Coexistence of magnetism and ferroelectricity in perovskites

We present a first-principles comparison of BaTiO3,CaMnO3, and YMnO3 which reveals the fundamental role of the d electron occupation in preventing or favoring the simultaneous presence of magnetic and electric polarization. The ferroelectric compounds are distinguished from the nonferroelectrics by an ultrasensitivity of the p-d charge hybridization to the atomic displacements. To be effective these hybridization changes require the d orbitals oriented in the direction of the ferroelectric distortion to be formally empty. This orbital directionality can explain the coexistence of antiferromagnetism and ferroelectricity in the hexagonal phase of YMnO3, which is ferroelectric along the c axis: in the Mn3+ ion, four filled d orbitals provide a magnetic moment, and one empty d orbital (d(z)(2)) can participate in hybridization changes and induce a ferroelectric distortion. The Born effective charges are found to be highly anomalous in CaMnO3, which is far from being ferroelectric.