JP2933397B2 - Method for producing ferromagnetic iron oxide powder for magnetic recording - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improved magnetic powder used for a coating type high recording density magnetic recording medium and a method for producing the same, and more particularly, to a method for treating a surface of maghemite particles or intermediate particles with a trace amount of metal ions. The present invention relates to a ferromagnetic iron oxide powder having an improved saturation magnetization by being denatured and a method for producing the same.

Problems to be Solved by the Present Invention With the recent reduction in size and weight of magnetic recording / reproducing equipment and the increase in the amount of information processing in magnetic recording, higher recording density, higher output, and lower noise of magnetic recording media have been achieved. Increasingly, the direction of the shift is increasing. In conjunction with this, with respect to the magnetic iron oxide powder used for the magnetic recording medium, demands for improving various properties such as high coercive force, high saturation magnetization, fine particles and high orientation and high filling have been further increased.

[0003] By the way, in order to realize low noise, fine particles are the most effective means, but there is a problem that a decrease in the saturation magnetization cannot be avoided. In addition, a decrease in the orientation and the filling property also occur at the same time. In other words, in order to satisfy both the low noise and high output directions, it is necessary to realize the production of magnetic iron oxide that is compatible with the opposite properties of fine particles and high saturation magnetization.

[0004] Various attempts have been made to solve this problem. For example, there is a method in which the starting material of the cobalt-coated ferromagnetic iron oxide powder is converted from maghemite particles to intermediate particles or magnetite particles to achieve a high saturation magnetization. However, the saturation magnetization is still insufficient.

An object of the present invention is to provide a method for producing a ferromagnetic iron oxide powder suitable for a coating type high-density recording medium having fine particles and a high saturation magnetization in order to solve the above problems. It is in. This novel method for producing magnetic iron oxide powder has a process of applying manganese, zinc, iron or manganese, zinc to maghemite particles or hydrated iron oxide particles of a precursor to be maghemitized and heat-treating them under a specific atmosphere. There is an advantage in that maghemite particles or intermediate particles exhibiting a high saturation magnetization can be obtained because the particle crystals contain the metal ions and the surface of the particles is transformed with the manganese zinc ferrite layer. In this specification, “intermediate particles” refer to intermediate particles of maghemite and magnetite. Further, by applying a cobalt compound or a compound of cobalt and a ferrous salt, the magnetic properties can be further improved.

One preferred method for obtaining the magnetic iron oxide powder of the present invention is as follows:
It is as follows. (1) A specific amount of manganese, zinc and optionally iron is applied to the maghemite particles and heat-treated in a non-oxidizing atmosphere, or (2) a specific amount of manganese, zinc and optionally iron is applied to the maghemite particles. Treated and heat-treated in an oxidizing or non-oxidizing atmosphere and then re-reducing to form an intermediate, or (3) manganese and zinc are added to the hydrated iron oxide particles of the precursor to be maghemitized in a specific amount. Hematite particles obtained by deposition and heat dehydration are reduced and oxidized to maghemite, or reduced and slightly oxidized, or reduced and oxidized to maghemite and then re-reduced. By forming the particles into an intermediate, maghemite particles or intermediate particles having a high saturation magnetization and having a particle surface modified with a manganese zinc ferrite layer can be obtained. (4) The maghemite particles or intermediate particles of the above (1) to (3) are treated with a solution containing a cobalt compound or a compound of cobalt and a ferrous salt to further increase the coercive force and the saturation magnetization to achieve high recording. It can be finished to be very suitable for a magnetic tape. The cobalt-coated magnetic iron oxide powder of the present invention also has a feature that the coercive force development and orientation characteristics after the cobalt-depositing process are excellent as compared with those using maghemite particles that are not denatured by a conventional metal as a starting material. Have. Hereinafter, details of the method for obtaining the magnetic iron oxide powder of the present invention will be described in the order of (1) to (4).

(1) To an aqueous suspension of maghemite particles is added an aqueous solution containing manganese, zinc and optionally an iron compound. Various compounds can be used as the manganese, zinc and iron components used in the present invention. For example, water-soluble compounds such as sulfates, nitrates and chlorides thereof are suitable. It is necessary to use ferrous manganese for the manganese component, but the effect of the present invention does not change even if ferrous or ferric iron is used for the iron component. Regarding the added amount, the manganese ion is 0.40 to 2.50 as Mn / Fe in maghemite particles modified with manganese, zinc and optionally iron ions.
Atomic weight%, preferably containing 0.80 to 2.30 atomic weight%, while zinc ion is 0.4% as Zn / Fe.
0 to 2.50 atomic weight%, preferably 0.80 to 2.30
It is an amount containing atomic weight%. Iron ions can be added without addition or up to twice the total addition amount of manganese and zinc ions on a molar basis. Further, the pH is adjusted to 10 to 11 by adding an alkali, and the metal ion is deposited as a hydroxide on the surface of the base particles, and then filtered, washed with water, and any excess alkali or the like remaining in the suspension is removed. Remove. Then, drying at 50 to 110 ° C. is performed in the air. Subsequently, heat treatment is performed in a non-oxidizing atmosphere. The reason why the atmosphere is made non-oxidizing is to prevent oxidation of the manganese ion. Usually, nitrogen and carbon dioxide are used, but helium,
A rare gas such as argon may be used. The heat treatment temperature is 350 to 525 ° C, preferably 400 to 500 ° C. When the heat treatment temperature is lower than the above range, a predetermined improvement in the saturation magnetization cannot be obtained, and when the heat treatment temperature is high, a transition to α-hematite may occur and the saturation magnetization may be greatly reduced. The heat treatment time is 0.5 to 6 hours, preferably 1.5 to 3 hours. If the heat treatment time is shorter than the above range, a predetermined saturation magnetization cannot be obtained. Conversely, even if the length is increased, the saturation magnetization cannot be further increased. In the stage before the heat treatment, the metal ions only exist as non-magnetic hydroxides or oxides on the surface of the base particles. However, the heat treatment causes the metal ions to form a solid solution inside the crystal. Therefore, it is considered that the saturation magnetization is improved because the manganese zinc ferrite layer is formed on the surface of the base particles.

(2) The process from the metal deposition process to the drying process is the same as in (1). The subsequent steps are as follows. Unlike the case (1), the heat treatment atmosphere may be a non-oxidizing atmosphere or an oxidizing atmosphere such as air. The reason why the oxidizing atmosphere may be used is that even if part of the manganese ions is oxidized to manganese ions or tertiary manganese ions by the heat treatment and the improvement of the saturation magnetization does not appear, This is because dimanganese ions and the like are reduced again to manganese ions and the effect of improving the saturation magnetization appears. However, when an attempt is made to obtain intermediate particles having a low content of ferrous ion by reducing the degree of re-reduction, reduction of manganese ion or the like to manganese ion in the re-reduction treatment is not possible. It may be sufficient, and the desired effect of improving the saturation magnetization may not be achieved. Therefore, the heat treatment atmosphere is preferably a non-oxidizing atmosphere.
The heat treatment temperature is 350 to 525 ° C as in the case of (1),
Preferably it is 400-500 degreeC. Subsequently, by applying a known method, a re-reduction treatment is performed to obtain Fe ²⁺
Finish to an intermediate containing up to 0.45 ferrous ions as / Fe ³⁺ . The method is, for example, a method of performing a reduction treatment at 300 to 350 ° C. under a flow of hydrogen gas. The content of ferrous ions can be adjusted to a desired level by adjusting the reduction time.

(3) An aqueous solution of a manganese and zinc compound is added to an aqueous suspension of hydrated iron oxide particles. Regarding the amount of manganese ion added, the manganese ion contains 0.40 to 2.50 at.%, Preferably 0.80 to 2.30 at.% As Mn / Fe when heated and dehydrated to be hematite. Similarly, zinc ions are made to contain 0.40 to 2.50 atomic weight%, preferably 0.80 to 2.30 atomic weight% as Zn / Fe at the stage of hematite formation. Further, the pH is adjusted to 10 to 11 by adding an alkali, and the metal ion is applied as a hydroxide on the surface of the base particles, followed by filtration and washing with water to remove excess alkali remaining in the suspension. . The post-process can be finished to maghemite particles or intermediate particles by applying a known method. For example, when finishing to maghemite particles is as follows. 500-600 ° C-0.5 hour heat dehydration, 300-350 ° C under hydrogen gas flow
A maghemite is formed by performing a reduction treatment for 3 hours and an oxidation treatment at 250 ° C. for 3 hours under flowing air. On the other hand, the case of finishing to intermediate particles is as follows. 500-600 ° C-
Heating dehydration treatment for 0.5 hour, 300 ~ under hydrogen gas flow
A reduction treatment at 350 ° C. for 3 hours and a slight oxidation treatment in a stream of air are performed to take out before the oxidation is completed, or an oxidation treatment at 250 ° C. for 3 hours is performed to form maghemite, and then a stream of hydrogen gas is passed. At 300 to 350 ° C. to form an intermediate. The ferrous ion content of the intermediate particles can be adjusted to a desired level by adjusting the time of the micro-oxidation or the time of the re-reduction. When the hydrated iron oxide particles are used as the starting material, it is not necessary to add a heat treatment step after the metal deposition treatment, unlike the maghemite particles (1) and (2). It is considered that the heat dehydration treatment gives the same effect as the heat treatment.

(4) Manganese according to (1) to (3),
Maghemite particles or intermediate particles modified with zinc and iron or manganese and zinc iron ions are subjected to a coating treatment with a solution containing a cobalt compound or a compound of cobalt and a ferrous salt on the particle surface by a known method, By increasing the coercive force and the saturation magnetization, the magnetic properties can be further improved. The method of the deposition process is, for example, as follows. To an aqueous suspension of maghemite particles or intermediate particles modified with manganese, zinc and optionally iron ions, alkali and cobalt compounds or alkali, cobalt compounds and ferrous compounds are added and reacted.
The amount of cobalt ion to be applied is 1.0-6.
0 wt%, preferably 2.0 to 6.0 wt%, when a ferrous compound is used in combination, the amount of ferrous ion deposited is 10.0
wt% or less, preferably 3.0 to 7.0 wt%.
Further, the mixture is heated to 80 to 95 ° C. and aged for 2 to 7 hours. After the deposition reaction, the resultant is filtered, washed with water, and dried to obtain a cobalt-deposited ferromagnetic iron oxide powder. According to the present invention, the surface of the maghemite particles or the hydrated iron oxide particles of the precursor to be maghemitized is coated with a trace amount of metal and denatured by the manganese zinc ferrite layer, whereby the saturation magnetization is reduced. Improved maghemite particles or intermediate particles can be obtained. Further, by applying a cobalt compound or a compound of cobalt and a ferrous salt, the magnetic properties are further improved, and the ferromagnetic oxidation which is very suitable for a high recording density magnetic tape which shows a high saturation magnetization while being a fine particle is further preferable. Iron powder can be manufactured. The effects of the present invention will be specifically described below with reference to examples and comparative examples.

EXAMPLES Example 1 Coercive force Hc250Oe, saturation magnetization σs68.5 emu
/ G, maghemite having a specific surface area of 62.0 m ² / g (γ-
After dispersing 100 g of Fe ₂ O ₃ ) in 1.5 l of pure water,
18.8 m of 1 mol / l manganese sulfate aqueous solution under stirring
A mixed solution of 18.8 ml of a 1 mol / l zinc sulfate aqueous solution and 75.2 ml of a ferric sulfate aqueous solution was added. Then, a 1.0N aqueous sodium hydroxide solution was gradually added, and pH =
After adjusting to 10.0, hold for 30 minutes, filter, wash and wash
Dry at 0 ° C. for 10 hours. Then, heat treatment was carried out in a stationary tubular furnace at 450 ° C. for 3 hours in nitrogen to obtain a target magnetic iron oxide powder. (Sample A-1) Comparative Example 1 A magnetic iron oxide powder of a comparative sample was obtained in the same manner as in Example 1, except that the heat treatment at 450 ° C. in nitrogen gas was changed to the heat treatment at 450 ° C. in air. (Sample A-2) Example 2 In Example 1, a 1 mol / l manganese sulfate aqueous solution 1 was used.
The same treatment was carried out except that 8.8 ml was changed to 7.5 ml, and 18.8 ml of a 1 mol / l aqueous solution of zinc sulfate was changed to 30.1 ml, to obtain a target magnetic iron oxide powder. (Sample B-1) Example 3 In Example 1, a 1 mol / l manganese sulfate aqueous solution 1 was used.
The same treatment was carried out except that 8.8 ml was changed to 30.1 ml and 18.8 ml of a 1 mol / l aqueous solution of zinc sulfate was changed to 7.5 ml, to obtain a target magnetic iron oxide powder. (Sample B-2) Comparative Example 2 In Example 1, the addition of a 1 mol / l manganese sulfate aqueous solution was omitted, and a 1 mol / l zinc sulfate aqueous solution 1 was added.
The same treatment was performed except that 8.8 ml was changed to 37.6 ml to obtain a magnetic iron oxide powder of a comparative sample. (Sample B
-3) Example 4 A target magnetic iron oxide powder was obtained in the same manner as in Example 1, except that the heat treatment at 450 ° C in nitrogen gas was changed to the heat treatment at 400 ° C in nitrogen gas. (Sample C-1) Example 5 A target magnetic iron oxide powder was obtained in the same manner as in Example 1, except that the heat treatment at 450 ° C in nitrogen gas was changed to the heat treatment at 475 ° C in nitrogen gas. (Sample C-2) Example 6 A target magnetic iron oxide powder was obtained in the same manner as in Example 1, except that the heat treatment at 450 ° C in nitrogen gas was changed to the heat treatment at 500 ° C in nitrogen gas. (Sample C-3) Example 7 A heat treatment at 450 ° C. in nitrogen gas at 350 ° C. in Example 1 was performed.
The same treatment was performed except that the heat treatment was changed to obtain the desired magnetic iron oxide powder. (Sample C-4) Comparative Example 3 A magnetic iron oxide powder of a comparative sample was obtained in the same manner as in Example 1, except that the heat treatment at 450 ° C in nitrogen gas was changed to the heat treatment at 550 ° C in nitrogen gas. (Sample C-5) Example 8 A target magnetic iron oxide powder was obtained in the same manner as in Example 5, except that ferric sulfate was not added. (Sample D-1) Example 9 In Example 5, a 1 mol / l ferric sulfate aqueous solution 7 was used.
The same treatment was carried out except that 5.2 ml was changed to 37.6 ml to obtain a target magnetic iron oxide powder. (Sample D-
2) Example 10 In Example 5, a 1 mol / l manganese sulfate aqueous solution 1 was used.
The same treatment was performed except that 8.8 ml was changed to 9.4 ml, a 1 mol / l aqueous solution of zinc sulfate 18.8 ml was changed to 9.4 ml, and a 1 mol / l aqueous solution of ferric sulfate 75.2 ml was changed to 37.6 ml. Thus, the desired magnetic iron oxide powder was obtained. (Sample E-1) Example 11 In Example 5, a 1 mol / l manganese sulfate aqueous solution 1 was used.
The same treatment was performed except that 8.8 ml was changed to 28.2 ml, 1 mol / l aqueous solution of zinc sulfate 18.8 ml was changed to 28.2 ml, and 15.2 ml of 1 mol / l aqueous solution of ferric sulfate was changed to 112.8 ml. Thus, the desired magnetic iron oxide powder was obtained. (Sample E-2) Comparative Example 4 A magnetic iron oxide powder of a comparative sample was obtained in the same manner as in Example 5, except that manganese sulfate, zinc sulfate and ferric sulfate were not added. (Sample E-3) Example 12 A target magnetic iron oxide powder was obtained in the same manner as in Example 5, except that ferric sulfate was changed to ferrous sulfate. (Sample F-1) Example 13 Coercive force Hc384Oe, saturation magnetization σs72.6 emu
/ G, maghemite having a specific surface area of 25.0 m ² / g (γ-
After dispersing 100 g of Fe ₂ O ₃ ) in 1.51 of pure water,
18.8 m of 1 mol / l manganese sulfate aqueous solution under stirring
A mixed solution of 18.8 ml of an aqueous 1 mol / l zinc sulfate solution and 75.2 ml of an aqueous ferric sulfate solution was added. Then, a 1.0 N aqueous sodium hydroxide solution was gradually added to the mixture to adjust the pH to
After adjusting to 10.0, hold for 30 minutes, filter, wash and wash
Dry at 0 ° C. for 10 hours. Then, heat treatment was performed in a stationary tubular furnace at 500 ° C. for 3 hours in nitrogen to obtain a target magnetic iron oxide powder. (Sample G-1) Example 14 The sample obtained in Comparative Example 1 was subjected to a reduction treatment at 300 ° C. for 30 minutes in a stationary tubular furnace under a flow of hydrogen gas containing nitrogen gas to obtain Fe ²⁺ / Fe ^3+. The desired magnetic iron oxide powder containing 0.20 ferrous iron was obtained. (Sample H-1) Example 15 The sample obtained in Example 5 was subjected to a reduction treatment at 300 ° C. for 30 minutes in a stationary tubular furnace under flowing hydrogen gas containing nitrogen gas to obtain Fe ²⁺ / Fe ^3+. The desired magnetic iron oxide powder containing 0.20 ferrous iron was obtained. (Sample H-2) Comparative Example 5 The maghemite particles used in Example 1 were placed in a stationary tubular furnace at 300 ° C. under a flow of hydrogen gas containing nitrogen gas.
For 30 minutes to obtain a Fe ²⁺ / Fe ³⁺ of 0.2
A comparative magnetic iron oxide powder containing 0 ferrous iron was obtained.
(Sample H-3) Example 16 The sample obtained in Example 13 was subjected to reduction treatment at 300 ° C. for 30 minutes in a stationary tubular furnace under a flow of hydrogen gas containing nitrogen gas to obtain Fe ²⁺ / Fe ^3+. The desired magnetic iron oxide powder containing 0.20 ferrous iron was obtained. (Sample I-1) Comparative Example 6 The maghemite particles used in Example 13 were placed in a stationary tubular furnace under flowing hydrogen gas containing nitrogen gas for 300 times.
Reduction treatment at 30 ° C. for 30 minutes to obtain Fe ²⁺ / Fe ³⁺ .
Thus, a comparative magnetic iron oxide powder containing 20 ferrous iron was obtained.
(Sample I-2) Example 17 After dispersing 100 g of goethite (α-Fe ₂ O ₃ ) particles having a specific surface area of 68 m ² / g in pure water 31, 1 m of the dispersion was stirred.
ol / l manganese sulfate aqueous solution 16.9ml and 1mol
/ L zinc sulfate aqueous solution 16.9 ml was added. Then, a 1.0 N aqueous sodium hydroxide solution was gradually added to the mixture to adjust the pH to 1
After adjusting to 0.0, the phosphorus compound was treated, filtered and washed with water to remove alkali, and then dried at 110 ° C. for 10 hours. This goethite was heated and dehydrated at 620 ° C. for 3 hours to obtain manganese and zinc-containing hematite. This hematite was heated and reduced at 350 ° C. for 3 hours under a hydrogen gas containing carbon dioxide gas to form magnetite, and then maghemite was formed by oxidation at 250 ° C. for 3 hours under an air flow to obtain a target magnetic iron oxide powder. (Sample J-1) Comparative Example 7 A magnetic iron oxide powder of a comparative sample was obtained in the same manner as in Example 17, except that manganese sulfate and zinc sulfate were not added. (Sample J-2) Example 18 The maghemite particles obtained in Example 17 were subjected to a reduction treatment at 300 ° C. for 30 minutes under flowing hydrogen gas containing nitrogen gas to obtain a target magnetic iron oxide powder. (Sample K-1) Comparative Example 8 The maghemite particles obtained in Comparative Example 7 were subjected to a reduction treatment at 300 ° C. for 30 minutes under a flow of hydrogen gas containing nitrogen gas.
A magnetic iron oxide powder of a comparative sample was obtained. (Sample K-2) Example 19 After dispersing 100 g of the manganese- and zinc-containing magnetite obtained in Example 15 in 450 ml of pure water, 400 ml of 10N sodium hydroxide was added at 35 ° C with stirring while blowing nitrogen gas. Next, 88.2 ml of a 1 mol / l cobalt sulfate aqueous solution and 89.5 ml of a 1 mol / l ferrous sulfate aqueous solution were added, and the mixture was aged at 95 ° C. for 5 hours. The obtained precipitate was filtered and washed with water, and then dried in air.
Drying at 3 ° C. for 3 hours gave the desired cobalt-coated magnetic iron oxide powder. (Sample L-1) Comparative Example 9 Example 19 except that the magnetite obtained in Example 15 was changed to the magnetite obtained in Comparative Example 5.
In the same manner as described above, a comparative cobalt-coated magnetic iron oxide powder was obtained. (Sample L-2) Magnetic iron oxide samples (A-1) to (L-) prepared in Examples 1 to 18 and Comparative Examples 1 to 8.
Regarding 2), the coercive force (Hc) and the saturation magnetization (σs) were measured by an ordinary method. Table 1 and 2
It is shown in the table. In addition, magnetic iron oxide samples (L-1) and (L-2)
Was prepared using a small kneader and a small sand grinder mill according to the following mixing ratio.

Cobalt-coated magnetic iron oxide powder 100.0 (parts by weight) PVC / vinyl acetate copolymer resin 18.0 Polyurethane resin 12.0 Methyl ethyl ketone 180.0 Cyclohexanone 60.0 Toluene 60.0 Applicator using the above magnetic paint Was applied on a polyester film having a thickness of 25 μm, magnetically oriented, and then dried to finish a magnetic sheet having a dry coating thickness of 10 μm. Then, the coercive force (Hc), the squareness ratio (SQ), the orientation ratio (OR), and the switching field distribution (SF) of the magnetic sheet are obtained by a usual method.
D) was measured. Table 2 shows the results.

From the above results, it is apparent that the magnetic iron oxide sample of the embodiment according to the present invention has a high saturation magnetization and exhibits excellent magnetic properties. Cobalt-coated sample (L-
1) is higher than that of the conventional sample (L-2) in which maghemite particles without metal addition and without heat treatment are used as a starting material.
It has a high c-expressing property, and if the same Hc as that of the sample (L-2) is used, there is an advantage that the amount of deposited cobalt can be reduced, which is industrially advantageous. It is also clear that the magnetic sheet has a good squareness ratio and orientation ratio and exhibits excellent dispersibility.

[0014]

[Table 1]

[0015]

[0016]

[Table 2]

Continuation of the front page (72) Inventor: Kazu Watanabe 1978 Kogushi, Ube City, Yamaguchi Prefecture, within 25 Hitachi Industries Ltd. (56) References JP-A-55-141712 (JP, A) (58) Fields investigated ( Int.Cl. ⁶ , DB name) H01F 1/34

Claims (5)

(57) [Claims] 1. The maghemite particles are treated with a solution containing manganese, zinc and optionally iron, and further heat-treated at 350-525 ° C. in a non-oxidizing atmosphere to obtain 0.40-2.50 atom as Mn / Fe. Wt% manganese and Zn / F
e) containing 0.40-2.50 atomic% by weight of zinc as e, and obtaining maghemite particles having an improved saturation magnetization, wherein the surface of the particles is denatured by a manganese zinc ferrite layer. A method for producing ferromagnetic iron oxide powder. 2. The maghemite particles are treated with a solution containing manganese, zinc and optionally iron, and further heat-treated at 350-525 ° C. in an oxidizing atmosphere or a non-oxidizing atmosphere to obtain 0.40 as Mn / Fe. Manganese ions of -2.50 atomic weight% and 0.40-2.5 as Zn / Fe
Maghemite particles whose surface is denatured by a manganese zinc ferrite layer, and the particles are first reduced at a ratio of Fe ²⁺ / Fe ³⁺ of 0.45 or less. A method for producing ferromagnetic iron oxide powder for magnetic recording, comprising obtaining intermediate particles of maghemite and magnetite containing iron ions and having an improved saturation magnetization. 3. The hydrated iron oxide particles of the precursor to be maghemitized are treated with a solution containing manganese and zinc, and further subjected to heat dehydration treatment to obtain Mn / Fe of 0.40-2.
50 atomic% by weight of manganese ions and 0.40-2.50 atomic% by weight of zinc ions as Zn / Fe,
A method for producing a ferromagnetic iron oxide powder for magnetic recording, comprising reducing and oxidizing hematite particles whose particle surfaces have been modified with a manganese zinc ferrite layer to obtain maghemite particles having an improved saturation magnetization. 4. A hydrated iron oxide particle of a precursor to be converted into maghemite is treated with a solution containing manganese and zinc, and further subjected to a heat dehydration treatment to obtain Mn / Fe of 0.40-2.
50 atomic% by weight of manganese ions and 0.40-2.50 atomic% by weight of zinc ions as Zn / Fe,
The hematite particles whose particle surfaces have been modified with a manganese zinc ferrite layer are reduced and then slightly oxidized, or reduced and oxidized to maghemite and then re-reduced to give Fe ²⁺ / Fe ³⁺ of 0.1. A method for producing a ferromagnetic iron oxide powder for magnetic recording, comprising obtaining intermediate particles of maghemite and magnetite containing 45 or less ferrous ions and having improved saturation magnetization. 5. A maghemite particle or an intermediate particle having an improved saturation magnetization obtained by the method according to claim 1, which is treated with a solution containing a cobalt compound or a compound of cobalt and a ferrous salt. A method for producing a ferromagnetic iron oxide powder for magnetic recording.