Electrically driven ferroelastic domain walls, domain wall interactions, and moving needle domains

Abstract

Ferroelastic domains generate polarity near domain walls via the flexoelectric effect. Applied electric fields change the wall dipoles and generate additional dipoles in the bulk. Molecular dynamics simulations show that the thickness of domain walls changes when an electric field is applied to the sample. Fields parallel to the walls lead to expansion of the wall thickness while fields perpendicular to the wall lead to shrinking of the wall thickness. The interactions between polar domain walls expand over more than 45 unit cells, the resulting forces change the wall-wall distances if pinning effects are small. The interaction increases nonlinearly with decreasing wall-wall distances favoring equal wall distances as the consequence of energy minimization under the constraints of a constant number of domain walls. Even for small groups of three walls the sequence of walls is locally periodic: assemblies of three parallel domain walls arrange themselves so that the intermediate domain wall is located exactly in the middle between the two outer walls. The driving force is appreciable if the distance between the outer domain walls is below approximately 30 lattice units. Pairs of domain walls often form needle domains where the shaft of the needle is ca. 3 lattice units wide. The movement of needle domains under applied electric field was simulated. The advancement and retraction of needles is larger in finite samples with charge-free surfaces than under periodic boundary conditions in the bulk. The needle tip moves even more freely when the sample surface is charged.

The duration-energy-size enigma for acoustic emission

“…The energy of avalanches in Barkhausen noise is partially released by elastic waves during acoustic emission, AE. As AE measurements have unsurpassed sensitivity they became the method of choice for the investigation of field induced changes in ferroics and are hence at the heart of current research into the dynamics of switching processes 2,13,14,[17][18][19][20][21][22]30,[32][33][34][35][36] . Here we show why some results of AE deviate from the predictions of mean field theory, MFT 14,37 .…”

Current vortices and magnetic fields driven by moving polar twin boundaries in ferroelastic materials

“…Moving needle domains are extremely common in ferroelastic materials. Their dynamics have been described for several decades 28,29,37,[40][41][42][43][44][45] . The needle domains contain kinks near the tip 46 , where the domain walls appear mesoscopically bent.…”

“…Another typical microstructure in ferroelastic materials is the comb pattern 28,29,37,[40][41][42][43][44][45]48 , which consists of needle arrays with various distances between the needles. In our model, the stressdriven propagation of comb pattern consists of three vertical thin needles with 4 atomic layers for each ( Supplementary Fig.…”

Twisting of pre-twinned α-Fe nanowires: from mild to wild avalanche dynamics

“…For close enough TBs, a kink deformation nucleating within one TB can nucleate other kinks in other TBs through elastic stress redistributions within the matrix, generating avalanches. Owing to the fast decay of these elastic interactions with distance [54,55], such mechanism is expected to vanish for TB spacings above few nanometers (Fig. 8d).…”