JP2002133636A - Magnetic disk - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic disk which has a large capacity, shows a high reproduction output and a high output/noise ratio when an MR head is used. SOLUTION: In this magnetic disk, in which an undercoat layer of at least one layer and a magnetic layer are formed in this order on one of the surfaces of a non-magnetic base material, the thickness of the magnetic layer is in the range of 0.01 to 0.10 μm, and the variation in thickness of the magnetic layer is 15% or smaller. By this, a high-capacity magnetic disk can be obtained. Particularly, the magnetic disk, which has the high reproduction output and the high output/noise ratio when the MR head is used, can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic disk which has a high recording density capacity, an access speed, and a high data write / read speed, and particularly uses a reproducing head (hereinafter also referred to as an MR head) utilizing a magnetoresistive element. About.

[0002]

2. Description of the Related Art Magnetic recording media have various uses such as audio tapes, video tapes, computer tapes, disks, and the like. In particular, in the field of magnetic recording media typified by digital systems, the density is increasing year by year. In 3), with the increase in software, the capacity of hard disk, and the development of image recording of digital cameras, etc., 3.
Those with a storage capacity of 5 inches or more than 120 MB are commercialized. It is indispensable to increase the capacity of magnetic disks in the future, and it is also necessary to increase the access speed and the data writing / reading speed in order to process large-capacity data in a short time. Improving the performance.

In a magnetic disk corresponding to such high recording density, high capacity, and high speed processing, the recording wavelength is reduced (the thickness loss is reduced) by making the magnetic layer ultra-thin (0.02 to 0.5 μm). It is necessary to increase the recording density (reduce the recording wavelength and track width) by improving the magnetic layer by improving the magnetic properties and dispersibility of the ferromagnetic powder. When the magnetic layer thickness is reduced,
Since saturation recording is performed on the entire magnetic layer, variations in the magnetic layer thickness are directly linked to variations in output. Further, when the recording density is increased by reducing the recording wavelength and the track width, the magnetic flux leakage from the disk becomes smaller. Therefore, it is necessary to use an MR head capable of obtaining a high output even with a small magnetic flux as the reproducing head. Further, there is a need for means for reducing the spacing loss with the magnetic head by improving the surface properties of the magnetic disk, and for improving the durability of the magnetic disk by optimizing the mechanical properties of the undercoat layer and the magnetic layer.

[0004] Magnetic recording media compatible with MR heads include, for example, JP-A-11-238225 and JP-A-2000-40.
No. 217 and JP-A-2000-40218. In these conventional techniques, the magnetic flux (the product of the residual magnetic flux density and the thickness) of the magnetic recording medium is controlled to a specific value, and
The distortion of the output of the R head is prevented, and the thermal asperity of the MR head is reduced by reducing the dent on the surface of the magnetic layer to a specific value or less.

[0005]

However, in a conventionally known magnetic recording medium, the reproduction output of the MR head greatly fluctuates due to uneven thickness of the magnetic layer, and the output-to-noise ratio (C / N) is low. There was a problem of being small.

That is, when the thickness of the magnetic layer is extremely thin, such as 0.1 μm or less, the entire magnetic layer is magnetized, and when reproducing using an MR head, the output is a magnetic flux (the product of the residual magnetic flux density and the thickness of the magnetic layer). ), The output fluctuates due to uneven thickness of the magnetic layer. Therefore, when the thickness of the magnetic layer is 0.1
When the thickness is as thin as μm, the thickness unevenness of the magnetic layer must be 15% or less, but the thickness unevenness of the magnetic layer obtained by the conventional method is limited to about 20%.

[0007]

Means for Solving the Problems In order to solve the above problems, the present inventors have conducted intensive studies and found that when the thickness of the magnetic layer is as thin as 0.1 μm or less, the thickness unevenness of the magnetic layer is reduced. It has been clarified that the undercoat layer and the magnetic layer should be dried simultaneously by far-infrared heating in order to reduce the amount. That is, in the conventional heating and drying method, since the heat is transmitted from the surface, the surface of the magnetic layer is dried first, and when the solvent coming out of the inside evaporates, the magnetic layer is disturbed, which causes unevenness in the thickness of the magnetic layer. Cause. On the other hand, when dried by heating with far infrared rays, the carbon black of the undercoat layer and the magnetic powder of the magnetic layer are directly heated by far infrared rays. It has been found that even when the layer thickness is as thin as 0.1 μm or less, the thickness unevenness of the magnetic layer becomes 15% or less.

The present invention has been completed based on the above findings. That is, the present invention provides a magnetic disk in which at least one undercoat layer and a magnetic layer are formed in this order on at least one surface of a non-magnetic support, and the thickness of the magnetic layer is 0.01 to 0.10 μm. A magnetic disk wherein the thickness unevenness of the magnetic layer is 15% or less, a thickness of the magnetic layer (d1) and a thickness of the undercoat layer (d1).
(2) a magnetic disk whose ratio (d2 / d1) is 10 or more and 100 or less, and a magnetic disk on which a magnetic recording signal is reproduced by a reproducing head using a magnetoresistive element (claim 4). ) And the thickness of the magnetic layer is 0.0
1 to 0.1 μm, coercive force of 160 to 320 kA / m, product of residual magnetic flux density (Br) in the track longitudinal direction and thickness of magnetic layer (Br · d1) of 0.0018 μTm or more and 0.04 μTm
The present invention relates to the following magnetic disk (claim 4). The particle size referred to in the present invention means an average particle size per 100 particles unless otherwise specified. The track longitudinal direction refers to the direction in which the track extends, in other words, the disk rotation direction.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION In a magnetic disk in which at least one undercoat layer and a magnetic layer having a thickness of 0.01 to 0.10 μm are formed in this order on one surface of a nonmagnetic support, the magnetic layer As a result of examining a method for reducing the thickness unevenness of the undercoat, an undercoat layer was formed on the non-magnetic support, and a wet coating was formed thereon.
It has been found that when the magnetic layer formed on-wet is dried with far-infrared rays, a magnetic disk having a very small thickness of the magnetic layer can be obtained. In this case, the thickness unevenness of the magnetic layer is preferably 15% or less. 15% or less is preferable because 1
This is because if the thickness of the magnetic layer exceeds 5%, the output fluctuation when reproducing with the MR head becomes large. The smaller the thickness unevenness of the magnetic layer, the better. However, since it is difficult to realize the thickness unevenness of less than 1%, the thickness is usually 1 to 15%. 1-12% is more preferable, 1-10% is still more preferable. Thickness of magnetic layer (d1) and thickness of undercoat layer (d2)
(D2 / d1) is preferably 10 or more and 100 or less, more preferably 20 or more and 100 or less, and 40 or more and 10 or less.
0 or less is more preferable. This range is preferably 1
If it is less than 0, the durability tends to deteriorate, and if it exceeds 100, the thickness of the entire magnetic disk becomes thick, which not only causes an increase in manufacturing cost, but also makes it difficult to form a coating film on a wet-on-wet basis. It is because it becomes.

The thickness of the magnetic layer is preferably 0.01 to 0.10 μm, more preferably 0.02 to 0.10 μm, and more preferably 0.02 to 0.10 μm.
07 μm is more preferred. This range is preferred because
If the thickness of the magnetic layer is less than 0.01, it is difficult to reduce the thickness unevenness to less than 15%. If the thickness exceeds 0.10 μm, the output is reduced due to thickness loss. The coercive force of the magnetic layer is preferably 160 to 320 kA / m, and 200 to 32 kA / m.
0 kA / m is more preferred. This range is preferred because
If the recording wavelength is shorter than 160 kA / m, the output will decrease due to the demagnetizing field. If the recording wavelength exceeds 320 kA / m, it becomes difficult to perform recording with the magnetic head. The product (Br · d1) of the residual magnetic flux density (Br) in the track longitudinal direction and the magnetic layer thickness (d1) is 0.0018 μTm or more and 0.04 μm.
Tm or less, preferably 0.0036 μTm or more and 0.04 μT or less.
m or less, more preferably 0.004 μTm or more and 0.028 μm
Tm or less is more preferable. This range is preferred because
If it is less than 0.0018 μTm, the reproduction output by the MR head is small, and if it exceeds 0.04 μTm, the reproduction output by the MR head tends to be distorted. A magnetic disk comprising such a magnetic layer can shorten the recording wavelength,
This is preferable because it is possible to increase the reproduction output when reproducing with the R head, further reduce the distortion of the reproduction output, and increase the output-to-noise ratio.

In the undercoat layer, carbon black having a particle size of 10 to 100 nm is 15 to 35% by weight, particle size is 0.05 to 0.20 μm, based on the weight of the whole inorganic powder in the undercoat layer.
m to contain 35 to 83% by weight of nonmagnetic iron oxide,
Wet-on-wet is preferable because the thickness unevenness of the magnetic layer formed thereon is reduced. Hereinafter, each component will be described in detail.

<Nonmagnetic Support> The thickness of the nonmagnetic support is as follows.
Depending on the application, usually those having a thickness of 30 to 100 μm are used. If it is less than 30 μm, the strength of the magnetic disk is weak, and if it exceeds 100 μm, not only does the cost increase, but also the rigidity of the magnetic disk becomes too high. This is because the surface is easily damaged. Non-magnetic supports usually have a Young's modulus of 5.07-1.
5.20 GPa (500-1500 kg / mm ² ) is used.

As the nonmagnetic support, polyethylene terephthalate, polyethylene naphthalate, aromatic polyimide and aromatic polyamide are used.

<Undercoat layer> The thickness of the undercoat layer is determined by the ratio (d2 / d1) of the thickness (d1) of the magnetic layer to the thickness (d2) of the undercoat layer.
Is set to 10 or more and 100 or less. The ratio (d2 / d1) is more preferably 20 or more and 100 or less, and further preferably 40 or more and 100 or less. If the range is less than 10, the effect of reducing the unevenness of the thickness of the magnetic layer is small, and if it exceeds 100, the thickness of the entire magnetic disk is increased, which not only causes an increase in manufacturing cost, but also increases the wet / magnetic property. This is because it is difficult to form a coating film on wet.

Carbon black is added to the undercoat layer for the purpose of improving conductivity, and iron oxide is added for the purpose of controlling disk rigidity. In the undercoat layer, based on the weight of all the inorganic powders in the undercoat layer, 15 to 35% by weight of carbon black having a particle size of 10 to 100 nm, a major axis length of 0.05 to 0.20 μm,
It is preferable to contain 35 to 83% by weight of a nonmagnetic iron oxide having a short axis length of 5 to 200 nm, because the thickness unevenness of the magnetic layer formed on a wet-on-wet and dried by far-infrared radiation is reduced. . The non-magnetic iron oxide is usually acicular, but when granular or amorphous non-magnetic iron oxide is used, iron oxide having a particle size of 5 to 200 nm is preferable.

As carbon black (hereinafter, also referred to as CB) to be added to the undercoat layer, acetylene black, furnace black, thermal black and the like can be used.
Those having a particle size of 5 nm to 200 nm are used, and those having a particle size of 10 to 100 nm are preferred. This range is preferable because, because the carbon black has a structure, it is difficult to disperse CB when the particle size is 10 nm or less, and the smoothness is deteriorated when the particle size is 100 nm or more.
The amount of CB to be added varies depending on the particle diameter of CB.
35% by weight is preferred. This range is preferably 15
If the amount is less than 35% by weight, the effect of improving conductivity is poor, and if it exceeds 35% by weight, the effect is saturated. Particle size 15nm ~ 8
It is more preferable to use CB of 0 nm in 15 to 35% by weight, and it is even more preferable to use CB of 20 nm to 50 nm in particle size in 20 to 30% by weight. By adding carbon black having such a particle size and amount, the electric resistance is reduced, the generation of electrostatic noise is reduced, and the thickness unevenness of the magnetic layer dried by far infrared rays is reduced.

The nonmagnetic iron oxide to be added to the undercoat layer may have a major axis length of 0.05 to 0.20 μm and a minor axis length of
The particle size is preferably from 200 to 200 nm. In the case of granular or amorphous particles, the particle size is preferably from 5 to 200 nm. Needle-like ones are more preferable because the orientation of the magnetic layer is improved. The addition amount is preferably 35 to 83% by weight. The particle size in this range (short axis length in the case of needles) is preferable because if the particle size is less than 5 nm, uniform dispersion is difficult, and if it exceeds 200 nm, unevenness at the interface between the undercoat layer and the magnetic layer increases. . The addition amount in this range is preferable because the effect of improving the strength of the coating film is small when the amount is less than 35% by weight, and the strength of the coating film decreases when the amount exceeds 83% by weight.

<Lubricant> Lubricants having different roles are used in the coating layer composed of the undercoat layer and the magnetic layer. The undercoat layer contains 0.5 to 4.0% by weight of higher fatty acid based on the whole powder, and
It is preferable to contain 0 to 14.0% by weight of an ester of a higher fatty acid because the coefficient of kinetic friction between the tape and the rotating cylinder (hereinafter, simply referred to as the coefficient of friction) becomes small. The reason for the addition of higher fatty acids in this range is that if it is less than 0.5% by weight, the effect of reducing the friction coefficient is small, and if it exceeds 4.0% by weight, the undercoat layer is plasticized and the toughness is lost. . The addition of higher fatty acid ester in this range is preferable if less than 2.0% by weight, the effect of reducing the coefficient of friction is small, and if it exceeds 14.0% by weight, the amount of transfer to the magnetic layer is too large. This is because there are side effects such as sticking of the disk and the magnetic head.

The lubricant used for the undercoat layer can also be used for the magnetic layer. However, since the fatty acid has a lower leaching property to the magnetic layer than the fatty acid ester, the fatty acid ester is usually added to the undercoat layer at a lower ratio. Make it higher. The lubricant used for the undercoat layer / magnetic layer may be any of conventionally known fatty acids, fatty acid esters and the like, but among them, among them, those having 10 or more carbon atoms, preferably 12 to 3 carbon atoms.
It is preferable to use a fatty acid of 0 and a fatty acid ester having a melting point of 35 ° C. or lower, preferably 10 ° C. or lower.

As the fatty acid having 10 or more carbon atoms, straight chain,
Any of isomers such as branched and cis-trans may be used, but a linear type having excellent lubricating performance is preferred. Examples of such fatty acids include lauric acid, myristic acid, stearic acid, palmitic acid, behenic acid, oleic acid, linoleic acid and the like. Among these, myristic acid, stearic acid, palmitic acid and the like are preferable.

Fatty acid esters having a melting point of 35 ° C. or lower include n-butyl oleate, hexyl oleate, n-octyl oleate, 2-ethylhexyl oleate, oleyl oleate, n-butyl laurate, heptyl laurate, myristine N-butyl acid, n-butoxyethyl oleate, trimethylolpropane trioleate, n-butyl stearate, s-butyl stearate, isoamyl stearate, butyl cellosolve stearate and the like. These fatty acid esters can control the oil film strength and the amount of oil output depending on the difference in molecular weight, structure, and melting point, and may be optimized by combination. By having the above melting point, even when exposed to low temperature and low humidity, the magnetic layer easily oozes and migrates to the surface of the magnetic layer at the time of high-speed sliding contact between the magnetic layer and the magnetic head, and effectively exerts its excellent lubricating action. it can.

In the magnetic layer, 0.5 with respect to the ferromagnetic powder
Containing 3.0% by weight of fatty acid, 1.0 to 10.0% by weight
The higher fatty acid ester is preferred because the coefficient of friction between the magnetic disk and the head is reduced. Fatty acids in this range are preferred because if it is less than 0.5% by weight, direct contact at the head / magnetic layer interface is likely to occur and the effect of preventing seizure is small, and if it exceeds 3.0% by weight, bleed out and drop out will occur. This is because a defect occurs. The addition of higher fatty acid ester in the above range is preferable because the effect of reducing the coefficient of friction is small when it is less than 1.0% by weight, and when it exceeds 10.0% by weight, there are side effects such as sticking of the magnetic disk to the head. It is. The mutual movement between the lubricant of the magnetic layer and the lubricant of the undercoat layer is not excluded.

<Magnetic Layer> The thickness of the magnetic layer is as described above.
It is preferably from 0.01 μm to 0.1 μm, more preferably from 0.02 μm to 0.1 μm. This range is preferable because, when the thickness is less than 0.01 μm, the reproduction output by the MR head becomes small due to the small magnetic flux of the magnetic layer, and when it exceeds 0.1 μm, the reproduction output of the MR head becomes easily distorted. As described above, the coercive force of the magnetic layer in the track longitudinal direction is preferably 160 to 320 kA / m, and 200 to 32 kA / m.
0 kA / m is more preferred. This range is preferred because
If the recording wavelength is shorter than 160 kA / m, the output will decrease due to the demagnetizing field. If the recording wavelength exceeds 320 kA / m, it becomes difficult to perform recording with the magnetic head. Product of residual magnetic flux density in the longitudinal direction of track and thickness of magnetic layer 0.0018μT
m to 0.04 μTm, preferably 0.0036 μTm
It is more preferably at least 0.04 μTm and at most 0.004 μTm.
It is more preferably at least 0.028 μTm. This range is preferable because the reproduction output by the MR head is small below 0.0018 μTm, and the reproduction output above 0.04 μTm.
This is because the reproduction output by the head is easily distorted. A magnetic recording medium comprising such a magnetic layer is preferable because the recording wavelength can be shortened, the reproduction output when reproducing with an MR head can be increased, the distortion of the reproduction output can be reduced, and the output-to-noise ratio can be increased.

As the magnetic powder to be added to the magnetic layer, ferromagnetic iron-based metal powder and hexagonal barium ferrite powder are used. The coercive force of the ferromagnetic iron-based metal powder and the hexagonal barium ferrite powder is preferably 160 to 320 kA / m, and the saturation magnetization is 120 to 200 for the ferromagnetic iron-based metal powder.
^{A · m 2 / kg (120~200emu} / g) are ^{preferable, 130~180A · m 2 / kg (} 130~180em
u / g) is more preferred. In the case of hexagonal barium ferrite powder, 50 to 70 A · m ² / kg (50 to 70 emu /
g) is preferred. Note that the magnetic properties of the magnetic layer and the magnetic properties of the ferromagnetic powder are measured values of an external magnetic field of 1.28 MA / m (16 kOe) using a sample vibrating magnetometer.

The average major axis length of the ferromagnetic iron-based metal powder of the present invention is preferably 0.03 to 0.2 μm, more preferably 0.03 to 0.2 μm.
0.18 μm is more preferable, and 0.04 to 0.15 μm is further preferable. This range is preferable because the average major axis length is
When it is less than 0.03 μm, the cohesive force of the magnetic powder increases, and it becomes difficult to disperse the magnetic powder in the coating. When it is more than 0.2 μm, the coercive force decreases, and the particle noise based on the particle size increases. Become. In the case of hexagonal barium ferrite powder, the plate diameter is preferably 5 to 200 nm for the same reason. The average major axis length and the particle diameter are determined by measuring the particle size of a photograph taken with a scanning electron microscope (SEM) and averaging 100 particles. Also,
The BET specific surface area of the ferromagnetic iron-based metal powder is 35 / g.
Or more, more preferably 40 / g or more, and 50 / g or more.
g or more is most preferable. The BET specific surface area of the hexagonal barium ferrite powder is preferably 1 to 100 / g or more.

As the binder contained in the undercoat layer and the magnetic layer, vinyl chloride resin, vinyl chloride-vinyl acetate copolymer resin, vinyl chloride-vinyl alcohol copolymer resin, vinyl chloride-vinyl acetate-vinyl alcohol copolymer resin , Vinyl chloride-vinyl acetate-maleic anhydride copolymer resin, vinyl chloride-hydroxyl-containing alkyl acrylate copolymer resin,
A combination of at least one selected from nitrocellulose and a polyurethane resin can be used. Among them, it is preferable to use a vinyl chloride-hydroxyl group-containing alkyl acrylate copolymer resin and a polyurethane resin in combination. Polyurethane resins include polyester polyurethane, polyether polyurethane, polyether polyester polyurethane, polycarbonate polyurethane, polyester polycarbonate polyurethane, and the like.

COOH, SO ₃ M, OSO
₂ M, P = O (OM) ₃ , OP = O (OM) ₂ [M is a hydrogen atom, an alkali metal base or an amine salt], OH, N
R′R ″, N ⁺ R ″ ′ R ″ ″ R ′ ″ ″ [R ′, R ″,
R ′ ″, R ″ ″, R ′ ″ ″ represent a hydrogen or hydrocarbon group],
A binder such as a urethane resin made of a polymer having an epoxy group is used. The use of such a binder is
This is because the dispersibility of the magnetic powder and the like is improved as described above.
When used in combination of two or more resins are preferably match the polarity of the functional groups, among them the combination of each other -SO ₃ M group.

These binders are used in an amount of 7 to 50 parts by weight, preferably 10 to 35 parts by weight, based on 100 parts by weight of the ferromagnetic powder. Particularly, as a binder, 5 to 30 parts by weight of a vinyl chloride resin and 2-2 to a polyurethane resin
It is most preferable to use 0 parts by weight in combination.

It is desirable to use together with these binders a thermosetting crosslinking agent that bonds to a functional group or the like contained in the binder to crosslink. Examples of the cross-linking agent include tolylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, and the reaction products of these isocyanates with those having a plurality of hydroxyl groups such as trimethylolpropane, and condensation products of the isocyanates. Various polyisocyanates are preferred. These crosslinking agents are generally used in a proportion of 3 to 50 parts by weight based on 100 parts by weight of the binder. More preferably, it is 3 to 20 parts by weight.

A conventionally known abrasive can be added to the magnetic layer. As these abrasives, α-alumina, β-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum, artificial diamond,
Silicon nitride, silicon carbide, titanium carbide, titanium oxide,
Those having a Mohs hardness of 6 or more, such as silicon dioxide and boron nitride, can be used alone or in combination. Among these, alumina is particularly preferable because of its high hardness and excellent head cleaning effect with a small amount of addition. As the particle size of the abrasive, in the case of a magnetic layer as thin as 0.01 to 0.1 μm, it is usually preferable to set the average particle size to 0.002 to 0.15 μm,
The particle size is more preferably 0.005 to 0.10 μm. The addition amount is preferably 5 to 20% by weight based on the ferromagnetic powder. More preferably, it is 8 to 18% by weight.

Further, conventionally known carbon black (CB) can be added to the magnetic layer of the present invention for the purpose of improving conductivity and surface lubricity. Examples of such CB include acetylene black and furnace black. And thermal black. Particle size of 5 nm to 200
nm is used, and the particle size is 10 nm to 100 nm.
Are preferred. This range is preferred when the particle size is 5
If it is less than nm, it is difficult to disperse CB, and if it is more than 200 nm, it is necessary to add a large amount of CB, and in any case, the surface becomes coarse and the output is reduced. The addition amount is preferably 0.2 to 5% by weight based on the ferromagnetic powder. More preferably, it is 0.5 to 4% by weight.

The above magnetic disk has a magnetic layer thickness of 0.1.
Even when the thickness is as thin as μm or less, the thickness unevenness of the magnetic layer is 15% or less, and the reproduction output when using the MR reproducing head is
Excellent output-to-noise ratio.

[0033]

The present invention will be described in detail with reference to the following Examples, but it should not be construed that the present invention is limited thereto. In addition, the part of an Example and a comparative example shows a weight part.

Example 1 << Coating Component for Undercoat Layer >> (1) Iron oxide powder (particle size: 0.11 × 0.02 μm) 68 parts Alumina (gelatinization ratio: 50%, particle size: 0.07 μm) 8 parts Carbon black (particle size: 25 nm) 24 parts Stearic acid 2.0 parts Vinyl chloride copolymer 8.8 parts (Contains -SO ₃ Na group: 0.7 × 10 ^-4 equivalents / g) Polyester polyurethane resin 4.4 parts (Tg: 40 ° C., containing —SO ₃ Na group: 1 × 10 ⁻⁴ equivalent / g) cyclohexanone 25 parts methyl ethyl ketone 40 parts toluene 10 parts (2) butyl stearate 2 parts oleyl oleate 6 parts cyclohexanone 70 Parts methyl ethyl ketone 50 parts toluene 20 parts (3) polyisocyanate 1.4 parts cyclohexanone 10 parts methyl ethyl ketone 15 parts toluene 10 parts

<< Coating Component for Magnetic Layer >> (1) Ferromagnetic iron-based metal powder (Co / Fe: 30 at%, Y / (Fe + Co): 3 at%, Al / (Fe + Co): 5 wt%, Ca / Fe: 0) , Σs: 155 Am ² / kg, Hc: 188.2 kA / m, pH: 9.4, major axis length: 0.10 μm) 100 parts Vinyl chloride-hydroxypropyl acrylate copolymer 12.3 parts (content- SO ₃ Na group: 0.7 × 10 ⁻⁴ equivalent / g) Polyester polyurethane resin 5.5 parts (contained-SO ₃ Na group: 1.0 × 10 ⁻⁴ equivalent / g) α-alumina (average particle size: 0.2 parts) 8 parts α-alumina (average particle diameter: 0.07 μm) 2 parts Carbon black 2.0 parts (average particle diameter: 75 nm, DBP oil absorption: 72 cc / 100 g) Methyl acid phosphate 2 parts Stearic acid 1. 5 parts Oleyl oleate 5 parts Tetrahydro Furan 65 parts Methyl ethyl ketone 245 parts Toluene 85 parts (2) Polyisocyanate 2.0 parts Cyclohexanone 167 parts

After kneading (1) in the above undercoat layer coating composition with a kneader, adding (2), stirring and dispersing the mixture with a sand mill with a residence time of 60 minutes, and adding (3) to this. After stirring and filtration, the mixture was used as a coating for an undercoat layer.
Separately, the above-mentioned magnetic layer coating component (1) is kneaded with a kneader, dispersed by a sand mill with a residence time of 45 minutes, and the magnetic layer coating component (2) is added thereto, followed by stirring and filtration. It was a magnetic paint. Apply the above-mentioned undercoat paint to a thickness of 6
On a support made of a 2 μm polyethylene terephthalate film, the thickness after drying and calendering is 1.1 μm on one side.
Then, on the undercoat layer, the above magnetic paint is wet-on-wet so that the thickness of the magnetic layer after random orientation treatment, drying and calendering treatment is 0.08 μm on one side. , And after a random orientation treatment, dried using a dryer and far-infrared rays to obtain a magnetic sheet.

The magnetic sheet obtained in this manner was heated at a temperature of 80.degree.
After mirror-finish treatment under the condition of 0 kg / cm, and aging at 60 ° C. for 24 hours with the magnetic sheet wound on the core, 3.5
Punched to inch size. This was subjected to a surface polishing treatment (air pressure 0.25MP) using an alumina polishing tape (WA-6000: average particle diameter of alumina 2 µm, surface roughness 0.4 µm).
a, polishing time 1 second, disk rotation speed 2,000 rpm)
After aging treatment at 70 ° C. for 24 hours, the resultant was adhered to a hub having a surface runout of 60 μm, and then inserted into a jacket with a nonwoven fabric without a lifter to produce a magnetic disk.

Examples 2 to 13: Magnetic disks of Examples 2 to 13 were produced in the same manner as in Example 1 except that some of the conditions were changed to those shown in Tables 1 to 3.

Example 14 A magnetic disk was produced in the same manner as in Example 1 except that iron oxide was changed to 76 parts by weight, except for 8 parts by weight of alumina in the undercoat layer.

Comparative Examples 1-4: Comparative Examples 1-4 were carried out in the same manner as in Example 1 except that some of the conditions were changed to those shown in Table 4.
Was manufactured.

The above Examples 1 to 14 and Comparative Examples 1 to 4
Each of the magnetic disks was measured by the following method. Tables 1 to 4 show the measurement results.

<Magnetic Layer Thickness (d1) and Undercoat Layer Thickness (d2)> The thickness was measured by embedding a magnetic tape in a resin, cutting it out with a diamond cutter, and observing the cross section with a transmission electron microscope. Was measured, and the thickness of the magnetic layer (d1) and the thickness of the undercoat layer (d2) were determined from the average value.

<Magnetic Layer Thickness Unevenness> The magnetic layer thickness unevenness is determined by calculating the maximum value M (ma) of the magnetic layer thickness (d1).
x), the minimum value is defined as M (min), and is defined as follows. Magnetic layer thickness unevenness (%) = (| M (max) −M (min) | / d
1) x 100

<Magnetic Characteristics> The magnetic characteristics of the magnetic disk are as follows.
Using a sample vibration type magnetometer VSM-3 manufactured by Toei Industry Co., Ltd.
The measurement was performed at a magnetic field of 0.80 MA / m (10 kOe). The measurement sample was a disk of 8 mmφ, and the coercive force (H
c) The residual magnetic flux density (Br) was measured.

<Surface Roughness of Magnetic Surface> The surface roughness of the magnetic surface was measured using a light interference three-dimensional surface roughness meter.

<Reproduction output and output versus noise> Spindle motor SPU-MUX158G made by NTN for disk rotation
Central Precision Machinery Precision Stage MM- for V3 and Head Position Adjustment
40X · Y, MM-40Z, MM-40GU and MM
Using an inductive head having a track width of 3 μm as a write head and a track width of 1.5 μm MR as a read head, recording (recording wavelength: 0.37 μm) and reproduction were performed on an electromagnetic conversion characteristic evaluation device consisting of −40 GL. The reproduction output and output-to-noise are values when the tape of Comparative Example 1 is set to 0 dB.

[0047]

[Table 1]

[0048]

[Table 2]

[0049]

[Table 3]

[0050]

[Table 4]

[0051]

Examples 1 to 14 shown in Tables 1 to 4
As is clear from the evaluation results of Comparative Examples 1 to 4, the magnetic disk of the present invention in which the thickness of the magnetic layer is 0.01 to 0.10 μm and the unevenness of the thickness of the magnetic layer is 15% or less has a large storage capacity. High and excellent magnetic disk. Particularly, the reproduction output and the output-to-noise ratio when the MR head is used are high and excellent.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Kazuhiko Nagiri, 1-1-88 Ushitora, Ibaraki City, Osaka Prefecture Inside Hitachi Maxell Co., Ltd. (72) Kenji Tanaka 1-1-88 Ushitora, Ibaraki City, Osaka Prefecture F-term (reference) in Hitachi Maxell, Ltd. 5D006 BA19 CA05 FA09 5E040 AA11 AA19 CA06 NN06 NN12 NN13

Claims (4)

[Claims] 1. A magnetic disk in which at least one undercoat layer and a magnetic layer are formed on at least one surface of a nonmagnetic support in this order, the thickness of the magnetic layer is 0.01 to
10. A magnetic disk, wherein the thickness of the magnetic layer is 0.10 μm or less and 15% or less. 2. The magnetic disk according to claim 1, wherein a ratio (d2 / d1) of the thickness (d1) of the magnetic layer to the thickness (d2) of the undercoat layer is 10 or more and 100 or less. 3. A magnetic recording signal is reproduced by a reproducing head using a magnetoresistive element.
The magnetic disk as described. 4. A coercive force in the magnetic layer of 160 to 3
The magnetic disk according to any one of claims 1 to 3, wherein the magnetic disk has a residual magnetic flux density in the longitudinal direction of the track of 20 kA / m and a product of thickness and 0.0018 µTm or more and 0.04 µTm or less.