We have investigated the magnetoelectric coupling in the lone pair containing piezoelectric ferrimagnet ${\text{Cu}}_2{\text{OSeO}}_3$. Significant magnetocapacitance develops in the magnetically ordered state $\left(T_c=60\text{K}\right)$. We find critical behavior near $T_c$ and a divergence near the metamagnetic transition at 500 Oe. High-resolution x-ray and neutron powder diffraction measurements show that ${\text{Cu}}_2{\text{OSeO}}_3$ is metrically cubic down to 10 K but that the ferrimagnetic ordering reduces the symmetry to rhombohedral $R3$. The metric cubic lattice dimensions exclude a magnetoelectric coupling mechanism involving spontaneous lattice strain, and this is unique among magnetoelectric and multiferroic materials.

• Received 24 June 2008

DOI:https://doi.org/10.1103/PhysRevB.78.094416