Anomalous Magnetization of Two-Dimensional S=1/2 Frustrated Square-Lattice Antiferromagnet (CuCl)LaNb_2O_7(Condensed Matter : Electronic Structure, Electrical, Magnetic and Optical Properties)

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High-field magnetization measurements have been performed up to 56 T for a two-dimensional S=1/2 frustrated square-lattice antiferromagnet (CuCl)LaNb_2O_7, a recently discovered spin gap system (Δ/k_B=26.7 K). It is found that the spin gap closes at a surprisingly low field H_<c1>=10.3 T compared with that expected from the zero-field spin gap (Δ/gμ_B=18.4 T). For H>H_<c1>, the magnetization exhibits a linear increase without any trace of anomalies such as fractional plateaus until it saturates at H_<c2>=30.1 T. This means that the gapless phase, where the field-induced magnetic ordering is expected to occur at low temperatures, is stable over a wide field region. These results suggest strong correlations of triplet excitations in the layer and the proximity of the spin-liquid phase to the magnetically ordered phase.
社団法人日本物理学会の論文
2005-12-15