Magnon Raman spectroscopy and in-plane dielectric response in BiFeO3: Relation to the Polomska transition

We report strong dielectric loss anomalies near 450 K in bismuth ferrite, together with anomalous magnon damping, and suggest that since this temperature is coincident with the mysterious "Polomska transition" [M. Polomska et al., Phys. Status Solidi A 23, 567 (1974)] at 458 +/- 5 K, this may indicate a surface phase transition. In bismuth ferrite thin films the cycloidal spiral spin structure is suppressed, and as a result the spin-wave magnon branches of long wavelength are reduced from a dozen to one, a omega = 19.2 cm(-1) (T = 4 K). This spin wave has not been measured previously in thin-film bismuth ferrite above room temperature, but in the present work we show via Raman spectroscopy that it is an underdamped propagating wave until 455 K. The data show that omega(T) follows an S = 5/2 Brillouin function, and hence its Fe+3 ions are in the high-spin 5/2 state and not the low-spin S = 1/2 state. The surface spin wave cannot be measured as a propagating wave above the Polomska transition at 458 K.