Insulating FerromagneticLaCoO3−δFilms: A Phase Induced by Ordering of Oxygen Vacancies

The origin of ferromagnetism in strained epitaxial ${\mathrm{LaCoO}}_{3}$ films has been a long-standing mystery. Here, we combine atomically resolved $Z$-contrast imaging, electron-energy-loss spectroscopy, and density-functional calculations to demonstrate that, in epitaxial ${\mathrm{LaCoO}}_{3}$ films, oxygen-vacancy superstructures release strain, control the film's electronic properties, and produce the observed ferromagnetism via the excess electrons in the Co $d$ states. Although oxygen vacancies typically dope a material $n$-type, we find that ordered vacancies induce Peierls-like minigaps which, combined with strain relaxation, trigger a nonlinear rupture of the energy bands, resulting in insulating behavior.