Competing exchange interactions on the verge of a metal-insulator transition in the two-dimensional spiral magnet Sr₃Fe₂O₇

We report a neutron scattering study of the magnetic order and dynamics of the bilayer perovskite Sr₃Fe₂O₇, which exhibits a temperature-driven metal-insulator transition at 340 K. We show that the Fe⁴⁺ moments adopt incommensurate spiral order below T_N = 115 K and provide a comprehensive description of the corresponding spin wave excitations. The observed magnetic order and excitation spectra can be well understood in terms of an effective spin Hamiltonian with interactions ranging up to third nearest-neighbor pairs. The results indicate that the helical magnetism in Sr₃Fe₂O₇ results from competition between ferromagnetic double-exchange and antiferromagnetic superexchange interactions whose strengths become comparable near the metal-insulator transition. They thus confirm a decadesold theoretical prediction and provide a firm experimental basis for models of magnetic correlations in strongly correlated metals.