Magnetic and vibronic THz excitations in multiferroic Fe$_{1.8}$Zn$_{0.2}$Mo$_3$O$_8$.

We report on optical excitations in the magnetically ordered phases of Fe$_{1.8}$Zn$_{0.2}$Mo$_3$O$_8$ in the frequency range from 10-110 cm$^{-1}$ (0.3-3.3\,THz). In the collinear easy-axis antiferromagnetic phase below $T_N=50$ K eleven optically active modes have been observed in finite magnetic fields, assuming that the lowest-lying mode is doubly degenerate. The large number of modes reflects either a magnetic structure much more complex than in pure Fe$_{2}$Mo$_3$O$_8$ or that spin stretching modes become active in addition to the usual spin precessional modes. Their magnetic field dependence, for fields applied along the easy axis, reflects the irreversible magnetic-field driven phase transition from the antiferromagnetic ground state to a ferrimagnetic state, while the number of modes remains unchanged in the covered frequency region. We determined selection rules for some of the AFM modes by investigating all polarization configurations and identified magnetic- and electric-dipole active modes as well. In addition to these sharp resonances, a broad electric-dipole active excitation band occurs below $T_N$, which is not influenced by the external magnetic field. We assign this absorption band to a vibronic excitation related to the lowest-lying Fe$^{2+}$ electronic states in tetrahedral environment.