Lattice dynamics of cubic Laves phase ferromagnets

Abstract

The phonon dispersion curves have been measured in ${\mathrm{YFe}}_{2},{\mathrm{UFe}}_{2},$ and ${\mathrm{CeFe}}_{2}$ Laves phases by inelastic neutron scattering. The phonon dispersion curves and the generalized phonon densities of states are calculated by a Born--von K\'arm\'an model taking into account longitudinal and transverse forces up to the fifth-nearest neighbors. Many differences in the phonon spectrum are found in the itinerant ferromagnet ${\mathrm{UFe}}_{2}$ with respect to the isostructural $\mathrm{RE}{\mathrm{Fe}}_{2}$ compounds $(RE=\mathrm{rare}$ earth). The most surprising effects are the negative transverse forces acting between the first neighbors, a situation not encountered in the isostructural systems, and probably responsible for the magnetoelastic anomalies found in ${\mathrm{UFe}}_{2}.$ The strong reduction of U-U longitudinal forces between the uranium atoms is attributed to the collapse of the U-metallic radius induced by the itineracy of uranium outer electron clouds. Our results are compared with the available phonon data on ${\mathrm{ZrFe}}_{2}$ and the isostructural $M{\mathrm{Al}}_{2}$ compounds $(M=\mathrm{Y},$Ca,Ce,La).