Fluctuations on the magnetic response of superconducting thin films of Nb and MgB2 - Percolation limit of vortex mobility

Application of a magnetic field of sufficient intensity orthogonal to superconducting thin films may produce dendritic patterns, where penetrated and Meissner regions coexist. The dendritic mode can be detected by AC-susceptibility measurements, since fingers penetrated by the magnetic field act as intergranular material. Measurements of the AC-susceptibility have the conventional shape for smaller values of magnitude and frequency of the excitation field. However, for frequencies in the vicinity of 1 kHz and AC-fields around 3 Oe and above, the curve deviates from its canonical behavior and fluctuates, the excursion becoming wider as the amplitude is increased. In this contribution we present results of a systematic study conducted to determine the threshold between smooth and fluctuating regimes of the magnetic response of the film, which is interpreted as the percolation limit of vortex mobility throughout the sample.