Hexagonal ${\mathrm{UNi}}_4\mathrm{B}$ is magnetically frustrated, yet it orders antiferromagnetically at $T_N=20\mathrm{}\mathrm{K}$. However, one-third of the U spins remains paramagnetic below this temperature. In order to track these spins to lower temperature, we measured the specific heat $C$ of ${\mathrm{UNi}}_4\mathrm{B}$ between 100 mK and 2 K, in applied fields up to 9 T. A sharp kink in $C$ at $T^{*}\approx 330\mathrm{mK}$ is observed at zero field, which we interpret as an indication of a second phase transition involving paramagnetic U. We attribute the rise in $\gamma =C/T$ between 7 K and 330 mK and the absence of a large entropy liberated at $T^{*}$ to a combination of Kondo screening effects and frustration that strongly modifies the low $T$ transition.

• Received 5 February 1999

DOI:https://doi.org/10.1103/PhysRevLett.83.2065