  It is shown that in antiferromagnetic open or closed corrals of magnetic
adatoms grown on surfaces, the attachment of a single extra adatom anywhere in
the corral impacts on the geometrical topology of the nanosystem and generates
complex magnetic structures when a magnetic field is applied or a magnetic
coupling to a ferromagnetic substrate exists. The spin configuration of the
corral can be tuned to a non-planar state or a planar non-collinear or
ferrimagnetic state by adjusting its number of sites, the location of the extra
adatom or the strength of the coupling to the ferromagnetic substrate. This
shows the possibility to generate non-trivial magnetic textures with atom by
atom engineering anywhere in the corral and not only at the edges.

Strongly Correlated Electrons;Mesoscale and Nanoscale Physics