Theoretical Study of Magnetic Behavior in Ru-based Ferromagnetic Superconductors

A microscopic theory of interplay of superconductivity and ferromagnetism in hybrid rutheno-cuprate superconductors RuSr₂RECu₂O₈ (Ru-1212) (RE=Gd, Eu) is developed from first principles considering s–f model. Self-consistent equations for superconducting order parameter Δ and ferromagnetic order parameter 〈S ^Z 〉 are obtained using Green’s function technique and equation of motion method. The theory is applied to explain the experimental results for RuSr₂RECu₂O₈ system. This model clearly predicts the coexistence of superconductivity and spatially uniform ferromagnetism in RuSr₂RECu₂O₈. The theory shows that it is possible to become superconducting if the system is already ferromagnetic. Study of specific heat, density of states, free energy, critical field, and ac spin susceptibility is also presented. Agreement between experimental results and theory is quite convincing.