Inelastic neutron scattering experiments on an ordered Invar alloy ${\mathrm{Fe}}_3$Pt show large softening of the [100]TA and [110]T${\mathrm{A}}_1$ acoustic shear modes below the Curie temperature. In addition, a central peak is observed below 100 K, which, however, behaves noncritically. The phonon softening is discussed in terms of the magnetoelastic coupling caused by the electron-phonon interaction.

• Received 28 October 1975

DOI:https://doi.org/10.1103/PhysRevLett.37.519