Critical Field Enhancement in Hybrid Superconductor/Ferromagnet Mesoscopic Disks

We investigated experimentally the nucleation of superconductivity in a mesoscopic hybrid structure, consisting of a thin superconducting disk covered with a ferromagnetic layer with an in-plane magnetic moment. By applying a magnetic field in the plane of the structure, the remanent magnetic state of the ferromagnet can be switched from a flux-closure state where field lines are confined inside the ferromagnet to a polarized state with non zero stray fields at the edges. This change of the magnetic state causes a drastic modification on the superconductor/normal state phase boundary of the hybrid sample. In the polarized state a reentrant transition line and a strong broadening of the phase boundary is observed.