Ferromagnetic Kondo behavior inUAuBi2single crystals

Abstract

We combine magnetization, pressure-dependent electrical resistivity, and heat capacity measurements to investigate the physical properties of the novel compound ${\mathrm{UAuBi}}_{2}$. Our single crystals, grown by the self-flux method, share the same tetragonal ${\mathrm{HfCuSi}}_{2}$-type structure as their Ce-based counterparts. ${\mathrm{UAuBi}}_{2}$ shows ferromagnetic ordering at ${T}_{c}=22.5$ K, in contrast with the antiferromagnetic transition found in ${\mathrm{CeAuBi}}_{2}$ $({T}_{N}=12\text{K})$ but closely related to ${\mathrm{UAuSb}}_{2}$ $({T}_{c}=31\text{K})$. Despite the differences, all compounds display an easy axis of magnetization along the $c$ axis and a large magnetocrystalline anisotropy. The heat capacity and pressure-dependent resistivity suggest that ${\mathrm{UAuBi}}_{2}$ exhibits moderately heavy-fermion behavior $(\ensuremath{\gamma}\ensuremath{\sim}100\phantom{\rule{0.28em}{0ex}}\mathrm{mJ}/\mathrm{mol}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{K}}^{2})$ with strongly localized $5f$ electrons. An intricate competition between crystalline electric field (CEF) effects and two anisotropic exchange interactions $({J}_{\mathrm{RKKY}})$ persists in the $5f$ system, which leads to the striking difference between ground states. A systematic analysis of our macroscopic data using a mean-field model including anisotropic ${J}_{\mathrm{RKKY}}$ interactions and the tetragonal CEF Hamiltonian allows us to extract the CEF scheme and the values of ${J}_{\mathrm{RKKY}}$. Our results suggest a general trend in this family of compounds and shed light on the similarities and differences between $4f$ and $5f$ members.