  We study the vortex structure in a Pb film deposited on top of a periodic
array of ferromagnetic square microrings by combining two high resolution
imaging techniques: Bitter decoration and scanning Hall probe microscopy
(SHPM). The periodicity and strength of the magnetic pinning potential
generated by the square microrings are controlled by the magnetic history of
the template. When the square rings are in the magnetized dipolar state, known
as the onion state, the strong stray field generated at the domain walls
prevents the decoration of vortices. SHPM images show that the stray field
generated by the dipoles is much stronger than the vortex field in agreement
with the results of simulations. Real space vortex imaging has revealed that,
in the onion state, the corners of the square rings act as effective pinning
centers for vortices.
