Temperature-dependent anisotropy of Cu(2) nuclear-relaxation rate in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ below ${\mathit{T}}_{\mathit{c}}$

We have measured the anisotropy of the Cu(2) nuclear-spin–lattice relaxation rate in the superconducting state of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ in a small magnetic field (0.44 T) and the temperature range 30–210 K. The anisotropy is almost temperature independent in the normal state but develops a nonmonotonic temperature dependence below ${\mathit{T}}_{\mathit{c}}$ with a minimum near 70–80 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.