JPH0243255B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a perpendicular magnetic recording medium, and more particularly to an improvement in a method for manufacturing a recording medium by applying a paint containing magnetic fine particles onto a substrate. Magnetic recording generally relies on a method that uses magnetization in the in-plane longitudinal direction of a recording medium. However, in a recording method that uses magnetization in the in-plane longitudinal direction,
If an attempt is made to increase the recording density, the demagnetizing field within the recording medium will increase, so the recording density cannot be improved that much. Therefore, in order to eliminate such problems,
In recent years, a perpendicular magnetization recording method that uses magnetization in a direction perpendicular to the surface of a recording medium has been proposed. In this perpendicular magnetization recording method, the demagnetizing field in the recording medium decreases as the recording density increases, so it can be said to be a recording method essentially suitable for high-density recording. However, in order to employ such a perpendicular magnetization recording method, a magnetic recording medium having an axis of easy magnetization in a direction perpendicular to the surface is required. Conventionally, as a recording medium that satisfies these demands, the recording film was made of Co-Cr.
There have been proposed methods in which the recording film is formed from a sputtered film and a recording film formed from a coated layer of magnetic fine particles. By the way, in the case where the recording film is formed by a coating layer of magnetic fine particles, the following manufacturing method can be considered. That is, as magnetic fine particles, for example
A hexagonal ferrite such as BaFe ₁₂ O ₁₉ is used. The reason for using hexagonal ferrite is that this ferrite has a flat plate shape and the axis of easy magnetization is perpendicular to the plate surface, so it can be easily vertically aligned by magnetic field alignment treatment or mechanical treatment. It is. After mixing such magnetic fine particles of hexagonal ferrite and a binder and coating the mixture on the surface of a non-magnetic tape, the coating layer is passed through a magnetic field so that the surface is perpendicular to the direction of the magnetic field. By arranging the easy axis of magnetization of each magnetic fine particle to match the direction of the magnetic field, and then drying the coating, a recording medium suitable for perpendicular magnetic recording can be obtained. However, when manufacturing a perpendicular magnetic recording medium using hexagonal ferrite fine particles as magnetic fine particles by the so-called coating method described above, the following points need to be taken into consideration. In other words, in order to clarify the advantages of perpendicular magnetization recording method over conventional in-plane magnetization recording method, it is necessary to make the minimum recording unit on the order of submicron.
The average particle size of submicron magnetic powder is 0.3
It is necessary to use magnetic fine particles of micron to 0.01 micron. Such minute magnetic particles have a single-domain structure, that is, become minute magnets, and therefore are likely to be magnetically coupled to each other. Therefore, care must be taken to ensure uniform distribution within the binder. Furthermore, even if a desired magnetic paint with uniform dispersion is obtained, if such a magnetic paint is applied onto a substrate and vertically oriented using a magnetic field orientator, the following method may occur. Phenomena often occur. In other words, when a substrate coated with magnetic paint is placed between the magnetic poles of a magnetic field orientator with NS poles facing each other, and its surface is placed perpendicular to the magnetic field, the axis of easy magnetization of the magnetic fine particles in the paint coincides with the direction of the magnetic field. Rotate and orient as shown. In this way, it takes several milliseconds to several hundred milliseconds for the magnetic particles to rotate and complete their orientation in the magnetic field, depending on the viscosity or composition of the paint. If the above-mentioned perpendicular magnetic field continues to be applied, magnetic fine particles will magnetically aggregate, resulting in unevenness on the coating surface. These irregularities cannot be removed by calendering the coating after drying. In this way, when unevenness occurs, even when the orientation rate is high, the reproduction output characteristics are impaired. Therefore, it is necessary to somehow prevent the occurrence of magnetic aggregation. For this reason, during the alignment process, at least
High orientation rate of 80% or more can be obtained and surface roughness is improved.
The reality is that there is a strong desire for a manufacturing method that can provide a coating surface with a thickness of 0.1 μm or less. The present invention has been made in view of the above-mentioned problems, and its purpose is to use hexagonal ferrite fine particles with a particle size of submicron order as magnetic fine particles to perform perpendicular magnetic recording by a coating method. An object of the present invention is to provide a method for manufacturing a perpendicular magnetic recording medium, which allows a recording film with a high perpendicular orientation rate and less surface roughness to be obtained in manufacturing the medium. That is, the present invention applies a paint containing hexagonal ferrite fine particles with an average particle size in the range of 0.3 microns to 0.01 microns to a substrate surface, and then applies a magnetic field in a direction perpendicular to the substrate surface to remove the magnetic fine particles. When orienting, the viscosity of the coating on the substrate before entering the magnetic field is set to 2000 cp (centipoise) or more, and the strength of the magnetic field is adjusted.
The above objective was achieved by setting the power to 2000 Oe or more. Various methods can be used to apply the paint having the above-mentioned viscosity onto the substrate. For example, when using a normal reverse roll coater or gravure coater, the appropriate viscosity is about 1000 cp at most, but even with these, if you make some changes to the method of supplying the paint to the coating roll, you can easily reach a viscosity of about 20000 cp. Can be applied. Furthermore, if an extrusion coating method is used, it is possible to coat a paint with a viscosity of 20,000 cp or more. A means for applying a magnetic field to the paint on the substrate from the perpendicular direction can be realized by arranging both the NS magnetic pole surfaces parallel to the coating surface. Furthermore, when a continuous coating method is adopted, the running coating web may pass between the magnetic pole surfaces while maintaining a parallel state. Further, the viscosity can be easily checked using, for example, a Brookfield viscometer. By using the above-described method, the present inventors were able to obtain a perpendicular magnetic recording medium in which submicron-order hexagonal ferrite fine particles in the paint have high vertical orientation and have almost no surface roughness. did it. The present invention will be explained below using specific examples. Example 1 First, an alkali was added dropwise to an aqueous solution containing barium salt, iron salt, cobalt salt, and titanium salt to obtain a coprecipitate. After that, the alkali was removed and Co-Ti was heated.
Substituted barium ferrite fine particles were obtained. These have a crystal grain size of 0.1 μm or less and plate-like properties, and the magnetic properties of this powder are a saturation magnetization of 60 emu/g and a coercive force of 1000.
It was (Oe). Next, 80 parts by weight of this magnetic powder was placed in a stainless steel ball mill pot, and 20 parts by weight of VAGH (vinyl chloride-vinyl acetate copolymer) was dissolved in 120 parts by weight of a 1:1 mixture of methyl isobutyl ketone and toluene. solution, 4 parts by weight of resistin, 0.5 parts of butyl stearate
After adding parts by weight and 1 part by weight of carbon black.
Ball milled for 50 hours. The viscosity of the resulting paint is
It was 4000 cp. This paint was coated on a polyester film as a substrate to a thickness of 25 μm using a knife edge coater, and then dried while passing through a perpendicular magnetic field to obtain a perpendicular magnetic recording medium. The vertical magnetic field was varied to 1000, 2000, 3000, and 5000 Oe. Next, a 1:1 mixed solvent of methyl ethyl ketone and toluene was added to the paint obtained using the ball mill, and the ball mill was again applied to obtain a paint viscosity of 2000 cp.
A 1000 cp paint was prepared, and a perpendicular magnetic recording medium was prepared in the same manner as above. After passing all the prepared coated samples through a calender roll to smooth the surface, the surface roughness was measured using a contact type surface roughness meter, and the degree of vertical orientation was determined using the VSM method. The degree of perpendicular orientation is the residual magnetization obtained by correcting the demagnetizing field from the magnetization curve obtained by mounting the surface of the coating medium perpendicular to the measuring magnetic field.
It was calculated from the ratio Mr/Ms of Mr to the saturation magnetization Ms. The results are shown in Table 1.

[Table] From Table 1, it can be seen that there is almost no orientation in a low magnetic field, and that when the surface roughness exceeds 0.2 μm, not only the output (C) decreases but also the noise (N) increases. It can be seen that N) decreases, that is, surface roughness has a significant effect on (C/N). Furthermore, it can be seen that when a paint with low viscosity is used, the surface roughness increases. However, the above (C/N) is the recording/reproducing characteristic when the recording wavelength is 0.6 μm. As you can see from Table 1, use magnetic paint of 2000 cp or more.
The recording medium obtained by the method of the present invention, which is vertically aligned under a high magnetic field of 2000 Oe or more, has a high degree of vertical alignment of 80% or more and a small surface roughness of 0.1 μm or less. Therefore, if manufactured using this method, the agglomeration phenomenon will be suppressed by the high viscosity, and the orientation will be promoted by the high magnetic field, resulting in high recording density characteristics and high reproduction output characteristics. . Example 2 After applying the magnetic paint with a viscosity of 1000 cp used in Example 1 onto a polyester film, it was allowed to stand still in an open state for 10 and 20 seconds before being passed through a vertical magnetic field to remove some of the solvent on the coated surface. After evaporation, the solid content of each coated film was measured, and the viscosity of the coated film was determined from the previously determined solid content-viscosity curve to be 3200 cp and 5000 cp. These coated films were then dried while passing through a perpendicular magnetic field of 4000 Oe and calendered to obtain a perpendicular magnetic recording medium. The perpendicular orientation rates of the obtained perpendicular magnetic recording media were 93% and 91%, respectively, and the surface roughness was
The diameter was good, at 0.05 μm or less, and the effectiveness of the method of the present invention could be reconfirmed. In addition, although each of the above examples shows an example in which the magnetic paint is coated on a flexible polyester film, the same applies to a coated film on a substrate that has no flexibility; for example, a coated film with a thickness of 0.7 mm.
A similarly high orientation rate and surface precision could be obtained in the recording film prepared on the Al disk using the method described in Example 2.

Claims (1)

[Claims] 1. Hexagonal ferrite fine particles with an average particle size in the range of 0.3 microns to 0.01 microns are used as magnetic fine particles, and after applying a paint containing these magnetic fine particles to a substrate surface, a magnetic field is applied in a direction perpendicular to the substrate surface. In the method for manufacturing a perpendicular magnetic recording medium, the viscosity of the paint on the substrate before entering the magnetic field is
A method for manufacturing a perpendicular magnetic recording medium, characterized in that the strength of the magnetic field is set to 2000 cp or more and the strength of the magnetic field is set to 2000 Oe or more.