Evidence for impurity-induced frustration in La2CuO4

Zero-field muon spin rotation and magnetization measurements were performed in La${}_{2}$Cu${}_{1\ensuremath{-}x}{M}_{x}$O${}_{4}$, for $0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.12$, where Cu${}^{2+}$ is replaced either by $M={\text{Zn}}^{2+}$ or by $M={\text{Mg}}^{2+}$ spinless impurity. It is shown that while the doping dependence of the sublattice magnetization [$M(x)$] is nearly the same for both compounds, the N\'eel temperature [${T}_{N}(x)$] decreases unambiguously more rapidly in the Zn-doped compound. This difference, not taken into account within a simple dilution model, is associated with the frustration induced by the Zn${}^{2+}$ impurity onto the Cu${}^{2+}$ antiferromagnetic lattice. In fact, from ${T}_{N}(x)$ and $M(x)$ the spin stiffness is derived and found to be reduced by Zn doping more significantly than expected within a dilution model. The effect of the structural modifications induced by doping on the exchange coupling is also discussed.