Nonequilibrium relaxation study of the anisotropic antiferromagnetic Heisenberg model on the triangular lattice

Effects of exchange anisotropy on the relaxation time of spin and vector chirality are studied for the antiferromagnetic classical Heisenberg model on the triangular lattice by the nonequilibrium relaxation Monte Carlo method. We identify the Berezinskii-Kosterlitz-Thouless (BKT) transition and the chiral transition in a wide range of the anisotropy, even for a very small anisotropy of ∼0.01%. As the anisotropy decreases, both critical temperatures steeply decrease, while the BKT critical region becomes divergently wide. We elucidate a sharp "V shape" of the phase diagram around the isotropic Heisenberg point, which suggests that the isotropic case is exceptionally singular and that the associated Z₂ vortex transition will be isolated from the BKT and chiral transitions. We discuss the relevance of our results to peculiar behavior of the spin relaxation time observed experimentally in triangular antiferromagnets.