The paper describes a study of the antiferromagnetic ordering of oxygen-deficient ${\mathrm{Nd}}_{1+y}{\mathrm{Ba}}_{2-y}{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ single crystals. In pure, stoichiometric samples, $y=0,$ with different oxygen contents x in the Cu(1) plane $\mathrm{}(0.02<x<\mathrm{}0.2\mathrm{}),$ the antiferromagnetic I (AFI) phase appears to be stable down to 316 mK. The magnetic ordering within the Cu(2) sublattice of the pure ${\mathrm{NdBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ system is therefore similar to that of the pure ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ parent compound. With increasing oxygen content, the Néel temperature drops significantly and the critical exponent changes from 0.26 for ${\mathrm{NdBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.09}$ to about 0.5 for ${\mathrm{NdBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.23}.$ Magnetic ordering of the ${\mathrm{Nd}}^{3+}$ moments sets in at 1.7 K with a critical wave vector ${\mathbf{q}}_{\mathrm{Nd}}=\left(\frac{1}{2}\frac{1}{2}\frac{1}{2}\right).\mathrm{}$ Reordering to the AFII phase is observed in a crystal, which has a significant amount (4%) of ${\mathrm{Nd}}^{3+}$ ions substituted on the Ba sites. The relatively high temperature $T_2=95\mathrm{}\mathrm{K}$ of this reordering suggests that the ${\mathrm{Nd}}^{3+}$ ions on Ba sites are very effective defects forcing the $\mathrm{AFI}\leftrightarrow \mathrm{AFII}$ reordering. The mechanism of reordering is explained in terms of the creation of ${\mathrm{Cu}}^{2+}$ moments within the Cu(1) layer due to the ${\mathrm{Nd}}^{3+}$ ions on Ba sites, which via polarization lead to an effective ferromagnetic coupling between the moments on next-nearest neighboring Cu(2) layers. The ${\mathrm{Nd}}^{3+}$ doping on the ${\mathrm{Ba}}^{2+}$ sites increases the Néel temperature compared to the stoichiometric compound.

• Received 29 December 1997

DOI:https://doi.org/10.1103/PhysRevB.59.3870