JP2002217586A - Radio wave absorber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a radio wave absorber, having wide absorbing characteristics up to high-frequency electromagnetic waves and small environmental load. SOLUTION: The radio wave absorber is constituted, by mixing any of magnetic material particles having a discaidal or elliptical shape or magnetic material particles, generated by coating a magnetic material having a saturated magnetization of 0.5 T or higher on a surface of a magnetic material, having an in-plane magnetic anisotropy, with an organic coupling material having biodegradability or photodecomposition properties. This absorber is realized, for example, as a radio wave absorbing sheet 3, formed into a sheet-like state or a radio wave absorbing housing formed through injection molding or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radio wave absorber for absorbing unnecessary electromagnetic waves, and more particularly to a radio wave absorber having an absorption characteristic for high frequency electromagnetic waves.

[0002]

2. Description of the Related Art In recent years, as the frequency of signals handled by electronic devices has become higher, the problem of unnecessary radiation emitted from these electronic devices has become more prominent. As a method of suppressing this unnecessary radiation, it is conceivable to change the circuit design, use countermeasures, and the like.However, these methods are becoming more and more difficult due to shortened product spans and increased costs. is there. For this reason, a method of using a countermeasure sheet or the like made of a sheet of a composite soft magnetic material having magnetic loss even with high-frequency electromagnetic waves has been adopted.

In recent years, wireless LAN (Local Area Network)
rk) and communication systems that use high-frequency radio waves, such as automatic tolling systems for expressways, have been developed. However, in equipment that uses radio waves for these, radio waves other than the target signal radio waves are interfering waves. There has been a demand for the development of a radio wave absorber to absorb waves and perform communication smoothly. For example, electromagnetic waves in the 2.45 GHz band are used in various electronic devices such as a microwave oven, a portable information terminal, a wireless LAN, and bluetooth, and these electronic devices communicate smoothly without mutual malfunction. is important.

[0004]

The radio wave absorber converts the energy of the incident radio wave into heat and absorbs it. This energy conversion includes a loss term ε ″ of the relative dielectric constant of the radio wave absorber.
(The imaginary component of the complex relative permittivity) and the loss term μ ″ of the relative permeability (the imaginary component of the complex relative permeability). When a radio wave is incident on a material having such a loss, the energy of the radio wave is as follows. Is converted into heat and absorbed according to the equation (1) shown in FIG.

[0005]

(Equation 1)

According to equation (1), a material having a higher magnetic loss has a higher radio wave absorbing ability. Further, since the intensity of a high-frequency magnetic field from a small loop current such as common mode noise becomes higher as it is closer to the generation source, it is desirable to arrange the radio wave absorber closer to the generation source.

Heretofore, as such a radio wave absorber, powder of a soft magnetic material having a relatively high value of magnetic loss has been compounded with an organic binder such as a synthetic resin, and these have been processed into a sheet to form electronic equipment. Is known, or is formed as a housing of an electronic device by injection molding or the like. As the soft magnetic material to be used, ferrites such as MnZn and NiZn for electromagnetic waves in the MHz band,
Flake powders such as Y, Z and M type hexagonal ferrite, Fe-based or Co-based soft magnetic materials are widely used. For example, a soft magnetic material powder having a diameter of 10 to 30 μm and a thickness of not more than skin depth is used for a FeSi-based alloy by a pulverizer such as a ball mill.

However, these materials do not have good absorption performance for electromagnetic waves in the GHz band. For example, ferrite causes resonance at a lower frequency as a material having higher magnetic permeability due to the limit of snake, and its electromagnetic wave absorption performance does not reach the GHz band. In addition, hexagonal ferrite has a higher resonance frequency than ferrite because it has in-plane magnetic anisotropy, but since the value of saturation magnetization is about 0.5T,
The band that can be used as a radio wave absorber has been limited to several GHz. On the other hand, in recent years, a soft magnetic material such as an Fe-based material has been further processed into a disc shape or an elliptical shape from an irregularly shaped flake shape to provide good electromagnetic wave absorption for electromagnetic waves having a high frequency of several GHz or more. It is known that performance can be obtained. Furthermore, by coating the above-mentioned hexagonal ferrite as a center material and thinly coating a magnetic material having a saturation magnetization value of 0.5 T or more as a surface material covering the same as the center material, it is as high as the above-mentioned soft magnetic material in a disk shape It is known that good radio wave absorption performance can be obtained at a frequency.

On the other hand, as the organic binder in which these magnetic particles are mixed, polystyrene, ABS resin, polycarbonate, ABS / polycarbonate composite resin, modified polyphenylene ether resin and the like are used. However, these synthetic resin materials have good moldability, but when left as waste, there is a problem in that they are not decomposed into nature and have a large environmental load.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a radio wave absorber having a wide absorption characteristic up to high-frequency electromagnetic waves and a small environmental load.

[0011]

According to the present invention, in order to solve the above-mentioned problems, in a radio wave absorber for absorbing unnecessary electromagnetic waves, magnetic particles having a disk-like or elliptical shape, biodegradable or optical There is provided a radio wave absorber characterized by comprising a mixture with a decomposable organic binder.

In such a radio wave absorber, a magnetic particle having a disc-like or elliptical shape is mixed with a biodegradable or photo-degradable organic binder to form a radio wave absorber. As a radio wave absorber, while having good radio wave absorption for high-frequency electromagnetic waves, it is possible to reduce the environmental load due to its natural decomposability and easily decomposable properties.

Further, according to the present invention, in a radio wave absorber for absorbing unnecessary electromagnetic waves, a magnetic material having in-plane magnetic anisotropy is formed by coating a surface of a magnetic material having a saturation magnetization of 0.5 T or more. There is provided a radio wave absorber comprising a mixture of magnetic particles and a biodegradable or photodegradable organic binder.

In such a radio wave absorber, magnetic particles formed by coating the surface of a magnetic material having in-plane magnetic anisotropy with a magnetic material having a saturation magnetization of 0.5 T or more are formed by:
By forming a radio wave absorber by mixing it with a biodegradable or photo-degradable organic binder, it has good radio wave absorbability against high-frequency electromagnetic waves as a radio wave absorber and naturally decomposes. The environmental load can be reduced due to the nature and the property of easily decomposing.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. The radio wave absorber provided in the present invention has a structure of a mixture of powdered magnetic particles and an organic binder. By having such a structure, for example, as described later, for example, it is used in the form of a sheet or paste, or injection molding becomes possible, and is excellent in moldability,
It can be used for various applications and shapes as a radio wave absorber.

In order for the magnetic particles used in such a radio wave absorber to exhibit good radio wave absorption performance up to high frequency electromagnetic waves of several GHz or more, a material exhibiting a high magnetic loss, that is, a high magnetic permeability. Must be used. In the present invention, as a first method for obtaining a magnetic material having such a high magnetic permeability, a method of processing magnetic material particles having a high saturation magnetization value into a disk shape or an elliptical shape is used.
As a second method, a method is used in which magnetic material particles having a high saturation magnetization are thinly coated on the surface of a magnetic material having in-plane magnetic anisotropy. The magnetic material used in these methods needs to have both a crystal magnetic anisotropy constant and magnetostriction of zero.

FIG. 1 conceptually shows the structure of the magnetic particles formed by the above-mentioned first method among the magnetic particles mixed with the organic binder. The principle of exhibiting good absorption performance for electromagnetic waves will be described.

Magnetic particles 1 according to the first method shown in FIG.
Has a disk shape or an elliptical shape having an aspect ratio (ratio between diameter and thickness) of, for example, 10 or more. Generally, in magnetic materials, the magnetic moment is arranged to minimize energy. In a magnetic material having a three-dimensional structure,
It can be oriented in each of the X, Y, and Z axes. However, in the case of the disk-shaped or elliptical magnetic particles 1 as shown in FIG. 1, when the magnetic moment is directed in the direction perpendicular to the plane, the shape magnetic anisotropy energy is increased. It is difficult to arrange magnetic moments in the plane. Here, the in-plane anisotropy is denoted by H _A1 , and the anisotropy when the magnetic moment is directed in the direction perpendicular to the plane is denoted by H _A2 .

At this time, since the magnetic moment has a disk shape in the plane, the magnetic moment can be arranged isotropically in any direction and is easy to move. That is, the in-plane magnetic anisotropy H _A1 is small. On the other hand, since a large energy is required to direct the magnetic moment in the direction perpendicular to the plane, H _A2 takes a large value. Here, the resonance frequency fr
And the relative magnetic permeability μ ′ (the real component of the complex relative magnetic permeability) is expressed by the following equation (2).

[0020]

(Equation 2)

Here, since H _A1 is small and H _A2 is large, the value of the square root shown in the equation (2) takes a large value. As a result, the particles are formed into a disk shape using a material having a high saturation magnetization I _S. Alternatively, by forming the filter into an elliptical shape, it becomes possible to extend the frequency limit that can be absorbed to a high frequency.

One example of a method for producing such a disk-shaped or elliptical magnetic particle 1 is as follows. First, spherical fine particles having a diameter of about several hundred nm to several tens μm are formed by an atomizing method or a chemical precipitation method. By applying a physical force such as a rolling roll or a stamp mill,
Crush and process into a flat shape. Further, as another example, a molten metal is sputtered, vapor-deposited or CVD (Chemical Vapor Depth) on a base film through a mask in which a large number of holes forming the outer shape of the target magnetic particles are formed.
osition) or the like, and then removing the mask, and peeling the disk-shaped magnetic particles 1 from the base film.

A material that can be used as the magnetic particles 1 has a saturation magnetization of 0.5 T
All of the above ferromagnetic materials or ferrimagnetic materials containing at least one ferromagnetic element of Fe, Co, and Ni, such as pure iron, an iron-cobalt alloy (permendur), and an iron nitride compound, are mentioned. Further, an alloy containing Mn, which does not contain a ferromagnetic element but becomes a ferromagnetic material or a ferrimagnetic material, such as a Heusler alloy such as a MnAl alloy, can be used.

Next, FIG. 2 conceptually shows the structure of the magnetic particles formed by the above-described second method. The magnetic particles 2 shown in FIG. 2 are constituted by a center member 21 and a surface member 22 coating the surface thereof. As the center member 21, a magnetic material having in-plane magnetic anisotropy is used, and FIG. 2 shows a case where hexagonal ferrite is used as an example. The surface material 22 is coated with a magnetic material having a saturation magnetization value of 0.5 T or more with a thickness on the order of nanometers. With such a configuration, the magnetic moment of the surface member 22 is pulled by the magnetic moment of the center member 21, has the same properties as the center member 21, and has in-plane magnetic anisotropy. For this reason, a high saturation magnetization is obtained along with a high resonance frequency in the surface layer, and the frequency limit that can be absorbed can be extended to a high frequency.

As an example of a method for producing such magnetic particles 2, first, as a powder material having a size of several hundred nm to several tens μm, the central material 21 is formed by a solid phase reaction method or a chemical precipitation method. Is formed, and a surface material 22 having a thickness of about several nm to several tens nm is coated by a thin film forming method such as sputtering, vapor deposition or CVD, or by plating. Further, as the center member 21, Y or Z-type ferrite (hexagonal ferrite) can be used, and as the surface member 22, the saturation magnetization is set to 0.1.
A ferromagnetic or ferrimagnetic material containing at least one ferromagnetic element of Fe, Co, Ni, such as pure iron, an iron-cobalt alloy, an iron nitride compound or the like having a temperature of 5 T or more;
An alloy containing Mn, which does not contain a ferromagnetic element but becomes a ferromagnetic or ferrimagnetic material, such as a Heusler alloy such as an Al alloy, can be used.

On the other hand, in the radio wave absorber of the present invention, a biodegradable or photodegradable resin material is used as the organic binder in which the magnetic particles 1 or 2 described above are mixed. . As the biodegradable material, for example, resin materials such as polylactic acid, polybeta-hydroxybutyrate, polybutylene succinate, polycaprolactone can be used, and when these are left in nature as waste, Decomposes into water and carbon dioxide by the action of natural microorganisms and decomposing enzymes, and does not release harmful substances. Further, the resin material having photodegradability, for example, a functional group or a chromophore capable of taking in light energy necessary for causing a photochemical reaction is introduced into a molecular chain,
Alternatively, it can be obtained by adding such a reagent. Such a resin material shows high degradability by irradiation with light such as ultraviolet rays, and is easily decomposed into a state that can be easily treated, for example, in a waste treatment process, and can significantly reduce the amount of carbon dioxide emitted. Becomes

Next, FIG. 3 shows a radio wave absorbing sheet as a first embodiment of the radio wave absorber of the present invention, and shows a structure thereof. The electromagnetic wave absorbing sheet 3 shown in FIG. 3 is formed by forming a mixture of the above-described magnetic particles and the organic binder in a sheet shape. For example, the magnetic particles have a volume filling ratio of 30 to 60.
%, Dispersed in an organic binder, kneaded with three rolls to produce a paste-like sample, adjusted to a predetermined thickness by a doctor blade method, and processed into a sheet. You. This radio wave absorbing sheet 3
For example, by being provided locally near the wave source of an electromagnetic wave in a housing of an electronic device or the like, it functions as an absorber of unnecessary electromagnetic waves in the electronic device.

FIG. 4 shows a measurement graph of the frequency dependence of the relative magnetic permeability μ ′ in the radio wave absorbing sheet 3. In the graph shown in FIG. 4, the magnetic particles formed by the first method, that is, the magnetic particles processed into a disk shape or an elliptical shape, are mixed with a biodegradable resin to form a radio wave absorption formed into a sheet. Along with the measured values of sheet 3, the results of measurements on a radio wave absorbing sheet using ferrite and hexagonal ferrite as flake-shaped magnetic particles as conventional products A and B for comparison are also shown. ing. According to this graph, it can be seen that the radio wave absorbing sheet 3 of the present invention has a higher relative magnetic permeability than the conventional products A and B, and has an absorbing performance up to a high frequency band of 10 GHz or more. . As described above, in the case where the magnetic particles having a disk shape or an elliptical shape are used for the radio wave absorbing sheet 3, the magnetic permeability characteristics can be obtained by changing the aspect ratio and the thickness of the disk or the mixed composition of the mixture. Can be changed to control the value of the frequency of the electromagnetic wave to be absorbed. Similarly, when magnetic particles composed of the center material 21 and the surface material 22 are used, the magnetic material particles can be absorbed by changing the thickness and material of the surface material 22 or the mixed composition of the mixture. It is possible to control the frequency.

FIG. 5 shows a measurement graph of the noise reduction effect when the radio wave absorbing sheet 3 is mounted on a notebook personal computer case. In the graph shown in FIG. 5, radio wave absorption formed by mixing the magnetic particles produced by the first method, that is, the magnetic particles processed into a disk shape or an elliptical shape, with a biodegradable resin to form a sheet. The figure shows a value obtained by measuring the level of an electromagnetic wave radiated from the inside of the apparatus for each frequency when the sheet 3 is attached to the housing of a notebook personal computer using the sheet 3. In addition, the measured values of the level of the radiated electromagnetic wave when the radio wave absorbing sheet 3 is not attached are also shown. Note that FIG. 5 shows a polygonal line obtained by combining peaks in each measurement value for simplicity. The polygonal line shown by a solid line shows the case where the radio wave absorbing sheet 3 is attached, and the polygonal line shown by a dotted line shows the case where it is not attached. Are shown respectively. According to this graph, it can be seen that when the radio wave absorbing sheet 3 is attached, unnecessary radiation of electromagnetic waves is reduced over almost the entire frequency band.

Next, FIG. 6 shows the structure of a radio wave absorbing casing according to a second embodiment of the radio wave absorber of the present invention. The electromagnetic wave absorbing casing 4 shown in FIG. 6 is a paste-like material in which the above magnetic particles are adjusted to be in a state of not losing fluidity with a volume filling rate of about 20 to 50% and dispersed in an organic binder. And formed by injection molding or the like. The radio wave absorbing casing 4 is used, for example, as a part of an external casing of an electronic device, and absorbs unnecessary electromagnetic waves from inside.
In this radio wave absorbing housing 4, since the external housing itself can have a radio wave absorbing function, it is not necessary to provide a radio wave absorber separately, so that the space in the housing can be used more effectively. become. Further, it is possible to easily form a large-area radio wave absorber covering the entire device.

The electromagnetic wave absorber having a structure having only an electromagnetic wave absorbing layer, such as the above-described electromagnetic wave absorbing sheet 3 and the electromagnetic wave absorbing casing 4, has a relatively short electromagnetic distance at a distance from the wave source of λ / 6. Has absorption performance for the world. On the other hand, in order to obtain electromagnetic wave absorption performance for an electromagnetic field from a relatively distant place, in the above-described electromagnetic wave absorbing sheet 3 and the electromagnetic wave absorbing casing 4, for example, sputtering, evaporation, A thin layer of a conductor may be provided by a thin film forming method such as plating or plating. This makes it possible to effectively prevent radio interference in a building such as a communication facility.

As described above, in the radio wave absorber of the present invention,
Either a magnetic particle having a disk-like or elliptical shape, or a magnetic material particle having a surface of a magnetic material having in-plane magnetic anisotropy, and a magnetic material having a high saturation magnetization value coated thinly, By being mixed with an organic binder having biodegradability or photodegradability, as a radio wave absorber, it has good radio wave absorption performance up to high frequency electromagnetic waves such as 10 GHz or more, and is naturally degradable. Due to its easily decomposable nature, it is possible to reduce the environmental load.

Further, the radio wave absorber of the present invention has a structure in which the magnetic particles are mixed with the organic binder, so that the radio wave absorber having good absorption performance for high-frequency electromagnetic waves can be obtained.
It can be easily formed into various shapes according to the installation method, and it is possible to efficiently absorb unnecessary electromagnetic waves in a small space as compared with the related art, and to reduce the weight of the device. For example, the radio wave absorbing sheet 3 shown in FIG. 3 can be installed and used inside a housing of various electronic devices or the like to prevent unnecessary radiation. Further, it can be used as a prepreg used for bonding substrates together. This makes it possible to efficiently take measures against unnecessary radiation in a lightweight, space-saving, and also has an effect of attenuating conducted noise.

When a mixture of the magnetic particles and the organic binder is produced in the form of a paste, for example, the paste can be inserted into a dispenser and used as a sealing resin for an IC or the like. Further, by making the mixture in a state of increased fluidity, for example, a radio wave absorber can be formed by a method such as spraying on a surface of a target structure with a sprayer or applying with a brush or the like. It is possible.

In recent years, as a measure of the amount of electromagnetic waves emitted by electronic devices absorbed by the human body, a specific absorption rate SAR (Spec
(Absorption Rate) is defined, but the radio wave absorber of the present invention can be used as a SAR suppressor. Hereinafter, a description will be given of a result of measurement of an application example of the radio wave absorber of the present invention to a SAR suppressor. First, FIG. 7 shows an attachment position of the SAR suppressor to the mobile phone.

The mobile phone 5 for measurement shown in FIG. 7 has a sheet-shaped SAR suppressor made of the radio wave absorber of the present invention inside the outer casing formed of polycarbonate and near the feeding point of the antenna 51. 52 are mounted. The SAR suppressor 52 is formed by mixing disk-shaped magnetic particles with a biodegradable organic mixture, and is about 1 cm in length and width. The mobile phone 5 has a structure in which a mounting claw is provided inside the outer housing, and the SAR suppressor 52 is fitted into the claw, so that the SAR suppressor 52 can be easily attached and detached. I have.

FIG. 8 shows an SA using this portable telephone.
1 shows a schematic configuration of an apparatus for measuring the amount of R radiation. The SAR radiation amount measuring device 6 using the above-mentioned mobile phone 5 is made of plastic, and sugar water is injected into the phantom 61 and the dielectric constant of the SAR is adjusted to be substantially the same as that of a human body. An electrolytic probe 62 for detecting the SAR
It is constituted by a robot arm 63 that transports the electrolysis probe 62. In the measurement of the SAR radiation amount, the phantom 61
The mobile phone 5 is installed at the front of the phantom 61, and the electrolytic probe 62 is
The SAR radiation amount is measured by measuring the absorption amount of the SAR in the phantom 61 by performing the detection by being moved dimensionally.

Next, FIG. 9 shows the SAR by this measuring device 6.
The measurement results of the radiation amount are shown. FIG. 9 (a) shows SAR suppressor 5
(B) shows the SAR suppressor 5
2 shows the measurement results in the case where No. 2 is provided.

In the measurement results of FIG. 9, the measurement results are shown two-dimensionally by plotting the maximum value of the measurement values in the vertical direction with respect to the front surface of the mobile phone 5 from the measurement results of the measurement device 6. In addition, this measurement value is
In the measurement results shown in FIG. 9A by the mobile phone 5 not having the SAR 2, the values when the maximum value of the SAR radiation amount is 100 are shown. According to this measurement result, the SAR
When the SAR suppressor 52 is provided near the feeding point of the antenna 51 of the mobile phone 5 having the largest radiation amount,
It can be seen that the SAR radiation amount is reduced by about 50% as compared with the case where it is not provided, and the range in which the radiation amount of 10% or more is detected is also reduced. In the mobile phone 5, the gain of the antenna 51 hardly changes due to the mounting of the SAR suppressor 52. From this result,
The radio wave absorber according to the present invention suppresses only the SAR without hindering the characteristics of the antenna 51 and has a very high performance SA.
It can be used as the R suppressor 52.

[0040]

As described above, the radio wave absorber of the present invention is obtained by mixing magnetic particles having a disk shape or an elliptical shape with a biodegradable or photodegradable organic binder. By configuring the absorber, it is possible to reduce the environmental load as a radio wave absorber while having good radio wave absorption for high frequency electromagnetic waves, and due to its natural decomposability and easy decomposability. Become.

Further, the radio wave absorber of the present invention is capable of biodegrading magnetic particles formed by coating a surface of a magnetic material having in-plane magnetic anisotropy with a magnetic material having a saturation magnetization of 0.5 T or more. By forming a radio wave absorber by mixing it with an organic binder having thermal or photodegradable properties, it has good radio wave absorption for high frequency electromagnetic waves as a radio wave absorber, The easily decomposed nature makes it possible to reduce the environmental load.

[Brief description of the drawings]

FIG. 1 is a view conceptually showing a structure of a magnetic particle formed by a first method.

FIG. 2 is a view conceptually showing a structure of magnetic particles formed by a second method.

FIG. 3 is a view showing the structure of a radio wave absorbing sheet according to a first embodiment of the radio wave absorber of the present invention.

FIG. 4 is a graph showing the measurement results of the frequency dependence of the relative magnetic permeability μ ′ in the radio wave absorbing sheet.

FIG. 5 is a graph showing a measurement result of a noise reduction effect when the radio wave absorbing sheet is attached to a notebook personal computer housing.

FIG. 6 is a view showing the structure of a radio wave absorbing casing according to a second embodiment of the radio wave absorber of the present invention.

FIG. 7 is a diagram showing a mounting position of a SAR suppressor to a mobile phone.

FIG. 8 shows S using a mobile phone equipped with a SAR suppressor.
It is a figure showing the schematic structure of the measuring device of AR radiation.

FIG. 9 shows a measurement result of an absorption amount of SAR.

[Explanation of symbols]

1, 2, ... magnetic particles, 21 ... central material, 22 ... surface material, 3 ... radio wave absorbing sheet, 4 ... radio wave absorbing casing, 5 ...
... mobile phone, 51 ... antenna, 52 ... SAR suppressor, 6 ... measuring device, 61 ... phantom, 62 ... electrolytic probe, 63 ... robot arm

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Junichi Ogasawara 6-7-35 Kita-Shinagawa, Shinagawa-ku, Tokyo F-term within Sony Corporation (reference) 5E321 BB23 BB32 BB35 BB53 GG11

Claims (7)

[Claims] 1. An electromagnetic wave absorber for absorbing unnecessary electromagnetic waves, comprising a mixture of magnetic particles having a discoid or elliptical shape and an organic binder having biodegradability or photodegradability. Characteristic radio wave absorber. 2. The method according to claim 1, wherein the magnetic particles are made of a material containing at least one of Fe, Co, and Ni, or an alloy containing Mn, and are a ferromagnetic material or a ferrimagnetic material. The radio wave absorber according to claim 1, wherein 3. A radio wave absorber that absorbs unnecessary electromagnetic waves, wherein a magnetic material particle formed by coating a surface of a magnetic material having in-plane magnetic anisotropy with a magnetic material having a saturation magnetization of 0.5 T or more is provided. An electromagnetic wave absorber comprising a mixture with a biodegradable or photodegradable organic binder. 4. The magnetic particles are any one of a material containing at least one of Fe, Co, and Ni, or an alloy containing Mn, and are a ferromagnetic material or a ferrimagnetic material. The radio wave absorber according to claim 3, wherein 5. The radio wave absorber according to claim 1, wherein the mixture is formed into a paste. 6. A radio wave absorbing casing comprising the radio wave absorber according to claim 1. 7. A radio wave absorbing sheet comprising the radio wave absorber according to claim 1.