Theoretical study of magnetic excitations in Ni3Al

Abstract

Inelastic neutron scattering studies of the transition metal alloy Ni3Al have revealed highly unusual spin-wave behavior, that spin waves have been observed only in a small region around the Brillouin zone center (q≂0). Results from calculations of the inelastic neutron scattering cross section based on itinerant electron theory for Ni3Al have led to a relatively simple explanation of this phenomenon. To our knowledge, this is the first calculation of this type for an alloy system. The calculations yield the well-defined Goldstone mode (spin wave) at q=0 but no spin-wave peaks were found for the smallest calculable nonzero momentum transfer, which was just outside the range of q where spin waves were observed experimentally. The reason is simply that the spin wave runs immediately into a region of high density of Stoner excitations (single-particle spin-flip excitations) as q is increased from zero. This system, therefore, represents the extreme limiting case of the itinerant electron theory prediction of spin waves disappearing into the Stoner continuum.