Optically Induced Phase Transition in Compressively Strained BiFeO₃ on LaAlO₃

The structural and electronic properties of multiferroics are highly sensitive to external perturbations when the material has a strain state that places it near the phase boundary between two structural phases. Recent advances in epitaxial growth techniques have allowed epitaxial strain to be used to drive BiFeO₃ to the phase boundary between tetragonal (T-like) and rhombohedral (R-like) phases. Here, we report an optically induced phase transition from the R-like phase to T-like phase following illumination by an ultrafast optical pulse. The transition is observed in a time-resolved synchrotron x-ray diffraction study of a compressively strained BiFeO₃ thin film grown on a LaAlO₃ substrate. The experimental signatures of the phase transition are an increase in the intensity of the x-ray reflection associated with the T-like phase and an accompanying decrease in the intensity of the R-like phase. We have conducted a study of optically induced strain and resolved the intensity changes in BiFeO₃ at the submicron-scale through the use of x-ray nanobeams and x-ray full-field imaging. These probes reveal that (i) the intensity changes are proportional to the magnitude of the strain and (ii) the degree of the phase transition varies across the lateral extent of the BiFeO₃ layer.