The magnetic properties of (20 Å Pt/15 Co/20 Å Pt) triple layer deposited on a thick Nb(110) substrate are measured by vibrating sample magnetometry and the polar magneto-optical Kerr effect (p-MOKE). Above the superconducting transition temperature Tc 9.1 K of Nb an out-of-plane magnetization curve M(H) is observed by p-MOKE due to the perpendicular magnetic anisotropy of the Pt/Co/Pt triple layer. Cooling the sample below Tc gives rise to a modification of the M(H) hysteresis with a sudden change of the magnetization for magnetic fields smaller than the coercive field Hcoerc of the ferromagnetic triple layer. This is due to the penetration of magnetic vortices into the center of the weak-pinning superconducting Nb disk for fields above the lower critical field. These vortices generate an enhanced local magnetic field at the Nb surface which is larger than Hcoerc of the Pt/Co/Pt triple layer and locally switches isolated domains already for applied fields smaller than Hcoerc. © 2010 IOP Publishing Ltd.