  The Nd1-xCexCoIn5 alloys evolve from local moment magnetism (x = 0) to heavy
fermion superconductivity (x =1). Magnetic order is observed over a broad range
of x. For a substantial range of x (0.83 <= x <= 0.95) in the temperature -
composition phase diagram we find that superconductivity may coexist with spin
- density wave magnetic order at the Fermi surface. We show that a delicate
balance betwen superconducting and magnetic instabilities can be reversibly
tuned by both the Ce/Nd ratio and magnetic field, offering a new and unique
model electronic system.
