  We show that it is possible to engineer magnetic multi-domain configurations
without domain walls in a prototypical rare earth/transition metal ferrimagnet
using keV He+ ion bombardment. We additionally shown that these patterns
display a particularly stable magnetic configuration due to a deep minimum in
the free energy of the system which is caused by flux closure and the
corresponding reduction of the magnetostatic part of the total free energy.
This is possible because light-ion bombardment differently affects an elements
relative contribution to the effective properties of the ferrimagnet. The
impact of bombardment is stronger for rare earth elements. Therefore, it is
possible to influence the relative contributions of the two magnetic subsystems
in a controlled manner. The selection of material system and the use of
light-ion bombardment open a route to engineer domain patterns in continuous
magnetic films much smaller than what is currently considered possible.
