Dipolar Interactions in the Critical Dynamics of Heisenberg Ferromagnets

It is shown that in contrast to previous assumptions the real magnetic dipole interaction has to be considered as a possible source of deviations of the dynamical exponent z from 5/2 as observed recently on Fe and Ni. While the dipolar crossover temperatures agree with experiment, at Tc the dynamic crossover around the dipolar wave vector qd as predicted by current theories is absent. It is argued that this effect is related to a hitherto overlooked dynamical decoupling between transverse and noncritical longitudinal fluctuations if q exceeds the inverse correlation range κ.