Critical dynamics of a uniaxial and dipolar ferromagnet

We study the critical dynamics of three-dimensional ferromagnets with uniax ial anisotropy by taking into account exchange and dipole-dipole interactio n. The dynamic spin-correlation functions and the transport coefficients ar e calculated within a mode coupling theory. It is found that the crossover scenario is determined by the subtle interplay between three length scales: the correlation length, the dipolar, and uniaxial wave vector. We compare our theoretical findings with hyperfine interaction experiments on Gd and f ind quantitative agreement. This analysis allows us to identify the univers ality class for Gd. It also turns out that the mu SR relaxation rate can be best fitted if it is assumed that muons occupy octahedral interstitials si tes within the Gd lattice. [S0163-1829(99)04237-X].