Hertz-to-infrared electrodynamics of single-crystalline barium-lead hexaferrite Ba1-xPbxFe12O19

Broadband electrodynamic response of single-crystalline lead-substituted barium hexaferrite Ba_1-xPb_xFe₁₂O₁₉ is studied at temperatures from 5 to 300 K in the range from 1 Hz to 240 THz that includes radio, sub-terahertz, terahertz and infrared frequencies and altogether spans over 14 frequency decades. Discovered phenomena include relaxational radio-frequency dynamics of domains and domain walls, temperature-unstable terahertz excitations connected with electric dipoles induced by off-center displacements in the ab-plane of the lead ions, narrow terahertz excitations associated with electronic transitions between the fine-structure components of the Fe²⁺ground state, dielectric gigahertz resonances presumably of magneto-electric origin and polar lattice vibrations.