Structural Anomalies at the Magnetic and Ferroelectric Transitions in $RM{n}_2{O}_5$ (R=Tb, Dy, Ho)

Strong anomalies of the thermal expansion coefficients at the magnetic and ferroelectric transitions have been detected in multiferroic $RM{n}_2{O}_5$. Their correlation with anomalies of the specific heat and the dielectric constant is discussed. The results provide evidence for the magnetic origin of the ferroelectricity mediated by strong spin-lattice coupling in the compounds. Neutron scattering data for $HoM{n}_2{O}_5$ indicate a spin reorientation at the two low-temperature phase transitions.