JPH06340426A - Production of hematite particle and production of ferromagnetic powder for magnetic recording using the same and magnetic recording medium - Google Patents

Abstract

PURPOSE:To provide a producing method of a monodisperse spindle form hematite particle useful as a pigment, a coating material, a ferromagnetic powder for magnetic recording medium or the like and extremely small in particle size distribution, a producing method of ferromagnetic powder using the hematite particle and a magnetic recording medium. CONSTITUTION:The monodisperse spindle form hematite particle is obtained by using a hematite fine particle as a crystalline nucleus and hydrolyzing ferric hydroxide in the presence of sulfate ion or phosphate ion. The monodisperse spindle form hematite particle is reduced to make magnetite, and the magnetite is oxidized or reduced to obtain ferromagnetic powder excellent in magnetic property. The magnetic recording medium excellent in output particularly in a short wave length band is obtained by using this ferromagnetic powder and, in this case, the further excellent ferromagnetic powder is obtained by annealing the hematite or the magnetite.

Description

Detailed Description of the Invention

[0001]

The present invention relates to the production of monodisperse spindle-type hematite particles which have a very small particle size distribution and are useful as pigments, paints, catalysts, raw materials for ferrites, raw materials for magnetic iron oxides for magnetic recording media, etc. The present invention relates to a method, a method for producing a ferromagnetic powder for magnetic recording using the same as a raw material, and a magnetic recording medium, and in particular, a method for producing monodisperse spindle-type hematite particles which are fine particles and have excellent particle size distribution, and the hematite. The present invention relates to a method for producing a ferromagnetic powder for magnetic recording having excellent magnetic properties by using particles, and a magnetic recording medium.

[0002]

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

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

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

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

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

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

However, the metal thin film type magnetic recording medium has problems in characteristics such as durability, running property and corrosion resistance.
The original excellent characteristics are not fully utilized to deal with the problem. Further, the manufacturing cost is high and there is a problem in economic efficiency. On the other hand, in a so-called coating type magnetic recording medium having a magnetic layer mainly composed of a ferromagnetic powder and a binder resin on a non-magnetic support, durability, running property and corrosion resistance are better than those of a metal thin film type magnetic recording medium. However, many attempts have been made to improve the recording density of the metal thin film magnetic recording medium, which has excellent electromagnetic characteristics but has electromagnetic conversion characteristics.

For example, there are a method of improving the surface property of the magnetic layer, a method of improving the magnetic properties of the ferromagnetic powder, and a method of increasing the dispersibility of the ferromagnetic powder in the magnetic layer.

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

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

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

Further, as a method for reducing the self-demagnetization loss and the recording demagnetization loss of the magnetic layer to improve the recording density, the magnetic layer is made as thin as a ferromagnetic metal thin film type magnetic recording medium and its magnetic property is reduced. A magnetic recording medium provided with a relatively thick non-magnetic layer as a lower layer in order to secure the flatness and strength of the layer is disclosed in Japanese Patent Laid-Open No.
7-198536, JP 62-154225, JP 63-187428, JP 4-3.
No. 25915, Japanese Patent Application Laid-Open No. 4-325916, Japanese Patent Application Laid-Open No. 4-325917, etc., the electromagnetic conversion characteristics are similar to those of a ferromagnetic metal thin film type magnetic recording medium with excellent output particularly in a high range. However, further attempts to improve the electromagnetic conversion characteristics have been limited due to the following problems related to the particle shape of the ferromagnetic powder.

That is, when the magnetic characteristics of the coating type medium using the metal thin film type and the fine particle metal (ferromagnetic metal) powder are compared, the SFD (Swit) of the coating type medium using the metal powder is compared.
It has been recently revealed that the ching field (Destination Field) has a large numerical value and is inferior in characteristics to metal thin films, while Nakamura et al. conducted simulations to improve the electromagnetic conversion characteristics of metal thin film media. It has been pointed out that the short wavelength output is improved by improving the distribution of the magnetic field (Journal of Applied Magnetics, Vol. 115, 155-158 (199).
1)). The anisotropic magnetic field distribution has a strong correlation with the SFD, and if the anisotropic magnetic field distribution is large, the SFD (Hc distribution) will be large.

It is known that the ferromagnetic metal powder has a large shape anisotropy with respect to Hc (coercive force), and the particle size distribution of the ferromagnetic powder becomes wide.
It is known that the distribution of is also wide. For this reason, attempts to reduce the particle size distribution of needle-like particles, which are the raw materials of ferromagnetic metal powders, such as goethite, rapid crosite, and hematite, have been continuously made. When the fine ferromagnetic metal powder is used to reduce the roughness of the surface of the tape and improve the short wavelength output, variations in the major axis length and the minor axis length have a great influence on the Hc distribution.

As described above, in order to further improve the characteristics and improve the recording density of the conventional magnetic recording medium, the ferromagnetic metal powder should have a small particle size and the particle size and shape should be as uniform as possible. Is desired. Particles that are well separated and have a very small particle size distribution are called monodisperse particles. Studies are underway to synthesize monodisperse particles to study particle size and catalytic performance and particle-particle interactions (eg, surface,
29,978 (1991), 23,177 (198).
4)). The method for synthesizing monodisperse spindle-type hematite is
M. Ozaki et al. Colloid Inte
rface Sci. 102 146-151 (198)
4). As a first method, they use 0.
02 mol ferric chloride and 1.0 × 10 ^{−4 to} 4.0 × 10
A liquid containing ⁴ mol of NaH ₂ PO ₄ or NaH ₂ PO ₂ was kept at 100 ± 2 ° C. for 2 days to give a particle length of 0.1
Particles with a needle ratio of 2 to 0.55 μm and a needle ratio of 2 to 5.5 are synthesized. As a second method, sodium hydroxide is added to ferric nitrate to form ferric hydroxide, and washing with distilled water is repeated,
5 to 20 in 200 cm ^{3 of} 0.02 mol of ferric hydroxide
0.1 of 1N HCl and 0.5～2.5Cm ³ of cm ³
M NaH ₂ PO ₄ was added, and the whole was diluted with distilled water to 500c.
m ³ and hold at 100 ± 2 ℃ for 2 days, particle length 0.2
Particles having a particle size of 7 to 0.33 μm and an acicular ratio of 3 to 6 are synthesized. It has been reported that if the concentrations of ferric chloride and NaH ₂ PO ₄ or NaH ₂ PO ₂ are increased in order to increase the yield, βFeOOH is mixed and a single-phase monodisperse spindle hematite cannot be obtained.

M. Ozaki et al. Reduced the obtained monodisperse spindle hematite with hydrogen at 340 to 400 ° C. for 1 to 3 hours and then flowing air to oxidize at 240 ° C. for 1 to 2 hours to obtain γFe ₂ O
_The result set to ₃ is J. Colloid Interfac
e Sci. 107 199-203 (1985). In addition, one of the inventors first started with an extremely concentrated Fe (OH) ₃ gel (about 1 mol / liter) and then, through the intermediate product βFeOOH, a pseudo-cube (average particle size of about 2 μm) single Dispersed hematite particles were synthesized (J. Colloid Interface Sci.
152, 587 (1992)).

M. According to the method of Ozaki et al., ΓFe ₂ O ₃ is obtained from monodisperse spindle hematite. However, since the concentration of the reaction solution for producing monodisperse spindle hematite is low, the yield of monodisperse spindle hematite is small and it is economically large-scale. Can not be synthesized. J. Colloid Interf
ace Sci. According to the method of 152 587 (1992), spindle-type hematite cannot be obtained and fine particles cannot be obtained.

Further, the fine monodisperse spindle type hematite particles as described above are produced, and thereby excellent in practical properties such as running property, durability and storability and economical efficiency, good electromagnetic conversion property and high recording. It is desired to obtain a ferromagnetic powder suitable for a magnetic recording medium having a high density, and even a coating type magnetic recording medium has a magnetic recording medium having an electromagnetic conversion characteristic comparable to that of a metal thin magnetic recording medium. It is desired to obtain.

[0020]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems of the prior art. The first object of the present invention is to provide an average particle length of 1 μm or less and an acicular ratio of 2 to 2. It is an object of the present invention to provide a method for industrially producing monodisperse spindle type hematite particles of No. 8. A second object of the present invention is to produce a ferromagnetic powder for magnetic recording using the above monodisperse spindle type hematite particles, and particularly to reduce or reduce the above monodisperse spindle type hematite particles after annealing. It is intended to provide a method for producing a ferromagnetic powder for a magnetic recording medium, which is excellent in magnetic characteristics by post-annealing and oxidizing or reducing the obtained magnetite so that the particle size distribution is almost the same as that of the starting material. .

A third object of the present invention is to provide a magnetic recording medium suitable for high density recording using the ferromagnetic powder.

[0022]

[Means for Solving the Problems] One of the inventors has conducted extensive studies on the method for producing iron oxide, and as a result, by hydrolyzing ferric hydroxide in the presence of certain ions, it has hitherto been possible. Inventors of the following production method for obtaining fine monodisperse particles of hematite at a higher concentration than by the method. Then, the present inventors examined the production method of the ferromagnetic powder as a raw material of the magnetic recording medium using hematite as a raw material, and as a result, the obtained ferromagnetic powder was used as a raw material of the magnetic recording medium for high density recording. The inventors have found that they have optimum characteristics and have invented the following ferromagnetic powder and magnetic recording medium.

That is, the object of the present invention is to disperse the hematite fine particles as nuclei crystals in the ferric hydroxide gel suspension in the presence of sulfate radical ions (S0 ₄ ²⁻ ) or phosphate radical ions. After that, the method for producing monodisperse spindle type hematite particles by hydrolyzing the ferric hydroxide (claim 1), the monodisperse spindle obtained by the method according to claim 1 or 2 above. Type hematite particles are reduced to magnetite and then the magnetite is oxidized to produce FeO _x.
(1.33 <x ≤ 1.5) The method for producing ferromagnetic iron oxide, wherein the monodisperse spindle hematite particles obtained by the method according to claim 1 or 2 are reduced. A method for producing a ferromagnetic metal powder by reducing the magnetite after the magnetite is formed, and a magnetic recording medium having a magnetic layer mainly composed of the ferromagnetic powder and a binder resin on a non-magnetic support, A magnetic recording medium in which the magnetic powder is a ferromagnetic iron oxide obtained by the method according to claim 3 or 5 or a ferromagnetic metal powder obtained by the method according to claim 4 or 6. To be achieved. Furthermore, after the hematite fine particles are dispersed as nuclei in the ferric hydroxide gel suspension in the presence of the above-mentioned sulfate radical ion (S0 ₄ ²⁻ ) or phosphate radical ion, the ferric hydroxide hydroxide is dispersed. When the monodisperse spindle-type hematite particles are produced by hydrolyzing bisulfite, the concentration of sulfate ion in the gel suspension of ferric hydroxide is 0.01 mol / liter to 0.1 mol / liter. If the phosphate ion concentration is 0.001 to 0.01 mol / liter, it is possible to more effectively produce monodisperse spindle-shaped hematite particles of fine particles. Further, the monodisperse spindle-type hematite particles obtained by the method according to claim 1 or 2 are reduced to magnetite, and then the magnetite is oxidized to produce FeO _x (1.33 <x
≦ 1.5), or the monodisperse spindle-type hematite particles obtained by the method according to claim 1 or 2 are reduced to magnetite, and then the magnetite is obtained. By the method for producing ferromagnetic iron oxide or ferromagnetic metal powder by the reduction of, by annealing monodisperse spindle type hematite particles or magnetite, a more complete crystal growth can be achieved. It is possible to obtain a ferromagnetic powder having excellent magnetic properties.

Further, a ferromagnetic powder and a magnetic layer mainly composed of a binder resin are provided on a non-magnetic support, and the ferromagnetic powder is obtained by the method according to claim 3 or 5. A layer structure of a magnetic recording medium, which is iron oxide or a ferromagnetic metal powder obtained by the method according to claim 4 or 6, is formed on a non-magnetic layer mainly composed of a non-magnetic powder and a binder resin, and a non-magnetic layer formed thereon. The ferromagnetic powder according to the present invention is sufficiently characterized by forming a magnetic layer mainly composed of a ferromagnetic powder and a binder resin and forming a thin film magnetic layer having a thickness of 1.0 μm or less. Therefore, it is possible to obtain a magnetic recording medium that is suitable for use as a magnetic recording medium for high density recording, which has a particularly high output at a short wavelength.

Furthermore, the present inventors have found that in the above-mentioned step, even if the phosphate radical ion is used instead of the sulfate radical ion,
It was found that similar monodisperse spindle type hematite particles were obtained. Further, the present inventors, using the above monodisperse spindle hematite particles, as a result of repeated studies to obtain a magnetic recording ferromagnetic powder, the monodisperse spindle hematite particles are reduced to magnetite, By oxidizing or reducing this, we succeeded in obtaining a ferromagnetic powder for magnetic recording having a small particle size distribution.

In particular, since the above-mentioned monodisperse spindle hematite particles are polycrystalline, the obtained monodisperse spindle hematite particles are reduced in hydrogen to form magnetite, and it is not always necessary to simply oxidize or reduce the magnetite. Ferromagnetic powder with excellent magnetic properties could not be obtained, and the magnetic properties were remarkably improved by annealing the monodisperse spindle hematite particles prior to reducing them or annealing them after reducing them to magnetite. Was successfully obtained.

Further, by using the ferromagnetic powder annealed as described above, it has been possible to obtain a magnetic recording medium having extremely excellent magnetic characteristics. In particular, a ferromagnetic metal powder is used as the ferromagnetic powder, Succeeded in obtaining a magnetic recording medium comparable to the above-mentioned metal thin type (evaporation type) by applying a multilayer structure, an upper layer being a magnetic layer having a thickness of 1.0 μm or less and a lower layer being a non-magnetic layer did.

That is, the first invention of the present application is to disperse the hematite fine particles as nuclei crystals in the gel suspension of ferric hydroxide in the presence of sulfate radical ion (S0 ₄ ²⁻ ) or phosphate radical ion. After that, the ferric hydroxide is hydrolyzed and then the monodisperse spindle-type hematite particles are produced.

In the second invention of the present application, the monodisperse spindle type hematite particles are reduced to magnetite and then the magnetite is oxidized to produce FeO _x (1.33 <x ≦ 1.
5) A method for producing a ferromagnetic iron oxide and a method for producing a ferromagnetic metal powder characterized by reducing the magnetite. According to a preferred aspect of the second invention, in the second invention, the monodisperse spindle type hematite particles or the magnetite is annealed.

A third invention of the present application is a magnetic recording medium comprising a nonmagnetic support and a magnetic layer comprising a ferromagnetic powder and a binder resin as a main component, wherein the ferromagnetic powder is the strong magnetic recording medium of the second invention. A magnetic recording medium characterized by being magnetic iron oxide or ferromagnetic metal powder.

Further, according to a preferred aspect of the third invention, the magnetic recording medium comprises a non-magnetic support and a non-magnetic layer mainly composed of a binder resin and a ferromagnetic powder thereon. It has a magnetic layer having a thickness of 1.0 μm mainly composed of a binder resin, and the ferromagnetic powder of the magnetic layer is the ferromagnetic iron oxide or the ferromagnetic metal powder. When the monodisperse spindle-type hematite particles obtained by the method of the present invention are used for the nonmagnetic powder used in the nonmagnetic layer that is the lower layer of the magnetic recording medium having the above thin magnetic layer, the particle size is small Since the particles are particles, the interface with the magnetic layer can be made uniform, and a magnetic recording medium having excellent electromagnetic conversion characteristics can be obtained. At this time, the monodisperse spindle-type hematite particles may be annealed as necessary, or may be surface-treated with silica, alumina, silica-alumina or the like.

The above-mentioned ferromagnetic powder of the present invention retains the monodisperse characteristic of the monodisperse spindle type hematite particles as it is, and is annealed to give SFD (sw
Good itch-ing field distribution and coercive force (H
c) The distribution is narrow, so that the recording demagnetization loss is particularly improved when used in a magnetic recording medium, and the short wavelength recording characteristics are excellent.

The present invention will be described in detail below. In the present invention, monodisperse refers to particles having a sharp particle size distribution, and the spread of the size distribution is within 10% in standard deviation with respect to the average size. The concentration range of the ferric hydroxide gel used is limited to 2 mol / liter or less because the solubility of the ferric salt is limited and stirring is difficult if the viscosity of the obtained gel is too high. Although monodisperse spindle type hematite can be obtained even if the concentration is low, the lower limit of the concentration is determined from the economical point of view because the yield decreases as the concentration decreases. If the amount of alkali such as NaOH added during the preparation of the hydroxide gel is not more than the equivalent of Fe ³⁺ , the intermediate product β-FeOOH cannot be obtained, and the hematite produced will not be monodispersed. In order to obtain hematite having high monodispersity, the excess concentration of Fe ³⁺ ions is preferably 0.05 to 0.2 mol / liter. In the present invention, the concentration of the above ferric hydroxide in the gel suspension of sulfate is preferably 0.01 mol / liter to 0.1 mol / liter, more preferably 0.015 mol / liter.
It is preferably about 0.08 mol / liter. or,
The concentration of the phosphate group is preferably 0.001 to 0.01 mol / liter, more preferably 0.0015 to
It is 0.008 mol / liter. In the case of using a phosphate root ion in the present invention, PO ₄ ³⁻ as a phosphate root ion,
It can be used in the form of H ₂ PO ^4- or HPO ^2- . The particle size of the hematite particles obtained by hydrolyzing ferric hydroxide used as nuclei is preferably 30 to 200 angstrom. If the particle size is too small, the effect as nuclei is poor, and if it is too large, fine particle monodisperse spindle hematite cannot be obtained. It is effective to make the particle size distribution of the nuclei uniform, and it is desirable to grind the hydrolysis product with mechanical force to remove coarse particles before use. When adding a nuclei crystal to ferric hydroxide gel and conducting a hydrolysis reaction, if the reaction rate is too high, new nucleation occurs and monodisperse particles may not be obtained. It is necessary to set the temperature, pressure and time within the range where re-nucleation does not occur.

The present inventors have found that the monodisperse spindle type hematite fine particles obtained as described above can be used to obtain a ferromagnetic powder for magnetic recording having excellent magnetic characteristics. In producing a ferromagnetic iron oxide powder, the obtained monodisperse spindle type hematite particles are reduced, before being oxidized, it is effective to subject the hematite to a sintering preventing treatment, and as a sintering inhibitor, silica, Phosphoric acid, aluminum-silica, or a combination thereof can be used.

The amount of the sintering inhibitor to be used is preferably 0.05 to 3.0% by weight, more preferably 0.1 to 1.5% by weight, based on hematite. If the amount of the sintering inhibitor is too small, the shape retention effect during reduction and oxidation will be poor, and if it is too large, the saturation magnetization will be small and the magnetic properties of the obtained ferromagnetic powder will be deteriorated.

Further, in order to prepare a ferromagnetic iron oxide powder from the above monodisperse spindle type hematite particles, the hematite particles are reduced to magnetite and then the magnetite is oxidized to obtain FeOx (1.33 <x The ferromagnetic iron oxide is ≦ 1.5). In the present invention, a ferromagnetic powder having good magnetic properties can be obtained as described above, but since the monodisperse spindle hematite particles obtained in the present invention are polycrystalline, the obtained monodisperse spindle hematite particles are By reducing in hydrogen to form magnetite, and simply oxidizing or reducing this, a ferromagnetic powder with excellent magnetic properties is not always obtained, and prior to reducing the monodisperse spindle type hematite particles,
By annealing this, or reducing it to magnetite and then annealing, the magnetic properties of the obtained ferromagnetic iron oxide and ferromagnetic metal powder can be remarkably improved.

When annealing is performed prior to the reduction, the annealing temperature is preferably 450 to 800 ° C. and 500 to 75.
It is 0 ° C. The annealing time is preferably about 1 to 5 hours, and it is desirable to select it in relation to the processing temperature. If the annealing is performed for a long time at a high temperature, the particle shape will change. As a method of reducing the particles, a well-known hydrogen gas reduction method, a method of using a reducing gas generated when an organic substance is thermally decomposed in an inert gas, and the like can be used. In the case of annealing after making magnetite, it is desirable to perform the treatment at a slightly lower temperature than that in the case of treating with hematite, and it is preferable to perform the treatment at 400 to 600 ° C.

After reduction, air is flowed at 200 to 300 ° C. to obtain γ.
In addition to Fe ₂ O ₃ , FeOx (1.
33 <x ≦ 1.5). As a method of adjusting the degree of oxidation, it can be adjusted by lowering the oxygen concentration in the oxidizing gas, or by setting the temperature during oxidation to 60 to 150 ° C. A known method can be used as a method for depositing the Co compound on the surface after forming the magnetic iron oxide. These methods are described, for example, in JP-A-49-74399 and JP-A-50-37667. Japanese Patent Publication 4
No. 9-49475.

In order to convert the monodisperse spindle type hematite obtained in the present invention into a ferromagnetic metal (alloy) powder, it is necessary to treat it with a sintering inhibitor, and the sintering inhibitor is A
Conventionally known materials such as l, Si, Al-Si, B, Ca, Nd, Y, La and combinations thereof can be used. The processing amount of the sintering inhibitor is 1-2 with respect to iron.
0 atom% is preferable, and 3 to 15 atom% is more preferable.
Is. The method for applying the sintering inhibitor is to disperse the hematite particles that have been sufficiently washed with water, add an aqueous solution of a compound containing an element used for preventing sintering, and adsorb it, or deposit on the particle surface by neutralization treatment, etc. Then, it is preferable to wash with water to remove impurities. In order to improve the magnetic properties and weather resistance, it is also preferable to alloy after the reduction by depositing a metal element before the treatment with the sintering inhibitor.
Elements used for the deposition treatment include Co, Ni, Mn,
Cu, Sn, Cr, Sb, Nd, Y, La, Sc, B
a, Sr, Ca, etc., which may be used alone or in combination.

The amount of metal used for alloying depends on the alloyable composition region, but is generally 0.1 to 30% by weight. In the case of a non-magnetic alloy, the addition amount cannot be increased so much, but the addition amount of transition metals such as Co and Ni can be changed in the range of 0.1 to 30% by weight.
Co has the effect of increasing saturation magnetization and improving weather resistance,
It is a preferred additive element. On the other hand, Ni is an element that promotes reduction, and therefore, is an element that can be reduced at low temperature and is effective in terms of shape retention effect.

Hydrogen is used to reduce to 3
The reduction is carried out at 50 to 550 ° C, preferably 400 to 500 ° C. The progress of the reduction can be determined by monitoring the water content in the hydrogen gas after the reduction with a dew point meter or the like. It is necessary to oxidize and stabilize the surface of the obtained ferromagnetic metal powder. For this reason, after the reduction, the surface is gradually oxidized by flowing a gas whose oxygen concentration is controlled or by passing an oxygen-containing gas in an organic solvent. In this case, monitor the calorific value with a thermometer to prevent sudden oxidation. Further, it is desirable that the water content in the gas used for the slow oxidation be as low as possible. It is also effective to treat the compound described in JP-A-63-52327 prior to deoxidation and then deoxidize to increase the saturation magnetization σs of the ferromagnetic powder.

Even if the magnetic recording medium of the present invention has a single magnetic layer, the SFD is superior to the conventional magnetic recording medium using ferromagnetic powder, and the short wavelength output is excellent. There is. The coating layer formed on the non-magnetic support is a ferromagnetic powder dispersed in a binder or a non-magnetic powder dispersed in a binder.
The magnetic layer having the following thickness has an increased short-wavelength output, and a magnetic recording medium having higher performance than that of a single layer can be obtained.

As a method for forming two or more coating layers on the non-magnetic support, the coating solution for the second layer is applied while the first coating layer on the non-magnetic support is still wet. Coating by a so-called wet-on-wet method (simultaneous multilayer coating method, JP-A-61-139929, JP-A-61-54992), that is, the first coating layer is formed. If the upper magnetic layer is applied and formed while it is in a wet state, it becomes easy to make the upper magnetic layer thin, and coating defects can be reduced, and further, the adhesion with the magnetic layer can be improved. It is preferable because it can be increased.

In the magnetic recording medium of the present invention, the reason for using the above-mentioned ferromagnetic powder of the present invention in the uppermost layer having two or more coating layers is to maximize the high density magnetic recording performance. The content is, firstly, that a non-magnetic powder such as carbon black, which is necessary for the practical characteristics of the magnetic recording medium, can be added only to the lower layer to secure the necessary electric conductivity. This is because it is possible to increase the saturation magnetic flux density per unit volume in the portion of 1 μm or less of the uppermost layer of the recording medium, and secondly, because the unevenness of the surface of the uppermost layer can be finished smoothly. Further, since it is possible to reduce the amount of a relatively expensive ferromagnetic material obtained by reducing monodisperse spindle hematite as a material, there is an economical advantage.

The binder resin for the magnetic layer in the magnetic recording medium of the present invention may be a thermoplastic resin, a thermosetting resin, a reactive resin, or a mixture thereof, which has been conventionally used in this field. The thermoplastic resin has a glass transition temperature of −100 to 150 ° C. and a number average molecular weight of 1000 to 2
00000, preferably 10,000 to 100,000,
The degree of polymerization is about 50 to 1000.

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

As the thermosetting resin or the reactive resin, 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, Examples thereof include epoxy-polyamide resin, a mixture of polyester resin and isocyanate prepolymer, a mixture of polyester polyol and polyisocyanate, and a mixture of polyurethane and polyisocyanate.

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

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

The filling degree of the ferromagnetic powder in the magnetic layer can be calculated from the maximum saturation magnetization σs and the maximum magnetic flux density Bm of the used ferromagnetic powder (Bm / 4πσs), and in the present invention, the value is It is preferably 1.7 g / cc or more, more preferably 1.9 g / cc or more, and most preferably 2.1 g / cc or more.

In the present invention, when polyurethane is used, the glass transition temperature is −50 to 100 ° C., the elongation at break is 100 to 2000%, and the stress at break is 0.05 to 10 k.
The g / cm ² and the yield point are preferably 0.05 to 10 kg / cm ² .

The polyisocyanate used in the present invention includes tolylene diisocyanate, 4-4'-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate and naphthylene.
Isocyanates such as 1,5-diisocyanate, o-toluidine diisocyanate, isophorone diisocyanate and triphenylmethane triisocyanate, and also
Products of these isocyanates and polyalcohols, polyisocyanates produced by condensation of isocyanates, and the like can be used. Commercially available trade names of these isocyanates are Coronate L, Coronet HL, manufactured by Nippon Polyurethane Co., Ltd.
Coronate 2030, Coronet 2031, Millionaire
To MR Millionate MTL, Takeda Pharmaceutical Co., Ltd., Takenet D-102, Takenet D-110N, Takenet D-
200, Takenet D-202, Sumitomo Bayer Co., Ltd., Desmodule L, Desmodule IL, Desmodule N Desmodule HL, etc., and these are used alone or by utilizing the difference in curing reactivity. You can use one or more combinations.

In the magnetic layer of the magnetic recording medium of the present invention, various functions such as a lubricant, an abrasive, a dispersant, an antistatic agent, a dispersant, a plasticizer and an antifungal agent are usually included. The material to have is contained according to the purpose.

As the lubricant used in the magnetic layer of the present invention, dialkyl polysiloxane (alkyl has 1 to 5 carbon atoms), dialkoxy polysiloxane (alkoxy has 1 to 4 carbon atoms), monoalkyl monoalkoxy poly Siloxane (alkyl has 1 to 5 carbon atoms, alkoxy has 1 to 4 carbon atoms), phenyl polysiloxane, fluoroalkyl polysiloxane (alkyl has 1 to 5 carbon atoms)
Silicone oil such as); conductive fine powder such as graphite; inorganic powder such as molybdenum disulfide and tungsten disulfide; plastic fine powder such as polyethylene, polypropylene, polyethylene vinyl chloride copolymer, polytetrafluoroethylene; α -Olefin polymer; unsaturated aliphatic hydrocarbon that is liquid at normal temperature (a compound in which an n-olefin double bond is bonded to a terminal carbon, carbon number about 20); monobasic fatty acid and carbon having 12 to 20 carbon atoms Number 3 to 12
Fatty acid esters, fluorocarbons, etc., each consisting of a monohydric alcohol can be used.

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

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

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

The following compounds can also be used as the lubricant. That is, silicone oil, graphite, molybdenum disulfide, boron nitride, fluorinated graphite, fluoroalcohol, polyolefin, polyglycol, alkyl phosphate, tungsten disulfide and the like.

As the abrasive used for the magnetic layer of the present invention, generally used materials are fused alumina, silicon carbide,
Chromium oxide (Cr ₂ O ₃ ), corundum, artificial corundum, diamond, artificial diamond, garnet, emery (main components: corundum and magnetite) are used.
These abrasives have a Mohs hardness of 6 or more. Specific examples are AKP-10 and AKP-1 manufactured by Sumitomo Chemical Co., Ltd.
2, AKP-15, 20AKP-30, AKP-50,
AKP-1520, AKP-1500, HIT-50,
HIT60A, HIT-100, manufactured by Nippon Chemical Industry Co., G
5, G7, S-1, chromium oxide K, UB manufactured by Uemura Kogyo Co., Ltd.
40B, WA8000, WA1000 manufactured by Fujimi Abrasives Co., Ltd.
0, TF140 and TF180 manufactured by Toda Kogyo Co., Ltd. are listed. Those having an average particle size of 0.05 to 3 μm are effective, and preferably 0.05 to 1.0 μm.

These abrasives are added in an amount of 1 to 20 parts by weight, preferably 1 to 15 parts by weight, based on 100 parts by weight of the magnetic material. If it is less than 1 part by weight, sufficient durability cannot be obtained, and if it is more than 20 parts by weight, the surface property and filling degree are deteriorated. These abrasives may be dispersed in a binder in advance and then added to the magnetic paint.

The magnetic layer of the magnetic recording medium of the present invention may contain conductive particles as an antistatic agent in addition to the non-magnetic powder. However, in order to maximize the saturation magnetic flux density of the uppermost layer, it is preferable to add as little as possible to the uppermost layer and to add it to a coating layer other than the uppermost layer. As the antistatic agent, it is particularly preferable to add carbon black in order to reduce the surface electric resistance of the entire medium. The carbon black that can be used in the present invention may be a furnace for rubber, thermal for rubber, black for color, conductive carbon black, acetylene black or the like. Specific surface area of 5 to 500 m ² / g, DBP
Oil absorption is 10 to 1500ml / 100g, particle size is 5
mμ to 300 mμ, PH 2 to 10 and water content 0.
It is preferably 1 to 10% and the tap density is 0.1 to 1 g / cc. A specific example of the carbon black used in the present invention is BLACKPEARL manufactured by Cabot Corporation.
S2000, 1300, 1000, 900, 800, 7
00, VULCAN XC-72, manufactured by Asahi Carbon Co., Ltd., #
80, # 60, # 55, # 50, # 35, Mitsubishi Kasei Kogyo Co., Ltd., # 3950B, # 3250B, # 2400B, #
2300, # 900, # 1000, # 30, # 40, #
10B, CONLON CARBON, CONDUCTE
X SC, RAVEN 150, 50, 40, 15, Ketjen Black EC, Ketjen Black ECDJ-500, Ketjen Black ECD manufactured by Lion Aguzo
Examples include J-600. The carbon black may be surface-treated with a dispersant or the like, the carbon black may be oxidized, or may be grafted with a resin, or a part of the surface may be graphitized. Further, the carbon black may be dispersed with a binder in advance before being added to the magnetic paint. When carbon black is used for the magnetic layer, the amount based on the magnetic material is preferably 0.1 to 30% by weight. Further, the nonmagnetic layer preferably contains 3 to 20% by weight based on the total amount of nonmagnetic powder.

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

When the nonmagnetic layer is formed below the magnetic layer of the magnetic recording medium of the present invention, the nonmagnetic layer is a layer in which nonmagnetic powder is dispersed in a binder resin. Various non-magnetic powders can be used for the non-magnetic layer. For example,
α-alumina, β-alumina, γ- with an alpha conversion rate of 90% or more
Alumina, silicon carbide, chromium oxide, cerium oxide, α
-Iron oxide, corundum, silicon nitride, titanium carbide,
Titanium oxide, silicon dioxide, boron nitride, zinc oxide, calcium carbonate, calcium sulfate, barium sulfate and the like are used alone or in combination. The particle size of these non-magnetic powders is preferably 0.01 to 2μ, but if necessary, non-magnetic powders having different particle sizes may be combined, or a single non-magnetic powder may be used to broaden the particle size distribution to obtain the same effect. You can also The non-magnetic powder used may be surface-treated in order to increase the interaction with the binder resin used and improve the dispersibility. The surface-treated product may be treated with an inorganic substance such as silica, alumina or silica-alumina, or may be treated with a coupling agent. Tap density is 0.3 to 2 g / cc, water content is 0.1 to 5% by weight, pH is 2 to 11, specific surface area is 5 to 100 m ^2.
/ G is preferred. The shape of the non-magnetic powder may be needle-like, spherical, dice-like or plate-like. Specific examples of the non-magnetic powder used in the present invention include AKP-20, AKP-30, AKP-50 and HI manufactured by Sumitomo Chemical Co., Ltd.
T-50, Nippon Kagaku Kogyo KK, G5, G7, S-1, Toda Kogyo TF-100, TF-120, TF-14
0, TT055 series made by Ishihara Sangyo, ET300W,
Examples include STT30 manufactured by Titanium Industry Co., Ltd., ferromagnetic iron oxide, and hematite particles, which are an intermediate raw material of ferromagnetic metal powder.

When a ferromagnetic layer is formed under the coating layer of the magnetic recording medium of the present invention, the ferromagnetic substance is ferromagnetic iron oxide, cobalt modified ferromagnetic iron oxide, CrO ₂ , hexagonal ferrite, Various metal ferromagnetic materials dispersed in resin,
Various things can be used

Regarding the coating method for forming two or more coating layers on the non-magnetic support, a specific method of the wet-on-wet method is as follows.

(1) First, the lower layer is coated by a gravure coating, roll coating, blade coating, or extrusion coating apparatus that is generally used for magnetic coating, and while the layer is still in a wet state, for example, Japanese Patent Publication No. 1 -46186, JP-A-60-238179 and JP-A-2-2
Japanese Patent Publication No. 65672 discloses a method for coating an upper layer by a non-magnetic support pressure type extrusion coating apparatus,

(2) JP-A-63-88080, JP-A-2-17971 and JP-A-2-265672
A coating head having two built-in coating liquid passage slits as disclosed in Japanese Patent Publication, a method for coating a lower layer coating liquid and an upper layer coating liquid almost simultaneously,

(3) A method of coating the upper layer and the lower layer almost at the same time by using the extrusion coating apparatus with a backup roll disclosed in Japanese Patent Laid-Open No. 2-174965.

In the case of applying by the wet-on-wet method, the flow characteristics of the coating liquid for the magnetic layer and the coating liquid for the non-magnetic layer should be as close as possible so that the interface between the coated magnetic layer and the non-magnetic layer is not disturbed. It is possible to obtain a magnetic layer having a uniform thickness and less variation in thickness. Since the flow characteristics of the coating liquid strongly depend on the combination of the powder particles and the binder resin in the coating liquid,
In particular, it is necessary to pay attention to the selection of the non-magnetic powder used for the non-magnetic layer.

The non-magnetic support of the present magnetic recording medium is usually
The thickness is 1 to 100 μm, preferably 3 to 20 μm, and the nonmagnetic layer is 0.5 to 10 μm. Further, formation of layers other than the magnetic layer and the non-magnetic layer according to the purpose is allowed as long as the magnetic layer is the uppermost layer and the non-magnetic layer is the lower layer. For example,
An undercoat layer may be provided between the non-magnetic support and the lower layer to improve adhesion. This thickness is 0.01 to 2μ
m, preferably 0.05 to 0.5 μm. Also,
A backcoat layer may be provided on the side opposite to the side of the nonmagnetic support magnetic layer. This thickness is 0.1 to 2 μm, preferably 0.3 to 1.0 μm. Known intermediate layers and back coat layers can be used. In the case of a disk-shaped magnetic recording medium, the above layer structure can be provided on both sides or both sides.

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

In order to effectively achieve the object of the present invention, the surface roughness of the non-magnetic support is 0.03 μm or less, preferably 0.02 μm, as the center line average surface roughness Ra (cutoff value 0.25 mm). Hereafter, it is more preferably 0.01 μm or less. Further, it is preferable that these non-magnetic supports not only have a small center line average surface roughness but also that they have no coarse protrusions of 1 μm or more. Further, the surface roughness shape can be freely controlled depending on the size and amount of the filler added to the non-magnetic support, if necessary. Examples of these fillers include oxides and carbonates of Ca, Si, Ti and the like, as well as fine powders of organic resins such as acrylic resins. The F-5 value in the web running direction of the non-magnetic support used in the present invention is preferably 5 to 50 kg / m.
m ² , the F-5 value in the web width direction is preferably 3 to 30.
It is kg / mm ² , and the F-5 value in the long web direction is generally higher than the F-5 value in the web width direction.
This is not the case especially when it is necessary to increase the strength in the width direction.

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

The organic solvent used in the present invention may be any ratio of ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone, tetrahydrofuran, etc., and methanol.
Alcohols such as alcohol, ethanol, propanol, butanol, isobutyl alcohol, isopropyl alcohol, methylcyclohexanol, etc., methyl acetate, butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate, glycoacetate -Esters such as glycol, dimethyl ether, glycol monoethyl ether, glycol ethers such as dioxane, aromatic hydrocarbons such as benzene, toluene, xylene, cresol, chlorobenzene, methylene chloride, ethylene chloride, etc. Chlorinated hydrocarbons such as carbon tetrachloride, chloroform, ethylene chlorohydrin, dichlorobenzene, N,
Those such as N-dimethylformamide and hexane can be used. These organic solvents are not necessarily 100% pure, and may contain impurities such as isomers, unreacted substances, by-products, decomposition products, oxides, and water in addition to the main components. The content of these impurities is preferably 30% or less, more preferably 10% or less. If necessary, the type and amount of the organic solvent used in the present invention may be changed in the magnetic layer and the intermediate layer. , A highly volatile solvent is used for the first layer to improve the surface property, a solvent having a high surface tension (cyclohexanone, dioxane, etc.) is used for the first layer to improve the coating stability, and a solubility parameter for the second layer As an example, there is a method of increasing the filling degree by using a solvent having a high solvent, but it is needless to say that the present invention is not limited to these examples.

The magnetic recording medium of the present invention is kneaded and dispersed in an organic solvent together with the ferromagnetic powder, the binder resin, and other additives if necessary, and the magnetic coating material is applied onto a non-magnetic support, It is obtained by orienting and drying if necessary.

The process for producing the magnetic coating material for the magnetic recording medium of the present invention comprises at least a kneading process, a dispersing process, and a mixing process provided before and after these processes, if necessary. Each process may be divided into two or more stages. All the raw materials such as the magnetic material, binder, carbon black, abrasive, antistatic agent, lubricant and solvent used in the present invention may be added at the beginning or in the middle of any step. In addition, individual raw materials may be divided and added in two or more steps. For example, polyurethane may be divided and added in the kneading step, the dispersing step, and the mixing step for adjusting the viscosity after dispersion.

Various kneading machines are used for kneading and dispersing the magnetic paint. For example, two-roll mill, three-roll mill, ball mill, pebble mill, tron mill, sand grinder, Szegvari, attritor, high speed impeller disperser, high speed stone mill, high speed impact mill, disper, kneader, high speed mixer, homogenizer, ultra A sonic disperser or the like can be used.

In order to achieve the object of the present invention, it goes without saying that the conventionally known manufacturing techniques can be used as a part of the steps, but in the kneading step, a continuous kneader or a pressure kneader is used. High Br of the magnetic recording medium of the present invention can be obtained only by using a material having a strong kneading force such as a dam. When a continuous kneader or a pressure kneader is used, all or a part of the magnetic substance and the binder (however, 30% or more of the total binder is preferable) and 15 to 500 parts by weight per 100 parts by weight of the magnetic substance. The kneading is performed within the range. Details of these kneading treatments are described in JP-A-1-10.
6338 and JP-A-64-79274. In the present invention, the simultaneous multi-layer coating method as shown in Japanese Patent Laid-Open No. 62-1212933 can be used for more efficient production.

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

The porosity of the lower layer and the uppermost layer of the magnetic layer is preferably 30% by volume or less, more preferably 10% by volume or less. The porosity of the nonmagnetic layer is preferably larger than that of the magnetic layer, but may be small as long as the porosity of the nonmagnetic layer is 5% by volume or more.

The magnetic recording medium of the present invention has a lower layer and an uppermost layer, but it is easily presumed that the physical properties of the lower layer and the uppermost layer can be changed according to the purpose.
For example, the elastic modulus of the uppermost layer is increased to improve running durability, and at the same time, the elastic modulus of the lower layer is made lower than that of the magnetic layer to improve the contact of the magnetic recording medium with the head.

By such a method, the magnetic layer coated on the support is optionally subjected to a treatment for orienting the ferromagnetic powder in the layer, and then the formed magnetic layer is dried. If necessary, the surface is smoothed or cut into a desired shape to manufacture the magnetic recording medium of the present invention. The above-mentioned composition for the uppermost layer and the composition for the lower layer are dispersed together with a solvent, and the obtained coating liquid is coated on a non-magnetic support and oriented and dried to obtain a magnetic recording medium.

The elastic modulus at 0.5% elongation of the magnetic layer is preferably 100 to 2000 in both the web coating direction and the width direction.
kg / mm ² , breaking strength is preferably 1 to 30 kg /
cm ² , the elastic modulus of the magnetic recording medium is preferably 100 to 1500 kg / mm ² in both the web application direction and the width direction, the residual spread is preferably 0.5% or less, and the heat shrinkage rate at any temperature of 100 ° C. or less is desirable. Is 1% or less, more preferably 0.5% or less, most preferably 0.1%
It is the following.

The magnetic recording medium of the present invention may be a tape for video use, audio use or the like, or a floppy disk or magnetic disk for data recording.
It is particularly effective for a medium for digital recording in which the loss of a signal due to the occurrence of drop-out is fatal.
Furthermore, by setting the lower layer to be a non-magnetic layer and the thickness of the uppermost layer to be 1 μm or less, it is possible to obtain a high density, large capacity magnetic recording medium having high electromagnetic conversion characteristics.

[0085]

EXAMPLES Examples of the present invention will be specifically described below in comparison with comparative examples. Example 1 Preparation of Nucleus Crystals 200 ml of 2M FeCl ₃ aqueous solution and 200 ml of 5.94 N NaOH aqueous solution in a sealable 1 liter glass container.
Was added with stirring for 5 minutes, and after the addition was completed, stirring was continued for 10 minutes, and the container was completely sealed.

The sample was placed in an oven which had been heated to 100 ° C. in advance and kept for 48 hours. After 48 hours, quench with running water, and centrifuge the reaction solution at 18,000 rpm for 15 hours.
After centrifuging, the supernatant was discarded. Distilled water was added to this to redisperse it, and the mixture was centrifuged again and the supernatant was discarded. Thus, the washing with water was repeated 4 times using the centrifuge. The precipitate of hematite particles (average particle size of about 0.1 μm) that had been washed with water was freeze-dried. 0.5 ml to 4 g of this dry powder
Distilled water is added and crushed with a planetary mill for 30 minutes, then 10
It was washed in a beaker using 0 ml of distilled water, and then ultrasonically dispersed for another 30 minutes. 10000 this dispersion
After centrifugation at rpm for 30 minutes, a supernatant liquid in which ultrafine hematite (average particle size: about 100 angstroms) was dispersed was taken out to obtain a nuclear crystal liquid (nuclear crystal A). The electron micrograph of the obtained nuclei A is shown in FIG. The iron concentration in the nuclear crystal liquid A was 1800 ppm.

Size control of monodisperse spindle hematite 2M / liter FeCl _{3 in a} 1 liter glass beaker
150m of aqueous solution 150m of 4.8N NaOH aqueous solution
1 with stirring for 5 minutes, and after addition is complete, 10 more
After stirring for 1 minute, the resulting ferric hydroxide gel was added to 40 m.
6 samples of each 1 were taken out into a 200 ml container capable of being sealed. To each of them, the nucleus crystal A shown in Table 1 was added with stirring, and after 5 minutes, 0.12 M / liter of N 2 was added.
20 ml of a ₂ SO ₄ aqueous solution was added to each and stirred for 5 minutes. It was put in an oven preheated to 100 ° C. and kept for 48 hours or 72 hours. After holding for a certain period of time, quench with running water, and centrifuge the reaction solution 1
After centrifugation at 8000 rpm for 15 minutes, the supernatant was discarded.
Distilled water was added to this to redisperse it, and the mixture was centrifuged again and the supernatant was discarded. In this way, wash with water using a centrifuge.
Repeated times. Next, 1 M / liter of ammonia water was added and redispersed, and the mixture was centrifuged and the supernatant was discarded. Distilled water was added to this to redisperse it, and the mixture was centrifuged again and the supernatant was discarded. Thus, the washing with water was repeated three times using the centrifuge. The product was freeze dried. The particles thus obtained were observed with a transmission electron microscope to determine the average major axis length and the acicular ratio (major axis / minor axis). The results are shown in Table 1. In addition, electron micrographs of the spindle-type hematite obtained in (1-A) and (1-D) of Table 1 are shown in FIGS. 2 (a) and 2 (b), respectively.

Method for producing ferromagnetic iron oxide Monodisperse spindle type hematite particles 5 thus obtained
An amount of g was spread thinly in a firing tube and heated at 500 ° C. for 2 hours in an electric furnace while flowing air at a rate of 2 l / min for annealing treatment. Next, while lowering the temperature of the electric furnace to 380 ° C., nitrogen gas was flowed at 2 liters / minute for 30 minutes. After reaching 380 ° C., the gas was switched to hydrogen gas, hydrogen gas was flowed at 2 liters / minute, and reduction was performed for 1 hour. After reduction, switch to nitrogen gas,
The temperature of the electric furnace was lowered to 240 ° C. Next, the nitrogen gas was switched to air, and air was flowed at 2 liters / minute for 1 hour to oxidize and produce γFe ₂ O ₃ . Obtained γFe ₂ O ₃
Was measured with a vibrating sample magnetometer (VSM-5 type, manufactured by Toei Industry Co., Ltd.) at a measurement magnetic field strength of 5 KOe. The particles thus obtained were observed with a transmission electron microscope to determine the average major axis length and the acicular ratio (major axis / minor axis). The results are shown in Table 1.

[0089]

[Table 1]

[0090] To investigate the effect of Example ₂ Na 2 SO _4, except for changing the concentration of Na ₂ SO ₄ added in Example 1-A Example 1
The same reaction as in A was performed to produce monodisperse spindle type hematite particles, and then the same reaction as in Example 1 was performed to produce monodisperse spindle type hematite, followed by firing under the same firing conditions as in Example 1. Then, γFe ₂ O ₃ was obtained. ΓF obtained
The magnetic properties of e ₂ O ₃ were measured with a vibrating sample magnetometer (VSM-5 type, manufactured by Toei Industry Co., Ltd. with a magnetic field strength of 5 KOe. The particles thus obtained were observed with a transmission electron microscope to obtain an average length. The axial length and the acicular ratio (major axis / minor axis) were determined, and the results are shown in Table 2.

[0091]

[Table 2]

[0092] Further, in order to examine the effect of NaH ₂ PO _4, except for changing the concentration of NaH ₂ PO ₄ in Example 1-A was reacted in the same manner as in Example 1, generates monodisperse spindle-shaped hematite particles Then, the obtained particles were observed with a transmission electron microscope to determine the average major axis length and the acicular ratio (major axis / minor axis). The results are shown in Table 3.

[0093]

[Table 3]

Example 3 In order to investigate the effect of the annealing treatment of the monodisperse spindle type hematite prepared under the conditions of Example 1-A, the heating conditions for hematite were changed and reduced and reoxidized. Example 1-A
The monodisperse spindle-type hematite prepared under the conditions described above was bubbled in water and hydrogen gas containing water vapor was added at 350 ° C. for 1 hour.
It was reduced for an hour, switched to nitrogen gas, changed the heat treatment temperature, annealed, and then oxidized with air at 240 ° C. The magnetic properties of the obtained γFe ₂ O ₃ were measured with a vibrating sample magnetometer (VSM-5 type manufactured by Toei Industry Co., Ltd.) at a magnetic field strength of 5 KOe. The particles thus obtained were observed with a transmission electron microscope to determine the average major axis length and the acicular ratio (major axis / minor axis). The specific surface area was measured by using Cantersorb (manufactured by Canterchrome) at 250 ° C. for 30 minutes for dehydration. The results are shown in Table 4.

[0095]

[Table 4]

Example 4 5 g of the magnetic iron oxide obtained in Examples 1-A, 1-C, and 3-C was kneaded with a vinyl chloride vinyl acetate binder (5 g of a cyclohexanone solution of 20 wt%) by a kneader and then the solvent was added. (15 g of a 1: 1 mixed solvent of toluene and cyclohexanone) was added and dispersed, and the dispersion was applied to PET by an applicator and then 15,000.
The magnetic field was oriented by the facing magnet of e. Magnetic properties of the obtained sheet sample were measured with a vibrating sample magnetometer (VSM-5 type manufactured by Toei Industry Co., Ltd.) at a magnetic field strength of 2 KOe. The results obtained are shown in Table 5.

Comparative Examples 7 and 8 Magnetic iron oxide obtained in Comparative Example 3, magnetic iron oxide M manufactured by Toda Kogyo Co., Ltd.
X450 (Hc 3500e, σs 72.5emu /
g, average major axis length 0.5 μm, acicular ratio 10, specific surface area 20
m ² / g) was used to prepare a magnetic sheet in the same manner as in Example 4, and the magnetic properties were measured. The results obtained are shown in Table 5.

[0098]

[Table 5]

Example 5 The monodisperse spindle-type hematite obtained in Example 1-A was dispersed in water, and cobalt sulfate and nickel sulfate were set to 100 atom% of Fe in the hematite, Co of 6 atom% and Ni of Three
The mixture was added to the suspension so that the concentration became atomic%, and the mixture was sufficiently stirred and mixed. While monitoring the pH of this suspension, ammonia water was added to the suspension to adjust the pH to 8.0 and C on the hematite surface.
o, Ni compound was deposited. The suspension was centrifuged, the supernatant was discarded, and then distilled water was added to suspend the suspension again. An aqueous solution of sodium aluminate and sodium silicate makes Fe in hematite 100 atomic%, Al 3 atomic%, S
i was added to the suspension so as to be 1 atomic%, and p of the suspension was added.
Carbon dioxide was bubbled through the suspension while monitoring H to adjust the pH to 7.0, and Al and Si compounds were deposited on the hematite surface. The suspension was centrifuged, the supernatant was discarded, distilled water was added, and the suspension was resuspended three times to wash and dry the suspension.

5 g of the surface-treated monodisperse spindle type hematite was thinly spread in a firing tube and treated under various conditions shown in the table under annealing conditions. The annealing with hematite was carried out by heating at a predetermined temperature for 2 hours in an electric furnace while flowing nitrogen at a rate of 2 liters / minute. Next, after the temperature of the electric furnace reached 450 ° C., the hydrogen gas was changed to hydrogen gas at a rate of 2 liters / minute for reduction for 6 hours. When annealing with magnetite, surface-treated hematite is bubbled in water and reduced with hydrogen gas containing water vapor at 350 ° C for 1 hour, switched to nitrogen gas and annealed by changing the heat treatment temperature and the temperature of the electric furnace. To 450 ° C and switch to hydrogen gas
It was flowed in liter / minute and reduced for 6 hours. After completion of the reduction, the atmosphere was changed to nitrogen gas and cooled to room temperature. Using a gas mixer, mix oxygen-containing gas into nitrogen gas, adjust the oxygen concentration in nitrogen to 0.1% by volume, contact for 5 hours, and then monitor the temperature of the upper part of the metal powder so that it does not exceed 50 ° C. The oxygen concentration was increased while increasing the oxygen concentration to 21%, and the metal was gradually oxidized. The obtained metal powder was applied to a vibrating sample magnetometer (V
The magnetic characteristics were measured with a magnetic field strength of 10 KOe (SM-5 type manufactured by Toei Industry Co., Ltd.). The specific surface area was measured by using a cantersorb (manufactured by Canterchrome Co., Ltd.) at 250 ° C. for 30 minutes for dehydration. Further, the obtained particles were observed with a transmission electron microscope to determine the average major axis length and the acicular ratio (major axis / minor axis).
The obtained results are shown in Table 6.

[0101]

[Table 6]

Example 6 Hematite nucleus crystal formation

2M / in a sealable 2 liter glass container
500 ml of 5.94 N NaOH aqueous solution in 500 ml of FeCl ₃ aqueous solution of 1 liter
Add l while stirring for 5 minutes, then add 20 more
After stirring for a minute, the container was completely sealed. 100 ℃ in advance
It was put in an oven heated to 0 ° C and kept for 48 hours. After 48 hours, it was rapidly cooled with running water, the reaction solution was collected and centrifuged at 15000 rpm for 15 minutes in a centrifuge, and the supernatant was discarded. Distilled water was added to this to redisperse it, and the mixture was centrifuged again and the supernatant was discarded. Thus, washing with water was repeated 4 times using the centrifuge. The precipitate of hematite particles (average particle size of about 0.1 μm) that had been washed with water was filtered and dried. This dry powder 5
5 ml of distilled water was added to 0 g, crushed for 30 minutes with a raikai machine, washed in a beaker with 500 ml of distilled water, and 700 ml of distilled water was added, followed by ultrasonic dispersion for 30 minutes. This dispersion was collected and centrifuged at 10,000 rpm for 30 minutes to take out a supernatant liquid in which ultrafine hematite (average particle size: about 100 angstroms) was dispersed to obtain a nuclear crystal liquid. The iron concentration in the nucleating solution was 2000 ppm.

Size control of monodisperse spindle type hematite 2M / liter FeCl 2 in 5 liter stainless beaker
₍₃₎ 1500 ml of 4.8 N NaOH aqueous solution was added to 1500 ml of aqueous solution with stirring for 10 minutes, and after stirring for 30 minutes after the addition, the obtained ferric hydroxide gel was taken out in 1000 ml aliquots of 2000 ml. Add the following amount of nucleation solution to each of them with stirring, and add 0.12 after 10 minutes.
500 ml of Na ₂ SO ₄ aqueous solution of M was added to each and stirred for 10 minutes.
It was put in an oven preheated to 100 ° C and kept for 72 hours. After holding for a predetermined time, it was quenched with running water.
The reaction solution was filtered, distilled water was added thereto to redisperse, and the solution was filtered again. In this way, filtration and redispersion were carried out, and washing with water was repeated 3 times. Next, 1 M / liter of ammonia water was added and redispersed, heated to 80 ° C., held for 1 hour and cooled. After filtration and addition of distilled water, redispersion was repeated 3 times. The product obtained was dried. The particles thus obtained were observed with a transmission electron microscope to determine the average major axis length and the acicular ratio (major axis / minor axis). The results are shown in Table 6. Synthesis of ferromagnetic metal powder (metal powder) Obtained monodisperse spindle type hematite (6-A, 6-B, 6
-C) is dispersed in distilled water so that the concentration of hematite is 2%, and cobalt sulfate and nickel sulfate are used so that Fe in the hematite is 100 at%, Co is 6 at%, and Ni is 3 at%. The mixture was added and thoroughly mixed with stirring. While monitoring the pH of this suspension, aqueous ammonia was added to the suspension to adjust the pH to 8.0 and Co and Ni compounds were deposited on the surface of hematite. An aqueous solution of sodium aluminate and sodium silicate was added to this solution to obtain 100 atomic% of Fe in hematite.
Is added to the suspension so that Al is 3 atomic% and Si is 1 atomic%, and carbon dioxide is bubbled through the suspension while monitoring the pH of the suspension to adjust the pH to 7.0 and Co- Al and Si compounds were deposited on the surface of the Ni-attached spindle type hematite.
The suspension was filtered and washed with distilled water to remove impurities. The obtained surface-treated spindle-type hematite was passed through a molding plate having a diameter of 3 mm, molded into a cylindrical shape, and dried.

Surface-treated monodisperse spindle type hematite
500 g was put in a rotary reduction furnace and annealed in nitrogen at 500 ° C. for 2 hours. Next, the temperature was set to 450 ° C., the hydrogen gas was changed to a hydrogen gas flow rate of 20 l / min, and reduction was performed for 10 hours.
After completion of the reduction, the atmosphere was changed to nitrogen gas and cooled to room temperature. Using a gas mixer, mix oxygen-containing gas in nitrogen gas,
The oxygen concentration in nitrogen was set to 0.1% by volume and allowed to contact for 5 hours,
Then, the temperature of the upper part of the metal powder was monitored, and the oxygen concentration was increased so as not to exceed 50 ° C., the oxygen concentration was increased to 21%, and the metal powder was gradually oxidized. The magnetic properties of the obtained metal powder were measured with a vibrating sample magnetometer (VSM-5 type manufactured by Toei Industry Co., Ltd.) at a magnetic field strength of 10 KOe. In addition, using a specific surface area of Canter Sorb (manufactured by Canter Chrome),
It was dehydrated at 250 ° C. for 30 minutes and measured. Further, the obtained particles were observed with a transmission electron microscope to determine the average major axis length and the acicular ratio (major axis / minor axis). The results are shown in Table 7.

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[Table 7]

Preparation of Coating Liquid Commercially available Co—Ni-containing metal powder (Hc1640O) made of goethite via the above ferromagnetic powder and iron carbonate.
e, σs126 emu / g, major axis length 0.13 μm, needle ratio 1
0, specific surface area 58 m ² / g) was used as a magnetic paint under the following conditions. (Upper layer composition) 100 parts of ferromagnetic powder binder resin of vinyl chloride copolymer 12 parts (-SO ³ Na groups and containing 1 × 10 ^-4 eq / g polymerization degree: 300 Polyester polyurethane resin 3 parts (neopentyl glycol / Caprolactone polyol / MD I = 0.9 / 2.6 / 1-SO ³ Na group 1 × 10 ^-4 eq / g contained) α-alumina (average particle size 0.1 μm) 1.5 parts carbon black (average particle Size 100 nm) 0.5 part Butyl stearate 1 part Stearic acid 2.5 parts Methyl ethyl ketone and cyclohexanone 1: 1 mixed solvent 200 parts

(Composition for lower layer) 80 parts of needle-shaped α-Fe ² O ₃ (average particle length 0.15 μm, average needle-shaped ratio 12, Al-Si treatment, surface area by BET method 55 m ² / g pH 6.4) carbon black 20 parts (average primary particle diameter 16 nm, DBP and oil amount 80 ml / 100 g BET specific surface area 55 m ² / g pH 6.4) Binder resin Vinyl chloride-vinyl acetate-vinyl alcohol copolymer 10 parts (-N (CH3 ) Containing 5 × 10 ⁻⁶ eq / g of polar group of ³⁺ Cl ⁻ Monomer composition ratio 86: 13: 1 Polymerization degree 400) Polyester polyurethane resin 8 parts (basic skeleton: 1,4-BD / phthalic acid / HMDI molecular weight : 10200 hydroxyl: 0.23 × 10 ^-3 eq / g containing -SO ₃ Na group: 1 × 10 ^-4 eq / g containing butyl stearate 1 part stearic acid 2.5 Methyl ethyl ketone and cyclohexanone 1: 1 mixed solvent 200 parts

Composition for Lower Layer) A needle-shaped C instead of the needle-shaped α-Fe ₂ O _{3 in} the composition for the lower layer.
o Modified iron oxide (specific surface area 45m ² / g, Hc850O
e, an axial length of 0.15 μm, an axial ratio of 8, and an Al-Si surface-treated product) were used. The lower layer composition and the upper layer composition were each kneaded with a kneader and then dispersed using a sand grinder. 5 parts by weight of polyisocyanate was added to the lower layer coating solution,
6 parts were added to the upper layer coating solution, and 20 parts of a mixed solvent of methyl ethyl ketone and cyclohexanone 1: 1 was further added,
Filtration was performed using a filter having an average pore size of 1 μm to prepare coating solutions for forming a non-magnetic layer and for forming a magnetic layer.

The obtained lower layer coating solution was applied so that the thickness after drying was 2 μm, and immediately after that, while the lower layer coating layer was still in a wet state, the thickness of the magnetic layer thereon was When wet simultaneous multi-layer coating is performed on a polyethylene terephthalate support having a thickness of 7 μm so as to have a predetermined thickness, and when orientation treatment is performed, Sm should be applied while both layers are still in a wet state.
Magnetic field orientation of the ferromagnetic powder was performed using a Co magnet (surface magnetic flux 3000 gauss) and a solenoid electromagnet (surface magnetic flux 1500 gauss), and dried. Then, a seven-stage calender composed of metal rolls was used for calendering at a roll temperature of 90 ° C. to obtain a web-shaped magnetic recording medium.
The magnetic characteristics of a sample of 8 mm video tape prepared by slitting it into a width of 8 mm were measured with an applied magnetic field of 5 kOe using a vibrating sample magnetometer VSM-5 manufactured by Toei Industry. The tape using magnetic iron oxide whose Hc is lower than that of metal is SF for the Hc and high Hc parts corresponding to the metal.
D was calculated.

A FUJIX8 8 mm video deck manufactured by Fuji Photo Film Co., Ltd. was used to record signals of 5 MHz and 10 MHz, and the reproduction output when these signals were reproduced was read from an oscilloscope and measured. The output was expressed as a relative value to a single layer tape using a commercially available metal powder. The measurement results are shown in Table 8 below.

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[Table 8]

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The monodisperse spindle hematite can be obtained even at a concentration 10 to 100 times higher than that obtained by the conventional hydrolysis method, which is an economical production method. Further, when the weight-proof hematite of the present invention was converted into magnetic iron oxide, the magnetic properties (particularly Hc) could be increased by applying the annealing treatment. Also obtained g
Ferromagnetic iron oxide using ferromagnetic iron oxide, SQ and SF
D (a differential curve of the BH curve and a measure of Hc distribution) is significantly superior to the magnetic iron oxide produced by the conventional goethite method.

The magnetic recording medium obtained by dispersing the ferromagnetic metal powder obtained from the monodisperse spindle type hematite as the raw material in the present invention together with the binder resin and coating it on the non-magnetic support has the squareness ratio and the SFD of the conventional method. Compared with magnetic recording media using ferromagnetic metal powder, it shows significantly superior characteristics. In particular, the improvement of the short wavelength output of a magnetic recording medium in which a thin magnetic layer (in particular, a ferromagnetic metal powder layer) is formed on a non-magnetic layer is remarkably improved.

Further, as is clear from the above results, even with the metal powder (ferromagnetic metal powder) synthesized from monodisperse hematite, the particle size is large and 1 / of the recording wavelength.
It can be seen that when the number is 2 or more, the output near that frequency is lower than that according to the present invention.

[Brief description of drawings]

FIG. 1 is an electron micrograph of fine particle hematite (nuclear crystal A) used as a nuclei crystal prepared in Example 1,

2 (a) and 2 (b) are electron micrographs of monodisperse spindle-type hematite particles obtained by using (1-A) and (1-D) of Example 1, respectively.

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[Procedure amendment]

[Submission date] September 10, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Brief description of the drawing

[Correction method] Change

[Correction content]

[Brief description of drawings]

FIG. 1 is an electron micrograph of the particle structure of fine grain hematite (nuclear crystal A) particles used as a nuclei crystal prepared in Example 1,

2 (a) and 2 (b) are electron micrographs of the particle structure of monodisperse spindle-type hematite particles obtained by using (1-A) and (1-D) of Example 1, respectively. is there. ─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] June 28, 1994

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0041

[Correction method] Change

[Correction content]

Hydrogen is used to reduce to 3
The reduction is carried out at 50 to 550 ° C, preferably 400 to 500 ° C. The progress of the reduction can be determined by monitoring the water content in the hydrogen gas after the reduction with a dew point meter or the like. It is necessary to oxidize and stabilize the surface of the obtained ferromagnetic metal powder. For this reason, after the reduction, the surface is gradually oxidized by flowing a gas whose oxygen concentration is controlled or by passing an oxygen-containing gas in an organic solvent. In this case, monitor the calorific value with a thermometer to prevent sudden oxidation. Further, it is desirable that the water content in the gas used for the slow oxidation be as low as possible. Further, after the treatment with compounds described in JP-63-52327 Publication before the slow oxidation, it is also effective in enhancing the saturation magnetization σs of the ferromagnetic powder to be slow oxidation.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Name of item to be corrected] 0112

[Correction method] Change

[Correction content]

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[Table 8]

Claims (8)

[Claims] 1. A after dispersing the hematite particles as seed crystals in the gel suspension of the sulfate ion (S0 ₄ ^2-) or ferric hydroxide in the presence of phosphate root ions, aqueous oxide second
A method for producing monodisperse spindle-type hematite particles, which comprises hydrolyzing iron. 2. The concentration of sulfate ion in the ferric hydroxide gel suspension is from 0.01 mol / liter to 0.
The method for producing hematite particles according to claim 1, wherein the content is 1 mol / liter or the concentration of phosphate root ions is 0.001 to 0.01 mol / liter. 3. The monodisperse spindle-type hematite particles obtained in claim 1 or 2 are reduced to magnetite, and the magnetite is then oxidized to produce FeO _x (1.33 <
x ≦ 1.5), a method for producing a ferromagnetic iron oxide. 4. A method for producing a ferromagnetic metal powder, which comprises reducing the monodisperse spindle-type hematite particles obtained in claim 1 or 2 to magnetite, and then reducing the magnetite. 5. The method for producing ferromagnetic iron oxide according to claim 3, wherein the monodisperse spindle type hematite particles or the magnetite is annealed. 6. The method for producing a ferromagnetic metal powder according to claim 4, wherein the monodisperse spindle type hematite particles or the magnetite is annealed. 7. A magnetic recording medium comprising a magnetic layer mainly comprising a ferromagnetic powder and a binder resin on a non-magnetic support,
A magnetic recording medium characterized in that the ferromagnetic powder is the ferromagnetic iron oxide obtained in claim 3 or 5, or the ferromagnetic metal powder obtained in claim 4 or 6. 8. A magnetic recording medium comprising a non-magnetic layer containing a non-magnetic powder and a binder resin as a main component on a non-magnetic support, and a magnetic layer mainly containing a ferromagnetic powder and a binder resin provided on the non-magnetic layer. The film thickness of the magnetic layer is 1.0 μm or less, and the ferromagnetic powder is the ferromagnetic iron oxide obtained in claim 3 or 5, or the ferromagnetic metal powder obtained in claim 4 or 6. A magnetic recording medium characterized by: