Ferromagnetic insulating state in tensile-strained LaCoO ₃ thin films from LDA + U calculations

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 (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 the HS/LS state is stabilized via the superexchange interactions between adjacent LS and HS Co. We also show that the Co spin state can be identified by measuring the electric field gradient at the Co nucleus via nuclear magnetic resonance spectroscopy.