  We investigate the ground state of a classical two-dimensional system of
hard-sphere dipoles confined between two hard walls. Using lattice sum
minimization techniques we reveal that at fixed wall separations, a first-order
transition from a vacuum to a straight one-dimensional chain of dipoles occurs
upon increasing the density. Further increase in the density yields the
stability of an undulated chain as well as nontrivial buckling structures. We
explore the close-packed configurations of dipoles in detail, and we find that,
in general, the densest packings of dipoles possess complex magnetizations
along the principal axis of the slit. Our predictions serve as a guideline for
experiments with granular dipolar and magnetic colloidal suspensions confined
in slit-like channel geometry.
