MAGNETIC ORDER OF THE CU PLANES AND CHAINS IN RBA2CU3O6+X

Polarized and unpolarized neutron-diffraction measurements have been taken to determine the magnetic structure of the Cu spins in the {ital R}Ba{sub 2}Cu{sub 3}O{sub 6+{ital x}} system as a function of temperature. Most of the results have been obtained on two (semiconducting) oxygen-deficient single crystals of composition NdBa{sub 2}Cu{sub 3}O{sub 6.1} and NdBa{sub 2}Cu{sub 3}O{sub 6.35}. On cooling from the paramagnetic state, the Cu-O plane'' layers first order at a Neel temperature {ital T}{sub {ital N}1} in a simple antiferromagnetic arrangement of spins, with {ital T}{sub {ital N}1}=430 K for {ital x}=0.1 and {ital T}{sub {ital N}1}=230 K for {ital x}=0.35, and with a saturated moment of {similar to}0.65{mu}{sub {ital B}}. At a lower temperature {ital T}{sub {ital N}2} the Cu chain'' layer also orders, with {ital T}{sub {ital N}2}=80 and 10 K for {ital x}=0.1 and 0.35, respectively. Hence both transition temperatures decrease with increasing oxygen concentration in this range of {ital x}, and the qualitative behavior for these two samples is found to be identical.