Structure refinement and dielectric relaxation of M-type Ba, Sr, Ba-Sr, and Ba-Pb hexaferrites

Abstract

M-type hexaferrites with compositions BaFe12O19 (BFO), SrFe12O19 (SFO), Ba0.5Sr0.5Fe12O19 (BSFO), and Ba0.5Pb0.5Fe12O19 (BPFO) were synthesized by commercial solid state reaction method. The Rietveld refinement of x-ray powder diffraction revealed a single hexagonal phase with space group P63/mmc for BFO, SFO, and BSFO samples, whereas BPFO sample contains hematite (α-Fe2O3) phase with space group R3c along with the M-type main phase. All the samples show dispersion in dielectric constant (ɛ′) and dielectric loss (tan δ) values with frequency. The values of ɛ′ and tan δ increase with increase in temperature due to increase in the number of charge carriers and their mobilities, which are thermally activated. The reciprocal temperature dependence of conductivity (σac) and the most probable relaxation time (τM″) satisfies the Arrhenius relation. A perfect overlapping of the normalized plots of modulus isotherms on a single “super curve” for all the studied temperatures reveals a temperature independence of dynamic processes involved in conduction and for relaxation. Further, the complex plots of M* (M″ vs M′) indicate that dc conductivity dominates in the region below the M″max point. Above M″max, the variations follow Jonscher power law (σ = Aωs) implying that ac conductivity is dominating in this region. Among the prepared samples, SFO hexaferrite has lowest values of σac, ɛ′, and tan δ making it suitable for use in microwave devices.