Ultrasmall-moment static magnetism inCeRu2Si2

Muon-spin-rotation experiments have been carried out in single crystals of the heavy-fermion compound ${\mathrm{CeRu}}_2$ ${\mathrm{Si}}_2$. The muon-stopping site has been determined from an analysis of the muon Knight shift. The zero-field relaxation rate of the muon polarization exhibits a small but significant increase below ∼2 K which can be suppressed in longitudinal-field experiments. This result provides definite evidence for the occurrence of static electronic magnetism involving ultrasmall moments of the order of ${10}^{\mathrm{-}3}$ ${\mathrm{\mu }}_{\mathit{B}}$.