Domains and supercurrents
Singlet superconductivity can exist in ferromagnetic superconductors like erbium rhodium boride (ErRh4B4) thanks to the formation of a domain-like structure. ErRh4B4 is a superconductor below 8.7 K, but when it is cooled to about 1 K this modulated magnetic structure appears with a period d. This period is smaller than the size of the Cooper pairs, xi.
All the magnetic moments in each domain point in the same direction, but the spin of neighbouring domains alternates. Strictly speaking, ErRh4B4 is not a ferromagnet until it is cooled to 0.8 K. Below this temperature the domain walls break down, all the spins point in the same direction and the superconductivity is destroyed.