Anomalies in vortex lattice dynamics driven by induced ac currents in superconducting films with magnetic arrays of two-fold symmetry

We study the dynamics of the vortex lattice driven by ac induced currents in the critical state regime, for T > 0.70 T_C. 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 J_C(T) behavior. Moreover, the temperature dependence of the pinning force F_P(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. (paper)