JPH04210403A - Acicular alloy magnetic powder and magnetic recording medium using this - Google Patents

Abstract

PURPOSE:To obtain acicular alloy magnetic powder having excellent corrosion resistant characteristic and high saturation magnetization under atmosphere of high temp. and high humidity by mainly containing iron and the specific ratio of cobalt and forming this to the acicular powder having the specific major axis particle diameter. CONSTITUTION:In the acicular alloy magnetic powder containing >= about 90wt% iron and cobalt, the major axis particle diameter is made to be <=0.22mum and wt. ratio of the cobalt to the iron is made to be 8-50%, preferably 10-30%. The above powder can be formed with method, which is reduced with hydrogen gas, etc., after adding Co<2+> to goethite arranging particle shape, etc. By this method, the acicular alloy magnetic powder having >=120emu/g value of saturation magnetization after letting alone for 7 days under atmosphere of 60 deg.C and 90% humidity, is obtd. By using this magnetic powder, magnetic recording medium of tapestate, etc., having magnetic layer of >=2800G residual magnetic flux density and <=40nm surface flatness (P-V) after letting alone for 7 days under atmosphere of 60 deg. and 90% humidity, is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an acicular fine particle alloy magnetic powder mainly composed of iron and cobalt, and to improvement of the output characteristics of a magnetic recording medium using the same.

(Prior Art) Metallic iron magnetic powder is characterized by having larger coercive force and saturation magnetization than conventional iron oxide magnetic powder, and is suitable for high-density recording, so it has been put into practical use.

However, since the particle surface of metallic iron magnetic powder is active, it is extremely susceptible to corrosion, which not only makes it inconvenient to handle, but also causes the output characteristics of magnetic recording media using it to deteriorate in hot and humid environments. There is a drawback. This is clear from the fact that when a metallic iron magnetic powder is left in an environment of, for example, a temperature of 60° C. and a humidity of 90%, its saturation magnetization rapidly decreases within a few hours.

In order to improve this corrosivity of metal iron magnetic powder,
Conventionally, attempts have been made to form a passive film on the surface of particles by alloying iron with a metal such as cobalt.

For example, as a standard method for producing alloy magnetic powder, 1) a coprecipitate obtained from an iron salt and a cobalt salt added to an oxalic acid aqueous solution is reduced; ■ Add a reducing agent to a solution containing iron salts and cobalt salts. ■ Evaporate metal in an inert gas and collide with gas molecules to obtain alloy magnetic powder. ■There is a method of reducing Fe or Co chloride to metal by flowing the vapor of Fe or Co chloride in a mixed gas of hydrogen and nitrogen or argon gas.

However, in ■■, it is difficult to control the composition of the particles, and in ■■■, the particles are not needle-like (but bead-like), which poses a problem in terms of orientation.

Therefore, a method has been proposed in which cobalt-containing acicular goethite powder obtained from an aqueous aqueous suspension of iron salt and cobalt salt is heated and reduced.

However, although the alloy magnetic powder obtained using this method has a certain degree of corrosion resistance (4-1) compared to standard metal iron magnetic powder, the cobalt M present inside the particles is limited to about 7% by weight. Not only that, but this amount of cobalt in the alloy often does not provide sufficient corrosion resistance. Although the reason for this is not clear, it is thought that the amount of cobalt in the alloy magnetic powder is insufficient, making it impossible to form a sufficient passive film on the particle surface.

Therefore, in order to ensure a sufficient amount of cobalt in the alloy magnetic powder, it has been proposed that cobalt hydroxide be attached to the surface of acicular goethite and reduced to contain 6% by weight or more of cobalt relative to iron. (Unexamined Japanese Patent Publication No. 1-2573
No. 09).

However, when this method is applied to goethite of a certain size or more, it is certainly possible to form a passivation film that is sufficient for practical use.
If particles as small as 2 μm or less are used, it becomes difficult to uniformly adhere hexagonal plate-shaped cobalt hydroxide to the particle surface, resulting in the formation of a uniform cobalt-containing passive film with excellent corrosion resistance. It becomes difficult to do so.

Therefore, in the present invention, the final particle size is 0. The aim is to develop an alloy magnetic powder that is as small as 2 μm or less, in some cases 0.17 μm or less, yet has sufficient corrosion resistance.

(Problem to be solved by the invention) The present inventors
In order to supply sufficient cobalt at 1°C to the method of thermally reducing cobalt-containing acicular goethite powder obtained from an alkaline aqueous suspension of iron salt and cobalt salt, cobalt was added to the aqueous suspension. Attempts were made to add too much salt, but the shape of the goethite would collapse, irregularly shaped particles would be mixed in the goethite powder, and the uniformity of the goethite's particle shape and composition would be impaired, resulting in a loss of metal magnetic powder. Cobalt could not be sufficiently dissolved in the solid solution.

After conducting various studies on the cause of this problem, the inventor realized that a fundamental solution could not be achieved by following the conventional method of adding cobalt salt to the suspension during goethite formation. .

Firstly, the iron that makes up goethite is trivalent and is not equivalent to divalent cobalt, so it cannot freely undergo ion exchange reactions.Secondly, the cobalt concentration in the aqueous suspension is is thought to control the growth rate of goethite crystals. Thirdly, the shape of the goethite particles determines the shape of the metal magnetic powder particles through subsequent processing, so the formation stage of goethite particles In fact, they realized that it would be better not to have cobalt ions, which affect the growth rate of crystals.

Based on this basic idea, goethite with a well-defined particle shape is generated in advance, and this is converted into magnetite with divalent iron ions, and the - part of this iron knee ion is equivalently converted to magnetite. We discovered that by performing ion exchange with divalent cobalt ions and dissolving cobalt into these magnetite particles from the outside, we were able to secure an acicular shape and also dissolve a high concentration of cobalt. A new alloy magnetic powder was obtained by this method. The new alloy magnetic powder has a beautiful needle shape and a particle size of 0022 μm.
Although it is a fine particle, the saturation magnetization is 12 Q emu/g even if it is left for 7 days in an environment with a temperature of 60°C and a humidity of 90%.
It maintains the above properties and has extremely excellent corrosion resistance. The reason for this is thought to be that the solid solution of cobalt could be made uniform even though the particles contained more than 8% by weight of cobalt relative to iron.

This method was particularly effective when cobalt was 15% by weight or more based on iron.

In addition, we found a method in which goethite with a uniform particle shape is generated in advance when cobalt is in the range of 8% to 20% by weight relative to iron, and fine Co ferrite particles can be uniformly deposited on the surface of this goethite. It has been found that similar alloys can also be obtained by reducing .

This invention solves the problem of poor corrosion resistance of the conventional metallic iron magnetic powder, and creates a magnetic recording medium that satisfies the conditions for cavity density recording with high saturation magnetization using acicular fine particles. It also makes it possible to provide

(Means for Solving the Problems) In order to solve these problems, the present invention has realized that it is particularly important to obtain a group of acicular alloy magnetic powders with uniform particle size and uniform cobalt concentration, First, a Fe-based particle group with uniform particle size was obtained. In order to uniformly adjust the particle size of the Fe-based particles, it is preferable to use nickel at the stage of crystal synthesis.

That is, the Fe-based particle group obtained by reacting an iron salt and a nickel salt in an alkaline aqueous solution to obtain a co-precipitate of iron hydroxide and nickel hydroxide and oxidizing this has a small particle size distribution (
Become. When the amount of nickel to act is 2% by weight or more, F
It is possible to reduce the particle size distribution of the e particle group.If the nickel content is 2% by weight or less, it is difficult to make the particle size distribution uniform, so even if the amount of cobalt added after this is increased, the cobalt composition concentration will decrease. The corrosion resistance becomes uneven between particles, and no improvement in corrosion resistance is observed.

From the viewpoint of durability and corrosion resistance, it is desirable that nickel be present on the surface of the alloy magnetic powder. It can also be present in the form of an iron-nickel intermetallic compound.

Furthermore, if the amount of cobalt is small, the effect of improving corrosion resistance performance will be insufficient.

Furthermore, 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 large amounts, the saturation magnetization decreases. In order not to impair saturation magnetization, it is preferable that the proportion of iron and cobalt be 90% by weight or more in the metal elements constituting the magnetic powder.

According to our study results, in order to obtain alloy magnetic powder with excellent cavity saturation magnetization and corrosion resistance, it is important to uniformly dissolve cobalt in iron and nickel. If it is melted, it will be difficult to obtain high saturation magnetization because local batteries will occur and corrosion will occur.

As a method for uniformly dissolving cobalt in iron and nickel, for example, (a) nickel-containing magnetic iron oxide powder is dispersed in polyhydric alcohol in which cobalt chloride is dissolved, and this suspension is Cobalt is uniformly dissolved in magnetic iron oxide by heating, and then heated and reduced with hydrogen gas to obtain iron-copal I/-nickel alloy powder (Japanese Patent Application Laid-open No. 146900/1982) and ( b) In an alkaline solution in which gaugeite fine particles are dispersed (
2- Adding the mixed solution of Co2+ salt and Fe” salt dropwise
After depositing a cobalt ferrite film on the surface of goethite, it is heated and reduced with hydrogen gas to form an iron-cobalt film.
~ There is a method to obtain nickel alloy powder.

In the (a) method, if there is a spread in the particle size distribution of magnetic iron oxide, which is the raw material for the cobalt solid solution reaction, a difference will naturally appear in the specific surface area between each particle, and a difference will occur in the rate of the cobalt exchange reaction on the particle surface. occurs, and the composition among each particle becomes non-uniform.

As a result, a potential difference occurs between each particle, causing a local battery and progressing corrosion.

In the (b) method, if there is a spread in the particle size distribution of the raw goethite, there will naturally be a difference in the specific surface area between each particle, and a difference will occur in the amount of cobalt ferrite particles deposited on the goethite surface. The composition between each particle becomes non-uniform, resulting in the same result as in method (a).

Therefore, the particle size distribution of the raw material goethite in which cobalt is dissolved is very important in forming a metal magnetic powder with excellent corrosion resistance. Such particle size distribution can be controlled by adjusting the alkaline earth metal ion concentration in the suspension.

In other words, as a metal magnetic powder with excellent corrosion resistance, it is preferable to regulate the particle distribution ratio to a range of 0.5 to 1.0.

Moreover, such a particle size distribution is S of magnetic powder.

It provides good coercive force distribution characteristics with F, D, and values of O15 or less.

In order to form such metal magnetic powder, it is possible to obtain goethite with a small particle size distribution by incorporating nickel in the formation stage of the raw material goethite. This is in order to improve the corrosion resistance of the alloy magnetic powder obtained later. is essential.

In addition, in method (a), an exchange reaction occurs between divalent iron ions and cobalt ions on the surface of the magnetic iron oxide powder, and cobalt becomes solid solution in the magnetic iron oxide powder. control becomes very effective.

The iron oxide used for the nucleus crystal is magnetite, which has a spinel structure and has the highest amount of Fe2 (Fe2"/Fe3" = 50
wt%) is expressed as Fe34[Fe3'Fe2''] Oa.

In addition, 7 Fe2O3 is Fe” where Fe” does not exist.
It is expressed as [Fe"5°301/3]04, where the mouth represents a hole.

Therefore, as Fe24B increases, the number of vacancies decreases. The solid solution reaction of cobalt into particles of iron oxide caused by polyhydric alcohol causes an exchange reaction between Fe2'' ions on the surface of magnetic iron oxide and Co2'' in the polyhydric alcohol at the solid-liquid interface. Then, the Co2'' taken into the surface diffuses into the inside of the particle using the pores inside the particle.

Therefore, in order to efficiently dissolve cobalt in this method, the amount of Fe24 or the amount of pores may be controlled.

That is, when Fe''/Fe'' is in the range of 5 wt% to 45 wt%, cobalt replaces Fe'' and diffuses into the inside of the particles (.

However, if it is less than 5wt%, cobalt cannot be replaced (
The amount of solid solution decreases. On the other hand, when it exceeds 45 wtχ, the amount of pores that contribute to diffusion decreases, making it difficult for Co2'' that has replaced Fe2' to diffuse into the interior, so that cobalt segregates on the surface and the amount of solid solution of cobalt decreases.

As the polyhydric alcohol, polyethylene glycol, ethylene glycol, propylene glycol, glycerin, etc. can be used.

In addition, in the (b) method, it is important to uniformly deposit cobalt ferrite on the goethite surface.

In order to uniformly form cobalt ferrite on the goethite surface, Co24 ions and Fe34 ions are gradually added to a solution in which goethite is suspended, and these hydrated ions are adsorbed on the surface of the goethite. It is effective to subsequently dehydrate and condense these ions on the surface.

Therefore, in order to proceed with the co-ferrite production reaction, it is necessary to add alkali and gradually neutralize the generated H'' ions.If the alkali concentration is too low, metal ions are limited to specific OH groups on the surface of goethite. If the metal ions are not adsorbed and deposited uniformly on the surface of the particles, and the alkali concentration is too high, a dehydration condensation reaction occurs instantaneously before the metal ions are adsorbed onto the surface of keezite, forming nuclei of ferrite particles in the reaction solution. Does not adhere uniformly to the surface.

In addition, in any of these methods, the thermal reduction can be carried out according to a known thermal reduction method (the reduction temperature is usually 150"
C to about 500″C.

As described above, according to the present invention, it is possible to obtain a fine particle alloy magnetic powder that has high saturation magnetization, has a coercive force suitable for magnetic recording, and has excellent corrosion resistance.

The obtained magnetic powder is cross-dispersed together with a binder resin and a solvent to be used in the production of magnetic paint. However, in order to achieve a good dispersion state, the alloyed magnetic powder must be placed in an N H3 gas, CO2 gas, or CO gas atmosphere in advance. By treating with , active points of the alloy magnetic powder can be modified, oxidation reactions can be further suppressed, and adsorption of acid components can be reduced. The amount of gas used in such treatment is approximately 0.01 to 10.0% by weight based on the alloy magnetic powder, preferably 0.9% to 10.0% by weight based on the alloy magnetic powder.
01 to 5% by weight.

In order to obtain as high a coercive force as possible, ordinary sword-shaped magnetic powder for magnetic recording has an axial ratio of 10 or more, and the coercive force is increased by shape anisotropy. However, when the axial ratio of the alloy magnetic powder mainly composed of saturation magnetized pit iron and cobalt is increased to 10 or more, the coercive force increases to about 17,000e or more.

In order to reduce the coercive force to about 16,000e or less, the particle size is usually made thicker, but in such alloy magnetic powders, fine particles to coarse particles are mixed together, resulting in a wide particle size distribution. .

Therefore, even if the coercive force value is appropriate, the anisotropic magnetic field distribution becomes wide (as a result, magnetic recording media using this acicular alloy magnetic powder have a problem of poor erasing characteristics. However, there is a problem of high noise due to the large particle size of the magnetic powder, and the current situation is that acicular alloy magnetic powder that can withstand practical use has not yet been obtained.

Therefore, in order to control the coercive force to a value suitable for magnetic recording while maintaining a small particle size and anisotropic magnetic field distribution at a small value, it is effective to control the axial ratio of the particles.

The coercive force value is about 0.78 when the axial ratio is 4, and about 0.96 when the axial ratio is 8, assuming that the alloy magnetic powder having an axial ratio of 10 or more is 1.

Therefore, in order to reduce the coercive force to about 16,000e or less, the axial ratio of the particles should be in the range of 4 to 8, preferably 4.5 to 7.
Setting it in the range of 0 is most effective. Also, if the coercive force is set to 1600-17000e, the axial ratio becomes 8-10, 1
In order to achieve 7000e or more, it is suitable to have an axial ratio of 10 or more.

In addition, since the coercive force of the alloy powder of the present invention is controlled by the axial ratio, it is not necessary to make the particle size thick, and usually the major axis particle diameter is (L) 1 μm to 0.22 μm heavy alloy powder is preferably used.

All of these alloy powders have a narrow anisotropic magnetic field distribution with a quotient of 3.2 or less when dividing the half-width value of the anisotropic magnetic field distribution by the coercive force, and the erasing characteristics are improved. A magnetic recording medium using this has low noise.

The t1-like alloy magnetic powder obtained by the present invention is, for example,
Even when left for 70 minutes in an environment with a temperature of 60°C and humidity of 90%, there is no saturation magnetization! It still maintains a richness of more than 20 e;su/g and is extremely resistant to corrosion.This is not only due to the inclusion of an appropriate amount of cobalt, but as a result, nickel is
This is also due to the fact that it is contained in more than % by weight.

Such magnetic recording media are generally made by coating a support such as a polyester film with a magnetic paint made by cross-dispersing magnetic powder, binder, inorganic powder lubricant, etc. in an organic solvent, and then drying the magnetic coating film. It is made up of

In addition, in order to reduce noise, it has already been proposed in Japanese Patent Publications No. 52-17404 and No. 60-12688 to reduce noise by smoothing the surface of the magnetic layer by performing surface finishing treatment on the magnetic layer using supercalender treatment. has been done.

However, the magnetic recording medium obtained by using the acicular alloy magnetic powder according to the present invention as a magnetic recording element and undergoing surface finishing treatment has low noise initially, but after running for a long time, especially in a corrosive environment, There were problems with dropouts and increased noise when left for a long period of time.

This problem is noticeable when an alloy magnetic powder containing less than 8% cobalt is used, and dropouts and noise for signals at frequencies corresponding to recording wavelengths of about 0.8 μ or less occur after long-term running. It is clear that it will rise.

This problem occurred almost universally with various prototypes (although it did occur), but according to the inventors' study, as an acicular alloy magnetic powder containing mainly iron and cobalt, the long-axis particle diameter is 0.22 μm or less, the weight ratio of cobalt to iron is in the range of 8 to 50% by weight, and a magnetic recording medium using a ferrite film containing mainly iron and cobalt on the surface is particularly suitable. The result was that the original reduced noise was maintained even after storage.

Therefore, we thought that this problem was caused not only by the corrosion resistance of the alloy magnetic powder (but also by the wear resistance), and various studies were conducted.

Therefore, the present inventors further found that the fragility of the magnetic layer depends on the particle size of the powder contained in the coating film, and if the particle size is smaller than a certain level, the reinforcing effect of the coating film decreases. It was discovered that the strength of the coating film could be maintained by increasing the hardness, and in light of this fact, after various studies, as mentioned earlier, in order to simultaneously impart hardness and corrosion resistance to the powder itself, ■ alloy Three means have been provided: (1) formation of a passive film on the surface of the powder particles, (2) a protective film with a small amount of Si and AI (both types).

Materials for the non-magnetic support used in the present invention include polyesters such as polyethylene terephthalate and polyethylene-2゜6-naphthalate, polyolefins such as polyethylene and polypropylene, cellulose derivatives such as cellulose triacetate and cellulose diacetate, and polychlorinated Examples include vinyl resins such as vinyl and polyvinylidene chloride, polycarbonates, polyimides, and polyamides, among which polyesters such as polyethylene terephthalate and polyethylene-2,6-naphthalate are preferred.

Then, a magnetic paint containing a magnetic powder and an inorganic powder having a Mohs hardness of 5 or more dispersed in a binder resin dissolved in an organic solvent is applied onto the support and dried to form a magnetic coating film.

The major axis particle diameter of the ferromagnetic alloy powder will be 0.8 μm or less in relation to the central recording wavelength, but from the viewpoint of resolution it will be 0.8 μm or less.
It is desirable that the thickness be 22 μm or less.

Alternatively, it is preferable to adjust the composition of an alloy powder containing metal elements such as nickel and cobalt 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.

Inorganic powders with a Mohs hardness of 5 or more that can be included in the magnetic layer include metal oxides, metal carbides, metal nitrides, etc. Among them, α-Fe2e3■, Al2O3■, Cr2
0a■, 5i02■, TiO2■, Z"02■, SiC
(2), TiC (2), hBN (2), Si3N4 (2), etc. (for example, ■ indicates a Mohs hardness of 6) are listed as more preferable. Since these inorganic powders are easily available in various particle sizes, they can be appropriately selected based on the above findings of the present invention.

As the carbon black, any of channel black, furnace black, acetylene black, and thermal black can be used, but acetylene black is particularly preferred.

It is also possible to use graphitized carbon black whose surface is covered with a graphite layer as disclosed in JP-A No. 61-22424.

Specific examples of commercially available carbon blacks include Black Pearl 7000, which has a particle size of 18 mμ, Mogal Shi1, which has a particle size of 20 mμ, and Black Pearl 7001, which has a particle size of 20 mμ, manufactured by Cabot Corporation in the United States.
u's ELFTEXpe lle tsl15, same-Gal 3001, particle size 30ml Palcan X C-
72, Sterling NS1 with a particle size of 75 mμ, Stirling R1 manufactured by Columbia Carbon Co., Ltd., and Raven 8000 with a particle size of 13 mμ. Raven 5250° with Kff diameter of 20 mμ, Raven 890 with particle diameter of 30 mμ, particle size 6
2mμ Raven 450, particle size 70mμ Raven 41
0, Raven MT-P with particle size 280 mμ (0.28u)
Beads, Raven Sebacarb MT-CL with a particle size of 300 mμ (0.30 μ) As those from Asahi Carbon Co., Ltd., H5-500 with a particle size of 75 mμ, #60 with a particle size of 35 mμ
H, those manufactured by Tokai Carbon Co., Ltd. include Giest 5H with a particle size of 20 mμ, and those manufactured by Akzo of the Netherlands,
Ketchen Black EC with a particle size of 30 mμ, manufactured by Mitsubishi Kasei, #4040 with a particle size of 20 mμ, particle size 23
mμ #4330 B S, particle size 45m, cz #4
350B81 #4010 with a particle size of 80 mμ can be suitably used.

As described above, since carbon black is easily available in various particle sizes, in accordance with the above findings of the present invention, the carbon black can be appropriately selected in consideration of the long axis diameter of the metal magnetic powder. However, when the particle size is relatively small, it is desirable to utilize the structure-forming ability of the carbon black itself to form an aggregate of several primary particles. This is because in this case, the aggregates function as if they were a single carbon black particle.

These inorganic powders and carbon black may be mixed into the paint so as to satisfy the above particle size relationship.

The binder resin used for the coating film is vinyl chloride.
Vinyl acetate copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-acrylic ester copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylo-nitrile copolymer, vinyl chloride- Examples include vinyl chloride resins such as vinyl acetate-maleic acid copolymers, thermoplastic polyurethane resins, thermosetting polyurethane resins, polyester resins, phenoxy resins, polyvinyl butyral resins, cellulose derivatives, epoxy resins, and mixtures thereof.

In addition, hydrophilic polar groups such as carboxylic acid, sulfonic acid, sulfonate, phosphoric acid, phosphate, amine, and ammonium salt are introduced into these resins to improve the dispersibility of powder particles as coating film constituent materials. Alternatively, an acrylic double bond may be introduced and cured by electron beam irradiation.

Solvents used to create paints to form these coatings include alcohols such as ethanol, propatool, and butanol, esters such as methyl acetate, ethyl acetate, and butyl acetate, and ketones such as methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone. Examples include ethers such as tetrahydrofuran and dioxane, aromatic hydrocarbons such as benzene, toluene, and xylene, aliphatic hydrocarbons such as heptane, hexane, and cyclohexane, and chlorinated hydrocarbons such as methylene chloride, ethylene chloride, and chloroform. However, a mixed solvent of cyclohexanone and toluene is preferable.

However, when using such cyclohexanone, the cyclohexanone may be altered on the surface of the alloy magnetic powder and its binding with the binder resin may be hindered.To prevent this and achieve a good dispersion state, It is extremely effective to treat the obtained alloy magnetic powder in a CO2 gas or CO gas atmosphere.

Furthermore, the coating film may contain other additives such as saturated and unsaturated higher fatty acids, higher fatty acid amides, fatty acid esters,
High alcohol, silicone oil, mineral oil, edible oil,
Lubricants such as fluorine oil can be used.

For example, when trying to obtain a magnetic layer with a residual magnetic flux density of 2000 G or more and a surface smoothness (PV) of 40 nm or less even after being left in an environment at a temperature of 60° C. and a humidity of 90% for 7 days, Using such a composition, disperse it in a ball mill or sand mill, etc. to make a magnetic paint,
It is preferable to leave this on a predetermined non-magnetic support for 5 seconds or more (it is preferable to apply it).

As described above, it is possible to obtain a magnetic recording medium with controlled dropout and noise increase even by storing it in a corrosive environment at high output.

The details of the present invention will be described below along with examples.

(2) Preparation of acicular alloy magnetic powder A (a) Goethite production process (hydroxide with a concentration of 5 mol/N) IJium aqueous solution 1.5
9 to 0. 72 mol/Q of ferrous sulfate and O-03
A mixed solution of 1.59 mol/Ω of nickel sulfate is added at room temperature with stirring, and reacted to obtain a co-precipitate of ferrous hydroxide and nickel hydroxide.

1.69/wi while keeping this precipitate suspension at 40°C.
Blow air at a rate of n and stir for 8 hours, filter, wash with water,
After drying, needle-shaped goethite A1 with a major axis particle diameter of 0.22 μm and an axial ratio of 7 was obtained.

The particle shape depends on the concentration of alkaline earth metal salts, etc., and by slightly changing these concentrations, the axial ratio can be changed.

(b) Silicic acid compound and alumina adhesion process Next, 100 g of goethite A1 was dispersed in 3 g of water, and in this suspension, 1 mol/2111 degrees of sodium hydroxide aqueous solution 29 and Imol/R concentration were added. Is Or-I strium silicate water soluble? (126 m2 was added and t9: Acid gas was blown in to neutralize the pH to 8, followed by washing with water and drying to deposit a silicon compound on the surface of Goethite A1.Next, this silicate compound-coated iron oxide was To this suspension, add a sodium hydroxide aqueous solution 29 with a concentration of 1 n+ol/Q and a sodium aluminate aqueous solution IM135 mR with a concentration of 0.5 mol/Ω, and blow carbon dioxide gas to dissolve the PI (in medium until the PI becomes 8). After being washed with water and dried, Goethite A2 coated with a silicic acid compound and alumina was obtained.

(e) Magnetic iron oxide production process Next, goethite A2 was fired at 750°C for 4 hours and then reduced in a hydrogen gas flow containing water vapor at 300°C for 8 hours to obtain magnetic iron oxide.

Then, 20g of the magnetic iron oxide powder obtained was heated in an oxygen-containing gas to partially oxidize it to form 20w of Fe2+/Fe3+.
Controlled to t%. Magnetic iron oxide A-1.

(d) Cobalt solid solution step Next, 25 g of cobalt chloride hexahydrate was dissolved in 300 mΩ of polyethylene glycol, and the Fe2+/Fe3
320 g of magnetic iron oxide A with controlled +ffl was dispersed and heated at 200° C. for 6 hours with stirring to obtain magnetic powder A4 in which cobalt was uniformly dissolved in magnetic iron oxide.

Next, this magnetic powder A4 was washed with water.

(e) Reduction process Magnetic powder A4 is heated and reduced in hydrogen gas at 450°C for 2 hours, then a gas containing 11,000 pp of oxygen in an inert gas is flowed at 60°C for 2 hours to perform gradual oxidation, and then the temperature is lowered. Cool to 25℃ and flow CO2 gas for 30 minutes to cover the surface with C.
O2 gas was adsorbed. In this way, the major axis particle size of the particles having a ferrite layer mainly composed of iron and cobalt on the surface is 0.
Acicular alloy magnetic powder A having a diameter of 2 μm was obtained.

■Preparation of acicular magnetic alloy powder B I - 100g of goethite B+100g whose axial ratio was adjusted to 6.5 by changing the concentration of the alkaline solution in step (a)
9 in water, add sodium hydroxide solution and adjust the pH to 1.
1 and disperse well with a homomixer. Next, while stirring this suspension, a mixed solution of 73.1 g of cobal nitrate and 202.9 g of ferric nitrate was gradually added dropwise. At this time, in order to keep the pH of the suspension constant, a sodium hydroxide solution was also dropped at the same time, and fine cobalt ferrite was evenly deposited on the surface of the goethite, thereby obtaining cobalt ferrite-coated goethite B2.

Next, a silicic acid compound and alumina were deposited on the surface of Gesai) B2 by step I-(b), and after washing with water,
It was baked at 50°C for 4 hours. Thereafter, it was reduced in step I-(e) to obtain acicular alloy magnetic powder B having a ferrite layer mainly composed of iron and cobalt on the surface and having a major axis particle diameter of 0.2 μm.

(2) Preparation of acicular alloy member powder C 1100 g of goethite C was prepared by changing the concentration of the alkaline solution in step I-(a) and adjusting the axial ratio to 9.
By step b), a silicic acid compound and alumina are deposited,
After washing with water, this was calcined at 750"C for 4 hours. Thereafter, it was reduced by the step I-(d) to obtain a long-axis particle size of 0.2μ.
Acicular alloy magnetic powder C was obtained.

For these 11-like alloy magnetic powders AXB and C, the weight ratio (%) of COIN is Alz Si to Fe was measured using fluorescent X-rays. The results are shown in Table 1.

Table 1゜Powder Co/Fe Ni/Fe Al/Fe
Si/Fe powder A 21.0 4-2 3.
5 1.6 Powder B 14.0 4.2 3.
5 1.6 Powder C04, 23, 51, 6 In addition, the coercive force He (Oe) of the acicular magnetic alloy powder was measured using a sample vibrating magnetometer (manufactured by Toei Kogyo Co., Ltd.) under the measurement conditions of an applied magnetic field of 10 KOe. Saturation magnetization σs (em u / g
), saturation magnetization σs' (emu/g), and anisotropic magnetic field distribution ΔHr4 after being left in an environment of 60° C. and 90% humidity for 7 days were measured. The anisotropic magnetic field distribution is expressed as the quotient of the half width of the anisotropic magnetic field distribution curve divided by the coercive force.

The axial ratio was measured using a transmission electron microscope. These results are shown in Table 2.

Table 2゜Powder Main component Hc σS σS゛Axis ratio△He powder A
Fe-Co 1550 158 137 6.5
2.6 Powder B Fe-Co 1550 150
132 7.0 2.6 Powder CFe 1495 1
25 105 9.0 2.6 ■, magnetic recording medium A,
BXC writing needle-shaped alloy magnetic powder A, B, C,
Using each, 100 parts by weight of acicular alloy magnetic powder,
11 parts by weight of a phosphoric acid ester group-containing vinyl chloride resin with a degree of polymerization of 400, 6.6 parts by weight of a thermoplastic polyurethane resin, 15 parts by weight of alumina with a particle size of 0,043 μm, and Colombian carbon with a particle size of 300 μm as carbon black. A composition prepared by blending 1 part by weight of Ravensebacarb MTCI (trade name) with 110 parts by weight each of cyclohexanone and toluene was prepared in a ball mill at 96°C.
After time mixing and dispersion, 4.4 parts by weight of a trifunctional polyisocyanate compound was further added and stirred to prepare a magnetic paint. This magnetic paint was applied to both sides of a 33 μm thick polyethylene terephthalate film support, dried, and then calendered to reduce the thickness of the paint film. 0μ
A double-sided magnetic recording medium AX BX C was manufactured in such a manner that the magnetic recording medium AX BX C was set to m.

(2) Preparation of a magnetic recording medium The magnetic recording medium is different from the magnetic recording medium except that in the manufacturing process of (2) above, the acicular alloy magnetic powder B was mixed and dispersed for 8 hours using a sand grind mill instead of being kneaded for 96 hours in a ball mill. A magnetic recording medium was produced in the same manner.

(2) Measurement results The magnetic recording medium prepared as described above was measured using a sample vibrating magnetometer (manufactured by Toei Kogyo Co., Ltd.) in an applied magnetic field of 10 KOe, with a coercive force Hc (Os) and residual magnetic flux density Br (G
), and the surface smoothness (peak-to-valley) PV (nm) of the magnetic coating film was measured using a non-stylus surface roughness meter (TOP (1
-3D, %WYKO) using 250 μm x 25
Measurements were made in the 0μ knee range. In addition, the residual magnetic flux density Br' was measured after being left in an environment of 60℃90% for 7 days, and the surface slope (peak-to-valley) PV' of the magnetic coating film at that time was measured. Measurement was carried out in an area of 250 μm×250 μm.

In addition, the reduction rate ΔBr (%) of the residual magnetic flux density after being left in an environment of 60° C. 90% for 7 days was calculated.

The results are shown in Table 3.

Table 3゜Medium Hc Br Br' P-V P
-V'△BrA 1400 2320 2200
33 34 5.2B 1400 2200
2110 30 31 4. ICI430 1710
1550 42 45 9.9D 1400 2
200 2110 36 37 4.1 This media sheet is punched out into a donut-shaped disk with an outer diameter of 47 mm and an inner diameter of 10 mm, and a 9 MHz signal is input and reproduced. Output C9 (dB) and noise level at 8 MHz N5 (dB) was measured and the C9/N8 ratio was determined.

In addition, similarly to the magnetic powder, the anisotropic magnetic field distribution △H1 of the medium
11 was measured. The anisotropic magnetic field distribution is the anisotropic magnetic field component i1
) It is expressed as the quotient of half the width of the curve divided by the magnetic force of 1. These results are shown in Table 4.

Table 4゜Medium C9C9/N8△HQ A +2.3 +2.5
2.1B ÷2.5 +2.4 2.1C00
1,8 D +2.7 +2.6 1
．． From the results of 8 or above, magnetic recording medium A in the present invention,
It can be seen that BXD exhibits better characteristics than magnetic recording medium C.

Patent applicant: Hitachi Maxell, Ltd. Representative Hiroshi Watanabe Procedure report (spontaneous) December 43, 1990 Patent application 2 filed on November 30, 1990, name of the invention Acicular alloy magnetic powder and magnetic recording medium using the same 3, Relationship with the M-correction case Patent applicant address: 1-88 Ushitora-chome, Ibaraki City, Osaka Name (581
) Hitachi Maxell, Ltd. (1) The statement will be amended as per the attached amended statement.

Amendment Statement 1, Name of the invention: Acicular alloy magnetic powder and magnetic recording medium using the same 2, Claims (1) In acicular alloy magnetic powder mainly containing iron and cobalt, long-axis particles The diameter is 0.22 μm or less, and the weight ratio of cobalt to iron is 8 to 50! i% range, and has a saturation magnetization value of 120 en+u/g or more after being left in an environment at a temperature of 60° C. and a humidity of 90% for 7 days.

(2) An acicular alloy magnetic powder containing mainly iron and cobalt, whose major axis particle diameter is 0.20 μm or less, whose surface is covered with a ferrite film containing mainly iron and cobalt, and whose temperature is 600 μm or less. An acicular alloy magnetic powder having a saturation magnetization value of 120 emu/g or more after being left in an environment of 90% humidity at °C for 7 days.

(3) The acicular alloy magnetic powder according to claim 2, wherein the ferrite coating has a thickness of 5 to 30 angstroms.

(4) Claim characterized in that the weight ratio of cobalt to iron is in the range of 10 to 30% by weight (IL (2)
L The acicular alloy magnetic powder according to any one of (3).

(5) Contains acicular alloy magnetic powder mainly containing iron and cobalt, and has a residual magnetic flux density of 2000G or more after being left in an environment of 60"C and 90% humidity for 7 days, and has a surface smoothness (
A magnetic recording medium having a magnetic layer having a P-V) of 40 nm or less.

(6) Contains acicular alloy magnetic powder mainly containing iron and cobalt, and has a residual magnetic flux density of 2800G or more after being left in an environment of 60°C and 90% humidity, and has a surface smoothness (P
-V) is a tape-shaped magnetic recording medium having a magnetic layer having a thickness of 40 nm or less.

(7) Coercive force 16000e containing mainly iron and cobalt
A claim (5L (
6) The magnetic recording medium according to any one of item 6).

(8) The acicular alloy magnetic powder according to any one of claims (1), (2), (3), and (4) is included, and the value of the half-value width of the anisotropic magnetic field distribution is 2.15 or less. A magnetic recording medium characterized by summing the quotient when divided by coercive force.

3. Detailed Description of the Invention (Field of Industrial Application) This invention relates to an acicular fine particle alloy magnetic powder mainly composed of iron and cobalt, and to improvement of the output characteristics of a magnetic recording medium using the same.

(Prior Art) Metallic iron magnetic powder is characterized by having larger coercive force and saturation magnetization than conventional iron oxide magnetic powder, and is suitable for high-density recording, so it has been put into practical use.

However, since the particle surface of metallic iron magnetic powder is active, it is extremely susceptible to corrosion, which not only makes it inconvenient to handle, but also causes the output characteristics of magnetic recording media using it to deteriorate in hot and humid environments. There is a drawback. This is clear from the fact that when a metallic iron magnetic powder is left in an environment of, for example, a temperature of 60° C. and a humidity of 90%, its saturation magnetization rapidly decreases within a few hours.

In order to improve this corrosivity of metal iron magnetic powder,
Conventionally, attempts have been made to form a passive film on the surface of particles by alloying iron with a metal such as cobalt.

For example, as a standard method for producing alloy magnetic powder, 1) a coprecipitate obtained from an iron salt and a cobalt salt added to an oxalic acid aqueous solution is reduced; ■ Add a reducing agent to a solution containing iron salts and cobalt salts. ■ Evaporate metal in an inert gas and collide with gas molecules to obtain alloy magnetic powder. ■There is a method of reducing Fe or Co chloride to metal by flowing the vapor of chloride in a mixed gas of hydrogen and nitrogen or argon gas.

However, in ■■, it is difficult to control the composition of the particles, and in ■■■, the particles are not needle-shaped but bead-shaped, which poses a problem in terms of orientation.

Therefore, a method is generally used in which cobalt-containing acicular goethite powder obtained from an aqueous aqueous suspension of iron salt and cobalt salt is heated and reduced.

However, although the Boo alloy magnetic powder obtained using this method has a certain degree of corrosion resistance compared to standard metal PN magnetic powder, the amount of cobalt present inside the particles is limited to about 7% by weight. However, in many cases, sufficient 1Fj4 corrosion performance cannot be achieved with this amount of cobalt alloy.

Although the reason for this is not clear, it is thought that the amount of cobalt in the alloy magnetic powder is insufficient, making it impossible to form a sufficient passive film on the particle surface.

Therefore, in order to ensure a sufficient amount of cobalt in the alloy magnetic powder, it has been proposed that cobalt hydroxide be attached to the surface of acicular goethite and reduced to contain 6% by weight or more of cobalt relative to iron. (Unexamined Japanese Patent Publication No. 1-2573
No. 09).

However, when applying this method to goethite with a size of 0.25 μm or more, that is, when obtaining an alloy magnetic powder with a size of about 0.23 μm or more, as described in the same document,
It is certainly possible to form a passivation film that is sufficient for practical use, but if you are trying to obtain a small alloy magnetic powder with a particle size of about 0.22 μm or less and the goethite used is too small, it is possible to form a hexagonal plate on the particle surface. It becomes difficult to uniformly adhere cobalt hydroxide, and as a result, it becomes difficult to form a uniformly cobalt-containing passive film with excellent corrosion resistance.

Therefore, in the present invention, the final particle size is 0. 22-0
．． The aim is to develop an alloy magnetic powder that is as small as 21 μm or less, preferably 0.20 μm or less, and in some cases 0.017 μm or less, yet has sufficient corrosion resistance.

(Problem to be solved by the invention) The present inventors
In order to supply sufficient cobalt in the method of thermally reducing cobalt-containing acicular goethite powder obtained from an alkaline aqueous suspension of iron salt and cobalt salt, an excess of cobalt salt is added to the aqueous suspension. However, the shape of the goethite is distorted, irregularly shaped particles are mixed in the goethite powder, the uniformity of the goethite particle shape and composition is impaired, and cobalt is mixed in the metal magnetic powder. It was not possible to form a sufficient solid solution.

After conducting various studies on the cause of this problem, the inventor realized that a fundamental solution could not be achieved by following the conventional method of adding cobalt salt to the suspension during goethite formation. .

Firstly, the iron that makes up goethite is trivalent and is not equivalent to divalent cobalt, so it cannot freely undergo ion exchange reactions.Secondly, the cobalt concentration in the aqueous suspension is is thought to control the growth rate of goethite crystals. Thirdly, the shape of the goethite particles determines the shape of the metal magnetic powder particles through subsequent processing, so the formation stage of goethite particles In fact, they realized that it would be better not to have cobalt ions, which affect the growth rate of crystals.

Based on this basic idea, goethite with a uniform particle shape is generated in advance, and this is converted into magnetite with divalent iron ions, and a part of the divalent iron ions is converted into magnetite. Perform ion exchange with equivalent cobalt divalent ions,
It was discovered that by injecting cobalt into the magnetite grains J~ from the outside as a solid solution, an acicular shape can be ensured, and a high concentration of cobalt can be dissolved in the solid solution.By reducing this, a new alloy with ε properties can be created. A powder was obtained. The new alloy magnetic powder has a fine needle shape and a particle size of 0°22μ.
Although it is a fine particle with a temperature of 60"C and a humidity of 90%, the saturation magnetization is 120 emu/
The corrosion resistance was extremely excellent as it maintained a value of 1.5 g or more. The reason for this is thought to be that the solid solution of cobalt could be made uniform even though the particles contained more than 8% by weight of cobalt relative to iron.

This method was particularly effective when cobalt was 15% by weight or more based on iron.

In addition, when cobalt is in the range of 8% to 20% by weight relative to iron, goethite with a uniform particle shape is generated in advance, and fine Co ferrite particles are uniformly coated on the surface and reduced. Similar results can also be obtained by the method of

This invention solves the problem of poor corrosion resistance of the conventional metallic iron magnetic powder, and provides a magnetic recording medium that satisfies the requirements for high-density recording with acicular fine particles and high saturation magnetization. It also makes it possible to

(Means for Solving the Problem) In order to solve the problem, the inventors have found that it is particularly important to obtain a group of acicular alloy magnetic powders with uniform particle size and cobalt concentration. In view of this, we have come to realize that it is necessary to uniformly adjust the particle size of the Fe-based particle group before reduction.

In order to uniformly adjust the particle size of the Fe-based particle group, it is preferable to use nickel at the stage of crystal synthesis.

That is, the iron salt and nickel salt are reacted in an alkaline aqueous solution to obtain a co-precipitate of iron hydroxide and nickel hydroxide, and the Fe-based particle group obtained by oxidizing this co-precipitate has a small particle size distribution ( Become.

When the amount of nickel to be applied is 2% by weight or more, the particle size distribution of the Fe particle group can be reduced.

If the nickel content is less than 2% by weight, it is difficult to make the particle size distribution uniform, so even if the amount of cobalt added after this is increased, the compositional concentration of cobalt tends to become uneven among each particle, resulting in significant corrosion resistance. It is difficult to expect performance improvement.

From the viewpoint of durability and corrosion resistance, it is desirable that nickel be present on the surface of the alloy magnetic powder. It can also be present in the form of an iron-nickel intermetallic compound.

Furthermore, if the amount of cobalt is small, the effect of improving corrosion resistance performance will be insufficient.

Further, 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 large amounts, the saturation magnetization decreases.

In order not to impair saturation magnetization, it is preferable that the proportion of iron and cobalt be 90 m4 percent or more in the metal elements constituting the magnetic powder.

According to the results of our study, in order to obtain alloy magnetic powder with high saturation magnetization and excellent corrosion resistance, it is important to uniformly dissolve other elements other than cobalt into nickel, and cobalt is not uniformly dissolved in nickel. If this occurs, local batteries may occur and corrosion may occur, making it difficult to obtain high saturation magnetization.

As a method for uniformly dissolving cobalt in iron and nickel, for example, (a) nickel-containing magnetic iron oxide powder is dispersed in polyhydric alcohol in which cobalt chloride is dissolved, and this suspension is After heating to make cobalt a uniform solid solution in magnetic iron oxide,
Method for obtaining iron-cobalt-nickel alloy powder by thermal reduction with hydrogen gas (JP-A-52-146900) and (b) Mixing co24' salt and Fe3'' salt in an alkaline solution in which goethite fine particles are dispersed. After depositing a cobalt ferrite film on the surface of goethite by dropping the solution, it is heated and reduced with hydrogen gas to form iron-cobalt-
There is a method to obtain nickel alloy powder.

In the (a) method, if there is a spread in the particle size distribution of magnetic iron oxide, which is the raw material for the cobalt solid solution reaction, a difference will naturally appear in the specific surface area between each particle, and a difference will occur in the rate of the cobalt exchange reaction on the particle surface. occurs, and the composition among each particle becomes non-uniform.

As a result, a potential difference occurs between each particle, causing a local battery and progressing corrosion.

In the (b) method, if there is a spread in the particle size distribution of the goethite that is the raw material, a difference will naturally appear in the specific surface area between each particle, and a difference will occur in the amount of cobalt ferrite particles deposited on the goethite surface. The composition between particles becomes non-uniform, resulting in the same result as in method (a).

Therefore, the particle size distribution of the raw material goethite in which cobalt is dissolved is very important in forming a metal magnetic powder with excellent corrosion resistance. Such particle size distribution can be controlled by adjusting the alkaline earth metal ion concentration in the suspension.

That is, a metal magnetic powder with excellent corrosion resistance has a coarse distribution ratio of 0. 5-1. It is preferable to regulate it within the range of 0~).

Moreover, such a particle size distribution is caused by S,
F, D, value is 0. It provides good coercive force distribution characteristics of 5 or less.

In order to form such metal magnetic powder, the raw material goethite is impregnated with nickel during the formation stage to obtain goethite with a small particle size distribution.One company has developed a method to improve the corrosion resistance of the alloy magnetic powder obtained later. It is essential for doing so.

In addition, in method (a), an exchange reaction occurs between divalent iron ions and cobalt ions on the surface of the magnetic iron oxide powder, and cobalt becomes solid solution in the magnetic iron oxide powder. control becomes very effective.

The iron oxide used for the nucleus crystal has a spinel structure and has the highest amount of Fe2◆ in magnetite (Fe2'''/Fe3''=50w
t%) is expressed as Fe"[Fe"Fe"] 04.

In addition, 7-Fe2O3 where Fe2" does not exist is Fe"
It is expressed as [Fe"5z301/3] Ot. Here, the mouth represents a hole.

Therefore, as the amount of Fe2'' increases, the number of vacancies decreases.The solid solution reaction of cobalt into iron oxide particles by polyhydric alcohol is caused by the Fe2'' ions on the surface of magnetic iron oxide and Co'' in the polyhydric alcohol. causes an exchange reaction at the solid-liquid interface.

The CO2 taken into the surface then diffuses into the particle using the pores inside the particle.

Therefore, in order to efficiently dissolve cobalt in this method, it is sufficient to control Fe2''ffi or the amount of pores.

That is, when Fe"/Fe" is in the range of 5 wt% to 45 wt%, cobalt replaces Fe2+ and diffuses into the inside of the particles.

However, if it is less than 5wtX, cobalt cannot be replaced (
The amount of solid solution decreases. On the other hand, if it exceeds 45 wt%, the amount of pores that contribute to diffusion decreases, so that Co2'', which has replaced Fe2'', is less likely to diffuse into the interior, so that cobalt segregates on the surface and the amount of solid solution of cobalt decreases.

As the polyhydric alcohol, polyethylene glycol, ethylene glycol, propylene glycol, glycerin, etc. can be used.

- Also, in the (b) method, it is important to uniformly deposit cobalt ferrite on the goethite surface.

In order to uniformly form cobalt ferrite on the goethite surface, Co" ions and Fe3" ions are gradually added to a solution in which goethite is suspended, and these hydrated ions are adsorbed to the OH groups on the goethite surface. It is effective to subsequently dehydrate and condense these ions on the surface.

Therefore, in order to advance the co-ferrite production reaction, it is necessary to add an alkali and gradually neutralize the produced H4 ions. If the alkali concentration is too low, metal ions will adsorb only to specific OH groups on the surface of goethite, and will not be uniformly deposited on the particle surface.On the other hand, if the alkali concentration is too high, metal ions will be dehydrated before adsorbing to the goethite surface. The condensation reaction occurs instantaneously, and ferrite particle nuclei are formed in the reaction solution, which does not adhere uniformly to the surface.

In addition, in any of these methods, thermal reduction can be carried out according to known thermal reduction methods (the reduction temperature is usually 150°C).
to about 500°C.

As described above, according to the present invention, it is possible to obtain a fine particle alloy magnetic powder that has high saturation magnetization, has a coercive force suitable for magnetic recording, and has excellent corrosion resistance.

The obtained magnetic powder is cross-dispersed together with a binder resin and a solvent to be used in the production of magnetic paint. However, in order to achieve a good dispersion state, the alloyed magnetic powder must be placed in an N H3 gas, CO2 gas, or CO gas atmosphere in advance. By treating with , it is possible to modify the active bonds of the alloy magnetic powder, further suppress the oxidation reaction, and reduce the adsorption of acid components. The amount of gas used in such treatment is approximately 0.01 to 10.0% by weight, preferably 0.01 to 10.0% by weight based on the alloy magnetic powder.
It is 01 to 5% by weight.

In order to obtain as high a coercive force as possible, ordinary acicular magnetic powder for magnetic recording has an axial ratio of 10 or more, and the coercive force is increased by shape anisotropy. However, when the axial ratio of alloy magnetic powder mainly composed of iron and cobalt with high saturation magnetization is increased to 10 or more, the coercive force increases to about 17,000e or more.

In order to reduce the coercive force to about 16,000e or less, the particle size is usually increased (but in such alloy magnetic powder, fine particles to coarse particles are mixed together, resulting in a wide particle size distribution.

Therefore, even if the coercive force value is appropriate, the anisotropic magnetic field distribution becomes wide, and as a result, a magnetic recording medium using this acicular alloy magnetic powder has a problem of poor erasing characteristics. In addition, since the particle size of magnetic powder in such magnetic recording media is large,
However, there is a problem of high IZ, and the current situation is that acicular alloy magnetic powder that can withstand practical use has not yet been obtained.

Therefore, in order to control the coercive force to a value suitable for magnetic recording while maintaining a small particle size and anisotropic magnetic field distribution at a small value, it is effective to control the axial ratio of the particles.

The coercive force value is about 0.78 when the axial ratio is 4, and about 0.96 when the axial ratio is 8, assuming that the alloy magnetic powder having an axial ratio of 10 or more is 1.

Therefore, in order to reduce the coercive force to about 16,000e or less, the axial ratio of the particles should be in the range of 4 to 8, preferably 4.5 to 7.
Setting it in the range of 0 is most effective. Also, if the coercive force is set to 1600-17000e, the axial ratio becomes 8-10, 1
In order to achieve 7000e or more, it is suitable to have an axial ratio of 10 or more.

In addition, since the coercive force of the alloy powder of the present invention is controlled by the axial ratio, there is no need to increase the particle size, and alloy powders with a major axis particle diameter of 0.1μI11 to 0.22μm are usually suitable. used as something.

All of these alloy powders have a narrow anisotropic magnetic field distribution with a quotient of 3.2 or less when dividing the half-width value of the anisotropic magnetic field distribution by the coercive force, and the erasing characteristics are improved. A magnetic recording medium using this has low noise.

The acicular alloy magnetic powder obtained according to the present invention maintains a high saturation magnetization of 120 ersu/g or more even when left for 7 days in an environment with a temperature of 60° C. and a humidity of 90%, for example, and is extremely resistant to corrosion. This is due not only to the inclusion of an appropriate amount of cobalt (but also to the fact that nickel is contained in an amount of 2% by weight or more relative to iron).

Such magnetic recording media are generally made by coating a support such as a polyester film with a magnetic paint made by cross-dispersing magnetic powder, binder, inorganic powder, lubricant, etc. in an organic solvent, and then drying the magnetic coating film. It is made up of

In addition, in order to reduce noise, it has already been proposed in Japanese Patent Publications No. 52-17404 and No. 60-12688 to reduce noise by smoothing the surface of the magnetic layer by performing surface finishing treatment on the magnetic layer using supercalender treatment. has been done.

However, the magnetic recording medium obtained by using the acicular alloy magnetic powder according to the present invention as a magnetic recording element and undergoing surface finishing treatment has low noise initially, but after running for a long time, especially in a corrosive environment, There are problems with dropouts and increased noise when left for a long period of time.

This problem is noticeable when an unvortexed magnetic alloy powder containing 8% cobalt by weight is used, and dropout and noise for signals at frequencies corresponding to recording wavelengths of about 0.8μ or less increase after long-term running. It became clear that.

This problem occurred almost universally with various prototypes (although it did occur), but according to the inventors' study, as an acicular alloy magnetic powder containing mainly iron and cobalt, the long-axis particle diameter 0.22 μm or less, the weight ratio of cobalt to iron is in the range of 8 to 50% by weight, and magnetic recording media using a ferrite film containing mainly iron and cobalt on the surface are particularly difficult to store. The result was that the original reduced noise was maintained.

Therefore, we considered that this problem was caused by not only the corrosion resistance but also the wear resistance of the alloy magnetic powder, and various studies were conducted.

Therefore, the present inventors further found that the fragility of the magnetic layer depends on the particle size of the powder contained in the coating film, and if the particle size is smaller than a certain level, the reinforcing effect of the coating film decreases. It was discovered that the strength of the coating film could be maintained by increasing the hardness, and in light of this fact, after various studies, as mentioned earlier, in order to simultaneously impart hardness and corrosion resistance to the powder itself, ■ alloy Three means have been provided: (1) the formation of a passive film on the surface of the powder particles; and (2) a protective film with a small amount of 51% AI.

Materials for the non-magnetic support used in the present invention include polyesters such as polyethylene terephthalate and polyethylene-2゜6-natsuta 1 note, polyolefins such as polyethylene and polypropylene, cellulose derivatives such as cellulose triacetate and cellulose diacetate, Examples include vinyl resins such as polyvinyl chloride and polyvinylidene chloride, polycarbonate, polyimide, and polyamide, among which polyesters such as polyethylene terephthalate and polyethylene-2,6-naphthalate are preferred.

Then, a magnetic paint containing a magnetic powder and an inorganic powder having a Mohs hardness of 5 or more dispersed in a binder resin dissolved in an organic solvent is applied onto the support and dried to form a magnetic coating film.

The long-axis particle diameter of the ferromagnetic alloy powder will be 0.8 μm or less in relation to the central recording wavelength, but from the viewpoint of resolution it will be 0.8 μm or less.
It is desirable that the thickness be 22 μm or less, or even smaller.

Further, it is desirable to adjust the composition of the alloy powder containing the metal elements nickel and cobalt 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.

Inorganic powders with a Mohs hardness of 5 or more that can be included in the magnetic layer include metal oxides, metal carbides, metal nitrides, etc. Among them, α-Fe203■, Al2O3■, Cr2
(h■, 5i02■, TiO2■, ZrO2■, SiC
2, TiC2, hBN2, 5iaNa2, etc. (for example, 2 indicates a Mohs hardness of 6) are listed as more preferable. Since these inorganic powders are easily available in various particle sizes, they can be appropriately selected based on the above findings of the present invention.

As the carbon black, any of channel black, furnace black, acetylene black, and thermal black can be used, but acetylene black is particularly preferred.

It is also possible to use graphitized carbon black whose surface is covered with a graphite layer as disclosed in JP-A No. 61-22424.

Carbon black can be easily produced manually in various particle sizes, but when the particle size is relatively small, it is possible to make aggregates of several primary particles by utilizing the carbon black's own stracture-forming ability. It is desirable to form it. In this case, the aggregate acts as if it were a single carbon black particle.

The binder resin used for the coating film is vinyl chloride.
Vinyl acetate copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-acrylic acid ester copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, vinyl chloride-vinyl acetate - PVC resins such as maleic acid copolymers, thermoplastic polyurethane resins, thermosetting polyurethane resins, polyester resins, phenoxy resins, polyvinyl butyral resins, cellulose derivatives, epoxy resins, or mixtures thereof.

In addition, hydrophilic polar groups such as carboxylic acid, sulfonic acid, sulfonate, phosphoric acid, phosphate, amine, and ammonium salt are introduced into these resins to improve the dispersibility of powder particles as coating film constituent materials. Alternatively, an acrylic double bond may be introduced and cured by electron beam irradiation.

Solvents used to create paints to form these coatings include alcohols such as ethanol, propatool, and butanol, esters such as methyl acetate, ethyl acetate, and butyl acetate, and ketones such as methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone. Examples include ethers such as dioxane, aromatic hydrocarbons such as benzene, toluene, and xylene, aliphatic hydrocarbons such as heptane, hexane, and cyclohexane, and chlorinated hydrocarbons such as methylene chloride, ethylene chloride, and chloroform. However, a mixed solvent of cyclohexanone and toluene is preferable.

However, when such cyclohexanone is used as a solvent, the cyclohexanone may deteriorate on the surface of the alloy magnetic powder and its binding with the binder resin may be hindered.
In order to prevent this and achieve a good dispersion state, it is extremely effective to treat the obtained alloy magnetic powder in a CO2 gas, CO gas atmosphere, or the like.

Furthermore, the coating film may contain other additives such as saturated and unsaturated higher fatty acids, higher fatty acid amides, fatty acid esters,
High alcohol, silicone oil, mineral oil, edible oil,
Lubricants such as fluorine oil can be used.

For example, when trying to obtain a magnetic layer with a residual magnetic flux density of 2000 G or more and a surface smoothness (PV) of 40 nm or less even after being left in an environment at a temperature of 60° C. and a humidity of 90% for 7 days, Using such a composition, disperse it in a ball mill or sand mill, etc. to make a magnetic paint, and leave it on a prescribed non-magnetic support for more than 5 seconds.
It is desirable to apply it. For this purpose, it is convenient to apply the magnetic paint while stirring.

As described above, it is possible to obtain a magnetic recording medium with controlled dropout and noise increase even by storing it in a corrosive environment at high output.

The details of the present invention will be described below along with examples.

(2) Preparation of acicular alloy magnetic powder A (a) Goethite explosion step 5 mol/2 concentration sodium hydroxide aqueous solution 1.5 Ω diode. 72 a+ol/9 Ferrous sulfate 0.03
A mixed solution of 1.52 mol/'2 of nickel sulfate is added with stirring at room temperature and reacted to obtain a co-precipitate of ferrous hydroxide and nickel hydroxide.

This precipitate suspension was heated to 1.6Q/a while keeping it at 40℃.
The mixture was stirred for 8 hours by blowing air into the mixture at a rate of 1.5 in., filtered, washed with water, and dried to obtain needle-shaped goethite A1 with a long-axis particle diameter of 0.22 μm and a shoulder-to-axis ratio of 7.

The particle shape depends on the concentration of alkaline earth metal salts, etc., and by slightly changing these concentrations, the axial ratio can be changed.

(b) Silicic acid compound and alumina adhesion process Next, 100 g of goethite A1 is dispersed in water 39, and in this suspension, a sodium hydroxide aqueous solution 29 with a concentration of 1 mol/Q and fIaol/Q concentration are added. Add 26 mff of sodium orthosilicate aqueous solution and blow carbon dioxide gas to make P1.
After neutralizing it until it reaches 18, it is washed with water and dried to obtain Goethite A1.
A silicon compound was deposited on the surface. Next, this silicate compound-coated iron oxide was dispersed in 3Ω of water, and 1
mol/Q concentration of sodium hydroxide aqueous solution 2Ω and 0.
5mo[/! Q concentration sodium aluminate aqueous solution 1
35m51 was added and carbon dioxide gas was blown into the solution to neutralize it until the pH reached 8, followed by washing with water and drying to obtain goethite A2 coated with a silicate compound and alumina.

(c) Magnetic iron oxide production process Next, goethite A2 was fired 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, 20 g of the obtained magnetic iron oxide powder was heated in an oxygen-containing gas to partially oxidize it, and Fe2+/Fe3+ was converted to -20
The wt% was controlled. Magnetic iron oxide A3.

(d) Cobalt solid solution step Next, 25 g of cobalt chloride hexahydrate was dissolved in 300 mΩ of polyethylene glycol, and the Fe2+/Fe3
320 g of magnetic iron oxide A with a controlled amount was dispersed and heated at 200° C. for 6 hours with stirring to obtain magnetic powder A4 in which cobalt was uniformly dissolved in magnetic iron oxide. Next, this magnetic powder A4 was washed with water.

(e) Reduction process Magnetic powder A4 is heated and reduced in hydrogen gas at 450°C for 2 hours, and then a gas containing 11,000 pp of oxygen in an inert gas is flowed at 60°C for 2 hours to perform gradual oxidation, and then the temperature is lowered to 25°C. Cool to ℃ and flow CO2 gas for 30 minutes to release CO on the surface.
2 gases were adsorbed. In this way, the major axis particle diameter is 0.2 with a ferrite layer mainly composed of iron and cobalt on the surface.
Acicular alloy magnetic powder A having a diameter of 0 μι was obtained.

■Preparation of acicular magnetic alloy powder B I - 100g of goethite B+100g whose axial ratio was adjusted to 6.5 by changing the concentration of the alkaline solution in step (a)
g of water, and add sodium hydroxide solution to adjust the pH to 1.
1 and dispersed using a homomixer. Next, while stirring this suspension, a mixed solution of 73.1 g of cobal nitrate and 202.9 g of ferric nitrate was gradually added dropwise. At this time, in order to keep the pH of the suspension constant, a sodium hydroxide solution was also dropped at the same time, and fine cobalt ferrite was uniformly deposited on the surface of the goethite, thereby obtaining cobalt ferrite-coated goethite B2.

Next, a silicic acid compound and alumina were deposited on the surface of Gesai) B2 by step I-(b), and after washing with water,
It was baked at 50°C for 4 hours. After that, it is reduced by the process of ■-(e) to produce acicular alloy magnetic powder B with a long axis particle diameter of 0° and 20 μm, which has a ferrite layer mainly composed of iron and cobalt on the surface.
I got it.

(2) Preparation of acicular alloy magnetic powder C I-(
By step b), a silicic acid compound and alumina are deposited,
After washing this with water, it was fired at 750°C for 4 hours. After that, it was reduced by the step I-(e) and the major axis particle diameter was 0.20 μI.
Acicular alloy magnetic powder C was obtained.

For these acicular alloy magnetic powders A, B, and C, the weight ratio (%) of CO, NIXA I, and Si to Fe was measured using fluorescent X-rays. The results are shown in Table 1.

Table f.

Powder Co/Fe Ni/Fe Al/Fe
Si/Fe powder A 21.0 4゜2 3.
5 16 powder B 14.0 4.2 3.5
1.6 powder C04, 23-51, S Coercive force Hc (Oe), saturation magnetization of acicular magnetic alloy powder under measurement conditions of applied magnetic field 10 KOe using sample vibrating magnetometer (manufactured by Toi Kogyo Co., Ltd.) σs(em u/g
), saturation magnetization σs' (emu/g), and anisotropic magnetic field distribution ΔHa after being left in an environment of 60° C. and 90% humidity for 7 days were measured. The anisotropic magnetic field distribution was expressed as the quotient of the half width of the anisotropic magnetic field distribution curve divided by the coercive force. The axial ratio was measured using a transmission electron microscope. These results are shown in Table 2.

Table 2゜Powder Main component He σS σS' axial ratio △Hc powder A
Fe-Co 1550 158 137 6.5
2-6 Powder B Fe-Go 1550 150
132 7.0 2.6 Powder CFe 1495 1
25 105 9.0 2.6■, Magnetic recording medium A,...
Preparation of acicular alloy magnetic powders B and C Using acicular alloy magnetic powders A and BSC, respectively, 100 parts by weight of acicular alloy magnetic powders and 1 part of phosphate group-containing vinyl chloride resin with a degree of polymerization of 400.
1 part by weight of thermoplastic polyurethane resin, 6.6 parts by weight of thermoplastic polyurethane resin, 15 parts by weight of alumina with a particle size of 0.43 μm, and 1 part by weight of Columbian Carbon Co.'s Raven Sebacalf MTCI with a particle size of 300 μm as carbon black. A mixture of 110 parts by weight of cyclohexanone and toluene was mixed and dispersed in a ball mill for 96 hours, and 4.4 parts by weight of a trifunctional polyisocyanate compound was added and stirred. This was applied to a support of polyethylene prephthalate film 12 having a thickness of 33 μm, and dried while stirring constantly. The coating was carried out on both sides of the film, which was calendered so that the thickness of each coating film was 4.0 Itm, thereby producing double-sided magnetic recording media A and BXC.

(2) Preparation of magnetic recording medium Magnetic recording medium A except that in the manufacturing process of (2) above, acicular alloy magnetic powder B was mixed and dispersed for 8 hours using a sand grind mill instead of being kneaded for 96 hours in a ball mill.
A magnetic recording medium was produced in the same manner as in , B, and C.

(2) Measurement results The magnetic recording medium prepared as described above was measured using a sample vibrating magnetometer (manufactured by Toi Kogyo Co., Ltd.) in an applied magnetic field of 10 KOe, and the residual magnetic flux density Br (G
), and the surface smoothness (peak to Φ valley) PV (nm) of the magnetic coating film was measured using a non-stylus surface roughness meter (TOPO-
3D, manufactured by WYKO) to 250 μm x 250 μm
Measured in the range of In addition, the residual magnetic flux density Br'' was measured after being left in an environment of 60'C90% for 7 days, and the surface smoothness (peak to φ valley) of the magnetic coating was measured.
Measurement was performed using PV' in a range of 250 .mu.m x 250 .mu.-.

In addition, the reduction rate ΔBr (%) of the residual magnetic flux density after being left in an environment of 60° C. 90% for 7 days was calculated.

The results are shown in Table 3.

Table 3゜Medium Hc Br Br' PV
P-V'ΔBrA 1400 2320 220
0 33 34 5.2B 1400 2200
2110 30 31 4-1CI430 1710
1550 42 45 9.9D 1400
2200 2110 36 37 4.1 Punch out this medium sheet into a donut-shaped disk with an outer diameter of 47 m rl'h and an inner diameter of 10 mm, input a 9 MHz signal, and reproduce the output C9 (dB) and 8 MHz.
Measured for noise level N8 (dB) at z, C9
/Ne ratio was determined.

In addition, similarly to the magnetic powder, the anisotropic magnetic field distribution ΔHf of the medium
l+ was measured. The anisotropic magnetic field distribution was expressed as the quotient of the half width of the anisotropic magnetic field distribution curve divided by the coercive force. These results are shown in Table 4.

Table 4゜Medium C9C9/N8△H Self A +2.3 +2.5 2.1B
+2.5 +2.4 2.1C001,8 D +2.7 +2.6 1.8 From the above results, magnetic recording media A, B, D in the present invention
It can be seen that the magnetic recording medium C exhibits better characteristics than the magnetic recording medium C.

Patent applicant: Hitachi Maxell, Ltd. Representative Hiroshi Watanabe

Claims (8)

[Claims] (1) An acicular alloy magnetic powder mainly containing iron and cobalt, the major axis particle diameter is 0.22 μm or less, the weight ratio of cobalt to iron is in the range of 8 to 50% by weight, and the temperature is 60 ° C. An acicular alloy magnetic powder having a saturation magnetization value of 120 emu/g or more after being left in an environment with 90% humidity for 7 days. (2) An acicular alloy magnetic powder containing mainly iron and cobalt, whose major axis particle diameter is 0.22 μm or less, whose surface is covered with a ferrite film containing mainly iron and cobalt, and whose temperature is 600 μm or less. An acicular alloy magnetic powder having a saturation magnetization value of 120 emu/g or more after being left in an environment of 90% humidity at °C for 7 days. (3) The acicular alloy magnetic powder according to claim 2, wherein the ferrite coating has a thickness of 5 to 30 angstroms. (4) Claims (1) and (2) characterized in that the weight ratio of cobalt to iron is in the range of 10 to 30% by weight.
, (3) The acicular alloy magnetic powder according to any one of (3). (5) Contains acicular alloy magnetic powder mainly containing iron and cobalt, and has a residual magnetic flux density of 2000G or more after being left in an environment of 60°C and 90% humidity, and has a surface smoothness (P
-V) is a magnetic recording medium having a magnetic layer of 40 nm or less. (6) Contains acicular alloy magnetic powder mainly containing iron and cobalt, and has a residual magnetic flux density of 2800G or more after being left in an environment of 60°C and 90% humidity, and has a surface smoothness (P
-V) is a tape-shaped magnetic recording medium having a magnetic layer having a thickness of 40 nm or less. (7) Coercive force 1600 Oe, mainly containing iron and cobalt
Claims (5) and (6) using acicular alloy magnetic powder exceeding
) magnetic recording medium according to any one of the above. (8) The acicular alloy magnetic powder according to any one of claims (1), (2), (3), and (4) is included, and the value of the half-width of anisotropic magnetic field distribution is 2.15 or less. 1. A magnetic recording medium characterized by having a quotient obtained by dividing the magnetic field by the coercive force.