Analysis of Electronic and Magnetic Properties at the Interfaces of Transition Metal Heterostructures

Material science is a field of physics that bridges the gap between the microscopic properties of materials and how these properties manifest as tangible, observable characteristics that can be observed or harnessed for applications. Material scientists grow, analyze, modify, and model complex, exotic materials to understand emergent, novel phenomena and create devices which can readily be employed for electronic, magnetic, and other applications. However, many material properties cannot be measured or observed directly; it is not feasible to measure the orbital energies of an atom directly or quantify angstrom-scale magnetic variations in a thin film sample directly. Since these properties are frequently what create meaningful effects at a macroscopic level, an understanding of them is required that only material science techniques can provide. This thesis concerns two separate studies of complicated material systems that require an understanding of their underlying structure and properties that cannot be directly discerned by experiment alone. These samples belong to a class of materials known as heterostructures, structures formed by layering multiple materials with different chemical/elemental compositions. The junctions where different materials in a heterostructure connect are called interfaces, and serve as the sites of emergent physical and chemical phenomena with a myriad of electronic and magnetic applications. The first material system studied is the interface of bulk LaAlO3 and bulk CaTiO3, often simply abbreviated as LAO/CTO. Systems containing interfaces between transition metal compounds have been intensely studied within the past two decades because certain phenomena such as two-dimensional electron gases (2DEG), magnetism, and other effects tend to appear specifically around the interface. Historically, a combination of bulk LaAlO3 and bulk SrTiO3 (LAO/STO) was studied instead, but the interfacial effects can be changed by swapping out various elements in the compound. This variation is due to the crystal structure near the interface being distorted according to the element introduced, altering the Ti orbital energies near the interface as well. These near-interface orbital energies directly correlate to observed interfacial phenomena, so swapping elements is expected to affect the macroscopic electronic and magnetic properties of the system. The difficulty resides in the fact that orbital energies and 2DEG charge densities cannot be measured directly by any experiment; rather, they need to be extracted from experimental data via sophisticated modelling. The purpose of this study was to probe two samples of LAO/CTO with varying thicknesses, use the extracted experimental data to generate models of the two samples, and finally use this model to discern orbital energies. Special consideration was given to comparing and contrasting the difference between the LAO/STO interfacial electronic structure with that obtained for LAO/CTO. It was found that the orbital energies of LAO/CTO maintain a significantly different configuration from those of LAO/STO, and suggest that LAO/CTO may be more promising for magnetic applications. Furthermore, this difference will foster more investigation into interfaces of this kind, particularly in designing new configurations of different metals to observe what macroscopic effects they produce. The second material system studied is thin films of Fe3GeTe2, often abbreviated FGT. FGT as a bulk material has been studied since the turn of the millennium for potential magnetic applications, and attention has recently moved towards growing the substance in thin film form on the order of angstroms thick. Several sources have found that the electronic and magnetic properties of these thin films vary dramatically vary dynamically as more FGT film layers are grown sequentially in one sample. However, this information is only known at a high level; the magnetic effects are known to differ, but how exactly this manifests on a microscopic level is unknown. Given the angstrom level thickness of the films, it is extremely difficult to probe the magnetic properties in detail. A more sophisticated technique is needed, so here we apply resonant X-ray reflectometry. This study considered two samples: a single FGT layer film (monolayer) and a combination of two FGT layers as a film (bilayer). Experimental results were used to synthesize a model of each film and the magnetized iron distribution was quantified in each case. The two samples were found to have differing magnetized iron distributions, further lending credence to observations that FGT films of various layers will produce different magnetic properties and effects. These two studies represent intriguing but very limited applications of material science. New exotic materials are being actively discovered all the time, each with their own unique need for a method that probes their microscopic properties to understand macroscopic phenomena. Material science techniques are and will continue to be important for these reasons; advancement of technology is now reliant on synthesizing and understanding new materials that improve electronic and magnetic infrastructures, an understanding that material science provides