Iron spin waves in YFe2 and UFe2

We report the results of neutron inelastic scattering measurements on single crystals of YFe2 and UFe2 with special emphasis on the energy region above similar to 7 meV, which was not reported in our earlier work on UFe2. Iron spin waves have been observed up to 55 meV in YFe2, but in UFe2 such modes become broad at energies above similar to 35 meV. The most surprising aspects in UFe2 are the absence of any observable mode involving the U 5f electrons, and the strong Fe-Fe interactions as judged by the dispersion of the Fe only mode. Since the spectrum in YFe2 can be considered as representing "diluted" Fe in the cubic Laves phase structure, it is particularly useful to compare the results for UFe2 and YFe2. In addition to the increased Fe-Fe interaction (resulting in a larger spin-wave stiffness;than found in pure Fe), there is evidence that the Fe spin wave in UFe2 interacts with a Lambda(1) optic phonon mode in the [111] direction. This phonon represents a breathing between the U and Fe sublattices, and we suggest that it is mediated by the hybridization between the U 5f and Fe 3d electrons. [S0163-1829(99)00510-X].