  We show results from Monte Carlo simulations of a two dimensional Heisenberg
model for ultrathin films with perpendicular anisotropy. A complete phase
diagram is obtained as a function of anisotropy and temperature, spanning a
wide range of behavior. We discuss our results in relation with experimental
findings in different ultrathin films. We observe and characterize a line of
Spin Reorientation Transitions . This transition from out of plane stripe order
to in plane ferromagnetic order presents a paramagnetic gap in between in a
finite region in parameter space, as reported in experiments. For large
anisotropies direct transitions from a low temperature stripe phase to a
paramagnetic or tetragonal phase with dominant perpendicular magnetization is
observed, also in agreement with experiments.
  We also show the phase diagram for a system without exchange, i.e. with pure
dipolar and anisotropy interactions. It shows a similar behavior to the
ferromagnetic case with antiferromagnetic instead of stripe phases at low
temperatures. A Spin Reorientation Transition is also found in this case.
