Dilution in volborthite S=1/2 frustrated magnet:: a μSR and NMR study

Volborthite, Cu3V2O7(OH)(2)(.)2H(2)O, is a natural frustrated antiferromagnet (theta(CW) similar or equal to 130 K) with S = 1/2 spins (Cu2+) sitting at the vertices of a Kagome-like lattice built on isosceles triangles. We report on the static (SQUID, V-51 NMR) magnetic properties of the pure and 5% Zn/Cu substituted compounds and on an extensive muSR study of the dilution effect (up to 15% Zn substitutions) on the spin dynamics. Although volborthite shares most of the unusual features already exhibited in Kagome bilayer compounds, namely a dynamical state as T --> 0 and a low temperature maximum in the local susceptibility, we found some surprising specificities. The T --> 0 dynamical state is less robust against dilution and the low temperature local susceptibility studied by means of NMR also strongly depends on dilution. Such a sensitivity to dilution questions the role of the asymmetry of the exchange constants.