Zero Sound Anomaly in a Ferromagnetic Invar Alloy Fe65Ni35

The phonon dispersion of a single crystal of ferromagnetic Invar alloy, Fe 65 Ni 35 has been measured along the high symmetry directions at several temperatures in the small momentum region, q ≤0.35 Å -1 , by the inelastic neutron scattering. Marked softening of the [110] T 1 and [111] T acoustic modes is found below the Curie temperature, T c =499±2 K. On the contrary, temperature dependent character of the longitudinal modes is quite normal such that their elastic constants increase upon cooling. These results present a striking contrast to the ultrasonic results, which show the anomalous depression for [110] L mode near T c . Zero sound anomaly of the transverse mode in the Fe 65 Ni 35 alloy is well interpreted by the coupling of the transverse phonons and the electrons of the 3d band. The specific relation between the elastic softening and the Invar properties is discussed.