CONICET | Buscador de Institutos y Recursos Humanos

The vortex lattice (VL) in type II superconductors is known to model numerous other elastic systems in interaction with pinning landscapes. In most cases, pinning centers largely outnumber the vortex lines, and provide random weak potentials that give rise to complex dynamics, metastability and history eects. A new generation of patterned samples containing articial arrays of strong pinning centers reveals new VL dynamics. The fingerprint of strong periodic pinning centers isan anomalous increase in critical current which generally occurs for applied matching fields producing a number of vortex lines equal to the number of articial pinning sites. An open question is the effect of competing random defects on the matching anomaly. In this context we have studied the VL mobility in patterned Nb lms containing periodic arrays of sub-50 nm magnetic nanodots or holes, by means of ac susceptibility measurements. Nb films are chosen because comparedto other unpatterned lms, show quite strong random disorder and may lead to competing interactions. We observe matching effects in the patterned lms within a wide temperature range, determined by the periodicity of the strong articial pinning potential. In addition to these eects, we nd a novel hysteretic behavior. Below a crossover temperature T*= 0.75 Tc, matching field not only depends on the geometry of the pinning array but depends on the sample thermo-magnetic history. This hysteretic response is examined in detail and we have consistently ascribed it to metastable VL congurations that arise from the competition between pinning by random intrinsic and periodic articial arrays. By means of dierent measuring protocols, including small magnetic eld loops, eld cooling procedures or the application of a perturbation as an ac shaking eld, we examine the response of the dierent VL congurations that are accessible with these pinning landscapes.