Atomic and microstructure of CMR materials

Abstract

The local structure of bulk and thin films of different perovskite-based CMR materials has been studied by high-resolution electron microscopy. The structure of Ln 1− x A x MnO 3 is not only a function of temperature and A-doping, but also of the thickness of the film. Evidence is produced for a slight monoclinic distortion at room temperature in most Ln 1− x A x MnO 3 compounds. For epitaxial La 0.7Sr 0.3MnO 3(LSMO) films on a LaAlO 3 (0 0 1) the evolution of stress in the film is studied as a function of film thickness and thermal treatment. Close to the interface both film and substrate are elastically strained in opposite sense such that the interface is perfectly coherent for thin films not exceeding 30–35 nm. In thicker films the stress is partially relieved after annealing by the formation of misfit dislocations with an edge character. Thin films of La 1− x Ca x MnO 3 on a SrTiO 3 substrate, exhibit a remarkable microstructure. In direct contact with the SrTiO 3 substrate a thin featureless perfectly coherent La 1− x Ca x MnO 3 layer is formed. Subsequently, on top of this first layer a second thicker layer is deposited; it has a columnar microstructure. These columns, parallel to the interface normal, are in fact prismatic anti-phase domains. Their formation is attributed to the introduction of chemical faults during the film growth process. Islands of rocksalt-type MnO structure, nucleated within the regular La–O layer of the LCMO structure, initiate the formation of the prismatic anti-phase domains. Models of the domain boundaries and of the interface film/substrate are proposed. A growth mechanism for the domain structure is suggested.