Abstract
The superexchange theory predicts dominant antiferromagnetic kinetic interaction when the orbitals accommodating magnetic electrons are covalently bonded through diamagnetic bridging atoms/groups. Here 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 principle calculations show that it is comparable in strength with antiferromagnetic superexchange in a number of magnetic materials with diamagnetic metal bridges. In particular, it is responsible for a very large ferromagnetic coupling ($-10$ meV) between the iron ions in a Fe$^{3+}$-Co$^{3+}$-Fe$^{3+}$ complex.
Highlights
Anderson’s superexchange theory [1] plays a central role in the description of exchange interactions in correlated magnetic insulators
We show that in materials not exhibiting ferromagnetism, the kinetic ferromagnetic contribution is crucial for the annihilation of the antiferromagnetic superexchange
In order to achieve a realistic description of exchange contributions, the results of first-principles calculations were mapped into an extended three-site model which, contrary to the basic model in Eq (1), includes all relevant localized bridging orbitals (LBOs) on the diamagnetic bridging site and the Coulomb and potential exchange interactions between the localized magnetic orbitals (LMOs) and LBOs: H = H0 + Ht + HCoul + HPE, (13)
Summary
Anderson’s superexchange theory [1] plays a central role in the description of exchange interactions in correlated magnetic insulators It provides in particular an explanation of phenomenological Goodenough-Kanamori rules [2,3,4]. A different extension of the theory was proposed by Geertsma [43], Larson et al [44], and Zaanen and Sawatzky [45] through explicit consideration of the orbitals of bridging diamagnetic atoms or groups along with the orbitals accommodating the magnetic electrons Such an extension allowed for a concomitant description of high-energy excitations and exchange interaction in charge-transfer insulators [46]. The situation changes crucially when the metal-to-metal electron transfer is added to the model In this case a strong ferromagnetic contribution of kinetic origin can arise [48,49,50]. We show that in materials not exhibiting (strong) ferromagnetism, the kinetic ferromagnetic contribution is crucial for the annihilation of the antiferromagnetic superexchange
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