Broadband free space impedance in Co2Z hexaferrites by substitution of high valency heavy transition metal ions for miniaturized RF devices

Abstract

Polycrystalline samples of Z-type hexaferrites, having nominal compositions Ba3Co2+xFe24−2xMxO41, where M = Ir, Hf, or Mo and x = 0 and 0.05, were processed via ceramic protocols in pursuit of low magnetic and dielectric losses, as well as equivalent permittivity and permeability over the VHF–UHF band. Fine process control was conducted to ensure optimal magnetic properties. Organic dispersants, i.e., isobutylene and maleic anhydride, were employed to achieve maximum densities. Crystallographic structure, characterized by x-ray diffraction, revealed that the dopants did not adversely affect the crystal structure and phase purity of the Z-type hexaferrite. An increase in permeability and size reduction factor is shown to be linearly proportional to the ionic radii of the dopants. This trend is consistent with local bonding distortions giving rise to increased exchange energy (J) as predicted by the Goodenough–Kanamori–Anderson rules and superexchange theory. We posit that these distortions increase the magnetocrystalline anisotropy energy affecting the frequency dependent complex permeability. For Mo doping of x = 0.05, a bandwidth of 400 MHz exists at a center frequency of 650 MHz where the permittivity and permeability are very nearly equal, i.e., Z = 377 ± 5 Ω. These results give rise to low loss, i.e., tan δε/ε′ = 0.0006 and tan δμ/μ′ = 0.038 at 650 MHz, with considerable size reduction (×9). The miniaturization and optimized performance of magnetodielectric materials for antenna at VHF–UHF frequencies using cost-effective and volumetric processing methodologies are demonstrated.