  We investigate magnetization reversal processes of magnetic nanorings. Using
a recently developed efficient cartesian coordinates fast multipole method,
magnetization switching phase diagrams are constructed for such structures. We
find that the dominant switching mechanisms strongly depend on the relative
spatial dimensions of the rings, i.e. height, width and radius. Furthermore,
they are found to depend on a characteristic exchange length scale, which is
determined by the competing short-range ferromagnetic and long-range dipolar
interactions in these systems. Detailed simulations allow us to identify three
novel switching mechanisms, which should be useful for building high density
storage systems.
