Anomalous transport properties in highly epitaxial complex oxide thin film

Perovskite oxides have already attracted great attention owing to such fascinating features as spin-state and metal-insulator transitions, charge and orbital ordering phenomena and colossal magnetoresistance. Apparently, the perovskite cobalt oxides, which are still much less studied than cuprates or manganites, are meant to become the next primary field in investigation of strongly correlated electron systems and colossal magnetoresistance, because of the occurrence of the high concentration oxygen vacancies and different spin state of Co, which result in intriguing and unique magneto-transport properties. The main theme of this thesis is to investigate the influence of interface strain on electrical and magneto transport properties of highly epitaxial LaBaCo₂O_5.5+&deta (LBCO) thin film.

It is the first time that the metal-insulator transition is only found along one direction of (110) NdGaO₃ substrate, but along another direction, this phenomenon doesn't happen. A strained layer (7nm) was observed in the film on (001) MgO substrate. Magnetoresistance is not associated with the type of strain (compressive or tensile), but is dependent on the magnitude of strain and the terrace of substrate change the conductivity behavior from semiconductor to semimetallic. Finally, the oxygen annealed thin film shows ferromagnetic semimetal behavior, while the as-grown thin film exhibit the weakly ferromagnetic semiconductor behavior. An ultra high magnetoresistance (54%) is achieved at 40K when the applied magnetic field is parallel to c or b axis of the thin film, indicating that magnetic anisotropy exists in the film, associated with the Co spins have a preferred spin orientation in the interface region.