Magnetization plateaux of the S=1/2 two-dimensional frustrated antiferromagnet Cs2CuBr4

The field induced magnetic phase transitions of Cs2CuBr4 were investigated by means of a magnetization process and neutron scattering experiments. This system undergoes a magnetic phase transition at Neel temperature T-N = 1.4 K at zero field, and exhibits a magnetization plateau at approximately one third of the saturation magnetization for the field directions H parallel to b and H parallel to c. In the present study, an additional symptom of the two-third magnetization plateau was found in the field derivative of the magnetization process. The magnetic structure was found to be incommensurate with the ordering vector Q = (0, 0.575, 0) at zero field. With increasing magnetic field parallel to the c-axis, the ordering vector increases continuously and is locked at Q = (0, 0.662, 0) in the plateau field range 13.1 T < H < 14.4 T. This indicates that the collinear up-up-down spin structure is stabilized by quantum fluctuation at the magnetization plateau.