Neutron scattering, muon spin relaxation, and dc susceptibility studies have been carried out on polycrystalline Tb 2 Ti 2 O 7 , a pyrochlore antiferromagnet in which the Tb 31 moments reside on a network of corner-sharing tetrahedra. Unlike other geometrically frustrated systems, Tb 2 Ti 2 O 7 remains paramagnetic down to ϳ0.07 K, rather than ordering into a conventional Néel or spin-glass-like state, despite the fact that short-range antiferromagnetic correlations (AFC) develop at ϳ50 K. At the first AFC wave vector, its low-lying, relatively flat magnetic excitation spectrum softens partially below 30 K.

We compare the effects of in-plane non magnetic Li + and Zn 2+ impurities on the normal state of high-Tc cuprates. 89 Y NMR shows that the extra hole introduced by Li is not localized in its vicinity. The Tc depression and induced moments on near neighbour Cu sites of Zn or Li are found identical. These effects of spinless impurities establish the major influence of the spin perturbation with respect to the charge defect. The susceptibility of the induced moment measured by 7 Li NMR displays a 1/(T+Θ) behavior. Θ increases with doping up to about 200 K in the overdoped regime. We attribute this to a "Kondo like" effect.Increasingly, impurities are used to probe the magnetic properties of correlated systems. For instance, in cuprates, substitution of the Cu sites of the CuO 2 planes directly reveals the existence of magnetic correlations in the planes and probes their interplay with superconductivity. In particular, Zn 2+ substitution has been studied thoroughly because it was unexpectedly found to strongly affect both the normal and superconducting states. Above T c , in a metallic picture, Zn 2+ should only weakly affect both magnetism and transport properties; the former because it is a spinless impurity and the latter because it has the same charge as Cu 2+ . In contrast, Zn acts as a very strong scattering center [1]. The fact that T c is depressed by this scattering is primarily a consequence of the now well established d-wave anisotropy of the superconducting order parameter [2]. Furthermore, Zn induces local magnetic moments on its near neighbor (n.n.) coppers, as shown by NMR [3] and macroscopic SQUID measurements [4]. Zn, as a spin vacancy, creates indeed a perturbation of the local antiferromagnetic correlations, as also observed in undoped low dimensional spin chains or ladders [5]. Such effects were anticipated on theoretical grounds [6] [7]. However, until now no experiment could clearly expose the relation between the magnetic correlations and the scattering effects on T c . Another interesting problem is the evolution of these anomalies with hole doping. Recent macroscopic experiments showed that the local moment susceptibility falls rapidly, though it still exists at optimal doping [4]. Such local moments have also been found in Al 3+ substituted LaSrCuO at optimal doping, despite some qualitative differences with Zn [8]. In this later work, NMR of 27 Al itself was used to probe locally the susceptibility of its n.n. Cu sites. However, no experiment has yet been dedicated to probing the evolution of this moment into the overdoped regime. Such an experiment should help to clarify wether the cuprates exhibit an uncorrelated Fermi Liquid behavior at high doping.In order to address both problems, we have undertaken a study of Li in YBaCuO which substitutes within the CuO 2 planes [9]. Li + is not magnetic like Zn 2+ but has a different valence. Comparing the local magnetism and the effect of T c between Li + and Zn 2+ will elucidate the respective roles of charge and spin in the impurity response of...

has previously been observed in measurements of the nuclear spin-lattice relaxation rate. Both the uncertainties in our analysis and the implications for the mechanism of high-temperature superconductivity are discussed.