Structural, Chemical, and Electronic State on La-0.7Sr_0.3MnO_3 Dense Thin-film Surfaces at High Temperature - Surface Segregation

摘要

The evolution of the surface topographic and electronic structure and chemical state of the La_0.7Sr_0.3MnO_3 (LSMO) thin films were probed using Scanning Tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) at elevated temperature and oxygen gas environment to identify the structural nature of surface segregation of Sr on LSMO. The as-prepared films had a layer-by-layer structure with a step height of 3.9 A, close to the lattice parameter of LSMO. Up to 500℃, the topography and the step heights remained the same, statistically within 2-4%, implying that no phase separation took place on the top layers of the film. The low oxygen pressures, down to 10~(-10) mbar, at elevated temperatures promoted segregation of Sr by 12-20% on the A-site, accompanied by a smaller increase of (La+Sr)/Mn. Our results suggests two possible structures for Sr segregation; the replacement of La by Sr on the AO-surface of the LSMO which retains a perovskite termination, or a separate AO-oxide phase nucleating on the defected lower layers.