Abstract
Because of the frustration the spin structure of hexagonal Heisenberg antiferromagnets, which have weak XY-like anisotropy and ferromagnetic interlayer coupling, is determined by a delicate balance. The Monte Carlo method (for classical spins) is applied to obtain the phase diagram. It is shown that a competition between weak XY-like anisotropy and entropy effect (i.e.,thermal fluctuations) results successively in various phases, which show up in the magnetization process. At high fields a coplanar spin structure is stabilized at finite temperatures, while only one phase with the umbrella-type spin structure is present in the ground state. For a comparison the phase diagram of the Heisenberg model without anisotropy is also studied to find the same field-induced phase at high fields. In the low field region, the spin structure in the Heisenberg model is different from that in the XY-like case. We find entropy-induced phase transitions to occur due to thermal fluctuations, which overcome the weak anisotropy.
