Magnetization of New Kagome Lattice Antiferromagnets : Cr-Jarosites, ACr_3(OH)_6(SO_4)_2 [A = Na, K, Rb, NH_4]

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The jarosite-type compounds, KFe_3(OH)_6(SO_4)_2 and KCr_3(OH)_6(S O_4)_2, are model materials of the frustrated antiferromagnet on the kagome lattice. We have synthesized new Cr-jarosites, ACr_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 kagom6 lattice, which indicates that the new Cr-jarosites are good examples of the Heisenberg kagome 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 l20° spin structure in the ordered state.
社団法人日本物理学会の論文
2003-08-15