High-resolution electron microscopy study of strained epitaxial La_0.7Sr_0.3MnO₃ thin films

The microstructure of strained epitaxial La_0.7Sr_0.3MnO₃(LSMO) films on a LaA103(001) substrate has been investigated by means of electron diffraction and high-resolution electron microscopy in cross-section and plan view. Courier transforms of the high-resolution images allow us to deduce the local lattice parameters at increasing distance from the interface. The evolution of stress in the film is studied as a function of film thickness and thermal treatment. It is found that, close to the interface, both the film and the substrate are elastically strained in the opposite sense such that the interface is perfectly coherent for thin films not exceeding a certain thickness (about 30-35 nm). In thicker films, inhomogeneities develop in the as-grown films. In thicker (about 120nm) films the stress is partially relieved after annealing by the formation of misfit dislocations with an edge character. In annealed thick films we also provide some evidence for the formation of coherent ‘precipitates’ that contribute to the relief of stress. It is suggested that part of the mismatch strain is accommodated by the isomorphous substitution of ions of a different size and/or a dilferent charge.