ZERO AND LONGITUDINAL-FIELD MU-SR IN MONOCRYSTALLINE PRNI5

ZF-muSR measurements between 37 mK and 50 K were performed on a single crystal sample of the orbital singlet ground state system PrNi5. A st rong increase of the relaxation rate was observed below 20 K, with a s aturation behaviour below 1.5 K at a value of sigma(ZF) congruent-to 1 7 mus-1 corresponding to a field spread of approximately 200 G. At lea st 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 a t the Pr-sites or at the Ni-sites (PrN)5 is a well known nuclear cooli ng agent, with no nuclear ordering down to 0.1 mK). The observed sigma (ZF) = 17 mus-1 is an order of magnitude larger than what can be expec ted to be induced by the Pr-141-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 t he mu+, which changes the crystal field splitting of the nearest neigh bor 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 'st atic' features cannot be explained ID a simple manner by this conjectu re.