We have examined the magnetic structure of the kagomé lattice antiferromagnet potassium jarosite (K jarosite: ${\mathrm{KFe}}_3{\mathrm{}\left(\mathrm{OH}\right)\mathrm{}}_6{(\mathrm{S}\mathrm{O}}_4{)}_2)$ by means of powder neutron diffraction. Extremely high degeneracy of the ground states prevents the long-range magnetic ordering at any temperature and the $\sqrt{3}\times \sqrt{3}$ structure is predicted theoretically to be favored rather than the $q=0\mathrm{}$ structure at $T=0\mathrm{}$ 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 $q=0\mathrm{}$ structure. In addition, although the $q=0\mathrm{}$ 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.

• Received 27 August 1999

DOI:https://doi.org/10.1103/PhysRevB.61.12181