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

Abstract

We have studied the electronic structure of multiferroic BiCoO${}_{3}$ using x-ray photoemission spectroscopy (XPS), x-ray absorption spectroscopy (XAS), and subsequent model calculations. The XAS results show that the Co${}^{3+}$ ion takes the high-spin ${d}^{6}$ configuration which usually prefers G-type antiferromagnetic state. The XPS results and model Hartree-Fock calculations show that, in case of BiCoO${}_{3}$, 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 3$d$ ${t}_{2g}$ and the Co--O--O--Co superexchange interaction gives rise to C-type antiferromagnetic state in BiCoO${}_{3}$. 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 BiCoO${}_{3}$.