Global Optimization of Resonant X-ray Reflectometry Models: Analysis of Perovskite Oxide Heterostructures
Date
2023-12-19
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0009-0007-1329-6065
Type
Thesis
Degree Level
Masters
Abstract
Resonant x-ray reflectometry is an emerging synchrotron technique used to characterize the depth-dependent structure of quantum materials. The main challenge impeding the success of resonant x-ray
reflectometry is the extreme difficulty of analyzing the data because the process involves both large-scale
computational quantum mechanics simulations and the fitting of many independent variables. This leads
to prolonged analysis periods that require a significant amount of engagement. As a part of this thesis, a
new data analysis software named Global Optimization of Resonant X-ray Reflectometry was developed for
researchers to use to more effectively analyze resonant x-ray reflectometry data and to mitigate some of these
challenges. It has been shown throughout this thesis that multiple features in the software have been able
to ease the data analysis process. A large focus will be put on the customizable objective function because
the boundaries and weights and total variation features have been proven to be integral components to the
success of the software.
The developed software was used to analyze resonant x-ray reflectometry (RXR) data of the catalyst
La0.7Sr0.3MnO3/SrTiO3 (LSMO/STO) for electrochemical water splitting. Resonant x-ray reflectometry is
used to develop a new enhanced understanding of the structural, electronic, and magnetic depth profiles of
thin LSMO films by characterizing the depth-dependence of such materials for varying film thickness and
measurement temperature. The results provide evidence of a magnetically dead layer at the surface and
demonstrate a decrease in the magnetic moment near the Curie temperature. These findings are significant
because they help understand the mechanisms involved in the oxygen evolution reaction and methods that
can be used to improve water splitting efficiency.
Resonant x-ray reflectometry is also employed to study the thickness relationship between film thickness
and the presence of ferromagnetism in the LaMnO3/SrTiO3 heterostructure. The electronic reconstruction
due to polar catastrophe is the leading theory for the mechanism involved in the magnetic phase transition,
but this study provides a new understanding of the emergence of magnetism in ultra-thin films of LaMnO3
that contradict the polar catastrophe mechanism. Notably, ferromagnetism is detected below the critical
thickness, as supported by density functional theory calculations. Moreover, this study provides evidence
that the magnetic moment is related to the distortions in the material. It is possible that octahedral distortions are formed and are the proposed cause for the observed ferromagnetism.
Description
Keywords
Resonant X-ray Reflectometry, Global Optimization, Perovskite Oxide Heterostructures
Citation
Degree
Master of Science (M.Sc.)
Department
Physics and Engineering Physics
Program
Physics