Kim, J.-H.; Jain, A.; Reehuis, M.; Khaliullin, G.; Peets, D.C.; Ulrich, C.; Park, J.T.; Faulhaber, E.; Hoser, A.; Walker, H.C.; Adroja, D.T.; Walters, A.C.; Inosov, D.S.; Maljuk, A.; Keimer, B.: Competing Exchange Interactions on the Verge of a Metal-Insulator Transition in the Two-Dimensional Spiral Magnet Sr3Fe2O7. Physical Review Letters 113 (2014), p. 147206/1-5
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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 TN=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 decades-old theoretical prediction and provide a firm experimental basis for models of magnetic correlations in strongly correlated metals.