Determination of the wavevector and temperature dependence of the `forbidden' mode in Invar using inelastic neutron scattering

In previous polarized neutron inelastic scattering experiments on Fe - Ni alloys [1] a `forbidden' mode was found at the Invar composition. This mode has the same dispersion as the transverse acoustic phonon, but was observed in longitudinal scans in which the momentum transfer was along the ( ) direction. The origin of this mode, which seems to be confined to the Invar composition, is not yet understood. This paper reports the results of an inelastic neutron scattering experiment using high-energy incident neutrons in which the mode has been found, and the wavevector dependence of its energy measured, in the first Brillouin zone. The intensity of the mode is greater in the first zone than in that centred on (200) and since the phonon cross section is minimized at small wavevectors it was concluded that the mechanism giving rise to the excitation is predominantly magnetic in origin. This conclusion receives support from the decrease in intensity of the mode with increasing temperature. On the basis of these results it is postulated that the `forbidden' mode arises from a dynamic modulation of the magnetic moments associated with the compressive strain in the phonon excitations rather than from magnetovibrational scattering. These results together with the earlier polarized neutron measurements, indicating a frequency-dependent moment, are able to account for the anomalous thermal variation of the magnetization.