JP2000030920A - Magnetic powder, its manufacture, and magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture magnetic powder which can suppress an increase in noise after a magnetic recording medium using this magnetic powder is run for a long time and has higher durability and a higher corrosion resistance by forming a ferrite layer composed mainly of iron and cobalt on the surface of each acicular particle containing iron and cobalt. SOLUTION: In a method for manufacturing magnetic powder, acicular particles containing iron and cobalt are first reduced. Then a ferrite layer composed mainly of iron and cobalt is formed on the surface of each particle by oxidizing the reduced particles in a high-temperature atmosphere higher than a room temperature. In the acicular alloy magnetic powder, the mixing ratio of the cobalt against the iron is adjusted to about 5-50 wt.%. The major- axis and aspect ratio of each particle are respectively adjusted to about <=0.25 μm and about 4-8 respectively. Moreover, the ferrite layer is coated with at least either one of Si and Al. Therefore, the occurrence of such a phenomenon that dropout or noise increases can be suppressed when a magnetic recording medium using the magnetic powder is kept in a corrosive environment and the output is high.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an acicular fine particle alloy magnetic powder mainly composed of iron and cobalt, a method for producing the same, and a magnetic recording medium.

[0002]

2. Description of the Related Art Metallic iron magnetic powders have the characteristic that their coercive force and saturation magnetization are larger than conventional iron oxide magnetic powders, and are suitable for high-density recording and have been put to practical use.

[0003] However, the metallic iron magnetic powder is very susceptible to corrosion due to the active surface of the particles, which is not only inconvenient to handle, but also the magnetic recording medium using the same has an output characteristic under a high temperature and high humidity environment. However, there is a drawback that is deteriorated. This is apparent from the fact that when the metallic iron magnetic powder is left in an environment of, for example, a temperature of 60 ° C. and a humidity of 90%, the saturation magnetization rapidly decreases within several hours.

[0004] In order to improve such corrosiveness of the metallic iron magnetic powder, it has heretofore been attempted to alloy a metal such as cobalt with iron to form a passivation film on the particle surface.

For example, as a standard method for producing an alloy magnetic powder, The coprecipitate obtained from the iron salt and the cobalt salt added to the aqueous oxalic acid solution is reduced. . The iron oxide powder having cobalt deposited on the surface is reduced by heating. . A reducing agent is added to the solution containing the iron salt and the cobalt salt. . The metal is evaporated in an inert gas and collided with gas molecules to obtain an alloy magnetic powder. . F in a mixed gas of hydrogen and nitrogen or argon gas
There is a method of reducing to metal while flowing vapor of chloride of e or Co.

However, it is difficult to control the composition of the particles, and it is difficult to maintain the needle-like shape of the fine particles because the cobalt compound is formed on the surface.
In the case of, the particles are not acicular but bead-shaped, which is problematic in terms of orientation.

Accordingly, a method has been proposed in which acicular goethite powder containing cobalt obtained from an aqueous suspension of an alkali of an iron salt and a cobalt salt is reduced by heating.

[0008] However, although the alloy magnetic powder obtained by this method has a certain degree of corrosion resistance as compared with the standard metallic iron magnetic powder, the amount of cobalt present inside the particles is limited to about 7% by weight. In addition to this, a sufficient amount of cobalt alloy cannot provide sufficient corrosion resistance in many cases. Although the reason for this has not been clarified, it is considered that a sufficient passivation film cannot be formed on the particle surface because the amount of cobalt in the metal magnetic powder is insufficient.

[0009]

Accordingly, the present inventors have thought that it is necessary to protect the passivation film or to sufficiently supply cobalt, and have made the following attempts. In other words, in the method of heating and reducing the acicular goethite powder containing cobalt obtained from an aqueous suspension of an alkali of an iron salt and a cobalt salt in order to sufficiently supply cobalt, the cobalt salt is excessively added to the aqueous suspension. To increase the cobalt content of the resulting product, but the uniformity of the goethite grain shape and composition, such as the collapse of the goethite shape and the incorporation of amorphous particles in the goethite powder Was impaired, and cobalt could not be sufficiently dissolved in the metal magnetic powder.

The present inventor has made various studies on the cause and found that if the conventional method of adding a cobalt salt to the suspension during the formation of goethite was followed, the fundamental solution could be permanently solved. I noticed that there was no.

[0011] First, since iron constituting goethite is trivalent in the first place and is not equivalent to divalent cobalt, the ion exchange reaction cannot be freely performed. Third, it is considered that the cobalt concentration of goethite crystals governs the growth rate of goethite crystals. Third, since the shape of goethite particles determines the shape of metal magnetic powder particles through subsequent processing, It has been found that it is rather desirable that no cobalt ions that affect the growth rate of the crystal be present at the stage of the formation of.

Based on such basic philosophy, goethite having a uniform particle shape is generated in advance, and this is converted into magnetite having divalent iron ions. It was found that ion exchange was carried out with a cobalt divalent ion equivalent to this, and cobalt was solid-dissolved in the magnetite particles from the outside to ensure a needle-like shape and that a high concentration of cobalt could be dissolved. This was reduced to obtain a new alloy magnetic powder. The new alloy magnetic powder has a fine acicular shape and is fine particles having a particle size of 0.25 μm or less.
Saturation magnetization is 120e even after 7 days in 0% environment
Mu / g or more was maintained and the corrosion resistance was extremely excellent. It is considered that the reason for this is that even though cobalt contained 8% by weight or more with respect to iron inside the particles, the solid solution of cobalt could be made uniform.

The new needle-shaped alloy magnetic powder is
When it is included in a magnetic coating film as a magnetic recording medium, it still exhibits excellent corrosion resistance even after a severe recording / reproducing process. Although the reason for this is not clear, it is considered that cobalt is a solid solution in iron in a large amount inside the particle, and is much harder than conventional magnetic iron magnetic powder.

In fact, when the hardness of this new needle-shaped alloy magnetic powder is further increased by forming a ferrite coating on the surface or coating with silicon or alumina, severe running This is because even under use under conditions, corrosion is reduced. Therefore, as a result, the novel acicular alloy magnetic powder provides good durability corrosion resistance even in a corrosion test after the magnetic recording medium is placed in a wear environment for a certain period of time under running conditions of a constant load.

The present invention solves the problem that the conventional metallic iron magnetic powder has inferior corrosion resistance and has a condition for high-density recording with needle-like fine particles having a predetermined axial ratio and high saturation magnetization. It is another object of the present invention to provide a magnetic powder having a high durability and a high corrosion resistance, a method for producing the same, and a magnetic recording medium, which suppress noise rise after running for a long time while satisfying the above conditions.

[0016]

According to a first aspect of the present invention, a ferrite layer mainly composed of iron and cobalt is formed on the surface of needle-like particles containing iron and cobalt. It is characterized by the following.

According to a second aspect of the present invention, there is provided a method for producing a magnetic powder, the method comprising the steps of: reducing a needle-like particle containing iron and cobalt; A ferrite layer forming step of forming a ferrite layer mainly composed of iron and cobalt on the particle surface by oxidizing in a high temperature atmosphere.

According to a third aspect of the present invention, there is provided a magnetic recording medium in which a magnetic coating is provided on one surface of a support, wherein the magnetic coating comprises needle-like particles containing iron and cobalt. It is characterized by containing acicular magnetic powder having a ferrite layer mainly composed of iron and cobalt formed on the surface.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS That is, the present invention realizes that it is particularly important to obtain a group of acicular alloy magnetic powders having a uniform particle size and a uniform cobalt concentration. As a result of conducting various studies from the viewpoint that it is effective to obtain a particle group and then solid-dissolve cobalt, and furthermore, to uniformly dissolve cobalt, it is preferable that the solid-solution reaction proceeds uniformly. In order to uniformly adjust the particle size of the Fe-based particles, it is preferable to act nickel at the stage of crystal synthesis. Nickel itself has a function of improving the corrosion resistance of the alloy magnetic powder, but also has the effect of uniformly dissolving cobalt.

That is, an iron salt and a nickel salt are reacted in an aqueous alkali solution to obtain a coprecipitate of iron hydroxide and nickel hydroxide, and the Fe-based particles obtained by oxidizing the precipitate have a particle size distribution. Become smaller. When the amount of nickel to act on is 2% by weight or more, the particle size distribution of the Fe particle group can be reduced.

In a magnetic iron oxide obtained by reducing the Ni-containing Fe-based particles having a small particle size distribution in a hydrogen gas stream containing water vapor, cobalt, which contributes to corrosion resistance by being uniformly contained in the particles, is added. It has been found that the saturation magnetization of the alloy magnetic powder is increased and the corrosion resistance is improved by uniformly reducing the content of iron in each particle of the iron compound in a predetermined range and then reducing the content. If the content of nickel is less than 2% by weight, the particle size distribution cannot be made uniform. Therefore, even if the amount of cobalt added thereafter is increased, the compositional concentration of cobalt becomes uneven among the particles, and the corrosion resistance is improved. unacceptable.

It is desirable that nickel be present on the surface of the alloy magnetic powder from the viewpoint of durable corrosion. In addition, iron-
It can be present in the form of a nickel intermetallic compound.
On the other hand, if the amount of cobalt is small, the effect of improving the corrosion resistance becomes insufficient.

It is possible to add metal elements other than iron, nickel and cobalt, such as chromium and manganese, for the purpose of controlling the shape of the magnetic powder, but if they are added in a large amount, the saturation magnetization decreases. To keep the saturation magnetization,
It is preferable that the ratio between iron and cobalt be 90% by weight or more in the metal elements constituting the magnetic powder.

According to our investigation results, it is important to uniformly dissolve cobalt in iron and nickel in order to obtain alloy magnetic powder having high saturation magnetization and excellent corrosion resistance.
If cobalt segregates on the surface of the magnetic powder, high saturation magnetization is hardly obtained, for example, a local battery is generated and corrosion is induced.

As a method for uniformly dissolving cobalt in iron and nickel as described above, for example, nickel-containing magnetic iron oxide powder is dispersed in a polyhydric alcohol in which cobalt chloride is dissolved, and this suspension is dispersed. Is heated to form a solid solution of cobalt in the magnetic iron oxide, and then reduced by heating with hydrogen gas to obtain an iron-cobalt-nickel alloy powder. Also, in this method, if the particle size distribution of the magnetic iron oxide, which is a raw material of the cobalt solid solution reaction, has a wide distribution, the specific surface area of each particle naturally appears, and the particle surface is different. There is a difference in the rate of the cobalt exchange reaction between the particles, and the composition of each particle becomes non-uniform. As a result, a potential difference is generated between the particles, a local battery is generated, and corrosion proceeds. Therefore, the particle size distribution of the raw material for forming a solid solution of cobalt is very important for forming a metal magnetic powder having excellent corrosion resistance. That is, by adding nickel at the stage of forming the raw material goethite, obtaining goethite having a small particle size distribution can improve the corrosion resistance of the metal magnetic powder obtained later.

In this method, the exchange reaction between the divalent iron ions and the cobalt ions on the surface of the magnetic iron oxide powder causes the solid solution of cobalt in the magnetic iron oxide powder. Controls become very effective.

The iron oxide used in the core crystal is most often amount of Fe ²⁺ has a spinel structure magnetite (Fe ²⁺ / Fe ³⁺ = 5
0% by weight) is represented as Fe ³⁺ [Fe ³⁺ Fe ²⁺ ] O ₄ .
Further, γ-Fe ₂ O ₃ in which Fe ²⁺ does not exist is Fe ³⁺ [F
e ³⁺ _5/3 □ _1/3 ] O ₄ . Here, □ represents a hole. Therefore, when the amount of Fe ²⁺ increases, vacancies decrease. In the solid solution reaction of cobalt into iron oxide particles by the polyhydric alcohol, an exchange reaction occurs between Fe ²⁺ ions on the surface of the magnetic iron oxide and Co ²⁺ in the polyhydric alcohol at the solid-liquid interface. Then, the Co ²⁺ taken into the surface is diffused into the inside using the vacancy inside the particle.

Therefore, in order to efficiently form a solid solution of cobalt in this method, the amount of Fe ²⁺ or the amount of vacancies may be controlled.

That is, the content of Fe ²⁺ / Fe ³⁺ is 5% by weight to 4%.
In the range of 5% by weight, cobalt replaces Fe ²⁺ and diffuses inside the particles. However, when the content is less than 5% by weight, cobalt cannot be replaced and the amount of solid solution decreases. On the other hand, 45
If the content is more than 10% by weight, the amount of vacancies contributing to diffusion is reduced, so that Co ²⁺ substituted with Fe ²⁺ is not easily diffused into the interior. Therefore, cobalt segregates on the surface and the amount of solid solution of cobalt decreases.

As a general method for appropriately controlling the divalent iron ions of the magnetic iron oxide powder, there is a method in which the partial reduction of the magnetic iron oxide powder is reduced by heating in hydrogen gas. However, according to this method, 1) it is difficult to set the reduction conditions and it is difficult to obtain a predetermined Fe ²⁺ / Fe ³⁺ , and 2) it is easy to cause sintering between the magnetic iron oxide powder particles. Since the solid solution reaction speed of cobalt differs between the unsintered portion and the unsintered portion, the amount of cobalt solid solution between the particles becomes non-uniform. As a result, there is a problem that the erasing characteristics of the iron-cobalt alloy obtained by reducing the cobalt-dissolved iron oxide powder are deteriorated.

On the other hand, by dispersing the magnetic iron oxide powder having a controlled shape in a polyhydric alcohol containing an iron salt and heating the solution, the magnetic iron oxide powder as a raw material can be produced without sintering. Control of divalent iron ions has been proposed in JP-A-53-90198 and the like. As a result of various studies, the inventors have found that this proposal is particularly useful for solving the above problems.

That is, the control of the divalent iron ions of the magnetic iron oxide, which greatly affects the cobalt composition, can be performed without performing heat reduction in a reducing atmosphere that causes sintering.
When the reaction is carried out in the same polyhydric alcohol as the solution used for the cobalt solid solution reaction to obtain an iron-cobalt alloy as a raw material, a product with less sintering can be obtained.

When a magnetic iron oxide powder is dispersed in a polyhydric alcohol in which an iron salt is dissolved, and this solution is heated, divalent iron ions are introduced into the magnetic iron oxide powder to form an intermediate iron oxide. The amount of divalent iron ions introduced increases with an increase in the concentration of iron ions in the solution and the heating temperature. As the polyhydric alcohol, polyethylene glycol, ethylene glycol, propylene glycol, glycerin and the like can be used.

The heat reduction may be performed according to a known heat reduction method, and the reduction temperature is usually about 150 ° C. to 500 ° C. As described above, according to the present invention, it is possible to obtain a fine-particle alloy magnetic powder having high saturation magnetization, a coercive force suitable for magnetic recording, and excellent corrosion resistance.

Incidentally, in order to obtain the highest possible coercive force, the needle-like magnetic powder for ordinary magnetic recording has an axial ratio of 10 or more.
The coercive force is increased by the shape anisotropy. However, when the axial ratio of the alloy magnetic powder mainly composed of iron and cobalt having a high saturation magnetization is set to 10 or more, the coercive force becomes about 1700 Oe or more, which is too high for a magnetic recording medium. In order to reduce the coercive force to about 1600 Oe or less suitable for magnetic recording, the particle size is usually increased. However, in such an alloy magnetic powder, fine particles to coarse particles are mixed and the particle size distribution is increased. Becomes wider. Therefore, even if the coercive force value is appropriate, the anisotropic magnetic field distribution is widened, and as a result, a recording medium using the acicular alloy magnetic powder has a problem that the erasing characteristics are inferior. Further, such a medium has a problem that noise is high due to the large particle size of the magnetic powder, and at present, needle-like alloy magnetic powder that can be used practically has not been obtained.

Therefore, in order to reduce the coercive force to a value suitable for magnetic recording while keeping the particle size small and the anisotropic magnetic field distribution at a small value, the axial ratio of the particles is in the range of 4 to 8, preferably 4 to 8. It is most effective to set it in the range of 0.5 to 7.0.

Assuming that the alloy magnetic powder having an axial ratio of 10 or more is 1, the value of the coercive force is approximately 0.78 when the axial ratio is 4 and approximately 0.96 when the axial ratio is 8. When the axial ratio is smaller than 4, it is difficult to control the coercive force. When the axial ratio is larger than 8, the effect of reducing the coercive force is hardly recognized.

Since the coercive force of the alloy powder of the present invention is reduced by the axial ratio, it is not necessary to increase the particle size. Usually, the long axis particle diameter is 0.1 μm to 0.25 μm.
Are preferably used.

All of these alloy powders have a narrow anisotropic magnetic field distribution with a quotient value of 3.2 or less when the value of the half width of the anisotropic magnetic field distribution divided by the coercive force, and the erasing characteristic is low. Is improved, and a magnetic recording medium using the same has reduced noise.

The acicular alloy magnetic powder obtained according to the present invention has a high saturation magnetization of 120 emu / g or more even when left for 7 days in an environment of a temperature of 60 ° C. and a humidity of 90%, and is extremely corroded. Hateful. This is not only due to the proper amount of cobalt, but also to the fact that nickel is contained in an amount of 2% by weight or more with respect to iron.

In order to obtain a magnetic recording medium having good corrosion resistance even under a certain wear environment, that is, to obtain a magnetic recording medium excellent in durable corrosion, using the above-described novel needle-shaped alloy magnetic powder, as follows. can do.

That is, the surface of the acicular alloy magnetic powder is brought into contact with a heated oxygen-containing gas to form a ferrite layer, or water is applied to an Al or Si alcohol solution to hydrolyze these compounds into acicular alloys. By coating at least one of Al and Si by forming on the surface of the alloy magnetic powder, it is possible to impart durable corrosion resistance to the needle-shaped alloy magnetic powder itself even with a relatively small amount of cobalt.

In particular, the effect is recognized when Al or Si is coated in a specific quantitative range, and it is preferable that the coating amount of at least one of Al and Si is in the range of 1 to 9 atoms / nm ² . If the coating amount is less than 1 atom / nm ² , Al, Si
When the coating amount exceeds 9 atoms / nm ² , non-magnetic components unnecessarily increase to impair the magnetic properties, and the needle-like alloy magnetic powder having a high saturation magnetization only has a high saturation magnetization. In some cases, the characteristics may be impaired.

Further, by imparting a protective effect of the acicular alloy magnetic powder to the magnetic coating film, excellent durability corrosion resistance can be obtained. That is, as described later, an inorganic powder having a Mohs hardness of 5 or more is added as a filler for the magnetic coating film, or a fatty acid or fatty acid ester mineral oil which gives lubricity to the surface of the magnetic coating film is added as a lubricant. A protective action can be imparted to the magnetic coating film by, for example, selecting a specific binder as a binder for imparting strength against rubbing of the film. Further, by setting the surface of the magnetic coating film to have a center line average roughness of 0.004 μm or less, it is possible not only to prevent the corrosive gas from entering the inside of the coating film from the surface but also to reduce the friction coefficient. Therefore, the coating film is less likely to be damaged and the durability corrosion resistance can be increased as a result. When the magnetic recording medium is used in the form of a tape, the durability of the magnetic coating film may be deteriorated due to scratching of the magnetic coating film due to rubbing with the back coat during winding. By 0.01
It is preferable to employ one containing primary particles or aggregated carbon black having a surface smoothness of not more than μm and having a diameter larger than the major axis particle diameter of the acicular alloy magnetic powder.

In particular, a magnetic recording type of winding a tape having a magnetic layer such as a magnetic tape and a back layer and having a total thickness of a support, a magnetic coating film and a back coat layer of 14 μm or less in total. The medium has good corrosion resistance even in a severe wear environment, and can ensure reliability so that dropout and noise do not increase.

Such a magnetic recording medium is generally provided with a magnetic powder, a binder,
It is formed by applying a magnetic paint obtained by kneading and dispersing an inorganic powder, a lubricant and the like in an organic solvent, and forming a dried magnetic coating film. In particular, in the case of video tapes and the like, a magnetic recording medium having a back coat layer of a coating film containing a binder, an inorganic powder, a lubricant, etc. formed on the back surface of the support in order to improve running properties. It has been proposed that both the magnetic layer and the back coat layer contain an inorganic powder having a Mohs hardness of 5 or more in order to prevent abrasion when coming into contact with the head or the guide roller.

The inclusion of an inorganic powder having a Mohs hardness of 5 or more in the back coat layer is described in, for example,
No. 12, JP-A-62-38525, JP-A-62-38
No. 526, JP-A-62-38527 and JP-A-62-385
No. 8528, etc., for the magnetic layer, see, for example,
-18572, JP-B48-15003, JP-B-4
No. 9-39402, Japanese Patent Publication No. 52-28642, Japanese Patent Publication No. 52-49961, Japanese Patent Publication No. 55-15771, and the like.

Further, in order to reduce noise, it is necessary to perform a surface finishing treatment on the magnetic layer by a super calender treatment to smooth the surface of the magnetic layer to reduce noise.
No. 17404, Japanese Patent Publication No. 60-12688, and the like.

However, the magnetic recording medium obtained by using the needle-shaped alloy magnetic powder according to the present invention as a magnetic recording element and undergoing a surface finishing treatment has a low noise at first, but after running for a long period of time, and particularly in a corrosive environment. There has been a problem that dropouts and noise rise when placed under a long period of time.

This problem is remarkable when a metal magnetic powder containing less than 8% by weight of cobalt is used. Dropout or noise for a signal having a frequency corresponding to a recording wavelength of about 0.8 μm or less occurs after a long running. It was clear that it would rise.

Although this problem occurred almost evenly for various prototypes, according to the study of the present inventors, in the case of a needle-shaped alloy magnetic powder mainly containing iron and cobalt,
A major axis particle diameter of 0.25 μm or less and an axial ratio of 4 to 8,
In addition, a magnetic recording medium in which the weight ratio of cobalt to iron is in the range of 8 to 50% by weight, a ferrite coating mainly containing iron and cobalt is formed on the surface, and at least one of Si and Al is coated on the surface, In particular, the result was that the initially reduced noise was maintained even after storage.

Therefore, various investigations have been made on the assumption that this problem is caused not only by the corrosion resistance but also the wear resistance of the metal magnetic powder. This phenomenon was found to be particularly remarkable in a tape having a total thickness of the support, the magnetic coating film, and the back coat layer of 14 μm or less. This is considered to be one of the causes of the fact that the number of windings per unit winding diameter is larger than that of a thicker one having a total thickness of about 20 μm, and particularly the tightening of the windings in the vicinity of the innermost side of the hub is remarkable.

In the VHS system in which the total thickness of the support, the magnetic coating film, and the back coat layer is about 20 μm, the shortest recording wavelength of the white level is 1.3 μm, whereas the ferromagnetic alloy powder is used as the magnetic recording element. The center recording wavelength is 0.
This is particularly noticeable in a magnetic recording medium of 8 μm or less.

Therefore, the present inventors further concluded that the susceptibility of the magnetic layer to damage depends on the particle size of the powder contained in the coating film. We found the fact that the strength of the coating film can be maintained by increasing the hardness of the powder itself, and in light of this fact, as a result of various investigations, as described earlier, an alloy that simultaneously imparts the hardness of the powder itself and corrosion resistance Thus, three means are provided: a predetermined amount of cobalt therein, the formation of a passivation film on the surface of the powder particles, and at least one kind of protective film of Si and Al.

On the other hand, the corrosion resistance of the magnetic coating film also depends on the back coat layer which is apparently unrelated to the magnetic layer. That is, when an inorganic powder having a Mohs hardness of 5 or more, which is necessarily included in the back coat layer when the magnetic recording tape is wound, damages the surface of the magnetic layer, the metal magnetic powder is exposed to a corrosive environment in some cases. Therefore, it has been found that it is desirable that the particle diameter of the inorganic powder having a Mohs hardness of 5 or more in the back coat layer be smaller than the major axis particle diameter of the acicular alloy magnetic powder.

In many cases, it is necessary to include carbon black in the back coat layer in order to improve running properties. This is itself, including carbon black of fine particles of 10 to 30 mμ, carbon black of coarse particles of 200 to 500 mμ and non-magnetic powder of 0.2 μm or less,
It has already been described in Japanese Patent Application Laid-Open No. 62-8328, for example, that the increase in dropout is suppressed and the running durability is improved.

However, this document does not refer to the relationship between the particle size of the primary particles of carbon black, that is, the diameter of the aggregate, and the long-axis particle size of the ferromagnetic alloy powder in the magnetic coating film. Further, no study has been made on the relationship between a signal having a frequency corresponding to a recording wavelength of 0.8 μm or less and noise with respect to a signal when a needle-shaped alloy magnetic powder having a small major axis particle diameter is used.

Therefore, in the present invention, the particle diameter of the primary particles or the diameter of the aggregates of the carbon black in the back coat layer is
As a result of examining the relationship with the major axis particle diameter of the needle-shaped alloy magnetic powder in the magnetic coating film, when the particle diameter of the primary particles or the diameter of the aggregates of the carbon black is larger than the magnetic powder,
As a result, it has become clear that the demand for high output and low noise can be sufficiently satisfied.

This fact becomes particularly remarkable in a magnetic recording medium having a magnetic layer having a thickness of about 3.0 μm or less corresponding to a demand for a high recording density, particularly, a thinner magnetic coating film.

Therefore, in the present invention, a magnetic layer and a back coat layer are provided on both surfaces of a non-magnetic support, respectively, so that a high output can be obtained. In order to provide a magnetic recording medium capable of suppressing the rise of noise as much as possible despite the preservation of the magnetic recording medium, the magnetic coating film should have a squareness ratio of 0.85 or more under the measurement condition of the applied magnetic field of 10 KOe. Is preferable,
Preferably, the coercive force of the magnetic layer is 1500 Oe or more, and the surface smoothness of the magnetic layer is 0.0
It is preferable to regulate the surface smoothness of the back coat layer to 0.01 μm or less, and it is preferable to regulate the surface smoothness of the back coat layer to be smaller than the long axis particle diameter of the acicular alloy magnetic powder. I found that.

In general, it has not been expected that carbon black or the like is soft and that what is included in the back coat damages the surface of the magnetic layer due to the winding of the tape. In order to achieve high-density recording with a wavelength of about 0.8 μm or less, the major axis diameter of the magnetic powder is reduced to 0.2 μm or less, and the magnetic layer mainly containing metal magnetic powder has a reinforcing effect on the coating film. In addition to the fact that it is not enough, the strong tightening force can be applied under the harsh conditions of many turns of thin tape required by high density recording, so even carbon black with a small particle size is a magnetic layer Have found the fact that the surface may be damaged.

In this case, when carbon black having a diameter larger than the major axis particle diameter of the needle-shaped alloy magnetic powder is contained as primary particles or aggregates in the back coat layer, those having a small particle diameter enter the gap. Therefore, it does not protrude to the surface of the back coat layer, which is preferable. Further, the particle size of the inorganic powder having a Mohs hardness of 5 or more contained in the back coat coating film,
It is preferable to make the magnetic tape having such a configuration smaller than the long axis particle diameter of the needle-shaped alloy magnetic powder. It has been found that it is preferable to house it in a cassette body containing a pigment having a Mohs' hardness of 2 to 4.

As the material of the nonmagnetic support used in the present invention, polyethylene terephthalate, polyethylene
Polyesters such as 2,6-naphthalate, polyolefins such as polyethylene and polypropylene, cellulose derivatives such as cellulose triacetate and cellulose diacetate, vinyl resins such as polyvinyl chloride and polyvinylidene chloride, polycarbonate, polyimide, polyamide and the like. And biaxially oriented polyethylene terephthalate, polyethylene-2,6
A polyester such as naphthalate having an elastic modulus in the longitudinal direction of 700 kg / mm ² or more and an elastic modulus in the width direction of 40;
It is desirable that the surface roughness is 0 kg / mm ² or more and the surface roughness is 0.01 μm or less.

Then, the magnetic powder and the inorganic powder having a Mohs hardness of 5 or more are both contained in the binder resin dissolved in the organic solvent on the support, and a magnetic coating material in which the magnetic powder and the inorganic powder are dispersed is applied. Dry to form a magnetic coating.

The major axis particle diameter of the ferromagnetic alloy powder may be 0.8 μm or less in relation to the central recording wavelength, but is preferably 0.25 μm or less from the viewpoint of resolution. Alternatively, an alloy powder containing a metal element of nickel or cobalt,
If the composition is adjusted so that the weight ratio of nickel is 2% by weight or more and the weight ratio of cobalt to nickel is 110% by weight or more, saturation occurs even when left in an environment of 60 ° C. and 90% humidity for 7 days. When the value of magnetization is 120 emu /
g or more is preferable. In addition, it is desirable that the value of the quotient obtained by dividing the half width of the anisotropic magnetic field distribution by the coercive force is 3.2 or less for maintaining the erasing characteristics.

Examples of the inorganic powder having a Mohs hardness of 5 or more contained in the magnetic layer or the back coat layer include metal oxides, metal carbides, and metal nitrides.
₂ O ₃ (Mohs hardness 6), Al ₂ O ₃ (Mohs hardness 9), Cr ₂ O ₃ (Mohs hardness 9), SiO ₂ (Mohs hardness 6), TiO ₂ (Mohs hardness 6), ZrO ₂ (Mohs hardness) 6), SiC (Mohs hardness 9), TiC (Mohs hardness 9), hBN (Mohs hardness 9), Si ₃ N ₄ (Mohs hardness 9) and the like are listed as more preferable ones. Since these inorganic powders having various particle diameters can be easily handled manually, they can be appropriately selected in accordance with the above findings of the present invention.

In the present invention, any of channel black, furnace black, acetylene black and thermal black can be used as the carbon black used in the back coat layer, but acetylene black is particularly preferred.

Further, a graphitized carbon black whose surface is covered with a graphite layer as disclosed in JP-A-61-22424 can also be used.

Specific examples of commercially available carbon black products include those manufactured by Cabot Corporation in the United States, Black Pearl 700 having a particle size of 18 μm, Mogal L having a particle size of 20 μm, ELFTEXpellets-115 having a particle size of 27 μm, and Legal 3001 having the same particle size. Vulcan XC-7 with a diameter of 30mμ
2. Sterling NS, Sterling R having a particle size of 75 μm, manufactured by Columbian Carbon Co., Ltd.
Raben 8000 with 3 μm, raben 5 with 20 μm particle size
250, Raben 890 with a particle size of 30 mμ, particle size of 62 mμ
Raben 450 with a particle size of 70 mμ, raben MT-P beads with a particle size of 280 mμ (0.28 μ), rabensebacalp MT-CI with a particle size of 300 mμ (0.30 μ), and those manufactured by Asahi Carbon Co., Ltd. , Particle size 7
HS-500 having a particle size of 5 μm, # 60H having a particle size of 35 μm, Seat 5H having a particle size of 20 μm for Tokai Carbon Co., Ltd.
For Ketjen Black EC of mμ, manufactured by Mitsubishi Kasei Co., Ltd., {4040 having a particle size of 20 μm and {
4330BS, # 4350BS with 45mμ particle size, 8 particle size
0 μm of # 4010 or the like can be preferably used. As described above, since carbon blacks having various particle diameters are easily available, they can be appropriately selected in design in consideration of the major axis diameter of the metal magnetic powder according to the above findings of the present invention. However, if the particle size is relatively small, it is desirable to form an aggregate of several primary particles using the structure forming ability of the carbon black itself. In this case, the aggregated aggregate functions as if it were a single carbon black particle. These inorganic powders and carbon black may be mixed in the coating material of the magnetic layer and the back coat layer so as to satisfy the above-mentioned relationship of the particle size.

The binder resin used for the magnetic coating film and the back coat coating film includes vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-vinyl alcohol-
PVC resin such as vinyl copolymer, vinyl chloride-acrylate copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, vinyl chloride-vinyl acetate-maleic acid copolymer, heat Plastic polyurethane resin, thermosetting polyurethane resin, polyester resin, phenoxy resin, polyvinyl butyral resin,
Examples include cellulose derivatives, epoxy resins or mixtures thereof.

Further, a hydrophilic polar group such as carboxylic acid, sulfonic acid, sulfonic acid salt, phosphoric acid, phosphate, amine, ammonium salt or the like is introduced into these resins to disperse powder particles as a coating material. It is also possible to improve the properties or to introduce an acrylic double bond to cure by irradiation with an electron beam.

Solvents used for preparing paints for forming these coating films include alcohols such as ethanol, propanol and butanol, esters such as methyl acetate, ethyl acetate and butyl acetate, methyl ethyl ketone, methyl isobutyl ketone and cyclohexane. Ketones such as xanone, ethers such as tetrahydrofuran and dioxane, aromatic hydrocarbons such as benzene, toluene, xylene, heptane, hexane, aliphatic hydrocarbons such as cyclohexane, methylene chloride, ethylene chloride,
Chlorinated hydrocarbons such as chloroform are mentioned, and a mixed solvent of cyclohexanone-toluene is preferable.

In addition, the above-mentioned coating film may further include, as an additive,
Lubricants such as saturated and unsaturated higher fatty acids, higher fatty acid amides, fatty acid esters, higher alcohols, silicone oils, mineral oils, edible oils, fluorine oils and the like can be used.

Such a composition can be achieved by dispersing a predetermined amount in a ball mill or a sand mill to obtain a magnetic paint and a back coat paint, and applying the magnetic paint and the back coat paint on the non-magnetic support.

When dispersing the paint, one of the most important points is to apply an extra force to the particles of the inorganic powder and the carbon black so as not to change the particle size of the carbon black compounded as designed. It is to consider.

In the application, it is preferable to apply the magnetic layer first, orient the magnetic field before drying, smooth the surface of the coating film, wind it up, and apply the back coat layer on the back surface. The order of this process is described in
No. 23647.

The magnetic layer and the back coat layer may be applied alone or as a plurality of layers, or may be applied simultaneously. The coated magnetic recording medium is cut into a predetermined tape width by a slit device, usually wound around an individual hub with the backcoat layer inside, and can be housed in the tape cassette body.

When the total thickness of the magnetic coating film, the support and the back coat layer is 14 μm or less, the elastic modulus in the longitudinal direction is larger than the elastic modulus in the width direction, and the elastic modulus in the width direction is 2 μm.
It is desirable not to exceed twice. Further, it is desirable that the elastic modulus in the longitudinal direction is 1000 kg / mm ² or more. The magnetic coating film is also made such that when a steel ball having a diameter of 6 mm is pressed with a load of 5 g and run for 23 m, the volume reduction due to wear of the steel ball is 20 × 10 ⁻⁵ mm ³ or less. It is preferable to regulate the magnetic layer surface of the magnetic recording medium, which is slit to a width of 3.81 mm, using a sapphire blade having a vertex angle of 45 ° and a load of 50 g / (3.81 mm width). Even when the magnetic film is reciprocated 1000 times, the magnetic coating film which causes corrosion in storage in a corrosive environment thereafter does not occur.

Making the modulus of elasticity in the longitudinal direction larger than the modulus of elasticity in the width direction and not exceeding twice the modulus of elasticity in the width direction depends on the selection of an appropriate support and the magnetic field orientation of magnetic particles. This can be achieved by mechanical control. Further, setting the elastic modulus in the longitudinal direction to 1000 kg / mm ² or more can be achieved by appropriately selecting the curing conditions of the binder resin of each coating film. In order to regulate the amount of volume reduction due to abrasion of the magnetic coating film to 20 × 10 ⁻⁵ mm ³ or less, an appropriate amount of inorganic powder having a Mohs hardness of 5 or more is included, and conditions for surface smoothing treatment are selected. This can be achieved by selecting dry conditions.

Since a strong radial tightening force is applied to the hub serving as the core, it is desirable to control the sink of the tape winding peripheral surface within 5 μm in order to prevent curling of the tape. .

Since the back coat side of such a magnetic recording tape is in contact with the tape guide member of the tape cassette main body, in order to reduce noise, the tape cassette main body or at least a portion of the tape guide must be A
It is preferable to use a BS resin, and it is desirable to include a pigment having a Mohs hardness of 2 to 4 such as calcium carbonate and barium sulfate to prevent damage to the back coat. It is preferable to include an antistatic agent such as quaternary ammonium salt such as carbon black and polyoxyethylene alkylamine, and a molten resin fluidizing agent such as ethylene bis stearamide. To manufacture such a cassette body, these compositions are usually melted and injected into a mold by injection molding.

As described above, in short-wavelength recording with a center recording wavelength of about 0.8 μm or less, which is required for a small and high-quality video tape, etc., dropout and noise are caused by storage in a high-output corrosive environment. Magnetic recording medium in which the phenomenon that the temperature rises as much as possible can be obtained.

Hereinafter, details of the present invention will be described with reference to examples.

[0084]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preparation of alloy magnetic powder 5 mol / liter sodium hydroxide aqueous solution 1.5
0.72mol / liter of ferrous sulfate and (A)
A mixed solution of 1.5 mol / liter and nickel sulfate was added thereto with stirring at room temperature while stirring to obtain a coprecipitate of ferrous hydroxide and nickel hydroxide. The precipitate suspension was blown with air at a rate of 1.6 liter / min while maintaining at 40 ° C., stirred for 8 hours, filtered, washed with water, and dried to obtain a needle-shaped mold having a long axis particle diameter of 0.22 μm and an axis ratio of 7. I got a game site. The particle shape depends on the concentrations of the alkaline solution, metal salt, and the like, and the axial ratio (G) can be changed by slightly changing these concentrations.

Next, 100 g of this goethite was dispersed in 3 liters of water, and 2 liters of a 1 mol / liter sodium hydroxide aqueous solution and 26 ml of a 1 mol / liter sodium orthosilicate aqueous solution were added to 3 liters of water. Carbon dioxide gas was blown into the mixture to neutralize it to PH8, followed by washing and drying, and a silicon compound was adhered to the particle surface.

Next, the silicate compound-coated iron oxide was dispersed in 3 liters of water, and 2 liters of a 1 mol / liter sodium hydroxide aqueous solution and 0.5 mol / liter aqueous sodium aluminate solution were added to the suspension. After adding 135 milliliters and blowing in carbon dioxide gas to neutralize to PH8, washed with water and dried, alumina was adhered to the surface of the silicate compound-coated particles. Next, the goethite coated with the silicate compound and alumina was calcined at 750 ° C. for 4 hours and then reduced in a hydrogen gas stream containing water vapor at 300 ° C. for 8 hours to obtain magnetic iron oxide. Then, the Fe ²⁺ / Fe ³⁺ amount in the magnetic iron oxide is controlled by one of the following methods (B).

(B-A) Method 20 g of the magnetic oxidized pig iron powder obtained by the above method is heated in an oxygen-containing gas to perform partial oxidation to control Fe ²⁺ / Fe ³⁺ to (D)% by weight.

(B-B) Method Next, ferrous chloride tetrahydrate (C) g was dissolved in 300 ml of ethylene glycol, and 40 g of the magnetic iron oxide obtained by the above method was dispersed.
Heat at 180 ° C. for 4 hours with stirring to ^obtain Fe ²⁺ / Fe ³⁺
Is controlled to (D)% by weight, and washed with water and dried.

Next, g of cobalt chloride hexahydrate (E) was dissolved in 300 milliliter of polyethylene glycol,
Magnetic iron oxide 2 in which the amount of Fe ²⁺ / Fe ³⁺ is controlled
0 g was dispersed and heated at 200 ° C. for 6 hours with stirring to obtain a magnetic powder in which cobalt was uniformly dissolved in the magnetic iron oxide. Next, the magnetic powder is washed with water, and then washed with hydrogen gas.
Reduce by heating at 0 ° C for 2 hours.
A gas containing 0 ppm oxygen was flowed at 60 ° C. for 2 hours to perform slow oxidation to obtain a needle-shaped alloy magnetic powder having a long axis particle diameter of 0.2 μm and having a ferrite layer mainly composed of iron and cobalt on the surface.

Next, the surface treatment (F) of the acicular alloy magnetic powder was performed as follows. 100 g of the needle-shaped alloy magnetic powder obtained above was dispersed in 2 liters of ethanol,
7.5 g of Si (OC ₂ H ₅ ) ₄ was added thereto, the temperature was raised to 60 ° C. with stirring, and 7.8 g of water was gradually added dropwise to hydrolyze the Si (OC ₂ H ₅ ) _4. The surface of the acicular alloy magnetic powder was coated with a 7 atom / nm ² hydroxide of Si.

The production conditions (A) to (G) of the acicular alloy magnetic powders of Samples 1 to 12 thus obtained are shown in FIG. In the drawing, the mark “○” indicates that the processing was performed, and the mark “X” indicates that the processing was not performed.

II. Preparation of magnetic layer 100 parts by weight of the needle-shaped alloy magnetic powder obtained above, 10 parts by weight of a hydroxyl group-containing vinyl chloride resin having a degree of polymerization of 340,
7 parts by weight of thermoplastic polyurethane resin and particle size 0.2 μm
8 parts by weight of alumina, 2 parts by weight of myristic acid, 2 parts by weight of bengara (α-Fe ₂ O ₃ ) having a particle size of 0.8 μm,
A mixture of 1 part by weight of # 4010 manufactured by Mitsubishi Kasei Co., Ltd. having a particle size of 80 μm and 2 parts by weight of Seat 5H manufactured by Tokai Carbon Co., Ltd. having a particle size of 20 μm was used as a carbon black, and 70 parts by weight of cyclohexanone was used. And a mixture of toluene and kneaded and dispersed in a ball mill for 96 hours, and 5 parts by weight of a trifunctional polyisocyanate compound was further added thereto and stirred to prepare a magnetic paint. This magnetic paint was applied on a support of a biaxially oriented polyethylene terephthalate film having a thickness of 10 μm, applied to a thickness of 2.5 μm, dried and calendered to produce a magnetic recording medium.

III. Preparation of Back Coat Layer On the other hand, 60 parts by weight of a seam 5H manufactured by Tokai Carbon Co., Ltd. having a particle size of 20 mμ as carbon black, and a particle size of 280 m
7.5 parts by weight of Raven MT-P beads (μm (0.28 μm) manufactured by Columbian Carbon Co., Ltd.) and a particle size of 0.05 μm
m, 30 parts by weight of calcium carbonate, 2.5 parts by weight of bengara having a particle size of 0.1 μm,
Parts by weight, 40 parts by weight of nitrocellulose, and 15 parts by weight of a trifunctional isocyanate crosslinking agent were mixed with 330 parts by weight of cyclohexanone and toluene, and the composition was kneaded and dispersed in a ball mill for 96 hours for backing. A paint was prepared. This was applied on the back surface of the magnetic recording medium coated with the magnetic layer to a thickness of 1.0 μm and dried.
Time cured. The total thickness of this magnetic recording medium is 13.5 μ
m.

The tape was cut into a predetermined width to form a tape.
Wound around a hub made of injection molded polyoxymethylene resin processed to within μm.

IV. Preparation of Cassette Main Body On the other hand, 100 parts by weight of ABS resin NA-1060 manufactured by Denki Kagaku Kogyo Co., 23 parts by weight of a coloring agent obtained by treating carbon black with ferrocyanine blue, and 35 parts by weight of calcium carbonate (particle size: 0.5 μm) , Polyoxyethylene alkylamine 12 parts by weight and ethylene bis stearamide 3
1 part by weight at 110 ° C. with a Henschel mixer.
After kneading for 220 minutes, the mixture was extruded at 220 ° C. using a twin-screw extruder and pelletized. Further, together with 1500 parts by weight of the ABS resin, it was melted at 240 ° C. in a heating furnace, and the cassette body was injection-molded at a mold temperature of 30 ° C. The magnetic recording medium was assembled by housing the hub-wound tape in the cassette body.

The squareness ratios of the magnetic tapes prepared as described above under a measurement condition of an applied magnetic field of 10 KOe were all 0.8.
5 or more. The surface smoothness of the magnetic coating film and the back coat layer was measured using a stylus type surface roughness meter under the conditions of a stylus R = 2 μm and a cutoff of 0.08 mm. As a result, the center line average roughness of the magnetic coating film was 0.004 μm.
Hereinafter, the center line average roughness of the back coat layer is 0.01 μm
It was below.

For the magnetic powders of Samples 1 to 12, the weight ratios of Co, Ni, Al, and Si to Fe, ie, Co / Fe (H) wt%, Ni / Fe (I) wt%, Al / Fe (J) weight% and Si / Fe (K) weight% were measured, and the axial ratio (L) was measured using a transmission electron microscope. Further, using a sample vibration magnetometer (manufactured by Toei Kogyo Co., Ltd.), the coercive force (M) Oe, the saturation magnetization (N) emu / g, and the saturation magnetization after being left in an environment of a temperature of 60 ° C. and 90% for 7 days. (O) emu / g and anisotropic magnetic field distribution were measured. The anisotropic magnetic field distribution was represented by a quotient (P) obtained by dividing the half width of the anisotropic magnetic field distribution by the coercive force. These results are shown in FIG.
It was shown to.

The magnetic recording media obtained for Samples 1 to 6 and Sample 12 were placed in an environment at a temperature of 60 ° C. and a humidity of 90%.
The saturation magnetic flux density after standing for one day (Q)% and the surface of the magnetic layer of the magnetic recording medium slit long and narrow to 3.81 mm width was subjected to a load of 50 g / (3.81 mm width) using a sapphire blade having a vertex angle of 45 °. ), And immediately after being allowed to stand in an environment of a temperature of 60 ° C. and a humidity of 90% for 7 days, the reduction rate (R)% of the saturation magnetic flux density is determined by VSM.
Was measured by

Regarding the noise level at 6 MHz when a signal of 7 MHz was input and reproduced, the sapphire blade having a vertex angle of 45 ° was formed on the magnetic layer surface of the magnetic recording medium immediately after winding the hub and slit to a width of 3.81 mm. After reciprocating 1000 times with a load of 50 g / (3.81 mm width) using a tester, immediately stand for 7 days in an environment of a temperature of 60 ° C. and a humidity of 90%, and measure the noise level. With the noise level of 0 dB as standard
The surface of the magnetic layer was reciprocated 1000 times with a load of 50 g / (3.81 mm width) 1000 times using a sapphire blade having a vertex angle of 45 °, and immediately afterward, under an environment of a temperature of 60 ° C. and a humidity of 90%.
It was expressed as a noise level ratio (S) dB after standing for a day. These results are shown in FIG.

As a result of observing the surfaces of the magnetic powders of Samples 1 and 8 with a transmission electron microscope at a high magnification,
The thickness of the ferrite layer on the surface was 28 angstroms for sample 1 and 35 angstroms for sample 10. The observation state is shown in the figure. FIG. 1 is an explanatory diagram showing the state of the particle structure of the magnetic powder of Sample 1, and FIG. 2 is an explanatory diagram showing the state of the particle structure of the magnetic powder of Sample 8. A in the figure is a solid solution layer of iron and cobalt, and B is a ferrite layer of iron and cobalt.

[0101]

As is apparent from the above, the needle-shaped alloy magnetic powder according to the present invention has high saturation magnetization and excellent corrosion resistance, and at the same time, the magnetic recording medium produced by using this alloy has excellent corrosion resistance and durable corrosion resistance. There was almost no increase in noise even after running after the corrosion test.

In the present invention, since the ferrite layer mainly composed of iron and cobalt is formed on the surface of the needle-like particles containing iron and cobalt as described above, excellent corrosion resistance is obtained even under severe conditions. I have.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a state of a particle structure of a magnetic powder of a sample 1;

FIG. 2 is an explanatory diagram showing a state of a particle structure of a magnetic powder of a sample 8;

FIG. 3 is a diagram summarizing manufacturing conditions of a magnetic powder.

FIG. 4 is a diagram summarizing measured values of a magnetic powder.

FIG. 5 is a diagram summarizing measured values of a magnetic powder.

[Explanation of symbols]

 A Solid solution layer of iron and cobalt B Ferrite layer of iron and cobalt

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Toshinobu Sueyoshi 1-88 Ushitora, Ibaraki-shi, Osaka Hitachi Maxell Co., Ltd. (72) Inventor Masayoshi Kawarai 1-188 Ushitora, Ibaraki-shi, Osaka Hitachi Maxell, Inc.

Claims (14)

[Claims] 1. A magnetic powder, wherein a ferrite layer mainly composed of iron and cobalt is formed on the surface of needle-like particles containing iron and cobalt. 2. The magnetic powder according to claim 1, wherein the weight ratio of cobalt to iron is 5 to 50% by weight, the major axis particle diameter is 0.25 μm or less, and the surface of the ferrite layer contains Si or Al. A magnetic powder comprising at least one kind of protective film formed thereon. 3. The magnetic powder according to claim 1, wherein the weight ratio of cobalt to iron is 5 to 50% by weight, the major axis particle diameter is 0.25 μm or less, and the axial ratio is 4 to 8. A magnetic powder, wherein at least one kind of protective film of Si or Al is formed on the surface of the ferrite layer. 4. A reduction step of reducing needle-like particles containing iron and cobalt, and oxidizing the particles obtained by the reduction in a high-temperature atmosphere of room temperature or higher to form a ferrite mainly composed of iron and cobalt on the particle surface. A method for producing a magnetic powder, comprising a ferrite layer forming step of forming a layer. 5. A reducing step for reducing needle-like particles containing iron and cobalt, and oxidizing the particles obtained by the reduction in a high-temperature atmosphere at room temperature or higher to form a ferrite mainly composed of iron and cobalt on the particle surface. A method for producing a magnetic powder, comprising: a ferrite layer forming step of forming a layer; and a protective film forming step of forming at least one kind of protective film of Si or Al on the surface of the ferrite layer. 6. The method for producing a magnetic powder according to claim 4, wherein the weight ratio of cobalt to iron is 5%.
A method for producing a magnetic powder, wherein the long axis particle diameter is 0.25 μm or less at 50 to 50% by weight. 7. The method for producing a magnetic powder according to claim 4, wherein the weight ratio of cobalt to iron is 5%.
A method for producing magnetic powder, characterized in that the long axis particle diameter is 0.25 μm or less and the axial ratio is in the range of 4 to 8 at ５０50% by weight. 8. A magnetic recording medium in which a magnetic coating is provided on one surface of a support, wherein the magnetic coating is formed on a surface of needle-like particles containing iron and cobalt by a ferrite layer mainly composed of iron and cobalt. A magnetic recording medium characterized by comprising a needle-like magnetic powder formed by: 9. The magnetic recording medium according to claim 8, wherein
The weight ratio of cobalt to iron is 5 to 50% by weight.
A magnetic recording medium having a long axis particle diameter of 0.25 μm or less and at least one kind of protective film of Si or Al formed on the surface of the ferrite layer. 10. The magnetic recording medium according to claim 8, wherein the weight ratio of cobalt to iron is 5 to 50% by weight, the major axis particle size is 0.25 μm or less, and the axial ratio is 4 to 8%.
Wherein at least one kind of protective film of Si or Al is formed on the surface of the ferrite layer. 11. The magnetic recording medium according to claim 8, wherein a center line average roughness of the surface of the magnetic coating film is 0.004 μm or less, and a thickness of the entire magnetic recording medium is 14 μm or less. A magnetic recording medium characterized by the above-mentioned. 12. The magnetic recording medium according to claim 8, wherein said magnetic coating film contains an inorganic powder having a Mohs hardness of 5 or more. 13. The magnetic recording medium according to claim 8, wherein a back coat layer is formed on the other surface of the support, and the center line average roughness of the back coat layer surface is 0.01 μm or less. A magnetic recording medium characterized by the following. 14. The magnetic recording medium according to claim 8, wherein a backcoat layer is formed on the other surface of the support, and the backcoat layer has a size smaller than the major axis particle diameter of the acicular magnetic powder. A magnetic recording medium comprising primary particles or aggregates of carbon black having a large diameter.