  Epitaxial strain imposed in complex oxide thin films by heteroepitaxy is
recognized as a powerful tool for identifying new properties and exploring the
vast potential of materials performance. A particular example is LaCoO3, a zero
spin, nonmagnetic material in the bulk, whose strong ferromagnetism in a thin
film remains enigmatic despite a decade of intense research. Here, we use
scanning transmission electron microscopy complemented by X-ray and optical
spectroscopy to study LaCoO3 epitaxial thin films under different strain
states. We observed an unconventional strain relaxation behavior resulting in
stripe-like, lattice modulated patterns, which did not involve uncontrolled
misfit dislocations or other defects. The modulation entails the formation of
ferromagnetically ordered sheets comprising intermediate or high spin Co3+,
thus offering an unambiguous description for the exotic magnetism found in
epitaxially strained LaCoO3 films. This observation provides a novel route to
tailoring the electronic and magnetic properties of functional oxide
heterostructures.
