Magnetoelectric Cr2O3 and relativity theory F. W. Hehl, Y. N. Obukhov, J.-P. Rivera and H. Schmid Eur. Phys. J. B, 71 3 (2009) 321-329 Published online: 19 June 2009 DOI: 10.1140/epjb/e2009-00203-7