  We study a stacked triangular lattice Ising model with both intra- and
inter-plane antiferromagnetic interactions in a field, by Monte Carlo
simulation. We find only one phase transition from a paramagnetic to a
partially disordered phase, which is of second order and 3D XY universality
class. At low temperatures we identify two highly degenerate phases: at smaller
(larger) fields the system shows long-range ordering in the stacking direction
(within planes) but not in the planes (stacking direction). Nevertheless,
crossovers to these phases do not have a character of conventional phase
transitions but rather linear-chain-like excitations.
