${}_{}{}^{139}{}_{}{}^{}\mathrm{La}$ NQR and zero-field ${\mathrm{\mu }}^{+}$SR in antiferromagnetic (AF) ${\mathrm{La}}_2$${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$${\mathrm{O}}_4$, for x up to 0.13 and in the temperature range 1.6–350 K, are used to study the effects related to the substitution of magnetic ${\mathrm{Cu}}^{2+}$ S=1/2 with homovalent diamagnetic S=0 ${\mathrm{Zn}}^{2+}$ in ${\mathrm{La}}_2$${\mathrm{CuO}}_4$. We report measurements both of static magnetic properties, such as Néel temperatures ${\mathit{T}}_{\mathit{N}}$, sublattice magnetization and field ‖h‖ at the La nucleus or at the ${\mathrm{\mu }}^{+}$ site, as well as of NQR relaxation rates W. These quantities are used to study the effects of Zn doping on the low-energy ${\mathrm{Cu}}^{2+}$ spin excitations. It is found that ${\mathit{T}}_{\mathit{N}}$ decreases with x in a way close to the one expected by diluting quasi-two-dimensional Heisenberg magnets on square lattice, while the sublattice magnetization is slightly affected 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≃0.08 the temperature dependence of the AF field ‖h‖ is close to the one in pure ${\mathrm{La}}_2$${\mathrm{CuO}}_4$, with a sharp decrease for T→${\mathit{T}}_{\mathit{N}}^{\mathrm{-}}$ indicative of a continuous transition with a small critical exponent β.

For strong doping the low-temperature dependence of ‖h‖ appears to depart from the one in pure ${\mathrm{La}}_2$${\mathrm{CuO}}_4$. For x≥0.05 both NQR spectra and μSR reveal the presence of regions where the long-range AF order is suppressed. For temperature above 100 K up to ${\mathit{T}}_{\mathit{N}}$ the ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}$ relaxation rate W due to the ${\mathrm{Cu}}^{2+}$ spin fluctuations shows only slight corrections with respect to pure ${\mathrm{La}}_2$${\mathrm{CuO}}_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≤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 Cu orbitals, interacting via the underlying AF matrix. The maxima in W occur when the fluctuation frequencies of the anomalous spins become of the order of the NQR frequency, thus driving the system to a spin-glass state superimposed to the AF matrix.

• Received 13 February 1995

DOI:https://doi.org/10.1103/PhysRevB.52.4226