Zn-doped $La_2CuO_4$ from $^{139}La$ NQR relaxation

Özet

$^{139}La$ NQR spectra and relaxation rates W are used to study the effects on static magnetic properties (inertial hyperfine field, sublattice magnetization and Neel temperatures $T_N$) and the spin wave excitations in antiferromagnetic upon substitution of $Zn^{+2}$ (S = 0) for $Cu^{+2}$ (S = —1/2). The measurements have been carried out in the temperature range 1.6K — 350K for Zn doping x up to 0.13. It is found that $T_N$ decreases with a; in a way close to the one expected for quasi 2D Heisenberg magnets on square lattice while the sublattice magnetization is only slightly affected by Zn doping. The temperature dependence of the AF field |h| is close to one in pure La2CuO4 up to x = 0.08 with a sudden decrease which can be described in terms of a small critical exponent 0, while for strong doping a departure from the one in pure La2CuO4 is observed. The relaxation rates W are affected by Zn doping in two ways. For $100K\leq T\leq TN $(and $x\leq 0.09$) W decrease rapidly with decreasing T, as expected for Raman two magnon relaxation rates; W at a given T slightly increases with x, possibly as a result of the modifications in the spin wave velocity. A second effect is the appearance, for $T\leq 100K$, of large and marked maxima in the temperature behaviour of W, with x dpendent $T_{max}$ and $W_{max}$.