Observation of a bi-domain state and nucleation free switching in mesoscopic ring magnets

We present the results of a study of the magnetic properties of an array of 34-nm thick Co(100) epitaxial ring magnets, with inner and outer diameters of d(in) = 1.3 mum and d(out) = 1.6 mum, respectively. Magnetic measuremen ts and micromagnetic simulations show that a two step switching process occ urs at high fields, indicating the existence of two different stable states . In addition to the vortex state, which occurs at intermediate fields, we have identified a new bi-domain state, which we term the onion state, corre sponding to opposite circulation of the magnetization in each half of the r ing. The onion state is stable at remanence and undergoes a simple and well characterized nucleation free switching.