Magnetism Of Pd1-Xnix Alloys Near The Critical Concentration For Ferromagnetism

We report results of a muon spin rotation and relaxation (mu SR) study of dilute Pd1-xNix alloys, with emphasis on Ni concentrations x = 0.0243 and 0.025. These are close to the critical value x(cr) for the onset of ferromagnetic long-range order (LRO), which is a candidate for a quantum critical point. Additional control data were taken for pure nonmagnetic Pd, and for an alloy where ferromagnetism is well established (x = 0.05). The 2.43 and 2.5 at.% Ni alloys exhibit similar mu SR properties. Both samples are fully magnetic, with average zero-temperature muon local fields < B-loc(T = 0)> = 2.0 and 3.8 mT and Curie temperatures T-C = 1.0 and 2.03 K for 2.43 and 2.5 at.% Ni, respectively. The temperature dependence of < B-loc > suggests ordering of Ni spin clusters rather than isolated spins. Just above T-C, the temperature where LRO vanishes, a two-phase region is found with coexisting separate volume fractions of quasistatic short-range order (SRO) and paramagnetism. The SRO fraction decreases to zero with increasing temperature a few kelvin above T-C. This mixture of SRO and paramagnetism is consistent with the notion of an inhomogeneous alloy with Ni clustering. The measured values of T-C extrapolate to x(cr) = 0.0236 +/- 0.0027. The dynamic muon spin relaxation in the vicinity of T-C differs for the two samples: a relaxation-rate maximum at T-C is observed for x = 0.0243, reminiscent of critical slowing down, whereas for x = 0.025 no dynamic relaxation is observed within the mu SR time window. The data suggest a mean-field-like transition in this alloy.