Field-induced two-step phase transitions in the singlet ground state triangular antiferromagnet CsFeBr3

The ground state of the stacked triangular antiferromagnet CsFeBr3 is a spin singlet due to the large single ion anisotropy D(S-z)(2). The field-induced magnetic ordering in this compound was investigated by the magnetic susceptibility, the magnetization process and specific heat measurements for an external field parallel to the c-axis. Unexpectedly, two phase, transitions were observed in the magnetic field H higher than 3 T. The phase diagram for temperature vs. magnetic field vas obtained. The mechanism leading to the successive phase transitions is discussed.