Doctor of Philosophy (PhD)
Physics And Astronomy
The study of magnetism has been a rich playground in condensed matter physics due to the multiple mechanisms capable of producing the effect and its relationship to multiple characteristics of a material. Transition metal oxides (TMOs) have been of particular interest for ongoing research into magnetic phenomenon due to the abundance of interesting physical phenomena found in member systems such as colossal magnetoresistance, skyrmion formation, and interface-driven 2D electron gases. Thin films introduce an additional thickness-dependent element, where reduction below a critical thickness eliminates the magnetic coherence of a system and FM order is lost. The atomic structure of these materials can also affect the formation of coherent spin alignments due to hybridization change and charge doping. Finally, heteroepitaxy of multiple materials in strained systems can introduce new interactions that allow for novel FM states, such as the giant magnetoresistance found in 3d TMO superlattices.
This thesis work will aim probe the evolution of certain FM materials through each of these manipulations, namely the dependence of FM behavior on thickness, structural change, and heterointerfacing. The relationship between defect evolution and epitaxial strain will be examined in heterostructures between strong spin-orbit coupled SrIrO3 and ferroelectric BaTiO3, finding that strong strain is the primary driver of defect formation and not symmetry mismatch at interfaces. Thickness dependent studies of ferromagnetic phase [La.67Ca.33]MnO3 (LCMO) heterostructures will show that a novel FM state can be induced in LCMO layers by introducing buffer SrRuO3 (SRO) layers. This SRO induced onset occurs well below the dead layer thickness of monolithic LCMO and exists in completely insulating films, contrary to normal double-exchange manganite FM models. The SRO system is compared structurally and electronically with the lower enhancement effects seen from buffer layers CaRuO3 and SrTiO3. Multiple causes of this FM are considered, and through resistivity modelling and structural analysis we find that an incipient FM phase in SRO coupled with structural change and an interfacially-mediated AFM pinning are the most likely driving factors. Through this research we find a new route for magnetism in transition metal oxides that pushes the dimensional control of magnetic functionalities in artificial heterostructures.
Howe, David, "Interface-Induced Lattice Structure and Magnetism in Ultrathin Transition Metal Oxide Trilayers" (2021). LSU Doctoral Dissertations. 5697.