Ferroelectricity promoted by cation/anion divacancies in SrMnO$_3$

TL;DR

This study shows that cation/anion divacancies can induce and stabilize ferroelectricity in SrMnO$_3$ thin films, even without strain, by creating local electric dipoles that couple with atomic relaxations.

Contribution

It demonstrates that intrinsic defect pairs can promote ferroelectricity in SrMnO$_3$, offering a defect engineering approach to control ferroelectric properties in complex oxides.

Findings

Divacancies induce local polarization in unstrained SrMnO$_3$.

Defect pairs stabilize ferroelectricity below critical strain.

Weaker defect polarization coupling in metallic ferromagnetic SrMnO$_3$.

Abstract

We investigate the effect of polar Sr-O vacancy pairs on the electric polarization of SrMnO$_3$ (SMO) thin films using density functional theory (DFT) calculations. This is motivated by indications that ferroelectricity in complex oxides can be engineered by epitaxial strain but also \textit{via} the defect chemistry. Our results suggest that intrinsic doping by cation and anion divacancies can induce a local polarization in unstrained non-polar SMO thin films and that a ferroelectric state can be stabilized below the critical strain of the stoichiometric material. This polarity is promoted by the electric dipole associated with the defect pair and its coupling to the atomic relaxations upon defect formation that polarize a region around the defect. This suggests that polar defect pairs affect the strain-dependent ferroelectricity in semiconducting antiferromagnetic SMO. For metallic ferromagnetic SMO we find a much weaker coupling between the defect dipole and the polarization due to much stronger electronic screening. Coupling of defect-pair dipoles at high enough concentrations along with their switchable orientation thus makes them a promising route to affect the ferroelectric transition in complex transition metal oxide thin films.