WO2022190742A1

WO2022190742A1 - Method for producing hexagonal strontium ferrite powder

Info

Publication number: WO2022190742A1
Application number: PCT/JP2022/004898
Authority: WO
Inventors: 嵩潟口; 奈津貴市瀬; 健高橋
Original assignee: ソニーグループ株式会社
Priority date: 2021-03-11
Filing date: 2022-02-08
Publication date: 2022-09-15
Also published as: JPWO2022190742A1

Abstract

Provided is a method for producing hexagonal strontium ferrite powder capable of both ensuring thermal stability and achieving high-density recording. This method for producing hexagonal strontium ferrite powder includes: dissolving a mixture including sodium tetraborate (Na₂B₄O₇) as a glass raw material and a magnetic raw material containing strontium oxide and iron oxide to obtain a dissolved product; quenching the dissolved product to room temperature and generating an amorphous body including an amorphous component; and generating a crystallized product by firing the amorphous body at a firing temperature of 570-630°C, inclusive, and precipitating the crystallized hexagonal strontium ferrite.

Description

Method for producing hexagonal strontium ferrite powder

The present disclosure relates to a method for producing hexagonal strontium ferrite powder used, for example, as a constituent material of magnetic layers of magnetic recording media.

Tape-shaped magnetic recording media are widely used to store electronic data. For example, Patent Document 1 proposes a magnetic recording medium and a hexagonal strontium ferrite powder for magnetic recording used in the magnetic recording medium.

JP 2019-3715 A

By the way, magnetic recording media are expected to have both high magnetic properties that can ensure, for example, thermal stability and high-density recording.

Therefore, a method for producing hexagonal strontium ferrite powder that can be used in magnetic recording media with high reliability and high recording density is desired.

A method for producing hexagonal strontium ferrite powder as an embodiment of the present disclosure includes the following (1) to (3).
(1) A melt is obtained by melting a mixture containing sodium tetraborate (Na2B4O7) as a glass raw material and a magnetic raw material containing strontium (Sr) oxide and iron (Fe) oxide.
(2) quenching the melt to room temperature to produce an amorphous body containing an amorphous component in the mixture;
(3) sintering the amorphous body at a sintering temperature of 570° C. or higher and 630° C. or lower to produce a crystallized product and precipitate crystallized hexagonal strontium ferrite;

The method for producing a hexagonal strontium ferrite powder as an embodiment of the present disclosure includes the above (1) to (3), so high magnetic properties that can ensure thermal stability and high magnetic recording density can be obtained.

1 is a cross-sectional view of a magnetic recording medium according to an embodiment of the present disclosure; FIG. 2 is a flow chart showing a method for producing hexagonal strontium ferrite powder used in the magnetic layer of the magnetic recording medium shown in FIG. 1. FIG. FIG. 4 is a characteristic diagram showing the relationship between particle volume and coercive force in an experimental example regarding the magnetic recording medium of the present disclosure;

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The description will be given in the following order.
1. Background 2. One embodiment 2-1. Configuration of Magnetic Recording Medium 2-2. Manufacturing method of magnetic recording medium 2-3. Method for producing hexagonal strontium ferrite particles 2-4. Effect 3. Experimental example

<1. History>
First, the circumstances leading to the creation of the technology of the present disclosure will be described. Various magnetic powders have been developed so far as magnetic substances constituting magnetic layers of tape-shaped or disk-shaped magnetic recording media. Recently, there has been a demand for further improvement in the recording density of magnetic recording media. In order to improve the recording density of a magnetic recording medium, it is desirable to reduce the size of the magnetic particles that make up the magnetic powder. However, as the magnetic particles become finer, that is, as the volume of the magnetic particles becomes smaller, the thermal stability of the magnetic particles decreases. The thermal stability of the magnetic particles can be improved by increasing the magnetocrystalline anisotropy constant Ku of the magnetic particles. However, when the magnetocrystalline anisotropy constant Ku of the magnetic particles increases, it becomes difficult to perform a recording operation on the magnetic layer using the magnetic particles. Therefore, there has been a demand for measures to overcome the trilemma concerning such magnetic particles for magnetic recording. The thermal stability of the magnetic particles is evaluated by a parameter Ku×V/kB×T including the magnetocrystalline anisotropy constant Ku. That is, it can be said that the higher the value of the parameter Ku×V/kB×T, the higher the thermal stability. By the way, the magnetocrystalline anisotropy constant Ku of the magnetic particles tends to be proportional to the coercive force Hc and the mass magnetization σs. Therefore, it is considered that the higher the numerical values of the magnetic properties such as the coercive force Hc and the mass magnetization σs, the higher the thermal stability of the magnetic particles.

Therefore, the present disclosure provides a method for producing a hexagonal strontium ferrite powder that exhibits high magnetic properties equal to or higher than those of conventional magnetic powder (for example, barium ferrite powder) while miniaturizing the magnetic material.

<2. one embodiment>
[2-1 Configuration of Magnetic Recording Medium 10]
FIG. 1 shows a cross-sectional configuration example of a magnetic recording medium 10 according to an embodiment of the present disclosure. As shown in FIG. 1, the magnetic recording medium 10 has a laminated structure in which multiple layers are laminated. Specifically, the magnetic recording medium 10 includes a long tape-shaped substrate 11, an underlayer 12 provided on one main surface 11A of the substrate 11, and a magnetic layer provided on the underlayer 12. 13 and a back layer 14 provided on the other main surface 11B of the base 11 . The surface 13S of the magnetic layer 13 is the surface on which the magnetic head travels in contact. The base layer 12 and the back layer 14 are provided as required, and may be omitted. The average thickness of the magnetic recording medium 10 is preferably 5.6 μm or less, for example.

The magnetic recording medium 10 has a long tape shape, and travels along its own longitudinal direction during recording and reproduction operations. The magnetic recording medium 10 is preferably used in a recording/reproducing apparatus having a ring-shaped head as a recording head, for example.

(Substrate 11)
The base 11 is a nonmagnetic support that supports the underlayer 12 and the magnetic layer 13 . The substrate 11 has a long film shape. The upper limit of the average thickness of the substrate 11 is preferably 4.2 μm or less, more preferably 4.0 μm or less. When the upper limit of the average thickness of the substrate 11 is 4.2 μm or less, the recording capacity that can be recorded in one data cartridge can be made higher than that of a general magnetic recording medium. The lower limit of the average thickness of the substrate 11 is preferably 3 μm or more, more preferably 3.2 μm or more. When the lower limit of the average thickness of the substrate 11 is 3 μm or more, a decrease in the strength of the substrate 11 can be suppressed.

The average thickness of the substrate 11 is obtained as follows. First, a magnetic recording medium 10 having a width of 1/2 inch is prepared and cut into a length of 250 mm to prepare a sample. Subsequently, layers other than the substrate 11 of the sample, that is, the underlayer 12, the magnetic layer 13 and the back layer 14 are removed with a solvent such as MEK (methyl ethyl ketone) or dilute hydrochloric acid. Next, using a Mitutoyo laser hologram (LGH-110C) as a measuring device, the thickness of the substrate 11, which is a sample, is measured at five or more points. Then, these measured values are simply averaged (arithmetic average) to calculate the average thickness of the substrate 11 . It is assumed that the measurement position is randomly selected from the sample.

The base 11 contains, for example, polyesters as a main component. Alternatively, the substrate 11 may contain PEEK (polyetheretherketone) as a main component. Substrate 11 may contain at least one of polyolefins, cellulose derivatives, vinyl resins, and other polymer resins in addition to polyesters or PEEK. When the substrate 11 contains two or more of the above materials, the two or more materials may be mixed, copolymerized, or laminated.

Polyesters contained in the substrate 11 include, for example, PET (polyethylene terephthalate), PEN (polyethylene naphthalate), PBT (polybutylene terephthalate), PBN (polybutylene naphthalate), PCT (polycyclohexylenedimethylene terephthalate), PEB (polyethylene-p-oxybenzoate) and at least one of polyethylene bisphenoxycarboxylate.

The polyolefins contained in the substrate 11 include, for example, at least one of PE (polyethylene) and PP (polypropylene). Cellulose derivatives include, for example, at least one of cellulose diacetate, cellulose triacetate, CAB (cellulose acetate butyrate) and CAP (cellulose acetate propionate). Vinyl-based resins include, for example, at least one of PVC (polyvinyl chloride) and PVDC (polyvinylidene chloride).

Other polymer resins contained in the substrate 11 include, for example, PA (polyamide, nylon), aromatic PA (aromatic polyamide, aramid), PI (polyimide), aromatic PI (aromatic polyimide), PAI (polyamideimide ), aromatic PAI (aromatic polyamideimide), PBO (polybenzoxazole, e.g. Zylon®), polyether, PEK (polyetherketone), polyetherester, PES (polyethersulfone), PEI ( polyetherimide), PSF (polysulfone), PPS (polyphenylene sulfide), PC (polycarbonate), PAR (polyarylate) and PU (polyurethane).

(Magnetic layer 13)
The magnetic layer 13 is a recording layer for recording signals. The magnetic layer 13 contains, for example, magnetic powder, binder and lubricant. The magnetic layer 13 may further contain additives such as conductive particles, abrasives, and rust preventives, if necessary.

The magnetic layer 13 has, for example, a surface 13S provided with a large number of recesses. Lubricant is stored in these numerous recesses. A large number of recesses preferably extend in a direction perpendicular to the surface of the magnetic layer 13 . This is because the ability to supply the lubricant to the surface 13S of the magnetic layer 13 can be improved. In addition, a part of many recesses may extend in the vertical direction.

(Magnetic powder)
The magnetic powder contained in the magnetic layer 13 preferably has a mass magnetization σs of, for example, 30 emu/g or more and 60 emu/g or less. When the magnetic powder has a mass magnetization σs of 30 emu/g or more, an improvement in the output of the magnetic recording medium 10 can be expected. Examples of the magnetic powder contained in the magnetic layer 13 include hexagonal strontium ferrite powder. The hexagonal strontium ferrite powder will be described later.

The average particle volume of the magnetic powder is, for example, 2500 nm 3 or less, preferably 1800 nm ³ or less, more preferably 1200 nm ³ or less. This is because excellent electromagnetic conversion characteristics (for example, SNR) can be obtained.

(Binder)
As the binder, a resin having a structure obtained by imparting a cross-linking reaction to a polyurethane-based resin, a vinyl chloride-based resin, or the like is preferable. However, the binder is not limited to these, and other resins may be blended as appropriate depending on the physical properties required for the magnetic recording medium 10 . The resin to be blended is not particularly limited as long as it is a resin commonly used in the coating type magnetic recording medium 10 .

For example, polyvinyl chloride, polyvinyl acetate, vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, acrylate-acrylonitrile copolymer, acrylate-chloride Vinyl-vinylidene chloride copolymer, vinyl chloride-acrylonitrile copolymer, acrylate-acrylonitrile copolymer, acrylate-vinylidene chloride copolymer, methacrylate-vinylidene chloride copolymer, methacrylate-chloride Vinyl copolymer, methacrylate-ethylene copolymer, polyvinyl fluoride, vinylidene chloride-acrylonitrile copolymer, acrylonitrile-butadiene copolymer, polyamide resin, polyvinyl butyral, cellulose derivatives (cellulose acetate butyrate, cellulose dye acetate, cellulose triacetate, cellulose propionate, nitrocellulose), styrene-butadiene copolymer, polyester resin, amino resin, synthetic rubber, and the like.

Examples of thermosetting resins or reactive resins include phenol resins, epoxy resins, urea resins, melamine resins, alkyd resins, silicone resins, polyamine resins, and urea-formaldehyde resins.

In addition, polar functional groups such as -SO3 M, _-OSO3 M, -COOM and P=O(OM) ₂ are introduced into each of the binders described above for the purpose of improving the dispersibility of the magnetic powder. may Here, M in the above chemical formula is a hydrogen atom or an alkali metal such as lithium, potassium or sodium.

Further, the polar functional groups include side chain types having end groups of -NR1R2, -NR1R2R3+X-, and main chain types of >NR1R2+X-. Here, R1, R2 and R3 in the above formula are hydrogen atoms or hydrocarbon groups, and X- is a halogen element ion such as fluorine, chlorine, bromine or iodine, or an inorganic or organic ion. Further, the polar functional group includes -OH, -SH, -CN, epoxy group and the like.

(lubricant)
The lubricant contained in the magnetic layer 13 contains, for example, fatty acid and fatty acid ester. The fatty acid contained in the lubricant preferably contains, for example, at least one of a compound represented by the following general formula <1> and a compound represented by the following general formula <2>. Moreover, the fatty acid ester contained in the lubricant preferably contains at least one of the compound represented by the following general formula <3> and the compound represented by the following general formula <4>. The lubricant contains the compound represented by the general formula <1> and the compound represented by the general formula <3>, so that the compound represented by the general formula <2> and the compound represented by the general formula <3> By including two types of the compound represented by the general formula <1> and the compound represented by the general formula <4>, the compound represented by the general formula <2> and the general formula <4> By including two types of compounds represented by general formula <1>, by including three types of compounds represented by general formula <2> and general formula <3>, general The compound represented by the general formula <1>, the compound represented by the general formula <3, by including three kinds of the compound represented by the formula <1>, the compound represented by the general formula <2>, and the compound represented by the general formula <4>> and the compound represented by the general formula <4>, the compound represented by the general formula <2>, the compound represented by the general formula <3>, and the compound represented by the general formula <4> or by including three types of compounds represented by the general formula <1>, the compound represented by the general formula <2>, the compound represented by the general formula <3> and the general formula <4> By including the four compounds, it is possible to suppress an increase in the coefficient of dynamic friction due to repeated recording or reproduction in the magnetic recording medium 10 . As a result, the running properties of the magnetic recording medium 10 can be further improved.
_CH3 ( _CH2 ) _kCOOH <1>
(However, in general formula <1>, k is an integer selected from the range of 14 or more and 22 or less, more preferably 14 or more and 18 or less.)
_CH3 ( _CH2 ) _nCH =CH( _CH2 ) _mCOOH <2>
(However, in general formula <2>, the sum of n and m is an integer selected from the range of 12 or more and 20 or less, more preferably 14 or more and 18 or less.)
_CH3 ( _CH2 ) _pCOO ( _CH2 ) _qCH3 < ₃ >
(However, in general formula <3>, p is an integer selected from the range of 14 to 22, more preferably 14 to 18, and q is 2 to 5, more preferably 2 to 4 An integer selected from the following range.)
_CH3 ( _CH2 ) _pCOO- ( _CH2 ) _qCH ( _CH3 ) ₂ <4>
(However, in the general formula <4>, p is an integer selected from the range of 14 or more and 22 or less, and q is an integer selected from the range of 1 or more and 3 or less.)

(Additive)
The magnetic layer 13 contains nonmagnetic reinforcing particles such as aluminum oxide (α, β or γ alumina), chromium oxide, silicon oxide, diamond, garnet, emery, boron nitride, titanium carbide, silicon carbide, titanium carbide, titanium oxide ( Rutile-type or anatase-type titanium oxide) and the like may be further included.

(underlying layer 12)
The underlayer 12 is a nonmagnetic layer containing nonmagnetic powder and a binder. The underlayer 12 may further contain at least one additive selected from lubricants, conductive particles, hardeners, rust preventives, and the like, if necessary. Further, the underlayer 12 may have a multi-layer structure in which a plurality of layers are laminated. The average thickness of the underlayer 12 is preferably 0.5 μm or more and 0.9 μm or less, more preferably 0.6 μm or more and 0.7 μm or less. By reducing the average thickness of the underlayer 12 to 0.9 μm or less, the Young's modulus of the magnetic recording medium 10 as a whole is more effectively reduced than when the thickness of the substrate 11 is reduced. Therefore, tension control for the magnetic recording medium 10 is facilitated. Further, by setting the average thickness of the underlayer 12 to 0.5 μm or more, the adhesive force between the substrate 11 and the underlayer 12 is ensured. Moreover, variations in the thickness of the underlayer 12 can be suppressed, and an increase in the roughness of the surface 13S of the magnetic layer 13 can be prevented.

The average thickness of the underlying layer 12 is obtained, for example, as follows. First, a magnetic recording medium 10 having a width of 1/2 inch is prepared and cut into a length of 250 mm to prepare a sample. Subsequently, the underlayer 12 and the magnetic layer 13 of the sample magnetic recording medium 10 are peeled off from the substrate 11 . Next, using a laser hologram (LGH-110C) manufactured by Mitutoyo as a measuring device, the thickness of the laminated body of the underlayer 12 and the magnetic layer 13 peeled off from the substrate 11 is measured at five or more positions. do. Then, these measured values are simply averaged (arithmetic average) to calculate the average thickness of the laminate of the underlayer 12 and the magnetic layer 13 . It is assumed that the measurement position is randomly selected from the sample. Finally, the average thickness of the underlayer 12 is obtained by subtracting the average thickness of the magnetic layer 13 measured using the TEM as described above from the average thickness of the laminate.

The underlayer 12 may have pores, that is, the underlayer 12 may be provided with a large number of pores. The pores of the underlayer 12 may be formed, for example, by forming pores (recesses) in the magnetic layer 13 . can be formed by pressing the portion against the surface on the magnetic layer side. That is, by forming depressions corresponding to the shapes of the protrusions on the surface 13S of the magnetic layer 13, pores can be formed in the magnetic layer 13 and the underlayer 12, respectively. Pores may also be formed as the solvent evaporates during the drying process of the coating material for forming the magnetic layer. In addition, when the magnetic layer-forming paint was applied to the surface of the underlayer 12 to form the magnetic layer 13, the solvent in the magnetic layer-forming paint applied and dried the lower layer. It can permeate into the underlying layer 12 through the pores. After that, when the solvent that has penetrated into the underlayer 12 evaporates in the drying process of the magnetic layer 13, the solvent that has penetrated into the underlayer 12 migrates from the underlayer 12 to the surface 13S of the magnetic layer 13, resulting in fine particles. Holes may be formed. The pores formed in this manner can communicate the magnetic layer 13 and the underlayer 12, for example. The average diameter of the pores can be adjusted by changing the solid content or solvent type of the magnetic layer-forming paint and/or the drying conditions of the magnetic layer-forming paint. By forming pores in both the magnetic layer 13 and the underlayer 12, a particularly suitable amount of lubricant appears on the magnetic layer side surface for good running stability, and dynamic friction due to repeated recording or reproduction is reduced. An increase in the coefficient can be further suppressed.

From the viewpoint of suppressing a decrease in the coefficient of dynamic friction after repeated recording or reproduction, it is preferable that the pores of the underlayer 12 and the recesses of the magnetic layer 13 are connected. Here, the connection between the pores of the underlayer 12 and the recesses of the magnetic layer 13 means that some of the pores of the underlayer 12 and some of the recesses of the magnetic layer 13 are connected. It includes the state in which some things are connected.

From the viewpoint of improving the supply of lubricant to the surface 13S of the magnetic layer 13, it is preferable that the large number of recesses include those extending in the direction perpendicular to the surface 13S of the magnetic layer 13. In addition, from the viewpoint of improving the supply of the lubricant to the surface 13S of the magnetic layer 13, the pores of the underlayer 12 extending in the direction perpendicular to the surface 13S of the magnetic layer 13 and the surface of the magnetic layer 13 It is preferable that the recesses of the magnetic layer 13 extending in the direction perpendicular to 13S are connected.

(Non-Magnetic Powder for Underlayer 12)
The non-magnetic powder includes, for example, at least one of inorganic powder and organic powder. Also, the non-magnetic powder may contain carbon powder such as carbon black. One type of non-magnetic powder may be used alone, or two or more types of non-magnetic powder may be used in combination. Inorganic particles include, for example, metals, metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, metal sulfides, and the like. Examples of the shape of the non-magnetic powder include various shapes such as acicular, spherical, cubic, and plate-like, but are not limited to these.

(Binder for base layer 12)
The binder in the underlayer 12 is the same as in the magnetic layer 13 described above.

(back layer 14)
The back layer 14 contains, for example, a binder and non-magnetic powder. The back layer 14 may further contain at least one additive selected from lubricants, curing agents, antistatic agents, and the like, if necessary. The binder and non-magnetic powder in the back layer 14 are the same as the binder and non-magnetic powder in the underlayer 12 described above.

[2-2 Manufacturing Method of Magnetic Recording Medium 10]
Next, a method of manufacturing the magnetic recording medium 10 having the above configuration will be described. First, a non-magnetic powder, a binder, a lubricant, etc. are kneaded and dispersed in a solvent to prepare a paint for forming an underlayer. Next, magnetic powder, a binder, a lubricant, etc. are kneaded and dispersed in a solvent to prepare a coating material for forming a magnetic layer. Next, a binder, non-magnetic powder, etc. are kneaded and dispersed in a solvent to prepare a paint for forming a back layer. For the preparation of the magnetic layer-forming paint, undercoat layer-forming paint and back layer-forming paint, for example, the following solvents, dispersing devices and kneading devices can be used.

Examples of the solvent used for preparing the above paint include ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone, alcohol solvents such as methanol, ethanol and propanol, methyl acetate, ethyl acetate, butyl acetate and propyl acetate. , ester solvents such as ethyl lactate and ethylene glycol acetate, ether solvents such as diethylene glycol dimethyl ether, 2-ethoxyethanol, tetrahydrofuran and dioxane, aromatic hydrocarbon solvents such as benzene, toluene and xylene, methylene chloride, ethylene chloride, Examples thereof include halogenated hydrocarbon solvents such as carbon tetrachloride, chloroform, and chlorobenzene. These may be used alone, or may be used by mixing them as appropriate.

As the kneading device used for the preparation of the above paint, for example, a continuous twin-screw kneader, a continuous twin-screw kneader capable of multistage dilution, a kneader, a pressure kneader, a roll kneader, or the like can be used. , and are not particularly limited to these devices. Dispersing devices used for the above coating preparation include, for example, roll mills, ball mills, horizontal sand mills, vertical sand mills, spike mills, pin mills, tower mills, pearl mills (e.g. "DCP Mill" manufactured by Eirich), homogenizers, ultra A dispersing device such as a sonic disperser can be used, but it is not particularly limited to these devices.

Next, the base layer 12 is formed by applying a base layer forming coating material to one main surface 11A of the substrate 11 and drying it. Subsequently, the magnetic layer 13 is formed on the underlayer 12 by coating the underlayer 12 with a magnetic layer-forming paint and drying it. During drying, it is preferable to magnetically orient the magnetic powder in the thickness direction of the substrate 11 using, for example, a solenoid coil. Further, during drying, the magnetic powder may be magnetically oriented in the running direction (longitudinal direction) of the substrate 11 by, for example, a solenoid coil, and then magnetically oriented in the thickness direction of the substrate 11 . By performing such a magnetic field orientation treatment, the degree of perpendicular orientation (that is, the squareness ratio S1) of the magnetic powder can be improved. After forming the magnetic layer 13, the back layer 14 is formed by applying a back layer forming coating material to the other main surface 11B of the substrate 11 and drying it. Thus, the magnetic recording medium 10 is obtained.

After that, the obtained magnetic recording medium 10 is calendered to smooth the surface 13S of the magnetic layer 13. Next, the calendered magnetic recording medium 10 is wound into a roll, and in this state, the magnetic recording medium 10 is subjected to a heat treatment so that a large number of projections on the surface 14S of the back layer 14 become magnetic. It is transferred to surface 13S of layer 13. FIG. As a result, a large number of recesses are formed on the surface 13S of the magnetic layer 13. As shown in FIG.

The temperature of the heat treatment is preferably 50°C or higher and 80°C or lower. When the heat treatment temperature is 50° C. or higher, good transferability can be obtained. On the other hand, if the temperature of the heat treatment is 80° C. or less, the number of pores may become too large, and the lubricant on the surface 13S of the magnetic layer 13 may become excessive. Here, the temperature of the heat treatment is the temperature of the atmosphere that holds the magnetic recording medium 10 .

The heat treatment time is preferably 15 hours or more and 40 hours or less. When the heat treatment time is 15 hours or more, good transferability can be obtained. On the other hand, when the heat treatment time is 40 hours or less, a decrease in productivity can be suppressed.

Also, the range of pressure applied to the magnetic recording medium 10 during heat treatment is preferably 150 kg/cm or more and 400 kg/cm or less.

Finally, the magnetic recording medium 10 is cut into a predetermined width (eg, 1/2 inch width). As described above, the intended magnetic recording medium 10 is obtained.

[2-3 Hexagonal Strontium Ferrite Powder]
The hexagonal strontium ferrite powder of this embodiment is a ferromagnetic material for magnetic recording. The hexagonal strontium ferrite powder of the present embodiment can be used, for example, as the magnetic powder of the magnetic layer 13 of the coating type magnetic recording medium 10 . In addition, "magnetic powder" in the present disclosure refers to an aggregate of a plurality of magnetic particles. An aggregate of a plurality of magnetic particles refers to a mode in which the plurality of magnetic particles forming the aggregate are in direct contact with each other, and a binder, lubricant, additive, or the like is added to the plurality of magnetic particles. It also means an intervening aspect.

The crystal structure of hexagonal ferrite contains at least iron atoms, divalent metal atoms and oxygen atoms as constituent atoms. A bivalent metal atom is a metal atom that can become a divalent cation as an ion. Examples of bivalent metal atoms include alkaline earth metal atoms such as strontium (Sr) atoms, barium (Ba) atoms and calcium (Ca) atoms, and lead (Pb) atoms. Strontium ferrite is hexagonal ferrite containing strontium atoms as divalent metal atoms. The hexagonal strontium ferrite powder of the present disclosure refers to hexagonal strontium ferrite powder in which the main divalent metal atoms contained are strontium atoms. The main divalent metal atom means the divalent metal atom having the highest abundance ratio (atomic %) among the divalent metal atoms contained in the hexagonal strontium ferrite powder. In the hexagonal strontium ferrite powder, the strontium atom content may be, for example, in the range of 2.0 atomic % or more and 15.0 atomic % or less with respect to 100 atomic % of iron atoms. The hexagonal strontium ferrite powder of the present disclosure may contain only strontium atoms as divalent metal atoms. Alternatively, the hexagonal strontium ferrite powders of the present disclosure may contain one or more other divalent metal atoms in addition to the strontium atoms. For example, at least one of barium atoms and calcium atoms can be included. When other divalent metal atoms other than strontium atoms are contained, the barium atom content and calcium atom content in the hexagonal strontium ferrite powder are, for example, 0.05 to 0.05 to 100 atomic % iron atoms. It can be about 5.0 atomic %.

As crystal structures of hexagonal ferrite, magnetoplumbite type (also called “M type”), W type, Y type and Z type are known. The hexagonal strontium ferrite powder may have any crystal structure. The crystal structure can be confirmed by X-ray diffraction analysis. The hexagonal strontium ferrite powder may have a single crystal structure or two or more crystal structures detected by X-ray diffraction analysis. For example, hexagonal strontium ferrite powder may be one in which only the M-type crystal structure is detected by X-ray diffraction analysis. For example, M-type hexagonal ferrite is represented by the composition formula MFe ₁₂ O ₁₉ . Here, M represents a divalent metal atom. Therefore, when M in the composition formula MFe ₁₂ O ₁₉ is only a strontium (Sr) atom, an M-type hexagonal strontium ferrite powder is obtained. Alternatively, when a plurality of divalent metal atoms are included as M in the composition formula MFe ₁₂ O ₁₉ , when the strontium (Sr) atom has the highest abundance ratio (atomic %) among the plurality of divalent metal atoms, M type hexagonal strontium ferrite powder. The divalent metal atom content of the hexagonal strontium ferrite powder is usually determined by the type of crystal structure of the hexagonal ferrite, and is not particularly limited. The same applies to the iron atom content and the oxygen atom content. The hexagonal strontium ferrite powder contains at least iron atoms, strontium atoms, oxygen atoms and rare earth atoms, and may or may not contain atoms other than these atoms.

Thermal fluctuation of hexagonal strontium ferrite powder, that is, indices of thermal stability include, for example, coercive force Hc and mass magnetization σs. In order to reduce the thermal fluctuation of the hexagonal strontium ferrite powder and improve its thermal stability, it is desirable to increase its coercive force Hc and mass magnetization σs.

[2-3 Method for producing hexagonal strontium ferrite powder]
Next, a method for producing hexagonal strontium ferrite powder according to the present embodiment will be described with reference to FIG. The method for producing the hexagonal strontium ferrite powder of the present embodiment uses a so-called glass crystallization method.

The method for producing hexagonal strontium ferrite powder according to the present embodiment specifically includes the operations of steps S101 to S106 below.

(S101) Mixing a glass raw material with a magnetic raw material containing strontium (Sr) oxide and iron (Fe) oxide (raw material mixing step).
(S102) dissolving the mixture to obtain a melt (dissolving step);
(S103) Rapidly cooling the melt to room temperature to generate an amorphous component in the melt (amorphization step).
(S104) sintering the melt in which the amorphous component is produced at a sintering temperature of 570° C. or higher and 630° C. or lower to produce a crystallized product and precipitate crystallized hexagonal strontium ferrite (sintering step).
(S105) removing the component of the glass raw material from the crystallized product by acid treatment (acid treatment step).
(S106) Drying the crystallized product to obtain hexagonal strontium ferrite powder (drying step).

The above steps will be explained in detail below.

(Raw material mixing process)
A raw material mixture used in a glass crystallization method for obtaining hexagonal strontium ferrite powder contains a hexagonal strontium ferrite-forming component and a glass-forming component. Here, the glass-forming component is a glass raw material that exhibits a glass transition phenomenon and can be amorphized, that is, vitrified. Specifically, sodium tetraborate (Na ₂ B ₄ O ₇ ) is used.

The hexagonal strontium ferrite-forming component contained in the raw material mixture is, for example, a magnetic raw material containing strontium (Sr) oxide and iron (Fe) oxide. Things, etc. Specific examples include SrCO ₃ as strontium oxide and Fe ₂ O ₃ as iron oxide. Here _, it is desirable that the content of SrCO3 in the magnetic raw material is higher than the content of _Fe2O3 in the magnetic raw material.

The contents of various components in the raw material mixture are determined according to the composition of the hexagonal strontium ferrite powder to be obtained. For example, the content of glass raw materials in the raw material mixture is 30 mol % or less. The raw material mixture can be prepared by weighing various components and then mixing them. A glass raw material such as sodium tetraborate (Na ₂ B ₄ O ₇ ) and a magnetic material raw material containing SrCO ₃ and Fe ₂ O ₃ are mixed, for example, by placing them in a plastic container and then using a powder mixer. It runs for 60 minutes.

(Melting process)
The raw material mixture can be melted, for example, in a glass melting furnace. For example, the raw material mixture is put into a crucible of a glass melting furnace and melted at a melting temperature of 1300° C. to 1500° C., for example. The dissolution time may be appropriately set so that the raw material mixture is sufficiently melted. The melting time can be, for example, 80 minutes when 1 kg of the raw material mixture is put into the glass melting furnace. Moreover, it is desirable to dissolve the raw material mixture in the melting furnace while stirring it with a stirring device. This is for reducing the temperature unevenness in the melting furnace and promoting the amorphization of the melt obtained by melting the raw material mixture. In particular, in the present disclosure, when the content of glass raw materials in the raw material mixture is reduced to, for example, 30 mol % or less, the content of components containing Fe ₂ O ₃ becomes relatively high. In that case, since the melting point of the raw material mixture rises, the stirring operation becomes important in order to homogenize the temperature distribution in the furnace and eliminate uneven melting. In addition, the agitation can prevent clogging of the outlet with the melted material when the melted material is discharged from the melting furnace. The stirrer should stir at a rotational speed of, for example, 30 rpm or higher.

(quenching process)
Next, by rapidly cooling the melt obtained by dissolving the raw material mixture, an amorphous body containing an amorphous component is produced. The quenching can be carried out in the same manner as the quenching step usually performed to obtain an amorphous body by the glass crystallization method. For example, a method of rapidly cooling the melt while rolling it using a pair of cooling rolls rotated at high speed is suitable. It is preferable that the pair of cooling rolls maintain a constant surface temperature, for example, by circulating cooling water through internal channels. This is for stabilizing the quenching efficiency and promoting the amorphization of the melt. The temperature of the surface of the cooling roll is set at 20° C., for example. Further, the gap between the pair of cooling rolls is, for example, 1 mm or less, and the discharge speed is, for example, 0.5 g/sec or more and 1.0 g/sec or less. The term "rapid cooling" means to rapidly cool the melted raw material mixture to around room temperature to bring the melt into a disordered state (hereinafter referred to as an amorphous state). It is considered that one of the conditions for the amorphous state is that the cooling rate exceeds the crystal growth rate. By making it amorphous, it becomes possible to control the growth of nanoparticles and control the particle size of nanoparticles. If the cooling rate is slower than the crystal growth rate, crystal growth of the particles will occur before transitioning to the amorphous state, and the amorphous state and the crystalline state will coexist in the melt. Therefore, when rapid cooling is not successful and the melt of the raw material mixture does not become sufficiently amorphous, the melt contains both an amorphous state and a crystalline state. For this reason, the hexagonal strontium ferrite powder contains particles that grow from an amorphous state in the subsequent firing process and particles that grow from a crystalline state having a certain size in the firing process. Therefore, it is considered that the particle size distribution and magnetic properties of the hexagonal strontium ferrite powder to be obtained are varied.

(Baking process)
The amorphous body obtained by quenching is put into, for example, an electric furnace and fired. Through this firing step, hexagonal strontium ferrite particles and crystallized glass components can be precipitated. The particle size of the deposited hexagonal strontium ferrite particles can be controlled by the firing conditions. Increasing the firing temperature for crystallization (crystallization temperature) leads to an increase in the particle size of the precipitated hexagonal strontium ferrite particles. Therefore, it is desirable that the temperature be as low as possible and above the temperature at which crystallization of hexagonal strontium ferrite occurs. Specifically, it is desirable to produce a crystallized product by firing an amorphous body at a firing temperature of 570° C. or higher and 630° C. or lower. The baking time for crystallization (holding time at the crystallization temperature) is, for example, 1 to 24 hours, preferably 8 hours or longer. Also, the rate of temperature increase until reaching the firing temperature was 1.0° C./min. above 10.0°C/min. and, for example, 5.0° C./min. It is below.

(Acid treatment step)
A crystallized material obtained by heating an amorphous body contains a crystallized glass component together with hexagonal strontium ferrite particles. Therefore, the hexagonal strontium ferrite particles are extracted by subjecting the crystallized product to an acid treatment. In the acid treatment, the crystallized product is put into an acid such as acetic acid and washed with a ball mill. Since the acid treatment dissolves and removes the glass component surrounding the hexagonal strontium ferrite particles, the hexagonal strontium ferrite particles can be collected. In addition, it is preferable to subject the crystallized product to pulverization treatment before the acid treatment. This is for increasing the efficiency of the acid treatment. Coarse pulverization may be carried out by either a dry method or a wet method. After that, by centrifuging with a centrifuge and decanting, the glass component as an impurity is removed.

(Drying process)
Hexagonal strontium ferrite particles can be obtained by washing the crystallized product from which the glass component has been removed and then drying the crystallized product.

[2-4 Effect]
As described above, the method for producing hexagonal strontium ferrite powder according to the present embodiment includes the following operations <1> to <3>.
<1> To obtain a melt by melting a mixture containing a glass raw material and a magnetic raw material containing strontium (Sr) oxide and iron (Fe) oxide.
<2> Rapidly cooling the melt to room temperature to produce an amorphous body containing an amorphous component.
<3> Firing the amorphous body at a firing temperature of 570° C. or higher and 630° C. or lower to produce a crystallized product and deposit crystallized hexagonal strontium ferrite.

According to the method for producing hexagonal strontium ferrite powder of the present embodiment, by including each of the above treatments, it is possible to obtain hexagonal strontium ferrite particles that are finely divided while having a high coercive force. Therefore, the magnetic recording medium 10 containing the hexagonal strontium ferrite powder thus obtained in the magnetic layer 13 can be expected to achieve both excellent electromagnetic conversion characteristics and high long-term reliability.

In the method for producing hexagonal strontium ferrite powder according to the present embodiment, Na ₂ B ₄ O ₇ is used as the glass raw material, and the content of Na ₂ B ₄ O ₇ as the glass raw material in the raw material mixture is 30 mol % or less. . In this way, by keeping the glass raw material content in the raw material mixture low, the number of nucleation particles that serve as nuclei of hexagonal strontium ferrite particles in the raw material mixture relatively increases. The nucleation particles are, for example, Sr atoms contained in SrCO ₃ and Fe atoms contained in Fe ₂ O ₃ as the magnetic raw material. It is believed that the relative increase in the number of nucleation particles produces a large number of hexagonal strontium ferrite particles, suppressing coarsening of individual hexagonal strontium ferrite particles. Also, the use of Na ₂ B ₄ O ₇ as a glass raw material has the following advantages over the use of, for example, H ₃ BO ₃ . The boiling point of H ₃ BO ₃ is as low as 300°C. Therefore, H ₃ BO ₃ may evaporate when the raw material mixture is put into the melting furnace. Therefore, the melting point of the melt is increased, making it difficult to melt. Since Na ₂ B ₄ O ₇ has a relatively high boiling point of 1575° C., it is difficult for Na ₂ B ₄ O ₇ to evaporate when the raw material mixture is put into the melting furnace. Therefore, the melting point of the melt can be kept low, and the raw material mixture can be sufficiently melted. Also, by using Na ₂ B ₄ O ₇ , it is easier to bring the melt into an amorphous state during quenching, as compared with the case of using H ₃ BO ₃ . Therefore, it is possible to obtain the effects of suppressing variations in particle growth and suppressing coarsening of particles during firing.

Further, in the method for producing the hexagonal strontium ferrite powder of the present embodiment, the SrCO3 content (molar ratio) in the magnetic raw material is set higher than the _Fe2O3 content ₍ molar ratio) in the magnetic raw material. I have to. That is, the Sr content (molar ratio) is made higher than the Fe content (molar ratio). Therefore, a large number of hexagonal strontium ferrite particles are produced. Therefore, it is considered that coarsening of individual hexagonal strontium ferrite particles is suppressed. Strontium has a high ionization tendency and dissolves in glass to some extent. Therefore, if the Sr content (molar ratio) is equal to or less than the Fe content (molar ratio), the strontium is insufficient, and the number of hexagonal strontium ferrite particles produced decreases. As a result, individual hexagonal strontium ferrite particles tend to be coarse.

<3. Experimental example>
The present disclosure will be specifically described below with some experimental examples. However, the present disclosure is not limited only to the following experimental examples.

(1) Production of ferrite powder Ferrite powder samples of Experimental Examples 1 to 14 were produced in the following manner.

[Experimental example 1]
First, Na ₂ B ₄ O ₇ , Fe ₂ O ₃ , SrCO ₃ , and TiO ₂ were weighed so as to have the composition ratio shown in Table 1, and mixed with a powder mixer. A raw material mixture was obtained. Mixing time was 60 minutes. Table 1 shows the composition ratio of the raw material mixture.

Next, 1 kg of the raw material mixture was put into a crucible of a glass melting furnace and melted to obtain a melt. The melting temperature was 1400° C. and the melting time was 80 minutes. At the time of melting, the raw material mixture placed in the crucible was stirred with a stirring rod rotating at 30 rpm.

Next, by rapidly cooling the melt while flowing it out of the crucible, an amorphous body containing an amorphous component was produced. Here, a pair of cooling rolls whose surface temperature was set to 20° C. was used to rapidly cool the molten material while rolling it. At that time, the gap between the pair of cooling rolls was 1 mm or less, and the discharge speed was 0.5 g/sec or more and 1.0 g/sec or less.

The amorphous body obtained by quenching was put into an electric furnace and fired. Here, the sintering temperature was 620° C., and the rate of temperature increase from room temperature to the sintering temperature was 5.0° C./min. and In addition, the firing temperature of 620° C. was maintained for 8 hours after reaching the firing temperature of 620° C. As a result, a crystallized product containing hexagonal strontium ferrite particles was obtained.

Next, the obtained crystallized product was subjected to an acid treatment to remove the glass component and extract the hexagonal strontium ferrite particles. Acetic acid was used for the acid treatment, and ball mill cleaning was performed. Thereafter, centrifugal separation was performed using a centrifuge, followed by decantation to obtain hexagonal strontium ferrite powder. Finally, the hexagonal strontium ferrite powder was put into an electric furnace and dried in an environment of 120° C. until the water content of the strontium ferrite became 2.0 (wt %) or less.

[Experimental Examples 2 to 14]
A ferrite powder sample was prepared in the same manner as in Experimental Example 1, except that the composition ratio of the raw material mixture was changed. However, in Experimental Examples 7 to 9, instead of adding SrCO ₃ to the raw material mixture, BaCO ₃ was added to prepare samples of hexagonal barium ferrite powder.

(2) Evaluation of Ferrite Powder Next, the crystal structures of the ferrite powder samples of Experimental Examples 1 to 14 produced as described above were confirmed by X-ray diffraction analysis. Furthermore, mass magnetization σs, coercive force Hc, and particle volume were measured. Those results are shown in Table 1. Furthermore, FIG. 3 shows the relationship between the particle volume and the coercive force of the ferrite powders of Experimental Examples 1-14.

(X-ray diffraction analysis)
When each sample was subjected to X-ray diffraction analysis, it was confirmed that each sample had a magnetoplumbite-type (M-type) hexagonal ferrite crystal structure. The X-ray diffraction analysis was carried out under the following setting conditions using "Ultima IV" manufactured by RIGAKU Co., Ltd.
・X-ray tube: Co tube,
・Measuring range: 20 to 80 (deg.)
・Scan speed: 1 (deg./min.)
・X-ray voltage: 40 kV

(mass magnetization σs and coercive force Hc)
Mass magnetization σs and coercive force Hc of each sample were measured using a high-sensitivity vibrating sample magnetometer "VSM-P7-15" manufactured by Toei Industry Co., Ltd. The measurement conditions are as follows.
・Maximum external magnetic field: 15 (kOe)
・Measurement mode: full loop, maximum magnetic field: 15kOe, magnetic field step: 40bit, Time constant of Locking amp: 0.3sec, Waiting time: 1sec, MH average number: 20

(particle volume)
The volume of the hexagonal ferrite particles of each sample was calculated from the plate diameter and plate thickness of the particles, assuming that each particle was a hexagonal column. The tabular diameter of the particles as used herein refers to the dimension along the crystal plane (220) of strontium ferrite (or barium ferrite). Further, the plate thickness of a particle refers to the dimension along the crystal plane (006) of strontium ferrite (or barium ferrite). The plate diameter and plate thickness of the ferrite particles were calculated using the fundamental parameter method from the X-ray diffraction pattern obtained by the X-ray diffraction analysis.

As shown in Table 1 and FIG. 3, in Experimental Examples 2 to 6, the particle volume could be reduced while maintaining good mass magnetization σs and coercive force Hc, that is, while maintaining high magnetic properties. .

On the other hand, in Experimental Examples 7 to 9, barium oxide was used instead of strontium oxide in the raw material mixture. tended to be For example, in Experimental Example 7, the coercive force Hc was 2700 [Oe], which is about the same as in Experimental Examples 4 and 6, but the particle volume was 2700 [nm ³ ], while the particle volume of Experimental Example 4 was 1994 [nm ³ ]. And compared with the particle volume of Experimental Example 6, 982 [nm ³ ], the particle volume was greatly increased. Further, in Experimental Examples 8 and 9, the coercive force Hc was 2300 [Oe] and 2200 [Oe], respectively, which was about the same as 2191 [Oe] in Experimental Example 5, but the particle volume was 1800 [nm]. ³ ], 1500 [nm ³ ], which is significantly larger than the particle volume of Experimental Example 5, 1196 [nm ³ ].

In Experimental Examples 1 to 6 and 12 to 14, the content of Na ₂ B ₄ O ₇ as a glass raw material in the raw material mixture was 30 mol % or less. The particle volume could be reduced while maintaining a high magnetic force Hc. In particular, in Experimental Examples 2 to 6, the content of strontium oxide (SrCO ₃ ) is high and the content of Na ₂ B ₄ O ₇ as a glass raw material is low. Therefore, as shown in FIG. 3, the particle volume could be reduced while maintaining higher magnetic properties than in other Experimental Examples 1 and 7-14. Also, in Experimental Examples 10 and 11, the content of Na ₂ B ₄ O ₇ as a glass raw material is high. Therefore, compared with other experimental examples, the particle volume is large and the magnetic properties are low. Also, in Experimental Examples 12 and 13, the content of strontium oxide (SrCO ₃ ) is relatively low. For this reason, the number of nucleation particles is relatively small, and it is considered that the particle growth is uneven. It was also confirmed that the addition of TiO ₂ inhibits the crystal growth of hexagonal ferrite grains, resulting in a decrease in coercive force Hc.

Although the present disclosure has been specifically described above with reference to the embodiments and modifications thereof, the present disclosure is not limited to the above-described embodiments and the like, and various modifications are possible.

For example, the configurations, methods, steps, shapes, materials, numerical values, etc. given in the above-described embodiments and modifications thereof are merely examples, and different configurations, methods, steps, shapes, materials, numerical values, etc., may be used if necessary. etc. may be used. Specifically, the magnetic recording medium of the present disclosure may contain components other than the substrate, underlayer, magnetic layer, back layer and barrier layer. Also, the chemical formulas of compounds and the like are representative ones, and the valence numbers and the like are not limited as long as they are common names of the same compound.

Also, the configurations, methods, steps, shapes, materials, numerical values, etc. of the above-described embodiments and modifications thereof can be combined with each other without departing from the gist of the present disclosure.

In addition, in the numerical ranges described stepwise in this specification, the upper limit or lower limit of the numerical range at one stage may be replaced with the upper limit or lower limit of the numerical range at another stage. The materials exemplified in this specification can be used singly or in combination of two or more unless otherwise specified.

As described above, according to the method for producing a hexagonal strontium ferrite powder as an embodiment of the present disclosure, fine hexagonal strontium ferrite particles can be obtained while having a high coercive force.
Note that the effect of the present disclosure is not limited to this, and may be any of the effects described in this specification. In addition, the present technology can take the following configurations.
(1)
Melting a mixture containing sodium tetraborate (Na ₂ B ₄ O ₇ ) as a glass raw material and a magnetic raw material containing strontium (Sr) oxide and iron (Fe) oxide to obtain a melt. When,
quenching the melt to room temperature to produce an amorphous body comprising an amorphous component;
A method for producing a hexagonal strontium ferrite powder, comprising: sintering the amorphous material at a sintering temperature of 570° C. or higher and 630° C. or lower to produce a crystallized product, and precipitating the crystallized hexagonal strontium ferrite.
(2)
using SrCO3 as the strontium oxide _,
Using Fe ₂ O ₃ as the iron oxide,
The method for producing a hexagonal strontium ferrite powder according to (1) above, wherein the SrCO ₃ content in the magnetic raw material is higher than the Fe ₂ O ₃ content in the magnetic raw material.
(3)
The method for producing a hexagonal strontium ferrite powder according to any one of (1) or (2) above, wherein the content of the glass raw material in the mixture is 30 mol % or less.
(4)
5°C/min. The method for producing a hexagonal strontium ferrite powder according to any one of (1) to (3) above, wherein the temperature is raised from the room temperature to the firing temperature at the following temperature elevation rate.
(5)
The method for producing a hexagonal strontium ferrite powder according to any one of (1) to (4) above, wherein the amorphous body is fired at the firing temperature for 8 hours or longer.
(6)
The method for producing a hexagonal strontium ferrite powder according to any one of the above (1) to (5), wherein the quenching is performed while rolling the melt using cooling rolls.
(7)
The method for producing a hexagonal strontium ferrite powder according to any one of (1) to (6) above, further comprising removing the glass raw material component from the crystallized product by acid treatment.

This application claims priority based on Japanese Patent Application No. 2021-039757 filed on March 11, 2021 at the Japan Patent Office, and the entire contents of this application are incorporated herein by reference. to refer to.

Depending on design requirements and other factors, those skilled in the art may conceive various modifications, combinations, subcombinations, and modifications that fall within the scope of the appended claims and their equivalents. It is understood that

Claims

Melting a mixture containing sodium tetraborate (Na 2 B 4 O 7 ) as a glass raw material and a magnetic raw material containing strontium (Sr) oxide and iron (Fe) oxide to obtain a melt. When,
quenching the melt to room temperature to produce an amorphous body comprising an amorphous component;
A method for producing a hexagonal strontium ferrite powder, comprising: sintering the amorphous material at a sintering temperature of 570° C. or higher and 630° C. or lower to produce a crystallized product, and precipitating the crystallized hexagonal strontium ferrite.
Using SrCO3 as the strontium ( Sr) oxide,
Using Fe 2 O 3 as the iron (Fe) oxide,
2. The method for producing hexagonal strontium ferrite powder according to claim 1 , wherein the SrCO3 content in the magnetic raw material is higher than the Fe2O3 content in the magnetic raw material.
The method for producing hexagonal strontium ferrite powder according to claim 1, wherein the content of the glass raw material in the mixture is 30 mol% or less.
5°C/min. The method for producing a hexagonal strontium ferrite powder according to claim 1, wherein the temperature is raised from the room temperature to the firing temperature at a temperature elevation rate of:
2. The method for producing hexagonal strontium ferrite powder according to claim 1, wherein said amorphous body is fired at said firing temperature for 8 hours or longer.
The method for producing a hexagonal strontium ferrite powder according to claim 1, wherein the quenching is performed while the melt is rolled using cooling rolls.
2. The method for producing a hexagonal strontium ferrite powder according to claim 1, further comprising removing the glass raw material component from the crystallized product by acid treatment.