Zero and longitudinal fieldμSR in monocrystalline PrNi₅

Abstract

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 ofσ _ZF17μs⁻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 observedσ _ZF=17μs ⁻¹ is an order of magnitude larger than what can be expected to be induced by the¹⁴¹Pr-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 theμ ⁺, 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.