Normal-state magnetic properties of Ni and Zn substituted in YBa2Cu3O6+x: Hole-doping dependence

We present SQUID susceptibility data on Zn and Ni substituted YBa2Cu3O6+x. Cross-checks with NMR yield an unprecedented accuracy in the estimate of th e magnetic susceptibility associated with the substituents, from the underd oped to the lightly overdoped case. This allows us to determine the Weiss t emperature theta for YBCO: its value is very small for all hole dopings n(h ). Since in conventional metals the Kondo temperature T-K < theta, magnetic screening effects would not be expected for T >> theta(i) in contrast, inc reasing n(h) produces a reduction of the small moment induced by Zn2+ and a nearly constant effective moment for Ni2+ corresponding to a spin 1/2 rath er than to a spin 1.