Scanning Hall probe microscopy observation of flux pinning in a Pb film covering an array of Co dots

Abstract

High-resolution scanning Hall probe microscopy (SHPM) has been used to investigate the superconducting properties of a thin Pb film covering a square array of rectangular submicron Co dots. After magnetization, all the dots are shown to be in a single-domain magnetic state with dipole stray field distributions. As the sample is cooled through the critical temperature of the Pb film at B = 0 we observe strong screening of the dipole fields that we attribute to fluxoid quantization in the superconducting state. Upon cooling in applied perpendicular fields we show that the first flux line is selectively pinned at the pole of the magnet with the opposite sign of stray field. Ordered vortex structures, which are commensurate with the underlying pinning array, are observed at fields up to twice the matching field. At fields in excess of the first matching field, each pinning site appears to be capable of trapping at least two flux lines and no evidence is seen for interstitial vortices up to twice the matching field at temperatures well below the critical temperature of the covering Pb film. Copyright © 2000 John Wiley & Sons, Ltd.