Antiferromagnetism in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi mathvariant="normal">Y</mml:mi><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Ba</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Cu</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">O</mml:mi></mml:mrow><mml:mrow><mml:mn>6</mml:mn><mml:mo>+</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>

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.

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