JP2007004842A - Magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic recording medium having excellent electromagnetic transducing characteristics and running durability. <P>SOLUTION: In the magnetic recording medium formed by providing a non-magnetic layer containing non-magnetic powder and a binder on a supporting body and providing a magnetic layer containing ferromagnetic powder and a binder on the non-magnetic layer, when the magnetic recording medium is cut in a direction orthogonal to its surface and in a direction parallel to its thickness direction, the sectional area of abrasive contained in the magnetic recording medium and measured by the following steps of (i) to (iii) is 50% or more. (i) A cut surface is observed, the abrasive existing over the non-magnetic layer and the magnetic layer is detected and the sum total sectional area (T) of the abrasive is calculated. (ii) A sectional area (E) of a part buried in the non-magnetic layer of the abrasive detected in the step (i) is calculated. (iii) The ratio of the sectional area (E) obtained in the step (ii) to the sum total sectional area (T) obtained in the step (i) is calculated as a percentage. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a magnetic recording medium, and more particularly to a magnetic recording medium having good electromagnetic conversion characteristics and durability.

  In recent years, a means for transmitting information such as an optical fiber at a high speed has rapidly developed, and an image or data having a huge amount of information can be transferred. In this situation, advanced techniques for recording, reproducing and storing such an enormous amount of information have been required. Examples of the recording / reproducing medium include a flexible disk, a hard disk, and a magnetic tape. In particular, magnetic tape is in great demand for computer data backup. In this magnetic tape for data backup, with the increase in capacity of the hard disk to be backed up, a recording capacity of several tens to several hundreds GB per volume has already been commercialized, and further increase in the recording capacity is required. ing.

  Along with the increase in recording capacity, advanced technology is required in the tribology of the drive head and magnetic tape surface, and even if a very small amount of foreign matter adheres to the head, a problem occurs in data recording and playback, which is a big problem. May be a problem.

In order to remove the foreign matter, it is effective to increase the grain size of the abrasive on the surface of the magnetic tape or increase the amount of addition. However, these are electromagnetic conversions in magnetic tapes for high density recording in which the magnetic layer is a thin layer. It causes the characteristics, especially the output to decrease.
On the other hand, in order to achieve both good electromagnetic conversion characteristics (smoothness) and durability, there is a means of adding a fine particle abrasive of about 0.2 to 0.3 μm into the magnetic layer. However, as the magnetic layer becomes thinner, the fine particle abrasive tends to fall off during driving, and durability cannot be ensured unless a considerable amount is added. In that case, since the magnetic material density is lowered, the electromagnetic conversion characteristics are significantly lowered.

In the following Patent Document 1, a magnetic recording medium in which a nonmagnetic layer and a magnetic layer are provided on one surface of a support and a backcoat layer containing carbon black is provided on the other surface, the support having a thickness of 3 is provided. Less than .9 μm, the total thickness of the medium is 5.4 μm or less, and when the medium is run under specific running conditions, the wear width of the wear width of the AlFeSil prism is a, and after that run, the same running location of the medium When the wear width of the AlFeSil prism is b in contact with a new AlFeSil prism under the same conditions as b, the change rate b / a of the wear width is 0.6 or more and 1.0 or less. A recording medium is disclosed. However, the magnetic recording medium disclosed in Patent Document 1 has a problem that the running durability is insufficient and the error rate increases in a low humidity environment.
From the above, in the development of magnetic recording media that require high reliability, means for further improving electromagnetic conversion characteristics and durability have been demanded.

JP 2004-288332 A

  Accordingly, an object of the present invention is to provide a magnetic recording medium having good electromagnetic conversion characteristics and durability.

The present invention is as follows.
1) A magnetic recording medium in which a nonmagnetic layer containing a nonmagnetic powder and a binder is provided on a support, and a magnetic layer containing a ferromagnetic powder and a binder is provided on the nonmagnetic layer.
When the magnetic recording medium is cut in a direction perpendicular to the surface and parallel to the thickness direction, the cross-sectional area of the abrasive contained in the magnetic recording medium measured by the following steps (i) to (iii) Is 50% or more, a magnetic recording medium characterized by:
(I) Observe the cut surface, detect the abrasive that exists across the nonmagnetic layer and the magnetic layer, and determine the total cross-sectional area (T) of the abrasive.
(Ii) The cross-sectional area (E) of the portion of the abrasive detected in the step (i) buried in the nonmagnetic layer is obtained.
(Iii) The ratio of the cross-sectional area (E) obtained in the step (ii) to the total cross-sectional area (T) obtained in the step (i) is obtained as a percentage.
2) The magnetic recording medium as described in 1) above, wherein the magnetic layer has a thickness of 0.03 to 0.15 μm.
3) The magnetic recording according to 1) or 2), wherein the nonmagnetic layer and the magnetic layer are provided on one surface of the support, and a back layer is provided on the other surface of the support. Medium.

  According to the present invention, since the form of the abrasive present in the nonmagnetic layer and the magnetic layer is specified, a magnetic recording medium excellent in electromagnetic conversion characteristics and durability can be provided.

Hereinafter, the present invention will be described in more detail.
The magnetic recording medium of the present invention is characterized by the presence form of the abrasive contained therein. That is, in the present invention, when the magnetic recording medium is cut in a direction perpendicular to the surface and parallel to the thickness direction, the magnetic recording medium is included in the magnetic recording medium measured by the following steps (i) to (iii). The cross-sectional area of the abrasive is 50% or more.
(I) Observe the cut surface, detect the abrasive that exists across the nonmagnetic layer and the magnetic layer, and determine the total cross-sectional area (T) of the abrasive.
(Ii) The cross-sectional area (E) of the portion of the abrasive detected in the step (i) buried in the nonmagnetic layer is obtained.
(Iii) The ratio of the cross-sectional area (E) obtained in the step (ii) to the total cross-sectional area (T) obtained in the step (i) is obtained as a percentage.

Furthermore, it demonstrates with reference to drawings. FIG. 1 is a cross-sectional view of a magnetic recording medium for explaining how to determine the cross-sectional area of an abrasive in the present invention.
As shown in FIG. 1, the magnetic recording medium 10 of the present invention is provided with a nonmagnetic layer 12 containing a nonmagnetic powder and a binder on a support 11, and coupled with a ferromagnetic powder on the nonmagnetic layer 12. For example, spherical abrasives A, B, and C are present in the nonmagnetic layer 12 and the magnetic layer 13.

First, in the step (i), the magnetic recording medium is cut in a direction perpendicular to the surface and parallel to the thickness direction. As a cutting means, for example, a microtome can be used. Then, the cut surface in the thickness direction is observed, the abrasive that exists across the nonmagnetic layer and the magnetic layer is detected, and the total cross-sectional area (T) of the abrasive is obtained. In the present invention, the cut surface is observed at a magnification of 100,000 using a transmission electron microscope. In FIG. 1, the abrasive that exists across the nonmagnetic layer and the magnetic layer refers to the abrasives A and B, and polishing in which at least a part of the abrasive protrudes from the nonmagnetic layer 12 toward the magnetic layer 13. It is a material. The abrasive may protrude beyond the magnetic layer 13 to the air side (Abrasive B) or may not protrude (Abrasive A). The abrasive C does not belong to the “abrasive that exists across the nonmagnetic layer and the magnetic layer” in the present invention because the entire cross section is buried in the nonmagnetic layer 12.
The total cross-sectional area of the abrasive detected by the observation can be easily analyzed from the magnification of the microscope by, for example, taking and printing an electron micrograph.
Next, in the step (ii), the cross-sectional area (E) of the portion of the detected abrasive embedded in the nonmagnetic layer 12 is obtained. The portion buried in the nonmagnetic layer 12 is a hatched portion of the abrasives A and B in FIG. The interface between the magnetic layer 12 and the nonmagnetic layer 13 can be determined by paying attention to the shape difference between the ferromagnetic powder and the nonmagnetic powder. Similarly to the above, the cross-sectional area (E) of the portion embedded in the nonmagnetic layer 12 of the abrasive can be determined in consideration of the magnification of the microscope, for example, by taking and printing an electron micrograph.
In the step (iii), the ratio of the cross-sectional area (E) obtained in the step (ii) to the total cross-sectional area (T) obtained in the step (i) is obtained as a percentage (cross-sectional area (E) / Cross-sectional area (T) × 100 (%)). In the present invention, the cross-sectional area (E) / (T) of the abrasive obtained by the step (iii) needs to be 50% or more. In the present invention, since the detected abrasive exists across the nonmagnetic layer and the magnetic layer, the cross-sectional area (E) / (T) is always less than 100%. When the cross-sectional area (E) / (T) is less than 50%, the abrasive easily falls off during driving, and the durability cannot be maintained. Further, the smoothness of the surface cannot be maintained, and the electromagnetic conversion characteristics deteriorate. The cross-sectional area (E) / (T) is preferably 55% or more, and more preferably 60% or more.

  In order to increase the cross-sectional area (E) / (T) to 50% or more, as described below, set the size of the abrasive to an appropriate range, and add an abrasive to the coating solution for the nonmagnetic layer. And controlling the calendar processing conditions.

Next, the layer structure of the magnetic recording medium of the present invention will be described.
[Support]
The support in the present invention is usually a nonmagnetic support, and a polyester support (hereinafter simply referred to as polyester) is preferable. Such a polyester is a polyester composed of a dicarboxylic acid and a diol, such as polyethylene terephthalate and polyethylene naphthalate.
The main constituent dicarboxylic acid components include terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, diphenylsulfone dicarboxylic acid, diphenyl ether dicarboxylic acid, diphenylethanedicarboxylic acid, Examples thereof include cyclohexane dicarboxylic acid, diphenyl dicarboxylic acid, diphenyl thioether dicarboxylic acid, diphenyl ketone dicarboxylic acid, and phenylindane dicarboxylic acid.
Examples of the diol component include ethylene glycol, propylene glycol, tetramethylene glycol, cyclohexanedimethanol, 2,2-bis (4-hydroxyphenyl) propane, 2,2-bis (4-hydroxyethoxyphenyl) propane, bis ( 4-Hydroxyphenyl) sulfone, bisphenol fluorene hydroxyethyl ether, diethylene glycol, neopentyl glycol, hydroquinone, cyclohexanediol and the like.
Among the polyesters having these as main constituents, terephthalic acid and / or 2,6-naphthalenedicarboxylic acid as a dicarboxylic acid component and ethylene glycol as a diol component from the viewpoint of transparency, mechanical strength, dimensional stability, etc. And / or a polyester mainly composed of 1,4-cyclohexanedimethanol is preferred.
Among them, a polyester mainly composed of polyethylene terephthalate or polyethylene-2,6-naphthalate, a copolymer polyester composed of terephthalic acid, 2,6-naphthalenedicarboxylic acid and ethylene glycol, and two or more kinds of these polyesters A polyester having a mixture as a main constituent is preferred. Particularly preferred is a polyester having polyethylene-2,6-naphthalate as a main constituent.
The polyester used in the present invention may be biaxially stretched or a laminate of two or more layers.
The polyester may be further copolymerized with other copolymerization components, or may be mixed with other polyesters. Examples of these include the dicarboxylic acid components and diol components mentioned above, or polyesters composed thereof.
In the polyester used in the present invention, an aromatic dicarboxylic acid having a sulfonate group or an ester-forming derivative thereof, a dicarboxylic acid having a polyoxyalkylene group or an ester-forming derivative thereof, in order to make it difficult for delamination during film formation. A diol having a polyoxyalkylene group may be copolymerized.
Of these, 5-sodium sulfoisophthalic acid, 2-sodium sulfoterephthalic acid, 4-sodium sulfophthalic acid, 4-sodium sulfo-2,6-naphthalenedicarboxylic acid, and the like in terms of polyester polymerization reactivity and film transparency. Compounds in which sodium is substituted with other metals (for example, potassium, lithium, etc.), ammonium salts, phosphonium salts, etc., or ester-forming derivatives thereof, polyethylene glycol, polytetramethylene glycol, polyethylene glycol-polypropylene glycol copolymers and their A compound obtained by oxidizing the hydroxy groups at both ends to form a carboxyl group is preferred. The proportion of copolymerization for this purpose is preferably 0.1 to 10 mol% based on the dicarboxylic acid constituting the polyester.
For the purpose of improving heat resistance, a bisphenol compound, a compound having a naphthalene ring or a cyclohexane ring can be copolymerized. The copolymerization ratio is preferably 1 to 20 mol% based on the dicarboxylic acid constituting the polyester.

In the present invention, the method for synthesizing the polyester is not particularly limited, and can be produced according to a conventionally known polyester production method. For example, a direct esterification method in which a dicarboxylic acid component is directly esterified with a diol component. First, a dialkyl ester is used as a dicarboxylic acid component, this is transesterified with the diol component, and this is heated under reduced pressure. A transesterification method of polymerizing by removing excess diol component can be used. At this time, if necessary, a transesterification catalyst or a polymerization reaction catalyst can be used, or a heat-resistant stabilizer can be added.
In addition, anti-coloring agents, antioxidants, crystal nucleating agents, slipping agents, stabilizers, anti-blocking agents, UV absorbers, viscosity modifiers, antifoaming and clearing agents, antistatic agents, pH adjustments in each process during synthesis One or more of various additives such as an agent, a dye, a pigment, and a reaction terminator may be added.

  Further, a filler may be added to the polyester. Examples of the filler include inorganic powders such as spherical silica, colloidal silica, titanium oxide, and alumina, and organic fillers such as crosslinked polystyrene and silicone resin.

  In the present invention, the thickness of the polyester as the support is preferably 3 to 80 μm, more preferably 3 to 50 μm, and particularly preferably 3 to 10 μm. The center line average roughness (Ra) of the support surface is 8 nm or less, more preferably 6 nm or less. This Ra was measured with TOPO-3D manufactured by WYKO.

  In the magnetic recording medium of the present invention, a magnetic layer in which a ferromagnetic metal powder is dispersed in a binder is provided as a coating layer on the above support, and substantially between the support and the magnetic layer. A nonmagnetic layer (lower layer) that is nonmagnetic in nature is provided.

[Magnetic layer]
The ferromagnetic powder contained in the magnetic layer preferably has a volume of (0.1 to 8) × 10 ⁻¹⁸ ml, and more preferably (0.5 to 5) × 10 ⁻¹⁸ ml. By setting it within this range, it is possible to effectively suppress a decrease in magnetic characteristics due to thermal fluctuations, and to obtain good C / N (S / N) while maintaining low noise. The ferromagnetic powder is not particularly limited, but ferromagnetic metal powder or hexagonal ferrite powder is preferable.
The volume of the acicular powder is obtained from the major axis length and minor axis length assuming that the shape is a cylinder.
In the case of plate-like powder, the volume is determined from the plate diameter and axial length (plate thickness) assuming that the shape is a prism (hexagonal prism in the case of hexagonal ferrite powder).
As for the size of the magnetic material, an appropriate amount of the magnetic layer is peeled off. N-Butylamine is added to 30 to 70 mg of the peeled magnetic layer, sealed in a glass tube, set in a thermal decomposition apparatus, and heated at 140 ° C. for about 1 day. After cooling, the contents are taken out from the glass tube and centrifuged to separate the liquid and solids. The separated solid is washed with acetone to obtain a powder sample for TEM. This sample is photographed with a Hitachi transmission electron microscope H-9000, and the particles are photographed at a photographing magnification of 100000 times and printed on a photographic paper to obtain a total magnification of 500,000 times to obtain a particle photograph. A target magnetic substance is selected from the particle photograph and placed on an image analyzer KS-400 digitizer manufactured by kontron, and the size of each particle is measured by tracing the outline of the powder. The size of 500 particles is measured, and the measured values are averaged to obtain an average size.

<Ferromagnetic metal powder>
The ferromagnetic metal powder used for the magnetic layer in the magnetic recording medium of the present invention is not particularly limited as long as it contains Fe as a main component (including an alloy), but is ferromagnetic with α-Fe as a main component. Alloy powder is preferred. These ferromagnetic powders include Al, Si, S, Sc, Ca, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, other than predetermined atoms. It may contain atoms such as W, Re, Au, Hg, Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, and B. It is preferable that at least one of Al, Si, Ca, Y, Ba, La, Nd, Co, Ni, and B is included in addition to α-Fe, and it is particularly preferable that Co, Al, and Y are included. More specifically, it is preferable that Co is contained in an amount of 10 to 40 atomic%, Fe is contained in an amount of 2 to 20 atomic%, and Y is contained in an amount of 1 to 15 atomic%.

The ferromagnetic metal powder may be previously treated with a dispersant, a lubricant, a surfactant, an antistatic agent or the like, which will be described later. The ferromagnetic metal powder may contain a small amount of water, hydroxide or oxide. The moisture content of the ferromagnetic metal powder is preferably 0.01-2%. It is preferable to optimize the moisture content of the ferromagnetic metal powder depending on the type of the binder. The pH of the ferromagnetic metal powder is preferably optimized depending on the combination with the binder used. The range is usually from 6 to 12, but preferably from 7 to 11. The ferromagnetic metal powder may contain inorganic ions such as soluble Na, Ca, Fe, Ni, Sr, NH ₄ , SO ₄ , Cl, NO ₂ and NO ₃ . These are preferably essentially absent. If the total of each ion is about 300 ppm or less, the characteristics are not affected. The ferromagnetic metal powder used in the present invention preferably has fewer vacancies, and its value is 20% by volume or less, more preferably 5% by volume or less.

  The average major axis length of the ferromagnetic metal powder is preferably 30 to 150 nm, more preferably 30 to 100 nm, and particularly preferably 30 to 60 nm. The crystallite size of the ferromagnetic metal powder is preferably 8 to 20 nm, more preferably 10 to 18 nm, and particularly preferably 12 to 16 nm. This crystallite size is an average value obtained by a Scherrer method from a half-value width of a diffraction peak using an X-ray diffractometer (RINT2000 series manufactured by Rigaku Corporation) under the conditions of a radiation source CuKα1, a tube voltage 50 kV, and a tube current 300 mA. .

The specific surface area by BET method of the ferromagnetic metal powder _{(S BET)} is preferably at least ^{50 m} 2 / g, more preferably from 55~100m ² / g, and most preferably 60~80m ² / g. Within this range, both good surface properties and low noise can be achieved. The pH of the ferromagnetic metal powder is preferably optimized depending on the combination with the binder used. The range is 4-12, preferably 7-10. The ferromagnetic metal powder may be surface-treated with Al, Si, P, or an oxide thereof as required. The amount thereof is 0.1 to 10% with respect to the ferromagnetic metal powder. When the surface treatment is performed, the adsorption of a lubricant such as a fatty acid is preferably 100 mg / m ² or less. The ferromagnetic metal powder may contain soluble inorganic ions such as Na, Ca, Fe, Ni, and Sr. However, if it is 200 ppm or less, it does not particularly affect the characteristics. Further, the ferromagnetic metal powder used in the present invention preferably has fewer vacancies, and its value is 20% by volume or less, more preferably 5% by volume or less.

The shape of the ferromagnetic metal powder may be needle-like, granular, rice-grained or plate-like as long as the particle volume shown above is satisfied, but it is particularly preferable to use a needle-like ferromagnetic powder. In the case of acicular ferromagnetic metal powder, the average acicular ratio [arithmetic average of (major axis length / minor axis length)] is preferably 4 to 12, more preferably 5 to 12. The coercive force (Hc) of the ferromagnetic metal powder is preferably 159.2 to 238.8 kA / m (2000 to 3000 Oe), more preferably 167.2 to 230.8 kA / m (2100 to 2900 Oe). . The saturation magnetic flux density is preferably 150 to 300 mT (1500 to 3000 G), and more preferably 160 to 290 mT. The saturation magnetization (σs) is preferably 140 to 170 A · m ² / kg (140 to 170 emu / g), and more preferably 145 to 160 A · m ² / kg. The SFD (switching field distribution) of the magnetic material itself is preferably small and is preferably 0.8 or less. When the SFD is 0.8 or less, the electromagnetic conversion characteristics are good, the output is high, the magnetization reversal is sharp, the peak shift is small, and it is suitable for high-density digital magnetic recording. In order to reduce the Hc distribution, there are methods such as improving the goethite particle size distribution in the ferromagnetic metal powder, using monodispersed αFe ₂ O ₃ , and preventing sintering between particles.

  As the ferromagnetic metal powder, those obtained by a known production method can be used, and the following methods can be mentioned. Reduction of hydrous iron oxide and iron oxide with anti-sintering treatment using iron or other reducing gas to obtain Fe or Fe-Co particles, reduction of complex organic acid salt (mainly oxalate) and hydrogen A method of reducing with a reactive gas, a method of thermally decomposing a metal carbonyl compound, a method of reducing a ferromagnetic metal aqueous solution by adding a reducing agent such as sodium borohydride, hypophosphite or hydrazine, a metal at a low pressure For example, the powder is obtained by evaporating in an inert gas. The ferromagnetic metal powder thus obtained is subjected to a known slow oxidation treatment. A method of reducing the amount of demagnetization by reducing the hydrous iron oxide and iron oxide with a reducing gas such as hydrogen and controlling the partial pressure, temperature and time of the oxygen-containing gas and inert gas to form an oxide film on the surface. preferable.

<Hexagonal ferrite powder>
Hexagonal ferrite powders include, for example, barium ferrite, strontium ferrite, lead ferrite, calcium ferrite, and their substitutes such as Co. More specifically, magnetoplumbite type barium ferrite and strontium ferrite, magnetoplumbite type ferrite whose particle surface is coated with spinel, and magnetoplumbite type barium ferrite and strontium ferrite partially containing a spinel phase, etc. Is mentioned. In addition to predetermined atoms, Al, Si, S, Sc, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg , Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, B, Ge, and Nb may be included. In general, elements such as Co—Zn, Co—Ti, Co—Ti—Zr, Co—Ti—Zn, Ni—Ti—Zn, Nb—Zn—Co, Sb—Zn—Co, and Nb—Zn are added. You can use things. Some raw materials and production methods contain specific impurities. Other preferable atoms and the content thereof are the same as those of the ferromagnetic metal powder.

The particle size of the hexagonal ferrite powder is preferably a size that satisfies the above volume, but the average plate diameter is 10 to 50 nm, preferably 10 to 30 nm, and more preferably 10 to 20 nm.
The average plate ratio {average of (plate diameter / plate thickness)} is 1-15, and preferably 1-7. If the average plate ratio is 1 to 15, sufficient orientation can be obtained while maintaining high filling properties in the magnetic layer, and noise increase can be suppressed by stacking between particles. Further, the specific surface area (S _BET ) by the BET method within the above particle size range is preferably 40 m ² / g or more, more preferably 40 to 200 m ² / g, and 60 to 100 m ² / g. Is most preferred.

  The distribution of the particle plate diameter and plate thickness of the hexagonal ferrite powder is generally preferably as narrow as possible. Quantifying the particle plate diameter and plate thickness can be compared by randomly measuring 500 particles from a particle TEM photograph. In many cases, the distribution of the particle plate diameter and plate thickness is not a normal distribution, but when calculated and expressed as a standard deviation with respect to the average size, σ / average size = 0.1 to 2.0. In order to sharpen the particle size distribution, the particle generation reaction system is made as uniform as possible, and the generated particles are subjected to a distribution improvement process. For example, a method of selectively dissolving ultrafine particles in an acid solution is also known.

  The coercive force (Hc) of the hexagonal ferrite powder can be in the range of 159.2 to 238.8 kA / m (2000 to 3000 Oe), preferably 175.1 to 222.9 kA / m (2200 to 2800 Oe). More preferably, it is 183.1-214.9 kA / m (2300-2700 Oe). However, when the saturation magnetization (σs) of the head exceeds 1.4T, it is preferably 159.2 kA / m or less. The coercive force (Hc) can be controlled by the particle size (plate diameter / plate thickness), the type and amount of the contained element, the substitution site of the element, the particle generation reaction conditions, and the like.

The saturation magnetization (σs) of the hexagonal ferrite powder is 40 to 80 A · m ² / kg (emu / g). Higher saturation magnetization (σs) is preferable, but tends to be smaller as the particles become finer. In order to improve the saturation magnetization (σs), it is well known to combine spinel ferrite with magnetoplumbite ferrite, and to select the type and amount of contained elements. It is also possible to use W-type hexagonal ferrite. When the magnetic material is dispersed, the surface of the magnetic material particles is also treated with a material suitable for the dispersion medium and the polymer. As the surface treatment agent, inorganic compounds and organic compounds are used. Typical examples of the main compound include oxides or hydroxides such as Si, Al, and P, various silane coupling agents, and various titanium coupling agents. The addition amount is 0.1 to 10% by mass with respect to the mass of the magnetic substance. The pH of the magnetic material is also important for dispersion. Usually, about 4 to 12 has optimum values depending on the dispersion medium and polymer, but about 6 to 11 is selected from the chemical stability and storage stability of the medium. Water contained in the magnetic material also affects the dispersion. Although there is an optimum value depending on the dispersion medium and the polymer, 0.01 to 2.0% is usually selected.

The hexagonal ferrite powder is manufactured by mixing (1) barium oxide / iron oxide / metal oxide replacing iron and boron oxide as a glass forming substance so as to have a desired ferrite composition, and then melting and quenching. After being reheated, washed and ground to obtain barium ferrite crystal powder, glass crystallization method, (2) neutralizing barium ferrite composition metal salt solution with alkali, by-product Hydrothermal reaction method to obtain barium ferrite crystal powder by liquid phase heating at 100 ° C. or higher after removal of water, (3) neutralization of barium ferrite composition metal salt solution with alkali, by-product There is a coprecipitation method in which the product is removed, dried, treated at 1100 ° C. or lower, and pulverized to obtain a barium ferrite crystal powder, but the present invention does not select a production method. The hexagonal ferrite powder may be subjected to surface treatment with Al, Si, P, or an oxide thereof as required. The amount thereof is 0.1 to 10% with respect to the ferromagnetic powder, and the surface treatment is preferable because the adsorption of a lubricant such as a fatty acid is 100 mg / m ² or less. The ferromagnetic powder may contain inorganic ions such as soluble Na, Ca, Fe, Ni, and Sr. Although it is preferable that these are essentially not present, if they are 200 ppm or less, they do not particularly affect the characteristics.

<Binder>
The binder (binder) used in the magnetic layer of the present invention is a conventionally known thermoplastic resin, thermosetting resin, reactive resin, or a mixture thereof. Examples of the thermoplastic resin include vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic ester, vinylidene chloride, acrylonitrile, methacrylic acid, methacrylic ester, styrene, butadiene, ethylene, vinyl butyral, vinyl acetal. And polymers or copolymers containing vinyl ether as a constituent unit, polyurethane resins, and various rubber resins.

  Examples of thermosetting resins or reactive resins include phenolic resins, epoxy resins, polyurethane curable resins, urea resins, melamine resins, alkyd resins, acrylic reactive resins, formaldehyde resins, silicone resins, and epoxy-polyamide resins. And a mixture of polyester resin and isocyanate prepolymer, a mixture of polyester polyol and polyisocyanate, a mixture of polyurethane and polyisocyanate, and the like. The thermoplastic resin, thermosetting resin and reactive resin are all described in detail in “Plastic Handbook” issued by Asakura Shoten.

  Further, when an electron beam curable resin is used for the magnetic layer, not only the coating film strength is improved and the durability is improved, but also the surface is smoothed and the electromagnetic conversion characteristics are further improved. These examples and their production methods are described in detail in JP-A No. 62-256219.

  The above resins can be used alone or in combination. Among them, it is preferable to use a polyurethane resin, and further, a cyclic structure such as hydrogenated bisphenol A or a polypropylene oxide adduct of hydrogenated bisphenol A, a polyol having an alkylene oxide chain with a molecular weight of 500 to 5000, and a chain extender. A polyurethane resin having a cyclic structure and a molecular weight of 200 to 500 reacted with an organic diisocyanate and having a polar group introduced, or an aliphatic dibasic acid such as succinic acid, adipic acid, or sebacic acid; Fat having no cyclic structure having an alkyl branched side chain such as dimethyl-1,3-propanediol, 2-ethyl-2-butyl-1,3-propanediol, 2,2-diethyl-1,3-propanediol Polyester polyol composed of a group diol and 2 as a chain extender Reaction of an aliphatic diol having a branched alkyl side chain having 3 or more carbon atoms such as ethyl-2-butyl-1,3-propanediol and 2,2-diethyl-1,3-propanediol with an organic diisocyanate compound And a polyurethane resin into which a polar group is introduced, or a cyclic structure such as dimer diol, a polyol compound having a long-chain alkyl chain, and an organic diisocyanate, and a polyurethane resin into which a polar group is introduced are used. Is preferred.

  The average molecular weight of the polar group-containing polyurethane resin used in the present invention is preferably 5,000 to 100,000, and more preferably 10,000 to 50,000. If the average molecular weight is 5,000 or more, it is preferable because the obtained magnetic coating film is not brittle and the physical strength is not lowered, and the durability of the magnetic recording medium is not affected. Further, when the molecular weight is 100,000 or less, the solubility in the solvent does not decrease, and the dispersibility is also good. Further, since the viscosity of the paint at a predetermined concentration does not increase, the workability is good and the handling is easy.

Examples of the polar group contained in the polyurethane resin, for _{example, -COOM, -SO 3 M, -OSO} 3 M, -P = O (OM) 2, -O-P = O (OM) per ₂ (or more, M is a hydrogen atom or an alkali metal base), —OH, —NR ₂ , —N ⁺ R ₃ (R is a hydrocarbon group), an epoxy group, —SH, —CN, etc., and at least one of these polar groups One in which two or more are introduced by copolymerization or addition reaction can be used. Moreover, when this polar group-containing polyurethane-based resin has an OH group, it preferably has a branched OH group from the viewpoint of curability and durability, and preferably has 2 to 40 branched OH groups per molecule. It is more preferable to have 3 to 20 molecules per molecule. The amount of such polar group is from ¹⁰ -1 ^{to 10 -8} mol / g, preferably ¹⁰ -2 ^{to 10 -6} mol / g.

  Specific examples of the binder include, for example, VAGH, VYHH, VMCH, VAGF, VAGD, VROH, VYES, VYNC, VMCC, XYHL, XYSG, PKHH, PKHJ, PKHC, PKFE, Nissin Chemical Industry Co., Ltd. MPR-TA, MPR-TA5, MPR-TAL, MPR-TSN, MPR-TMF, MPR-TS, MPR-TM, MPR-TAO, Denki Kagaku 1000W, DX80, DX81, DX82, DX83, 100FD, Japan MR-104, MR-105, MR110, MR100, MR555, 400X-110A manufactured by ZEON Co., Ltd. NIPPOLAN N2301, N2302, N2304 manufactured by Nippon Polyurethane Co., Ltd. Pandex T-5105, T-R3080, T-5201 manufactured by Dainippon Ink, Inc. , Barnock D 400, D-210-80, Crisbon 6109, 7209, Byron UR8200, UR8300, UR-8700, RV530, RV280, Toyobo Co., Ltd. Daiferamin 4020, 5020, 5100, 5300, 9020, 9022, 7020, Mitsubishi Chemical company MX5004, Sanyo Chemical Co., Ltd. sampler SP-150, Asahi Kasei Co., Ltd. Saran F310, F210, etc. can be mentioned.

The addition amount of the binder used in the magnetic layer of the present invention is in the range of 5 to 50% by mass, preferably in the range of 10 to 30% by mass with respect to the mass of the ferromagnetic powder. When using polyurethane, it is preferable to use a combination of 2 to 20% by mass and polyisocyanate in the range of 2 to 20% by mass. For example, when head corrosion occurs due to a small amount of dechlorination, only polyurethane or It is also possible to use only polyurethane and isocyanate. When using a vinyl chloride resin as the other resin, it is preferably in the range of 5 to 30% by mass. In the present invention, when polyurethane is used, the glass transition temperature is −50 to 150 ° C., preferably 0 to 100 ° C., the breaking elongation is 100 to 2000%, and the breaking stress is 0.49 to 98 MPa (0.05 to 10 kg / mm). ² ) The yield point is preferably 0.49 to 98 MPa (0.05 to 10 kg / mm ² ).

  When the magnetic recording medium used in the present invention is, for example, a flexible disk, it can be composed of two or more layers on both sides of the support. Therefore, the amount of the binder, the amount of vinyl chloride resin, polyurethane resin, polyisocyanate, or other resin in the binder, the molecular weight of each resin forming the magnetic layer, the polar group amount, or the resin described above It is of course possible to change the physical characteristics and the like between the non-magnetic layer and each magnetic layer as required, and rather it should be optimized for each layer, and a known technique relating to a multilayer magnetic layer can be applied. For example, when changing the amount of binder in each layer, it is effective to increase the amount of binder in the magnetic layer in order to reduce scratches on the surface of the magnetic layer. The amount of binder in the magnetic layer can be increased to provide flexibility.

  Examples of the polyisocyanate usable in the present invention include tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate, o-toluidine diisocyanate, isophorone diisocyanate, triol. Examples thereof include isocyanates such as phenylmethane triisocyanate, products of these isocyanates and polyalcohols, and polyisocyanates generated by condensation of isocyanates. Commercially available product names of these isocyanates include Coronate L, Coronate HL, Coronate 2030, Coronate 2031, Millionate MR Millionate MTL, Takeda Pharmaceutical Takenate D-102, Takenate D-110N, Takenate D-200, Takenate D-202, Sumika Bayer's Death Module L, Death Module IL, Death Module N, Death Module HL, etc., these may be used alone or two or more using the difference in curing reactivity Each layer can be used in the above combination.

Additives can be added to the magnetic layer in the present invention as necessary. Examples of the additive include an abrasive, a lubricant, a dispersant / dispersion aid, an antifungal agent, an antistatic agent, an antioxidant, a solvent, and carbon black. Examples of these additives include molybdenum disulfide, tungsten disulfide, graphite fluoride, silicone oil, silicone having a polar group, fatty acid-modified silicone, fluorine-containing silicone, fluorine-containing alcohol, fluorine-containing ester, polyolefin, polyglycol, Polyphenyl ether, phenylphosphonic acid, benzylphosphonic acid, phenethylphosphonic acid, α-methylbenzylphosphonic acid, 1-methyl-1-phenethylphosphonic acid, diphenylmethylphosphonic acid, biphenylphosphonic acid, benzylphenylphosphonic acid, α-cumylphosphonic acid , Toluylphosphonic acid, xylylphosphonic acid, ethylphenylphosphonic acid, cumenylphosphonic acid, propylphenylphosphonic acid, butylphenylphosphonic acid, heptylphenylphosphonic acid, octyl Aromatic ring-containing organic phosphonic acids such as phenylphosphonic acid and nonylphenylphosphonic acid and alkali metal salts thereof, octylphosphonic acid, 2-ethylhexylphosphonic acid, isooctylphosphonic acid, isononylphosphonic acid, isodecylphosphonic acid, isoun Alkylphosphonic acids such as decylphosphonic acid, isododecylphosphonic acid, isohexadecylphosphonic acid, isooctadecylphosphonic acid, isoeicosylphosphonic acid and alkali metal salts thereof, phenyl phosphate, benzyl phosphate, phenethyl phosphate, phosphoric acid α-methylbenzyl, 1-methyl-1-phenethyl phosphate, diphenylmethyl phosphate, biphenyl phosphate, benzylphenyl phosphate, α-cumyl phosphate, toluyl phosphate, xylyl phosphate, ethylphenyl phosphate, phosphoric acid Cumenyl, propylene phosphate Aromatic phosphates such as ruphenyl, butylphenyl phosphate, heptylphenyl phosphate, octylphenyl phosphate, nonylphenyl phosphate and their alkali metal salts, octyl phosphate, 2-ethylhexyl phosphate, isooctyl phosphate, phosphoric acid Isononyl, isodecyl phosphate, isoundecyl phosphate, isododecyl phosphate, isohexadecyl phosphate, isooctadecyl phosphate, isoeicosyl phosphate, etc., phosphoric acid alkyl esters and alkali metal salts thereof, alkylsulfonic acid esters and alkali metal salts thereof, Fluorine-containing alkyl sulfates and alkali metal salts thereof, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, butyl stearate, oleic acid, linoleic acid, linolenic acid, elaidic acid, erucic acid, etc. Monobasic fatty acids which may contain or be branched to 24 unsaturated bonds and their metal salts, or butyl stearate, octyl stearate, amyl stearate, isooctyl stearate, octyl myristate, butyl laurate, A monobasic fatty acid which may contain or be branched, such as butoxyethyl stearate, anhydrosorbitan monostearate, anhydrosorbitan tristearate, etc. Either a 1-6 hexahydric alcohol which may contain or be unsaturated, an alkoxy alcohol which may contain or be unsaturated having 12 to 22 carbon atoms, or a monoalkyl ether of an alkylene oxide polymer Mono-fatty acid ester, di-fatty acid ester or polyvalent Fatty acid esters, fatty acid amides having 2 to 22 carbon atoms, and aliphatic amines having 8 to 22 carbon atoms can be used. In addition to the above hydrocarbon groups, nitro groups and alkyl groups, aryl groups, and aralkyl groups in which groups other than hydrocarbon groups such as halogen-containing hydrocarbons such as F, Cl, Br, CF ₃ , CCl ₃ , and CBr ₃ are substituted. You may have something.

  In addition, nonionic surfactants such as alkylene oxide, glycerin, glycidol, and alkylphenol ethylene oxide adducts, cyclic amines, ester amides, quaternary ammonium salts, hydantoin derivatives, heterocyclics, phosphonium or sulfoniums, etc. Amphoteric interfaces such as cationic surfactants, anionic surfactants containing acidic groups such as carboxylic acid, sulfonic acid, and sulfate ester groups, amino acids, aminosulfonic acids, sulfuric or phosphate esters of aminoalcohols, and alkylbetaines An activator or the like can also be used. These surfactants are described in detail in “Surfactant Handbook” (published by Sangyo Tosho Co., Ltd.).

  The lubricants, antistatic agents, and the like are not necessarily pure, and may contain impurities such as isomers, unreacted materials, side reaction products, decomposition products, and oxides in addition to the main components. These impurities are preferably 30% by mass or less, more preferably 10% by mass or less.

  Specific examples of these additives include, for example, manufactured by NOF Corporation: NAA-102, castor oil hardened fatty acid, NAA-42, cation SA, Naimine L-201, Nonion E-208, Anon BF, Anon LG, Takemoto Oil and fat: FAL-205, FAL-123, Shin Nippon Chemical Co., Ltd .: NJ Lube OL, Shin-Etsu Chemical Co., Ltd .: TA-3, Lion Corporation: Armide P, Duomin TDO, Nisshin Oilio Co., Ltd .: BA-41G, Sanyo Kasei Co., Ltd .: Profan 2012E, New Pole PE61, Ionette MS-400 and the like.

In addition, carbon black can be added to the magnetic layer in the present invention as necessary. Examples of carbon black that can be used in the magnetic layer include rubber furnace, rubber thermal, color black, and acetylene black. Specific surface area is 5 to 500 m ² / g, DBP oil absorption is 10 to 400 ml / 100 g, particle diameter is 5 to 300 nm, pH is 2 to 10, water content is 0.1 to 10%, tap density is 0.1 to 1 g / ml is preferred.

  Specific examples of carbon black used in the present invention include Cabot's BLACKPEARLS 2000, 1300, 1000, 900, 905, 800, 700, VULCAN XC-72, Asahi Carbon Co., Ltd. # 80, # 60, # 55. # 50, # 35, Mitsubishi Chemical Corporation # 2400B, # 2300, # 900, # 1000, # 30, # 40, # 10B, Columbian Carbon Corporation CONDUCTEX SC, RAVEN150, 50, 40, 15, RAVEN- MT-P, Ketjen Black International Ketjen Black EC, etc. are mentioned. Carbon black may be surface-treated with a dispersant, or may be used after being grafted with a resin, or may be obtained by graphitizing a part of the surface. Carbon black may be dispersed with a binder in advance before being added to the magnetic coating. These carbon blacks can be used alone or in combination. When using carbon black, it is preferable to use 0.1-30 mass% with respect to the mass of a magnetic body. Carbon black functions to prevent the magnetic layer from being charged, reduce the friction coefficient, impart light-shielding properties, and improve the film strength. These differ depending on the carbon black used. Therefore, these carbon blacks used in the present invention have different types, amounts, and combinations in the magnetic layer and the nonmagnetic layer, and are based on the above-mentioned characteristics such as particle size, oil absorption, conductivity, pH, etc. Of course, it is possible to use them properly according to the purpose, but rather they should be optimized in each layer. For the carbon black that can be used in the magnetic layer of the present invention, for example, “Carbon Black Handbook” edited by Carbon Black Association can be referred to.

  Known organic solvents can be used in the present invention. The organic solvent used in the present invention is an arbitrary ratio of ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone, tetrahydrofuran, methanol, ethanol, propanol, butanol, isobutyl alcohol, isopropyl alcohol, methyl Alcohols such as cyclohexanol, esters such as methyl acetate, butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate, glycol acetate, glycol ethers such as glycol dimethyl ether, glycol monoethyl ether, dioxane, benzene, toluene, xylene, Aromatic hydrocarbons such as cresol and chlorobenzene, methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, ethyl Nkuroruhidorin, chlorinated hydrocarbons such as dichlorobenzene, N, N- dimethylformamide, may be used hexane.

  These organic solvents are not necessarily 100% pure, and may contain impurities such as isomers, unreacted materials, side reaction products, decomposition products, oxides, and moisture in addition to the main components. These impurities are preferably 30% or less, more preferably 10% or less. The organic solvent used in the present invention is preferably the same in the magnetic layer and the nonmagnetic layer. The amount added may be changed. Use a solvent with high surface tension (such as cyclohexanone or dioxane) in the nonmagnetic layer to increase the coating stability. Specifically, the arithmetic average value of the upper layer solvent composition may not fall below the arithmetic average value of the nonmagnetic layer solvent composition. It is essential. In order to improve dispersibility, it is preferable that the polarity is somewhat strong, and it is preferable that 50% or more of a solvent having a dielectric constant of 15 or more is included in the solvent composition. Moreover, it is preferable that a solubility parameter is 8-11.

  These dispersants, lubricants, and surfactants used in the present invention can be properly used in the magnetic layer and further in the nonmagnetic layer described later as needed. For example, of course, the dispersant is not limited to the example shown here, but the dispersant has a property of adsorbing or binding with a polar group, and in the magnetic layer, mainly on the surface of the ferromagnetic metal powder and nonmagnetic. In the layer, it is presumed that the organic phosphorus compound adsorbed or bonded mainly to the surface of the non-magnetic powder with the above-mentioned polar group, for example, is difficult to desorb from the surface of metal or metal compound. Accordingly, the surface of the ferromagnetic metal powder or the nonmagnetic powder of the present invention is in a state of being coated with an alkyl group, an aromatic group, etc., so that the binder resin component of the ferromagnetic metal powder or nonmagnetic powder is used. The affinity is improved and the dispersion stability of the ferromagnetic metal powder or nonmagnetic powder is also improved. In addition, since the lubricant exists in a free state, fatty acids with different melting points are used in the nonmagnetic layer and magnetic layer to control the bleeding to the surface, and leaching to the surface using esters with different boiling points and polarities. It is conceivable to improve the coating stability by controlling the amount of the surfactant, to improve the lubrication effect by increasing the additive amount of the lubricant in the nonmagnetic layer. All or part of the additives used in the present invention may be added in any step during the production of the coating solution for the magnetic layer or nonmagnetic layer. For example, when mixing with a ferromagnetic powder before the kneading step, when adding at a kneading step with a ferromagnetic powder, a binder and a solvent, when adding at a dispersing step, when adding after dispersing, when adding just before coating, etc. There is.

[Nonmagnetic layer]
Next, detailed contents regarding the nonmagnetic layer will be described. The magnetic recording medium of the present invention has a nonmagnetic layer containing a binder and a nonmagnetic powder on a support. The nonmagnetic powder that can be used in the nonmagnetic layer may be an inorganic substance or an organic substance. Carbon black or the like can also be used. Examples of the inorganic substance include metals, metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides.

Specifically, titanium oxide such as titanium dioxide, cerium oxide, tin oxide, tungsten oxide, ZnO, ZrO ₂ , SiO ₂ , Cr ₂ O ₃ , α-alumina, β-alumina having an α conversion of 90 to 100%, γ-alumina, α-iron oxide, goethite, corundum, silicon nitride, titanium carbide, magnesium oxide, boron nitride, molybdenum disulfide, copper oxide, MgCO ₃ , CaCO ₃ , BaCO ₃ , SrCO ₃ , BaSO ₄ , silicon carbide Titanium carbide or the like is used alone or in combination of two or more. Preferable are α-iron oxide and titanium oxide.

  The shape of the nonmagnetic powder may be any of acicular, spherical, polyhedral and plate shapes. The crystallite size of the nonmagnetic powder is preferably 4 nm to 1 μm, and more preferably 40 to 100 nm. A crystallite size in the range of 4 nm to 1 μm is preferred because it does not become difficult to disperse and has a suitable surface roughness. The average particle size of these non-magnetic powders is preferably 5 nm to 2 μm. However, if necessary, non-magnetic powders having different average particle sizes may be combined, or even a single non-magnetic powder may have a wide particle size distribution. It can also have an effect. The average particle size of the particularly preferred nonmagnetic powder is 10 to 200 nm. The range of 5 nm to 2 μm is preferable because the dispersion is good and the surface roughness is suitable.

The specific surface area of the nonmagnetic powder is 1 to 100 m ² / g, preferably 5 to 70 m ² / g, and more preferably 10 to 65 m ² / g. A specific surface area in the range of 1 to 100 m ² / g is preferred because it has a suitable surface roughness and can be dispersed with a desired amount of binder. The oil absorption using dibutyl phthalate (DBP) is 5 to 100 ml / 100 g, preferably 10 to 80 ml / 100 g, and more preferably 20 to 60 ml / 100 g. The specific gravity is 1 to 12, preferably 3 to 6. The tap density is 0.05 to 2 g / ml, preferably 0.2 to 1.5 g / ml. When the tap density is in the range of 0.05 to 2 g / ml, there are few particles to be scattered, the operation is easy, and there is a tendency that it is difficult to adhere to the apparatus. The pH of the nonmagnetic powder is preferably 2 to 11, but the pH is particularly preferably between 6 and 9. When the pH is in the range of 2 to 11, the friction coefficient does not increase due to high temperature, high humidity, or liberation of fatty acids. The moisture content of the nonmagnetic powder is 0.1 to 5% by mass, preferably 0.2 to 3% by mass, and more preferably 0.3 to 1.5% by mass. A water content in the range of 0.1 to 5% by mass is preferable because the dispersion is good and the viscosity of the paint after dispersion is stable. The ignition loss is preferably 20% by mass or less, and the ignition loss is preferably small.

When the nonmagnetic powder is an inorganic powder, the Mohs hardness is preferably 4-10. If the Mohs hardness is in the range of 4 to 10, durability can be ensured. The nonmagnetic powder has a stearic acid adsorption amount of 1 to 20 μmol / m ² , more preferably ² to 15 μmol / m ² . The heat of wetting of the nonmagnetic powder into water at 25 ° C. is preferably in the range of 200 to 600 erg / cm ² (200 to 600 mJ / m ² ). Moreover, the solvent which exists in the range of this heat of wetting can be used. The amount of water molecules on the surface at 100 to 400 ° C. is suitably 1 to 10 / 100Å. The pH of the isoelectric point in water is preferably between 3 and 9. It is preferable that Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ , SnO ₂ , Sb ₂ O ₃ , and ZnO are present on the surface of these nonmagnetic powders by surface treatment. Particularly preferred for dispersibility are Al ₂ O ₃ , SiO ₂ , TiO ₂ , and ZrO ₂ , but more preferred are Al ₂ O ₃ , SiO ₂ , and ZrO ₂ . These may be used in combination or may be used alone. Further, a surface-treated layer co-precipitated according to the purpose may be used, or a method of treating the surface layer with silica after first treating with alumina, or vice versa may be employed. The surface treatment layer may be a porous layer depending on the purpose, but it is generally preferable that the surface treatment layer is homogeneous and dense.

  Specific examples of the non-magnetic powder used in the non-magnetic layer of the present invention include, for example, Showa Denko Nanotite, Sumitomo Chemical HIT-100, ZA-G1, Toda Kogyo DPN-250, DPN-250BX, DPN-245, DPN-270BX, DPB-550BX, DPN-550RX, Ishihara Sangyo Titanium oxide TTO-51B, TTO-55A, TTO-55B, TTO-55C, TTO-55S, TTO-55D, SN-100, MJ -7, α-iron oxide E270, E271, E300, STT-4D, STT-30D, STT-30, STT-65C manufactured by Titanium Industry, MT-100S, MT-100T, MT-150W, MT-500B manufactured by Teika T-600B, T-100F, T-500HD, etc. are mentioned. FINEX-25, BF-1, BF-10, BF-20, ST-M, Dowa Mining DEFIC-Y, DEFIC-R, Nippon Aerosil AS2BM, TiO2P25, Ube Industries 100A, 500A, Titanium Industry Y-LOP manufactured and what baked it are mentioned. Particularly preferred nonmagnetic powders are titanium dioxide and α-iron oxide.

Carbon black can be mixed in the nonmagnetic layer together with nonmagnetic powder to lower the surface electrical resistance, reduce the light transmittance, and obtain a desired micro Vickers hardness. The micro-Vickers hardness of the nonmagnetic layer is generally 25~60kg ^/ mm 2 (245~588MPa), preferably in order to adjust the head ^contact, a 30~50kg / mm 2 (294~490MPa), thin film hardness meter ( Using a HMA-400 manufactured by NEC, measurement can be performed using a diamond triangular pyramid needle having a ridge angle of 80 degrees and a tip radius of 0.1 μm at the tip of the indenter. It is standardized that the light transmittance is generally 3% or less for absorption of infrared rays having a wavelength of about 900 nm, for example, 0.8% or less for a VHS magnetic tape. For this purpose, rubber furnace, rubber thermal, color black, acetylene black and the like can be used.

Non specific surface area of the carbon black used in the magnetic layer is 100 to 500 m ² / g, preferably 150~400m ² / g, DBP oil absorption of the present invention are 20 to 400 ml / 100 g, preferably 30 to 200 ml / 100 g . The particle size of carbon black is 5 to 80 nm, preferably 10 to 50 nm, and more preferably 10 to 40 nm. Carbon black preferably has a pH of 2 to 10, a water content of 0.1 to 10%, and a tap density of 0.1 to 1 g / ml.

  Specific examples of carbon black that can be used in the non-magnetic layer of the present invention include BLACKPEARLS 2000, 1300, 1000, 900, 800, 880, 700, VULCAN XC-72, manufactured by Cabot Corporation, # 3050B, manufactured by Cabot Corporation. # 3150B, # 3250B, # 3750B, # 3950B, # 950B, # 650B, # 970B, # 850B, MA-600, CONDUCTEX SC, Raven8800, 8000, 7000, 5750, 5250, 3500, 2100, Columbia Carbon Co., Ltd. 2000, 1800, 1500, 1255, 1250, and Ketjen Black EC manufactured by Ketjen Black International.

  Carbon black may be surface-treated with a dispersant, or may be grafted with a resin, or may be obtained by graphitizing a part of the surface. Moreover, before adding carbon black to a coating material, you may disperse | distribute with a binder beforehand. These carbon blacks can be used in a range not exceeding 50% by mass with respect to the inorganic powder and in a range not exceeding 40% of the total mass of the nonmagnetic layer. These carbon blacks can be used alone or in combination. The carbon black that can be used in the nonmagnetic layer of the present invention can be referred to, for example, “Carbon Black Handbook” edited by Carbon Black Association.

  Further, an organic powder can be added to the nonmagnetic layer according to the purpose. Examples of such organic powder include acrylic styrene resin powder, benzoguanamine resin powder, melamine resin powder, and phthalocyanine pigment, but polyolefin resin powder, polyester resin powder, polyamide resin powder, polyimide resin, and the like. Resin powder and polyfluorinated ethylene resin can also be used. As the production method, those described in JP-A Nos. 62-18564 and 60-255827 can be used.

  As the binder resin, lubricant, dispersant, additive, solvent, dispersion method and the like of the nonmagnetic layer, those of the magnetic layer can be applied. In particular, with respect to the binder resin amount, type, additive, and dispersant addition amount and type, known techniques relating to the magnetic layer can be applied.

An abrasive is preferably added to the nonmagnetic layer in the present invention.
As an abrasive, α-alumina, β-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum, artificial diamond, silicon nitride, silicon carbide, titanium carbide having an α conversion rate of 90 to 100%, Examples thereof include titanium oxide, silicon dioxide, boron nitride and the like, and materials having a Mohs hardness of 6 to 10, preferably a Mohs hardness of 9 to 10 are used alone or in combination. Further, a composite of these abrasives (a product obtained by surface-treating an abrasive with another abrasive) may be used. These abrasives may contain compounds or elements other than the main component, but the effect is not affected if the main component is 90 to 100%. The average particle size of the abrasive is 0.1 to 0.8 μm, preferably 0.2 to 0.7 μm, more preferably 0.3 to 0.6 μm. The cross-sectional area (E) / (T) can be controlled within the range defined by the present invention by the average particle size range of the abrasive. Note that, if necessary, abrasives having different average particle diameters can be combined, or even a single abrasive can have the same effect by widening the particle size distribution. The tap density is preferably 0.3 to ² g / ml, the water content is 0.1 to 5%, the pH is 2 to 11, and the specific surface area is preferably 1 to 30 m ² / g. The shape of the abrasive used in the present invention may be any of a needle shape, a spherical shape, and a dice shape, but those having a corner in a part of the shape are preferable because of high polishing properties. Specific examples of the abrasive include trade names HIT-60A and AKP-30 manufactured by Sumitomo Chemical Co., Ltd.
Moreover, the addition amount of the abrasive in the nonmagnetic layer is 30 to 200% by mass, preferably 50 to 150% by mass with respect to the nonmagnetic powder.
The type of abrasive in the non-magnetic layer partially overlaps with the non-magnetic powder.In this case, the non-magnetic layer has a large occupied volume, and the non-magnetic layer has a small occupied volume. It will be called an abrasive.

  The magnetic recording medium of the present invention may be provided with an undercoat layer. By providing the undercoat layer, the adhesive force between the support and the magnetic layer or the nonmagnetic layer can be improved. As the undercoat layer, a solvent-soluble polyester resin is used.

[Layer structure]
In the thickness structure of the magnetic recording medium used in the present invention, the preferable thickness of the support is 3 to 80 μm. When an undercoat layer is provided between the support and the nonmagnetic layer or the magnetic layer, the thickness of the undercoat layer is 0.01 to 0.8 μm, preferably 0.02 to 0.6 μm.

  The thickness of the magnetic layer is optimized depending on the saturation magnetization amount, the head gap length, and the recording signal band of the magnetic head to be used. In the present invention, the magnetic field with the cross-sectional area (E) / (T) defined is used. Since it is a recording medium, a particularly remarkable effect is achieved when a thin magnetic layer is employed. That is, the thickness of the magnetic layer of the present invention is preferably 0.03 to 0.15 μm, more preferably 0.05 to 0.13 μm, and particularly preferably 0.07 to 0.10 μm. Further, the thickness variation rate of the magnetic layer is preferably within ± 50%, and more preferably within ± 40%. There may be at least one magnetic layer, and the magnetic layer may be separated into two or more layers having different magnetic characteristics, and a configuration related to a known multilayer magnetic layer can be applied.

  The thickness of the nonmagnetic layer of the present invention is 0.5 to 2.0 μm, preferably 0.8 to 1.8 μm, and more preferably 1.0 to 1.5 μm. The non-magnetic layer of the magnetic recording medium of the present invention exhibits its effect if it is substantially non-magnetic. For example, even if it contains a small amount of magnetic material as an impurity or intentionally, This shows the effect of the invention and can be regarded as substantially the same configuration as the magnetic recording medium of the invention. Note that “substantially the same” means that the residual magnetic flux density of the nonmagnetic layer is 10 mT or less or the coercive force is 7.96 kA / m (100 Oe) or less, and preferably has no residual magnetic flux density and coercive force. Means.

[Back layer]
The magnetic recording medium of the present invention is preferably provided with a back layer on the other surface of the support. The back layer preferably contains carbon black and inorganic powder. For the binder and various additives, the formulation of the magnetic layer and the nonmagnetic layer is applied. The thickness of the back layer is preferably from 0.1 to 1.0 μm, more preferably from 0.3 to 0.6 μm.

[Production method]
The step of producing the magnetic layer coating liquid for the magnetic recording medium used in the present invention comprises at least a kneading step, a dispersing step, and a mixing step provided before and after these steps. Each process may be divided into two or more stages. All raw materials such as ferromagnetic metal powder, non-magnetic powder, binder, carbon black, abrasive, antistatic agent, lubricant, and solvent used in the present invention may be added at the beginning or middle of any step. In addition, individual raw materials may be added in two or more steps. For example, polyurethane may be divided and added in a kneading step, a dispersing step, and a mixing step for adjusting the viscosity after dispersion. In order to achieve the object of the present invention, a conventional known manufacturing technique can be used as a partial process. In the kneading step, it is preferable to use a kneading force such as an open kneader, a continuous kneader, a pressure kneader, or an extruder. Details of these kneading treatments are described in JP-A-1-106338 and JP-A-1-79274. Further, glass beads can be used to disperse the liquid for the magnetic layer and the liquid for the nonmagnetic layer. Such glass beads are preferably zirconia beads, titania beads, and steel beads, which are high specific gravity dispersion media. The particle diameter and filling rate of these dispersion media are optimized. A well-known thing can be used for a disperser.

  In the method for producing a magnetic recording medium of the present invention, for example, a magnetic layer is applied to the surface of a support under running so as to have a predetermined film thickness. Here, a plurality of magnetic layer coating solutions may be applied sequentially or simultaneously, and a nonmagnetic layer coating solution and a magnetic layer coating solution may be applied sequentially or simultaneously. The coating machine for applying the above magnetic coating solution or non-magnetic layer coating solution includes 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, spray coat, spin coat, etc. can be used. As for these, for example, “Latest Coating Technology” (May 31, 1983) issued by General Technology Center Co., Ltd. can be referred to.

  In the case of a magnetic tape, the magnetic layer coating solution is subjected to magnetic field orientation treatment in the longitudinal direction using a cobalt magnet or a solenoid on the ferromagnetic metal powder contained in the magnetic layer coating solution. In the case of a disk, a sufficiently isotropic orientation may be obtained even without non-orientation without using an orientation device, but known random methods such as alternately arranging cobalt magnets obliquely and applying an alternating magnetic field with a solenoid. It is preferable to use an alignment device. In the case of a ferromagnetic metal powder, the isotropic orientation is generally preferably in-plane two-dimensional random, but can also be three-dimensional random with a vertical component. Further, isotropic magnetic characteristics can be imparted in the circumferential direction by using a well-known method such as a counter-polarized magnet and making it vertically oriented. In particular, when performing high density recording, vertical alignment is preferable. Moreover, circumferential orientation can also be achieved using spin coating.

  It is preferable that the drying position of the coating film can be controlled by controlling the temperature, air volume, and coating speed of the drying air, the coating speed is preferably 20 m / min to 1000 m / min, and the temperature of the drying air is preferably 60 ° C. or higher. Also, moderate pre-drying can be performed before entering the magnet zone.

After drying, the coating layer is usually subjected to a surface smoothing treatment. For the surface smoothing process, for example, a super calendar 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 metal powder in the magnetic layer is improved, so that a magnetic recording medium having high electromagnetic conversion characteristics can be obtained. Can do. As the calendering roll, a heat-resistant plastic roll such as epoxy, polyimide, polyamide, polyamideimide or the like is used. Moreover, it can also process with a metal roll.
The magnetic recording medium of the present invention has a surface having extremely excellent smoothness with a center surface average roughness of 0.1 to 4 nm, preferably 1 to 3 nm at a cutoff value of 0.25 mm. preferable. As the method, for example, as described above, a magnetic layer formed by selecting a specific ferromagnetic metal powder and a binder is subjected to the calendar treatment. In the present invention, in order to control the cross-sectional area (E) / (T) to a specific range, the calender roll temperature is in the range of 60 to 100 ° C., preferably in the range of 70 to 100 ° C., as the calendering condition. Particularly preferably, it is in the range of 80 to 100 ° C., and the pressure is in the range of 100 to 500 kg / cm (98 to 490 kN / m), preferably in the range of 200 to 450 kg / cm (196 to 441 kN / m), It is particularly preferable to perform the operation under conditions in the range of 300 to 400 kg / cm (294 to 392 kN / m).

  The obtained magnetic recording medium can be cut into a desired size using a cutting machine or the like. The cutting machine is not particularly limited, but is preferably provided with a plurality of pairs of rotating upper blades (male blades) and lower blades (female blades). The slitting speed, the engagement depth, and the upper blade (male blade) are appropriately selected. The peripheral speed ratio (upper blade peripheral speed / lower blade peripheral speed) between the blade and the lower blade (female blade), the continuous use time of the slit blade, and the like are selected.

[Physical properties]
The saturation magnetic flux density of the magnetic layer of the magnetic recording medium used in the present invention is preferably 100 to 300 mT. The coercive force (Hr) of the magnetic layer is preferably 143.3 to 318.4 kA / m (1800 to 4000 Oe), more preferably 159.2 to 278.6 kA / m (2000 to 3500 Oe). The coercive force distribution is preferably narrow, and SFD and SFDr are 0.6 or less, more preferably 0.2 or less.

The friction coefficient with respect to the head of the magnetic recording medium used in the present invention is 0.50 or less, preferably 0.3 or less in the range of temperature -10 to 40 ° C. and humidity 0 to 95%. The surface resistivity is preferably 10 ⁴ to 10 ¹² Ω / sq of the magnetic surface, and the charging position is preferably within −500 V to +500 V. The elastic modulus at 0.5% elongation of the magnetic layer is preferably 0.98 to 19.6 GPa (100 to 2000 kg / mm ² ) in each in-plane direction, and the breaking strength is preferably 98 to 686 MPa (10 to 70 kg). / Mm ² ), the elastic modulus of the magnetic recording medium is preferably 0.98 to 14.7 GPa (100 to 1500 kg / mm ² ) in each in-plane direction, and the residual spread is preferably 0.5% or less, 100 ° C. The thermal shrinkage at any of the following temperatures is preferably 1% or less, more preferably 0.5% or less, and most preferably 0.1% or less.

The glass transition temperature of the magnetic layer (maximum point of loss elastic modulus measured by dynamic viscoelasticity measured at 110 Hz) is preferably 50 to 180 ° C, and that of the nonmagnetic layer is preferably 0 to 180 ° C. The loss elastic modulus is preferably in the range of 1 × 10 ⁷ to 8 × 10 ⁸ Pa (1 × 10 ⁸ to 8 × 10 ⁹ dyne / cm ² ), and the loss tangent is preferably 0.2 or less. If the loss tangent is too large, adhesion failure is likely to occur. These thermal characteristics and mechanical characteristics are preferably almost equal within 10% in each in-plane direction of the medium.

The residual solvent contained in the magnetic layer is preferably 100 mg / m ² or less, more preferably 10 mg / m ² or less. The porosity of the coating layer is preferably 30% by volume or less, more preferably 20% by volume or less for both the nonmagnetic layer and the magnetic layer. The porosity is preferably small in order to achieve high output, but it may be better to ensure a certain value depending on the purpose. For example, in the case of a disk medium in which repeated use is important, a larger void ratio is often preferable for running durability.

The maximum height SR _max of the magnetic layer is 0.5 μm or less, the ten-point average roughness SRz is 0.3 μm or less, the center plane peak height SRp is 0.3 μm or less, and the center plane valley depth SRv is 0.3 μm or less. The center surface area ratio SSr is preferably 20 to 80%, and the average wavelength Sλa is preferably 5 to 300 μm. These can be easily controlled by controlling the surface properties with the filler of the support or the surface of the calendered roll. The curl is preferably within ± 3 mm.

  When the magnetic recording medium of the present invention is composed of a nonmagnetic layer and a magnetic layer, these physical characteristics can be changed between the nonmagnetic layer and the magnetic layer according to the purpose. For example, the elastic modulus of the magnetic layer can be increased to improve running durability, and at the same time, the elastic modulus of the nonmagnetic layer can be made lower than that of the magnetic layer to improve the contact of the magnetic recording medium with the head.

EXAMPLES Hereinafter, although an Example demonstrates this invention, this invention is not limited to the following example.
In addition, the display of "part" in an Example shows "mass part".

Example 1
1. Preparation of magnetic layer coating solution Preparation of coating solution for magnetic layer 100 parts of ferromagnetic acicular metal powder Composition: Fe / Co / Al / Y = 65/20/8/7 (atomic ratio)
Surface treatment layer: Al ₂ O ₃ , Y ₂ O ₃
Coercive force (Hc): 183 kA / m
Crystallite size: 12.5nm
Average long axis length: 45 nm
Average needle ratio: 6
BET specific surface area (S _BET ): 60 m ² / g
Saturation magnetization (σs): 140 A · m ² / kg (140 emu / g)
Vinyl chloride copolymer 12 parts Polyurethane resin 8 parts Branched side chain-containing polyester polyol / diphenylmethane diisocyanate,
Hydrophilic polar group: —SO ₃ Na = 70 eq / ton-containing phenylphosphonic acid 3 parts carbon black (average particle size 20 nm) 2 parts cyclohexanone 110 parts methyl ethyl ketone 100 parts toluene 100 parts butyl stearate 2 parts stearic acid 1 part

2. Preparation of nonmagnetic layer coating liquid Nonmagnetic inorganic powder 85 parts α-iron oxide Surface treatment layer: Al ₂ O ₃ , SiO ₂
Average major axis diameter: 0.15 μm
Tap density: 0.8g / ml
Average needle ratio: 7
BET specific surface area (S _BET ): 52 m ² / g
pH 8
DBP oil absorption: 33ml / 100g
Abrasive material α-Al ₂ O ₃ 15 parts (manufactured by Sumitomo Chemical Co., Ltd., trade name: AKP-30, average particle size = 0.4 μm)
Carbon black 20 parts DBP oil absorption: 120ml / 100g
pH: 8
BET specific surface area (S _BET ): 250 m ² / g
Volatile content: 1.5%
Vinyl chloride copolymer 10 parts Polyurethane resin 8 parts Branched side chain-containing polyester polyol / diphenylmethane diisocyanate,
Hydrophilic polar group: —SO ₃ Na = 70 eq / ton-containing phenylphosphonic acid 3 parts α-Al ₂ O ₃ (average particle size 0.2 μm) 1 part cyclohexanone 140 parts methyl ethyl ketone 170 parts butyl stearate 2 parts stearic acid 1 part

  The components for forming the magnetic layer and nonmagnetic layer coating solutions were kneaded with an open kneader and then dispersed using a sand mill. 6 parts of trifunctional low molecular weight polyisocyanate compound (Coronate 3041 made by Nippon Polyurethane) and 40 parts of methyl ethyl ketone were added to each of the obtained dispersions, and further stirred and mixed for 20 minutes, and then a filter having an average pore size of 1 μm was used. Then, a magnetic layer coating solution and a nonmagnetic layer coating solution were prepared.

3. Preparation of back layer coating liquid 1
Carbon black A Average particle size 40 nm 100 parts Nitrocellulose RS1 / 2 50 parts Polyurethane resin 40 parts (glass transition temperature: 50 ° C.)
Dispersant Copper Oleate 5 parts
Copper phthalocyanine 5 parts
Precipitated barium sulfate 5 parts Methyl ethyl ketone 500 parts Toluene 500 parts

  The kneaded material 1 was pre-kneaded with a roll mill.

Kneaded product 2
Carbon black B 100 parts Average particle size 270 nm, S _BET 8.5 m ² / g
DBP oil absorption 36ml / 100g, pH10
Nitrocellulose RS1 / 2 40 parts Polyurethane resin 10 parts Methyl ethyl ketone 300 parts Toluene 300 parts

  The kneaded materials 1 and 2 were dispersed with a sand grinder, and after completion, the following was added.

Polyester resin 5 parts Polyisocyanate 5 parts

  In addition, the back layer coating solution was prepared.

4). Production of Magnetic Tape The obtained magnetic layer coating solution and nonmagnetic layer coating solution were mixed with a polyethylene naphthalate (PEN) support (thickness: 6 μm, length (MD) Young's modulus 7.84 GPa (800 kg / mm ² ), Young's modulus in the width (TD) direction of 7.35 GPa (750 kg / mm ² ), center line average surface roughness Ra of the magnetic layer coating surface: 2 nm (cutoff value: 0.25 mm)), and nonmagnetic layer The magnetic layers were coated in the same order so that the thickness after drying was 1.2 μm and 0.15 μm, respectively. Next, while the magnetic layer was still wet, orientation treatment was performed using a cobalt magnet having a magnetic flux density of 300 mT (3000 gauss) and a solenoid having a magnetic flux density of 150 mT (1500 gauss). Thereafter, the magnetic layer was formed by drying. Thereafter, the back layer coating solution was applied to the other side of the support (the side opposite to the magnetic layer) so that the thickness after drying was 0.5 μm, and dried to form a back layer. A magnetic recording laminate roll having a magnetic layer on one side of the support and a back layer on the other side was obtained. The above roll was subjected to a surface smoothing treatment at 245 kN / m (250 kg / cm) and a temperature of 80 ° C. with a seven-stage calendar composed only of metal rolls, followed by a heat treatment at 70 ° C. for 48 hours. Slit to inch width. Thereafter, the cartridge was wound into a cartridge to produce a magnetic tape according to the present invention, and the following evaluation was performed.

<Measurement of cross-sectional area (E) / (T)>
As defined above, the cross-sectional area (E) / (T) was determined.

<Initial error rate (ER)>
Output during driving: A magnetic tape was set in an LTO2 drive manufactured by IBM under an environment of 13 ° C. and 15% RH, and the initial error rate was examined.

<Error rate (ER) during driving at 300 ffp>
The error rate (ER) at the time of driving at 300 ffp was examined under the same conditions as the initial error rate (ER).

  The results are shown in Table 1.

(Example 2)
In Example 1, Example 1 was repeated except that the calendar temperature was changed to 90 ° C.

(Example 3)
In Example 1, Example 1 was repeated except that the average particle diameter of the abrasive in the nonmagnetic layer coating solution was changed to 0.3 μm and the thickness of the magnetic layer was changed to 0.1 μm.

Example 4
In Example 1, Example 1 was repeated except that the thickness of the magnetic layer was changed to 0.08 μm.

(Comparative Example 1)
In Example 1, except that 10 parts of α-Al ₂ O ₃ having an average particle diameter of 0.2 μm was added as an abrasive to the magnetic layer coating solution, and no abrasive was added to the nonmagnetic layer coating solution. Repeated Example 1.

(Comparative Example 2)
In Example 1, Example 1 was repeated except that 10 parts of α-Al ₂ O ₃ having an average particle diameter of 0.2 μm was added as an abrasive to the magnetic layer coating solution.

(Comparative Example 3)
In Example 1, 10 parts of α-Al ₂ O ₃ having an average particle diameter of 0.25 μm was added as an abrasive to the magnetic layer coating solution and the thickness of the magnetic layer was changed to 0.2 μm. Example 1 was repeated except that no abrasive was added to the coating solution.

  From Table 1, the magnetic recording medium of the example of the present invention in which the cross-sectional area (E) / (T) of the abrasive is 50% or more suppresses the increase in error rate as compared with the comparative example, and also runs It turns out that it is excellent also in durability.

It is sectional drawing of the magnetic recording medium for demonstrating how to obtain | require the cross-sectional area of the abrasives in this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Magnetic recording medium 11 Support body 12 Nonmagnetic layer 13 Magnetic layer A, B, C Abrasive material

Claims (3)

A magnetic recording medium in which a nonmagnetic layer containing a nonmagnetic powder and a binder is provided on a support, and a magnetic layer containing a ferromagnetic powder and a binder is provided on the nonmagnetic layer.
When the magnetic recording medium is cut in a direction perpendicular to the surface and parallel to the thickness direction, the cross-sectional area of the abrasive contained in the magnetic recording medium measured by the following steps (i) to (iii) Is 50% or more, a magnetic recording medium characterized by:
(I) Observe the cut surface, detect the abrasive that exists across the nonmagnetic layer and the magnetic layer, and determine the total cross-sectional area (T) of the abrasive.
(Ii) The cross-sectional area (E) of the portion of the abrasive detected in the step (i) buried in the nonmagnetic layer is obtained.
(Iii) The ratio of the cross-sectional area (E) obtained in the step (ii) to the total cross-sectional area (T) obtained in the step (i) is obtained as a percentage.   The magnetic recording medium according to claim 1, wherein the magnetic layer has a thickness of 0.03 to 0.15 μm.   3. The magnetic recording medium according to claim 1, wherein the nonmagnetic layer and the magnetic layer are provided on one surface of the support, and a back layer is provided on the other surface of the support.

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