  Pb films embedded with homogeneously distributed cobalt (Co) nanoparticles
(mean size 4.5 nm) have been prepared. Previous transport investigations have
shown that Co particles induce spontaneous vortices below the superconducting
transition temperature (T$_{c}$) in zero external magnetic field. In this paper
we study in detail the influence of the Co volume franction and an external
magnetic field on the superconducting transition in such composites. The large
difference in T$_c$-reduction between the as-prepared and annealed samples can
be attributed to the different superconducting coherence lengths and the
resulting different diameters of the spontaneous vortices in these samples.
