A model of magnetic order in hexagonal HoMnO3.

Symmetry arguments are used to develop a spin Hamiltonian for the description of the complex magnetic ordering in HoMnO(3). Using a novel application of the Landau-Lifshitz-Gilbert dynamic torque equations to this model of the frustrated Mn ions on an AB stacked triangular antiferromagnetic, it is shown that the four principal spin configurations observed in this compound are stabilized. Ho-Mn coupling is found to be a consequence of an unusual trigonal anisotropy term which is responsible for simultaneous Mn spin reorientation and the onset of Ho magnetic order. On the basis of these microscopic considerations, a mean-field Landau-type free energy is derived which reproduces the succession of observed temperature-driven magnetic phase transitions at zero field, including re-entrant behavior. In addition, our analysis suggests that the basal-plane magnetic order should be slightly incommensurate with the lattice.