Altermagnets versus Antiferromagnets

Altermagnets are metals with a momentum-dependent spin splitting of electron bands due to a specific crystal structure that is invariant under time reversal only in combination with rotations and reflections, or lacks time reversal altogether. The developed phenomenological approach allows one to obtain a spectrum of electron bands in an altermagnet corresponding to an antiferromagnet with the same symmetry. There are considered collinear antiferromagnets, weak ferromagnetic antiferromagnets, and ferrimagnetic structures. It is shown that only a weak ferromagnetic antiferromagnet has a nonzero Berry curvature.