Origin of lowered magnetic moments in epitaxially strained thin films of multiferroic Bi$_2$FeCrO$_6$

TL;DR

This study uses density-functional theory to explore how epitaxial strain affects magnetic ordering and cation arrangement in Bi$_2$FeCrO$_6$ thin films, revealing strain-induced magnetic phase transitions and stability of disordered structures.

Contribution

It provides the first detailed theoretical analysis of strain effects on magnetic and cation ordering in Bi$_2$FeCrO$_6$ thin films, highlighting the stability of disordered structures and magnetic phase transitions.

Findings

Epitaxial strain induces a transition from C to A-type AFM order.

Disordered B-site cation structures are more stable under strain.

Strain influences the Fe-Cr superexchange coupling, affecting magnetic moments.

Abstract

We have investigated the effect of epitaxial strain on the magnetic properties and $B$-site cation ordering in multiferroic Bi$_2$FeCrO$_6$ (001) thin films using a density-functional theory approach. We find that in thin films with rock-salt ordering of Fe and Cr the ground state is characterised by C-type anti-ferromagnetic (AFM) order. This is in contrast to the bulk form of the material which was predicted to be a ferrimagnet with G-type AFM order. Furthermore, the cation ordered thin-films undergo a transition with epitaxial strain from C to A-type AFM order. Other magnetic orders appear as thermally accessible excited states. We also find that $B$-site cation disordered structures are more stable in coherent epitaxial strains thereby explaining the lowered magnetic moments observed in these samples at room temperature. Strain varies both the sign as well as strength of the Fe-Cr superexchange coupling resulting in a very interesting phase diagram for Bi$_2$FeCrO$_6$ thin films.