Zero and longitudinal field μSR in monocrystalline PrNi5

ZF-μSR measurements between 37 mK and 50 K were performed on a single crystal sample of the orbital singlet ground state system PrNi5. A strong increase of the relaxation rate was observed below 20 K, with a saturation behaviour below 1.5 K at a value of σZF ≅ 17 μs-1 corresponding to a field spread of ∼ 200 G. At least below 2 K the relaxation is well described by a gaussian function, and some 'static' nature of the field spread is confirmed by decoupling in longitudinal fields up to 0.1 T. The results are rather surprising since no static electronic moments are expected to be formed either at the Pr-sites or at the Ni-sites (PrNi5 is a well known nuclear cooling agent, with no nuclear ordering down to 0.1 mK). The observed σZF = 17 μs-1 is an order of magnitude larger than what can be expected to be induced by the 141Pr-nuclear moments, even when taking into account the large enhancement due to core polarization. Therefore we speculate that the observed phenomenon is induced by the presence of the μ+, which changes the crystal field splitting of the nearest neighbor Pr3+-ions in such a way as to allow the formation of a non-singlet or near degenerate ground state level. However, the appearance of 'static' features cannot be explained in a simple manner by this conjecture. (orig.)