Effects of Zn and Ni substitution on the Cu spin dynamics and superconductivity in La_2−xSr_xCu_1−y(Zn,Ni)_yO₄ (x=0.15–0.20): Muon spin relaxation and magnetic susceptibility study

We have investigated effects of Zn and Ni on the Cu spin dynamics and superconductivity from the zero-field muon spin relaxation (ZF-μSR) and magnetic susceptibility χ measurements for La_2−xSr_xCu_1−y(Zn,Ni)_yO₄ with x=0.15–0.20, changing y up to 0.10 in fine step. In the optimally doped x=0.15, it has been concluded that the formation of a magnetic order requires a larger amount of Ni than of Zn, which is similar to our previous results of x=0.13. From the estimation of volume fractions of superconducting (SC) and magnetic regions, it has been found for x=0.15 that the SC region is in rough correspondence to the region where Cu spins fluctuate fast beyond the μSR frequency window for both Zn- and Ni-substituted samples. According to the stripe model, it follows that, even for x=0.15, the dynamical stripe correlations of spins and holes are pinned and localized around Zn and Ni, leading to the formation of the static stripe order and the suppression of superconductivity. These may indicate an importance of the dynamical stripe in the appearance of the high-T_c superconductivity in the hole-doped cuprates. In the overdoped regime of x=0.18 and 0.20, on the other hand, the SC region seems to be in rough correspondence to the region where Cu spins fluctuate fast beyond the μSR frequency window, although it appears that the Cu spin dynamics and superconductivity are affected by the phase separation into SC and normal-state regions.