Terahertz spin-wave excitations in the transverse conical phase of BiFeO3

Although ${\mathrm{BiFeO}}_{3}$ is one of the most studied multiferroic materials, recent magnetization and neutron scattering studies have revealed a new magnetic phase in this compound---the transverse conical phase. To study the collective spin excitations of this phase, we performed terahertz spectroscopy in magnetic fields up to 17 T at and above room temperature. We observed five spin-wave branches in the magnetic phase with long-wavelength conical modulation. Using a numerical spin dynamics model, we found two kinds of excitations with magnetic moments oscillating either along or perpendicular to the static fields. Remarkably, we detected strong directional dichroism, an optical manifestation of the magnetoelectric effect, for one spin-wave mode of the conical phase. According to our experiments, the stability of the conical state is sensitive to the magnetic-field history, and it can become (meta)stable at or close to zero magnetic field, which may allow exploiting its magnetoelectric properties at room temperature.