Background:symmetry argument Magnetism Ferroelectricity Partially filled d shells Empty d shells break break Time reversal Space reversal symmetry symmetry t-→-t,M→-M x→-x,P→-P H-f(M,P)=f(-M,-P)=f(-M,P)=f(M,-P) N.A.Hill,Why are there so few magnetic ferroelectrics?J.Phys.Chem.B 104:6694(2000).Background: symmetry argument ++ - ++ Partially filled d shells break Time reversal symmetry t-t, M-M N. A. Hill, Why are there so few magnetic ferroelectrics? J. Phys. Chem. B 104: 6694 (2000). Empty d shells break Space reversal symmetry x-x, P-P Magnetism Ferroelectricity H=f(M, P)=f(-M, -P) = f(-M, P)=f(M, -P)