Microscopic origin of the magnetic interactions and their experimental signatures in altermagnetic La2O3Mn2Se2

Altermagnets (AM) are a recently introduced type of magnets, with no net magnetization like antiferromagnets, but displaying a non-relativistic Zeeman splitting in reciprocal space like ferromagnets. One of the lately discussed models to realize AM is the inverse Lieb lattice (ILL). Initially suggested as a purely theoretical construct, the ILL occurs in real materials such as La ₂ O ₃ Mn ₂ Se ₂ . However, AM on the ILL requires 90 ^◦ nearest-neighbor superexchange to be antiferromagnetic and dominant over the 180 ^◦ next-nearest-neighbor superexchange, in apparent contradiction to the Goodenough-Kanamori-Anderson (GKA) rules. Yet, AM ordering was found to be the ground state in La ₂ O ₃ Mn ₂ Se ₂ . Combining ab initio and analytical methods, we determine how direct exchange and superexchange act together to produce a large antiferromagnetic nearest-neighbor coupling. The seeming contradiction with the GKA rules is traced back to the multiorbital character of Mn ⁺² ions. By calculating magnon bands, we identify universal signatures of the exchange interactions, suggesting experimental fingerprints.