Measurement of spin waves and activation volumes in superparamagnetic Fe   films on GaAs(100)
  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. 
R. L. Stamps, A. Stollo, M.Madami, S. Tacchi, G. Carlotti, G.   Gubbiotti, M. Fabrizioli, J. Fujii