Spin Configuration in the 1/3 Magnetization Plateau of Azurite Determined by NMR

High magnetic field (63,65)Cu NMR spectra were used to determine the local spin polarization in the 1/3 magnetization plateau of azurite, Cu(3)(CO(3))(2)(OH)(2), which is a model system for the distorted diamond antiferromagnetic spin-1/2 chain. The spin part of the hyperfine field of the Cu2 (dimer) sites is found to be field independent, negative and strongly anisotropic, corresponding to approximate to 10% of fully polarized spin in a d orbital. This is close to the expected configuration of the quantum plateau, where a singlet state is stabilized on the dimer. However, the observed nonzero spin polarization points to some triplet admixture, induced by strong asymmetry of the diamond bonds J(1) and J(3).