JPS614202A - Manufacture of magnetic powder made of acicular ferromagnetic iron oxide - Google Patents

Abstract

PURPOSE:To provide magnetic powder made of acicular ferromagnetic iron oxide which has large coercive force and saturated magnetization and extremely small variation of magnetization with time, by a method in which, after acicular gamma-Fe2O3 particle surfaces are coated with iron oxide layers containing zinc and bivalent iron and cobalt compound layers, they are heat-treated in an inert gas atmosphere. CONSTITUTION:After acicular gamma-Fe2O3 particle surfaces are coated with iron oxide layers containing zinc and bivalent iron and cobalt compound layers, are dehydrated and dried, they are heat-treated at a temperature of 100-250 deg.C for a time from 30 min-3hr in an inert gas atmosphere. As the inert gas, nitrogen, argon, or neon, etc. is preferable to be utilized, but nitrogen can be pratically used, which is low cost and can be easily available. Variation of coercive force Hc of the magnetic powder made of acicular ferromagnetic iron oxide produced in this way with time is extremely small. When a ratio Zn/Fe<2+> of zinc to bivalent iron being contained in the iron oxide layers, which is important, is 8-25atom%, magnetic powder having large saturated magnetization can be produced, thereby enabling the increase of obtained saturated magnetization value sigmas of magnetic powder.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for producing acicular ferromagnetic iron oxide magnetic powder used in coated magnetic recording media.

[Background technology and its problems]

Conventionally, magnetic powders for coated magnetic recording media include γ, -F.
EzO3 particles, especially acicular γ-F0203 particles having high coercive force due to shape anisotropy, are widely used. These acicular γ-Fe 20-3 particles have advantages such as excellent chemical and magnetic stability and low cost.

By the way, in general, in magnetic recording media, the coercive force He of magnetic powder is an important factor that influences the recording and reproducing characteristics, and by increasing this coercive force He, demagnetization can be suppressed and the recording density can be improved. It is known that this is possible. In response to demands for improved performance of video tapes, audio tapes, etc., the coercive force H of the magnetic powder
There is a need to further improve e.

Therefore, conventionally, it has been proposed that cobalt ions are dissolved (doped) in the γ-Fe2e3 particles to greatly increase the coercive force He due to the magnetocrystalline anisotropy of cobalt ferrite. However, in γ-Fe203 particles in which cobalt is dissolved in solid solution, cobalt ions rearrange within the particles due to the induced magnetic anisotropy generated in the particles, and as a result, the coercive force He However, it has not been put into practical use due to drawbacks such as the inability to suppress the phenomenon of increase over time and the increased temperature dependence of magnetic properties.

In order to improve these drawbacks, so-called cobalt-adhered γ-Fe2O3 particles, in which a cobalt compound is adsorbed only on the surface of the γ-Fe203 particles, have been considered. In this cobalt-coated γ-F8203 particle, it is possible to increase the coercive force Hc by concentrating the effect of cobalt ions on the particle surface and improve the above-mentioned drawbacks, but the amount of adsorbed cobalt is It was found that although the coercive force He increases with increase, the saturation magnetization σS per unit weight decreases. This saturation magnetization σS
When this decreases, the recording/reproducing output decreases, which adversely affects electromagnetic characteristics.

Therefore, the present inventor previously disclosed in Japanese Patent Application No. 59-20764 and Japanese Patent Application No. 59-63202 that an iron oxide layer containing zinc and divalent iron is added to the cobalt-coated γ-FezQa particles. Acicular 4'i'J@**Ke*'1n with improved both coercive force He and saturation magnetization σS by providing
=i'15) t**Lfc-c')**'i'
J-Wfia iron oxide magnetic powder is made by forming a cobalt compound layer on the surface of γ-Fe2es particles as a core, and further forming an iron oxide layer containing zinc and divalent iron on the cobalt compound layer. Alternatively, an iron oxide layer containing zinc and divalent iron is formed on the surface of γ-Fe203 particles as a core, and a cobalt compound layer is further formed on the iron oxide layer, and the iron oxide is It has been found that by providing the layer, the saturation magnetization σS can be improved while maintaining the high coercive force due to cobalt deposition, and the change in the coercive force He over time can be reduced to some extent.

By the way, for magnetic powder used in magnetic recording media, it is necessary to suppress the change in coercive force Hc over time as much as possible, and if this change over time can be eliminated, it will be extremely effective in improving the quality of magnetic recording media. It is advantageous for Therefore, it is desirable to further reduce the change over time in the coercive force Hc of the above-mentioned acicular ferromagnetic iron oxide magnetic powder.

[Purpose of the invention]

Therefore, the present invention has been proposed in view of the above-mentioned circumstances, and has an acicular shape that makes it possible to produce magnetic powder with large coercive force HC and saturation magnetization σS, and extremely little change over time in coercive force He. The purpose of the present invention is to provide a method for producing ferromagnetic iron oxide magnetic powder.

[Summary of the invention]

In order to achieve the above-mentioned objects, the present invention provides γ-Fe
After forming an iron oxide layer containing zinc and divalent iron and a cobalt compound layer on the surface of the 2e3 particles as a core,
It is characterized by heat treatment in an inert gas atmosphere.

〔Example〕

Hereinafter, a method for producing acicular ferromagnetic iron oxide magnetic powder to which the present invention is applied will be explained.

゛In the present invention, first, the core acicular r-Fe203
Particles are prepared, and an iron oxide layer containing zinc and divalent iron and a cobalt compound layer are deposited on the surfaces of the acicular γ-Fe203 particles.

Here, either the iron oxide layer or the cobaltized 6S layer may be formed first, so that the acicular r-Fe
After forming an iron oxide layer on the surface of the ze3 particles, a cobalt compound layer may be formed on the surface of the iron oxide layer, or alternatively, after forming a cobalt compound layer on the surface of the acicular γ-Fe20a particles, the cobalt compound layer may be formed on the surface of the iron oxide layer. An iron oxide layer may be formed on the layer surface.

- The method for forming the cobalt compound layer may be any conventional method used when producing cobalt-coated γ-F8203 particles. For example, an aqueous solution in which a cobalt salt is dissolved is added to an alkaline suspension in which acicular γ-Feze particles or acicular γ-Fe203 particles with iron oxide dust formed on the surface are dispersed, and the mixture is heated and stirred at a temperature below the boiling point. The cobalt compound layer is formed on the surface by holding it for a predetermined period of time. In this case, examples of the alkali used include sodium hydroxide, potassium hydroxide, lithium hydroxide, etc., and examples of the cobalt salt include cobalt chloride, cobalt bromide,
Examples include cobalt sulfate.

On the other hand, the iron oxide scrap is made of acicular γ-Fe particles with a cobalt compound layer formed on the surface of the acicular γ-F0203 particles.
An aqueous solution of a zinc salt and an aqueous solution of a ferrous salt are added at a predetermined ratio into an alkaline suspension in which ZES particles are dispersed, and the predetermined amount is heated and stirred at a temperature below the boiling point as in the previous cobalt compound layer. Formed by holding time.

At this time, the ratio of zinc and divalent iron contained in the iron oxide layer is important, and whether the ratio of zinc is too small or too large, the saturation magnetization σS can be improved. cannot be expected. Figure 1 shows the composition of the iron oxide layer as Znx
It shows the change in saturation magnetization σS when Fe 3-x04 is used. As shown in Figure 1, although saturation magnetization σS gradually increases as the proportion of zinc increases,
It can be seen that when the proportion of zinc exceeds 20 atomic %, that is, when X exceeds 0.6, the saturation magnetization σS decreases. A practical range is that the ratio of zinc to divalent iron, Zn/Fe2+, is 8 to 25 at.%.

By adding zinc within the above range, the saturation magnetization σS of the obtained magnetic powder can be increased. Figure 2 is a graph showing changes in saturation magnetization σS depending on the amount of iron oxide layer deposited.
The change is shown when the concentration is 10 atomic % with respect to 2+, and the straight line shows the change when only divalent iron is added to form an iron oxide layer having the composition Fe3O4. In addition, the above coating amount is
Fe2+ contained in the iron oxide layer and γ-Fe2 which is the nucleus crystal
Atomic ratio Fe2+/Fe with Fe3+ contained in 03 particles
Shown as 3+. According to this Figure 2, the atomic ratio of zinc is Zn/
When an iron oxide layer is formed by adding Fe to 10 atomic %, the amount of Fe2+ deposited is the same as when an iron oxide layer is formed by adding only Fe2+. It was found that a saturation magnetization σS higher by about 28 mV1 can be obtained with the Fe2+
This is presumed to be due to the ferrite-like properties of the iron oxide layer containing Zn'f and Zn'f.

As described above, an iron oxide layer and a cobalt compound layer were formed on the surface of the acicular γ-Fe2e3 particles, dehydrated and dried, and then heated in an inert gas atmosphere at a temperature of 100 to 250°C for 30 minutes. Heat treatment for ~3 hours. As the inert gas, nitrogen N2, argon Ar, neon Ne, etc. can be used, but nitrogen N2, which is inexpensive and easily available, is used in practice.

In the present invention, the heat treatment in the inert gas atmosphere is important, and the change over time in the coercive force Hc of the acicular ferromagnetic iron oxide magnetic powder obtained by this heat treatment is significantly reduced.

Figure 3 shows the magnitude of the saturation magnetization σB of the obtained acicular ferromagnetic iron oxide magnetic powder and the increase in coercive force due to aging ΔHe
In the figure, curve C shows the relationship between Z and Z.
VCZ n and 0 so that n/Fe2+ is 10 atomic%
: When an iron oxide layer is formed by adding Fe-2+, curve e shows that after adding Zn and Fe-2+ to form an iron oxide layer so that Zn/Fe2+ is 10 atomic %, the iron oxide layer is formed in a nitrogen atmosphere. Each figure shows the case of heat treatment at 150°C. As shown in Fig. 3, when heat treatment is performed in a nitrogen atmosphere, the change over time (increase ΔHe) in the coercive force He of the obtained acicular ferromagnetic oxidation and iron magnetic powder is significantly increased (approximately 15 to 50%). )
It can be seen that this decreases. The aging conditions were as follows: temperature 60° C., 12 days, and the cobalt coating amount Co/Fe of each magnetic powder was 4 atomic %.

The reason why the change over time in the coercive force He of the acicular ferromagnetic iron oxide magnetic powder obtained by the heat treatment in an inert gas is reduced is that the heat treatment shortens the reaction that causes the change over time in the coercive force He. It is possible that things happen over time, but
In fact, as shown in FIG. 4, it was found that the coercive force He of the magnetic powder increased somewhat by applying the above heat treatment.・
Incidentally, the heat treatment time can be shortened if the heat treatment temperature is high; for example, if the heat treatment temperature is f25.0°C, the heat treatment time of about 30 minutes is sufficient.

As described above, after forming an iron oxide layer containing zinc and divalent iron and a cobalt compound layer on the surface of acicular γ-F8203 particles, heat treatment is performed in an inert gas atmosphere. , it is possible to produce acicular ferromagnetic iron oxide magnetic powder with extremely little change in coercive force He over time.

Next, specific examples of the present invention will be described, but it goes without saying that the present invention is not limited to these examples.

Example 1゜Coercive force He3670e, saturation magnetization σB72.3emu/
γ-F62Q3100 of f! After dispersing P in an aqueous solution containing 115.2 t of sodium hydroxide, 100 g of an aqueous solution containing 2.23 t of ferrous sulfate S and 6.41 t of zinc sulfate was added, and the mixture was stirred at 70°C for 30 minutes. Then, the temperature was further raised to 100°C and stirred for 3e minutes.

Next, an aqueous solution containing 1-11.91 cobal chloride 100
After adding fnl and stirring for 7 hours, the mixture was dehydrated and dried.

Furthermore, this magnetic powder was heated at 150° C. in a nitrogen gas atmosphere.
Heat treatment was performed at ℃ for 2 hours.

The coercive force He of the acicular ferromagnetic iron oxide magnetic powder thus obtained is 5900e, the saturation magnetization σS is 8Q, l em
It was u/7. In addition, the coercive force Hc of this magnetic powder after being stored in air at -60°C for 300 hours was 6000e, and the change in coercive force over time was 1°Oe, which was significantly reduced compared to the comparative example described below. was.

- Comparative example.

Coercive force Hc3570e, saturation magnetization σ872.3emu/
S' (7) γ-Fe203100? Sodium hydroxide 115. : After dispersing in an aqueous solution containing 860% of H', 100% of an aqueous solution containing 52.23P of ferrous sulfate and 6.44F of zinc sulfate was added, and the mixture was stirred at 70°C for 30 minutes.
The temperature was further raised to 100°C and stirred for 30 minutes.

Next, an aqueous solution containing 11.92 M' of cobal chloride
After adding 0- and stirring for 7 hours, the mixture was dehydrated and dried.

The coercive force Hc of the acicular ferromagnetic iron oxide magnetic powder thus obtained is 5800e, the saturation magnetization σS is 8Q, l) e
mu/? Met. In addition, this magnetic powder was heated to 60°C.
Coercive force after storage in air for 300 hours with Heba 5950
e, and the change in coercive force over time was 150e.

Comparative Example 2 After dispersing the same γ-Fe2e31'00t as in Example 1 in an aqueous solution containing 115.2F of sodium hydroxide, 69.64% of ferrous sulfate was added. 100°C of an aqueous solution was added, stirred at 70°C for 30 minutes, and
The temperature was raised to 00°C and stirred for 30 minutes.

Then an aqueous solution containing 10:1 (cobal chloride) of 11.9:l (cobal chloride)
After adding 1* and stirring for 7 hours, the mixture was dehydrated and dried.

The coercive force Hc of the acicular ferromagnetic iron oxide magnetic powder thus obtained is 5 g 50e, the saturation magnetization σsI′isO,
2emu/? Met. In addition, this magnetic powder was heated at a temperature of 6
The coercive force He after being stored in air at 0°C for 300 hours is 61
00e, and the change in coercive force over time was 250e.

Comparative Example 3 Acicular ferromagnetic iron oxide magnetic powder obtained by the same method as Comparative Example 2 was heat-treated at 150° C. for 2 hours in a nitrogen gas atmosphere.

The magnetic powder thus obtained has a coercive force He of 598,
Oe, saturation magnetization σS was 3 Q, l emu/f. Further, after this magnetic powder was stored in air at a temperature of 60° C. for 300 hours, the coercive force Hci (i) was 150e, and the change in coercive force over time was 170e.

Example 2 Coercive force HC364 Oersted, saturation magnetization σS72.6
r-Fe2u31QQf of emu/y was dispersed in an aqueous solution 86θ- containing 115.2r of sodium hydroxide, and further 100m of an aqueous solution containing 10.769 cobal chloride was added and stirred at 100°C for 4 hours.

Then ferrous sulfate 52.23r and zinc sulfate 11.3
After adding an aqueous solution containing 8 t and stirring for 1 hour, the mixture was dehydrated and dried.

Furthermore, this magnetic powder was heated at 150° C. in a nitrogen gas atmosphere.
Heat treatment was performed at ℃ for 2 hours.

The coercive force HcI/'16810e1 saturation magnetization as of the acicular ferromagnetic iron oxide magnetic powder thus obtained is 79.9e
It was mu/l. Further, after this magnetic powder was stored in air at a temperature of 60° C. for 300 hours, the coercive force Hc was 6900e, and the change in coercive force over time was extremely small at 90e.

Comparative Example 4 Coercive force Hc 364 oersted, saturation magnetization σS 72.
6 emu/f of γ-Fezes 100 t hydroxide of thorium 115.2. The mixture was dispersed in 860 tnl of an aqueous solution containing 10.761 (cobal chloride), and 100 mA of an aqueous solution containing 10.761 (cobal chloride) was added thereto, followed by stirring at 100°C for 4 hours.

Then 52.23 f of ferrous sulfate and 11.5 f of zinc sulfate.
After adding an aqueous solution containing 38 F and stirring for 1 hour, the mixture was dehydrated and dried.

The thus obtained acicular ferromagnetic iron oxide magnetic powder has a coercive force He of 673 oersted and a saturation magnetization σS of 79.9.
emu/li'. In addition, this magnetic powder was heated at a temperature of 6
After storage at 0° C. for 300 hours, the coercive force Hc was 689 oersted, and the change in coercive force over time was 160e.

〔Effect of the invention〕

As is clear from the description of the above embodiments, according to the present invention, heat treatment is performed in an inert gas atmosphere.
Coercive force He and saturation magnetization σS are large, and coercive force He
It is possible to produce acicular ferromagnetic iron oxide magnetic powder with extremely little change over time.

[Brief explanation of drawings]

Figure 1 shows the proportion of zinc contained in the iron oxide layer and the saturation magnetization σS
Figure 2 shows the relationship between the amount of iron oxide layer deposited and the saturation magnetization σS when the proportion of zinc contained in the iron oxide layer is Zn/Fe'' = IQ atomic %. Figure 3 shows the relationship between the saturation magnetization σS of the magnetic powder obtained by heat treatment in an inert gas atmosphere and the amount of change (increase) ΔHe in the coercive force He over time. FIG. 4 is a characteristic diagram showing the relationship between the heat treatment temperature in a nitrogen gas atmosphere and the coercive force He of the obtained acicular ferromagnetic iron oxide magnetic powder.

Claims (1)

[Claims] Acicular ferromagnetism characterized by forming an iron oxide layer containing zinc and divalent iron and a cobalt compound layer on the surface of γ-Fe_2O_3 particles as a core, and then heat-treating in an inert gas atmosphere. Manufacturing method of iron oxide magnetic powder.