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

Abstract 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 La2O3Mn2Se2. 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 La2O3Mn2Se2. 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.