Strongly Correlated Oxides - Half-metallicity in Chromium-based Rutiles and Quantum Magnetism in Copper-based Double Perovskites

Strongly correlated oxides are materials with significant electron-electron correlation effects arising from the Coulomb repulsion between localized electrons. While very difficult to model by theory, these materials exhibit a wide range of novel properties and phenomena of both fundamental and technological interest in solid state chemistry and physics. In this thesis two types of strongly correlated materials are investigated: half-metallic ferromagnets and quantum spin liquids.  Chromium dioxide is an archetypical half-metallic ferromagnet. It is simultaneously ferromagnetic and metallic, a property of significance in spintronics applications. In this thesis, chemical substitution tuning was investigated in the CrO2-VO2 system as a route for discovering new half-metals. While density functional theory calculations predicted the V-for-Cr substituted phases to be half-metallic similar to CrO2, in practice the materials became antiferromagnetic. This revealed a significant potential pitfall in the search for new half-metals by entirely ab initio methods: the difficulty of correctly predicting oxidation states in materials with multiple transition metals.  Quantum magnetism on spin-1/2 square lattices has been of significant theoretical and experimental interest in condensed matter physics for three decades. This is due to the discovery of high-temperature superconductivity in these materials in the 1980's. Specifically, a quantum disordered ground state has been predicted to emerge when nearest-neighbor and next-nearest neighbor interactions compete. In this dissertation, the first experimental evidence of such a state is presented.  The perovskite compounds A2CuB''O6 are an ideal system for studying spin-1/2 square-lattice antiferromagnetism. A method for tuning magnetic exchange using diamagnetic d10/d0 cations on the B'' site was developed in this dissertation. This d10/d0 method allowed the tuning of the ground state into the quantum critical regime for the first time. Spin-liquid-like behavior was observed in the compound Sr2Cu(Te0.5W0.5)O6. A thorough investigation of the Sr2Cu(Te1-xWx)O6 system revealed the suppression of magnetic order in a wide composition range.