Thermal evolution of the full three-dimensional magnetic excitations in the multiferroic BiFeO ₃

We present neutron inelastic scattering measurements of the full three-dimensional spin-wave dispersion in the multiferroic material BiFeO ₃ for temperatures from 5 to 700 K. Despite the presence of strong electromagnetic coupling, the magnetic excitations behave as conventional magnons over all parts of the Brillouin zone. At low temperature the spin waves are well-defined coherent modes, described by a classical model for a G-type antiferromagnet. A softening of the spin-wave velocity and broadening in energy is already present at room temperature, which is well below the Néel temperature T _N∼640 K, and increases on heating. In addition, a strong hybridization of the Fe 3d and O 2p states is found to modify significantly the distribution of the spin-wave spectral weight, which implies that the spins are not restricted to the Fe atomic sites as previously believed.