Co-O-O-Co superexchange pathways enhanced by small charge-transfer energy in multiferroic BiCoO₃

We have studied the electronic structure of multiferroic BiCoO₃ using x-ray photoemission spectroscopy (XPS), x-ray absorption spectroscopy (XAS), and subsequent model calculations. The XAS results show that the Co3 ⁺ ion takes the high-spin d6 configuration which usually prefers G-type antiferromagnetic state. The XPS results and model Hartree-Fock calculations show that, in case of BiCoO₃, small charge-transfer energy plays an essential role to enhance the Co-O-O-Co superexchange pathways. It is found that the combination of ferro-type orbital ordering of Co 3d t _2g and the Co-O-O-Co superexchange interaction gives rise to C-type antiferromagnetic state in BiCoO₃. The present analysis suggests that, in addition to the Bi-O bonds responsible for the noncentrosymmetric deformation, the O-O bonds are important to stabilize the multiferroic phase of BiCoO₃.