Successive magnetic phase transitions in tetragonal ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathrm{x}}$

Neutron diffraction experiments on a tetragonal single crystal of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathrm{x}}$ (with x∼0.35) reveal a second magnetic phase transition associated with appearance of superlattice peaks at (1/2 1/2l)(l=half-integer) at about 40 K, which is well below the Néel temperature 405±5 K. This transition is associated with the ordering of the ${\mathrm{Cu}}^{2+}$ moments on the oxygen-deficient layers. The observed magnetic intensities at 9 K are consistent with ground-state spin configurations of a model which takes account of frustration.