Artificial pinning arrays investigated by scanning Hall probe microscopy

High resolution scanning Hall probe microscopy has been used to study flux line pinning in a thin superconducting ph film evaporated on top of an orde red array of ferromagnetic Co dots. After magnetisation along the easy axis , all the nanomagnets are seen to be in single domain magnetic states with dipole stray field distributions (T > T-e). Upon cooling through the critic al temperature of the superconducting film at H = 0, we observe strong scre ening of the dipole fields which we attribute to fluxoid quantisation. Afte r field cooling, we find that flux lines are selectively Dinned at poles wi th the opposite sign of field and ordered structures which are commensurate with the underlying pinning array are observed. Each pinning site appears to be capable of trapping at least two nux lines, and no evidence has yet b een seen for interstitial vortices at low temperatures. Results will be com pared with those from thr disordered pinning system found naturally in Cu-r ich epitaxial YBCO thin films. These typically contain micron-sized normal Cu-rich precipitates, which have a similar density to other artificially pa tterned systems;. We show that the precipitates act as strong pinning sites close to T-c, and also give rise to weak magnetisation peaks at average ma tching fields. (C) 2000 Elsevier Science B,V. All rights reserved.