JPH07114015B2 - Magnetic recording medium - Google Patents

Description

The present invention relates to a magnetic recording medium, and more particularly to a magnetic recording medium having good electromagnetic conversion characteristics and suitable for high density recording.

[Conventional technology]

Generally, in magnetic recording media, magnetic properties are improved by orienting acicular magnetic powder having magnetic anisotropy in the acicular direction in the magnetic layer in the longitudinal direction of the magnetic layer and utilizing the longitudinal magnetization component. However, in the case of utilizing such a magnetization component in the longitudinal direction of the magnetic layer, the demagnetizing field in the magnetic recording medium increases as the magnetic recording density increases, so that the decay and rotation of the residual magnetization are reduced. As a result, it becomes difficult to detect the recording signal, and there is a limit to the improvement of the recording density.

Therefore, in recent years, the perpendicular magnetic recording method, in which the influence of the demagnetizing field becomes smaller as the recording density becomes higher, is attracting attention as being suitable for high-density recording. For example, a ferromagnetic metal thin film layer for perpendicular magnetic recording made of a Co--Cr alloy is used. Of the hexagonal ferrite powder, γ-Fe ₂ O ₃ powder, CrO ₂ powder, Co epitaxy γ-F.
It has been attempted to use magnetic powder such as e ₂ O ₃ powder and metal powder, and to orient the easy axis of magnetization in the direction perpendicular to the surface of the magnetic layer to utilize the perpendicular magnetization component. Among these, a coating type magnetic recording medium formed by coating a magnetic coating material prepared by using a magnetic powder such as hexagonal ferrite powder on a substrate and drying the coating type magnetic recording medium is excellent in durability and therefore generally practical. Although adopted, it has a problem that the saturation magnetic flux density is low and the reproduction output is low as compared with a metal thin film layer type magnetic recording medium in which a ferromagnetic metal thin film layer for perpendicular magnetic recording is formed.

Therefore, in order to solve such problems and increase the reproduction output of the coating type magnetic recording medium using magnetic powder such as hexagonal ferrite powder, in order to increase the reproduction output, a magnetic layer under the low coercive force is formed between the substrate and the magnetic layer. Is proposed to be formed. (JP-A-56-98718) [Problems to be solved by the invention] However, in a coating type magnetic recording medium using magnetic powder such as hexagonal ferrite powder, a low coercive force between the substrate and the magnetic layer. In the case where the lower magnetic layer is formed, there is a drawback that the overwrite characteristic, which is an overwriting characteristic, is poor. As a result of studying such overwrite characteristic, the present inventors have found that the upper magnetic layer for perpendicular magnetic recording is It was found that the overwriting property is improved by 5 dB or more when the layer thickness is 0.7 μm or less, but at the same time, when the layer thickness of the upper magnetic layer is 0.7 μm or less, the surface roughness of the magnetic layer is deteriorated, It was revealed that the recording density characteristics were low.

[Means for solving problems]

The present invention has been made as a result of various studies in view of the present situation. The saturation magnetic flux density is 200 to 150 between the upper magnetic layer having a layer thickness of 0.7 μm or less for perpendicular magnetic recording and the substrate.
By forming the lower magnetic layer of 0 Gauss, the magnetic interaction between the magnetic powder in the lower magnetic layer and the magnetic powder in the upper magnetic layer for perpendicular magnetic recording during the formation of the upper magnetic layer can be improved. The surface roughness of the upper magnetic layer for perpendicular magnetic recording is effectively prevented, and the surface roughness of the upper magnetic layer having a layer thickness of 0.7 μm or less is 0.01 μm in terms of center line average roughness.
Below, the electromagnetic conversion characteristics are sufficiently improved so that high-density recording can be favorably performed.

In the present invention, the lower magnetic layer has a saturation magnetic flux density of 20
It is preferable to set it within the range of 0 to 1500 gauss, sufficient output cannot be obtained if the saturation magnetic flux density of the lower magnetic layer is less than 200 gauss, and if it is greater than 1500 gauss, the magnetic properties in the lower magnetic layer The magnetic interaction between the powder and the magnetic powder in the magnetic layer for perpendicular magnetic recording on the magnetic layer during formation of the upper magnetic layer is too strong, and the upper magnetic layer with a thickness of 0.7 μm or less for perpendicular magnetic recording is formed. The layer is roughened and the surface smoothness is deteriorated. Thus, the saturation magnetic flux density of the lower magnetic layer is 200
It is preferable to set the thickness within the range of ~ 1500 gauss, and if it is within this range, the layer thickness of the upper magnetic layer for perpendicular magnetic recording is 0.7.
Even if the thickness is less than μm, the surface smoothness of the upper magnetic layer is not deteriorated, and a magnetic layer having a center line average roughness of 0.01 μm or less is formed. The saturation magnetic flux density of the lower magnetic layer is 300 to 110
It is more preferably within the range of 0 gauss, and even more preferably within the range of 500 to 1000 gauss.

If the thickness of the lower magnetic layer is less than 0.5 μm, good durability cannot be obtained, and if the thickness is more than 10 μm, the cost becomes high. Therefore, it is preferable to set the thickness within the range of 0.5 to 10 μm. And more preferably within the range of 0.5 to 2 μm.

If the magnetic powder used for such a lower magnetic layer has a coercive force of more than 200 Oersted, the effect of improving the reproduction output is reduced, and thus the coercive force is reduced.
A material having a coercive force of 200 oersted or less is preferably used, and a coercive force of 200 oersted or less is used. Γ-Fe ₂ O ₃ , Fe ₃ O ₄ powder, Co
Any conventionally used magnetic powder such as γ-Fe ₂ O ₃ powder containing, Fe ₃ O ₄ powder containing Co, CrO ₂ powder, Fe powder, Co powder, and Fe-Ni alloy powder are preferably used. When the volume filling rate of the lower magnetic layer of these magnetic powders is less than 2% by volume, the saturation magnetic flux density of the lower magnetic layer becomes small, so that the effect of improving the reproduction output becomes small. Since the saturation magnetic flux density of the magnetic layer becomes excessive and the surface roughness of the upper magnetic layer becomes large, the range is preferably 2 to 20% by volume, and the range is 3 to 15% by volume, and further 5 to More preferably, it is within the range of 13% by volume.

Further, in the lower magnetic layer, when the pores (volume in which the lubricant can enter) in the magnetic layer is less than 6 volume% in volume occupancy, the pressure absorbing effect becomes small and the durability deteriorates, resulting in 12 volume%. If the amount is larger, the lubricant is absorbed in the lower magnetic layer, and the amount of the lubricant present on the surface of the upper magnetic layer is too small, which deteriorates the durability. Therefore, it has pores in the range of 6 to 12% by volume. Is preferred. Further, in order to further improve the durability of the magnetic recording medium together with the magnetic powder in the magnetic layer having such pores, a non-magnetic powder or a lubricant may be used in combination. Black, metal powder, metal oxide powder, synthetic resin powder, etc. may be used. If the volume filling rate of such a non-magnetic powder in the magnetic layer is less than 13% by volume, the durability will deteriorate and the volume filling rate will be 30% by volume.
Since the surface roughness of the upper magnetic layer increases as the amount is increased, it is preferable to set it in the range of 13 to 30% by volume. As the lubricant, oleyl oleate, myristic acid, zinc stearate, stearic acid amide, aliphatic lubricants such as stearyl alcohol, fluorine-based lubricants such as perfluoroalkyl polyether, silicone-based lubricants such as silicone oil, Lubricants generally used for magnetic recording media such as hydrocarbon-based lubricants such as liquid paraffin and squalane are preferably used, and the volume filling rate in the magnetic layer is within the range of 4 to 7% by volume. Is preferred.

When the upper magnetic layer is formed by superposing on the lower magnetic layer as described above, the overwrite property, which is the overwriting property, is deteriorated when the layer thickness is thicker than 0.7 μm, and when the magnetic paint is applied, the overwrite property becomes 0.1 μm. Since it is difficult to make it thinner, 0.1 ~
The thickness is preferably in the range of 0.7 μm, and if it is 0.7 μm or less, a sufficiently good overwrite characteristic can be obtained. However, in order to improve the overwrite characteristics, 0.1-0.7
The thickness is preferably in the range of μm, more preferably in the range of 0.2 to 0.6 μm, and even more preferably in the range of 0.3 to 0.5 μm.

The surface roughness of the upper magnetic layer is 0.01 μm in terms of center line average roughness so that high density recording can be performed well.
The surface roughness of such a magnetic layer is preferably equal to or less than 20 as described above, as described above.
It is achieved by being in the range of 0 to 1500 Gauss. Thus, the surface roughness of the magnetic layer is 0 at the centerline average roughness.
If it is set to 01 μm or less, high density recording can be satisfactorily performed. However, in order to perform high density recording better, the surface roughness of the magnetic layer is further reduced by the center line average roughness, and 0.008 μm or less, It is more preferably 0.005 μm or less.

As the magnetic powder used for such an upper magnetic layer,
γ-Fe ₂ O ₃ , Fe ₃ O ₄ powder, Co-containing γ-Fe ₂ O ₃ powder, Co-containing F
e ₃ O ₄ powder, CrO ₂ powder, Fe powder, Co powder, Fe-Ni alloy powder, such as hexagonal ferrite powder, although the magnetic powder used in the conventional general are both suitably used, upper magnetic layer A hexagonal ferrite powder capable of achieving the smoothest surface roughness (1) is particularly preferably used. As such hexagonal ferrite powder, barium ferrite powder, strontium ferrite powder, lead ferrite powder, calcium ferrite powder, and those obtained by substituting a part of Fe element of these ferrite powders with other metal elements are preferable. Used as a thing. The coercive force of the magnetic powder used for these upper magnetic layers is in the range of 200 to 1500 Oersted because the recording density characteristic becomes poor at less than 200 Oersted and it becomes difficult to record by the magnetic head at higher than 1500 Oersted. Those are preferably used. When the saturation magnetization is less than 45emu / g, the reproduction output becomes small,
When it is larger than emu / g, it becomes difficult to make the surface roughness of the magnetic layer 0.1 μm or less, so that the range of 45 to 140 emu / g is preferably used. If the volume filling rate of the magnetic layer above these magnetic powders is less than 20% by volume, the reproduction output becomes small, and if it exceeds 40% by volume, the durability becomes small. The preferred range is 23 to 35% by volume, and 25 to 33%
More preferably, it is within the range of volume%.

The upper magnetic layer preferably has pores within the range of 20 to 35% by volume. Furthermore, in order to further improve the durability of the magnetic recording medium together with the magnetic powder in the magnetic layer having such pores, a non-magnetic powder or a lubricant may be used in combination. Al ₂ O ₃ powder, Cr ₂ O ₃ powder, α-Fe ₂ O ₃ harder than magnetic powder
Powder or the like is used. If the volume filling rate of such a non-magnetic powder in the magnetic layer is less than 2% by volume, the durability will be low, and if it is more than 20% by volume, the wear of the magnetic head will be large. It is preferable to set it within. As the lubricant, the same lubricants that are generally used for magnetic recording media such as aliphatic lubricants, fluorine-based lubricants, silicone-based lubricants, and hydrocarbon-based lubricants used in the lower magnetic layer are used. Is preferably used, and the volume filling rate in the magnetic layer is preferably in the range of 6 to 20% by volume.

Further, in order to sufficiently reduce electrostatic noise, the upper magnetic layer preferably has an electric resistance of the surface of 1 × 10 ¹⁰ Ω / inch ² or less, and 1 × 10 ⁹ Ω / inch ² or less. More preferable.

The upper and lower magnetic layers may be formed according to a conventional method. For example, first, on a substrate such as a polyester film, a magnetic powder having a coercive force of 200 Oersted or less, a binder resin, an organic solvent and other additives are added. Apply a magnetic coating material containing it by ordinary means and dry it to form a lower magnetic layer with a saturation magnetic flux density of 200 to 1,500 gauss, and then, on this lower magnetic layer, for perpendicular magnetic recording such as hexagonal ferrite powder. A magnetic paint containing a suitable magnetic powder, a binder resin, an organic solvent and other additives is applied by a usual means, dried,
The layer thickness is 0.7 μm or less, and the surface roughness is 0.0 at the center line surface roughness.
The upper magnetic layer having a thickness of 1 μm or less may be formed in multiple layers.

When forming the upper magnetic layer, if hexagonal ferrite powder is used, the shape of this type of magnetic powder is plate-shaped, and the easy magnetization direction is perpendicular to the plate surface.
The magnetic powder can be oriented sufficiently so that its plate surface is parallel to the magnetic layer surface by merely applying mechanical shearing force during coating, and the easy magnetization direction is oriented perpendicular to the magnetic layer. Therefore, magnetic field orientation by applying a magnetic field is not always necessary, but when magnetic field orientation is performed by applying a magnetic field in the direction perpendicular to the magnetic layer surface, the easy magnetization direction of the magnetic powder used in the upper magnetic layer is changed. It is preferable to perform such magnetic field orientation because the orientation in the vertical direction is better and the surface smoothness of the magnetic layer is also better.

As the binder resin used in the upper and lower magnetic layers, vinyl chloride-vinyl acetate copolymer, polyvinyl butyral resin,
Conventionally widely used binder resins such as fibrin resin, polyurethane resin, isocyanate compound, and radiation curable resin are widely used.

As the organic solvent, conventionally used organic solvents such as toluene, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, tetrahydrofuran and ethyl acetate may be used alone or in combination of two or more.

〔Example〕

 Next, an embodiment of the present invention will be described.

Example 1 <Preparation of Magnetic Coating Material for Lower Layer> Granular γ-Fe ₂ O ₃ magnetic powder (average particle diameter 0.15 μm, coercive force 2
0 Oersted, specific gravity about 5) 100 parts by weight S-REC A (vinyl chloride-vinyl acetate-vinyl alcohol copolymer manufactured by Sekisui Chemical Co., Ltd., specific gravity about 5) 24〃 Pandex T5250 (Dainippon Ink and Chemicals, polyurethane elastomer) , Specific gravity about 1.2) 24〃 Coronate L (Nippon Polyurethane Industry Co., Ltd., trifunctional low molecular weight isocyanate compound, specific gravity about 1.2) 12〃 Sterling NS (Cabot Co., carbon black,
Specific gravity of about 1.8) 43 〃 oleyl oleate (specific gravity of about 0.8) 18 〃 Cyclohexanone 100 〃 Toluene 100 〃 Composition was mixed and dispersed in a ball mill for 150 hours to prepare a magnetic paint.

<Preparation of magnetic coating for upper layer> Hexagonal plate-shaped Co, Ti modified barium ferrite magnetic powder (average particle size 0.07 μm, coercive force 420 oersted, saturation magnetization 55 emu / g) 100 parts by weight S-REC A (manufactured by Sekisui Chemical Co., Ltd.) Vinyl chloride-vinyl acetate-vinyl alcohol copolymer, specific gravity of about 1.2) 8〃Takelac E-551 (Takeda Pharmaceutical Co., Ltd., polyurethane, specific gravity of about 1.2) 8〃 Coronate L (Nippon Polyurethane Industry Co., Ltd., trifunctional low Molecular weight isocyanate compound, specific gravity about 1.2) 4〃 AOS-28 (Sumitomo Chemical Co., Ltd., Al ₂ O ₃ , average particle size 0.4 μm) 5
A magnetic paint was prepared by mixing and dispersing a composition of oleyl oleate (specific gravity of about 0.8) 10 〃 cyclohexanone 100 〃 toluene 100 〃 for 72 hours in a ball mill.

<Production of magnetic tape> The above-mentioned magnetic coating material for the lower layer is applied onto a polyester base film having a thickness of about 12 μm, dried, calendered at 50 ° C., and further cured at 60 ° C. for 16 hours to obtain 2.0 μm.
A lower magnetic layer having a saturation magnetic flux density of 1100 gauss and having a thickness of m was formed. Next, the above magnetic coating material for the upper layer was further applied onto this magnetic layer and dried, and a magnetic field of 3000 oersted was applied in the vertical direction of the polyester base film during drying.
Then, calendering was performed at 50 ° C. to form an upper magnetic layer having a thickness of 0.6 μm, which was then cut into a predetermined width to prepare a magnetic tape in which the upper and lower magnetic layers were laminated. The surface electric resistance of this magnetic tape was 2 × 10 ⁷ Ω / inch ² .

Example 2 In the magnetic coating composition of the lower magnetic layer of Example 1, the amount of granular γ-Fe ₂ O ₃ powder used was 100 parts by weight to 18 parts by weight, and the amount of S-REC A used was 24 parts by weight to 30 parts by weight. Parts, the amount of Pandex T5250 used was changed from 24 parts by weight to 18 parts by weight, and the amount of Stirling NS used was changed from 43 parts by weight to 72 parts by weight. 2.5μ
A lower magnetic layer having a saturation magnetic flux density of 200 gauss is formed at m, and a thickness of 0.5 is formed on the lower magnetic layer.
The upper magnetic layer of μm was formed to make a magnetic tape.

Example 3 In the magnetic coating composition of the lower magnetic layer of Example 1, the amount of granular γ-Fe ₂ O ₃ powder used was changed from 100 parts by weight to 136 parts by weight,
Except that the amount of S-REC A was changed from 24 to 27 parts by weight, the amount of Pandex T5250 was changed from 24 to 21 parts by weight, and the amount of Sterling NS was changed from 43 to 30 parts by weight. The thickness is 3.0 as in Example 1.
A lower magnetic layer having a saturation magnetic flux density of 1,500 gauss and a thickness of 0.7 is formed on the magnetic layer.
The upper magnetic layer of μm was formed to make a magnetic tape.

Example 4 In the magnetic coating composition of the lower magnetic layer of Example 1, the amount of granular γ-Fe ₂ O ₃ powder used was 100 parts by weight to 27 parts by weight, and the amount of S-REC A used was 24 parts by weight to 30 parts by weight. Parts, the amount of Pandex T5250 used was changed from 24 parts by weight to 18 parts by weight, and the amount of Stirling NS used was changed from 43 parts by weight to 69 parts by weight. 1.5μ
A magnetic layer having a saturation magnetic flux density of 300 gauss is formed at m, and a thickness of 0.5 is formed thereon in the same manner as in Example 1.
The upper magnetic layer of μm was formed to make a magnetic tape.

Example 5 In the magnetic coating composition of the lower magnetic layer of Example 1, the amount of granular γ-Fe ₂ O ₃ powder used was 100 parts by weight to 45 parts by weight, and the amount of Stirling NS used was 43 parts by weight to 62 parts by weight. The thickness is 1 μm in the same manner as in Example 1 except that the respective portions are changed.
m, a saturation magnetic flux density is 500 Gauss, and a lower magnetic layer of 500 gauss is formed.
The upper magnetic layer of μm was formed to make a magnetic tape.

Example 6 In the magnetic coating composition of the lower magnetic layer of Example 1, the amount of granular γ-Fe ₂ O ₃ powder used was 100 parts by weight to 63 parts by weight, and the amount of S-REC A used was 24 parts by weight to 30 parts by weight. The amount of Pandex T5250 used was changed from 24 to 18 parts by weight, the amount of Coronate L used was changed from 12 to 14 parts by weight, and the amount of Stirling NS used was changed from 43 to 56 parts by weight. Other than the above, a lower magnetic layer having a thickness of 2.0 μm and a saturation magnetic flux density of 700 gauss is formed in the same manner as in Example 1, and further, in the same manner as in Example 1, a thickness of 0.4 μm is formed. An upper magnetic layer was formed to make a magnetic tape.

Example 7 A magnetic tape was produced in the same manner as in Example 1 except that the orientation magnetic field was not applied when the upper magnetic layer was dried.

Comparative Example 1 In the magnetic coating composition of the lower magnetic layer of Example 1, the amount of granular γ-Fe ₂ O ₃ powder used was changed from 100 parts by weight to 250 parts by weight,
S-Lec A usage from 24 to 20 parts by weight, Pandex T5250 usage from 24 to 20 parts by weight, Coronate L usage from 12 to 10 parts by weight, Sterling NS usage The thickness was 2 μm in the same manner as in Example 1 except that the amount was changed from 43 parts by weight to 4 parts by weight.
Then, a lower magnetic layer having a saturation magnetic flux density of 1800 gauss is formed, and a layer thickness of 0.6 is formed on the lower magnetic layer.
The upper magnetic layer of μm was formed to make a magnetic tape.

Comparative Example 2 In the same manner as in Example 1, except that the granular γ-Fe ₂ O ₃ powder was omitted from the magnetic coating composition of the lower magnetic layer of Example 1.
An undercoat layer having a thickness of 2 μm and a saturation magnetic flux density of 0 Gauss is formed, and a layer thickness of 0.6 is formed on the undercoat layer.
The upper magnetic layer of μm was formed to make a magnetic tape.

Comparative Example 3 In Example 1, the lower magnetic layer was omitted and the thickness was replaced by
A magnetic tape was produced in the same manner as in Example 1 except that a permalloy vapor deposition layer having a saturation magnetic flux density of 6000 gauss and a thickness of 0.5 μm was formed.

Regarding the magnetic tapes obtained in the respective examples and comparative examples,
The surface roughness of the magnetic layer was measured by Talysurf and expressed as the center line average roughness. The recording wavelength λ is 2.0 μm and 0.5 μm.
The output at m was measured.

The results are shown in Table 1 below.

[Effects of the Invention] As is clear from Table 1 above, the magnetic tapes obtained in the present invention (Examples 1 to 7) were recorded in comparison with the conventional magnetic tapes (Comparative Examples 1 to 3). The output at the wavelength λ of 0.5 μm is particularly good, and therefore, the magnetic recording medium of the present invention obtained by setting the saturation magnetic flux density of the lower magnetic layer within the range of 200 to 1500 Gauss is obtained when the recording density is increased ( It can be seen that the output at a recording wavelength λ of 0.5 μm) is dramatically improved and is suitable for high density recording.

Claims (7)

[Claims] 1. A lower magnetic layer having a saturation magnetic flux density of 200 to 1,500 gauss is formed on a substrate, and the layer thickness of the magnetic anisotropy in the direction perpendicular to the magnetic layer surface is 0.7. μ
A magnetic recording medium characterized in that an upper magnetic layer having a surface roughness of 0.01 μm or less and a center line average roughness of 0.01 μm or less is formed in multiple layers. 2. The magnetic recording medium according to claim 1, wherein the thickness of the lower magnetic layer is in the range of 0.5 to 10 μm. 3. The magnetic recording medium according to claim 1 or 2, wherein the volume filling rate of the magnetic powder contained in the lower magnetic layer is 2 to 20% by volume. 4. The volume filling ratio of the non-magnetic powder contained in the lower magnetic layer is 13 to 30% by volume.
The magnetic recording medium according to item 2 or 3. 5. The magnetic recording medium according to claim 1, wherein the magnetic powder used for the upper magnetic layer is hexagonal ferrite powder. 6. The volume filling rate of the magnetic powder contained in the upper magnetic layer is 20 to 40% by volume, and the first, second, third, fourth or fifth claim. Magnetic recording medium described 7. The volume filling ratio of the non-magnetic powder contained in the upper magnetic layer is 2 to 20% by volume.
The magnetic recording medium according to item 2, item 3, item 4, item 5, or item 6.