Neutron scattering study of magnetic excitations in a 5d-based double-perovskite Ba₂FeReO₆

Motivated by exploring spin-orbit-coupled magnetism in 5d-based transition metal oxides (TMOs) beyond the iridates, we present a powder inelastic neutron scattering study of magnetic excitations in Ba₂FeReO₆ - a member of the double-perovskite family of materials which exhibit half-metallic behavior and high Curie temperatures T_c. We find clear evidence of two well-defined dispersing magnetic modes in its low-temperature ferrimagnetic state. We develop a local moment model, which incorporates the interaction of Fe spins with spin-orbital locked magnetic moments on Re and show that it captures our experimental observations. This allows us to extract moment sizes and exchange couplings, explain the magnitude of T_c, and infer that magnetostructural locking terms are weak. Our study further opens up Re-based compounds as model systems to explore the interplay of strong correlations and spin-orbit coupling in 5d TMOs.