Magnetization of New Kagomé Lattice Antiferromagnets: Cr-Jarosites,<i>A</i>Cr₃(OH)₆(SO₄)₂[<i>A</i>= Na, K, Rb, NH₄]

Abstract

The jarosite-type compounds, KFe 3 (OH) 6 (SO 4 ) 2 and KCr 3 (OH) 6 (SO 4 ) 2 , are model materials of the frustrated antiferromagnet on the kagomé lattice. We have synthesized new Cr-jarosites, A Cr 3 (OH) 6 (SO 4 ) 2 , with other monovalent cations A + [ A = Rb, NH 4 , Na] and have measured the magnetization between 2 K and 300 K. The measured susceptibilities above 20 K agree well with the calculation based on the high-temperature expansion and with the Monte Carlo simulation for the two-dimensional Heisenberg antiferromagnet on the kagomé lattice, which indicates that the new Cr-jarosites are good examples of the Heisenberg kagomé lattice antiferromagnets with S = 3/2. The exchange interactions for these Cr-jarosites are estimated to be about 5 K. The susceptibilities of the Cr-jarosites with K, Rb, and NH 4 increase steeply below about 4 or 8 K, although they are antiferromagnets. The susceptibilities after zero-field-cooling and those after field-cooling are different below this temperature, and the magnetization curves at 2.0 K show small hysteresis loops, which indicates the existence of small ferromagnetic components. The anomalies are considered to be due to the magnetic ordering and not to the spin-glass-like freezing. The weak ferromagnetic moment is caused by the canted 120° spin structure in the ordered state.