Method for manufacturing hexagonal barium ferrite magnetic powder

Abstract

To achieve simultaneous improvement of both corrective force Hc and a magnetocrystalline anisotropy constant Ku in finely pulverized hexagonal barium ferrite magnetic powder.SOLUTION: Provided is a method for producing hexagonal barium ferrite magnetic powder, which includes a crystallization step of synthesizing hexagonal barium ferrite crystals by subjecting a precursor to a crystallization heat treatment, the precursor containing a constituent element of hexagonal barium ferrite magnetic powder. As the precursor, there is used a material having a mass increase rate of 0.08% or less at 700°C based on a mass at 300°C when subjected to thermogravimetry which includes heating with a heating rate of 10°C / min from room temperature to a temperature of 700°C or higher in an air atmosphere and also has a saturation magnetization σs of 5.0 Am2 / kg or less.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] This invention relates to a method for producing magnetoplanbite-type (M-type) hexagonal barium ferrite magnetic powder suitable for high-density recording on magnetic recording media. Furthermore, it relates to magnetoplanbite-type (M-type) hexagonal barium ferrite magnetic powder in which both coercivity (Hc) and crystal magnetic anisotropy constant (Ku) are simultaneously improved. [Background technology]

[0002] M-type hexagonal barium ferrite magnetic powder is known as a magnetic powder suitable for high-density recording used in magnetic recording media. From the viewpoint of improving recording density, miniaturization of magnetic particles (miniaturization of the Dx volume, described later) is advantageous. However, generally, as the size of magnetic particles decreases, the coercivity Hc decreases. In other words, there is a trade-off relationship between miniaturization of magnetic particles and increasing coercivity.

[0003] Patent Document 1 discloses a manufacturing technology for hexagonal ferrite magnetic powder having a particle size distribution with fewer excessively fine particles, in light of the problem that the SFD (coercivity distribution) increases with the miniaturization of hexagonal ferrite magnetic powder (paragraph 0004). According to this, a two-stage heat treatment is effective, consisting of a first heat treatment step to precipitate nuclei and a second heat treatment step to grow the precipitated nuclei (paragraphs 0021-0022). Specifically, in the glass crystallization method, examples are shown in which the first heat treatment step is 500°C for 30 hours and the second heat treatment step is 780°C for 2 hours (Example 1) or 850°C for 2 hours (Example 2), and in which the first heat treatment step is 400°C for 10 hours and the second heat treatment step is 780°C for 2 hours (Example 3). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 10-92618 [Overview of the project] [Problems that the invention aims to solve]

[0005] From the perspective of improving the reliability of magnetic recording media, it is advantageous to use magnetic materials with high coercivity. However, if the coercivity of the magnetic material is excessively high, exceeding the capacity of the magnetic head, demagnetization becomes difficult, and the recorded content cannot be rewritten. Magnetic recording media use magnetic materials that are adjusted to have the highest possible coercivity depending on the usage conditions. On the other hand, increasing the recording density is important for improving the performance of magnetic recording media. As mentioned above, miniaturization of magnetic particles (miniaturization of the Dx volume, described later) is effective in increasing the recording density, but generally, miniaturizing magnetic particles reduces coercivity. It is desirable to establish manufacturing technology for magnetic powder that maintains the highest possible coercivity even when magnetic particles are miniaturized.

[0006] Patent Document 1 does not show any examples of synthesizing fine hexagonal ferrite particles that can adequately meet the current demands for higher recording density. Furthermore, even if one attempts to refine the magnetic powder using the two-stage heat treatment crystallization process disclosed in Patent Document 1, it is difficult to improve the impact on the reduction in coercivity associated with refinement.

[0007] To improve the reliability of magnetic recording media, it is important to use magnetic powders with a high crystalline magnetic anisotropy constant Ku, which is an indicator of thermal stability. When magnetic particles are miniaturized, the crystalline magnetic anisotropy constant Ku usually decreases. When magnetic particles are miniaturized, if the degree of miniaturization is the same (i.e., the Dx volume is the same), magnetic powders with higher coercivity Hc and crystalline magnetic anisotropy constant Ku are advantageous in achieving both high recording density and improved reliability of magnetic recording media. The present invention aims to simultaneously improve coercivity Hc and crystalline magnetic anisotropy constant Ku in miniaturized hexagonal barium ferrite magnetic powder. [Means for solving the problem]

[0008] The above objective is to provide a method for producing hexagonal barium ferrite magnetic powder, which includes a crystallization step of synthesizing hexagonal barium ferrite crystals by subjecting a precursor material containing the constituent elements of hexagonal barium ferrite magnetic powder to a crystallization heat treatment, The aforementioned precursor material is subjected to thermogravimetric analysis in which it is heated from room temperature to a temperature of 700°C or higher at a heating rate of 10°C / min in an atmospheric environment. The mass increase rate at 700°C, based on the mass at 300°C, is 0.08% or less, and the saturation magnetization σs is 5.0 Am 2 This is achieved by a method for producing hexagonal barium ferrite magnetic powder using materials with a weight of less than / kg.

[0009] As the aforementioned precursor material, a substance obtained by subjecting an amorphous body, which is obtained by rapidly cooling a molten material containing the constituent elements of hexagonal barium ferrite magnetic powder, more specifically a molten material containing Ba and B as amorphous-forming components, to a preheat treatment in which it is held in an oxidizing atmosphere at a temperature range of 450 to 560°C, can be applied. The preheat treatment is more effective when it is performed under conditions such that the relationship between the average holding temperature Tm (°C) and the holding time t (h) at 450 to 560°C satisfies the following equation (1). t(h)≧0.0005×exp[8720 / (273+Tm(℃))] …(1)

[0010] In the crystallization step, the crystallization heat treatment can be carried out at, for example, 600 to 700°C. The hexagonal barium ferrite magnetic powder may contain Bi in a Bi / Fe molar ratio in the range of 0.005 to 0.05.

[0011] Furthermore, in the present invention, a preferred magnetic powder obtained by the above manufacturing method is a magnetic powder composed of hexagonal ferrite particles having the chemical formula AO·6Fe2O3 as its basic structure, where element A is Ba, the Co / Fe molar ratio is 0.005 or more, and the ratio of [total content of Fe site-substituted elements (moles)] / [Fe content (moles)] is 0 to 0.06, and the Dx volume represented by the following formula (2) is 1100 to 2000 nm. 3 The coercivity Hc (kA / m) and the Dx volume (nm) are given. 3 The relationship between ) satisfies equation (3) below, and the crystal magnetic anisotropy constant Ku (MJ / m 3 ) and Dx volume (nm 3Hexagonal barium ferrite magnetic powder is provided in which the relationship of ( ) satisfies the following formula (4). Instead of the formula (4), hexagonal barium ferrite magnetic powder that satisfies the following formula (5) is a more suitable target. 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 pi. Hc (kA / m) > 0.11 × Dx volume (nm 3 ) - 20 …(3) Ku (MJ / m 3 ) > 3.083 × 10 -5 × Dx volume (nm 3 ) + 0.079 …(4) Ku (MJ / m 3 ) > 3.083 × 10 -5 × Dx volume (nm 3 ) + 0.083 …(5)

Advantages of the Invention

[0012] According to the present invention, in hexagonal barium ferrite magnetic powder composed of refined magnetic particles with a Dx volume of, for example, 2400 nm 3 or less, more preferably 2000 nm 3 or less, both the coercive force Hc and the crystal magnetic anisotropy constant Ku can be improved. That is, compared with conventional hexagonal barium ferrite magnetic powder having the same composition and particle diameter (Dx volume), both the coercive force Hc and the crystal magnetic anisotropy constant Ku can be increased, leading to an improvement in the reliability of the magnetic recording medium. Further, when adjusting the coercive force Hc or the crystal magnetic anisotropy constant Ku to a predetermined characteristic value, the particle diameter can be made finer than before, leading to an improvement in the recording density of the magnetic recording medium.

Brief Description of the Drawings

[0013] [Figure 1]This graph illustrates the TG curves for amorphous materials (Ref.) and precursor materials obtained by preheating them under appropriate conditions, when heated at a rate of 10°C / min in an atmospheric environment. [Figure 2] This graph illustrates the TG curve for each precursor substance when the temperature is increased at 10°C / min in an atmospheric environment. [Figure 3] A graph plotting the relationship between the crystallization heat treatment temperature and the Dx volume of the resulting hexagonal ferrite magnetic powder, allowing for the distinction of the precursor material used. [Figure 4] A graph plotting the relationship between the Dx volume and coercivity Hc of hexagonal ferrite magnetic powder, allowing for distinction of the precursor material used. [Figure 5] A graph plotting the relationship between the Dx volume and the crystal magnetic anisotropy constant Ku of hexagonal ferrite magnetic powder, allowing for distinction of the precursor material used. [Figure 6] A graph plotting the relationship between temperature and time during preheating. [Modes for carrying out the invention]

[0014] [Precursor] This invention employs a manufacturing process for synthesizing hexagonal barium ferrite crystals by subjecting a precursor material containing the constituent elements of hexagonal barium ferrite magnetic powder to a crystallization heat treatment. The "precursor material" is a substance obtained by subjecting an amorphous body to the preliminary heat treatment described later, in which crystallization into hexagonal ferrite is incomplete. Here, the saturation magnetization σs is 5.0 Am 2 The precursor material is identified by having a saturation magnetization σs of 5.0 Am 2 For materials with a value of less than / kg, it is considered that crystallization into hexagonal ferrite has hardly occurred, or if it has occurred partially, the amount is extremely small. Saturation magnetization σs is 5.0 Am 2 Using a precursor material with a coercivity Hc exceeding / kg makes it difficult to stably obtain an increase in coercivity Hc. There is no particular limit to the lower limit of the saturation magnetization σs of the precursor material, but for example, 0.1 Am 2 It is acceptable as long as it is adjusted within a range of / kg or more.

[0015] As described above, among the precursor materials with sufficiently small saturation magnetization σs, we particularly apply those in which the mass increase rate at 700°C, relative to the mass at 300°C, is 0.08% or less when subjected to thermogravimetric analysis in an atmospheric environment (air) where the temperature is raised from room temperature to 700°C or higher at a heating rate of 10°C / min. Here, the mass increase rate (%) at 700°C relative to the mass at 300°C is calculated by 100 × (mass at 700°C (g) - mass at 300°C (g)) / mass at 300°C (g).

[0016] When synthesizing hexagonal ferrite by firing amorphous materials using the so-called glass crystallization method, the divalent Fe contained in the amorphous material needs to be oxidized to trivalent Fe during the crystallization process. Precursors with a mass increase rate of 0.08% or less in the thermogravimetric measurement when heated to 700°C are considered to be in a state where most of the divalent Fe has already been oxidized to trivalent Fe. The inventors have found that when such precursors are heated to the temperature range in which hexagonal ferrite crystals are formed and crystallized, it is possible to improve both the coercivity Hc and the crystal magnetic anisotropy constant Ku, under conditions in which the particle size (Dx volume) of the resulting magnetic powder is equivalent to that of the conventional glass crystallization method, in which hexagonal ferrite is synthesized directly from amorphous materials containing divalent Fe.

[0017] [Preheat treatment] The above-mentioned precursor material can be obtained, for example, by subjecting an amorphous body containing the constituent elements of hexagonal barium ferrite magnetic powder to a heat treatment held in an oxidizing atmosphere such as air at a temperature range of 450-560°C. In this specification, this heat treatment is referred to as "pre-heat treatment". When the temperature of the pre-heat treatment exceeds 560°C, the saturation magnetization σs of the resulting precursor material is 5.0 Am 2The coercivity may exceed 1 kg / kg. In that case, the coercivity Hc improvement effect will not be sufficiently obtained. Also, if the preheat treatment temperature is below 450°C, the oxidation of divalent Fe contained in the amorphous component will be insufficient, and the coercivity Hc improvement effect will not be sufficiently obtained. The above amorphous material can be obtained by the amorphousization method used in conventionally known glass crystallization methods. Specifically, it can be amorphousized by rapidly cooling a molten material containing Ba and B as amorphous-forming components.

[0018] In the preheat treatment, the holding time in the temperature range of 450-560°C is adjusted so that a precursor material is obtained in which the mass increase rate in the thermogravimetric measurement described above is 0.08% or less, more preferably 0.05% or less. The appropriate conditions can be determined by preliminary experiments, but it is particularly effective to adopt conditions in which the relationship between the average holding temperature Tm (°C) and the holding time t (h) in the range of 450-560°C satisfies equation (1) below. t(h)≧0.0005×exp[8720 / (273+Tm(℃))] …(1) Here, the average holding temperature Tm (°C) can be considered as the time-averaged temperature when the temperature is in the range of 450 to 560°C in a heat curve represented by a graph with time (h) on the horizontal axis and temperature (°C) on the vertical axis. In cases where the material is held in a furnace at a constant temperature, such as in the examples described later, that holding temperature can be considered as the average holding temperature Tm.

[0019] Figure 1 illustrates the TG curves for an amorphous material (Ref.) with a similar composition to the examples described later, and a precursor material obtained by subjecting the amorphous material to appropriate preheat treatment (490°C × 72h, 510°C × 72h), when heated at 10°C / min in an atmospheric environment. The amorphous material (Ref.) shows a significant mass increase between 300 and 700°C. This is thought to be due to the presence of divalent Fe in the amorphous material (Ref.), and the subsequent oxidation of this divalent Fe to trivalent Fe. On the other hand, the precursor material that underwent appropriate preheat treatment shows little to no mass increase between 300 and 700°C. It is presumed that most of the divalent Fe has already been converted to trivalent Fe by the preheat treatment. Note that the values ​​on the vertical axis of Figure 1 are based on the mass at the start of heating (35°C). In this invention, considering the presence of moisture and other volatile components, the mass at the time of heating to 300°C is used as the basis.

[0020] [Crystallization Heat Treatment] The above-mentioned precursor material is crystallized by calcination to synthesize hexagonal barium ferrite. In this specification, this calcination is referred to as "crystallization heat treatment". Crystallization heat treatment can be carried out at, for example, 600 to 700°C. From the viewpoint of obtaining the finest possible magnetic particles, it is preferable to carry it out at 600 to 650°C. The holding time in the temperature range from 600°C to the maximum attainable temperature Tmax (°C) (however, 600°C ≤ Tmax ≤ 700°C) can be set in the range of, for example, 50 to 240 minutes.

[0021] [Hexagonal barium ferrite magnetic powder] The hexagonal barium ferrite magnetic powder obtained through the above-described preheat treatment and crystallization heat treatment is composed of hexagonal ferrite particles in which element A of the hexagonal ferrite with the basic structure of chemical formula AO·6Fe2O3 is Ba. Part of the Fe site may be substituted with one or more divalent, tetravalent, or pentavalent metal elements. Examples of the divalent metal elements include Co and Zn, examples of the tetravalent metal elements include Ti and Sn, and examples of the pentavalent metal elements include Nb and V. Metal elements that substitute for part of the Fe site in this way are called "Fe site substituting elements". Regarding the amount of substitution, for example, the ratio of [total content of Fe site substituting elements (moles)] / [Fe content (moles)] can be exemplified as being in the range of 0 to 0.06, more preferably in the range of 0.001 to 0.04. Among the Fe site substituting elements, for example, Co is effective in improving the SNR of magnetic recording media. When Co is included, it is effective to have a Co / Fe molar ratio of 0.005 or higher.

[0022] The hexagonal barium ferrite magnetic powder targeted by this invention may contain Bi. Although Bi is not an element that constitutes the crystal structure of hexagonal ferrite (an element that occupies any of the atomic sites in the chemical formula AO·6Fe2O3), it is an effective additive element for refining the hexagonal ferrite crystal grains and improving the electromagnetic conversion characteristics of magnetic recording media using this magnetic powder. In particular, it has the effect of minimizing the decrease in magnetic properties even when aiming to refine the crystal grains by lowering the firing temperature. When Bi is included, it is effective to set the Bi / Fe molar ratio in the range of 0.005 to 0.05.

[0023] Furthermore, depending on the required characteristics, one or more rare earth elements such as Nd, Y, Sm, Y, Er, Ho, or Al may be included. These elements do not constitute the crystal structure of hexagonal ferrite. When one or more rare earth elements are included, it is preferable that the R / Fe molar ratio be 0.001 to 0.010, where the rare earth element is denoted as R. When Al is included, it is preferable that the Al / Fe molar ratio be 0.001 to 0.050.

[0024] The hexagonal barium ferrite magnetic powder targeted in this invention has a Dx volume represented by the following formula (2) of, for example, 1100 to 2400 nm. 3 And more preferably 1300~2000nm 3 The Dx volume can be adjusted by the temperature of the crystal heat treatment, etc. Its characteristics include coercivity Hc (kA / m) and Dx volume (nm). 3 The relationship between ) satisfies equation (3) below, and the crystal magnetic anisotropy constant Ku (MJ / m 3 ) and Dx volume (nm 3 Examples of suitable materials include those that satisfy the following equation (4) in relation to (4). Instead of equation (4) above, hexagonal barium ferrite magnetic powder that satisfies the following equation (5) is a more suitable material. Dx volume (nm) 3 ) = Dxc × π × (Dxa / 2) 2 …(2) Here, Dxc is the crystallite diameter in the c-axis direction (nm) of the hexagonal ferrite crystal lattice, Dxa is the crystallite diameter in the a-axis direction (nm) of the same crystal lattice, and π is pi. Hc(kA / m)>0.11 × Dx volume(nm) 3 )-20 …(3) Ku(MJ / m 3 )>3.083×10 -5 ×Dx volume (nm) 3 ) + 0.079 …(4) Ku(MJ / m 3 )>3.083×10 -5 ×Dx volume (nm) 3 ) + 0.083 …(5) [Examples]

[0025] [Example 1] Boric acid (H3BO3, industrial grade), barium carbonate (BaCO3, industrial grade), iron oxide (α-Fe2O3, industrial grade), cobalt oxide (CoO, reagent grade, purity 90% or higher), titanium dioxide (TiO2, reagent grade, purity 98.5%), bismuth oxide (Bi2O3, industrial grade, purity 99% or higher), neodymium oxide (Nd2O3, industrial grade, purity 99% or higher), and aluminum hydroxide (Al(OH)3, industrial grade, purity 99% or higher) were weighed to obtain the raw material formulations shown in Table 1. These were mixed using a Mitsui Miike FM mixer to obtain a raw material mixture. The above raw material mixture was placed in a pelletizer, granulated by forming it into spheres while spraying water, and then dried at 270°C for 14 hours to obtain granulated products with particle sizes of 1 to 50 mm.

[0026] The above granulated material was melted in a melting furnace using a platinum crucible. The temperature was raised to 1400°C and maintained for 60 minutes while stirring to completely melt each raw material. Then, the molten material was dispensed from a nozzle and rapidly cooled by gas atomization to obtain an amorphous material.

[0027] The obtained amorphous material was subjected to preheat treatment by holding it at 490°C in an atmospheric environment for 72 hours to obtain a precursor material. Thermogravimetric (TG) measurements were performed on this precursor material using the following method. (Thermogravimetric (TG) measurement of precursor material) 20 mg of the precursor was packed into a φ5 mm alumina (Al2O3) container, and the mass change of the sample was measured in the range of 35°C to 1000°C at a heating rate of 10°C / min while introducing dry air at a flow rate of 0.2 SL / min in an atmospheric environment using a differential thermogravimetric analyzer (NEXTA STA300) manufactured by Hitachi High-Tech Science Co., Ltd. The mass increase rate was calculated at 700°C, using the mass at 300°C as the baseline. The precursor obtained in this example showed a mass increase of 0.03% at 700°C, relative to its mass at 300°C. Furthermore, when magnetic measurements were performed on this precursor material according to the "Measurement of Powder Magnetic Properties" described later, the saturation magnetization σs was found to be 0.5 Am 2 It was / kg.

[0028] Next, the above-mentioned precursor material was subjected to a crystallization heat treatment under conditions of being held at 630°C for 60 minutes in an atmospheric environment to induce crystallization.

[0029] The powder obtained by the crystallization heat treatment contains hexagonal ferrite as well as residual substances mainly composed of barium borate. To remove the residual substances, the powder obtained by the crystallization heat treatment was immersed in a 10% by mass aqueous solution of acetic acid heated to 60°C and held for 1 hour while stirring to dissolve the residual substances in the liquid. After that, solid-liquid separation was performed by filtration, and the powder was washed with pure water. After washing with water, it was dried in the air at 110°C to obtain hexagonal barium ferrite magnetic powder. This hexagonal barium ferrite magnetic powder was used as the test powder for the following investigation. The main items of the magnetic powder manufacturing conditions are shown in Table 1.

[0030] (Compositional analysis of magnetic powder) The composition of the test powder was analyzed using an ICP (720-ES) high-frequency induction plasma emission spectrometer manufactured by Agilent Technologies, Inc. Measurement wavelengths (nm) were set as follows: Fe: 259.940 nm, Ba: 233.527 nm, Co: 231.160 nm, Ti: 334.941 nm, Bi: 222.821 nm, Nd: 406.108 nm, and Al: 396.152 nm. The measurement wavelengths for each metal element were selected to avoid interference from the spectra of other elements and to ensure linearity of the calibration curve, depending on the composition of the magnetic powder being analyzed. From the obtained quantitative values, the molar ratio of each element to Fe was calculated.

[0031] (Measurement of powder magnetic properties) The test powder was packed into a φ6mm plastic container, and measured using a vibrating sample type magnetometer (VSM-P7-15, manufactured by Toei Kogyo Co., Ltd.) with an external magnetic field of 795.8 kA / m (10 kOe) and an M measurement range of 0.010 A·m. 2 The coercivity Hc, saturation magnetization σs, and square aspect ratio SQ were measured under the following conditions: (10 emu), step bit 198 (bit), time constant 0.03 sec, and weight time 0.1 sec. The coercivity Hc of the powder used in this example was 143 kA / m.

[0032] (Measurement of BET specific surface area) The specific surface area of ​​the test powder was determined using the BET single-point method with a fully automated specific surface area measuring device (Macsorb HM Model-1210, manufactured by Mountec Co., Ltd.).

[0033] (Evaluation of activation volume Vact and crystal magnetic anisotropy constant Ku) A pulsed magnetic field generator (TESLA, TP15326) and a vibrating sample magnetometer (Toei Kogyo Co., Ltd., VSM-5) were used. The activation volume Vact and crystal magnetic anisotropy constant Ku were evaluated by the following procedures (1) to (10). However, procedures (2) to (10) were performed at 25±1℃. The remanent magnetization was measured using the M measurement range 0.005 A·m 2 The measurement was performed at (5emu) and a time constant of 0.03 sec. (1) The hexagonal barium ferrite magnetic powder to be tested was packed into a φ6 mm plastic container. (2) A magnetic field of 1034.54 kA / m (13 kOe) was applied using a vibrating sample magnetometer to saturate the magnetization, and the magnetic field was then returned to zero. At this time, the step bit was set to 240 bits, the wait time to 0.8 seconds, and the magnetic field was applied in Return mode. (3) The sample was removed from the vibrating sample magnetometer and attached to the pulsed magnetic field generator. At this time, the sample was attached so that a magnetic field (called a reverse magnetic field) was applied in the opposite direction to the saturation magnetization direction. (4) A magnetic field was applied for a reverse magnetic field duration of 0.40 ms, and then the magnetic field was returned to zero. For the first application, the magnetic field should be approximately Hc + 23.88 kA / m. For subsequent applications, a different reverse magnetic field should be set from the first application, using the results of the first application as a reference, so that the remanent magnetization is near zero. (5) The sample was removed from the pulsed magnetic field generator and attached to the vibrating sample type magnetometer so that the orientation of the sample was the same as in (2). (6) The amount of remanent magnetization was measured using a vibrating sample type magnetometer. The procedure was performed within 20 seconds from the completion of the operation in (2) to the measurement of the amount of remanent magnetization. (7) The value of the reverse magnetic field applied in (4) was changed, and the operations from (2) to (6) were repeated four or more times. (8) If the remanent magnetization is 0 Am 2Select five or more measurement points and perform linear approximation so that the reverse magnetic field value Hr(0.40ms) which corresponds to / kg can be interpolated, and the coefficient of determination R 2 Steps (2) to (7) were repeated until the value of was 0.990 or greater. From this approximate straight line, the remanent magnetization was found to be 0 Am 2 The value of the reverse magnetic field Hr (0.40 ms) when the value is / kg was determined. This Hr will be called the residual coercivity. The value of the applied reverse magnetic field can be appropriately set depending on the Hr value of the magnetic material. (9) Perform the same procedure as in (2) to (8) with a reverse magnetic field application time of 6.1 ms, and when the remanent magnetization is 0 Am 2 The residual coercivity Hr(6.1ms) was calculated when the value was / kg. (10) The reverse magnetic field application time is set to 17 s, the device for applying the magnetic field is changed to an oscillating sample type magnetometer, and the same procedure as in (2) to (8) is performed, and the remanent magnetization is 0 Am 2 The residual coercivity Hr(17s) was determined when the value was / kg. During this process, the sample attachment and removal procedures in (3) to (5) were not performed. Furthermore, the number of repetitions in (7) was set to two or more, and in (8), three measurement points were selected and linearly approximated, with a coefficient of determination R 2 The value was set to 0.997 or higher.

[0034] Data analysis was performed for Hr(0.40ms), Hr(6.1ms), and Hr(17s) using data analysis software (Origin, OriginLab Corporation). Using the Curve Fit (nonlinear) function, H0 and KuV / kT in equation (6) below were used as fitting parameters, and the values ​​of H0 and KuV / kT were obtained by optimization using the least squares method. Initial values ​​of 5000 and 50 were input for H0 and KuV / kT, respectively. The activation volume Vact was calculated by substituting the H0 and KuV / kT obtained by the least squares method into equation (7) below. The crystal magnetic anisotropy constant Ku was calculated by substituting H0 into equation (8) below. Hr(t)=H0{1-[(kT / KuV)ln(f0t / ln2)] 0.77} …(6) Here, k: Boltzmann constant (J / K), T: measurement temperature (K), Ku: crystal magnetic anisotropy constant (J / m 3), V=Vact: activation volume (nm 3 ), Hr(t): residual coercivity at reverse magnetic field application time t (kA / m), H0: 10 -9 Residual coercivity in seconds (kA / m), f0: spin precession frequency (s -1 ), t: reverse magnetic field application time (s). The value of f0 here is 10 9 (s -1 ) Vact(nm 3 ) = 1.249 × 10 4 ×KuV / kT / H0…(7) Ku(J / m 3 ) = 331 × H0 (kA / m) …(8) Here, the coefficient of equation (7) is 1.249 × 10 4 The coefficient 331 in equation (8) is a sum of the individual values ​​and unit conversion factors used in the calculation process. The crystalline magnetic anisotropy constant Ku of the powder used in this example is the Ku value calculated using equation (8) in units of J / m 3 to MJ / m 3 When converted, it becomes 0.130 MJ / m 3 That was the case.

[0035] (Evaluation of Dx volume and Dx ratio) Using an X-ray diffractometer (Rigaku, Ultima IV), the crystallite size Dxc (nm) in the c-axis direction and the crystallite size Dxa (nm) in the a-axis direction of the hexagonal ferrite crystal lattice were determined using a Cu tube according to equation (6) below. Crystallite diameter (nm)=Kλ / (β·cosθ) …(9) Here, K is the Scherrer constant of 0.9, λ is the Cu-Kα line wavelength (nm), β is the full width at half maximum (in radians) of the diffraction peak of the hexagonal (006) plane in the Dxc measurement, and the full width at half maximum (in radians) of the diffraction peak of the hexagonal (220) plane in the Dxa measurement, and θ is the Bragg angle of the diffraction peak (1 / 2 of the diffraction angle 2θ) (in radians). Dxc was measured by scanning within the range of 2θ: 20.5 to 25°, and Dxa was measured by scanning within the range of 2θ: 60 to 65°. The measurement method was a continuous measurement method using a concentrated method, and a one-dimensional semiconductor detector (D-tex) was used. The divergence slit was 1 / 2°, the scattering slit was 8 mm, and the receiving slit was left open during the measurement. The sampling interval was Dxc: 0.05°, Dxa: 0.02°, the scanning speed was Dxc: 0.1° / min, Dxa: 0.4° / min, and the number of integrations was 1. The Dx volume was calculated by substituting the measured values ​​of Dxc (nm) and Dxa (nm) into the following equation (2). Dx volume (nm) 3 ) = Dxc × π × (Dxa / 2) 2 …(2) Here, π represents the ratio of a circle's circumference to its diameter (pi). The Dx volume of the powder used in this example is 1458 nm. 3 That was the case. The results are shown in Table 1.

[0036] [Example 2] Hexagonal barium ferrite magnetic powder was prepared under the same conditions as in Example 1, except that the crystallization heat treatment temperature was set to 660°C. This magnetic powder was used as the test powder for the same investigation as in Example 1. The manufacturing conditions and investigation results are shown in Table 1. The coercivity Hc of the test powder in this example was 222 kA / m, and the crystal magnetic anisotropy constant Ku was 0.150 MJ / m 3 The Dx volume is 2099 nm 3 That was the case.

[0037] [Example 3] Hexagonal barium ferrite magnetic powder was prepared under the same conditions as in Example 1, except that the preheat treatment was performed by holding the powder at 510°C for 72 hours. This magnetic powder was used as the test powder for the same investigation as in Example 1. The manufacturing conditions and investigation results are shown in Table 1. The precursor obtained by the preheat treatment showed a mass increase rate of -0.01% at 700°C based on the mass at 300°C, as measured by thermogravimetric (TG) measurement, and a saturation magnetization σs of 0.9 Am 2 The values ​​were / kg. The coercivity Hc of the powder used in this example was 144 kA / m, and the crystal magnetic anisotropy constant Ku was 0.131 MJ / m 3 The Dx volume is 1428 nm3 That was the case.

[0038] [Example 4] Hexagonal barium ferrite magnetic powder was prepared under the same conditions as in Example 3, except that the crystallization heat treatment temperature was set to 660°C. This magnetic powder was used as the test powder for the same investigation as in Example 1. The manufacturing conditions and investigation results are shown in Table 1. The coercivity Hc of the test powder in this example was 220 kA / m, and the crystal magnetic anisotropy constant Ku was 0.150 MJ / m 3 The Dx volume is 2056 nm 3 That was the case.

[0039] [Example 5] Hexagonal barium ferrite magnetic powder was prepared under the same conditions as in Example 1, except that the preheat treatment was performed by holding the powder at 530°C for 72 hours. This magnetic powder was used as the test powder for the same investigation as in Example 1. The manufacturing conditions and investigation results are shown in Table 1. The precursor obtained by the preheat treatment showed a mass increase rate of -0.01% at 700°C based on the mass at 300°C, as measured by thermogravimetric (TG) measurement, and a saturation magnetization σs of 1.2 Am 2 The values ​​were / kg. The coercivity Hc of the powder used in this example was 145 kA / m, and the crystal magnetic anisotropy constant Ku was 0.132 MJ / m 3 The Dx volume is 1468 nm 3 That was the case.

[0040] [Example 6] Hexagonal barium ferrite magnetic powder was prepared under the same conditions as in Example 5, except that the crystallization heat treatment temperature was set to 660°C. This magnetic powder was used as the test powder for the same investigation as in Example 1. The manufacturing conditions and investigation results are shown in Table 1. The coercivity Hc of the test powder in this example was 224 kA / m, and the crystal magnetic anisotropy constant Ku was 0.151 MJ / m 3 The Dx volume is 2114 nm 3 That was the case.

[0041] [Example 7] Hexagonal barium ferrite magnetic powder was prepared under the same conditions as in Example 1, except that the preheat treatment was performed by holding the powder at 550°C for 72 hours. This magnetic powder was used as the test material for the same investigation as in Example 1. The manufacturing conditions and investigation results are shown in Table 2. The precursor material obtained by the preheat treatment showed a mass increase rate of -0.01% at 700°C based on the mass at 300°C, as measured by thermogravimetric (TG) measurement, and a saturation magnetization σs of 3.5 Am 2 The values ​​were / kg. The coercivity Hc of the powder used in this example was 147 kA / m, and the crystal magnetic anisotropy constant Ku was 0.133 MJ / m 3 The Dx volume is 1415 nm 3 That was the case.

[0042] [Example 8] Hexagonal barium ferrite magnetic powder was prepared under the same conditions as in Example 7, except that the crystallization heat treatment temperature was set to 660°C. This magnetic powder was used as the test powder for the same investigation as in Example 1. The manufacturing conditions and investigation results are shown in Table 2. The coercivity Hc of the test powder in this example was 232 kA / m, and the crystal magnetic anisotropy constant Ku was 0.153 MJ / m 3 The Dx volume is 2172 nm 3 That was the case.

[0043] [Example 9] Hexagonal barium ferrite magnetic powder was prepared under the same conditions as in Example 1, except that the preheat treatment was performed by holding the powder at 530°C for 30 hours. This magnetic powder was used as the test material for the same investigation as in Example 1. The manufacturing conditions and investigation results are shown in Table 2. The precursor material obtained by the preheat treatment showed a mass increase rate of 0.01% at 700°C based on the mass at 300°C, as measured by thermogravimetric (TG) measurement, and a saturation magnetization σs of 1.0 Am 2 The values ​​were / kg. The coercivity Hc of the powder used in this example was 151 kA / m, and the crystal magnetic anisotropy constant Ku was 0.132 / m 3 The Dx volume is 1453 nm 3 That was the case.

[0044] [Example 10] Hexagonal barium ferrite magnetic powder was prepared under the same conditions as in Example 9, except that the crystallization heat treatment temperature was set to 660°C. This magnetic powder was used as the test powder for the same investigation as in Example 1. The manufacturing conditions and investigation results are shown in Table 2. The coercivity Hc of the test powder in this example was 227 kA / m, and the crystal magnetic anisotropy constant Ku was 0.150 MJ / m 3 The Dx volume is 2128 nm 3 That was the case.

[0045] [Example 11] Hexagonal barium ferrite magnetic powder was prepared under the same conditions as in Example 1, except that the preheat treatment was performed by holding the powder at 470°C for 72 hours and the crystallization heat treatment temperature was set to 620°C. This magnetic powder was used as the test powder for the same investigation as in Example 1. The manufacturing conditions and investigation results are shown in Table 2. The precursor material obtained by the preheat treatment showed a mass increase rate of 0.05% at 700°C based on the mass at 300°C as measured by thermogravimetric (TG) measurement, and a saturation magnetization σs of 0.3 Am 2 The values ​​were / kg. The coercivity Hc of the powder used in this example was 154 kA / m, and the crystal magnetic anisotropy constant Ku was 0.129 / m 3 The Dx volume is 1551 nm 3 That was the case.

[0046] [Example 12] Hexagonal barium ferrite magnetic powder was prepared under the same conditions as in Example 9, except that the crystallization heat treatment temperature was set to 630°C. This magnetic powder was used as the test powder for the same investigation as in Example 1. The manufacturing conditions and investigation results are shown in Table 2. The coercivity Hc of the test powder in this example was 168 kA / m, and the crystal magnetic anisotropy constant Ku was 0.132 MJ / m 3 The Dx volume is 1679 nm 3 That was the case.

[0047] [Example 13] Hexagonal barium ferrite magnetic powder was prepared under the same conditions as in Example 9, except that the crystallization heat treatment temperature was set to 660°C. This magnetic powder was used as the test powder for the same investigation as in Example 1. The manufacturing conditions and investigation results are shown in Table 2. The coercivity Hc of the test powder in this example was 235 kA / m, and the crystal magnetic anisotropy constant Ku was 0.151 MJ / m3 The Dx volume was 2275 nm 3 It was as follows.

[0048] [Comparative Example 1] Hexagonal barium ferrite magnetic powder was produced under the same conditions as in Example 1, except that the preliminary heat treatment was carried out under the condition of holding at 570 °C for 72 hours and the crystallization heat treatment temperature was 620 °C. This magnetic powder was used as a test powder and subjected to the same investigation as in Example 1. The production conditions and investigation results are shown in Table 3. The precursor obtained by the preliminary heat treatment had a mass increase rate at 700 °C based on the mass at 300 °C by thermogravimetric (TG) measurement of -0.02%, and the saturation magnetization σs was 15.7 Am 2 / kg. The coercive force Hc of the test powder in this example was 132 kA / m, and the crystal magnetic anisotropy constant Ku was 0.125 / m 3 The Dx volume was 1398 nm 3 It was as follows.

[0049] [Comparative Example 2] Hexagonal barium ferrite magnetic powder was produced under the same conditions as in Comparative Example 1, except that the crystallization heat treatment temperature was 630 °C. This magnetic powder was used as a test powder and subjected to the same investigation as in Example 1. The production conditions and investigation results are shown in Table 3. The coercive force Hc of the test powder in this example was 145 kA / m, and the crystal magnetic anisotropy constant Ku was 0.129 MJ / m 3 The Dx volume was 1548 nm 3 It was as follows.

[0050] [Comparative Example 3] Hexagonal barium ferrite magnetic powder was produced under the same conditions as in Comparative Example 1, except that the crystallization heat treatment temperature was 660 °C. This magnetic powder was used as a test powder and subjected to the same investigation as in Example 1. The production conditions and investigation results are shown in Table 3. The coercive force Hc of the test powder in this example was 227 kA / m, and the crystal magnetic anisotropy constant Ku was 0.151 MJ / m 3 The Dx volume was 2256 nm 3 It was as follows.

[0051] [Comparative Example 4] Hexagonal barium ferrite magnetic powder was prepared under the same conditions as in Example 1, except that the preheat treatment was performed by holding the powder at 490°C for 1 hour and the crystallization heat treatment temperature was set to 620°C. This magnetic powder was used as the test powder for the same investigation as in Example 1. The manufacturing conditions and investigation results are shown in Table 3. The precursor obtained by the preheat treatment showed a mass increase rate of 0.09% at 700°C based on the mass at 300°C as measured by thermogravimetric (TG) measurement, and a saturation magnetization σs of 0.3 Am 2 The values ​​were / kg. The coercivity Hc of the powder used in this example was 158 kA / m, and the crystal magnetic anisotropy constant Ku was 0.130 / m 3 The Dx volume is 1683 nm 3 That was the case.

[0052] [Comparative Example 5] Hexagonal barium ferrite magnetic powder was prepared under the same conditions as in Comparative Example 4, except that the crystallization heat treatment temperature was set to 630°C. This magnetic powder was used as the test powder for the same investigation as in Example 1. The manufacturing conditions and investigation results are shown in Table 3. The coercivity Hc of the test powder in this example was 166 kA / m, and the crystal magnetic anisotropy constant Ku was 0.131 MJ / m 3 The Dx volume is 1750 nm 3 That was the case.

[0053] [Comparative Example 6] Hexagonal barium ferrite magnetic powder was prepared under the same conditions as in Comparative Example 4, except that the crystallization heat treatment temperature was set to 660°C. This magnetic powder was used as the test powder for the same investigation as in Example 1. The manufacturing conditions and investigation results are shown in Table 3. The coercivity Hc of the test powder in this example was 238 kA / m, and the crystal magnetic anisotropy constant Ku was 0.152 MJ / m 3 The Dx volume is 2363 nm 3 That was the case.

[0054] [Table 1]

[0055] [Table 2]

[0056] [Table 3]

[0057] Figure 2 shows the TG curves for each precursor material when heated at a rate of 10°C / min in an atmospheric environment. The legend indicates the conditions for the preheat treatment. The precursor materials used in Comparative Examples 4-6 had insufficient thermal energy transfer during the preheat treatment, resulting in a large mass increase between 300 and 700°C. Note that the values ​​on the vertical axis of Figure 2 are displayed based on the mass at the temperature at the start of heating (35°C).

[0058] Figure 3 shows a graph plotting the relationship between the crystallization heat treatment temperature and the Dx volume of the obtained hexagonal ferrite magnetic powder, with the used precursor material distinguishable. The legend indicates the conditions for the preheat treatment. It can be seen that each example shows a greater effect on the refinement of magnetic particles at the same crystallization heat treatment temperature compared to Comparative Examples 1-3, where the thermal energy input in the preheat treatment was insufficient.

[0059] Figure 4 shows a graph plotting the relationship between the Dx volume and coercivity Hc of hexagonal ferrite magnetic powder, with the precursor material used being distinguishable. The legend indicates the conditions for the preheat treatment. Each example satisfies equation (3) above, and it can be seen that the improvement in coercivity Hc for the same Dx volume is greater compared to Comparative Examples 1-3, where the thermal energy input in the preheat treatment was insufficient, and Comparative Examples 4-6, where the thermal energy input was excessive.

[0060] Figure 5 shows a graph plotting the relationship between the Dx volume and the crystal magnetic anisotropy constant Ku of hexagonal ferrite magnetic powder, with the precursor material used being distinguishable. The preheat treatment conditions are shown in the legend. Each example satisfies equation (4) above, and it can be seen that the improvement effect on the crystal magnetic anisotropy constant Ku for the same Dx volume is greater compared to Comparative Examples 1-3, where the thermal energy input in the preheat treatment was insufficient. It is also possible to create different products that satisfy equation (5) above. Note that Comparative Examples 4-6, where the thermal energy input in the preheat treatment was excessive, also satisfy equation (4), but as shown in Figure 5 above, no improvement in coercivity Hc is observed.

[0061] Figure 6 shows a graph plotting the relationship between the temperature and time of the preheat treatment. It can be seen that the hexagonal barium ferrite magnetic powder (example) exhibiting the effects of the present invention can be realized by performing preheat treatment under conditions that satisfy equation (1) above.

Claims

1. A method for producing hexagonal barium ferrite magnetic powder, comprising a crystallization step of synthesizing hexagonal barium ferrite crystals by subjecting a precursor material containing the constituent elements of hexagonal barium ferrite magnetic powder to a crystallization heat treatment, The aforementioned precursor material, when subjected to thermogravimetric analysis in an atmospheric atmosphere where it is heated from room temperature to a temperature of 700°C or higher at a heating rate of 10°C / min, has a mass increase rate of 0.05% or less at 700°C relative to its mass at 300°C, and a saturation magnetization σs of 5.0 Am ２ A method for producing hexagonal barite magnetic powder, using a material with a density of less than / kg.

2. The method for producing hexagonal barium ferrite magnetic powder according to claim 1, wherein the precursor material is a substance obtained by subjecting an amorphous body, which is obtained by rapidly cooling a molten material containing the constituent elements of hexagonal barium ferrite magnetic powder, to a preheat treatment in which it is held at a temperature range of 450 to 560°C in an oxidizing atmosphere.

3. The method for producing hexagonal barium ferrite magnetic powder according to claim 1, wherein the precursor material is a substance obtained by subjecting an amorphous body, which is obtained by rapidly cooling a molten material containing the constituent elements of hexagonal barium ferrite magnetic powder, to a preheat treatment in an oxidizing atmosphere, where the temperature range is 450 to 560°C, under conditions that the relationship between the average holding temperature Tm (°C) and the holding time t (h) at 450 to 560°C satisfies the following equation (1). t(h)≧0.0005×exp[8720 / (273+Tm(℃))]…(1)

4. The method for producing hexagonal barium ferrite magnetic powder according to claim 2 or 3, wherein the molten material contains Ba and B as amorphous forming components.

5. A method for producing hexagonal barium ferrite magnetic powder according to any one of claims 1 to 4, wherein the crystallization step is performed at a crystallization heat treatment of 600 to 700°C.

6. The method for producing hexagonal barium ferrite magnetic powder according to any one of claims 1 to 5, wherein the hexagonal barium ferrite magnetic powder contains Bi in a Bi / Fe molar ratio of 0.005 to 0.05.

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