Second low-temperature phase transition in frustrated UNi_4B

Abstract

Hexagonal UNi_4B is magnetically frustrated, yet it orders antiferromagnetically at T_N = 20 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 \UNib between 100 mK and 2 K, and in applied fields up to 9 T. For zero field there is a sharp kink in C at T* ≈ 330 mk, which we interpret as an indication of a second phase transition involving paramagnetic U. The rise in γ = C/T between 7 K and 330 mK and the absence of a large entropy liberated at T* may be due to a combination of Kondo screening effects and frustration that strongly modifies the low T transition. © 1999 The American Physical Society.