Periodic vortex pinning with magnetic and nonmagnetic dots: The influence of size

Periodic pinning arrays can give rise to commensurability effects for the vortex pinning in superconductors. We studied the influence of size and separation of the individual pinning centers made out of magnetic and nonmagnetic dots. While there is some dependence on the dot size, the periodic pinning changes its character drastically, as soon as the separation between dots becomes comparable to the coherence length of the superconductor. This can be understood by a crossover from a weak pinning regime to a superconducting wire network regime with decreasing dot separation. In addition, the comparison between the periodic pinning of magnetic dot arrays and nonmagnetic dot arrays shows that the former generally show more pronounced periodic pinning. This indicates conclusively a magnetic contribution to the pinning mechanism.