NMR study of S= quasi-two-dimensional antiferromagnet Cs2CuBr4

Abstract 133 Cs -NMR experiments on the S= 1 2 quasi-two-dimensional antiferromagnet Cs2CuBr4 have been performed up to magnetic fields of 15.9 T , which covers the field range for the 1 3 magnetization plateau. Outside the plateau region, the NMR spectrum at T=0.5 K shows a double-horn type broad line shape characteristic of an incommensurate spin structure. On the other hand, discrete two peaks were observed in the plateau phase, indicating a commensurate up–up–down spin structure. The nuclear spin–lattice relaxation rate T1−1 for commensurate phase decreases much more steeply with decreasing temperature than that for incommensurate phase, which indicates the different low lying excitations of the two phases. Experimental results in the paramagnetic phase are also discussed.