Determination of the Universality Class of Gadolinium

We resolve a long-standing puzzle for the static and dynamic critical behavior of gadolinium by a combined theoretical and experimental investigation. It is shown that the spin dynamics of a ferromagnet with hcp lattice structure and a spin-spin interaction given by both exchange and dipole-dipole interaction belongs to a new dynamic universality class, model J*. Comparing results from mode coupling theory with results from three different hyperfine interaction probes we find quantitative agreement.