Comprehensive study of the ferroelectricity induced by the spin-dependent d-p hybridization mechanism in Ba2XGe2O7 (X = Mn, Co, and Cu)

We have investigated the magnetoelectric properties originating from the spin-dependent hybridization between transitionmetal (X) d orbital and ligand ion p orbitals in the noncentrosymmetric crystals Ba2XGe2O7 (X = Mn, Co, Cu). The novelmagnetic-field responses of the electric polarization observed in the staggered antiferromagnets Ba2MnGe2O7 (S = 5/2) and Ba2CoGe2O7 (S = 3/2) can be accounted for by the mean-field approximation based on the d-p hybridization mechanism. In the case of the S = 1/2 helimagnet Ba2CuGe2O7, we have found the metamagnetic transition to the staggered spin state in the high magnetic field region and observed an electric polarization originating from the d-p hybridization mechanism. Especially, the observation of electric polarization not only in the fully spin-polarized state at high magnetic field in Ba2MnGe2O7 but also in all the compounds, under applied external magnetic field, strongly supports the single-spin mechanism of the d-p hybridization as the origin of ferroelectricity in this class of materials. We have examined the transition-metal dependence of the polarization magnitude in Ba2XGe2O7 in terms of the electronic configuration in d levels and found that the unquenched orbital angular momentum determines the magnitude of the electric polarization.