Local Electronic Structure of Magnetic and Superconducting Thin Films

Date of Completion

January 2010

Keywords

Physics, Condensed Matter

Degree

Ph.D.

Abstract

Complete understanding of many materials used in current technologies, like tunneling magneto-resistance (TMR) and superconductivity requires the knowledge of the local electronic structure near the surface, at interfaces and in the bulk of these systems. This work presents three studies of the local electronic structure of transition metal based films grown by pulsed Laser deposition (PLD), utilizing x-ray photoelectron (XPS) and x-ray absorption (XAS) spectroscopies. ^ The existence of an interfacial Fe-oxide layer in unannealed textured FM/MgO (FM=Fe, FeCo) bilayers was confirmed and a transformation of the oxide layer from Fe3O4 and/or Fe2O3 - like to a more FeO-like oxide as a function of thermal annealing is discovered from analysis of the O-K XAS spectra. Changes to this interfacial oxide layer, with post-growth thermal annealing are suggested to be responsible for the increased TMR ratios in magnetic tunnel junctions. ^ Local electronic structure changes in FeSe and FeTe films, resulting from oxygen incorporation, and their correlations to superconductivity in these materials are studied. XPS and XAS measurements indicate that Fe in both parent films experience similar nominal valence changes whereas the valence changes of the respective chalcogenides (Te, Se) are different. Valence changes of Fe and Te are examined as a function of different oxygen incorporation methods: air exposure, low temperature oxygen annealing and in-situ dry oxygen exposure. The rates of valence change of both Fe and Te vary depending on the type of oxygen treatment. The valence states associated with superconductivity observed in this work should be included in any proper theory describing superconductivity of these systems. ^ Optimization of PLD growth parameters for La2NiO4 films was quantified from analysis of XPS spectra and it was found that it is difficult to grow films with stoichiometric surface termination. Structural reorientation in La2NiO4 films as a function of thickness is found which could be used for growth of epitaxial, single domain a-axis oriented films. The increased ionic transport in the a/b plane can be exploited in solid-oxide fuel cell cathodes. ^

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