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

Abstract

We present SQUID susceptibility data on Zn and Ni substituted YBa₂Cu₃O_{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 Zn²⁺ and a nearly constant effective moment for Ni²⁺ corresponding to a spin 1/2 rather than to a spin 1.