Temperature-dependent infrared reflectivity studies of multiferroic TbMnO3: Evidence for spin-phonon coupling

Abstract

We have measured near normal incidence far-infrared (FIR) re°ectivity spec- tra of a single crystal of TbMnO3 from 10 K to 300 K in the spectral range of 50 cm¡1{700 cm¡1. Fifteen transverse optic (TO) and longitudinal optic (LO) modes are identi¯ed in the imaginary part of the dielectric function "2(!) and energy loss function Im(¡1/"(!)), respectively. Some of the observed phonon modes show anomalous softening below the magnetic transition temperature TN (»46 K). We attribute this anomalous softening to the spin-phonon coupling caused by phonon modulation of the superexchange integral be- tween the Mn3+ spins. The e®ective charge of oxygen (ZO) calculated using the measured LO{TO splitting increases below TN.