Flexoelectricity in Amorphous Hafnium Oxide (HfO2)

TL;DR

This paper presents a novel method to measure flexoelectricity accurately in amorphous HfO2, establishing the first such measurement in an amorphous material and revealing a quadratic relationship with permittivity.

Contribution

It introduces a groundbreaking technique to isolate flexoelectric effects from other phenomena and provides the first measurement of flexoelectricity in amorphous hafnium oxide.

Findings

Measured a flexoelectric coefficient of 105 ± 10 pC/m in amorphous HfO2

Revealed a quadratic relationship between flexoelectric coefficient and permittivity

Demonstrated a detection threshold below 1 fC/m

Abstract

Flexoelectricity, inherent in all materials, offers a promising alternative to piezoelectricity for nanoscale actuation and sensing. However, its widespread application faces significant challenges: differentiating flexoelectric effects from those of piezoelectricity and other phenomena, verifying its universality across all material structures and thicknesses, and establishing a comprehensive database of flexoelectric coefficients across different materials. This work introduces a groundbreaking methodology that accurately isolates flexoelectricity from piezoelectric, electrostrictive and electrostatic effects, with a detection threshold extending below 1 fC/m. The robustness of this method is demonstrated through its application to amorphous hafnium oxide (HfO$_2$), successfully measuring a flexoelectric coefficient of 105 $\pm$ 10 pC/m. This measurement signifies the first measurement of flexoelectricity in hafnia, as well as in any amorphous material. Additionally, the study compiles a list of published flexoelectric coefficients, revealing an important insight. The relationship between the flexoelectric coefficient and the material's relative permittivity is better approximated by a quadratic proportionality. This challenges the traditional linear assumption proposed in Kogan's work and opens new avenues for future research in flexoelectric materials.