Phonon and magnon excitations in Raman spectra of an epitaxial bismuth ferrite film

An epitaxial film of bismuth ferrite BiFeO$_3$ on a MgO(001) single-crystal substrate has been prepared by pulsed laser deposition using SrTiO$_3$ and SrRuO$_3$ buffer layers. At room temperature, the polarization characteristics of the Raman spectra of the BiFeO$_3$ film under study suggest a monoclinic symmetry. The high-temperature (295–1100 K) investigations of the Raman spectra have been performed in the frequency range 20 cm$^{-1}$ $<\nu<$ 1600 cm$^{-1}$. Particular attention has been paid to the high-frequency region with a band observed at 610 cm$^{-1}$, which corresponds to the maximum density of states of the magnon branch at the Brillouin zone boundary, and an intense band in the second-order Raman spectra with the maximum at $\sim$1250 cm$^{-1}$, which corresponds to the density of states of two-magnon excitations. It has been found that the intensity of the band at $\sim$1250 cm$^{-1}$ decreases linearly with an increase in the temperature and, above 650 K, this band is absent. The extrapolation of the temperature dependence of the integrated intensity of the band at 1250 cm$^{-1}$ suggests that this film undergoes an antiferromagnetic phase transition at a temperature of $\sim$670 K.