Electronic correlations arising from anti-Stoner spin excitations: An ab initio study of itinerant ferro- and antiferromagnets

The anti-Stoner excitations are a spin flips in which an electron is effectively promoted from a minority to a majority spin state, i.e., complementary to Stoner excitations and spin waves. Since their spectral power is negligible in strong itinerant ferromagnets and they are entirely absent in the ferromagnetic Heisenberg model, their properties and role in correlating electrons have scarcely been investigated. However, they are present in weak ferromagnets, fcc Ni being a prominent example. Both types of spin flips (down-to-up and up-to-down) must be treated equally in systems with degenerate spin-up and spin-down bands, particularly in antiferromagnets, for which we choose CrSb as a model system. For these two materials, we evaluate the strength of the effective interaction between the quasiparticles and the gas of virtual spin-flip excitations. To this end, we compute the corresponding self-energy taking advantage of our efficient ab initio numerical scheme [Phys. Rev. B 107, 134410 (2023)]. In Ni, we find that the band-structure renormalization caused by the anti-Stoner processes is weaker than that caused by Stoner-type magnons in the majority spin channel, but the two become comparable for minority spin carriers. The effect can be traced back primarily to the spectral strength of the respective spin excitations and the densities of the final available quasiparticle states in the scattering process. In the antiferromagnet, the situation is more complex and we observe that the electronmagnon interaction is sensitive not only to these densities of states but also critically to the spatial shapes of the coupling magnonic modes.