Magnetization switching in Permalloy square ring magnets

We present the results of a study of magnetization switching in Permalloy square rings using the diffraction magneto-optic Kerr effect combined with numerical micromagnetic simulations. Diffracted loops show a two-step switching process as the external field is swept along both the ring edge and the diagonal. The hysteresis loops calculated from micromagnetic simulations show that switching should occur via a single step transition from a one onion to the reversed onion state for both directions of field applied. We have found that in order to reproduce the observed two-step reversal slight asymmetry has to be introduced into the ring shape. In this case a stable intermediate states appear during switching, accounting for the shape of diffracted loops.