Ni-substituted sites and the effect on Cu electron spin dynamics of YBa₂Cu_3-xNi_xO_7-δ

We report a Cu nuclear quadrupole resonance experiment on magnetic impurity Ni-substituted YBa₂Cu_3-xNi_xO_7-δ. The distribution of Ni-substituted sites and its effect on the Cu electron spin dynamics are investigated. Two samples with the same Ni concentration x = 0.10 and nearly the same oxygen content but different T_c's were prepared: One is an as-synthesized sample (7-δ=6.93) in an oxygen gas (T_c≈80 K), and the other is a quenched one (7-δ= 6.92) in a reduced oxygen atmosphere (T_c≈70 K). The plane-site ⁶³Cu(2) nuclear spin-lattice relaxation for the quenched sample was faster than that for the as-synthesized sample, in contrast to the ⁶³Cu(1) relaxation that was faster for the as-synthesized sample. This indicates that the density of plane-site Ni(2) is higher in the quenched sample, contrary to the chain-site Ni(1) density, which is lower in the quenched sample. From the analysis in terms of the Ni-induced nuclear spin-lattice relaxation, we suggest that the primary origin of suppression of T_c is associated with the nonmagnetic depairing effect of the plane-site Ni(2).