Neutron polarization analysis of the anisotropic spin wave excitations in multiferroic BiFeO₃

A multiferroic material BiFeO3 shows a cycloidal spin structure below 640 K. Anisotropy of the magnetic excitations below 6 meV was previously studied using neutron polarization analysis (Zhang et al. 2022 Phys. Rev. B 105 144426). The magnetic excitations in and out of the cycloidal plane were separated and compared to those calculated based on the linear spin-wave theory. The overall feature of the observed excitations was found to be consistent with those calculated. Further polarized neutron study with extended energy range has been performed. The anisotropy of the magnetic excitations is present at least up to 15 meV. The out-of-plane mode is constantly more intense by ∼50% than the in-plane mode in the energy range between 3 and 15 meV. The anisotropic excitations in the wide energy range are unexpected since multiple in-plane and out-of-plane modes are mixed at higher energies and the excitations are expected to become more isotropic.