Atomic-resolution imaging of spin-state superlattices in nanopockets within cobaltite thin films

Certain cobalt oxides are known to exhibit ordered Co spin states, as determined from macroscopic techniques. Here we report real-space atomic-resolution imaging of Co spin-state ordering in nanopockets of La _0.5Sr_0.5CoO_3-δ thin films. Unlike the bulk material, where no Co spin-state ordering is found, thin films present a strain-induced domain structure due to oxygen vacancy ordering, inside of which some nanometer sized domains show high-spin Co ions in the planes containing O vacancies and low-spin Co ions in the stoichiometric planes. First-principles calculations provide support for this interpretation.