Far-IR magnetospectroscopy of magnons and electromagnons in TbFe O3 single crystals at low temperatures

Transmittance spectra of TbFeO3 single crystals have been studied in the far-infrared spectral range 15-120cm-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 TNTb=3.3K. Both the quasiferromagnetic and quasiantiferromagnetic modes of AFMR demonstrate hardening at T<TNTb. The quasiantiferromagnetic mode gains electric-dipole activity along the c axis below TNTb and thus behaves as a hybrid, i.e., both electric- and magnetic-dipole active, mode. In addition to AFMR modes, below TNTb we observed the appearance of electromagnon excitation at about 27cm-1, which is electric-dipole active along the c axis. The electromagnon is optically active only in a narrow temperature range, 2.7K<T<TNTb 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.