  A micromagnetic study of epitaxial micron-sized iron dots is reported through
the analysis of Fresnel contrast in Lorentz Microscopy. Their use is reviewed
and developed through analysis of various magnetic structures in such dots.
Simple Landau configuration is used to investigate various aspects of
asymmetric Bloch domain walls. The experimental width of such a complex wall is
first derived and its value is discussed with the help of micromagnetic
simulations. Combination of these two approaches enables us to define what is
really extracted when estimating asymmetric wall width in Lorentz Microscopy.
Moreover, quantitative data on the magnetization inside the dot is retrieved
using phase retrieval as well as new informations on the degrees of freedom of
such walls. Finally, it is shown how the existence and the propagation of a
surface vortex can be characterized and monitored. This demonstrates the
ability to reach a magnetic sensitivity a priori hidden in Fresnel contrast,
based on an original image treatment and backed-up by the evaluation of
contrasts obtained from micromagnetic simulations.
