MATERIALS SCIENCE SEMINAR

ABSTRACT: The vortex lattice (VL) dynamics in patterned Nb films containing dense periodic arrays of sub 50 nm magnetic nanodots or holes is studied by means of ac susceptibility measurements. Near the critical temperature, TC, reversible magnetic field matching effects are related to the periodicity of the strong artificial pinning potentials. However, below a characteristic cross over temperature T* ~ 0.75 TC an anomalous hysteretical behavior is observed, independent of the type of periodic pinning, that leads to a history dependent apparent matching field value. This behavior, that occurs for enhanced low temperature intrinsic pinning, together with the absence of hysteresis in a reference unpatterned Nb film and observations in FC samples, suggests that the interplay between intrinsic random and periodic pinning induces VL metastability below T* which leads to the observed irreversibility.