Microscopic origin of the magnetic interactions and their experimental signatures in altermagnetic La$_2$O$_3$Mn$_2$Se$_2$

TL;DR

This paper investigates the microscopic magnetic interactions in the altermagnetic material La$_2$O$_3$Mn$_2$Se$_2$, revealing how exchange mechanisms produce its unique magnetic order and identifying experimental signatures.

Contribution

It combines ab initio and analytical methods to explain the origin of altermagnetism in La$_2$O$_3$Mn$_2$Se$_2$, challenging GKA rules due to multiorbital effects.

Findings

Large antiferromagnetic nearest-neighbor coupling explained by combined exchange mechanisms.

Multiorbital character of Mn$^{2+}$ ions accounts for GKA rule deviations.

Magnon band calculations reveal universal signatures of exchange interactions.

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 La$_2$O$_3$Mn$_2$Se$_2$. However, AM on the ILL requires 90$^\circ$ nearest-neighbor superexchange to be {\it antiferromagnetic} and dominant over the 180$^\circ$ 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-neigbor 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.