Improper flexoelectricity in hexagonal rare-earth ferrites

TL;DR

This paper investigates improper flexoelectricity in hexagonal ferrite h-YbFeO3, revealing a significant coupling between strain gradients and polarization mediated by structural distortions, with potential applications in sensors and energy devices.

Contribution

It introduces the concept of improper flexoelectricity in hexagonal ferrites and demonstrates a novel microscopic mechanism linking strain gradients to polarization via structural distortions.

Findings

Strain gradients of ~10^6 m^-1 occur near grain boundaries and dislocations.

A phenomenological model suggests an indirect flexoelectric effect of ~10 nC/m.

The coupling mechanism is mediated by structural distortion, termed improper flexoelectricity.

Abstract

Flexoelectricity is a universal effect that generates electric polarization due to broken inversion symmetry caused by local strain gradient. The large strain gradient at nanoscale makes flexo-electric effects, especially in nanoscopic ferroelectric materials, promising in sensors, actuator, energy harvesting, and memory applications. In this work, we studied flexoelectricity in hexagonal ferrites h-YbFeO3, an improper ferroelectric expected to have weak piezoelectricity and low sensitivity to depolarization field, which are advantageous for studying flexoelectric effects. We show that in h-YbFeO3 epitaxial thin films, strain gradient on the order of 10^6 m-1 occurs near grain boundaries and edge dislocation, which has a significant impact on the non-polar K3 structural distortion that induces spontaneous polarization. The phenomenological model based on the Landau theory of improper ferroelectricity suggests an indirect flexoelectric effect on the order of 10 nC/m in h-YbFeO3, which is substantially larger than the expectation from Kogan mechanism. These results reveal a novel microscopic mechanism of coupling between strain gradient and polarization mediated by structural distortion, which we call improper flexoelectricity.