Crossover from divalent to valence fluctuating state of Eu in EuCu2(Ge1-xSix)2 probed by 63,65Cu NMR

(CU)-C-63,65 field sweep NMR investigations on a series of EuCu2(Ge1-x Si-x)(2) powder samples (x = 0.3,0.7, 1) are presented. EuCu2(Ge,Si)(2) is unique among Eu-intermetallics because, upon tuning from a divalent antiferromagnetically ordered state for x = 0 to a valence fluctuating state for x = 1, Heavy Fermion behaviour was found around x approximate to 0.7. The Knight shift (63,65) K is negative for all stoichiometries. Hyperfine fields are largest for the antiferromagnetically ordered Ge-rich compound (x = 0.3) and smallest for the valence fluctuating pure silicon compound (x = 1). For the x = 0.3 compound, above T-N approximate to 19 K K-63(T) exhibits a Curie-Weiss like behaviour (approximate to -10% at 20K) whereas for EuCu2Si2 a temperature-independent shift of K-63(7) approximate to-1.5% is found. Our results are discussed in terms of formation of heavy quasiparticles and/or presence of valence fluctuations of Eu. (c) 2006 Elsevier B.V. All rights reserved.