Semiconductor-to-metal transition in BaFe12−<i>x</i>(Ru, Re)<i>x</i>O19 (<i>x</i> = 0.1) hexaferrite at high temperatures

Abstract

Semiconductor-to-metal-like behavior was observed in the BaFe11.9(Ru,Re)0.1O19 ceramic hexaferrite at temperatures above 450 K. X-ray diffraction analysis confirmed its hexagonal structure with minor α-Fe2O3 impurities, while Rietveld refinement revealed significant changes in the lattice parameters, particularly an expansion along the c axis. Atomic positions at the 2b, 4f2, and 12k sites were altered due to the partial substitution of Fe3+ by Ru4+ and Re4+ cations, which have smaller ionic radii than Fe3+. These substitutions modified the bond lengths within the crystal structure, as evidenced by increased Fe–O distances, and led to a partial reduction of Fe3+ to Fe2+, increasing electron density. The AC conductivity, σac(T), showed a transition from semiconducting to metallic-like behavior above ∼450 K. The resistivity, ρac(T), exhibited a plateau near 500 K, indicating a change in the conduction mechanism. The transport mechanism below 450 K was dominated by the non-overlapping small polaron tunneling model, characterized by thermally activated hopping with minimal Coulomb interactions, while above 450 K, the correlated barrier hopping model became dominant. The activation energy for high temperatures, Ea=0.31±0.06 eV, was consistent with the sum of the hopping energy WH=0.20±0.01 eV and half the disorder energy, Ed/2, which increased significantly with temperature. The partial substitution of Ru4+ and Re4+ significantly weakened the magnetic interactions, resulting in reduced saturation magnetization and modifications in coercivity.