Magnetic ordering and fluctuation in kagome lattice antiferromagnets, Fe and Cr jarosites

Jarosite family compounds, KFe_3(OH)_6(SO_4)_2, (abbreviate Fe jarosite), and KCr_3(OH)_6(SO_4)_2, (Cr jarosite), are typical examples of the Heisenberg antiferromagnet on the kagome lattice and have been investigated by means of magnetization and NMR experiments. The susceptibility of Cr jarosite deviates from Curie-Weiss law due to the short-range spin correlation below about 150 K and shows the magnetic transition at 4.2 K, while Fe jarosite has the transition at 65 K. The susceptibility data fit well with the calculated one on the high temperature expansion for the Heisenberg antiferromagnet on the kagome lattice. The values of exchange interaction of Cr jarosite and Fe jarosite are derived to be J_Cr = 4.9 K and J_Fe = 23 K, respectively. The 1H-NMR spectra of Fe jarosite suggest that the ordered spin structure is the q = 0 type with positive chirality of the 120 degrees configuration. The transition is caused by a weak single-ion type anisotropy. The spin-lattice relaxation rate, 1/T_1, of Fe jarosite in the ordered phase decreases sharply with lowering the temperature and can be well explained by the two-magnon process of spin wave with the anisotropy.