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We study the dynamics of the vortex lattice driven by ac induced currents in the critical state regime, for T > 0.70 TC. The samples are superconducting films grown on top of two-fold symmetry array of magnetic dots. In these heterostructures, the induced ac currents flow parallel to the short and to the long side of the pinning array in different areas of the samples simultaneously. This behavior produces remarkable effects in the vortex lattice dynamics. First of all, periodic features are observed in the ac susceptibility versus applied magnetic field measurements which are related to matching effects between the vortex lattices and the magnetic array. However, the vortex lattice reconfiguration observed in magnetotransport experiments is absent. Some of these features are revealed as maxima instead of being minima, indicating higher mobility at certain matching fields. Competing unstable vortex configurations could lead to increase vortex mobility precluding the reconfiguration transition. At high temperatures, where the matching effects show up, the magnetic permeability of the dots is the mechanism that governs the JC(T) behavior. Moreover, the temperature dependence of the pinning force FP(T) shows a temperature crossover related to an unexpected enhancement in vortex mobility. Vortex–vortex interaction and the interplay between trapped and interstitial vortices are a hint to explain these phenomena. [ABSTRACT FROM AUTHOR]

We study vortex lattice (VL) dynamics in patterned Nb films containing dense periodic arrays of sub 50 nm magnetic nanodots or holes, by means of ac susceptibility measurements. For both types of samples, we observe matching effects within a wide temperature range, determined by the periodicity of the strong artificial pinning potential. Below a crossover reduced temperature t* = 0.75, the ac vortex mobility and matching fields are different for increasing or decreasing fields. Low temperature field cooling experiments indicate that the hysteretic behavior is not related to different intensities of magnetic induction B in the ac penetrated region of the samples. The fact that hysteresis appears in both patterned samples when the intrinsic random pinning becomes stronger, together with the absence of hysteresis in a reference unpatterned Nb film, suggests that the interplay between random and periodic pinning, independent of its origin, induces VL metastability below t*, which leads to the observed irreversibility. [ABSTRACT FROM AUTHOR]