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 {\bf Q}$_1$ = $(\pi,0)$ and $s^{+?}$ superconductivity. The onset of superconductivity leads to the appearance of a magnetic resonance near the wave vector {\bf Q}$_2$ = $(0,\pi)$, where magnetic order is absent. This resonance is isotropic in spin space, unlike the excitations near {\bf Q}$_1$, where the magnetic Goldstone mode resides. We discuss several features which can be observed experimentally.