This thesis presents neutron and x-ray scattering measurements on quasi-two-dimensional honeycomb antiferromagnets A2IrO<sub>3</sub> (A=Na, Li) and the solid-solution intermediate material (Na<sub>1-x</sub>Li<sub>x</sub>)<sub>2</sub>IrO<sub>3</sub>. The aim is to study the magnetic order and excitations of 5d Ir<sup>4+</sup> ions in a honeycomb lattice, where unusual magnetic properties have been theoretically predicted to be stabilised by the combinations of strong spin-orbit coupling and honeycomb lattice geometry with 90 degree Ir-O-Ir bonding. By using an optimised setup to minimise the strong neutron absorption by Ir nuclei, inelastic neutron scattering measurements on powder sample of Na<sub>2</sub>IrO<sub>3</sub> observed dispersive excitations below 5meV with a dispersion that can be accounted for by including substantial further-neighbor exchanges that stabilize zigzag magnetic order. The onset of long-range magnetic order was confirmed by the observation of oscillations in zero-field muon-spin rotation experiments. Higher-resolution inelastic neutron data found features consistent with a spin gap of 1.8meV and the data was parameterised by including Ising-type exchange anisotropy. Combining single-crystal diffraction and density functional calculations, a revised crystal structure model with significant departures from the ideal 90 degree Ir-O-Ir bonds required for dominant Kitaev exchange was proposed. Various "idealised'' crystal structures were constructed to emphasize the departures between the actual structure and structures with cubic IrO<sub>6</sub> octahedra. The magnetic excitations from the isostructural Li<sub>2</sub>IrO<sub>3</sub> revealed strongly dispersive magnetic excitations, qualitatively different from Na<sub>2</sub>IrO<sub>3</sub>. Elastic neutron diffraction detected a magnetic Bragg peak with a wavevector consistent with spiral orders. To explain the observed neutron data, the spiral H2 phase in the Heisenberg J<sub>1</sub>-J<sub>2</sub>-J<sub>3</sub> model was proposed, and a full calculation was performed with strong in-plane anisotropic interaction. A further measurement for improving the lower-energy excitation found no clear evidence for a spin gap down to E=0.7meV. Lastly, the crystal structure of (Na<sub>1-x</sub>Li<sub>x</sub>)<sub>2</sub>IrO<sub>3</sub> was investigated with single-crystal x-ray diffraction, revealing a site-mixing of Ir and Na ions in the honeycomb lattice and insensitivity of the refinement to the Li positions. Ab initio calculations suggested that up to x=0.25 Li ions replaced Na in the honeycomb centre and phase separation occurred beyond that, which is consistent with the evolution of observed lattice parameters.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:647607 |
| Date | January 2014 |
| Creators | Choi, Sungkyun |
| Contributors | Coldea, Radu |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:d47c4e67-14c6-43ea-a8ba-47b9201b5002 |
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