Mesoscopic FCCCo ring magnets

We present the results of a systematic study of the magnetic properties of micrometer size arrays of Co(100) epitaxial ring magnets grown on prepatterned Si substrates. The Co thickness varied between 5 and 34 nm and the outer diameter of the rings varied between 0.9 and 1.6 mum. Magnetic measurements and micromagnetic simulations show that for appropriate ring structures a two-step switching process occurs, indicating the existence of two different stable states. In addition to the vortex state, we have identified a new bi-domain state, the 'onion state', corresponding to opposite circulation of the magnetisation in each half of the ring. The onion state is stable at remanence and undergoes a simple and well-characterised nucleation-free switching. Using micromagnetic simulations we show also that the magnetisation reversal in narrow rings can take place via a nucleation-free domain wall motion process when a field pulse is applied in the plane of the film and perpendicular to the net magnetisation. Switching times of the order of 400ps are predicted with this approach. (C) 2002 Elsevier Science B.V. All rights reserved.