Pinning effects by arrays of magnetic dots on niobium film

The interaction between an ordered array of submicron magnetic particles and a superconducting thin film can lead to important pinning effects due to the synchronized interaction with the vortex lattice. Triangular and square lattices of submicrometer magnetic dots have been fabricated by electron beam lithography, with typical spacings in the range 400-600 nm and dot radius around 100-200 nm. The influence of these lattices of small particles on the superconducting properties of niobium films has been studied. The resistivity vs. magnetic field curves exhibit a series of sharp regular minima close to the transition temperature. These minima appear at integer multiples of the matching field, which is determined by the lattice parameter of the dot array, thus, revealing a synchronized pinning effect due to the magnetic dots.