Non-linearity in engineered lead magnesium niobate (PbMg_1/3Nb_2/3O₃) thin films

The local compositional heterogeneity associated with the lack of long-range ordering of Mg²⁺and Nb⁵⁺in PbMg_1/3Nb_2/3O₃(PMN) is correlated with its characteristic "relaxor" ferroelectric behavior. Earlier work [Shetty et al., Adv. Funct. Mater. 29, 1804258 (2019)] examined the relaxor behavior in PMN thin films grown at temperatures below 1073 K by artificially reducing the degree of disorder via synthesis of heterostructures with alternate layers of Pb(Mg_2/3Nb_1/3)O₃and PbNbO₃, as suggested by the "random-site model." This work confirmed the development of ferroelectric domains below 150 K in long-range-ordered films, epitaxially grown on (111) SrTiO₃substrates using alternate target timed pulsed-laser deposition of Pb(Mg_2/3Nb_1/3)O₃and PbNbO₃targets with 20% excess Pb. In this work, the first through third-harmonic dielectric charge displacement densities and complex dielectric susceptibilities were analyzed as a function of temperature and frequency in zero-field-cooled PMN films with short- A nd long-range ordering. The long-range ordering decreased the dispersion in the first- A nd third-harmonic dielectric charge displacement densities relative to short-range-ordered films. It was found that the one-dimensional ordering achieved in the long-range-ordered film is insufficient to achieve a fully normal ferroelectric state. In the presence of quenched random electric fields, these films require a small ac field to facilitate percolation of the polar nano-regions, enabling normal ferroelectric-like behavior at lower temperature (T < 240 K). The films behave like a typical relaxor near room temperature. With reduced ordering, the short-range films exhibit greater dispersion in linear and higher order harmonic dielectric charge displacement density.