Dissertation/Thesis Abstract

Synchrotron x-ray studies on structural and chemical ordering in group IV-based magnetic epitaxial films
by Collins, Brian Akira, Ph.D., The University of North Carolina at Chapel Hill, 2009, 207; 3366472
Abstract (Summary)

In this thesis, complementary synchrotron x-ray techniques have been used to study compositional and epitaxial effects on structural and chemical ordering of Ge-based magnetic semiconductor films and Heusler alloys Co 2MnGe and Co2MnSi on the atomic scale. Combinatorial x-ray methods have been developed in this work to enhance the study of these materials. The study of transition metal doping in Ge reveals that the stability of magnetic dopants can be enhanced when two magnetic dopants (Mn and Co or Fe and Co) are used to reduce epitaxial strains and alter energetics and kinetics of epitaxial growth, resulting in suppression of metallic precipitates and significantly higher doping levels. Codoping is also shown to be responsible for an increase in populations of substitutional states over those in films with a single dopant. The study also reveals the strong interplay between dopant states and structural, magnetic, and electronic properties. In CoxMn yGez films, epitaxial constraints are shown to play an important role in stabilizing the Heusler structure over a large region of chemical composition. Additionally, it is found that structural and chemical ordering of these alloys are sensitive to the atomic ratio between Co and Mn and are strongly affected by epitaxial strains. Experiments and analysis of an anomalous diffraction technique have been developed in order to quantitatively probe structural and chemical disorders that can contribute to the suppression of the half metallic state. These studies reveal that the primary defects occurring in these films are Mn-Ge site swapping, as Co vacancies and Ge antisites constitute secondary defects that depend strongly on composition and epitaxial constraints. However, defects previously predicted to suppress the half metallic state, specifically Co-Mn swapping and antisites, are not present. These findings demonstrate that compositional and epitaxial effects can be used as viable means to control structure and magnetism on the atomic scale.

Indexing (document details)
Advisor: Tsui, Frank
Commitee: Chu, Yong S., Lopez, Rene, Lu, Jianping, Washburn, Sean
School: The University of North Carolina at Chapel Hill
Department: Physics & Astronomy
School Location: United States -- North Carolina
Source: DAI-B 70/07, Dissertation Abstracts International
Subjects: Condensed matter physics
Keywords: Epitaxy, Heusler alloys, Materials, Spintronics, Synchrotron, X-rays
Publication Number: 3366472
ISBN: 978-1-109-27757-9
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