Magnetic order in RBa₂Cu₂O_6+x (invited)

Neutron scattering results on the RBa₂ Cu₃ O _6+x (R=rare earth) class of systems are reviewed. For the oxygen-deficient systems (x<0.5), which are semiconducting, the Cu ions are observed to order with Néel temperatures as high as 500 K. The magnetic structure consists of strong nearest-neighbor antiferromagnetic couplings of the Cu spins within the Cu-O₂ layers, while nearest-neighbor spins in adjacent layers are also aligned antiferromagnetically. The moments are constrained in the tetragonal plane. At lower temperatures the Cu moments in the oxygen-deficient "chain" layers also order antiferromagnetically, with a moment that can be quite substantial (∼ 1/2 μ_B). Both transition temperatures are very sensitive to the oxygen concentration. The magnetic heavy rare-earth compounds, on the other hand, order antiferromagnetically at very low temperatures, analogous to the "conventional" magnetic-superconductor systems such as RMo₆ S₈ and RRh₄ B₄. The ordering is not very sensitive to the oxygen concentration, and the antiferromagnetic state coexists with superconductivity. Differences in the ordering behavior are ascribed to crystal-field effects, which are briefly discussed. Finally, small-angle neutron scattering results in the vicinity of the superconducting transition temperature are also discussed.