  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, at \omega = 19.2 cm-1 (T=4K). This spin wave
has not been measured previously above room temperature, but in the present
work we show via Raman spectroscopy that it is an underdamped propagating wave
until 455 K. This has important room-temperature device implications. 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 spin wave
cannot be measured as a propagating wave above 455 K. We also suggest that
since this temperature is coincident with the mysterious "Polomska transition"
(M. Polomska et al., Phys. Stat. Sol. A 23, 567, (1974)) at 458+/-5 K, that
this may be due to overdamping.
