Tuning the terahertz electrodynamics in Ba-Pb hexaferrite single crystals

First detailed measurements of broad-band (2-8000 cm-1, i.e. 0.06-240 THz) temperature-dependent (5-300 K) spectra of complex permittivity of single-crystalline barium-lead hexaferrites Ba1-xPbxFe12O19 covering full in-terval of lead concentrations, x(Pb)=0...1, were performed to study the effect of Ba for Pb substitution on electrodynamic response. It is found that variation of Ba-Pb relative content allows "tuning" of dielectric losses epsilon"(0.3 THz; 300 K) from 0.1 to 45, and dielectric constant epsilon'(0.3 THz; 300 K) from 10 to 50. To account for the discovered concentration evolution of terahertz dielectric properties, a microscopic model is suggested that considers interaction of 6s2 lone pair of Pb2+ with nearest FeO5 clusters. Pronounced anomaly in concentration -dependent oscillator strength of terahertz bands is observed at x(Pb)=0.5-0.6 and interpreted by considering redistribution of Fe2+/Fe3+ ions over five iron sublattices. It is pointed out that the revealed "tunability" of terahertz characteristics of hexaferrites makes them promising materials for designing devices operating at terahertz frequencies.