Field-Induced Two-Step Phase Transitions in the Singlet Ground State Triangular Antiferromagnet CsFeBr3

The ground state of the stacked triangular antiferromagnet CsFeBr 3 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 was obtained. The mechanism leading to the successive phase transitions is discussed.