  Cu/Co/Cu trilayers have been deposited on nanoporous alumina membranes.
Magnetic properties of the resulting Co antidot arrays are investigated using
SQUID magnetometry. Hysteresis loops of these arrays show two-step
magnetization reversal. In addition, exchange bias is observed, whether the
cooling field is applied within or perpendicular to the surface plane. In the
former case, the exchange bias changes sign close to the blocking temperature,
and becomes positive. We attribute these effects to the local, crescent shape
of the Co films, induced by the surface morphology of the alumina membranes.
This morphology leads to a three-dimensional magnetization distribution at the
nanoscale.
