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

Abstract

The transition between onion states and vortex states in exchange-coupled Ni–Fe∕Mn–Ir asymmetric ring dots has been investigated. A direction of domain wall motion, during the transition from the single-domain state to the vortex state via the onion state, depends on a sweep direction of an external field. This dependence fixes the directions of vortical magnetizations in the vortex states. The derivative of the amount of the domain wall motion with respect to the external field depends on the sweep direction of external field, and thus the hysteresis loop becomes asymmetric.