Since the discovery of the metallic antiferromagnetic (AF) ground state near superconductivity in iron pnictide superconductors 1-3 , a central question has been whether magnetism in these materials arises from weakly correlated electrons 4,5 , as in the case of spin density wave in pure chromium 6 , requires strong electron correlations 7 , or can even be described in terms of localized electrons 8,9 such as the AF insulating state of copper oxides 10 . Here we use inelastic neutron scattering to determine the absolute intensity of the magnetic excitations throughout the Brillouin zone in electron-doped superconducting BaFe 1.9 Ni 0.1 As 2 (T c = 20 K), which allows us to obtain the size of the fluctuating magnetic moment m 2 , and its energy distribution 11,12 . We find that superconducting BaFe 1.9 Ni 0.1 As 2 and AF BaFe 2 As 2 (ref. 13) both have fluctuating magnetic moments m 2 ≈ 3.2 µ 2 B per Fe(Ni), which are similar to those found in the AF insulating copper oxides 14,15 . 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 superconductivity 16 .In the undoped state, iron pnictides such as BaFe 2 As 2 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 phases 13 . On Co-and Ni-doping to induce optimal superconductivity via electron doping, the orthorhombic structural distortion and static AF order in BaFe 2 As 2 are suppressed and the system becomes tetragonal and paramagnetic at all temperatures 18 . In previous inelastic neutron scattering experiments on optimally electron-doped Ba(Fe, Co, Ni) 2 As 2 superconductors 11,12,[19][20][21][22] , spin excitations up to ∼120 meV were observed. However, the lack of spin excitation data at higher energies in absolute units precluded a comparison with spin waves in undoped BaFe 2 As 2 . 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 oxides 14,15 . For the experiments, we chose to study well-characterized electron-doped BaFe 1.9 Ni 0.1 As 2 (refs 20,22) because large single crystals were available 23 and their properties are similar to Co-doped BaFe 2 As 2 (refs 11,12,19,21,24).By comparing spin excitations in BaFe 1.9 Ni 0.1 As 2 and BaFe 2 As 2 throughout the Brillouin zone, we were able to probe how electron doping and superconductivity affect the overall spin

We report inelastic neutron scattering experiments on single crystals of superconducting Ba0.67K0.33Fe2As2 (Tc = 38 K). In addition to confirming the resonance previously found in powder samples, we find that spin excitations in the normal state form longitudinally elongated ellipses along the QAFM direction in momentum space, consistent with density functional theory predictions. On cooling below Tc, while the resonance preserves its momentum anisotropy as expected, spin excitations at energies below the resonance become essentially isotropic in the in-plane momentum space and dramatically increase their correlation length. These results suggest that the superconducting gap structures in Ba0.67Ka0.33Fe2As2 are more complicated than those suggested from angle resolved photoemission experiments.

We use polarized inelastic neutron scattering (INS) to study spin excitations of optimally holedoped superconductor Ba0.67K0.33Fe2As2 (Tc = 38 K). In the normal state, the imaginary part of the dynamic susceptibility, χ ′′ (Q, ω), shows magnetic anisotropy for energies below ∼7 meV with c-axis polarized spin excitations larger than that of the in-plane component. Upon entering into the superconducting state, previous unpolarized INS experiments have shown that spin gaps at ∼5 and 0.75 meV open at wave vectors Q = (0.5, 0.5, 0) and (0.5, 0.5, 1), respectively, with a broad neutron spin resonance at Er = 15 meV. Our neutron polarization analysis reveals that the large difference in spin gaps is purely due to different spin gaps in the c-axis and in-plane polarized spin excitations, resulting resonance with different energy widths for the c-axis and in-plane spin excitations. The observation of spin anisotropy in both opitmally electron and hole-doped BaFe2As2 is due to their proximity to the AF ordered BaFe2As2 where spin anisotropy exists below TN .

We use inelastic neutron scattering to study temperature dependence of the paramagnetic spin excitations in iron pnictide BaFe2As2 throughout the Brillouin zone. In contrast to a conventional local moment Heisenberg system, where paramagnetic spin excitations are expected to have a Lorentzian function centered at zero energy transfer, the high-energy (hω > 100 meV) paramagnetic spin excitations in BaFe2As2 exhibit spin-wave-like features up to at least 290 K (T = 2.1TN ). Furthermore, we find that the sizes of the fluctuating magnetic moments m 2 ≈ 3.6 µ 2 B per Fe are essentially temperature independent from the AF ordered state at 0.05TN to 2.1TN , which differs considerably from the temperature dependent fluctuating moment observed in the iron chalcogenide Fe1.1Te [I. A. Zaliznyak et al., Phys. Rev. Lett. 107, 216403 (2011).]. These results suggest unconventional magnetism and strong electron correlation effects in BaFe2As2. The elementary magnetic excitations (spin waves and paramagnetic spin excitations) in a ferromagnet or an antiferromagnet can provide direct information about the itinerancy of the unpaired electrons contributing to the ordered moment. In a local moment system, spin waves are usually well-defined throughout the Brillouin zone and can be accurately described by a Heisenberg Hamiltonian in the magnetically ordered state. The total moment sum rule requires that the dynamical structure factor S(q, ω), when integrated over all wave vectors (q) and energies (E =hω), is a temperature independent constant and equals to m 2 = (gµ B ) 2 S(S + 1), where g is the Landé g factor (≈ 2) and S is the spin of the system [1]. Upon increasing temperature to the paramagnetic state, spin excitations in the low-q limit can be described by a simple Lorentzian scattering function, where κ 1 is the temperature dependent inverse spin-spin correlation length and Γ is the wave vector dependent characteristic energy scale [2][3][4]. At sufficiently high temperatures above the magnetic order, spin excitations should be purely paramagnetic with no spin-wave-like correlations. Therefore, a careful investigation of the wave vector and energy dependence of spin excitations across the magnetic ordering temperature can provide important information concerning the nature of the magnetic order and spin-spin correlations. For example, a recent inelastic neutron scattering study of spin excitations in one of the parent compounds of iron-based superconductors, the iron chalcogenide Fe 1.1 Te which has a bicollinear antiferromagnetic (AF) structure and Néel temperature of T N = 67 K [5][6][7][8][9][10][11], reveals that the effective spin per Fe changes from S ≈ 1 in the AF state to S ≈ 3/2 in the paramagnetic state, thus providing evidence that Fe 1.1 Te is not a conventional Heisenberg antiferromagnet but a nontrivial local moment system coupled with itinerant electrons [12].Since antiferromagnetism may be responsible for electron pairing and superconductivity in iron-based superconductors [13,14], it is important to determine...

We use polarized inelastic neutron scattering to study low-energy spin excitations and their spatial anisotropy in electron-overdoped superconducting BaFe1.85Ni0.15As2 (Tc = 14 K). In the normal state, the imaginary part of the dynamic susceptibility, χ ′′ (Q, ω), at the antiferromagnetic (AF) wave vector Q = (0.5, 0.5, 1) increases linearly with energy for E ≤ 13 meV. Upon entering the superconducting state, a spin gap opens below E ≈ 3 meV and a broad neutron spin resonance appears at E ≈ 7 meV. Our careful neutron polarization analysis reveals that χ ′′ (Q, ω) is isotropic for the in-plane and out-of-plane components in both the normal and superconducting states. A comparison of these results with those of undoped BaFe2As2 and optimally electron-doped BaFe1.9Ni0.1As2 (Tc = 20 K) suggests that the spin anisotropy observed in BaFe1.9Ni0.1As2 is likely due to its proximity to the undoped BaFe2As2. Therefore, the neutron spin resonance is isotropic in the overdoped regime, consistent with a singlet to triplet excitation.