Electron doping evolution of the magnetic excitations in BaFe _2-xNi_xAs₂

We use inelastic neutron scattering (INS) spectroscopy to study the magnetic excitations spectra throughout the Brillouin zone in electron-doped iron pnictide superconductors BaFe_2-xNi_xAs₂ with x=0.096,0.15,0.18. While the x=0.096 sample is near optimal superconductivity with T_c=20 K and has coexisting static incommensurate magnetic order, the x=0.15,0.18 samples are electron overdoped with reduced T_c of 14 and 8 K, respectively, and have no static antiferromagnetic (AF) order. In previous INS work on undoped (x=0) and electron optimally doped (x=0.1) samples, the effect of electron doping was found to modify spin waves in the parent compound BaFe₂As₂ below ∼100 meV and induce a neutron spin resonance at the commensurate AF ordering wave vector that couples with superconductivity. While the new data collected on the x=0.096 sample confirm the overall features of the earlier work, our careful temperature dependent study of the resonance reveals that the resonance suddenly changes its Q width below T_c similar to that of the optimally hole-doped iron pnictides Ba_0.67K_0.33Fe ₂As₂. In addition, we establish the dispersion of the resonance and find it to change from commensurate to transversely incommensurate with increasing energy. Upon further electron doping to overdoped iron pnictides with x=0.15 and 0.18, the resonance becomes weaker and transversely incommensurate at all energies, while spin excitations above ∼100 meV are still not much affected. Our absolute spin excitation intensity measurements throughout the Brillouin zone for x=0.096,0.15,0.18 confirm the notion that the low-energy spin excitation coupling with itinerant electron is important for superconductivity in these materials, even though the high-energy spin excitations are weakly doping dependent.