Normal-state magnetic properties of Ni and Zn substituted in YBa ₂ Cu ₃ O 6 + x : Hole-doping dependence

We present SQUID susceptibility data on Zn and Ni substituted YBa 2 Cu 3 O 6 + x . Cross-checks with NMR yield an unprecedented accuracy in the estimate of the magnetic susceptibility associated with the substituents, from the underdoped to the lightly overdoped case. This allows us to determine the Weiss temperature θ for YBCO: its value is very small for all hole dopings n h . Since in conventional metals the Kondo temperature T K < θ, magnetic screening effects would not be expected for T >> θ; in contrast, increasing n h produces a reduction of the small moment induced by Zn 2+ and a nearly constant effective moment for Ni 2+ corresponding to a spin 1/2 rather than to a spin 1.