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Title: A study of experimental Kagomé antiferromagnets based on the jarosite series
Author: Wills, Andrew S.
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1996
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Regent theoretical interest in Kagomé antiferromagnets arose from the prediction that they possess a new type of magnetic ground state in which there is no magnetic long-range order even at T = 0. Prior to this work no detailed investigations had bee made of the magnetic properties of real materials that provide models for this class of magnet. Members of the series AB3(SO4)2(OH)6 (A+ = Na+, K+, Rb+, Ag+, NH4+, and H3O+; B3+ = V3+, Cr3+, and Fe3+) have been studied using a combination of experimental techniques- ac and dc susceptibility, specific heat, neutron diffraction, and MuSR. The hydronium compounds (A+ = H3O+; B3+ = V3+, Cr3+, and Fe3+) have been shown to provide the best model Kagomé antiferromagnets with S = 1, 3/2 and 5/2 respectively. (H3O)Fe3(SO4)2(OH)6 has been shown to possess only short-range magnetic order with a correlation length of 19 ± 2Å, below a spin glass transition temperature, Tf ˜17K. Unlike normal spin glasses, muon spin relaxation measurements show that this magnetic phase is dynamic below Tf (H3O)Fe3(SO4)2(OH)6 is a spin fluid: its dynamic and thermodynamic behaviour are completely unlike those seen in normal spin glass or long-range ordered ground states; the specific heat varies as T2 below Tf, rather than the linear T dependence observed in canonical spin glasses. Deliberate reduction of the concentration of magnetic atoms destabilises the spin fluid ground state and induces the formation of long-range order of the type seen in the other less magnetically concentrated members of the series. This is the first example of a system in which diamagnetic dilution causes it to order classically. (H3O)Cr3(SO4)2(OH)6 has a broad transition to a dynamic short-range ordered state at ˜ 30K, similar to that seen in (H3O)Fe3(SO4)2(OH)6, but with no observed spin glass behaviour. A weak ferromagnetic transition at 2.2K is ascribed to a canting of antiferromagnetically coupled sublattices. Specific heat and dc susceptibility measurements show that (H3O)V3(SO4)2(OH)6 undergoes a sharp antiferromagnetic transition at 21K to an ordered ground state. However, neutron diffraction data taken at 4.2K reveals no evidence for long-range magnetic order. It is suggested that the magnetic ground state could be a highly correlated spin fluid.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available