Carrier concentration dependence of the spin pseudo-gap behaviors in YBa2Cu3Oy

Oxygen content (y) dependence of the NMR relaxation rates (1/T-1, 1/T-2G) of the Cu(2) site of YBa2Cu3Oy has been studied to investigate magnetic and spin pseudo-gap phase diagrams of this system. The characteristic spin gap temperature (T-sg) defined by the peak position of (1/T1T) has a plateau as a function of oxygen content (y), at y similar to 6.6, so-called ''60 K'' phase where T-c also has a plateau. This plateau means that T-sg may be a monotonic function of hole concentration. On the other hand, it has been confirmed from the Curie-Weiss T-dependence of T-2G(2) well above T-sg that the ''Weiss Temperature (theta)'' for chi(Q) changes its sign from negative to positive at y similar to 6.85. This means that, unless the spin gap and the superconductivity occurred, the real ground state would change from the weak antiferromagnetic metallic state to the nearly antiferromagnetic One at y similar to 6.85, which is far from the real critical point, y = 6.42.