In situ observation of atomic movement in a ferroelectric film under an external electric field and stress

Atomic movement under application of external stimuli (i.e., electric field or mechanical stress) in oxide materials has not been observed due to a lack of experimental methods but has been well known to determine the electric polarization. Here, we investigated atomic movement arising from the ferroelectric response of BiFeO₃ thin films under the effect of an electric field and stress in real time using a combination of switching spectroscopy, time-resolved X-ray microdiffraction, and in situ stress engineering. Under an electric field applied to a BiFeO₃ film, the hysteresis loop of the reflected X-ray intensity was found to result from the opposing directions of displaced atoms between the up and down polarization states. An additional shift of atoms arising from the linearly increased dielectric component of the polarization in BiFeO₃ was confirmed through gradual reduction of the diffracted X-ray intensity. The electric-field-induced displacement of oxygen atoms was found to be larger than that of Fe atom for both ferroelectric switching and increase of the polarization. The effect of external stress on the BiFeO₃ thin film, which was controlled by applying an electric field to the highly piezoelectric substrate, showed smaller atomic shifts than for the case of applying an electric field to the film, despite the similar tetragonality.