INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER J. Phys.: Condens. Matter 16 (2004) S829–S834 PII: S0953-8984(04)74229-0 Dilution in volborthite S =1/2 frustrated magnet: a μSR and NMR study F Bert 1 , D Bono 1 , P Mendels 1 , J-C Trombe 2 , P Millet 2 , A Amato 3 , C Baines 3 and A Hillier 4 1 Laboratoire de Physique des Solides, Universit´ e Paris-Sud, Bˆ atiment 510, 91405 Orsay, France 2 Centre d’ ´ Elaboration des Mat´ eriaux et d’ ´ Etudes Structurales, 31055 Toulouse Cedex 4, France 3 Paul Scherrer Institut, Laboratory for Muon Spin Spectroscopy, CH-5232 Villigen PSI, Switzerland 4 ISIS Facility, CLRC Rutherford Appleton Laboratory Chilton, Didcot, Oxon OX11 0QX, UK Received 7 January 2004 Published 4 March 2004 Online at stacks.iop.org/JPhysCM/16/S829 (DOI: 10.1088/0953-8984/16/11/037) Abstract Volborthite, Cu 3 V 2 O 7 (OH) 2 ·2H 2 O, is a natural frustrated antiferromagnet (θ CW ≃ 130 K) with S = 1/2 spins (Cu 2+ ) sitting at the vertices of a Kagom´ e- like lattice built on isosceles triangles. We report on the static (SQUID, 51 V NMR) magnetic properties of the pure and 5% Zn/Cu substituted compounds and on an extensive μSR 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 Kagom´ e 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. 1. Introduction Over the last decade, a great effort was made in material science to develop a new compound meeting the theoretical requirements for the existence of a spin liquid ground state. Two main features have been put forward for destabilizing any magnetic ordering: first, a depleted triangular (Kagom´ e) or tetrahedral (pyrochlore) lattice to increase dramatically the ground state degeneracy and, second, S = 1/2 Heisenberg spins to enhance the quantum fluctuations. The recent rediscovery of volborthite [1], a natural frustrated antiferromagnet, could be an important step forward in this quest. The magnetic sublattice of volborthite Cu 3 V 2 O 7 (OH) 2 ·2H 2 O consists of S = 1/2 (Cu 2+ ) spins sitting at the vertices of well-separated (c = 7.2 Å) Kagom´ e-like planes. In contrast to the ideal Kagom´ e case, the lattice is rather built on isosceles triangles, which probably leads to a dissymmetry in the Cu–Cu super-exchange constants J 1 = J 2 . However, a former 0953-8984/04/110829+06$30.00 © 2004 IOP Publishing Ltd Printed in the UK S829