Shape functions of dipolar ferromagnets at and above the Curie point

The critical dynamics of dipolar ferromagnets are investigated by a mode-coupling theory. The dynamical scaling functions for the Kubo relaxation functions and the damping coefficients are computed. The shape of the transverse scaling functions shows a crossover at the dipolar wave vector qD, while the characteristic time has its crossover at about a tenth of qD. For wave vectors smaller than qD the critical transverse correlation function decays nearly exponentially with time. On the basis of this theory, we interpret neutron scattering experiments (constant-wave-vector and constant-energy scans) on EuO, EuS, and Fe. The agreement of the theory can be extended beyond the immediate vicinity of the critical point by including van Hove terms in the damping coefficients