JPS5941289B2 - Manufacturing method of magnetic iron oxide - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to magnetic iron oxide powder for magnetic recording media suitable for obtaining magnetic recording tapes. Magnetic iron oxide powder is suitable for obtaining magnetic recording media with little change over time, and in particular, divalent metals such as cobalt (hereinafter referred to as cobalt) are added to impart magnetic anisotropy and create a large coercive force. The present invention relates to a method for producing magnetic iron oxide powder.

Conventionally, as magnetic iron oxide powder for magnetic recording media, magnetic iron oxide powder added with cobalt (or the like) has been proposed.

This is an attempt to obtain magnetic powder with larger coercive force by incorporating cobalt (or the like) into magnetic iron oxide powder, and to obtain a magnetic recording medium capable of higher density recording.
For this reason, various methods of adding cobalt (etc.) have been proposed. (For example, Tokuko 15638-15638, Tokuko 48
-10994, Japanese Patent Publication No. 41-6538, U.S. Patent No. 3,
573, 980, etc.) However, when a magnetic recording medium such as a magnetic tape is formed using gamma iron monoxide or magnetite to which cobalt (etc.) is added, these magnetic recording bodies are mechanically It has the disadvantage of being magnetically unstable against external pressure, impact, etc.

That is, a phenomenon occurs in which the information signal recorded on the magnetic recording medium is weakened by mechanical pressure and impact.

This is because when a recorded magnetic tape is played back many times, the S/N ratio deteriorates with each playback, which is an extremely undesirable phenomenon for magnetic recording media. This phenomenon is small in iron oxides, chromium oxides, etc. that do not contain cobalt (or the like), but is particularly remarkable in magnetic iron oxides that contain cobalt (or the like). These phenomena are described in detail in the following literature: (i) Suzuki, Akashi: "Television"
18, P767 (1964) (i■) Kuroe, Kobayashi, Sugaya: "Magnetic Recording Research Group Materials" Material No. MR72-19 (1
(September 972).

Cobalt (etc.) has thermal and mechanical instability.
Various studies have been conducted to improve added magnetic iron oxide. Regarding the thermal stability of cobalt (etc.)-doped magnetic iron oxide, by adding metal ions other than cobalt ions, stable cobalt (etc.)-doped magnetic iron oxide with less reversible change in coercive force during heating can be achieved. I'm getting iron.
(For example, Japanese Patent Publication No. 43-12175, Japanese Patent Publication No. 482729
8, JP-A No. 48-27299, etc.).

Further, in US Pat. No. 3,573,980, by improving the method of adding cobalt (etc.) and reducing it, a material with less decrease in magnetization upon temperature rise was obtained. However, sufficient measures have not yet been taken to improve magnetic instability due to mechanical shock, pressure, etc., and to improve changes in coercive force over time, which is the biggest problem with cobalt (etc.)-added magnetic iron oxide. Improvements were desired.

The first object of the present invention is to provide a new method for producing cobalt (etc.)-added magnetic iron oxide that improves the instability against mechanical pressure and impact, which was the biggest drawback of conventional cobalt (etc.)-added magnetic iron oxide. There is a particular thing.

The second object is to provide a method for producing magnetic iron oxide with little change in coercive force over time.

While researching cobalt (etc.)-added magnetic iron oxide, the present inventors gradually oxidized cobalt (etc.)-added magnetite, and at a certain appropriate degree of oxidation, cobalt (etc.) is extremely stable against mechanical pressure and impact. It was discovered that there is an added magnetic iron oxide.

Here, the oxidation degree is y: the amount of cobalt (etc.) contained in magnetic iron oxide (AlO
m%) ・”R: Defined as the ratio of divalent iron ions to all iron ions constituting magnetic iron oxide.

One of the features of the present invention is that cobalt (etc.)-containing magnetite is gradually oxidized at a relatively low temperature (room temperature to 100°C or less).

According to the method described in U.S. Patent No. 3,573,980, the coercive force of magnetic powder is increased by adding a small amount of cobalt to γ-Fe2O3 and reducing a part of it to FeO. is listed.

This obtains a large coercive force with a small amount of cobalt, and the coercive force is further increased by making a part of it FeO. The present invention is based on the discovery that when cobalt-containing magnetite is gradually oxidized, the demagnetization stability against mechanical pressure is significantly improved at an appropriate degree of oxidation. This is a completely new fact.

That is, one of the aspects of the present invention is that magnetic iron oxide containing a divalent metal has an oxidation degree of 30 to 80 as represented by the following general formula.
In a magnetic iron oxide formula characterized by %, y
is the amount of divalent metal contained in magnetic iron oxide (AtOm%)
, R represents the ratio of divalent iron ions to the total iron ions constituting the magnetic iron oxide.

].

Another aspect of the present invention is to obtain magnetite containing divalent metals by dehydrating and reducing goethite containing divalent metals, and gradually oxidizing this at a relatively low temperature (below 100°C). This is a method for producing magnetic iron oxide containing a divalent metal with an oxidation degree of 30 to 80%.

If the cobalt (etc.)-added magnetic iron oxide according to the present invention has a cobalt (etc.) content of 0.5at0m% or more, it will be compared with the conventionally used cobalt (etc.)-added gamma iron monoxide or cobalt (etc.)-added magnetite. , the amount of demagnetization was significantly reduced.

In this case, when the cobalt (or the like) content is 0.5 at 0 m% or less, even if cobalt (or the like) is added, the effect of increasing the coercive force of the magnetic iron oxide is small, and the effect of the present invention was not so recognized. Therefore, the coercive force of the magnetic iron oxide obtained by the present invention is 3000e or more, and from a practical standpoint, the upper limit is 15
The amount of cobalt added can be controlled up to about 000e.

For example, when the degree of oxidation is about 50%, cobalt is 1.5at
When adding 0m%, it becomes 5100e. When adding 0m%, it becomes 5100e. When adding 0m%, it becomes 6400e. When adding 0m%, it becomes 7800e.
ferromagnetic powder was obtained. The magnetic material used in the present invention includes 0.5at0 of cobalt in magnetic iron oxide fine powder.
In addition to adding m% or more, Mg and Cu are added to cobalt.
, Cr, Mn, Ni, Zn, MO, Sn, Sb, Te,
It is also possible to use metals such as Rh, Ba, La, Ce, W, and Bi, individually or in combination.

Magnetic iron oxide fine particles containing these metals may be either acicular or granular.

As described above, the magnetic material used in the present invention is magnetic iron oxide fine particles containing 0.5 at 0 m% or more of cobalt (etc.), and may be either acicular or granular.

In the case of needle-like shape, it is desirable that the major axis is 0.1 μm or more and 1 μm or less. In the case of granular particles, the upper limit of the size is limited by an increase in noise when used as a magnetic recording medium, and the lower limit is limited by a decrease in the transfer effect or an increase in the washing and permeation time during production. Acicular iron oxide is disclosed in U.S. Patent No. 3,117,933.
No. 3,720,618, Special Publication No. 41-6538
Manufactured by methods such as

Granular iron oxide is prepared by adding an alkali such as sodium hydroxide, potassium hydroxide, ammonium hydroxide, etc. to an aqueous solution containing Fe(), CO(), etc. mixed in the desired ratio and keeping the pH of the solution at 10 or higher at 20 to 100°C. It is obtained by oxidizing with air, oxygen, oxidizing agents such as nitrates, etc. at a liquid temperature of .

The iron oxides thus obtained (Fe3O4・NH2O, α-FeOO, each containing a predetermined amount of cobalt (etc.))
H or α-Fe2O3, Fe3O4 fired from it
, γ-Fe2O3 with hydrogen, city gas, etc.
Reduce it to Fe3O4 state at 0°C, preferably around 350 elemental C, cool it to near room temperature while flowing N2 gas to prevent it from coming into contact with air, take it out, and cover it all with water. .

For example, it is effective to remove excess water by spraying water, or by immersing the entire body in water and then passing it through the water. The moisture content of this treated material is usually 50 to 200% based on dry iron oxide. Instead of this water, an aqueous solution of an oxidizing agent such as nitric acid, sodium nitrate, or ammonium nitrate, an aqueous solution of a dilute alkali such as sodium hydroxide or ammonium hydroxide, or an aqueous solution of a dilute acid such as hydrochloric acid or sulfuric acid may be used. In this case, it is desirable to wash thoroughly with water after treatment so that these salts do not remain in the next step. The thus prepared water-wet magnetite containing cobalt (etc.) is treated in a normal pressure air constant temperature bath kept at a temperature of 40 to 100°C.

At temperatures below 40°C, it takes more than one week to produce the effects of the present invention, which is not industrially practical, and at temperatures above 100°C, oxidation proceeds rapidly and the heat generated further oxidizes, making it impossible to obtain uniform properties.

Under these conditions necessary to obtain the effects of the present invention, the treatment time in the constant temperature bath is 10 minutes or more and one week or less. As the binder used in the heather invention, conventionally known thermoplastic resins, thermosetting resins, reactive resins, and mixtures thereof are used.

The thermoplastic resin has a softening temperature of 150°C or less, an average molecular weight of 10,000 to 200,000, and a degree of polymerization of about 20.
0 to about 1000, such as vinyl chloride monovinyl acetate copolymer, vinyl chloride monovinylidene chloride copolymer,
Vinyl chloride-acrylonitrile copolymer, acrylic ester-acrylonitrile copolymer, ester allylate-vinylidene chloride copolymer, acrylic ester-styrene copolymer, methacrylate ester-acrylonitrile copolymer, ethyl methacrylate-chloride vinylidene copolymer, methacrylic acid ester-styrene copolymer,
Urethane elastomer, nylon-silicon resin, nitrocellulose-polyamide resin, polyvinyl fluoride, vinylidene chloride-acrylic acid copolymer, butadiene-acrylonitrile copolymer, polyamide resin, polyvinyl butyral, cellulose derivative (cellulose) acetate butyrate, cellulose diacetate, cellulose triacetate, cellulose propionate, nitrocellulose, etc.), styrene-butadiene copolymer, polyester resin, chlorovinyl ether-acrylic acid ester copolymer, amino resin, various synthetic rubber-based heat Plastic resins, mixtures thereof, etc. are used.

Examples of these resins are given in Japanese Patent Publication No. 37-6877, 39-
No. 12528, No. 39-19282, No. 40-5349, No. 40-20907, No. 41-9463, No. 41-14
No. 059, No. 41-16985, No. 42-6428, 4
2-11621, 43-4623, 43-1520
No. 6, No. 44-2889, No. 44-17947, 44-
No. 18232, No. 45-14020, No. 45-14500
No. 47-18573, No. 47-22063, 47-2
It is described in publications such as No. 2064, No. 47-22068, No. 47-22069, No. 47-22070, and No. 47-27886.

The thermosetting resin or reactive resin has a molecular weight of 200,000 or less in the state of a coating liquid, and when heated after coating and drying, the molecular weight becomes infinite due to reactions such as condensation and addition.

Also, among these resins, those that do not soften or melt before the resin is thermally decomposed are preferred. Specifically, examples include phenolic resins, epoxy resins, polyurethane curable resins, urea resins, melamine resins, alkyd resins, silicone resins, acrylic reaction resins, epoxy-polyamide resins, nitrocellulose melamine resins, high molecular weight polyester resins, and isocyanate resins. Mixtures of polymers, mixtures of methacrylate copolymers and diisocyanate prepolymers, mixtures of polyester polyols and polyisocyanates, urea formaldehyde resins, mixtures of low molecular weight glycols/high molecular weight diols/triphenylmethane triincyanate, polyamine resins and mixtures thereof. These resins are disclosed in Japanese Patent Publications No. 39-8103, No. 40-9779, No. 41-7192, No. 41-810.
No. 6, No. 41-14275, No. 42-18179, 43
-12081, 44-28023, 4514501
No. 45-24902, 46-13103, 47-
No. 22065, No. 47-22066, No. 47-22067
No., 4722072, 47-22073, 47-2
It is described in publications such as No. 8045, No. 47-28048, and No. 47-28922.

In addition to the use of these binders alone or in combination, additives may be added.

The mixing ratio of the ferromagnetic powder and the binder is in the range of 10 to 200 parts by weight of the binder per 100 parts by weight of the ferromagnetic powder. Additives include dispersants, lubricants, abrasives, antistatic agents, and the like.

Dispersants include chifurylic acid, chipurinic acid, lauric acid,
Fatty acids with 12 to 18 carbon atoms such as myristic acid, palmitic acid, stearic acid, oleic acid, elaidic acid, linoleic acid, linolenic acid, and stearolic acid
, Rl is an alkyl group or alkenyl group having 11 to 17 carbon atoms): an alkali metal of the fatty acid (Li, Na,
K, etc.) or alkaline earth metals (Mg, Ca, Ba, etc.)
Publication, Japanese Patent Application No. 42-28641, Japanese Patent Application No. 438154
No. 3, US Pat. No. 3,423,233, Japanese Patent Publication No. 47-28043, etc.

As the abrasive, fused aluminum nine silicon carbide, chromium oxide, corundum, artificial corundum, diamond, artificial diamond, garnet, emery (main components: corundum and magnetite), etc. are generally used.

These abrasives have an average particle diameter of 0.05 to 2 μm, particularly preferably 0.1 to 2 μm.
belongs to. These abrasives are added in an amount of 7 to 20 parts by weight based on 100 parts by weight of the binder. These are described in Japanese Patent Application No. 48-26749. Antistatic agents include natural surfactants such as saponin, nonionic surfactants such as alkylene oxide, glycerin, and glycidol, higher alkylamines, quaternary ammonium salts, pyridine and other heterocyclic rings, phosphonium, or sulfonate. Metal soap consisting of lecithin, etc. is used. In addition, higher alcohols having 12 or more carbon atoms and sulfuric esters thereof can also be used. These dispersants are added in an amount of 1 to 20 parts by weight per 100 parts by weight of the binder. Such materials are disclosed, for example, in Japanese Patent Publication No. 39-28369, Japanese Patent Publication No. 44-17945, Japanese Patent Publication No. 15001-1982, US Patent No. 3387993, US Pat.
This is indicated in the specification etc.

Lubricants include silicone oil, graphite, molybdenum disulfide, tungsten disulfide, fatty acid esters consisting of monobasic fatty acids with 12 to 16 carbon atoms and monohydric alcohols with 3 to 12 carbon atoms, and 17 carbon atoms. The total number of carbon atoms is 21 or more monobasic fatty acids and the number of carbon atoms in the fatty acids.
Fatty acid esters consisting of ~23 monohydric alcohols, etc. can be used.

These lubricants contain 0.00 parts by weight per 100 parts by weight of binder.
It is added in an amount of 2 to 20 parts by weight. Regarding these, cationic surfactants such as Japanese Patent Publication No. 43-23889, anionic surfactants containing acidic groups such as carboxylic acids, sulfonic acids, phosphoric acids, sulfuric acid ester groups, phosphoric ester groups, amino acids, amino sulfones, etc. Acids, amphoteric activators such as sulfuric acid or phosphoric acid esters of amino alcohols, etc. are used. Some examples of surfactant compounds that can be used as antistatic agents are disclosed in U.S. Patent No. 2,271,623, U.S. Pat.
No. 2,676,122, No. 2,676,92
No. 4, No. 2,676,975, No. 2,691,5
No. 66, No. 2,727,860, No. 2,730,
No. 498, No. 2,742,379, No. 2,739
, No. 891, No. 3,068,101, No. 3,15
No. 8,484, No. 3,201,253, No. 3,2
No. 10,191, No. 3,294,540, No. 3,
No. 415,649, No. 3,441,413, No. 3
, 442,654, 3,475,174, 3,545,974, West German Patent Publication No. 1,942
, 665, British Patent No. 1,077,317, British Patent No. 1
, No. 198, 450, "Synthesis of Surfactants and Their Applications" by Ryohei Oda et al. (Maki Shoten 1964 edition), A. W. Perry, "Surf S Act 4 Agents" (Interscience Publishing, Inc. 1958 edition), T. P. Encyclopedia of Surf S Active Agents Vol. 2 by Hajime Sisli (Chemical Publishing Company, 1964 edition), Surfactant Handbook, 6th edition (Sangyo Tosho Co., Ltd., December 20, 1964) It is written in books such as

These surfactants may be added alone or in combination.

These are used as antistatic agents, but are sometimes used for other purposes, such as dispersion, improving magnetic properties,
It may also be applied as a coating aid to improve lubricity. To form the magnetic recording layer, the above-mentioned composition is dissolved in an organic solvent and applied as a coating solution onto a support.

When used as a tape, the thickness of the support is 2.5 to 1
The thickness is about 00 μm, preferably about 3 to 40 μm. Materials include polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyolefins such as polypropylene, cellulose derivatives such as cellulose triacetate and cellulose diacetate, vinyl resins such as polyvinyl chloride, plastics such as polycarbonate, and aluminum. Metals such as copper, ceramics such as glass, etc. are also used. Organic solvents used during kneading and coating include ketone types such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; alcohol types such as methanol, ethanol, P-CJ/manol, and butanol; methyl acetate, ethyl acetate, and butyl acetate. , ethyl lactate, glycol monoethyl acetate, and other esters; ether, glycol dimethyl ether, glycol monoethyl ether, dioxane, and other glycol ethers; benzene, toluene, xylene, and other aromatic hydrocarbons; methylene chloride, ethylene chloride, Chlorinated hydrocarbons such as carbon chloride, chloroform, ethylene chlorohydrin, and dichlorobenzene can be used.

Methods for applying the magnetic recording layer onto the support include air doctor coating, blade coating, air knife coating, squeeze coating, impregnation coating, reverse roll coating, transfer roll coating, gravure coating, scratch coating, cast coating, and spray coating. can be used, and other methods are also possible, and their specific explanations are described in detail in "Coating Engineering" published by Asakura Shoten, pages 253 to 277 (published on March 20, 1972).

Next, the characteristics of the magnetic iron oxide described above will be explained with reference to the drawings.

FIG. 1 shows the relationship between the degree of oxidation and the demagnetization ratio of cobalt-containing magnetic iron oxide that was mildly oxidized at 60°C.

The amount of demagnetization is the amount of demagnetization in a magnetic tape made using magnetic iron oxide with a certain degree of oxidation compared to the amount of demagnetization in a magnetic tape made using magnetic iron oxide with a certain degree of oxidation in the same manner as the magnetic tape. It shows the percentage. Here, the amount of demagnetization refers to the playback output after recording a signal on a magnetic tape and running it for a certain period of time while pressurizing the magnetic tape with a roller adjusted to a constant pressure, and the playback output immediately after recording. It shows the ratio of the difference in playback output immediately after recording. The experimental results shown in Figure 1 show that cobalt is 4at0
This is an example of magnetic iron oxide containing cobalt (or the like) at 0 m% or more, but a magnetic iron oxide containing cobalt (or the like) that contains cobalt (or the like) at 0 m% or more exhibits the same tendency as shown in FIG.

That is, it was observed that the amount of demagnetization became smaller at an oxidation degree of 30 to 80%. The cobalt (etc.)-containing magnetic iron oxide according to the present invention is a cobalt (etc.)-added γ
- Compared to magnetite doped with iron oxide or cobalt (etc.), the amount of demagnetization is significantly smaller and information signals can be stably recorded and stored.

Next, the present invention will be explained using specific examples.

In each example, all parts indicate parts by weight. Example 1
All of the magnetic tapes obtained in Examples 1 to 5 and Comparative Examples 1 to 2 were subjected to a demagnetization tester to measure the amount of demagnetization. Example 1 Ferrous sulfate for reagent 200f! Dissolved in 600ml of water, 28.89ml of reagent grade sodium hydroxide and 180ml of water.
The mixture was added to a solution that had been dissolved to a volume of 0 ml and reacted at room temperature by blowing air into it, and after the solution turned yellow, the precipitate was filtered and dispersed again in water in Step 21.

In this case, the core crystal has a diameter of 0.05 μm and a length of 0.5 μm, and contains 249 goethites. Next, the bath temperature was set to 50°C, and a 2N solution of sodium hydroxide was added to pH 6. Ferrous sulfate containing 5.2 mol% of cobalt sulfate while adjusting to O.
Molar solution 21 was added at a constant rate over 8 hours to grow crystals. When the crystal growth is finished, the diameter is 0.2 μm and the length is 1.
1 μm, and the yield of goethite is 1129. Analysis of the obtained goethite using fluorescent X-rays revealed that the cobalt content was 4.0%. The obtained cobalt-containing goethite was converted to cobalt-containing α-Fe2O3 at 300°C in an air stream, and then heated at 350°C in a hydrogen stream.
The mixture was reduced with water, cooled to room temperature, and taken out into the air. When taken out into the air, the surface layer of the cobalt-containing magnetite was stabilized by being gently oxidized at a low oxygen partial pressure to prevent rapid oxidation. The degree of oxidation of the cobalt-containing magnetite in this state was 5%. This cobalt-containing magnetite was placed in a dryer kept at 60°C and gradually oxidized for about 40 hours.

In this way, magnetic iron oxide with a coercive force of 7200e and an oxidation degree of 25% was obtained. This is called magnetic material A. Example 2 Cobalt-containing goethite obtained in the same manner as in Example 1 was heated to cobalt-containing α-F at 300°C in a flow of air.
After converting to e2O3, it was reduced at 350° C. in a hydrogen stream and cooled.

This was immersed in water without being exposed to air, then placed in a dryer at 60°C and dried for 40 hours, while being subjected to mild oxidation.
In this way, magnetic iron oxide with a coercive force of 7800e and an oxidation degree of 45% was obtained. This is called magnetic material B. Example 3 Cobalt-containing magnetite 60f obtained by the same method as Example 1! was dispersed in 300 ml of water, and 309 ml of ammonium nitrate was added.

A sodium hydroxide solution was added to this solution, and the mixture was stirred for 1 hour while keeping the pH at 9, and then washed with water. This was dried at 60°C for 24 hours. In this way, the coercive force 7500e
Magnetic iron oxide (containing cobalt) with an oxidation degree of 35% was obtained. This is called magnetic material C. Example 4 Cobalt-containing magnetite obtained by the same method as in Example 1 was immersed in a 0.1N hydrochloric acid solution for 30 minutes and then washed with water.

This was placed in a dryer at 60°C and dried for 40 hours, resulting in mild oxidation. In this way, magnetic iron oxide (containing cobalt) with a coercive force of 7800e and an oxidation degree of 55% was obtained. This is called magnetic material D. Example 5 Cobalt-containing magnetite obtained in the same manner as in Example 1 was placed in a dryer kept at 60°C and gradually oxidized for about 140 hours.

In this way, magnetic iron oxide (containing cobalt) with a coercive force of 6400e was obtained. This is called magnetic material E. Comparative Example 1 After completion of reduction to magnetite containing 4% cobalt, it was cooled and immersed in a solvent while avoiding exposure to air.

This is called magnetic material F. Comparative Example 2 Magnetite containing 4% cobalt was air oxidized at 250°C to obtain cobalt-containing γFe2O3.

This is called magnetic material G. Example 6 A composition was prepared for 300 parts each of magnetic iron oxide (cobalt-containing) A-G used in Examples 1 to 5 and Comparative Examples 1 to 2, and the mixture was sufficiently kneaded in a ball mill, and then added and mixed uniformly. It was dispersed and made into a magnetic paint.

This magnetic material was coated on a polyethylene terephthalate base having a thickness of 25 μm to a thickness of 101 tm after drying, and was oriented in a magnetic field at 10,000 e and dried to obtain a magnetic tape. The completed magnetic tape was subjected to a demagnetization tester and the amount of demagnetization after running 300 times was measured.

Table 1 shows the amount of demagnetization for each magnetic tape. Examples 1 to 5 (magnetic material names A, B, C, D, and E) are typical examples of slow oxidation, and by adjusting the temperature, time, atmosphere, etc. of slow oxidation, any degree of oxidation can be obtained. can be obtained.

Also, regardless of the method used, the relationship between the demagnetization amount ratio and the oxidation degree shown in FIG. 1 is the same. As can be seen from the above examples, when the magnetic iron oxide containing cobalt (etc.) of the present invention is used, it is magnetically stable against mechanical pressure impact, and a magnetic recording medium with little change over time such as coercive force can be obtained. suitable for getting.

It goes without saying that the magnetic iron oxide containing cobalt (etc.) of the present invention can be applied to magnetic recording tapes and magnetic sheet magnetic disks for video signals, audio signals, digital information, etc.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the degree of oxidation and the demagnetization amount ratio of magnetic iron oxide containing cobalt.

Claims (1)

[Scope of Claims] 1. A method for producing cobalt-containing magnetic iron oxide having an oxidation degree of 30 to 80% as represented by the following formula, which comprises gradually oxidizing cobalt-containing magnetite at 100° C. or lower. Oxidation degree = (1-(300-y/100-y)・R)×1
00 (%) [wherein, y is the amount of cobalt (atom%) contained in the magnetic iron oxide, and R is the ratio of divalent iron ions to the total iron ions constituting the magnetic iron oxide. ]