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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 onset of Ho magnetic order. Based on 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.