Ferromagnetic insulating state in tensile-strained LaCoO$_3$ thin films

TL;DR

This study uses LDA+U calculations to identify the ferromagnetic insulating state in tensile-strained LaCoO$_3$ thin films as a HS/LS mixture, revealing the underlying magnetic interactions and potential experimental detection methods.

Contribution

It demonstrates that the ferromagnetic insulating state in strained LaCoO$_3$ is a HS/LS mixture stabilized by superexchange, providing insights into its electronic and magnetic structure.

Findings

HS/LS mixture state is the most stable ferromagnetic insulator

Superexchange interactions stabilize FM order in HS/LS state

NMR can identify Co spin states via electric field gradient

Abstract

With local density approximation + Hubbard $U$ (LDA+$U$) calculations, we show that the ferromagnetic (FM) insulating state observed in tensile-strained LaCoO$_3$ epitaxial thin films is most likely a mixture of low-spin (LS) and high-spin (HS) Co, namely, a HS/LS mixture state. Compared with other FM states, including the intermediate-spin (IS) state (\textit{metallic} within LDA+$U$), which consists of IS Co only, and the insulating IS/LS mixture state, the HS/LS state is the most favorable one. The FM order in HS/LS state is stabilized via the superexchange interactions between adjacent LS and HS Co. We also show that Co spin state can be identified by measuring the electric field gradient (EFG) at Co nucleus via nuclear magnetic resonance (NMR) spectroscopy.