Structural anomalies at the magnetic transition in centrosymmetric BiMnO3

The structural properties of BiMnO3 were determined from neutron powder diffraction data as a function of temperature and magnetic field. The structure at all temperatures was found to be cen- trosymmetric with space group C2/c, which is incompatible with ferroelectricity. At TC ? 100K, we observed the onset of a large magnetoelastic strain, proportional to the square of the magnetization. We interpret this structural rearrangement, together with the previously observed magnetodielectric anomalies, as due to the need to optimize the partially frustrated magnetic interactions.