Polarization switching dynamics in PbFe1/2Nb1/2O3 ceramics as seen via the frequency dependence of hysteresis loops

Abstract

Our studies have shown that both the shape of hysteresis loops and the value of the remnant polarization Pr of highly-resistive Li-doped PbFe1/2Nb1/2O3 (PFN) ceramics, depend substantially on the frequency of the measuring field. With decreasing of the field frequency, the value of Pr increases. We propose a model explaining this phenomenon, taking into account the relaxation of the charge localized at the grain-boundary surface states. In polycrystalline ferroelectrics-semiconductors, Schottky-type depletion layers are formed at the grain boundaries. The presence of such barrier layers in the PFN ceramics is evidenced by a small positive temperature coefficient of resistivity anomaly near the Curie temperature. Electric field in the Schottky layer is very large and it usually determines the direction of polarization Pgb in the layer. This direction may be opposite to the direction of polarization Pg in the bulk of the grain. If the measuring electric field changes sinusoidally with time, the temporal changes of Pgb are determined by the relaxation time τ of the charge redistribution in the Schottky layers. For high enough values of the external field frequency (υ ~ 70 Hz), this charge has not enough time to relax and spontaneous polarization in the part of the grain adjacent to the grain boundary does not manage to switch. Computer simulations have shown that this model gives rather good agreement with experiment for different frequencies of the external field. © 2017, Springer International Publishing AG.