Commensurate and incommensurate phases of the distorted triangular antiferromagnet Cs2CuBr4 studied using 133Cs NMR

A quasi- two dimensional antiferromagnet Cs2CuBr4 is characterized as a frustrated spin system on a distorted triangular lattice with easy- plane type anisotropy. Cs-133 NMR experiments on Cs2CuBr4 have been performed in the range of magnetic field up to 15.9 T, applied along the b- axis, which covers the field range for the 1/3 magnetization plateau, in order to clarify the properties of the field- induced incommensurate - commensurate transition at both ends of the plateau. We found that the NMR spectrum shows hysteresis and two phase coexistence around the transition field. We also present a field dependence of T-1(-1) at 0.5 K showing a hysteresis around each end of the plateau, indicating a first order incommensurate - commensurate phase transition.