Second Low Temperature Phase Transition in Frustrated UNi_4B

TL;DR

This study investigates a second low-temperature phase transition in frustrated UNi_4B, revealing a new magnetic transition below 330 mK likely influenced by Kondo effects and frustration.

Contribution

It reports the discovery of a second phase transition in UNi_4B at very low temperatures, expanding understanding of its magnetic behavior.

Findings

Identification of a sharp kink in specific heat at 330 mK indicating a second phase transition.

Observation of increased gamma (C/T) between 7 K and 330 mK suggesting Kondo screening.

Absence of large entropy change at the transition point.

Abstract

Hexagonal UNi_4B is magnetically frustrated, yet it orders antiferromagnetically at T_N = 20 K. However, one third of the U-spins remain 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 $T^\ast\approx$ 330 mK, which we interpret as an indication of a second phase transition involving paramagnetic U. The rise in $\gamma = C/T$ between 7 K and 330 mK and the absence of a large entropy liberated at $T^\ast$ may be due to a combination of Kondo screening effects and frustration that strongly modifies the low T transition.