We report the direct observation of a "resonance" mode in the lowest-energy optic phonon very near the zone center around (111) in the multiferroic BiFeO$_3$ using neutron scattering methods. The phonon scattering intensity is enhanced when antiferromagnetic (AFM) order sets in at T$_N = 640$~K, and it increases on cooling. This "resonance" is confined to a very narrow region in energy-momentum space where no spin-wave excitation intensity is expected, and it can be modified by an external magnetic field. Our results suggest the existence of a novel coupling between the lattice and spin fluctuations in this multiferroic system in which the spin-wave excitations are mapped onto the lattice vibrations via the Dzyaloshinskii-Moriya (DM) interaction.

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.