Neutron scattering and scaling behavior in URu₂ Zn₂₀ and YbFe₂ Zn₂₀

The dynamic susceptibility χ⊃″ (ΔE), measured by inelastic neutron-scattering measurements, shows a broad peak centered at E _max =15meV for the cubic actinide compound URu₂ Zn ₂₀ and 7 meV at zone center and at the (1/2, 1/2, 1/2) zone boundary for the rare-earth counterpart compound YbFe₂ Zn₂₀. For URu₂ Zn₂₀, the low-temperature susceptibility and magnetic specific-heat coefficient γ= C_mag /T take the values χ=0.011emu/mole and γ=190mJ/mole K2 at T=2K. These values are roughly three times smaller, and E_max is three times larger, than recently reported for the related compound UCo₂ Zn₂₀, so that χ and γ scale inversely with the characteristic energy for spin fluctuations, T_sf = E_max / k_B. While χ (T), C_mag (T), and E_max of the 4f compound YbFe₂ Zn₂₀ are very well described by the Kondo impurity model, we show that the model works poorly for URu₂ Zn₂₀ and UCo ₂ Zn₂₀, suggesting that the scaling behavior of the actinide compounds arises from spin fluctuations of itinerant 5f electrons.