Spin gap and magnetic resonance in superconducting BaFe_1.9Ni_0.1As₂

We use neutron spectroscopy to determine the nature of the magnetic excitations in superconducting BaFe_1.9Ni_0.1As₂(T_c=20 K). Above T_c the excitations are gapless and centered at the commensurate antiferromagnetic wave vector of the parent compound, while the intensity exhibits a sinusoidal modulation along the c axis. As the superconducting state is entered a spin gap gradually opens, whose magnitude tracks the T dependence of the superconducting gap as observed by angle-resolved photoemission. Both the spin-gap and magnetic-resonance energies are temperature and wave-vector dependent, but their ratio is the same within uncertainties. These results suggest that the spin resonance is a singlet-triplet excitation related to electron pairing and superconductivity.