NMR-STUDIES OF THE ELECTRONIC AND MAGNETIC-PROPERTIES OF THE 123 COMPOUNDS

A good knowledge of the magnetic properties of cuprates, from their insulating AF state to their metallic state, is required prior to a clarification of the origin of high temperature superconductivity. NMR techniques have proven to be quite successful in studying the magnetic properties on a local scale. The Y-89 NMR, which can be detected even in the AF and paramagnetic phases of YBa2Cu3O6+x, allows one to study the influence of oxygen content or impurity substitutions on the 3D AF ordering. The associated changes of spin correlation length and interlayer exchange couplings will be discussed. In the metallic state of YBa2Cu3O6+x, NMR shift data taken on the various nuclear sites, Y-89, O-17, and Cu-63 have allowed one to conclude that the covalency between Cu(3d(x2 - y2)) and O(2p-sigma) holes result in a spin system which behaves as a single spin fluid. The large decrease of the susceptibility of the CuO2 planes at low T for oxygen depleted samples, as well as the different T dependencies of the Y-89, O-17, and Cu-63 relaxation rates, is attributed to the existence of AF correlations at the wave vector Q of the AF structure of YBa2Cu3O6. However, the Y-89 and O-17 relaxation rates, which filter the Q component of the dynamic susceptibility, exhibit a behavior similar to that of a Fermi liquid, for large enough hole doping. The T variations of the spin susceptibility and of the AF correlations are discussed.