JP2013042047A - Hexagonal ferrite magnetic powder, manufacturing method of the same, and magnetic recording medium using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide fine magnetic powder having high magnetic characteristics and an appropriate surface condition in order to make hexagonal ferrite magnetic powder adaptable to high recording density.SOLUTION: For magnetic powder used for high density magnetic recording, hexagonal ferrite magnetic powder is used whose pores of 6.5 nm or smaller calculated from a ratio of a specific surface area value by a BET method and a specific surface area value by a mercury penetration method are 35% or lower. This type of ferrite magnetic powder is made by producing a fine glass material so that saturation magnetization of the fine glass material becomes low, in producing the glass material from glass made by fusing a material.

Description

  The present invention relates to a hexagonal ferrite magnetic powder for a magnetic recording medium, and also relates to a coating type magnetic recording medium using the magnetic powder.

  At present, metal magnetic powder is mainly used as magnetic powder used in coating type high-density magnetic recording media. Metal magnetic powder has evolved to become smaller and have higher magnetic force with the aim of lower noise and higher output. Originally, metal magnetic powder is mainly composed of metal, so it is necessary to avoid loss of magnetic force due to aging oxidation.

  Usually, this is done by forming an oxide film on the surface of the magnetic powder to prevent oxidation. However, as the magnetic powder becomes fine particles, the ratio of the oxide film to the particle volume has increased. As a result, the ratio of the metal portion that controls the magnetic force is reduced, and the magnetic force of the particles themselves cannot be avoided. In other words, it can be said that the means for increasing the magnetic force while maintaining the trade-off relationship between the improvement of the magnetic force and the prevention of oxidation is becoming the limit.

  Therefore, materials other than metal magnetic powder have been studied as magnetic powder for next-generation high-density magnetic recording. A typical example is magnetic powder made of hexagonal ferrite. Since the structure itself of the hexagonal ferrite magnetic powder is an oxide, it is possible to avoid the problem of aged deterioration of magnetic force due to oxidation. In addition, although it is not magnetized as metal magnetic powder, it has a feature that a large coercive force can be imparted by controlling crystal anisotropy. Therefore, it is expected as a magnetic powder for high-density magnetic recording.

  In addition, according to the technique described in the prior art document 1 or 2, if attention is paid to the state of the glass body that is an intermediate during ferrite formation, hexagonal ferrite magnetic powder having excellent magnetic properties can be obtained and high density can be obtained. It is disclosed that a magnetic recording medium suitable for recording can be formed.

Japanese Patent No. 3251653 JP 2010-024113 A

However, when the desired particles are hexagonal ferrite magnetic powder having a particle diameter of 30 nm or less, even if manufactured by the method described in Patent Document 1, it is difficult to produce magnetic characteristics suitable as magnetic powder used for high-density recording. However, it became clear by the indication of patent document 2. Further, according to the study by the inventors, even if the saturation magnetization of the magnetic powder described in Patent Document 2 is controlled to 0.6 Am ² / kg or less, the magnetic properties are insufficient as particles for achieving ultrahigh density recording. It turns out that there are cases where you can only get things.

Here, the hexagonal ferrite magnetic powder for ultra-high density recording is, as one standard, a state of a magnetic powder having a plate surface particle diameter of 30 nm or less, saturation magnetization of 40 Am ² / kg or more, and SFD of 0.8. It has the following characteristics. That is, in order to obtain a magnetic powder that achieves ultra-high density recording, it is necessary to devise a manufacturing method that is further advanced than the above manufacturing method.

  Further, according to the study by the inventors, it has been found that a magnetic powder capable of achieving ultra-high density recording may or may not be obtained even with the same composition. The cause of such a phenomenon was presumed to be largely due to the surface state of the particles. Until now, there is no known method for appropriately controlling the surface of fine ferrite particles. If a method for appropriately controlling these can be found, it is expected that a higher-density magnetic recording medium can be obtained.

  Therefore, the technical problem of the present invention is to provide a hexagonal ferrite magnetic powder capable of quantifying the surface state of magnetic powder particles capable of achieving ultra-high density magnetic recording, and stably obtaining magnetic characteristics, and a method for producing the same. It is to be.

  According to a study by the inventors, the composition ratio of the fine pores of 6.5 nm or less calculated by the formula (2) from the ratio of the specific surface area value by the BET method and the specific surface area value by the mercury intrusion method is 35%. It has been found that the following problems can be achieved with the following hexagonal ferrite magnetic powder, and the present invention has been completed.

  Composition ratio of fine pore diameter = 100− (specific surface area value by mercury intrusion method) / (specific surface area value by BET method) × 100 (2)

  In addition, the magnetic powder more suitably has an average pore diameter (median diameter: surface area standard) calculated by a mercury intrusion method of 24 nm or less.

  If the magnetic powder having the above physical properties is applied to a magnetic recording medium, a medium capable of high density recording can be obtained.

When the glass crystallization method is used, in order to obtain a magnetic powder suitable for high-density magnetic recording as described above, the saturation magnetization σs in the glass body before the heat treatment is set to a range of 0.25 Am ² / kg or less. Is preferred. The heat treatment for precipitating ferrite from such a glass body is performed in the range of 450 to 700 ° C., and the rate of temperature rise when reaching a predetermined heat treatment temperature of 450 to 700 ° C. is 0.2 to 4.0 ° C. / Min is preferable.

  In order to form appropriate ferrite at such a heat treatment temperature, bismuth (Bi) and niobium (Nb) or a rare earth element may be included as a component for forming the hexagonal ferrite magnetic powder.

  In the ferrite magnetic powder having the above properties, the ferrite particle surface is formed smoothly, so that the compatibility with the binder is good and the dispersibility is also high. Therefore, hexagonal ferrite magnetic powder suitable for ultra high density magnetic recording can be obtained.

It is a graph which shows the relationship between the porosity of 6.5 nm or less, and SFD in a powder state. It is a graph which shows the relationship between the magnitude | size (pore diameter) of a hole, and the intrusion amount of mercury.

  The inventors of the present invention have prepared a fine hexagonal ferrite magnetic powder (hereinafter, also simply referred to as “ferrite magnetic powder”) under the production conditions shown below. It was also found that a ferrite magnetic powder having a porosity of 35% or less was obtained, and the present invention was completed.

<Synthesis of particles>
The magnetic powder having the properties according to the present invention can be produced using, for example, the following method. In this specification, an example using a so-called glass crystallization method will be described.

  First, glass base material, main constituent raw material (also called “main constituent component”) iron (Fe), barium (Ba) and additive niobium (Nb), bismuth (Bi), rare earth elements, etc. Mix. The addition ratio of the main component is set to an amount that matches the set target amount for iron. However, only the rare earth element is equimolar or less with respect to the input amount of iron for the reason described later, and an excess amount is added from the amount expected to be finally contained.

  When the rare earth element is added, it is preferable that the amount is in the range of equimolar or less, preferably 15 mol% or less, more preferably 1.5 to 12.5 mol% with respect to the charged amount of iron. When such an amount is added, when an amorphous glass body (hereinafter simply referred to as “glass body”) is formed, a heat treatment is performed after the formation of the amorphous glass body. By doing so, when forming the ferrite magnetic powder, the particles are individually independent, and particles having a small volume as in the present invention can be formed. The raw material is preferably in the form of a salt. More specifically, it can be selected from nitrates, sulfates, acetates or oxides, but it is suitable to add them as oxides.

  The mixing is not limited as long as the raw materials (main constituent raw materials and additives) and the glass base material are uniformly mixed, and the mixing method is not limited, but it is preferable to adopt a dry method.

  These mixtures are melted in an electric furnace. The melting temperature at this time is 1250 to 1500 ° C, preferably 1300 to 1500 ° C, and more preferably 1350 to 1450 ° C. The melting at this time may be performed while mixing. Since melting is sufficient if the glass base material, main constituent material and additive components are uniformly melted, the melting time is within 6 hours, preferably within 4 hours, and more preferably within 2 hours.

  The obtained molten metal is rapidly cooled to form a glass body. The quenching method at this time is not particularly limited, but a twin roll method, a water atomizing method, and a gas atomizing method having a rapid quenching rate can be employed. In particular, the gas atomization method and the water atomization method are suitable. In addition to boron compounds and silicon compounds, alkali metal oxides such as sodium oxide and potassium oxide may be added and melted to the extent that they do not affect the magnetic properties. The addition amount at this time is at most 10% by mass, preferably 5% by mass or less, more preferably less than 2% by mass with respect to the whole.

The inventors obtain ferrite magnetic powder particles having uniform crystallinity (having uniform magnetic properties) if the saturation magnetization of the glass body obtained here is in the range of 0.08 to 0.25 Am ² / kg. I found out that I could do it. More preferably 0.08~0.20Am ² / kg, more preferably in the range of 0.08~0.15Am ² / kg. In order to adjust the saturation magnetization of the glass body, it is important to adjust the cooling rate when forming the glass body. In particular, the atomizing method is preferable for adjusting the cooling rate. Although it is possible to adjust the cooling rate to some extent even with the twin roll method, the atomizing method can adjust the blowing speed and the like, and it is easy to adjust the cooling rate of the amorphous glass body. Because.

  The obtained glass body may be pulverized. A known method can be used for the pulverization at this time. For example, a crushing method using a ball mill can be easily used, but can be appropriately changed depending on the scale of the ferrite magnetic powder to be produced. Thereafter, coarse particles remaining at the time of pulverization are removed by sieving to obtain a pulverized product. In the case of a pulverized product, it is preferable to arrange the sizes in order to obtain a ferrite magnetic powder having uniform magnetic characteristics.

  The thus obtained glass body or a pulverized product thereof is subjected to heat treatment to precipitate ferrite magnetic powder particles in the glass body. The glass on which the particles of the ferrite magnetic powder are deposited is called “ferrite-containing glass body”. In the heat treatment at this time, the glass body may be left still, or in some cases, the heat treatment may be performed while rolling.

  The temperature of the heat treatment only needs to be such that particles of ferrite magnetic powder can be formed in the glass body. More specifically, the temperature is 450 ° C. or higher and 700 ° C. or lower, preferably 500 ° C. or higher and 700 ° C. or lower. The heat treatment may be performed at a single temperature, that is, heating in a single stage, or may be a so-called multi-stage process performed in several stages at different processing temperatures. The heat treatment time is 30 minutes or longer, preferably 1 hour or longer.

  What is important in this heat treatment step is to gradually increase the temperature increase rate of the heat treatment. More specifically, the rate of temperature rise to the ultimate temperature is in the range of 0.2 to 4.0 ° C./min, preferably in the range of 0.2 to 3.0 ° C./min. If the rate of temperature increase is too large, many small pores of 6.5 nm or less are formed on the particle surface, which is not preferable. On the other hand, when the rate of temperature rise is smaller than the above range, the generation of pores of 6.5 nm or less is relatively suppressed, but this is not preferable because productivity deteriorates.

  Next, the glass component is removed from the obtained ferrite-containing glass body. What removed the glass component is hexagonal ferrite magnetic powder. To remove the glass component, dilute acetic acid diluted to about 10% by mass is preferably used, and the treatment temperature is preferably 50 ° C. or higher. Since it is sufficient that the glass component can be removed, acetic acid may be boiled in some cases or stirred for uniform removal. At this time, the pH of the treatment liquid is preferably 4.0 or less acidic.

  Acetic acid and the like adhering to the surface is removed by washing from the obtained ferrite magnetic powder. The adhering components may be removed by washing with pure water or boiling water, but in some cases, acetic acid adhering during washing with aqueous ammonia, aqueous sodium hydroxide, aqueous potassium hydroxide, etc. may be removed. It is also preferred to wash while neutralizing. In the case of an aqueous sodium hydroxide solution, an aqueous solution having a concentration of 0.01 to 1.5 mol / L, preferably 0.05 to 1.2 mol / L, more preferably 0.1 to 1.0 mol / L is used. Is preferred. If the concentration is low, there is no cleaning effect, and if the concentration is high, the cleaning effect is saturated and the risk of contamination is increased.

  Thereafter, the cleaning liquid is pure water, and the filtrate is sufficiently washed until the electric conductivity of the filtrate is 1.0 mS / m or less, preferably 0.8 mS / m or less. Particles often take the form of agglomerates, and acetic acid and reaction residues may exist in the gaps between the particles, so the glass body is removed and washed through the removal and washing process. It is also preferable.

  The obtained ferrite magnetic powder after the washing treatment can be obtained as a dry powder by subjecting it to a moisture removal treatment under conditions of 100 ° C. or higher in the air. Then, about 0.5-5.0 mass% of moisture may be adhered to the surface of the dry magnetic powder by exposing it to a wet environment of about 80% RH.

(Particle composition)
In addition to iron and alkaline earth metal (A) as main components, the particles according to the present invention control divalent and tetravalent additive elements (M ₁ and M ₂ ) for adjusting the coercive force and the shape. Including addition of bismuth (Bi), which is an additive element, and niobium (Nb), for thermal stability. Further, rare earth elements are included as further additive elements. In particular, the addition of rare earth elements is preferable because it makes it easier to make particles finer and makes it relatively easy to adjust the volume reduction (high specific surface area), which is one of the problems described above.

In particular, when rare earth elements are used, neodymium (Nd), samarium (Sm), yttrium (Y), erbium (Er), holmium (Ho), etc. are suitable selection targets. Among these, it is preferable to select Nd, Sm, and Y. Moreover, these content is 0.2-1.0 at. %, That is, (Ba, Sr, Ca, Pb) aFebBicNbdM ₁ eM ₂ fRg, g / b is 0.002 to 0.01. In addition, “(Ba, Sr, Ca, Pb) aFebBicNbdM ₁ eM ₂ fRg” means that (Ba, Sr, Ca, Pb), Fe, Bi, Nb, M ₁ , M ₂ , and R each have a molar ratio of a : B: c: d: e: f: g. R represents a rare earth element.

  If it is a manufacturing method which does not use said component, the thing of a small particle may be obtained depending on the case. However, the ferrite particles easily sinter and easily form particles having a remarkably poor particle size distribution, resulting in poor production stability. As a result, the dispersibility at the time of coating is poor, and the surface of the medium at the time of forming the medium is deteriorated.

  By adding bismuth (Bi), the temperature of ferritization can be lowered, so that sintering between particles can be reduced, and as a result, the particles become smaller. . It is also possible to control the thickness of the ferrite magnetic powder particles by adjusting the amount of bismuth added. Therefore, an excessive amount of bismuth added may produce ferrite magnetic powder particles having a large plate diameter, and as a result, the particle diameter may increase.

(Particle properties)
The ferrite magnetic powder particles of the present invention have the following physical properties. That is, the average particle diameter (corresponding to the plate diameter when it is plate-like, and the diameter when it is spherical) is 10 to 50 nm, preferably 10 to 25 nm. If it is larger than 50 nm, the noise becomes high when the recording medium is formed, so that it is not suitable for high-density recording. On the other hand, particles smaller than 10 nm are not preferable because thermal stability is deteriorated.

In the present invention, the size in the plate surface direction is the crystal diameter calculated from the (220) diffraction surface of hexagonal ferrite with X-rays, and the plate thickness direction is calculated from the (006) diffraction surface with X-rays. Calculated as the crystal diameter. The crystallite diameter volume at this time was calculated by crystallite diameter volume = (crystal diameter in the plate thickness direction) × π × (crystal diameter in the plate surface direction / 2) ² . In Examples described later, this is simply referred to as “particle volume”.

Particle volume calculated in such calculation method 100 to 3000 nm ^3, it is preferable preferably is 500~2500nm ^3. If the particle is smaller than this range, the thermal stability is deteriorated and it is not suitable for magnetic recording. This is because the recorded information may disappear. On the other hand, when the particle size is too large, the particle size becomes large, which is a cause of generation of particulate noise, which is not preferable.

  The measurement of the specific surface area using nitrogen is performed using the BET single point method (also simply referred to as “BET method”). The BET one-point method is a method of calculating a specific surface area using a BET equation from an isothermal adsorption curve of nitrogen gas under liquid nitrogen.

The specific surface area calculated by the BET single point method of the ferrite magnetic powder is 50 to 120 m ² / g, preferably 55 to 115 m ² / g, more preferably 60 to 110 m ² / g. When it is smaller than the lower limit, the particles are aggregated or condensed, and the particles are difficult to disperse. As a result, unevenness may occur in the medium after coating, and as a result, the medium characteristics deteriorate, which is not preferable. On the other hand, if it is too large, the presence of superparaparticles having no magnetism is suspected, and the medium characteristics generally deteriorate, which is not preferable.

  The specific surface area measured using the mercury intrusion method is a measure of the amount of mercury that enters the sample voids according to the indentation pressure. In other words, the measurement is performed by first covering the sample with mercury in a sealed container and applying pressure to the mercury liquid surface. And the pressure at that time and the amount of penetration of mercury into the powder surface are measured. Therefore, the direct observations are pressure and mercury penetration into the powder. More specifically, the pressure and the amount of decrease in the mercury liquid level when pressure is applied to the mercury liquid level with the powder covered with mercury. For the measurement, “Autopore IV 9500 series” manufactured by Micromeritics Instrument Corporation was used.

The void amount is evaluated using the Washburn equation [(mercury intrusion pressure: pc) = − 2φm cos θm / r]. Here, r is the radius of the void modeled as a cylindrical shape into which mercury can intrude according to the indentation pressure, θm is the contact angle between mercury and the cylindrical void interface, and φm is the interfacial energy of mercury (interfacial tension). In general, values of about θm = 141 ° and φm = 0.48 [J / m ² ] are used.

In order to evaluate the specific surface area from the test results, the calculation of equation (1) is further performed.

Here, V _m0 indicates the amount of mercury intrusion at the maximum press-fitting pressure in the test.

  The specific surface area calculated from the mercury intrusion method is smaller than the specific surface area calculated from the BET method. This is because when the mercury intrusion method is used, only pores through which mercury can penetrate can be measured, and pores through which mercury cannot penetrate due to the surface tension cannot be measured. It is practically difficult to measure pores having a pore diameter of 6.5 nm or less by the mercury intrusion method because a considerable pressure is required. Therefore, the specific surface area by the mercury intrusion method is a specific surface area obtained by setting the pores having a pore diameter larger than 6.5 nm as the surface shape.

  Therefore, if the specific surface area value by the BET method and the specific surface area value by the mercury intrusion method are compared, it can be estimated to some extent how much fine pores are present. For example, the method shown by Formula (2) is mentioned.

  Composition ratio of fine pore diameter = 100− (specific surface area value by mercury intrusion method) / (specific surface area value by BET method) × 100 (2)

  At this time, the proportion of fine pores is 35% or less, more preferably 33% or less in the above formula (2). When this value is large, there are many fine pores having a pore diameter of 6.5 nm or less, and the particle surface is rough. If the particle surface is rough in this way, it is considered that the solvent or resin does not reach the inside of the pores (does not get wet) unless a considerable share is added at the time of dispersion when producing the magnetic paint. That is, it is suggested that it is difficult to make a paint with the usual paint forming method. Moreover, when this value is large, many bubbles may remain in the paint, which is not preferable because the storage stability of the paint decreases.

  Moreover, it is considered that the large value of the expression (2) (that is, the ratio of fine pores is large) also indicates that the crystal itself is not generated without defects. This is because the grain surface roughness is considered to be caused by disorder of crystal growth. In addition, when the particle surface is rough in this way, projection points are formed on the particle surface, and unnecessary magnetic pole points are formed, which may reduce the magnetic characteristics of the entire particle. That is, it is difficult to obtain magnetic characteristics when the medium is used, which is not preferable. In addition, the poor crystallinity is preferable for ferrite magnetic powder whose coercive force originates from magnetocrystalline anisotropy, because it lowers saturation magnetization and SFD (Switching Field Distribution), which is the distribution of coercive force. Absent.

  In the calculation of the pore distribution using mercury, the contact angle between mercury and another substance is larger than 90 °, and therefore mercury cannot enter the pores existing on the particle surface unless pressure is applied. The pressure and the pore radius are given by the Kelvin equation of −2σcos θ = PRp. Here, σ is the surface tension of mercury, θ is the contact angle, P is the pressurized pressure, and Rp is the pore radius. Using this equation, the pore distribution is calculated from the mercury penetration amount and pressure.

Here, the pores are present on the particle surface, and the average value of the pore diameter should normally be smaller than the primary particle diameter. However, if this value is significantly different from the particle size measured by TEM, it is suspected that the particles are not monodispersed and the voids formed by aggregation or sintering of a plurality of particles are measured. It is presumed that the particles cannot exist in a monodisperse form. According to the study by the inventors, in the ferrite magnetic powder in the region where the BET specific surface area exceeds 70 m ² / g, if the average pore diameter in terms of surface area calculated by the mercury intrusion method is larger than 24 nm, the particles It was suspected that sintering and coagulation occurred during this period, and it turned out to be undesirable.

  That is, if this value is greatly different from the average particle diameter confirmed by TEM or the like, it is considered that the particles are sintered. When ferrite particles are sintered, there is a possibility that magnetization reversal hardly occurs simultaneously in the medium, and it is considered that the ferrite particles are not suitable for magnetic recording. Further, since the sintered particles, not the primary particles, behave as one lump, it may cause particle noise as a result, which is not preferable. Considering this, the pore diameter is preferably 22 nm or less, more preferably 20 nm or less.

  However, the average pore diameter used here is the median diameter based on the specific surface area. The specific surface area-based median diameter refers to a pore diameter at which the cumulative surface area is halved when a graph is drawn with the horizontal axis representing the pore diameter (reciprocal of pressure) and the vertical axis representing the cumulative surface area. This can be obtained by data processing of the pressure applied to mercury and the liquid level change data of mercury by the mercury intrusion method. This data processing was performed using a measurement program of “Autopore IV 9500 Series” manufactured by Micromeritics Instrument Corporation used for measurement. In Table 1, which will be described later, it is expressed as “average pore diameter (surface area)”.

<Evaluation of magnetic powder>
The obtained magnetic powder was evaluated for physical properties by the following methods.

<Particle composition>
The composition of the obtained ferrite magnetic powder was evaluated by the method shown below for the finally obtained ferrite magnetic powder. The iron dissolved the sample and was quantified using a Hiranuma automatic titrator (CONTIME-980 type) manufactured by Hiranuma Sangyo Co., Ltd. The other components were quantified by dissolving the powder and using a high frequency induction plasma emission analyzer ICP (IRIS / AP) manufactured by Nippon Jarrel Ash Co., Ltd.

<Calculation of specific surface area by nitrogen adsorption>
The calculation of the specific surface area using nitrogen was measured using the BET single point method, and the measuring device was measured using 4 Sorb US made by Your Sonics Co., Ltd.

<Evaluation of pore size distribution using mercury and calculation of specific surface area>
The calculation of pore size distribution and specific surface area using mercury was measured using mercury intrusion method. As a measuring device, Auto Pore IV 9500 manufactured by Micromeritics Instrument Corporation was used. The pressurization during measurement was in the range of 0.4 to 30000 psi. The pore size that can be measured by this corresponds to a range of approximately 440000 to 6.5 nm.

  In addition, 0.50 ± 0.05 g was sampled for each evaluation item. The specific surface area calculated by the mercury intrusion method is calculated using the value when the mercury intrusion pressure is the highest. That is, it calculated using the value at the time of 30000 psi pressurization (however, the value changed a little with the Example and the comparative example by the pressurization condition by an apparatus).

<Calculation of converted particle diameter from specific surface area>
It was approximated that the particles were spherical, and it was calculated between the specific surface area and the particle diameter (converted particle diameter) = 6 / barium ferrite density / specific surface area. The specific surface area value calculated from the mercury intrusion method and the BET method, and the density of barium ferrite were calculated as 5.3 (g / cm ³ ), respectively. In Table 1 to be described later, the converted particle diameter obtained by the mercury intrusion method is called “injection particle diameter”, and the converted particle diameter obtained by the BET method is called “BET particle diameter”.

<Powder magnetic property evaluation>
Ferrite magnetic powder or glass body is packed in a plastic container of φ6 mm, and using a VSM device (VSM-P7-15) manufactured by Toei Kogyo Co., Ltd., with an external magnetic field of 795.8 kA / m (10 kOe), a coercive force Hc (kA / m, Oe), saturation magnetization σs (Am ² / kg), squareness ratio SQ, and BSFD (SFD value in the bulk state) of the powder were measured.

<Single layer magnetic tape evaluation>
0.35 g of the obtained magnetic powder (ferrite magnetic powder as final product) is weighed and placed in a pot (inner diameter: 45 mm, depth: 13 mm), left for 10 minutes with the lid open, and then the vehicle with a micropipette (Vinyl chloride resin MR-555 (20 mass%) manufactured by Nippon Zeon Co., Ltd., Byron (registered trademark) UR-8200 (30 mass%), cyclohexanone (50 mass%), and acetylacetone (manufactured by Toyobo Co., Ltd.) 0.3 mass%) and 1.1 mL of a mixed solution of stearic acid-n-butyl (0.3 mass%)), and immediately after that, 30 g of steel balls (2φ) and 10 nylon balls (8φ) were added. In addition to the pot, it was allowed to stand for 10 minutes with the lid closed.

  Thereafter, the pot was set on a centrifugal ball mill (FRITSCH P-6), and the number of revolutions was slowly increased to 600 rpm and dispersed for 60 minutes. After stopping the centrifugal ball mill, the pot was taken out, and 0.7 mL of a preliminarily mixed solution of methyl ethyl ketone and toluene was added by a micropipette. Thereafter, the pot was set again on the centrifugal ball mill, and dispersed at 600 rpm for 5 minutes to produce a magnetic paint.

  Next, the pot lid was opened to remove the nylon balls, and the magnetic paint together with the steel balls was placed in an applicator (550 μm) and applied onto a base film (polyethylene film 15C-B500 manufactured by Toray Industries, Inc., film thickness 15 μm). . A magnetic tape was prepared by quickly placing the magnetic coil at the center of the solenoid coil having a center magnetic flux density of 5.5 kG (kilo gauss) and orienting it in a short time without drying. The coating thickness after drying is 3 μm. Here, in order to confirm only the effect of the magnetic powder, a non-magnetic layer was not provided, and a magnetic layer single layer tape was produced. Also, calendar processing is not performed.

  The magnetic tape as a medium thus produced was subjected to magnetic measurement using a VSM device (VSM-P7-15) manufactured by Toei Kogyo Co., Ltd., and the coercive force Hcx (Oe, kA / m), magnetic The coercive force distribution SFDx, the maximum energy product BHmax in the direction parallel to the layer surface, the squareness ratio SQx in the direction parallel to the magnetic layer surface, the squareness ratio SQz in the direction perpendicular to the magnetic layer surface, and the orientation ratio OR were obtained.

  Examples of the present invention and comparative examples will be described below.

Example 1
As main constituents, iron oxide 162.0 g (manufactured by Tetsugen Co., Ltd./HRT), barium carbonate (manufactured by Sakai Chemical Industry Co., Ltd./BW-P) 290.0 g are weighed, and boron oxide (manufactured by Borax / 89.4 g for industrial use, 6.1 g of cobalt oxide (manufactured by Wako Pure Chemical Industries, Ltd./special grade reagent), 6.5 g of titanium dioxide (manufactured by Wako Pure Chemical Industries, Ltd./special grade reagent), bismuth oxide (Kanto) 18.9 g of Chemical Co./reagent) and 27.3 g of niobium oxide (Kishida Chemical / reagent) were weighed.

  The resulting mixture was treated in an automatic mortar for 10 minutes to make the mixture uniform. The mixture thus obtained was inserted into a platinum crucible, dissolved at 1400 ° C., and maintained for 60 minutes to completely dissolve the mixture.

  The obtained molten metal was discharged from the nozzle, and a glass body was formed by a water atomization method. At this time, the pressure of jet water used for atomization was set to 30 MPa to form a glass body. Here, the glass body before heat treatment was fractionated and the saturation magnetization was measured (initial glass σs). Next, the obtained glass body is screened with a mesh having a mesh size of 250 μm to remove coarse particles, and then inserted into a furnace adjusted to a temperature of 450 ° C. from room temperature to 450 ° C. to stabilize the furnace temperature. I left it until.

  From 450 to 550 ° C., the temperature was slowly raised at a rate of 0.2 ° C./min. Thereafter, the temperature was maintained at 550 ° C. for 2.5 hours, and then the temperature was increased to 650 ° C. at a temperature increase rate of 2 ° C./min, and heat treatment was performed for 1 hour. That is, two-stage heat treatment at 550 ° C. and 650 ° C. was performed. Thereafter, it was gradually cooled to obtain a ferrite-containing glass body in a powder state.

  The ferrite-containing glass body powder was immersed in 10% by mass acetic acid heated to 60 ° C. and held for 60 minutes to remove the glass component. Thereafter, acetic acid adhering to the surface of the particles was removed using pure water to obtain a ferrite magnetic powder. This is washed with 1.0 mol / L of caustic soda, repeatedly washed with pure water until the filtrate conductivity becomes 0.8 mS / m or less, and the resulting powder is dried at 110 ° C. in the atmosphere for 4 hours. Thus, a ferrite magnetic powder was obtained. The physical properties of the obtained ferrite magnetic powder are shown in Table 1, and the magnetic properties are shown in Table 2.

  Further, when the state of pore distribution on the surface of the ferrite magnetic powder was measured by a mercury intrusion method, the presence of pores as shown in FIG. 2 was obtained. In FIG. 2, the horizontal axis represents the pore diameter (nm), and the vertical axis represents the mercury intrusion amount (mL / g) per unit ferrite magnetic powder.

(Examples 2 to 4)
The glass body was formed in the same manner, and the ferrite magnetic powder was formed in the same manner except that the heating rate during the heat treatment was changed. The physical properties of the obtained ferrite magnetic powder are shown in Table 1, and the magnetic properties are shown in Table 2. Moreover, when the surface pore distribution state of the ferrite magnetic powder of Example 4 was measured by the mercury intrusion method, the presence state of the pores as shown in FIG. 2 was obtained.

(Examples 5-8, Comparative Examples 1-2)
In the method of forming a glass body, the values were changed by changing the blowing speed and adjusting the cooling speed of the glass body to change the saturation magnetization (initial glass σs) of the glass body. In addition, Example 8 shows the temperature increase rate difference during the heat treatment of Example 7. The physical properties of the obtained ferrite magnetic powder are shown in Table 1, and the magnetic properties are shown in Table 2.

Example 9
As main constituents, 484.4 g of iron oxide (manufactured by Tetsugen Co., Ltd./HRT) and 810.2 g of barium carbonate (manufactured by Sakai Chemical Industry Co., Ltd./BW-P) are weighed, and boron oxide (manufactured by Borax / 436.6 g for industrial use, 2.9 g of cobalt oxide (manufactured by Wako Pure Chemical Industries, Ltd./special grade reagent), 8.8 g of zinc dioxide (manufactured by Wako Pure Chemical Industries, Ltd./special grade reagent), bismuth oxide (Kanto) 56.5 g of Chemical Co./reagent) and 9.6 g of niobium oxide (Kishida Chemical / reagent) were weighed.

  The resulting mixture was treated in an automatic mortar for 10 minutes to make the mixture uniform. The mixture thus obtained was transferred to a platinum crucible, dissolved at 1400 ° C., and maintained for 60 minutes to completely dissolve the mixture.

  The obtained molten metal was discharged from the nozzle, and a glass body was formed by a gas atomization method. At this time, the pressure of the injection gas used for atomization was set to 0.4 MPa to form a glass body. Here, the saturation magnetization was measured by separating the glass body before the heat treatment. The obtained glass body is screened with a mesh having a mesh size of 250 μm, and after removing coarse particles, it is inserted into a furnace adjusted to a temperature of 450 ° C. until normal temperature to 450 ° C. until the furnace temperature is stabilized. I left it alone. From 450 to 550 ° C., the temperature was slowly raised at a rate of 0.5 ° C./min. Then, after holding at 550 ° C. for 2.5 hours, the temperature was raised to 650 ° C. at a temperature rising rate of 2 ° C./min, and heat treatment was performed for 1 hour as it was to obtain a ferrite-containing glass body.

  The ferrite-containing glass body was immersed in 10% by mass acetic acid heated to 60 ° C. and held for 60 minutes to remove the glass component. Thereafter, acetic acid adhering to the surface of the particles was removed using pure water to obtain a ferrite magnetic powder. This ferrite magnetic powder powder was washed with 1.0 mol / L of caustic soda, and then repeatedly washed with pure water until the filtrate conductivity was 0.8 mS / m or less. And dried for 4 hours. The physical properties of the ferrite magnetic powder obtained as described above are shown in Table 1, and the magnetic properties are shown in Table 2. Further, when the state of pore distribution on the surface of the ferrite magnetic powder of Example 9 was measured by mercury porosimetry, the presence of pores as shown in FIG. 2 was obtained.

(Comparative Example 3)
Example 1 was repeated except that the heating rate (0.2 ° C./min) during heat treatment in Example 1 was increased (30 ° C./min). The physical properties of the obtained ferrite magnetic powder are shown in Table 1, and the magnetic properties are shown in Table 2.

  In Table 2, the composition ratio of the fine pores determined by the equation (2) is described as “6.5 nm or less porosity”. This porosity of 6.5 nm or less exceeds 36% in the comparative example. Such magnetic powder has a high SFD (0.8 or more) even in the magnetic properties of the powder state. This shows that the distribution of the coercive force is broad, so it can be said that the uniformity of the magnetic properties is low. In the case of hexagonal ferrite magnetic powder, the magnetic properties are considered to depend on the crystallinity of the particles, so the nonuniformity of the magnetic properties suggests the nonuniformity of the crystallinity. In the saturation magnetization (σs) and the coercive force (Hc), the value of the comparative example is lower than that of the example. This also suggests that the comparative example has poor crystallinity of the particles.

  FIG. 1 shows the relationship between the porosity of 6.5 nm or less and the SFD in the powder state. The vertical axis is SFD (unitless), and the horizontal axis is 6.5 nm or less porosity (%). It can be seen that when the porosity of 6.5 nm or less decreases, the SFD decreases and the variation in coercive force of the powder as a whole decreases.

  Moreover, also in the evaluation with the medium using these ferrite magnetic powders (see Table 2), the SFD of the medium produced with the ferrite magnetic powder of the example is the SFD of the medium when the ferrite magnetic powder of the comparative example is used. Lower (0.55 or less). Further, the SQx, which is the squareness ratio in the orientation direction, is 0.7 or more in the example, whereas it is less than 0.7 in the comparative example.

  This is presumably because, in Examples where the number of pores on the surface of the ferrite magnetic powder is small, it is familiar with the binder and rotates well in the orientation magnetic field when it is made into a paint. That is, it can be said that the magnetic powder is suitable for a magnetic recording medium.

When Example 4 and Comparative Example 3 are compared, the initial glass σs (0.50 Am ² / kg) of Comparative Example 3 is the initial of Example 4 despite the heat treatment under the same conditions and the composition of the same conditions. It was higher than glass σs (0.11 Am ² / kg). This has affected the magnetic characteristics when the recording medium is formed (coercivity, SFD, SQx, etc.). From this, it can be seen that for the formation of ferrite magnetic powder by the glass crystallization method, it is necessary to prepare the glass body so that the magnetization (σs) of the glass body before the heat treatment becomes sufficiently small.

  The ferrite magnetic powder according to the present invention can provide a magnetic recording medium suitable for high-density magnetic recording.

Claims (7)

A hexagonal ferrite magnetic powder in which the composition ratio of fine pores of 6.5 nm or less calculated from the specific surface area value by the BET method and the specific surface area value by the mercury intrusion method is 35% or less.
Composition ratio of fine pore diameter = 100− (specific surface area value by mercury intrusion method) / (specific surface area value by BET method) × 100 (2)   The hexagonal ferrite magnetic powder according to claim 1, wherein an average pore diameter (median diameter: specific surface area standard) calculated by a mercury intrusion method is 24 nm or less.   A magnetic recording medium using the hexagonal ferrite magnetic powder according to claim 1 as a magnetic layer. Melting a glass component containing hexagonal oxide and a hexagonal ferrite-forming component;
Quenching the melt to obtain a solid adjusted to a saturation magnetization of 0.25 Am ² / kg or less;
A method of producing hexagonal ferrite magnetic powder, comprising the step of heat treating the solid at a heat treatment temperature of 450 to 700 ° C. to precipitate ferrite in the solid.   The method for producing hexagonal ferrite magnetic powder according to claim 4, wherein the rapid cooling of the melt is performed by an atomizing method.   The method for producing a hexagonal ferrite magnetic powder according to any one of claims 4 to 5, wherein the heat treatment temperature is reached at a temperature rising rate of 0.2 to 4.0 ° C / min.   The method for producing a hexagonal ferrite magnetic powder according to any one of claims 4 to 6, wherein the forming component of the hexagonal ferrite magnetic powder contains bismuth and niobium or a rare earth element.