Narrow Results

We have prepared Fe–Ni particles coated with 3 wt% SiO₂ by chemical synthesis. The precursor compounds have been reduced in hydrogen gas at different temperatures. The dependence of complex permeability μ on different reduction temperatures is studied by using an impedance analyzer. The experiment results indicate that the domain wall pinning strength, the average domain thickness and the resistivity depend on different heat treatment conditions. The magnetization process is mostly contributed by the domain wall bulging and displacement.

The microstructure and magnetic properties of Mg–Cu–Zn ferrites prepared by using solid-state reaction method have been investigated. X-ray diffraction (XRD), a scanning electron microscope (SEM), impedance analyzer and a vibrating sample magnetometer (VSM) were utilized in order to study the effect of copper substitution and its impact on the crystal structure, grain size, microstructure and magnetic properties of the Mg–Cu–Zn ferrite. The formation of cubic spinel phase was identified using XRD technique. The microstructures of the samples show that the grain growth is greatly enhanced by the addition of CuO which is attributed to the liquid phase during sintering. The average grain size (D_m) increases significantly with increasing Cu content. The initial permeability (μ') of the samples increases appreciably with increasing Cu content which is attributed to the increase of grain size and density of the samples. The resonance frequency (f_r) of the samples shifts toward the lower frequency as the Cu content increases. The sharp fall of μ' in μ'-T curves is observed for all the samples which indicate the homogeneity of the samples. The saturation magnetization (M_s) of the Mg–Cu–Zn ferrites increases slightly with increasing Cu concentration.

Magnetic, dielectric and DC conductive properties of Ni${}_{0.95-x}$Zn_xCo${}_{0.05}$Fe${}_{1.90}$Mn${}_{0.02}$O₄ (with $x=0$-0.20 at an interval of 0.05) ferrite ceramics were studied, in order to develop magneto-dielectric materials with almost equal values of relative permeability and permittivity, for the miniaturization of HF (3–30MHz) and VHF (30–90MHz and 100–300MHz) antennas. The ferrite ceramics were prepared by using the conventional two-step sintering process. The real part of relative permeability is increased almost linearly with increasing concentration of Zn, while that of relative permittivity keeps nearly unchanged. It is found that promising magneto-dielectric materials, with close values of real permeability and permittivity over 30–90 MHz (VHF), can be obtained for the samples at Zn concentrations between $x=0.05$ and $x=0.10$.

The high frequency magnetic properties (Complex intrinsic permeability µ_i and effective permeability µ_e) have been studied for iron based magnetic particulate materials and composite with non-magnetic matrix. The complex relative intrinsic permeability spectra of iron based magnetic particulate powders were calculated with the Soohoo model. Effective permeability of composite consisting of Fe inclusions embedded in a nonmagnetic matrix (epoxy resin) was calculated using Bruggeman’s effective medium theory. The theoretically calculated permeability spectra were then compared with experimentally measured permeability spectra of the composite. The measured permeability µ_max´ and resonant frequency is 2.5-2.8 and around several GHz, respectively, for composites with 30% (by volume) carbonyl iron powders. The parameters indicate that iron based composites possess suitable high frequency magnetic properties for use as electromagnetic materials with low reflectivity at microwave frequencies.

The results also showed that the Soohoo model is suitable for simulation of high frequency magnetic properties of ferromagnetic iron based powders.