mu SR-spectroscopy on superconducting Chevrel phase compounds

We report on a systematic study of the London penetration depth lambda(0) i n a series of stoichiometric Chevrel phase compounds XMo6S8 (X = Ag, La, Y, Sc, Yb, Nd, Pr, Tm) in search for a possible correlation of lambda(0) with T-c as has been found before in solid solution of SnMo6S8-xSex and PbMo6S8 -xSex (Birrer et al., Phys. Rev. B 48 (1993) 16589 [1]). In contrast to the solid solution compounds it seems that lambda(0) is more or less the same for all stoichiometric compounds with T-c greater than or equal to 7 K and very much larger for compounds with T-c less than or equal to 7 K. In NdMo6 S8 and PrMo6S8 fast relaxation due to fluctuations of the 4f-electron momen ts masked completely the effect of the flux line lattice on the mu(+)-polar ization in transverse field experiments, but it seems that the onset of sup erconductivity has some minute effect on the 4f-spin dynamics in NdMo6S8. P reliminary TF-studies in TmMo6S8 show the relaxation to be induced by the A ux line lattice and the 4f-spin dynamics. We do not find any evidence for m agnetic ordering of the Nd, Tm, Yb or Pr sublattice at low temperature. For Yb our results clearly show that Yb is in a nonmagnetic state in contrast to some previous reports (K.N. Shrivastava, K.P. Sinha, Phys. Rep. 115 (198 4) 93). (C) 2000 Elsevier Science B.V. All rights reserved.