Interplay of anisotropy and frustration: Triple transitions in a triangular-lattice antiferromagnet

The classical Heisenberg antiferromagnet on a triangular lattice with the single-ion anisotropy of the easy-axis type is studied theoretically. The phase diagram in an external magnetic field is constructed from the mean-field analysis. Three successive Berezinskii-Kosterlitz-Thouless transitions are found by Monte Carlo simulations in zero field. Two upper transitions are related to the breaking of the discrete Z(6) symmetry while the lowest transition is associated with a quasilong-range ordering of transverse components. The intermediate collinear phase between the first and second transition is the critical phase predicted by Joseacute [Phys. Rev. B 16, 1217 (1977)].