Avalanches and the Propagation and Retraction of Ferroelastic Needle Domains

Abstract

Time-lapse optical microscopy of single crystal LaAlO3 allows the propagation and retraction of individual needle domains to be observed under conditions of slowly varying shear stress. The propagation of a single ferroelastic needle domain consists of two parts: a continuous front propagation and jerky avalanches. Optical observation and themodynamic analysis show that the continuous behavior is thermally activated. The avalanches follow power law behavior with an energy exponent ε = −1.8 ± 0.2 in agreement with self-similar avalanches close to the depinning threshold. Twin walls remain smooth during propagation, whereas the one-dimensional front line of the needle tip shows “wiggles” caused by interactions with defect fields. The front line becomes highly distorted during approach to, or retraction from, the sample surface. Singularities of the characteristic (∼Larkin) length occur when the front line breaks. Elastic forces produce planar twin walls with very large Larkin lengths, whereas the front line is not restrained by the compatibility energy and displays considerably shorter Larkin lengths.