JPH04299505A - Manufacture of needlelike magnetic ion oxide particle and powder for magnetic recording - Google Patents

Abstract

PURPOSE:To provide the title magnetic ion oxide particles and powder with high coersive force and the magnetic with-time stability by containing a moderate and specific compound in a suspension. CONSTITUTION:When a Si compound is contained in the suspension, Co (OH)2 or Fe(OH)2 reacts to Si ions to produce a complex compound. Later, the reaction can be restrained by the resultant adherent reaction. Even if a needlelike magnetic particles powder are used as a precursor or a lot of Fe<+2> salt is added to enhance high coersive force and black color reaction, the adherent layer shall be composed of a Co compound containing Si produced by denaturizing a Si compound or an Fe compound containing Si. Through these procedures, the magnetic with-time stability can be enhanced while when the uppermost adherent layer is composed of the Co compound containing Si, the oxygen in the air does not come into contact with Fe<+2> so as to further enhance the effect.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is directed to the production of acicular magnetic iron oxide particles having high coercive force, excellent erasing properties and coercive force distribution, and good magnetic stability over time. It is about law.

0002

2. Description of the Related Art In recent years, as magnetic recording and reproducing equipment has become smaller and lighter, there has been an increasing need for higher performance magnetic recording media such as magnetic tapes and magnetic disks. That is, the recording density characteristics are improved.

In order to improve the recording density characteristics of a magnetic recording medium, it is important that the magnetic material particles used have as high a coercive force as possible. This fact can be seen, for example, in IEICE Technical Research Report MR77-
36 (published in 1978), page 37, "In order to increase the recording density of a magnetic tape, it is necessary to increase the coercive force of the magnetic powder used in the tape."

Currently, so-called Co-doped acicular magnetic iron oxide particles and so-called Co-coated acicular magnetic iron oxide particles are known as acicular magnetic iron oxide particles having high coercivity. The coercive force of these acicular magnetic iron oxide particles tends to increase as the amount of Co increases. In the former case, Co-containing acicular goethite particles are generated by adding Co salt in advance during the production reaction of acicular goethite particles, which are the starting materials, and then reduced to produce Co-containing acicular magnetite particles, or as necessary. The latter is further oxidized to Co-containing acicular maghemite particles by reducing the acicular goethite particles that are the starting material, or converting the acicular magnetite particles or acicular maghemite particles obtained by oxidizing the starting material into precursors. The precursor particles as particles are Co or Co and F.
Obtained by metamorphosis with e2+.

On the other hand, magnetic recording media are required to have excellent erasing characteristics because they are used for long periods of time by repeated recording and erasing.

In order to satisfy the above requirements for magnetic recording media, it is necessary that the magnetic iron oxide particles used have excellent erasing properties.

Conventionally, in order to improve various properties of Co-coated acicular magnetic iron oxide particles, a special method has been developed in which Co salt and Fe2+ salt are added separately to coat Co compound and Fe compound. Publication No. 58-29605, Special Publication No. 1-14
There are methods described in JP-A No. 176, JP-A-56-59629, JP-A-59-151402, JP-A-61-10031, and JP-A-61-212003.

Furthermore, methods for adding a Si compound together with a Co salt include methods described in JP-A-55-72007 and JP-A-63-303817.

[0009]

[Problems to be Solved by the Invention] Acicular magnetic iron oxide particles having a high coercive force and excellent erasing properties are currently most in demand, but the above-mentioned C
O-doped acicular magnetic iron oxide particles have a high coercive force, but on the other hand, due to the diffusion of Co into the crystal, the coercive force distribution becomes wider, and as a result, the erasing property It has the disadvantage that it deteriorates.

[0010] This phenomenon is explained in the above-mentioned ``Technical Research Report of the Institute of Electronics and Communication Engineers'', which states, ``Co solid solution type (doped type) iron oxide magnetic powder has a coercive force that easily changes thermally and over time. When used, it has a major drawback of poor transfer and erasing properties.These drawbacks are thought to be caused by the movement of Co ions within the crystal even at room temperature."

[0011] Furthermore, the Co-coated acicular magnetic iron oxide particles as described above have a high coercive force and are also characterized in that they have superior erasing properties compared to Co-doped acicular magnetic iron oxide particles. It has the following. This phenomenon is explained in the above-mentioned ``IEICE Technical Research Report'' as ``...Co epitaxial (Co-adhered) iron oxide magnetic powder has a double structure, so these drawbacks are eliminated and heat resistance is reduced. It is stable over time, and tapes using this magnetic powder have excellent transfer and erasing properties.

However, in recent years, there has been an unrelenting demand for improvement in erasing properties, and even the above-mentioned Co-coated acicular magnetic iron oxide particles still have a wide coercive force distribution and excellent erasing properties. It has been pointed out that it is difficult to say.

[0013] This fact is, for example, disclosed in Japanese Patent Laid-Open No. 174-1983
Publication No. 26 states, ``In the magnetic powder using the above-mentioned γ-Fe2 O3 particles, the coercive force distribution broadens as the γ-Fe2 O3 particles become finer particles, and when cobalt is further deposited, this coercive force increases. It was found that the distribution tends to become even wider. When the cobalt-coated γ-Fe2O3 particles are made finer in order to achieve high-density recording, even if a predetermined coercive force Hc is obtained, the coercive force Hc becomes smaller. Only magnetic powder with poor magnetic force distribution and poor erasing properties can be obtained.''

The spread of the coercive force distribution of Co-coated acicular magnetic iron oxide particles increases as the amount of Co-coated increases, and as a result, the erasing characteristics tend to deteriorate, and the coercive force cannot be improved. There is an inverse correlation.

On the other hand, since video equipment usually employs a mechanism for detecting the end of a running tape using light, malfunctions may occur if the light transmittance of the video tape used becomes high. However, as the Co-coated γ-Fe2O3 particles, which are currently most commonly used in videotapes, become finer, the light transmittance increases.

Attempts have been made to improve the light transmittance by adding a large amount of non-magnetic filler such as carbon black, but the use of non-magnetic materials such as carbon black hinders high-density recording.

Therefore, as a method for improving light transmittance without adding a non-magnetic filler, magnetic iron oxide coated with Co on the particle surface of acicular magnetite particles or acicular bertolide compound particles with excellent blackness was proposed. Particulate powders began to be used.

The technical means of depositing Co on the particle surface of the acicular magnetite particles or acicular beltolide compound particles described above is well known, and magnetic iron oxide particles have a high coercive force and excellent blackness. It is also well known that

However, magnetic iron oxide particle powder in which Co is coated on acicular magnetite particles and acicular beltolide compound particles has the advantage of obtaining a high saturation magnetization value and excellent electrical resistance. There is a problem that the magnetic properties are gradually oxidized and the magnetic properties deteriorate over time.
This tendency becomes more pronounced as the particles become finer.

Even with the technical means of dividing and adding Co salt and Fe2+ salt disclosed in the above-mentioned publications, the erasing characteristics and coercive force distribution are still insufficient, and the precursor particles are not mixed with acicular magnetite particles or The stability of magnetic properties over time when acicular bertolide compound particles are used is also insufficient.

[0021] Furthermore, even the technical means disclosed in JP-A-63-303817 in which a coating layer of a Si compound or the like is formed after depositing Co has insufficient effects. In addition, C disclosed in Japanese Patent Application Laid-Open No. 57-72007
o In the technical means of adding a silicic acid compound at the time of deposition, it is said that it is desirable to add it one hour after heating in order to prevent part of the Co deposition from being destroyed by alkali. and their actions and effects are different.

Furthermore, JP-A-1-290534 discloses a technical means of depositing Co in the presence of an Al salt, which is often used because it has the same effect as a Si compound. It is said that the presence of the Fe2+ compound before or during the deposition significantly reduces the coercive force, and as will be described later, the present invention differs in its action and effect in that Fe2+ is actively present.

Therefore, the present invention further improves the erasing characteristics and coercive force distribution while maintaining the coercive force of Co-coated acicular magnetic iron oxide particles, and also improves the magnetic stability over time. The technical challenge is to achieve this goal.

[0024]

[Means for Solving the Problems] The above-mentioned technical problems can be achieved by the present invention as follows.

That is, in the present invention, an aqueous Co salt solution is added to and mixed with an aqueous dispersion of acicular magnetic iron oxide particles at a temperature of 60° C. or less in a non-oxidizing atmosphere, and an aqueous alkali hydroxide solution is added to the mixed solution. is added to form an alkaline suspension, and this suspension is heated and stirred in a non-oxidizing atmosphere at a temperature below the boiling point to modify the surface of the acicular magnetic iron oxide particles with a Co compound. After bringing the temperature of the suspension to 60°C or lower, an aqueous Fe2+ salt solution is added and mixed, and the surface of the acicular magnetic iron oxide particles is further coated with Fe compounds by heating and stirring in a non-oxidizing atmosphere at a temperature below the boiling point. Then, after bringing the suspension to a temperature of 60 ° C. or less, adding and mixing a Co salt aqueous solution and a Si compound,
The top layer of the surface of the acicular magnetic iron oxide particles is modified with a Co compound containing Si by heating and stirring in a non-oxidizing atmosphere at a temperature below the boiling point, and then filtered, washed with water, and dried to form acicular magnetic iron oxide particles. A method for producing acicular magnetic iron oxide particles for magnetic recording, comprising obtaining magnetic iron oxide particles;

[0026]
Co salt aqueous solution and Fe2+ salt aqueous solution or Co salt aqueous solution and Si compound or Co salt aqueous solution, Fe2+ salt aqueous solution and Si compound are added to an aqueous dispersion of acicular magnetic iron oxide particle powder in a non-oxidizing atmosphere at a temperature of 60°C or less. Add and mix,
An aqueous alkali hydroxide solution is added to this mixed solution to form an alkaline suspension, and this suspension is heated and stirred in a non-oxidizing atmosphere at a temperature below the boiling point to coat the surface of the acicular magnetic iron oxide particles with a Co compound. and Co containing Fe compound or Si
compound or a Co compound containing Si and an Fe compound, and then the suspension is brought to a temperature of 60°C or lower, and then an aqueous Fe2+ salt solution is added and mixed, and the temperature is lowered to the boiling point in a non-oxidizing atmosphere. The surface of the acicular magnetic iron oxide particles is further modified with the Fe compound by heating and stirring, and then, after bringing the suspension to a temperature of 60 ° C. or less, an aqueous Co salt solution and a Si compound are added and mixed. death,
The top layer of the surface of the acicular magnetic iron oxide particles is modified with a Co compound containing Si by heating and stirring in a non-oxidizing atmosphere at a temperature below the boiling point, and then filtered, washed with water, and dried to form acicular magnetic iron oxide particles. A method for producing acicular magnetic iron oxide particles for magnetic recording, comprising obtaining magnetic iron oxide particles;

[0027]
Adding and mixing a Co salt aqueous solution to an aqueous dispersion of acicular magnetic iron oxide particle powder at a temperature of 60 ° C. or less in a non-oxidizing atmosphere,
An aqueous alkali hydroxide solution is added to this mixed solution to form an alkaline suspension, and this suspension is heated and stirred in a non-oxidizing atmosphere at a temperature below the boiling point to coat the surface of the acicular magnetic iron oxide particles with a Co compound. Then, after bringing the suspension to a temperature of 60° C. or lower, an aqueous Fe2+ salt solution and an aqueous Co salt solution, or an aqueous Fe2+ salt solution and a Si compound, or an aqueous Fe2+ salt solution, an aqueous Co salt solution, and a Si compound are added and mixed. Then, by heating and stirring at a temperature below the boiling point in a non-oxidizing atmosphere, the surface of the acicular magnetic iron oxide particles is further coated with an Fe compound and a Co compound, or an Fe compound containing Si, or an Fe compound containing Si and a Co compound. Then, after bringing the suspension to a temperature of 60 ° C. or less, adding and mixing a Co salt aqueous solution and a Si compound,
The top layer of the surface of the acicular magnetic iron oxide particles is modified with a Co compound containing Si by heating and stirring in a non-oxidizing atmosphere at a temperature below the boiling point, and then filtered, washed with water, and dried to form acicular magnetic iron oxide particles. A method for producing acicular magnetic iron oxide particles for magnetic recording, which comprises obtaining magnetic iron oxide particles, and

0028
] Add Co salt aqueous solution and Fe2+ salt aqueous solution or Co salt aqueous solution and Si compound or Co salt aqueous solution, Fe2+ salt aqueous solution and Si compound to an aqueous dispersion of acicular magnetic iron oxide particle powder in a non-oxidizing atmosphere at a temperature of 60°C or less. The above-mentioned acicular magnetic oxidation agent is added and mixed, and an aqueous alkali hydroxide solution is added to this mixed solution to form an alkaline suspension, and this suspension is heated and stirred in a non-oxidizing atmosphere at a temperature below the boiling point. The surface of the iron particles is coated with Co compound and Fe compound or C containing Si.
o compound or a Co compound containing Si and an Fe compound, and then, after the suspension is brought to a temperature of 60°C or less, an aqueous Fe2+ salt solution and an aqueous Co salt solution or an F
By adding and mixing the e2+ salt aqueous solution and the Si compound or the Fe2+ salt aqueous solution, the Co salt aqueous solution, and the Si compound, and heating and stirring in a non-oxidizing atmosphere at a temperature below the boiling point, the surface of the acicular magnetic iron oxide particles is Furthermore, Fe compound and C
o compound or a Fe compound containing Si or a Fe compound containing Si and a Co compound, and then
After bringing the temperature of the suspension to 60° C. or lower, a Co salt aqueous solution and a Si compound are added and mixed, and the acicular magnetic iron oxide particles are heated and stirred in a non-oxidizing atmosphere at a temperature below the boiling point. A method for producing acicular magnetic iron oxide particle powder for magnetic recording, which comprises modifying the uppermost layer of the surface with a Co compound containing Si, followed by filtering, washing with water, and drying to obtain acicular magnetic iron oxide particle powder. It is.

Next, various conditions for implementing the present invention will be described.

The acicular magnetic iron oxide particles used as the precursor in the present invention include acicular maghemite particles, acicular magnetite particles, acicular bertolide compound particles which are intermediate oxides between maghemite and magnetite, and these particles containing Ni and Si. ,
1 such as Al, Zn, P, Zr, Ba, Sr, Ca, Pb, etc.
Particles containing a species or two or more species can also be used.

As the Co salt aqueous solution in the present invention, an aqueous solution of cobalt sulfate, cobalt chloride, cobalt nitrate, etc. can be used.

[0032] The amount of Co salt aqueous solution added in the present invention is:
The amount is 0.1 to 15.0% by weight, preferably 0.1 to 10.0% by weight, based on Co, based on the precursor particles. 0.
If it is less than 1% by weight, the Co modification effect will not be noticeable. If it exceeds 15.0% by weight, the coercive force distribution of the obtained acicular magnetic iron oxide particles becomes large, making it difficult to improve the erasing characteristics.

[0033] When the Co salt aqueous solution is added in portions, the total amount added is within the above range, and the addition ratio can be selected arbitrarily, but it is better if the amount of Co in the upper layer is larger than the amount of Co in the lower layer. The ratio is preferably about 2 to 20 times, preferably 3 to 15 times. If it is less than 2 times, the stability of the obtained acicular magnetic iron oxide particles over time will be poor, and if it is more than 20 times, it will not be possible to obtain a high coercive force, and the Fe2+ compound to be transformed on the particle surface will be Since the amount is relatively small, the electrical resistance becomes high, which is not preferable.

As the Fe2+ salt aqueous solution in the present invention, an aqueous solution of ferrous sulfate, ferrous chloride, etc. can be used.

The amount of the Fe2+ salt aqueous solution added is 0.1 to 15.0% by weight in terms of Fe based on the precursor particles. 0.
If it is less than 1% by weight, the effect of improving coercive force will be low and the electrical resistance will increase. If it exceeds 15.0% by weight, the coercive force distribution of the obtained acicular magnetic iron oxide particles becomes large, making it difficult to improve the erasing characteristics.

[0036] The aqueous Fe2+ salt solution may be added at once or may be added in portions. In that case, the total amount added is within the above range, and the addition ratio can be arbitrarily selected.

As the aqueous alkali hydroxide solution in the present invention, aqueous solutions of sodium hydroxide, potassium hydroxide, aqueous ammonia, etc. can be used.

The OH group concentration after addition of the aqueous alkali hydroxide solution is preferably 0.05 to 3.0 mol/l. If it is less than 0.05 mol/l, metamorphosis will not occur sufficiently. If it exceeds 3.0 mol/l, cobalt hydroxide starts to dissolve, which is not preferable.

[0039] When carrying out deposition treatment by adding Co salt, Fe2+ salt, etc. in portions, an aqueous alkali hydroxide solution corresponding to the above addition amount is additionally added in order to maintain the deposition treatment conditions. Good too.

As the Si compound in the present invention, water glass, colloidal silica, sodium silicate, potassium silicate, silicon dioxide, etc. can be used.

The amount of the Si compound added is 0.01 to 2.00% by weight, preferably 0.05 to 1.00% by weight, based on SiO2 based on the precursor particles. If it is less than 0.01% by weight, the effect of the present invention cannot be obtained, and the
If it exceeds 0% by weight, the adhesion reaction will be inhibited and high coercive force will not be obtained.

As will be described later, the Si compound is added to suppress the precipitation of Co ferrite on surfaces other than the particle surface of the precursor, so it may be added all at once.
It is preferable to add the Co salt in portions depending on the reaction temperature of the Co salt. In that case, the total amount added is within the above range, and as a guide, when the temperature of the suspension is 60°C or less, 0.05 to 0.50 in terms of SiO2 is added to the precursor particles.
% by weight is preferred, and when the temperature exceeds 60°C and is below the boiling point, 0.10 to 1.00% by weight is preferred. Even when the coating is repeated several times, it is desirable that the total amount added be 2.00% by weight or less.

The Co salt aqueous solution, the Fe2+ salt aqueous solution, and the alkali hydroxide aqueous solution may be added in any order or at the same time.

Furthermore, there is no particular order in which the Si compounds are added;
The purpose of this treatment is to suppress the precipitation of o-ferrite, so the treatment temperature may be adjusted appropriately using the above treatment temperature as a guide.

[0045] Co is added to the suspension of precursor particles in the present invention.
When adding and mixing the salt aqueous solution, Fe2+ salt aqueous solution and alkali hydroxide aqueous solution, each temperature is 60°C or less,
Preferably it is 40°C or lower. The reason for keeping the temperature below 60°C is to suppress rapid reactions and ensure that the Co compound and Fe compound are sufficiently mixed with the aqueous alkali hydroxide solution to form Co(
This is to obtain a high coercive force and an excellent coercive force distribution by generating colloids of OH)2 and Fe(OH)2 and then transmuting them onto the particle surface of the precursor.

[0046] In this case, the stirring time is preferably selected from a range of 30 to 120 minutes. If the time is less than 30 minutes, sufficient mixing may not be possible, and if the time is more than 120 minutes, there is no industrial significance.

[0047] In the present invention, each heating and stirring process for strongly modifying the Co compound and Fe compound on the particle surface of the precursor is carried out at a temperature below the boiling point. Even if the temperature exceeds the boiling point, it is possible to carry out the deposition treatment, but since equipment such as an autoclave is required, industrially it is preferable to carry out the process at a temperature below the boiling point. Although the adhesion process can be carried out at temperatures below 60°C, it is preferable to carry out the process at temperatures above 60°C since the adhesion reaction becomes extremely slow and sufficient magnetic properties cannot be obtained.

[0048] In this case, the stirring time is preferably selected from a range of 30 to 900 minutes. If the duration is less than 30 minutes, the adhesion will not be sufficient, and if it exceeds 900 minutes, there is no industrial significance. Practically, the desirable range is 30 to
It is 600 minutes.

[0049] In the present invention, each addition and each stirring treatment are carried out under a non-oxidizing atmosphere. Fe2+ under non-oxidizing atmosphere
This is to suppress the oxidation of the metal and obtain high coercive force and excellent coercive force distribution through the adhesion treatment. In addition, the non-oxidizing atmosphere is
It is desirable to carry out the process under a flow of an inert gas such as N2 or Ar gas.

[0050]

[Function] As described above, the conventional Co-coated acicular magnetic iron oxide particles have a wide coercive force distribution and cannot be said to have excellent erasing characteristics. Further, in the case of acicular magnetic iron oxide particles containing a large amount of Fe2+ ions and having excellent blackness, there is a problem in magnetic stability over time.

[0051] The present inventor discovered that Co and Fe could not be uniformly deposited on the surfaces of individual precursor particles using conventional technical means, and that fine Co and Fe having a high coercive force, which is different from the precursor particles, It was thought that the coercive force distribution would increase due to the formation of ferrite, and the erasing characteristics would also deteriorate.

In addition, the magnetic stability over time was determined by the obtained C
o Fe2 is present near the particle surface of the adhering type acicular magnetic iron oxide particles.
If there is a large amount of +, it will be oxidized by contact with oxygen in the air and become F.
It is thought that this is because Fe3+ penetrates into the inside of the particles to become e3+, and the magnetic change over time progresses.

[0053] Therefore, we conducted studies on a method that can control the deposition state of Co and Fe, and arrived at the present invention.

That is, by having an appropriate amount of Si compound in the suspension, the rapid reaction of Co salt and Fe2+ salt can be suppressed.
)2 and Fe(OH)2 colloids are produced, Co and Fe can be uniformly deposited on the particle surfaces of the precursor particles, and independent Co ferrite can be produced on surfaces other than the particle surfaces of the precursor particles. I was able to prevent it from happening. As a result, it was possible to improve the erasing characteristics and coercive force distribution while maintaining a high coercive force.

[0055] Although it is unclear why the presence of a Si compound in the suspension can suppress the rapid reaction, the present inventor has found that Co(OH)2 or Fe(OH)2 and Si ions react. It is believed that the reaction was able to be suppressed because a complex compound was produced and then a deposition reaction occurred.

Furthermore, even when using acicular magnetite particles or acicular beltolide compound particles as a precursor, or when adding a large amount of Fe2+ salt in order to improve coercive force and blackness, in the presence of Si compounds, The magnetic stability over time is improved by using a Co compound containing Si obtained through modification treatment, an Fe compound containing Si, or a Co compound containing Si and an Fe compound as an adhesion layer. When a Co compound containing Si was used as the coating material, it became difficult for Fe2+ to come into contact with oxygen in the air, and the effect was dramatically improved.

These excellent effects are due to Co salt and Fe2
It can also be obtained when +salt is added in two or more portions.

The present invention is a breakthrough that overturns the conventional wisdom that adding a Si compound to deposition not only inhibits high coercive force but also has various negative effects on the deposition reaction itself. It can be said that it is something like that.

It should be noted that Si, Al, Ca, Ti, V, Mn, N are added to the acicular magnetic iron oxide particle powder after coating Co according to the present invention.
Coating treatment with one or two compounds such as i, Zn, P, and Zr does not impede the effects of the present invention; rather,
Furthermore, effects such as improved magnetic stability over time and improved dispersibility with the binder resin can be expected.

[0060]

[Examples] Next, the present invention will be explained with reference to Examples and Comparative Examples.

[0061] In the following Examples and Comparative Examples, the average diameter of the particles is the average value of numerical values measured from electron micrographs, and the specific surface area is shown as the value measured by the BET method. The magnetic properties of the acicular magnetic iron oxide particles were measured using a "vibrating sample magnetometer VSM-3S-15" (Toei Kogyo Co., Ltd.).
These are the values measured using an external magnetic field of up to 10 KOe.

[0062] To measure the content of Fe2+, put the magnetic iron oxide particles into a flask, replace with inert gas, add a mixed acid of sulfuric acid and phosphoric acid while aerating, heat and dissolve, and then measure the Fe2+ content in the solution. was determined by redox titration method.

[0063] The erasing property is determined by the Powder Metallurgy Association of Japan.
The values are shown as values measured according to the ``Measurement method for erasure magnetization of magnetic powder'' described on pages 152-153 of ``Collection of Abstracts of Lectures at the 1986 Spring Conference.'' That is, the erasing characteristic is determined by applying a DC magnetic field of 10K to the sample.
After applying Oe, the residual magnetization Mr was measured, and then the eraser was set in an eraser and an erase magnetic field was applied from 800 Oe to zero, and the residual magnetization Me was measured.
The value of r (dB) was determined and an actual value was obtained. The value of the erase characteristic is closely related to the coercive force value, and the higher the coercive force value, the worse the erase characteristic tends to be. Therefore, the above measured value was corrected to the erase characteristic value at a coercive force of 700 Oe. .

ΔHc, which indicates the stability of magnetic properties over time, is the value obtained by measuring the coercive force Hc after leaving the sample in a constant temperature bath at a temperature of 60° C. and a relative humidity of 90% for two weeks, and subtracting the initial Hc value.

To measure the coercive force distribution (S.F.D.), use a sheet sample piece, use the differential circuit of the magnetic measuring machine to obtain a differential curve of coercive force, and measure the half-width of this curve. death,
It was calculated by dividing this value by the coercive force of the peak value.

[0066] The sheet-like sample piece was prepared by placing magnetic iron oxide particles, resin, and solvent in a 140 cc glass bottle in the proportions shown below, and then mixing and dispersing the mixture with paint conditioner for 2 hours. using an applicator to a thickness of 40 μm on a polyethylene terephthalate film of 1900G.
It was obtained by orienting it in the magnetic field of AUSS and drying it.

1.5mmφ glass beads
100 parts by weight magnetic iron oxide particle powder
15 parts by weight Vinyl chloride-vinyl acetate copolymer resin
3 parts by weight Nitrile rubber

0.75 parts by weight cyclohexanone
11.5 parts by weight Methyl ethyl ketone
11.5 parts by weight toluene

11.5 parts by weight lecithin

0.03 parts by weight Phosphate ester (Gafac RE-610) 0.
03 parts by weight

Example 1 Acicular magnetite particle powder (average major axis diameter 0.19 μm, axial ratio (major axis diameter/minor axis diameter) 8.0, BE
T specific surface area 32m2/g, coercive force 400Oe, saturation magnetization value 80.5emu/g, Fe2+18.7% by weight)8
00 g in 3200 ml of water to obtain an aqueous dispersion, add 2.0 mol/l CoSO4 aqueous solution 67.9
ml (corresponding to 1.0% by weight in terms of Co relative to the precursor particles) was added and mixed, and after flowing N2 gas to create a non-oxidizing atmosphere, the temperature of this mixed solution was set to 30°C. did. Thereafter, it was treated under a non-oxidizing atmosphere.

[0069] 1230 ml of 18 mol/l NaOH aqueous solution was added to the above mixture, and the OH group concentration was 1.5 mol/l.
After stirring for 10 minutes at a temperature of 40°C,
Furthermore, 1.723 mol/l FeSO4 aqueous solution 466
ml (corresponding to 5.6% by weight in terms of Fe based on the precursor particles) was added, the temperature was set at 40° C., and stirring was continued for 30 minutes after the addition to form blackish brown precipitated particles.

Next, black precipitate particles were added at 2.0 mol/l.
176.5 ml of CoSO4 aqueous solution (corresponding to 2.6% by weight in terms of Co based on the precursor particles) and 3.2 g of No. 3 water glass (0.1% in terms of SiO2 based on the precursor particles)
This corresponds to 14% by weight. ) and water to bring the total volume to 130.
00ml. Subsequently, stirring was continued for 30 minutes at a temperature of 40°C, and the temperature was raised to 98°C while adding 8.0 g of the above-mentioned No. 3 water glass (corresponding to 0.285% by weight of SiO2 based on the precursor particles). After raising the temperature, the mixture was maintained for 360 minutes and stirred under heating to generate black precipitated particles.

The produced black precipitated particles were filtered, washed with water, and dried in a conventional manner to obtain black particle powder.

The obtained black particles are Co-coated acicular magnetic iron oxide particles, with an average major axis diameter of 0.20 μm,
Axial ratio (long axis diameter/short axis diameter) 7.5, coercive force 750 Oe, saturation magnetization value 84.4 emu/g, Fe2+ content 17.8
% by weight. Further, the erasing characteristic was 54.3 dB, and ΔHc was -22 Oe, which showed little deterioration over time.

[0073] Using the obtained Co-coated magnetic iron oxide particle powder, a sheet sample was prepared and the sheet properties were determined as follows:
Coercive force 770Oe, squareness ratio (Br/Bm) 0.811,
Coercive force distribution (S.F.D.) 0.422, electrical resistance 1.
It was 6×10 9 Ω/sq.

Examples 2 to 10, Comparative Examples 1 to 4 Types of precursor particles, amount and timing of addition of aqueous Co salt solution, amount and timing of addition of aqueous Fe2+ solution, amount and timing of addition of Si compound, aqueous alkali hydroxide solution Co-coated acicular magnetic iron oxide particle powder was obtained in the same manner as in Example 1, except that the OH group concentration, the temperature during addition and heating stirring, and the addition stirring time and heating stirring time were varied. .

[0075] The main manufacturing conditions and various properties at this time are shown in Tables 1 to 3.

[0076]

[Table 1]

[0077]

[Table 2]

[0078]

[Table 3]

[0079]

[Effects of the Invention] The Co-coated acicular magnetic iron oxide particles produced according to the present invention have the following properties as shown in the above examples:
It has high coercive force, excellent erasing characteristics and coercive force distribution, and good magnetic stability over time, making it suitable for high-density recording.

Claims (4)

[Claims] Claim 1: Add and mix a Co salt aqueous solution to an aqueous dispersion of acicular magnetic iron oxide particle powder in a non-oxidizing atmosphere at a temperature of 60° C. or lower, and add an alkali hydroxide aqueous solution to this mixture to make it alkaline. The surface of the acicular magnetic iron oxide particles is modified with a Co compound by heating and stirring this suspension at a temperature below the boiling point in a non-oxidizing atmosphere, and then the suspension is heated to 60°C. After bringing the temperature below ℃,
Add and mix Fe2+ salt aqueous solution, under non-oxidizing atmosphere,
Further modifying the surface of the acicular magnetic iron oxide particles with an Fe compound by heating and stirring at a temperature below the boiling point,
Next, after bringing the temperature of the suspension to 60°C or less, Co
An aqueous salt solution and a Si compound were added and mixed, and the top layer on the surface of the acicular magnetic iron oxide particles was modified by a Co compound containing Si by heating and stirring in a non-oxidizing atmosphere at a temperature below the boiling point. A method for producing acicular magnetic iron oxide particles for magnetic recording, the method comprising the following steps: filtering, washing with water, and drying to obtain acicular magnetic iron oxide particles. [Claim 2] Co salt aqueous solution and Fe2+ salt aqueous solution, or Co salt aqueous solution and Si compound or Co salt aqueous solution, Fe2+ salt are added to an aqueous dispersion of acicular magnetic iron oxide particle powder in a non-oxidizing atmosphere at a temperature of 60° C. or less. An aqueous solution and a Si compound are added and mixed, an alkali hydroxide aqueous solution is added to this mixed solution to obtain an alkaline suspension, and this suspension is heated and stirred at a temperature below the boiling point in a non-oxidizing atmosphere. After denaturing the surface of the acicular magnetic iron oxide particles with a Co compound and a Fe compound, or a Co compound containing Si, or a Co compound containing Si and a Fe compound, and then bringing the temperature of the suspension to 60° C. or lower, The surface of the acicular magnetic iron oxide particles is further modified with the Fe compound by adding and mixing an aqueous Fe2+ salt solution and heating and stirring at a temperature below the boiling point in a non-oxidizing atmosphere, and then the suspension is
After bringing the temperature to 0° C. or lower, a Co salt aqueous solution and a Si compound are added and mixed, and heated and stirred in a non-oxidizing atmosphere at a temperature lower than the boiling point to form the top layer on the surface of the acicular magnetic iron oxide particles. A method for producing acicular magnetic iron oxide particles for magnetic recording, which comprises modifying the powder with a Co compound containing Si, followed by filtering, washing with water, and drying to obtain acicular magnetic iron oxide particles. 3. Add and mix a Co salt aqueous solution to an aqueous dispersion of acicular magnetic iron oxide particle powder in a non-oxidizing atmosphere at a temperature of 60° C. or less, and add an alkali hydroxide aqueous solution to this mixture to make it alkaline. The surface of the acicular magnetic iron oxide particles is modified with a Co compound by heating and stirring this suspension at a temperature below the boiling point in a non-oxidizing atmosphere, and then the suspension is heated to 60°C. After bringing the temperature below ℃,
Add and mix Fe2+ salt aqueous solution and Co salt aqueous solution or Fe2+ salt aqueous solution and Si compound or Fe2+ salt aqueous solution, Co salt aqueous solution and Si compound, under a non-oxidizing atmosphere,
By heating and stirring at a temperature below the boiling point, the surface of the acicular magnetic iron oxide particles is further coated with Fe compound and Co compound or S.
Fe compound containing i or Fe compound containing Si and C
o compound, and then the suspension was denatured with 6
After bringing the temperature to 0° C. or lower, a Co salt aqueous solution and a Si compound are added and mixed, and heated and stirred in a non-oxidizing atmosphere at a temperature lower than the boiling point to form the top layer on the surface of the acicular magnetic iron oxide particles. A method for producing acicular magnetic iron oxide particles for magnetic recording, which comprises modifying the powder with a Co compound containing Si, followed by filtering, washing with water, and drying to obtain acicular magnetic iron oxide particles. 4. Co salt aqueous solution and Fe2+ salt aqueous solution, or Co salt aqueous solution and Si compound or Co salt aqueous solution, Fe2+ salt in an aqueous dispersion of acicular magnetic iron oxide particle powder at a temperature of 60° C. or less in a non-oxidizing atmosphere. An aqueous solution and a Si compound are added and mixed, an alkali hydroxide aqueous solution is added to this mixed solution to obtain an alkaline suspension, and this suspension is heated and stirred at a temperature below the boiling point in a non-oxidizing atmosphere. After denaturing the surface of the acicular magnetic iron oxide particles with a Co compound and a Fe compound, or a Co compound containing Si, or a Co compound containing Si and a Fe compound, and then bringing the temperature of the suspension to 60° C. or lower, Fe2+ salt aqueous solution and Co
Salt aqueous solution or Fe2+ salt aqueous solution and Si compound or F
By adding and mixing e2+ salt aqueous solution, Co salt aqueous solution and Si compound, and heating and stirring at a temperature below the boiling point in a non-oxidizing atmosphere, the surface of the acicular magnetic iron oxide particles is further coated with Fe compound and Co compound or The suspension is denatured with a Fe compound containing Si or a Fe compound containing Si and a Co compound, and then the temperature of the suspension is lowered to 60°C, and then an aqueous Co salt solution and a Si compound are added and mixed to form a non-oxidized By heating and stirring in a magnetic atmosphere at a temperature below the boiling point, the top layer on the surface of the acicular magnetic iron oxide particles is converted to carbon containing Si.
A method for producing acicular magnetic iron oxide particles for magnetic recording, which comprises modifying the powder with an o compound, followed by filtering, washing with water, and drying to obtain acicular magnetic iron oxide particles.