Single- and multi-magnon 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 = 5/2 α-Fe₂O₃ by combining resonant inelastic x-ray scattering, Anderson impurity model plus dynamical mean-field theory, and the 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 ?S_z = 1, 2, 3, 4, and 5 magnetic excitations corresponding to multiple, non-interacting 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. On a broader perspective, we generalize the use of L-edge RIXS as a multi-spin excitation probe up to ?S_z = 2S. Our analysis identifies the spin-orbital mixing in the valence shell as the key element for accessing excitations beyond ?S_z = 1, and up to e.g. ?S_z = 5. At the same time, we elucidate the novel origin of the spin excitations beyond the ?S_z = 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.