ZN AND NI DOPING EFFECTS ON T-C AND SPIN GAP BEHAVIOR IN YBA2CU4O8 AND YBA2CU3O6.6

The existence of local moments in both Zn- and Ni-doped YBa2Cu4O8 (Y12 48) is confirmed by a Curie-law T-dependence of NMR linewidth and the Cu relaxation at low T. The reduction of Cu-NQR signal intensity in Ni -doped Y1248 suggests that, in contrast to the case of Ni-doped YBa2Cu 3O7 (Y1237), the Cu spin correlation around Ni is destroyed. This effe ct may be responsible for the larger reduction of T-c by Ni-doping tha n in Y1237. For Cu sites which are away from an impurity by two lattic e constants, 1/T1T continues to increase below T, at which it has a m aximum in the non-doped material. The effect in reducing T, which is sometimes called the spin gap temperature, is much more pronounced in Zn-doped samples, suggesting that the reduction of T is mainly due to the induction of magnetic moments in the d(x2-y2) orbital, which may modify Cu spin fluctuation.