MAGNETIC PINNING OF THE VERTEX 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 superconduct ing length scales. The transport properties of Nb films, grown on top of these dots, were studied in a wide temperature and magnetic-field r ange. The Ni dots act as pinning centers. thereby producing oscillatio ns in the field dependence of the resistance and critical current of t he Nb film. The periodicity of the oscillation corresponds to the matc hing field of the dots lattice spacing. Such an oscillatory behavior i s absent for arrays of nonmagnetic Ag dots. The comparison between the two types of arrays implies a magnetic origin of the enhanced pinning effect.