Reentrant superconductivity

Reentrant superconductivity is an effect observed in systems that lie close to the boundary between ferromagnetic and superconducting. By its very nature (normal) superconductivity (condensation of electrons into the BCS ground state) cannot exist together with ferromagnetism (condensation of electrons into the same spin state, all pointing in the same direction). Reentrance is when while changing a continuous parameter, superconductivity is first observed, then destroyed by the ferromagnetic order, and later reappears. An example is the changing of the thickness of the ferromagnetic layer in a bilayer of a superconductor and a ferromagnet. At a certain thickness superconductivity is destroyed by the Andreev reflected electrons in the ferromagnet, but if the thickness increases, this effect disappears again.