JPH03189925A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To obtain a recording medium having excellent electromagnetic conversion characteristics and high recording density by using ferromagnetic metal powder which has excellent magnetic characteristics and can be densely packed in a magnetic layer. CONSTITUTION:The ferromagnetic metal powder is an acicular one having the specific surface area SSA of 40-75 m<2>/g, 100-250 Angstrom crystallite measured by X-ray diffraction method and true density calculated by >= -0.015 X (SSA) + 6.4 (g/cm<3>). The true density of the powder can be increased by forming an oxide layer on the surface of the metallic magnetic powder particle and then treating the powder in a short time at temp. as high as >=1000 deg.C to completely crystallize the oxide layer and sintering preventive layer on the particle surface to form a dense surface layer. By forming the ferromagnetic metal powder in small grain size while varying the true density of the powder according to the specific surface area, the powder shows good dispersibility in a magnetic layer to increase the packing density, and the obtd. medium has good electromagnetic conversion characteristics and excellent storage durability.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a ferromagnetic metal powder suitable as a material for a magnetic recording medium for high-density recording, and in particular, it can be highly packed into a magnetic layer, and as a result, magnetic This invention relates to ferromagnetic metal powder that can achieve high output in recording media.

(Prior Art and its Problems) Ferromagnetic metal powders mainly composed of Fe, Co, Ni, etc. are used exclusively as materials for magnetic recording media for high-density recording because of their excellent magnetic properties.

In particular, by using a ferromagnetic metal powder with a small particle size, a magnetic recording medium with high electromagnetic conversion characteristics can be obtained.

Ferromagnetic metal powders are industrially synthesized by reducing acicular oxides with hydrogen. Constituent elements are removed from the oxide in the form of oxygen and water and it turns into a metal, so the volume changes drastically until it becomes a metal, and the resulting ferromagnetic metal powder has pores and is extremely bulky. It becomes a thing. Therefore, the degree of filling in the magnetic layer cannot be made very high, and in this respect there is a limit to the improvement of the electromagnetic conversion characteristics of a magnetic recording medium using ferromagnetic metal powder.

In addition, in order to prevent the rapid reaction of ferromagnetic metal powder with oxygen in the air, Japanese Patent Publication No. 35-3862, Japanese Patent Application Laid-Open No. 125205-1982, and Japanese Patent Application Laid-Open No. 59-27.371
Publication No. 1, JP-A-60-26602 and JP-A-6
It has been proposed to form an oxide layer on the surface of a ferromagnetic metal powder by various methods such as those disclosed in Japanese Patent No. 0-162708.

In the above method, an oxide layer is formed on the surface of the metal magnetic powder particles by bringing them into contact with oxygen under relatively mild conditions, so the structure of the resulting oxide film is insufficiently crystallized and the oxygen There was a problem in that atoms diffused inside the particles and deteriorated the magnetic properties of the ferromagnetic metal powder. In particular, as the particle size became smaller, the influence of the oxide layer on the particle surface was more likely.

(Problems to be Solved by the Invention) The present invention has been made in consideration of the problems of the prior art, and uses ferromagnetic metal powder that has excellent magnetic properties and can be highly filled in the magnetic layer. The purpose is to provide a magnetic recording medium with large electromagnetic conversion characteristics and excellent high-density recording performance.

(Means for Solving the Problems) An object of the present invention is to provide a magnetic recording medium having a magnetic layer mainly composed of ferromagnetic powder and binder resin, in which the ferromagnetic powder has a specific surface area determined by the BET method. (SSA)
is 40 to 75 (nf/gr), and the crystallite size determined by X-ray diffraction is 100 to 250 crystallites, and its true density (ρ) is ρ−0.015.
X(SSA)+6. This is achieved by a magnetic recording medium characterized by being made of acicular ferromagnetic metal powder of 4 (gr/cJ) or more.

The ferromagnetic metal powder used in the magnetic recording medium of the present invention is
The magnetic recording medium of the present invention has a relatively small particle size with a specific surface area of 40 to 75 (rd/gr) determined by the BET method and a crystallite size of 100 to 250 particles determined by the X-ray diffraction method. has high magnetic properties and excellent electromagnetic conversion properties.

In addition, the ferromagnetic metal powder is further heated at 100° after forming an oxide layer on the surface of the metal magnetic powder particles in order to prevent the generation of pores during synthesis from oxides and to stabilize the particles after synthesis. The true density is higher than that of conventional ferromagnetic metal powders by processing at a high temperature of C or higher for an extremely short time to fully crystallize the surface oxide layer and anti-sintering agent layer and densify the surface layer. ing.

The ferromagnetic powder used in the magnetic recording medium of the present invention has a smooth particle surface and excellent dispersion, making it easy to obtain a smooth magnetic layer.

In addition, since the ferromagnetic metal powder has a relatively high true density, when it is dispersed with a binder resin or the like to form a magnetic layer on a non-magnetic support, it is filled in the magnetic layer with a higher filling degree than before. , the magnetic recording medium of the present invention can increase output.

As the true density ρ, the specific surface area is SS
A (-0,015x (SS
A) +6. 4 (g r/c+d) or more -L is preferable.

That is, it is preferable that the particles consist of a dense metal particle portion and a surface oxide film whose true density satisfies the above relational expression even if the specific surface area increases as the particles become finer.

The ferromagnetic metal powder in the magnetic recording medium of the present invention is
By suppressing the generation of vacancies during synthesis, the true density is increased, and the size is related to the specific surface area using the above formula, but as a method for suppressing the generation of vacancies,
This can be achieved by selecting the composition of the oxide as a starting material, selecting the sintering inhibitor, and strengthening the dehydration conditions, and annealing in the oxide state.

For example, B as an element that reduces vacancies in ferromagnetic powder.
P, Si, Sb, Se, Ti, Cu, Ni, etc. may be added as a solid solution to the oxide of the starting material or by surface treatment of the starting material.

Furthermore, in order to prevent sintering between particles, it is necessary to treat the above-mentioned starting material with an anti-sintering agent such as silica or alumina by a well-known method and reduce it with hydrogen to form a ferromagnetic metal.

Densifying the oxide or sintering inhibitor layer provided on the surface of the metal magnetic powder particles by divisor treatment can also increase the true density of the ferromagnetic metal powder used in the magnetic recording medium of the present invention. It is valid. For example, 1000 ferromagnetic metal powder
There are methods such as passing through a heating section heated to a temperature higher than 0.degree. C., passing through a high-temperature plasma region, or treating with low-temperature plasma or ozone.

By densifying the particle surface layer, the oxidation number of the elements on the particle surface of the ferromagnetic metal powder becomes high, resulting in an oxide with good crystallinity and uniform thickness, and the true density of the ferromagnetic metal powder increases. This treatment improves the magnetic properties and improves the oxidation stability of the ferromagnetic metal powder.
It is not easily affected by oxygen or moisture and does not deteriorate its magnetic properties.

Therefore, the storage durability of the magnetic recording medium of the present invention is excellent.

Furthermore, when the particles of the ferromagnetic metal powder used in the present invention were observed with a transmission electron microscope, the particle surfaces were found to be smooth. This is probably due to the smoothness of the particle surface, but because the ferromagnetic powder has good dispersibility, good tape surface properties, excellent squareness ratio, and high true density of the ferromagnetic powder, it can be highly packed in the magnetic layer. As a result, a magnetic recording medium with high output, high S/N ratio, and excellent storage durability can be obtained.

The effect of densification of the oxide in the ferromagnetic metal powder treated by the above method becomes more pronounced as the ferromagnetic metal powder becomes finer. However, since it includes a process of processing at high temperatures, if it is made too fine, it will sinter, resulting in deterioration of magnetic properties and dispersibility. Therefore, the particle size of the ferromagnetic metal powder is 35 in terms of specific surface area.
The crystallite size determined by chi-ray diffraction is preferably 100 to 250.

The magnetic recording medium of the present invention is produced by uniformly mixing and dispersing the ferromagnetic metal powder and binder resin using a kneader and a disperser,
The obtained magnetic coating liquid is applied onto a non-magnetic support, oriented and dried to form a magnetic layer. The structure of the magnetic layer is not limited to a single layer, and may have a multilayer structure of two or more layers. In addition, lubricants, abrasives, antistatic agents, etc. are also added to the magnetic layer.

The material used in the magnetic recording medium of the present invention is not particularly limited, and materials conventionally used in magnetic recording media can be used.

Lubricants that can be used in the magnetic recording medium of the present invention include various silicone oils such as polysilogysan, graphite, inorganic powders such as molybdenum disulfide, fine plastic powders such as polyethylene and polytetrafluoroethylene, higher fatty acids, higher Alcohol, higher fatty acid ester, fluorocarbons, etc. are added in a proportion of 0.1 to 20 parts by weight based on 100 parts by weight of the binder.

As the abrasive, fine powder of α-alumina, fused alumina, silicon carbide, chromium oxide (Cr20.), corundum, diamond, etc. with an average particle diameter of 0.05 to 0.5μ is used. .5 to 20 parts by weight are added.

As the antistatic agent, conductive powders such as graphite, carbon black, and carbon black graft polymers, nonionic surfactants, anionic surfactants, cationic surfactants, and the like are used.

As the binder resin for the magnetic layer in the magnetic recording medium of the present invention, known thermoplastic resins, thermosetting resins, radiation curable resins, reactive resins, which have been conventionally used as binder resins for magnetic recording media, or a mixture thereof. More preferably, the binder resin contains a substance that can be crosslinked or polymerized by radiation irradiation.

The thermoplastic resins include acrylic acid ester acrylonitrile copolymer, acrylic acid ester vinylidene chloride copolymer, acrylic acid ester styrene copolymer, methacrylic acid ester acrylonitrile copolymer, methacrylic acid ester vinylidene chloride copolymer, Methacrylate styrene copolymer, vinyl chloride copolymer (details below), polyurethane resin (details below),
Urethane elastomer, nylon-silicon resin, nitrocellulose-polyamide resin, polyvinyl fluoride,
Vinylidene chloride acrylonitrile copolymer, butadiene acrylonitrile copolymer, polyamide resin, polyvinyl butyral, cellulose derivatives (cellulose acetate butyrate, cellulose diacetate, cellulose triacetate, cellulose propionate, nitrocellulose, etc.), styrene butadiene copolymer Polyester resins, chlorovinyl ether acrylic acid ester copolymers, amino resins, various synthetic rubber-based thermoplastic resins, and the like can be mentioned.

In addition, the thermosetting resin or reactive resin has a molecular weight of 200,000 or less in the state of a coating liquid, and its molecular weight becomes extremely large when heated after coating and drying, such as phenol resin. , phenoxy resin,
Epoxy resin, polyurethane curable resin, urea resin, melamine resin, alkyd resin, silicone resin, acrylic reaction resin, epoxy-polyamide resin, nitrocellulose melamine resin, mixture of high molecular weight polyester resin and isocyanate prepolymer, methacrylate Mixture of copolymer and diisocyanate prepolymer, mixture of polyester polyol and polyisocyanate, urea formaldehyde resin, low molecular weight glycol/high molecular weight diol/triphenylmethane triisocyan yarn-1.
mixtures thereof, polyamine resins, and mixtures thereof.

Further, as the radiation-curable resin, a resin having at least one carbon-carbon unsaturated bond in the molecule that can be cured by radiation irradiation can be used. Examples of radiation-curable resins include using a compound having at least one carbon-carbon unsaturated bond in the molecule in the vinyl chloride copolymer or polyurethane resin as a copolymerization component during polymerization, or Examples include those produced by reacting with resins and containing them. Compounds having at least one carbon-carbon unsaturated bond include at least one (
Compounds containing a meta)acroyl group are preferred, and such compounds may further contain a glycidyl group or a hydroxyl group.

Furthermore, a compound that can be polymerized by radiation irradiation may be added to the binder resin. Such compounds include (meth)acrylic esters, (meth)acrylamides, allyl compounds, vinyl ethers, vinyl esters, vinyl heterocyclic compounds, N-vinyl compounds, styrenes, acrylic acid, methacrylic acid, Examples include crotonic acids, itaconic acids, olefins, and the like. Among these, particularly preferred compounds are - compounds containing two or more (meth)acryloyl groups in the molecule, such as:
Diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, reaction product of polyisocyanate and poly(meth)acrylate,
etc. can be mentioned.

The vinyl chloride copolymer has a softening temperature of 150
℃ or less and an average molecular weight of about 10,000 to 300,000 can be used.

Specific examples of preferred vinyl chloride copolymers include vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-maleic acid copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, and vinyl chloride-vinyl acetate. -Maleic acid-vinyl alcohol copolymer, vinyl chloride-vinyl propionate-maleic acid copolymer, vinyl chloride-
Vinyl propionate-vinyl alcohol copolymer, vinyl chloride-vinyl acetate-acrylic acid copolymer, vinyl chloride-vinyl acetate-acrylic acid-vinyl alcohol copolymer, and oxidized products of these copolymers, etc. can be mentioned.

In particular, vinyl chloride copolymers having polar groups such as carboxylic acid groups or their salts, sulfonic acid groups or their salts, phosphoric acid groups or their salts, amino groups, hydroxyl groups, etc. can improve the dispersibility of ferromagnetic powders. preferred for.

Further, as the polyurethane, a polyurethane manufactured by a known polyurethane manufacturing method from a polyol, a diisocyanate, and further a chain extender if necessary can be used.

Further, the binder may further contain a compound having two or more isocyanate groups (polyisocyanate). Examples of such polyisocyanates include tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate-1, and naphthylene diisocyanate.
Isocyanates such as 1,5-diisocyane-1., 0-toluidine diisocyanate-1., isophorone diisocyanate, triphenylmethane triisocyanate, reaction products of these isocyanates and polyalcohols, and products of these isocyanates. Examples include polyisocyanates produced by condensation. The polyisocyanes 1-1 are, for example, Coronate, Coronate 1-HL, Coronate H1, Coronate EH, available from Nippon Polyurethane Kogyo (■).

Coronate 2030. Coronate) 2031, Coronate 2036, Coronate 3015, Coronate) 3041,
Coronet 1-2014, Millionate MR, Millionate MTL, Daltosec 1350, Daltosec 217
0, Dalto Sesoku 2280, from Takeda Pharmaceutical Company, Takenate) D-102, Takenate D-11ON, Takenate D-200, Takenate D-202, from Sumitomo Bayer, Sumidur-N75, from West German Bayer, Desmodor ■ 1. Desmodule IL, Desmodule N1 Desmodule HL, sold by Dainippon Ink and Chemicals under trade names such as Burnock-D850 and Parnock D802.

The mixing ratio of the ferromagnetic metal powder and the binder resin in the magnetic layer of the magnetic recording medium of the present invention is preferably 18 to 30 parts by weight of the binder per 100 parts by weight of the ferromagnetic metal powder.

Examples of the solvent for the coating liquid that can be used in the magnetic recording medium of the present invention include ketones such as methyl ethyl ketone and cyclohexanone, alcohols, esters such as ethyl acetate and butyl acetate, cellosolves, ethers, toluene, etc. There are organic solvents such as aromatic solvents, chlorinated hydrocarbon solvents such as carbon tetrachloride, and chloroform.

The non-magnetic support used in the magnetic recording medium of the present invention includes:
Synthetic resins (e.g., polyester, polyamide, polyolefin, cellulose derivatives), non-magnetic metals, glass, ceramics, paper, etc. can be used, and the forms used are films, tapes, sheets, cards, disks, drums, etc. .

The ferromagnetic metal powder, the above-mentioned organosilane compound (I), the dispersion liquid, the lubricant, the abrasive, the antistatic agent, the solvent, etc. are kneaded to obtain a coating solution. Each component may be added to the kneading machine either all at the same time or one after the other.

Various types of kneading machines are used for crosstalk dispersion, but for details, see T, C, Patton Point Flow
andPigment Dispersion (]
964, John Willey & 5ons Co.).

Various methods are possible for applying the magnetic layer onto the non-magnetic support, and are specifically described in detail in "Coating Engineering" (1971) published by Asakura Shoten.

By such a method, the magnetic layer coated on the nonmagnetic support is optionally treated to orient the ferromagnetic metal powder in the layer, and then the formed magnetic layer is dried. Also,
In order to enhance the magnetic properties, a smoothing treatment (for example, a smoothing treatment before drying or a calendering treatment after drying) may be performed.

The thickness of the magnetic layer is from 1 to 6 μm, preferably from 2 to 4 μm.
m is desirable.

(Effect of the invention) In a magnetic recording medium having a magnetic layer mainly composed of ferromagnetic metal powder and binder resin, the particle size of the ferromagnetic metal powder can be reduced and its true density can be adjusted to the specific surface area. By changing it accordingly, it is possible to obtain a magnetic recording medium that has good dispersibility in the magnetic layer, can increase the degree of filling, has good electromagnetic conversion characteristics, and has excellent storage durability.

The novel effects of the present invention will be explained in more detail below using Examples.

(Example) (Synthesis of ferromagnetic metal powder 1) Transition metal sulfuric acid was added to 100 parts by weight of iron as metal components, 5 parts by weight of cobalt, 2.5 parts of nickel, and 2 parts by weight of boron. Salt and boric acid were added to water and dissolved while bubbling with nitrogen. Add twice the amount of neutralization equivalent of sodium hydroxide aqueous solution to the solution bubbling with nitrogen,
While maintaining the temperature at 30'C, air (0,1β/aqueous solution IA
) was aerated to produce goethite containing the metal.

After washing with water to remove salts, an aqueous aluminum sulfate solution is added so that the ratio of aluminum is 2 parts by weight to 100 parts by weight of iron, and an aqueous sodium hydroxide solution is added in an amount to neutralize the aluminum sulfate to coat the goethite particle surface with AN. After wearing, washing with water, filtering and passing through a shaping machine, it is dried to a diameter of approximately 5 m.
A cylindrical cake of m was obtained.

The cake thus obtained was dehydrated in air at 350°C for 1 hour, then annealed at 600°C for 1 hour while bubbling with nitrogen, then brought to a temperature of 470°C and further heated with hydrogen for 4 hours.
After time reduction, ferromagnetic metal powder was obtained.

Next, nitrogen gas containing 1% by volume of dry air was added to 20 parts by weight of the ferromagnetic metal powder at a constant flow rate of 21% per minute.
Oxidation treatment was carried out over a period of time to form an oxide layer on the surface of the ferromagnetic metal powder particles.

(Synthesis 2 of ferromagnetic metal powder) In the synthesis 1, the conditions were the same as in the synthesis 1 of the ferromagnetic metal powder, except that the air aeration amount when generating goethite was 0.02β/aqueous solution 11, A ferromagnetic metal powder with an oxide layer on the particle surface was synthesized.

(Synthesis of ferromagnetic metal powder 3) Transition metal sulfate, antimony sulfate, and phosphoric acid are added so that the metal components are 100 parts by weight of iron, 5 parts by weight of cobalt, 2.5 parts by weight of SB, and 2 parts by weight of phosphorus. Sl and Lium were added to water and dissolved while bubbling with nitrogen. Add thorium hydroxide aqueous solution twice the neutralization equivalent to the solution bubbling with nitrogen, and add air (0,1,
n/aqueous solution 1p) was aerated to produce goethite.

After washing with water and removing salts, a water glass aqueous solution was added so that silicon was 5 parts by weight per 100 parts by weight of iron, and dilute sulfuric acid was added in an amount to neutralize the water glass, so that S
After washing with water, filtering, passing through a shaping machine, and drying, a cylindrical cake with a diameter of about 5 mm was obtained.

The cake thus obtained was dehydrated in air at 400°C for 1 hour, then annealed at 600°C for 1 hour while bubbling with nitrogen, and then reduced to 430°C with hydrogen for 4 hours to form a ferromagnetic metal powder. I got it.

Nitrogen gas containing 1% by volume of dry air was introduced into 20 parts by weight of the obtained ferromagnetic metal powder at a constant flow rate of 21/min for 5 hours to perform a deoxidation treatment, and the surface of the particles of the ferromagnetic metal powder was removed. An oxide layer was formed.

The ferromagnetic metal powders obtained in the above Synthesis I, Synthesis 2, and Synthesis 3 were treated in the following manner to densify the surface layer.

(Densification treatment) A transparent quartz tube is installed at a predetermined angle with respect to the horizontal plane.
On the outside is an infrared gold image furnace (manufactured by Shinku Riko ■)
It was used as a heating furnace. The quartz tube step-in section and the storage section for the processed material were connected, and these heat treatment sections were replaced with nitrogen gas. A container containing ferromagnetic metal powder whose internal atmosphere was replaced with nitrogen gas was connected to the input port, and heat treatment was performed by passing the powder through the quartz tube through the input port. The temperature of the infrared furnace was 1100° C., and the heat treatment time was controlled by the number of infrared furnaces and the angle of the quartz tube.

The heat treatment time was selected in the range of 3 seconds to 20 seconds.

(Example 1) A sample of a magnetic recording medium was prepared under the following conditions using ferromagnetic metal powder synthesized by the synthesis method 1 and subjected to densification treatment under a heat treatment time of 3 seconds. Note that the demagnetization rate of this ferromagnetic metal powder was 0.15.

(Composition for magnetic coating liquid) Ferromagnetic metal powder 100 parts by weight Vinyl chloride copolymer (contains SOx Na group) (MR110 manufactured by Nippon Zeon) 12 parts by weight Polyurethane resin 8 parts by weight (Japan Polyurethane type N2304) α Alumina 7 parts by weight (Sumitomo Chemical HIT50 average particle diameter 0.2 μm) Carbon blank 3 parts by weight (Mitsubishi Kasei side type 3250B) Stearic acid 2 parts by weight Butyl stearate 2 parts by weight Methyl ethyl ketone 125 parts by weight Cyclohexanone
After dispersing 125 parts by weight or more of the composition with a sand grinder for 3 hours, 8 parts by weight of an isocyanate compound (Japan Polyurethane side-type Coronate L) was added, and an additional 3 parts by weight was added.
After 0 minutes of dispersion, the mixture was filtered using a filter with an average particle size of 1 μm to obtain a magnetic coating liquid. After coating to a dry thickness of 2.7 μm and orienting in a magnetic field, it was dried and a bank coat layer was provided on the opposite side of the magnetic layer. Further, after being subjected to pressure forming treatment using a super calender, it was cut into 8 mm width to obtain a sample of an 8 mm video magnetic recording medium.

(Example-2) Magnetic recording was performed under the same conditions as in Example-1, except that in Example-1, a ferromagnetic metal powder with a heat treatment time of 5 seconds during densification treatment was used as the ferromagnetic metal powder. A sample of the medium was prepared.

The demagnetization rate of this ferromagnetic metal powder was 0.13.

(Example-3) Magnetic recording was performed under the same conditions as in Example-1, except that in Example-1, a ferromagnetic metal powder whose heat treatment time during densification treatment was 10 seconds was used. A sample of the medium was prepared.

The demagnetization rate of this ferromagnetic metal powder was 0.16.

(Example-4) Example-1 except that the ferromagnetic metal powder synthesized by the synthesis method 2 and heat-treated for 5 seconds during the densification treatment was used as the ferromagnetic metal powder in Example-1. A sample of a magnetic recording medium was prepared under the same conditions as described above.

The demagnetization rate of this ferromagnetic metal powder was 0.12.

(Example-5) Example-1 except that the ferromagnetic metal powder synthesized by the synthesis method 3 and heat-treated for 5 seconds during the densification treatment was used as the ferromagnetic metal powder in Example-1. A sample of a magnetic recording medium was prepared under the same conditions as described above.

The demagnetization rate of this ferromagnetic metal powder was 0.15.

(Comparative Example-1) In Example-1, the same conditions as Example 1 were used, except that ferromagnetic metal powder that was synthesized by the synthesis method 1 and not subjected to densification treatment was used as the ferromagnetic metal powder. A sample of a magnetic recording medium was created using the following steps.

The demagnetization rate of this ferromagnetic metal powder was 0.40.

(Comparative Example-2) The same conditions as in Example 1 were used, except that in Example-1, ferromagnetic metal powder that was synthesized by the synthesis method 2 and not subjected to densification treatment was used as the ferromagnetic metal powder. A sample of a magnetic recording medium was created using the following steps.

The demagnetization rate of this ferromagnetic metal powder was 0.37.

(Comparative Example-3) Same as Example-1 except that in Example-1, ferromagnetic metal powder synthesized by the synthesis method 3 and not subjected to densification treatment was used as the ferromagnetic metal powder. Samples of magnetic recording media were prepared under these conditions.

The demagnetization rate of this ferromagnetic metal powder was 0.39.

(Comparative Example-4) Example-1 except that the ferromagnetic metal powder synthesized by the synthesis method 1 and heat-treated for 20 seconds during the densification treatment was used as the ferromagnetic metal powder in Example-1. A sample of a magnetic recording medium was prepared under the same conditions as described above.

The demagnetization rate of this ferromagnetic metal powder was 0.30.

Ferromagnetic metal powder and 8 mm obtained as above
Each characteristic of the sample of the video magnetic recording medium was evaluated under the following conditions.

(Evaluation of properties of ferromagnetic metal powder) Magnetic properties (coercive force Hc, saturation magnetization σS) were measured using a Toei Kogyo VSM with a measuring magnetic field of 10 kOe.

The specific surface area was measured by the BET-point method using Cantersorb manufactured by Cantertarom, and the crystallite size was determined by calculation using X-ray diffraction. Further, the true density was measured using a multi-polywoom densitometer model 1305 manufactured by Micromeritex Co., Ltd. using a He gas displacement method.

Furthermore, the demagnetization rate of each ferromagnetic metal powder was set to 60"C90%.
Saturation magnetization (σS) after being left in an RH atmosphere for 7 days
') was measured and expressed as σS - σS')/σS.

(Evaluation of magnetic recording media) Magnetic properties (coercive force Hc, saturation magnetic flux density Bm, residual magnetic flux density Br) were measured with a Toei Kogyo VSM at a magnetic field of 1.0 k.
Measured in Oe.

The electromagnetic conversion characteristics were determined using a commercially available old gh-8 video recorder Fujix hi-8M830HR model.
MHz video sensitivity (read the peak value of RF output with an oscilloscope), S/N (Shibaku 925R-I)
Noise was measured using an NTSC color video noise meter. The tape obtained using the sample prepared in Synthesis 1 of Ferromagnetic Metal Powder was defined as QdB (reference).

The surface roughness was measured using a WYKO (Alicina, USA) type light wave interference three-dimensional roughness meter "TOPO-3D".

In calculating the measured values, corrections such as inclination correction, spherical correction, cylindrical correction, etc. were added in accordance with JIS-B2O2, and the center surface average roughness RaT was taken as the value of the surface roughness. The demagnetization rate is determined by measuring the saturation magnetic flux density (Bm') after leaving a sample of each magnetic recording medium at 60°C, 90% RH for 7 days, and calculating the demagnetization rate by (Bm - Bm'
)/B m.

Furthermore, the ratio of the residual magnetic flux density to the saturation magnetic flux density was defined as the squareness ratio.

The above evaluation results are shown in Tables 1 and 2.

Claims (1)

[Claims] In a magnetic recording medium having a magnetic layer mainly composed of ferromagnetic powder and binder resin on a non-magnetic support, the ferromagnetic powder has a specific surface area (SSA) determined by the BET method.
is 40 to 75 (m^2/gr), and the crystallite size determined by X-ray diffraction is 100 to 250 Å.
And its true density (ρ) is ρ=-0.01
A magnetic recording medium characterized in that it is an acicular ferromagnetic metal powder of 5×(SSA)+6.4(gr/cm^3) or more.