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

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The behavior of the so-called weak moment antiferromagnetic states, observed in the heavy-fermion superconductors UPt₃ and URu₂Si₂, is discussed in view of recent μSR results obtained as function of control parameters like 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, μSR 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 μSR, it is deduced that the respective order parameters have different symmetries.