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*.