Field-dependent evolution of magnetic states in Co ring arrays

We present a systematic study of field-dependent evolution in the magnetization reversal process of elongated Co ring arrays using in-situ field magnetic force microscopy. We observed that, the rings typically undergo a uniform→onion→vortex→reverse onion→reverse uniform spin-state transition as the field is swept along the major axis of the rings. However, the switching field distribution in the arrays generally leads to each transition occupying a wide range of fields, which results in the observation of coexistence of different magnetization states at lower field. The magnetization reversal sequence has also been verified by micromagnetic simulations, which show good agreement with our experimental data.