Spin-charge coupling in a band ferromagnet: magnon-energy reduction, anomalous softening, and damping

TL;DR

This paper investigates how correlation-induced spin-charge coupling affects spin-wave excitations in band ferromagnets, revealing magnon energy reduction, anomalous softening, and damping, aligning with experimental observations.

Contribution

It introduces a systematic inverse-degeneracy expansion scheme that preserves the Goldstone mode to analyze spin-charge coupling effects in ferromagnets.

Findings

Magnon energies are substantially reduced due to spin-charge coupling.

Zone-boundary magnons exhibit anomalous softening and damping.

Results qualitatively agree with experimental measurements in manganites and transition metal films.

Abstract

The effects of correlation-induced coupling between spin and charge fluctuations on spin-wave excitations in a band ferromagnet are investigated by including self-energy and vertex corrections within a systematic inverse-degeneracy expansion scheme which explicitly preserves the Goldstone mode. Arising from the scattering of a magnon into intermediate spin-excitation states (including both magnon and Stoner excitations) accompanied with charge fluctuations in the majority spin band, this spin-charge coupling results not only in a substantial reduction of magnon energies but also in anomalous softening and significant magnon damping for zone-boundary modes lying within the Stoner gap. Our results are in good qualitative agreement with recent spin-wave excitation measurements in colossal magneto-resistive manganites and ferromagnetic ultrathin films of transition metals.