Phonon anomalies in multiferroic BiFeO3 epitaxial thin films with rhombohedral (R3c) symmetry

Raman spectrum of (111) oriented BiFeO₃ (BFO) thin film was studied in temperature range between 27 °C - 500 °C. Observed three sharp A₁ Raman modes at 136, 162, and 212 cm^-1 show anomalous changes in frequencies, line width and integrated intensity between span of temperature range 250-400 °C i.e. around magnetic phase transition (TN). The sign and magnitude of such anomalous behavior appears to be an experimental evidence of perturbation in antiferromagnetic ordering coupled with octahedral tilting reveals strong spin-phonon coupling around TN. The soft mode behavior of A1 Raman modes at 136 and 162 cm^-1 was observed directly and reveals a decrease of the Curie temperature (T_C) of the strained film, which was originally suggested by M. S. Kartavtseva and co workers [Thin Solid Films, 515 (2007) 6416].