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

Abstract

Ferroelastic twin boundaries often have properties that do not exist in bulk, such as superconductivity, polarity etc. Designing and optimizing domain walls can hence functionalize ferroelastic materials. Using atomistic simulations, we report that moving domain walls have magnetic properties even when there is no magnetic element in the material. The origin of a robust magnetic signal lies in polar vortex structures induced by moving domain walls, e.g., near the tips of needle domains and near domain wall kinks. These vortices generate displacement currents, which are the origin of magnetic moments perpendicular to the vortex plane. This phenomenon is universal for ionic crystals and holds for all ferroelastic domain boundaries containing dipolar moments. The magnetic moment depends on the speed of the domain boundary, which can reach the speed of sound under strong mechanical forcing. We estimate that the magnetic moment can reach several tens of Bohr magnetons for a collective thin film of 1000 lattice planes and movements of the vortex by the speed of sound. The predicted magnetic fields in thin slabs are much larger than those observed experimentally in SrTiO3/LaAlO3 heterostructures, which may be due to weak (accidental) forcing and slow changes of the domain patterns during their experiments. The dynamical multiferroic properties of ferroelastic domain walls may have the potential to be used to construct localized magnetic memory devices in future.

Ferroelastic Twinning in Minerals: A Source of Trace Elements, Conductivity, and Unexpected Piezoelectricity

“…Magnetic signals of moving kinks in twin walls were computer simulated and they are now predicted for perovskite structures [109]. These results confirm the idea that twins and twin walls commonly produce vortex structures [110,111], as shown in Figure 11.…”

Crackling noise and avalanches in minerals

“…We know today that a transistor, as an example, does not need bulk materials to operate but is often localized in tiny areas inside twin boundaries or near junctions between boundaries. The same holds for ferroic memories and memristive conductors (Salje et al 2017a;He et al 2019;Bak et al 2020;Lu et al 2020a;Zhang et al 2020;Salje 2021) where only a few atoms near domain boundaries move. The diameters or thicknesses of these functional regions are a few inter-atomic distances (Lu et al 2019(Lu et al , 2020bMcCartan et al 2020).…”

Dynamic domain boundaries: chemical dopants carried by moving twin walls

“…Insight into the atomic mechanism was initially based on simple analytical models. Great progress was made by the development of simple toy models where the local mechanisms can be studied in systems with some 100 000 particles [122][123][124]. Future work needs now to extend the models to more realistic interatomic potentials and larger simulation boxes.…”

“…Each domain boundary may carry specific chemical loading, which can be tailored to change the percolation point for the memristive carrier transport. Even additional magnetic interactions, generated by the polar and rough domain boundaries, have been postulated [114] but not confirmed experimentally.…”