Inelastic neutron scattering studies on spin excitations of Fe Pnictides

We used inelastic neutron scattering to study the evolution of spin excitations in the FeAs superconductors and their parent compounds. We show here not only the antiferromagnetic order is suppressed by the doping; the spin excitations also change dramatically with doping. We observed sharp spin-wave excitations in the antiferromagnetically ordered parent compound. Based on the observed dispersion relation, we estimate the effective magnetic exchange coupling using a Heisenberg model. We also studied the spin excitation spectrum in the superconducting sample and its relationship to the superconductivity.