Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.649056
Title: Iron jarosite and zinc paratacamite as models of the classical and the quantum kagomé antiferromagnet
Author: de Vries, Mark Alexander
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2007
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Abstract:
In this thesis two experimental antiferromagnets with the particularly interesting lattice resembling the kagomé basket-weaving pattern, are studied. The kagomé topology frustrates the classical antiferromagnetic Néel state, the commonly encountered magnetic groundstate in transition-metal compounds. Extensive theoretical work has shown that in particular the S = 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 S = 1/2 kagomé antiferromagnet has been discovered, the x = 1 phase of zinc-paratacamite ZnxCu(4-x)(OH)6Cl2. Here samples of stoichiometry 0.15 ≤ x ≤ 1 were synthesised and characterised. Using muon-spin relaxation spectroscopy on these samples, it was found that for x > 0.6 the spins do not freeze, even at 50 mK. From neutron powder diffraction for x = 1 and heat capacity measurements on zinc paratacamite with 0.5 ≤ x ≤ 1, it was found that ~6% antisite disorder is present in the x = 1 phase, and that samples with Zn stoichiometry 0.8 ≤ x ≤ 1 model the S = 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 x > 0,8. The field dependence of the heat capacity provides additional evidence that the total magnetic quantum number Stot 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 (S = 5/2) kagomé antiferromagnet iron jarosite KFe3 (SO4)2(OH)6, it is shown that the Fe3+ ion with a 6S free-ion configuration can acquire a large orbital angular momentum in the solid state. The high-resolution spectra are reproduced in excellent detail.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.649056  DOI: Not available
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