New magnetic phase transitions in BiFeO3

We observed Raman scattering from magnons in the 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 (similar to 280 K) at which the magnon becomes overdamped. The diverging cross section and the frequency shift at similar to 140 and similar to 200 K implies a spin-reorientation transition as in orthoferrites.