  Ordering in realistic elongated box-shape crystals of the molecular magnet
Mn_12 Ac is investigated with the site-resolved mean-field approximation that
does not assume a uniform ordering. It is shown that ferromagnetic ordering
should not occur in crystals with the aspect ratio up to 12. Instead, for the
aspect ratio about 6, the inner and outer regions of the crystal order in
different directions, uniformly along its length. Finding ordering temperature
by extrapolating the inverse susceptibility curve does not provide a correct
T_C.

Statistical Mechanics