We present a model of changing Fermi surface geometry in a ferromagnetic, spin-split environment, where the control parameter is the Stoner exchange energy. A two-peak density of states, here obtained from a quasi-one-dimensional bandstructure allows two jumps in magnetisation. The jump at finite magnetisation can be first order, and may occur near a maximum in the transition temperature for a triplet superconducting instability. Our motivation is the ferromagnetic superconductor, UGe₂.

PACS numbers: 74.20.Mn, 75.10.Lp, 74.70.Tx