Theoretical investigation of magnetoelectric effects in Ba2CoGe2O7

A joint theoretical approach, combining macroscopic symmetry analysis with microscopic methods (density functional theory and a model cluster Hamiltonian), is employed to shed light on magnetoelectricity in Ba${}_{2}$CoGe${}_{2}$O${}_{7}$. The recently reported experimental behavior of polarization guided by a magnetic field [Murakawa et al., Phys. Rev. Lett. 105, 137202 (2010)] can be predicted on the basis of phenomenological Landau theory. On the microscopic side, two main ingredients are needed for the cross-coupling between magnetic and dipolar degrees of freedom: on-site spin-orbit coupling and the spin-dependent hybridization between O-$p$ and transition metal $d$ states. Structural constraints related to the noncentrosymmetric symmetry and the particular configuration of CoO${}_{4}$ tetrahedrons provide additional features for a peculiar magnetoelectricity to develop.