CRITICAL-DYNAMICS OF MAGNETS

We review our current understanding of the critical dynamics of magnet s above and below the transition temperature with focus on the effects due to the dipole-dipole interaction present in all real magnets. Sig nificant progress in our understanding of real ferromagnets in the vic inity of the critical point has been made in the last decade through i mproved experimental techniques and theoretical advances in taking int o 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 of the theory with experimental result s obtained by different measuring techniques, such as neutron scatteri ng, hyperfine interaction, muon spin resonance, electron spin resonanc e, and magnetic relaxation, in various materials. Furthermore we discu ss the effects of dipolar interaction on the critical dynamics of thre e-dimensional isotropic antiferromagnets and uniaxial ferromagnets. Sp ecial attention is also paid to a discussion of the consequences of di polar anisotropies on the existence of magnetic order and the spin-wav e spectrum in two-dimensional ferromagnets and antiferromagnets. We cl ose our review with a formulation of critical dynamics in terms of non linear Langevin equations.