Semester

Fall

Date of Graduation

2019

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Mikel Holcomb

Committee Member

Aldo Romero

Committee Member

Edward Flagg

Committee Member

Loren Anderson

Committee Member

Ken Showalter

Abstract

The valence of atoms often has a strong effect on the properties of materials, such as magnetism, conductivity, and superconductivity. The atomic valence is often perturbed at the surface and/or interface and this deviation may play a strong role in many physical phenomena such as interfacial coupling and dead layers, both magnetic and electric. In this dissertation, I present a non-destructive approach of combining two X-ray absorption detection modes, electron yield and fluorescence, with very different probing depths in conjunction with theory to map out the layer-by-layer valence of a thin film.

The weighted average Mn atomic valence as measured from the two modes are simultaneously fitted using a model for the layer-by-layer variation of valence based on theoretical model Hamiltonian calculations. Using this model, the Mn valence profile in thin film La0.7Sr0.3MnO3 (LSMO) is extracted and the valence within each layer is determined to within an uncertainty of a few percent. It was found that while the bulk averaged valence hovers around its expected value of 3.3, a significant deviation occurs within several unit cells of the surface and interface. The surface valence increases to up to Mn3.7+, whereas the interface valence reduces down to Mn2.5+. These results were supported by density functional theory calculations. The change in valence from the expected bulk value is consistent with charge redistribution due to the polar discontinuity at the film-substrate interface. The comparison with theory employed here illustrates how this layer-by-layer valence evolves with film thickness and allows for a deeper understanding of the microscopic mechanisms at play in this effect. These results offer insight on how the two-dimensional electron gas is created in thin film oxide alloys and how the magnetic ordering is reduced with dimensionality. Preliminary magnetic depth profiling measurements, discussed in the last section, show an enhancement and suppression of the material’s net magnetization at the interface and surface, respectively. This correlation indicates that the observed change in valence may well play a role in the magnetic properties at the film boundaries.

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