Polarization switching in the PbMg1/3Nb2/3O3 relaxor ferroelectric: An atomistic effective Hamiltonian study

Abstract

An atomistic effective Hamiltonian technique is employed to investigate the polarization switching mechanism of the (metastable but long-living) ferroelectric state of ${\mathrm{PbMg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_{3}$ relaxor ferroelectric, resulting from the application of a dc electric field along the $[\overline{1}\overline{1}\overline{1}]$ direction---that is, opposite to the initial polarization. It is predicted that such switching is of inhomogeneous type. It involves the creation of intermediate short-range ordered, relaxor-like phases in between long-range ordered states inside which an infinite cluster exists and has dipoles near either the initial polarization direction (for shorter times) or the field's direction (for longer times). In contrast, dipoles belonging to finite clusters or being isolated can deviate away from [111] and $[\overline{1}\overline{1}\overline{1}]$, and, in fact, rotate in average from [111] to $[\overline{1}\overline{1}\overline{1}]$ when time increases. Such rotations govern the reversal of the polarization from [111] to $[\overline{1}\overline{1}\overline{1}]$ occurring within the intermediate relaxor-like states' region, while always resulting in the overall cancellation of any Cartesian component of the polarization that is perpendicular to [111]. These rotations occurring at the atomic scale also naturally imply that some fundamental assumptions of the original nucleation-limited-switching (NLS) model are not valid, despite the fact that we numerically further find that the whole temporal behavior of the macroscopic polarization can be well fitted by the general formula associated with NLS. In other words, we have stumbled into a novel switching or at least a switching that should be denoted as a generalized NLS model. Finally, three different electric-field regimes are predicted, with each of them having its own dependency of the switching time on the magnitude of the applied electric field and only one of them obeying the Merz's law. The existence of these three regimes is explained in terms of specific microscopic features.