SPIN FLUCTUATIONS IN DOPED ANTIFERROMAGNETS FROM LI-7 NMR RELAXATION IN CUOLI

Li-7 NMR relaxation is used to derive information on the low-energy spin excitations induced in antiferromagnetic (AF) CuO by the heterovalent substitution of one percent of Cu2+ by Li+. The energy gap E(g) between localized and itinerant states, where the extra hole diffuses in the AF matrix, is obtained as E(g) congruent-to 600 K while the effective correlation time of the hole is tau(h) congruent-to (1/omega(exch)/xe(-E(g)/T), in agreement with the main conclusions of a previous work in Cu1-xLixO for larger doping [P. Carretta, M. Corti, and A. Rigamonti, Phys. Rev. B 48, 3433 (1993)]. The experimental findings are discussed in comparison with the ones in lightly doped AF La2-xSrxCuO4, where a drastic changeover in the x dependence of the spin fluctuation properties has been recently found for x less than or similar to 0.02 [F. C. Chou et al., Phys. Rev. Lett. 71, 2323 (1993)]. Finally, mostly on the basis of Li-7 relaxation rates at different measuring frequencies, it is conjectured that in Cu0.99Li0.01O one has a localization of the extra hole, possibly in the CuO4 plaquette [R. J. Gooding, Phys. Rev. Lett. 66, 2266 (1991)], establishing a condition of fast spin fluctuations.