Muon spin rotation and magnetic order in the heavy-fermion compoundURu2Si2

Muon spin rotation and relaxation experiments have been carried out in the paramagnetic and magnetically ordered states of the magnetic heavy-fermion superconductor ${\mathrm{URu}}_2$ ${\mathrm{Si}}_2$. The positive-muon (${\mu }^{+}$) linewidth ${\sigma }_{\mu }$ in zero and low applied fields (≲100 Oe) was consistent with nuclear dipolar broadening at temperatures above the ordering temperature $T_N$, but increased below $T_N$ to a value of ∼0.1 μ${\mathrm{s}}^{\mathrm{-}1}$. This is definite evidence for magnetic ordering. However, the value of ${\sigma }_{\mu }$ corresponds to a rms local field of only ∼1 Oe, which is more than an order of magnitude smaller than expected from an antiferromagnetic (AF) array of U moments μ∼0.03${\mu }_B$, as obtained from neutron diffraction. Either the ${\mu }^{+}$ site is approximately symmetrically placed between the AF sublattices, or the muon suppresses AF ordering in its vicinity. The isotropic component of the observed ${\mu }^{+}$ frequency shift is small, and the anisotropic shift is of the order of the calculated dipolar contribution for several candidate ${\mu }^{+}$ sites (including the symmetric one).