Magnetic resonance from the interplay of frustration and superconductivity

Motivated by iron-based superconductors, we develop a self-consistent electronic theory for the itinerant spin excitations in the regime of coexistence of the antiferromagnetic stripe order with wave vector Q₁=(π,0) and s^+- superconductivity. The onset of superconductivity leads to the appearance of a magnetic resonance near the wave vector Q₂=(0,π), where magnetic order is absent. This resonance is isotropic in spin space, unlike the excitations near Q₁, where the magnetic Goldstone mode resides. We discuss several features which can be observed experimentally.