JPS6149251B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing cobalt-modified acicular maghemite particles coated with cobalt, which is used as a magnetic recording medium material. Specifically, the present invention has high coercive force and a small spread of coercive force distribution. It has high residual magnetic flux density and stability against changes in magnetic properties over time, pressure, and temperature.
A further object of the present invention is to provide a method for producing cobalt-modified acicular maghemite particles coated with cobalt that have excellent dispersibility in a vehicle, excellent orientation and filling properties in a coating film. In recent years, various magnetic recording media have become increasingly dense and
High fidelity is required, and in order to meet this requirement, it is necessary to increase the magnetic properties, particularly the coercive force and residual magnetic flux density, of the magnetic particles used as the magnetic recording medium material coated into the coating film. is necessary. As is well known, the magnitude of the coercive force of magnetic particles depends on shape anisotropy, crystal anisotropy, strain anisotropy, exchange anisotropy, etc., or the interaction thereof. It is already known that when the magnetic particles are fine particles and are single-domain particles, the coercive force is further increased due to the rotating magnetization mechanism. In addition, the residual magnetic flux density of magnetic particles depends on the magnetic field orientation, and the larger the axial ratio of the acicular crystals, the better the magnetic field orientation. It is known that the residual magnetic flux density also increases. Known acicular maghemite particles utilize their shape anisotropy to obtain a high coercive force, and utilize their excellent magnetic field orientation to obtain a high residual magnetic flux density. It is known that the coercive force of these acicular maghemite particles can be further improved by adding cobalt. Examples of literature that specifically explain this include ``Cobalt Substitution γ-Fe <sub>2</sub> O <sub>3</sub> as High-Density Magnetic Tape,'' IEEE TRANSACTIONS ON ELECTRONIC COMPUTERS.
COMPUTERS) Vol EC−15, No.5 (October 1966
month) etc. Methods for obtaining acicular maghemite particles by adding cobalt are roughly divided into the following two methods depending on the method of adding cobalt. (1) By adding cobalt at the stage when acicular crystal hydrated ferric oxide particles are produced, acicular crystal hydrated ferric oxide particles containing cobalt at a uniform concentration are obtained, and the particles are reduced and oxidized. method, the so-called cobalt doping method. (For example, Tokuko Sho 37
(2) A method in which cobalt is added to acicular ferric oxide particles as a cobalt ion or a cobalt compound, and then the particles are oxidized as a ring element, so-called cobalt How to coat. (For example, Tokuko Sho 41-
Publication No. 6538, Publication No. 48-10994, Japanese Patent Publication No. Sho
48-20098, Japanese Patent Application Laid-open No. 47-22707) Cobalt-modified acicular maghemite particles doped with cobalt obtained by the method (1) have a high coercive force, but the magnetic properties change over time. It has been pointed out that it is extremely unstable with respect to change, pressure, and temperature. On the other hand, the cobalt-modified acicular maghemite particles coated with cobalt obtained by the method (2) have various properties that improve the above-mentioned drawbacks of the cobalt-doped cobalt-modified acicular maghemite particles. It has recently been revealed that this is the case, and it is attracting attention in the industry. The above-mentioned matters are described in detail on pages 16 to 24 of the summary collection of the 3rd Symposium on Magnetic Materials for Recording under the title "Improvement of magnetic properties by post-treatment of magnetic iron oxide for recording." The gist of this is as follows. ``In order to achieve a record high density, we made it from α-FeOOH doped with Co ²⁺ at a uniform concentration.
High Hc is seen in Fe ₃ O ₄ and γ-Fe ₂ O ₃ , but Hc
K in ∝K/Is (K is the magnetocrystalline anisotropy constant)
has a large temperature dependence, and good magnetic properties cannot be obtained. "However, recently, once produced Fe ₃ O ₄ and γ-Fe ₂ O ₃ (or α-FeOOH)
By post-treating with a Co ²⁺ compound to change the surface condition, it has become possible to obtain excellent magnetic powder with a high Hc. "Those using cobalt doping are referred to as γ-Fe ₂ O ₃ (doped), and those resulting from surface treatment are referred to as γ-Fe ₂ O ₃ (adhered), to distinguish between the two." Among these, it belongs to (2), and obtains cobalt-modified acicular maghemite particles coated with cobalt that have stability against changes in magnetic properties over time, pressure, and temperature. The present inventor has been involved in the production of cobalt-modified acicular maghemite particles for many years, and in the course of his research, he has already developed a method for depositing cobalt on acicular maghemite particles. There is. For example, as described below. That is, acicular maghemite particles are used as a precursor, and dispersed in an aqueous cobalt salt solution or water containing cobalt hydroxide so that 50 atomic % or less of cobalt is deposited on the iron in the precursor. Cobalt is deposited by adding an alkali such as caustic soda so that the OH group concentration of the dispersion is 0.05 to 3.0 mol/, maintaining the temperature at 50 to 100°C, and processing in a non-oxidizing atmosphere. Cobalt modified acicular maghemite particles can be obtained. In this case, when the OH group concentration of the dispersion is 0.05 mol/or less, sufficient results are obtained judging from the magnetic properties of the cobalt-modified acicular maghemite particles to which the target cobalt is deposited. can't get it. This is clear from curve C in FIG. In other words, Figure 1 is a graph showing that the OH group concentration of the dispersion has a large effect on the magnetic properties (coercive force Hc) of the product. This is an example showing the influence of the OH group concentration when the temperature is kept constant at 95°C. still,
The OH group concentration for curve A is 2.5 mol/, and for curve B is 0.1 mo.
l/, curve C is when it is 0.01 mol/. On the other hand, when the OH group concentration of the dispersion is 3.0 mol/or more, cobalt hydroxide begins to dissolve.
Therefore, the OH group concentration should be selected from the range of 0.05 mol/ to 3.0 mol/. Next, the temperature of the dispersion liquid is related to the processing time, and if the temperature is set to 50°C or less, cobalt-modified acicular maghemite particles coated with cobalt, which is the object of the present invention, will be produced. This is difficult, and even if it were to be generated, it would require extremely long processing time. This is also clear from curve C in FIG. That is, Fig. 2 is a graph showing the relationship between the temperature of the dispersion, the treatment time, and the magnetic properties (coercive force Hc) of the product. This is an example showing the effect of temperature when is kept constant at 2.5 mol/. Furthermore, the temperature of curve A is 95
℃, Curve B is at 50℃, Curve C is at 20℃. In the above method for obtaining cobalt-modified acicular maghemite particles coated with cobalt, the acicular maghemite particles used as the precursor are in a state in which individual particles are independently dispersed, that is, particles It is important that there is no entanglement between them. This is because when acicular maghemite particles in such a state are used as a precursor, cobalt can be uniformly and effectively deposited on the precursor particles, and the resulting cobalt is In the process of manufacturing magnetic recording media such as magnetic tapes using cobalt modified acicular maghemite particles, it has excellent dispersibility in vehicles, orientation and filling properties in coating films, and therefore has high This is because a magnetic recording medium having coercive force and high residual magnetic flux density can be obtained. By the way, acicular maghemite particles are usually produced by reducing acicular crystal hydrated ferric oxide particles obtained by a wet reaction between a ferrous salt solution and an alkali in a reducing gas such as hydrogen. It is obtained by forming magnetite particles and then oxidizing them in air to form acicular maghemite particles. However, in the acicular maghemite particles obtained by the above method, the particles are generally tightly entangled with each other, and through heat treatment processes such as reduction and oxidation, the particles are sintered and aggregated. It's summery. Cobalt-modified acicular maghemite particles obtained by using such acicular maghemite particles as a precursor and depositing cobalt are particles that are tightly intertwined with each other or aggregated by sintering etc. Since the cobalt is deposited as is, it is not possible to deposit the cobalt uniformly and effectively, which in particular causes the coercive force distribution to widen. Furthermore, in the process of manufacturing magnetic recording media, it causes poor dispersion in the vehicle and inhibits orientation and filling properties in the coating film. Therefore, in order to obtain high-performance magnetic recording media, that is, various magnetic tapes, magnetic disks, etc., it is necessary to sufficiently orient the raw material magnetic particles and at the same time increase the filling rate. Similarly, the particles are entangled with each other as raw magnetic particle powder, or
When using cobalt-modified acicular maghemite particles to which cobalt is adhered while remaining agglomerated by sintering or the like, a satisfactory degree of orientation and filling rate cannot be obtained as is. Suppose that various dispersants are used to disperse the cobalt-modified acicular maghemite particles coated with cobalt in a vehicle, and furthermore, if the particles are forcibly dispersed using a powerful mixer, In the obtained magnetic recording medium, the distribution of coercive force becomes wider, which causes insufficient recording and reproducing sensitivity to be obtained. Furthermore, the needle crystals of the needle crystal magnetic particles also deteriorate, making it impossible to obtain a preferable degree of orientation and filling rate. In view of the above-mentioned conventional methods, the present inventors have diligently investigated the manufacturing process of cobalt-modified acicular maghemite particles to which cobalt is adhered, the timing and causes of entanglement between particles, aggregation, etc. As a result of repeated research, the present invention was arrived at. That is, in the process of producing acicular crystal hydrated ferric oxide particles by a wet reaction between a ferrous salt and an alkali, the present invention is directed to a reaction in which the production reaction of acicular crystal hydrated ferric oxide particles has been completed. Adding and mixing 0.1 to 15 wt% of phosphate in terms of (PO ₃ ) ₆ to the acicular crystalline hydrated ferric oxide particles in the reaction mother liquor,
Next, the acicular crystal hydrous ferric oxide particles obtained by washing with water, filtering, and drying are reduced at a temperature of 350°C to 600°C, and further oxidized to form acicular maghemite particles. Using maghemite particles as a precursor, this precursor is dispersed in an aqueous cobalt salt solution or water containing cobalt hydroxide, and an alkali such as caustic soda is added so that the OH group concentration of the dispersion is 0.05 to 3.0 mol/. Cobalt-modified acicular maghemite in which 0.1 to 50 atomic % of cobalt is deposited on the iron in the precursor by maintaining the temperature at 50 to 100 °C and processing in a non-oxidizing atmosphere. A method of producing cobalt-modified acicular magnetic iron oxide particle powder, which consists of producing particles, and washing the product with water, filtering, and drying by a conventional method to obtain cobalt-modified acicular maghemite particles coated with cobalt. It is a manufacturing method. Next, the background that led to the completion of the present invention and the structure of the present invention will be described. In order to obtain the final target product, cobalt-modified acicular maghemite particles coated with cobalt, the acicular maghemite particles, which are the precursor, must be separated one by one, that is, tangled. I learned that it is necessary to use a powder of acicular crystal particles that do not match. Furthermore, the present inventor has discovered that when producing acicular hydrated ferric oxide particles as a starting material by a wet method and separating them from the reaction mother liquor, the acicular hydrated ferric oxide particles are separated from the reaction mother liquor. In this case, the particles are already entangled with each other with considerable force, and even if various dispersion treatments using conventional methods are applied, the particles cannot be separated into one unit.
They learned that it is difficult to create individual particles, that is, particles without entanglement.
In other words, the acicular crystalline hydrated ferric oxide particles have already become partially entangled and bonded particles in the process of crystal growth in the reaction mother liquor. Based on the facts explained above, the present inventor has conducted repeated research on loosening the entanglements and bonds of acicular crystal hydrated ferric oxide particles generated during the crystal growth process in a wet reaction, and By adding phosphate to the reaction mother liquor in which the production reaction of diiron particles has been completed and stirring and mixing, it is possible to loosen the needle-shaped hydrous ferric oxide particles one by one into pieces. We also obtained the knowledge that the acicular maghemite particles obtained by reducing and oxidizing these acicular crystalline hydrous ferric oxide particles are also in a state of being separated one by one. Considering the above phenomenon, the phosphate added to the reaction mother liquor in which needle-shaped hydrated ferric oxide particles are produced first has the effect of loosening the aggregated particles in the liquid, and secondly has the effect of loosening the aggregated particles in the liquid. The particles obtained by washing, filtering, and drying the above-mentioned treated particles are particles containing the added phosphate remaining as a phosphorus compound, and these are used as a starting material in a reducing gas.
After thermal reduction at 350°C or higher and 600°C or lower, the acicular maghemite particles are heated and oxidized in an oxidizing gas, and the contained phosphorus compound suppresses the reduction reaction of iron oxide particles and suppresses the particle growth rate. It is thought that it has the effect of suppressing the In general, the shape of acicular crystal particles is easily affected by the heating temperature, and the higher the temperature, especially when the atmosphere is reducing, the more remarkable the particle growth becomes, and the size of a single particle exceeds that of a skeleton particle. The outer shape of the skeleton particles gradually disappears, causing deformation of the particle shape and sintering of the particles and each other. However, if it contains a phosphorus compound, the outer shape of the skeleton particle gradually disappears, but if it contains a phosphorus compound, 600℃ especially 400℃～600
Even when heat-treated at a high temperature of .degree. C., the particles still inherit the acicular crystal structure of the starting material acicular crystal hydrous ferric oxide particles. FIG. 3 shows the relationship between the reduction temperature of acicular crystal hydrous ferric oxide particles and the retention force of acicular maghemite particles obtained by later oxidizing them at 300°C. In the figure, A and B are treated with sodium hexametaphosphate (A is 14.57wt in terms of (PO ₃ ) ₆₎
%, B indicates the case where 6.47wt% sodium hexametaphosphate treated product) was used as the starting material, and C indicates the case where the final treated product was used as the starting material. curve A,
For B, the coercive force increases until the reduction temperature reaches 550°C and 450°C, respectively, and gradually decreases after this temperature is exceeded. The above fact is that the phosphorus compound contained in the acicular hydrated ferric oxide particles suppresses the reduction reaction and particle growth rate of the iron oxide particles, depending on the content. This shows that particles with sufficient crystal structure can be obtained. In curve C, the coercive force decreases rapidly when the reduction temperature is increased above 330°C. This indicates that particles with broken needle crystals are obtained due to promotion of the reduction reaction and particle growth. Next, various conditions for carrying out the method of the present invention will be described. First, the phosphate in the present invention will be explained. The effect of adding phosphate in the present invention is exerted in two stages throughout the entire process. It is to loosen the bonds and break them apart one by one. For this purpose, the amount of phosphate added varies depending on the properties of the reaction matrix, especially the pH value, but the amount of (PO ₃ ) ₆ to acicular hydrated ferric oxide particles in the reaction mother liquor is This is converted to 0.1 to 15 wt%. If the amount of phosphate added is less than 0.1wt%, the purpose of the present invention cannot be fully achieved, and if it is more than 15wt%, the purity of the iron oxide particles will decrease and the desired magnetic properties will not be achieved. can't get it. In the range of 0.1 to 15 wt%, the greater the amount added, the greater the effect of loosening the entanglements and bonds of the acicular crystal hydrated ferric oxide particles, but later the acicular crystal hydrated ferric oxide particles containing the phosphorus compound When diiron particles are reduced to acicular magnetite particles and further oxidized to obtain acicular maghemite particles, if the reduction temperature is considered, the relationship between the content of phosphorus compounds and the reduction temperature is proportional. Therefore, as the content of phosphorus compounds increases, the reduction temperature must be increased. It is not industrially possible to raise the reduction temperature excessively. Therefore, the target amount of phosphate is preferably 0.1 to 5 wt% in terms of (PO ₃ ) ₆ based on the acicular hydrated ferric oxide particles. When the pH value of the reaction mother liquor is about 3 to 4, the effect of adding phosphate is particularly significant. When the reaction mother liquor is alkaline, phosphate can be added directly to the alkaline reaction mother liquor, but a suitable acid such as sulfuric acid is added to the alkaline reaction mother liquor to adjust the pH value to 3 to 4. It is also possible to add post-phosphate. In addition, the method of adding phosphate is as follows:
The powder or the like may be directly added to the reaction mother liquor, or it may be added after being dissolved in water. Note that examples of the phosphate to be added include sodium hexametaphosphate, sodium pyrophosphate, and sodium metaphosphate. Second, the reduction of the acicular crystal hydrous ferric oxide particles containing the phosphorus compound obtained by treatment with a phosphate in a reducing gas can be carried out at 350°C to 600°C. When the acicular hydrated ferric oxide particles contain a phosphorus compound, the reduction is carried out at a high temperature, especially between 400°C and 600°C, depending on the content of the phosphorus compound.
It can be carried out at ℃, and since the particles retain acicular crystals, a high coercive force can be obtained. If the reduction temperature is below 350°C, the reduction reaction will not proceed sufficiently, and if it is above 600°C, the reduction reaction will be promoted and the particles will grow, resulting in broken acicular crystals, which will result in a high coercive force. I can't.
This is as shown in FIG. Moreover, reducing at high temperatures of 600°C or higher in a reducing gas requires highly precise equipment and advanced technology, which is not industrially or economically viable. industrial,
Considering economic efficiency, the temperature is preferably 550°C or lower. Next, the amount of cobalt deposited when producing cobalt modified acicular maghemite particles to which cobalt is deposited will be explained. Cobalt deposition occurs no matter how much cobalt is deposited, as long as it is less than 50 atomic % of iron in the precursor acicular maghemite particles. However, when considering the magnetic stability (temperature dependence of coercive force, pressure demagnetization, etc.) as a magnetic recording medium, the amount of cobalt deposited is
0.1 to 10 atom% is preferred. According to the present invention as described above, the particle shape of the acicular crystalline hydrated ferric oxide particles, which is the starting material, is maintained and inherited, and there is no crosslinking state between the particles and between the particles, and the particles are substantially Cobalt-modified acicular maghemite particles having independent cobalt deposits can be obtained. In addition, looking at the difference between the coercive force of the obtained powder and the coercive force of the coating film obtained using the same as shown in Table 5,
In the case of the comparative example, the difference in coercive force is more than 90 Oe, while in the case of the example it is 35 Oe.
It is as follows. This small difference in coercive force is considered to indicate that the particle powder essentially has a narrow coercive force distribution. The cobalt-modified acicular maghemite particles coated with cobalt thus obtained are:
High coercive force, small spread of coercive force distribution, high residual magnetic flux density, and stability against changes in magnetic properties over time, pressure and temperature, good dispersibility in vehicles, and good orientation in coatings. Since it also has excellent filling properties, it can be used as a magnetic material for high-density, high-fidelity recording, which is currently most required. Next, the present invention will be explained with reference to Examples and Comparative Examples. <Production of starting material particles> Examples 1 to 5, Comparative Example 1; Example 1 250 aqueous solution containing 22.3 mol of FeSO ₄ was added to 196 2.63-N NaOH aqueous solution prepared in advance in the reactor, and each time Ferrous hydroxide colloid was produced at pH 12.8 and temperature 45° C. for 45 minutes while bubbling N ₂ gas at a rate of 1000 min. Thereafter, the above colloidal solution was subjected to an oxidation reaction for 14 hours by passing 1800 air per minute at a temperature of 50°C.
Acicular α-FeOOH particles were produced. The liquid containing the acicular α-FeOOH particles is referred to as reaction mother liquid 1.
The above reaction mother liquor was mixed and stirred to form needle-shaped α-FeOOH crystals.
Add reaction mother liquor 1 while dispersing the particles sufficiently and uniformly.
20 samples were taken. 49% sulfuric acid in the collected reaction mother liquor
2.16 was gradually added to adjust the pH to 3.4, and then 800 ml of an aqueous solution containing 6.55 g of sodium hexametaphosphate (converted to (PO ₃ ) ₆ based on acicular hydrated ferric oxide particles) was added.
0.73wt%) and stirred for 60 minutes. after that,
Wash with water, filter, and dry to obtain needle-shaped α-
855 g of FeOOH granular powder was obtained. As a result of chemical analysis, the obtained acicular α-FeOOH particles were found to be (PO ₃ ) ₆
Contains 0.51wt% of phosphorus compounds in terms of
According to electron microscopy, the average value of the long axis is 0.65μ,
They were acicular crystal particles with a long axis/short axis ratio of 10:1. Examples 2 and 3 Acicular α-FeOOH particles were obtained in the same manner as in Example 1, except that the amount of sulfuric acid and the amount of sodium hexametaphosphate added were varied.
Table 1 shows the main production conditions and powder properties of this acicular α-FeOOH particle powder. Example 4 Reaction mother liquor 1 obtained in Example 1 was mixed and stirred to sufficiently and uniformly disperse the needle-shaped α-FeOOH particles, while reaction mother liquor 20 was collected. To the collected reaction mother liquor, 2500 ml of an aqueous solution containing 57.9 g of sodium hexametaphosphate (6.43 wt% in terms of (PO ₃ ) ₆ based on the acicular hydrated ferric oxide particles) was added and stirred for 60 minutes. Thereafter, the mixture was washed with water, filtered, and dried in a conventional manner to obtain 900 g of acicular α-FeOOH particles. Table 1 shows the powder characteristics of the obtained acicular α-FeOOH particles. Example 5 Ferrous sulfate solutions 12.5 and 2.14 containing 2 mol of α-FeOOH particles as fine nuclei and 8 mol of Fe ²⁺
Using a 7.5 volume of -N NaOH solution, a total volume of 20 of the reaction mother liquor was heated at 60° C. and PH of 4.2, and air was blown at a rate of 50 per minute to generate needle-shaped α-FeOOH particles. After 12 hours, the reaction mother liquor contains 0.5 mol/acicular crystals α
-Contained FeOOH particles. This needle crystal α-
The liquid containing FeOOH particles is referred to as reaction mother liquid 2. To the above reaction mother liquor 2, 2500 ml of an aqueous solution containing 58.5 g of sodium hexametaphosphate (6.50 wt% in terms of (PO ₃ ) ₆ based on the acicular hydrated ferric oxide particles) was added and stirred for 60 minutes. Thereafter, the mixture was washed with water, filtered, and dried in a conventional manner to obtain 900 g of acicular α-FeOOH particles. Table 1 shows the powder characteristics of the obtained acicular α-FeOOH particles. Comparative Example 1 Reaction mother liquor 140 was collected while mixing and stirring the reaction mother liquor 1 obtained in Example 1 to sufficiently and uniformly disperse the needle-shaped α-FeOOH particles. The collected reaction mother liquor was washed with water, filtered, and dried in a conventional manner to obtain acicular α-FeOOH particles. The obtained needle crystal α-
Table 1 shows the powder properties of FeOOH particles. <Manufacture of precursor particles> Examples 6 to 16, Comparative Examples 2 to 7; Example 6 Acicular α-FeOOH particles obtained in Example 2
1000g was reduced in a reduction furnace by passing _H2 gas at a rate of 3 per minute at a reduction temperature of 350℃ for 120 minutes, and then further oxidized in air at 300℃ for 90 minutes to obtain 860g of acicular maghemite particles. . As a result of electron microscopic observation, the obtained acicular maghemite particles inherited the shape of the acicular α-FeOOH particles, which were the starting material particles, and had a long axis of 0.60μ and an axial ratio of 10:1. . Further, as a result of magnetic measurement, the coercive force Hc343Oe and the saturation magnetization (σs) were 72emu/g. Examples 7 to 16, Comparative Examples 2 to 7 Acicular maghemite particles were obtained in the same manner as in Example 6, except that the type of starting material particles and the reduction temperature were varied. The main manufacturing conditions and various properties of the acicular maghemite particles are shown in Tables 2 and 3. <Production of cobalt-modified acicular maghemite particles> Examples 17 to 23, Comparative Examples 8 to 9; Example 17 260 g of the acicular maghemite particles obtained in Example 7 was mixed with cobalt using cobalt sulfate.
Pour into 2400ml of water in which 0.051mol is dissolved,
It was dispersed until it became a fine slurry. The temperature of the resulting dispersion was raised to 95°C, and 7.61-N NaOH was added to the dispersion while stirring well while preventing air from entering as much as possible.
1065 ml of the solution was added to form a dispersion with an OH group concentration of 2.3 mol/. The temperature of the dispersion was maintained at 95°C, stirring was continued, and the slurry was taken out after 200 minutes, washed with water, separated, and dried at 100°C to obtain cobalt-modified acicular maghemite particles. As a result of electron microscopic observation, the obtained particles inherited the shape and particle size of the precursor acicular maghemite particles, and the long axis
They were acicular crystal particles of 0.50μ and an axial ratio of 10:1. Further, as a result of magnetic measurement, the coercive force Hc was 486 Oe, and the saturation magnetization σs was 78 emu/g. As a result of atomic absorption analysis, the particles contained 1.65 at.% of cobalt. The above cobalt-modified acicular maghemite particles were blended with a binder composition as shown below, and then mixed and dispersed in a ball mill for 10 hours to obtain a magnetic paint. Cobalt modified acicular maghemite particles powder
100 g Vinyl resin (vinyl acetate: vinyl chloride =
3:91 copolymer 20 g Nitrile rubber (acrylonitrile copolymer)
50 g Toluene 100 g Methyl ethyl ketone 75 g Methyl isobutyl ketone 75 g Dispersant (lecithin) 0.2 g Add a solvent (toluene: methyl ethyl ketone: methyl isobutyl ketone = 1:1:
1) was added to adjust the paint viscosity to an appropriate level, and the mixture was coated on a polyester resin film in a conventional manner and dried to produce a magnetic coating film. The coercive force Hc of this magnetic coating is 468 Oe, and the residual magnetization Br is
1430 Gauss, square type Br/Bm was 0.90. Examples 18-23, Comparative Examples 8-9 The amount of acicular maghemite particles as a precursor was
Cobalt-modified acicular maghemite was prepared in the same manner as in Example 17, except that the total volume of the treatment solution was 3500 ml, and the type of precursor, amount of cobalt added, amount of NaOH added, temperature, and time were varied. Particles were obtained. The main manufacturing conditions are shown in Table 4, and
Table 5 shows various properties of the obtained particles.

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【table】 [Brief explanation of the drawing]

FIG. 1 is a relationship diagram showing the influence of the OH group concentration of the dispersion on the properties of the product, with the vertical axis representing the coercive force Hc of the product and the horizontal axis representing the treatment time. In addition, A in the figure indicates the OH group concentration of the dispersion liquid of 2.5 mol/
, B is 0.1 mol/, and C is 0.01 mol/. FIG. 2 is a relationship diagram showing the influence of dispersion temperature on processing time and product properties;
The vertical axis shows the coercive force Hc of the product, and the horizontal axis shows the processing time. In addition, A in the figure indicates the temperature of the dispersion liquid.
The temperature was 95°C, B was 50°C, and C was 20°C. FIG. 3 shows the relationship between the reduction temperature of acicular crystal hydrous ferric oxide particles and the coercive force Hc of acicular maghemite particles obtained by subsequently oxidizing them at 300°C. In addition, A and B in the figure are C when the starting materials were treated with sodium hexametaphosphate of 14.57 wt% and 6.47 wt% in terms of (PO ₃ ) ₆ , respectively.
shows the case where untreated material was used as a starting material.

Claims (1)

[Scope of Claims] 1. In the step of producing acicular hydrated ferric oxide particles through a wet reaction between a ferrous salt and an alkali, the reaction for producing acicular hydrated ferric oxide particles is completed. 0.1 to 0.1 to (PO ₃ ) ₆ based on the acicular hydrated ferric oxide particles in the reaction mother liquor.
15 wt% of phosphate is added and mixed, then washed with water, filtered, and dried to obtain acicular crystal hydrated ferric oxide particles, which are reduced at a temperature of 350°C to 600°C, and further oxidized to form acicular crystals. After forming crystalline maghemite particles, using these acicular crystalline maghemite particles as a precursor,
This precursor is dispersed in a cobalt salt aqueous solution or water containing cobalt hydroxide, and an alkali such as caustic soda is added so that the OH group concentration of the dispersion is 0.05 to 3.0 mol/, and the temperature is raised to 50 to 100°C. hold,
By processing in a non-oxidizing atmosphere, cobalt-modified acicular maghemite particles are produced, in which 0.1 to 50 at. Wash with water, filter,
A method for producing cobalt-modified acicular magnetic iron oxide particle powder, which comprises drying to obtain cobalt-modified acicular maghemite particles to which cobalt is adhered. 2 The amount of cobalt deposited relative to the iron in the precursor
A method for producing a cobalt-modified acicular magnetic iron oxide particle powder according to claim 1, wherein the cobalt content is 0.1 to 10 atomic %. 3 The amount of phosphate added is 0.1 to (PO ₃ ) ₆ based on the acicular crystal hydrated ferric oxide particles in the reaction mother liquor.
A method for producing cobalt-modified acicular magnetic iron oxide particles according to claim 1 or 2, wherein the cobalt content is 5 wt%. 4. The method for producing cobalt modified acicular magnetic iron oxide particle powder according to any one of claims 1 to 3, wherein the reduction temperature is 400°C or higher and 550°C or lower.