Phonon spectroscopy near phase transition temperatures in multiferroic BiFeO3 epitaxial thin films

We report a Raman-scattering investigation of multiferroic bismuth ferrite (BiFeO3) epitaxial (c-axis-oriented) thin films from -192 to 1000 degrees C. Phonon anomalies have been observed in three temperature regions: in the gamma phase from 930 to 950 degrees C; at similar to 370 degrees C, Neel temperature (T-N), and at similar to-123 degrees C, due to a phase transition of unknown type (magnetic or structural). An attempt has been made to understand the origin of the weak phonon-magnon coupling and the dynamics of the phase sequence. The disappearance of several Raman modes at similar to 820 degrees C (T-c) is compatible with the known structural phase transition and the Pbnm orthoferrite space group assigned by Arnold et al. [Phys. Rev. Lett. 102, 027602 (2009)]. The spectra also revealed a noncubic beta phase from 820-930 degrees C and the same noncubic phase extends through the gamma phase between 930-950 degrees C, in agreement with Arnold et al. [Phys. Rev. B (to be published)], and an evidence of a cubic delta phase around 1000 degrees C in thin films that is not stable in powder and bulk. Such a cubic phase has been theoretically predicted by and Gonzalez-Vazquez and Iniguez [Phys. Rev. B 79, 064102 (2009)]. Micro-Raman scattering and x-ray diffraction showed no structural decomposition in thin films during the thermal cycling from 22-1000 degrees C.