Phonon Spectroscopy Near Phase Transition Temperatures in Multferroic BiFeO3 Epitaxial Thin Films

TL;DR

This study uses Raman spectroscopy to investigate phonon behavior near phase transition temperatures in BiFeO3 thin films, revealing multiple phase transitions, phonon anomalies, and the existence of a cubic delta-phase at high temperatures.

Contribution

It provides new insights into the phase transitions and phonon dynamics of BiFeO3 thin films, including evidence of a cubic delta-phase not observed in bulk materials.

Findings

Phonon anomalies observed near 370°C, 123°C, and 950°C.

Disappearance of Raman modes at 820°C indicating structural transition.

Evidence of a cubic delta-phase around 1000°C in thin films.

Abstract

We report a Raman scattering investigation of multiferroic bismuth ferrite BiFeO3 epitaxial (c-axis oriented) thin films from -192 to 1000C. Phonon anomalies have been observed in three temperature regions: in the gamma-phase from 930C to 950C; at ~370C, Neel temperature (TN), and at ~123C, 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 ~820C (Tc) is compatible with the known structural phase transition and the Pbnm orthoferrite space group assigned by Arnold {\it et al.} \cite{arnold:09}. The spectra also revealed a {\it non-cubic} $\beta$-phase from 820-930\dc and the same {\it non-cubic} phase extends through the $\gamma$-phase between 930-950\dc, in agreement with Arnold {\it et al.} \cite{arnold2:09}, and an evidence of a cubic $\delta$-phase around 1000\dc in thin films that is not stable in powder and bulk. Such a cubic phase has been theoretically predicted in \cite{vasquez:prb09}. Micro-Raman scattering and X-ray diffraction showed no structural decomposition in thin films during the thermal cycling from 22-1000\dc.