Spectroscopic Evidence for Superexchange in the Ferrimagnetic Spinel FeCr2S4

Low-energy orbital excitations in spinel FeCr2S4have been studied element specifically at both Fe and Cr 2p–3d resonances by resonant inelastic X-ray scattering (RIXS). As a first step X-ray absorption spectroscopy measurements confirmed that iron, located at the tetrahedral (Td) sites of the spinel structure, is in a Fe2+(d6) state and chromium, located at the octahedral (Oh) sites, is in a Cr3+(d3) state. We have identified RIXS spectral features with the help of crystal-field simulations that account consistently for on-site dd excitations and provide 10Dq = 0.30 ± 0.05 eV for Fe Tdsites and 10Dq = 1.60 ± 0.05 eV for Cr Ohsites. The giant magneto-optical Kerr effect previously observed for photons with 0.3 eV can be unambiguously attributed to the lowest possible dd excitations on the iron site Fe(e3t23) → Fe(e2t24). A 1.6 eV energy-loss feature, measured in the Fe 2p–3d RIXS spectra, is found to be a two-site orbital excitation directly related to superexchange (SE) interaction between Fe and Cr cations. It opens new perspectives for applying RIXS to study SE in a broad range of 3d transition metal compounds.