Local visualization of asymmetric flux pinning by magnetic dots with perpendicular magnetization

Field polarity-dependent flux pinning is investigated in a superconducting Pb film on an array of Co/Pt dots with out-of-plane magnetization. With the magnetic moments m of all dots aligned, the pinning force strongly depends on the mutual orientation of m and the magnetic field H. By scanning Hall probe microscopy, we have revealed the origin of this field polarity-dependent pinning. For antiparallel m and H, vortices are loosely caged at interstitial positions, while for parallel m and H, vortices are strongly pinned on the dots, thus providing a strong pinning asymmetry.