MAGNETIC-FIELD EFFECT ON CU-63 NUCLEAR-RELAXATION RATES IN YBA2CU3O7 AND IN LA1.85SR0.15CUO4 ABOVE AND BELOW TC

Nuclear electric quadrupole measurements of Cu-63 nuclear spin-lattice relaxation rates (R1) in zero applied magnetic field and nuclear magnetic resonance R1 in 8.2 T are reported for YBa2Cu3O7 and La1.85Sr0.15CuO4. A direct effect of the magnetic field is seen for the first time both above and below T(c). Above T(c) the effect is ascribed to the partial suppression of antiferromagnetic (AF) spin fluctuations. For T < T(c) the effect goes in the opposite sense and may be related to the spin-temperature diffusion from the core of the fluxons or to the enhancement of AF spin fluctuations persisting in the superconducting phase.