Anisotropic spin-wave excitations in multiferroic BiFeO3

Polarized inelastic neutron-scattering experiments have been performed to elucidate the anisotropic behavior of the low-energy spin-wave excitations in a multiferroic BiFeO3, which shows a cycloidal spin structure below 640 K. Using neutron polarization analysis for single magnetic domain crystals, magnetic excitation modes in and out of the cycloidal plane below 6 meV were separated successfully. The magnetic excitation spectra were analyzed using linear spin-wave theory. The low-energy magnon density of states consist of several magnon modes, including the two anisotropic modes, Phi and Psi modes, distributed in and out of the cycloidal plane, respectively, which were previously observed using optical spectroscopies. Furthermore, there are other magnon modes that are not active in optical measurements. A model spin Hamiltonian, which reproduces the spin-wave frequencies observed using optical spectroscopies, explains the overall spectra reasonably well.