Strong spin-lattice coupling in multiferroicHoMnO3: Thermal expansion anomalies and pressure effect

Evidence for a strong spin-lattice coupling in multiferroic $\mathrm{Ho}\mathrm{Mn}{\mathrm{O}}_3$ 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 $\left(T_N\right)$ and the spin rotation $\left(T_{\mathit{SR}}\right)$ 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 $T_{\mathit{SR}}$ various physical quantities show discontinuities that are thermodynamically consistent with a first-order phase transition.