Strong spin-lattice coupling in multiferroic HoMnO ₃: Thermal expansion anomalies and pressure effect

Evidence for a strong spin-lattice coupling in multiferroic HoMnO ₃ is derived from thermal expansion measurements along the a and c axes. The magnetoelastic effect results in sizable anomalies of the thermal expansivities at the antiferromagnetic (T _N) and the spin rotation (T _SR) transition temperatures as well as in a negative c-axis expansivity below room temperature. The coupling between magnetic orders and dielectric properties below T _N is explained by the lattice strain induced by the magnetoelastic effect. At T _SR various physical quantities show discontinuities that are thermodynamically consistent with a first-order phase transition.