  We review models for the nucleation of magnetisation reversal, i.e. the
formation of a region of reversed magnetisation in an initially magnetically
saturated system. For small particles models for collective reversal, either
uniform (Stoner-Wohlfarth model) or non-uniform like curling, provide good
agreement between theory and experiment. For microscopic objects and thin
films, we consider two models, uniform (Stoner-Wohlfarth) reversal inside a
nucleation volume and a droplet model, where the free energy of an inverse
bubble is calculated taking into account volume energy (Zeeman energy) and
surface tension (domain wall energy). In macroscopic systems, inhomogeneities
in magnetic properties cause a distribution of energy barriers for nucleation,
which strongly influences effects of temperature and applied field on
magnetisation reversal. For these systems, macroscopic material parameters like
exchange interaction, spontaneous magnetisation and magnetic anisotropy can
give an indication of the magnetic coercivity, but exact values for nucleation
fields are, in general, hard to predict.
