  In this theoretical investigation we address the effect of defects on thermal
excitations in square-lattice dipolar arrays. The geometry of the nanomagnets,
adopted from recent experiments [A. Farhan \textit{et al.}, Nature Phys.\
\textbf{9} (2013) 375], allows for thermal activation at room temperature. It
turns out that excitations can efficiently be pinned at defects. Furthermore,
ferromagnetic strings of nanomagnets that connect a pair of defects can be
produced; their lengths are closely related to the distance of the defects and
the topology of the magnetic ground-state configuration. We discuss various
types of defects, all of which can be produced by microstructuring techniques.
