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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. [ABSTRACT FROM AUTHOR]

Ni–Fe asymmetric ring dots with partially planed outer sides were investigated as candidates for high-density magnetic memory cells. The magnetic states, which were measured with magnetic force microscopy, show that in-plane magnetic fields can control the chirality, either clockwise or counterclockwise, of vortical magnetizations of the Ni–Fe asymmetric ring dots. This control facilitates applying ring dots to the magnetic random access memories. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]