  We observed Raman scattering from magnon in frequency range from 10 to 65
cm-1 in BiFeO3 single crystals at cryogenic temperatures; the temperature
dependence of the magnon frequency at 18.2 cm-1 approximates an S=5/2 Brillouin
function up to the temperature (280 K) at which the magnon becomes overdamped.
The diverging cross-section and the frequency-shift at 140K and 200 K implies a
magnon-reorientation transition as in orthoferrites. Magnons in polar materials
such as BiFeO3 are often termed electromagnons meaning that they possess an
electric dipole moment due to magnetoelectric coupling.
