Antiferromagnetism inYBa2Cu3O6+x
Neutron diffraction has been used to study antiferromagnetic ordering in $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ as a function of $x$. Evidence for such ordering has been observed only in the tetragonal, nonsuperconducting phase ($x\ensuremath{\lesssim}0.4$). The magnetic origin of the superlattice reflections has been confirmed with polarized beam measurements. A single antiferromagnetic structure was observed throughout the ordered phase, with a maximum N\'eel temperature of \ensuremath{\sim}500 K and average ordered moment of ${(0.66\ifmmode\pm\else\textpm\fi{}0.07)}_{\mathrm{\ensuremath{\mu}}\mathit{B}}$ per magnetic Cu atom at $x\ensuremath{\approx}0$. Magnetic susceptibility measurements suggest that two-dimensional correlations are strong, as in ${\mathrm{La}}_{2}\mathrm{Cu}{\mathrm{O}}_{4\ensuremath{-}y}$, and may survive into the orthorhombic phase.