Evidence for a Magnetically Driven Superconducting Q Phase of CeCoIn5

We have studied the magnetic order inside the superconducting phase of CeCoIn₅ for fields along the [100] crystallographic direction using neutron diffraction. We find a spin-density wave order with an incommensurate modulation Q=(q,q,1/2) and q=0.45(1), which within our experimental uncertainty is indistinguishable from the spin-density wave found for fields applied along [1-10]. The magnetic order is thus modulated along the lines of nodes of the d_x²-y² superconducting order parameter, suggesting that it is driven by the electron nesting along the superconducting line nodes. We postulate that the onset of magnetic order leads to reconstruction of the superconducting gap function and a magnetically induced pair density wave.