  The influence of long-range dipole interactions on two dimensional magnetism
has been studied extensively in the spin-reorientation transition of
ferromagnetic ultrathin films. Although there is a great deal of experimental
information on the perpendicular domain phase that is stabilized by dipole
interactions, the transitions to or from the domain phase are subtle and
difficult to characterize experimentally. Magnetic susceptibility measurements
show no divergence in the vicinity of the spin-reorientation transition as a
function of thickness -- a null result that is difficult to interpret with
confidence. This article reports separate dynamical and equilibrium versions of
the reorientation transition in Fe/2ML Ni/W(110) films, using measurements of
the magnetic susceptibility as the films are grown. The dynamical version
occurs when increasing the film thickness causes the domain walls to depin, and
the system moves from a configuration that minimizes local energetics to one
that minimizes global energetics. The dynamical transition \textit{is} marked
by a divergent magnetic susceptibility measured with a field applied along the
in-plane W[001] direction. A comparative study of the two versions of the same
spin-reorientation transition aids in the experimental characterization of the
effects of dipole interactions on the phase transitions. This comparison
confirms the original null result found in magnetic susceptibility measurements
of the equilibrium transition; despite its name, the spin-reorientation
transition in ferromagnetic ultrathin films has no critical phase transition in
either the magnetization or its orientation.
