JPH0458501A - Ferromagnetic superfine particle and manufacture thereof and magnetic storage medium - Google Patents

Abstract

PURPOSE:To easily obtain a ferromagnetic superfine particle which is the material of highly dense magnetic storage medium by heating the material powder, which has Co for its main ingredient, in gas phase by plasma jet or the like, and quenching it after instantaneous evaporation. CONSTITUTION:Cobalt oxide powder 100mum or less in average grain diameter and carbon black are smashed and mixed to make granular material particles, wherein the total content of C, B, W, Al, Si, P, Cr and Mn is 10wt.% or less. Next, it is put in a reactor 1, and is evaporated by plasma jet or the like. And it is quenched and condensed by cooling gas to make approximately spherical ferromagnetic superfine particles which have Co for their main ingredients and are 0.1mum in average grain diameter. Hereby, the material of a highly dense storage medium can easily be made, and besides it can be prevented from taking a chain shape, and the corrosion resistance can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to ultrafine ferromagnetic particles, a method for producing the same, and a magnetic recording medium having a recording layer containing the ultrafine ferromagnetic particles.

<Prior Art> The range of applications of magnetic recording is showing a tendency to expand more and more, for audio or video recording, as well as for various information processing purposes.

The important points for the magnetic recording media used for these magnetic recordings are how faithfully the original signal can be recorded and reproduced, and how much information can be packed into a narrow area. For this reason, development of magnetic recording media and magnetic materials aiming at high fidelity, high reliability, and high density continues.

In order to improve the recording density of magnetic recording media, it is necessary to shorten the recording wavelength. The size approaches that of magnetic powder. Therefore, the contribution of the isotropic or perpendicular magnetization component becomes more important than the magnetization component in the longitudinal direction of the medium.

Therefore, for high-density recording, granular or spherical particles that are easier to align in the same direction or perpendicular direction are considered to be more suitable than nail-like particles that are easier to align in the longitudinal direction of the medium.

In addition, in the case of a flexible type magnetic disk, magnetic particles are applied in the length direction of the raw film, but since they are finally punched out into a circular shape, uniform characteristics in the circumferential direction are required. Therefore, granular or spherical particles, which are not oriented regardless of the coating direction, are superior to acicular particles, which tend to be oriented in the coating direction, in that uniformity of properties can be ensured.

From these viewpoints, Ba ferrite-based plate-like particles are excellent materials as high-density recording materials. However, this material is easily charged due to its oxide insulator particles (,
When made into a coating film, the electrical resistance of the magnetic layer increases,
In addition, since the particles are synthesized by heat treatment at a relatively high temperature, there is a drawback that agglomeration is large and it is difficult to disperse.

On the other hand, most of the magnetic particles currently used for magnetic recording generally have an acicular shape. This is to increase the coercive force. For example, in the case of metallic iron (α-Fe), a high coercive force cannot be expected in a granular or spherical state because the crystal magnetic anisotropy of iron is small. For this reason,
Acicular α-FeMi powder with increased coercive force by imparting shape magnetic anisotropy is used.

On the other hand, metallic cobalt has a relatively large magnetocrystalline anisotropy, so it is expected to generate a high coercive force even in the form of granular or spherical particles.

Furthermore, since cobalt has a high saturation magnetization, a magnetic recording medium with high output can be obtained.

As methods for producing cobalt particles, the following methods 1 to 2 are known.

■ A method for reducing cobalt salt in a high-temperature hydrogen stream ■ A method for reducing cobalt oxide or hydroxide in a high-temperature hydrogen stream ■ A method for reducing a cobalt salt aqueous solution by adding sodium borohydride ■ A method for reducing water-soluble cobalt salts in the presence of hypophosphite ions ■ A method for thermally decomposing cobalt carbonyl compounds in a polymer resin solution ■ A method for evaporating cobalt/metal in vacuum ■ A method for converting cobalt halide vapor into a hydrogen stream ■ Method of reducing cobalt hydroxide in liquid phase (Unexamined Japanese Patent Publication No. 6
3-6806) However, in the methods (1) and (2) above, cobalt fine particles are obtained by reducing fine particles generated in an aqueous solution in a dry state in a high-temperature hydrogen stream, resulting in volume reduction, porosity formation, There are disadvantages in that shape changes, sintering, etc. occur, resulting in insufficient dispersibility in the binder, and particle surfaces are activated and easily ignite.

In addition, in the methods (1), (2), (2), (2) and (2) above, the produced cobalt fine particles are chain-shaped and have basically the same characteristics as needle-shaped particles.

In the method (2) above, hexagonal plate-shaped ultrafine particles of hexagonal cobalt are produced. Since these particles have a thin plate shape with a platelet ratio of about 2 to 10, the main surface tends to be oriented in the direction of the coating film surface during coating film formation (the disadvantage is that it is difficult to obtain good properties as an isotropic medium). There is.

Further, the gas phase method also causes the following problems.

Examples of the gas phase method used for producing ultrafine metal or alloy particles include, for example, Japanese Patent Publication No. 52-21719, Japanese Patent Publication No. 55-44123, Japanese Patent Publication No. 50-5149, and Japanese Patent Publication No. 50-5665. , Special Public Service 1986-1986
The so-called vacuum evaporation method described in Japanese Patent Application Laid-open No. 48-55400, Japanese Patent Publication No. 57-4472, etc.
No. 5, JP-A-58-104103, JP-A-Sho 6
The so-called active plasma arc evaporation method described in Japanese Patent No. 0-162705 and the like is known.

These methods basically involve heating a raw material ingot metal to a high temperature using an electron beam, arc plasma, etc., and condensing and recovering atoms evaporated from the metal surface.

However, when these methods are applied to the production of Co fine particles, the following problems arise.

(2) When adding an element with a large difference in melting point or boiling point from CO to melt and evaporate the raw material ingot, continuous compounding or composite formation is difficult. For this reason, the elements that can be added are limited, making it difficult to improve various properties.

■ It is expensive because it uses ingot alloy as a raw material.

For example, Japanese Patent Application Laid-Open No. 60-149705
Although the publication succeeded in compounding some substances with different boiling points, the structure of the device is quite complicated and is considered unsuitable for mass production.

<Problem to be solved by the invention> The present invention has been made in view of the above circumstances.
The purpose of the present invention is to provide a magnetic recording medium that can perform high-density magnetic recording and has a high output, and also to provide ultrafine ferromagnetic particles for realizing such a magnetic recording medium. The object of the present invention is to provide a method for easily producing such ferromagnetic ultrafine particles.

<Means for Solving the Problems> Such objects are achieved by the following inventions (1) to (13).

(1) Ferromagnetic ultrafine particles containing Co as a main component and having a substantially spherical shape with an average particle size of 0.1- or less.

(2) The ferromagnetic ultrafine particles described in (1) above, which contain C.

(3) The ferromagnetic ultrafine particles described in (1) or (2) above, containing one or more elements selected from B, N, Aj2, Si, P, Cr, and Mn.

(4) The ferromagnetic ultrafine particles according to (2) or (3) above, wherein the total content of C and the elements is 10% by weight or less.

(5) The ferromagnetic ultrafine particle according to any one of (1) to 4) above, which contains O.

(6) The above (1) whose saturation magnetization is 60 emu/g or more
Ferromagnetic ultrafine particles according to any one of ) to 5).

(7) The ferromagnetic ultrafine particle according to any one of (1) to 6) above, which has a coercive force of 3000e or more.

(8) The ferromagnetic ultrafine particles according to any one of (1) to 7) above, produced by a gas phase reaction method.

(9) A method for producing ferromagnetic ultrafine particles, which comprises evaporating raw material powder containing at least Co in a gas phase and then rapidly cooling it to obtain ferromagnetic ultrafine particles.

(10) The method for producing ultrafine ferromagnetic particles according to (9) above, wherein the evaporation is performed by heating with thermal plasma.

(11) The method for producing ferromagnetic ultrafine particles according to (9) or (10) above, wherein the raw material particles constituting the raw material powder have an average particle diameter of 100p or less.

(12) The method for producing ferromagnetic ultrafine particles according to any one of (9) to 11) above, in which the ferromagnetic ultrafine particles according to any one of (1) to 8) are produced.

(13) A magnetic recording medium comprising a magnetic layer containing the ferromagnetic ultrafine particles according to any one of (1) to 8) above.

Effect> Since the ferromagnetic ultrafine particles of the present invention mainly contain Co,
It has high coercive force and extremely high saturation magnetization. Therefore, when applied to a magnetic recording medium, high-density recording can be performed and extremely high reproduction speeds can be obtained.

Furthermore, since the ferromagnetic ultrafine particles of the present invention are spherical, isotropic or perpendicular magnetization components in short wavelength recording can be effectively utilized, making them suitable for high-density recording.

In the production method of the present invention, raw material powder is heated in a gas phase using a plasma jet or the like to instantaneously evaporate, and then rapidly cooled to obtain ferromagnetic ultrafine particles containing Co as a main component.

Therefore, chain formation is suppressed, and spherical ferromagnetic ultrafine particles are obtained.

Furthermore, in the present invention, if the above-mentioned additive elements are added to the ferromagnetic ultrafine particles, chain formation due to fusion of the ferromagnetic ultrafine particles can be almost completely prevented.

Furthermore, the addition of the above elements significantly improves the corrosion resistance of the ferromagnetic ultrafine particles.

Even if the melting point or boiling point of the additive element is different from that of CO, the production method of the present invention instantly evaporates the raw material particles, so the additive element can be uniformly contained in the ferromagnetic ultrafine particles. The effect of preventing corrosion and improving corrosion resistance is achieved stably.

<Specific configuration> Ferromagnetic ultrafine particles of the present invention (hereinafter abbreviated as ultrafine particles)
contains Co as a main component.

The ultrafine particles may be composed only of Co, but C,
B, N, 0. It is preferable to contain one or more elements selected from AA, Si, P, Cr and Mn as an additive element.

These elements have the effect of preventing chain-like formation of ultrafine particles due to fusion and sintering, producing spherical ultrafine particles, and also have the effect of improving corrosion resistance.

To prevent chain formation, C, B, A℃, Si, P, C
Addition of one or more of r and Mn is more effective, and C is particularly effective.

In addition, to improve corrosion resistance, C, N, O, Si and Cr
It is more effective to add one or more of these, and C is particularly effective.

In addition, the addition of CLy) is effective in reducing the electrical resistance and improving the dispersibility of ultrafine particles. Furthermore, when cobalt oxide is used as a raw material, C is effective in reducing and evaporating it. It is extremely effective.

Note that C exists in the ultrafine particles.

The content of the above additive element in the ultrafine particles is 10i1i
% or less, particularly preferably 7% by weight or less. If the content of the additive element exceeds the above range, the saturation magnetization will decrease.

Furthermore, although there is no particular lower limit for the content of the additive elements, in order to prevent chain formation and improve corrosion resistance, the content should be set to 0.
It is preferably 1% by weight or more, particularly 0.5% by weight or more, and more preferably 1% by weight or more.

Further, the ultrafine particles of the present invention may contain Ni, Fe, etc., as necessary, in addition to the above-mentioned additive elements. In this case, Co to the total content of C01Fe and Ni
The content ratio of is preferably 70% by weight or more.

Note that these elements usually exist as metals in the ultrafine particles, but some of them may be oxidized during the condensation reaction or after production. For this reason, the surfaces of ultrafine particles are sometimes coated with oxides. When the surface of ultrafine particles is coated with oxide, the self-combustibility of ultrafine particles decreases,
Spontaneous combustion is prevented.

Note that the oxygen content in the ultrafine particles is preferably 30% by weight or less.

The composition of ultrafine particles was determined by plasma emission analysis, fluorescent X-ray analysis,
It can be measured by other chemical analysis.

The ultrafine particles of the present invention are spherical particles, and this appearance can be confirmed using a transmission electron microscope or the like.

The average particle size of the ultrafine particles of the present invention is preferably 0.005
-01-, more preferably 0.01-0.05
It is a door.

When the average particle size is less than the above range, superparamagnetic behavior becomes significant and the coercive force decreases significantly. Moreover, if it exceeds the above range, the agglomeration effect between the particles becomes large, making it difficult to disperse, which is not preferable.

According to the production method of the present invention, which will be described later, ultrafine particles having such an average particle size can be obtained as a single, substantially spherical particle without the need for means such as pulverization. Therefore, highly dispersible ferromagnetic ultrafine particles can be easily realized.

The ultrafine particles of the present invention obtained by the production method of the present invention are
The coercive force can be 3000e or more, particularly 5000e or more.

In addition, the saturation magnetization is 60 emu/g or more, especially 90~
It can be about 150 emu/g.

Next, the manufacturing method of the present invention will be explained.

The ultrafine particles of the present invention are produced by a gas phase reaction method.

The gas phase reaction method preferably used in the present invention is a method in which raw material powder containing at least CO is evaporated in a gas phase and then rapidly cooled to obtain ultrafine particles.

In the raw material powder, Co may be contained alone or in the form of a compound. Alternatively, a mixture of these may be used.

The CO compound to be used is not particularly limited and may be either an oxide or carbonate, but oxides are particularly preferred.

Further, the additive element may be contained alone in the raw material powder, or may be contained in the form of a compound. Alternatively, a mixture of these may be used.

The compound used is not particularly limited and may be any oxide or carbonate, but carbon black, alumina, silica, phosphorus oxide, chromium oxide, manganese oxide, boron oxide, ammonium salts and the like are particularly preferred.

Further, the additive element can also be added to the ultrafine particles by introducing a gas containing the additive element into the reaction system.

Preferred gases used in this case include various hydrocarbon gases such as methane gas and propane gas, nitrogen gas, oxygen gas, ammonia gas, -carbon oxide gas, carbon dioxide gas, hydrogen phosphide gas, and various carbonyl compounds. Then, these gases may be included in the carrier gas for transporting the raw material powder, or these gases themselves may be used as the carrier gas or plasma gas.

Furthermore, C and N can also be supplied from materials constituting the reactor used for manufacturing.

Additive elements and their compounds need only be contained in the raw material powder to a desired content when made into ultrafine particles, and when the additive elements are supplied from a gas, they can be ultrafine by controlling the flow rate. The additive element content in the fine particles may be set to a desired value.

In addition, in the present invention, scrap, ore, mill scale, etc. can also be used as a mixture containing each of the above-mentioned elements. Even when such low-cost raw materials are used, substantially spherical ultrafine particles with good magnetic properties can be obtained according to the present invention.

The average particle diameter of the raw material particles constituting the raw material powder is preferably 100 or less, particularly preferably 10 or less.

By setting the average particle size to this level, C.

It is possible to increase the evaporation efficiency of additive elements and additive elements, and it is also possible to easily quantitatively supply raw material particles into the reactor.

Such raw material particles can be obtained by pulverizing and mixing raw materials such as the above-mentioned elements or compounds using a known pulverizing means such as a jet mill or a ball mill.

Furthermore, in order to improve the fluidity of the raw material particles, a known binder may be used to granulate the raw material particles. In addition, it is preferable to use spray drying etc. for granulation. Although there is no particular restriction on the binder used, examples of suitable binders include polyvinyl alcohol, polyvinylpyrrolidone, and ethylcellulose.

In the present invention, the raw material particles as described above are heated in a gas phase in a reactor to instantaneously evaporate the entire raw material particles, and then rapidly cooled and condensed to form ultrafine particles.

In this case, the entire reaction system is preferably carried out in an inert or reducing atmosphere at atmospheric pressure or lower.

The heating means to be used is not particularly limited as long as it can instantaneously evaporate raw material particles, but in the present invention, it is preferable to use thermal plasma, particularly plasma jet.

Examples of means for generating a plasma jet include:
DC plasma is an example of DC plasma, which generates a direct current arc discharge between the inner surface of the tip of a nozzle-shaped anode and the cathode tip provided within this anode, and heats the plasma gas supplied into the anode to an extremely high temperature. It is heated to create thermal plasma, which is ejected as a jet from a nozzle at the tip of the anode.

In addition to this, a plasma jet using inductively coupled plasma (hereinafter abbreviated as ICP) can also be preferably used.

In this method, plasma is generated inductively by a high-frequency magnetic field generated by flowing gas into a quartz tube and passing a high-frequency current through a coil wound around the quartz tube.

By introducing raw material particles into such a plasma jet, the raw material particles are instantaneously heated and instantaneously evaporated.

FIG. 1 and FIG. 2 show a preferred example of the apparatus for producing ultrafine particles of the present invention.

The reactor 1 shown in FIGS. 1 and 2 has an evaporation section 2, a cooling section 3, and a collection section 4 in series.

A plasma jet 211 is ejected into the furnace of the evaporation section 2 by the plasma jet generating means 21 . As the plasma jet generating means 21, a DC plasma generator is used in FIG. 1, and an ICP generator is used in FIG. 2.

Raw material powder is introduced into the plasma jet 211 by a carrier gas from the raw material powder supply means 22 .

In the case of the DC plasma shown in FIG. 1, the flow rate of the ultra-high temperature plasma gas is very fast, so the raw material powder does not reach the center of the plasma and is likely to be blown off on the outside of the fast-flowing flame. For this reason, it is better to bring the inner wall of the furnace in the evaporation section as close as possible to the plasma flame, maintain the inside of the furnace at a high temperature, and create a turbulent flow of plasma to prolong the residence time of the raw material powder under high temperature. .

For this reason, the inner wall surface of the furnace of the evaporation section 2 is covered with a heat-resistant material 23. As the material of the heat-resistant material 23, it is preferable to use graphite, boron nitride, tungsten, or other heat-resistant alloy materials. Note that when a carbon-containing material such as graphite is used as the heat-resistant material, C can be supplied to the ultrafine particles from this material.

The heat resistant material 23 is further covered with a heat insulating material 24.

Preferable materials for the heat insulating material 24 include fibrous carbon, alumina, and zirconia.

Heat is retained within the evaporation section by the heat resistant material 23 and the heat insulating material 24. In addition, in this case, the inner wall of the evaporation section 2 is
It is preferable that the temperature is maintained at a high temperature of 1.000° C. or higher.

On the other hand, in ICP, the diameter of the flame of plasma is larger than that of DC plasma, the gas flow rate is slower, and the raw material powder can be supplied from the central axis of the plasma. Residence time can be increased. Therefore, by increasing the inner wall diameter of the reactor shown in FIG. 2 and lowering the temperature of the reactor wall, the evaporation reaction can proceed effectively while preventing other substances from being mixed in. In this case, as shown in FIG. 2, the raw material powder supply means 22 is installed on the central axis of the plasma jet generation means 21, and the raw material can be directly conveyed to the center of the plasma jet 211.

Gas generated by evaporation of the raw material powder in the evaporation section 2 is carried to the cooling section 3 by the carrier gas.

Then, it is rapidly cooled and condensed by the cooling gas supplied from the cooling gas supply port 31, and becomes the target ultrafine particles 10. The obtained ultrafine particles of 1° are transported to the collection section 4 by plasma gas and carrier gas, and are discharged to the outside of the reactor 1.

The ultrafine particles obtained in this manner are free from fusion or chain formation between particles, and are monodispersed, substantially spherical particles.

As carrier gas for transporting plasma gas, cooling gas, raw material powder and its evaporated gas, Ar, N2, He
, N2, NH3, Co, various hydrocarbons, etc. may be appropriately selected depending on the purpose. As the plasma gas, Ar-H24 combined gas, Ar-N21 combined gas, N2H2 mixed gas, etc. Gas etc. are preferable, and as the cooling gas, N2, N2. NHZ, CO1, various hydrocarbons, etc. are preferable.

The magnetic recording medium of the present invention contains the above-mentioned ultrafine particles of the present invention as a magnetic recording material in a magnetic layer.

In this case, the ultrafine particles are kneaded with a known binder and other additives and coated on a nonmagnetic substrate. Therefore, the magnetic recording medium of the present invention is a so-called coated magnetic recording medium.

<Example> Hereinafter, the present invention will be explained in further detail by giving specific examples of the present invention.

[Example 1] Cobalt oxide powder and carbon black were pulverized and mixed, and then granulated by spray drying to obtain raw material particles.

The raw material powder composed of these raw material particles is put into the reactor 1 shown in FIG. 1, evaporated by a plasma jet,
Furthermore, the sample was rapidly cooled and condensed using a cooling gas to produce an ultrafine particle sample.

The C content in the raw material particles, the average particle size of the raw material particles, the plasma gas used, the plasma output, and the cooling gas are shown in Table 1 below.
Shown below.

Note that the plasma jet was generated using DC plasma.

Furthermore, graphite was used as the material for the heat-resistant material 23, and carbon fiber was used as the material for the heat insulating material 24.

Co content, C content, N content of the obtained sample,
Table 1 shows the average grain size, main crystal structure, coercive force Hc, and saturation magnetization O3.

The Co content was measured by ICP emission analysis, the C content by gas chromatography analysis, the N content by a CHN elemental analyzer, the average particle size by a transmission electron microscope, and the crystal structure by X-ray diffraction. did. In addition, the coercive force and saturation magnetization are VS
It was determined by measurement using M.

FIG. 3 shows a transmission electron micrograph of sample N011.

FIG. 4 shows an X-ray diffraction chart of sample No. 1. This chart clearly shows that it has a face-centered cubic (FCC) structure.

In each sample shown in Table 1, the components other than Co, C, and N are considered to be oxygen present in the surface layer of the ultrafine particles.

[Example 2] A magnetic recording disk having a recording layer containing ultrafine particle sample No. 3 obtained in Example 1 was produced.

Ultrafine particles 100 parts by weight α-alumina fine particles 10 parts by weight Epoxy resin
30 parts by weight of phenolic resin, 20 parts by weight of vinyl resin, and 300 parts by weight of cyclohexanone were kneaded in a ball mill for 24 hours.

Further, 200 parts by weight of cyclohexanone was added thereto and kneaded for 48 hours to prepare a magnetic paint.

The obtained magnetic paint was spin-coded onto a rotating aluminum substrate, and heated and cured at 200° C. for 2 hours to form a magnetic coating.

Next, polishing was performed until the coating thickness was 0.5-1, and 1% perfluoropolyether was added to the Freon solvent.
Immersed in a solution containing 0% by weight and pulled up at a speed of 15 m.
A lubricant layer was formed by pulling up at m/sec.

When the coercive force Hc and saturation magnetic flux density Bm of the thus obtained magnetic disk sample were measured, Hc was 11600e and Bm was 2200G.

Also, load this magnetic disk sample into the drive device,
When recording and reproducing were performed, almost no output fluctuation was observed.

Furthermore, this magnetic disk sample was heated at 60°C and 90% R.
After storage for 1 day at
m was 4%.

[Example 3] A magnetic tape having a magnetic layer containing ultrafine particle sample No. 3 obtained in Example 1 was produced.

Ultrafine particles 400 parts by weight Vinyl chloride Vinyl acetate resin 50 parts by weight Urethane resin
A magnetic paint was prepared by kneading 50 parts by weight of stearic acid, 2 parts by weight of butyl stearate, 300 parts by weight of methyl ethyl ketone, and 250 parts by weight each of MI BK and toluene.

A curing agent was added to this magnetic paint, and a dry film thickness of 3 was applied to a 12-thick polyester film using the gravure coating method.
After applying it to a thickness of 60μ and performing surface smoothing treatment,
The intermediate layer was cured by heating at .degree. C. for 48 hours to prepare a magnetic tape sample.

When the coercive force Hc and saturation magnetic flux density Bm of this magnetic tape sample were measured, He was 10900e.
and Bm was 3650G.

Furthermore, this magnetic tape sample was heated at 60°C and 90% RH.
After storage for one minute at , the saturation magnetic flux density was measured, and the rate of change ΔBm in the saturation magnetic flux density after storage was calculated, and ΔBm was 3%.

[Example 4] In place of C added in Examples 1 to 3, B, N, 0, Aff
When one or more elements selected from , Si, P, Cr, and Mn were added, almost spherical ultrafine particles without chain formation were obtained as in each of the above examples, and the magnetic properties were also almost the same. Met.

The effects of the present invention are clear from the above examples.

That is, as shown in FIG. 3, the ultrafine particles of the present invention have an extremely high degree of sphericity (and a rough surface).

In addition, as shown in Table 1, the saturation magnetization is high (, Example 2
As is clear from 3 and 3, by forming a coating film, it has a coercive force suitable for a magnetic recording medium.

Therefore, it is clear that a magnetic recording medium having a magnetic layer containing ultrafine particles of the present invention is suitable for high-density recording and can provide high reproduction output.

<Effects of the Invention> Since the ultrafine particles of the present invention are almost spherical, the perpendicular magnetization component can be effectively utilized when used as a coated magnetic recording medium, making it suitable for short wavelength recording, that is, high-density recording. suitable.

In addition, since no orientation is caused by coating, it is suitable as a recording material for magnetic disks, especially floppy disks.

The ultrafine particles of the present invention containing predetermined additive elements have good corrosion resistance.

According to the manufacturing method of the present invention, ferromagnetic ultrafine particles containing Co as a main component can be obtained in a substantially spherical shape.

Furthermore, in the present invention, since the raw material powder containing the raw material element or its compound is instantaneously evaporated, Co and various additional elements having different melting points and boiling points can be evaporated at the same time.

Therefore, it is possible to effectively add elements that have the effect of preventing chain-like formation of ferromagnetic ultrafine particles and improving corrosion resistance.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view of a reactor using C plasma, which is suitable for producing the ultrafine particles of the present invention. FIG. 2 is a schematic cross-sectional view of a reactor using ICP suitable for producing the ultrafine particles of the present invention. FIG. 3 is a photograph substituted for a drawing showing the particle structure, and is a transmission electron micrograph of the ferromagnetic ultrafine particles of the present invention. FIG. 4 is an X-ray diffraction chart of ultrafine ferromagnetic particles. Explanation of symbols] Reaction furnace 10 Ultrafine particles 2 Evaporation section 21 Plasma jet generation means 211 Plasma jet 22 Raw material powder supply means 23 Heat-resistant material 24...Insulating material 3...Cooling section 31...Cooling gas supply port 4...Collection section Application agent of I-K Co., Ltd.

Claims (13)

[Claims] (1) Ferromagnetic ultrafine particles containing Co as a main component and having a substantially spherical shape with an average particle diameter of 0.1 μm or less. (2) The ferromagnetic ultrafine particles according to claim 1, which contain C. (3) The ferromagnetic ultrafine particles according to claim 1 or 2, containing one or more elements selected from B, N, Al, Si, P, Cr, and Mn. (4) The ferromagnetic ultrafine particles according to claim 2 or 3, wherein the total content of C and the element is 10% by weight or less. (5) The ferromagnetic ultrafine particles according to any one of claims 1 to 4, which contain O. (6) The ferromagnetic ultrafine particles according to any one of claims 1 to 5, which have a saturation magnetization of 60 emu/g or more. (7) Claims 1 to 6, wherein the coercive force is 300 Oe or more.
Ferromagnetic ultrafine particles according to any one of. (8) The ferromagnetic ultrafine particles according to any one of claims 1 to 7, which are produced by a gas phase reaction method. (9) A method for producing ferromagnetic ultrafine particles, which comprises evaporating raw material powder containing at least Co in a gas phase and then rapidly cooling it to obtain ferromagnetic ultrafine particles. (10) The method for producing ultrafine ferromagnetic particles according to claim 9, wherein the evaporation is performed by heating with thermal plasma. (11) The method for producing ferromagnetic ultrafine particles according to claim 9 or 10, wherein the raw material particles constituting the raw material powder have an average particle diameter of 100 μm or less. (12) The method for producing ferromagnetic ultrafine particles according to any one of claims 9 to 11, wherein the ferromagnetic ultrafine particles according to any one of claims 1 to 8 are produced. (13) A magnetic recording medium comprising a magnetic layer containing the ferromagnetic ultrafine particles according to any one of claims 1 to 8.