Role of vortices in magnetization reversal of rectangular NiFe elements

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Abstract
Vortices are seen in the magnetization distributions of rectangular magnetic elements in both experiments and micromagnetic simulations. To investigate 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 lithography and studied by high-resolution magnetic imaging in the transmission electron microscope. During reversal, vortices appeared near the ends of the elements, grew under an increasing reverse field, and disappeared after rapid 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 occurred by means of vortices, however the simulated switching fields were much higher than the experimentally observed values. Lower switching fields were obtained in the simulations when vortex creation was assisted by `defects' at the edges of the elements. However, to successfully simulate the magnitude and thickness dependence of the switching fields, it was necessary to start from an initial magnetic state which already contained a vortex.