Field-induced magnetic instability within a superconducting condensate

The application of magnetic fields, chemical substitution, or hydrostatic pressure to strongly correlated electron materials can stabilize electronic phases with different organizational principles. We present evidence for a field-induced quantum phase transition, in superconducting Nd 0.05 Ce 0.95 CoIn 5 , that separates two antiferromagnetic phases with identical magnetic symmetry. At zero field, we find a spin-density wave that is suppressed at the critical field μ 0 H * = 8 T. For H > H *, a spin-density phase emerges and shares many properties with the Q phase in CeCoIn 5 . These results suggest that the magnetic instability is not magnetically driven, and we propose that it is driven by a modification of superconducting condensate at H *.