Pinning centers produced by magnetic microstructures

Abstract

We investigate the flux pinning and dynamic properties of superconducting vorticesin an Al film with an array of magnetic bars deposited on top. The dimensionsof each bar are chosen in such a way that they host a single magnetic domain.These micromagnets are distributed periodically in a rectangular array with0.5 µm separation parallel to the longest side of the bars and displaced laterally by a distancew. We showthat, for w>Λ, where Λ is the effective field penetration depth, the pinning strength is almost independent ofw whereas the critical temperature at zero field,Tc(0), decreasesas ∼w−1. Forw<Λ the opposite behavioris observed, i.e. Tc(0) seemsto saturate to a nearly w-independent value and the transition from large to smallw is accompanied by a large suppression of the critical currentjc together with a clear change in the shape of the current–voltage characteristics.In particular, the substantial weakening of the pinning potential forw<Λ gives rise to an unexpected flux flow response. The field evolution of this regime allows usto determine whether the magnetic bars induce vortex–antivortex pairs in the system. Thepresent findings suggest that practical application of magnetic pinning centers arerestricted to low field values.