Neutron inelastic scattering measurements of low-energy phonons in the multiferroic BiFeO3

We present neutron inelastic scattering measurements of the low-energy phonons in single crystal BiFeO3. The dispersions of the three acoustic phonon modes (LA along [100], TA1 along [010], and TA2 along [11¯0]) and two low-energy optic phonon modes (LO and TO1) have been mapped out between 300 and 700K. Elastic constants are extracted from the phonon measurements. The energy linewidths of both TA phonons at the zone boundary clearly broaden when the system is warmed toward the magnetic ordering temperature TN=640K. This suggests that the magnetic order and low-energy lattice dynamics in this multiferroic material are coupled.