  The half antivortex, a fundamental topological structure which determines
magnetization reversal of submicron magnetic devices with domain walls, has
been suggested also to play a crucial role in spin torque induced vortex core
reversal in circular disks. Here we report on magnetization reversal in
circular disks with nanoholes through consecutive metastable states with half
antivortices. In-plane anisotropic magnetoresistance and broadband
susceptibility measurements accompanied by micromagnetic simulations reveal
that cobalt disks with two and three linearly arranged nanoholes directed at 45
and 135 degrees with respect to the external magnetic field show reproducible
step-like changes in the anisotropic magnetoresistance and magnetic
permeability due to transitions between different intermediate states mediated
by vortices and half antivortices confined to the dot nanoholes and edges,
respectively. Our findings are relevant for the development of multi-hole based
spintronic and magnetic memory devices.
