Size dependence of switching fields of permalloy rings

The size dependence of switching fields of permalloy rings is studied by micromagnetic simulations and by magnetoresistance measurements. Experimentally, the switching fields can be obtained from the discontinuities of the magnetoresistance curve. From the simulated hysteresis loops and the magnetoresistance curves obtained by experiment it is shown that the switching field from the vortex state to the reverse onion state increases considerably as decreasing of the lateral size or as increasing of the thickness. There is, however, no significant correlation between the ring size and the switching field from the onion state to the vortex state.