JP2010239067A - Magnetic fine particle powder for magnetic recording medium, and manufacturing method of the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide magnetic fine particle powder for magnetic recording medium and method of manufacturing the same, and more particularly to provide magnetic fine particle powder for magnetic recording medium capable of reducing influence of heat fluctuation accompanied with making the magnetic particle powder to finer particles. <P>SOLUTION: A method for obtaining the magnetic fine particle powder for magnetic recording medium, wherein the average plane diameter of hexagonal ferrite particle powder is 10-50 nm and the existence ratio of super-fine particle having a plane diameter of less than 10 nm to whole particles is 15% or less, includes the steps of: dispersing the hexagonal ferrite particle powder in water to be aqueous suspension liquid; dissolving a super-fine particle component in the hexagonal ferrite particle powder on condition of 3.5 or less pH value and 20-100°C of temperature ranges; and washing and drying the remaining hexagonal ferrite particle powder. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a magnetic fine particle powder for a magnetic recording medium and a method for producing the same, and more specifically, a hexagonal ferrite particle powder having a mean plate surface diameter of 10 to 50 nm and a plate surface diameter of less than 10 nm. The present invention relates to a magnetic fine particle powder for a magnetic recording medium that can reduce the influence of thermal fluctuation accompanying the miniaturization of the magnetic particle powder by reducing the proportion of ultrafine particles.

  Conventionally, magnetic recording technology has been widely used in various fields including audio, video, and computer. In recent years, there has been a demand for smaller and lighter devices, longer recording time, increased recording capacity, and the like, and further improvement in recording density is desired for recording media.

  In order to perform high-density recording on a conventional magnetic recording medium, a high C / N ratio is required, noise (N) is low, and reproduction output (C) is required to be high. In recent years, high-sensitivity heads such as magnetoresistive heads (MR heads) and giant magnetoresistive heads (GMR heads) have been developed in place of the induction type magnetic heads used so far. Since these high-sensitivity heads are easier to obtain reproduction output than inductive magnetic heads, it is more important to reduce noise than to increase output in order to obtain a high C / N ratio. .

  The noise of the magnetic recording medium is roughly divided into particulate noise and surface noise generated due to the surface property of the magnetic recording medium. In the case of particulate noise, the influence of the particle size is large, and the finer the particle, the better the noise reduction. Therefore, the magnetic particle powder used in the magnetic recording medium needs to be as small as possible. Since the particle volume is reduced by miniaturization, the ratio of magnetic anisotropy energy and thermal energy (KuV / kT) representing the thermal stability of magnetization (Ku: magnetic anisotropy constant, V: particle volume) , K: Boltzmann constant, T: absolute temperature) becomes small, and is susceptible to thermal fluctuations.

  In addition, it is known that as the magnetic particle powder becomes finer, the volume of the crystal grain decreases, the crystal magnetization becomes unstable and the magnetism is lost (super paramagneticism). Reduction of ultrafine particle components is important.

  In general, as magnetic particles having fine particles and a high coercive force value, metal magnetic particles having iron as a main component, hexagonal ferrite particles and the like are known. In particular, the hexagonal ferrite particle powder has a feature that a high output can be obtained in a short wavelength region as compared with the acicular metal magnetic particle powder, and a magnetic recording medium for high density recording using an MR head or a GMR head for reproduction. It is very promising as a magnetic powder.

  However, the hexagonal ferrite particle powder has σs (saturation magnetization value) of about ½ that of the metal magnetic particle powder, and Ku (Ku is obtained by Hk · σs / 2) (Hk: anisotropic magnetic field) ) Is difficult to increase, and the influence of thermal fluctuation tends to be larger than that of metal magnetic particle powder.

By limiting the product of the anisotropic magnetic field (Hk) and the average particle volume (V) of the hexagonal ferrite particle powder to the range of 1.2 × 10 ^{6 to} 2.4 × 10 ⁶ kA / m · nm ³ A magnetic recording medium (Patent Document 1) in which the influence of thermal fluctuation is reduced is disclosed.

  In addition, in order to improve the particle size distribution of the hexagonal ferrite particle powder, when the hexagonal ferrite is produced by the glass crystallization method, the amorphous body is subjected to a heat treatment to precipitate the hexagonal ferrite. A hexagonal ferrite particle powder (Patent Document 2) is disclosed, which is characterized by subjecting to heat treatment.

JP 2005-129115 A JP 2006-120823 A

  The hexagonal ferrite particle powder that can reduce the influence of thermal fluctuation accompanying the miniaturization of the magnetic particle powder is the most demanded at present, but the hexagonal ferrite particle powder that sufficiently satisfies these characteristics is still in demand. Not obtained.

That is, in the aforementioned Patent Document 1, the product of the anisotropic magnetic field (Hk) and the average particle volume (V) of the hexagonal ferrite particle powder is 1.2 × 10 ^{6 to} 2.4 × 10 ⁶ kA / m. Although it is described that it is limited to the range of nm ^3, the purpose is to make the thermal fluctuation a specific range, and reduction of ultrafine particles having a plate surface diameter of less than 10 nm is not considered. The saturation magnetization value σs cannot be increased due to the presence of ultrafine particles that cause superparamagnetism, and therefore it is difficult to increase the magnetic anisotropy constant Ku, so that the influence of thermal fluctuation increases.

  Patent Document 2 discloses a process for precipitating hexagonal ferrite by heat-treating an amorphous body when producing hexagonal ferrite by a glass crystallization method in order to improve the particle size distribution of hexagonal ferrite particle powder. In the above, hexagonal ferrite particle powder obtained by heat-treating an amorphous body is described, but as described above, reduction of ultrafine particles having a plate surface diameter of less than 10 nm is not considered. The saturation magnetization value σs cannot be increased due to the presence of ultrafine particles that cause paramagnetism. Therefore, it is difficult to increase the magnetic anisotropy constant Ku, so that the influence of thermal fluctuation increases.

  Therefore, the present invention reduces the ratio of the ultrafine particles having a plate surface diameter of less than 10 nm while the average plate surface diameter of the hexagonal ferrite particle powder is 10 to 50 nm. A technical problem is to obtain hexagonal ferrite particle powder capable of reducing the influence of thermal fluctuations associated with miniaturization.

  The technical problem can be achieved by the present invention as follows.

  That is, according to the present invention, the average plate surface diameter of the hexagonal ferrite particle powder is 10 to 50 nm, and the existence ratio of ultrafine particles having a plate surface diameter of less than 10 nm with respect to all particles is 15% or less. (Part 1 of the present invention).

  In the present invention, hexagonal ferrite particles are dispersed in water to form an aqueous suspension, and an acid is added to the aqueous suspension so that the pH value is 3.5 or less and the temperature range is 20 to 100 ° C. The present invention is characterized in that after the treatment is carried out to dissolve the ultrafine particle component of the hexagonal ferrite particle powder present in the aqueous suspension, the remaining hexagonal ferrite particle powder is washed with water and dried. This is a method for producing a magnetic fine particle powder for magnetic recording media (Invention 2).

  The magnetic fine particle powder for a magnetic recording medium according to the present invention reduces the existence ratio of ultrafine particles having a plate surface diameter of less than 10 nm while the average plate surface diameter of the hexagonal ferrite particle powder is 10 to 50 nm. Thus, the influence of thermal fluctuation accompanying the miniaturization of the magnetic particle powder can be reduced, so that it is suitable as a magnetic particle powder for a high-density magnetic recording medium.

  The configuration of the present invention will be described in more detail as follows.

  First, the magnetic fine particle powder for a magnetic recording medium according to the present invention will be described.

The magnetic fine particle powder for magnetic recording media according to the present invention has an average plate surface diameter of hexagonal ferrite particle powder of 10 to 50 nm, and the presence ratio of ultrafine particles having a plate surface diameter of less than 10 nm with respect to all particles. Is 15% or less.

  The magnetic fine particle powder for a magnetic recording medium according to the present invention is a magnetoplumbite type (M type) ferrite fine particle powder or W type ferrite fine particle powder containing Ba, Sr or Ba and Sr, or a part of their atoms. It is a hexagonal ferrite particle powder substituted with these elements. Specific examples of substitution elements include Co, Ni, Zn, Mn, Mg, Ti, Sn, Zr, Cu, Mo, La, Ce, V, Si, S, Sc, Sb, Y, Rh, Pd, and Nd. Nb, B, P, Ge, Al, Ag, Au, Ru, Pr, Bi, W, Re, Te, or the like can be used alone or in combination.

  The average plate surface diameter of the magnetic fine particle powder for magnetic recording media according to the present invention is 10 to 50 nm, preferably 10 to 40 nm, more preferably 10 to 30 nm. When the average plate surface diameter of the magnetic fine particle powder for magnetic recording media exceeds 50 nm, the particle size of the magnetic particle powder is large, so it is difficult to reduce particulate noise, and the magnetic recording medium has a high C / N ratio. It becomes difficult to obtain. In addition, when the average plate surface diameter is less than 10 nm, aggregation is likely to occur due to an increase in intermolecular force due to the formation of fine particles, and as a result, coarse particles are generated and uniform in the vehicle during the production of the magnetic coating material. Dispersion becomes difficult and the average plate surface diameter is less than 10 nm means that a large number of ultrafine particles having a plate surface diameter of less than 10 nm exist, and the saturation magnetization value σs decreases, Since it is difficult to increase the magnetic anisotropy constant Ku, the influence of thermal fluctuation is increased.

  The average thickness of the magnetic fine particle powder for magnetic recording media according to the present invention is preferably 1.00 to 20 nm, more preferably 1.25 to 18 nm, and still more preferably 1.67 to 16 nm.

  The plate ratio (ratio of average plate surface diameter to average thickness) (hereinafter referred to as “plate ratio”) of the magnetic fine particle powder for magnetic recording media according to the present invention is preferably 1.5 to 10.0, more preferably. Is from 1.75 to 8.0, more preferably from 2.0 to 6.0. When the plate ratio exceeds 10, stacking between particles increases, dispersibility in the vehicle during production of the magnetic coating material decreases, and viscosity may increase, which is not preferable.

BET specific surface area of the magnetic fine powder for magnetic recording medium according to the present invention is preferably from 20 to 200 m ² / g, more preferably 25~200m ² / g, still more preferably 30 to 200 m ² / g. When the BET specific surface area value is less than 20 m ² / g, the magnetic fine particle powder for a magnetic recording medium is coarse, so that the surface smoothness of the magnetic recording medium obtained by using this decreases, resulting in that It becomes difficult to improve the output. In addition, the saturation magnetization value and the coercive force value in the short wavelength region are decreased, and particle noise is increased, which is not preferable. When the BET specific surface area value exceeds 200 m ² / g, aggregation tends to occur due to an increase in intermolecular force due to finer particles, so that the dispersibility in the vehicle during the production of the magnetic coating material decreases.

  In the magnetic fine particle powder for a magnetic recording medium according to the present invention, the proportion of particles having a plate surface diameter of less than 10 nm is 15% or less, preferably 12% or less, more preferably 10% or less, based on all particles. . When the presence ratio of particles having a plate surface diameter of less than 10 nm exceeds 15% with respect to all the particles, the presence ratio of ultrafine particles is high, so the saturation magnetization value σs becomes small and the magnetic anisotropy constant Ku. Since it is difficult to increase the value, the effect of thermal fluctuations will increase.

  The geometric standard deviation value of the plate surface diameter of the magnetic fine particle powder for magnetic recording media according to the present invention is preferably 1.8 or less, more preferably 1.7 or less, and even more preferably 1.6 or less. When the geometric standard deviation value of the plate surface diameter exceeds 1.8, the particle size distribution is widened, and the variation of the coercive force value Hc becomes large. Therefore, when this is used as a magnetic recording medium, noise is generated. It becomes difficult to reduce.

  The saturation magnetization increase rate (%) ((σs after acid treatment−σs before acid treatment) / σs × 100 after acid treatment) of the magnetic fine particle powder for magnetic recording medium according to the present invention is preferably 3.0% or more. More preferably, it is 4.0% or more.

The magnetic properties of the magnetic fine particle powder for magnetic recording media according to the present invention preferably have a coercive force value Hc of 63.7 to 397.9 kA / m, more preferably 79.6 to 318.3 kA / m, and saturation magnetization. The value σs is preferably 30 Am ² / kg or more, more preferably 35 Am ² / kg or more.

  In the magnetic fine particle powder for magnetic recording media according to the present invention, the surface of the hexagonal ferrite particle powder is selected from aluminum hydroxide, aluminum oxide, silicon hydroxide and silicon oxide, if necessary. Alternatively, it may be coated with one or more compounds (hereinafter referred to as “aluminum hydroxide, etc.”). By carrying out a coating treatment with aluminum hydroxide or the like, when dispersed in a magnetic paint, it is well-familiar with the binder resin and a desired degree of dispersion is more easily obtained.

The coating amount of aluminum hydroxide and the like in the coating treatment is Al conversion, SiO ₂ conversion or the sum of Al conversion amount and SiO ₂ conversion amount with respect to hexagonal ferrite particle powder coated with aluminum hydroxide and the like. 0.01 to 20% by weight is preferable.

  When the coating amount of aluminum hydroxide or the like exceeds 20% by weight, the effect of improving the dispersibility of the hexagonal ferrite particles in the vehicle during the production of the magnetic coating can be sufficiently obtained. has no meaning. Further, an increase in aluminum hydroxide, which is a nonmagnetic component, is not preferable because the magnetic properties of the magnetic fine particle powder for a magnetic recording medium are impaired.

  Next, a method for producing a magnetic fine particle powder for a magnetic recording medium according to the present invention will be described.

The magnetic fine particle powder for a magnetic recording medium according to the present invention can be obtained by subjecting an untreated hexagonal ferrite particle powder, which is a starting material, to dissolution treatment with an acid under specific conditions, and then washing and drying. .

  The untreated hexagonal ferrite particle powder used as a starting material in the present invention can be obtained by a conventionally known production method such as a wet synthesis method, a coprecipitation method, a heat firing method, or a glass crystallization method.

  The magnetic fine particle powder for a magnetic recording medium according to the present invention is obtained by treating an aqueous suspension of untreated hexagonal ferrite particles as a starting material under conditions of a pH value of 3.5 or less and a temperature range of 20 to 100 ° C. After the ultrafine particle component contained in the hexagonal ferrite particle powder present in the aqueous suspension is dissolved, the remaining hexagonal ferrite particle powder can be obtained by washing with water and drying.

  The untreated hexagonal ferrite particle powder as the starting material is subjected to an acid treatment, and then coarsely pulverized in advance to loosen the coarse particles. It is preferable to loosen it. The wet pulverization may be performed using various pulverizers, a disperser, a kneader, or the like so that at least the coarse particles of the secondary agglomerated particles of 44 μm or more are eliminated. The degree of wet pulverization is 10% or less, preferably 5% or less, and more preferably 0% for coarse particles of 44 μm or more. If coarse particles of 44 μm or more remain in excess of 10%, it is difficult to obtain the effect of treatment with an acid in the next step.

  The concentration of the hexagonal ferrite particle powder in the aqueous suspension used for the treatment with the acid is preferably 1 to 500 g / l, more preferably 5 to 250 g / l. If it is less than 1 g / l, the amount of treatment per treatment unit is too small, which is not industrially preferable. If it exceeds 500 g / l, it is difficult to perform uniform processing.

  As the acid used for the treatment with an acid, any of hydrochloric acid, acetic acid, sulfuric acid, nitric acid, sulfurous acid, chloric acid, perchloric acid, and oxalic acid can be used. In view of the removal efficiency of the ultrafine particle component of the hexagonal ferrite particles by the acid, hydrochloric acid is preferable.

  The initial pH value in the acid dissolution treatment is 3.5 or less, and considering the dissolution time and the like, the pH value is preferably 3.0 or less, more preferably 2.5 or less. When the pH value exceeds 3.5, it takes an extremely long time to dissolve the ultrafine particle component of the hexagonal ferrite particles, which is industrially disadvantageous.

  The temperature range of the aqueous suspension in the dissolution treatment with an acid is 20 to 100 ° C, preferably 25 to 100 ° C, more preferably 30 to 100 ° C. If it is less than 20 ° C., it takes a very long time to dissolve the ultrafine particle component of the hexagonal ferrite particles, which is industrially disadvantageous. When the temperature exceeds 100 ° C., dissolution of the particles proceeds rapidly, making it difficult to control, and an apparatus such as an autoclave is required, which is not industrially preferable.

  After the dissolution treatment with an acid, it can be washed with water and dried according to a conventional method.

  In the magnetic fine particle powder for magnetic recording media according to the present invention, the surface of the hexagonal ferrite particle powder is preferably coated with aluminum hydroxide or the like. The coating treatment with aluminum hydroxide or the like may be performed before or after the acid treatment, but is preferably after the acid treatment in consideration of the treatment effect by the surface coating.

  The hexagonal ferrite particle powder coated with an aluminum hydroxide is added to an aqueous suspension obtained by dispersing the hexagonal ferrite particle powder, and the aluminum compound, the silicon compound or both of the compounds are mixed and stirred. Or, if necessary, by adjusting the pH value after mixing and stirring, the particle surface of the hexagonal ferrite particle powder is oxidized with aluminum hydroxide, aluminum oxide, silicon hydroxide and silicon. It can be obtained by coating with one or two or more kinds of surface coatings selected from products, and then filtering, washing, drying and pulverizing.

  As the aluminum compound, aluminum salts such as aluminum acetate, aluminum sulfate, aluminum chloride, and aluminum nitrate, and alkali aluminates such as sodium aluminate can be used.

  As the silicon compound, No. 3 water glass, sodium orthosilicate, sodium metasilicate and the like can be used.

<Action>
The most important point in the present invention is that the magnetic fine particle powder for a magnetic recording medium according to the present invention is a fine particle having an average plate surface diameter of 10 to 50 nm of the hexagonal ferrite particle powder, but accompanying the refinement of the magnetic particle powder. This is the fact that the influence of thermal fluctuation can be reduced.

  The reason why the magnetic fine particle powder for a magnetic recording medium according to the present invention is a hexagonal ferrite fine particle powder having an average plate surface diameter of 10 to 50 nm, but the influence of thermal fluctuation accompanying the miniaturization of the magnetic particle powder is small. The person thinks as follows.

  In other words, it is known that when the magnetic particle powder becomes finer, the volume of the crystal grains decreases, the crystal magnetization becomes unstable, and the magnetism is lost (super paramagnetism). However, the saturation magnetization value σs can be increased by reducing the existence ratio of ultrafine particles having a plate surface diameter of less than 10 nm, which is considered to have an extremely low saturation magnetization value σs, to 15% or less. Accordingly, the magnetic anisotropy constant Ku can be increased, and it is considered that the influence of thermal fluctuation can be reduced.

  Examples of the present invention are shown below, and the present invention will be specifically described.

  The average plate surface diameter and average thickness of the hexagonal ferrite particle powder and the magnetic fine particle powder for a magnetic recording medium are obtained by taking a photograph of the particles using a transmission electron microscope and using the photograph for the plate surface of 360 or more particles. The diameter and the thickness were measured, and the average value of the average plate surface diameter and the average thickness of the particles was shown. In addition, as a selection criterion for particles, particles that overlap each other and whose boundaries are not clear are not measured.

  Further, the geometric standard deviation value of the plate surface diameter of the magnetic fine particle powder for magnetic recording medium is shown by the value obtained by the following method. That is, the value obtained by measuring the plate surface diameter of the particles shown in the above enlarged photograph, from the actual plate surface diameter and the number of particles obtained by calculating from the measured value, on a log normal probability paper according to a statistical method. The plate surface diameter of the particles is plotted on the horizontal axis, and the cumulative number of particles belonging to each of the predetermined particle diameter sections (under the integrated sieve) is plotted on the vertical axis as a percentage. Then, the values of the plate surface diameters corresponding to the number of particles of 50% and 84.13% are read from this graph, and the geometric standard deviation value = plate surface diameter under integrated fluid 84.13% / under integrated fluid 50 %, The value calculated according to the plate surface diameter (geometric mean diameter). The closer the geometric standard deviation value is to 1, the better the particle size distribution of the particles.

  The presence ratio of fine particles (less than 10 nm) was expressed as a ratio (%) to the total measured particles by calculating the number of particles having a plate surface diameter of less than 10 nm out of the total (number) of measured particles.

  The plate ratio was shown as the ratio between the average plate surface diameter and the average thickness.

  The specific surface area was shown by the value measured by BET method.

The amounts of Al, Si and various substitutional elements present in the inside and on the surface of the hexagonal ferrite particles and magnetic fine particles for magnetic recording media are “X-ray fluorescence analyzer 3063M type” (Rigaku Denki Kogyo Co., Ltd.) ) And use JIS
The measurement was carried out in accordance with K0119 "General X-ray fluorescence analysis rules".

  The magnetic properties of the hexagonal ferrite particle powder and the magnetic fine particle powder for a magnetic recording medium are as follows. Measured under conditions.

  The saturation magnetization increase rate (%) of the magnetic fine particle powder for a magnetic recording medium is expressed by multiplying σs after acid treatment by subtracting σs before acid treatment by σs after acid treatment and multiplying by 100. .

  In order to evaluate and confirm the characteristics of the magnetic fine particle powder for magnetic recording media according to the present invention in a magnetic recording medium, a magnetic tape was prepared and evaluated by the following method.

  9 g of magnetic particle powder, binder resin solution (1) (30% by weight of vinyl chloride copolymer resin having potassium sulfonate group and 70% by weight of cyclohexanone), binder resin solution (2) (polyurethane having sodium sulfonate group) 30% by weight of resin, 70% by weight of solvent (methyl ethyl ketone: toluene = 1: 1), cyclohexanone, methyl ethyl ketone and toluene are mixed in the 140 ml glass bottle together with 95 g of 0.5 mmφ glass beads at the following blending ratio, and mixed and dispersed for 6 hours with a paint shaker. Then, filtration was performed using a filter having an average pore diameter of 3 μm to prepare a magnetic paint for magnetic tape.

The composition of the obtained magnetic coating material for magnetic tape was as follows.
100.0 parts by weight of magnetic particle powder,
Binder resin solution (1)
(Vinyl chloride copolymer resin having potassium sulfonate group) 10.9 parts by weight,
Binder resin solution (2)
(Polyurethane resin having a sodium sulfonate group) 10.9 parts by weight,
62.7 parts by weight of cyclohexanone,
156.6 parts by weight of methyl ethyl ketone,
93.9 parts by weight of toluene.

  The obtained magnetic paint was applied on a polyethylene terephthalate film having a thickness of 12 μm to a thickness of 45 μm using an applicator, and then oriented and dried in a magnetic field (1193.7 kA / m) to obtain a magnetic tape.

  Among the magnetic properties of the magnetic tape, the coercive force value Hc and the squareness ratio Br / Bm were measured using an “oscillating sample magnetometer VSM SSM-5-15” (manufactured by Toei Kogyo Co., Ltd.) and an external magnetic field of 1193.7 kA / m. It measured on condition of this.

  Among the magnetic properties of the magnetic tape, the coercive force distribution SFD has a range of 0 to 397.9 kA / m applied magnetic field using “vibrating sample magnetometer VSM SSM-5-15” (manufactured by Toei Kogyo Co., Ltd.). Then, the sweep speed was 79.6 (kA / m) / min, and the sweep speed was measured at 397.9 (kA / m) / min in the range of 397.9 to 1,193.7 kA / m.

Ku of the magnetic tape showing the magnitude of the influence of the thermal fluctuation is measured using a magnetic torque meter (TRT-2-15-AUT) manufactured by Toei Industry Co., Ltd. in the range of 39.8 to 1,592 kA / m. Ku was determined.

  The thickness of each of the nonmagnetic support and the magnetic recording layer constituting the magnetic recording medium is measured by first measuring the film thickness (A) of the nonmagnetic support using a digital electronic micrometer K351C (manufactured by Anritsu Electric Co., Ltd.). Then, the thickness (B) of the magnetic layer formed on the nonmagnetic support (total of the thickness of the nonmagnetic support and the thickness of the nonmagnetic underlayer) was measured in the same manner. The thickness of the magnetic recording layer is indicated by (B)-(A).

  The glossiness of the coating surface of the magnetic tape was determined by measuring the 45 ° glossiness of the coating film using “Gloss meter UGV-5D” (manufactured by Suga Test Instruments Co., Ltd.).

  Surface roughness Ra measured the centerline average roughness of the coating film using "ZYGO NewView600S" (made by ZYGO Corporation).

<Example 1: Production of magnetic fine particle powder for magnetic recording medium>
Hexagonal ferrite particle powder 1 (type: magnetoplumbite type ferrite particle powder, Ti / Fe = 1.5 mol%, Ni / Fe = 1.4 mol%, average particle diameter: 24 nm, average thickness: 7 nm, plate ratio : 3.4, BET specific surface area value: 68.4 m ² / g, coercive force value Hc: 144.4 kA / m, saturation magnetization value σs: 47.2 Am ² / kg, presence of particles having a plate surface diameter of less than 10 nm In order to break up the agglomeration, 10 kg of the mixture was poured into 100 liters of pure water using a stirrer, and further, a line mill (product name: TK pipeline homomixer, manufactured by Tokushu Kika Kogyo Co., Ltd.) and vertical A slurry containing hexagonal ferrite particle powder was obtained through a type bead mill (product name: DSP, manufactured by Itonaga Iron Works Co., Ltd.). The residue of the sieve at 325 mesh (aperture 44 μm) of the obtained dispersion slurry was 0%.

  Water was added to the obtained dispersion slurry containing the hexagonal ferrite particle powder to adjust the concentration of the slurry to 40 g / l, and then a 35% hydrochloric acid solution was added with stirring to adjust the pH value of the slurry to 2. 0. Next, the slurry was held at 40 ° C. for 1 hour with stirring to dissolve the ultrafine component of the hexagonal ferrite particle powder.

  Next, the slurry is separated by filtration and the filtrate is separated. Then, pure water is passed by a decantation method and washed with water until the conductivity of the filtrate is 50 μs or less, then dried by a conventional method and pulverized. Magnetic fine particle powder 1 for magnetic recording medium was obtained.

The magnetic fine particle powder 1 for magnetic recording medium thus obtained has an average plate surface diameter of 25 nm, an average thickness of 7 nm, a plate ratio of 3.6, a BET specific surface area of 66.8 m ² / g, and a geometric standard of plate surface diameter. The deviation value consists of 1.54 particles, the abundance ratio of fine particle components (particles whose plate surface diameter is less than 10 nm) is 9.4%, the coercive force value Hc is 150.4 kA / m, and the saturation magnetization value σs Was 49.6 Am ² / kg, and the saturation magnetization increase rate was 5.1%.

  A magnetic fine particle powder for a magnetic recording medium was prepared according to Example 1. Various production conditions and various properties of the obtained magnetic fine particle powder for a magnetic recording medium are shown.

Hexagonal ferrite particles 1-3:
Various types of hexagonal ferrite particle powders were prepared, and hexagonal ferrite particle powders that were deagglomerated were obtained in the same manner as in Example 1.

  Table 1 shows various properties of these hexagonal ferrite particle powders.

Examples 2-3 and Comparative Examples 1 and 2:
A magnetic fine particle powder for a magnetic recording medium was obtained in the same manner as in Example 1 except that the kind of hexagonal ferrite particles, the kind of acid in the acid treatment, and the treatment conditions were variously changed.

  Table 2 shows the treatment conditions at this time, and Table 3 shows the characteristics of the obtained magnetic fine particle powder for magnetic recording medium.

Example 4:
After using the slurry washed with water after the acid treatment of Example 1 (slurry consisting of 10 kg of magnetic fine particle powder 1 for magnetic recording medium and 180 l of water), an aqueous sodium hydroxide solution was added to adjust the pH value to 9.0. The slurry was heated to 80 ° C., and 2,722 ml of a 1.0 mol / l sodium aluminate solution (corresponding to 1.2% by weight in terms of Al with respect to the magnetic fine particle powder for magnetic recording medium) ) And held for 30 minutes, and then the pH value was adjusted to 7.5 using acetic acid. After maintaining for 30 minutes in this state, filtration, washing with water, drying and pulverization were performed to obtain a magnetic fine particle powder for a surface-treated magnetic recording medium of Example 4 in which the particle surface was coated with an aluminum hydroxide or the like.

  Table 4 shows the manufacturing conditions at this time, and Table 5 shows the characteristics of the magnetic fine particle powder for surface-treated magnetic recording medium in which the obtained particle surface is coated with aluminum hydroxide or the like.

Examples 5-6:
Magnetic fine particle powder for surface-treated magnetic recording medium was obtained in the same manner as in Example 4 except that the kind of magnetic fine particle powder for magnetic recording medium and the kind and amount of surface coating in the surface treatment step were variously changed.

  Table 4 shows the manufacturing conditions at this time, and Table 5 shows the characteristics of the magnetic fine particle powder for surface-treated magnetic recording medium in which the obtained particle surface is coated with aluminum hydroxide or the like.

<Manufacture of magnetic tape>
Magnetic tapes 1-8, comparative magnetic tapes 1 and 3:
A magnetic tape was manufactured according to the above-described magnetic tape manufacturing method, except that the type of magnetic particles was variously changed.

Table 6 shows properties of the obtained magnetic tape.

  As shown in Table 6, the magnetic tape (magnetic tapes 1 to 6) using the magnetic fine particle powder for a magnetic recording medium according to the present invention has a higher squareness ratio (Br / Bm) than the comparative magnetic tapes 1 to 3. It has excellent coercivity SFD, excellent surface smoothness, and high magnetic anisotropy constant.

  The magnetic fine particle powder for a magnetic recording medium according to the present invention reduces the existence ratio of ultrafine particles having a plate surface diameter of less than 10 nm while the average plate surface diameter of the hexagonal ferrite particle powder is 10 to 50 nm. Thus, the influence of thermal fluctuation accompanying the miniaturization of the magnetic particle powder can be reduced, so that it is suitable as a magnetic particle powder for a high-density magnetic recording medium.

Claims (2)

Magnetic recording, characterized in that the hexagonal ferrite particle powder has an average plate surface diameter of 10 to 50 nm, and the proportion of ultrafine particles having a plate surface diameter of less than 10 nm is 15% or less with respect to all particles. Magnetic fine particle powder for medium. A hexagonal ferrite particle powder is dispersed in water to form an aqueous suspension, and an acid is added to the aqueous suspension to perform treatment under conditions of a pH value of 3.5 or less and a temperature range of 20 to 100 ° C. 2. The magnetic recording medium according to claim 1, wherein after the ultrafine particle component of the hexagonal ferrite particle powder existing in the aqueous suspension is dissolved, the remaining hexagonal ferrite particle powder is washed with water and dried. Manufacturing method of magnetic fine particle powder.