  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.
