Takanori Tsutaoka

Frequency dispersion of complex permeability in Mn–Zn and Ni–Zn spinel ferrites and their composite materials

Complex permeability spectra μ*=μ′−iμ″ for two types of spinel ferrites (Ni–Zn ferrite and Mn–Zn ferrite) and their composite materials have been investigated. The contribution of domain-wall and natural resonance to the permeability spectra was estimated by the numerical fitting of actual measurement data to a simple formula. Frequency dispersion type of each component, relaxation or resonance, can be estimated from one of the fitting parameters, damping factor. In sintered Mn–Zn ferrite, domain-wall contribution is dominant and gyromagnetic spin resonance or relaxation-type magnetization rotation is large in Ni–Zn ferrite. However, relaxation character is dominant in both Mn–Zn and Ni–Zn ferrite composite materials. In composite materials, the permeability value can be scaled by the ferrite particle content using a simple model concerning demagnetizing field. This analysis is useful in designing the permeability spectra of ferrite composite materials.

Frequency dispersion and temperature variation of complex permeability of Ni‐Zn ferrite composite materials

Permeability spectra in Ni‐Zn ferrite composite materials were studied at the volume loading of ferrite above 30% and at temperatures from 100 to 400 K. The permeability decreased with decreases in the volume loading of ferrite. This decrease was much larger than that expected from the empirical mixing law. This was attributed to the demagnetizing field, generated by the magnetic poles on the surface of the ferrite particles. Simultaneously, the demagnetizing field increased spin resonance frequency. For the sintered ferrite, the primary peak of the permeability was just below the Curie temperature. The peak becomes obscure and disappeared as the volume loading decreased. The temperature dependence of the spin resonance frequency was lower in the ferrite composite material than that in the sintered ferrite. These features were also discussed from the view point of the demagnetizing field.

Frequency dispersion of permeability in ferrite composite materials

Abstract Permeability spectra of Ni-Zn ferrite composite materials, prepared by mixing the ferrite particles with EVA resin, have been studied. In the sintered ferrite (volume fraction 1.0), the spin resonance is around 9 MHz and the static permeability about 1400. As the ferrite content decreases (composite materials), the static susceptibility of the spin component decreases and the spin resonance frequency shifts higher. The real part of the permeability in the ferrite composite materials becomes larger than that of the sintered ferrite in the rf frequency region. These features have been analyzed using the magnetic circuit model. The application of Snoeks limit extended to ferrite composite materials is also proposed.

Negative permeability spectra in Permalloy granular composite materials

Complex permeability spectra of Permalloy granular composite materials have been studied in the microwave frequency range. The heat-treated Permalloy particles in the air at several hundreds of °C have a high surface electrical resistance; the eddy current effect in the high frequency permeability spectra can be suppressed in the composite structure containing the percolated particles. A negative permeability has been obtained above 5GHz due to the natural magnetic resonance in the 70vol% particle content composite material. In this content, electrical permittivity spectra show a nonmetallic characteristic. This permeability dispersion can be applied for the left-handed media.

Permeability spectra of yttrium iron garnet and its granular composite materials under dc magnetic field

Relative complex permeability spectra ( μ r = μ r ′ - i μ r ′ ′ ) and the dc magnetic field effect on them for a yttrium iron garnet (YIG) and its granular composite materials have been studied to evaluate the negative permeability characteristics. In the sintered YIG, two distinct peaks corresponding to the domain wall and the gyromagnetic spin resonance were observed in the imaginary part μ r ′ ′ under zero magnetic field; the real part of complex permeability μ r ′ shows a small negative value in a certain frequency range. The Lorentz type magnetic resonance with the negative permeability dispersion was observed under dc magnetic field. Permeability spectra were evaluated by the numerical fitting of actual measurement data to a resonance formula using six parameters (resonance frequencies, static susceptibilities, and damping factors of the domain wall motion and the gyromagnetic spin rotation). The dc magnetic field suppresses the domain wall contribution and the spin component becomes dominant. In the YIG granular composite material, the permeability dispersion frequency shifts to higher frequency region due to demagnetizing field; the spin component becomes dominant. Negative permeability spectra were also observed in the high content YIG composites under the dc field. The negative permeability spectra of YIG composite materials can also be applied to the left-handed material as well as the sintered YIG.

Magnetic field effect on the complex permeability spectra in a Ni–Zn ferrite

Complex permeability spectra μ*(=μ′−iμ′′) in a Ni–Zn ferrite was studied in the frequency range from 10 kHz to 3 GHz under dc magnetic field up to 1000 Oe. In the absence of a dc magnetic field, the μ′ spectrum has a frequency dispersion above 1 MHz; the μ′′ spectrum has a maximum at about 2 MHz. This feature can be described by the superposition of the two types of magnetic resonance, domain wall motion with a resonance frequency ωdwr=3.5 MHz and spin rotation with a resonance frequency ωspinr=8.0 MHz. Under dc magnetic field, low frequency permeability μ′ decreases with increasing static field bias. On the other hand, the μ′′ spectrum is broadened and two distinct peaks appear in the external field of 606 Oe. Under about 900 Oe external field, this ferrite becomes to have single-domain structure and the dispersion of domain wall motion in the permeability spectra disappears. In the above 700 Oe external field, high frequency dispersion of μ* shows ferromagnetic resonance characteristics.Complex permeability spectra μ*(=μ′−iμ′′) in a Ni–Zn ferrite was studied in the frequency range from 10 kHz to 3 GHz under dc magnetic field up to 1000 Oe. In the absence of a dc magnetic field, the μ′ spectrum has a frequency dispersion above 1 MHz; the μ′′ spectrum has a maximum at about 2 MHz. This feature can be described by the superposition of the two types of magnetic resonance, domain wall motion with a resonance frequency ωdwr=3.5 MHz and spin rotation with a resonance frequency ωspinr=8.0 MHz. Under dc magnetic field, low frequency permeability μ′ decreases with increasing static field bias. On the other hand, the μ′′ spectrum is broadened and two distinct peaks appear in the external field of 606 Oe. Under about 900 Oe external field, this ferrite becomes to have single-domain structure and the dispersion of domain wall motion in the permeability spectra disappears. In the above 700 Oe external field, high frequency dispersion of μ* shows ferromagnetic resonance characteristics.

Low frequency plasmonic state and negative permittivity spectra of coagulated Cu granular composite materials in the percolation threshold

We have studied the relative complex permittivity (e r = e r′- ie r″) of copper granular composite materials containing coagulated Cu particles in the microwave range as well as the electrical conductivity. The insulator to metal transition was observed at the percolation threshold φ c = 16.0 vol. %. The enhancement of permittivity in the insulating state can be described by the Effective Cluster Model. Above the percolation threshold φ c, it was found that the Cu granular composites show negative permittivity spectra below a characteristic frequency f 0 indicating the low frequency plasmonic state. Characteristic frequency tends to increase with particle content.

Dehydriding reaction of metal hydrides and alkali borohydrides enhanced by microwave irradiation

Effects of microwave irradiation on metal hydrides (LiH, NaH, MgH2, CaH2, and TiH2) and alkali borohydrides (LiBH4, NaBH4, and KBH4) were systematically investigated for the first time. TiH2 was heated to 600K by microwave irradiation for 3.5min, at which less than 0.16mass% of hydrogen was desorbed from surface of the powder. On the other hand, LiBH4 was heated rapidly above 380K, at which almost all hydrogen, 13mass%, was desorbed. The rapid heating of TiH2 is mainly due to conductive loss, while that of LiBH4 is related to a structural transition at approximately 380K.

Negative permittivity and permeability spectra of Cu/yttrium iron garnet hybrid granular composite materials in the microwave frequency range

The relative complex permittivity and permeability spectra of the coagulated copper and yttrium iron garnet (Cu/YIG) hybrid granular composite materials have been studied in the microwave range. The insulator to metal transition was observed at the percolation threshold of Cu particle content (φ Cu  = 16.0 vol. %) in the electrical conductivity. In the percolation threshold, the low frequency plasmonic state caused by the metallic Cu particle networks was observed. The percolated Cu/YIG granular composites show simultaneous negative permittivity and permeability spectra under external magnetic fields.

Double negative electromagnetic properties of percolated Fe53Ni47/Cu granular composites

Electromagnetic properties of hybrid composite materials containing copper and permalloy (Fe53Ni47 alloy) particles have been investigated in the RF to microwave frequency range up to 20 GHz. Double negative permittivity and permeability spectra have been observed in the percolated state of the hybrid composite material. The negative permittivityspectra in this composite can be attributed to the low frequency plasmonic state produced by the percolatedCu and permalloy cluster chains as well as the dielectric resonance of the isolated metal clusters. The refractive indexspectra which were calculated from the measured permittivity and permeability data indicated the negative refraction from 200 MHz to 1.8 GHz. The near zero or zero refractive index state can be obtained near the two zero crossing frequencies in the refractive indexspectra.