  Using inelastic neutron scattering technique, we measured the spin wave
dispersion over the entire Brillouin zone of room temperature multiferroic
BiFeO3 single crystals with magnetic excitations extending to as high as 72.5
meV. The full spin waves can be explained by a simple Heisenberg Hamiltonian
with a nearest neighbor exchange interaction (J=4.38 meV), a next nearest
neighbor exchange interaction (J'=0.15 meV), and a Dzyaloshinskii-Moriya-like
term (D=0.107 meV). This simple Hamiltonian determined, for the first time, for
BiFeO3 provides a fundamental ingredient for understanding of the novel
magnetic properties of BiFeO3.
