On the observation of the spin resonance in superconducting CeCoIn₅

Recent observation of a resonance spin excitation at (1/2,1/2,1/2) in the superconducting state of CeCoIn_5 [C. Stock et al., Phys. Rev. Lett. {\bf 100} 087001 (2008)] was interpreted as an evidence for d_{x^2-y^2} gap symmetry, by analogy with the cuprates. This is true if the resonance is a spin exciton. We argue that such description is undermined by the three-dimensionality of CeCoIn_5. We show that in 3D systems the excitonic resonance only emerges at strong coupling, and is weak. We argue in favor of the alternative, magnon scenario, which does not require a d_{x^2-y^2} gap.