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Sign of the superexchange coupling between next-nearest neighbors in EuO

The sign of the superexchange coupling J 2 between next nearest neighboring Eu 2+ magnetic moments in EuO is a matter subject to debate. We have obtained evidence that this coupling is of antiferromagnetic nature (J 2 < 0). EuO thin films grown at different temperatures suggest that lattice expansion results in enhancement of T C as clearly observed in stoichiometric EuO films grown on CaF 2 substrates. Resonant photoemission spectroscopy provides compelling evidence of strong hybridization between O 2 p and Eu 5 d 6 s 6 p weighted bands, suggesting that strong superexchange may be mediated by oxygen thus consistent with the observed antiferromagnetic behavior between the next nearest neighboring Eu atoms via nearest neighbor oxygen in EuO.