Nature of magnetic excitations in superconducting BaFe1.9Ni0.1As2

Since the discovery of the metallic antiferromagnetic (AF) ground state near superconductivity in iron pnictide superconductors1–3, 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 μB 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. In the undoped state, iron pnictides such as BaFe2As2 form a metallic low-temperature orthorhombic phase with the antiferromagnetic (AF) structure as shown in Fig. 1a (ref. 17). Inelastic neutron scattering measurements have mapped out spin waves throughout the Brillouin zone in the AF orthorhombic and paramagnetic tetragonal phases13. On Coand Ni-doping to induce optimal superconductivity via electron doping, the orthorhombic structural distortion and static AF order in BaFe2As2 are suppressed and the system becomes tetragonal and paramagnetic at all temperatures18. In previous inelastic neutron scattering experiments on optimally electron-doped Ba(Fe,Co,Ni)2As2 superconductors11,12,19–22, spin excitations up to ∼120meV were observed. However, the lack of spin excitation data at higher energies in absolute units precluded a comparison with spin waves in undoped BaFe2As2. Only the absolute intensity measurements in the entire Brillouin zone can reveal the effect of electron doping on the overall spin excitation spectra and allow a direct comparison with the results in the AF insulating copper oxides14,15. For the experiments, we chose to study well-characterized electron-doped BaFe1.9Ni0.1As2 (refs 20,22) because large single crystals were available23 and their properties are similar to Co-doped BaFe2As2 (refs 11,12,19,21,24). By comparing spin excitations in BaFe1.9Ni0.1As2 and BaFe2As2 throughout the Brillouin zone, we were able to probe how electron doping and superconductivity affect the overall spin