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- W80385689 abstract "Jarosites are a family of minerals with the general formula AFe3(SO4)2(OH)6(A=H3O+, NH+4 , K+, Na+, Rb+, Ag+, 12 Pb2+). Long known to earth scientistsand the mining community, jarosites provide the most studied examples of modelkagome magnets: materials where the magnetic exchange interactions are frustratedby the kagome geometry of vertex-sharing triangles. In most jarosites this frustrationis insufficient to prevent the formation of magnetic order at low temperatures andtwo ordering transitions are observed. The primary transition at 60 < TN1=K < 65is to an umbrella spin structure, which ops into the kagome plane at the secondarytransition, TN2 , between 45 and 55 K. The exception to this behaviour is hydroniumjarosite (A=H3O+) which instead undergoes a critical freezing transition to anunconventional spin glass state at a much lower temperature, Tg ~ 17 K.This thesis presents studies of the chemistry, crystallography and magnetism ofthe jarosites with the general aim of relating the observed magnetic responses to theirchemistry and crystal structures. The investigations presented here concentrate onhydronium jarosite and relate the changes to the synthesis chemistry and crystalchemistry to the spin-glass transition temperature. A combination of powder andsingle-crystal X-ray diffraction and SQUID magnetometry are used to show that thespin-glass transition in hydronium jarosite is correlated to the degree of distortionof the coordination around the moment bearing Fe3+ ions. Samples with the mostsymmetric 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, thisobservation is remarkable. Further, these studies show that in hydronium jarosite thekey to the spin glass transition is a uniform (translationally invariant) energy scalethat is associated with the crystallographic distortion. In so doing, they support theproposal that the spin-glass transition in hydronium jarosite is driven by anisotropy.Further, elemental analyses show that the Fe stoichiometry has little effect uponthe displayed magnetic properties of the jarosites. Rather, these are found to bemost sensitive to small crystallographic changes in the Fe-O coordination octahedraassociated with substitution of the A-site cation. In terms of the ratio between theFe-O equatorial and apical bond lengths, greater deviation from Oh symmetry isshown to correlate with higher values for Tg and TN2 ." @default.
- W80385689 created "2016-06-24" @default.
- W80385689 creator A5043913930 @default.
- W80385689 date "2011-04-28" @default.
- W80385689 modified "2023-09-28" @default.
- W80385689 title "Crystal structures and magnetism in jarosites : model kagome antiferromagnets" @default.
- W80385689 hasPublicationYear "2011" @default.
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