We have examined the magnetic structure of the kagomé lattice antiferromagnet potassium jarosite "K jarosite: KFe 3 ͑OH͒ 6 ͑SO 4 ͒ 2 … by means of powder neutron diffraction. Extremely high degeneracy of the ground states prevents the long-range magnetic ordering at any temperature and the )ϫ) structure is predicted theoretically to be favored rather than the qϭ0 structure at Tϭ0 in a kagomé lattice Heisenberg antiferromagnet. Nevertheless, K jarosite shows long-range magnetic ordering at 65 K and the ordered magnetic structure was found to be the qϭ0 structure. In addition, although the qϭ0 structure has two degenerated states of ''positive'' and ''negative'' chirality, the observed magnetic structure contains only elemental triangles of positive chirality. We found that a weak single-ion-type anisotropy is crucial for selecting the observed magnetic structure. The long-range magnetic ordering at finite temperature in the jarosite family of compounds can be ascribed to this anisotropy. PHYSICAL

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