Evidence for impurity-induced frustration in La2CuO4

Zero-field muon spin rotation and magnetization measurements were performed in La2Cu1-xMxO4, for 0 <= x <= 0.12, where Cu2+ is replaced either byM = Zn2+ or by M = Mg2+ spinless impurity. It is shown that while the doping dependence of the sublattice magnetization [M(x)] is nearly the same for both compounds, the Neel 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 Zn2+ impurity onto the Cu2+ 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.