the Z direction was extremely high, about three times that of a normal tape. In addition, no abnormalities were observed on the surface of the tape without calendering, and the standard deviation of the surface roughness was σ =
It was as small as 0.12μ and had extremely good surface properties.

Example 3 Co-coated γ-Fe ₂ O ₃ with coercive force (Hc) of 625 Oe
The orientation treatment was performed in the same manner as in Example 2, except that the magnetic particles were used as magnetic particles by applying an alternating magnetic field using a solenoid alternating current magnet.

Even in the magnetic tape obtained in this manner, a phenomenon in which Rs in the Z direction increased near 635 Oe was observed, and the Rs was extremely high at about 60%. Furthermore, the surface roughness of the tape before calendering was extremely small, σ = 0.11μ.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing a specific example of the orientation treatment used in Example 1, FIG. 2 is a graph showing the Rs values in three directions of the magnetic tape obtained in Example 1, and FIG. 3 is a graph showing Example 2.
Figure 4 is a schematic perspective view showing a specific example of the orientation treatment used in Example 2.
It is a graph showing values. Furthermore, among the symbols used in the drawings, 1...
Solenoid AC magnet, 2... tape.

Claims (1)

[Claims] 1 After coating a magnetic paint containing magnetic particles on a non-magnetic support to form a coating film, the coating film is applied with a strength in the range of ±20% of the coercive force of the magnetic powder while the coating film is not dry. 1. A method for manufacturing a magnetic recording medium, which comprises obtaining a perpendicularly magnetized recording medium by applying an alternating current magnetic field from at least two directions perpendicular to the orientation direction.