JPH06131653A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To provide a magnetic recording medium excellent in practical characteristics such as running property, durability and preservability, also excellent in cost effectiveness and having satisfactory electromagnetic transducing characteristics and high recording density. CONSTITUTION:When a magnetic layer based on ferromagnetic powder and a resin binder is formed on a nonmagnetic substrate to obtain a magnetic recording medium, ferromagnetic metal powder formed by reducing cobalt ferrite and having 20-35 atomic % Co content, 100-500Angstrom average particle diameter and >=1.2 acicularity ratio is used as the ferromagnetic powder.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording medium for high density recording, and more particularly to improvement of a magnetic recording medium having a coating type magnetic layer.

[0002]

2. Description of the Related Art In magnetic recording technology, a medium can be repeatedly used, signals can be easily digitized, and a system can be constructed by combining with peripheral equipment.
Since it has excellent features not found in other recording methods, such as the ability to easily modify signals,
It has been widely used in various fields including computer applications.

In order to meet the demands for downsizing equipment, improving the quality of recording / reproducing signals, lengthening the recording time, increasing the recording capacity, etc., the recording density of the recording medium should be further improved. Has always been desired.

For example, in audio and video applications, in order to cope with the practical use of a digital recording method for improving the sound quality and image quality and the development of a recording method compatible with high-definition TV, it is more important than the conventional system. A magnetic recording medium capable of recording and reproducing a short wavelength signal has been required.

Further, even for a floppy disk having a magnetic layer on a flexible non-magnetic support which is an external storage medium of a microcomputer or a personal computer, the recent spread of personal computers, the sophistication of application software, the processing information Due to the increasing trend, there has been a strong demand for higher capacity of 10 Mbytes or more.

The magnetic recording medium is used in the form of a tape, a disk, a card or the like according to the application by forming a magnetic layer on a non-magnetic support. In the magnetic layer, a coating liquid in which ferromagnetic powder is dispersed is applied and dried on a non-magnetic support, so-called coating type magnetic layer mainly composed of ferromagnetic powder and binder resin, and vapor deposition or sputtering. There is a so-called metal thin film type magnetic layer composed of a ferromagnetic metal thin film obtained by vacuum film formation on a non-magnetic support by the vacuum film formation method.

From the viewpoint of recording density, the latter metal thin film type magnetic layer is excellent and has been studied for practical use for a long time, and has been put to practical use for 8 mm video applications and the like.

However, the metal thin film type magnetic recording medium has problems in characteristics such as durability, running property and corrosion resistance.
The original excellent characteristics are not fully utilized to deal with the problem. Further, the manufacturing cost is high and there is a problem in economic efficiency.

On the other hand, the coating type magnetic layer is superior to the metal thin film type in dealing with such a problem and has been put to practical use in various applications for a long time.
Many efforts have been made to improve the recording density, which is less than that of the metal thin film type, but there is a limit.

From the viewpoint of improving the magnetic characteristics of the magnetic layer, improving the dispersibility of the ferromagnetic powder, and improving the surface property of the magnetic layer for the purpose of achieving high density recording of the coating type magnetic recording medium. Have studied and proposed various methods.

For example, a method of using a ferromagnetic ferromagnetic metal powder or a hexagonal ferrite as a ferromagnetic powder for enhancing magnetic properties is disclosed in JP-A-58-122623 and JP-A-61-74137. Japanese Patent Publication No. 62-49656, Japanese Patent Publication No. 60-50323, US462965.
3, US Pat. No. 4,666,770, US Pat. No. 4,543,198, etc.

Further, in order to enhance the dispersibility of the ferromagnetic powder, various surfactants (for example, Japanese Patent Laid-Open No. 52-15660).
6, JP-A-53-15803, JP-A-53
It is disclosed in Japanese Patent Laid-Open No. 116114. ) Or various reactive coupling agents (see, for example, JP-A-4
9-59608, JP-A-56-58135, JP-B-62-28489 and the like. ) Is proposed.

Further, in order to improve the surface property of the magnetic layer,
A method (for example, disclosed in Japanese Patent Publication No. 60-44725) of improving the surface forming treatment method of the magnetic layer after coating and drying has been proposed.

The above-mentioned ferromagnetic metal powder and hexagonal ferrite are excellent as ferromagnetic powders for high density recording media, but the magnetic conversion characteristics of metal thin film type magnetic recording media are inferior. . In particular, hexagonal ferrite has a problem that its saturation magnetization is small and the magnetic flux density of the magnetic layer cannot be increased. Further, its dispersibility is poor, and it is difficult to obtain a magnetic layer having excellent surface properties.

On the other hand, in producing a magnetic recording medium for high-density recording, the following requirements are required: 1) small particle size 2) large coercive force 3) large residual magnetic flux density 4) weather resistance It is good and is being studied as an effective method for promoting higher density recording in the future. 5) It is required that the perpendicular component of the magnetization of the magnetic layer be utilized as a ferromagnetic powder. No magnetic recording medium has been obtained that satisfies all the above requirements. Most of this factor is considered to be due to the performance of the ferromagnetic powder itself.

For example, ferromagnetic metal powders have high magnetic properties and are excellent in coercive force and residual magnetic flux density, but they are not sufficient in weather resistance and are the mainstream of the manufacturing method even in terms of particle size. In the method using as a raw material, the major axis length was limited to 0.1 μm. In addition, the ferromagnetic metal powder produced by the vacuum evaporation method has a fibrous form in which 100 to 300Å spherical metal particles are connected, and the Co content,
Since it is difficult to control the grain size and the acicular ratio, and it is difficult to obtain the one with the upper limit of the acicular ratio suppressed, there is a problem that the perpendicular component of the magnetization of the magnetic layer cannot be used and there is a limit in terms of high density recording. It was Further, this ferromagnetic metal powder is produced by evaporating a ferromagnetic metal under vacuum, and has a drawback that the manufacturing cost is very high. In addition, barium ferrite has a very small particle size, and a magnetic recording medium having stable weather resistance can be obtained. The easy magnetization axis is in the direction perpendicular to the plate surface of the plate-like particles, and the particle size is small. In addition to this, the magnetization component in the perpendicular direction can be utilized, which is advantageous in high density recording. However,
As described above, it is impossible to obtain a magnetic recording medium having a high saturation output and a low output.

Therefore, in order to satisfy the above-mentioned requirements, various methods have been conventionally tried to improve the ferromagnetic powder. For example, as disclosed in Japanese Patent Publication No. 43-20117, cobalt ferrite (CoFe ₂ O ₄ )
Has been proposed to obtain a ferromagnetic metal powder having a high Co content by dry reduction. However, the particle shape of the cobalt ferrite powder is cubic, and a needle-like shape having high coercive force due to shape anisotropy cannot be obtained.
Only coercive force less than Oersted was obtained.

Further, Japanese Patent Application Laid-Open No. 55-133505 discloses a method of converting the surface of Fe ₃ O ₄ into cobalt ferrite and then reducing it into a ferromagnetic metal powder. Further, in the case of a method for producing a ferromagnetic metal powder using goethite as a starting material, it is possible to obtain a ferromagnetic metal powder in which the acicular shape of the particles of goethite is maintained, but dehydration and deoxygenation from goethite occur during the production process. However, there are many voids and branches in the particles, and the shape thereof is a mere form, and the particles are easily broken, the particle size distribution and the coercive force distribution are wide, and the coercive force that can be obtained is also limited.

In Japanese Patent Publication No. 60-42174, a specific organic substance is added as a crystallization controlling agent to a suspension of ferric hydroxide, and it has excellent acicularity without pores. A method of making ferric oxide is disclosed. It is also described that a metallic magnetic powder can be obtained by causing a reducing agent such as hydrogen gas to act on the ferric oxide. Furthermore, JP-A-51-
Japanese Patent No. 25454 and Japanese Patent Application Laid-Open No. 53-123898 disclose a method of obtaining a ferromagnetic metal powder having an excellent acicular ratio by reducing acicular iron oxide containing various chelate compounds.

Further, Japanese Patent Application Laid-Open No. 60-114509 discloses a method of reducing amino acid by attaching amino acid as a sintering inhibitor on the surface of iron oxide, and Japanese Patent Application Laid-Open No. 53-76959 also discloses a method for preventing sintering. As a method of using an organic substance such as triphenylene as an agent, Japanese Patent Publication No. 62-17362 discloses an example of using a chelate compound. In order to meet the demand for high-density recording, a so-called metal thin film type magnetic recording medium in which a ferromagnetic metal thin film having excellent electromagnetic conversion characteristics is used as a magnetic layer has also been proposed. However, the magnetic recording medium using the metal thin film as a magnetic layer has problems that the manufacturing cost is high and practical properties such as weather resistance and durability are poor.

Therefore, in the current coating type magnetic recording medium,
The above requirements could not be satisfied, and it was not possible to obtain a magnetic recording medium having a high density comparable to that of a metal thin film type magnetic recording medium. That is, there has been a demand for a coating type magnetic recording medium which is excellent in practical properties such as running property, durability and storability, is also excellent in economical efficiency and can meet the demand for high density recording in the future.

[0022]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and is excellent in practical properties such as running property, durability, and storability and economical efficiency, and electromagnetic conversion properties. Aiming to provide a good magnetic recording medium having a high recording density, and in particular to provide a magnetic recording medium having an electromagnetic conversion characteristic comparable to that of a metal thin film type in a coating type magnetic recording medium. There is.

[0023]

SUMMARY OF THE INVENTION An object of the present invention is to provide a magnetic recording medium having a magnetic layer mainly composed of a ferromagnetic powder and a binder resin on a non-magnetic support, the ferromagnetic powder reducing cobalt ferrite. And a Co content of 2 is obtained.
0-35 atom% and average particle size 100-500Å
In the magnetic recording medium, the acicular ratio is 1.2 or more.

The present invention is characterized in that Co--Fe alloy powder having a specific composition and shape formed by reducing cobalt ferrite is selected and used as the ferromagnetic metal powder. The Co-Fe alloy powder has a ferromagnetic metal content of 90% by weight or more, preferably 95% by weight or more, and a Co content of 20 to 35 atomic%, preferably 25%.
To 34 atom%, particularly preferably 32 to 34 atom%, and the average particle size (meaning the major axis length) is 100 to 500.
Å, preferably 100-300 Å, particularly preferably 1
The acicular ratio (average particle size / average minor axis length) is 1.2 or more, preferably 1.2 to 10, and more preferably 1.5 to 5.0 at 00 to 250Å. The Co—Fe alloy powder can have a saturation magnetization σ _S of 150 emu / g or more, preferably 150 to 180 emu / g. Also,
The coercive force is 1000 Oe or more, preferably 1000-20
00Oe, particularly preferably 1100 to 1800 Oe.

The magnetic recording medium of the present invention uses this Co--Fe
By using the alloy powder, the coercive force of the magnetic layer is reduced to 10
It can be improved to 00 Oe or more, preferably 1100 to 2000 Oe, and particularly preferably 1200 to 1900 Oe. The magnetic recording medium using the conventional cobalt ferrite powder has a maximum of 800 Oe.

In the present invention, a magnetic recording medium with less noise can be obtained (effect that the particle size is small) by controlling the needle ratio appropriately while keeping the particle size of the Co--Fe alloy powder small, and the conventional cobalt The coercive force can be made higher than that of the ferromagnetic metal powder with reduced ferrite (the effect that the acicular ratio is large), and the perpendicular magnetization component of the magnetic layer can be effectively used by suppressing the upper limit of the acicular ratio. Therefore, a magnetic recording medium capable of higher density recording can be obtained, and the object of the present invention can be achieved.

The coercive force of the perpendicular magnetization component of the magnetic recording medium of the present invention is 400 Oe or more, preferably 900 to 1800.
Oe, particularly preferably 1000 to 1700 Oe, and the saturation magnetization σ _S of the perpendicular magnetization component can be 1000 gauss or more, preferably 1000 to 3000 gauss. Further, as another advantage of the magnetic recording medium of the present invention, the stability of the ferromagnetic powder is good, and there is almost no ignitability which is a problem of the ferromagnetic metal powder in the manufacturing process of the magnetic recording medium (the ignition temperature is The temperature is 200 ° C. or higher.), And since the oxidation stability is also very high, the magnetic recording medium can be stable and have excellent weather resistance.

The method for producing the Co--Fe alloy powder used in the present invention is not particularly limited, and any method can be used as long as the above conditions are satisfied. Hereinafter, specifically Co-F
A method for producing the e-alloy powder will be described, but the method is not limited thereto. First, a Co ²⁺ salt aqueous solution and an Fe ³⁺ salt aqueous solution or a Co ²⁺ salt aqueous solution and an Fe ²⁺ salt aqueous solution are coprecipitated with an alkali, the pH of the reaction mother liquor is adjusted to 7 or more, and the reaction is performed at 40 ° C. or more to react with cobalt ferrite. To generate. Then, after washing with water and prevention of sintering, it is further washed with water and dried to obtain a cobalt ferrite powder. An organic crystallization inhibitor may be added during the cobalt ferrite formation reaction to make the particle shape acicular. Then, after being placed in an electric furnace, reduced with hydrogen gas and deoxidized, a Co—Fe alloy powder can be obtained.

The composition of ordinary cobalt ferrite is Co
/ (Fe + Co) is 33 at%, but the Co content of the Co—Fe alloy powder can be increased by converting a part of Co to a Fe ²⁺ salt at the time of reaction charging and increasing the Co content. It can be adjusted to 20 to 35 atomic%. When the Co content is 20 atomic% or less, the saturation magnetic flux density of the ferromagnetic metal powder becomes low, the output as a magnetic recording medium becomes low, and the improvement of the output as a magnetic recording medium becomes insufficient. If the amount of cobalt exceeds 35 atomic%, particles of a cobalt compound different from the cobalt ferrite particles will be mixed, which is not preferable.

The acicular ratio of the Co--Fe alloy powder can be controlled by the amount of the organic crystallization inhibitor added. As described above, the acicular ratio used in the present invention is 1.2 or more, preferably 1.2 or more.
10 to 10, more preferably 1.5 to 5.0. When the acicular ratio exceeds 10, the perpendicular component of the magnetic recording medium becomes low, which is not suitable for high density recording, which is not preferable. The organic crystallization inhibitor, various organic compounds can be used, for example, organic phosphonic acid described in JP-B-60-42174, hydroxycarboxylic acid, an organic compound selected from these salts and esters, Specifically, as the organic phosphonic acid, aminotri (methylenephosphonic acid), 1-hydroxyethylidene-1,1-diphosphonic acid, ethylenediaminetetra (methylenesulfonic acid),
Hydroxymethylenediphosphonic acid, methylenediphosphonic acid, ethylene-1,2-diphosphonic acid and the like, and as the hydroxycarboxylic acid, citric acid, tartaric acid, dihydroxyglutanic acid, glycolic acid, salicylic acid and the like mainly aliphatic hydroxycarboxylic acid Among them, citric acid and ethylenediaminetetramethylenesulfonic acid are preferable.

The amount of the organic crystallization inhibitor added is such that all the metals used in the reaction (the sum of Co, Fe and other added metals) are used.
The amount is 1 × 10 ^{−6 to} 3 mol, preferably 1 × 10 ^{−3 to} 0.5 mol per mol of metal. If the amount of the crystallization inhibitor added is too large, the acicular ratio becomes too large, and if it is too small, there is no effect and the Hc of the magnetic recording medium becomes low. From these studies, the above range is preferable when the crystallization inhibitor is added.

Other details of the method for producing the Co--Fe alloy powder are described in Japanese Patent Application No. 4-188350 filed by the present applicant.
The specification can be referred to. In the magnetic recording medium of the present invention, the structure of the coating layer formed on the non-magnetic support is not particularly limited as long as it has at least one magnetic layer in which a ferromagnetic powder is dispersed in a binder resin. It will not be done. In the present invention, it is preferable to form a nonmagnetic layer in which a nonmagnetic powder is dispersed in a binder resin between the nonmagnetic support and the magnetic layer, because higher density recording can be realized.

In the present invention, the thickness of the magnetic layer (in the case of a laminated structure of two or more layers, the sum thereof) is 1 μm or less, preferably 0.1 to 1.0 μm, and particularly preferably 0.2 to.
It is 0.8 μm. As a method for forming two or more coating layers on the non-magnetic support, the coating liquid for the second and subsequent layers is applied onto the non-magnetic support while the first coating layer on the non-magnetic support is still wet. By applying by a wet-on-wet method (simultaneous multilayer coating method, disclosed in JP-A-61-139929, JP-A-61-54992, etc.), that is, the first coating layer is wet. If the second and subsequent coating layers are formed while in the state, it becomes easy to make the upper magnetic layer thin, and coating defects can be reduced, and the adhesion of each coating layer can be increased. It is possible to do so, which is preferable.

In the above magnetic recording medium, particularly when the first layer is a non-magnetic layer, the Co of the present invention is used as the uppermost layer (magnetic layer).
The use of —Fe alloy powder can maximize the high density magnetic recording characteristics. Specifically, firstly, the non-magnetic powder such as carbon black necessary for the practical characteristics of the magnetic recording medium can be added only to the first layer other than the uppermost layer to ensure the necessary electrical conductivity. This is because the magnetic layer important for short wavelength magnetic recording can be formed as a thin layer of 1 μm or less and the saturation magnetic flux density per unit volume can be increased. Secondly, the unevenness of the surface of the uppermost layer is smooth. It is because it is finished in. Furthermore, C, which is relatively expensive as a material
Since the amount of o-Fe alloy powder used can be reduced, there is also an advantage that it is economical.

The binder resin for the magnetic layer or non-magnetic layer in the magnetic recording medium of the present invention may be a conventionally known thermoplastic resin, thermosetting resin, reactive resin or a mixture thereof. . The thermoplastic resin has a glass transition temperature of −100 to 150 ° C. and a number average molecular weight of 10
00 to 200,000, preferably 10,000 to 10
0000, and the degree of polymerization is about 50 to 1000.

Examples of such binder resin include vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic acid ester, vinylidene chloride, acrylonitrile, methacrylic acid, methacrylic acid ester, styrene, butadiene, ethylene, There are polymers or copolymers containing vinyl butyral, vinyl acetal, vinyl ether, etc. as constituent units, polyurethane resins, and various rubber resins.

Further, as the thermosetting resin or the reaction type resin, a phenol resin, an epoxy resin, a polyurethane curing type resin, a urea resin, a melamine resin, an alkyd resin, an acrylic type reaction resin, a formaldehyde resin, a silicone resin, Examples thereof include epoxy-polyamide resin, a mixture of polyester resin and isocyanate prepolymer, a mixture of polyester polyol and polyisocyanate, and a mixture of polyurethane and polyisocyanate.

In order to obtain more excellent dispersion effect of the ferromagnetic powder and durability of the magnetic layer in the above-mentioned binder resin, COOM, SO ₃ M, OSO ₃ M, P = O (OM) is added as necessary.
₂ , OP = O (OM) ₂ , (wherein M is a hydrogen atom or an alkali metal base), OH, NR ₂ , N ⁺ R
₃ (R is a hydrocarbon group) It is preferable to use one having at least one polar group selected from an epoxy group, SH, CN and the like introduced by copolymerization or addition reaction. The amount of such polar group is 10 ^{-1 to} 10 ^-8 mol / g, preferably 10 ^{-2 to} 10 ^-6 mol / g.

The binder resin used in the magnetic recording medium of the present invention is in the range of 5 to 50% by weight with respect to the ferromagnetic powder in the magnetic layer and the nonmagnetic powder in the nonmagnetic layer. It is preferably used in the range of 10 to 30% by weight. It is preferable to use a combination of 5 to 100% by weight when using a vinyl chloride resin, 2 to 50% by weight when using a polyurethane resin, and 2 to 100% by weight of polyisocyanate.

The filling degree of the ferromagnetic powder in the magnetic layer can be calculated from the maximum saturation magnetization σ _S and the maximum magnetic flux density Bm of the used ferromagnetic powder (Bm / 4πσs), and in the present invention, the value is It is preferably 1.7 g / cc or more, more preferably 1.9 g / cc or more, and most preferably 2.1 g / cc or more. In the present invention, when polyurethane is used, the glass transition temperature is -50 to 10
Preferably, the elongation at break is 100 to 2000%, the stress at break is 0.05 to 10 kg / cm ² , and the yield point is 0.05 to 10 kg / cm ² .

The polyisocyanate used in the present invention includes tolylene diisocyanate, 4-4'-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate and naphthylene-1. ，
5-diisocyanate, o-toluidine diisocyanate
, Isophorone diisocyanate, triphenylmethane triisocyanate, and the like, products of these isocyanates with polyalcohols, and polyisocyanates formed by condensation of isocyanates. -, Etc. can be used. Commercially available trade names of these isocyanates are Nippon Polyurethane Co., Ltd., Coronate L, Coronet HL, Coronet 2030, Coronet 2031, Millionate MR.
Millionate MTL, Takeda Pharmaceutical Co., Takenet D-1
02, Takenet D-110N, Takenet D-20
0, Takenet D-202, Sumitomo Bayer Co., Ltd., Desmodule L, Desmodule IL, Desmodule N,
Desmodule HL, etc. can be used alone or in combination of two or more by utilizing the difference in curing reactivity.

In the magnetic layer and / or nonmagnetic layer of the magnetic recording medium of the present invention, a lubricant, an abrasive, a dispersant, and
Materials having various functions such as an antistatic agent, a dispersant, a plasticizer, an antifungal agent and the like can be contained according to the purpose. The lubricant used in the magnetic layer of the present invention is a dialkylpolysiloxane (where alkyl is 1 carbon atom).
To 5), dialkoxy polysiloxane (alkoxy has 1 to 4 carbons), monoalkyl monoalkoxy polysiloxane (alkyl has 1 to 5 carbons, alkoxy has 1 to 4 carbons), phenyl polysiloxane, Silicon oil such as fluoroalkyl polysiloxane (where alkyl is 1 to 5 carbon atoms); Conductive fine powder such as graphite; Inorganic powder such as molybdenum disulfide and tungsten disulfide; Polyethylene, polypropylene, polyethylene vinyl chloride copolymer, Plastic fine powder such as polytetrafluoroethylene; α-olefin polymer; unsaturated aliphatic hydrocarbon liquid at normal temperature (compound in which n-olefin double bond is bonded to terminal carbon, carbon number about 20); carbon number 12 to 20 monobasic fatty acids and 3 to 12 carbon atoms
Fatty acid esters, fluorocarbons, etc., each consisting of a monohydric alcohol can be used.

Of these, fatty acid esters are more preferred.
The alcohol used as the raw material of the fatty acid ester is ethanol, butanol, phenol, benzyl alcohol, 2-methylbutyl alcohol, 2-hexyldecyl alcohol, propylene glycol monobutyl ether, ethylene glycol monobutyl ether, dipropylene glycol monobutyl ether, diethylene glycol monobutyl ether, Examples thereof include monoalcohols such as s-butyl alcohol, and polyhydric alcohols such as ethylene glycol, diethylene glycol, neopentyl glycol, glycerin, and sorbitan derivatives. Similarly, as fatty acids, acetic acid, propionic acid, octanoic acid, 2-ethylhexanoic acid, lauric acid, myristic acid, stearic acid, palmitic acid, behenic acid, arachidic acid, oleic acid, linoleic acid, linolenic acid, elaidic acid, palmitoleic acid. And aliphatic carboxylic acids or mixtures thereof.

Specific examples of the fatty acid ester include butyl stearate, s-butyl stearate, isopropyl stearate, butyl oleate, amyl stearate, 3-methylbutyl stearate, 2-ethylhexyl stearate, 2-hexyldecyl. Stearate, butyl palmitate, 2-ethylhexyl myristate,
Mixture of butyl stearate and butyl palmitate, butoxyethyl stearate, 2-butoxy-1-propyl stearate, dipropylene glycol monobutyl ether acylated with stearic acid, diethylene glycol dipalmitate, hexamethylene diol myristic acid And various ester compounds such as glycerin oleate and the like.

Further, in order to reduce the hydrolysis of the fatty acid ester that often occurs when the magnetic recording medium is used under high humidity, branched / straight chain, cis / trans, etc. isomer structures of branched fatty acids and alcohols as raw materials, and branch positions are used. Is made. These lubricants are added in the range of 0.2 to 20 parts by weight based on 100 parts by weight of the binder.

The following compounds may be used as the lubricant. That is, silicone oil, graphite, molybdenum disulfide, boron nitride, fluorinated graphite, fluoroalcohol, polyolefin, polyglycol, alkyl phosphate, tungsten disulfide and the like. The abrasive used in the magnetic layer of the present invention is a commonly used material, such as fused alumina, silicon carbide and chromium oxide (Cr ₂
O ₃ ), corundum, artificial corundum, diamond,
Artificial diamond, garnet, emery (main components: corundum and magnetite) are used. These abrasives have a Mohs hardness of 6 or more. Specific examples are AKP-10, AKP-12, AKP-15, and 2 manufactured by Sumitomo Chemical Co., Ltd.
0AKP-30, AKP-50, AKP-1520, A
KP-1500, HIT-50, HIT-100, Nippon Kagaku Kogyo Co., Ltd., G5, G7, S-1, chromium oxide K, Uemura Kogyo UB40B, Fujimi Abrasives WA8000,
WA10000, TF140, TF180 manufactured by Toda Kogyo Co., Ltd.
And so on. Those having an average particle size of 0.05 to 3 μm are effective, and preferably 0.1 to 1.
It is 5 μm.

These abrasives are added in an amount of 1 to 20 parts by weight, preferably 1 to 15 parts by weight, based on 100 parts by weight of the ferromagnetic powder. If it is less than 1 part by weight, sufficient durability cannot be obtained, and if it is more than 20 parts by weight, the surface property and filling degree are deteriorated. These abrasives may be dispersed in a binder in advance and then added to the magnetic paint. The magnetic layer of the magnetic recording medium of the present invention may contain conductive particles as an antistatic agent in addition to the non-magnetic powder. However, in order to maximize the saturation magnetic flux density of the uppermost layer, it is preferable to add as little as possible to the uppermost layer and to add it to a coating layer other than the uppermost layer. As the antistatic agent, it is particularly preferable to add carbon black in order to reduce the surface electric resistance of the entire medium. The carbon black that can be used in the present invention may be a furnace for rubber, thermal for rubber, black for color, acetylene black, or the like. Specific surface area is 5 to 5
00 m ² / g, DBP oil absorption is 10 to 1500 ml /
100 g, particle size 5 to 300 mμ, pH 2 to 10, water content 0.1 to 10%, tap density 0.1.
It is preferably 1 to 1 g / cc. As a specific example of the carbon black used in the present invention, manufactured by Cabot Corporation,
BLACKPEARLS 2000, 1300, 100
0, 900, 800, 700, VULCAN XC-7
2, Asahi Carbon Co., Ltd., # 80, # 60, # 55, # 5
0, # 35, Mitsubishi Kasei Kogyo Co., Ltd., # 3950B, # 24
00B, # 2300, # 900, # 1000, # 30,
# 40, # 10B, Columbia Carbon Co., COND
UCTEX SC, RAVEN 150, 50, 40,
15, Ketjen Black EC, Ketjen Black ECDJ-500, Ketjen Black ECDJ-600 and the like manufactured by Lion Aguzo are listed. Carbon black may be surface-treated with a dispersant or the like, may be grafted with a resin, or may be partially graphitized. Further, the carbon black may be dispersed with a binder in advance before being added to the magnetic paint. When carbon black is used in the magnetic layer, the amount thereof based on the ferromagnetic powder is preferably 0.1 to 30% by weight. Further, the nonmagnetic layer preferably contains 3 to 20% by weight based on the total amount of nonmagnetic powder.

In general, carbon black not only functions as an antistatic agent, but also functions to reduce the friction coefficient, impart light-shielding properties, improve the film strength, etc. These differ depending on the carbon black used. Therefore, these cards used in the present invention
It is of course possible to change the type, amount, and combination of the bonblacks and use them in each coating layer according to the purpose based on the above-mentioned characteristics such as particle size, oil absorption, electric conductivity, and pH. . The carbon black that can be used can be referred to, for example, in "Carbon Black Handbook" edited by Carbon Black Association.

When the nonmagnetic layer is formed below the magnetic layer of the magnetic recording medium of the present invention, the nonmagnetic layer is a layer in which nonmagnetic powder is dispersed in a binder resin. Various non-magnetic powders can be used for the non-magnetic layer. For example,
α-alumina, β-alumina, γ- with an alpha conversion rate of 90% or more
Alumina, silicon carbide, chromium oxide, cerium oxide, α
-Iron oxide, corundum, silicon nitride, titanium carbide,
Titanium oxide, silicon dioxide, boron nitride, zinc oxide, calcium carbonate, calcium sulfate, barium sulfate and the like are used alone or in combination. The particle size of these non-magnetic powders is preferably 0.01 to 2μ, but if necessary, non-magnetic powders having different particle sizes may be combined, or a single non-magnetic powder may be used to broaden the particle size distribution to obtain the same effect. You can also Tap density is 0.3 to 2 g / cc,
The water content is preferably 0.1 to 5% by weight, the pH is 2 to 11, and the specific surface area is preferably 1 to 60 m ² / g. The shape of the non-magnetic powder may be needle-like, spherical, or dice-like. Specific examples of the non-magnetic powder used in the present invention include AKP-20, AKP-30 and AK manufactured by Sumitomo Chemical Co., Ltd.
P-50, HIT-50, Nippon Kagaku Kogyo Co., Ltd., G5, G
7, S-1, Toda Kogyo Co., Ltd., TF-100, TF-12
0, TF-140, TT055 series made by Ishihara Sangyo Co., Ltd.,
Examples include ET300W and STT30 manufactured by Titanium Industry Co., Ltd.

When the lower magnetic layer is formed under the uppermost magnetic layer of the magnetic recording medium of the present invention by stacking layers, the ferromagnetic powder is iron oxide ferromagnetic powder, cobalt modified iron oxide ferromagnetic powder, or CrO. _2. Various hexagonal ferrites and various metal ferromagnetic powders (the Co—Fe alloy powder used in the present invention may be used) dispersed in resin can be used.

As a coating method for forming two or more coating layers on a non-magnetic support, the above-mentioned wet-on-wet method can be mentioned. As a concrete method, (1) magnetic coating is used. Commonly used gravure coating,
The first layer (lower layer) is first coated by a roll coating, blade coating, or extrusion coating apparatus, and while the layer is still in a wet state, for example, Japanese Patent Publication No. 46186/1989, JP-A-60-238179. Gazette and Japanese Patent Laid-Open No. 2-
No. 265672, a method of coating an upper layer by a non-magnetic support pressure type extrusion coating apparatus, (2) JP-A-63-88080, JP-A 2-1
A method of coating a lower layer coating liquid and an upper layer coating liquid almost at the same time by a coating head having two slits for passing the coating liquid as disclosed in JP-A-7971 and JP-A-2-265672. ) A method of coating the upper layer and the lower layer almost at the same time by using the extrusion coating apparatus with a backup roll disclosed in JP-A-2-174965.

In particular, when a magnetic recording medium having a structure in which the lower layer is a non-magnetic layer and the upper layer is a magnetic layer is coated on a non-magnetic support by a wet-on-wet method, a magnetic layer coating solution and a non-magnetic layer coating solution. When the fluidity characteristics of the above are as close as possible, it is possible to obtain a magnetic layer having a uniform thickness and a small thickness variation without disturbance of the interface between the coated magnetic layer and the non-magnetic layer. Since the flow characteristics of the coating liquid strongly depend on the combination of the powder particles and the binder resin in the coating liquid, it is necessary to pay particular attention to the selection of the nonmagnetic powder used in the nonmagnetic layer.

The non-magnetic support of the present magnetic recording medium is usually
The thickness is 1 to 100 μm, preferably 3 to 20 μm, and the nonmagnetic layer is 0.5 to 10 μm. Further, layers other than the magnetic layer and the non-magnetic layer can be formed according to the purpose. For example, an undercoat layer for improving adhesion may be provided between the non-magnetic support and the lower layer. This thickness is 0.01 to 2 μm, preferably 0.05 to 0.5 μm. Further, a back coat layer may be provided on the side of the non-magnetic support opposite to the side of the magnetic layer. This thickness is 0.1 to 2 μm, preferably 0.3 to 1.0 μm
Is. Known intermediate layers and back coat layers can be used. In the case of a disk-shaped magnetic recording medium, the above layer constitution can be provided on one side or both sides.

The non-magnetic support used in the present invention is not particularly limited, and those commonly used can be used. Examples of materials forming the non-magnetic support include polyethylene terephthalate, polyethylene, polypropylene, polycarbonate, polyethylene naphthalate, polyamide, polyamide imide, polyimide, polysulfone, films of various synthetic resins such as polyethersulfone, and aluminum foil. , And metal foils such as stainless steel foil.

In order to effectively achieve the object of the present invention, the surface roughness of the non-magnetic support is 0.03 μm or less in the center line average surface roughness Ra (cutoff value 0.25 mm), preferably 0. It is not more than 02 μm, more preferably not more than 0.01 μm. Further, it is preferable that these non-magnetic supports not only have a small center line average surface roughness but also that they have no coarse protrusions of 1 μm or more. Further, the surface roughness shape can be freely controlled depending on the size and amount of the filler added to the non-magnetic support, if necessary. Examples of these fillers include oxides and carbonates of Ca, Si, Ti and the like, and fine organic resin powders of acrylic and the like.

The F-5 value of the non-magnetic support used in the present invention in the web running direction is preferably 5 to 50 kg / mm.
² , F-5 value in the web width direction is preferably 3 to 30k
It is g / mm ² , and the F-5 value in the long direction of the web is generally higher than the F-5 value in the width direction of the web, but as long as it is necessary to increase the strength in the width direction, that is the case. Not.

The heat shrinkage ratio of the support in the web running direction and width direction at 100 ° C. for 30 minutes is preferably 3% or less, more preferably 1.5% or less, and the heat shrinkage ratio at 80 ° C. for 30 minutes is It is preferably 1% or less, more preferably 0.
It is 5% or less. Breaking strength is 5 to 100k in both directions
The g / mm ² and the elastic modulus are preferably 100 to 2000 kg / mm ² .

The organic solvent used in the present invention may be any ratio of ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone and tetrahydrofuran, and methanol.
Alcohols such as alcohol, ethanol, propanol, butanol, isobutyl alcohol, isopropyl alcohol, methylcyclohexanol, etc., methyl acetate, butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate, glycoacetate -Esters such as glycol, dimethyl ether, glycol monoethyl ether, glycol ethers such as dioxane, aromatic hydrocarbons such as benzene, toluene, xylene, cresol, chlorobenzene, methylene chloride, ethylene chloride, Chlorinated hydrocarbons such as carbon tetrachloride, chloroform, ethylene chlorohydrin, dichlorobenzene, N,
Those such as N-dimethylformamide and hexane can be used. These organic solvents are not necessarily 100% pure, and may contain impurities such as isomers, unreacted substances, by-products, decomposition products, oxides, and water in addition to the main components. The content of these impurities is preferably 30% or less, more preferably 10% or less. If necessary, the type and amount of the organic solvent used in the present invention may be changed between the magnetic layer and the lower layer. Use a highly volatile solvent for the magnetic layer to improve surface properties, use a solvent with high surface tension (cyclohexanone, dioxane, etc.) for the lower layer to increase coating stability, use a solvent with high solubility parameters for the lower layer Examples of such methods include increasing the filling degree, but it goes without saying that they are not limited to these examples.

The magnetic recording medium of the present invention is kneaded and dispersed with an organic solvent together with the above-mentioned ferromagnetic powder, binder resin and, if necessary, other additives, and a magnetic coating material is applied onto a non-magnetic support, It is obtained by orienting and drying if necessary. The process for producing the magnetic coating material or the non-magnetic coating material for the magnetic recording medium of the present invention comprises at least a kneading step, a dispersing step, and a mixing step provided before and after these steps, if necessary. Each process may be divided into two or more stages. Ferromagnetic powder, non-magnetic powder, binder, carbon black, abrasive, antistatic agent used in the present invention,
All raw materials such as lubricants and solvents may be added at the beginning or in the middle of any step. In addition, individual raw materials may be divided and added in two or more steps. For example,
Polyurethane may be added separately in the kneading step, the dispersing step, and the mixing step for adjusting the viscosity after dispersion.

Various kneading machines are used for kneading and dispersing the magnetic paint and the non-magnetic paint. For example, two roll mill, three roll mill, ball mill, pebble mill, tron mill, sand grinder, Segvar (Szegvar)
i), an attritor, a high speed impeller disperser, a high speed stone mill, a high speed impact mill, a disper, a kneader, a high speed mixer, a homogenizer, and an ultrasonic disperser can be used.

In order to achieve the object of the present invention, it goes without saying that the conventionally known manufacturing techniques can be used as a part of the steps, but in the kneading step, a continuous kneader or a pressure kneader is used. It is possible to obtain high Br of the magnetic recording medium of the present invention by using a material having a strong kneading force. When a continuous kneader or a pressure kneader is used, all or part of the magnetic substance and binder (however, 3 of all binders are used)
0% or more) and 15 to 500 parts by weight with respect to 100 parts by weight of the ferromagnetic powder. For details of these kneading treatments, refer to JP-A-1-10633.
No. 8 and Japanese Patent Laid-Open No. 64-79274. In the present invention, the simultaneous multi-layer coating method as disclosed in JP-A-62-212933 can be used for more efficient production.

The residual solvent contained in the magnetic layer of the magnetic recording medium of the present invention is preferably 100 mg / m ² or less, more preferably 10 mg / m ² or less, and the residual solvent contained in the magnetic layer is contained in the non-magnetic layer. It is preferably less than the residual solvent contained.

The porosity of the magnetic layer is preferably 30% by volume or less, more preferably 10% by volume in both the non-magnetic layer and the magnetic layer.
Volume% or less. The porosity of the nonmagnetic layer is preferably larger than that of the magnetic layer, but may be small as long as the porosity of the nonmagnetic layer is 5% by volume or more. The magnetic recording medium of the present invention preferably has a non-magnetic layer and a magnetic layer,
It is easily presumed that these physical properties can be changed between the non-magnetic layer and the magnetic layer depending on the purpose. For example, the elastic modulus of the magnetic layer is increased to improve running durability, and at the same time, the elastic modulus of the nonmagnetic layer is made lower than that of the magnetic layer to improve the contact of the magnetic recording medium with the head.

By such a method, the magnetic layer coated on the support is optionally subjected to a treatment for orienting the ferromagnetic powder in the layer, and then the formed magnetic layer is dried. If necessary, the surface is smoothed or cut into a desired shape to manufacture the magnetic recording medium of the present invention. The elastic modulus at 0.5% elongation of the magnetic layer is preferably 100 to 2000 kg / mm ^{2 in} the web application direction and the width direction, the breaking strength is preferably 1 to 30 kg / cm ² , and the elastic modulus of the magnetic recording medium is Desirably 1 in the web application direction and the width direction
00 to 1500 kg / mm ² , the residual spread is preferably 0.5% or less, and the heat shrinkage at any temperature of 100 ° C. or less is preferably 1% or less, more preferably 0.5%.
Hereafter, it is most preferably 0.1% or less.

The magnetic recording medium of the present invention is preferably a tape for video use, audio use, etc., but may be a floppy disk or a magnetic disk for data recording, and a signal loss due to drop out occurs. It is also effective for a disk-shaped medium for fatal data recording. Further, the lower layer is a non-magnetic layer and the thickness of the magnetic layer is 1 μm or less, so that the electromagnetic conversion characteristics are high.
It is possible to obtain a high-density and high-capacity magnetic recording medium having excellent overwrite characteristics.

The novel features of the present invention will be specifically described in the following examples.

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EXAMPLES The present invention will be specifically described according to examples.
In addition, "part" in an Example shows a "weight part." (Production of ferromagnetic alloy powder) Ferromagnetic powder A 8.4 kg of cobalt sulfate heptahydrate and ferric sulfate heptahydrate 1
Dissolve 5.7 kg in 100 liters of water and add 500
The temperature was raised to 75 ° C. in a liter reactor, and a 75 ° C. aqueous solution prepared by dissolving 7.2 kg of caustic soda in 200 liters of water was added while stirring at 300 rpm. Then, a solution prepared by dissolving 170 g of citric acid in 20 liters of water was added. The reaction solution was brought to 80 ° C. and reacted for 10 hours to obtain cobalt ferrite. After returning to room temperature, solid-liquid separation and washing with water, 200 liters of water is added again to make a dispersion slurry, and while stirring, water glass is Si / (Fe + Co) with an atomic ratio of 0.05.
And treated for 2 hours. Further, after solid-liquid separation and washing with water, it was dried at 100 ° C.

The above powder was placed in a rotary electric furnace having a volume of 50 liters, and nitrogen gas was passed through the rotary electric furnace at 100 liters / min.
Heat treatment was performed at 500 ° C. for 3 hours. Then the temperature is 400 ℃
And hydrogen gas was aerated at 100 liter / min for reduction for 6 hours. After returning to room temperature, a mixed gas of nitrogen and air with an oxygen concentration of 0.1% was aerated for 5 hours, after which the oxygen concentration was gradually increased and finally air was aerated, and then taken out into the atmosphere, and ferromagnetic Powder A was obtained.

Ferromagnetic powder B, C Stirring speed during reaction between cobalt sulfate heptahydrate and ferric sulfate heptahydrate was 500 rpm (B) or 700 rpm (C)
Other than the above, the ferromagnetic powder A or C was obtained under the same conditions as the above-mentioned ferromagnetic powder A. Ferromagnetic powders D and E Ferromagnetic powders D and E were obtained under the same conditions as those for the ferromagnetic powder A except that no citric acid was added during the formation of cobalt ferrite (D) or the amount of citric acid was changed to 850 g (E). .

Ferromagnetic powders F and G Ferromagnetic powders F and G were prepared under the same conditions as ferromagnetic powder A except that methylenephosphonic acid was changed to 400 g (F) or 800 g (G) instead of citric acid when cobalt ferrite was produced.
Got Ferromagnetic powder H. Average particle size (major axis length) in place of cobalt ferrite.
Ferromagnetic powder H was obtained by reduction under the same conditions as ferromagnetic powder A except that goethite having a needle-like ratio of 15 μm was used.

Table 1 shows the characteristics of the ferromagnetic powders A to H.

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[Table 1]

Incidentally, the specific surface area is measured by a cantersorb (US,
It was degassed in a nitrogen atmosphere at 250 ° C. for 30 minutes and then measured by the BET one-point method (partial pressure 0.30). The average particle diameter and the average minor axis length are measured by a transmission electron microscope (T
EM). The magnetic characteristics (Hc, σ _S ) were measured with a vibrating sample magnetometer (VSM) with an applied magnetic field of 10 kOe (kilo-Oersted).

A magnetic paint was prepared using the above ferromagnetic powder under the following conditions. (Magnetic layer composition) ferromagnetic powder (Table 2 described) 100 parts binder resin of vinyl chloride copolymer 12 parts (-SO ₃ Na groups and containing 1 × 10 ^-4 eq / g polymerization degree: 300) Polyester polyurethane resin 3 parts (neopentyl glycol / caprolactone polyol / MD I = 0.9 / 2.6 / 1 -SO 3 Na group containing 1 × 10 ^-4 eq / g) alpha-alumina (average particle size 0.2 [mu] m) 1. 5 parts Carbon black (average particle size 100 nm) 0.5 part Butyl stearate 1 part Stearic acid 2 parts Methyl ethyl ketone and cyclohexanone 1: 1 mixed solvent 200 parts (composition for non-magnetic layer) non-magnetic powder acicular α-Fe ₂ O ₃ 80 parts (average particle length 0.25 [mu] m, the surface area by an average acicular ratio 12 BET method 35m ² / g pH4.0) carbon black 20 parts (Rights The primary particle diameter of 16 nm, DBP surface area by及油amount 80 ml / 100 g BET method 235m ² / g pH8.0) binder resin a vinyl chloride - vinyl acetate - vinyl alcohol copolymer 10 parts (-N (CH ₃₎ ³⁺ Cl ^- polar groups 5 × 10 ^-6 eq / g containing monomer composition ratio 86: 13: 1 degree of polymerization of 400) 8 parts polyester polyurethane resin (basic skeleton: l, 4-BD / phthalate / HMDI molecular weight: 10200 hydroxyl: 0.23 × 10 ^-3 eq / g content-SO ₃ Na group: 1 × 10 ^-4 eq / g content butyl stearate 1 part Stearic acid 2.5 parts Methyl ethyl ketone and cyclohexanone 1: 1 mixed solvent 200 parts (non-magnetic Layer composition) Needle-like α-of non-magnetic layer composition
Acicular Co-modified iron oxide instead of Fe ₂ O ₃ (specific surface area 5
It was prepared under the same conditions except that 0 m ² / g, Hc850, axial length 0.15, and axial ratio 8) were used.

The above-mentioned non-magnetic layer composition and magnetic layer composition were each kneaded with a kneader and then dispersed using a sand grinder. Polyisocyanate was added to the resulting dispersion in an amount of 5 parts for the coating liquid for the non-magnetic layer, 6 parts for the coating liquid for the magnetic layer, and further 20 parts of the mixed solvent of methyl ethyl ketone and cyclohexanone 1: 1 to give an average pore diameter of 1 μm. It filtered using the filter which has and prepared the coating liquid for nonmagnetic layer formation and magnetic layer formation.

The obtained coating liquid for the non-magnetic layer was coated so that the thickness after drying was 2 μm, and immediately thereafter, while the coating layer for the non-magnetic layer was still in a wet state, the magnetic layer was applied onto it. Wet simultaneous multilayer coating was performed on a polyethylene terephthalate support having a thickness of 7 μm so that the layers had a predetermined thickness, and while both layers were still in a wet state, an Sm-Co magnet (surface magnetic flux 3000 gauss) and a solenoid were applied. The magnetic field of the ferromagnetic powder was oriented by an electromagnet (surface magnetic flux: 1,500 gauss), and then the magnetic layer was dried, and then calendered with a seven-stage calender consisting of metal rolls at a roll temperature of 90 ° C. to form a web. The magnetic recording medium of No. 1 was obtained, and it was slit into a width of 8 mm to prepare 8 mm video tape samples (Tape Nos. 1 to 19 and Tape No. 4 were not subjected to orientation treatment, Tape N was used). o16 to 19 are single layers without a lower layer).

The performance of the obtained tape was evaluated as follows, and the results are shown in Table 2. Demagnetization is the rate of change in Bm before and after storage at 60 ° C. and 90% for 10 days, and the magnetic characteristics were measured using a vibrating sample magnetometer VSM-5 manufactured by Toei Industry Co., Ltd. with an applied magnetic field of 5 kOe. The reproduction output was measured by recording signals of 1 MHz and 7 MHz by using FUJIX8, an 8 mm video deck manufactured by Fuji Photo Film Co., Ltd., and reading the reproduction outputs when reproducing these signals from an oscilloscope.

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[Table 2]

From Table 2, the magnetic recording tapes (Tapes Nos. 11, 15, 19) and the Co--Fe alloy powders using the conventionally used metal ferromagnetic powder mainly composed of .alpha.Fe, which are out of the scope of the present invention. The output of the magnetic recording medium of the present invention is 1M as compared with the magnetic recording mediums of Tapes Nos. 7, 13, and 18 using a ferromagnetic powder having a small acicular ratio of 1.0.
It can be seen that both Hz and 7 MHz are excellent. The reason for this is considered to be the high saturation magnetization and low acicular ratio of the Co—Fe alloy powder, which are the features of the present invention. The magnetic recording tape of the present invention is a magnetic recording tape (Tape No. 11,
It has better weather resistance than 15 and 19).

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INDUSTRIAL APPLICABILITY The present invention is obtained by reducing cobalt ferrite in a magnetic recording medium having a magnetic layer mainly composed of a ferromagnetic powder and a binder resin on a non-magnetic support. Co with 20 to 35 atomic% and an average particle size of 100 to 500Å and a needle ratio of 1.2 or more
By using the —Fe alloy powder, it is possible to provide a magnetic recording medium having excellent electromagnetic conversion characteristics and excellent weather resistance.

Claims (3)

[Claims] 1. A magnetic recording medium having a magnetic layer mainly composed of a ferromagnetic powder and a binder resin on a non-magnetic support, wherein the ferromagnetic powder is a ferromagnetic metal powder formed by reducing cobalt ferrite. The magnetic recording medium is characterized in that the ferromagnetic metal powder has a Co content of 20 to 35 atomic%, an average particle size of 100 to 500Å, and an acicular ratio of 1.2 or more. 2. The magnetic layer has a thickness of 1.0 μm or less, and has a nonmagnetic layer mainly composed of nonmagnetic powder and a binder resin between the magnetic layer and the nonmagnetic support. The magnetic recording medium according to claim 1. 3. The ferromagnetic metal powder has an acicular ratio of 1.2 to
10. The method according to claim 1 or 2, characterized in that
The magnetic recording medium according to 1.