Onset of antiferromagnetism in ${\mathrm{UPt}}_3$ via Th substitution studied by muon spin spectroscopy

Muon spin spectroscopy has been used to study in detail the onset of large-moment antiferromagnetism (LMAF) in ${\mathrm{UPt}}_3$ as induced by Th substitution. Zero-field experiments have been carried out on a series of polycrystalline ${\mathrm{U}}_{1-x}{\mathrm{Th}}_x{\mathrm{Pt}}_3$ $(0\mathrm{}<~x<~0.05)$ samples in the temperature range 0.04–10 K. At low Th content $(x<~0.002)$ magnetic ordering on the time scale of the μSR experiment ${(10\mathrm{}}^{-8}\mathrm{s})$ is not detected. For $x=0.005\mathrm{}$ a weak magnetic signal appears below $T=2\mathrm{}\mathrm{K},$ while for $0.006<~x<~0.05,$ spontaneous oscillations in the μSR spectra signal the presence of the LMAF phase. The data are well described by a two-component depolarization function, combining the contribution of a polycrystalline antiferromagnet and a Kubo-Lorentzian response. However, the transition into the antiferromagnetic phase is quite broad. For $x=0.01\mathrm{}$ and 0.02, a weak magnetic signal appears below about 7 K, which is well above the mean-field transition temperatures. The broadening may be a result of the effects of disorder on the time fluctuations associated with anomalous small-moment antiferromagnetism.