阅读说明：本技术 铁系氧化物磁性粉及其制造方法 (Iron-based oxide magnetic powder and method for producing same ) 是由 大越慎一 生井飞鸟 坂根坚之 川人哲也 于 2019-03-26 设计创作，主要内容包括：本发明提供粒度分布狭窄、且无助于磁记录特性的微细粒子的含量少,其结果矫顽力分布狭窄、适于磁记录介质的高记录密度化的铁系氧化物磁性粉的制造方法。通过湿式法得到ε型的铁系氧化物磁性粉后,向包含该磁性粉的浆料中添加作为表面改性剂的0.009mol/kg以上且1.0mol/kg以下的四烷基铵盐,并且将pH设为11以上且14以下,实施分散处理后进行分级,由此得到粒度分布狭窄、且矫顽力分布狭窄的铁系氧化物磁性粉。(The present invention provides a method for producing an iron-based oxide magnetic powder having a narrow particle size distribution, a small content of fine particles that do not contribute to magnetic recording characteristics, a narrow coercive force distribution as a result, and a high recording density suitable for a magnetic recording medium. A magnetic powder of an iron oxide of a wet type is obtained, and then a tetraalkylammonium salt of 0.009mol/kg or more and 1.0mol/kg or less is added as a surface modifier to a slurry containing the magnetic powder, and the slurry is subjected to a dispersion treatment with a pH of 11 or more and 14 or less, and then classified, thereby obtaining a magnetic powder of an iron oxide having a narrow particle size distribution and a narrow coercive force distribution.)

1. The iron oxide magnetic powder comprises iron oxide particles having an average particle diameter of 8-30 nm and a coefficient of variation of particle diameter of 30% or less as measured by a transmission electron microscope, and is defined by I_L/I_HThe value of (A) is 0.55 or less,

wherein, I_HThe magnetic field is 3979kA/M (50kOe), and the M measurement range is 0.005A · M²(5emu), a B-H curve obtained by measuring the applied magnetic field change speed of 13(kA/m · s), a time constant of 0.03sec and a waiting time of 0.8sec, and the intensity of a peak appearing on the high magnetic field side in a differentiated B-H curve obtained by numerically differentiating the B-H curve, and I_LIs the strength of the longitudinal axis intercept at zero field of the differentiated B-H curve.

2. An iron-based oxide magnetic powder comprising particles of iron oxide having an average particle diameter of 8nm to 30nm as measured by a transmission electron microscope and having a coefficient of variation of particle diameter of 30% or less in which a part of Fe sites is substituted with another metal element, wherein I is defined below_L/I_HThe value of (A) is 0.55 or less,

wherein, I_HThe magnetic field is applied in a range of 1035kA/M (13kOe) and M of 0.005A · M²(5emu), a B-H curve obtained by measuring the applied magnetic field change rate of 15(kA/m · s), a time constant of 0.03sec, and a waiting time of 0.1sec, and the intensity of a peak appearing on the high magnetic field side in a differentiated B-H curve obtained by numerically differentiating the B-H curve, and further,I_Lis the strength of the longitudinal axis intercept at zero field of the differentiated B-H curve.

3. The iron-based oxide magnetic powder according to claim 2, wherein the metal element that substitutes for a part of the Fe sites is one or more of Ga, Co, and Ti.

4. The iron-based oxide magnetic powder according to claim 1 or 2, wherein the squareness ratio is 0.54 or more.

5. A method for producing an iron-based oxide magnetic powder, comprising:

a step of preparing a slurry containing iron oxide particles having an average particle diameter of 5nm to 100nm as measured by a transmission electron microscope and a coefficient of variation in particle diameter of 70% or less or iron oxide particles in which a part of Fe sites is substituted with another metal element,

a step of adding a quaternary ammonium salt as a surface modifier to the slurry at a concentration of 0.009mol/kg to 1.0mol/kg inclusive and a pH of 11 to 14 inclusive,

subjecting the slurry containing the surface modifier to a dispersion treatment to obtain a dispersion slurry of particles of iron oxide or iron oxide in which a part of the Fe sites is replaced with another metal element, and

and classifying the dispersed slurry of the iron oxide particles in which a part of the iron oxide or Fe sites is substituted with another metal element.

6. The method for producing an iron-based oxide magnetic powder according to claim 5, wherein the quaternary ammonium salt is a tetraalkylammonium salt.

7. The method for producing iron-based oxide magnetic powder according to claim 5, wherein the quaternary ammonium salt is tetraalkylammonium hydroxide.

8. The method for producing an iron-based oxide magnetic powder according to claim 5, wherein the average secondary particle diameter of the particles of iron oxide in which iron oxide or Fe sites are partially substituted with another metal element, as measured by a dynamic light scattering particle size distribution measuring apparatus, in the dispersed slurry of the particles of iron oxide in which iron oxide or Fe sites are partially substituted with another metal element is 65nm or less.

9. The method for producing iron-based oxide magnetic powder according to claim 5, wherein a slurry containing the iron oxide or particles of iron oxide in which a part of Fe sites is replaced with another metal element has a conductivity of 15mS/m or less.

10. The method of producing iron-based oxide magnetic powder according to claim 5, wherein the classifying step is a step of applying a slurry of the iron oxide or a dispersion of iron oxide particles in which a part of Fe sites is replaced with another metal element to a centrifuge and removing a supernatant liquid thereof.

11. The method for producing an iron-based oxide magnetic powder according to claim 10, wherein a centrifugal acceleration when applied to the centrifugal separator in the classifying step is 40000G or more.

Technical Field

The present invention relates to an iron-based oxide magnetic powder suitable for high-density magnetic recording media, electromagnetic wave absorbers, and the like, and particularly to a magnetic powder having an average particle diameter of a nanometer order and a method for producing the same.

Background

-Fe₂O₃Is a very dilute phase in iron oxide, but the nano-sized particles exhibit 20kOe (1.59X 10) at room temperature⁶A/m) and a large coercive force (Hc), so that it has been conventionally carried out to synthesize-Fe in a single phase₂O₃Study of the production method (patent document 1, non-patent documents 1 and 2). In addition, mixing-Fe₂O₃In the case of magnetic recording media, since there is no material for magnetic heads having a high level of saturation magnetic flux density corresponding thereto, substitution of-Fe with trivalent metals such as Al, Ga, and In has also been carried out₂O₃The coercive force was also adjusted by adjusting a part of the Fe sites in (b), and the relationship between the coercive force and the electromagnetic wave absorption characteristics was also investigated (patent document 2, non-patent documents 3, 4, and 5).

On the other hand, in the field of magnetic recording, magnetic recording media having a high Ratio of reproduction signal level to granular Noise (C/N Ratio) are being developed, and for higher recording densities, miniaturization of magnetic grains constituting a magnetic recording layer is required. However, in general, the miniaturization of magnetic particles tends to deteriorate the environmental stability and thermal stability, and there is a fear that the magnetic properties of the magnetic particles in the environment of use or storage are lowered, and therefore, Fe is substituted with another metal having excellent heat resistance₂O₃F of (A)Part of the e site, developed as a general formula-A_xB_yFe_2-x-yO₃or-A_xB_yC_zFe_2-x-y-zO₃(wherein A is a divalent metal element such as Co, Ni, Mn, Zn, etc., B is a tetravalent metal element such as Ti, etc., and C is a trivalent metal element such as In, Ga, Al, etc.) -Fe having a reduced particle size, a variable coercive force, and excellent environmental stability and thermal stability₂O₃A partial substitution body of (1) (patent document 3, non-patent document 6).

-Fe₂O₃Obtained as a stable phase in the nanoscale dimensions, and therefore require special processes for their manufacture. Patent documents 1 to 3 describe-Fe obtained by using fine crystals of iron oxyhydroxide produced by a liquid phase method as a precursor, coating the precursor with a silicon oxide by a sol-gel method, and then performing a heat treatment₂O₃The production methods of (1) disclose a reverse micelle method using an organic solvent as a reaction medium and a method using only an aqueous solution as a reaction medium, respectively, as liquid phase methods. However, the-Fe obtained by these methods₂O₃or-Fe₂O₃Because the magnetic properties of the partial substitution body of (2) are not uniform (vary), it is proposed to remove the silicon oxide coating after the heat treatment and to perform a classification treatment to improve the magnetic properties thereof.

For example, patent documents 4 and 5 disclose that-Fe is produced by a reverse micelle method₂O₃or-Fe₂O₃The method (4) is a technique of subjecting a part of the displaced product of (1) to centrifugal separation in ultrapure water. The reason why ultrapure water is used as the dispersion medium is to reduce the ionic strength of the medium and to improve the dispersibility of the slurry. However, -Fe₂O₃or-Fe₂O₃Since the partial substitution product of (2) does not exhibit good dispersibility in the vicinity of neutral, the classification methods disclosed in patent documents 4 and 5 have the following problems: in order to obtain good dispersibility in the neutral region, it is necessary to partially leave the silicon oxide coating in the-Fe region₂O₃or-Fe₂O₃Partial substitution of (2) is required after the classification treatmentThe silicon oxide coating was removed again.

Patent document 6 discloses, as a method for improving the dispersion treatment method, the following technique: to a catalyst containing-Fe produced using an aqueous solvent₂O₃or-Fe₂O₃NaOH is added to the slurry of the particles of the partial substitution body of (2), and the pH of the dispersion is adjusted to 10 to 11, followed by classification treatment. The particle size distribution and coercive force distribution of the type of iron-based oxide magnetic powder obtained by this method are narrower and the content of the nonmagnetic component is small compared to the magnetic powder obtained by the method described in patent document 4 or patent document 5, but the particle size distribution and coercive force distribution cannot be said to be sufficient when used in a magnetic recording medium.

Disclosure of Invention

Problems to be solved by the invention

The iron-based oxide magnetic powder of the type produced by the conventional production method disclosed in the above patent documents 1 to5 has excellent magnetic properties, but depending on the production conditions, a variation in the coercive force distribution may be observed. According to the study of the present inventors, it was found that the variation in the coercivity distribution is caused by the presence of fine particles having a small coercivity included in the iron-based oxide magnetic powder. Further, the content of the fine particles in the type iron-based oxide magnetic powder obtained by the production method disclosed in patent document 6 is smaller than that in the magnetic powder produced by the production methods of patent documents 1 to5, but when it is considered to be used as a coating type high recording density magnetic recording medium, it is still necessary to further reduce the content of the fine particles.

That is, two peaks are observed in a curve obtained by numerically differentiating a hysteresis curve (B-H curve) obtained by measuring a magnetic powder obtained by a conventional method (hereinafter, referred to as a differentiated B-H curve). Of these peaks, the magnetic powder corresponding to the high Hc component, which is the peak appearing at a position where the applied magnetic field is high, is considered to contribute to magnetic recording, but the magnetic powder corresponding to the low Hc component, which is the peak appearing at a position where the applied magnetic field is low, is considered to contribute to no magnetic recording. When the content of the fine particles is reduced, the peak of the low Hc component of the differential B-H curve is reduced, and therefore, the magnetic properties of the magnetic powder can be improved by reducing the content of the fine particles.

The reason why the Hc of the fine particles is low is not clear at present, and it is presumed that the fine particles contain a heterogeneous phase for the type iron oxide such as an α -type iron oxide or a γ -type iron oxide, or exhibit superparamagnetism due to a small particle diameter.

In view of the above problems, the present invention provides a method for producing an iron-based oxide magnetic powder having greatly improved characteristics of particle size distribution and coercive force distribution.

Means for solving the problems

The inventor finds that: the present inventors have found that a magnetic powder having a significantly reduced content of fine particles can be obtained by classifying a slurry containing a type of iron-based oxide magnetic powder in a basic region in which the magnetic powder is well dispersed, and further classifying the magnetic powder in a state in which a quaternary ammonium salt, preferably a tetraalkylammonium salt, dissociated in the basic region is present in the slurry as a surface modifier, and have completed the present invention described below.

In order to solve the above problems, the present invention provides an iron-based oxide magnetic powder comprising iron oxide particles having an average particle diameter of 8nm to 30nm as measured by a transmission electron microscope and a coefficient of variation in particle diameter of 30% or less, wherein I is defined as_L/I_HThe value of (A) is 0.55 or less.

Wherein, I_HThe magnetic field is 3979kA/M (50kOe), and the M measurement range is 0.005A · M²(5emu), a change rate of an applied magnetic field of 13(kA/m · s), a time constant of 0.03sec, and a waiting time of 0.8sec, and the intensity of a peak appearing on the high magnetic field side in a differentiated B-H curve obtained by numerically differentiating the B-H curve. In addition, I_LIs the strength of the longitudinal axis intercept at zero field of the differentiated B-H curve.

The present invention also provides an iron-based oxide magnetic powder comprising particles of iron oxide having an average particle diameter of 8nm to 30nm as measured by a transmission electron microscope and a coefficient of variation of particle diameter of 30% or less in which a part of Fe sites is substituted with another metal element, preferably one or two or more of Ga, Co and Ti (hereinafter, iron oxide or iron oxide having Fe sites partially substituted with another metal element is collectively referred to as a type iron-based oxide), wherein I defined below_L/I_HThe value of (A) is 0.55 or less.

Wherein, I_HIs toThe magnetic field 1035kA/M (13kOe) was applied, and the M measurement range was 0.005A · M²(5emu), a change rate of an applied magnetic field of 15(kA/m · s), a time constant of 0.03sec, and a waiting time of 0.1sec, and the intensity of a peak appearing on the high magnetic field side in a differentiated B-H curve obtained by numerically differentiating the B-H curve. In addition, I_LIs the strength of the longitudinal axis intercept at zero field of the differentiated B-H curve.

The squareness ratio SQ of these iron-based oxide magnetic powders is preferably 0.54 or more.

Further, the present invention provides a method for producing an iron-based oxide magnetic powder, comprising: preparing a slurry containing particles of a type iron oxide having an average particle diameter of 5nm to 100nm as measured by a transmission electron microscope and a coefficient of variation in particle diameter of 70% or less; adding a quaternary ammonium salt, preferably a tetraalkylammonium salt, more preferably a tetraalkylammonium hydroxide as a surface modifier to the slurry so as to have a concentration of 0.009mol/kg or more and 1.0mol/kg or less, and setting the pH to 11 or more and 14 or less; subjecting the slurry containing the surface modifier to a dispersion treatment to obtain a type iron oxide dispersion slurry; and a step of classifying the iron-based oxide dispersion slurry.

Preferably, the average secondary particle diameter of the iron-based oxide in the iron-based oxide dispersion slurry of the above type, as measured by a dynamic light scattering particle size distribution measuring apparatus, is 65nm or less. Further, it is preferable that the conductivity of the slurry containing the particles of the iron-based oxide is 15 mS/m.

Preferably, the classification step is a step of applying the iron-based oxide dispersion slurry to a centrifuge and then removing a supernatant liquid, and a centrifugal acceleration when the slurry is applied to the centrifuge is 40000G or more.

Effects of the invention

By using the production method of the present invention, it is possible to obtain an iron-based oxide magnetic powder having a narrow particle size distribution, particularly a small content of fine particles that do not contribute to improvement of magnetic recording properties, and as a result, having a narrow coercive force distribution and being suitable for increasing the recording density of a magnetic recording medium.

Drawings

Fig. 1 is a TEM photograph of the iron-based oxide magnetic powder obtained in example 1.

FIG. 2 is a differential B-H curve of the iron-based oxide magnetic powder obtained in comparative example 2.

FIG. 3 is a differential B-H curve of the iron-based oxide magnetic powder obtained in example 4.

FIG. 4 is a differential B-H curve of the iron-based oxide magnetic powder obtained in example 10.

Detailed Description

[ iron-based oxide magnetic powder ]

The production method of the present invention is used for producing a magnetic powder of an iron-based oxide of a type having an improved particle size distribution and coercive force distribution, and the following are examples of iron-based oxides that can be used as a starting material.

In the general formula-Fe₂O₃An iron-based oxide containing no substitution element.

Is represented by the general formula-C_zFe_2-zO₃(wherein C is a trivalent metal element).

Is represented by the general formula-A_xB_yFe_2-x-yO₃(wherein A is a divalent metal element and B is a tetravalent metal element).

Is represented by the general formula-A_xC_zFe_2-x-zO₃(wherein A is a divalent metal element and C is a trivalent metal element).

With the general formula-B_yC_zFe_2-y-zO₃(wherein B is a tetravalent metal element, and C is a trivalent metal element).

Is represented by the general formula-A_xB_yC_zFe_2-x-y-zO₃(wherein A is a divalent metal element, B is a tetravalent metal element, and C is a trivalent metal element).

Here, the type of substitution with only C element can arbitrarily control the coercive force of the magnetic particle, and has the advantage of being easily available with-Fe₂O₃Advantages of the same space group, but thermal stabilityThe qualitative is slightly poor, and therefore, it is preferable to simultaneously replace the elements with A or B.

The type substituted with two elements, a and B, is excellent in thermal stability and capable of maintaining the coercive force of the magnetic particles at room temperature to a high level, but it is slightly difficult to obtain-Fe₂O₃A single phase of the same space group.

A. The type of substitution with the three elements B and C is the most favorable type in which the above properties are well balanced, and is excellent in heat resistance, easy to obtain a single phase, and controllability of coercive force, and is most preferable.

The substitution elements include Co, Ni, Mn, Zn, etc. as A, Ti, Sn, etc. as B, In, Ga, Al, etc. as C. In the case of the three-element substitution type, from the viewpoint of practical balance of characteristics, it is preferable that the metal elements substituting a part of the Fe sites be Ga, Co, and Ti.

The preferable ranges of the substitution amounts x, y and z of the three-element substituent are as follows.

x and y may be in any ranges of 0 < x < 1 and 0 < y < 1, but considering the use for magnetic recording, it is necessary to make the coercive force of the magnetic particles of the three-element displacement body equal to that of-Fe without displacement₂O₃Since the coercive force of (b) varies to some extent, it is preferable to set 0.01 ≦ x ≦ 0.2 and 0.01 ≦ y ≦ 0.2. z may be in the range of 0 < z < 1, as with x and y, but is preferably in the range of 0 < z ≦ 0.5 from the viewpoint of coercivity control and easy acquisition of a single phase.

In the production method of the present invention, a high coercive force can be maintained at normal temperature by appropriately adjusting y or values of x and y for the type of iron-based oxide used as a starting material, and the coercive force can be controlled to a desired value by adjusting x, y, and z.

In the iron oxide magnetic powder of the present invention, in addition to the type iron oxide crystal, α -type iron oxide, γ -type iron oxide, and Fe may be present as impurities₃O₄These crystals are included and are referred to as iron-based oxide magnetic powder.

[ average particle diameter ]

In the production method of the present invention, the magnetic particles of the iron-based oxide of the type used as the starting material are preferably fine to the extent that each particle has a single-domain structure. The average particle diameter thereof measured by a transmission electron microscope is preferably 100nm or less, more preferably 30nm or less. However, when the average particle diameter is too small, the proportion of fine particles that do not contribute to the improvement of the magnetic properties is increased, and the magnetic properties per unit weight of the magnetic particle powder are deteriorated, and therefore, it is preferable that the average particle diameter is 5nm or more. The iron-based oxide to be a starting material in the present invention can be prepared by any method including the production methods described in patent documents 1 to 6. The average particle diameter after classification by the production method of the present invention is preferably 8nm or more and 30nm or less.

[ coefficient of variation ]

The coefficient of variation (hereinafter referred to as CV value) of the average particle diameter of magnetic particles of a type iron oxide obtained by a conventional production method is usually about 35% to 70%.

The CV value decreases by classification using the production method of the present invention, and the classified CV value is preferably 30% or less, more preferably 25% or less, and still more preferably 20% or less. The lower limit of the CV value is preferably as small as possible, but since it is necessary to increase the load on the production process in order to reduce the CV value, it is usually 10% or more.

The reason why the magnetic particles of the iron-based oxide having a small CV value can be obtained by the production method of the present invention is as follows: when a quaternary ammonium salt is used as a surface modifier described later, aggregates of particles having an average particle diameter of 8nm or less, which have been difficult to disperse in the past, can be dispersed, and particles having an average particle diameter of 8nm or less can be removed by a subsequent classification operation.

[I_L/I_H]

When the B-H curve is measured for an iron-based oxide magnetic powder produced by the liquid phase method, when demagnetization is completed and an external magnetic field is increased, a small shoulder (dent) is present in the increase curve of the magnetic flux density in the vicinity of zero magnetic field. Therefore, two peaks are observed in the differentiated B-H curve obtained by numerically differentiating the B-H curve. This means that the B-H curve measured for the iron-based oxide magnetic powder is a curve synthesized from two B-H curves having different coercive forces Hc, and the iron-based oxide magnetic powder contains two components having different magnetic properties.

Here, the low Hc side component is a component that does not contribute to an increase in recording density when the iron-based oxide magnetic powder is used for a magnetic recording medium. When the proportion of particles having a larger number of particles than the average particle diameter contained in the iron-based oxide magnetic powder is reduced by changing the production conditions, classification, or the like, the height of the peak on the low Hc side of the differential B-H curve is observed to decrease, and therefore, it is judged that the fine particles are a low Hc component.

Now, considering the use of iron-based oxide magnetic powder for a magnetic recording medium, the intercept of the vertical axis at 0 magnetic field of the differential B-H curve is set to I_LLet the peak height on the high Hc side be I_HThe ratio of the peak heights I_L/I_HThe lower the value of (A), the less particles do not contribute to magnetic recording, and the higher the recording density.

In the conventional production method, I is usually obtained as magnetic particles of type iron-based oxide_L/I_HThe value of (b) is usually 3.0 or less. I of magnetic particles of iron-based oxide of the type used as a starting material in the production method of the present invention_L/I_HThe value of (d) is preferably 2.7 or less.

[ rectangle ratio SQ ]

The squareness ratio SQ of the iron-based oxide magnetic powder obtained by the present invention is preferably 0.54 or more. In the field of coating-type magnetic recording media, from the viewpoint of magnetic powder suitable for a system for performing recording, the squareness ratio (SQ) of the magnetic powder is required to be large. Since this squareness ratio (SQx ═ σ r/σ s) is a ratio of the residual magnetization σ r of the magnetic powder to the saturation magnetization σ s of the magnetic powder when a magnetic field is applied, the output is improved when SQ is high, and therefore, a magnetic powder with high SQ is required to produce a high-performance coating-type recording medium.

[ slurry containing iron-based oxide particles ]

In the production method of the present invention, first, a slurry is prepared in which magnetic particles of a starting material type iron-based oxide are dispersed in pure water. In this case, from the viewpoint of suppressing aggregation of the iron-based oxide and reducing impurities, it is preferable to set the conductivity of the solvent constituting the slurry to ≦ 15 mS/m. When a slurry containing iron-based oxide magnetic particles of a type produced by a wet method immediately after removal of a silicon oxide coating in an alkaline solution is used as a starting material, the slurry is preferably cleaned with pure water until the conductivity of the solvent becomes ≦ 15 mS/m. Then, a quaternary ammonium salt is added to the slurry to adjust the pH of the slurry to 11 or more and 14 or less. As described later, when a tetraalkylammonium salt hydroxide is used as the quaternary ammonium salt, the pH of the slurry is in the above range even without adding another base.

The pH value described in the present specification was measured using a glass electrode according to JIS Z8802. The pH standard solution is a value measured by a pH meter calibrated by using an appropriate buffer corresponding to the measured pH region. The pH described in the present specification is a value obtained by directly reading a measurement value displayed by a pH meter compensated by a temperature compensation electrode under reaction temperature conditions.

[ Quaternary ammonium ion ]

The quaternary ammonium ion being ammonium ion (NH)₄ ⁺) The cation wherein all of the 4 hydrogen atoms are substituted with an organic group is usually an alkyl group or an aryl group. In the production method of the present invention, as a surface modifier for magnetic particles of a type iron-based oxide, a quaternary ammonium ion stable in a strongly basic region where the magnetic particles have good dispersibility is used. Among the quaternary ammonium ions, tetraalkylammonium ions which are industrially readily available are preferably used. The tetraalkylammonium ion is quaternary ammonium cation with molecular formula of NR₄ ⁺(R is an arbitrary alkyl group). The tetraalkylammonium salt is a completely dissociated salt, and the tetraalkylammonium ion exists as a stable ion in an aqueous alkaline solution, and therefore, is used as a surface modifier for improving the dispersibility of the type iron-based oxide in the slurry in the production method of the present invention. As the tetraalkylammonium ion supply source, there are hydroxides, chlorides, bromides, etc., and since the tetraalkylammonium ion hydroxide itself is a strong base, when it is added to the slurry, the slurry is not added with other basesThe pH of (b) is also in the above-described preferable pH range, and therefore, it is further preferable to use tetraalkylammonium hydroxide as the surface modifier.

As the tetraalkylammonium ion supply source, chloride or bromide of tetraalkylammonium ion can be used, but in this case, the increase in pH is suppressed when these salts are added to the slurry, and an additional base is required for pH adjustment, and the ionic strength of the system increases, so that the dispersibility of the type iron oxide is slightly inferior to that when hydroxide is used.

The tetraalkylammonium salt may be a quaternary ammonium salt having the same alkyl group such as a tetramethylammonium salt, a tetrapropylammonium salt, or a tetrabutylammonium salt, or a quaternary ammonium salt having a different alkyl group, and any of them can be used, and it is preferable to use a hydroxide such as tetramethylammonium hydroxide (tetramethylammonium hydroxide), tetrapropylammonium hydroxide (tetrapropylammonium hydroxide), or tetrabutylammonium hydroxide (tetrabutylammonium hydroxide).

The concentration of tetraalkylammonium ion is 0.009mol/kg to 1.0mol/kg in order to modify the surface of the type iron oxide particles in the slurry and improve dispersibility. When the concentration is less than 0.009 mol% or exceeds 1.0mol/kg, the dispersibility-improving effect is insufficient, and particles having a CV value of 30% or less after classification cannot be obtained.

[ Dispersion treatment ]

In the production method of the present invention, the slurry containing the surface modifier to which the surface modifier is added is subjected to a dispersion treatment to obtain an iron-based oxide dispersion slurry. Here, the dispersion treatment is a treatment of removing aggregation of the type iron-based oxide included in the slurry containing the surface modifier. As the dispersion treatment method, known methods such as dispersion using an ultrasonic disperser, pulverization using a medium such as a bead mill, a stirring blade, and stirring using an oscillator and a shaker can be used.

The dispersion treatment of the slurry containing the surface modifier is preferably performed until the average secondary particle diameter (DLS particle diameter) of the iron-based oxide measured by a dynamic light scattering particle size distribution measuring apparatus becomes 65nm or less. When the DLS particle size is larger than 65nm, particles having a CV value of 30% or less cannot be obtained when the TEM average particle size is small such as 20nm or less, which is not preferable.

[ fractionation operation ]

In the production method of the present invention, the surface modifier-containing slurry to which the surface modifier is added is subjected to a dispersion treatment, and the iron-based oxide dispersion slurry is subjected to a classification operation. As the classification operation, a known wet classification method such as a centrifugal separation method can be used. When the supernatant is removed after application to a centrifuge, fine particles can be removed, and when the precipitate is removed after application to a centrifuge, coarse particles can be removed. The gravitational acceleration during centrifugal separation is preferably 40000G or more. In addition, the classification operation is preferably repeated 3 times or more. After the classification operation, the iron-based oxide magnetic powder is recovered by a known solid-liquid separation method, washed with water as necessary, and dried.

[ Transmission Electron Microscope (TEM) Observation ]

TEM observation of the iron-based oxide magnetic powder obtained by the production method of the present invention was performed under the following conditions. For TEM observation, JEM-1011 made by Nippon electronic Co., Ltd was used. For the particle observation, TEM photographs taken at 10000 times and 100000 times magnification were used. (photograph obtained by removing silicon oxide coating was used).

Measurement of average particle diameter and particle size distribution evaluation (coefficient of variation (%))) -

For TEM average particle size and particle size distribution evaluation (coefficient of variation (%)), a digitizer (デジタイズ) was used. As image processing software, Mac-View Ver.4.0 was used. When the image processing software is used, the particle diameter of a certain particle is calculated as the length of the long side of the rectangle having the smallest area among the rectangles circumscribing the particle. The number of the cells was 200 or more.

Among the particles projected on the transmission electron micrograph, the selection criteria of the particles to be measured are as follows.

[1] Particles having a portion of the particles exposed outside the field of view of the photograph were not measured.

[2] The particles were measured as well defined, independently present.

[3] Particles that can be measured independently as individual particles even if they deviate from the average particle shape are measured.

[4] The particles overlap each other, but the boundary between the particles is clear, and it is also possible to determine the shape of the particles as a whole, and measure each particle as an individual particle.

[5] Particles that overlap each other and have no clear boundary and no overall shape of the particles are determined, and the particles whose shape cannot be determined are not measured.

The number average of the particle diameters of the particles selected by the above-mentioned reference was calculated and used as the average particle diameter of the iron oxide magnetic powder observed by TEM. Further, a value obtained by dividing "the standard deviation of the particle size of the selected particles" by "the number average value (average particle size) of the particle size of the selected particles" was calculated as a variation coefficient of the particle size of the iron oxide magnetic powder observed by TEM.

[ measurement of average Secondary particle diameter ]

The average secondary particle diameter (DLS particle diameter) of the slurry containing the surface modifier obtained by the production method of the present invention was measured under the following conditions.

As the dynamic light scattering particle size distribution measuring apparatus, a tsukamur electronic co ltd (FPAR-1000K high sensitivity standard) was used, and a thin probe (thin プローブ) was used as the optical fiber probe. As for the measurement conditions, at the measurement time (sec): 180 seconds, repetition number: once, setting a solvent: the method is implemented in water. The analytical mode is the Cumulant method.

[ composition analysis by high-frequency inductively coupled plasma emission Spectroscopy (ICP) ]

The composition of the obtained type iron-based oxide magnetic powder was analyzed. For composition analysis, ICP-720ES (アジレントテクノロジー, product name) was used, and Fe was used as a measurement wavelength (nm); 259.940nm, Ga; 294.363nm, Co; 230.786nm and Ti; 336.122 nm.

[ measurement of hysteresis Curve (B-H Curve) ]

The magnetic properties of the obtained iron-based oxide magnetic powder were measured under the following conditions.

In the case of iron-based oxide magnetic powder composed of iron oxide particles, a Vibration Sample Magnetometer (VSM) (VSM-5 HSC manufactured by east English Co., Ltd.) was used as a magnetic characteristic measuring device, and a magnetic field of 3979kA/M (50kOe) was applied thereto, and the M measuring range was 0.005A · M²(5emu), applied magnetic field change rate 13(kA/m · s), time constant 0.03sec, and waiting time 0.8 sec. Coercive force Hc, saturation magnetization σ s, SFD, and squareness ratio SQ were evaluated by B-H curves, and low Hc components that did not contribute to magnetic recording were evaluated by differentiating B-H curves. In this case, the intercept of the vertical axis at 0 magnetic field of the differential B-H curve is represented as I_LLet the peak height on the high Hc side be I_H. SFD is a value obtained by dividing the full width at half maximum (half value of ピーク) of the peak on the high Hc side by the coercivity Hc. In addition, software "high-temperature superconducting VSM measurement program" (ver.1.0.0.1) was used for the measurement and evaluation.

In the case of an iron-based oxide magnetic powder composed of iron oxide particles in which a part of the Fe sites is replaced with another metal element, a Vibration Sample Magnetometer (VSM) (VSM-5, manufactured by Dongxin industries, Ltd.) was used as a magnetic property measuring device, and a magnetic field 1035kA/M (13kOe) was applied thereto, and the M measuring range was 0.005A · M²(5emu), applied magnetic field change rate 15(kA/m · s), time constant 0.03sec, and waiting time 0.1 sec. Coercive force Hc, saturation magnetization σ s, SFD, and squareness ratio SQ were evaluated by B-H curves, and low Hc components that did not contribute to magnetic recording were evaluated by differentiating B-H curves. In this case, the intercept of the vertical axis at 0 magnetic field of the differential B-H curve is represented as I_LLet the peak height on the high Hc side be I_H. Note that SFD is a value obtained by dividing the full width at half maximum of the peak on the high Hc side by the coercive force Hc. In addition, ancillary software (Ver.2.1) manufactured by east English Industrial Co., Ltd was used for the measurement and evaluation. The reason why the measurement conditions differ between iron oxide and iron oxide in which a part of the Fe site is replaced with another metal element is that the coercive force Hc changes due to the difference in the amount of replacement of the metal element in the Fe site, and the applied magnetic field that can be magnetically saturated differs. The squareness ratio SQ is a ratio to the entire B-H curve.