  We investigate a spin reorientation transition (SRT) in ultrathin magnetic
films by Monte-Carlo simulations. We assume that the lateral size of the film
is relatively small and it has a single-domain structure. To gain insights into
the SRT, we measure a free-energy as a function of perpendicular and in-plane
magnetizations. As a result, we find that the system is in a superparamagnetic
state at the SRT temperature. The disappearance of magnetization around the SRT
temperature, which is observed in experiments, emerges due to dynamical
fluctuations in magnetization which are inherent in the superparamagnetic
state. This observation is in contrast to that in large ultrathin magnetic
films that the disappearance of magnetization is caused by a static magnetic
structure with many complex domains.
