Antiferromagnetism in YBa2Cu3O/sub 6+//sub x/

Neutron diffraction has been used to study antiferromagnetic ordering in YBa2Cu3O/sub 6+//sub x/ as a function of x. Evidence for such ordering has been observed only in the tetragonal, nonsuperconducting phase (xapprox. <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 Neel temperature ∼500 K and average ordered moment of (0.66 +- 0.07)μ/sub B/ per magnetic Cu atom at x≅0. Magnetic susceptibility measurements suggest that two-dimensional correlations are strong, as in La2CuO/sub 4-//sub y/, and may survive into the orthorhombic phase