Since the discovery of the metallic antiferromagnetic (AF) ground state near superconductivity in iron pnictide superconductors1, 2, 3, a central question has been whether magnetism in these materials arises from weakly correlated electrons4, 5, as in the case of spin density wave in pure chromium6, requires strong electron correlations7, or can even be described in terms of localized electrons8, 9 such as the AF insulating state of copper oxides10. Here we use inelastic neutron scattering to determine the absolute intensity of the magnetic excitations throughout the Brillouin zone in electron-doped superconducting BaFe1.9Ni0.1As2 (Tc=20 K), which allows us to obtain the size of the fluctuating magnetic moment m2, and its energy distribution11, 12. We find that superconducting BaFe1.9Ni0.1As2 and AF BaFe2As2 (ref. 13) both have fluctuating magnetic moments m2≈3.2 μB2 per Fe(Ni), which are similar to those found in the AF insulating copper oxides14, 15. The common theme in both classes of high-temperature superconductors is that magnetic excitations have partly localized character, thus showing the importance of strong correlations for high-temperature superconductivity16.
At a glance
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