JPH11100213A - Production of hematite, production of ferromagnetic powder using the hematite and magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce fine hematite particles excellent in particle size distribution and improved in coercive force distribution by adding a specific α-Fe2 O3 nucleus crystal in a ferric compound solution, heating them under the presence of a crystal form controlling agent thereby depositing α-Fe2 O3 on the nucleus crystal to form a mono disperse fusiform type α-Fe2 O3 . SOLUTION: The α-Fe2 O3 nucleus crystal having <=100 Åaverage particle diameter and >=100% coefficient of variation is added into the ferric compound solution having a concentration of 0.005-1 mol, preferably 0.0075-0.5 mol. The mono disperse fusiform type α-Fe2 O3 having <=0.2 μm, preferably 0.05-0.18 μm average major axis length and an acicular ratio (average malor axis length/ average minor axis length) of >=4, preferably 4-8 is obtained by adding 0.001-0.1 mol, preferably 0.005-0.05 mol crystal form controlling agent per 1 mol ferric compound into the solution, adjusting to >=pH 10 when the crystal form controlling agent is an organic phosphoric acid or to <=pH 2.5 when the crystal form controlling agent is a phosphate compound and heating at 100-200 deg.C for 24-240 hr to deposit α-Fe2 O3 on the nucleus crystal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an improvement in a method for producing a monodisperse spindle type hematite having a very small particle size distribution useful as a raw material for a pigment, a paint, a catalyst, a ferrite, a ferromagnetic powder for magnetic recording, and the like. The present invention relates to a magnetic recording medium for high-density recording using a ferromagnetic powder for magnetic recording using this material as a raw material, and particularly to a method for producing monodisperse spindle type αFe ₂ O ₃ particles having fine particles and excellent particle size distribution. This αFe ₂
The present invention relates to a method for producing a magnetic powder for magnetic recording which is excellent in magnetic properties, particularly a coercive force distribution by using O ₃ , and a magnetic recording medium.

[0002]

2. Description of the Related Art Magnetic recording technology requires that a medium can be used repeatedly, that signals can be easily digitized, and that a system can be constructed in combination with peripheral devices.
Video, audio, video, audio, etc.
It has been widely used in various fields including computer applications.

In order to respond to demands for downsizing equipment, improving the quality of recording / reproducing signals, extending recording time, increasing recording capacity, etc., the recording density of recording media has been further improved. Always wanted.

[0004] For example, in audio and video applications, in order to cope with the practical use of a digital recording system for realizing an improvement in sound quality and image quality, and the development of a recording system compatible with a high-definition TV, a conventional system is required. Therefore, a magnetic recording medium capable of recording and reproducing a short wavelength signal has been required. Even for computer applications, there is an increasing demand for higher capacity to cope with the increase in large capacity data.

In addition, floppy disks having a magnetic layer on a flexible support, which is an external storage medium of a personal computer, have been used in recent years due to the spread of personal computers, the sophistication of application software, and the increase in processing information. High-capacity products of 100 MB or more have appeared.

[0006] The magnetic recording medium is used in the form of a tape, disk or card depending on the application by forming a magnetic layer on a support. The magnetic layer is coated with a coating liquid in which a ferromagnetic powder is dispersed and dried on a support. The so-called coating-type magnetic layer mainly composed of a ferromagnetic powder and a binder resin, and a vacuum such as evaporation or sputtering. There is a so-called metal thin film type magnetic layer formed of a ferromagnetic metal thin film obtained by forming a vacuum film on a support by a film forming method.

[0007] From the viewpoint of recording density, the latter metal thin film type magnetic layer is excellent, and its practical use has been studied for a long time. Used in the system.

However, the metal thin film type magnetic recording medium has problems in characteristics such as durability, running property, corrosion resistance and the like.
In order to deal with the problem, the original excellent characteristics have not been fully utilized. In addition, the production cost is high and there is a problem in economy. On the other hand, so-called coated magnetic recording media having a magnetic layer mainly composed of a ferromagnetic powder and a binder resin are superior to the metal thin film type in terms of running property, durability, and corrosion resistance, and have been used for a long time. Although it has been put to practical use for various uses, with regard to the electromagnetic conversion characteristics, many efforts have been made to improve the recording density, which is inferior to that of the metal thin film type, but there is a limit.

From the viewpoints of improving the magnetic properties of the magnetic layer, improving the dispersibility of the ferromagnetic powder, and improving the surface properties of the magnetic layer, etc., for increasing the recording density of the coating type magnetic recording medium. Various methods have been studied and proposed.

For example, a method of using a 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-61-7.
No. 4137, Japanese Patent Publication No. 62-49656, Japanese Patent Publication No. 60-50323, US Pat.
No. 4,666,770 and US Pat. No. 4,543,198.

In order to enhance the dispersibility of the ferromagnetic powder, various surfactants (for example, JP-A-52-15660) are used.
No. 6, JP-A-53-15803, JP-A-53-15803
-116114. ) Or various reactive coupling agents (for example,
Nos. 9-59608, JP-A-56-58135, and JP-B-62-28489. ) Has been proposed.

Further, in order to improve the surface properties of the magnetic layer,
There has been proposed a method of improving the surface forming treatment method of a magnetic layer after coating and drying (for example, disclosed in Japanese Patent Publication No. 60-47725).

The above-mentioned ferromagnetic metal powder and hexagonal ferrite are excellent as ferromagnetic powders for high-density recording media, but their electromagnetic conversion properties are inferior to metal thin-film magnetic recording media. . 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. In addition, the dispersibility was poor, and it was difficult to obtain a magnetic layer having excellent surface properties.

As a method for improving the recording density by reducing the self-demagnetizing loss and the recording demagnetizing loss of the magnetic layer, the thickness of the magnetic layer is made as thin as a ferromagnetic metal thin film type magnetic recording medium and A magnetic recording medium provided with a relatively thick non-magnetic layer as a lower layer in order to ensure the flatness and strength of the layer is disclosed in
7-198536, JP-A-62-154225, JP-A-63-187428, JP-A-4-3.
No. 25915, JP-A-4-325916, JP-A-4-325917, and the like, which have particularly excellent output in a high frequency range and exhibit electromagnetic conversion characteristics close to those of a ferromagnetic metal thin film type magnetic recording medium. However, there is a limit to further improve the electromagnetic conversion characteristics due to the following problems related to the particle shape of the ferromagnetic powder.

That is, metal thin film type and metal (ferromagnetic metal)
When comparing the magnetic properties of the coating type media using powder,
SFD (Switching) of coating type medium using metal powder
Field Destribution) has a large numerical value compared to metal thin films, and it has recently been revealed that the properties are inferior.On the other hand, Nakamura et al. Performed simulations to improve the electromagnetic conversion characteristics of metal thin film media, It has been pointed out that improving the distribution of the inductive magnetic field improves the short-wavelength output (Journal of the Japan Society of Applied Magnetics, Vol. 115, 155-158 (1991)). The anisotropic magnetic field distribution has a strong correlation with the SFD. If the anisotropic magnetic field distribution is large, the SFD (Hc distribution) increases.

It is known that ferromagnetic metal powder has a large effect on shape anisotropy with respect to Hc (coercive force).
It is known that the distribution of c also increases. For this purpose, goethite, which is acicular particles, which is a raw material of ferromagnetic metal powder,
Attempts to reduce the particle size distribution of lipid closite, hematite and the like are continually ongoing. When a fine-grained ferromagnetic metal powder is used to reduce the tape surface roughness and improve the short-wavelength output, variations in the major axis length and minor axis length have a large effect on the Hc distribution.

As described above, in order to improve the recording density by further improving the characteristics as compared with the conventional magnetic recording medium, it is necessary that the ferromagnetic metal powder has a small particle size and a uniform particle size and shape as much as possible. Is desired. Particles in which each particle is well separated and whose particle size distribution is very small are called monodisperse particles. Research on synthesizing monodisperse particles to study particle size and catalytic performance and particle-particle interactions (eg, surface,
29, 978 (1991) and 23, 177 (198)
4)). The method of synthesizing monodisperse spindle type hematite is as follows.
M. Ozaki et al. Colloid Inte
rface Sci. 102 146-151 (198
Reported in 4). They use the first method, 0.
02 mol of 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 is kept at 100 ± 2 ° C. for 2 days, and a particle length of 0.2
Particles having a diameter of 6 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 make 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 distilled water 500 c.
m ³ and kept at 100 ± 2 ° C. for 2 days, particle length 0.2
Particles having a diameter of 7 to 0.33 μm and an acicular ratio of 3 to 6 are synthesized. It is reported that if the concentration of ferric chloride and NaH ₂ PO ₄ or NaH ₂ PO ₂ is increased to increase the yield, βFeOOH is mixed in, and a single-phase monodisperse spindle type hematite cannot be obtained.

M. Ozaki et al. Hydrogen-reduce the obtained monodisperse spindle type hematite at 340 to 400 ° C. for 1 to 3 hours, and then oxidize at 240 ° C. for 1 to 2 hours by flowing air to obtain γFe ₂ O.
₃ and the results are described in J. Colloid Interfac
e Sci. 107 199-203 (1985). Sugimoto (Tohoku University) started with an extremely dense Fe (OH) ₃ gel (about 1 mol / l) as a starting material, then passed through an intermediate product βFeOOH to form a pseudo-cube (average particle size about 2 μm) monodisperse. Hematite particles were synthesized (J. Colloid Interface Sci.
152, 587 (1992)).

M. In the method of Ozaki et al., ΓFe ₂ O ₃ is obtained from monodisperse spindle type hematite. However, the yield of monodisperse spindle type hematite is low because the concentration of the reaction solution for producing monodisperse spindle type hematite is low, so that it is economically large. Cannot 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.

[0020] Sugimoto, Muramatsu considers monodisperse alpha iron ₂ O ₃ generation mechanism in dilute FeCl ₃ solution, co-exist in alpha iron ₂ O ₃ iron solute core crystal surface (the iron hydroxide and systems in the liquid BetaFeOOH
Dissolves into iron solutes), and adsorbs and grows. They coexist with phosphate ions and have an average major axis length.
ΑFe ₂ O ₃ particles having 0.22 μm to 0.67 μm and a needle ratio of を 得 6 have been obtained (J. Colloid & Interface Sci. 184, 626-638 (199
6)).

The present inventor has proposed that monodisperse αFe ₂ O ₃ is synthesized, used as a magnetic powder for magnetic recording, and further used as a magnetic recording medium (JP-A-6-340426, JP-A-7-109122). , Japanese Patent Application No. 8-23644. In order to cope with higher C / N and higher capacity of the magnetic recording medium, fine particles are further required. Further, in order to improve the overwrite characteristics in digital recording, it is necessary to make the particle size distribution uniform and to reduce the coercive force distribution. Monodisperse αFe ₂ O ₃ having an average major axis length of 0.20 μm or less and a needle ratio of 4 or more
It is necessary to generate However, in the manufacturing method studied conventionally, the variation in the major axis length of αFe ₂ O ₃ (coefficient of variation (%)) is caused by reducing the average major axis length of αFe ₂ O ₃ particles.
= 100 x standard deviation of major axis length / average major axis length).

[0022]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and has an average particle size of 0.1.
A method for producing monodisperse hematite particles having a needle ratio of 4 or more at 2 μm or less is provided, and the obtained spindle-type hematite particles are used as ferromagnetic powder, which is suitable for high-density recording using the ferromagnetic powder. It is to provide a magnetic recording medium.

[0023]

Means for Solving the Problems The present inventors have found that the single crystal α
The present inventors have conducted intensive studies on the reaction conditions for reducing the average particle size of Fe ₂ O ₃ and not increasing the variation in the particle size, and reached the present invention. JP-A-6-340426 proposes to add a nucleus crystal to produce monodisperse αFe ₂ O ₃ . That is, the present inventors have focused on the size and size distribution of the nucleus crystal used in the reaction, and have achieved the present invention.

That is, the present invention has the following constitution. 1. ΑFe having an average particle diameter of 100 ° or less and a coefficient of variation represented by the following formula (1) of 100% or less in the ferric compound solution:
Production of hematite characterized by adding a ₂ O ₃ nucleus, coexisting with a crystal morphology control agent, and precipitating αFe ₂ O ₃ on the nucleus under heating to form a monodisperse spindle type αFe ₂ O ₃ Method.

Coefficient of variation (%) = 100 × standard deviation of particle diameter / average particle diameter (1) When the crystal form controlling agent is a phosphate compound, the pH is adjusted to 2.
5. The method for producing hematite as described in 1 above, wherein the pH is 5 or less, and when the crystal morphology controlling agent is an organic phosphonic acid compound, the pH is 10 or more to obtain a monodisperse spindle type αFe ₂ O ₃ . 3. After reducing the monodisperse spindle type αFe ₂ O ₃ obtained in the above 1 or 2 to Fe ₃ O ₄ , the Fe ₃ O ₄ is oxidized to FeOx (1.33 <x
≦ 1.50) ferromagnetic iron oxide powder.

4. The monodisperse spindle type αFe ₂ O ₃ obtained in the above 1 or 2 is subjected to a sintering prevention treatment and, if necessary, an annealing treatment, and then reduced to a metal powder, and the metal powder is gradually oxidized to obtain a ferromagnetic metal. A method for producing a ferromagnetic powder, which is a powder. 5. A magnetic recording medium comprising a magnetic support and a magnetic layer mainly comprising a ferromagnetic powder and a binder resin, wherein the ferromagnetic powder is the ferromagnetic powder obtained in the above item 3 or 4. Medium.

6. In a magnetic recording medium in which a nonmagnetic layer mainly composed of a nonmagnetic powder and a binder resin is provided on a support and a magnetic layer mainly composed of a ferromagnetic powder and a binder resin is provided thereon, the thickness of the magnetic layer is A magnetic recording medium having a thickness of 1.0 μm or less, wherein the ferromagnetic powder is the ferromagnetic powder obtained in the above item 3 or 4. The invention of claim 1, which is the basis of the present invention, is characterized in that a monodisperse spindle type αFe ₂ O ₃ is obtained by using a specific nucleus crystal and a crystal morphology controlling agent having a particle diameter and a variation coefficient thereof.

According to the present invention, αFe having a fine and excellent particle size distribution
₂ O ₃ seed crystal, i.e. the average particle diameter of 10nm or less, preferably 8nm or less, more preferably should variation coefficient of less and particle diameter thereof 5nm to create a 100% or less of alpha iron ₂ O ₃ seed crystals. Here, the variation coefficient is expressed by the following equation (1). Coefficient of variation (%) = 100 × standard deviation of particle size / average particle size (1) A method for preparing a nucleus crystal having such a coefficient of variation of 100% or less, preferably 90% or less, more preferably 80% or less is particularly preferred. Although not to be limited, the following method is preferably used.

Usually, the average particle size is 500 to 3000 nm.
Polycrystalline αFe ₂ O ₃ is a mechanical force, for example, crushed by a planetary mill, a raikai machine, a ball mill, etc., dispersed in water and centrifuged to remove coarse particles, and further centrifuged at high speed A nucleus crystal having a uniform grain size can be produced. In this case, the degree of pulverization of the polycrystalline αFe ₂ O ₃ is appropriately adjusted according to the desired average particle size of the nucleus crystal, and the coefficient of variation is 100%.
The following nucleus crystal population containing αFe ₂ O ₃ is a compound having a function of, for example, changing the pH of water, preferably the isoelectric point by ultrasonic dispersion or the like, and stabilizing the dispersion of particles, for example, HClO.
₄ is dispersed in an aqueous solution such as
The speed is sequentially increased in the range of 0000 rpm to 30 to 300
By centrifuging the supernatant liquid for one minute, nuclei having a small average particle diameter and a small coefficient of variation can be sequentially obtained.

Further, a compound having a function of adsorbing on the particle surface during the pulverization of polycrystalline αFe ₂ O ₃ and separating the particles, for example, organic phosphonic acid (for example, phenylphosphonic acid, etc.), aminomethylene phosphonic acid, etc. The compound can also be adsorbed on the surface of the nucleus crystal by mixing them. The morphology of the nucleus obtained by the above method depends on the conditions for forming the starting polycrystalline αFe ₂ O ₃ . Here, since the polycrystalline αFe ₂ O ₃ is obtained by aging under heating from mixing and stirring of a ferric salt solution and an alkali solution based on a known gel-sol method, conditions at this time are appropriately adjusted. Thereby, a desired polycrystalline αFe ₂ O ₃ can be obtained.

The obtained nucleus αFe ₂ O ₃ is spindle-shaped (spheroidal) or spherical. In the case of a spindle shape, the major axis length is defined as the particle diameter, and the average value is defined as the average particle diameter. The needle ratio (average particle diameter / average minor axis length) of the nucleus particles used in the present invention is preferably in the range of 1 to 6, more preferably 1 to 4, but is preferably a monodisperse spindle type αFe ₂ O _3. The control of the acicular ratio has a large contribution from the crystal form controlling agent described later.

In order to obtain a monodisperse spindle type αFe ₂ O ₃ from the nucleus of the present invention, it is also important to select a crystal morphology controlling agent to be coexistent with the nucleus in the ferric compound solution. The crystal form controlling agent dissociates in solution and has a coordinating ability for iron, and the function of controlling the growth direction and the rate of the crystal of αFe ₂ O ₃ generated by this coordinating ability, that is, αFe ₂ O ₃ There is no particular limitation as long as it produces ions having the function of controlling the average major axis length and the acicular ratio, mainly anions, but phosphoric acid compounds and organic phosphonic acid compounds are preferred.

When a phosphate compound is used, the pH of the solution
Is preferably 2.5 or less, more preferably 1.3 to 1.
8, when an organic phosphonic acid is used, the pH is preferably 10 or more, more preferably 11 to 13, preferably 100 to 200 ° C, preferably 24 to 240 hours.
More preferably, a monodisperse spindle type αFe ₂ O is formed by depositing αFe ₂ O ₃ on a nucleus through formation of β-FeOOH from the ferric compound while heating the ferric compound under the conditions of 24 to 120 hours. O ₃ can be obtained. If the respective pH setting ranges deviate, new nucleation occurs or the adsorption / desorption ratio of the crystal form controlling agent changes, and the variation coefficient of the generated αFe ₂ O ₃ deteriorates. Acids for pH adjustment include hydrochloric acid, nitric acid, sulfuric acid and the like, and alkalis for the adjustment include hydroxides of alkali metals such as sodium hydroxide.

Examples of the organic phosphonic acid usable in the present invention include compounds having an aminomethylene phosphonic acid group such as aminotrimethylenephosphonic acid and diethylenetripentaaminomethylenephosphonic acid, phenylphosphonic acid, naphthylphosphonic acid, diphenylphosphonic acid and the like. And alkylenediphosphonic acid compounds such as aromatic phosphonic acid, methylene diphosphonic acid, and ethylene diphosphonic acid. As the phosphoric acid compound, an inorganic phosphoric acid compound such as orthophosphoric acid, metaphosphoric acid, hypophosphorous acid, phosphorous acid, pyrophosphoric acid and salts thereof can be used.

The amount of the crystal form controlling agent used is αFe ₂ O
₃ to control the growth direction and speed of the crystal, the average major axis length to be described later, it is preferable that the acicular ratio and major axis of the variation coefficient is adjusted to the obtained amount, usually ferric compound 1 mole 0.001 to 0.1 mol, preferably 0.005 to
It is in the range of 0.05 mole. Further, in the present invention, coexisting ions in a heating reaction system to obtain a monodisperse spindle type αFe ₂ O ₃ , for example, ions mainly dissociated from a ferric compound, ions used for preparing the nucleus, and the like. Has the same function as the above-mentioned crystal morphology controlling agent, and therefore, is preferably an ion which has a relatively weak adsorption power to iron and adsorbs and desorbs during the growth process.

The ferric compound used in the present invention is not particularly limited as long as it is a compound containing ferric iron. In particular, halides, preferably chlorides, hydroxides, nitrates,
Perchlorates, sulfates and the like can be mentioned. The concentration range of the ferric compound solution used in the present invention is limited to the solubility of the ferric compound, and the monodisperse spindle type αFe ₂ O ₃ can be obtained at a low concentration, but the yield decreases as the concentration decreases. Therefore, the lower limit of the concentration is determined from the viewpoint of economy. Here, the solution of the ferric compound solution is a concept including a colloidal dispersion system. The concentration of the ferric compound solution is usually 0.005 to 1 M (mol / l), preferably 0.0075 to 0.5 M.

Monodisperse spindle type αFe ₂ O obtained by the present invention
₃ , the average major axis length is preferably 0.2 μm or less, more preferably the average major axis length is in the range of 0.05 to 0.18 μm, and the acicular ratio (average major axis length / average minor axis length) is preferably Is 4
As described above, the value is more preferably in the range of 4 to 8, and the coefficient of variation of the major axis length defined by the following formula (1 ') is preferably 0 to 20%, more preferably 0 to 15%.

Coefficient of variation (%) = 100 × standard deviation of major axis length / average major axis length (1 ′)

The present inventors have found that the use of the monodisperse spindle type αFe ₂ O ₃ fine particles obtained as described above makes it possible to obtain a ferromagnetic powder for magnetic recording having excellent magnetic properties. That is, the present invention reduces the monodisperse spindle type αFe ₂ O ₃ obtained as described above to Fe ₃ O ₄ ,
_A method for producing a ferromagnetic powder, which comprises oxidizing ₃ O ₄ to obtain a ferromagnetic iron oxide of FeOx (1.33 <x ≦ 1.50), and performing a sintering prevention treatment on the monodisperse spindle type αFe ₂ O ₃ A method for producing a ferromagnetic powder, characterized by reducing to a metal powder after performing an annealing treatment as required, and gradually oxidizing the metal powder to obtain a ferromagnetic metal powder.

The ferromagnetic powder of the present invention retains the monodispersity characteristic of the monodisperse spindle type αFe ₂ O ₃ particles, that is, the low variation coefficient of the major axis length, and has a good SFD. The Hc distribution is narrow, so that when used for a magnetic recording medium, the recording demagnetization loss is particularly improved, and the recording medium has an advantage of excellent short-wavelength recording characteristics. In producing ferromagnetic iron oxide powder, it is effective to perform a sintering prevention treatment on hematite before reducing and oxidizing the obtained monodisperse spindle type αFe ₂ O ₃ particles. , Silica, phosphoric acid, aluminum-silica, or a combination thereof.

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

In order to produce a ferromagnetic iron oxide powder from the above-mentioned monodisperse spindle type αFe ₂ O ₃ particles, the hematite particles are reduced to magnetite, and then the magnetite is oxidized to obtain FeOx (1. 33 <x ≦ 1.5). In the present invention, a ferromagnetic powder having good magnetic properties can be obtained as described above.However, prior to reducing the monodisperse spindle type αFe ₂ O ₃ particles obtained in the present invention, this is annealed, By performing annealing after reducing to magnetite, the magnetic properties of the resulting ferromagnetic iron oxide and ferromagnetic metal powder can be further improved.

In the case of performing the annealing treatment prior to the reduction, the annealing temperature is preferably 450 to 1000 ° C., more preferably 500 to 900 ° C. in the production of both ferromagnetic iron oxide and ferromagnetic metal powder. Annealing time is 1-5
It is preferable that the time be about the time, and it is desirable to select the time in relation to the processing temperature. If the annealing treatment is performed at a high temperature for a long time, the particle shape changes. As a method for reducing particles, a known hydrogen gas reduction method, a method using a reducing gas generated when an organic substance is thermally decomposed in an inert gas, or the like can be used. In the case where annealing is performed after magnetite is applied, it is desirable to perform the treatment at a slightly lower temperature than in the case of treating hematite in the production of ferromagnetic iron oxide and ferromagnetic metal powder, and to perform treatment at 400 to 600 ° C. Is preferred.

In the method for producing ferromagnetic iron oxide, monodisperse spindle type αFe ₂ O ₃ particles are reduced to magnetite, and this is
In addition to flowing air at 0 to 300 ° C. to oxidize to γFe ₂ O ₃ , the degree of oxidation is adjusted and FeOx (1.33 <x ≦
1.5). The degree of oxidation can be adjusted by lowering the oxygen concentration in the oxidizing gas or by adjusting the temperature at the time of oxidation to 60 to 150 ° C.

After the magnetic iron oxide is formed, a known method can be used as a method for applying a Co compound to the surface. These methods are described, for example, in JP-A-49-7439.
No. 9, 50-37667. Tokiko 49-49475
No. in each publication.

The FeOx obtained according to the present invention (1.33)
(110) crystal by X-ray diffraction measurement of <x ≦ 1.5)
The child size is usually 150-300 mm, preferably 18
0-270 °, average by observation with a transmission electron microscope
The major axis length is usually 0.15 to 0.25 μm, preferably
0.18 to 0.20 μm, and the acicular ratio is usually 4 to
8, preferably 6-8. Also, the variation coefficient of the long axis length
Is a monodisperse spindle type αFe _TwoO_ThreeIt is almost the same as Also,
The Hc of FeOx is usually 150 to 300 Oe, preferably
200 to 300 Oe, more preferably 250 to 30 Oe
0 Oe and σ_S(Saturation magnetization) is usually 60 to 70
emu / g, preferably 62 to 70 emu / g, more preferably
More preferably, it is 65 to 70 emu / g.

In order to convert the monodisperse spindle type hematite into ferromagnetic metal powder, it is preferable to treat with a sintering inhibitor, such as Al, Si, Al-Si,
Compounds containing B, Y and rare earth elements and combinations thereof can be used. The treatment amount of metal atoms of the sintering inhibitor is preferably 1 to 20 atomic% with respect to iron,
More preferably, it is 3 to 15 atomic%. The method of applying the sintering inhibitor is to disperse hematite particles that have been sufficiently washed in water, add an aqueous solution of a compound containing an element used for preventing sintering, adsorb the particles, or deposit them on the particle surface by neutralization, etc. It is preferable to remove the impurities by washing with water. It is also preferable to form an alloy after reduction by applying a metal element before treating with a sintering inhibitor to improve magnetic properties and weather resistance. Elements used for the deposition process include Co, Ni, Mn, Cu, Sn, Cr, and S.
There are b, Nd, Y, La, Sm, Ba, Sr, Ca, Mg and the like, which may be used alone or in combination.

The amount of metal used for alloying depends on the alloyable composition range, but is generally 0.1 to 4 with respect to Fe.
0% by weight. In the case of a non-magnetic alloy, the addition amount cannot be too large, but the addition amount of a transition metal such as Co or Ni can be changed in the range of 0.1 to 40% by weight based on Fe. Co is a preferable additive element because it has the effect of increasing the saturation magnetization and improving the weather resistance. On the other hand, Ni and Cu are elements that promote reduction and can be reduced at a low temperature, and are effective elements in terms of shape retention effect. When the metal element is applied, annealing is also effective before the reduction in order to distribute the element uniformly in the particles. As the annealing temperature, there is no choice in the range of 400 to 900 ° C. After the annealing treatment, a sintering prevention treatment may be performed.

As a method for reducing particles, a well-known hydrogen gas reduction method, a method using a reducing gas generated when an organic substance is thermally decomposed in an inert gas, or the like can be used. When annealing after magnetite,
It is desirable to treat at a slightly lower temperature than when treating with hematite, and it is preferred to treat at 400 to 600 ° C.

Pure hydrogen is used to reduce to metal,
The reduction is carried out at 350-550 ° C, preferably at 400-500 ° C. The progress of the reduction can be determined by monitoring the moisture in the hydrogen gas after the reduction using a dew point meter or the like. In order to use the obtained ferromagnetic metal powder as a raw material for a magnetic recording medium, it is necessary to oxidize and stabilize the surface of the obtained ferromagnetic metal powder.
For this reason, after completion of the reduction, the surface is gradually oxidized by flowing a gas having a controlled oxygen concentration or passing an oxygen-containing gas in an organic solvent. At this time, it is necessary to monitor the calorific value with a thermometer so that rapid oxidation does not occur, and it is desirable that the moisture in the gas used for slow oxidation is as low as possible. Prior to slow oxidation, JP-A-61-52327,
It is also effective to gradually oxidize after treating with the compound described in JP-A-94310, because the saturation magnetization s of the ferromagnetic alloy powder can be increased.

X of the ferromagnetic metal powder obtained according to the present invention
The crystallite size of (110) by X-ray diffraction measurement is usually
100 to 250 °, preferably 110 to 200 °, and the average major axis length by observation with a transmission electron microscope is usually
0.04 to 0.15 μm, preferably 0.04 to 0.1
2 μm, and the acicular ratio is usually 4 to 8, preferably 5
８8. The coefficient of variation of the major axis length is almost the same as that of the monodisperse spindle type αFe ₂ O ₃ . The Hc of the ferromagnetic metal powder is usually 1400 to 3000 Oe, preferably 2 to 3000 Oe.
000-3000 Oe, more preferably 2200-28
00 Oe, and σ _S is usually 100 to 180 emu.
/ G, preferably 110-170 emu / g, more preferably 115-160 emu / g.

The magnetic recording medium of the present invention can be manufactured by applying the ferromagnetic powder prepared from the monodisperse spindle type αFe ₂ O ₃ to the magnetic layer of the magnetic recording medium. Hereinafter, the magnetic recording medium of the present invention will be described. The magnetic layer of the magnetic recording medium of the present invention may be a magnetic layer provided on a support, and may be composed of a single layer or multiple layers. Further, a non-magnetic layer may be provided between the magnetic layer and the support. In this case, the magnetic layer is also called an upper layer or a second layer, and the nonmagnetic layer is also called a lower layer or a first layer.

The magnetic recording medium containing the ferromagnetic powder obtained by the present invention in the magnetic layer has a SFD even when the magnetic layer has a single layer as compared with the conventional magnetic recording medium using the ferromagnetic metal powder. The short-wavelength output is excellent, reflecting that it is excellent. Further, as a coating layer formed on the support,
The magnetic layer in which the ferromagnetic powder is dispersed in the binder is preferably overlaid on the non-magnetic layer in which the non-magnetic powder is dispersed in the binder.
A magnetic recording medium provided with a magnetic layer thickness of not more than μm, more preferably 0.04 to 0.3 μm can increase the short-wavelength output and provide a magnetic recording medium with higher performance than a single layer.

As a method of forming two or more coating layers on the support, a wet coating method is used in which the second layer coating solution is coated thereon while the first coating layer on the support is still wet. On-wet method (simultaneous multilayer coating method,
No. 139929 and Japanese Patent Application Laid-Open No. 61-54992. ), That is, if the upper layer is formed while the first coating layer is in a wet state, it becomes easy to make the upper layer thinner and coating defects can be reduced. This is preferable because the adhesion can be increased. In the magnetic recording medium of the present invention, the lower layer and the ultrathin magnetic layer may be provided on only one side or both sides of the support. The upper and lower layers can be provided either after the lower layer is applied and the lower layer is in a wet state (W / W) or after drying (W / D). From the viewpoint of production yield, simultaneous or sequential wet coating is preferable, but in the case of a disk medium having a small area, coating after drying can be sufficiently used.

In the magnetic recording medium of the present invention, the reason why the above-mentioned ferromagnetic powder is used as the upper layer having two or more coating layers is to maximize the high-density magnetic recording performance.
First, if necessary, non-magnetic powder such as carbon necessary for practical characteristics of the magnetic recording medium can be added only to the lower layer or a larger amount to secure the necessary electric conductivity.
1.0 μm of the upper layer of the magnetic recording medium important for short-wavelength magnetic recording
This is because it is possible to increase the speed density at the time of saturation per unit volume of a portion of m or less, and secondly, it is possible to finish the unevenness of the surface of the upper layer smoothly. Another economical reason is that the amount of relatively expensive ferromagnetic powder obtained by reducing monodisperse spindle type hematite as a material can be reduced.

The magnetic recording medium of the present invention exhibits high Hc, high SQ (square ratio), low SFD, and excellent surface properties (ie, low center plane average surface roughness Ra). When the upper layer is a magnetic layer and the ferromagnetic metal powder prepared from the monodisperse spindle type αFe ₂ O ₃ is used for the magnetic layer, Hc is usually 1400 to 3000 Oe, preferably 2000 to 3000 Oe. , SQ are typically 0.
70-0.95, preferably 0.80-0.92, and the SFD is usually 0.15-0.40, preferably 0.15-0.35. The high Hc component (%) defined in the examples is usually 10 to 25%, preferably 1 to 25%.
0 to 15%, and Ra is usually 1.0 to 3.5 nm,
Preferably it is in the range of 1.0 to 2.5 nm.

As the binder resin for the magnetic layer in the magnetic recording medium of the present invention, conventionally known thermoplastic resins, thermosetting resins, reactive resins and mixtures thereof can be used. As the thermoplastic resin, the glass transition temperature is -100 to 150
° C, the number average molecular weight is 1,000 to 200,000, preferably 10,000 to 100,000, and the degree of polymerization is about 50 to 100.
It is about 0.

Examples of such a 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, and vinyl. There are polymers or copolymers containing butyral, vinyl acetal, vinyl ether, and the like as constituent units, polyurethane resins, and various rubber resins.

Examples of the thermosetting resin or reactive resin include phenol resin, epoxy resin, polyurethane curable resin, urea resin, melamine resin, alkyd resin, acrylic-based reactive resin, formaldehyde resin, silicone resin, and epoxy-polyamide resin. , A mixture of a polyester resin and an isocyanate prepolymer, a mixture of a polyester polyol and a polyisocyanate, and a mixture of a polyurethane and a polyisocyanate.

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

The binder resin used in the magnetic recording medium of the present invention is used in an amount of 5 to 50% by weight, preferably 10 to 30% by weight, based on the ferromagnetic powder. It is preferable to use 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 in combination.

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 ferromagnetic powder used (Bm / 4πσs). It is preferably at least 1.7 g / cc, more preferably at least 1.9 g / cc, most preferably at least 2.1 g / cc.

In the present invention, when polyurethane is used, the glass transition temperature is -50 to 100 ° C., and the elongation at break is 1
00 to 2000%, breaking stress 0.05 to 10 kg / c
m 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-isocyanate.
1,5-diisocyanate, o-toluidine diisocyanate, isophorone diisocyanate, isocyanates such as triphenylmethane triisocyanate,
Products of these isocyanates and polyalcohols, and polyisocyanates formed by condensation of isocyanates can be used. Commercially available trade names of these isocyanates include Nippon Polyurethane Co., Ltd., Coronate L, Coronate HL,
Coronate 2030, Coronate 2031, Millionate MR, Millionate MTL, Takenate D-102, Takenate D-102, Takenate D-110N, Takenate D
-200, Takenate D-202, manufactured by Sumitomo Bayer,
There are death module L, death module IL, death module N, death module HL, etc. These can be used alone or in combination of two or more by utilizing the difference in curing reactivity.

The magnetic layer of the magnetic recording medium of the present invention usually has various functions including a lubricant, an abrasive, a dispersant, an antistatic agent, a dispersant, a plasticizer, an antifungal agent and the like. Materials to be contained according to the purpose.

As the lubricant used in the magnetic layer of the present invention, a dialkylpolysiloxane (where alkyl is
5), dialkoxypolysiloxane (alkoxy has 1 to 4 carbon atoms), monoalkylmonoalkoxypolysiloxane (alkyl has 1 to 5 carbon atoms, alkoxy has 1 to 4 carbon atoms), phenylpolysiloxane, fluoro Silicone oil such as alkyl polysiloxane (alkyl has 1 to 5 carbon atoms); conductive fine powder such as graphite; inorganic powder such as molybdenum disulfide and tungsten disulfide; polyethylene, polypropylene, polyethylene vinyl chloride copolymer, poly Plastic fine powder such as tetrafluoroethylene; α-olefin polymer; unsaturated aliphatic hydrocarbon liquid at room temperature (compound having n-olefin double bond bonded to terminal carbon, about 20 carbon atoms); ~ 20
Fatty acid esters, fluorocarbons, and the like, each of which comprises one monobasic fatty acid and a monohydric alcohol having 3 to 12 carbon atoms, can be used.

Among them, fatty acid esters are more preferred.
The alcohols used as raw materials for fatty acid esters include 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 monoalcohols such as s-butyl alcohol, and polyhydric alcohols such as ethylene glycol, diethylene glycol, neopentyl glycol, glycerin, and sorbitan derivatives. Similarly, fatty acids include acetic acid, propionic acid, octanoic acid, 2-ethylhexanoic acid, lauric acid, myristic acid, stearic acid, palmitic acid, behenic acid, arachiic acid, oleic acid, linoleic acid, linolenic acid, elaidic acid, and palmitoleic acid And the like or a mixture thereof.

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-hexyl decyl Stearate, butyl palmitate, 2-ethylhexyl myristate,
A 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, and hexamethylene diol myristic acid And various ester compounds such as glycerin oleate.

Furthermore, in order to reduce the hydrolysis of fatty acid esters, which often occur when the magnetic recording medium is used under high humidity, the fatty acid and alcohol used as raw materials are branched / straight chain, isomer structure such as cis / trans, etc., branch position. A choice 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 be further used as the lubricant. That is, silicon oil, graphite, molybdenum disulfide, boron nitride, graphite fluoride, fluorinated alcohol, polyolefin, polyglycol, alkyl phosphate, tungsten disulfide, and the like.

Examples of the abrasive used for the magnetic layer of the present invention include commonly used materials such as α-alumina, silicon carbide, chromium oxide (Cr ₂ O ₃ ), corundum, artificial corundum, diamond, artificial diamond, garnet, Emery (main component: corundum and magnetite) or the like is used.
These abrasives have a Mohs hardness of 6 or more. Specific examples include AKP-10 and AKP-1 manufactured by Sumitomo Chemical Co., Ltd.
2, AKP-15, AKP-20, AKP-30, AK
P-50, AKP-1520, AKP-1500, HI
T-50, HIT-60A, HIT-70, HIT-1
00, manufactured by Nippon Chemical Industry Co., Ltd., G5, G7, S-1, chromium oxide K, UB40B manufactured by Uemura Kogyo Co., Ltd., W manufactured by Fujimi Abrasive Co., Ltd.
A8000, WA10000, TF14 manufactured by Toda Kogyo
0, TF180, etc. Average particle size is 0.0
Those having a size of 5 to 3 μm are effective, and preferably 0.05 to 1.0 μm.

These abrasives are usually 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 ferromagnetic powder. If the amount is less than 1 part by weight, it is difficult to obtain sufficient durability, and if it is more than 20 parts by weight, the surface properties and the degree of filling tend to deteriorate. These abrasives may be added to the magnetic paint after dispersion treatment with a binder in advance.

The magnetic layer of the magnetic recording medium of the present invention may contain conductive particles as an antistatic agent in addition to the nonmagnetic powder. However, in order to maximize the saturation magnetic flux density of the upper layer, it is preferable to add as little as possible to the upper layer and to add it to the coating layer other than the upper layer.
It is particularly preferable to add carbon black as an antistatic agent in terms of reducing the surface electric resistance of the entire medium. As the carbon black that can be used in the present invention, furnace for rubber, thermal for rubber, black for color, conductive carbon black, acetylene black and the like can be used. Specific surface area is 5 to 500 m ² / g, DBP oil absorption is 10 to 1500 ml / 100 g, particle size is 5 to 300
nm, pH 2-10, water content 0.1-10%, tap density 0.1-1 g / cc. Specific examples of the carbon black used in the present invention include BLACKPEARLS 2000, 130 manufactured by Cabot Corporation.
0, 1000, 900, 800, 700, VULCAN
XC-72, manufactured by Asahi Carbon Co., Ltd., # 80, # 60, # 5
5, # 50, # 35, manufactured by Mitsubishi Chemical Corporation, # 3950B, #
2400B, # 2300, # 900, # 1000, # 3
0, # 40, # 10B, manufactured by Columbian Carbon Co., C
ONDUCTEX SC, RAVEN 150, 50,
40, 15, Ketchen Black E manufactured by Lion Aguso
C, Ketjen Black ECDJ-500, Ketjen Black ECDJ-600 and the like. Carbon black may be surface-treated with a dispersing agent or the like, carbon black may be oxidized, or grafted with a resin, or may be used in which a part of the surface is graphitized. Before adding the carbon black to the magnetic paint, the carbon black may be dispersed in a binder in advance. When carbon black is used for the magnetic layer, it is preferable to use 0.1 to 30% by weight based on the ferromagnetic powder. Further, the non-magnetic layer preferably contains 3 to 20% by weight based on the total non-magnetic powder.

In general, carbon black not only acts as an antistatic agent, but also has a function of reducing friction coefficient, imparting light-shielding properties, improving film strength, and the like, and these differ depending on the carbon black used. Therefore, these carbon blacks used in the present invention may be used in different types, amounts, and combinations depending on the purpose based on the above-mentioned properties such as particle size, oil absorption, conductivity, and pH. Is of course possible. Carbon black that can be used can be referred to, for example, "Carbon Black Handbook" edited by Carbon Black Association.

Next, in the magnetic recording medium of the present invention, the non-magnetic
Detailed contents on the lower layer mainly composed of powder and binder resin
Will be described. Non-magnetic powder used for lower layer of the present invention
Powder, for example, metal oxides, metal carbonates, metal nitrides,
Can be selected from inorganic compounds such as metal carbides
You. As the inorganic compound, for example, α-90% or more α-
Alumina, β-alumina, γ-alumina, θ-aluminum
Na, silicon carbide, chromium oxide, cerium oxide, α-oxidation
Iron, goethite, silicon nitride, titanium oxide, silicon dioxide,
Tin oxide, magnesium oxide, zirconium oxide, oxidation
Zinc, barium sulfate, etc. used alone or in combination
It is. Particularly preferred are small particle size distribution and function addition.
Titanium dioxide, zinc oxide, α
-Iron oxide, barium sulfate, more preferably diacid
Titanium oxide and α-iron oxide. Normally, titanium dioxide is light
Radiation is generated when exposed to light due to its catalytic properties
There is a concern that it reacts with binders and lubricants. others
Therefore, the titanium dioxide used in the present invention is Al, Fe, etc.
It is necessary to lower the photocatalytic property by making solid solution 1-10%
It is. Furthermore, the surface is treated with an Al or Si compound,
It is preferable to reduce the action. These non-magnetic powders
The particle size of is preferably 0.005 to 1 μm,
Combined non-magnetic powders with different particle sizes depending on
Even with a single non-magnetic powder, the same effect can be obtained by broadening the particle size distribution.
It can also have fruit. Particularly preferred is non-magnetic
The particle size of the conductive powder is 0.01 μm to 0.5 μm.
In particular, when the nonmagnetic powder is a particulate metal oxide, the average
The particle size is preferably not more than 0.08 μm, and is a needle-like metal oxide.
In some cases, the particle size is preferably 0.3 μm or less.
It is more preferably 2 μm or less. Tap density is 0.1-2g
/ ml, preferably 0.2-1.5 g / ml. Non-magnetic powder
The water content of the powder is 0.1 to 5% by weight, preferably 0.2 to 3% by weight.
%, More preferably 0.3-1.5% by weight.
The pH of the non-magnetic powder is 2 to 12, but the pH is 5.5 to 5.5.
Particularly preferred is between 11. The specific surface area of the nonmagnetic powder is 1 to
100m^Two/ G, preferably 5 to 80 m^Two/ G, even better
Preferably 10-70m^Two/ G. Nonmagnetic powder crystals
The size of the child is preferably 40 to 1000 mm, and 40 to 800 mm.
Å is more preferred. Use DBP (dibutyl phthalate)
Oil absorption was 5-100 ml / 100g, preferably 10-8
0 ml / 100 g, more preferably 20 to 60 ml / 100 g.
The specific gravity is 1.5 to 7, preferably 3 to 6. The shape is a needle
Shape, spherical shape, polyhedral shape, or plate shape. Mohs hardness
The degree is preferably from 4 to 10. SA of non-magnetic powder
(Tearic acid) adsorption amount is 1 to 20 μmol / m^Two, Preferably
Is 2 to 15 μmol / m^Two, More preferably 3 to 8 μmol /
m^TwoIt is. Use non-magnetic powder with high stearic acid adsorption
The surface with an organic substance that strongly adheres to the surface
It is preferable to create a recording medium. These non-magnetic powders
Al, Mg, Si, Ti, Zr, Sn, S
It is preferable to perform surface treatment with b, Zn, and Y compounds. Special
Al is preferable for dispersibility by being present on the surface_TwoO_Three,
SiO_Two, TiO_Two, ZrO_TwoAnd their hydrous content
But more preferred is Al _TwoO_Three, SiO_Two,
ZrO_TwoAnd hydrated oxides thereof. These are pairs
Can be used in combination or can be used alone
You. Also, using a surface treatment layer coprecipitated according to the purpose
After treating with alumina, the surface layer is
Mosquito treatment and vice versa.
Wear. Also, the surface treatment layer can be made porous depending on the purpose.
Although it does not matter, it is generally preferred that the material be homogeneous and dense.

Specific examples of the non-magnetic powder used in the lower layer of the present invention include Nanotite manufactured by Showa Denko, HIT-100, ZA-G1 manufactured by Sumitomo Chemical, and α iron oxide DP manufactured by Toda Kogyo.
N-250BX, DPN-245, DPN-270B
X, DPN-550BX, DPN550RX, DBN-
650RX, DAN-650RX, titanium oxide TTO-51B, TTO-55A, TTO-55B, manufactured by Ishihara Sangyo
TTO-55C, TTO-55S, TTO-55D, S
N-100, titanium industrial STT-4D, S
TT-30D, STT-30, STT-65C, α iron oxide α-40, Titanium oxide MT-100S, MT manufactured by Teika
-100T, MT-150W, MT-500B, MT-
600B, MT-100F, MT-500HD, FINEX-25, BF-1, BF-10, BF-2 manufactured by Sakai Chemical
0, ST-M, Dowa Mining iron oxide DEFIC-Y, DE
FIC-R, Nippon Aerosil AS2BM, TiO2 P
25, Ube Industries 100A, 500A, and those obtained by firing the same.

The magnetic recording medium according to claims 5 and 6 of the present invention is characterized in that, as the ferromagnetic powder contained in the magnetic layer, iron oxide other than the ferromagnetic powder prepared from the monodisperse spindle type αFe ₂ O _{3 according} to the present invention is used. Magnetic material, cobalt-modified iron oxide ferromagnetic material, CrO
_2. Hexagonal ferrite and various metal ferromagnetic materials can be used together, and various magnetic layers in which they are dispersed can be provided other than the magnetic layer containing the ferromagnetic powder prepared according to the present invention.

Regarding the coating method for forming two or more coating layers on a support, specific examples of the wet-on-wet method include (1) gravure coating generally used for magnetic coatings, First, the lower layer is applied by a roll coating, blade coating, or extrusion coating device, and while the layer is still wet, for example,
No. 46186, Japanese Unexamined Patent Publication No. 60-238179 and Japanese Unexamined Patent Publication No. 2-265672 discloses a method of applying an upper layer by a support pressure type extrusion coating apparatus.

(2) A coating head having two built-in coating liquid passage slits as disclosed in JP-A-63-88080, JP-A-2-17971 and JP-A-2-265672. A method of applying the lower layer coating solution and the upper layer coating solution almost simultaneously,

(3) A method in which an upper layer and a lower layer are coated almost simultaneously by an extrusion coating apparatus with a backup roll disclosed in JP-A-2-174965.

In the case of coating by the wet-on-wet method, the flow characteristics of the coating solution for the magnetic layer and the coating solution for the non-magnetic layer should be as close as possible to avoid disturbance of the interface between the coated magnetic layer and the non-magnetic layer. A magnetic layer having a uniform thickness and a small thickness variation can be obtained. Since the flow characteristics of the coating solution strongly depend on the combination of the powder particles and the binder resin in the coating solution,
In particular, it is necessary to pay attention to the selection of the non-magnetic powder used for the non-magnetic layer.

The support of the magnetic recording medium is usually 1 to 1
00 μm, preferably 2 to 15 μm for tape, 20 to 80 μm for flexible disk
m, the thickness of the nonmagnetic layer is 0.5 to 10 μm. Further, layers other than the magnetic layer and the nonmagnetic layer can be formed according to the purpose. For example, an undercoat layer for improving adhesion may be provided between the support and the lower layer. This thickness is 0.01 to 2 μm, preferably 0.05
０．５0.5 μm. Further, a back coat layer may be provided on the side of the support opposite to the magnetic layer side. This thickness is 0.1 to 2 μm, preferably 0.3 to 1.0 μm. Known undercoating layers and backcoat layers can be used. In the case of a disk-shaped magnetic recording medium, the above layer configuration can be provided on both sides or one side.

There is no particular limitation on the support used in the present invention, and those commonly used can be used, but a flexible and non-magnetic support is preferred. Examples of the material forming the support include polyethylene terephthalate, polyethylene, polypropylene, polycarbonate, polyethylene naphthalate, polyamide, polyamideimide, polyimide, polysulfone, films of various synthetic resins such as polyethersulfone, and aluminum foil,
A metal foil such as a stainless steel foil can be used.

In order to effectively achieve the object of the present invention, the surface roughness of the support should be not more than 0.03 μm, preferably not more than 0.03 μm in terms of center line average surface roughness Ra (cutoff value 0.25 mm).
It is 2 μm or less, more preferably 0.01 μm or less.
Further, it is preferable that these supports not only have a small center line average surface roughness but also have no coarse protrusions of 1 μm or more. The surface roughness shape can be freely controlled by the size and amount of the filler added to the support as needed. Examples of these fillers include oxides and carbonates of Ca, Si, Ti, and the like, and fine particles of an organic resin such as an acrylic resin. The F-5 value of the support used in the present invention in the web running direction is preferably 5 to 50 kg / mm ² , and the F-5 value in the web width direction is preferably 3 to 30 kg / mm ² . Generally, the F-5 value is higher than the F-5 value in the web width direction, but this is not particularly necessary 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 is 30%. Preferably it is 1% or less, more preferably 0.1%.
5% or less. Breaking strength is 5-100Kg in both directions
/ Mm ² and an elastic modulus of 100 to 2000 kg / mm ² are desirable.

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 and tetrahydrofuran, methanol, ethanol, propanol, butanol, isobutyl alcohol and isopropyl alcohol. Alcohols such as methylcyclohexanol, esters such as methyl acetate, butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate, and glycol acetate; glycol ethers such as glycol dimethyl ether, glycol monoethyl ether, and dioxane; benzene; Aromatic hydrocarbons such as toluene, xylene, cresol, chlorobenzene, methylene chloride, ethylene chloride, carbon tetrachloride, Chloroform, ethylene chlorohydrin, dichlorobenzene, chlorinated hydrocarbons and the like, N,
N-dimethylformamide, hexane and the like can be used. These organic solvents are not necessarily 100% pure, and may contain impurities such as isomers, unreacted products, by-products, decomposed products, oxides, and moisture in addition to the main components. These impurities are preferably 30% or less, more preferably 10% or less. The kind and amount of the organic solvent used in the present invention may be changed between the magnetic layer and the intermediate layer if necessary. Use a highly volatile solvent for the first layer to improve surface properties, use a solvent with a high surface tension (cyclohexanone, dioxane, etc.) for the first layer to increase application stability, and use a high solubility parameter for the second layer Examples include raising the degree of filling using a solvent, but it is a matter of course that the present invention is not limited to these examples.

The magnetic recording medium of the present invention is prepared by kneading and dispersing the above-mentioned ferromagnetic powder, binder resin and, if necessary, other additives together with an organic solvent, applying a magnetic paint on a support,
It is obtained by orientation and drying as required.

The step of producing the magnetic paint of the magnetic recording medium of the present invention comprises at least a kneading step, a dispersing step, and a mixing step provided before and after these steps as necessary. Each step may be divided into two or more steps. All the raw materials such as the ferromagnetic powder, binder, carbon black, abrasive, antistatic agent, lubricant, and solvent used in the present invention may be added at the beginning or during any step. Further, individual raw materials may be added in two or more steps in a divided manner. For example, polyurethane may be divided and supplied in a kneading step, a dispersing step, and a mixing step for adjusting the viscosity after dispersion.

Various kneaders 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, Segvari, attritor, high-speed impeller disperser, high-speed stone mill, high-speed impact mill, disper, kneader, high-speed mixer, homogenizer, ultra A sound disperser or the like can be used.

The lower layer paint can be prepared according to the magnetic paint.

In order to achieve the object of the present invention, it is a matter of course that a known manufacturing technique can be used as a part of the process. However, in the kneading step, a strong kneading such as a continuous kneader or a pressure kneader is used. It is preferable to use one having power. When a continuous kneader or a pressure kneader is used, all or a part of the ferromagnetic powder and the binder (however, 30% by weight or more of the total binder is preferable) and 15 to 500 parts by weight based on 100 parts by weight of the ferromagnetic powder. It is kneaded in the range. Details of these kneading processes are described in JP-A-1-106338 and JP-A-64-79274. In the present invention, JP-A-62-21
By using the simultaneous multilayer coating method as shown in 2933, it is possible to produce more efficiently.

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 preferable that the amount is smaller than the contained residual solvent.

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

When the magnetic recording medium of the present invention has a lower layer and an upper layer, it is easily presumed that these physical properties can be changed between the lower layer and the upper layer according to the purpose. For example, the elastic modulus of the upper 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.

According to such a method, the magnetic layer applied on the support is subjected to a treatment for orienting the ferromagnetic powder in the layer if necessary, and then the formed magnetic layer is dried. If necessary, the magnetic recording medium of the present invention is manufactured by performing a surface smoothing process or cutting into a desired shape. The composition for the upper layer and the composition for the lower layer described above are dispersed together with a solvent, and the obtained coating solution is applied on a support and orientation dried to obtain a magnetic recording medium.

The elastic modulus of the magnetic layer at 0.5% elongation is preferably 100 to 2000 kg in both the web coating direction and the width direction.
/ Mm ² , and the breaking strength is desirably 1 to 30 kg / c.
m ² , the elastic modulus of the magnetic recording medium is preferably 100 to 1500 kg / mm ^{2 in} both the web coating direction and the width direction, the residual elongation is preferably 0.5% or less, and the thermal shrinkage at any temperature of 100 ° C. or less is desirable. Is 1% or less, more preferably 0.5% or less, and most preferably 0.1% or less.

The magnetic recording medium of the present invention may be a tape for video use, computer use, audio use, or a floppy disk or magnetic disk for data recording use. This is particularly effective for a medium for digital recording in which the lack of a file is fatal. Further, by making the lower layer a non-magnetic layer and setting the thickness of the upper layer to 1.0 μm or less, a high-density, large-capacity magnetic recording medium having high electromagnetic conversion characteristics can be obtained.

[0098]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The novel features of the present invention will be specifically described in the following embodiments.

Hereinafter, examples of the present invention will be specifically described in comparison with comparative examples. The indication of “part” indicates “part by weight”. Example Preparation of nucleus crystal 2M / L FeCl ₃ aqueous solution 50 in a sealable 2 liter glass container
500 ml of a 5.94N NaOH aqueous solution was added to 0 ml with stirring for 5 minutes. After the addition was completed, the mixture was further stirred for 20 minutes, and the vessel was completely sealed.

[0100] The sample was placed in an oven which had been heated to 100 ° C in advance, and kept for 48 hours. After 48 hours, the mixture was rapidly cooled with running water, the reaction solution was separated, centrifuged at 15,000 rpm for 15 minutes using a centrifugal separator, and the supernatant was discarded. Distilled water was added to this to re-disperse, and the mixture was centrifuged again to remove the supernatant. The washing with water was repeated four times using the centrifuge as described above. The precipitate of αFe ₂ O ₃ particles (average particle size of about 0.1 μm), which had been washed with water, was filtered and dried (sample A).

5 ml of distilled water was added to 50 g of sample A, pulverized for 30 minutes in a planetary mill, washed in a beaker using 500 ml of distilled water, and washed with 700 ml of a 0.02 mol / l HClO ₄ solution.
, And ultrasonically dispersed for another 30 minutes. This dispersion was collected and centrifuged at 10,000 rpm for 30 minutes to obtain a supernatant (α concentration: 4300 ppm) in which αFe ₂ O ₃ having an average particle diameter of 85 ° and a variation coefficient of 120% was dispersed, and a nucleating liquid of Sample 1 And After further centrifugation at 18000 rpm for 30 minutes, the average particle diameter is 65 ° and the coefficient of variation is 85% (Sample 2: Fe concentration 2500pp
A nuclear crystal liquid in which the ultrafine particles αFe ₂ O _{3 of} m) were dispersed was obtained. The supernatant was further centrifuged at 30,000 rpm for 30 minutes to obtain an average particle diameter of 40 ° and a coefficient of variation of 60% (sample 3: Fe concentration of 18).
Alpha iron ₂ O ₃ of 00Ppm) was obtained KakuAkiraeki dispersed. The iron concentration in each nucleate solution was determined by dissolving the nucleus crystals with hydrochloric acid, ICP
(Inductive Coupled Prasma: emission spectrometer).

50 g of sample A, phenylphosphonic acid 1
g of water, add 5 ml of distilled water, pulverize for 30 minutes with a planetary mill, wash in a beaker using 500 ml of distilled water,
After adding 00 ml, the mixture was ultrasonically dispersed for another 30 minutes. This dispersion was collected and centrifuged at 10,000 rpm for 30 minutes to obtain an average particle diameter of 75 ° and a coefficient of variation of 90% (sample 4: Fe concentration of 3300).
(ppm) of αFe ₂ O ₃ dispersed therein was taken out as a nucleus crystal liquid. Further, the mixture was centrifuged at 30,000 rpm for 30 minutes to obtain an average particle diameter of 30 ° and a variation coefficient of 50% (Sample 5; Fe concentration;
(00 ppm) of ultrafine particles αFe ₂ O ₃ were dispersed. Synthesis of Monodisperse Spindle Type αFe ₂ O ₃ (1) In a reaction vessel equipped with a stirrer, add 3000 ml of 0.025 mol / l ferric chloride, and add 0.1 mol / l of NaH ₂ PO ₄
30 ml was added and stirred. 400 m of the obtained reaction solution
Five samples were taken at a time. The nucleus crystal liquid shown in Table 1 and additional water as needed were added and stirred for 10 minutes. P of reaction solution
H ranged from 1.5 to 1.8. The reaction vessel was sealed and kept in an oven preheated to 100 ° C. for 48 hours.

After holding for a predetermined time, the reaction solution was rapidly cooled with running water, and the reaction solution was centrifuged at 18,000 rpm for 15 minutes using a centrifugal separator, and the supernatant was discarded. Distilled water was added to this to re-disperse, and the mixture was centrifuged again to remove the supernatant. Thus, washing with water was repeated three times using the centrifuge. Next, 1 M aqueous ammonia was added to re-disperse, centrifuged, and the supernatant was removed. Distilled water was added to this to re-disperse, and the mixture was centrifuged again to remove the supernatant. Thus, washing with water was repeated three times using the centrifuge. A portion of the product was lyophilized.

The particles thus obtained were measured for specific surface area (degassed in nitrogen at 250 ° C. for 30 minutes and measured with a cantersorb) and observed with a transmission electron microscope to find the average particle diameter, needle ratio and particle size. The variation coefficient (%) of the diameter was determined. Table 1 shows the results.

[0105]

[Table 1]

Synthesis of Monodisperse Spindle Type αFe ₂ O ₃ (2) 162 g of FeCl ₃ .6H ₂ O was dissolved in 1 liter of distilled water, and distilled water was added to make 1.4 liter. 72.0 g of NaOH in distilled water 1.
Dissolved in 4 liters. 1.2 liters of distilled water was placed in a beaker, a jacket was placed, and cooling water was allowed to flow to prevent temperature rise due to heat of neutralization. The iron salt solution and the sodium hydroxide solution were added with stirring at 20 mL / min while adjusting the pH to 7.5. Due to the effect of cooling, the liquid temperature during neutralization was 20 to 25 ° C. Filter the precipitate, recover the ferric hydroxide colloid,
Distilled water was added, and repulping and filtration were repeated three times. Measured with ion conductivity meter, 100ppm as chloride ion in suspension
It was made to be as follows.

0.299 mol of Fe (OH) ₃ colloid as Fe was dispersed in 0.4 liter of distilled water to make the total liquid volume 0.5 liter. The pH of the suspension was adjusted to 7.5 with an aqueous NaOH solution, and the colloid was aged while stirring at 55 ° C. for 3 hours. After adding the nucleus crystal liquid described in Table 2 to the aged liquid and the additional water 400 ml and the crystal form controlling agent (0.005 mol to Fe) in Example 2-1,
An aqueous NaOH solution was added to adjust the pH to 10.5. 180 ° C with stirring
And kept at this temperature for 90 minutes to obtain αFe ₂ O ₃ . The reaction solution was cooled, filtered and washed with water. Dry a part, measure specific surface area (degas in nitrogen at 250 ° C for 30 minutes and measure with cantersorb), and use transmission electron microscope to measure the average major axis length, needle ratio, and variation coefficient of major axis length (%) Was observed. Table 2 shows the results.

[0108]

[Table 2]

Production Example of Ferromagnetic Iron Oxide Powder Monodisperse spindle type αFe obtained in Example 1-1_TwoO_ThreeThe rotary
At 380 ° C for 1 hour while flowing hydrogen gas
After reducing to nitrogen and cooling to 150 ° C, the oxygen concentration
The surface was oxidized and stabilized with 1% gas. Obtained particles
Is measured with a vibrating sample magnetometer (VSM-5, manufactured by Toei Kogyo).
Magnetic properties were measured at a constant magnetic field strength of 5 KOe. Redox titration
Fe in particles by the method²⁺/ Fe³⁺Was measured. In addition, the specific surface area
250 ° C using an Intersorb (manufactured by Cantarchrome)
For 30 minutes and measured. Further penetrate the obtained particles
Observation was made with a scanning electron microscope to determine the average major axis length and the needle ratio. H
c340Oe, σs74.3emu / g, Fe²⁺/ Fe³⁺= 0.38, ratio
Surface area is 48.5m^Two/ G. The average major axis length is 0.
14 μm, needle ratio 6.5, coefficient of variation of major axis length 11%
Met. Production Examples of Ferromagnetic Metal Powder Examples 1-3, 1-4, 1-5, Examples as Starting Materials
2-2, 2-3, spindle type αFe obtained under the conditions of Comparative Example 2
_TwoO_ThreeSo that the hematite concentration becomes 2% by weight in distilled water.
And the cobalt sulfate is converted to Fe atoms in the hematite.
Co is added so as to be 10 atomic%, or
Without stirring (in this case, the added Co is
10 atomic% with respect to Fe of the generated ferromagnetic metal powder
It is. The same applies to Y and Al described later.
You. ). While monitoring the pH of this suspension,
Add ammonia water to adjust the pH to 8.0, and hematite surface
And a hydrothermal treatment at 200 ° C. for 1 hour.
Was. After cooling, yttrium nitrate and aluminum sulfate
Aqueous solution of chromium with respect to the Fe atoms in the hematite
Is added to the suspension so that 3% by atom and 5% by atom Al
And monitor the pH of the suspension while adding ammonia to the suspension.
PH adjusted to 7.0 with water and monodispersed spindle type αFe with Co _TwoO_Three
An Al compound and a Y compound were deposited on the surface. Suspension
After filtration and washing with distilled water, impurities were removed. The resulting surface
Treated monodisperse spindle type αFe_TwoO_ThreePassed through a 3 mm diameter molded plate
It was molded into a cylindrical shape and dried.

The surface-treated monodisperse spindle type αFe ₂ O ₃
g was placed in a static reduction furnace and annealed in nitrogen at 600 ° C. for 1 hour. Next, the temperature was set to 480 ° C., the gas was switched to hydrogen gas, and the hydrogen gas was reduced at a flow rate of 50 l / min for 5 hours. After the reduction was completed, the gas was switched to nitrogen gas and cooled to room temperature. Using a gas mixer, mix the oxygen-containing gas into the nitrogen gas, adjust the oxygen concentration in the nitrogen to 0.1% by volume, and contact for 5 hours, then monitor the upper temperature of the ferromagnetic metal powder to exceed 50 ° C. The oxygen concentration was increased to 21% while keeping the temperature constant, and the metal was gradually oxidized and kept in this state for 5 hours.
The obtained ferromagnetic metal powder was placed in a vibrating sample magnetometer (VSM-
The magnetic properties were measured at a measured magnetic field strength of 10 KOe using a No. 5 type manufactured by Toei Kogyo. The specific surface area was measured by dehydration treatment at 250 ° C. for 30 minutes using Cantersorb (manufactured by Cantarchrome). Furthermore, the obtained particles were observed with a transmission electron microscope, and the average major axis length, the acicular ratio, and the variation coefficient (%) of the major axis length were determined.
From the X-ray diffraction measurement of the obtained ferromagnetic metal powder, (110)
Was calculated. Table 3 shows the results.

[0111]

[Table 3]

Preparation of Magnetic Recording Medium Using the ferromagnetic metal powder thus obtained, a tape was formed under the following conditions, and the magnetic characteristics and electromagnetic conversion characteristics were measured. (Composition for upper layer) Ferromagnetic metal powder (described in Table 4) 100 parts Vinyl chloride copolymer 14 parts (containing 1 × 10 ⁻⁴ eq / g of —SO ₃ Na group, polymerization degree 300) Polyester polyurethane resin 4 parts (Neopentyl glycol / caprolactone polyol / MDI = 0.9 / 2.6 / 1, containing 1 × 10 ⁻⁴ eq / g of —SO ₃ Na group) α-alumina (average particle size 0.1 μm) 3.5 parts Carbon black (average particle size 70 nm) 0.5 part Butyl stearate 1 part Stearic acid 2 parts Methyl ethyl ketone and cyclohexanone 1: 1 mixed solvent (for kneading) 100 parts Methyl ethyl ketone and cyclohexanone 1: 1 mixed solvent 200 parts (composition for lower layer) acicular α-Fe ₂ O ₃ 80 parts (average particle length 0.15 [mu] m, average acicular ratio of 8, for the entire Al ₂ O ₃ particles on the surface There 2 wt%, the surface area 53m ² / g pH 9.2 by the BET method) Carbon black 20 parts (average primary particle diameter 16 nm, DBP及油amount 80 ml / 100 g BET method surface area of 250 meters ² / g pH 8.0 by) vinyl chloride copolymer Polymer 14 parts (containing 1 × 10 ⁻⁴ eq / g of —SO ₃ Na group, degree of polymerization: 300) Polyester polyurethane resin 4 parts (neopentyl glycol / caprolactone polyol / MDI = 0.9 / 2.6 / 1, −) SO ₃ Na group 1 × 10 ⁻⁴ eq / g) Butyl stearate 1 part Stearic acid 2.5 parts Methyl ethyl ketone and cyclohexanone 1: 1 mixed solvent (for kneading) 100 parts Methyl ethyl ketone and cyclohexanone 1: 1 mixed solvent 200 parts In the upper layer composition and the lower layer composition, the upper layer is composed of a magnetic powder, an abrasive, Black and binder resin and kneading the solvent, the lower non-magnetic powder, and kneaded by a kneader with carbon black and a binder resin and kneading solvent. The kneading liquid, the remaining additives and the solvent for dispersion were dispersed using a sand grinder. To the resulting dispersion, 5 parts of polyisocyanate is added to the lower layer coating solution, 6 parts to the upper layer coating solution, and 20 parts of a 1: 1 mixed solvent of methyl ethyl ketone and cyclohexanone is added. The solution was filtered using a filter to prepare a coating solution for forming a nonmagnetic layer and a magnetic layer.

The obtained lower layer coating solution was applied so that the thickness after drying became 2 μm. Immediately thereafter, while the lower layer coating layer was still wet, the thickness of the magnetic layer was reduced. Wet simultaneous multi-layer coating was performed on a 7 μm-thick polyethylene terephthalate support so as to have a thickness of 0.2 μm.
(00 gauss) and a solenoid electromagnet (surface flux of 1500 gauss), and the ferromagnetic powder was magnetically oriented and dried. Then, a calender treatment was performed at a roll temperature of 90 ° C. using a seven-stage calender composed of metal rolls to obtain a web-shaped magnetic recording medium, which was slit to a width of 8 mm to prepare a sample of an 8 mm video tape.

The magnetic characteristics were measured using a vibrating sample magnetometer VSM-5 manufactured by Toei Kogyo Co., with an applied magnetic field of 10 kOe.
The high Hc component was measured by the following method. The vibration sample type magnetometer manufactured by Toei Kogyo Co., Ltd. was set so that the orientation direction of the measurement sample of the magnetic recording medium was the same as that of the magnetic field.
After DC saturation with applying Oe, the magnetic field is returned to zero and the residual magnetization (-Mrmax) is measured. After applying a magnetic field of 3000 Oe in the opposite direction, the magnetic field is returned to zero and the residual magnetization Mr is measured. After that, 10 kOe is applied and DC saturation occurs in the reverse direction, the magnetic field is set to zero and the residual magnetization Mrmax is measured. It was calculated by the following equation from each of the obtained residual magnetizations.

High Hc component (%) = 100 × (Mrmax−Mr)
/ (Mrmax-(-Mrmax)) The magnitude of the magnetic field applied in the opposite direction can be set arbitrarily.
In the present application, 3000 Oe was adopted from the viewpoint of detection sensitivity.
The high Hc component represents a component that undergoes magnetization reversal at a set applied magnetic field or higher. For the surface roughness, a 250 μm square sample area was measured using an optical interference three-dimensional roughness meter “TOPO-3D” manufactured by WYKO (US, Arizona). In calculating the measured values, corrections such as tilt correction, spherical surface correction, and cylinder correction are performed in accordance with JIS-B601, and the center plane average roughness Ra is calculated.
Was taken as the value of the surface roughness.

The method for measuring the overwrite characteristics was as follows. Using a drum tester, the relative speed of the TSS head (head gap 0.2 μm, track width 14 μm, saturation magnetic flux density 1.1 Tesla) was 10.2 m / sec,
The optimum recording current is determined from the input / output characteristics of 1 / 2Tb (λ = 0.5 μm) and the optimum recording current is determined by this current.
The overwrite characteristics were measured from the erasure rate of 1/90 Tb when the signal of m) was recorded and overwritten at 1/2 Tb. The output was expressed as a relative value with respect to Fuji Photo Film's Super DC tape. Overwriting is Comparative Example 3-
It was expressed as a relative value to the tape produced in 1. Table 4 shows the results.

[0117]

[Table 4]

[0118]

EFFECT OF THE INVENTION A nucleus crystal which is fine and has an excellent particle size distribution
Using αFe with crystal morphology control agent _TwoO_ThreeCan be synthesized
Ferromagnetic powder with fine particle size distribution
Could be produced. Magnetic recording, especially as ferromagnetic metal powder
When producing a recording medium, the surface
Bar light can produce good magnetic recording media
Was. In terms of magnetic properties, the high Hc component is
And reflect improved particle size distribution.

Claims (6)

[Claims] 1. The ferric compound solution has an average particle diameter of 100
変 動 The coefficient of variation below and represented by the following formula (1) is 100%
The following αFe ₂ O ₃ nucleus is added, a crystal form controlling agent is coexisted, and αFe ₂ O ₃ is precipitated on the nucleus under heating to form a monodisperse spindle type αFe ₂ O ₃ Production method of hematite. Coefficient of variation (%) = 100 × standard deviation of particle diameter / average particle diameter (1) 2. When the crystal form controlling agent is a phosphate compound,
The production of hematite according to claim 1, wherein the pH is 2.5 or less, and when the crystal morphology controlling agent is an organic phosphonic acid compound, the pH is 10 or more to obtain a monodisperse spindle type αFe ₂ O _3. Method. 3. The monodisperse spindle type αFe ₂ O ₃ obtained in claim 1 or 2 is reduced to Fe ₃ O ₄ , and then the Fe ₃ O ₄ is oxidized to FeOx (1.33 <x ≦ 1.50) A method for producing a ferromagnetic powder, wherein the ferromagnetic iron oxide is used. 4. The monodisperse spindle type αFe ₂ O ₃ obtained in claim 1 or 2 is subjected to a sintering prevention treatment, and if necessary, an annealing treatment, and then reduced to a metal powder. A ferromagnetic metal powder by gradually oxidizing the ferromagnetic powder. 5. A magnetic recording medium comprising a support and a magnetic layer mainly composed of a ferromagnetic powder and a binder resin, wherein the ferromagnetic powder is the ferromagnetic powder obtained in claim 3 or 4. Characteristic magnetic recording medium. 6. A magnetic recording medium comprising a non-magnetic layer mainly composed of a non-magnetic powder and a binder resin on a support and a magnetic layer mainly composed of a ferromagnetic powder and a binder resin provided thereon. A magnetic recording medium, wherein the layer has a thickness of 1.0 μm or less, and the ferromagnetic powder is the ferromagnetic powder obtained in claim 3 or 4.