Low-temperature magnetization study ofU6X (X=Mn,Fe,Co,Ni) compounds

Magnetization and susceptibility measurements have been performed on ${\mathrm{U}}_6$Fe and ${\mathrm{U}}_6$Co at temperatures 4 K≤T≤290 K and applied magnetic fields ${10}^{\mathrm{-}2}$ T≤H≤5 T. Low-temperature magnetization measurements at H${=10\mathrm{}}^{\mathrm{-}2}$ T were also made on ${\mathrm{U}}_6$Mn, ${\mathrm{U}}_6$Ni, and ${\mathrm{U}}_6$ ${\mathrm{Mn}}_{0.5}$ ${\mathrm{Co}}_{0.5}$. All of these materials have strongly paramagnetic, weakly-temperature-dependent susceptibilities at T≤290 K. Low-field magnetization and metallography results provide information concerning the nature of small concentrations of ferromagnetic impurity phases found in all of the samples studied. An analysis of the data with use of paramagnon models implies that all of the ${\mathrm{U}}_6$X compounds are strongly-exchange-enhanced paramagnets. Recent Fermi-liquid models are discussed in relation to predictions of p-wave superconductivity, exchange-enhanced paramagnetism, and electron localization in heavy-fermion systems. An empirical correlation is examined between the ratio of the total magnetic susceptibility to the electronic specific heat and the occurrence of either superconductivity or ferromagnetism for a broad class of metallic solids. The correlation suggests that the ${\mathrm{U}}_6$X compounds are near the boundary between superconducting and ferromagnetic ground states.