NMR-STUDY OF THE LOW-TEMPERATURE MAGNETIC EXCITATIONS IN RBA(2)(CU1-YNIY)(3)O-6 (R=Y,TM)

(Cu(1) NQR, Cu(2) Zero Field NMR and Tm NMR spin-lattice relaxation da ta have been found quite different in the two AF states of the 123 com pounds. In the usual AF-I phase of the pure compounds T-1 is short and nearly T-independent at low T. With Ni or Co substitutions in the abo ve by about 2% concentration, the Cu(2) bilayers are ordered ferromagn etically with respect to each other (AF-II phase), which is evidenced by a splitting of the Cull) NQR. In this case, T-1(-1) much smaller an d exhibits a strong T dependence, which might be associated with a 5 K gap in the magnetic excitation spectrum. We suggest that magnetic def ects apparently unavoidable on the Cu(1) sites govern the spin dynamic s. While they are frustrated in the ''pure samples'', they are not in the AF-II phase, which suppresses partly their spin Fluctuations.