Electron doping evolution of the magnetic excitations in BaFe2-xNixAs2

We use inelastic neutron scattering (INS) spectroscopy to study the magnetic excitations spectra throughout the Brioullion zone in electron-doped iron pnictide superconductors BaFe$_{2-x}$Ni$_{x}$As$_{2}$ 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 K 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 BaFe$_2$As$_2$ below $\sim$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 confirms 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$_{0.67}$K$_{0.33}$Fe$_2$As$_2$. 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 $\sim$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.