Noninvasive magnetic imaging and magnetization measurement of isolated mesoscopic Co rings

A high-resolution scanning Hall probe microscope was used as a noninvasive technique to visualize the magnetization reversal in an array of micron-size Co rings. Two stable “onion” states at remanence and “vortex” states at switching fields were found. To rule out a possible influence of dipole–dipole interaction between ring elements on remagnetization processes, an isolated Co ring was deposited on top of a Hall magnetometer and extremely sharp transitions from onion to vortex and from vortex to onion state of opposite polarity were resolved. Our results were supported by MOKE magnetization measurements and micromagnetic simulations.