Magnetic Phase Transition and Magnetization Plateau in Cs${}_2$CuBr${}_4$

The crystal structure of Cs${}_2$CuBr${}_4$ is the same as that of Cs${}_2$CuCl${}_4$, which has been characterized as a spin-1/2 quasi-two-dimensional frustrated system. The magnetic properties of Cs${}_2$CuBr${}_4$ were investigated by magnetization and specific heat measurements. The phase transition at zero magnetic field was detected at $T_{\mathrm{N}}=1.4$ K. It was observed that the magnetization curve has a plateau at about one-third of the saturation magnetization for magnetic field $H$ parallel to the $b$- and $c$-axes, while no plateau was observed for $H\parallel a$. The field-induced phase transition to the plateau state appears to be of the first order. The mechanism leading to the magnetization plateau is discussed.