S. Raghuraman, K. Kelley, S. Jesse
Oak Ridge National Laboratory,
Keywords: ferroelectrics, piezo response force microscopy
Summary:Quantifying domain wall dynamics in ferroelectric materials is crucial to predict its functional properties. Moreover, nudging, erasing and writing domain walls on demand provides an efficient method to engineer conductive nanostructures. Especially, piezo-response force microscopy has been widely used to observe and manipulate bias and thermally induced domain wall motion at the nanometer scale. It has been shown that domain wall motion is sensitive to several factors such as pre-existing domain configurations, defects, dopants, local wall curvature, and thermal fluctuations. However, in order to obtain statistically significant dynamics as a function of spatial and thermal perturbations, there is a need to develop a fast, repeatable, and reliable technique to map displacement and velocities of domain walls under applied external stimuli. Here, we demonstrate a novel technique named “Scanning Oscillator Microscopy” to quantify domain wall oscillations in Lead Titanate thin film (epitaxial PbTiO3). The scanning oscillator simultaneously applies a high frequency AC bias which drives piezo-response and a low frequency AC bias that modulates the instantaneous domain wall displacement. The resulting three-dimensional dataset contains both spatial and temporal effects of domain wall oscillations. The suite of analyses modules specifically developed to visualize the dataset uncovers several interesting facets of wall motion on Lead Titanate. Notably, the 180° domain walls between the antiparallel c+/c- domains show significant expansion and contraction under voltage cycles. Since the voltage magnitudes explored were sub-coercive field, the domain wall motion was repeatable with several cycles with no hysteresis. The overall displacement and velocities of the 180° wall depends greatly on its orientation relative to pre-existing large a- (in-plane) and c- (out-of-plane) domains and periodic a-c domain structures. The local curvature of the domain walls tends to affect their susceptibility to move, where, regions of higher curvature in general show larger displacements. In addition, shrinking and expansion was also observed within the periodic a-c structures due to differences in response of in-plane and out-of-plane domains to voltage regimes. Access to time-synchronized displacement data at every pixel further allows us to quantify wall motion dynamics by programmatically identifying heavily pinned and de-pinned regions. In general, obtaining several distinct observations such as in Scanning Oscillator Microscopy allows fast and detailed investigation of domain wall dynamics on ferroelectric materials and other heterostructures under a wide range of external stimuli such as heat, bias, light, and stress.