μSR studies of the kagomé antiferromagnet (H3O)Fe3(OH)6(SO4)2

The Heisenberg kagomé antiferromagnet is a frustrated system that provides a simple model for the study of strongly fluctuating magnets with a highly degenerate ground state. Hydronium jarosite, (H3O)Fe3(OH)6(SO4)2 possesses a kagomé lattice of Fe3+ ions that behave as S=52 Heisenberg moments coupled through strong antiferromagnetic exchange. DC susceptibility measurements reveal a spin-glass-like transition at Tf≅15K. We have performed muon-spin relaxation (μSR) measurements on this material to elucidate the nature of this transition. Well above Tf, the muon depolarisation adopts a simple exponential form, becoming a stretched exponential with exponent β approaching 0.5 as T→Tf. On cooling further, β drops further to 13, and the depolarisation adopts a dynamic Kubo–Toyabe form which may be decoupled by applied longitudinal fields of the order of 5kG. We estimate the upper limit of the static component of the moment to be 3.4μB per iron ion, compared with a value of ≅5.92μB expected for high-spin Fe3+.