The phase diagram of temperature versus exchange field is obtained within a BCS model for d-wave superconductivity in ${\mathrm{CuO}}_2$ layers which is coupled to ferromagnetic ${\mathrm{RuO}}_2$ layers in ${\mathrm{RuSr}}_2{\mathrm{GdCu}}_2{\mathrm{O}}_8.$ 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.

• Received 23 June 2000

DOI:https://doi.org/10.1103/PhysRevB.62.11369