Spin wave measurements over the full Brillouin zone of multiferroic BiFeO ₃

Using the inelastic neutron scattering technique, we measured the spin wave dispersion over the entire Brillouin zone of room temperature multiferroic BiFeO ₃ single crystals with magnetic excitations extending to as high as 72.5meV. The full spin waves can be explained by a simple Heisenberg Hamiltonian with a nearest-neighbor exchange interaction (J=4.38meV), a next-nearest-neighbor exchange interaction (J'=0.15meV), and a Dzyaloshinskii-Moriya-like term (D=0.107meV). This simple Hamiltonian determined, for the first time, for BiFeO ₃ provides a fundamental ingredient for understanding the novel magnetic properties of BiFeO ₃.