LATTICE-DYNAMICS OF CUBIC LAVES PHASE FERROMAGNETS

The phonon dispersion curves have been measured in YFe2, UFe2, and CeF e2, Laves phases by inelastic neutron scattering. The phonon dispersio n curves and the generalized phonon densities of states are calculated by a Born-von Karman model taking into account longitudinal and trans verse forces up to the fifth-nearest neighbors. Many differences in th e phonon spectrum are found in the itinerant ferromagnet UFe2 with res pect to the isostructural REFe2 compounds (RE = rare earth). The most surprising effects are the negative transverse forces acting between t he first neighbors, a situation not encountered in the isostructural s ystems, and probably responsible for the magnetoelastic anomalies foun d in UFe2. 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 ZrFe2 and the isostruc tural MAl2 compounds (M=Y,Ca,Ce,La). [S0163-1829(98)06342-5].