Phase transitions induced by easy-plane anisotropy in the classical Heisenberg antiferromagnet on a triangular lattice: A Monte Carlo simulation

The antiferromagnetic classical $\mathrm{XXZ}$ model with easy-plane exchange anisotropy on the triangular lattice, which causes frustration of the spin alignment, is studied by means of Monte Carlo simulations. The system 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 temperature. Data for internal energy, specific heat, magnetic susceptibility, correlation length, and some properties associated with the chirality are reported in a broad temperature range for lattice sizes ranging from $24\times 24$ to $120\times 120$; 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 behavior, the two transition temperatures decrease by about 25% and 22%, respectively.