Hexagonal ferrite magnetic powder and method for producing the same

Abstract

The present invention provides hexagonal ferrite magnetic powder which is extremely useful in achieving both an increase in recording density and an increase in SNR of a magnetic recording medium, is crystalline and has a high saturation magnetization. The hexagonal ferrite magnetic powder contains Bi in a range of a Bi / Fe molar ratio of 0.035 or less, has a saturation magnetization σs of 42.0 Am 2 / kg or more, and a Dx volume of 1800 nm 3 or less, based on a microcrystal diameter. The magnetic powder can be produced by a method for producing hexagonal ferrite magnetic powder, comprising a process of immersing hexagonal ferrite magnetic powder containing Bi in a solution in which a compound X that forms a complex with Bi is dissolved, thereby performing a treatment of dissolving a part of the Bi present in the hexagonal ferrite magnetic powder into the solution.

Description

Technical Field

[0001] This invention relates to magnetite-type (M-type) hexagonal ferrite magnetic powder suitable for high-density recording in magnetic recording media and its manufacturing method. Background Technology

[0002] M-type hexagonal ferrite magnetic powder is known as a suitable magnetic powder for high-density recording in magnetic recording media. From the viewpoint of increasing recording density, refining the magnetic particles (miniaturization of the Dx volume, discussed later) is advantageous. On the other hand, from the viewpoint of improving the SNR (S / N ratio) of the recording medium, thinning the magnetic layer is advantageous. In order to ensure high recording density even when the magnetic phase is thinned, it is necessary to increase the saturation magnetization σs of the magnetic powder. However, it is not easy to maintain a high saturation magnetization σs while achieving significant refining of the magnetic particles.

[0003] Patent Document 1 discloses a hexagonal ferrite magnetic powder with small particle size and high magnetic properties, containing iron, divalent metals, tetravalent metals, Ba, Bi, and rare earth elements, wherein the content of Bi is greater than the content of the rare earth elements. It is disclosed that by adding Bi, the sintering between hexagonal ferrite particles can be reduced, enabling particle miniaturization.

[0004] In Patent Document 2, a hexagonal ferrite magnetic powder with a Ba / Fe molar ratio of 8.0% or more (0.080 or more), a Bi / Fe molar ratio of 2.5% or more (0.025 or more), and an Al / Fe molar ratio of 3.0 to 6.0% (0.030 to 0.060) was disclosed as a magnetic powder capable of simultaneously improving the magnetic properties, including SNR, and the durability of the magnetic recording medium.

[0005] Patent document 3 discloses a hexagonal Ba ferrite magnetic powder, wherein the Fe site valence number XFe is 3.005 to 3.030, the R / M molar ratio (M is Fe and its substitutional elements) is 0.001 to 0.020, and the Dx volume is 1150 to 1450 nm. 3 This magnetic powder employs a method of narrowing the distribution of coercivity Hc within a magnetic powder composed of fine magnetic particles, with the aim of improving the magnetic properties (especially the S / N ratio) of the magnetic recording medium. Regarding Bi, it is described as effective for both particle size reduction and improved magnetic properties (Patent Document 3, Paragraph 0023).

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2011-178654

[0009] Patent Document 2: Japanese Patent Application Publication No. 2015-111484

[0010] Patent Document 3: Japanese Patent Application Publication No. 2016-171264 Summary of the Invention

[0011] The problem that the invention aims to solve

[0012] In hexagonal ferrite powder, lowering the sintering temperature during crystallization is effective in achieving finer magnetic particles (miniaturization of the Dx volume, discussed later). However, if the sintering temperature is lowered to achieve a Dx volume refinement of, for example, 2000 nm... 3 At levels below approximately 100 nm, there is a significant decrease in saturation magnetization σs. Adding Bi is effective in mitigating this problem. Specifically, when Bi is added in an appropriate amount (e.g., 0.005 or more in Bi / Fe ratio) to the raw material mixture used to manufacture hexagonal ferrite, even by lowering the sintering temperature at which the amorphous (amorphous) portion of the raw material mixture is sintered to crystallize it, the Dx volume can be controlled to, for example, 2000 nm. 3 The following methods can also reduce the degree of decrease in σs as the firing temperature decreases. However, even under these conditions, it is impossible to obtain the original high σs exhibited by hexagonal ferrite crystals synthesized by sufficiently increasing the firing temperature. In recent years, the demand for further improvement in the performance of magnetic recording media has been continuously increasing. Even using the Bi addition method described above, it is difficult to fully meet this requirement.

[0013] In view of the need for further improvements in the performance of magnetic recording media, the present invention aims to provide hexagonal ferrite magnetic powder that is extremely useful in simultaneously achieving improvements in recording density and SNR. Furthermore, it aims to provide an efficient manufacturing technique for obtaining such hexagonal ferrite magnetic powder.

[0014] Methods for solving problems

[0015] To achieve the above objectives, the following invention is disclosed in this specification.

[0016] [1] Hexagonal ferrite magnetic powder, wherein the content of Bi is in the range of Bi / Fe molar ratio of 0.035 or less, and the saturation magnetization σs is 42.0 Am. 2 / kg or more, the volume of Dx expressed by the following equation (1) is 1800nm 3 the following.

[0017] Dx volume (nm) 3 )=Dxc×π×(Dxa / 2) 2 …(1)

[0018] Where Dxc is the crystallite diameter (nm) along the c-axis of the hexagonal ferrite lattice, Dxa is the crystallite diameter (nm) along the a-axis of the lattice, and π is pi.

[0019] [2] The hexagonal ferrite magnetic powder described in [1] above contains Bi in the range of 0.005 to 0.035 with a Bi / Fe molar ratio.

[0020] [3] According to the hexagonal ferrite magnetic powder described in [1] or [2] above, a portion of the Fe sites of the hexagonal ferrite crystals are replaced by one or more of the divalent, tetravalent or pentavalent metal elements.

[0021] [4] The hexagonal ferrite magnetic powder according to any one of [1] to [3] above, wherein the hexagonal ferrite magnetic powder is hexagonal Ba ferrite magnetic powder.

[0022] [5] A method for manufacturing hexagonal ferrite magnetic powder includes the following steps: immersing hexagonal ferrite magnetic powder containing Bi in a solution containing a compound X that forms a complex with Bi, thereby performing a treatment to dissolve a portion of the Bi present in the hexagonal ferrite magnetic powder into the solution (hereinafter referred to as "Bi dissolution treatment").

[0023] [6] In the method for manufacturing hexagonal ferrite magnetic powder described in [5] above, the compound X is a chelating agent.

[0024] [7] According to the manufacturing method of hexagonal ferrite magnetic powder described in [6] above, wherein the chelating agent satisfies the following formula (2).

[0025] logK Bi -logK Fe ≥0.5…(2)

[0026] Among them, K Bi For Bi 3+ The chelation stability constant, K Fe For Fe 3+ The chelation stability constant.

[0027] [8] The method for manufacturing hexagonal ferrite magnetic powder according to any one of [5] to [7] above, wherein the hexagonal ferrite magnetic powder supplied for the Bi leaching treatment is referred to as "raw powder" and the hexagonal ferrite magnetic powder obtained by the Bi leaching process is referred to as "treated powder", and the volume of Dx expressed by the formula (1) is 1800 nm. 3 For raw powder with a Bi / Fe molar ratio of 0.020 to 0.100, the Bi residual ratio defined by the following formula (3) is set to 0.2 to 0.8.

[0028] Bi residual ratio = [Bi / Fe molar ratio of treated powder] / [Bi / Fe molar ratio of raw powder]...(3)

[0029] [9] The method for manufacturing hexagonal ferrite magnetic powder according to any one of [5] to [8] above, wherein the pH of the solution in the Bi dissolution treatment is set to 2.0 to 10.0.

[0030]

[10] The method for manufacturing hexagonal ferrite magnetic powder according to any one of [5] to [9] above, wherein the total amount of compound X used in the Bi leaching treatment is A K (molar) and the amount of Bi A contained in the hexagonal ferrite magnetic powder supplied for the Bi dissolution treatment Bi Bi leaching treatment is performed under the condition that the relationship between (moles) satisfies the following equation (4).

[0031] N×A k / A Bi ≥1.0…(4)

[0032] Where N is the maximum number of Bi atoms that can coordinate with one molecule of compound X.

[0033]

[11] The method for manufacturing hexagonal ferrite magnetic powder according to any one of [5] to

[10] above, wherein the hexagonal ferrite magnetic powder supplied for the Bi leaching treatment is a hexagonal ferrite magnetic powder formed by replacing a portion of the Fe sites of the hexagonal ferrite crystal with one or more of divalent, tetravalent or pentavalent metal elements.

[0034]

[12] The method for manufacturing hexagonal ferrite magnetic powder according to any one of [5] to

[11] above, wherein the hexagonal ferrite magnetic powder supplied for the Bi leaching treatment is hexagonal Ba ferrite magnetic powder.

[0035] Invention Effects

[0036] According to the present invention, the saturation magnetization σs can be significantly improved in hexagonal ferrite magnetic powder with small magnetic particle size. When this magnetic powder is used in magnetic recording media, the small size of the magnetic particles is beneficial for increasing the recording density, and the high saturation magnetization σs allows for a thinner magnetic layer, which in turn is beneficial for improving the SNR. In other words, the present invention contributes to improving the performance of magnetic recording media. Attached Figure Description

[0037] Figure 1 This is a graph illustrating the relationship between firing temperature and Dx volume for Bi-containing hexagonal ferrite magnetic powder.

[0038] Figure 2This is a graph illustrating the relationship between the volume Dx and the saturation magnetization σs for Bi-containing hexagonal ferrite magnetic powder. Detailed Implementation

[0039] The hexagonal ferrite used in this invention is a magnetolite-type (magnetopalite type, M type) ferrite with the basic chemical formula AO·6Fe2O3. The element A in the above chemical formula is one or more elements selected from Ba, Sr, Pb, and Ca; some types may have a portion of it replaced by elements such as La. A portion of the Fe site can be replaced by one or more divalent, tetravalent, or pentavalent metal elements. Examples of divalent metal elements include Co and Zn; examples of tetravalent metal elements include Ti and Sn; and examples of pentavalent metal elements include Nb and V. Metal elements that replace a portion of the Fe site are called "Fe site substitution elements." By substituting with these metal elements, the coercivity Hc can be adjusted.

[0040] The hexagonal ferrite powder targeted in this invention contains Bi. Bi is not an element constituting the crystal structure of hexagonal ferrite (an element entering any atomic site in the chemical formula AO·6Fe2O3), but it is an effective additive element for refining the grains of hexagonal ferrite and improving the electromagnetic conversion characteristics of the magnetic recording medium using this powder. In particular, even when grain refinement is sought by lowering the firing temperature, it has the effect of reducing the decrease in magnetic properties. The mechanism for obtaining such useful effects of Bi is not yet fully understood, but it is considered that the presence of Bi changes the particle shape and / or the deviation (fluctuation) of the particle shape of the hexagonal ferrite powder, and improves the crystallinity of the hexagonal ferrite. In this invention, hexagonal ferrite powder containing Bi is targeted to enjoy the advantages of Bi addition described above. A predetermined amount of Bi also remains in the modified hexagonal ferrite powder obtained by the Bi dissolution treatment described later.

[0041] Furthermore, the hexagonal ferrite powder targeted in this invention may contain one or more rare earth elements such as Nd, Y, Sm, Y, Er, Ho, and Al. These elements do not constitute the crystal structure of the hexagonal ferrite.

[0042] [Modification method of Bi-containing hexagonal ferrite magnetic powder]

[0043] The manufacturing method of the present invention produces modified hexagonal ferrite magnetic powder by subjecting a Bi-containing hexagonal ferrite magnetic powder synthesized using a Bi-containing raw material mixture to a treatment that dissolves a portion of the Bi contained in the powder. In this specification, the treatment that dissolves a portion of the Bi is referred to as "Bi dissolution treatment." Furthermore, the hexagonal ferrite magnetic powder supplied for the Bi dissolution treatment is referred to as "raw powder," and the hexagonal ferrite magnetic powder obtained by the Bi dissolution treatment is referred to as "treated powder." Hereinafter, the raw powder, the Bi dissolution treatment, and the treated powder will be described.

[0044] [Original powder]

[0045] From the viewpoint of obtaining hexagonal ferrite magnetic powder with small grain size and concentrated grain size distribution, the glass crystallization method is preferred as the manufacturing process for the raw powder applicable to the present invention. The glass crystallization method is a method of crystallizing an amorphous mixture of raw materials by firing it. When applying the glass crystallization method, Bi-containing hexagonal ferrite powder obtained by known methods as shown in Patent Documents 1 to 3 can be used as the raw powder. As the Bi source in the glass crystallization method, Bi oxide powder, metallic Bi powder, etc. can be used. Any synthesis method capable of synthesizing Bi-containing hexagonal ferrite magnetic powder can also be used, except for the glass crystallization method. In addition, in the process of manufacturing hexagonal ferrite magnetic powder using the glass crystallization method, in order to extract the hexagonal ferrite grains from the powder obtained by the crystallization process (the process of precipitating ferrite by heat treatment), an "acid washing treatment" is usually performed to dissolve and remove the residual material, mainly barium borate, by acid. In the case of using hexagonal ferrite crystallized by glass crystallization in this invention, a cleaning process including acid washing is required to thoroughly remove unwanted residual substances from the hexagonal ferrite magnetic powder, which is then used as the raw powder. Conventionally known hexagonal ferrite magnetic powder containing Bi, which can be used as a magnetic material for magnetic recording media, can be used as the raw powder in this invention.

[0046] Regarding the range of Bi content in the raw powder, a relatively small lower limit can be set, for example, a Bi / Fe molar ratio of 0.001 or higher, and it is more preferable to use raw powder with a Bi / Fe molar ratio of 0.020 or higher. During the crystallization stage, hexagonal ferrite magnetic powder containing Bi with a Bi / Fe molar ratio of 0.020 or higher is more effective in improving the electromagnetic conversion characteristics of magnetic recording media due to the moderate variation in grain shape and the degree of deviation of its grain shape, as well as good crystallinity. Using raw powder with a Bi / Fe molar ratio of 0.030 or higher is even more effective. However, if the raw powder contains a large amount of Bi, a large amount of excess Bi will remain in the treated powder obtained after the Bi dissolution treatment described later. Since Bi is a non-magnetic component, the less excess (unwanted) Bi residue there is, the more beneficial it is to improving the magnetic properties of the magnetic recording medium. A Bi content in the raw powder with a Bi / Fe molar ratio of 0.100 or lower is effective.

[0047] Regarding the Fe-substituted elements in the constituent elements of the raw powder, the molar ratio relative to Fe (hereinafter sometimes referred to as the "Fe molar ratio") is set to be the same as the target composition of the treated powder. The Fe molar ratio of the Fe-substituted elements is approximately maintained before and after the Bi leaching treatment. Regarding rare earth elements in the constituent elements of the raw powder, a tendency to dissolve through the Bi leaching treatment has been observed. Therefore, when it is necessary for the treated powder to contain a specified amount of rare earth elements, the rare earth element content in the raw powder is set by estimating the amount dissolved during the Bi leaching treatment. The extent of rare earth element loss during the Bi leaching treatment can be determined by conducting preliminary experiments based on actual manufacturing conditions. Regarding Al in the constituent elements of the raw powder, the Al / Fe molar ratio is approximately maintained before and after the Bi leaching treatment. However, regarding Al, a treatment to coat the particles with Al can be performed before or after the Bi leaching treatment, thereby adding all or part of the necessary Al to the powder. In this case, the Al content in the raw powder is set based on the estimated amount of Al added through coating. It should be noted that when synthesizing hexagonal ferrite crystals using the glass crystallization method, the composition of the raw material mixture (hereinafter sometimes referred to as the "feed composition") is roughly reflected in the composition of the synthesized hexagonal ferrite powder. Therefore, when obtaining raw powder using the glass crystallization method, the content of each component element in the raw powder only needs to be adjusted in the feed composition.

[0048] For high recording density in magnetic recording media, fine hexagonal ferrite grains are advantageous. As a grain size parameter, the volume Dx, which is derived from the crystallite diameter, can be used. The volume Dx is calculated according to the following equation (1).

[0049] Dx volume (nm) 3 )=Dxc×π×(Dxa / 2) 2…(1)

[0050] Where Dxc is the crystallite diameter (nm) along the c-axis of the hexagonal ferrite lattice, Dxa is the crystallite diameter (nm) along the a-axis of the lattice, and π is pi.

[0051] The crystallite diameter is determined by the full width at half maximum (FWHM) of the diffraction peaks measured using X-ray diffraction (XRD) with Cu-Kα rays, according to the Scherrer formula shown in equation (5) below.

[0052] Crystallite diameter (nm) = Kλ / (β·cosθ)…(5)

[0053] Wherein, K: Scherer constant 0.9, λ: wavelength of Cu-Kα rays (nm), β: full width at half maximum (FWHM) of the diffraction peak of the hexagonal (006) plane in the determination of Dxc, full width at half maximum (FWHM) of the diffraction peak of the hexagonal (220) plane in the determination of Dxa, and θ: Bragg angle of the diffraction peak (1 / 2 of the diffraction angle 2θ) (radians).

[0054] According to the inventors' research, when applying Dx with a volume of 1800nm... 3 When the following hexagonal ferrite magnetic powder is used as the raw material, in magnetic recording media using powder that has undergone Bi dissolution treatment, in addition to the increased recording density due to the small Dx volume, a very high improvement in SNR can also be expected. Regarding the lower limit of the Dx volume, it is not necessarily necessary to impose a restriction; however, when the coercivity of the magnetic powder is also important, it is preferable to set the Dx volume of the raw powder to 1000 nm. 3 The above is more preferably set to 1300nm. 3 above.

[0055] [Bi dissolution treatment]

[0056] As described above, Bi is effective in refining (micro-refining) the grains of hexagonal ferrite and improving the electromagnetic conversion characteristics of magnetic recording media using this magnetic powder. Furthermore, even when grain refinement is achieved by lowering the sintering temperature, it also reduces the decrease in magnetic properties. This useful effect of Bi is exerted by Bi present in the raw materials during the crystallization of hexagonal ferrite. After exerting the aforementioned effect during crystallization synthesis, Bi remains in the hexagonal ferrite magnetic powder. Since Bi is a non-magnetic component, it is believed that reducing the content of excess Bi present in the magnetic powder is effective for improving magnetic properties. Therefore, the inventors have repeatedly studied methods for removing excess Bi present in hexagonal ferrite magnetic powder. The results showed that when a "wet treatment" was performed, in which Bi-containing hexagonal ferrite magnetic powder was impregnated in a solution containing a compound that forms a complex with Bi (hereinafter referred to as "compound X"), the dissolution of Fe and its substituted metal elements was significantly suppressed, while the Bi content in the magnetic powder was greatly reduced. Furthermore, it was confirmed that the saturation magnetization σs of the magnetic powder increased with the significant reduction in Bi content. Therefore, in the method for manufacturing the modified hexagonal ferrite magnetic powder of the present invention, the treatment of impregnating Bi-containing hexagonal ferrite magnetic powder in a solution containing compound X is applied as a Bi dissolution treatment. It should be noted that the above-mentioned acid washing treatment, which is usually performed when manufacturing hexagonal ferrite magnetic powder using the glass crystallization method, cannot dissolve Bi from the hexagonal ferrite magnetic powder. Furthermore, when strong acids such as hydrochloric acid and sulfuric acid are mixed with the hexagonal ferrite magnetic powder, not only Bi but also the ferrite crystals dissolve, making it difficult to selectively dissolve Bi only.

[0057] The inventors conducted a comparative study on conventional Bi-containing hexagonal ferrite magnetic powder products that have undergone thorough cleaning treatment in the aforementioned cleaning process including acid washing, resulting in a state suitable for use as a magnetic recording medium. The study compared the total Bi content in the magnetic powder determined by chemical analysis with the Bi concentration in the surface layer of the powder particles determined by XPS (X-ray photoelectron spectroscopy). This revealed that Bi in Bi-containing hexagonal ferrite magnetic powder tends to be concentrated in the surface layer of the magnetic grains. In other words, it can be said that each magnetic grain of the Bi-containing hexagonal ferrite powder is covered by a Bi-thickened layer. The reason for the significant reduction in Bi content in the magnetic powder by immersing it in a solution containing compound X is that the Bi, which is abundant in the particle surface layer, dissolves into the liquid by forming a metal complex with compound X. Compound X can coordinate not only with Bi but also with other metallic elements such as Fe. However, since Bi is enriched in the surface layer of magnetic grains, when the magnetic powder is immersed in a solution of compound X, the Bi in the surface layer preferentially combines with the molecules of compound X and dissolves. This significantly reduces the Bi content while roughly maintaining the proportions of elements (Ba, Fe, and Fe site substitution elements) that constitute the hexagonal ferrite crystal structure. In terms of the effectiveness of this Bi dissolution treatment, as long as Bi-containing hexagonal ferrite magnetic powder with the chemical formula AO·6Fe2O3 as the basic structure is used as the raw powder, this effect can be achieved regardless of the types of elements constituting the A site, the types of substitution elements at the Fe site, the types of rare earth elements, and the presence or absence of Al.

[0058] Reducing the Bi content enriched in the particle surface layer through Bi leaching treatment is effective in improving magnetic properties. Therefore, it can be said that Bi leaching treatment is generally effective in improving the performance of magnetic recording media regardless of grain size. On the other hand, in fine-grained hexagonal ferrite magnetic powder, as mentioned above, a decrease in saturation magnetization σs is easily generated, but by containing Bi, the degree of σs reduction, which is a problem in fine-grained hexagonal ferrite magnetic powder, has been suppressed. Therefore, if fine-grained Bi-containing hexagonal ferrite magnetic powder is used as raw material for Bi leaching treatment, the synergistic effect of the aforementioned σs reduction suppression effect and the σs increase effect generated by Bi leaching treatment can achieve a high level of σs that was previously difficult to achieve for fine-grained hexagonal ferrite magnetic powder. Specifically, in a Dx volume of 1800 nm... 3 The following hexagonal ferrite magnetic powder can stably achieve a saturation magnetization σs of 42.0 Am. 2 Magnetic powder of / kg or above.

[0059] (Compound X that forms a complex with Bi)

[0060] Compound X, which has the property of forming complexes with Bi, can be categorized in addition to various chelating agents, such as lactic acid and thiourea.

[0061] Chelating agents are water-soluble compounds that coordinate with metal ions such as alkaline earth metals and transition metals to form chemically stable chelate complexes. In this invention, known chelating agents such as ethylenediaminetetraacetic acid (EDTA), trans-1,2-cyclohexanediaminetetraacetic acid (CyDTA), diethylenetriaminepentaacetic acid (DTPA), hydroxyethylenediaminetriacetic acid (EDTA-OH), ethylene glycol ether diaminetetraacetic acid (GEDTA), and their alkali metal salts can be used, and their chemical types are not particularly limited. From the perspectives of ease of acquisition and cost, EDTA, 1-4 sodium salts of EDTA, and 1-4 potassium salts of EDTA are preferred. A chelating agent solution containing two or more chelating agents can also be used.

[0062] The chelation stability constant K, which represents the stability of the chelated complex in a liquid, is expressed by the following equation (6).

[0063] K = [M N L] / ([M] N [L])…(6)

[0064] Here, [L] is the molar concentration of chelating agent molecule L, [M] is the molar concentration of metal ion M, and N is the number of moles of metal ion M coordinated to 1 mole of chelating agent molecule. N L] is a chelate complex M N The molar concentration of L molecules in the liquid. A larger chelation stability constant K indicates higher stability of the chelated complex. In this specification, the stability relative to Bi will be considered higher. 3+ and Fe 3+ The above-mentioned chelation stability constant K is expressed as K Bi and K Fe .

[0065] When comparing the stability of chelated complexes in liquids, the common logarithm of the chelation stability constant K mentioned above is often used. Regarding representative chelating agents, the chelation stability constant K for various metal ions was investigated, and logK can be obtained. Bi logK Fe The value of logK is given in Table 1. Table 1 illustrates the examples of the chelating agents shown above. Bi logK Fe The value of .

[0066] [Table 1]

[0067] Table 1

[0068]

[0069] In the manufacturing method of the present invention, when using a chelating agent as compound X, from the viewpoint of preventing the dissolution of hexagonal ferrite crystals during the Bi dissolution treatment, it is advantageous to use a chelating agent with greater chelating stability for Bi ions than for Fe ions. Based on various studies, a substance satisfying the following formula (2) is preferred.

[0070] logK Bi -logK Fe ≥0.5…(2)

[0071] Among them, K Bi For Bi 3+ The chelation stability constant, K Fe For Fe 3+ The chelation stability constant.

[0072] More preferably, a chelating agent that satisfies the following formula (2)' is preferred, and even more preferably, a chelating agent that satisfies the following formula (2)” is preferred.

[0073] logK Bi -logK Fe ≥1.0…(2)'

[0074] logK Bi -logK Fe ≥2.0…(2)”

[0075] Additionally, logK is applied. Bi Chelating agents with a strength of 20.0 or higher are more effective.

[0076] (Processing conditions)

[0077] The pH of the solution used in the Bi dissolution treatment is preferably maintained in the range of 2.0 to 10.0, more preferably in the range of 3.0 to 9.0. If the pH of the liquid is too low, the solubility of hexagonal ferrite crystals increases. If the pH of the liquid is too high, the Bi dissolution capacity provided by compound X decreases. This pH can be adjusted by adding an acid such as acetic acid or sulfuric acid, or a base such as sodium hydroxide, to the compound X solution. It should be noted that the pH values ​​described in this specification refer to values ​​measured using a pH meter calibrated according to JIS Z8802, using an appropriate buffer solution corresponding to the measured pH range as the pH standard solution. In addition, the pH values ​​described in this specification are obtained by directly reading the values ​​shown by a pH meter compensated by a temperature-compensated electrode under dissolution temperature conditions.

[0078] In order to implement Bi leaching treatment in industry, it is advantageous to significantly increase the leaching rate of Bi. Therefore, the relationship between the amount of compound X and the amount of raw powder in Bi leaching treatment is preferably set to satisfy the following condition (4).

[0079] N×A k / A Bi ≥1.0…(4)

[0080] Among them, A K A represents the total amount (in moles) of compound X (e.g., chelating agent) used in the Bi leaching treatment. Bi The amount of Bi (in moles) contained in the hexagonal ferrite magnetic powder supplied for Bi leaching treatment, where N is the maximum number of Bi atoms that can be coordinated in one molecule of the compound X.

[0081] Regarding the upper limit of the left side of equation (4), since it is limited by the solubility of compound X, it does not need to be specifically limited. For example, the conditions that satisfy equation (4)' below can be exemplified.

[0082] 100≥N×A k / A Bi ≥1.0…(4)'

[0083] It should be noted that for each chelating agent exemplified in Table 1, the value of N is 1.

[0084] In order to fully utilize the effect of the increased magnetic properties as the Bi content decreases, it is effective to carry out Bi leaching treatment with a Bi residual ratio of 0.2 to 0.8 as defined by the following formula (3).

[0085] Bi residual ratio = [Bi / Fe molar ratio of treated powder] / [Bi / Fe molar ratio of raw powder]...(3)

[0086] In particular, the Dx volume mentioned above is 1800nm. 3 When raw powder with a Bi / Fe molar ratio of 0.020 to 0.100 is subjected to Bi leaching treatment with a Bi residual ratio of 0.2 to 0.8, it is possible to achieve a high saturation magnetization σs that was previously difficult to achieve in regions with small Dx volumes.

[0087] In the Bi leaching process, water is typically used as the liquid medium for the solution containing compound X (compound X solution). Alternatively, a mixed liquid medium of water and a solvent other than water (e.g., alcohols such as ethanol) may be used as needed. The solution in the Bi leaching process may contain compound X, an acid or alkali for pH adjustment, and substances other than the original powder, provided it does not impair the effects of the invention.

[0088] The temperature of the solution in the Bi dissolution treatment can be set in the range of 10 to 90°C, for example. From the viewpoint of increasing the dissolution rate of Bi, it is more preferable to set it in the range of 40 to 90°C. Regarding the concentration of compound X in the compound X solution, it can be set in the range of, for example, 0.001 to 0.2 mol / kg, relative to the total amount of the liquid medium (water or a mixture of water and other solvents) and compound X. Regarding the amount of raw powder impregnated in the solution during the Bi dissolution treatment, it can be set in the range of, for example, 1.0 to 50.0% by mass, relative to the total amount of compound X solution and raw powder.

[0089] In the manufacturing method of the present invention, the step of impregnating the hexagonal ferrite magnetic powder (raw powder) containing Bi into the compound X solution is not particularly limited, as long as the mixed state of the raw powder particles is ensured to be in contact with the compound X solution. The order in which the various substances in the liquid are added is also not particularly limited. Specific steps may include, for example, adding the raw powder to the compound X solution, adding the compound X solution to a container containing the raw powder, or adding the compound X to the liquid after impregnating the raw powder in a liquid medium such as water.

[0090] Bi leaching treatment can be repeated multiple times. That is, the solid components recovered from the slurry after Bi leaching treatment can be washed and dried in pure water as needed, and then subjected to Bi leaching treatment again in a solution in which compound X is dissolved. In this case, the compound X used in each Bi leaching treatment can be the same compound or different types of compounds.

[0091] [Processed powder]

[0092] Through the above Bi leaching treatment, hexagonal ferrite magnetic powder with reduced Bi content (treated powder) can be obtained. In particular, it can stably produce Bi-containing powder with a Bi / Fe molar ratio of 0.035 or less and a saturation magnetization σs of 42.0 Am. 2 / kg or more, the above Dx volume is 1800nm 3 The following is an example of hexagonal ferrite magnetic powder. Thus, despite its small grain size, hexagonal ferrite magnetic powder exhibiting high saturation magnetization σs is highly useful in magnetic recording media for achieving a high balance between high recording density and high SNR. Regarding the Bi contained in the treated powder, it is believed that its enrichment in the grain surface is avoided, with most of it being absorbed into the interior of the grain. It is speculated that this form of Bi effectively functions in maintaining the good crystallinity of hexagonal ferrite, rather than contributing to improved magnetic properties. The Bi / Fe molar ratio of the treated powder can also be suppressed to below 0.025.

[0093] If we exemplify the preferred target content of elements other than Bi in the processed powder, then for Fe site substitution elements, the ratio of [total content of Fe site substitution elements (moles)] to [Fe content (moles)] is preferably set to 0.001 to 0.060. When the powder contains one or more rare earth elements, and the rare earth element is denoted as R, the R / Fe molar ratio is preferably set to 0.001 to 0.010. When the powder contains Al, the Al / Fe molar ratio is preferably set to 0.001 to 0.050.

[0094] Example

[0095] [Comparison Example]

[0096] Prepare raw powder for Bi leaching treatment and examine its properties.

[0097] As raw materials, 661.5g of boric acid (H3BO3, industrial grade), 1285.1g of barium carbonate (BaCO3, industrial grade), 765.9g of iron oxide (Fe2O3, industrial grade), 14.4g of cobalt oxide (CoO, reagent grade 90% or higher), 15.3g of titanium oxide (TiO2, reagent grade 1), 89.4g of bismuth oxide (Bi2O3, industrial grade), 116.2g of neodymium oxide (Nd2O3, industrial grade), and 52.1g of aluminum hydroxide (Al(OH)3, reagent grade 1) were prepared. The above raw materials were mixed using an FM mixer manufactured by Mitsui Miike to obtain a raw material mixture. Co is equivalent to a divalent Fe site substitution element, and Ti is equivalent to a tetravalent Fe site substitution element.

[0098] The above raw material mixture was placed in a granulator, and granulated into spheres while spraying water. The granules were then dried at 270°C for 14 hours to obtain granules with a particle size of 1–50 mm. The granules were melted in a melting furnace using a platinum crucible. The temperature was raised to 1400°C and maintained while stirring for 60 minutes to ensure all raw materials were completely molten. The melt (molten metal) was then rapidly cooled using a gas atomization method to obtain an amorphous substance. The amorphous substance was then sintered at 630°C for 60 minutes to crystallize it, forming hexagonal ferrite.

[0099] The powder obtained by the above calcination process contains residual substances, mainly barium borate, in addition to hexagonal ferrite. This powder is immersed in a 10% by mass aqueous solution of acetic acid heated to 60°C for 1 hour with stirring, thereby dissolving the residual substances in the liquid. Solid-liquid separation is then performed by filtration to recover the solid components. This solid component is referred to as the "acid-washed solid component".

[0100] The acid-washed solid components were washed with pure water to remove acetic acid and other components adhering to the grain surface. Washing continued until the conductivity of the resulting liquid (filtrate) was below 10 μS / cm. After washing, the sample was dried in air at 110°C to obtain a sample of hexagonal ferrite magnetic powder containing Bi. This sample corresponds to the "raw powder" used in Examples 1-6 described later.

[0101] (Composition analysis of magnetic powder)

[0102] The compositional analysis of the hexagonal ferrite magnetic powder sample was performed using an ICP (720-ES) system manufactured by Advanced Technology Co., Ltd. The elemental composition was calculated from the obtained quantitative values ​​as the molar ratio relative to Fe. The magnetic powder sample obtained in this example (the "raw powder" used in Examples 1-6 described below) was calculated to have Fe: 50.0 wt%, Bi: 7.12 wt%, and a Bi / Fe molar ratio of 0.038.

[0103] (Determination of magnetic properties)

[0104] The hexagonal ferrite magnetic powder sample was loaded into... In a plastic container, a VSM apparatus (VSM-P7-15) manufactured by Toei Kogyo Co., Ltd. was used to measure the coercivity Hc, saturation magnetization σs, squareness ratio SQ, and coercivity distribution SFD under an external magnetic field of 795.8 kA / m (10 kOe) and a magnetic field scanning speed of 795.8 kA / m / min (10 kOe / min). The results showed that the coercivity Hc of the magnetic powder sample in this example (the "raw powder" used in Examples 1-6 described later) was 174 kA / m, and the saturation magnetization σs was 41.1 Am. 2 / kg, the squareness ratio (SQ) is 0.513, and the coercivity distribution (SFD) is 0.764.

[0105] (Determination of BET specific surface area)

[0106] For the hexagonal ferrite magnetic powder sample, the specific surface area was determined using the 4-socket US material manufactured by Yuasa Ionicus Co., Ltd., based on the BET one-point method. As a result, the BET specific surface area of ​​the magnetic powder sample in this example (the "original powder" used in Examples 1-6 described below) was 101.1 m². 2 / g.

[0107] (Determination of the volume of Dx)

[0108] Using an X-ray diffraction apparatus (Rigaku, Ultima IV), and a Cu tube sphere, the crystallite diameters Dxc (nm) along the c-axis and Dxa (nm) along the a-axis of the hexagonal ferrite lattice were determined according to equation (5) above. Dxc was measured by scanning 2θ: 20.5–25°, and Dxa was measured by scanning 2θ: 60–65°. The measurement method was a continuous measurement method, with sampling intervals of Dxc: 0.05°, Dxa: 0.02°, scanning speeds of Dxc: 0.1° / min, Dxa: 0.4° / min, and a cumulative number of measurements of 1. The volume of Dx was calculated by substituting the measured values ​​of Dxc and Dxa into equation (1) above. The volume of Dx in the magnetic powder sample of this example (the “original powder” used in Examples 1–6 described later) was 1690 nm. 3 .

[0109] The results are shown in Table 2.

[0110] [Example 1]

[0111] The hexagonal ferrite magnetic powder obtained in the above comparative example was used as the raw powder for the following Bi leaching treatment.

[0112] (Bi dissolution treatment)

[0113] As compound X, which forms a complex with Bi, ethylenediaminetetraacetic acid disodium dihydrate (manufactured by Dojin Chemical Research Institute, reagent) is prepared as a chelating agent. The logK of this chelating agent... Bi -logK Fe The value is 2.8.

[0114] In a 1L beaker, 793.2g of pure water, 16.8g of the above-mentioned chelating agent, and 16.0g of a 90% by mass aqueous acetic acid solution were mixed to obtain a solution containing the chelating agent (hereinafter referred to as the "chelating agent solution"). After the chelating agent solution was heated to 60°C, 90g of the hexagonal ferrite magnetic powder (raw powder) obtained in the above comparative example was added to the chelating agent solution and impregnated in the solution. The liquid temperature was maintained at 60°C and stirred for 6 hours to obtain a slurry containing magnetic powder. Under these impregnation conditions, N×A, which is the left side of the above equation (4), k / A Bi The value was 1.5. The pH of the liquid at the start of impregnation was 3.6, and the pH of the liquid at the end of impregnation, after stirring and maintaining for 6 hours, was 4.8. The obtained slurry was filtered to recover the solid components. The Bi leaching treatment was completed following the above steps. The recovered solid components were washed with pure water to remove chelating agents and other components adhering to the particle surface. The washed solid components were dried in air at 110°C to obtain a sample of hexagonal ferrite magnetic powder (treated powder) that had undergone Bi leaching treatment.

[0115] The obtained magnetic powder sample (treated powder) was subjected to the same measurements as the control example described above. The results showed that the magnetic powder sample obtained in this example contained 51.2% by mass Fe, 4.37% by mass Bi, and a Bi / Fe molar ratio of 0.023. Based on the Bi residual ratio in equation (3) above, 0.023 / 0.038≈0.61, confirming that the Bi content was significantly reduced due to the Bi leaching treatment. The coercivity Hc of the magnetic powder sample (treated powder) obtained in this example was 182 kA / m, and the saturation magnetization σs was 42.0 Am. 2 / kg, with a squareness ratio (SQ) of 0.517 and a coercivity distribution (SFD) of 0.692. Additionally, the specific surface area of ​​BET is 104.6 m². 2 / g, Dx volume is 1750nm 3 The results above are shown in Table 2 (the same applies to the following examples).

[0116] [Example 2]

[0117] The hexagonal ferrite magnetic powder obtained in the comparative example above was used as the raw powder, and Bi leaching treatment was performed. In this example, the amount of chelating agent (disodium ethylenediaminetetraacetate dihydrate) added was changed so that N×A, which is the left side of the above equation (4), was... k / A Bi The value was 2.0, and the experiment was conducted using the same method as in Example 1. The pH of the liquid at the start of the impregnation was 3.6, and the pH of the liquid at the end of the impregnation, after stirring and holding for 6 hours, was 4.5.

[0118] In this example, the Bi / Fe molar ratio of the magnetic powder sample (treated powder) obtained was 0.019. According to the above formula (3), the residual Bi ratio was 0.019 / 0.038≈0.50, confirming that the Bi content was significantly reduced due to the Bi leaching treatment. Furthermore, the coercivity Hc was 184 kA / m, and the saturation magnetization σs was 42.3 Am. 2 / kg, squareness ratio (SQ) is 0.519, coercivity distribution (SFD) is 0.679, and BET specific surface area is 106.7 m². 2 / g, Dx volume is 1680nm 3 .

[0119] [Example 3]

[0120] The hexagonal ferrite magnetic powder obtained in the comparative example above was used for Bi leaching treatment of the original powder. In this example, the amount of chelating agent (disodium ethylenediaminetetraacetate dihydrate) added was changed to make the left side of the above equation (4), i.e., N×A, more suitable for Bi leaching treatment. k / A BiThe experiment was conducted using the same method as in Example 1, except that the pH of the liquid at the start of the impregnation was 2.0 and the amount of 90% acetic acid aqueous solution added was changed from 16.0 g to 8.0 g. The pH of the liquid at the start of the impregnation was 4.1, and the pH of the liquid at the end of the impregnation, after stirring and holding for 6 hours, was 8.2.

[0121] In this example, the Bi / Fe molar ratio of the magnetic powder sample (treated powder) obtained was 0.023. According to the above formula (3), the residual Bi ratio was 0.023 / 0.038≈0.61, confirming that the Bi content was significantly reduced due to the Bi leaching treatment. Furthermore, the coercivity Hc was 180 kA / m, and the saturation magnetization σs was 42.1 Am. 2 / kg, squareness ratio (SQ) is 0.516, coercivity distribution (SFD) is 0.699, and BET specific surface area is 104.1 m². 2 / g, Dx volume is 1680nm 3 .

[0122] [Example 4]

[0123] The hexagonal ferrite magnetic powder obtained in the comparative example above was used for Bi leaching treatment of the original powder. In this example, the amount of chelating agent (disodium ethylenediaminetetraacetate dihydrate) added was changed to make the left side of the above equation (4), i.e., N×A, more suitable for Bi leaching treatment. k / A Bi The experiment was conducted using the same method as in Example 1, except that the pH of the liquid at the start of the impregnation was 4.0 and the amount of 90% acetic acid aqueous solution added was changed from 16.0 g to 8.0 g. The pH of the liquid at the start of the impregnation was 4.2, and the pH of the liquid at the end of the impregnation, after stirring and holding for 6 hours, was 7.4.

[0124] In this example, the Bi / Fe molar ratio of the magnetic powder sample (treated powder) obtained was 0.012. According to the above formula (3), the residual Bi ratio was 0.012 / 0.038≈0.32, confirming that the Bi content was significantly reduced due to the Bi leaching treatment. Furthermore, the coercivity Hc was 181 kA / m, and the saturation magnetization σs was 43.3 Am. 2 / kg, squareness ratio (SQ) is 0.519, coercivity distribution (SFD) is 0.676, and BET specific surface area is 108.0 m². 2 / g, Dx volume is 1750nm 3 .

[0125] [Example 5]

[0126] The hexagonal ferrite magnetic powder obtained in the comparative example above was used for Bi leaching treatment of the original powder. In this example, except for changing the amount of chelating agent (disodium ethylenediaminetetraacetate dihydrate) added so that the left side of the above equation (4), i.e., N×A k / ABi The experiment was conducted using the same method as in Example 1, except that the pH of the liquid was set to 4.0 and the temperature of the immersion solution was changed from 60°C to 40°C. The pH of the liquid at the beginning of immersion was 3.6, and the pH of the liquid at the end of immersion, after stirring and holding for 6 hours, was 4.3.

[0127] In this example, the Bi / Fe molar ratio of the magnetic powder sample (treated powder) obtained was 0.019. According to the above formula (3), the residual Bi ratio was 0.019 / 0.038≈0.50, confirming that the Bi content was significantly reduced due to the Bi leaching treatment. Furthermore, the coercivity Hc was 184 kA / m, and the saturation magnetization σs was 42.3 Am. 2 / kg, squareness ratio (SQ) is 0.519, coercivity distribution (SFD) is 0.668, and BET specific surface area is 105.8 m². 2 / g, Dx volume is 1660nm 3 .

[0128] [Example 6]

[0129] The hexagonal ferrite magnetic powder obtained in the comparative example above was used for Bi leaching treatment of the original powder. In this example, the amount of chelating agent (disodium ethylenediaminetetraacetate dihydrate) added was changed to make the left side of the above equation (4), i.e., N×A, more suitable for Bi leaching treatment. k / A Bi The experiment was conducted in the same manner as in Example 1, except that the pH of the liquid at the beginning of the impregnation was 4.0, the amount of 90% acetic acid aqueous solution added was changed from 16.0 g to 8.0 g, and the liquid temperature during impregnation was changed from 60°C to 40°C. The pH of the liquid at the beginning of the impregnation was 4.1, and the pH of the liquid at the end of the impregnation after stirring and maintaining for 6 hours was 5.9.

[0130] In this example, the Bi / Fe molar ratio of the magnetic powder sample (treated powder) obtained was 0.016. According to the above formula (3), the residual Bi ratio was 0.016 / 0.038≈0.42, confirming that the Bi content was significantly reduced due to the Bi leaching treatment. Furthermore, the coercivity Hc was 184 kA / m, and the saturation magnetization σs was 42.8 Am. 2 / kg, squareness ratio (SQ) is 0.521, coercivity distribution (SFD) is 0.650, and BET specific surface area is 106.1 m². 2 / g, Dx volume is 1680nm 3 .

[0131] [Table 2]

[0132]

[0133] In each embodiment, the hexagonal ferrite magnetic powder obtained underwent a Bi dissolution treatment, resulting in a significant reduction in Bi content compared to the original powder of the control example. Regarding the Ba and Fe site substitution elements (Co, Ti) constituting the hexagonal ferrite lattice, it was confirmed that the Fe molar ratio remained almost unchanged before and after the Bi dissolution treatment, enabling preferential dissolution of Bi. Furthermore, the Dx volume of the original powder was approximately maintained after the Bi dissolution treatment. On the other hand, the saturation magnetization σs was increased by the Bi dissolution treatment.

[0134] For reference, the following experimental results are briefly presented: Three feed mixtures with Bi / Fe molar ratios of 0.001, 0.010, and 0.040 were prepared. The changes in Dx volume and saturation magnetization were examined when hexagonal ferrite magnetic powder was synthesized by varying the firing temperature. Except for the varying firing temperature, the experiments were conducted under conditions largely the same as the control example described above, without Bi leaching treatment.

[0135] exist Figure 1 The relationship between firing temperature and Dx volume is shown. Graphs are also provided for the above embodiments.

[0136] As the firing temperature decreases, a tendency for the volume of Dx to decrease is observed.

[0137] exist Figure 2 The relationship between Dx volume and saturation magnetization σs is shown in the figure. Plots are also provided for the above embodiments. The curve near Dx volume 1700 on the line where Bi / Fe = 0.040 corresponds to the original powder obtained in the comparative example described above. It can be seen that the samples of the embodiments that underwent Bi leaching treatment originally exhibited high σs even in the region of small Dx volume where the reduction in saturation magnetization became significant.

Claims

1. Hexagonal ferrite magnetic powder containing Bi in a Bi / Fe molar ratio ranging from 0.005 to 0.025, with a saturation magnetization σs of 42.0 Am. 2 / kg or more, the volume of Dx expressed by the following equation (1) is 1800nm 3 the following, Dx volume (nm) 3 ) = Dxc × π × (Dxa / 2) 2 …(1) in, Dxc is the crystallite diameter (nm) along the c-axis of the hexagonal ferrite lattice, Dxa is the crystallite diameter (nm) along the a-axis of the lattice, and π is pi.

2. The hexagonal ferrite magnetic powder according to claim 1, wherein, In hexagonal ferrite crystals, a portion of the Fe sites are replaced by one or more of divalent, tetravalent, or pentavalent metal elements.

3. The hexagonal ferrite magnetic powder according to claim 1 or 2, wherein, The hexagonal ferrite magnetic powder is hexagonal Ba ferrite magnetic powder.

4. A method for manufacturing hexagonal ferrite magnetic powder, comprising the following steps: immersing hexagonal ferrite magnetic powder containing Bi in a solution containing a compound X that forms a complex with Bi, thereby performing a treatment to dissolve a portion of the Bi present in the hexagonal ferrite magnetic powder into the solution, hereinafter referred to as "Bi dissolution treatment".

5. The method for manufacturing hexagonal ferrite magnetic powder according to claim 4, wherein, The compound X is a chelating agent.

6. The method for manufacturing hexagonal ferrite magnetic powder according to claim 5, wherein, The chelating agent satisfies the following formula (2): logK Bi -logK Fe ≥0.5 …(2) Among them, K Bi For Bi 3+ The chelation stability constant, K Fe For Fe 3+ The chelation stability constant.

7. The method for manufacturing hexagonal ferrite magnetic powder according to any one of claims 4 to 6, wherein, When the hexagonal ferrite magnetic powder supplied for the Bi dissolution treatment is referred to as "raw powder" and the hexagonal ferrite magnetic powder obtained through the Bi dissolution process is referred to as "treated powder", the volume of Dx expressed by the following formula (1) is 1800 nm. 3 For the following raw powder with a Bi / Fe molar ratio of 0.020 to 0.100, the Bi residual ratio defined by the following formula (3) is set to 0.2 to 0.

8. Dx volume (nm) 3 ) = Dxc × π × (Dxa / 2) 2 …(1) Where Dxc is the crystallite diameter (nm) along the c-axis of the hexagonal ferrite lattice, Dxa is the crystallite diameter (nm) along the a-axis of the lattice, and π is pi. Bi residual ratio = [Bi / Fe molar ratio of treated powder] / [Bi / Fe molar ratio of original powder] ... (3).

8. The method for manufacturing hexagonal ferrite magnetic powder according to any one of claims 4 to 7, wherein, The pH of the solution in the Bi leaching treatment is set to 2.0–10.

0.

9. The method for manufacturing hexagonal ferrite magnetic powder according to any one of claims 4 to 8, wherein, The total amount A of compound X used in the Bi dissolution treatment K (molar) and the amount of Bi A contained in the hexagonal ferrite magnetic powder supplied for the Bi leaching treatment Bi Bi leaching treatment is performed under the condition that the relationship between (moles) satisfies the following equation (4). N×A k / A Bi ≥1.0 …(4) Where N is the maximum number of Bi atoms that can coordinate with one molecule of compound X.

10. A method for manufacturing hexagonal ferrite magnetic powder according to any one of claims 4 to 9, wherein, The hexagonal ferrite magnetic powder supplied for the Bi leaching treatment is a hexagonal ferrite magnetic powder formed by replacing part of the Fe sites of the hexagonal ferrite crystal with one or more of the divalent, tetravalent or pentavalent metal elements.

11. The method for manufacturing hexagonal ferrite magnetic powder according to any one of claims 4 to 10, wherein, The hexagonal ferrite magnetic powder supplied for the Bi leaching treatment is hexagonal Ba ferrite magnetic powder.

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