Role of vortices in magnetization reversal of rectangular NiFe elements

Vortices are seen in the magnetization distributions of rectangular magneti c elements in both experiments and micromagnetic simulations, To investigat e the role of vortices during magnetization reversal Ni80Fe20 elements 100 nm and 200 nm wide and 5-60 nm thick were fabricated by electron beam litho graphy and studied by high-resolution magnetic imaging in the transmission electron microscope. During reversal, vortices appeared near the ends of th e elements, grew under an increasing reverse field, and disappeared after r apid switching. Maximum switching fields of 400 Oe for 100 nm wide elements and 200 Oe for 200 nm wide elements occurred for film thicknesses of 25-30 nm and above. Simulations showed that reversal in these elements always oc curred by means of vortices, however the simulated switching fields were mu ch higher than the experimentally observed values. Lower switching fields w ere obtained in the simulations when vortex creation was assisted by 'defec ts' at the edges of the elements. However, to successfully simulate the mag nitude and thickness dependence of the switching fields, it was necessary t o start from an initial magnetic state which already contained a vortex.