Date of Completion

12-6-2013

Embargo Period

12-6-2013

Major Advisor

Barrett Wells

Associate Advisor

Joseph Budnick

Associate Advisor

Pamir Alpay

Field of Study

Physics

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

SrCoOx has generated interest due to its high oxygen mobility and unique magnetic properties. Above x = 2.75, SrCoOx is a cubic perovskite and orders ferromagnetically. The magnetic ordering temperature is dependent on the oxygen content of the material. Distinct phases have been noted at x = 3.0 (TC = 280), x = 2.88 (TC = 220) and x = 2.75 (TC = 160 K) in bulk samples. At oxygen contents between x = 2.88 and 3.0, the transitions of both end points are seen, though the sample retains a single crystal structure. While magnetic phase separation has been described in a number of similar complex oxides, separation between phases of the same magnetic order is unexpected. Additionally, the oxygen concentrations at which end phases occur (3.0 – n/8, n = 0, 1, 2) imply an ordering commensurate with the lattice.

We explore this system through a series of magnetic and structural investigations, beginning with bulk samples and extending to an extensive investigation of thin films. Using pulsed laser deposition, we have successfully grown epitaxial films of SrCoOx and detail a method to select oxygen concentration using an electrochemical cell. We report on the evolution of magnetic properties as a function of oxygen content. A thickness dependent study of these epitaxial films, oriented along the (1 0 0) and (1 1 1) directions, revealed the effects of finite dimensionality and determined a critical length scale for the magnetic regions.

Resonant X-ray diffraction was used to determine the presence of more subtle orderings not detected in standard structural analyses. By tuning to important Co and O absorption edges, arrangements particular to the electric states of those elements are enhanced. We have discovered, for the first time, an ordering commensurate to the lattice at the (¼ ¼ ¼) position. Incommensurate peaks near this position were also observed. The intensity of these peaks depends on the oxygen concentration of the sample, and can be suppressed at temperatures above 350 K. Regular orderings of charge density which closely match the underlying lattice may help to explain the observed propensity for SrCoOx to stabilize at particular phases.

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