Date of Award

2023

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Applied Physics (PhD)

Administrative Home Department

Department of Physics

Advisor 1

Miguel Levy

Committee Member 1

Ramy El-Ganainy

Committee Member 2

Jae Yong Suh

Committee Member 3

Durdu Guney

Committee Member 4

Ravindra Pandey

Abstract

This dissertation presents the results of a study investigating the physical mechanisms underlying an unexpectedly large increase in magneto-optic efficiency observed in iron garnet. Such materials are technologically important for telecommunications due to their nonreciprocal optical action. In the past, our group had found evidence of an enhanced Faraday rotation in bismuth-substituted iron garnet films less than 50 nm thick. Subsequent investigation revealed that this enhancement could be traced to surface effects. This is significant because understanding these phenomena could be used to formulate engineering solutions for device miniaturization. In this dissertation, we present the result of a research project investigating the physical mechanisms underlying this unexpectedly large increase in Faraday response. Both experimental and theoretical techniques were brought to bear, including spectroscopic analysis and density-functional theory calculations, to understand how surface reconstruction effects impact the quantum origins of these curious phenomena. High-energy X-ray magnetic circular dichroism (XMCD) and X-ray absorption (XAS) measurements were used to analyze electronic transitions, while scanning transmission electron microscopy yielded concomitant structural analysis. The XAS and XMCD allowed us to produce a comparative analysis of surface and bulk densities of state impacting the 3d Fe orbitals responsible for Faraday rotation. This investigation was supplemented with surface-sensitive XPS analysis and ultraviolet and visible light absorption studies of the electronic distribution near the Fermi-level bands. These results were used to build and verify a density functional theory (DFT) model for iron garnet materials. Vienna ab-initio simulation (VASP) techniques yielded density-of-state (DOS) and band structure information for these systems. These allowed us to compare differences in bulk and surface band structures and DOS and to trace the surface-induced electronic origin of changes in Faraday rotation. DFT calculations based on these results showed that, indeed, the Faraday effect is enhanced in the visible and near-infrared regimes. Transition dipole matrix analysis was used to predict an accurate Faraday rotation response based on DFT calculations. Surface symmetry-breaking and surface reconstruction were thus found to be responsible for the effects under investigation. The combined evidence from this comprehensive study shows that a decrease in band gap at the surface plays a decisive role in the enhanced Faraday rotation.