Oxygen vacancy ordering in transition-metal-oxide LaCoO$_{3}$ films*

Oxygen vacancies in complex oxides affect the structure and the electronic and magnetic properties. Here we use atomically-resolved Z-contrast imaging, electron-energy-loss spectroscopy and densityfunctional calculations to demonstrate that ordered oxygen vacancies may act as the controlling degree of freedom for the structural, electronic, and magnetic properties of LaCoO$_{3}$ thin films. We find that epitaxial strain is released through the formation of O vacancy superlattices. The O vacancies donate excess electrons to the Co $d$-states, resulting in ferromagnetic ordering. The appearance of Peierls-like minigaps followed by strain relaxation triggers a nonlinear rupture of the energy bands, which explains the observed insulating behavior. We conclude that oxygen vacancy ordering constitutes a degree of freedom that can be used to engineer novel behavior in complex-oxide films.