  We have determined the full magnetic dispersion relations of multiferroic
BiFeO3. In particular, two excitation gaps originating from magnetic
anisotropies have been clearly observed. The direct observation of the gaps
enables us to accurately determine the Dzyaloshinskii-Moriya (DM) interaction
and the single ion anisotropy. The DM interaction supports a strong
magneto-electric coupling in this compound.
