Magnetic order of the Cu planes and chains in R${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$

Polarized and unpolarized neutron-diffraction measurements have been taken to determine the magnetic structure of the Cu spins in the R${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$ system as a function of temperature. Most of the results have been obtained on two (semiconducting) oxygen-deficient single crystals of composition ${\mathrm{NdBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.1}$ and ${\mathrm{NdBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.35}$. On cooling from the paramagnetic state, the Cu-O ‘‘plane’’ layers first order at a Néel temperature ${\mathit{T}}_{\mathit{N}1}$ in a simple antiferromagnetic arrangement of spins, with ${\mathit{T}}_{\mathit{N}1}$=430 K for x=0.1 and ${\mathit{T}}_{\mathit{N}1}$=230 K for x=0.35, and with a saturated moment of ∼0.65${\mathrm{\mu }}_{\mathit{B}}$. At a lower temperature ${\mathit{T}}_{\mathit{N}2}$ the Cu ‘‘chain’’ layer also orders, with ${\mathit{T}}_{\mathit{N}2}$=80 and 10 K for x=0.1 and 0.35, respectively. Hence both transition temperatures decrease with increasing oxygen concentration in this range of x, and the qualitative behavior for these two samples is found to be identical.