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

TL;DR

This study investigates anti-Stoner spin excitations in weak ferromagnets and antiferromagnets using ab initio methods, revealing their role in electron correlations and their interaction strength compared to traditional magnons.

Contribution

It introduces a novel ab initio numerical scheme to evaluate the effects of anti-Stoner excitations on electron correlations in itinerant magnetic materials.

Findings

In Ni, anti-Stoner processes cause weaker band-structure renormalization than Stoner magnons in the majority spin channel.

In Ni, anti-Stoner effects are comparable to Stoner effects in the minority spin channel.

In CrSb, electron-magnon interactions depend on the spatial shape of magnonic modes.

Abstract

The anti-Stoner excitations are a spin-flips in which, effectively, an electron is 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 identically absent in the ferromagnetic Heisenberg model, their properties and role in correlating electrons were hardly investigated so far. On the other hand, they are present in weak ferromagnets, fcc Ni being a prominent example, and both types of spin-flips (down-to-up and up-to-down) must be treated on the equal footing in systems with the degenerate spin up and down bands, in particular antiferromagnets in which case 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 novel efficient \textit{ab initio} numerical scheme. We find that in Ni the band-structure renormalization due to the anti-Stoner processes is weaker than the one due to Stoner-type magnons in the majority spin channel but the two become comparable in the minority one. 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. For the antiferromagnet, the situation is more complex and we observe that the electron-magnon interaction is sensitive not only to these densities of states but critically to the spatial shapes of the coupling magnonic modes as well.