  We address a systematic microscopic theory of spin-dependent electric
polarization in 3d oxides starting with a generic three-site two-hole cluster.
A perturbation scheme realistic for 3d oxides is applied which implies the
quenching of orbital moments by low-symmetry crystal field, strong intra-atomic
correlations, the dp-transfer effects, and rather small spin-orbital coupling.
An effective spin operator of the electric dipole moment is deduced
incorporating both nonrelativistic and relativistic terms. The nonrelativistic
electronic polarization mechanism related with the effects of the
redistribution of the local on-site charge density due to $pd$ covalency and
exchange coupling is believed to govern the multiferroic behaviour in 3d
oxides. The relativistic exchange-dipole moment is mainly stems from the
nonrelativistic one due to the perturbation effect of Dzyaloshinsky-Moriya
coupling and is estimated to be a weak contributor to the electric polarization
observed in the most of 3d multiferroics.
