Phonon spectroscopy near phase transition temperatures in multiferroicBiFeO3epitaxial thin films

Abstract

We report a Raman-scattering investigation of multiferroic bismuth ferrite $\left({\text{BiFeO}}_3\right)$ epitaxial ($c$-axis-oriented) thin films from $-192$ to $1000°\text{C}$. Phonon anomalies have been observed in three temperature regions: in the $\gamma$ phase from 930 to $950°\text{C}$; at $\sim 370°\text{C}$, Néel temperature $\left(T_{\text{N}}\right)$, and at $\sim -123°\text{C}$, 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 $\sim 820°\text{C}$ $\left(T_{\text{c}}\right)$ 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 $820–930°\text{C}$ and the same noncubic phase extends through the $\gamma$ phase between $930–950°\text{C}$, in agreement with Arnold et al. [Phys. Rev. B (to be published)], and an evidence of a cubic $\delta$ phase around $1000°\text{C}$ 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 $22–1000°\text{C}$.