  Shape asymmetries in nominally circular nanomagnets provide a potential means
for vortex chirality control. However, in realistic arrays their effects are
challenging to probe since asymmetric magnetization reversal processes are
often averaged to include distributions over all angles. Here we investigate
how shape asymmetry influences the vortex reversal in arrays of sub-micron
edge-cut Co dots. We find that the vortices can be manipulated to annihilate at
particular sites under different field orientations and cycle sequences. The
vortex annihilation field and degree of chirality control depend sensitively on
the angular position of the applied field relative to the flat edge of the
dots. For small angles, the major loop annihilation field is significantly
larger than that found from the half loop and the vortex chirality can be well
controlled. At intermediate angles the chirality control is lost and an
interesting crossover in the annihilation field is found: the half loop
actually extrudes outside of the major loop, exhibiting a larger vortex
annihilation field. At large angles the annihilation fields along major and
half loops become degenerate.
