Far-IR magnetospectroscopy of magnons and electromagnons in TbFeO3 single crystals at low temperatures

Transmittance spectra of TbFeO3 single crystals have been studied in the far-infrared spectral range 15-120 cm(-1) in the external magnetic fields up to 9 T and at low temperatures down to 1.5 K. The temperature and magnetic-field dependencies of the antiferromagnetic resonance (AFMR) modes have been measured below and above the magnetic ordering temperature of Tb moments T-N(Tb) = 3.3 K. Both the quasiferromagnetic and quasiantiferromagnetic modes of AFMR demonstrate hardening at T < T-N(Tb). The quasiantiferromagnetic mode gains electric-dipole activity along the c axis below T-N(Tb) and thus behaves as a hybrid, i. e., both electricand magnetic-dipole active, mode. In addition to AFMR modes, below T-N(Tb) we observed the appearance of electromagnon excitation at about 27 cm(-1), which is electric-dipole active along the c axis. The electromagnon is optically active only in a narrow temperature range, 2.7K < T < T-N(Tb) at H = 0, and in a narrow range of magnetic fields 2.4-2.7 T applied along the b axis at T = 1.5K. We argue that this electromagnon appears in magnetic phases, which are compatible with a spontaneous electric polarization along the b axis.