It is shown that the critical temperature for spin-triplet, $p$-wave superconductivity mediated by spin fluctuations is generically much higher in a Heisenberg ferromagnetic phase than in a paramagnetic one, due to the coupling of the magnons to the longitudinal magnetic susceptibility. Together with the tendency of the low-temperature ferromagnetic transition in very clean Heisenberg magnets to be of first order, this qualitatively explains the phase diagram recently observed in ${\mathrm{UGe}}_2$.

• Received 1 June 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.127003