JP7547445B2 - Magnetoplumbite-type hexagonal ferrite magnetic powder for radio wave absorbers and its manufacturing method, and radio wave absorbers and its manufacturing method - Google Patents

Description

The present invention relates to magnetoplumbite-type hexagonal ferrite magnetic powder and a manufacturing method thereof, and also to a radio wave absorber containing magnetoplumbite-type hexagonal ferrite magnetic powder and a manufacturing method thereof.

In recent years, with the advancement of information and communication technology, radio waves in the GHz band have come to be used for a variety of purposes. Examples of applications using such high-frequency technology include mobile phones, wireless LANs, satellite broadcasting, intelligent road transport systems, non-stop automatic toll collection systems (ETC), and automated highway systems (AHS). As the use of radio waves in the high-frequency range becomes more diverse, there are concerns about failures, malfunctions, and malfunctions due to interference between electronic components, and measures to ensure electromagnetic compatibility (EMC) become important. One effective method is to use a radio wave absorber to absorb unnecessary radio waves and prevent them from being reflected or entering.

As such a magnetic powder for a radio wave absorber, Patent Document 1 discloses a strontium ferrite particle powder which is a magnetoplumbite-type hexagonal ferrite powder represented by the composition formula AFe _(12-x) Al _x O ₁₉ (where A is one or more of Sr, Ba, Ca and Pb, and x: 1.0 to 2.2) and has a peak particle size of 10 μm or more in a laser diffraction scattering particle size distribution, and discloses that the powder is used in a radio wave absorber for absorbing radio waves in the vicinity of 76 GHz. In addition, in the examples, a strontium ferrite particle powder in which A in the composition formula is Sr is disclosed. In addition, as a method for producing the strontium ferrite particle powder, a production method is disclosed in which raw material powders for the strontium ferrite particle powder are mixed, fired, and then pulverized.

JP 2007-250823 A

As mentioned above, frequency bands are assigned according to various applications, and radio wave absorbers corresponding to each frequency band are being developed. In addition, radio wave absorbers for absorbing radio waves (millimeter waves) in the 60 to 90 GHz band, including the 76 GHz band, are being considered for practical use in on-board radar applications that detect information such as vehicle distance. However, in order to put radio wave absorbers into practical use, they are required to exhibit stable radio wave absorption performance over a wide temperature range, from room temperature to high temperatures of 100°C or higher. In general, radio wave absorption performance has a tendency to reach a maximum value in a certain frequency range, and the amount of absorption gradually decreases when the frequency deviates from that frequency range. Therefore, in order to exhibit stable radio wave absorption performance over a wide temperature range, it is desirable for the change in the maximum frequency value with respect to temperature to be small.

In this regard, the magnetoplumbite-type hexagonal ferrite magnetic powder of Patent Document 1 is a material that has radio wave absorption capacity around 76 GHz, but has a problem in that the peak frequency at high temperatures differs significantly from that at room temperature. The object of the present invention is to provide a magnetoplumbite-type hexagonal ferrite magnetic powder that has radio wave absorption capacity in the 60 to 90 GHz band, including the 76 GHz band, and has a small change in peak frequency over a wide temperature range, a manufacturing method thereof, and a radio wave absorber that uses the magnetic powder and a manufacturing method thereof.

In order to solve the above problems, the present inventors have conducted extensive research. The resonance frequency of a radio wave absorber is a value specific to the composition, and magnetoplumbite-type hexagonal ferrite magnetic powder also shows different resonance frequencies depending on the substitution elements and substitution ratios. It has been confirmed that magnetoplumbite-type hexagonal ferrite magnetic powder that satisfies the general formula A _(1-x) La _x Fe _(ny-z) Al _y Co _z (wherein A is one or more selected from the group consisting of Sr, Ba, and Ca, and 0.01≦x≦0.70, 1.00≦y≦2.20, 11.00≦n≦12.50, and 0.00≦z≦1.00) showing the atomic ratio of metal elements contained in the single-phase crystal structure of magnetoplumbite-type hexagonal ferrite magnetic powder shows a small change in peak frequency over a wide temperature range. That is, the gist of the present invention is as follows.

(1) A magnetoplumbite-type hexagonal ferrite magnetic powder, which satisfies the general formula showing the atomic ratio of metal elements: A _{(1-x) LaxFe (ny-z) AlyCoz} (wherein A is one or more selected from the group consisting of Sr, Ba, and Ca, and 0.01≦x _≦ 0.70, 1.00≦y≦2.20, 11.00≦n≦12.50, and 0.00≦z≦1.00).

(2) The range of x is
0.03≦x≦0.70;
The magnetoplumbite-type hexagonal ferrite magnetic powder according to (1) above.

(3) The range of x is
0.10≦x≦0.70;
The magnetoplumbite-type hexagonal ferrite magnetic powder according to (1) above.

(4) The magnetoplumbite-type hexagonal ferrite magnetic powder according to any one of (1) to (3), wherein the cumulative 50% particle size (D ₅₀ ) is 1.0 μm or more and 10.0 μm or less in a volume-based particle size distribution measured with a laser diffraction particle size distribution measuring device.

(5) The magnetoplumbite-type hexagonal ferrite magnetic powder according to (4) above, having a cumulative 50% particle size (D ₅₀ ) of 1.0 μm or more and 5.0 μm or less in a volume-based particle size distribution measured with a laser diffraction particle size distribution measuring device.

(6) The magnetoplumbite hexagonal ferrite magnetic powder according to any one of (1) to (5), wherein 0.36 g of the magnetoplumbite hexagonal ferrite magnetic powder and 0.84 g of microcrystalline cellulose are mixed to obtain a mixed powder, which is then pressure-molded at 151 MPa to produce a green compact having a diameter of 13 mm. The green compact thus obtained is subjected to measurement of transmission attenuation at temperatures of ₃₀ ° C., ₆₀ ° C., _{90° C.} and ₁₂₀ ° C. using terahertz wave time domain spectroscopy, and the respective peak frequencies are X ₃₀ , X ₆₀ , X ₉₀ and X _120. The frequency range R, which is the difference between the maximum and minimum values of X 30 , X 60 , X 90 and X 120, is 2.5 GHz or less.

(7) The magnetoplumbite-type hexagonal ferrite magnetic powder according to (6), in which the frequency range R is 2.5 GHz or less.

(8) The magnetoplumbite-type hexagonal ferrite magnetic powder according to any one of (1) to (7), wherein the metal element A is one or more selected from Sr and Ba.

(9) The magnetoplumbite-type hexagonal ferrite magnetic powder according to any one of claims 1 to 8, having a specific surface area of 0.5 m ² /g or more and 8.0 m ² /g or less.

(10) In a particle size distribution based on the number of particles measured by a laser diffraction particle size distribution measuring device, the most frequent diameter is 1.0 μm or less and the most frequent diameter is smaller than a cumulative 50% particle size (d ₅₀ ). The magnetoplumbite hexagonal ferrite magnetic powder according to any one of claims 1 to 9.

(11) The range of n is
11.00≦n<12.00;
The magnetoplumbite type hexagonal ferrite magnetic powder according to any one of claims 1 to 10.

(12) A radio wave absorber comprising the magnetoplumbite-type hexagonal ferrite magnetic powder described in any one of (1) to (11) above and a resin.

(13) A method for producing magnetoplumbite type hexagonal ferrite magnetic powder, comprising: a raw material mixing step of mixing powders that are raw materials for magnetoplumbite type hexagonal ferrite magnetic powder to obtain a raw material mixture; a firing step of firing the raw material mixture to obtain a fired product; and a pulverization step of pulverizing the fired product to obtain the magnetoplumbite type hexagonal ferrite magnetic powder, wherein the magnetoplumbite type hexagonal ferrite magnetic powder satisfies the general formula A _{(1-x) LaxFe (ny-y-z) AlyCoz (} wherein A is one or more selected from the group consisting of Sr, Ba, and Ca, and 0.01≦x≦0.70, 1.00≦y≦2.20, 11.00≦n≦12.50, and 0.00≦z≦1.00), which indicates the atomic ratio of metal elements.

(14) The method for producing magnetoplumbite-type hexagonal ferrite magnetic powder according to (13), wherein the range of x is 0.03≦x≦0.70.

(15) A method for producing magnetoplumbite-type hexagonal ferrite magnetic powder according to (13), in which the range of x is 0.10≦x≦0.70.

(16) The method for producing magnetoplumbite-type hexagonal ferrite magnetic powder according to any one of (13) to (15), wherein in the pulverization step, the magnetoplumbite-type hexagonal ferrite magnetic powder is pulverized so that the cumulative 50% particle size (D ₅₀ ) is 1.0 μm or more and 10.0 μm or less in a particle size distribution on a volume basis measured by a laser diffraction particle size distribution measuring device.

(17) A method for producing magnetoplumbite-type hexagonal ferrite magnetic powder according to any one of (13) to (16), further comprising a heat treatment after the grinding step.

(18) A method for producing a radio wave absorber, comprising a step of kneading the magnetoplumbite-type hexagonal ferrite magnetic powder obtained by the method described in (13) to (17) above with a resin and then molding the mixture.

The present invention provides magnetoplumbite-type hexagonal ferrite magnetic powder that has radio wave absorption capability in the 60 to 90 GHz band, including the 76 GHz band, and exhibits little change in peak frequency over a wide temperature range, a method for producing the same, and a radio wave absorber that uses the magnetic powder and a method for producing the same.

FIG. 2 is a diagram illustrating a manufacturing flow according to an embodiment of the present invention. 1 is a profile showing radio wave absorption in a frequency band including the 76 GHz band for Comparative Example 2, Examples 7, and 12.

(Magnetoplumbite type hexagonal ferrite magnetic powder)
The magnetoplumbite hexagonal ferrite magnetic powder of the present invention is _a magnetoplumbite hexagonal ferrite magnetic powder as a composite oxide, which satisfies the general formula A _{(1-x) LaxFe (ny-z) AlyCoz} (wherein A is at least one selected from the group consisting of Sr, Ba and Ca, and 0.01≦x≦0.7, 1.0≦y≦2.2, 11.0≦n≦12.5, 0.0≦z≦1.0, more specifically 0.01≦x≦0.70, 1.00≦y≦2.20, 11.00≦n≦12.50, 0.00≦z≦1.00). The composition, particle size distribution, and other aspects of the magnetoplumbite hexagonal ferrite magnetic powder of the present invention will be described below.

[Atomic ratio]
The magnetoplumbite-type hexagonal ferrite magnetic powder of the present invention is a hexagonal ferrite magnetic powder having a magnetoplumbite _- type crystal structure represented by the general formula A _{(1-x) LaxFe (ny-y-z) AlyCoz} (wherein A is one or more selected from the group consisting of Sr, Ba and Ca, and 0.01≦x≦0.70, 1.00≦y≦2.20, 11.00≦n≦12.50, and 0.00≦z≦1.00), which indicates the atomic ratio of metal elements.

According to the present invention, in the case of a commonly known strontium ferrite (SrFe ₁₂ O ₁₉ ) having a magnetoplumbite crystal structure as a skeleton, the effect of substitution by each element shown in the above general formula can be explained as follows. First, the absorption frequency can be adjusted by substituting Al for the Fe site. Here, while the absorption frequency shifts to the high frequency side with increasing temperature when only Al is substituted, the temperature dependency of the adjusted absorption frequency can be reduced by further substituting La for the Sr site. In addition, the temperature dependency can be reduced by substituting Co for the Fe site while substituting La for the Sr site. Based on this principle, it is possible to obtain a magnetoplumbite-type hexagonal ferrite magnetic powder having radio wave absorption capability in the 60 to 90 GHz band including the 76 GHz band and having a small change in peak frequency over a wide temperature range.
In order to obtain the effect of controlling the peak frequency and reducing the temperature dependency of the peak frequency, in the general formula showing the above atomic ratio, for La substitution, the value of x is 0.01 or more, for Al and Co substitution, the value of y is 1.0 or more, and the value of z is 0.0 or more, and more specifically, for La substitution, the value of x is 0.01 or more, and for Al and Co substitution, the value of y is 1.00 or more, and the value of z is 0.00 or more.
On the other hand, if La, Al, or Co is added in excess, it becomes difficult to maintain the crystal structure, so in the general formula showing the above atomic ratio, for La substitution, the value of x is 0.7 or less, for Al and Co substitution, the value of y is 2.2 or less, and the value of z is 1.0 or less, and more specifically, for La substitution, the value of x is 0.70 or less, for Al and Co substitution, the value of y is 2.20 or less, and the value of z is 1.00 or less.
x is preferably 0.03 or more, more preferably 0.1 or more, even more preferably 0.3 or more, and preferably 0.6 or less, and more preferably 0.5 or less, and more specifically, it is preferably 0.03 or more, more preferably 0.10 or more, even more preferably 0.30 or more, and preferably 0.60 or less, and more preferably 0.50 or less. Here, since the effect of La substitution is remarkable, even if the value of x is in the range of 0.01 to 0.10, a certain effect is obtained.
The numerical range of y is preferably 1.1 or more and 1.9 or less, more preferably 1.4 or more and 1.7 or less, and more specifically, it is preferably 1.10 or more and 1.90 or less, and more preferably 1.40 or more and 1.70 or less.
The numerical range of z is preferably 0.0 or more and 0.5 or less, more preferably 0.0 or more and 0.4 or less, and more specifically, it is preferably 0.00 or more and 0.50 or less, and more preferably 0.00 or more and 0.40 or less.

To obtain a hexagonal ferrite magnetic powder having a magnetoplumbite-type crystal structure, the value of n in the above atomic ratio is 11.0 or more and 12.5 or less, more specifically, 11.00 or more and 12.50 or less. In order to suppress the amount of unreacted material remaining after firing, the value of n is preferably 11.00 or more and less than 12.00, and may be 11.20 or more and 12.30 or less, or 11.40 or more and 12.20 or less.

The absorption frequency of the radio wave absorber is specific to the composition formula of the magnetic powder constituting the radio wave absorber, and the element ratio indicated by the composition formula does not refer to the element ratio as a mixture, but refers to the element ratio determined by a single crystal structure. The ratio indicated in the general formula of the atomic ratio of metal elements in the present invention refers to the atomic ratio of metal elements contained in the single-phase crystal structure of magnetoplumbite-type hexagonal ferrite magnetic powder, and each metal element is substituted at the Sr site and the Fe site as a component of the magnetoplumbite-type crystal structure. Note that if each metal element mixed as a raw material is not substituted at each site of magnetoplumbite-type hexagonal ferrite but remains outside it in another form (for example, Al ₂ O ₃ , etc.), the frequency of the magnetic powder mixture finally obtained will be significantly outside the desired absorption frequency range.

The magnetoplumbite-type hexagonal ferrite magnetic powder of the present invention may contain unavoidable components such as impurities contained in the raw materials and impurities derived from the manufacturing equipment. Examples of such components include oxides of Mn, etc. It is preferable to suppress the content of these to 0.40 mass% or less. The atomic ratios of the above metal elements are atomic ratios excluding unavoidable components.

[Particle size distribution]
In the magnetoplumbite hexagonal ferrite magnetic powder of the present invention, the cumulative 50% particle size (D ₅₀ ) on a volume basis measured by a laser diffraction particle size distribution measuring device can be used as an index of particle size. In the magnetoplumbite hexagonal ferrite magnetic powder of the present invention, in order to make the electromagnetic wave absorber sheet using the magnetic powder thinner, the cumulative 50% particle size (D ₅₀ ) is preferably 1 μm or more and 10 μm or less, more preferably 1 μm or more and 5 μm or less, and even more preferably 2 μm or more and less than 3 μm. In addition, in the magnetoplumbite hexagonal ferrite magnetic powder of the present invention, the most frequent diameter and cumulative 50% particle size on a number basis measured by a laser diffraction particle size distribution measuring device (for convenience, this is referred to as d ₅₀ to distinguish it from the cumulative 50% particle size on a volume basis. Hereinafter, the lower case letter d is used to indicate the particle size on a number basis) can also be used as an index of particle size. In order to easily uniformize the thickness of the thin film when it is formed into a sheet, the magnetoplumbite-type hexagonal ferrite magnetic powder of the present invention preferably has a mode diameter in the number distribution of 1.0 μm or less and a mode diameter smaller than the cumulative 50% particle diameter (d ₅₀ ). Here, the specific surface area (BET) is preferably 0.5 m ² /g or more and 8.0 m ² /g or less. The specific surface area (BET) is more preferably 0.6 m ² /g or more and 3.0 m ² /g or less, and even more preferably 0.9 m ² /g or more and 2.2 m ² /g or less. As the particles are crushed or become small, the specific surface area increases, but if the specific surface area exceeds 8.0 m ² /g, the viscosity increases when mixed with the resin, making it difficult to mix and making it difficult to disperse uniformly, which is not preferable. Moreover, a specific surface area of less than 0.5 m ² /g means that there are many coarse particles, and when the powder is made into a sheet, there are many areas where the packing density is low, which is not preferable. Similarly, in the particle size distribution based on the number of particles, if the mode diameter is 1.0 μm or more, or if the mode diameter is greater than the cumulative 50% particle diameter (d ₅₀ ), there are fewer small particles, but conversely, considering the packing property of the particles when the powder is made into a sheet, there are fewer particles that fill the gaps between the larger particles, which is not preferable. By measuring the specific surface area, it is possible to confirm the sintering state of the fine powder, which is difficult to confirm with a laser diffraction type particle size distribution measuring device, and by setting the specific surface area within the above numerical range, it is expected to have the effect of uniformly dispersing the magnetoplumbite type hexagonal ferrite magnetic powder in resin or rubber when producing a radio wave absorber.

[Transmission attenuation]
The magnetoplumbite hexagonal ferrite magnetic powder of the present invention has radio wave absorption capacity in the 60-90 GHz band including the 76 GHz band as a compact, and is used as a radio wave absorber with small change in peak frequency over a wide temperature range. Here, the radio wave absorption capacity is measured by pressing a mixed powder obtained by mixing 0.36 g of magnetoplumbite hexagonal ferrite magnetic powder and 0.84 g of microcrystalline cellulose at 151 MPa to produce a compact with a diameter of 13 mm, using terahertz wave time domain spectroscopy. When the peak frequencies of the transmission attenuation measured at 30°C, 60°C, 90°C, and 120°C are _X30 , _X60 , _X90 , and _X120 , respectively, it is preferable that _X30 is 60 GHz or more and 90 GHz or less. In addition, when the difference between the maximum and minimum values of X ₃₀ , X ₆₀ , X ₉₀ and X ₁₂₀ is defined as the frequency range R, the frequency range R is preferably 2.5 GHz or less. In addition, when the difference between the maximum and minimum values of X ₃₀ , X ₆₀ , X ₉₀ and X ₁₂₀ is defined as the frequency range R, the frequency range R is more preferably 2.4 GHz or less, even more preferably 2.0 GHz or less, and particularly preferably 1.0 GHz or less. The compact made of magnetoplumbite-type hexagonal ferrite magnetic powder of the present invention is used mainly as a radio wave absorber in the 60 to 90 GHz band including the 76 GHz band, so it is required to have stable radio wave absorption characteristics in a wide range. In addition, since it is used as a radio wave absorber, it is preferable that the transmission attenuation is 6 dB or more at all peak frequencies.

(Preparation and evaluation of radio wave absorbers)
Moreover, a radio wave absorber can be manufactured by kneading the magnetoplumbite hexagonal ferrite magnetic powder of the above-mentioned embodiment with a resin. This radio wave absorber can be formed into various shapes depending on the application, but when manufacturing a sheet-shaped radio wave absorber (radio wave absorber sheet), the radio wave absorber material (kneaded product) obtained by kneading the magnetoplumbite hexagonal ferrite magnetic powder with a resin may be rolled to a desired thickness (preferably 0.1 to 4.0 mm, more preferably 0.2 to 2.5 mm) using a rolling roll or the like. Moreover, the content of the magnetoplumbite hexagonal ferrite magnetic powder in the radio wave absorber material (kneaded product) is preferably 70 to 95% by mass in order to obtain a radio wave absorber having radio wave absorbing ability in the 60 to 90 GHz band including the 76 GHz band. Moreover, the content of the resin in the radio wave absorber material (kneaded product) is preferably 5 to 30% by mass in order to sufficiently disperse the magnetoplumbite hexagonal ferrite magnetic powder in the radio wave absorber material (kneaded product). The total content of the magnetoplumbite hexagonal ferrite magnetic powder and the resin in the radio wave absorber material (kneaded product) is preferably 99 mass % or more.

(Method of manufacturing magnetoplumbite-type hexagonal ferrite magnetic powder)
The method for producing magnetoplumbite type hexagonal ferrite magnetic powder of the present invention includes at least a raw material mixing step of mixing raw material powders to obtain a raw material mixture, a sintering step of sintering the raw material mixture to obtain a sintered product, and a pulverizing step of pulverizing the sintered product to obtain the magnetoplumbite type hexagonal ferrite magnetic powder, wherein the magnetoplumbite type hexagonal ferrite magnetic powder has a general formula: A _(1-x) La _x Fe _(ny-y-z) Al _y Co _z which indicates the atomic ratio of metal elements. (wherein A is one or more selected from the group consisting of Sr, Ba, and Ca, and satisfies 0.01≦x≦0.7, 1.0≦y≦2.2, 11.0≦n≦12.5, and 0.0≦z≦1.0, more specifically, 0.01≦x≦0.70, 1.00≦y≦2.20, 11.00≦n≦12.50, and 0.00≦z≦1.00, and further, x is preferably 0.03 or more, and more preferably 0.10 or more).

With reference to Figure 1, each step of the manufacturing method of magnetoplumbite-type hexagonal ferrite magnetic powder according to the present invention will be described in detail below.

[Raw material mixing process]
First, in the raw material mixing step, powders that are raw materials for magnetoplumbite- _type hexagonal ferrite magnetic powder and satisfy the general formula A _{(1-x) LaxFe (ny-z) AlyCoz} (where A is at least one selected from the group consisting of Sr, Ba, and Ca, and 0.01≦x≦0.70, 1.00≦y≦2.20, 11.00≦n≦12.50, and 0.00≦z≦1.00) that indicates the atomic ratio of metal elements are mixed to obtain a raw material mixture. In addition, x is preferably 0.03 or more, and more preferably 0.10 or more. The raw material powder is not particularly limited, and powders such as SrCO ₃ , BaCO ₃ , BaCl _{2.2H 2} O, CaCO ₃ , La(OH) ₃ , Fe ₂ O ₃ , Al ₂ O ₃ and Co ₃ O ₄ can be used. In addition, the method for mixing the raw material powder is not particularly limited, and mixing can be performed using a known mixing device such as a Henschel mixer.

In addition, a process may be provided in which the obtained raw material mixture is granulated to obtain a molded body. The granulation method is not particularly limited, and any method may be used to mold the molded body into pellets. If the granulated molded body contains moisture, a drying process may be further provided after the process.

[Firing process]
The resulting raw material mixture is then fired in a firing step to obtain a fired product. The firing is preferably carried out in any firing furnace at a temperature of 1150°C to 1400°C, more preferably 1170°C to 1350°C, and even more preferably 1200°C to 1300°C. For example, when a box-type firing furnace is used, the raw materials can be filled into a firing container. The firing atmosphere is preferably an oxidizing atmosphere, and the oxidizing atmosphere is preferably an atmosphere such as air, oxygen, a mixed gas of oxygen and nitrogen, or a mixed gas of oxygen and a rare gas.

[Crushing process]
Next, the obtained sintered product is pulverized in a pulverization step to obtain magnetoplumbite-type hexagonal ferrite magnetic powder. In the pulverization step, coarse pulverization and fine pulverization may be performed. Here, coarse pulverization refers to disintegrating the sintered product, and any pulverization method such as impact pulverization using a hammer mill can be used. In addition, fine pulverization refers to further reducing the sintered product after coarse pulverization to a finer state, and any method such as wet pulverization using an attritor can be used. The slurry after wet pulverization is subjected to solid-liquid separation and drying by any method to obtain magnetoplumbite-type hexagonal ferrite magnetic powder.

In addition, the magnetoplumbite-type hexagonal ferrite magnetic powder obtained in the pulverization process can be heat-treated by any heat treatment method in the heat treatment process. The heat treatment temperature is preferably 850°C or higher and 1000°C or lower, more preferably 870°C or higher and 930°C or lower. The atmosphere during the heat treatment is preferably an oxidizing atmosphere, more preferably an air atmosphere. By performing heat treatment, the fine magnetoplumbite-type hexagonal ferrite magnetic powder is sintered and the specific surface area is reduced, so that the magnetoplumbite-type hexagonal ferrite magnetic powder can be uniformly dispersed in resin or rubber when manufacturing a radio wave absorber. In addition, although the direct effect on the absorption frequency is unknown, the heat treatment removes the crystal distortion generated in the pulverization process, and magnetic properties such as coercive force Hc are restored.

(Fabrication of radio wave absorber)
Moreover, the obtained magnetoplumbite hexagonal ferrite magnetic powder can be kneaded with a resin to produce a radio wave absorber. This radio wave absorber can be formed into various shapes depending on the application, but when producing a sheet-shaped radio wave absorber (radio wave absorber sheet), the radio wave absorber material (kneaded product) obtained by kneading the magnetoplumbite hexagonal ferrite magnetic powder with a resin may be rolled to a desired thickness (preferably 0.1 to 4.0 mm, more preferably 0.2 to 2.5 mm) using a rolling roll or the like.

The magnetoplumbite-type hexagonal ferrite magnetic powder and its manufacturing method according to the present invention will be described in detail below with reference to examples.

The evaluations in the examples were carried out as follows.
[Particle size distribution and cumulative 50% particle size]
The particle size distribution of the magnetoplumbite-type hexagonal magnetic powder was measured by dry dispersion using a laser diffraction particle size distribution measuring device (HELOS & RODOS, manufactured by JEOL Ltd.) at a focal length of 200 mm, a dispersion pressure of 1.7 bar, and a suction pressure of 130 mbar. From the measurement results obtained, the cumulative 50% particle size ( _D50 ) based on volume and the cumulative 50% particle size ( _d50 ) based on number were obtained. Similarly, the cumulative 10% particle size ( _D10 and _d10 ) and the cumulative 90% particle size ( _D90 and _d90 ) were obtained, and the most frequent diameter was also confirmed.

[Specific surface area measurement]
The specific surface area of the magnetoplumbite type hexagonal magnetic powder was measured by the BET single point method using a specific surface area measuring device (Macsorb model-1210 manufactured by Mountec Co., Ltd.).

[Composition Analysis]
The composition analysis was performed using a high-frequency induction plasma emission spectrometer ICP (720-ES) manufactured by Agilent Technologies, Inc. The measurement wavelengths were Sr: 216.596 nm, La: 408.671 nm, Fe: 259.940 nm, Al: 396.152 nm, Ba: 233.527 nm, and Co: 230.786 nm.

[Magnetic properties]
As the magnetic properties of the magnetoplumbite-type hexagonal magnetic powder, a vibrating sample magnetometer (VSM) (VSM-P7 manufactured by Toei Kogyo Co., Ltd.) was used to measure the M-H curve at an applied magnetic field of 1193 kA/m (15 kOe), and the coercive force Hc, saturation magnetization σs, squareness ratio SQ, and coercive force distribution SFD were obtained.

[Crystal structure]
The X-ray diffraction measurement of the magnetoplumbite-type hexagonal magnetic powder was performed by powder X-ray diffraction (XRD) using a powder X-ray diffractometer (horizontal multipurpose X-ray diffractometer Ultima IV manufactured by Rigaku Corporation) with a CuKα ray source, a tube voltage of 40 kV, a tube current of 40 mA, and a measurement range of 2θ = 20° to 50°.

[Measurement of radio wave absorption characteristics]
0.36 g of magnetoplumbite-type hexagonal magnetic powder and 0.84 g of microcrystalline cellulose were mixed and the resulting mixed powder was pressed at 151 MPa to obtain a compact with a diameter of 13 mm. The resulting compact was subjected to transmission attenuation measurement by terahertz wave time domain spectroscopy to obtain the peak frequency of the compact. Specifically, a terahertz spectroscopy system TAS7400SL manufactured by Advantest Corporation was used to perform measurements when the compact was placed on a sample holder and when it was blank. Using a temperature control module TAS1030 manufactured by Advantest Corporation, the compact was heated to 30°C, 60°C, 90°C, and 120°C, and the transmission attenuation was measured at each temperature to obtain the frequency (frequency peak value, unit: GHz) showing the maximum transmission attenuation at 50 to 100 GHz. The conditions used are listed below.
Sample holder diameter: φ10mm
・MeasurementMode:Transmission
・Frequency Resolution: 1.9GHz
・Vertical Axis: Absorption
・Horizontal Axis: Frequency [THz]
・Cumulated Number (Sample): 2048
・Cumulated Number (Background): 2048

The observed signal waveform of the sample and the blank reference waveform were expanded to 8448 ps and Fourier transformed, and the ratio (Ssig/Sref) of the obtained Fourier spectra (referred to as Ssig and Sref, respectively) was obtained, and the transmission attenuation of the powder compact placed in the sample holder was calculated. As an example of the main measurement results, the profiles showing the radio wave absorption in the frequency band including the 76 GHz band for Examples 7, 12, and Comparative Example 2 are shown in Figure 2.

Example 1
First, 636 _g of SrCO3 with a purity of 99% by mass, 392 _g of _Al2O3 with a purity of 99.9% by mass, 3880 _g of _Fe2O3 with a purity of 99% by mass, and 90 g of La(OH) ₃ with a purity of 99.99% by mass were weighed as raw material powders. The raw material powders were mixed using a Henschel mixer, and then further mixed using a vibrating mill by a dry method. The molar ratio of Sr, La, Fe, and Al in the raw material powder was Sr:La:Fe:Al = 0.90:0.10:10.15:1.62. The mixed powder thus obtained was granulated into pellets to obtain a molded body, and then 2 kg of the molded body was filled into a sintering sheath, which was then placed in a box-shaped sintering furnace and sintered at 1279 ° C. for 4 hours in the atmosphere. The sintered body obtained by this sintering was coarsely pulverized in a hammer mill, and the resulting coarse powder was then wet-pulverized for 70 minutes in an attritor using water as a solvent, the resulting slurry was subjected to solid-liquid separation, and the resulting cake was dried and crushed to obtain magnetoplumbite-type hexagonal ferrite magnetic powder (hereinafter simply referred to as "magnetic powder").

The magnetic powder obtained in this manner was first subjected to composition analysis to evaluate the physical properties, determining the BET specific surface area and particle size distribution, and then subjected to X-ray diffraction (XRD) measurement. After measuring the magnetic properties and the transmission attenuation of the powder compact, the frequency range R was determined. As a result of the XRD measurement, it was confirmed that the magnetic powder obtained in this example has a magnetoplumbite-type crystal structure, and no crystal phases other than magnetoplumbite-type crystals were confirmed. Similar results were also obtained for Examples 2 to 32.

Example 2
The magnetic powder obtained in Example 1 was further heated in an electric muffle furnace (FUW253PB manufactured by Advantec Toyo Co., Ltd.) at 900°C for 20 minutes in an air atmosphere to obtain a heat-treated magnetic powder. The magnetic powder thus obtained was subjected to composition analysis in the same manner as in Example 1 to determine the BET specific surface area and particle size distribution, and was also subjected to X-ray diffraction (XRD) measurement. Then, the magnetic properties were measured and the transmission attenuation of the compact was measured, and the frequency range R was then obtained.

Example 3
A magnetic powder was prepared under the same conditions as in Example 1, except that the molar ratio of Sr, La, Fe, and Al in the raw powder was Sr:La:Fe:Al = 0.80:0.20:10.15:1.62, and the wet grinding time after sintering was 60 minutes. The magnetic powder was prepared under the same conditions as in Example 1, and evaluated under the same conditions as in Example 1.

Example 4
The magnetic powder obtained in Example 3 was further heat-treated at 900° C. for 20 minutes to obtain a heat-treated magnetic powder. The obtained magnetic powder was evaluated under the same conditions as in Example 1.

Example 5
A magnetic powder was prepared under the same conditions as in Example 1, except that the molar ratio of Sr, La, Fe, and Al in the raw powder was Sr:La:Fe:Al = 0.71:0.29:10.15:1.62, and the wet grinding time after sintering was 35 minutes. The magnetic powder was prepared under the same conditions as in Example 1, and evaluated under the same conditions as in Example 1.

Example 6
The magnetic powder obtained in Example 5 was further heat-treated at 900° C. for 20 minutes to obtain a heat-treated magnetic powder. The obtained magnetic powder was evaluated under the same conditions as in Example 1.

(Example 7)
A magnetic powder was prepared under the same conditions as in Example 1, except that the molar ratio of Sr, La, Fe, and Al in the raw powder was Sr:La:Fe:Al = 0.61:0.39:10.15:1.62, and the wet grinding time after sintering was 50 minutes. The magnetic powder was prepared under the same conditions as in Example 1, and evaluated under the same conditions as in Example 1.

(Example 8)
The magnetic powder obtained in Example 7 was further heat-treated at 900° C. for 20 minutes to obtain a heat-treated magnetic powder. The obtained magnetic powder was evaluated under the same conditions as in Example 1.

Example 9
The molar ratio of Sr, La, Fe, and Al in the raw material powder was Sr:La:Fe:Al=0.61:0.39:9.45:1.56, _{La2O3 was} used as the La raw material instead of La(OH) ₃ , and the wet grinding time after firing was 50 minutes. Except for this, the magnetic powder was produced under the same conditions as in Example 1, and evaluated under the same conditions as in Example 1.

Example 10
A magnetic powder was produced under the same conditions as in Example 1, except that the molar ratio of Sr, La, Fe, and Al in the raw material powder was Sr:La:Fe:Al = 0.51:0.49:10.15:1.62, the sintering temperature was 1270°C, and the wet-pulverizing time after sintering was 55 minutes, and the magnetic powder was evaluated under the same conditions as in Example 1.

(Example 11)
A magnetic powder was produced under the same conditions as in Example 1, except that the molar ratio of Sr, La, Fe, and Al in the raw material powder was Sr:La:Fe:Al = 0.41:0.59:10.15:1.62, the sintering temperature was 1270°C, and the wet-pulverizing time after sintering was 60 minutes. The magnetic powder was produced under the same conditions as in Example 1, and evaluated under the same conditions as in Example 1.

Example 12
As raw material powders, 595 _g of SrCO3 with a purity of 99% by mass, 469 _g of _Al2O3 with a purity of 99.9% by mass, 4487 _g of _Fe2O3 with a purity of 99% by mass, 316 g of La(OH) ₃ with a purity of 99.99% by mass, and 133 _g of _Co3O4 with a purity of 99.99% by mass were weighed. After mixing the raw material powders with a Henschel mixer, they were further mixed by a dry method with a vibration mill. The molar ratio of Sr, La, Fe, and Al in the raw material powder was Sr:La:Fe:Al:Co=0.71:0.29:9.84:1.63:0.29. Except for the time of wet grinding after firing being 60 minutes, magnetic powders were produced under the same conditions as in Example 1 and evaluated under the same conditions as in Example 1.

(Example 13)
The magnetic powder obtained in Example 12 was further heat-treated at 900° C. for 20 minutes to obtain a heat-treated magnetic powder. The obtained magnetic powder was evaluated under the same conditions as in Example 1.

(Example 14)
As raw material powders, 467 g of SrCO ₃ with a purity of 99% by mass, 454 g of Al ₂ O ₃ with a purity of 99.9% by mass, 4508 g of Fe ₂ O ₃ with a purity of 99% by mass, 410 g of La(OH) ₃ with a purity of 99.99% by mass, and 160 g of BaCl _{2.2H 2} O with a purity of 99% by mass were weighed. After mixing these raw material powders with a Henschel mixer, they were further mixed by a dry method with a vibration mill. The molar ratio of Sr, La, Fe, and Al in this raw material powder was Sr:La:Ba:Fe:Al=0.53:0.36:0.11:9.41:1.50. Other than that, the time of wet grinding after firing was 55 minutes and the firing temperature was 1270° C., and the magnetic powder was produced under the same conditions as in Example 1 and evaluated under the same conditions as in Example 1.

(Example 15)
A magnetic powder was produced under the same conditions as in Example 1, except that the molar ratio of Sr, La, Fe, and Al in the raw material powder was Sr:La:Fe:Al = 0.71:0.29:10.59:1.18, the sintering temperature was 1270°C, and the wet grinding time after sintering was 10 minutes. The magnetic powder was produced under the same conditions as in Example 1, and evaluated under the same conditions as in Example 1.

(Example 16)
A magnetic powder was produced under the same conditions as in Example 15, except that the time for wet pulverization after firing was 20 minutes, and evaluated under the same conditions as in Example 1.

(Example 17)
A magnetic powder was produced under the same conditions as in Example 15, except that the time for wet pulverization after firing was 30 minutes, and evaluated under the same conditions as in Example 1.

(Example 18)
A magnetic powder was produced under the same conditions as in Example 15, except that the time for wet pulverization after firing was 40 minutes, and evaluated under the same conditions as in Example 1.

(Example 19)
A magnetic powder was produced under the same conditions as in Example 15, except that the time for wet pulverization after firing was 55 minutes, and evaluated under the same conditions as in Example 1.

(Example 20)
A magnetic powder was prepared under the same conditions as in Example 1, except that the molar ratio of Sr, La, Fe, and Al in the raw material powder was Sr:La:Fe:Al = 0.53:0.47:9.80:1.96, the sintering temperature was 1270°C, and the wet grinding time after sintering was 10 minutes. The magnetic powder was prepared under the same conditions as in Example 1, and evaluated under the same conditions as in Example 1.

(Example 21)
A magnetic powder was produced under the same conditions as in Example 20, except that the time for wet pulverization after firing was 20 minutes, and evaluated under the same conditions as in Example 1.

(Example 22)
A magnetic powder was produced under the same conditions as in Example 20, except that the time for wet pulverization after firing was 30 minutes, and evaluated under the same conditions as in Example 1.

(Example 23)
A magnetic powder was produced under the same conditions as in Example 20, except that the time for wet pulverization after firing was 40 minutes, and evaluated under the same conditions as in Example 1.

(Example 24)
A magnetic powder was produced under the same conditions as in Example 20, except that the time for wet pulverization after firing was 50 minutes, and evaluated under the same conditions as in Example 1.

Furthermore, tests were conducted and evaluations were performed on the following Examples 25 to 32.

(Example 25)
A magnetic powder was produced under the same conditions as in Example 1, except that wet pulverization after firing was not carried out, and the magnetic powder was evaluated under the same conditions as in Example 1.

(Example 26)
As raw material powders, 595 _g of SrCO3 with a purity of 99% by mass, 469 _g of _Al2O3 with a purity of 99.9% by mass, 4596 _g of _Fe2O3 with a purity of 99% by mass, 316 g of La(OH) ₃ with a purity of 99.99% by mass, and 22 _g of _Co3O4 with a purity of 99.99% by mass were weighed. After mixing the raw material powders with a Henschel mixer, they were further mixed by a dry method with a vibration mill. The molar ratio of Sr, La, Fe, and Al in the raw material powder was Sr:La:Fe:Al:Co=0.71:0.29:10.08:1.63:0.05. A magnetic powder was produced under the same conditions as in Example 1, except that the time of wet grinding after firing was 60 minutes, and evaluated under the same conditions as in Example 1.

(Example 27)
As raw material powders, 507 g of SrCO ₃ with a purity of 99% by mass, 454 g of Al ₂ O ₃ with a purity of 99.9% by mass, 4508 g of Fe ₂ O ₃ with a purity of 99% by mass, 410 g of La(OH) ₃ with a purity of 99.99% by mass, and 84 g of BaCl _{2.2H 2} O with a purity of 99% by mass were weighed. After mixing these raw material powders with a Henschel mixer, they were further mixed by a dry method with a vibration mill. The molar ratio of Sr, La, Fe, and Al in this raw material powder was Sr:La:Ba:Fe:Al=0.58:0.37:0.06:9.47:1.51. Other than that, the time of wet grinding after firing was set to 55 minutes, and magnetic powders were produced under the same conditions as in Example 1, and evaluated under the same conditions as in Example 1.

(Example 28)
As raw material powders, 378 g of SrCO ₃ with a purity of 99% by mass, 454 g of Al ₂ O ₃ with a purity of 99.9% by mass, 4508 g of Fe ₂ O ₃ with a purity of 99% by mass, 410 g of La(OH) ₃ with a purity of 99.99% by mass, and 299 g of BaCl _{2.2H 2} O with a purity of 99% by mass were weighed. After mixing the raw material powders with a Henschel mixer, they were further mixed by a dry method with a vibration mill. The molar ratio of Sr, La, Fe, and Al in the raw material powder was Sr:La:Ba:Fe:Al=0.43:0.37:0.21:9.47:1.51. A magnetic powder was produced under the same conditions as in Example 1, except that the time of wet grinding after firing was 55 minutes, and evaluated under the same conditions as in Example 1.

(Example 29)
A magnetic powder was produced under the same conditions as in Example 15, except that wet pulverization after firing was not carried out, and was evaluated under the same conditions as in Example 1.

(Example 30)
A magnetic powder was produced under the same conditions as in Example 15, except that the time for wet pulverization after firing was 120 minutes, and evaluated under the same conditions as in Example 1.

(Example 31)
A magnetic powder was produced under the same conditions as in Example 20, except that wet pulverization after firing was not carried out, and evaluated under the same conditions as in Example 1.

(Example 32)
A magnetic powder was prepared under the same conditions as in Example 1, except that the molar ratio of Sr, La, Fe, and Al in the raw powder was Sr:La:Fe:Al = 0.95:0.05:10.14:1.62, the sintering temperature was 1270°C, and the wet grinding time after sintering was 60 minutes. The magnetic powder was prepared under the same conditions as in Example 1, and evaluated under the same conditions as in Example 1.

(Comparative Example 1)
First, 515 g of _SrCO3 with a purity of 99% by mass, 284 _g of _Al2O3 with a purity of 99.9% by mass, and 2701 _g of _Fe2O3 with a purity of 99% by mass were weighed as raw material powders. After mixing these raw material powders with a Henschel mixer, they were further mixed by a dry method with a vibration mill. The molar ratio of Sr, Fe, and Al in this raw material powder was Sr:Fe:Al=1.00:9.71:1.61. The mixed powder thus obtained was used to prepare magnetic powders in the same manner as in Example 1, except that the firing temperature was set to 1260° C., and the magnetic powders were evaluated under the same conditions as in Example 1.

(Comparative Example 2)
First, 470 g of _SrCO3 with a purity of 99% by mass, 290 _{g of Al2O3} with a purity of 99.9% by mass, 2646 _g of _Fe2O3 with a purity of 99% by mass, and 93 g of _BaCl2.2H2O with a purity of 99 _% by mass were weighed as raw material powder. After mixing the raw material powder with a Henschel mixer, the raw material powder was further mixed by a dry method with a vibration mill. The molar ratio of Sr, Ba, Fe, and Al in the raw material powder was Sr:Ba:Fe:Al = 0.89:0.11:9.30:1.61. The mixed powder thus obtained was used to prepare a magnetic powder by the same method as in Example 1, except that the wet grinding was performed for 60 minutes, and the magnetic powder was evaluated under the same conditions as in Example 1.

The manufacturing conditions, evaluation results of the magnetic powder, and measurement results of the radio wave absorption characteristics in the above examples and comparative examples are shown in Table 1. In addition, while Table 1 shows only the volume-based particle size distribution D ₅₀ , the measurement results of volume-based D ₁₀ , D ₉₀ and most frequent diameter, and number-based d ₁₀ , d ₅₀ , d ₉₀ and most frequent diameter for Examples 29, 15 to 19, and 30 are shown in Table 2 as representative examples. Note that there is a slight discrepancy between the molar ratio in the raw material powder shown in each example and comparative example and the molar ratio in the composition formula shown in the evaluation results of the magnetic powder in Table 1, due to the unavoidable mixing of impurities during the manufacturing process, and these are essentially the same.

From the results in Table 1, when comparing Example 32 and Comparative Example 1 in the Sr system, it is seen that by substituting even a small amount of Sr with La, the frequency range R of the compact can be narrowed, and when the substitution amount x of La is 0.05, the frequency range R can be set to 2.5 GHz.
Furthermore, by comparing Example 1 and Comparative Example 1 in the Sr system, it is found that when the La substitution amount x is 0.10, the frequency range R can be set to 2.0 GHz.
Furthermore, in a comparison between Example 1 and Comparative Example 1 in the Sr system and in a comparison between Example 1 and Comparative Example 2 which further contains Ba, it is found that by substituting La, the frequency range R of the compact can be set to 2.0 GHz or less.
Furthermore, from the results of Examples 1 to 8, it was found that if the same feed composition was used, the frequency range R could be made 2.0 GHz or less even if heat treatment was performed on magnetic powder that had been treated in the same way up to the pulverization step.

Thus, it can be seen from the results of Examples 1 to 32 that the frequency peak can be controlled by the amount of Al substitution, while the frequency range R of the powder compact can be suppressed to 2.5 GHz or less.
It is also understood that by limiting the substitution amount of La, the frequency range R of the powder compact can be suppressed to 2.4 GHz or less, and by further limiting the substitution amount of La, the frequency range R of the powder compact can be suppressed to 2.0 GHz or less.

Furthermore, from the results in Table 2, it is evident that when the mode diameter in the particle size distribution on a number basis is 1.0 μm or less and is smaller than the cumulative 50% particle diameter (d ₅₀ ), a good radio wave absorber sheet can be obtained and the frequency range R of the compressed powder can be made small.

According to the present invention, by adjusting the substitution amounts of La, Al, and Co, it is possible to provide magnetoplumbite-type hexagonal ferrite magnetic powder that has radio wave absorption capacity in the 60 to 90 GHz band, including the 76 GHz band, and has small changes in peak frequency over a wide temperature range, a manufacturing method thereof, and a radio wave absorber using the magnetic powder and a manufacturing method thereof.

Claims (14)

A magnetoplumbite-type hexagonal ferrite magnetic powder,
General formula showing the atomic ratio of metal elements: A _(1-x) La _x Fe _(ny) Al _y
(where:
A is one or more selected from the group consisting of Sr, Ba, and Ca;
0.28 ≦x≦0.70,
1.00≦y≦2.20,
11.00≦n≦12.50)
Magnetoplumbite type hexagonal ferrite magnetic powder for radio wave absorbers. 2. The magnetoplumbite-type hexagonal ferrite magnetic powder for a radio wave absorber according to claim 1 , wherein the cumulative 50% particle size ( _D50 ) is 1.0 μm or more and 10.0 μm or less in a volume-based particle size distribution measured by a laser diffraction particle size distribution measuring device. 3. The magnetoplumbite-type hexagonal ferrite magnetic powder for a radio wave absorber according to claim 2 , wherein a cumulative 50% particle size ( _D50 ) is 1.0 μm or more and 5.0 μm or less in a volume-based particle size distribution measured with a laser diffraction particle size distribution measuring device. The magnetoplumbite-type hexagonal ferrite magnetic powder for a radio wave absorber according to any one of claims 1 to 3, wherein 0.36 g of the magnetoplumbite-type hexagonal ferrite magnetic powder and 0.84 g of microcrystalline cellulose are mixed to obtain a mixed powder, which is then pressure-molded at 151 MPa to produce a green compact having a diameter of 13 mm. The green compact thus obtained is subjected to measurement of transmission attenuation at each of temperatures of 30°C, ₆₀ °C, _90° C and ₁₂₀ °C using terahertz wave time domain spectroscopy, and the respective peak frequencies are _X30 , X60, X90 and X120. The _{magnetoplumbite} -type hexagonal ferrite magnetic powder for a radio wave absorber according to any one of claims 1 to 3 , wherein, when the respective peak frequencies are _X30 , X60, _X90 and _X120, a frequency range R which is a difference between the maximum and minimum values of X30, X60, X90 and X120 is 2.5 GHz or less. 5. The magnetoplumbite-type hexagonal ferrite magnetic powder for a radio wave absorber according to claim 4 , wherein the frequency range R is 2.4 GHz or less. 6. The magnetoplumbite-type hexagonal ferrite magnetic powder for a radio wave absorber according to claim 1 , wherein the metal element A is at least one element selected from the group consisting of Sr and Ba. The magnetoplumbite-type hexagonal ferrite magnetic powder for a radio wave absorber according to any one of claims 1 to 6 , having a specific surface area of 0.5 m ² /g or more and 8.0 m ² /g or less. The magnetoplumbite-type hexagonal ferrite magnetic powder for a radio wave absorber according to any one of claims 1 to 7, wherein in a particle size distribution on a number basis measured with a laser diffraction particle size distribution measuring device, the mode diameter is 1.0 µm or less and the mode diameter is smaller than a cumulative 50% particle diameter (d ₅₀ ) . The range of n is
11.00≦n<12.00;
The magnetoplumbite-type hexagonal ferrite magnetic powder for use in a radio wave absorber according to any one of claims 1 to 8 . A radio wave absorber comprising the magnetoplumbite hexagonal ferrite magnetic powder for radio wave absorbers according to any one of claims 1 to 9 and a resin. a raw material mixing step of mixing powders that are raw materials for the magnetoplumbite-type hexagonal ferrite magnetic powder to obtain a raw material mixture;
A firing step of firing the raw material mixture to obtain a fired product;
A pulverization step of pulverizing the sintered product to obtain the magnetoplumbite-type hexagonal ferrite magnetic powder,
The magnetoplumbite type hexagonal ferrite magnetic powder has a general formula showing the atomic ratio of metal elements: A _(1-x) La _x Fe _(ny) Al _y
(where:
A is at least one selected from the group consisting of Sr, Ba, and Ca;
0.28 ≦x≦0.70,
1.00≦y≦2.20,
11.00≦n≦12.50)
A manufacturing method for magnetoplumbite-type hexagonal ferrite magnetic powder for radio wave absorbers. 12. The method for producing a magnetoplumbite-type hexagonal ferrite magnetic powder for a radio wave absorber according to claim 11, wherein in the pulverization step, the magnetoplumbite-type hexagonal ferrite magnetic powder is pulverized so that a cumulative 50% particle size ( _D50 ) in a volume-based particle size distribution measured by a laser diffraction particle size distribution measuring device is 1.0 μm or more and 10.0 μm or less. The method for producing magnetoplumbite-type hexagonal ferrite magnetic powder for a radio wave absorber according to claim 11 or 12 , further comprising a heat treatment step after the pulverization step. A method for producing a radio wave absorber, comprising a step of kneading the magnetoplumbite-type hexagonal ferrite magnetic powder for a radio wave absorber obtained by the method according to any one of claims 11 to 13 with a resin, and then molding the mixture.

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