JP5124826B2 - Ε iron oxide powder with good dispersibility - Google Patents

Description

The present invention relates to an ε-Fe ₂ O ₃ based magnetic powder.

  In the field of magnetic recording, it is required to increase recording density while reducing noise. Therefore, on the magnetic recording medium side, it is important to increase the coercive force Hc of the medium as much as possible and promote magnetic separation while miniaturizing the magnetic particles constituting the medium. Furthermore, it is important to keep the recording state stable even when the magnetic particles are miniaturized.

For example, the magnetic energy (K _U × V) of the minimum unit of the magnetically coupled magnetic aggregate constituting the recording bit greatly exceeds the thermal energy (k _B × T) that tries to disturb the recording. It is done. Here, K _U is the magnetic anisotropy energy constant, V is the magnetic cluster volume, k _B is the Boltzmann constant, and T is the absolute temperature. Using (K _U × V) / (k _B × T) as an index for stably maintaining the recording state, it is a general goal that this ratio is approximately 60 or more (10-year service life). . This is in order to further increase the recording density lowers the magnetic cluster volume V, it can be said that the situation inevitably raise the magnetic anisotropy constant K _U. Since K _U has a relationship of K _U ∝Hc (Hc is a coercive force), in other words, a magnetic material having a higher Hc is required as a magnetic recording medium with a higher recording density is aimed.

In addition, even when the value of (K _U × V) / (k _B × T) is 100 or less, there has been reported an example in which the recording magnetization decreases with the passage of time. as the request to reduce the magnetic cluster volume V becomes stronger, which means that must have a high magnetic anisotropy constant K _U. Therefore, from the viewpoint of reducing noise, a magnetic material having a high Hc is required as a magnetic recording medium having a higher recording density is aimed.

As shown in Non-Patent Documents 1 to 3, recently, existence of ε-Fe ₂ O ₃ exhibiting a huge Hc of 20 kOe at room temperature with a nano-order particle size has been confirmed. Α-Fe ₂ O ₃ and γ-Fe ₂ O ₃ are the most universal polymorphs having a composition of Fe ₂ O ₃ but different crystal structures, and ε-Fe ₂ O ₃ is one of them. One. However, the crystal structure and magnetic properties of ε-Fe ₂ O ₃ were clarified because, as seen in Non-Patent Documents 1 to 3, ε-Fe ₂ O ₃ crystals were synthesized in a substantially single-phase state. This is only recently. Since this ε-Fe ₂ O ₃ shows huge Hc, it is expected to be applied to a magnetic recording medium having a high recording density as described above.

Jian Jin, Shinichi Ohkoshi and Kazuhito Hashimoto, ADVANCED MATERIALS 2004, 16, No. 1, January 5, p.48-51 Jian Jin, Kazuhito Hashimoto and Shinichi Ohkoshi, JOURNAL OF MATERIALS CHIMISTRY 2005, 15, p.1067-1071 Shunsuke Sakurai, Jian Jin, Kazuhito Hashimoto and Shinichi Ohkoshi, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, Vol.74, No.7, July, 2005, p.1946-1949

In order to put a magnetic material having a very high Hc into practical use as a recording medium, a magnetic head that generates a recording magnetic field capable of actually writing information on the recording medium is required. The magnetic field generated by the magnetic head is generally said to be proportional to the saturation magnetic flux density of the soft magnetic film used there. Currently, hard disks having Hc of about 1.5 to 4.5 kOe (1.19 × 10 ^{5 to} 3.58 × 10 ⁵ A / m) have been reported. In order to record information on such hard disks. In the magnetic head, a material having a high saturation magnetic flux density of 2.4 T is used.

As seen in Non-Patent Documents 1 to ₃ , in the case of ε-Fe ₂ O ₃ having a huge Hc of 20 kOe (1.59 × 10 ⁶ A / m) level, this is used as a magnetic recording material of a magnetic recording medium. However, information cannot be recorded unless there is a material having a higher saturation magnetic flux density than the current state. That is, magnetic recording cannot be performed with the current level of magnetic head material.

As a result of detailed studies, the inventors have found that when a part of the Fe site of the ε-Fe ₂ O ₃ crystal is substituted with the trivalent metal element M, the coercive force Hc can be reduced according to the amount of substitution. I found out that I could do it. The metal element M is, for example, Al, Ga, In or the like. By controlling the coercive force Hc using such a replacement method, a magnetic powder exhibiting extremely high Hc can be constructed within a range where magnetic recording can be performed with a magnetic head or the like, and it is expected to be put to practical use in various applications. The

  However, in order to actually use the magnetic powder as a magnetic material such as a magnetic recording medium or a radio wave absorber, [1] not only has a high coercive force Hc but also a high saturation magnetization σs and a large squareness ratio SQ. Therefore, it is required to have a comprehensively excellent magnetic property, and [2] it is required that the dispersibility of the powder particles in the liquid or the polymer base material is good.

In particular, the particle size distribution of the powder should be adjusted in order to sufficiently bring out the high coercivity characteristic inherent to the ε-Fe ₂ O ₃ crystal and to increase the filling rate of the powder particles into the polymer substrate. However, if the dispersibility of the particles in the liquid is not good, it is very difficult to adjust the particle size distribution by classification of the synthesized ε-Fe ₂ O ₃ crystal powder. In addition, when the performance of the magnetic recording medium or the radio wave absorber is improved by operations such as magnetic field orientation, it is an essential requirement that the dispersibility of the powder particles with respect to the polymer substrate is good.

As shown in Non-Patent Documents 1 to 3, the ε-Fe ₂ O ₃ crystal can be synthesized by a process combining a reverse micelle method and a sol-gel method. However, the powder of ε-Fe ₂ O ₃ crystal synthesized in such a manner does not always have stable and excellent magnetic properties, and it can be used in a liquid or a polymer substrate. Further improvement in dispersibility is also desired.
Therefore, the present invention is a powder that can be produced using the above-described synthesis process, and has an overall magnetic property, and an ε-Fe ₂ O ₃ crystal with improved dispersibility in a liquid or a polymer substrate. It aims to provide a powder.

In order to achieve the above object, in the present invention, an Si oxide is formed on the surface of an iron oxide whose main phase is an ε-Fe ₂ O ₃ crystal (including one in which a part of Fe site is substituted with a metal element M). There is provided a magnetic powder comprising a composite particle of iron oxide and Si oxide and having a Si content represented by Si / (Fe + M) × 100 adjusted to 0.1 to 30 mol%. Said M consists of 1 or more types, for example, Al, Ga, and In.
However, when the molar ratio of M to Fe in the iron oxide is expressed as M: Fe = x: (2-x), 0 ≦ x <1.

In the above-mentioned “iron oxide”, in this magnetic powder particle, (i) a crystal having the same space group as α-Fe ₂ O ₃ and (ii) a space group being the same as γ-Fe ₂ O ₃ (Iii) a crystal having the same space group as ε-Fe ₂ O ₃ , (iv) a crystal having the same space group as Fe ₃ O ₄ , and (v) a crystal having the same space group as FeO. Among these, it is a part comprised by 1 or more types. The “main phase” means a crystal having a molar ratio of 50 mol% or more to the whole iron oxide among the crystals of the above (i) to (v). The molar ratio of each of these crystals can be estimated by analysis by a lead belt method based on X-ray diffraction.

In the magnetic powder of the present invention, this main phase is ε-Fe ₂ O ₃ crystal. However, “ε-Fe ₂ O ₃ crystal” as used in this specification includes, unless otherwise specified, a pure ε-Fe ₂ O ₃ crystal in which the Fe site is not substituted with other elements, A crystal partially including a trivalent metal element M and having the same space group as the pure ε-Fe ₂ O ₃ crystal (that is, the space group is Pna2 ₁ ) is included.

The Si oxide present on the surface of the iron oxide is considered to be a silica component mainly composed of SiO ₂ , but the form is not necessarily crystalline, and an amorphous state exhibiting a broad X-ray diffraction peak. It doesn't matter. When the abundance is small, a broad X-ray diffraction peak may not be detected.

As for the particle diameter of the magnetic powder of the present invention, the average particle diameter measured by a TEM photograph is 5 to 200 nm. What has a coercive force of 1000-15000 Oe (7.96 * 10 < ⁴ > -1.19 * 10 < ⁶ > A / m) becomes a suitable object as a magnetic characteristic of powder.

  Measurement of the average particle diameter from a TEM (transmission electron microscope) photograph is obtained by measuring the largest diameter of each particle (major axis diameter in the case of a rod-shaped one) from a TEM photograph image magnified 600,000 times. Can do. Let the average value of the particle diameter calculated | required about 300 independent particle | grains be the average particle diameter of the powder. Hereinafter, this may be referred to as “TEM average particle diameter”.

(1) Since this magnetic powder has a very high coercive force Hc near room temperature, it can contribute to the improvement of the reliability of the magnetic recording medium. In addition, since the coercive force can be controlled by the content of the additive element M, even in magnetic applications where ε-Fe ₂ O ₃ cannot be used because the coercive force is too high, the coercive force is as high as possible within the usable range. It is possible to provide a magnetic material having the same.
(2) Since this magnetic powder consists of iron oxide particles in which iron is oxidized to trivalent, the corrosion resistance in the air environment is extremely good as compared with the conventional metal-based magnetic powder.
(3) Since this magnetic powder has an appropriate amount of Si oxide on the particle surface, the dispersibility of the powder particles in the liquid or in the polymer substrate is good. For this reason, it is possible to classify the synthesized powder particles to adjust the particle size, and to improve the magnetic properties and the magnetic properties or the radio waves by improving the filling properties of the powder particles in the magnetic recording medium or the radio wave absorber. Improvement of absorption characteristics is expected.
(4) This magnetic powder can be synthesized using a process combining the reverse micelle method disclosed in Non-Patent Documents 1 to 3 and a sol-gel method using Si, but Si oxidation present on the particle surface. Since nonmagnetic compounds such as substances are appropriately removed, adverse effects on magnetic properties caused by such nonmagnetic compounds are greatly suppressed.

As described in Non-Patent Documents 1 to 3, ε-Fe ₂ O ₃ nanoparticles can be synthesized by a process combining a reverse micelle method and a sol-gel method and a heat treatment (firing) step. This synthesis process can also be used in the invention. In the reverse micelle method, two types of micelle solution containing a surfactant, ie, micelle solution I (raw material micelle) and micelle solution II (neutralizer micelle) are mixed to precipitate iron hydroxide in the micelle. The gist of this is to proceed. The gist of the sol-gel method is to apply a silica coating to the surface of the iron hydroxide fine particles generated in the micelle. The iron hydroxide fine particles whose surface is covered with silica are separated from the liquid and then subjected to a heat treatment in an air atmosphere at a predetermined temperature (in the range of 700 to 1300 ° C.). By this heat treatment, ε-Fe ₂ O ₃ crystals are synthesized. A large amount of silica coat exists on the surface of the iron oxide having the crystal as a main phase. The magnetic powder of the present invention can be obtained by a method in which most of the silica coat on the particle surface is dissolved and removed using an alkaline aqueous solution.

More specifically, for example, the following is performed.
In the aqueous phase of the micelle solution I having n-octane as the oil phase, iron nitrate (III) as an iron source is used. When a part of iron is replaced with the metal element M, M nitrate (for example, Al Aluminum nitrate (III) 9 hydrate, Ga, gallium nitrate (III) n hydrate, In, indium nitrate (III) trihydrate), and surfactants (eg cetyl bromide) An aqueous ammonia solution is used for the aqueous phase of micelle solution II in which trimethylammonium) is dissolved and n-octane is used as the oil phase. At that time, an appropriate amount of alkaline earth metal (Ba, Sr, Ca, etc.) nitrate can be dissolved in the aqueous phase of the micelle solution I. These alkaline earth metal nitrates function as shape control agents. That is, when alkaline earth metal is present in the liquid, a rod-shaped ε-Fe ₂ O ₃ crystal can be finally obtained. When there is no shape control agent, granular ε-Fe ₂ O ₃ crystals can be obtained.

After combining both micelle solutions I and II, the sol-gel method is used in combination. That is, stirring is continued while dripping silane (for example, tetraethoxysilane, tetramethoxysilane) into the combined liquid, and the reaction of producing iron hydroxide proceeds in the micelle. As a result, the surface of fine particles of iron hydroxide produced in the micelle is coated with silica produced by hydrolysis of silane. Next, the particle powder obtained by separating, washing, and drying the silica-coated iron hydroxide particles from the liquid is charged into a furnace, and 700 to 1300 ° C, preferably 900 to 1200 ° C, more preferably in air. Is heat-treated (fired) in a temperature range of 950 to 1150 ° C. By this heat treatment, an oxidation reaction proceeds in the silica coat, and the fine iron hydroxide particles are changed to fine ε-Fe ₂ O ₃ particles. During this oxidation reaction, the presence of the silica coat contributes to the formation of ε-Fe ₂ O ₃ crystals, not α-Fe ₂ O ₃ or γ-Fe ₂ O ₃ crystals, and the particles are sintered together. Acts to prevent.

In the stage before the heat treatment (firing), the amount of the silica coat can be in the range of 50 to 1000 mol% in terms of the molar ratio expressed by Si / (Fe + M) × 100 in the Si content contained in the raw material. . It is desirable to increase the amount of silica coat as the average particle size is reduced. When the amount of the silica coat is less than 50 mol% in the above molar ratio, coarsening due to sintering of the particles becomes remarkable and α-Fe ₂ O ₃ crystals are easily formed, which is not preferable. For example, in order to obtain a magnetic powder having a particle size of 100 nm or less suitable for magnetic recording and having little sintering, it is preferable to apply a silica coat having a Si content of 100 mol% or more. On the other hand, an excessive silica coating exceeding 1000 mol% is not economically preferable because the particle diameter does not change significantly.

In order to obtain the magnetic powder of the present invention using the above process, it is important to remove the silica coat after ε-Fe ₂ O ₃ is synthesized by heat treatment (firing). The removal of the silica coat can be carried out by putting the magnetic powder after the heat treatment in an aqueous solution in which a strong alkali such as NaOH or KOH is dissolved and stirring. In order to increase the dissolution rate, the alkaline solution may be heated. Typically, when alkali such as NaOH is added 3 mol times or more with respect to the silica content, and the magnetic powder is added and stirred while the aqueous solution temperature is 60 to 70 ° C., the silica can be dissolved well. . The alkali is not limited to NaOH, and an aqueous solution of organic alkali such as NH ₃ or, in some cases, N (CH ₃ ) ₄ OH may be used. However, in order to adjust the amount of Si oxide present on the particle surface so that the Si content represented by Si / (Fe + M) × 100 is 0.1 to 30 mol%, it will be described in the examples below. It is extremely effective to perform the re-dissolution treatment as in [Procedure 6-2].

Using such a silica coat removal method, the amount of Si oxide on the surface of the magnetic particles is controlled so that the Si content represented by Si / (Fe + M) × 100 is 0.1 to 30 mol%. . Thereby, the magnetic powder of the present invention is obtained. Although it is considered that the Si oxide on the surface of the magnetic particles exists as SiO _2, as described above, the X-ray diffraction peak may exist in a broad amorphous state. SiO ₂ has an isoelectric point around pH 2 in water, and therefore exhibits high dispersibility in water in a wide pH range of pH 3 or higher. As the silica coating removal test proceeded, when the Si content by Si / (Fe + M) × 100 was less than 0.1 mol%, a decrease in dispersibility in the liquid was observed. In this case, it is considered that the Si oxide exists in an island shape on the surface of the iron oxide particles. For this reason, the silica coat is not completely removed, but the Si oxide is left so that the Si content according to the above formula becomes 0.1 mol% or more.

  When the Si content by Si / (Fe + M) × 100 is less than 0.1 mol%, the Si oxide is considered to be present in the form of islands on the surface of the iron oxide particles. However, in the range of 0.1 mol% or more, it is presumed that the surface of the iron oxide particles is almost completely covered with Si oxide, and the dispersibility in the liquid or in the polymer substrate is remarkable. Improved. Further, in this state, not only the dispersibility of the powder particles is simply improved, but also a large resistance to the reaction between Fe in the iron oxide and components such as a resin and a dispersant occurs, resulting in durability. Therefore, it can be a magnetic material with further excellent weather resistance and reliability. The Si oxide is more preferably present such that the Si content by Si / (Fe + M) × 100 is 0.5 mol% or more, and more preferably 1 mol% or more.

On the other hand, if the amount of Si oxide present on the surface of the iron oxide particles is excessively large, the Si oxides cross-link the magnetic particles, and if severe, the magnetic particles are contained in the Si oxide. It becomes a structure that is dispersed. In this case, it becomes extremely difficult to increase the filling rate of the magnetic phase and control the orientation of magnetization in the magnetic recording medium and the radio wave absorber, and sufficiently extract the characteristics inherent to the ε-Fe ₂ O ₃ crystal. I can't. As a result of various studies, it is necessary to control the amount of Si oxide deposited so that the Si content by Si / (Fe + M) × 100 is within a range of 30 mol% or less. Since Si oxide is a nonmagnetic component, its presence becomes a negative factor for magnetic properties. For this reason, it is desirable that the dispersibility, durability, weather resistance, and reliability are as low as possible. More preferably, it is within the range of 20 mol% or less, more preferably 10 mol% or less, and even more preferably 5 mol% or less.

  The surface coating material for magnetic particles is not limited to silica, but is a chemically stable, high melting point material that can be removed without dissolving the magnetic particles. It is thought that various things can be used. For example, alumina synthesized at a low temperature is preferable because it can be easily removed by alkali like silica. In addition, calcia and magnesia can be easily dissolved with a weak acid, so that the dissolution of magnetic particles can be minimized and used.

By the way, in the synthesis of the ε-Fe ₂ O ₃ crystal as described above, an ε-Fe ₂ O ₃ crystal may be mixed with an iron oxide crystal (impurity crystal) having a different space group. Examples of such impurity crystals include α-Fe ₂ O ₃ , γ-Fe ₂ O ₃ , FeO, and Fe ₃ O ₄ . When the metal element M is added, part of Fe in these impurity crystals may be substituted with M. Mixing of impurity crystals is not preferable for extracting as much of the characteristics of the ε-Fe ₂ O ₃ crystal as possible, but is allowed as long as the effects of the present invention are not impaired.

For example, when the ratio of ε-Fe ₂ O ₃ crystal in iron oxide is 75 mol% or more, it exhibits excellent magnetic properties that are difficult to realize with conventional magnetic materials, and is useful in various magnetic applications. is there. Even if the ratio of the ε-Fe ₂ O ₃ crystal in the iron oxide is less than 50 to 75 mol%, the magnetic material satisfies the saturation magnetization σs of 2 emu / g (2 A · m ² / kg) or more. If there is a GMR (giant magnetoresistive effect) head that is a high-sensitivity read magnetic head or a TMR head that uses the tunnel effect that is more sensitive, it is possible to read the written signal with high intensity, Make use.

Regarding the substitution element M, according to detailed examinations by the inventors, Al and Ga can be mentioned as the element M that can easily control the coercive force Hc of the ε-Fe ₂ O ₃ crystal according to the substitution amount. . For example, when the crystal after substitution is expressed as ε-M _x Fe _2−x O ₃ , when M is Al, when x = 0 (aluminum-free granular particle powder), Hc = 17. ⁶ kOe (1.40 × 10 ⁶ A / m), when x = 0.4, Hc = 11.8 kOe (0.94 × 10 ⁶ A / m), when x = 0.5, Hc = 11.1 kOe ( 0.88 × 10 ⁶ A / m), when x = 0.6, Hc = 9.7 kOe (0.77 × 10 ⁶ A / m), and when x = 0.7, Hc = 7.6 kOe (0.6). A change behavior of the coercive force Hc such as 61 × 10 ⁶ A / m) was observed. When M is Ga, Hc = 19.0 kOe (1.512 × 10 ⁶ A / m) and x = 0 when x = 0 (rod-shaped particle powder with no addition of Ga and shape control agent Ba) Hc = 15.3 kOe (1.22 × 10 ⁶ A / m) when .22, Hc = 10.7 kOe (0.851 × 10 ⁶ A / m) when x = 0.43, x = 0.62 Hc = 6.5 kOe (0.52 × 10 ⁶ A / m), and when x = 0.80, Hc = 1.3 kOe (0.10 × 10 ⁶ A / m).

In addition, alkaline earth metals (Ba, Sr, Ca, etc.) that are added to obtain rod-shaped ε-Fe ₂ O ₃ crystals are usually present in the surface layer of the crystals to be produced. When these alkaline earth metal elements are denoted as A, the abundance (content) is at most a blending ratio represented by A / (Fe + M) × 100 within a range of 20% by mass or less, 20 The inclusion of alkaline earth metal in excess of mass% is generally unnecessary in order to function as a shape control agent. More preferably, it is 10 mass% or less.

  The particle diameter of the particles constituting the magnetic powder of the present invention can be controlled, for example, by adjusting the amount of silica coat, the firing temperature, and the firing time in the stage before the heat treatment (firing) in the above process. The TEM average particle size of the magnetic powder is preferably in the range of 5 to 200 nm, more preferably in the range of 5 to 100 nm, and still more preferably in the range of 10 to 100 nm. Most of magnetic recording tapes for data backup currently on the market have an average particle diameter of 200 nm or less, and finer magnetic particles are required. The magnetic powder of the present invention can satisfy this requirement. When a magnetic layer for magnetic recording is formed using the magnetic powder of the present invention, a magnetic layer having a high magnetic recording density can be formed because each particle is fine enough to have a single domain structure.

  However, particles having a particle diameter of less than 5 nm are superparamagnetic and do not exhibit a hard magnetic behavior, and behave like a soft magnetic material. Therefore, if many particles smaller than 5 nm are contained, the particles are affected by the superparamagnetism. The body's magnetic properties are significantly reduced. In addition, the particle diameter in the range of 5 to 10 nm is also expected to be smaller than the critical radius already having a single domain structure, and a decrease in magnetic properties is observed. Therefore, it is preferable that particles having a particle diameter of less than 5 nm, preferably particles of less than 10 nm, are removed as much as possible.

As described above, for the synthesis of the ε-Fe ₂ O ₃ crystal of the present invention, an example in which ultrafine particles of iron hydroxide and aluminum hydroxide as precursors thereof are produced by the reverse micelle method has been given. If a similar precursor of nm or less can be produced, the production of the precursor is not particularly limited to the reverse micelle method.

Example 1
In this example, ε-Ga _0.38 Fe _1.62 O ₃ was synthesized. The following procedure was followed.

[Procedure 1]
Two kinds of micelle solutions, micelle solution I and micelle solution II, are prepared.
-Preparation of micelle solution I In a Teflon (registered trademark) flask, 6 mL of pure water, 18.3 mL of n-octane and 3.7 mL of 1-butanol are added. There, 0.0000 mol of iron (III) nitrate nonahydrate, gallium nitrate (III) n hydrate (99.9% purity by Wako Pure Chemical Industries, n = 7-9) Use and quantitatively analyze in advance and specify n, and then charge amount is calculated)) is added at 0.00060 mol and dissolved at room temperature with good stirring. Further, cetyltrimethylammonium bromide as a surfactant is added in such an amount that the molar ratio of pure water / surfactant becomes 30, and dissolved by stirring to obtain a micelle solution I.
The charged composition at this time is x = 0.40 when the molar ratio of Ga to Fe is expressed as Ga: Fe = x: (2-x).

-Preparation of micelle solution II 2 mL of 25% aqueous ammonia is mixed with 4 mL of pure water and stirred, and further 18.3 mL of n-octane and 3.7 mL of 1-butanol are added to the solution and stirred well. Cetyltrimethylammonium bromide as a surfactant is added to the solution in such an amount that the molar ratio of (pure water + water in ammonia) / surfactant is 30 and dissolved to obtain a micelle solution II. .

[Procedure 2]
While stirring the micelle solution I, the micelle solution II is added dropwise to the micelle I solution. After completion of the dropping, the mixture is continuously stirred for 30 minutes.

[Procedure 3]
While stirring the mixed solution obtained in procedure 2, 6.1 mL of tetraethoxysilane is added to the mixed solution. Continue stirring for about 1 day.

[Procedure 4]
The solution obtained in step 3 is set in a centrifuge and centrifuged. The precipitate obtained by this treatment is recovered. The collected precipitate is washed several times with a mixed solution of chloroform and methanol.

[Procedure 5]
After drying the precipitate obtained in procedure 4, heat treatment is performed at 1100 ° C. for 4 hours in an air atmosphere furnace.

[Procedure 6-1]
The heat-treated powder obtained in step 5 was carefully pulverized with an agate mortar, then placed in 1 L (liter) of 10 mol / L NaOH aqueous solution, stirred at a liquid temperature of 70 ° C. for 24 hours, and the particle surface The silica which will exist in this is removed. It is then filtered and washed thoroughly with water.

[Procedure 6-2]
The water-washed powder is dispersed in 1 L of pure water, pH is monitored while stirring at room temperature, and dilute nitric acid is added little by little to adjust to pH 2.5 to 3.0. Since the pH fluctuates when stirring is continued, the pH is always adjusted to 2.5 to 3.0. Stirring is carried out for 1 hour while adjusting the pH. When the heat-treated powder is treated with an alkali, the silica component is dissolved, but at the same time Fe is also slightly dissolved, and it has been confirmed that an amorphous Fe silicate which is difficult to dissolve in an alkaline solution is synthesized in the solution. Since this Fe silicate has high solubility in acid, it is removed by the above operation (re-dissolution treatment).

  Further, it has been confirmed that the metal element M is dissolved in the procedure 6-1 and may react with the silica component to synthesize the M element silicate. Further, it has been confirmed that Si and the metal element M may form a compound during the procedure 3 and the procedure 4. In particular, when M is Al, it tends to form a compound with Si. Thus, when silica forms a silicate with M element, it may happen that Si cannot be removed to the target Si content only by the alkali treatment in Procedure 6-1. Even in such a case, the procedure 6-2 is effective. That is, in order to obtain the powder composed of “composite particles of iron oxide and Si oxide” of the present invention, it is extremely possible to repeat Step 6-1 and Step 6-2 until the target Si content is reached. It is effective.

  FIG. 1 shows a TEM photograph of the sample powder obtained by the above procedure. The TEM average particle diameter was 17.4 nm, the standard deviation of the particle diameter was 9.1 nm, and the coefficient of variation represented by [Standard deviation of particle diameter] / [Average particle diameter] × 100 was 52.3%.

When the sample powder obtained by the above procedure was subjected to powder X-ray diffraction (XRD: RINT2000 manufactured by Rigaku, radiation source CoKα ray, voltage 40 kV, current 30 mA), the diffraction pattern shown in FIG. 2 was obtained. This diffraction pattern has a diffraction peak corresponding to the crystal structure of ε-Fe ₂ O ₃ (orthorhombic crystal, space group Pna2 ₁ ). Therefore, it is clear that the crystal is the main phase. Moreover, a broad peak due to amorphous SiO ₂ was slightly observed on the lower angle side of 2θ than 30 °. The peak height is clearly lower than the height of the diffraction peak (022) plane of the ε-Fe ₂ O ₃ crystal where 2θ is at a position of 32 to 33 °.

When the obtained sample was subjected to fluorescent X-ray analysis (JSX-3220 manufactured by JEOL Ltd.), when the molar ratio of Ga and Fe was expressed as Ga: Fe = x: (2-x), the charged composition was x = 0. Whereas it was .40, the analytical composition was x = 0.38, and it was confirmed that a Ga-containing ε-Fe ₂ O ₃ crystal having a composition of ε-Ga _0.38 Fe _1.62 O ₃ was synthesized. Moreover, Si content of this powder was 3.5 mol% in the molar ratio by Si / (Fe + Ga) × 100.

Moreover, about the obtained sample powder, the magnetic hysteresis loop in normal temperature (300K) was measured. The result is shown in FIG. The measurement of the magnetic hysteresis loop was performed under the condition of an applied magnetic field of 13 kOe (1.035 × 10 ⁶ A / m) using a vibration measurement type magnetometer (VSM) MODEL880 manufactured by Digital Measurement Systems.
Coercive force Hc ^{6774Oe (5.39 × 10 5 A /} m), saturation magnetization σs is ^{15.7emu / g (A · m 2} / kg), residual magnetization σr is ^{9.5emu / g (A · m 2} / kg), the squareness ratio SQ (= σr / σs) was 0.60.

<Control example>
In this example, ε-Ga _0.46 Fe _1.54 O ₃ was synthesized, and the amount of Si oxide on the particle surface was not controlled as defined in the present invention.

Specifically, the manufacturing process of the powder of Example 1 was changed as follows.
[1] In [Procedure 1], the addition amount of iron (III) nitrate nonahydrate used for the preparation of micelle solution I was changed from 0.0028 mol to 0.0000225 mol, and gallium (III) n The amount of hydrate added was changed from 0.0660 mol to 0.0707050 mol. The charge composition at this time is x = 0.47 when the molar ratio of Ga and Fe is expressed as Ga: Fe = x: (2-x).
[2] [Procedure 6-1] The powder after the heat treatment was put into the liquid without performing agate mortar pulverization. The concentration of the NaOH aqueous solution used was 2 mol / L, the liquid volume was 200 mL, and the solution temperature was changed to room temperature.
[3] A sample powder obtained by finishing [Procedure 6-1] without carrying out [Procedure 6-2] was used.
Except for the above, the same procedure as in Example 1 was repeated.

  A TEM photograph of the obtained sample powder is shown in FIG. The TEM average particle size was 16.4 nm, the standard deviation of the particle size was 8.9 nm, and the coefficient of variation was 54.3%. In addition, from the TEM photograph, it was observed that most of the iron oxide particles existed in the silica. It is clear that the powder in such a state is inferior in dispersibility in the liquid or in the polymer substrate as compared with the powder in Example 1.

The obtained sample powder was subjected to powder X-ray diffraction under the same conditions as in Example 1. The result is shown in FIG. It was confirmed that this powder also had a peak corresponding to the crystal structure of ε-Fe ₂ O ₃ (orthorhombic crystal, space group Pna2 ₁ ). However, the height of a broad peak due to amorphous SiO ₂ observed at a lower angle side than 30 ° of 2θ is the diffraction of ε-Fe ₂ O ₃ crystal at 2θ of 32 to 33 °. It is as high as the height of the peak (022) plane. It can be seen from the results of X-ray diffraction that the abundance of Si oxide is larger than in the case of Example 1.

The obtained sample was subjected to the same fluorescent X-ray analysis as described above. When the molar ratio of Ga to Fe was expressed as Ga: Fe = x: (2-x), the charged composition was x = 0.47. On the other hand, the analytical composition of the obtained powder was x = 0.46, and it was confirmed that Ga-containing ε-Fe ₂ O ₃ crystals having the composition of ε-Ga _0.46 Fe _1.1.54 O ₃ were synthesized. It was done. Moreover, Si content of this powder was 184.9 mol% in molar ratio by Si / (Fe + Ga) * 100. It can be seen that the amount of Si oxide is significantly greater than that of Example 1.

Moreover, the magnetic hysteresis loop of the obtained sample powder is shown in FIG. The measurement conditions are the same as in Example 1. The coercive force Hc is 4.2 kOe (3.35 × 10 ⁵ A / m), the saturation magnetization σs is 6.9 emu / g (A · m ² / kg), and the residual magnetization σr is 3.5 emu / g (A · m ² / kg) and the squareness ratio SQ (= σr / σs) was 0.50. Since the sample of this control example has a larger amount of Ga substitution than the sample of Example 1, the coercive force Hc tends to decrease inherently, but the saturation magnetization σs and the squareness ratio SQ also greatly decrease. Such a decrease in magnetic properties is thought to be due to the large amount of nonmagnetic compound (Si oxide) present on the surface of the magnetic particles.

  When TEM observation was performed over a wide range at a magnification lower than that in FIG. 4, a portion where the particles were relatively dispersed could be found. For reference, a TEM photograph of that portion is shown in FIG. However, since many particles of this powder exist so as to be surrounded by silica, this powder is not suitable for magnetic field orientation and is considered difficult to put into practical use as a magnetic material for magnetic recording media. .

4 is a TEM photograph of the sample powder obtained in Example 1. The X-ray diffraction pattern of the sample powder obtained in Example 1. The magnetic hysteresis loop of the sample powder obtained in Example 1. The TEM photograph of the sample powder obtained by the control example. The X-ray-diffraction pattern of the sample powder obtained by the control example. Magnetic hysteresis loop of the sample powder obtained in the control example. The TEM photograph of the location where the particle | grains of the sample powder obtained by the comparative example are disperse | distributing comparatively.

Claims (4)

It is composed of composite particles having Si oxide on the surface of iron oxide whose main phase is ε-Fe ₂ O ₃ crystal (including those in which part of Fe site is replaced by metal element M), and Si / (Fe + M ) Magnetic powder having a Si content represented by x100 of 0.1 to 30 mol%.
However, when the molar ratio of M to Fe in the iron oxide is expressed as M: Fe = x: (2-x), 0 ≦ x <1.   The magnetic powder according to claim 1, wherein the average particle size measured by a TEM photograph is 5 to 200 nm.   The magnetic powder according to claim 1, wherein M is one or more of Al, Ga, and In. The magnetic powder according to claim 1, which has a coercive force of 1000 to 15000 Oe (7.96 × 10 ^{4 to} 1.19 × 10 ⁶ A / m).