  We investigate spin precession in the presence of randomly distributed field
sources. Their fields h_i reorient by thermally activated transitions and
decrease as r^{-mu} with the distance from the spin probe. Based on analytical
calculations and scaling arguments we find that the polarization decay of a
spin ensemble exhibits a rich behavior characterized by stretched
exponenentials and power laws with exponents depending on mu and the dimension
d. The anomalous relaxation laws result from heavy tailed local field
distributions and are verified by computer simulations. Implications for
experiments are pointed out.
