Observation of Successive Phase Transitions in Kagomé Lattice Antiferromagnets RFe_<b>3</b>(OH)_<b>6</b>(SO_<b>4</b>)_<b>2</b>[<b><i>R</i>=<i>N</i><i>H</i>₄</b>, Na, K]

Abstract

Successive magnetic phase transitions were found in the Kagomé lattice antiferromagnets RFe 3 (OH) 6 (SO 4 ) 2 [ R=NH 4 , Na, K]. The temperature dependence of the susceptibility has two cusps around 60 K for each compound. The X-ray diffraction patterns and the quadrupole resonance spectra show that the transitions are not caused by a structural change. The presence of these transitions was confirmed by NMR experiments. The NMR spectra show the development of magnetic ordering below the upper transition temperature. In the low-temperature phase the nuclear magnetization does not relax exponentially, which may suggest a domain structure of 120° spin structure due to degeneracy on the Kagomé lattice. The successive transitions may be caused by a small Ising-like anisotropy in the Heisenberg antiferromagnet.