LA-139 NQR RELAXATION AND MU-SR STUDY OF ZN-DOPING EFFECTS IN LA2CUO4

La-139 NQR and zero-field mu(+)SR in antiferromagnetic (AF) La2Cu1-xZn xO4, for x up to 0.13 and in the temperature range 1.6-350 K, are used to study the effects related to the substitution of magnetic Cu2+ S = 1/2 with homovalent diamagnetic S = 0 Zn2+ in La2CuO4. We report meas urements both of static magnetic properties, such as Neel temperatures T-N, sublattice magnetization and field \h\ at the La nucleus or at t he mu(+) site, as well as of NQR relaxation rates W. These quantities are used to study the effects of Zn doping on the low-energy Cu2+ spin excitations. It is found that TN decreases with x in a way close to t he one expected by diluting quasi-two-dimensional Heisenberg magnets o n square lattice, while the sublattice magnetization is slightly affec ted by Zn doping. Mean-field arguments based on the dilution model for the interplanar interactions allow one to conclude that the in-plane magnetic correlation length is little sensitive to the Zn presence. Up to x similar or equal to 0.08 the temperature dependence of the AF fi eld \h\ is close to the one in pure La2CuO4, with a sharp decrease for T --> T-N(-) indicative of a continuous transition with a small criti cal exponent beta. For strong doping the low-temperature dependence of \h\ appears to depart from the one in pure La2CuO4. For x greater tha n or equal to 0.05 both NQR spectra and mu SR reveal the presence of r egions where the long-range AF order is suppressed. For temperature ab ove 100 K up to T-N the La-139 relaxation rate W due to the Cu2+ spin fluctuations shows only slight corrections with respect to pure La2CuO 4 which are possibly related to the disorder in the AF interactions or to finite-size effects. A novel and remarkable effect of Zn doping is the appearance in W, for T less than or equal to 100 K, of large and marked maxima, which are x dependent. This phenomenon is attributed to the cooperative freezing of local magnetic moments induced by Zn on C u orbitals, interacting via the underlying AF matrix. The maxima in W occur when the fluctuation frequencies of the anomalous spins become o f the order of the NQR frequency, thus driving the system to a spin-gl ass state superimposed to the AF matrix.