  An understanding of the pinning of magnetic skyrmions to defects is crucial
for the development of future spintronic applications. While pinning is
desirable for a precise positioning of magnetic skyrmions it is detrimental
when they are to be moved through a material. We use scanning tunneling
microscopy to study the interaction between atomic scale defects and magnetic
skyrmions that are only a few nanometers in diameter. The studied pinning
centers range from single atom inlayer defects and adatoms to clusters adsorbed
on the surface of our model system. We find very different pinning strengths
and identify preferred positions of the skyrmion. The interaction between a
cluster and a skyrmion can be sufficiently strong for the skyrmion to follow
when the cluster is moved across the surface by lateral manipulation with the
STM tip.
