Donnerer, Christian; (2018) X-ray studies of magnetic and structural transitions in iridates. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In this thesis, I describe x-ray experiments that rst detect the symmetry breaking at magnetic and structural transitions in iridium based transition metal oxides, and then characterise the ensuing state. The magnetic transition in pyrochlore iridates R2Ir2O7, where R = Sm, Nd and Tb, was studied with resonant x-ray scattering at the Ir L3 edge. In all samples, k = 0 magnetic order was discovered below 120 K (Sm), 5 K (Nd) and 78 K (Tb), respectively. It is demonstrated how resonant x-ray scattering can determine the size of the magnetic moment, yielding approximately 0:3 B (Sm) and 0:06 B (Tb). The magnetic dynamics of Sm and Nd pyrochlores could be well described by excitations from an all-in all-out magnetic structure within a minimal nearest-neighbour Hamiltonian of Heisenberg exchange (27 meV) and Dzyaloshinskii-Moriya interactions (5 meV). This provides a consistent description of the magnetic order and excitations, and suggests that a topological Weyl semimetal could be realised in pyrochlore iridates. A structural transition from tetragonal to monoclinic symmetry was discovered in the perovskite iridate Sr3Ir2O7 at 54 GPa using x-ray di raction. The high-pressure phase adopts an altered stacking sequence of perovskite layers, and may coupled to the emergence of a metallic state. X-ray absorption spectroscopy was used to characterise the resulting electronic state at high pressure. Compared to the spin-orbit induced insulating state at ambient conditions, the in uence of spin-orbit coupling in the high-pressure, metallic phase appears diminished.

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