Spin-Fermion model of UGe₂

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It is assumed that U atoms in UGe₂ have a number of f electrons appropriate to give them each a spin s=1 as well as one extra itinerant electron which may equally well be on one or other U atom. The dynamical degrees of freedom are spin-s operators of localized spins and spin-1/2 fermi operators of itinerant electrons. Applying hydrostatic pressure changes the bandwidths of spin-up and spin-down itinerant electrons in different way, which leads to decreasing of the contribution of the fermions to the magnetization keeping the spin-fermion interaction unchanged. In turn the local spin-fermion interaction leads to ferromagnetic superconductivity. The model accounts, in a quantitative and natural way, for the characteristics of the coexistence of superconductivity and ferromagnetism in UGe₂, including many of the key experimental results: metamagnetic transitions, quantum transition from ferromagnetism to ferromagnetic superconductivity, the position of the highest superconducting critical temperature etc.