Microscopic Study of Structure, Chemical Composition and Local Conductivity of La2/3Sr1/3MnO3 Films

Abstract

The colossal magnetoresistance (CMR) manganites have attracted intensive study due to their richness of underlying physics and potential technological applications. Of particular interest is half-metallic La2/3Sr1/3MnO3 (LSMO) because it possesses the highest known Curie temperature of the group ~ 370 K), which makes it a promising candidate for room temperature spintronic applications. On the other hand, LSMO ultrathin films exhibit a metal-insulator transition (MIT) when reducing film thickness. The origin of such a thickness-dependent MIT remains highly controversial, though understanding and controlling this kind of behavior is necessary for any possible device applications. An essential first step then, and the objective of this thesis project, is the characterization of the lattice structure and chemical composition. The chemical composition of LSMO films grown on TiO2-terminated SrTiO3 (001) is quantified with unit cell precision by combining in-situ angle-resolved x-ray photoelectron spectroscopy (ARXPS), ex-situ scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS). Substantial deviations in Sr doping concentrations from its bulk value are observed at both the interface and surface. Deviation at the interface is due mainly to single unit cell intermixing, while in proximity to the surface the segregation occurs in a wider thickness range. The surface undergoes a gradual conversion from MnO2 to (La/Sr)O layer termination with increasing thickness. To study the consequences of the surface Sr segregation, scanning tunneling spectroscopy (STS) is applied to study the local electronic properties. According to the STS results, the nonmetallic character and spontaneous polarization at the surface of both thin and thick LSMO films is revealed. The difference in surface behavior from the bulk is also confirmed by the temperature-dependent X-ray photoemission spectroscopy (XPS). Sr surface concentration deviation from the bulk value is unambiguously related to the nonmetallic behavior at the surface and interface, which is further verified by the thickness dependence of the film conductivity. The layer-by-layer variation in chemical composition generates an immense impact on the physical properties of the epitaxial oxide films and heterostructures. It naturally explains the existence of a 'dead' layer and the persistent nonmetallic behavior near the surface and interface of LSMO films, regardless their thickness.