JP2001216631A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic recording medium which can decrease the medium noise and show a high C/N even for high density magnetic recording. SOLUTION: The magnetic medium is produced by forming a nonmagnetic layer containing hematite and a binder and a magnetic layer containing a magnetic powder material and a binder in this order on a nonmagnetic supporting body. The hematite has <=0.05 μm major axial length, 10 at.% to 40 at.% Al content and 80 m2/g to 150 m2/g SBET. The surface roughness Ra of the magnetic layer is <=3.5 nm.

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 which exhibits high output and good C / N ratio in high density recording and is suitable for recording and reproducing digital signals at high density.

[0002]

2. Description of the Related Art In recent years, the recording wavelength tends to be shortened with the increase in density, and the problem of self-demagnetization loss at the time of recording and thickness loss at the time of reproduction, in which the output is reduced when the magnetic layer is thick, is increasing. For this reason, the thickness of the magnetic layer has been reduced. However, when a magnetic layer of 2 μm or less is directly applied to the support, the influence of the nonmagnetic support tends to appear on the surface of the magnetic layer. Tend to worsen. As one means for solving this problem, as described in JP-A-63-191315 and JP-A-63-187418, a nonmagnetic layer is formed on a support by using a simultaneous multilayer coating method. There is a method of applying a magnetic coating solution having a high concentration thinly by providing a magnetic layer in this order. According to these inventions, the yield is dramatically improved, and good electromagnetic conversion characteristics can be obtained.

In a consumer digital VCR system, a metal thin film-deposited tape, a so-called ME tape, has been put to practical use. However, in order to manufacture a coating type magnetic recording table, a so-called MP table, compatible with a consumer digital VCR system, it is necessary to ensure that the data signal has a high output and that the tracking signal output is maintained while the overwrite is performed. There is a need to improve erasing characteristics. The present inventors have proposed a coating type magnetic recording table that satisfies the above-mentioned requirements by increasing the degree of filling of the magnetic layer while reducing the thickness of the magnetic layer (Japanese Patent Application No. Hei 10-1998).
No. 1760).

[0004]

As described above, in the coating type magnetic recording table, good electromagnetic conversion characteristics can be maintained by devising the configuration of the nonmagnetic layer in addition to devising the configuration of the magnetic layer. Attempts have also been made to reduce the thickness. For example,
By adding needle-like hematite having a predetermined major axis length to the non-magnetic layer, the surface smoothness of the magnetic recording medium is improved, and as a result, the C / N is improved (Japanese Patent Application Laid-Open No. 9-1997).
-170003, JP-A-10-198948, JP-A-10-269553). However, in the magnetic recording medium described in the above publication, high C / N cannot be obtained and medium noise 低 減 cannot be reduced in high-density magnetic recording as employed in a system using MR head ゛. .

Accordingly, an object of the present invention is to provide a magnetic recording medium capable of reducing the medium noise 、 and exhibiting a high C / N even in high-density magnetic recording as employed in a system using the MR head ゛. Is to provide.

[0006]

According to the present invention, there is provided a magnetic recording medium wherein at least one of a nonmagnetic support is provided with a nonmagnetic layer containing hematite and a binder and a magnetic layer containing magnetic powder and a binder in this order. The hematite has a major axis length of 0.05 μm.
Below, the Al content is 10at% to 40at%, SBET is 80
m ² / g to 150 m ² / g, and the surface roughness Ra of the magnetic layer
Is 3.5 nm or less. Further, in the magnetic recording medium of the present invention, the standard deviation of the thickness of the magnetic layer is preferably 45 nm or less.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION In the magnetic recording medium of the present invention, the hematite contained in the non-magnetic layer has a major axis length of 0.05 μm or less, an Al content of 10 at% to 40 at%, and 80 m ² / g to 150 at%. m ² / g
One of the features is to have the SBET. The higher the SBET of hematite contained in the nonmagnetic layer, the lower the noise tends to be. The hematite SBET tends to increase as the major axis length decreases. Therefore, in the present invention, the major axis length of hematite contained in the nonmagnetic layer is set to 0.05 μm or less. If the major axis length of the hematite is too small, the dispersibility of the hematite is poor and the surface becomes rough. Therefore, it is preferably in the range of 0.01 to 0.05 μm.

As described above, S of hematite contained in the non-magnetic layer
As the BET increases, the noise 傾向 tends to be reduced. However, when the SBET is increased, the interface is easily disturbed, and as a result, the surface becomes rough and the output tends to decrease. Therefore, the Al content in the hematite is controlled for the purpose of suppressing the disturbance of the interface caused by increasing the SBET. As the Al content in hematite increases, the turbulence at the interface tends to be suppressed.
10at% or more. However, if the Al content exceeds 40 at%, the surface tends to be disordered, so the Al content is set to 40 at% or less. From the above, the Al content in hematite is in the range of 10 to 40 at%.

[0009] SBET hematite contained in the nonmagnetic layer, and 80 m ² / g or more because they tend be higher Neuss゛Wo reduced. However, if the SBET exceeds 150 m ² / g, there is a problem that the surface becomes rough. Therefore, the SBET is set to 150 m ² / g or less. That is, from the viewpoint that the interface between the magnetic layer and the nonmagnetic layer is smooth and the surface of the magnetic layer is smooth, the SBET of hematite is 80 to 1
It is preferably in the range of 50 m ² / g.

Hematite having the above-mentioned major axis length, Al content and SBET can be produced with reference to the methods described in JP-A-10-269553 and JP-A-10-198948.

The content of the hematite in the nonmagnetic layer can be appropriately determined in consideration of the thixotropic property and the like, and can be, for example, in the range of 50 to 100% based on the whole hematite.

When the surface roughness Ra of the magnetic layer is set to 3.5 nm or less, the output becomes high and the noise ゛ can be reduced. Ra of the magnetic layer is preferably 1.0 to 3.5 nm.

In the magnetic recording medium of the present invention, the standard deviation of the thickness of the magnetic layer is preferably 45 nm or less in order to reduce noise. Preferably the standard deviation of the thickness of the magnetic layer is 5 to 45 n
m. In order to make the standard deviation of the thickness of the magnetic layer 45 nm or less, it is effective to match the thixotropic properties of the magnetic layer liquid and the non-magnetic layer liquid. For that purpose, it is effective means to increase the SBET of hematite contained in the non-magnetic layer.However, if the SBET is too high, the dispersibility of hematite decreases, so that the SBET of hematite is 80 to 150 m ² / g. It is preferable that

Hereinafter, the magnetic recording medium of the present invention will be described in more detail. The ferromagnetic metal powder contained in the magnetic layer of the present invention is not particularly limited, but is preferably Fe or an alloy containing Fe as a main component. These ferromagnetic metal powders include Al, Mg, Si, S, Sc, C
a, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, A
g, Sn, Sb, Te, Ba, Ta, W, Re, Au,
Hg, Pb, Bi, La, Ce, Pr, Nd, P, C
It may contain atoms such as o, Mn, Zn, Ni, Sr, and B. In particular, Al, Mg, Si, Ca, Y, Ba,
It is preferable that at least one of La, Nd, Co, Ni, and B is contained other than Fe.

These ferromagnetic metal powders may be treated in advance with a dispersant, a lubricant, a surfactant, an antistatic agent and the like, which will be described later, before dispersion. Specifically, JP-B-44-14090, JP-B-45-18372, JP-B-47-22062,
JP-B-47-22513, JP-B-46-28466, JP-B-46-38755
No., JP-B-47-4286, JP-B-47-12422, JP-B-47-1
7284, JP-B-47-18509, JP-B-47-18573, JP-B
39-10307, JP-B-48-39639, U.S. Patent No. 3026215,
No. 3031341, No. 3100194, No. 3242005, No. 3389014 and the like.

The ferromagnetic metal powder may contain a small amount of hydroxide or oxide. A ferromagnetic metal powder obtained by a known production method can be used, and the following method can be used. A method of reducing a complex organic acid salt (mainly oxalate) with a reducing gas such as hydrogen, or reducing iron oxide with a reducing gas such as hydrogen to obtain Fe or Fe
-A method of obtaining Co particles and the like, a method of thermally decomposing a metal carbonyl compound, a method of reducing by adding a reducing agent such as sodium borohydride, hypophosphite or hydrazine to an aqueous solution of a ferromagnetic metal, and a method of reducing the pressure of a metal at a low pressure. And evaporating in an inert gas to obtain fine powder. The ferromagnetic metal powder thus obtained is subjected to a known slow oxidation treatment, that is, a method of immersing in an organic solvent and then drying, immersing in an organic solvent and then feeding an oxygen-containing gas to form an oxide film on the surface and drying. Any of the methods of forming an oxide film on the surface by adjusting the partial pressure of oxygen gas and inert gas without using an organic solvent can be used.

The specific surface area of the ferromagnetic metal powder used in the magnetic layer of the present invention by the BET method is preferably 30 to 50.
selected from m ² / g. This makes it possible to achieve both good surface properties and low noise.

The shape of the ferromagnetic metal powder is preferably acicular, particularly flat acicular, but granular, rice grain, and plate shapes are acceptable. The average major axis length of the ferromagnetic metal powder is preferably 0.05 to 0.15 μm, more preferably 0.08 to
0.12 μm. The length of the major axis is determined by taking a transmission electron microscope photograph and directly reading the short axis length and the long axis length of the ferromagnetic powder from the photograph, and using a transmission electron microscope photograph by an image analyzer Carl Zeiss IBASSI. It can be obtained by appropriately using the method of tracing and reading. The needle ratio of the ferromagnetic metal powder is preferably 4 or more and 18 or less, more preferably 5 or more and 12 or less. The water content of the ferromagnetic metal powder is preferably 0.01 to 2%. It is preferable to optimize the water content of the ferromagnetic metal powder depending on the type of the binder.

It is preferable that the pH of the ferromagnetic metal powder is optimized by a combination with the binder used. The range is from 4 to 12, but preferably from 7 to 10. If necessary, the ferromagnetic metal powder may have Al, Si, P, or an oxide thereof on the surface. The amount thereof is 0.1 to 10% by mass based on the ferromagnetic metal powder.
m ² or less, which is preferable. The ferromagnetic metal powder may contain inorganic ions such as soluble Na, Ca, Fe, Ni, and Sr, but if it is 200 ppm or less, there is little influence on the characteristics. The ferromagnetic metal powder used in the present invention preferably has a small number of vacancies, and its value is preferably 20% by volume or less, more preferably 5% by volume or less.

The thermoplastic resin which can be used as a binder in the magnetic layer and the non-magnetic layer includes a glass transition temperature of -1.
00-150 ° C, number average molecular weight 1,000-200,
000, preferably 10,000 to 100,000, and a degree of polymerization of about 50 to 1,000. Such examples include vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic esters,
Vinylidene chloride, acrylonitrile, methacrylic acid, methacrylic acid ester, styrene, butadiene, ethylene, vinyl butyral, vinyl acetal, vinyl ether
Polymers or copolymers containing, as constituent units,
There are polyurethane resins and various rubber resins.

The thermosetting resin or the reactive resin includes a phenol resin, an epoxy resin, a polyurethane curable resin, a urea resin, a melamine resin, an alkyd resin, an acrylic reaction resin, a formaldehyde resin, a silicone resin, and the like. Examples thereof include epoxy-polyamide resin, a mixture of polyester resin and isocyanate prepolymer, a mixture of polyester polyol and polyisocyanate, and a mixture of polyurethane and polyisocyanate. These resins are described in detail in "Plastic Handbook" (published by Asakura Shoten). Further, a known electron beam-curable resin can be used for the non-magnetic layer or the magnetic layer.

These examples and the production method thereof are described in detail in JP-A-62-256219. The above resins can be used alone or in combination, but preferred are vinyl chloride resin, vinyl chloride vinyl acetate resin,
Examples thereof include a combination of at least one selected from vinyl chloride vinyl acetate vinyl alcohol resin and vinyl chloride vinyl acetate maleic anhydride copolymer with a polyurethane resin, or a combination of these with a polyisocyanate. As the structure of the polyurethane resin, known materials such as polyester polyurethane, polyether polyurethane, polyether polyester polyurethane, polycarbonate polyurethane, polyester polycarbonate polyurethane, polycaprolactone polyurethane, and polyolefin polyurethane can be used. In particular, a polyurethane composed of the above-mentioned short-chain diol having a cyclic structure and a long-chain diol containing an ether group is preferable. In order to obtain better dispersibility and durability for all binders shown here, -COOM, -SO
_{3 M, -OSO 3 M, -P} = O (OM) 2, -O-P = O
(OM) ₂ , (M is a hydrogen atom or an alkali metal base), —OH, —NR ₂ , —N ⁺ R ₃ (R is a hydrocarbon group), epoxy group, —SH, —CN, sulfobetaine It is preferable to use one in which at least one or more polar groups selected from phosphobetaine, carboxybetaine and the like are introduced by a copolymerization or addition reaction. The amount of such a polar group is 10 ^-1 to 10 ^-8 mol / g, preferably 10 ^-2 to 10 ^-6 mol / g.

Specific examples of these binders used in the present invention include VAG manufactured by Union Carbide.
H, VYHH, VMCH, VAGF, VAGD, VRO
H, VYES, VYNC, VMCC, XYHL, XYS
G, PKHH, PKHJ, PKHC, PKFE, Nissin Chemical Industries' MPR-TA, MPR-TA5, MPR-
TAL, MPR-TSN, MPR-TMF, MPR-T
S, MPR-TM, MPR-TAO, manufactured by Denki Kagaku 1
000W, DX80, DX81, DX82, DX83,
100FD, ZEON Corporation MR-104, MR-1
05, MR110, MR100, 400X-110A,
Nipporan N2301, N23 manufactured by Nippon Polyurethane Co.
02, N2304, Pandex T manufactured by Dainippon Ink and Chemicals, Inc.
-5105, T-R3080, T-5201, Burnock D-400, D-210-80, Crisbon 610
9,7209, manufactured by Toyobo Co. Byron UR8200, U
R8300, RV530, RV280, manufactured by Dainichi Seika
Diferamine 4020, 5020, 5100, 530
0, 9020, 9022, 7020, Mitsubishi Kasei M
X5004, Sanyo Chemical Co., Ltd. sampler SP-150,
TIM-3003, Saran F310, F2 manufactured by Asahi Kasei Corporation
10 and the like. Among them, MR-104, MR1
10, UR-8200, UR8300, UR-870
0, and a reaction product containing a diol and an organic diisocyanate as main raw materials, and a polyurethane having a cyclic structure and an ether group is preferable.

In the present invention, when a polyurethane resin is used, the elongation at break is 100-2,000%, the stress at break is 0.05-10 kg / cm ² , and the yield point is 0.05-1.
It is preferable to use one having 0 kg / cm ² .

The polyisocyanate used in the present invention includes tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, naphthylene-1,
Isocyanates such as 5-diisocyanate, o-toluidine diisocyanate, isophorone diisocyanate, and triphenylmethane triisocyanate, products of these isocyanates and polyalcohols, and polyisocyanates formed by condensation of isocyanates are preferred. . Commercially available trade names of these isocyanates include Coronate L, Coronate HL, and Coronate 203 manufactured by Nippon Polyurethane Co.
0, Coronate 2031, Millionate MR, Millionate MTL, Takeda Pharmaceutical Takenate D-102, Takenate D-110N, Takenate D-200, Takenate D-202, Sumitomo Bayer Death Module L, Death Module IL, Death Module N , Death Module HL, and the like, and these can be used alone or in combination of two or more using the difference in curing reactivity as the nonmagnetic layer and the magnetic layer. These polyisocyanates are usually used for all binder resins in the magnetic layer.
0 to 50% by mass, preferably 0 to 30% by mass,
It is usually used in an amount of 0 to 40% by mass, preferably 0 to 25% by mass, based on all binder resins of the nonmagnetic layer.

When the magnetic recording medium of the present invention comprises two or more layers, the amount of the binder resin, the amount of the vinyl chloride resin, the polyurethane resin, the polyisocyanate or the other resin in the binder, and the formation of the magnetic layer It is of course possible to change the molecular weight and polar group content of each resin to be formed or the physical properties of the above-mentioned resin between the non-magnetic layer and the magnetic layer as required, and a known technique relating to a multilayer magnetic layer can be applied.

In the non-magnetic layer of the present invention, a non-magnetic inorganic powder other than hematite can be used as long as the effect of using hematite is not impaired. As such an inorganic powder, for example,
Inorganic compounds such as metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, metal sulfides and the like can be mentioned. Further, by mixing carbon black in the non-magnetic layer of the present invention, it is possible to lower Rs and reduce light transmittance, which are known effects, and to obtain a desired micro-Vickers hardness. .

The non-magnetic layer micro-Vickers hardness is usually
25-60 kg / mm ² , preferably 30-50 kg /
mm ² and measured with 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 using an NEC thin film hardness meter HMA-400. The light transmittance is generally such that the absorption of infrared rays having a wavelength of about 900 nm is 3% or less,
For example, in VHS, it is standardized that it is 0.8% or less. For this purpose, a furnace for rubber and a rubber
Black, color black, acetylene black, and the like can be used.

The carbon black used in the non-magnetic layer
Specific surface area is usually 100-500m ^Two/ G, preferably
150-400m^Two/ G, DBP oil absorption is usually 20 to
400 ml / 100 g, preferably 30-200 ml / 100 g
You. The particle size of carbon black is usually 5 μm to 80 μm.
m, preferably 10 to 50 μm, more preferably 10 to
40 μm. Usually, the pH of carbon black is 2
-10, water content 0.1-10%, tap density 0.1
１1 g / ml is preferred. Carbon used in the present invention
A specific example of black is BLA manufactured by Cabot Corporation
CK PEARLS 2000, 1300, 1000,
900, 800, 880, 700, VULCAN XC
-72, Mitsubishi Chemical Corporation # 3050B, 3150B, 3
250B, # 3750B, # 3950B, # 950, #
650B, # 970B, # 850B, MA-600,
CONDUCTEX SC manufactured by Rombian Carbon,
RAVE 8800,8000,7000,5750,5250,3500,2100,200
0,1800,1500,1255,1250, Akzo Ketchum
Rack EC and the like. Carbon black as dispersant
Surface treated with etc. or grafted with resin for use
Even if a part of the surface is graphitized,
I do not care. Before adding carbon black to paint
May be dispersed in advance with a binder. these
Of carbon black is 50
Over 40% of the total mass of the non-magnetic layer
Can be used in a range that does not exist. These carbon blacks are simply
Can be used alone or in combination.

The carbon black that can be used in the nonmagnetic layer can be referred to, for example, “Carbon Black Handbook” (edited by Carbon Black Association). Further, an organic powder can be added to the non-magnetic layer according to the purpose. For example, acrylic styrene resin powder, benzoguanamine resin powder,
Melamine-based resin powders and phthalocyanine-based pigments are exemplified, but polyolefin-based resin powders, polyester-based resin powders, polyamide-based resin powders, polyimide-based resin powders, and polyfluoroethylene resins can also be used. The manufacturing method is
Those described in JP-A-62-18564 and JP-A-60-255827 can be used.

The undercoat layer is provided on a general magnetic recording medium. The undercoat layer is provided to improve the adhesive strength between the support and the magnetic layer or the non-magnetic layer. Soluble polyester is used. The thickness is generally 0.5 μm or less. The binder resin, lubricant, dispersant, additive, solvent, dispersing method, etc. of the non-magnetic layer can be applied to those of the magnetic layer. In particular, with regard to the amount and type of the binder resin, the amount of the additive and the type of the dispersant, and the type of the dispersant, a known technique for the magnetic layer can be applied.

As the carbon black used in the magnetic layer of the present invention, those exemplified for the nonmagnetic layer can be applied. The carbon black may be dispersed in a binder before being added to the magnetic paint. These carbon blacks can be used alone or in combination. When carbon black is used, it is preferably used in an amount of 0.1 to 10% by mass, preferably 0.1 to 3% by mass, more preferably 0.5 to 1.5% by mass based on the ferromagnetic metal powder. . Carbon black has functions such as antistaticity of the magnetic layer, reduction of friction coefficient, provision of light-shielding properties, and improvement of film strength, and these differ depending on the carbon black used. Therefore, these carbon blacks used in the present invention are magnetic layers,
Change the type, amount and combination in the non-magnetic layer,
Of course, it is possible to use differently according to the purpose based on the above-mentioned various characteristics such as oil absorption, conductivity, pH and the like. Examples of the carbon black that can be used in the magnetic layer of the present invention include “Carbon Black Handbook” (edited by Carbon Black Association).
Can be referred to.

The abrasive used in the present invention includes α-alumina, β-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum, artificial diamond, silicon nitride, carbonized carbon having an α conversion of 90% or more. Known materials mainly having a Mohs hardness of 6 or more such as silicon, titanium carbide, titanium oxide, silicon dioxide, and boron nitride are used alone or in combination. Further, a composite of these abrasives (abrasive whose surface is treated with another abrasive) may be used. These abrasives may contain compounds or elements other than the main component, but the effect remains the same if the main component is 90% or more. The particle size of these abrasives is preferably from 0.01 to 2 μm. However, if necessary, abrasives having different particle sizes can be combined, or even a single abrasive can have the same effect by broadening the particle size distribution. . Tap density 0.3-2g / cc, water content 0.1
５5%, pH 2-11, specific surface area 1-30 m ² /
g is preferred.

The abrasive used in the present invention has a needle shape,
Any of a spherical shape and a dice shape may be used, but those having a corner in a part of the shape are preferable because of high abrasiveness. Specific examples of the abrasive used in the present invention include A manufactured by Sumitomo Chemical Co., Ltd.
KP-20, AKP-30, AKP-50, HIT-5
0, HIT-60, HIT-70, HIT-80, HI
T-80G, HIT-100, Nippon Kagaku Kogyo G
5, G7, S-1, TF-100, TF manufactured by Toda Kogyo
-140 and the like. The abrasive used in the present invention can be of different types, amounts, and combinations depending on the purpose of the non-magnetic layer and the magnetic layer. These abrasives may be added to the magnetic paint after dispersion treatment with a binder in advance. The abrasive present on the magnetic layer surface and the magnetic layer end face of the magnetic recording medium of the present invention is 5
Pcs / 100 μm ² or more is preferable.

As the additives used in the present invention, those having a lubricating effect, an antistatic effect, a dispersing effect, a plasticizing effect and the like are used. Molybdenum disulfide, tungsten graphite disulfide, boron nitride, graphite fluoride, silicone oil, silicone with polar groups, fatty acid-modified silicone, fluorine-containing silicone, fluorine-containing alcohol, fluorine-containing ester, polyolefin, polyglycol,
Alkyl phosphates and their alkali metal salts, alkyl sulfates and their alkali metal salts, polyphenyl ethers, fluorine-containing alkyl sulfates and their alkali metal salts, monobasic fatty acids having 10 to 24 carbon atoms (including unsaturated bonds) And may be branched), and their metal salts (Li, Na, K, Cu
Or monovalent, divalent, trivalent, having 12 to 22 carbon atoms,
Tetravalent, pentavalent, hexavalent alcohols (including unsaturated bonds,
It may be branched), alkoxy alcohol having 12 to 22 carbon atoms (which may contain an unsaturated bond or may be branched), monobasic fatty acid having 10 to 24 carbon atoms (unsaturated bond) Or may be branched) and any one of monohydric, divalent, trivalent, tetravalent, pentavalent, and hexavalent alcohols having 2 to 12 carbon atoms (including an unsaturated bond) Or a branched fatty acid), a monofatty acid ester, a difatty acid ester or a trifatty acid ester, a fatty acid ester of a monoalkyl ether of an alkylene oxide polymer, a fatty acid amide having 8 to 22 carbon atoms, and a fatty acid amide having 8 to 22 carbon atoms. 22, aliphatic amines, and the like.

Specific examples thereof include lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, butyl stearate, oleic acid, linoleic acid, linolenic acid, elaidic acid, octyl stearate, amyl stearate, and stearic acid. Isooctyl, octyl myristate, butoxyethyl stearate, anhydrosorbitan monostearate, anhydrosorbitan distearate, anhydrosorbitan tristearate, oleyl alcohol, lauryl alcohol. Also, nonionic surfactants such as alkylene oxides, glycerin, glycidol, alkylphenol ethylene oxide adducts, cyclic amines, ester amides, quaternary ammonium salts, hydantoin derivatives, heterocycles, phosphonium or sulfoniums, etc. Cationic surfactants, carboxylic acids, sulfonic acids,
Anionic surfactants containing acidic groups such as phosphoric acid, sulfate ester groups, phosphate ester groups, etc., amphoteric surfactants such as amino acids, aminosulfonic acids, sulfuric acid or phosphate esters of amino alcohols, alkylbedine type, etc. Can be used. These surfactants are described in detail in "Surfactant Handbook" (published by Sangyo Tosho Co., Ltd.). These lubricants, antistatic agents, etc. are not necessarily 100%
It is not pure and may contain impurities such as isomers, unreacted materials, by-products, decomposition products, oxides, etc. in addition to the main components.
The content of these impurities is preferably 30% by mass or less,
More preferably, the content is 10% by mass or less.

The types and amounts of these lubricants and surfactants used in the present invention can be properly used in the non-magnetic layer and the magnetic layer as required. For example, to control bleeding to the surface using fatty acids having different melting points in the non-magnetic layer and magnetic layer, to control bleeding to the surface using esters having different boiling points and polarities, and to adjust the amount of surfactant. It can be considered to improve the stability of coating and improve the lubricating effect by increasing the amount of the lubricant added to the non-magnetic layer. Of course, the present invention is not limited to the examples shown here.

Further, all or a part of the additives used in the present invention may be added at any step of the production of the magnetic paint. For example, when the additives are mixed with the ferromagnetic metal powder before the kneading step, the There are a case where it is added in a kneading step using a metal powder, a binder and a solvent, a case where it is added in a dispersion step, a case where it is added after dispersion, and a case where it is added just before coating. In some cases, the purpose may be achieved by applying a part or all of the additive simultaneously or sequentially after applying the magnetic coating layer according to the purpose. Depending on the purpose, a lubricant may be applied to the surface of the magnetic layer after calendering or after the slit is completed.

Examples of commercial products of these lubricants used in the present invention include NAA-102 and NAA-4 manufactured by NOF Corporation.
15, NAA-312, NAA-160, NAA-18
0, NAA-174, NAA-175, NAA-22
2, NAA-34, NAA-35, NAA-171, N
AA-122, NAA-142, NAA-160, NA
A-173K, castor hardened fatty acid, NAA-42, NA
A-44, Cation SA, Cation MA, Cation A
B, cation BB, Nimeen L-201, Nimeen L-202, Nimeen S-202, Nonion E-20
8, Nonion P-208, Nonion S-207, Nonion K-204, Nonion NS-202, Nonion NS-
210, nonion HS-206, nonion L-2, nonion S-2, nonion S-4, nonion O-2, nonion LP-20R, nonion PP-40R, nonion SP
-60R, Nonion OP-80R, Nonion OP-85
R, Nonion LT-221, Nonion ST-221, Nonion OT-221, Monogly MB, Nonion DS-6
0, Anone BF, Anone LG, butyl stearate, butyl laurate, erucic acid, oleic acid manufactured by Kanto Chemical Co., Ltd., FAL-205, FAL-123 manufactured by Takemoto Yushi Co., Ltd.
Engelbu LO, Enjorbu IP manufactured by Shin Nippon Rika Co., Ltd.
M, Sansocizer-E4030, TA- made by Shin-Etsu Chemical Co., Ltd.
3, KF-96, KF-96L, KF96H, KF41
0, KF420, KF965, KF54, KF50, K
F56, KF907, KF851, X-22-819,
X-22-822, KF905, KF700, KF39
3, KF-857, KF-860, KF-865, X-
22-980, KF-101, KF-102, KF-1
03, X-22-3710, X-22-3715, KF
-910, KF-3935, Alumide P, Alumide C, Armoslip CP, manufactured by Lion Armor Co., Ltd., Duyomin TDO manufactured by Lion Yushi Co., Ltd., BA manufactured by Nisshin Oil Co., Ltd.
-41G, Sanyo Kasei's Profan 2012E, New Pole PE61, Ionnet MS-400, Ionnet MO-200, Ionnet DL-200, Ionnet D
S-300, Ionnet DS-1000, Ionnet D
O-200 and the like.

The organic solvent used in the present invention includes ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone, tetrahydrofuran, methanol, ethanol, propanol, butanol, isobutyl alcohol, isopropyl alcohol, methyl cyclohexane. Alcohols such as hexanol, esters such as methyl acetate, butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate, and glycol acetate; glycol ethers such as glycol dimethyl ether, glycol monoethyl ether and dioxane; benzene, toluene and xylene , Cresol, chlorobenzene, etc., aromatic hydrocarbons, methylene chloride, ethylene chloride, carbon tetrachloride, chloroform Ethylene chlorohydrin, dichlorobenzene, chlorinated hydrocarbons and the like, N, N- dimethylformamide, dimethylacetamide, hexane or the like can be used alone or in combination in any ratio a. These organic solvents are not necessarily 100% pure, and may contain impurities such as isomers, unreacted products, by-products, decomposed products, oxides, and moisture in addition to the main components. The content of these impurities is preferably 30% by mass or less, more preferably 10% by mass or less. The type of the organic solvent used in the present invention is preferably the same for the magnetic layer and the non-magnetic layer.
The addition amount may be changed. Use a solvent with a high surface tension (such as cyclohexanone or dioxane) for the non-magnetic layer to improve coating stability. Specifically, the arithmetic mean of the solvent composition of the magnetic layer should not be lower than the arithmetic mean of the solvent composition of the non-magnetic layer. Is essential. In order to improve the dispersibility, it is preferable that the polarity is somewhat strong.
It is preferable that 5 to 20 solvents are contained in an amount of 50% by mass or more. Further, the dissolution parameter is preferably from 8 to 11.

The thickness structure of the magnetic recording medium of the present invention is such that the support is 1 to 100 μm, and is particularly effective when a thin support of 1 to 8 μm is used. The total thickness of the magnetic layer and the nonmagnetic layer is preferably in the range of 1/100 to 2 times the thickness of the support. Further, it is preferable to provide an adhesive layer between the support and the non-magnetic layer for improving adhesion. The thickness of the adhesive layer is 0.01 to 2 μm, preferably 0.02 to 2 μm.
0.5 μm.

A back coat layer may be provided on the side of the support opposite to the magnetic layer side. This thickness is 0.1-2μ
m, preferably 0.3 to 1.0 μm. Known adhesive layers and back coat layers can be used. The support used in the present invention has a micro Vickers hardness of 7
Known films such as polyethylene naphthalate, polyamide, polyimide, polyamide imide, aromatic polyamide, polybenzoxydazole and the like having a viscosity of 5 kg / mm ² or more that have been biaxially stretched can be used. In particular, a support using an aromatic polyamide or polyethylene naphthalate available as "Aramid" manufactured by Toray Industries or "Aramica" manufactured by Asahi Kasei is preferred.

On these supports, a corona discharge treatment, a plasma treatment, an easy adhesion treatment, a heat treatment, a dust removal treatment,
And so on. In order to achieve the object of the present invention, the center line average surface roughness of the surface of the support on which the magnetic layer is coated must be 1
0 nm or less 0.1 nm or more, preferably 6 nm or less.
It is preferable to use those having a thickness of 2 nm or more, more preferably 4 nm or less and 0.5 nm or more. In addition, these supports not only have a small center line average surface roughness,
It is preferable that there is no coarse projection of 1 μm or more. The surface roughness shape can be freely controlled by the size and amount of the filler added to the support as needed. Examples of these fillers include Al and C.
Examples thereof include oxides and carbonates of a, Si, Ti and the like, regardless of whether they are crystalline or amorphous, and organic fine powders such as acrylic and melamine. Also, in order to achieve compatibility with running durability, it is preferable that the surface on which the back layer is applied is rougher than the surface on which the magnetic layer is applied. The center line surface roughness of the back layer coating surface is preferably 1 nm or more and 20 nm or less, more preferably 2 nm or more and 8 nm or less. When changing the roughness between the magnetic layer application surface and the back layer application surface, a dual-structured support may be used, or may be changed by providing a coating layer.

The F-5 value of the support used in the present invention in the tape running direction is preferably 10 to 50 kg / mm ² ,
The F-5 value in the tape width direction is preferably 10 to 30 kg.
/ Mm ² , and the F-5 value in the longitudinal direction of the tape is generally higher than the F-5 value in the tape width direction. However, this is not particularly necessary when the strength in the width direction needs to be particularly increased. . In addition, 10 in the tape running direction and the width direction of the support.
The heat shrinkage at 0 ° C. for 30 minutes is preferably 3% or less, more preferably 1.5% or less, and the heat shrinkage at 80 ° C. for 30 minutes is preferably 1% or less, more preferably 0.5% or less. is there. The breaking strength is 5-100 kg / mm ^{2 in} both directions,
The elastic modulus is preferably from 100 to 2,000 kg / mm ² . Further, the light transmittance at 900 nm in the present invention is preferably 30% or less, more preferably 3% or less.

The step of producing the magnetic paint of the magnetic recording medium of the present invention comprises at least a kneading step, a dispersing step, and a mixing step provided before and after these steps as necessary. Each step may be divided into two or more steps. Ferromagnetic metal powder used in the present invention, binder, carbon black, abrasive, antistatic agent, lubricant,
All raw materials such as a solvent may be added at the beginning or during any step. In addition, each raw material may be divided and added in two or more steps. For example, the polyurethane resin may be divided and supplied in the kneading step, the dispersing step, and the mixing step for adjusting the viscosity after dispersion. In order to achieve the object of the present invention, it is a matter of course that a conventionally known manufacturing technique can be used as a part of the process, but in the kneading step, a material having a strong kneading force such as a continuous kneader or a pressure kneader. Is preferable because high Br can be obtained. When a continuous kneader or a pressure kneader is used, the ferromagnetic metal powder and all or a part of the binder (preferably 30% by mass or more of the total binder) and 15 to 500 parts per 100 parts of the ferromagnetic metal powder are used.
Parts are kneaded. Details of these kneading processes are described in JP-A-1-166338 and JP-A-64-79.
274. When preparing a magnetic layer liquid, a non-magnetic layer liquid, or an abrasive dispersion liquid, it is desirable to use a dispersion medium having a high specific gravity, and zirconia beads are preferable.

In the present invention, the following configuration can be proposed as an example of an apparatus and a method for simultaneously applying a multi-layered magnetic recording medium in a multi-layered manner. 1. First, apply the non-magnetic layer coating layer using a gravure coating, roll coating, blade coating, extrusion coating device, etc., which are generally used in the application of magnetic paint, and then apply the non-magnetic layer coating layer in a wet state. The upper magnetic layer is coated by a support-pressing-type extrusion coating device disclosed in Japanese Patent Publication No. 46186/1984, Japanese Patent Application Laid-Open No. 60-238179, and Japanese Patent Application Laid-Open No. 2-265672.

2, JP-A-63-88080, JP-A-2-17971,
The upper non-magnetic layer is applied almost simultaneously by one coating head having two built-in coating liquid passage slits as disclosed in JP-A-2-265672. 3. The upper non-magnetic layer is applied almost simultaneously by an extrusion coating apparatus with a backup roll disclosed in JP-A-2-174965.

Incidentally, in order to prevent the electromagnetic conversion characteristics of the magnetic recording medium from deteriorating due to the aggregation of the magnetic particles, Japanese Patent Application Laid-Open No. Sho 62-951
It is desirable to apply shear to the coating liquid inside the coating head by a method as disclosed in JP-A No. 74 and JP-A-1-236968. Further, regarding the viscosity of the coating liquid,
It is preferable to satisfy the numerical range disclosed in No. 8471.

In order to obtain the magnetic recording medium of the present invention, it is preferable to perform strong orientation. In the case of a magnetic tape, the magnetic tape is oriented in the longitudinal direction, and a solenoid and a cobalt magnet of 100 mT (1,000 G) or more, preferably 300 mT (3,000 G) or more are 200 mT (2,0
00G) or more, preferably 400 mT (4,000 G)
As described above, it is more preferable to use a magnetic field of 600 mT (6,000 G) or more, and it is preferable to provide an appropriate drying step before orientation so that the orientation after drying is the highest. In the case of a floppy disk, random orientation is performed. The orientation conditions are as follows: after orientation in the longitudinal direction of the tape as in the case of the magnetic tape, for example, at a frequency of 50 Hz and a magnetic field strength of 25 mT
(250G) In addition, a magnetic field strength of 12 mT (1
20G), and are passed through an AC magnetic field generator having two magnetic field strengths or the like, and are randomly oriented.

It is also known to improve adhesion by providing an adhesive layer containing a polymer as a main component before applying a non-magnetic layer and a magnetic layer at the same time and applying corona discharge, ultraviolet (UV) irradiation, and electron beam irradiation. It is preferable to combine the above methods. Further, a heat-resistant plastic roll such as epoxy, polyimide, polyamide, or polyimide amide, or a metal roll is used as the calendering roll. Further, the treatment can be performed with metal rolls, plastic rolls or a pair of metal roll and plastic roll. The processing temperature is preferably 70 to
It is 120 ° C, more preferably 80 to 100 ° C or higher. The linear pressure is preferably 200 to 500 Kg / cm,
More preferably, it is 300 to 400 kg / cm or more.

The magnetic layer surface of the magnetic recording medium of the present invention and its magnetic layer
The coefficient of friction of SUS420J on the opposite side of
Is 0.1 to 0.5, more preferably 0.2 to 0.3
is there. The surface resistivity is preferably 10^Four-10¹²Ohm
/ Sq, the elastic modulus at 0.5% elongation of the magnetic layer is
100 to 2,000 kg / m, preferably in the width direction
m ^Two, Breaking strength is preferably 1 to 30 kg / cm^Two, Magnetic
The elastic modulus of the recording medium is preferably 1 in both the running direction and the width direction.
00 ~ 1,500Kg / mm^Two, The residual elongation is preferably
Heat shrink at any temperature below 0.5% and below 100 ° C
The rate is preferably 1% or less, more preferably 0.5% or less.
Below, most preferably 0.1% or less, 0% is ideal
is there. Glass transition temperature of magnetic layer (dynamic viscosity measured at 110 Hz
The maximum point of the loss modulus of elasticity) is 30 ° C or more and 150 ° C or more.
The following is preferable, and that of the nonmagnetic layer is preferably 0 ° C to 100 ° C.
Good. Loss modulus is 1 × 10^Three~ 8 × 10^FourN / cm^Two(1
× 10⁸~ 8 × 10⁹dyne / cm^TwoPreferably in the range
New If the loss tangent is too large, adhesive failure is likely to occur
The loss tangent is preferably 0.2 or less.

The residual solvent contained in the magnetic layer is preferably 100 mg / m ² or less, more preferably 10 mg / m ^2.
It is preferable that the residual solvent contained in the magnetic layer be smaller than the residual solvent contained in the nonmagnetic layer. The porosity of both the nonmagnetic layer and the magnetic layer is preferably 30% by volume or less, more preferably 20% by volume or less. The porosity is preferably small in order to achieve high output, but it may be better to secure a certain value depending on the purpose. For example, in a magnetic recording medium for data recording in which repetitive use is emphasized, a higher porosity is often preferable in running durability. From the viewpoint of improving the calendering moldability, the porosity of the nonmagnetic layer and the magnetic layer in the pre-calendering sheet is 20% by volume or more, preferably 40% by volume or more.

The magnetic properties of the magnetic layer of the magnetic recording medium of the present invention, that is, Hc and SFD, and further, Bm and Br, were measured using a vibrating sample magnetometer (VSM) unless otherwise specified, using a magnetic field of 796 kA / m (10 kOe). ) Means the value measured in the in-plane direction of the magnetic layer. In the case of a magnetic tape, in the tape running direction, Hc is as described above, and the squareness ratio (SQ) is usually 0.85 or more, preferably 0.85 to 0.9.
5 The squareness ratio in two directions perpendicular to the tape running direction, that is, the two squareness ratios in the direction parallel to the tape surface and perpendicular to the tape running direction and in the direction perpendicular to the tape surface, is the squareness ratio in the running direction. It is preferably 80% or less. The longitudinal remanence coercive force Hr is 143,280 A / m
(1800 Oe) or more 238,800 A / m (3000 Oe)
e) The following are preferred. Hc and Hr in the vertical direction are 79,
600 A / m (1000 Oe) or more, 398,000 A / m (5
000 Oe) or less.

The root-mean-square roughness RRMS of the magnetic layer obtained by evaluation with an atomic force microscope (AFM) is preferably in the range of 2 nm to 15 nm.

Although the magnetic recording medium of the present invention preferably has a non-magnetic layer and a magnetic layer, it is easily presumed that the physical properties of the non-magnetic layer and the magnetic layer can be changed according to the purpose. is there. For example, the elastic modulus of the magnetic layer is increased to improve running durability, and at the same time, the elastic modulus of the non-magnetic layer is made lower than that of the magnetic layer to improve the contact of the magnetic recording medium with the head. It is also effective in the present invention to improve the head contact by changing the method of tensifying the support, and the support tensified in a direction perpendicular to the longitudinal direction of the tape has a better head contact. Often.

[0056]

[Example 1]

Single magnetic layer Ferromagnetic metal fine powder 100 parts Composition Fe / Co = 70/30 Hc 179, 100 A / m (2250 Oe) Specific surface area by BET method 45 m ² / g Crystallite size 170 Å Surface treatment agent Al ₂ O ₃ , Y ₂ O ₃ particle size (major axis diameter) 0.09 .mu.m acicular ratio ^{8 σs: 150A · m 2 /} kg (150emu / g)

Vinyl chloride copolymer 10 parts MR-110 polyurethane resin manufactured by Zeon Corporation (UR-8200 manufactured by Toyobo) 6 parts α-Al ₂ O ₃ (average particle diameter 0.15 μm) 5 parts carbon black (average particle diameter 0.08) 0.5 part Butyl stearate 1 part Stearic acid 5 parts Methyl ethyl ketone 90 parts Cyclohexanone 30 parts Toluene 60 parts

The above-mentioned paint was kneaded with an open kneader, and then dispersed using a sand mill.
To the resulting dispersion, 5 parts of a polyisocyanate (Coronate L, manufactured by Nippon Polyurethane Co., Ltd.) was added, and a mixed solvent of methyl ethyl ketone and cyclohexanone 40 was added to each.
The resulting mixture was filtered using a filter having an average pore size of 1 μm to prepare a coating solution.

Lower coating layer (non-magnetic) Non-magnetic powder 80 parts Hematite described in Table 1 pH 9.0 Tap density 0.8 DBP oil absorption 27-38 / g / 100 g Surface treatment agent Al ₂ O ₃ , SiO ₂ Carbon black 20 parts Average primary particle diameter 16 μm DBP oil absorption 120 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 α-Al ₂ O ₃ containing a cyclic structure and a polyether group (average particle size 0.2 μm) 1 part Ethyl stearate 1 Part Stearic acid 1 part Ethyl ethyl ketone 100 parts Cyclohexanone 50 parts Toluene 50 parts

Each of the above-mentioned paints was kneaded with an open kneader, and then dispersed using a sand mill. 5 parts of a polyisocyanate (Coronate L manufactured by Nippon Polyurethane Co., Ltd.) was added to the obtained lower layer dispersion, and 40 parts of a mixed solvent of methyl ethyl ketone and cyclohexanone were added to each of the lower layer coating liquid.
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 thickness of the obtained lower coating layer coating solution after drying is 1.0 μm.
m, and immediately thereafter, a polyethylene naphthalate having a thickness of 5.2 μm and a center line surface roughness of the magnetic layer applied surface of 0.001 μm so that the thickness of the magnetic layer is 0.15 μm thereon. Simultaneous multi-layer coating on the support, 500mT (5000G) while both layers are still wet
After being oriented and dried by a cobalt magnet having a magnetic force of 400 mT (4000 G) and a solenoid having a magnetic force of 400 mT (4000 G), a 7-stage calender composed of a metal roll and an epoxy resin roll at a temperature shown in Table 1 at a speed of 200 m / min. Do the processing,
Thereafter, a back layer having a thickness of 0.5 μm was applied. The sample was slit to a width of 8 mm to prepare a sample.

1. Electromagnetic conversion characteristics: VTR on sample tape
7 MHz using (Fuji Photo Film: FJIX8)
Was recorded and reproduced. 7MH described in Comparative Example 12
The relative reproduction output of the tape and the noise ゛ when the reproduction output of z and the noise ゛ were set to 0 dB were measured. For C / N, the noise spectrum was measured using a spectrum analyzer manufactured by Shibasoku. The ratio of the output of the carrier signal of 7 MHz to the noise level separated by 1 MHz was defined as C / N. 2. Surface Ra (nm) Measured using 3d-MIRAU which is measured in a non-contact manner by an optical interference method. Specifically, Ra of an area of 250 nm square was measured using TOPO3D (trade name) manufactured by WYKO. The measurement wavelength was 650 nm, and spherical and cylindrical corrections were performed. 3. Standard Depth of Magnetic Layer Thickness An end face was cut out and photographed with a transmission electron microscope, and then the magnetic layer thickness was read with a tissue sizer to determine the standard deviation of the magnetic layer thickness. Table 1 shows the measurement results of the major axis length, Al content, SBET, calendar temperature, surface roughness Ra, and electromagnetic conversion characteristics of hematite used for each sample.

[0063]

[Table 1]

From Table 1, it is found that SBET is in the range of 80 to 150 m ² / g and Al content is in the range of 10 to 40 at% (Example 1).
8) It can be seen that C / N increases. SBET 80-150
m ² / g, and when the Al content is less than 10 at% (Comparative Examples 10 to 13), the interface between the magnetic layer and the non-magnetic layer becomes smooth, and the noise の due to the interface decreases, Due to the high SBET, the dispersibility of hematite deteriorates and the surface becomes rough. As a result, the output decreases and the noise caused by the surface increases, so that the C / N does not improve. From the above, it can be seen that Al contained in hematite improves the dispersibility of hematite. However, the Al content
When the content exceeds 40 at% (Comparative Example 14), Al cannot be uniformly contained in hematite, so that the dispersibility of hematite is poor and C / N is not improved. When Al was in the range of 10 to 40 at% and SBET was less than 80 m ² / g (Comparative Examples 17 to 20), the interface was not smooth, and the improvement in C / N was not sufficient. When SBET exceeds 150 m ² / g (Comparative Example 15,
16 and 21) show that the effect of improving hematite dispersibility by Al cannot suppress the deterioration of dispersibility due to an increase in SBET, and as a result,
The surface was rough and C / N did not improve. SBET 80-150 m ² /
g, and even when the Al content is in the range of 10 to 40 at%, the calender temperature is low and the surface roughness Ra of the magnetic layer is 3.
When the surface is not smooth beyond 5 nm (Comparative Examples 22 and 23), the C / N ratio is reduced due to the decrease in output and the influence of noise ゛ caused by the surface.
Did not improve.

[0065]

According to the present invention, a nonmagnetic layer containing hematite and binder and a magnetic layer containing magnetic powder and binder are provided in this order on at least one of the nonmagnetic supports.
5μm or less, Al content is 10at% -40at%, SBET
Is 80 m ² / g to 150 m ² / g, and according to the magnetic recording medium of the present invention in which the surface roughness Ra of the magnetic layer is 3.5 nm or less, it is adopted in a system using an MR head. Even in such high-density recording, medium noise can be reduced and a high C / N can be exhibited.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4G002 AA03 AA06 AD01 AE03 5D006 BA19 CA04 5E040 AA11 AA19 AB02 AB09 BB04 CA06 CA12 NN00 NN06

Claims (2)

[Claims] 1. A magnetic recording medium comprising: a nonmagnetic layer containing hematite and binder and a magnetic layer containing magnetic powder and binder on at least one of the nonmagnetic supports in this order;
The hematite has a major axis length of 0.05 μm or less and an Al content of 10
an at% ~40at%, SBET is ^{80m 2 / g ~150 m 2 /} g,
A magnetic recording medium, wherein the surface roughness Ra of the magnetic layer is 3.5 nm or less. 2. The magnetic recording medium according to claim 1, wherein the standard deviation of the thickness of the magnetic layer is 45 nm or less.