Temperature dependence of the sublattice spontaneous magnetization ofYBa2Cu3O6

Zero-field copper NMR measurements on ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_6$ up to 380 K are reported. The temperature dependence of the local magnetic moment at the Cu(2) plane site is compared with the predictions of spin-wave theory, based on the exchange coupling constants obtained from inelastic neutron scattering. The agreement is satisfactory, within the limits of the theoretical approximations, but discrepancies with elastic neutron-scattering results are pointed out and justified in terms of three-dimensional stacking defects.