Coexistence of superconductivity and ferromagnetism in ferromagnetic metals

In this paper we address the question of coexistence of superconductivity and ferromagnetism in the high temperature superconductor RuSr${}_2$GdCu${}_2$O${}_{8-\delta }$. Using a field theoretical approach we study a one-fermion effective model of a ferromagnetic superconductor in which the quasiparticles responsible for the ferromagnetism form the Cooper pairs as well. We discuss the physical features which are different in this model and the standard BCS model and consider their experimental consequences.