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Title: Magnetic properties of the jarosites : model kagome antiferromagnets
Author: Pettigrew, Keith G.
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2002
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The long-range ordered ground state of KFe3(SO4)2(OH)6 has been determined unambiguously through Spherical Neutron Polarimetry on a natural single crystal. The magnetic arrangement has been shown to be the q = 0 structure with triangles of only positive chirality. A new transition to a state of higher magnetisation has been identified in a synthetic sample at T ~ 7K. The driving force for this has been attributed to anisotropy in the system and theory provides a possible ordering mechanism with weak ferromagnetic order. Magnetic neutron scattering confirms an intermediate phase at 52K < T < 65K with the coexistence of Bragg peaks and significant diffuse scattering. D3OFe3(SO4)2(OD)6 has been shown to exhibit spin-glass type behaviour with a freezing temperature, Tf, that is dependent on the timescale of the measurement. Intentional dilution of the magnetic lattice with Al3+ induces a transition to long-range order at a temperature dependent on the level of doping. Mössbauer spectroscopy shows two equally-weighed magnetic sites with local field value and probability in excellent agreement with theoretical predictions. A new mixed-cation compound, H3OFe2.4Cr0.3(SO4)2(OH)6, has been successfully prepared and represents the first experimental example of mixed magnetic ions on a kagome lattice. Field-cooled (FC) and zero-field-cooled (ZFC) dc susceptibility data cross at a temperature dependent on the magnitude of the applied field and hysteresis measurements suggest the presence of ferrimagnetism. High resolution neutron diffraction on D3OCr3(SO4)2(OD)6 has shown that the crystal structure is slightly distorted from the R ?m to the R 3 m space group. A magnetic transition is observed at 2K through a sharp peak in ac susceptibility accompanied by a small FC/ZFC splitting. Polarised neutron diffraction shows no change in the magnetic correlation length, ξ, on cooling from 15K to 0.25K where only short-range order exists with ξ ~ 6.6Å, about half the width of a kagome star.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available