  Topological magnetic textures such as vortex cores or skyrmions are key
candidates for non-volatile information processing. This exploits the texture
movement by current pulses that is typically opposed by pinning. A detailed
understanding of pinning is hence crucial with previous experiments being
either limited in terms of controlled magnetic texture positioning or in terms
of spatial resolution. Here, we use spin-polarized scanning tunneling
microscopy to track a magnetic vortex core that is deliberately moved by a 3D
magnetic field. The core covering about 10.000 Fe-atoms gets pinned by defects
that are only a few nm apart. Reproducing the vortex path via parameter fit, we
deduce the pinning potential of the defects as a mexican hat with short-range
repulsive and long-range attractive part. By comparison with micromagnetic
simulations, the attractive part is attributed to a local suppression of
exchange interaction. The novel approach to deduce defect induced pinning
potentials on the sub-nm scale is transferable to other non-collinear spin
textures eventually enabling an atomic scale design of defect configurations,
e.g., for reliable read-out in race-track type devices.
