Strategy to utilize transmission electron microscopy and X-ray diffraction to investigate biaxial strain effect in epitaxial BiFeO₃ films

The recent resurgence of bismuth ferrite (BiFeO₃) as a multiferroic material was triggered by the revelation of its true bulk physical properties in the mid 2000s. Subsequently, multiferroic properties of BiFeO₃ have been found to improve when it is grown as epitaxial film owing to the biaxial strain imposed by substrate materials. Since the crystal and microstructural modifications caused by the strain dominate the multiferroic property changes in BiFeO₃, tremendous efforts have been devoted to the investigation of structural changes in epitaxial BiFeO₃ films. However, details about strain-induced structural modifications remain elusive owing to the remarkably complex nature of BiFeO₃. In this review, we discuss the followings: (1) what are the pros and cons between transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques, (2) a noble methodology of how to apply TEM and XRD to unambiguously identify crystal symmetries in epitaxial BiFeO₃, and (3) once crystal symmetries are clearly identified, how can the misfit strain be accurately evaluated.