Starting with the pioneering work of Rossat-Mignod et al. in 1991, the thorough characterization of magnetic excitations in the high-Tc cuprates has contributed much to the understanding of superconductivity (SC) in these compounds. The resonance peak, which was later discovered to form part of the so-called hour-glass dispersion, is also observed in heavy-fermion superconductors, and carries information about the SC order parameter: In cuprates, it is a signature of d-wave superconductivity. An exciting observation in the recently discovered iron-based superconductors is the presence of a resonance peak - in this family, it was shown that the resonance is compatible with a sign-reversed s-wave order parameter.
In my talk, I will first briefly outline the properties of the magnetic excitations in the antiferromagnetically ordered parent compounds of iron arsenides and cuprates. Then, I will discuss the salient features of the excitation spectrum in the superconducting cuprate YBa2Cu3O6+x, with particular emphasis on the resonance feature and its dispersion. Then I will concentrate on the spin excitations in superconducting BaFe2-x(Co,Ni)xAs2, present the properties of the resonance and argue that this system is more likely to be explained within the framework of conventional weak-coupling and not too-strongly-correlated approaches. Finally, I will compare the out-of-plane dispersion of the resonance in cuprates and iron arsenides and draw conclusions about the dimensionality of the physics in the two systems.