  Dipolar coupling in crystals of the Mn$_{12}$Ac molecular magnet elongated
along the anisotropy axis favors ferromagnetic ordering below the mean-field
Curie temperature 0.71 K. With the help of Monte Carlo on crystals of up to one
million Mn$_{12}$ molecules, it is shown that ordering occurs at 0.36 K. The
resulting state is split into ferromagnetic domains with domain walls
preferring the diagonal orientation. Domain walls are pinned by the lattice at
low temperatures. Making the crystal shorter makes domains finer and smoothens
out the singularity at the transition, decreasing susceptibility in the domain
state. Hysteresis loops look ferromagnetic for prolate crystals and
antiferromagnetic for oblate crystals.
