Magnetization plateaux of S = 1/2 two-dimensional frustrated antiferromagnet Cs\(_2\)CuBr\(_4\)

The field induced magnetic phase transitions of Cs\(_2\)CuBr\(_4\) were investigated by means of magnetization process and neutron scattering experiments. This system undergoes magnetic phase transition at Ne\'{e}l temperature \(T_\mathrm{N}=1.4\) K at zero field, and exhibits the magnetization plateau at approximately one third of the saturation magnetization for the field directions \(H\parallel b\) and \(H\parallel c\). In the present study, 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 \(\boldsymbol{Q}=(0, 0.575, 0)\) at zero field. With increasing magnetic field parallel to the c-axis, the ordering vector increases continuously and is locked at \(\boldsymbol{Q}=(0, 0.662, 0)\) in the plateau field range \(13.1 \mathrm{T} < H < 14.4 \mathrm{T}\). This indicates that the collinear \textit{up-up-down} spin structure is stabilized by quantum fluctuation at the magnetization plateau.