In this thesis two experimental antiferromagnets with the particularly interesting lattice resembling the kagomé basket-weaving pattern, are studied. The kagomé topology frustrates the <i>classical </i>antiferromagnetic Néel state, the commonly encountered magnetic groundstate in transition-metal compounds. Extensive theoretical work has shown that in particular the <i>S</i> = 1/2 kagomé antiferromagnet should not show any symmetry-breaking transition to a classical long-range-ordered state. Hence, experimental realisations of this system should in theory allow a unique insight into properties of the symmetrical quantum-mechanical magnetic groundstate. Recently a viable experimental realisation of the <i>S</i> = 1/2 kagomé antiferromagnet has been discovered, the <i>x</i> = 1 phase of zinc-paratacamite Zn<sub>x</sub>Cu<sub>(4-x)</sub>(OH)<sub>6</sub>Cl<sub>2</sub>. Here samples of stoichiometry 0.15 ≤ <i>x</i> ≤ 1 were synthesised and characterised. Using muon-spin relaxation spectroscopy on these samples, it was found that for <i>x</i> > 0.6 the spins do not freeze, even at 50 mK. From neutron powder diffraction for <i>x</i> = 1 and heat capacity measurements on zinc paratacamite with 0.5 ≤ <i>x</i> ≤ 1, it was found that ~6% antisite disorder is present in the <i>x </i>= 1 phase, and that samples with Zn stoichiometry 0.8 ≤ <i>x</i> ≤ 1 model the <i>S</i> = 1/2 kagomé antiferromagnet equally well. No quantum critical phase transition to a quantum spin liquid is found. Instead, the groundstate of this model system is magnetic even for <i>x</i> > 0,8. The field dependence of the heat capacity provides additional evidence that the total magnetic quantum number <i>S</i><sub>tot </sub>is not a conserved quantity, despite the fact that no symmetry breaking transition occurs in the magnetic degrees of freedom. In a polarised X-ray spectroscopy experiment on synthesised single crystals of the classical (<i>S</i> = 5/2) kagomé antiferromagnet iron jarosite KFe<sub>3</sub> (SO<sub>4</sub>)<sub>2</sub>(OH)<sub>6</sub>, it is shown that the Fe<sup>3+</sup> ion with a <sup>6</sup><i>S</i> free-ion configuration can acquire a large orbital angular momentum in the solid state. The high-resolution spectra are reproduced in excellent detail.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:649056 |
| Date | January 2007 |
| Creators | de Vries, Mark Alexander |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/13603 |
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