The zero-field magnetism of a two-dimensional noncollinear antiferromagnet, κ-(BEDT-TTF)2Cu[N(CN)2]Cl, has been investigated by magnetization and zero-field muon spin rotation (μSR) measurements. Low-field magnetization measurements enabled us to determine the magnetic transition temperature TN as 22.80 ± 0.02 K. Distinct muon spin precession signals appeared below 21.4 K. μSR spectra below 21.4K were well described by two types of precession components and a relaxation one. The temperature dependence of internal field converted by μSR data was in good agreement with that of macroscopic residual magnetism. These results suggest that the tiny interlayer interaction, which has been suggested to be almost 106 times less than the intralayer exchange interaction, spontaneously causes the three-dimensional long-range order.
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