Anomalous magnetization of two-dimensional S=1/2 frustrated square-lattice antiferromagnet (CuCl)LaNb2O7

High-field magnetization measurements have been performed up to 56T for a two-dimensional S = 1/2 frustrated square-lattice antiferromagnet (CuCI)LaNb2O7, a recently discovered spin gap system (Delta/k(B) = 26.7 K). It is found that the spin gap closes at a surprisingly low field H-cl = 10.3 T compared with that expected from the zero-field spin gap (Delta/g mu(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.