Fresnel contrast in energy filtered images of magnetic domain walls in Nd_2Fe_(14)B is examined as a function of specimen thickness and defocus. The experimental contrast is compared with simulations of model structures with domain wall widths varying from 2 to 20nm. The form of the contrast is explained as a combination of interference between the beams passing through adjacent domains and Fresnel contrast from the boundary itself. Images from foils of thickness greater than 100nm show a greater sensitivity to the domain wall width. The experimental contrast was consistent with domain walls 5nm wide.
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