Imaging and magnetotransport in superconductor/magnetic dot arrays

Magnetoresistance and scanning Hall probe microscopy studies of Nb-film/Ni-dot structures are reported. The dots act as pinning sites for superconducting vortices. The transport measurements focus on the effects of introducing disorder into the positions of the pinning lattice near the superconducting critical temperature T-c in structures with 250 nm diameter Ni dots randomized about an ideal square lattice with lattice constant a = 560 nm. Features observable in the ordered arrays at higher multiples of the matching field H-0 = Phi (0)/a(2) are washed out in the disordered arrays, but those at H-0 remain. Scanning Hall probe microscope images were taken of the vortex configurations at fields up to 1.2H(0) in ordered arrays of 1-mum-diameter dots on a 5.2 mum x 4 mum rectangular lattice. These show that despite the relatively weak pinning of the magnetic dots, ordering commensurate with the dot lattice occurs even for fields below H-0. Both transport and imaging studies point to the importance of interstitial vortices in determining the properties of superconductor/magnetic dot arrays. (C) 2001 American Institute of Physics.