Band theoretic interpretation of neutron scattering experiments in metallic ferromagnets

Magnetic excitations in the 3-d transition metal ferromagnets nickel and iron have been found to possess unusual properties not found in other magnetic systems. For example, inelastic neutron scattering experiments have revealed that spin-waves do not exist at wave-vectors above a particular cut-off value while below this cut-off point they have been observed well above the Curie temperature. This behavior is not consistent with predictions of localized spin models but can be explained by the band theoretic, or itinerant electron theory. In fact, first principles calculations of the low temperature neutron scattering cross-section based on the itinerant model have been shown to be in excellent agreement with experiment. In addition, the prediction of an optical spin-wave mode has been recently confirmed. The finite temperature extrapolation of the low temperature theory based on the traditional concept of a temperature-dependent spin-splitting of the electronic energy bands appears to be inconsistent with experiment. A more realistic first principles approach to develop a correct finite temperature theory is under investigation