  We studied the temperature dependence of the magnetization reversal in
individual submicron permalloy disks with micro-Hall and bend-resistance
magnetometry. The nucleation field exhibits a nonmonotonic dependence with
positive and negative slopes at low and high temperatures, respectively, while
the annihilation field monotonically decreases with increasing temperature, but
with distinctly different slopes at low and high temperatures. Our analysis
suggests that at low temperatures vortex nucleation and annihilation proceeds
via thermal activation over an energy barrier, while at high temperatures they
are governed by a temperature dependence of the saturation magnetization.
