Charge Order Driven Multiferroic Behaviour in Sr$_4$Fe$_6$O$_{12}$: An $\textit{Ab-initio}$ Study

TL;DR

This study uses ab-initio calculations to reveal how charge ordering in Sr4Fe6O12 induces a phase transition and large electronic polarization, leading to multiferroic behavior with magneto-electric coupling.

Contribution

It demonstrates the charge order-driven phase transition and resulting multiferroic properties in Sr4Fe6O12 through first-principles calculations.

Findings

Charge ordering induces a phase transition from centrosymmetric to polar structure.

Electronic polarization is significantly larger than ionic polarization.

Magnetic ordering influences polarization directions and multiferroic behavior.

Abstract

In this letter, we report the structural, electronic and ferroelectric properties of the layered mixed-valent transition-metal compound, Sr$_{4}$Fe$_{6}$O$_{12}$ (SFO). We demonstrate how SFO undergoes a phase transition from a high-temperature (T) centrosymmetric tetragonal phase ($P4_{2}/mnm$) to a low-T polar orthorhombic phase ($Pmn2_{1}$). The transition is primarily driven by charge ordering at tetrahedral Fe-layer creating Fe$^{3+}$ and Fe$^{2+}$ cations between two edge sharing tetrahedra. This charge ordering induces electronic polarization, which is remarkably larger (3.5 times) in magnitude than ionic polarization and oppositely directed, giving a net polarization of 0.05 C/m$^2$ which is comparable to the state-of-the-art rare-earth nickelets and manganite perovskites. The direction of structural distortion, governed by the polar mode irrep $\Gamma_{5}^{-}$, depends sensitively on the type of magnetic ordering in the Fe-octahedral layer. Consequently, both the ionic and electronic polarization directions are influenced by magnetic ordering, suggesting the potential for multiferroic behavior with strong magneto-electric coupling in this material.