  Reproducible control of the magnetic vortex state in nanomagnets is of
critical importance. We report on chirality control by manipulating the size
and/or thickness of asymmetric Co dots. Below a critical diameter and/or
thickness, chirality control is achieved by the nucleation of single vortex.
Interestingly, above these critical dimensions chirality control is realized by
the nucleation and subsequent coalescence of two vortices, resulting in a
single vortex with the opposite chirality as found in smaller dots.
Micromagnetic simulations and magnetic force microscopy highlight the role of
edge-bound halfvortices in facilitating the coalescence process.
