Magnetic properties of the superconducting state of iron-based superconductors

We show that features of the dynamical spin susceptibility can unambiguously distinguish between different pairing symmetries of the superconducting state in iron-based superconductors. A magnetic resonance is a feature for the extended s x2 y2 -wave cos kx cos ky pairing symmetry. It is present in the pure superconducting (SC) state but weakens in the mixed SC and magnetically ordered state. We calculate the random-phase approximation correction to the NMR spin-relaxation rate 1/ T1 and the Knight shift in the above states and show a good agreement with experimental results. Moreover, we argue that the energy dispersion of the magnetic resonance along the c axis observed in neutron scattering experiments is also an indirect evidence supporting the s x2 y2 pairing symmetry.