Tetragonal domain structure and magnetoresistance of

La12xSr xCoO3 ~LSCO! has recently been found to exhibit large magnetoresistance ~MR! effect. In this paper, microstructures of LSCO thin films, which may provide important clues for understanding the unusual MR characteristics in these intrinsic magnetic oxides, were determined by transmission electron microscopy. An ordered, anisotropic perovskite-type structure n-LSCO was observed and its atomic model was determined. The ordered structure has a tetragonal cell with a La-Co-Sr-Co-@001# layered atom distribution along the c axis. This structure is intrinsic for the LSCO system and is resulted directly from the lattice substitution between La and Sr. The entire film is composed of @001#, @010#, and @100# anisotropic domains of sizes in the order of 30–200 nm. Microstructures of ~011!and ~001!-type domain boundaries were determined. Although n-La0.5Sr0.5CoO3 is the dominant structure, a tetragonal phase n-La0.33Sr0.67CoO3 has also been identified, which is produced by both the lattice substitution between La and Sr and the variation of local chemical composition. These structures could play a key role in determining the MR properties of the materials which are believed due to the intrinsic coupled spin scattering of Co atoms in different atomic layers. @S01631829~96!09726-3#