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Finite size effects in strongly correlated transition metal oxides probed by beta-detected NMR Karner, Victoria Louise

Abstract

In this thesis, the strongly correlated metal LaNiO3 was studied in bulk and heterostructures using 8Li beta-NMR. The main objective of the thesis was to probe the metallic state and how it changes on approach to a dimensionality-induced metal-insulator transition (MIT) in heterostructures. With its sensitivity to the metallic state, akin to conventional NMR, and ability to study thin films and heterostructures, 8Li beta-NMR is uniquely suited to tackle this challenging subject. In bulk LaNiO3, spin-lattice relaxation (SLR) measurements reveal two equally abundant components with linear temperature dependence below 200K. The linearity is consistent with a Korringa mechanism and is evidence of a conventional metallic state. The resonance is characterized by a single broad line with a small temperature independent Knight shift. The normalized Korringa product indicates substantial antiferromagnetic correlations. In LaNiO3 heterostructures with insulating LaAlO3, an MIT and Néel order has been observed when the thickness of LaNiO3 is reduced to 2 u.c.. We used 8Li beta-NMR to study heterostructures with varying thicknesses of LaNiO3. SLR measurements show two equally abundant components with distinct temperature dependences. One component is linear with temperature, and only weakly affected by LaNiO3 thickness. In contrast, the second component is non-linear, and strongly depends on thickness. We attribute the two component relaxation in the heterostructures and bulk LaNiO3 to microscopic phase separation of the electronic state. Finally, we studied some novel LaNiO3 heterostructures where LaAlO3 was replaced with the magnetic insulator La2CuO4. Bulk La2CuO4 is characterized by long-range antiferromagnetic order below TN = 300K which is extremely sensitive to doping. In a La2CuO4 film, SLR measurements reveal fast but measurable 1/T1. We see evidence of magnetic order not from a peak in 1/T1 but instead by a gradual loss of asymmetry beginning at 140K. We attribute the suppression of TN and the broad transition to inhomogeneous hole doping in the film. Below 40K, an upturn in 1/T1 suggests freezing of mobile holes from the more doped regions of the sample. The behavior in the heterostructures is similar to the La2CuO4 film.

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