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) posse
sses a kagome lattice of Fe3+ ions that behave as S = 5/2 Heisenberg moment
s coupled through strong antiferromagnetic exchange. DC susceptibility meas
urements reveal a spin-glass-like transition at T-f congruent to 15 K. We h
ave performed muon-spin relaxation (mu SR) measurements on this material to
elucidate the nature of this transition. Well above T-f, the muon depolari
sation adopts a simple exponential form, becoming a stretched exponential w
ith exponent beta approaching 0.5 as T --> T-f. On cooling further, beta dr
ops further to 1/3, and the depolarisation adopts a dynamic Kubo-Toyabe for
m which may be decoupled by applied longitudinal fields of the order of 5 k
C. 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) ex
pected for high-spin Fe3+. (C) 2000 Elsevier Science B.V. All rights reserv
ed.