OBSERVATION OF SUCCESSIVE PHASE-TRANSITIONS IN KAGOME LATTICE ANTIFERROMAGNETS RFE(3)(OH)(6)(SO4)(2) [R=NH4, NA, K]

Successive magnetic phase transitions were found in the Kagome lattice antiferromagnets RFe(3)(OH)(6)(SO4)(2) [R = NH4, Na, K]. The temperat ure dependence of the susceptibility has two cusps around 60 K for eac h compound. The X-ray diffraction patterns and the quadrupole resonanc e spectra show that the transitions are not caused by a structural cha nge. The presence of these transitions was confirmed by NMR experiment s. The NMR spectra show the development of magnetic ordering below the upper transition temperature. Ln the low-temperature phase the nuclea r magnetization does not relax exponentially, which may suggest a doma in structure of 120 degrees spin structure due to degeneracy on the Ka gome lattice. The successive transitions may be caused by a small Isin g-like anisotropy in the Heisenberg antiferromagnet.