Self-polarized terahertz magnon absorption in a single crystal of BiFeO3

We report the polarization dependence of terahertz magnon absorption in single crystals of BiFeO3 grown by a modified floating zone method. In a (111)(pc)-oriented crystal, two major magnon absorption signals were observed for all terahertz polarizations, which indicates that magnetic domains were not aligned in one of the three allowed directions. In contrast, the absorption modes in a (001)(pc)-oriented crystal showed significant polarization dependence, which was unchanged even after annealing the crystal at temperatures far above the Neel point to demagnetize it. This polarization dependence coincides with that of E mode phonons. Thus, we conclude that magnon and phonon in BiFeO3 are strongly coupled and the selection rules for magnon absorption are governed by the activity of E mode phonons, namely, the crystalline anisotropy originating from ferroelectric polarization.