Critical dynamics of magnets

We review our current understanding of the critical dynamics of
magnets above and below the transition temperature with focus on
the effects due to the dipole-dipole interaction present in all
real magnets. Significant progress in our understanding of real
ferromagnets in the vicinity of the critical point has been made in
the last decade through improved experimental techniques and
theoretical advances in taking into account realistic spin-spin
interactions. We start our review with a discussion of the
theoretical results for the critical dynamics based on recent
renormalization group, mode coupling and spin-wave theories. A
detailed comparison is made ot~ the theory with experimental
results obtained by different measuring techniques, such as neutron
scattering, hyperfine interaction, muon spin resonance, electron
spin resonance, and magnetic relaxation, in various materials.
Furthermore we discuss the effects of dipolar interaction on the
critical dynamics of three-dimensional isotropic antiferromagnets
and uniaxial ferromagnets. Special attention is also paid to a
discussion of the consequences of dipolar anisotropies on the
existence of magnetic order and the spin-wave spectrum in
two-dimensional ferromagnets and antiferromagnets. We close our
review with a formulation of critical dynamics in terms of
nonlinear Langevin equations.