Boseggia, S; (2015) Magnetic order and excitations in perovskite iridates studied with resonant X-ray scattering techniques. Doctoral thesis , UCL (University College London).

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Abstract

Transition metal oxides containing a 5d element are increasingly attracting attention as an arena in which to search for novel electronic states. These are proposed to derive from the interplay between the strong spin-orbit interaction (SOI) in the 5ds, the electronic correlations and crystal field effects. Iridium based compounds have featured predominantly in this quest, with considerable focus on the layered perovskites in which a novel “spin-orbit induced” Mott-like insulating state emerges. In this case, the SOI leads to the jeff = 1/2 ground state for the Ir4+ (5d5) ions observed in Sr2IrO4. In this thesis I demonstrate that resonant X-ray scattering techniques at the Ir L edges are a valuable tool to investigate the electronic and magnetic properties of iridium oxides. In particular the evolution of the jeff = 1/2 state as a function of structural distortions in the single layer iridates Sr2IrO4 and Ba2IrO4, and as a function of the dimensionality in the bilayer Sr3Ir2O7 is investigated. My findings show that the magnetic and electronic structures in the single layered perovskites are remarkably robust to structural distortions. Conversely, adding an extra IrO6 layer (Sr2IrO4 →Sr3Ir2O7) the ground and excited states change dramatically. Both these phenomena can be linked directly to the unique three-dimensional character of the jeff = 1/2 state. Furthermore, the X-ray resonant scattering cross-section of Ir4+ at the L2,3 edges, calculated in a single-ion model, shows a non-trivial dependence on the direction of the magnetic moment, μ. These results provide important insights into the interpretation of X-ray data from the iridates, including that a jeff = 1/2 ground state cannot be assigned on the basis of L2/L3 intensity ratio alone.

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