Superconductivity via Paramagnon and Magnon Exchange in a 2D Near-Ferromagnetic Full Metal and Ferromagnetic Half-Metal

We study superconductivity in paramagnetic and ferromagnetically ordered phases in a two-dimensional electron system with parabolic fermionic dispersion and short-range repulsive interaction. In the paramagnetic phase, we find that a weak momentum dependence of a paramagnon propagator parametrically reduces the onset temperature for the pairing compared to that in phenomenological theories, which assume a strong dispersion of a paramagnon, and also changes the topology of the gap function. In the ferromagnetic phase, we show that the order instantly polarizes low-energy fermionic excitations. We derive the fully renormalized pairing interaction between low-energy fermions, mediated by two transverse Goldstone modes, and show that it is attractive in a spatially odd channel. The pairing temperature in the ferromagnetic phase is found to be a fraction of the Fermi energy, significantly larger than in the paramagnetic phase near the transition. Our results are relevant for understanding superconductivity in proximity to itinerant ferromagnetism in multivalley graphene systems, particularly the ones with full valley and spin polarization.