Origin of ferromagnetic ordering in LaCoO$_{3}$ epitaxial thin films*

LaCoO$_{3}$ (LCO) film has received attention due to its unexpected ferromagnetic (FM) ordering, which is distinctly different from the bulk counterpart. Although the exact origin has not been understood, previous studies have suggested that the epitaxial strain should play an important role. In this work, we show that the FM ordering could be related to a locally-ordered microstructure. We used PLD to deposit epitaxial LCO thin films on various substrates in order to impose different degree of strain. XRD and XAS studies showed that the films are of high quality, without any secondary phases or changes in the Co valence. In addition, all the films were coherently-strained. From the STEM investigation, however, we noticed that some of the films had an unexpected stripe-like superstructure along the $<$100$>$ direction. While the microstructure resembles that of oxygen-vacancy or charge ordering, typically found in doped transition metal oxides, we could rule out such possibilities and interpret it as a nanoscale twin boundary. The strain induced structural change seems to originate the FM ordering.