3D magnetization profile and multi-axes exchange bias in Co antidot arrays

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.