Understanding insulating ferromagnetism in LaCoO3 films under tensile strain

TL;DR

This study uses density functional theory to reveal that tensile strain induces a ferromagnetic insulating state in LaCoO3 films, characterized by a specific high-spin and low-spin Co3+ ion ordering.

Contribution

It uncovers the microscopic origin of ferromagnetic insulating behavior in strained LaCoO3, highlighting a unique HS-LS Co3+ ion ordering and superexchange interactions.

Findings

Identified a ferromagnetic insulating ground state with HS-LS Co3+ ordering.

Revealed superexchange interactions favoring ferromagnetism via 90-degree paths.

Confirmed the insulating gap through electronic structure analysis.

Abstract

LaCoO3 thin films grown under epitaxial tensile strain exhibit a robust ferromagnetic insulating state that is absent in the bulk. Despite many studies, both experimental and computational, the microscopic origin of this phenomenon is not well understood. In this work, density functional theory calculations are used to systematically investigate the magnetic ground state of stoichiometric LaCoO3 under epitaxial strain equivalent to that imposed by a SrTiO3 substrate. The results identify a ferromagnetic insulating ground state characterized by a unique ordered array of high-spin (HS) and low-spin (LS) Co3+ ions. The spin state ordering is best described as 2 x 2 columns that consist of alternating HS and LS Co3+ ions, separated by planes of LS Co3+ ions. This leads to HS-LS-LS repeating sequence of Co3+ ions in both pseudocubic [100] and [010] directions. Analysis of the electronic structure confirms the presence of an insulating gap. Evaluation of the superexchange interactions reveal ferromagnetic interactions between HS Co3+ ions via 90 degree paths, and antiferromagnetic interactions via 180 degree paths, both of which are facilitated by empty sigma* (eg) orbitals on the diamagnetic LS Co3+ ions. The strength and number of 90 degree ferromagnetic interactions are sufficient to overcome the competing 180 degree antiferromagnetic interactions stabilizing a ferromagnetic insulating state.