  Finite-element micromagnetic simulations are employed to study the chiral
symmetry breaking of magnetic vortices, caused by the surface roughness of
thin-film magnetic structures. An asymmetry between vortices with different
core polarizations has been experimentally observed for square-shaped
platelets. E.g., the threshold fields for vortex core switching were found to
differ for core up and down. This asymmetry was however not expected for these
symmetrically-shaped structures, where both core polarizations should behave
symmetrically. Three-dimensional finite element simulations are employed to
show that a small surface roughness can break the symmetry between vortex cores
pointing up and down. A relatively small sample roughness is found sufficient
to reproduce the experimentally observed asymmetries. It arises from the lack
of mirror-symmetry of the rough thin-film structures, which causes vortices
with different handedness to exhibit asymmetric dynamics.
