Temperature-dependent anisotropy of Cu(2) nuclear-relaxation rate in YBa2Cu3O7 below Tc

We have measured the anisotropy of the Cu(2) nuclear-spinlattice relaxation rate in the superconducting state of YBa2Cu3O7 in a small magnetic field (0.44 T) and the temperature range 30210 K. The anisotropy is almost temperature independent in the normal state but develops a nonmonotonic temperature dependence below Tc with a minimum near 7080 K. The isotopic ratio of the relaxation rates shows that the nuclear relaxation is due to magnetic processes in this temperature range, although an extrinsic quadrupolar relaxation process appears below 30 K. © 1991 The American Physical Society.