Asymmetric Polarization Vortex in Compositionally Graded Ferroelectrics and Unusual Switching of Toroidal Ordering by an Irrotational Electric Field

Abstract

A reversal of polarization vortex in ferroelectric nanostructures plays important roles for next generations of electronic nanodevices. However, a direct switching of the polarization vortex in ferroelectrics is a nontrivial task since the toroidal moment of vortexis conjugated to a curled electric field rather than a homogeneous one.This work is dedicated to developing an approach to directly switch the vortex ordering under an irrotational (homogeneous) electric field with the use of compositionally graded ferroelectric (cgFE) nanodot. The variation in material compositions induces an additionally broken spatial inversion symmetry at a scale beyond unit-cell level, giving rise to a formation of asymmetric polarization vortex in cgFE nanodot. More interestingly, such an asymmetric character facilitates to a switch of the polarization vortex by an irrotational electric field. In particular,the vortex ordering can be directly switched from CCW to CW rotations and vice versa without a formation of intermediate domain structures during the switching process. This switching behaviour is distinguished from that in homogeneous counterparts. We further demonstrate that the variation in material compositions tailors the distribution of electrostatic and total free energies in the cgFE nanodot that can control the annihilation/initiation process of the polarization vortex under irrotational electric field, providing fundamental reason for the direct switching of the polarization vortex. Another interesting issue is found that both the amplitude and frequency of applied electric field strongly affect the switching behaviour of the polarization vortex incgFE nanodot.