  Spin wave frequencies are observed in ultra-thin Fe/GaAs(100) films at
temperatures where the spontaneous zero field magnetization is zero. The films
exhibit good cyrstalline structure, and the effect of magnetic anisotropies is
apparent even though no zero field spin wave energy gap exists. An analysis is
given in terms of a superparamagnetic model in which the film is treated as a
network of non-interacting single domain magnetic islands. A spin wave analysis
provides a means to separate measured values of anisotropy parameters from
products involving anisotropy and island volume. In this way, a measure of the
activation volume associated with superparamagnetic islands is obtained for
different Fe film thicknesses. Results suggest that the island lateral area
increases with increasing film thickness.
