Electric polarization of Sr$_{0.5}$Ba$_{0.5}$MnO$_{3}$: a multiferroic Mott insulator

TL;DR

This study uses advanced computational methods to predict and analyze the electric polarization in the Mott insulator Sr$_{0.5}$Ba$_{0.5}$MnO$_{3}$, revealing the effects of correlations and magnetic order on ferroelectricity.

Contribution

First ab initio calculations of electric polarization in a correlation-driven Mott insulator combining DFT and DMFT, explaining the role of magnetic order and correlations.

Findings

Predicted ferroelectric polarization of ~16.5 μC/cm² at high temperature.

Measured polarization of ~13.3 μC/cm² at low temperature matches predictions.

Correlations suppress hybridization, influencing ferroelectricity and magnetic order.

Abstract

Ab initio calculations of the electric polarization of correlation-driven insulating materials, namely Mott insulators, have not been possible so far. Using a combination of density functional theory and dynamical-mean-field theory we study the electric polarization of the Mott insulator Sr$_{0.5}$Ba$_{0.5}$MnO$_{3}$. We predict a ferroelectric polarization of $\simeq 16.5 \mu C/cm^2$ in the high temperature paramagnetic phase and recover the measured value of $\simeq 13.3 \mu C/cm^2$ in the low temperature antiferromagnetic phase. Our calculations reveal that the driving force for the ferroelectricity, the hybridization between Mn e$_g$ and O p orbitals, is suppressed by correlations, in particular by the Hund coupling and by the onset of magnetic order. They also confirm that the half-filled Mn $t_{2g}$ orbitals give rise to the antiferromagnetic Mott phase. This magnetic ordering leads to changes in the ionic polar displacement and in turn to the electronic polarization. In addition, for a fixed ionic displacement, we find that there is a reduction in the electronic contribution due to partial magnetic polarization of the Mn e$_g$ orbitals. The reduction of the polarization due to ionic displacement dominates over the additional electronic part, hence the net magneto-electric coupling is negative.