Weak-magnetism phenomena in heavy-fermion superconductors:: selected μSR studies

The behaviour of the so-called weak-moment antiferromagnetic states, observed in the heavy-fermion superconductors UPt3 and URu2Si2, is discussed in view of recent muSR results obtained as functions of control parameters such as chemical substitution and external pressure. In UPt3, the Pd substitution for Pt reveals the dynamical character of the weak-moment order. On the other hand, muSR measurements performed on samples in which Th substitutes U suggest that crystallographic disorder on the magnetic sites deeply affects the fluctuation timescale. In URu2Si2, 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 muSR, it is deduced that the respective order parameters have different symmetries.