Evidence for strong spin-lattice coupling in multiferroic R Mn ₂O ₅ (R=Tb,Dy,Ho) via thermal expansion anomalies

Thermal expansion measurements were done on single-crystal R Mn2 O5 (R=Tb,Dy,Ho) along the principal crystallographic axes. Distinctive anomalies were observed in the linear thermal expansivities at critical temperatures marking the onset of long-range antiferromagnetic order (TN1), ferroelectricity (TC1), as well as at temperatures when anomalous changes in the polarization, dielectric constant, and spin wave incommensurability have been previously reported. These observations suggest that the coupling between the magnetic orders and the dielectric properties is mediated by the lattice distortion. In particular, a possible scenario as to how ferroelectricity arises and subsequently is altered as a result of the subtle changes in the highly frustrated magnetic order, is discussed in the light of the strong spin-lattice coupling observed in these materials. Neutron diffraction measurements on Ho Mn2 O5 show a spin reorientation at 23 K corresponding to a step-like anomaly in the dielectric constant.