Bisson, W.G.; (2011) Crystal structures and magnetism in jarosites: model kagome antiferromagnets. Doctoral thesis , UCL (University College London).

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Abstract

Jarosites are a family of minerals with the general formula AFe3(SO4)2(OH)6 (A=H3O+, NH+ 4 , K+, Na+, Rb+, Ag+, 1 2 Pb2+). Long known to earth scientists and the mining community, jarosites provide the most studied examples of model kagome magnets: materials where the magnetic exchange interactions are frustrated by the kagome geometry of vertex-sharing triangles. In most jarosites this frustration is insufficient to prevent the formation of magnetic order at low temperatures and two ordering transitions are observed. The primary transition at 60 < TN1=K < 65 is to an umbrella spin structure, which flops into the kagome plane at the secondary transition, TN2 , between 45 and 55 K. The exception to this behaviour is hydronium jarosite (A=H3O+) which instead undergoes a critical freezing transition to an unconventional spin glass state at a much lower temperature, Tg ~ 17 K. This thesis presents studies of the chemistry, crystallography and magnetism of the jarosites with the general aim of relating the observed magnetic responses to their chemistry and crystal structures. The investigations presented here concentrate on hydronium jarosite and relate the changes to the synthesis chemistry and crystal chemistry to the spin-glass transition temperature. A combination of powder and single-crystal X-ray diffraction and SQUID magnetometry are used to show that the spin-glass transition in hydronium jarosite is correlated to the degree of distortion of the coordination around the moment bearing Fe3+ ions. Samples with the most symmetric coordination of the magnetic Fe3+ ions feature the lowest values of Tg. As the defining influence in spin glasses is typically thought to be disorder, this observation is remarkable. Further, these studies show that in hydronium jarosite the key to the spin glass transition is a uniform (translationally invariant) energy scale that is associated with the crystallographic distortion. In so doing, they support the proposal that the spin-glass transition in hydronium jarosite is driven by anisotropy. Further, elemental analyses show that the Fe stoichiometry has little effect upon the displayed magnetic properties of the jarosites. Rather, these are found to be most sensitive to small crystallographic changes in the Fe-O coordination octahedra associated with substitution of the A-site cation. In terms of the ratio between the Fe-O equatorial and apical bond lengths, greater deviation from Oh symmetry is shown to correlate with higher values for Tg and TN2 .

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