Magnetization reversal in nanostructured permalloy rings

Three different stable magnetization configurations of soft-type nanostructured permalloy rings are investigated in detail by micromagnetic simulation. During the reversal process, the abrupt change of the hysteresis loop corresponds to the evolution of these states. Beside the quasiuniform and circular states, we find a new pair-vortices state. We compare the magnetic pole density distributions transformed from the simulation patterns with the magnetic force microscopic (MFM) images and find very good agreement between them.