JPH10124848A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic recording medium which is excellent in traveling durability without the increase of the coefft. of friction and may be improved in electromagnetic conversion characteristics by improving the dispensability of particulate magnetic materials and the dispensability of particulate inorg. powder and decreasing low projections without changing the number of the relatively high projections of a magnetic layer surface. SOLUTION: This magnetic recording medium is formed by providing the surface of a nonmagnetic base with the magnetic layer formed by dispersing ferromagnetic powder into a binder or by further providing a lower layers coating layer contg. inorg. powder and the binder between the magnetic layer and the nonmagnetic base. In such a case, at least one binder of the binder in the magnetic layer and the binder in a lower layer coating layer are polyurethane reins including a cyclic structure and ether groups. When the projection height of every 10nm on the surface of the magnetic layer is measured by an atomic force microscope (AFM), the number M10 of the projections of the height of >=10 to <20nm in a square sized 30×30μm is <=4000 pieces and the number M20 of the projections of the height of >=20 to <40nm is >=20 pieces and the ratio of M10 /M20 is 10 to 35.

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, in particular, a recording medium having a minimum recording wavelength of 0.7 .mu.m or less and a track pitch of 25 .mu.m.
The present invention relates to a magnetic recording medium for recording and reproducing at a high density as described below, and more specifically to a coating type magnetic recording medium which is excellent in high-frequency output, excellent in CNR, has a low coefficient of friction, and is excellent in running properties.

[0002]

2. Description of the Related Art Magnetic recording media are widely used as recording tapes, video tapes, computer tapes, disks, and the like. The recording density of magnetic recording media has been increasing year by year, and the recording wavelength has been shortened, and recording methods from analog to digital are also being studied. In response to this demand for higher density, metal thin film media using a metal thin film formed under vacuum as a magnetic layer is being studied, but it is strongly considered in terms of practical reliability such as productivity and corrosivity. A so-called coating type magnetic recording medium in which magnetic powder is dispersed in a binder and coated on a support is excellent. However, since the coating type medium has a lower magnetic substance filling degree than the metal thin film type medium, the electromagnetic conversion characteristics are inferior. Coated magnetic recording media include ferromagnetic iron oxide, Co
A magnetic layer in which a modified ferromagnetic iron oxide, CrO ₂ , ferromagnetic metal powder or the like is dispersed in a binder is applied to a nonmagnetic support, and is widely used.

The improvement of the electromagnetic conversion characteristics of the coating type magnetic recording medium includes improvement of the magnetic characteristics of the ferromagnetic powder, smoothing of the surface of the magnetic layer, thinning of the magnetic layer, and improvement of the filling degree. A method has been proposed. In recent years, recording wavelengths have tended to be shorter with higher densities, and the contribution to output due to space loss has increased more than ever before. Therefore, a smoother magnetic layer surface is required. On the other hand, as the density of recording / reproducing increases, the use of magnetic particles and fine particles having a major axis length of 0.1 μm or less has recently been proposed. Specifically, Japanese Patent Application Laid-Open No. 7-22224 discloses that the major axis length is 0.08.
A μm magnetic material has been disclosed and is now readily available. In addition, it has been proposed to make abrasive particles finer.

These directions are very difficult to prepare a magnetic coating liquid dispersion for high-density recording magnetic media, and are disclosed in Japanese Patent Application Laid-Open Nos. 62-202321 and
With the conventional binders described in JP-A-195718 and JP-A-4-74312, dispersion for achieving a sufficient recording density is becoming difficult. In order to improve the dispersion of the magnetic substance, the abrasive, and the carbon, a vinyl chloride binder or a polyurethane binder containing a sulfonic acid group has been proposed as a high dispersion binder resin. The above contents are described in JP-A-62-20232.
No. 1, JP-A-4-195718, and JP-A-4-195718.
In U.S. Pat. No. 7,432,743, an invention in which a binder having a polar group is combined with carbon and an abrasive has been filed and published. However, the smoothing of the magnetic layer increases the coefficient of friction and causes problems in running durability and stability, as in the past, and various methods have been proposed. One is to propose a surface roughness that reduces the friction coefficient while reducing the spacing loss.

For example, Japanese Unexamined Patent Publication (Kokai) No. 5-135352 discloses a technique for defining the undulation and surface roughness of the surface of a non-magnetic support, and providing undulation on the surface of the magnetic layer to reduce space loss and reduce the friction coefficient. However, in a high-density system for performing short-wavelength recording, sufficient electromagnetic conversion characteristics and running properties cannot be obtained. It has been proposed to use various solid lubricants to reduce the coefficient of friction. For example, Japanese Patent Publication No. 4-70688 discloses a magnetic recording medium in which a magnetic layer contains spherical fine particles composed of a benzguanamine-formaldehyde resin or a benzoguanamine-melamine-formaldehyde resin.

Japanese Patent Publication No. 5-76699 discloses that the magnetic layer has a Mohs hardness of 5 or more and a benzoguanamine resin powder having a true specific gravity of 0.8 to 2.5 and a phthalocyanine pigment. There is disclosed a magnetic recording medium containing one kind of organic powder. JP-A-62-185235 discloses a magnetic recording medium characterized in that benzoguanamine particles are contained in alumina particles.

Japanese Patent Application Laid-Open No. 2-137119 discloses a magnetic recording medium containing particles having a Mohs' hardness of 5 or more and a powder of a benzoguanamine resin having an average particle diameter larger than that. However, in any case, in a high-density system having a track pitch of 25 μm or less, sufficient electromagnetic conversion characteristics could not be obtained. Also,
Because the coefficient of friction is not low enough, especially in systems with strict edge regulation recently, D.I. O. A phenomenon in which the increase was increased was observed, and running durability was not obtained.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a magnetic recording medium having good electromagnetic conversion characteristics and durability. An object of the present invention is to provide a magnetic recording medium capable of reducing low projections without changing the number of relatively high projections, improving running durability without increasing the friction coefficient, and improving electromagnetic conversion characteristics.

[0009]

Means for Solving the Problems The inventors of the present invention paid attention to urethane in a binder resin and examined the dispersibility of fine particle powders including a magnetic substance. It was found that when a polyurethane resin containing a base was used, the dispersibility of the magnetic material in the upper coating layer and the powder suitable for fine particles in the lower coating layer was improved, resulting in improved surface properties. Invented the invention. That is, the present invention is the following magnetic recording medium.

(1) A magnetic recording medium having a magnetic layer formed by dispersing a ferromagnetic powder in a binder on a nonmagnetic support or an inorganic powder between the magnetic layer and the nonmagnetic support. In a magnetic recording medium provided with a lower coating layer containing a binder, at least one binder of the binder in the magnetic layer or the binder in the lower coating layer is a polyurethane resin containing a cyclic structure and an ether group. The surface of the magnetic layer has a protrusion height of 10 nm or more in a 30 μm × 30 μm square, as measured by an atomic force microscope (AFM).
The number of the height of the projections of less than 0 nm: in M ₁₀ 4000 U or less,
The number of the height of the projections of less than 40nm over 20 nm: M ₂₀ is located more than 20 co, and the magnetic recording medium, wherein the ratio of M ₁₀ / M ₂₀ is 10 to 35. (2) The polyurethane resin comprises a polyurethane resin which is a reaction product mainly composed of a diol and an organic diisocyanate. The diol comprises a short-chain diol having a cyclic structure and a long-chain diol containing an ether group. The short-chain diol is added to the resin in an amount of 17 to 40.
(1) The magnetic recording medium according to the above (1), comprising 10 to 50% by weight of the long-chain diol and 1.0 to 5.0 mmol / g of the ether group.

(3) The ferromagnetic powder of the magnetic layer contains 10 to 40 atomic% of Co with respect to Fe, and has an average major axis length of 0.1%.
The magnetic recording medium according to (1) or (2), wherein the magnetic recording medium has an axis ratio of 2 to 8 and a crystallite size of 120 to 200 °. (4) The magnetic recording according to any one of (1) to (3), wherein the inorganic powder contained in the lower coating layer is in the form of particles or needles having an average particle diameter of 0.3 μm or less. Medium. (5) The magnetic recording medium according to any one of (1) to (4), wherein the lower coating layer and at least a magnetic layer in contact with the lower coating layer are formed by a wet-on-wet coating method.

The use of the polyurethane resin specified in the present invention as a binder for a magnetic layer or a non-magnetic layer improves the dispersibility of the magnetic material in the upper coating layer and the fine inorganic powder in the lower coating layer. surface properties, in particular the height of the projection density of less than 20nm over 10nm by AFM measurements: M ₁₀ greatly it is possible to reduce, and improved result electromagnetic conversion characteristics. Normally, when the surface property becomes smooth, the friction coefficient increases. However, even when the above polyurethane resin is used, a solid lubricant (filler) is used.
Since the number of protrusions of 20 nm or more formed by the above hardly changes, no increase in the friction coefficient is observed.

[0013] That is, conventionally, as a method for smoothing the surface properties, the dispersion is strengthened, the calendar is strengthened,
The amount of solid lubricant has been reduced. However, in this method, both the high projections and the low projections only increase or decrease at the same rate, and the only way to find the most balanced point among the phenomena where the friction coefficient increases as the electromagnetic conversion characteristics increase. By using the above polyurethane resin this time, the dispersion of fine particles of magnetic material and fine particles, which had been insufficient until now, is improved, so that only low projections close to the background can be greatly reduced, and electromagnetic conversion characteristics can be improved. It can be improved. When the polyurethane resin is a short-chain diol having a cyclic structure, the glass transition point can be increased, and the reliability can be improved with respect to durability.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The binder used for the magnetic layer or the non-magnetic layer of the magnetic recording medium of the present invention may be any one containing at least a polyurethane resin having a cyclic structure and an ether group. The polyurethane resin containing a cyclic structure and an ether group is not particularly limited, but specifically,
A polyurethane resin containing a short-chain diol having a cyclic structure and a long-chain diol containing an ether group. More specifically, a polyurethane resin which is a reaction product of a diol and an organic diisocyanate as main raw materials, and obtained by combining a short-chain diol having a cyclic structure with a long-chain diol containing an ether group as the diol. Resin. More specifically, a long-chain diol comprising a polyurethane resin, which is a reaction product containing a diol and an organic diisocyanate as main raw materials, containing a short-chain diol having a cyclic structure in an amount of 17 to 40% by weight in the polyurethane resin and containing an ether group 10 to 50 in polyurethane resin
%, And contains 1.0 to 5.0 mmol / g of ether groups based on the entire polyurethane resin.

The short-chain diol having a cyclic structure, which is a raw material of the polyurethane resin, contained in the binder used in the magnetic recording medium of the present invention is a saturated or unsaturated cyclic structure having a molecular weight of Mean less than 500 diols. For example, bisphenol A, hydrogenated bisphenol A represented by the following formula 1, bisphenol S, bisphenol P, and diols having an aromatic or alicyclic group such as ethylene oxide, propylene oxide adduct thereof, cyclohexanedimethanol, cyclohexanediol, etc. preferable.

[0016]

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More preferably, hydrogenated bisphenol A represented by the formula 1 and its ethylene oxide and / or propylene oxide adducts can be mentioned. Further, the short-chain diol having a cyclic structure has a molecular weight of 50 to 500.
Less than 100, more preferably 100 to 40
0, most preferably 100-300. If it is less than 50, the magnetic layer becomes brittle and the durability decreases. Also 500
In the case described above (that is, when the short-chain diol referred to in the present invention is not used), the glass transition temperature Tg of the magnetic layer decreases, and the magnetic layer becomes soft and the durability decreases.

The content of the short-chain diol in the polyurethane resin is preferably from 17 to 40% by weight, and more preferably from 20 to 30% by weight. If it is less than 17% by weight, the resulting coating film will be too soft, failing to obtain sufficient strength and deteriorating still durability. On the other hand, if it is more than 40% by weight, the solubility in a solvent is reduced and the dispersibility of the ferromagnetic powder is easily reduced, so that the electromagnetic conversion characteristics are easily reduced and the strength of the magnetic layer is reduced.

On the other hand, a long-chain diol containing an ether group is a diol having a molecular weight of 500 or more. Specifically, bisphenol A, hydrogenated bisphenol A, bisphenol S or bisphenol P is added to polyethylene oxide, propylene oxide or Those obtained by adding both of them, polypropylene glycol, polyethylene glycol, and polytetramethylene glycol are preferable, and a compound represented by the following formula 2 is particularly preferable.

[0020]

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The values of n and m are 3 to 24, preferably 3 to 20, and more preferably 4 to 1
5 If n and m are smaller than 3, the urethane bond concentration increases, the solubility in a solvent is reduced, the coating film is apt to be brittle, and the dispersibility and durability are further reduced. When it is larger than 24, the coating film becomes soft, and the still durability decreases. In the long-chain diol, R is preferably

[0022]

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Are more preferred. In the long-chain diol of the formula 2, X is preferably a hydrogen atom or a methyl group, more preferably a methyl group. Note that n or m
All the Xs in the parentheses need not be the same, and a hydrogen atom and a methyl group may be mixed. Since the polyurethane resin used in a particularly preferred embodiment of the present invention has a cyclic structure, the coating film strength is high, the durability is excellent,
Since it has branched CH ₃ of propylene, it has high solubility in solvents and excellent dispersibility. The weight average molecular weight (Mw) of the long-chain diol is from 500 to 5,000, and if it is 5,000 or more, the coating film strength is reduced and the coating becomes soft, so that the durability is reduced. Therefore, a more preferable Mw is selected from the range of 700 to 3000.

The content of the long-chain diol containing an ether group is preferably 10 to 50% by weight, more preferably 30 to 40% by weight in the polyurethane resin. When the content is less than 10% by weight, the solubility in a solvent is reduced, so that the dispersibility is reduced. On the other hand, if it is more than 50% by weight, the strength of the coating film is reduced, so that the durability is reduced. The content of the ether group in the long-chain diol is preferably 1.0 to 5.0 mmol / g, more preferably 2.0 to 4.0 mmol / g in the polyurethane resin. If it is less than 1 mmol / g, the adsorptivity to the magnetic substance is reduced, and the dispersibility is reduced. On the other hand, when it is 5.0 mmol / g or more, the solubility in a solvent is reduced, and the dispersibility is reduced.

The number average molecular weight (Mn) of the polyurethane resin of the present invention is preferably 5,000 to 100,000, more preferably 10,000 to 50,000, and particularly preferably 20,000 to 40,000. If it is less than 5,000, the strength of the magnetic layer decreases, and the durability decreases. On the other hand, if it is larger than 100,000, the solubility in a solvent is reduced and the dispersibility is reduced. The glass transition temperature Tg of the polyurethane resin of the present invention is from 50 to 200 ° C, preferably from 80 to 150 ° C, and more preferably from 100 to 130 ° C. When the temperature is lower than 50 ° C., the strength of the magnetic layer at a high temperature is reduced, so that the durability and the storability are reduced. If the temperature is higher than 200 ° C., calender moldability decreases, and electromagnetic conversion characteristics decrease. The content of OH groups in the polyurethane resin is preferably from 3 to 20 per molecule, more preferably from 4 to 5 per molecule. If the number is less than 3 per molecule, the reactivity with the isocyanate curing agent is reduced, so that the strength of the coating film is reduced and the durability tends to be reduced. On the other hand, when the number is more than 20, the solubility in a solvent is reduced, and the dispersibility tends to be reduced.

As the compound used for adjusting the content of the OH group in the polyurethane resin, a compound having three or more OH groups can be used. Specific examples include trimethylolethane, trimethylolpropane, trimellitic anhydride, glycerin, pentaerythritol, hexanetriol, and the like, which are used as polyester polyol raw materials described in Japanese Patent Publication No. 6-64726 described as a conventional technique. Examples include branched polyesters and polyetheresters having a tribasic or higher OH group obtained by using a dibasic acid and the compound as a glycol component. Preferably, it is a trifunctional one, and if it has four or more functionalities, it tends to gel in the reaction process.

In the binder comprising the polyurethane resin of the present invention, -SO ₃ M, -OSO ₃ M, -COO
_{M, -PO 3 M 2, -OPO} 3 M 2, -NR 2, -N R
₂ R'COO- (where M is a hydrogen atom, an alkali metal ion, an alkaline earth metal ion or an ammonium ion, and R and R 'independently represent an alkyl group) preferably containing a polar group, particularly _preferably, -SO 3 M, an -OSO ₃ M. The amount of these polar groups is preferably 1 × 10 ⁻⁵ to 2
× 10 ^-4 / g, particularly preferably 5 × 10 ^-5 to 1 ×
10 ⁻⁴ / g. If the amount is less than 1 × 10 ⁻⁵ / g, the adsorption to the ferromagnetic powder becomes insufficient, so that the dispersibility decreases, and
If the amount is more than × 10 ⁻⁴ / g, the solubility in a solvent is reduced, so that the dispersibility is reduced.

In the polyurethane resin of the present invention, other diols can be used together with the short-chain diol having a cyclic structure. Specifically, ethylene glycol, 1,3-propylene diol, 1,2-propylene glycol, 1,4-butane diol, 1,5-pentane diol, 1,6-hexane diol, 2,2-
Dimethylpropanediol, 1,8-octanediol, 1,9-nonanediol, diethylene glycol,
Cyclohexane-1,4-diol, cyclohexane-
Examples thereof include aliphatic diols such as 1,4-dimethanol.

Examples of the organic diisocyanate compound which is another component of the starting material for synthesizing the polyurethane resin of the present invention include 2,4-tolylene diisocyanate, 2,6
-Tolylene diisocyanate, xylene-1,4-diisocyanate, xylene-1,3-diisocyanate,
4,4'-diphenylmethane diisocyanate, 4,4
-Diphenyl ether diisocyanate, 2-nitrodiphenyl-4,4'-diisocyanate, 2,2'-diphenylpropane-4,4'-diisocyanate, 4,
Aromas such as 4'-diphenylpropane diisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate, naphthylene-1,4-diisocyanate, naphthylene-1,5-diisocyanate, 3,3'-dimethoxydiphenyl-4,4'-diisocyanate Aliphatic diisocyanates such as aliphatic diisocyanate and lysine diisocyanate, alicyclic diisocyanates such as isophorone diisocyanate, hydrogenated tolylene diisocyanate and hydrogenated diphenylmethane diisocyanate.

When the binder of the present invention is used for a magnetic layer, a vinyl chloride synthetic resin may be used in combination with the polyurethane resin of the present invention. The polymerization degree of the vinyl chloride resin that can be used in combination is preferably 200 to 600, and 250 to 600.
450 is particularly preferred. The vinyl chloride resin may be a copolymer of a vinyl monomer, for example, vinyl acetate, vinyl alcohol, vinylidene chloride, acrylonitrile and the like. Further, a cellulose derivative such as a nitrocellulose resin, an acrylic resin, a polyvinyl acetal resin, a polyvinyl butyral resin, an epoxy resin, a phenoxy resin, or the like may be used in combination, and these may be used alone or in combination. When another synthetic resin is used in combination, the polyurethane resin contained in the magnetic layer preferably contains 10 to 100% by weight of the binder, more preferably 20 to 100% by weight. .
Particularly preferred is an amount of 50 to 100% by weight. If the content is less than 10% by weight, the solubility in a solvent is reduced, and the dispersibility is reduced.

The vinyl chloride resin contains 10% in the binder.
Preferably, it is contained in an amount of 20 to 70% by weight, more preferably 20 to 70% by weight. Particularly preferred is an amount of 30 to 60% by weight.

The binder of the present invention may contain a polydiisocyanate as a curing agent. The polyisocyanate compound is selected from those usually used as a curing agent component such as a polyurethane resin. Examples of the polyisocyanate compound include 3 mol of tolylene diisocyanate and trimethylolpropane 1
The reaction product with a mole (eg, Desmodur L-75)
(Manufactured by Bayer AG)), a reaction product of 3 mol of a diisocyanate such as xylylene diisocyanate or hexamethylene diisocyanate with 1 mol of trimethylolpropane, a burette addition compound of 3 mol of hexamethylene diisocyanate, and an isocyanurate of 5 mol of tolylene diisocyanate Compounds, isocyanurate adducts of 3 moles of tolylene diisocyanate and 2 moles of hexamethylene diisocyanate, polymers of isophorone diisocyanate and diphenylmethane diisocyanate can be mentioned. The polyisocyanate compound contained in the magnetic layer is preferably contained in the binder in an amount of 10 to 50% by weight, more preferably 20 to 50% by weight.
It is in the range of 40% by weight.

In the case of performing a curing treatment by electron beam irradiation, a compound having a reactive double bond such as urethane acrylate can be used. The total weight of the resin component and the curing agent (that is, the binder) is preferably in the range of usually 15 to 40 parts by weight, more preferably 20 to 30 parts by weight, based on 100 parts by weight of the ferromagnetic powder.
Parts by weight. The ferromagnetic powder used in the magnetic recording medium of the present invention is not particularly limited.
e may contain 10 to 40 atom% of Co, an average major axis length of 0.03 μm or more and 0.1 μm or less, an axial ratio of 2 to 8, and a crystallite size of 120 ° or more and 20 ° or less. preferable.

The ferromagnetic powder has an SBET specific surface area of 35 to
80 m ² / g are preferred, more preferably of 40 to 60 ²
/ G. Fe, Fe-C as ferromagnetic metal powder
o, Fe-Ni, Co-Ni-Fe and the like, and aluminum, silicon, sulfur, scandium, titanium, vanadium, chromium, manganese, copper, zinc, yttrium, in a range of not more than 20% by weight of the metal component. molybdenum,
Rhodium, palladium, gold, tin, antimony, boron,
Barium, tantalum, tungsten, rhenium, silver,
Examples include alloys containing lead, phosphorus, lanthanum, cerium, praseodymium, neodymium, tellurium, and bismuth. Further, the ferromagnetic metal powder may contain a small amount of water, hydroxide or oxide. The method for producing these ferromagnetic powders is already known, and the ferromagnetic powder used in the present invention can also be produced according to a known method. The shape of the ferromagnetic powder is not particularly limited, but usually a needle shape, a granular shape, a dice shape, a rice grain shape, a plate shape or the like is used. In particular, it is preferable to use acicular or spindle-shaped ferromagnetic powder.

The ferromagnetic powder used in a particularly preferred embodiment of the present invention is based on Fe, with 10 to 40 at.% Co, 2 to 20 at.% Al, and Contains 1-15 atomic% Y. And
A more preferred range of the major axis length is 0.05 to 0.15 μm, and a still more preferred range is 0.06 to 0.1 μm. A more preferable range of the crystallite size is 100 to 2
00 °, more preferably 100-170 °. Further, the preferable range of the coercive force is 1800 to 2800.
Oersted, and a more preferred range is 2000-2.
500 Oersted.

The ferromagnetic alloy powder preferably has a spindle shape. The spindle-shaped ferromagnetic alloy powder can be synthesized by reducing spindle-type goethite. More specifically, an aqueous solution of a ferrous salt such as ferrous sulfate and an aqueous solution of an alkali carbonate such as ammonium carbonate are reacted in a pH range of 5 to 8 to obtain a suspension containing an iron-containing precipitate. The suspension is aged in a non-oxidizing atmosphere at a temperature of 40-60 ° C for a period of 2-7 hours. An aqueous solution of a cobalt salt such as cobalt sulfate or cobalt nitrate is added to the suspension immediately before the start of the ripening step or during the ripening step. In addition, a part of the aqueous solution of the cobalt salt may be added to the aqueous solution of the ferrous salt in advance. Thereafter, air is blown into the suspension to cause an oxidation reaction, thereby preparing spindle-shaped goethite particles containing Co. The goethite particles are separated by filtration, washed with water, and filter-pressed, and then suspended in water. If necessary, a cobalt salt such as cobalt acetate is added, and an aluminum salt aqueous solution such as aluminum sulfate and an yttrium salt aqueous solution such as yttrium nitrate are added. And, if necessary, at least one selected from an aqueous solution of sodium silicate such as water glass, an aqueous solution of neodymium salt such as neodymium nitrate, and an aqueous solution of boric acid. In addition, at least a part of these aqueous solutions may be added during the oxidation reaction step in the above goethite preparation process.

Next, if necessary, the pH of the suspension is adjusted.
After adding a known organic polymer flocculant, the mixture is filtered with a filter press and an Oliver filter, and the obtained cake is further granulated and formed, and then dried. Then
This is heat-treated in air at a temperature in the range of, for example, 250 to 500 ° C., to obtain hematite particles. Then, after reducing by heating in a temperature range of 300 to 550 ° C. with hydrogen gas, an oxidation treatment for covering the particle surface with an oxide film is performed. In this oxidation treatment, the above-mentioned heat-reduced metal alloy particles in a molded state are immersed in an organic solvent, and a method of blowing an oxygen-containing gas such as air therein, and
There is a method in which the heat-reduced metal alloy particles in a molded state are exposed and oxidized in an atmosphere of an oxygen gas and a confined gas while adjusting the partial pressure of the oxygen gas. The latter method is preferable.

The above resin component, curing agent and ferromagnetic powder are kneaded and dispersed together with a solvent such as methyl ethyl ketone, dioxane, cyclohexanone and ethyl acetate which are usually used in the preparation of a magnetic coating material to obtain a magnetic coating material. The kneading and dispersion can be performed according to a usual method. In addition, in the magnetic paint, in addition to the above-mentioned components, organic resin particles or carbon black, which are solid lubricants used for controlling minute projections, and abrasives such as α-Al ₂ O ₃ and Cr ₂ O ₃ , It may contain a commonly used additive or filler such as an antistatic agent such as carbon black, a lubricant such as a fatty acid, a fatty acid ester, and silicone oil, and a dispersant.

Next, the lower non-magnetic layer or lower magnetic layer in the case where the present invention has a multilayer structure will be described. The inorganic powder used for the lower layer of the present invention may be either a magnetic powder or a non-magnetic powder. For example, in the case of non-magnetic powder, it can be selected from inorganic compounds such as metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides. Examples of the inorganic compound include α-alumina, β-alumina, γ-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum, silicon nitride, titanium carbide, and titanium oxide having an α conversion of 90 to 100%. , Silicon dioxide,
Tin oxide, magnesium oxide, tungsten oxide, zirconium oxide, boron nitride, zinc oxide, calcium carbonate, calcium sulfate, barium sulfate, molybdenum disulfide and the like are used alone or in combination. Particularly preferred are titanium dioxide, zinc oxide, iron oxide and barium sulfate, and more preferred is titanium dioxide. The average particle size of these non-magnetic powders is preferably 0.005 to 2 μm,
In particular, it is preferably 0.3 μm or less. If necessary, non-magnetic powders having different average particle diameters may be combined, or a single non-magnetic powder may have a similar particle diameter distribution to achieve the same effect. Particularly preferably, the average particle size of the non-magnetic powder is 0.01 μm to 0.2 μm. The pH of the nonmagnetic powder is particularly preferably between 6 and 9. The specific surface area of the nonmagnetic powder is 1 to 100 m ² / g, preferably 5 to 50 m ² / g,
More preferably, it is 7 to 40 m ² / g. The crystallite size of the nonmagnetic powder is preferably 0.01 μm to 2 μm. DB
The oil absorption using P is 5 to 100 ml / 100 g, preferably 10 to 80 ml / 100 g, more preferably 20 to 100 ml / 100 g.
It is 60 ml / 100 g. The specific gravity is 1 to 12, preferably 3 to 6. The shape is granular, needle-like, spherical, polyhedral,
It may be in any form of a plate, but is preferably in the form of granules or needles.

At least a part of the surface of these nonmagnetic powders is coated with Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ , SnO ₂ , Sb ₂ O ₃ , and ZnO to improve dispersibility. Is preferred.
Particularly preferred for dispersibility are Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ ,
However, more preferred are Al ₂ O ₃ , SiO ₂ and ZrO ₂ . These may be used in combination or may be used alone. Although the surface coating layer may be a porous layer depending on the purpose, it is generally preferable that the surface coating layer be homogeneous and dense.

By mixing carbon black in the lower layer, it is possible to lower the surface resistance Rs, which is a known effect, and to obtain a desired micro Vickers hardness. For this purpose, furnace black for rubber, thermal black for rubber, carbon black for color, acetylene black and the like can be used. The specific surface area of carbon black is 100 to 500 m ² / g, preferably 15 to 500 m ² / g.
0-400m ² / g, DBP oil absorption 20-400ml
/ 100 g, preferably 30-200 ml / 100 g. The average particle size of the carbon black is 5 nm to 80 nm, preferably 10 to 50 nm, more preferably 10 to 40 nm. The carbon black has a pH of 2 to 10 and a water content of 0.1.
The tap density is preferably 1 to 10% and the tap density is preferably 0.1 to 1 g / ml. Specific examples of the carbon black used in the present invention include BLACKPEARLS 2 manufactured by Cabot Corporation.
000, 1300, 1000, 900, 800, 88
0,700, VULCAN XC-72, manufactured by Mitsubishi Chemical Corporation, # 3050B, 3150B, 3250B, # 375
0B, # 3950B, # 950, # 650B, # 970
B, # 850B, MA-600, manufactured by Columbia Carbon, CONDUCTEX SC, RAVEN 8800 00
0,7000 750,5 250 500,2100 000,1800 500,1255 25
0 and Ketjen Black EC manufactured by Akzo.

In the lower layer of the present invention, a magnetic powder can also be used. As the magnetic powder, γ-Fe ₂ O ₃ , Co
Modified γ-Fe ₂ O ₃ , alloy mainly composed of α-Fe, Cr
O ₂ or the like is used. Particularly, Co-modified γ-Fe ₂ O ₃ is preferable. The ferromagnetic powder used in the lower layer of the present invention preferably has the same composition and performance as the ferromagnetic powder used in the upper magnetic layer. However, it is known that the performance is changed between the upper and lower layers according to the purpose. For example, in order to improve long-wavelength recording characteristics, it is desirable that Hc of the lower magnetic layer is set lower than that of the upper magnetic layer, and that Br of the lower magnetic layer is higher than that of the upper magnetic layer. It is valid. In addition to the above, an advantage obtained by adopting a known multilayer structure can be provided.

The binder, lubricant, dispersant, additive, solvent, dispersing method and the like of the lower magnetic layer or the lower nonmagnetic layer can be the same as those of the magnetic layer. In particular, with respect to the amount and type of the binder, the amount of the additive and the type of the dispersant, and the type of the magnetic layer, known techniques for the magnetic layer can be applied. A magnetic coating prepared from the above materials is applied on a non-magnetic support to form a magnetic layer. As the non-magnetic support that can be used in the present invention, known materials such as biaxially stretched polyethylene naphthalate, polyethylene terephthalate, polyamide, polyimide, polyamide imide, aromatic polyamide, and polybenzoxdazole can be used. Preferred are polyethylene naphthalate and aromatic polyamide. These non-magnetic supports are pre-corona discharge, plasma treated,
Easy adhesion treatment, heat treatment, or the like may be performed. The nonmagnetic support that can be used in the present invention has a surface having excellent smoothness such that the center line average surface roughness is in the range of 0.1 to 20 nm, preferably 1 to 10 nm at a cutoff value of 0.25 mm. Is preferred. It is preferable that these nonmagnetic supports have not only a small center line average surface roughness but also no coarse protrusions of 1 μ or more.

In the method for producing a magnetic recording medium of the present invention, for example, a coating solution of a magnetic layer is preferably applied to the surface of a running nonmagnetic support, preferably in a thickness of the magnetic layer after drying of 0.05 to 5 μm. , More preferably 0.07-1 μm, further preferably 0.1-0.5 μm, most preferably 0.1-0.
It is applied so that the thickness becomes 3 μm. Here, a plurality of magnetic paints may be sequentially or simultaneously applied in a multilayer manner. Examples of the coating machine for applying the magnetic paint include air doctor coat, blade coat, rod coat, extrusion coat, air knife coat, squeeze coat, impregnation coat, reverse roll coat, transfer roll coat, gravure coat, kiss coat, cast coat, and spray. Coat, spin coat, etc. can be used. For these, for example, "Latest Coating Technology" (May 31, 1983) published by Sogo Gijutsu Center can be referred to.

When the present invention is applied to a magnetic recording medium having two or more layers, the following can be proposed as an example of a coating apparatus and method. (1) Gravure generally applied in the application of magnetic paint,
First, the lower layer is applied by a coating device such as a roll, a blade, an extrusion or the like, and while the lower layer is in an undried state, it is disclosed in Japanese Patent Publication No. 46186/1994, Japanese Patent Application Laid-Open No. 60-238179.
The upper layer is applied by a support pressurization type extrusion coating device as disclosed in Japanese Patent Application Laid-Open No. 2-265672 and JP-A-2-265672. (2) JP-A-63-88080, JP-A-2-179
No. 71 and Japanese Patent Application Laid-Open No. 2-265672 disclose the upper and lower layers almost simultaneously by one coating head having two coating liquid passage slits. (3) The upper and lower layers are coated almost simultaneously by an extrusion coating device with a backup roll as disclosed in JP-A-2-174965.

A back coat layer (backing layer) is formed on the surface of the non-magnetic support used in the present invention on which the magnetic paint is not applied.
May be provided. The back coat layer is provided by applying a back coat layer forming paint in which a particulate component such as an abrasive and an antistatic agent and a binder are dispersed in an organic solvent on a surface of the non-magnetic support on which the magnetic paint is not applied. Layer. Various inorganic pigments and carbon black can be used as the particulate component, and resins such as nitrocellulose, phenoxy resin, vinyl chloride resin, and polyurethane can be used alone or as a mixture of these as a binder. . Note that an adhesive layer may be provided on the surface of the non-magnetic support on which the magnetic paint and the back coat layer forming paint are applied.

The applied layer of the applied magnetic paint is dried after subjecting the ferromagnetic powder contained in the applied layer of the magnetic paint to a magnetic field orientation treatment. After being dried in this manner, the coating layer is subjected to a surface smoothing treatment. For the surface smoothing treatment, for example, a super calender roll or the like is used. By performing the surface smoothing treatment, voids generated by the removal of the solvent during drying disappear, and the filling rate of the ferromagnetic powder in the magnetic layer is improved, so that a magnetic recording medium having high electromagnetic conversion characteristics can be obtained. it can. As the calendering roll, a heat-resistant plastic roll such as epoxy, polyimide, polyamide, or polyamideimide is used. Moreover, it can also process with a metal roll.

The magnetic recording medium of the present invention has a center line average roughness of 0.1 to 0.1 at a cutoff value of 0.25 mm.
It is preferable that the surface has an extremely excellent smoothness of 4 nm, preferably 1 to 3 nm. As a method for this, for example, a magnetic layer formed by selecting a specific ferromagnetic powder and a binder as described above is subjected to the above calendering treatment. As the calendering conditions, the temperature of the calender roll is in the range of 60 to 100 ° C, preferably in the range of 70 to 100 ° C, particularly preferably 80 to 100 ° C.
The pressure is in the range of 100 to 500 kg / cm, preferably in the range of 200 to 450 kg / cm, and particularly preferably in the range of 300 to 400 kg / cm. preferable. The obtained magnetic recording medium can be used after being cut into a desired size using a cutting machine or the like.

[0049]

The present invention will be described below in more detail with reference to Examples of the present invention. In detail, the following magnetic recording media were prepared and their characteristics were tested. In the following description, “parts” represents “parts by weight”. Tables 1 and 2 show the filler for the upper layer and the polyurethane resin as the binder used in this example.

[0050]

[Table 1]

[0051]

[Table 2]

Hereinafter, a magnetic recording medium having a single magnetic layer will be described. Example 1-1 Ferromagnetic metal fine powder 100 parts Composition Fe / Co = 100/30 (atomic ratio) Al (atomic%): 7.1 Si (atomic%): 0.8 Y (atomic%): 6.5 Hc 2250 Oe, specific surface area by BET method 56 m ² / g Crystallite size 165Å Particle size (major axis diameter) 0.065 μm Needlelike ratio 5 σs: 142 emu / g

Vinyl chloride copolymer 11 parts MR-110 polyester polyurethane resin manufactured by Zeon Corporation 5 parts A α-Al ₂ O _{3 shown in} Table 2 (average particle size 0.15 μm) 10 parts Filler A 0 shown in Table 1 .5 parts sec-butyl stearate 1 part Stearic acid 5 parts Methyl ethyl ketone 120 parts Cyclohexanone 120 parts

After the above-mentioned paint was kneaded with an open kneader, it was dispersed using a sand mill. To the obtained magnetic layer dispersion, 5 parts of a polyisocyanate (Coronate L manufactured by Nippon Polyurethane Co., Ltd.) was added, 40 parts of a mixed solvent of methyl ethyl ketone and cyclohexanone was added, and the mixture was filtered using a filter having an average pore diameter of 1 μm. A coating solution for forming a layer was prepared. The obtained magnetic layer coating solution is 5.5 μm thick so that the thickness after drying becomes 0.7 μm.
The surface of the magnetic layer is coated on a polyethylene naphthalate support having a center line surface roughness of 0.001 μm, and while the coating layer is still wet, a cobalt magnet having a magnetic force of 3000 G and a magnetic force of 3000 G are sequentially applied. After being oriented and dried by a solenoid having the same, treatment was carried out at a temperature of 80 ° C. at a speed of 200 m / min. Using a seven-stage calender composed of only metal rolls, and then a 0.5 μm-thick back layer was applied. It was slit to a width of 8 mm to make a digital video tape. Comparative Example 1-1 A sample was prepared by changing the urethane resin to B.

Hereinafter, a magnetic recording medium having a lower nonmagnetic layer will be described. [Example 2-1] Upper magnetic layer Ferromagnetic metal fine powder 100 parts Composition Fe / Co = 100/30 (atomic ratio) Al (atomic%): 7.1 Si (atomic%): 0.8 Y (atomic %): 6.5 Hc 2250 Oe, specific surface area by BET method 56 m ² / g Crystallite size 165 粒子 Particle size (long axis diameter) 0.065 μm Needlelike ratio 5 σs: 142 emu / g

Vinyl chloride copolymer 11 parts MR-110 polyester polyurethane resin manufactured by Zeon Corporation 5 parts A α-Al ₂ O _{3 shown in} Table 2 (average particle size 0.15 μm) 10 parts Filler A 0 shown in Table 1 .3 parts sec-butyl stearate 1 part Stearic acid 5 parts Methyl ethyl ketone 120 parts Cyclohexanone 120 parts

Lower coating layer (non-magnetic layer) Non-magnetic powder 80 parts α-Fe ₂ O ₃ hematite (part of the surface is coated with Al ₂ O ₃ and SiO ₂ ) Long axis length 0.14 μm Specific surface area by BET method 55 m ² / g pH 8 Tap density 1.0 DBP oil absorption 27-38 g / 100 g,

Carbon black 20 parts Average primary particle size 16 mμ DBP oil absorption 80 ml / 100 g pH 8.0 Specific surface area by BET method 250 m ² / g Volatile content 1.5%

Vinyl chloride copolymer 12 parts MR-104 polyester polyurethane resin manufactured by Zeon Corporation 5 parts A α-Al ₂ O _{3 shown in} Table 2 (average particle size 0.15 μm) 1 part sec-butyl stearate 1 part 5 parts Stearic acid 1 part Methyl ethyl ketone 100 parts Cyclohexanone 100 parts Toluene 50 parts

Each of the above-mentioned paints was kneaded with an open kneader, and then dispersed using a sand mill. To the resulting lower dispersion, 5 parts of a polyisocyanate (Coronate L, manufactured by Nippon Polyurethane Co., Ltd.) was added to the lower coating layer, and 40 parts of a mixed solvent of methyl ethyl ketone and cyclohexanone were added to each.
The solution was filtered using a filter having an average pore size of μm to prepare a lower coating layer and a coating solution for forming a magnetic layer.
The obtained lower layer coating liquid is coated with a 5.5 μm thick magnetic layer so that the thickness after drying is 1 μm, and immediately thereafter, the magnetic layer is 0.2 μm thick thereon. Simultaneous multilayer coating is performed on a polyethylene naphthalate support having a center line surface roughness of 0.001 μm on the coated surface, and while both layers are still in a wet state, a cobalt magnet having a magnetic force of 3000 G and a solenoid having a magnetic force of 3000 G After drying at a speed of 200 m / min. At a temperature of 80 ° C. using a seven-stage calender consisting of only metal rolls, a 0.5 μm-thick back layer was applied. 8
It was slit to a width of mm to create a digital videotape.

Example 2-2 The amount of the upper filler A was 1.0 part by weight. [Example 2-3] The amount of the upper filler A was 2.0 parts by weight. Example 2-4 The polyurethane in the upper layer was changed to B, and the filler in the upper layer was the same as in Example 2-2. Example 2-5 The polyurethane in the lower layer was changed to B, and the filler in the upper layer was the same as in Example 2-2. [Comparative Example 2-1] The amount of the upper filler A was 0.05 part by weight. [Comparative Example 2-2] The amount of the upper filler A was 3.0 parts by weight. [Comparative Example 2-3] The polyurethane was changed to B for each of the upper layer and the lower layer, and the filler in the upper layer was the same as in Example 2-1. Comparative Example 2-4 Polyurethane was changed to B for each of the upper layer and the lower layer, and the filler in the upper layer was the same as in Example 2-2. [Comparative Example 2-5] The polyurethane was changed to B for each of the upper layer and the lower layer, and the filler in the upper layer was the same as in Example 2-3.

Example 3 A sample was prepared by replacing the filler (A) in the upper layer of Example 2 with (B). [Example 3-1] The amount of the upper filler B was 0.1 part by weight. [Example 3-2] The amount of the upper filler B was 1.5 parts by weight. [Comparative Example 3-1] Polyurethane was changed to B, and the upper filler was the same as in Example 3-1. [Comparative Example 3-2] The polyurethane was changed to B, and the upper filler was the same as in Example 3-2. [Example 4] An upper layer of Example 2-2 was formed with a thickness of 0.6 µm. [Comparative Example 4] The polyurethane of Example 4 was changed to B. [Example 5] A lower layer having a thickness of 3.0 µm was prepared in Example 2-3. [Comparative Example 5] The polyurethane of Example 5 was changed to B. [Example 6] The calender temperature of Example 2-3 was set to 100 ° C.
Was created. [Comparative Example 6] The polyurethane of Example 6 was changed to B.

Example 7 An upper magnetic material of Example 2-2 was prepared in the same manner as described above, except that the magnetic material was changed as follows. Ferromagnetic metal fine powder Composition Fe / Co = 100/30 (atomic ratio) Al (atomic%): 10.5 Si (atomic%): 0.8 Y (atomic%): 6.5 Hc 2300 Oe, BET method Specific surface area 55 m ² / g Crystallite size 150 粒子 Particle size (major axis diameter) 0.05 μm Needle ratio 5 σs: 145 emu / g Comparative Example 7 The polyurethane of Example 6 was changed to B.

The characteristics of the magnetic recording media of Examples and Comparative Examples obtained as described above were measured by the following measuring methods, and the results are shown in Tables 3 to 7. [Measurement Method] <Thickness of Magnetic Layer> A thickness of about 0.1 μm was cut out along the longitudinal direction of the magnetic recording medium with a diamond cutter, observed with a transmission electron microscope at a magnification of 30,000, and photographed. . The print size of the photo is A4 size. Thereafter, the interface was visually judged by focusing on the difference in shape between the ferromagnetic powder and the nonmagnetic powder of the magnetic layer and the nonmagnetic layer, and the surface of the magnetic layer was similarly blackened. Thereafter, the interval between the bordered lines was measured by an image processing apparatus IBAS2 manufactured by Zeiss. The length of the sample photograph was over a range of 21 cm, and measurement was performed by taking measurement points. The simple average value of the measured values at that time was defined as the thickness of the magnetic layer.

<Surface Roughness Ra of Magnetic Layer> WYKO (U.
S Arizona) Interferometric 3D roughness meter "TOPO3
D ”, and the surface of the magnetic layer is about 250 nm by MIRAU method.
Ra of an area of × 250 nm was measured. Measurement wavelength is about 65
At 0 nm, spherical and cylindrical corrections are added. <Number of Micro Protrusions> Using a digital instrument Nanoscope 3 (AFM: Atomic Force Microscope), a 30 μm square square (9
Within 100 μm ² ), the number of microprojections was measured every 10 nm up to a microprojection height of 40 nm, and the number of projections having a height of 10 nm or more and less than 20 nm: M ₁₀ , the number of projections having a height of 20 nm or more and less than 40 nm : it was calculated M ₂₀ and M ₁₀ / M ₂₀ ratio.

<Electromagnetic Conversion Characteristics> The “output at recording wavelengths of 0.488 μm and 22 μm” is a reference ME tape of our company, and the output is measured at a relative speed of 10.2 m / sec using an external drum tester. did. The head used was an Fe head and had a Bs of 1.5.
T. <Friction Coefficient> The tape was transferred to SUS420J of 4 mmφ at an angle of 180 °, slid at a load of 10 g and a speed of 18 mm / sec, and the friction coefficient was determined based on Euler's equation. μ = (1 / π) ln (T2 / 10) T2: Sliding resistance value (g) <Running durability> The tape was used at a temperature of 23 ° C. and 70% RH in an atmosphere of 10 mm for an 8 mm video deck FUJIX8 manufactured by Fuji Photo Film Co., Ltd.
Dropouts were measured after traveling 500 times for 10 minutes each on the table, and the number increased from the initial dropout was evaluated.

[0067]

[Table 3]

[0068]

[Table 4]

[0069]

[Table 5]

[0070]

[Table 6]

[0071]

[Table 7]

[0072]

[Table 8]

[0073]

[Table 9]

[0074] Comparing Example 1 and Comparative Example 1, Example 1 using the polyurethane resin A as defined in the present invention, M ₁₀
The fine protrusions number of M _20, and be in the range ratio both as defined in the present invention of M ₁₀ / M _20, output, friction coefficient, dropout increases were excellent in both. On the other hand, Comparative Example 1 using polyurethane resin B not specified in the present invention has M ₁₀ / M
The ratio of ₂₀ was out of the range specified in the present invention, and the output decreased. Comparing Comparative Example 2 and Example 2, using the polyurethane resin A as defined in the present invention, M ₁₀ and fine protrusions number of M _20,
In Example 2 where both the ratio of M ₁₀ and M ₂₀ were within the ranges specified in the present invention, the output, the coefficient of friction, and the increase in dropout were all excellent. On the other hand, a polyurethane resin B not specified in the present invention may be used, or M ₁₀ , M ₂₀ and / or M
₁₀ / M ₂₀ Comparative Example 2 was outside the range specified in the present invention is ,,
There has been a problem that the output is reduced or dropout is increased.

Examples 3, 4, 5, 6, 7 and Comparative Example 3,
Comparing 4, 5, 6, and 7 respectively,
Examples 3 to 7 using polyurethane resin A_TenAnd M
₂₀Number of microprojections and M_Ten/ M₂₀Of the present invention
Within a certain range, increase output, coefficient of friction, dropout
Were both excellent. On the other hand, points not specified in the present invention
Comparative Examples 3 to 7 using the urethane resin B_Ten, M₂₀
And / or M_Ten/ M ₂₀Is out of the range specified in the present invention.
Output decreased.

[0076]

The magnetic recording medium of the present invention uses a specified polyurethane resin as a binder for the magnetic layer or the non-magnetic layer to disperse the magnetic material in the upper coating layer and the fine inorganic powder in the lower coating layer. In order to improve the property, the surface property, particularly the number of protrusions having a height of 10 nm or more and less than 20 nm by AFM measurement can be greatly reduced. This makes it possible to improve running durability without increasing the coefficient of friction and to improve the electromagnetic conversion characteristics.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yuichiro Murayama 2-12-1, Ogimachi, Odawara-shi, Kanagawa Inside Fuji Photo Film Co., Ltd.

Claims (5)

[Claims] 1. A magnetic recording medium having a magnetic layer formed by dispersing a ferromagnetic powder in a binder on a nonmagnetic support, or further comprising an inorganic powder bound between the magnetic layer and the nonmagnetic support. A magnetic recording medium provided with a lower coating layer containing
At least one of the binder in the magnetic layer or the binder in the lower coating layer is a polyurethane resin containing a cyclic structure and an ether group, and the surface of the magnetic layer has a thickness of 10 nm by an atomic force microscope (AFM). When measuring the height of each protrusion, 10 nm or more and 20 nm in a 30 μm × 30 μm square
The number of the height of the projections of less than: with M ₁₀ 4000 U or less, 20
The number of the height of the projections of less than nm or 40 nm: M ₂₀ is located more than 20 co, and the magnetic recording medium, wherein the ratio of M ₁₀ / M ₂₀ is 10 to 35. 2. The polyurethane resin comprises a polyurethane resin which is a reaction product mainly composed of a diol and an organic diisocyanate. The diol comprises a short-chain diol having a cyclic structure and a long-chain diol containing an ether group, The short-chain diol is added to the polyurethane resin in an amount of 1
7 to 40% by weight, the long-chain diol is 10 to 50% by weight,
2. The magnetic recording medium according to claim 1, wherein said ether group is contained in an amount of 1.0 to 5.0 mmol / g. 3. The ferromagnetic powder of the magnetic layer contains 10 to 40 atomic% of Co with respect to Fe, and has an average major axis length of 0.03%.
3. The magnetic recording medium according to claim 1, wherein the magnetic recording medium has an axial ratio of 2 to 8 and a crystallite size of 120 to 200. 4. The magnetic recording medium according to claim 1, wherein the inorganic powder contained in the lower coating layer is in the form of particles or needles having an average particle size of 0.3 μm or less. . 5. The magnetic recording medium according to claim 1, wherein the lower coating layer and at least a magnetic layer in contact with the lower coating layer are formed by a wet-on-wet coating method.