Chirality and stability of vortex state in Permalloy triangular ring micromagnets

We present the results of a study of the magnetization reversal in Permalloy triangular rings using diffracted magneto-optics Kerr effect combined with numerical micromagnetic simulations and in-field magnetic force microscopy. Diffracted loops show a two-step switching process for external fields along or perpendicular to any of the ring edges. The diffracted loops calculated from micromagnetic simulations reproduce the measured ones and show that the switching occurs from one asymmetric onion state to the reversed state, for both directions of the applied field. In both cases a stable intermediate vortex state appears during the switching, accounting for the diffracted loop structures. The stability and the magnetization chirality of the vortex state depend on the direction of the applied field relative to the ring orientation. Magnetization configurations occurring during reversal imaged with magnetic force microscopy operated applying an external field confirm the above reversal process.