Magnetic pinning of the vortex lattice by arrays of submicrometric dots

We have prepared regular square arrays of Ni and Ag dots with typical diameter and thickness approaching characteristic low-$T_c$ superconducting length scales. The transport properties of Nb films, grown on top of these dots, were studied in a wide temperature and magnetic-field range. The Ni dots act as pinning centers, thereby producing oscillations in the field dependence of the resistance and critical current of the Nb film. The periodicity of the oscillations corresponds to the matching field of the dots lattice spacing. Such an oscillatory behavior is absent for arrays of nonmagnetic Ag dots. The comparison between the two types of arrays implies a magnetic origin of the enhanced pinning effect.