JPH0521321B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD OF THE INVENTION The present invention relates to a method for producing cobalt-containing magnetic iron oxide powder. (Technical background of the invention and its problems) Magnetic recording media are generally made by coating a magnetic paint, which is made by dispersing magnetic powder in a resin binder, on a plastic film or sheet substrate to form a magnetic layer. However, if the magnetic powder particles are not sufficiently dispersed in the paint due to secondary aggregation, it is not possible to form a uniform and smooth magnetic layer with a large degree of filling. Non-uniformity of the magnetic layer due to agglomeration of magnetic powder particles tends to significantly impair the magnetic properties and electromagnetic conversion properties of the magnetic recording medium. That is, a decrease in the squareness ratio and magnetic field orientation is unavoidable, which is likely to cause a decrease in output, an increase in noise, and dropout. Regarding the improvement of the dispersibility of magnetic powder, research has been carried out from various viewpoints, and many proposals have been made in this regard. Most commonly, various dispersants such as lecithin, fatty acids, fatty acid amides, fatty acid esters, metal soaps, higher alcohols and their esters, polyethylene oxide and its acid esters, organosilicones, etc. are blended into magnetic paints. However, in some cases, magnetic powder is surface-treated with these dispersants and turned into a paint. On the other hand, as the density of magnetic recording has increased in recent years, the demand for acicular cobalt-containing magnetic iron oxide powder with large coercive force and saturation magnetization has increased, but this powder is more expensive than ordinary magnetic iron oxide powder. magnetic agglomeration due to interaction between particles, making uniform dispersion in the binder resin very difficult, and as a result, the desired characteristics of the magnetic recording medium cannot be sufficiently improved. There are quite a few. However, the trend toward higher densities in magnetic recording media is becoming stronger, and with the recent trend towards 8mm video and microcassettes, cobalt-containing magnetic materials with high specific surface areas and small particle diameters are becoming more and more popular. Attempts have been made to increase the S/N ratio, output, information capacity, etc. by using iron oxide powder. However, when the specific surface area of the magnetic iron oxide particles exceeds, for example, 35 m ² /g, the dispersibility rapidly decreases as the surface energy increases, making it significantly difficult to form a magnetic recording medium with the desired characteristics. It is well known that this can be difficult. For this reason, proposals have been made to improve surface treatment methods, for example by using various dispersants, but no satisfactory improvement has yet been achieved, and a solution to this problem is desired. (Object of the Invention) The present invention provides a high specific surface area magnetic powder having high coercive force characteristics and excellent electromagnetic conversion characteristics, and a cobalt-containing magnetic oxide that can significantly improve properties such as dispersibility, orientation, and fillability. An object of the present invention is to provide a method for producing iron powder. (Summary of the Invention) As a result of various studies to achieve the above object, the inventors of the present invention applied a specific diamine fatty acid salt compound to the surface of cobalt-containing magnetic iron oxide particles with a high specific surface area by a specific treatment method. The present invention was completed based on the knowledge that the dispersibility of the magnetic iron oxide powder can be significantly improved by reaction-forming and adsorption treatment, and that all of the above-mentioned problems can be solved. That is, the present invention provides _an aqueous slurry of cobalt-containing magnetic iron oxide particles having a specific surface area of 35 m 2 /g or more with a BET value of ³⁵ m ² /g _or more.
At least one water-soluble diamine compound represented by NH ₂ (wherein R ¹ is an alkyl group, alkenyl group, or aryl group having 6 or more carbon atoms, and n is an integer of 1 to 10) and the general formula R ² COOH (wherein R ² is an alkyl group, alkenyl group, or aryl group having 6 or more carbon atoms) is reacted with at least one kind of water-soluble fatty acid compound to form the iron oxide particles on the surface of the iron oxide particles. This is a method for producing cobalt-containing magnetic iron oxide powder, which is characterized by adsorbing a fatty acid salt of diamine, which is a reaction product. In the method of the present invention, the cobalt-containing magnetic iron oxide particles treated with the dispersant include γ-Fe ₂ O ₃ ,
A bertolide compound such as that obtained by partially reducing Fe ₃ O ₄ or γ-Fe ₂ O ₃ can be used as the basic particles. This crystal usually has a needle-like crystal or a similar shape. A cobalt compound or a cobalt compound and another metal compound such as ferrous, manganese, zinc, chromium, or nickel is deposited on the surface of the basic particles to obtain cobalt-containing magnetic iron oxide particles. The amount of the cobalt-containing compound applied is 0.5 to 30% as cobalt, preferably 1% based on the weight of the basic particles.
~10%, for ferrous iron 1-30%, preferably 2-20%, and for other metals 0.05-10
%. In order to obtain a high coercive force, it is preferable to combine a cobalt compound and a ferrous compound. The method for depositing the cobalt-containing compound is as follows:
This can be carried out by various methods, but usually the basic particles are dispersed in water, an aqueous alkali solution, or an aqueous solution of a treatment agent containing at least cobalt to form an aqueous slurry, and the slurry is further mixed with an aqueous alkali solution or ( and) This can be carried out by adding part or all of the aqueous solution of the treatment agent simultaneously or sequentially to precipitate the hydroxide of the metal salt on the surface of the basic particles. The above-mentioned adhesion treatment is desirably carried out in a substantially non-oxidizing atmosphere, excluding the inclusion of air as much as possible, by reacting at a temperature below the boiling point, preferably below 50° C., for 0.1 to 10 hours. The alkali concentration in the reaction system is usually 0.01 to 3 mol/, preferably 0.5 to 2 mol/. In the method of the present invention, in order to perform a dispersant treatment on the cobalt-containing magnetic iron oxide particles obtained as described above, the slurry for adhering the cobalt-containing compound is filtered and washed with water to remove alkaline components, etc. The resulting wet cake can be either (1) suspended in water and served as an aqueous slurry, or (2) placed in a sealed container and heat-treated at 60 to 200°C in the presence of water vapor. Thereafter, this treated cake may be suspended in water to provide an aqueous slurry. Note that the heat treatment is preferably performed in a non-oxidizing atmosphere. Furthermore, (3) after performing normal drying to remove the adhering moisture, the
Dry heat treatment may be carried out at .degree. C., and the treated powder may be suspended in water to provide an aqueous slurry. Alternatively, (4) the product may be provided as an aqueous slurry after the steam treatment in (2) above and then the dry heat treatment in (3) above, and either method can be applied.
When heat treatment is performed as described above, a cobalt ferrite layer can be grown more uniformly on the surface of the basic particles, and a high coercive force with better squareness ratio, orientation, and saturation magnetic flux density can be obtained. By subjecting this material to a dispersant treatment, the properties of the cobalt-containing magnetic iron oxide are sufficiently maintained in the recording medium, making it possible to form an extremely high-performance recording medium. In the dispersant treatment in the method of the present invention, the magnetic iron oxide powder to which the dispersant treatment is applied is the magnetic iron oxide basic particle coated with the cobalt-containing compound as described above, but the specific surface area is smaller than the BET value. in
It is particularly suitable for fine particle powders such as 35 m ² /g or more. Common R ¹ NH for treatment
(CH ₂ ) _o NH ₂ (R ¹ is an alkyl group having 6 or more carbon atoms,
As the water-soluble diamine compound represented by an alkenyl group or an aryl group (n is an integer of 1 to 10), various compounds within the above range can be used, but preferably carbon atoms of the alkyl group, alkenyl group, or aryl group are used. Preferably, the number is from 6 to 20, and n is an integer from 2 to 6. Specific examples thereof include (1) C ₆ H ₁₃ NH (CH ₂ ) ₃ NH ₂ , (2) C ₈ H ₁₇ NH (CH ₂ ) ₃ NH ₂ , (3) C ₁₀ H ₂₁ NH (CH ₂ ) ₃ NH ₂ , (4) C ₁₂ H ₂₅ NH (CH ₂ ) ₃ NH ₂ , (5) C ₁₄ H ₂₉ NH (CH ₂ ) ₃ NH ₂ , (6) C ₁₄ H ₂₉ NH (CH ₂ ) ₄ NH ₂ , (7) C ₁₆ H ₃₃ NH (CH ₂ ) ₃ NH ₂ , (8) C ₁₈ H ₃₅ NH (CH ₂ ) ₃ NH ₂ , (9) C ₁₈ H ₃₇ NH (CH ₂ ) ₃ NH ₂ , (10) _{C18H37NH ( CH2} ) _{4NH2 , (} 11) _C20H41NH ( _CH2 ) _{4NH2 ,} (12 _{) C20H41NH} ( _CH2 ) _{6NH2 ,} (13)

Examples include hydrochloride, acetate, lactate, adipate, etc. of N-substituted tri-, tetra-, penta- or hexamethylenediamine such as [Formula]. The diamines may be used alone or as a mixture of two or more thereof. Furthermore, in the method of the present invention, the general formula
As the water-soluble fatty acid compound represented by R ² COOH (R ² is an alkyl group, alkenyl group, or aryl group having 6 or more carbon atoms), various compounds can be used, but preferably an alkyl group, an alkenyl group, or an aryl group is used. Preferably, the group has 6 to 20 carbon atoms. Specific examples thereof include, for example, C ₆ H ₁₃ COOH, C ₈ H ₁₇ COOH, C ₁₀ H ₂₁ COOH,
C ₁₁ H ₂₃ COOH, C ₁₃ H ₂₇ COOH, C ₁₅ H ₃₁ COOH,
C ₁₇ H ₃₃ COOH, C ₁₇ H ₃₅ COOH, C ₂₀ H ₄₁ COOH,

Examples include alkali metal salts, alkaline earth metal salts, and ammonium salts of fatty acids such as [Formula]. In the method of the present invention, in order to react the water-soluble compound of diamine and the water-soluble compound of fatty acid on the surface of the cobalt-containing magnetic iron oxide particles to coat the reaction product with the fatty acid salt of diamine,
First, the iron oxide particles are suspended in an aqueous medium such as water to form an aqueous slurry having a solid concentration of usually 100 to 200 g. Next, the water-soluble diamine compound and the water-soluble fatty acid compound described above are added to this aqueous slurry. The treatment agents may be added sequentially or simultaneously, but it is more preferable to add them sequentially and thoroughly mix each treatment agent into the aqueous slurry.The treatment is usually carried out at room temperature. However, the reaction may be carried out under heating if necessary. In addition, in the above reaction, the pH of the slurry in the reaction system is usually 6 to 13, especially
It is desirable that the pH range is from 6.5 to 12; if the pH is lower than 6, the deposited cobalt compound may elute, while if the pH is higher than 13, the reaction product may not be fully coated on the surface of the iron oxide particles. It tends to become uniform, which is undesirable. In the method of the present invention, the reaction product on the surface of the cobalt-containing magnetic iron oxide particles is formed by a reaction of about 1 to 2 moles of the fatty acid with respect to 1 mole of the diamine, and The reaction product is usually 0.1% based on the weight of the particles.
Preferably it is coated by ~10%, preferably 0.5-5%. The water-soluble diamine compound and the water-soluble fatty acid compound are added depending on the amount of coating, but the former is usually 0.2 to 2% and the latter 0.3 to 2% based on the weight of the iron oxide particles. Add 3%. In any case, if the amount added is too little than the above range, the desired effect cannot be obtained, and if the amount added is too much than the above range, for example, the performance of the magnetic coating film may be deteriorated due to blooming phenomenon, which is undesirable. . After the reaction treatment slurry has reacted for a predetermined period of time, it is filtered, washed with water, and dried to recover the treated product. In addition, if a dispersant treatment is performed after the cobalt-containing compound has been deposited without the heat treatment described above, the dry heat treatment may be followed by dry heat treatment in a non-oxidizing atmosphere, for example. The magnetic properties can be further enhanced by heat treatment such as the following. In addition, in the case of the dispersant treatment of the present invention, even if heat treatment is performed after the dispersant treatment, the treatment effect of the dispersant is not impaired in any way. Example 1 γ- with coercive force (Hc): 370 OE, axial ratio (L/W): 10, specific surface area (BET value): 40 m ² /g
Add 100g of acicular magnetic iron oxide fine powder of Fe ₂ O ₃ to 1 part of water.
to form an aqueous slurry, and while blowing nitrogen gas into the slurry at room temperature (30°C), add 63ml of a 0.85 mol/cobalt sulfate aqueous solution and 0.9 mol//
ferrous sulfate aqueous solution of 126 and then 10
Add 169 ml of an aqueous solution of sodium hydroxide (169 ml) and stir at room temperature (30°C) for 5 hours to precipitate cobalt and ferrous metal hydroxides on the surface of the iron oxide particles for deposition treatment. Ta. After filtering and washing the treated slurry with water, the resulting cake was placed in a container, sealed in an autoclave with water in another container, purged with nitrogen gas, and then placed in the presence of saturated steam at 130°C for 6 hours. heat treated. (The obtained cobalt-containing magnetic iron oxide powder had a specific surface area (BET value) of 35 m ² /g.) Next, the heat-treated wet cake was dispersed in water to form an aqueous slurry, and the main component was added to the slurry. Add 30 ml of an aqueous hydrochloride solution (concentration 2% by weight) of a diamine (DUOMIN T: manufactured by Lion Akzo) consisting of a mixture of exemplary compounds (7), (8) and (9), and at room temperature (30°C). Stir thoroughly. Next, 18.4 ml of an aqueous sodium oleate solution (concentration 5% by weight) was added and reacted to cause the reaction product to be adsorbed onto the surface of the cobalt-containing magnetic iron oxide particles. Note that the pH of the slurry during the reaction was 7. The reaction-treated slurry was filtered and washed with water, and then the wet cake was dried in a conventional manner to remove water content to obtain a product treated by the method of the present invention. (Sample A) Example 2 In Example 1, 16 ml of a sodium laurate aqueous solution (concentration 5% by weight) was used instead of the sodium oleate aqueous solution, and the hydrochloride aqueous solution of Duomine T (concentration 2% by weight) was used. A product treated by the method of the present invention was obtained in the same manner as in the same example except that 36 ml was used. (Sample B) Example 3 In Example 1, 12.7 ml of a sodium benzoate aqueous solution (concentration 5% by weight) was used instead of the sodium oleate aqueous solution, and the hydrochloride aqueous solution of Duomine T (concentration 2% by weight) was used. 41.8ml
A product treated by the method of the present invention was obtained by processing in the same manner as in the same example except for the use. (Sample C) Example 4 In Example 1, in place of the diamine water-soluble compound called DEUOMINE T, a diamine (DEUOMINE C, LION. 26.3 ml of a hydrochloride aqueous solution (concentration 2% by weight) of Akzo Corporation) and the above sodium oleate aqueous solution (concentration 5% by weight)
A product treated by the method of the present invention was obtained in the same manner as in the same example except that 19.9 ml was used.
(Sample D) Example 5 After coating a cobalt compound and a ferrous compound in the same manner as in Example 1, the treated slurry was filtered and washed with water, and the resulting wet cake was soaked in water. Dispersion formed an aqueous slurry of cobalt-containing magnetic iron oxide. Example 1 for the slurry
The adsorption treatment of the dispersant was carried out in the same manner as in the case of . Note that the pH of the slurry during the adsorption treatment was 10. Next, the adsorption-treated slurry was filtered and washed with water, and the resulting wet cake was heat-treated at 130°C for 6 hours in the presence of saturated steam in the same manner as in Example 1 to obtain a product treated by the method of the present invention. Obtained. (Sample E) Example 6 A product treated by the method of the present invention was treated in the same manner as in Example 5, except that the slurry pH of the dispersant adsorbent was adjusted to 12. Obtained. (Sample F) Example 7 In Example 1, the slurry coated with a cobalt compound and a ferrous compound was filtered and washed with water, and then the resulting wet cake was heated with saturated steam, 5 at 130℃ in nitrogen gas atmosphere
A product treated by the method of the present invention was obtained in the same manner as in the same example except that it was subjected to a dry heat treatment for a period of time. (Sample G) Example 8 In Example 5, after performing the adsorption treatment of the dispersant, the treated slurry was filtered and washed with water, the resulting wet cake was dried by a conventional method, and then placed in a nitrogen gas atmosphere. A product treated by the method of the present invention was obtained in the same manner as in the same example except that it was dry heat treated at 130°C for 5 hours. (Sample H) Example 9 γ- having coercive force (Hc): 340 Oe, axial ratio (L/W): 10, specific surface area (BET value): 45 m ² /g
Add 100g of acicular magnetic iron oxide fine powder of Fe ₂ O ₃ to 1 part of water.
to form an aqueous slurry, add 169 ml of a 10 mol/aqueous sodium hydroxide solution to the slurry while blowing nitrogen gas at room temperature (30°C),
Next, 126 ml of a 0.9 mol/aqueous ferrous sulfate solution was added, followed by 63 ml of a 0.85 mol/aqueous cobalt sulfate solution, and the mixture was stirred at room temperature (30°C) for 5 hours to coat the surface of the iron oxide particles with ferrous sulfate. The hydroxide of cobalt was precipitated and then the hydrate of cobalt was precipitated and deposited. The adhesion treatment slurry was filtered and washed with water, and the wet cake was dried by a conventional method, and then dry heat treated at 120° C. for 5 hours in a nitrogen gas atmosphere. 100 g of the obtained magnetic iron oxide powder coated with the cobalt-containing compound (specific surface area, BET value 41 m ² /g) was dispersed in 1 part of water to form an aqueous slurry, and the slurry was subjected to the above-mentioned procedure. In the same manner as in Example 1, an aqueous hydrochloride solution (concentration 2% by weight) of Duomine T (manufactured by Lion Akzo) 39.6
ml and 24.2 ml of a sodium oleate aqueous solution (concentration 2% by weight) were added and reacted, and the reaction product was adsorbed onto the surface of cobalt-containing magnetic iron oxide particles to obtain a treated product by the method of the present invention. Ta. (Sample) Example 10 In the above Example 9, 39.6 ml of an aqueous hydrochloride solution (concentration 2% by weight) of oleyl trimethylene diamine (exemplified compound (8)) was used in place of the diamine water-soluble compound called Duomine T. Besides,
A treated product was obtained by the method of the present invention by processing in the same manner as in the same example. (Sample J) Example 11 γ- having coercive force (Hc): 335 Oe, axial ratio (L/W): 10, specific surface area (BET value): 50 m ² /g
Add 100g of acicular magnetic iron oxide fine powder of Fe ₂ O ₃ to 1 part of water.
to form an aqueous slurry, and add 181 ml of a 10 mol/aqueous sodium hydroxide solution to the slurry under stirring while blowing nitrogen gas into the slurry at room temperature (30°C).
was added, then 139 ml of a 0.90 mol/ferrous sulfate aqueous solution, and further 70 ml of a 0.85 mol/cobalt sulfate aqueous solution, and the mixture was heated at room temperature (30°C) for 50 minutes.
The cobalt compound and ferrous compound were deposited by stirring for a period of time. Next, the adhesion treatment slurry was filtered and washed with water, and then the wet cake was dry heat treated at 125° C. for 4 hours in a nitrogen gas atmosphere. Obtained cobalt-containing magnetic iron oxide powder (specific surface area, BET value
46 m ² /g) was dispersed in 1 part of water to make an aqueous slurry, and to this slurry was added an aqueous hydrochloride solution of Duomine T (manufactured by Lion Akzo) in the same manner as in Example 1 above. 79.0 ml (concentration: 2% by weight) was added and thoroughly stirred at room temperature. Next, 48.4 ml of an aqueous sodium oleate solution (concentration 5% by weight) was added, and the reaction product was adsorbed onto the surface of the cobalt-containing magnetic iron oxide particles to obtain a product treated by the method of the present invention. Note that the pH of the slurry during the adsorption treatment was 7. (Sample K) Comparative Example 1 The same treatment as in Example 1 was carried out except that the dispersant treatment with diamine and fatty acid was not performed. (Sample L) Comparative Example 2 A sample was treated in the same manner as in Example 1 except that the pH of the slurry during the dispersant adsorption treatment was set to 5. (Sample M) Comparative Example 3 In Example 5, after applying the cobalt-containing compound, the treated slurry (alkali concentration;
The treatment was carried out in the same manner as in the same example, except that the dispersant was adsorbed without filtering 1 mol/OH group and washing with water. (Sample N) Comparative Example 4 The same treatment as in Example 9 was carried out except that the sodium oleate aqueous solution was not added during the dispersant adsorption treatment. (Sample P) Comparative Example 5 In Example 9, instead of treating the diamine and fatty acid with a dispersant, an aqueous hydrochloride solution of an alkyl monoamine compound (manufactured by Lion Akzo; Armin CD; coconut oil fatty acid-based alkyl-substituted monoamine) was used. The treatment was carried out in the same manner as in the same example except that 100 ml of (concentration 2% by weight) was used. (Sample Q) Comparative Example 6 The wet cake obtained by applying the cobalt-containing compound in Example 1 was heat-treated at 130°C for 6 hours in the presence of saturated steam, and the heat-treated cake was dispersed in water. To the aqueous slurry prepared, 75.9 ml of a hydrochloride aqueous solution (20% by weight) of a diamine fatty acid salt (manufactured by Lion Akzo; Duomin TDO; oleate salt of beef tallow fatty acid-based alkyl-substituted trimethylenediamine) was added. After thorough stirring at room temperature (30°C), the treated slurry was filtered and washed with water, and the resulting wet cake was dried in a conventional manner. (Sample R) Duomin TDO is water-insoluble and difficult to mix with the aqueous slurry, resulting in virtually no treatment effect. Also, the aqueous hydrochloride solution of Duomin TDO is viscous and difficult to mix with the aqueous slurry. Uniform mixing was not easy. Comparative Example 7 The same treatment as in Example 11 was carried out except that the dispersant treatment with diamine and fatty acid was not performed. (Sample S) Comparative Example 8 In Example 1, diamine fatty acid salt (manufactured by Lion Akzo; Duomin TDO) was added to the cobalt-containing magnetic iron oxide powder (Sample L).
When preparing the magnetic paint described later, 2 parts by weight were added to 100 parts by weight of magnetic iron oxide powder to form a paint. The samples obtained in the above Examples and Comparative Examples were mixed in the following proportions and kneaded with a sand grinder to prepare magnetic paints. (1) Magnetic iron oxide powder 100 parts by weight (2) Vinyl resin (vinyl chloride: vinyl acetate copolymer) 21〃 (3) Urethane resin 5〃 (4) Methyl ethyl ketone 123〃 (5) Toluene 123〃 (6) Cyclohexane 4. Each of the magnetic coatings prepared as described above was applied onto a polyester film, subjected to orientation treatment, and then dried to produce a magnetic recording medium having a magnetic coating film approximately 6 μm thick. Regarding this recording body, the coercive force (Hc), residual magnetic flux density (Br), squareness ratio (Br/Bm), and orientation ratio (OR) were determined as the magnetic properties by the usual method, and the 400Hz output for an input of -20dB was determined as the electromagnetic conversion characteristics. , were measured respectively. The results are shown in the table below.

【table】

[Table] Note that in the case of Comparative Example 2, the Hc was significantly lower than that before treatment with the dispersant, so no other measurements were performed. As is clear from the results in the above table, the cobalt-containing magnetic iron oxide powder obtained by the method of the present invention is
The dispersibility is improved compared to the comparative example, and as a result
It can be seen that the magnetic properties of Br, Br/Bm, and OR, and the electromagnetic conversion properties of 400Hz output are all improved, making it extremely suitable for producing high-density recording media. In addition, since the cobalt-containing magnetic iron oxide powder obtained by the method of the present invention has the dispersant strongly adsorbed, it is less prone to blooming and falling of the magnetic powder (magnetic head) than when a dispersant is added at the time of coating. clogging) was hardly observed. (Effects of the invention) As is clear from the above explanation, the method of the present invention has the following effects:
Fine particles of cobalt-containing magnetic iron oxide with a strong cohesive property and a large specific surface area are suspended in an aqueous solution, and a diamine fatty acid salt dispersant obtained by a reaction between a diamine water-soluble compound and a fatty acid water-soluble compound is added to the surface of the particles. Since the dispersant is reacted and adsorbed on the particles, the dispersant can be strongly supported, and the cohesiveness of the particles can be extremely effectively modified. When a magnetic recording medium is made using cobalt-containing magnetic iron oxide powder obtained by the method of the present invention,
High-density recording with significantly improved magnetic properties and electromagnetic conversion properties is achieved by forming a highly packed magnetic layer with good orientation due to good wettability with the binder resin of the recording medium and uniform dispersion. It can be a medium. Furthermore, in the method of the present invention, the aqueous slurry after the deposition treatment of the cobalt-containing compound of the magnetic iron oxide can be heat-treated and the adhesion layer of the cobalt-containing compound can be subjected to the transformation treatment at any time, thereby reducing the complexity of the process. In addition, agglomeration during heat treatment can be effectively avoided.

Claims (1)

[Claims] 1 In an aqueous slurry of cobalt-containing magnetic iron oxide particles having a specific surface area with a BET value of 35 m ² /g or more, the general formula R ¹ NH (CH ₂ ) _o NH ₂ (wherein R ¹ is carbon number 6
At least one water-soluble diamine compound represented by the above alkyl group, alkenyl group or aryl group, where n is an integer of 1 to 10) and the general formula R ² COOH (wherein R ² is the number of carbon atoms) 6 or more alkyl groups, alkenyl groups, or aryl groups) by reacting with at least one water-soluble fatty acid compound represented by 6 or more alkyl groups, alkenyl groups, or aryl groups, and adsorbing the reaction product fatty acid salt of diamine on the surface of the iron oxide particles. A method for producing cobalt-containing magnetic iron oxide powder, the method comprising: