Single- and Multimagnon Dynamics in Antiferromagnetic α−Fe2O3 Thin Films

Understanding the spin dynamics in antiferromagnetic (AFM) thin films is fundamental for designing novel devices based on AFM magnon transport. Here, we study the magnon dynamics in thin films of AFM S=52 α-Fe2O3 by combining resonant inelastic x-ray scattering, Anderson impurity model plus dynamical mean-field theory, and Heisenberg spin model. Below 100 meV, we observe the thickness-independent (down to 15 nm) acoustic single-magnon mode. At higher energies (100-500 meV), an unexpected sequence of equally spaced, optical modes is resolved and ascribed to ΔSz=1, 2, 3, 4, and 5 magnetic excitations corresponding to multiple, noninteracting magnons. Our study unveils the energy, character, and momentum dependence of single and multimagnons in α-Fe2O3 thin films, with impact on AFM magnon transport and its related phenomena. From a broader perspective, we generalize the use of L-edge resonant inelastic x-ray scattering as a multispin-excitation probe up to ΔSz=2S. Our analysis identifies the spin-orbital mixing in the valence shell as the key element for accessing excitations beyond ΔSz=1, and up to, e.g., ΔSz=5. At the same time, we elucidate the novel origin of the spin excitations beyond the ΔSz=2, emphasizing the key role played by the crystal lattice as a reservoir of angular momentum that complements the quanta carried by the absorbed and emitted photons.