Impurities and defects as probes of the original magnetic properties of the cuprates

Substitutional impurities in the CuO2 planes of the cuprates allow us to pr obe the electronic properties of the host material. The pseudo;gap in the u nderdoped regime is unmodified far from the impurities even though T-c is g reatly reduced. In the overdoped regime, it is not detected above T-c and t he data in presence of impurities seems to imply that the pseudo-gap and T- c lines cross each other. The spin polarisation induced by magnetic impurit ies has an oscillatory staggered behaviour reflecting the existing AF corre lations between the Cu spins. Its influence on the NMR spectra opens a way to determine the q dependence of the static spin susceptibility and the T d ependence of the AF correlation length. NMR measurements demonstrate that n on-magnetic impurities such as Zn or Li induce a local moment behaviour whi ch results from staggered magnetism on the near neighbour copper sites. Li NMR shift data allow to measure accurately the susceptibility associated wi th these local moments. It is found to display a Kondo like (T + Theta)(-1) dependence, with a Kondo Theta which increases abruptly near optimal dopin g. The magnetism revealed by spin-less sites can be understood on theoretical grounds in the case of undoped quantum spin systems. The present results re flect the influence of oxygen hole carriers on the AF correlations between hole spins in the cuprates. Another manifestation of this coupling is the l arge scattering of the carriers by spin-less impurities detected in transpo rt properties. Resistivity measurements on single crystals with point defec ts created by electron irradiation allow to demonstrate that the residual r esistivity and the reduction of T-c due to these defects are in agreement w ith scattering in a d-wave superconductor, and that the concentration of ca rriers remains the concentration n(h) of doped holes even in the overdoped regime. The large scattering due to spin-less defects should be related to the Kondo like resonance peak detected with local magnetic probes.