Polar modes in multiferroic Ba_2Mg_2Fe_(12)O_(22) hexagonal Y-type   hexaferrite
  Spectra of transmission and reflection coefficients of single crystalline Y-type hexaferrite Ba_(2)Mg_(2)Fe_(12)O_(22) are measured at Terahertz and infrared frequencies, 3 - 4500 c--1, and at temperatures 10 to 300 K, for polarization E prp c. Oscillator parameters of nineteen phonon lines allowed by the R3m crystal lattice symmetry are determined. Absorption lines assignment to vibrational modes of the lattice structural fragments is performed. Below the temperatures of 195 K and 50 K, corresponding to the zero field phase transitions to the proper screw and to the longitudinal-conical spin states, respectively, new absorption lines are discovered whose origin is assigned to the electric-dipole active magnetic excitations. 
G.A. Komandin, A.S. Prokhorov, V.I. Torgashev, A.A. Bush, E.S. Zhukova   and B.P. Gorshunov
cond-mat.mtrl-sci