This thesis describes the experimental investigation of three different strongly correlated transition-metal oxide systems. The magnetic behaviour of each has been probed using inelastic neutron spectroscopy. A distinctive hour-glass excitation spectrum has been observed in the layered cobaltate La<sub>1.75</sub>Sr<sub>0.25</sub>CoO<sub>4</sub>. This spectrum is similar to that measured in a related cobaltate La<sub>1.67</sub>Sr<sub>0.33</sub>CoO<sub>4</sub>, although it appears broader. The spectrum has been reproduced using a spin wave model derived from a disordered cluster spin glass ground state. Signatures of spin glass behaviour have also been observed in bulk magnetisation measurements of La<sub>1.75</sub>Sr<sub>0.25</sub>CoO<sub>4</sub>. These findings, once more, demonstrate the emergence of an hour-glass spectrum from a ground state that combines quasi-one dimensional magnetic correlations and disorder. Additionally, this study shows that charge and magnetic stripe order persists to lower dopings in La<sub>2-x</sub>Sr<sub>x</sub>CoO<sub>4</sub> than previously thought. The complete magnetic excitation spectrum of the multiferroic compound CuO has been measured for the first time. A high energy, one-dimensional magnetic spectrum is observed and modelled using the Muller ansatz derived for the S=1/2 Heisenberg antiferromagnetic chain. At lower energies, a three-dimension spectrum is observed. The measured spectrum is inconsistent with all previous theoretical estimates of the dominant inter-chain exchange interactions in CuO. The inter-chain dispersion is successfully described by a phenomenological model based on linear spin wave theory. The third material investigated, LuFe<sub>2</sub>O<sub>4</sub> demonstrates complex charge and magnetic order, the precise nature of which is still under debate. The full spectrum of in-plane excitations in LuFe<sub>2</sub>O<sub>4</sub> has been measured and a complicated dispersion consistent with six magnetic modes is observed. These findings are compatible with structures described by a magnetic unit cell containing six spins. The dispersion can be described by a spin wave model derived from a bilayer structure comprised of charge-rich and charge-poor monolayers. This structure is consistent with the original site-specific model for the 3D magnetic ordering in LuFe<sub>2</sub>O<sub>4</sub>.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:692862 |
| Date | January 2014 |
| Creators | Gaw, Stephen Michael |
| Contributors | Boothroyd, Andrew T. |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:c20676e1-b927-4ee5-a3fe-97f0f80cb141 |
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