Dielectric, magnetic, and lattice dynamics properties of Y-type hexaferrite Ba_0.5Sr_1.5Zn₂Fe₁₂O₂₂: Comparison of ceramics and single crystals

We prepared multiferroic Y-type hexaferrite Ba_0.5Sr_1.5Zn₂Fe₁₂O₂₂ ceramics and compared their magnetic and dielectric properties with single crystal. Magnetic susceptibility and microwave resonance measurement revealed magnetic phase transition at T_C=312 K, similar as in single crystal. Ferroelectric (FE) phase can be induced by external magnetic field in all investigated samples and the phase diagram in ceramics qualitatively resembles that of the single crystal. The range of magnetic fields, where the FE phase is induced, broadens after annealing of single crystal. Ceramics quenched after sintering exhibit several orders of magnitude lower conductivity than the single crystal. Heavily damped magnetic resonance was discovered in terahertz spectra at 10 K and its frequency softens below 5 GHz near T_C. Number and symmetry of observed infrared (IR) and Raman active phonons correspond to paraelectric phase with D_3d⁵ hexagonal structure. No evidence for a structural phase transition was found in the IR and Raman spectra on cooling (in zero magnetic field) or in the room-temperature IR spectra with external static magnetic field up to 0.3 T.