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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 versus magnetic field was obtained. The mechanism leading to the successive phase transitions is discussed.KEYWORDS: CsFeBr 3 , triangular antiferromagnet, singlet ground state, field-induced magnetic ordering, successive phase transitions, single ion anisotropy, magnetic susceptibility, magnetization process, specific heat §1. IntroductionThe spin frustration effect often plays an important role in the magnetic ordering process and magnetic excitations. In the hexagonal antiferromagnets of ABX 3 -type with the CsNiCl 3 structure, magnetic B 2+ ions form infinite chains along the c-axis and triangular lattices in the basal c-plane. Since the exchange interaction in the c-plane is antiferromagnetic, they behave as triangular antiferromagnets (TAF) at low temperatures. Because of the spin frustration effect being characteristic of TAF, together with the quantum effect, a rich variety of phase transitions have been observed in the hexagonal ABX 3 antiferromagnets. 1)Low-temperature magnetic properties of AFeX 3 systems are described by the pseudo spin S = 1 anisotropic XXZ model with the large easy-plane anisotropy of the form D(S z ) 2 due to the crystalline field.2) In RbFeCl 3 , exchange interactions overcome the anisotropy, so that RbFeCl 3 undergoes magnetic phase transition in the absence of magnetic field. On the other hand, CsFeCl 3 has a singlet ground state at zero field, because exchange interactions are not sufficiently strong to produce the magnetic ordering. The ordering process in RbFeCl 3 is not simple. The phase transition occurs from the paramagnetic state to the commensurate (C) ground state with the 120• structure through two different incommensurate (IC) states.3, 4) The IC-C phase transition occurs due to the competition between the antiferromagnetic exchange interaction in the c-plane and the dipole-dipole (D-D) interaction, the latter of which is enhanced by the ferromagnetic exchange interaction along the c-axis.5, 6) A similar IC-C phase transition was observed in CsFeCl 3 Table I. Interaction parameters for CsFeBr 3 in the unit of K.

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