Magnetic excitations in metallic ferro- and antiferromagnets

The recent twenty years have witnessed significant progress in the basic understanding of the dynamical magnetic properties in metallic and strongly correlated materials, mainly due to the progress in inelastic neutron scattering (INS) techniques and polarization analysis. One of the major achievements in this discipline is, in fact, the finding that the magnetic scattering in ferromagnetic metals shows a universal behavior independent of the itinerancy of the magnetic moment. The dynamic susceptibility in the ordered state at low temperatures is well described by spin-wave excitation. In addition, the magnetic scattering function in the paramagnetic phase above the Curie temperature, T-C, follows a simple double Lorentzian behavior in both momentum and energy. Then, the energy scale of the spin fluctuations can be expressed in terms of the ratio of the strength of the magnetic correlations defined by the molecular field Curie temperature with respect to T-C. The magnetic fluctuations in incommensurate antiferromagnets, i.e., in metallic Cr or in the high temperature superconductors of doped La2CuO4 compounds, show, on the other hand, a rather complicated dynamical structure factor. In contrast to ferromagnetic metals and localized antiferromagnets, the longitudinal fluctuations seem to play an important role deep in the ordered state of Cr.