Strain effects on ferroelectric polarization and magnetism in orthorhombic HoMnO3

TL;DR

This study uses computational methods to show that in-plane compressive strain significantly enhances ferroelectric polarization in orthorhombic HoMnO3 by altering orbital ordering and electron-lattice interactions, mainly through electronic contributions.

Contribution

It reveals how in-plane strain modifies orbital ordering and polarization in HoMnO3, highlighting the role of electron-lattice coupling and Jahn-Teller distortions in ferroelectric enhancement.

Findings

Net polarization increases under compressive strain.

Electronic contribution to polarization dominates over ionic.

Orbital ordering shifts from JT distortions to mixed states under strain.

Abstract

Aiming at increasing the ferroelectric polarization in AFM-E ortho-\hmo, we investigate the in-plane strain effects on both the magnetic configuration and the polarization by means of density functional theory calculations and model Hamiltonian approaches. Our results show that the net polarization is largely enhanced under compressive strain, due to an increase of the electronic contribution to the polarization, whereas the ionic contribution is found to decrease. We identify the electron-lattice coupling, due to Jahn-Teller (JT) distortions, and its response to strain to be responsible for the observed behavior. The JT-induced orbital ordering of occupied Mn-e$_g^1$ electrons in alternating $3x^2-r^2/3y^2-r^2$ orbital states at equilibrium changes to a mixture with $x^2-z^2/y^2-z^2$ states under in-plane compressive strain. The asymmetric hopping of e$_g$ electrons between Mn ions along zig-zag spin chains (typical of the AFM-E spin configuration) is therefore enhanced under strain, explaining the large value of the polarization. We reproduce the change in the orbital ordering pattern in a degenerate double-exchange model supplemented with electron-phonon interaction. In this picture, the orbital ordering change is related to a change of the Berry phase of the e$_g$ electrons, which in turn causes an increase of the polarization, whose origin is purely electronic.