NMR study of the low temperature magnetic excitations in RBa2(Cu1 - YNiy)3O6 (R = Y, Tm)

Abstract

Cu(1) NQR, Cu(2) Zero Field NMR and Tm NMR spin-lattice relaxation data have been found quite different in the two AF states of the 123 compounds. In the usual AF-I phase of the pure compounds T1 is short and nearly T-independent at low T. With Ni or Co substitutions in the above by about 2% concentration, the Cu(2) bilayers are ordered ferromagnetically with respect to each other (AF-II phase), which is evidenced by a splitting of the Cu(1) NQR. In this case, T1 -1 is much smaller and exhibits a strong T dependence, which might be associated with a 5 K gap in the magnetic excitation spectrum. We suggest that magnetic defects apparently unavoidable on the Cu(1) sites govern the spin dynamics. While they are frustrated in the "pure samples", they are not in the AF-II phase, which suppresses partly their spin fluctuations.