PHASE-TRANSITIONS INDUCED BY EASY-PLANE ANISOTROPY IN THE CLASSICAL HEISENBERG-ANTIFERROMAGNET ON A TRIANGULAR LATTICE - A MONTE-CARLO SIMULATION

The antiferromagnetic classical XXZ model with easy-plane exchange ani sotropy on the triangular lattice, which causes frustration of the spi n alignment, is studied by means of Monte Carlo simulations. The syste m shows the signature;of a Berezinskii-Kosterlitz-Thouless transition, associated with vortex-antivortex unbinding, and of an Ising-like one due to the chirality, the latter occurring at a slightly higher tempe rature. Data for internal energy, specific heat, magnetic susceptibili ty, correlation length, and some properties associated with the chiral ity are reported in a broad temperature range for lattice sizes rangin g from 24x24 to 120x120; four values of the easy-plane anisotropy are considered. Moving from the strongest towards the weakest anisotropy ( 1%), as the thermodynamic quantities tend to the isotropic model behav ior, the two transition temperatures decrease by about 25% and 22%, re spectively. [S0163-1829(98)00725-5].