Coupling between magnon and ligand-field excitations in magnetoelectric Tb₃Fe₅ O₁₂ garnet

The spectra of far-infrared transmission in Tb₃ Fe₅ O₁₂ magnetoelectric single crystals have been studied in the range between 15 and 100 cm^-1, in magnetic fields up to 10 T, and for temperatures between 5 and 150 K. We attribute some of the observed infrared-active excitations to electric dipole transitions between ligand-field split states of Tb3⁺ ions. Anticrossing between the magnetic exchange excitation and the ligand-field transition occurs at the temperature between 60 and 80 K. The corresponding coupling energy for this interaction is 6cm ^-1. Temperature-induced softening of the hybrid IR excitation correlates with the increase in the static dielectric constant. We discuss the possibility for hybrid excitations of magnons and ligand-field states and their possible connection to the magnetoelectric effect in Tb₃ Fe ₅ O₁₂.