mu SR studies of the kagome antiferromagnet (H3O)Fe-3(OH)(6)(SO4)(2)

The Heisenberg kagome 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)Fe-3(OH)(6)(SO4)(2) possesses a kagome lattice of Fe3+ ions that behave as S = 5/2 Heisenberg moments coupled through strong antiferromagnetic exchange. DC susceptibility measurements reveal a spin-glass-like transition at T-f congruent to 15 K. We have performed muon-spin relaxation (mu SR) measurements on this material to elucidate the nature of this transition. Well above T-f, the muon depolarisation adopts a simple exponential form, becoming a stretched exponential with exponent beta approaching 0.5 as T --> T-f. On cooling further, beta drops further to 1/3, and the depolarisation adopts a dynamic Kubo-Toyabe form which may be decoupled by applied longitudinal fields of the order of 5 kC. We estimate the upper limit of the static component of the moment to be 3.4 mu(B) per iron ion, compared with a value of congruent to 5.92 mu(B) expected for high-spin Fe3+. (C) 2000 Elsevier Science B.V. All rights reserved