Anomalous Magnetization of Two-Dimensional <i>S</i> = 1/2 Frustrated Square-Lattice Antiferromagnet (CuCl)LaNb₂O₇

Abstract

High-field magnetization measurements have been performed up to 56 T for a two-dimensional S = 1/2 frustrated square-lattice antiferromagnet (CuCl)LaNb 2 O 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/ g µ 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.