Onset of antiferromagnetism in UPt₃ 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 UPt₃ as induced by Th substitution. Zero-field experiments have been carried out on a series of polycrystalline U_1-xTh_xPt₃ (0<~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^-8 s) is not detected. For x=0.005 a weak magnetic signal appears below T=2 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 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.