  We report polarized neutron scattering studies of spin-wave excitations and
spin fluctuations in the S=5/2 kagome lattice antiferromagnet KFe3(OH)6(SO4)2
(jarosite). Inelastic polarized neutron scattering measurements at 10 K on a
single crystal sample reveal two spin gaps, associated with in-plane and
out-of-plane excitations. The polarization analysis of quasi-elastic scattering
at 67 K shows in-plane spin fluctuations with XY symmetry, consistent with the
disappearance of the in-plane gap above the Neel temperature T_N = 65 K. Our
results suggest that jarosite is a promising candidate for studying the 2D XY
universality class in magnetic systems.
