Critical state models for a granular ferromagnetic superconductor

Measurements of the magnetic moment M as a function of the magnetic field B can be used to extract information about the penetration behaviour of the magnetic field in a superconducting sample. In granular superconductors the response to an applied AC field is governed by both intra- and intergranular shielding currents. We have employed low-field AC susceptibility measurements to investigate the magnetic properties of a polycrystalline RuSr 2GdCu 2O 8 weak ferromagnet in its superconducting state. For fields below ≃1 mT the results are consistent with the Bean critical state model. For B>1 mT the Kim model is more suitable for the description of the sample's properties.