Abstract
This research aimed to investigate the effect of the Ni-concentration on the modulated crystallographic structure, constitutive electromagnetic properties, and microwave-absorbing behaviors of the MnZn ferrites. Substitution of Ni for Mn induced compressive strains in cubic spinel due to the smaller Ni than Mn. The activation energy of hopping carriers was determined from the plot of resistivity vs temperature. The conduction was considered to be small-polaron hopping among the spinel lattice which was distorted by Ni substitution. The ferrimagnetic properties of Ni <sub>0.5x</sub> Mn <sub>0.5-0.5x</sub> Zn <sub>0.5</sub> Fe <sub>2</sub> O <sub>4</sub> ferrites were improved with the increasing Ni concentration. The ferrite with x = 1 had the lowest intrinsic coercivity (H <sub>c</sub> = 7.4 Oe) and the highest saturation magnetization (M <sub>s</sub> = 68.9 emu/g). The increase of the saturation magnetization with the Ni substitution (x > 0.24) is ascribed to the dense microstructure with less pores and the decrease of Fe <sup>2+</sup> /Fe <sup>3+</sup> ratio. Additionally, a multilayer design of microwave-absorbing structure containing Ni <sub>0.5x</sub> Mn <sub>0.5-0.5x</sub> Zn <sub>0.5</sub> Fe <sub>2</sub> O <sub>4</sub> granules distributing in a contiguous polymeric matrix was also investigated. The constitutive electromagnetic properties of the composite such as relative permeability and relative permittivity within the whole frequency range were characterized by a combined transmission/reflection method using a coaxial airline fixture. The frequency responses to the electromagnetic field of the ferrite-polymer composites are ascribed to macroscopic magnetic loss and dielectric loss in association with domain-wall oscillation, spin relaxation, and dipolar polarizations. In additions, the microwave-absorbing behaviors of the ferrite-polymer composites were determined using the constitutive electromagnetic properties. The maximum return loss was estimated as -32 dB occurring at 2.3 GHz for 6 mm-thick composite for the composite with x = 0.24. For composites with ferrites containing high Ni concentration, the broadening of absorption bandwidth reduced the products of matching thickness and frequency which consequently reduced the required thickness of the microwave absorber. The substitution of Ni for Mn in MnZn ferrites is beneficial in fabricating thin microwave absorbers in GHz frequency range due to the improved electromagnetic lossy characteristics. © 2011 Elsevier Ltd. All rights reserved. All rights reserved.