  We demonstrate that the stray magnetic field generated beneath magnetic
vortex cores can be used to generate nano-scale, localized pinning sites for
magnetic domain walls in an underlying, perpendicularly magnetized nanostrip.
Moreover, the pinning strength can be tuned by toggling the vortex core
polarity. Indeed, switching the core polarity so that it is aligned with the
magnetization of the expanding domain (rather than against it) can be used to
reduce the vortex-mediated wall depinning field by between 40% and 90%,
depending on the system geometry. Significant reductions in the depinning field
can also be obtained in narrow strips by shifting the core away from the
strip's center.
