Multi-spin excitations in an antiferromagnetic S=5/2 system

Hematite (a-Fe₂O₃), an S=5/2 antiferromagnetic system, is a promising candidate in spintronic and magnonics due to its functional spin-related properties [1, 2], such as a weak in-plane magnetic anisotropy and a low spin dissipation.

A key element for the microscopic understanding of spin-wave-based transport phenomena in insulators is the knowledge of the intrinsic spin dynamics of the system, involved in both the transport mechanism as well as in the dissipation processes. The spin dynamics of bulk hematite has been studied in the early days by inelastic neutron scattering (INS) [3], revealing the presence of an acoustic and an optical single-magnon branch up to 100 meV. The evolution of the spin dynamics in thin films - as the ones used for devices - remains still unknown due to the lack of suitable probes.

Recently, resonant inelastic x-ray scattering (RIXS) has emerged as a highly performing technique for the study of the spin dynamics in magnetic thin films [4]. Additionally, it has shown great sensitivity in unraveling the higher ranked spin excitations such as ∆s=0 and 2 [5,6], elusive to conventional techniques like INS. Here, we extend this approach to hematite thin films by using Fe L₃-edge RIXS. Below 100 meV, we resolve two modes fully consistent with the ∆s=1 magnon branches detected by INS in the bulk whereas at higher energy (up to 200 meV), multi-spin excitation modes are identified. With the support of LDA+DMFT calculations built around the Anderson impurity model [7], we interpret these to be ∆s=2 and ∆s=3 spin excitations at single magnetic site, respectively. These observations shed new light on the high-energy eigenstates of the hematite magnetic Hamiltonian, providing novel information on the spin dynamics and suggesting new pathways for the transport dissipation processes.