Superconductivity in ferromagnetic RuSr₂GdCu₂O₈

The phase diagram of temperature versus exchange field is obtained within a BCS model for d-wave superconductivity in CuO₂ layers which is coupled to ferromagnetic RuO₂ layers in RuSr₂GdCu₂O₈. It is found that the Fulde-Ferrell-Larkin-Ovchinnikov state is very sensitive to the band filling factor. For strong exchange field, we point out that superconductivity could only exist in the interfaces between ferromagnetic domains. The magnetization curve is calculated and its comparison with experiment is discussed. We also propose the measuring of tunneling conductance near a single unitary impurity to detect the strength of the exchange interaction.