First-principles analysis of magnetic superexchange in cubic fluoro- and oxo-perovskites

We present a comprehensive first-principles analysis of magnetic ordering via superexchange in KMnF₃, CaMnO₃, and SrMnO₃, which constitutes an extension of our previous study on fluorides KNiF₃ and KVF₃. Using the metallic non-magnetic (NM) phase as a reference, we examine the transition to the insulating antiferromagnetic (AFM) phase from first principles calculations and compare it with the predictions of the Hubbard model, which only considers the antibonding magnetic orbitals. We show that spin-polarization promotes the charge transfer between atoms, localizing the spin density on each site and opening a gap at the Fermi level. The microscopic mechanism driving this transition, however, differs among the systems. A detailed analysis on energy contributions, charge redistribution and metal-ligand bonding reveals significant differences between divalent transition metal cations in fluorides and Mn⁴⁺ in oxides, due to the enhanced covalency in the latter. Despite these differences, a unifying, bonding-driven mechanism for AFM stabilization emerges: in all compounds, the energy gain relative to the FM phase arises from a slightly greater delocalization of minority-spin electrons associated with deep bonding ligand orbitals. This subtle redistribution strengthens the key role played by chemical bond in superexchange interactions and leads to a small but systematic kinetic-energy gain. Our findings highlight the limitations of canonical minimal-basis models that neglect deep bonding orbitals and charge transfer and demonstrate the need for quantitative first-principles approaches to accurately describe magnetism in transition-metal perovskites.