The coupling of electrons to spin excitations and the generation of magnons is essential for spin mixing in the ultrafast magnetization dynamics of 3d ferromagnets. Although magnon energies are generally much larger than phonon energies, until now their electronic band renormalization effect in 3d ferromagnets suggests a significantly weaker quasiparticle interaction. Using spin- and angle-resolved photoemission, we show an extraordinarily strong renormalization leading to two-branch splitting of an iron surface resonance at ∼200 meV. Its strong magnetic linear dichroism unveils the magnetic nature and momentum dependence of the energy renormalization. By determining the frequency- and momentum-dependent self-energy due to generic electron–boson interaction to compute the resultant electron spectral function, we suggest that the surface-state splitting can be described by strong coupling to an optical spin wave in an iron thin film.

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