Successive magnetic phase transitions in tetragonal YBa2Cu

Abstract

Neutron diffraction experiments on a tetragonal single crystal of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathrm{x}}$ (with x\ensuremath{\sim}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 N\'eel temperature 405\ifmmode\pm\else\textpm\fi{}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.