A model of magnetic order in hexagonal HoMnO3

TL;DR

This paper develops a theoretical model using symmetry and dynamic equations to explain the complex magnetic ordering and phase transitions in hexagonal HoMnO3, highlighting the role of Ho-Mn coupling and anisotropy.

Contribution

It introduces a novel application of Landau Lifshitz Gilbert equations to frustrated Mn ions and derives a free energy model explaining magnetic phases in HoMnO3.

Findings

Four principal spin configurations are stabilized.

Ho-Mn coupling arises from trigonal anisotropy.

The model reproduces temperature-driven phase transitions.

Abstract

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.