Inelastic Neutron Scattering from Itinerant Electron Antiferromagnets: Theory

Inelastic neutron scattering cross section for a simple model of itinerant electron antiferromagnets is calculated in both the ordered and the disordered temperature regions. The depairing effect due to impurity and phonon scattering is taken into account in terms of a phenomenological electron level width. It is found that at low temperatures, where the electron life-time is long, the neutrons interact with spin waves as described by earlier theories. As the temperature is raised, the spin-wave lines broaden continuously with very slight change of velocity. Near, but below the Néel temperature, the energy gap vanishes, and the spin-wave lines become ill-defined peaks having a flat top centered at the magnetic reciprocal-lattice point. Finally, above the Néel temperature, the excitations are the well-known paramagnons. The Stoner type of mode is found to be extremely broadened even at low temperatures ($T\cong 0.5T_N$), so that they are not observable by means of neutrons. The electron mass enhancement due to magnon and paramagnon interactions is also discussed.