Abstract
Oxygen content ( y ) dependence of the NMR relaxation rates (1/ T 1 , 1/ T 2G ) of the Cu(2) site of YBa 2 Cu 3 O y 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/ T 1 T ) has a plateau as a function of oxygen content ( y ), at y ∼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 (Θ)” for χ( Q ) changes its sign from negative to positive at y ∼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 ∼6.85, which is far from the real critical point, y =6.42.
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