Ferromagnetic kinetic exchange interaction in magnetic insulators

The superexchange theory predicts dominant antiferromagnetic kinetic interaction when the orbitals accommodating magnetic electrons are covalently bonded through diamagnetic bridging atoms or groups. In this work [1], we show that explicit consideration of magnetic and (leading) bridging orbitals, together with the electron transfer between the former, reveals a strong ferromagnetic kinetic exchange contribution. First-principles calculations show that it is comparable in strength with antiferromagnetic superexchange in a number of magnetic materials with diamagnetic metal bridges such as Fe-Co-Fe complex and quasi 1D Cu chain compound (La₄Ba₂Cu₂O₁₀). In particular, it is responsible for a very large ferromagnetic coupling (−10 meV) between the iron ions in a Fe³⁺−Co³⁺−Fe³⁺ complex. Furthermore, we find that the ferromagnetic exchange interaction turns into antiferromagnetic by substituting the diamagnetic bridge with magnetic one. The phenomenology is observed in two series of materials, supporting the significance of the ferromagnetic kinetic exchange mechanism.

[1] Z. Huang, D. Liu, A. Mansikkamäki, V. Vieru, N. Iwahara, and L. F. Chibotaru, Phys. Rev. Research 2, 033430 (2020).