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Abstract

This dissertation explores the manipulation of electronic and magnetic properties in materials structured as crystalline thin films. Each project follows a systematic approach: we synthesize crystal systems using the molecular beam epitaxy growth technique and characterize them through high-resolution spectroscopy, using both in-house sources and synchrotron radiation. We then interpret the underlying physics by comparing our findings with theoretical models. We present an in-depth study of N-substituted NiO, a prototypical strongly correlated oxide often used as a model to understand electron-electron interactions. We investigate defect centers in this material, which are imperfections in a crystal that host localized electronic and magnetic states distinct from the surrounding lattice. With growing interest in quantum technologies, efforts are ongoing to extend such centers to a broader class of materials. Here, we examine the formation of Ni-N-Ni centers via substitution of O by N in NiO. Each N introduces a hole localized at the anion site and modifies the magnetic moment of a neighboring Ni cation, while preserving its Ni²⁺ oxidation state. Such effects emerge due to the significant exchange interactions between Ni and N in this system. The resulting Ni-N-Ni centers are magnetically decoupled from the host lattice and exhibit degenerate spin states. They represent the first identification of defect centers in a strongly correlated antiferromagnetic oxide, with potential relevance for quantum device applications. We further study Ni-based compounds, including NiO, N-substituted NiO, and Ca-substituted LaNiO₃, focusing on their core-level and valence band electronic structures using photoemission spectroscopy. In N-substituted NiO, we investigate the states arising from N incorporation and their influence on non-local screening processes.