JP6532250B2 - Hexagonal Ba ferrite magnetic powder and method for producing the same - Google Patents

Description

  The present invention relates to a magnetoplumbite (M-type) hexagonal ferrite magnetic powder suitable for high density recording of a magnetic recording medium, and a method for producing the same.

  M-type hexagonal ferrite magnetic powder is known as a magnetic powder suitable for high density recording used for a magnetic recording medium. For example, Patent Documents 1 to 3 disclose examples in which hexagonal ferrite crystals are synthesized by the “glass crystallization method” in which an amorphous body of a raw material mixture is calcined to be crystallized. The glass crystallization method is considered to be a method that is easy to synthesize hexagonal particles with little aggregation. Patent Document 4 discloses an example in which hexagonal ferrite is synthesized by a coprecipitation-calcination method or a hydrothermal synthesis method.

JP, 2011-178654, A Unexamined-Japanese-Patent No. 2010-282671 JP 2012-204726 A JP, 2012-128904, A

  In recent years, while the use of digital data continues to increase, there is a large demand for further increase in recording density for magnetic recording media responsible for storing a vast amount of data. In order to increase the recording density, it is required to improve the resolution in magnetic recording, and for this purpose, it is desired to miniaturize the magnetic particles constituting the magnetic powder. Further, as the magnetic properties, it is particularly important to improve the S / N ratio in the magnetic recording medium. The magnetic powder used for the magnetic recording medium is required to have a predetermined coercive force Hc according to the required characteristics of the magnetic recording medium. The sharper the distribution of the coercivity Hc, the more advantageous it is to improve the S / N ratio. SFD (Switching Field Distribution) is generally employed as an index of coercivity distribution.

  SFD can be regarded as an index calculated by dividing the half value width (coercivity variation) of the differential curve of the hysteresis curve by the value of the coercivity. The coercivity of individual magnetic particles is influenced by the shape and size of the particles. In order to stably realize excellent magnetic characteristics (especially S / N ratio) in high density magnetic recording media, it is not always sufficient to select magnetic powder with small SFD, and magnetic powder with small variation in coercive force. It is desirable to use However, in the magnetic powder composed of fine magnetic particles, it is not easy to control the variation in coercive force of individual magnetic particles to a small level, and such a hexagonal ferrite magnetic powder has not been realized conventionally.

  An object of the present invention is to provide a hexagonal ferrite magnetic powder in which the magnetic particles constituting the powder are fine and the variation in coercive force of the individual magnetic particles is small.

  As a result of detailed studies, the inventors have found that when substituting part of trivalent Fe sites constituting the crystal lattice of M-type hexagonal Ba ferrite with a divalent or tetravalent metal element, Fe after substitution By controlling the component composition so that the average valence of the site atom falls within a narrow range slightly positive than 3, the miniaturization of the magnetic particles and the reduction of the coercivity variation of the individual magnetic particles are simultaneously performed. I found it possible to realize. The present invention has been completed based on such findings.

That is, in the magnetoplumbite-type hexagonal Ba ferrite in which a part of the Fe site is substituted by one or more kinds of divalent or tetravalent metal elements, the above object is to obtain a divalent element to be substituted for Fe M (II), Fe When the tetravalent element to be substituted is represented by M (IV), all elements of the Fe site (ie, Fe and its substitution elements) by M, and the rare earth element by R,
The Fe site valence X calculated by the following equation (1): _Fe 3.005 to 3.030, R / M molar ratio: 0.001 to 0.020, and Dx volume calculated by the following equation (2): Is achieved by a hexagonal Ba ferrite magnetic powder of 1150-1450 nm ³ .
X _Fe = (3 + 2 × [M (II) / Fe] + 4 × [M (IV) / Fe]) / (1+ [M (II) / Fe] + [M (IV) / Fe]) (1)
Dx volume (nm ³ ) = Dxc × π × (Dxa / 2) ² (2)
Here, [M (II) / Fe] is M (II) / Fe molar ratio, [M (IV) / Fe] is M (IV) / Fe molar ratio, and Dxc is c axis direction of hexagonal ferrite crystal lattice Is the crystallite diameter (nm), Dxa is the crystallite diameter in the a-axis direction of the same crystal lattice (nm), and π is the circumference ratio.

For example, one or more of Ni, Cu, and Co can be contained as M (II), and for example, Ti can be contained as M (IV). The M (II) / Fe molar ratio is preferably 0 to 0.02, and the M (IV) / Fe molar ratio is preferably 0.010 to 0.030. When M (II) / Fe molar ratio is 0, it means that a bivalent substitution element is not used.
As the rare earth element R, for example, one or more of Nd, Sm, Y, Pr, Er and Ho can be used.

Among the above magnetic powders, one having an Ha value of 143 kA / m or less, which is represented by the product of the coercive force Hc (kA / m) and the SFD, can be exemplified as a preferable target. The coercive force Hc is, for example, 159 to 223 kA / m.
The above-mentioned hexagonal Ba ferrite magnetic powder can be produced, for example, by a method (glass crystallization method) of firing and crystallizing an amorphous body of a raw material mixture. In that case, it is possible to crystallize an amorphous body adjusted to a composition in which the Fe site valence number X _Fe is 3.005 to 3.030 and the R / M molar ratio is 0.035 to 0.150. preferable.

  According to the present invention, it is possible to provide a hexagonal ferrite magnetic powder which has high coercivity Hc necessary for a magnetic recording medium, is fine in magnetic particles, and has a small variation in coercivity of individual magnetic particles. The magnetic powder contributes to the further improvement of the recording density and the improvement of the magnetic characteristics (particularly, the S / N ratio) in the high density magnetic recording medium.

The graph which shows the relationship of Dx volume and Ha value (= coercive force HcxSFD) about the hexagonal ferrite magnetic powder of an Example and a comparative example.

[Component composition]
The hexagonal ferrite of the present invention is of the magnetoplumbite type (M type) having a basic structure of BaO 6 Fe ₂ O ₃ . Some of the Fe sites are substituted with one or more of divalent or tetravalent metal elements. Conventionally, there is known a technique for adjusting particle size, particle shape, coercivity and the like by replacing a part of Fe sites of hexagonal ferrite with a metal element such as Ti. In this case, the particle size and the coercive force are adjusted to a predetermined value depending on the kind of substitution element and the substitution amount (molar ratio of substitution element to Fe).

  As a result of research, when substituting part of the Fe site with one or more of divalent or tetravalent metal elements, the average amount of atoms of the Fe site is not simply adjusted. It has been found that the variation in coercivity of individual magnetic particles can be stably reduced by shifting the valence slightly to the positive side more than trivalent. The preferable range of the substitution amount (molar ratio to Fe) necessary for that is in a range which is generally smaller than the substitution amount generally considered to be effective for adjusting the particle size and the coercive force.

Here, the Fe site valence number X _Fe is introduced as an index indicating the average valence of Fe site atoms. Fe site valence number X _Fe is represented by the following equation (1).
X _Fe = (3 + 2 × [M (II) / Fe] + 4 × [M (IV) / Fe]) / (1+ [M (II) / Fe] + [M (IV) / Fe]) (1)
Here, M (II) means a divalent element to replace Fe. Examples of M (II) include Ni, Cu, Co and the like. M (IV) means a tetravalent element replacing Fe. Examples of M (IV) include Ti and Sn. The magnetic powder targeted in the present invention contains Ba and Nd as essential components, and may contain Bi, Al, etc. as necessary. However, these Ba, Nd, Bi, Al are M (II), M Not applicable to (IV).

  [M (II) / Fe] means M (II) / Fe molar ratio, and [M (IV) / Fe] means M (IV) / Fe molar ratio. The denominator of the formula (1) corresponds to the number of moles of atoms occupying the Fe site (the total number of moles when Fe is 1 mole). The denominator number 1 means [Fe / Fe] = 1. The molecule of the formula (1) corresponds to the sum of valences of atoms occupying the Fe site. The molecular number 3 means 3 × [Fe / Fe] = 3 × 1 = 3 for trivalent Fe atoms.

As a result of various investigations, refinement of the magnetic particle size and individual magnetic particles by strictly controlling the Fe site valence number X _Fe in the range of 3.005 or more and 3.030 or less slightly on the plus side of 3 It is possible to realize both the reduction of the coercivity variation (Ha value described later) generated by the. As an embodiment when X _Fe falls within the above range,
(A) A pattern in which X _Fe is adjusted by the addition amount of M (IV) with the substitution element being only the tetravalent element M (IV),
(B) A pattern in which X _Fe is adjusted by subtraction of the valence of both M (II) and M (IV), wherein the substitutional element is a complex addition of divalent element M (II) and tetravalent element M (IV) ,
There is.

When the Fe site valence number X _Fe is smaller than 3.005, when forming hexagonal magnetic particles by firing, the magnetic particles are likely to be coarsened. As an aspect in this case,
(A) a pattern in which no substitution element is added,
(B) A pattern in which the amount of substitution is insufficient although the amount of substitution element is only tetravalent element M (IV),
(C) The substitution element is a complex addition of divalent element M (II) and tetravalent element M (IV), but X _Fe becomes too small by subtraction of the valence of both M (II) and M (IV) Pattern,
There is.
On the other hand, if the X _Fe exceeds 3.030, the miniaturization of the magnetic particles can be maintained, but the variation in coercivity of individual magnetic particles (Ha value described later) becomes large. This case is caused by the excessive addition of the tetravalent element M (IV).

Fe site valence number X The reason why miniaturization of the magnetic particle size can be realized when _Fe is shifted slightly to the positive side more than 3 is that when the hexagonal ferrite crystal is formed by firing, the crystal growth rate is suppressed It is believed that this is because the The mechanism for controlling the growth rate has not been clarified at present. It was confirmed that when X _Fe is less than 3.005, the particle growth rate is rapidly increased, and coarse particles are easily generated. On the other hand, if X _Fe becomes large, the influence of distortion of the crystal structure due to the substitution of Fe sites becomes large, which is considered to be a factor to increase the fluctuation of the magnetic characteristics. It was confirmed that when X _Fe exceeds 3.030, it becomes difficult to stably control the coercivity variation (Ha value described later) to a small value.

Fe site valence number X As content of the substitution element for adjusting _Fe to the above-mentioned range, M (II) / Fe molar ratio 0-0.02, M (IV) / Fe molar ratio 0.010 More preferably, it is in the range of -0.030. Here, M (II) / Fe molar ratio in the case of combining and adding a plurality of elements as the divalent element M (II) means the ratio of the total number of moles of these divalent elements to the number of moles of Fe. For example, in the case where Ni and Co are added in combination as M (II), the M (II) / Fe molar ratio is (Ni + Co) / Fe molar ratio. The same applies to the M (IV) / Fe molar ratio in the case where a plurality of elements are added in combination as the tetravalent element M (IV).

When producing the magnetic powder of the present invention, the rare earth element R is contained in the raw material mixture. According to the study of the present inventors, when the rare earth element R is not added, or when the R / M molar ratio is less than 0.035 even when added, the degree of progress of the growth of particles when crystallization is caused by variation. Is likely to occur, and the particle size distribution is broadened, so that the variation of the coercive force (Ha value described later) is undesirably increased. On the other hand, when the R / M molar ratio exceeds 0.150, in addition to saturation of the effect of reducing variation in coercive force, the mass ratio of nonmagnetic components in hexagonal ferrite magnetic powder increases and saturation magnetization decreases. Also, the cost of raw materials will increase. Therefore, it is extremely effective to contain the rare earth element R in an amount such that the R / M molar ratio is in the range of 0.035 to 0.150 in the amorphous body before crystallization. Most of the rare earth element R is removed by a process such as acid washing, and the R content in the finally obtained hexagonal ferrite magnetic powder is in the range of, for example, 0.001 to 0.020 in R / M molar ratio. As the rare earth element R, for example, one or more of Nd, Sm, Y, Pr, Er and Ho can be used.
Here, the element M means all elements of the Fe site (that is, Fe and its substitution elements). For example, if the rare earth element R is Nd, M (II) is Cu, and M (IV) is Ti, the R / M molar ratio is Nd / (Fe + Cu + Ti) molar ratio.

The hexagonal Ba ferrite magnetic powder to which the present invention is directed can contain Al, if necessary. Al does not constitute a hexagonal basic structure (BaO · 6Fe ₂ O ₃ ), but mainly exists as a surface coating layer of magnetic powder particles. The presence of Al can improve the dispersibility and the durability of the magnetic layer in the magnetic recording medium. Al can be added to the raw material mixture. Alternatively, Al may not be added to the raw material mixture, and Al may be contained by surface treatment with an Al-containing solution after acid cleaning is finished after crystallization to remove glass components.

  As another additive element, Bi can be contained as needed. Bi is an element effective for reducing particle size and improving magnetic properties. As a result of various investigations, in order to sufficiently obtain the above-mentioned action of Bi, it is effective to adjust the Bi addition amount in the raw material mixture so that the Bi / M molar ratio of the magnetic powder is 0.001 or more. It is more effective to adjust the Bi / M molar ratio to 0.025 or more. However, when the magnetic powder contains a large amount of Bi, which is a nonmagnetic component, depending on the application, the deterioration of the magnetic properties due to that may be a problem. The Bi / M molar ratio is preferably in the range of 0.10 or less, more preferably 0.06 or less.

Composition adjustment for optimizing Fe site valence number X _Fe can be performed by adjusting the compounding quantity of M (II) and M (IV) in a raw material mixture. For example, according to the glass crystallization method, as shown in Examples described later, the blending ratio (feed molar ratio) of the metal element for replacing the Fe site in the raw material is accurately reflected as the content ratio in the magnetic powder. Be done. Therefore, exact control of the Fe site valence X _Fe becomes possible by accurately adjusting the feed M (II) / Fe molar ratio and the feed M (IV) / mole ratio in the raw material mixture.

[Dx volume]
The present invention is directed to a magnetic powder in which hexagonal ferrite crystal particles are fine in order to cope with high recording density of a magnetic recording medium. Here, Dx volume determined from the crystallite diameter is adopted as a size parameter of crystal grains. The Dx volume is calculated by the following equation (2).
Dx volume (nm ³ ) = Dxc × π × (Dxa / 2) ² (2)
Here, Dxc is the crystallite diameter (nm) in the c-axis direction of the hexagonal ferrite crystal lattice, Dxa is the crystallite diameter (nm) in the a-axis direction of the same crystal lattice, and π is the circumference ratio.
The crystallite diameter is determined from the half-width of a diffraction peak measured by X-ray diffraction (XRD) using Co-Kα radiation, according to Schiller's equation shown in the following equation (3).
Crystallite diameter (nm) = Kλ / (β · cos θ) (3)
Here, K: Scherrer constant 0.9, λ: Co-Kα line wavelength (nm), β: half width of the diffraction peak of the hexagonal (006) plane (radian) in the measurement of Dxc, hexagonal in the measurement of Dxa The half-width (radian) of the diffraction peak on the (220) plane is the Bragg angle (1/2 of the diffraction angle 2θ) (radian) of the diffraction peak.

The hexagonal Ba ferrite magnetic powder composed of fine magnetic particles having a small Dx volume is advantageous for increasing the recording density and improving the S / N ratio of the magnetic recording medium. The present invention is directed to a hexagonal Ba ferrite magnetic powder having a Dx volume of 1450 nm ³ or less. Those with 1420 nm ³ or less are more preferable targets. However, if the Dx volume is excessively reduced, the coercivity is significantly reduced, and it becomes difficult to satisfy the characteristics required for magnetic recording. It is desirable Dx volume is 1150 nm ³ or more, may be managed to 1250 nm ³ or more. In the composition in which the Fe site valence number X _Fe is slightly shifted to the positive side of 3 as described above, the Dx volume can be adjusted to the above-described range.

The Dx ratio represented by Dxa / Dxc is more preferably 2.0 to 3.0.
Further, the BET specific surface area is desirably 100 m ² / g or more.

Magnetic property
In order to improve the S / N ratio of a high recording density magnetic recording medium, it is advantageous to use magnetic powder composed of magnetic particles in which the coercivities of the individual magnetic particles are as uniform as possible. In general, SFD is used as an index for evaluating the coercivity distribution of magnetic powder. The SFD is calculated by dividing the half value width (coercivity variation) of the derivative curve of the hysteresis curve by the value of the coercivity, whereby the magnetic characteristics of the magnetic powder at various coercivity levels can be a standard. Can be compared. However, focusing on the distribution width of the coercivity, that is, the variation itself of the coercivity of the individual magnetic particles, when the SFD is the same, the "magnetic particles have a larger variation in coercivity between magnetic particles" as the level of the coercivity increases. It will be. The peak width itself of the coercivity distribution curve (differential curve of the hysteresis curve) itself (SFD conversion) as an index to evaluate more strictly the influence of the "coercivity variation among the magnetic particles" on the S / N ratio of the magnetic recording medium It is considered effective to adopt a parameter corresponding to the one before division by the coercivity Hc in order to Here, the Ha value is adopted as the parameter. When the Ha value is defined using SFD, the following equation (4) is obtained.
Ha value (kA / m) = Hc (kA / m) × SFD (4)
The Ha value has the same dimension of kA / m as the coercive force Hc, and is an index for directly evaluating the peak width in the coercive force distribution curve.

According to the study of the inventors, in the small grained hexagonal Ba ferrite magnetic powder having a Dx volume within the defined range of the present invention, the recording density is increased when the Ha value is 143 kA / m (1800 Oe) or less It is very useful for improving the S / N ratio of the magnetic recording medium. More preferably, the Ha value is 140 kA / m (1760 Oe) or less. The Fe site valence X _Fe in the ferrite composition shifted slightly positive than three as described above, by the Dx volume get smaller magnetic powder as described above, it is possible to sufficiently reduce the Ha value . The smaller the Ha value, the better, but the reduction of the excessive Ha value is accompanied by an increase in cost. The Ha value may be adjusted in the range of, for example, 80 kA / m (1000 Oe) or more, and may be managed in the range of 111 kA / m (1400 Oe) or more.

  The coercive force Hc may be adjusted to an appropriate range in accordance with the required characteristics of the magnetic recording medium. For example, a magnetic powder exhibiting Hc of 159 to 223 kA / m (2000 to 2800 Oe) can be exemplified as a suitable target.

〔Production method〕
The hexagonal Ba ferrite magnetic powder according to the present invention can be manufactured using the “glass crystallization method” which crystallizes the amorphous body of the raw material mixture. Specifically, it can be produced, for example, through the following steps.

(Raw material mixing process)
The hexagonal ferrite magnetic powder according to the present invention has a basic structure of BaO · 6Fe ₂ O ₃ type, and a part of Fe is one or more of tetravalent metal elements (eg Ti, Sn), or the above-mentioned tetravalent metal element And a rare earth element R as an additive element, and further containing Bi, Al, etc. as necessary. . As the rare earth element R, for example, one or more of Nd, Sm, Y, Pr, Er and Ho can be used. A raw material mixed powder is obtained by mixing various elements including elements contained in these magnetic powders and Ba, B and the like necessary to obtain an amorphous substance. The various metal elements contained in the magnetic powder can be supplied by oxides, hydroxides, carbonates, oxalates, nitrates, halides and the like. Ba is a constituent element of hexagonal ferrite and is also an element for forming an amorphous body. B is an element for forming an amorphous body. Barium carbonate is exemplified as the Ba source. As the B source, boric acid is exemplified. When Al is contained for the purpose of improving the durability of the magnetic layer of the magnetic recording medium, it can also be added by surface treatment described later. The raw material compounding, the composition of the raw material mixture, wherein (1) a Fe site valence X _Fe is from 3.005 to 3.030, which is calculated by the formula, and R / M molar ratio is from 0.035 to 0.150 It is preferable to adjust to the composition which is

  Each raw material is stirred and mixed by a mixer to be a raw material mixture. It is desirable to perform shear mixing in a mixer having a stirring blade such as a Henschel mixer.

(Granulation process)
The raw material mixture obtained is generally made into a spherical granulated product having a predetermined particle diameter in consideration of the handleability and the like in the subsequent steps. For example, using a pan-type granulator, it is formed into a sphere while adding water or a binder component as needed, to form a granular material with a diameter of about 1 to 50 mm, which is heated to about 200 to 300 ° C. and dried. Granulated products are obtained.

(Amorphization process)
The raw material mixture after drying (the above-mentioned granulated product) is heated to a high temperature and melted to form a melt at 1350 to 1450 ° C. The melt is quenched to an amorphous form. The quenching method may, for example, be a twin roll method, a gas atomization method, a water atomization method, or a centrifugal atomization method. The obtained amorphous body can be adjusted in particle size after being crushed by a ball mill or the like as required.

(Crystallization process)
The above-mentioned amorphous body is heated and held at a temperature at which crystallization in the range of 600 to 630 ° C. occurs to precipitate hexagonal ferrite crystals. In the composition adjusted to the composition specified in the present invention, the effect of suppressing the growth rate of the magnetic crystal grains is sufficiently exhibited, and the particle size distribution is stably uniform in a relatively low temperature range of 600 to 630 ° C. Since hexagonal crystals can be grown, this is advantageous for reducing the Dx volume. The holding time at the temperature at which crystallization occurs may be usually 60 to 240 min. The powder obtained by the heat treatment (baking) of this crystallization includes, in addition to the hexagonal ferrite crystals, substances (mainly barium borate crystals) in which the remaining components contained in the amorphous body are crystallized. .

(Washing process)
Next, in order to extract hexagonal ferrite particles from the powder obtained in the crystallization step, the residual substance mainly composed of barium borate is dissolved and removed with an acid. This process is referred to herein as "acid wash". As the acid washing solution, an acetic acid aqueous solution having a concentration of 2 to 12% by mass is suitable. The powder obtained in the crystallization step is immersed in an acid cleaning solution and kept at a temperature not higher than the boiling point. It is effective to stir the solution. The pH of the solution is preferably 4.0 or less. After the dissolution of the remaining components is completed, solid-liquid separation is performed to extract hexagonal ferrite powder.

Since the acid washing solution adheres to the hexagonal ferrite powder extracted by the above solid-liquid separation, it is washed off with water. This process is referred to herein as "washing". As an initial stage of washing with water, processing for neutralization can be performed with an aqueous alkaline solution such as aqueous ammonia, aqueous sodium hydroxide solution, aqueous potassium hydroxide solution, etc., if necessary. The concentration of the alkaline aqueous solution may be adjusted, for example, in the range of 0.01 to 1.5 mol / L for sodium hydroxide. Finally, thoroughly wash with pure water. Specifically, it is desirable to carefully wash until the conductivity of the solution after washing (filtrate) becomes 10 μS / cm or less.
After washing with water, drying is usually performed in air at 100 ° C. or higher.

(Surface treatment process)
As needed, surface treatment for depositing Al on the surface of the magnetic particles can be applied. For example, the formation reaction of aluminum hydroxide can be generated by dispersing hexagonal Ba ferrite particles in an aqueous solution in which an aluminum salt is dissolved and adding an alkali. A hexagonal ferrite magnetic powder in which a coating layer of aluminum hydroxide is formed on the particle surface is excellent in dispersibility, and is extremely useful in forming a highly durable magnetic layer in a magnetic recording medium.

  Hexagonal ferrite magnetic powder was prepared by various raw material formulations, and component analysis, measurement of magnetic characteristics, measurement of specific surface area, and measurement of crystallite diameter were performed on the obtained magnetic powder. Below, the method and the result are shown.

[Production of hexagonal ferrite magnetic powder]
As raw materials, boric acid H ₃ BO ₃ (for industrial use), barium carbonate BaCO ₃ (for industrial use), iron oxide Fe ₂ O ₃ (for industrial use), cobalt oxide CoO (reagent 90% or more), nickel oxide NiO (reagent 99) %), Copper oxide CuO (reagent 95% or more), titanium oxide TiO ₂ (reagent first grade), bismuth oxide Bi ₂ O ₃ (for industrial use), neodymium oxide Nd ₂ O ₃ (for industrial use), aluminum hydroxide Al (OH) ₃ (reagent grade 1) was prepared. These were weighed to a predetermined composition, and mixed using a Mitsui Miike FM mixer to obtain a raw material mixture. Table 1 shows the preparation composition of the metal element based on the measured value, and the Fe site valence number X _Fe of the raw material mixture calculated from the preparation composition by the equation (1). Ni, Co and Cu correspond to the divalent substitution element M (II), and Ti corresponds to the tetravalent substitution element M (IV). As described above, M means all elements of the Fe site (here, Fe + Ni + Co + Cu + Ti).

  The raw material mixture was placed in a pelletizer, and the mixture was formed into spheres by spraying water and granulated, and then dried at 270 ° C. for 14 h to obtain granulated products with a particle size of 1 to 50 mm. The obtained granulated product was melted by a melting furnace using a platinum crucible. The temperature is raised to 1400 ° C. and held while stirring for 60 minutes to bring each raw material into a completely molten state, and then the molten material (molten metal) is poured from a nozzle and quenched by gas atomizing method I got The obtained amorphous substance was crystallized by heating at a predetermined temperature to form hexagonal ferrite. The heating holding temperature (baking temperature) is described in Table 1 as a crystallization treatment temperature. The holding time at that temperature was 60 min.

  The powder obtained by the above-described heating and holding contains, in addition to hexagonal ferrite, a residual substance mainly composed of barium borate. The said powder is immersed in a 6 to 10 mass% acetic acid aqueous solution heated to 60 to 85 ° C., and the above residual substance is dissolved in the liquid by holding for 1 to 2 hours while stirring, and then solid liquid by filtration. The separation was performed and the solid content was recovered. This solid content is called "acid-washed solid content".

  The above acid-washed solid content was washed with pure water to remove components such as acetic acid adhering to the particle surface. It was washed with water until the conductivity of the solution (filtrate) after washing became 10 μS / cm or less. After washing with water, drying was performed in the air at 110 ° C. to obtain a sample of hexagonal ferrite magnetic powder.

  In some examples (Examples 5 and 6), the surface treatment for depositing Al was applied as follows. Pure water was added to the above acid-washed solid content and stirred, and an aqueous solution of aluminum chloride was added. The addition amount of Al by aluminum chloride was 0.04 in Al / Fe molar ratio. Thereafter, sodium hydroxide was added to form a layer of aluminum hydroxide as a reaction product on the surface of solid particles (hexagonal ferrite magnetic particles). Thereafter, solid-liquid separation was carried out by filtration, pure water was added, and water washing was carried out until the conductivity of the washed solution (filtrate) became 10 μS / cm or less. After washing with water, the sample was dried in air at 110 ° C. for 12 hours to obtain a sample of hexagonal ferrite magnetic powder in which aluminum hydroxide was deposited on the particle surface.

[Component analysis of magnetic powder]
The component analysis of the hexagonal ferrite magnetic powder sample was analyzed using a high frequency inductive plasma emission analyzer ICP (720-ES) manufactured by Agilent Technologies, Inc. From the quantitative values obtained, the composition of the metal element was calculated as the molar ratio to Fe or the molar ratio to all elements M (here M = Fe + Co + Ni + Cu + Ti) of Fe sites. Moreover, Fe site valence number X _Fe was calculated from the composition by the above-mentioned equation (1). These values are shown in Table 2.

[Measurement of magnetic properties]
A hexagonal ferrite magnetic powder sample is packed in a plastic container of φ 6 mm, and an external magnetic field of 795.8 kA / m (10 kOe), a magnetic field sweep speed 795, using a VSM apparatus (VSM-P7-15) manufactured by Toei Kogyo Co., Ltd. The coercivity Hc, saturation magnetization σs, squareness ratio SQ, coercivity distribution SFD were measured at 8 kA / m / min (10 kOe / min). The Ha value represented by the product was calculated from Hc and SFD. These values are shown in Table 3.

[Measurement of specific surface area]
The specific surface area of the hexagonal ferrite magnetic powder sample was determined by the BET single-point method using 4 sorbs US manufactured by Yuasa Ionics. The results are shown in Table 3.

[Measurement of crystallite diameter]
The crystallite diameter Dxc (nm) in the c-axis direction of the hexagonal ferrite crystal lattice and the crystallite diameter Dxa (nm) in the a-axis direction using a Co tube with an X-ray diffractometer (RINT 2100-ULTIMA manufactured by Rigaku) ) Was obtained according to the above-mentioned equation (3). Dxc was scanned by measuring in the range of 2θ: 24 to 30 °, and Dxa was scanned in the range of 2θ: 73 to 77 °. The measurement method was the FT method, and the step was 0.01 °, the measurement time was 1.5 seconds, and the integration number was 3 times.
The Dx volume (nm ³ ) was calculated by substituting the obtained Dxc and Dxa into the equation (2). In addition, Dx ratio determined by Dxa / Dxc was determined. The results are shown in Table 3.

Small in range, Dx volume of 1150～1450Nm ³ those of the examples in which to adjust the Fe site valence X _Fe in the range of 3.005 to 3.030 in M-type hexagonal Ba ferrite magnetic powder containing Nd The Ha value, which is in the form of particles, indicates the variation of the coercivity stably and exhibits a low value of 143 kA / m (1800 Oe) or less. These hexagonal Ba ferrite magnetic powders are extremely useful for improving the S / N ratio of magnetic recording media with high recording density.

On the other hand, in Comparative Examples 1 to 3, the Fe site valence number X _Fe was too low, so that the growth rate of the particles was not sufficiently suppressed during the crystallization process, and the Dx volume was large. The increase in Dx volume is disadvantageous to the reduction of the S / N ratio. In Comparative Examples 4 and 5, since Nd was not added, the variation in the degree of progress of particle growth in the process of crystallization was large, and it was not possible to achieve both the reduction in particle size and the reduction in Ha value. In Comparative Examples 6 and 7, the Fe site valence number X _Fe was too high, and the reduction effect of the Dx volume was large, but the Ha value was increased.
For reference, FIG. 1 shows the relationship between Dx volume and Ha value for each example.

Claims (9)

In a magnetoplumbite-type hexagonal Ba ferrite in which a part of the Fe site is substituted with one or more of divalent or tetravalent metal elements, tetravalent in which the divalent element replacing Fe is substituted with M (II) or Fe When an element is represented by M (IV), all elements at the Fe site (ie, Fe and its substitution elements) by M, and a rare earth element by R,
The Fe site valence X calculated by the following equation (1): _Fe 3.005 to 3.030, R / M molar ratio: 0.001 to 0.020, and Dx volume calculated by the following equation (2): Hexagonal Ba ferrite magnetic powder of which is 1150 to 1450 nm ³ ;
X _Fe = (3 + 2 × [M (II) / Fe] + 4 × [M (IV) / Fe]) / (1+ [M (II) / Fe] + [M (IV) / Fe]) (1)
Dx volume (nm ³ ) = Dxc × π × (Dxa / 2) ² (2)
Here, [M (II) / Fe] is M (II) / Fe molar ratio, [M (IV) / Fe] is M (IV) / Fe molar ratio, and Dxc is c axis direction of hexagonal ferrite crystal lattice Is the crystallite diameter (nm), Dxa is the crystallite diameter in the a-axis direction of the same crystal lattice (nm), and π is the circumference ratio.   The hexagonal Ba ferrite magnetic powder according to claim 1, which contains Ti as M (IV).   The hexagonal Ba ferrite magnetic powder according to claim 1 or 2, wherein the M (IV) / Fe molar ratio is 0.010 to 0.030.   The hexagonal Ba ferrite magnetic powder according to any one of claims 1 to 3, which contains one or more of Ni, Cu and Co as M (II).   The hexagonal Ba ferrite magnetic powder according to any one of claims 1 to 4, wherein M (II) / Fe molar ratio is 0 to 0.02.   The hexagonal Ba ferrite magnetic powder according to any one of claims 1 to 5, wherein the rare earth element R is made of one or more of Nd, Sm, Y, Pr, Er and Ho.   The hexagonal Ba ferrite magnetic powder according to any one of claims 1 to 6, wherein an Ha value represented by the product of coercivity Hc (kA / m) and SFD is 143 kA / m or less.   The hexagonal Ba ferrite magnetic powder according to any one of claims 1 to 7, which has a coercive force Hc of 159 to 223 kA / m. Amorphous raw material mixture adjusted to a composition having an Fe site valence X calculated by the above equation (1) _Fe of 3.005 to 3.030 and an R / M molar ratio of 0.035 to 0.150 Forming a body,
Firing the amorphous body in a temperature range of 600 to 630 ° C. to form a hexagonal ferrite crystal;
A method for producing a hexagonal Ba ferrite magnetic powder according to any one of claims 1 to 8, which comprises

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