Magnetoelectric coupling in the cubic ferrimagnet Cu(2)OSeO(3)

We have investigated the magnetoelectric coupling in the lone pair containing piezoelectric ferrimagnet Cu(2)OSeO(3). Significant magnetocapacitance develops in the magnetically ordered state (T(c) = 60 K). 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 Cu(2)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.