  The interplay between superconductivity and magnetism gives rise to many
intriguing and exciting phenomena. In this Letter we report about a novel
manifestation of this interplay: a temperature induced phase transition between
different spontaneous vortex phases in lead superconducting films with embedded
magnetic nanoparticles. Unlike common vortices in superconductors the vortex
phase appears without any applied magnetic field. The vortices nucleate
exclusively due to the stray field of the magnetic nanoparticles, which serve
the dual role of providing the internal field and simultaneously acting as
pinning centers. As in usual superconductors, one can move the spontaneous
vortices with an applied electric current. Transport measurements reveal
dynamical phase transitions that depend on temperature (T) and applied field
(H) and support the obtained (H-T) phase diagram. In particular, we used a
scaling analysis to characterize a transition from a liquid to a novel
disordered solid resembling a vortex glass.
