Low Energy, Coherent, Stoner-like Excitations in CaFe${}_2$As${}_2$

Abstract

Using linear-response density-functional theory, magnetic excitations in the striped phase of CaFe${}_2$As${}_2$ are studied as a function of local moment amplitude. We find a new kind of excitation: sharp resonances of Stoner-like (itinerant) excitations at energies comparable to the N{é}el temperature, originating largely from a narrow band of Fe $d$ states near the Fermi level, and coexist with more conventional (localized) spin waves. Both kinds of excitations can show multiple branches, highlighting the inadequacy of a description based on a localized spin model.