In this dissertation, we utilized a wide range of complementary synchrotron-based X-ray spectroscopic and scattering techniques, notably X-ray absorption spectroscopy (XAS), hard X-ray photoelectron spectroscopy (HAXPES), standing-wave X-ray photoelectron spectroscopy (SW-XPS), and X-ray resonant magnetic reflectometry (XRMR), to understand and control the phenomenon of emergent interfacial ferromagnetism in strongly-correlated oxide heterostructures. This field holds great promise for the development of next-generation spintronic devices. In the heterostructures we investigated, neither of the parent oxide layers exhibits inherent ferromagnetism. Yet, when these layers are combined in an epitaxial film stack, charge-transfer phenomena give rise to an emergent ferromagnetic state at the interface. Throughout my graduate studies, I focused on studying such charge-transfer phenomena as the driving force for stabilizing interfacial ferromagnetism. This dissertation is structured around two main projects. The first project delves into the intriguing possibility of tuning the emergent interfacial ferromagnetism. More specifically, we investigated the mechanisms for suppressing interfacial charge transfer to gain control over and manipulate this magnetic phenomenon. In our second project, we explored a different facet of interfacial ferromagnetism, focusing on the origins of the imbalance in the magnitudes of the magnetic moment between the top and bottom interfaces in the same layer. Our investigation aimed to uncover the possible causes of this imbalance, ultimately leading us to scrutinize the role of defect states in this magnetic asymmetry. In the first part of this dissertation, we investigated the thickness-dependent metal-insulator transition within LaNiO3 and how it impacts the electronic and magnetic states at the interface between LaNiO3 and CaMnO3. We present a direct observation of a reduced effective valence state in the interfacial Mn cations. This reduction is most pronounced in the metallic LaNiO3/CaMnO3 superlattice, where the above-critical LaNiO3 thickness of 6-unit cells triggers this phenomenon, facilitated by the charge transfer of the itinerant Ni 3d eg electrons into the interfacial CaMnO3 layer. In contrast, when we examine the insulating superlattice with a LaNiO3 thickness below the critical value (2-unit cells), we observe a homogeneous effective valence state of Mn throughout the CaMnO3 layers. This homogeneity is attributed to the suppression of charge transfer across the interface. The second part of this dissertation delves deeply into the complexities of interfacial magnetism within the CaMnO3/CaRuO3 superlattices. Our experimental investigation unveiled an unexpected asymmetry in the strength of magnetism at these interfaces. Our findings suggest that within the superlattice CaMnO3/CaRuO3, the lower interface (CaRuO3/CaMnO3) exhibits a weaker magnetic moment when compared to the upper interface (CaMnO3/CaRuO3). This observation, supported by XRMR and XAS experimental data, was further clarified by first-principles density functional theory (DFT) calculations. Our calculations suggest that the observed magnetic asymmetry may be linked to the presence of oxygen vacancies at the interfaces. Our study significantly contributes to our understanding of interfacial ferromagnetism, potentially paving the way for controlling and manipulating this emergent property. This may be achieved by utilizing engineered defect states, offering exciting prospects for applications in the field of spintronics devices. / Physics

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We investigated the magnetic properties of hydrogen-plasma-treated ZnO single crystals by using superconducting quantum interferometer device magnetometry. In agreement with the expected hydrogen penetration depth, we found that ferromagnetic behavior is present in the first 20 nm of the H-treated surface of ZnO with magnetization at saturation up to 6 emu g−1 at 300K and a Curie temperature of Tc & 400 K. In the ferromagnetic samples, a hydrogen concentration of a few atomic per cent in the first 20 nm of the surface layer was determined by nuclear reaction analysis. The saturation magnetization of H-treated ZnO increases with the concentration of hydrogen.

<p>This thesis comprises studies on two ferromagnetic superconductors, URhGe (Tcurie=9.SK and Tsc=2S0mK) and UCoGe (Tcurie=2.SK and Tsc=800mK). These properties are interesting because the current theory to explain superconductivity predicts that ferromagnetism should destroy superconductivity. Not only is that not true in these materials, but ferromagnetism and superconductivity are thought to arise from a common mechanism. The studies conducted on these materials arise from that possibility, in an attempt to understand the unconventional nature of these materials.<br />Original work is contained in chapters 4, Sand 6. All of this work is currently not published in sources other than this thesis. <br /> Chapter 1 will give an introduction to these materials, and the work that has been done on them by other groups, and work done on related materials. <br /> Chapter 2 will give details of the various experimental methods used in measuring the structure and properties of the materials studied. This work was conducted by the author at McMaster University, with the assistance of individuals from the Brockhouse Institute for Materials Research, and the Center for Electron Microscopy at McMaster University.<br /> Chapter 3 will provide an introduction to the technique of muon Spin Resonance/Relaxation (μSR). This work was done at the TRIUMF facility in Vancouver, British Columbia, with the assistance of several TRIUMF staff. The data was collected by the author, and other members of Dr. Luke's research group as well as collaborators from TRlUMF and from Columbia University. <br /> Chapter 4 will present the measurements made on UCoGe, while Chapter 5 presents the measurements of URhGe. Details of the crystal growth and structure characterization measurements are included in these chapters, along with resistivity, bulk magnetization and μSR measurements. <br /> Both zero- field (ZF) and transverse field (TF) μSR has been performed. This work focuses on studying the magnetic moment size, and the magnetic volume fraction around the ferromagnetic transition, and to temperatures as low as 20mK. Consideration is also given to the magnetic and superconducting properties in the low-temperature region. <br /> In the Introduction, URhGe is presented first, followed by UCoGe, since this was the order in which they were discovered. The results obtained from UCoGe are presented first, since work on that compound was started before the work on URhGe. <br /> Chapter 6 focuses on the conclusions drawn from this work, comparing the measurements of both materials.</p> / Master of Science (MS)

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Heusler compounds are cubic intermetallics with a wide range of interesting properties, which are closely related to the structure of the material. In addition, several exotic physical phenomena have been predicted for different compositions in the family, but have not been experimentally realized. By and large, the lack of success in realization of various properties are due to the issues with intrinsic material attributes, which have been difficult to resolve as the relationship between them is not well understood. The aim of this work has been to unravel the entanglement between the intrinsic material attributes of cobalt-based quaternary Heusler compounds such as the structure, defects (disorder), chemical inhomogeneities etc., and the resulting physical properties.

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Neste trabalho foram produzidos filmes finos de Si[100]/buffer/NiFe/FeMn/Ta (buffer = Cu; Ta) e válvulas de spin de Si[100]/Cu/NiFe/Cu/NiFe/FeMn/Ta via Sputtering, e seus comportamentos magnéticos foram estudados analisando curvas de inversão de primeira ordem (First Order Reversal Curves, FORCs). Um Modelo de Preisach com Exchange-Bias foi desenvolvido e sua implementação computacional permitiu simular a histerese e as curvas forcas das bicamadas de NiFe/FeMn. O modelo também permitiu reproduzir os comportamentos assimétricos dos dois ramos da curva de histerese destes filmes. A análise dos resultados evidenciou os efeitos de campo médio sobre a camada de Py em função da espessura da camada de FeMn. A aplicação da análise FORC nas válvulas de spin se revelou interessante para estudar o grau de acoplamento entre as camadas livre e pressa, assim como os efeitos de campo médio em cada camada. / In this work, thin films of Si[100]/buffer/NiFe/FeMn/Ta (buffer = Cu; Ta) and spin valves of Si[100]/Cu/NiFe/Cu/NiFe/FeMn/Ta were produced via sputtering and their magnetic behavior were studied by First Order Reversal Curves (FORC\'s) analysis. A Preisach Model with Exchange-Bias was developed and applied in order to simulate the hysteresis and the first order reversal curves of the NiFe/FeMn bilayers. In addition, the model allowed to reproduce the asymmetric behaviors present in both branches of the hysteresis curve in these films. The study put in evidence the mean field effects on de Py layer as a function of the thickness of the FeMn layer. The application of the FORC analysis in the spin valves was interesting to study the degree of coupling between the free and the pressed layers, as well as the effect of mean field in each layer.