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

Transmittance spectra of $\mathrm{TbFe}{\mathrm{O}}₃$ single crystals have been studied in the far-infrared spectral range $15\ensuremath{-}120\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ 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 ${T}_N^{\mathrm{Tb}}=3.3\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Both the quasiferromagnetic and quasiantiferromagnetic modes of AFMR demonstrate hardening at $T<{T}_N^{\mathrm{Tb}}$. The quasiantiferromagnetic mode gains electric-dipole activity along the $c$ axis below ${T}_N^{\mathrm{Tb}}$ and thus behaves as a hybrid, i.e., both electric- and magnetic-dipole active, mode. In addition to AFMR modes, below ${T}_N^{\mathrm{Tb}}$ we observed the appearance of electromagnon excitation at about $27\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$, which is electric-dipole active along the $c$ axis. The electromagnon is optically active only in a narrow temperature range, $2.7\phantom{\rule{0.16em}{0ex}}\mathrm{K}<T<{T}_N^{\mathrm{Tb}}$ at $H=0$, and in a narrow range of magnetic fields 2.4--2.7 T applied along the $b$ axis at $T=1.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. We argue that this electromagnon appears in magnetic phases, which are compatible with a spontaneous electric polarization along the $b$ axis.