Title: The Magnetoelastic Distortion of Multiferroic BiFeO$_3$ in the Canted Antiferromagnetic State

Abstract: Using THz spectroscopy, we show that the spin-wave spectrum of multiferroic BiFeO$_3$ in its high-field canted antiferromagnetic state is well described by a spin model that violates rhombohedral symmetry. We demonstrate that the monoclinic distortion of the canted antiferromagnetic state is induced by the single-ion magnetoelastic coupling between the lattice and the two nearly anti-parallel spins. The revised spin model for BiFeO$_3$ contains two new single-ion anisotropy terms that violate rhombohedral symmetry and depend on the direction of the magnetic field.