Magnetic fluctuations in the magnetically ordering heavy-fermion compounds UCu5 and U2Zn/sub 17/

We report on the first direct observation of magnetic fluctuations in magnetically ordering heavy-fermion U systems above T/sub N/. They were measured for UCu5 and U2Zn/sub 17/ via the quasielastic lines by inelastic neutron scattering. Both compounds behave very similarly. We observe a common broad Lorentzian quasielastic line with width of order Γ/sub QE/≅10 meV, virtually unaffected by the magnetic phase transition, but also a narrow Gaussian quasielastic line, which we attribute to critical spin fluctuations. We discuss the occurrence of magnetic order in spite of the fast valence-fluctuation rate as in contrast to similar magnetically ordering 4f systems