Transition between onion states and vortex states in exchange-coupled Ni-Fe/Mn-Ir asymmetric ring dots

The transition between onion states and vortex states in exchange-coupled Ni-Fe/Mn-Ir asymmetric ring dots is investigated. A direction of domain wall motion, during the transition from the single-domain state to the vortex state through the onion state, depends on a sweep direction of an external field.