¹³⁹La NQR relaxation and μSR study of Zn-doping effects in La₂CuO₄

¹³⁹La NQR and zero-field μ⁺SR in antiferromagnetic (AF) La₂Cu_1-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 measurements both of static magnetic properties, such as Néel temperatures T_N, sublattice magnetization and field ‖h‖ at the La nucleus or at the μ⁺ 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 T_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 La₂CuO₄, with a sharp decrease for T→T_N^- 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 La₂CuO₄. 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 T_N the ¹³⁹La relaxation rate W due to the Cu²⁺ spin fluctuations shows only slight corrections with respect to pure La₂CuO₄ 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.