Weak magnetism phenomena in heavy-fermion superconductors: selected $\mu$SR studies

TL;DR

This paper reviews $$SR studies on heavy-fermion superconductors UPt$_3$ and URu$_2$Si$_2$, focusing on weak magnetic moments, their dynamics, and phase separation under varying conditions.

Contribution

It provides new insights into the dynamical nature of weak magnetic moments and phase separation in heavy-fermion superconductors based on recent $$SR experimental results.

Findings

Pd substitution reveals dynamical weak moment order in UPt$_3$

Crystallographic disorder affects fluctuation timescales in UPt$_3$

Phase separation between hidden order and antiferromagnetic state in URu$_2$Si$_2$

Abstract

The behavior of the so-called weak moment antiferromagnetic states, observed in the heavy-fermion superconductors UPt$_3$ and URu$_2$Si$_2$, is discussed in view of recent $\mu$SR results obtained as function of control parameters like chemical substitution and external pressure. In UPt$_3$, the Pd substitution for Pt reveals the dynamical character of the weak moment order. On the other hand, $\mu$SR measurements performed on samples in which Th substitutes U suggest that crystallographic disorder on the magnetic sites deeply affects the fluctuation timescale. In URu$_2$Si$_2$, a phase separation between the so-called hidden order state, present at ambient pressure, and an antiferromagnetic state, occurring under pressure, is observed. In view of the pressure-temperature phase diagram obtained by $\mu$SR, it is deduced that the respective order parameters have different symmetries.