We report the first observation of high wave vector magnon excitations in a ferromagnetic monolayer. Using spin-polarized electron energy loss spectroscopy, we observed the magnon dispersion in one atomic layer (ML) of Fe on W(110) at 120 K. The magnon energies are small in comparison to the bulk and surface Fe(110) excitations. We find an exchange parameter and magnetic anisotropy similar to that from static measurements. Our results are in sharp contrast to theoretical calculations, indicating that the present understanding of magnetism of the ML Fe requires considerable revision.

High wave-vector spin waves in ultrathin Fe/W͑110͒ films up to 20 monolayers ͑MLs͒ thick have been studied using spin-polarized electron energy-loss spectroscopy. An unusual nonmonotonous dependence of the spin wave energies on the film thickness is observed, featuring a pronounced maximum at 2 ML coverage. First-principles theoretical study reveals the origin of this behavior to be in the localization of the spin waves at the surface of the film, as well as in the properties of the interlayer exchange coupling influenced by the hybridization of the electron states of the film and substrate and by the strain.

High wave-vector magnon excitations in ferromagnetic Fe monolayer and doublelayer grown on W(110) are investigated using spin-polarized electron energy loss spectroscopy. The magnon dispersion relation is obtained up to the Brillouin zone boundary. A direct comparison among different systems shows that the magnons in the Fe monolayer are extremely soft and are even softer than the acoustic surface mode of Fe(110). By measuring the spectra in both energy loss and gain regions on a double-layer Fe film at room temperature and by reversing the sample magnetization, it is demonstrated that the magnon dispersion is asymmetric with respect to the sign of the wave-vector. The asymmetric dispersion relation is attributed to the degeneracy breaking of the magnons due to the presence of the Dzyaloshinskii-Moriya interaction.