Antiferromagnetic ordering of Cu ions in ${\mathrm{NdBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.1}$

Neutron-scattering experiments performed on single crystals ${\mathrm{NdBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.1}$ reveal antiferromagnetic order for T<$T_N$=385±2 K in which the structure is characterized by the magnetic wave vector ((1/2,1)/2,(1/2); this had previously been observed only at low temperature in single crystals of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathrm{x}}$ [the Y(1:2:3) compound]. The magnetic intensities are accounted for by ${\mathrm{Cu}}^{2+}$ spins coupled antiferromagnetically in the ${\mathrm{CuO}}_2$ planes as in the yttrium compounds ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathrm{x}}$ (x<0.5). However, unlike the Y(1:2:3) compound case, the oxygen-deficient planes in the Nd(1:2:3) compound exhibit small staggered magnetization εS which induces the spin ordering -S-εS-S+S+εS+S, along the tetragonal axis. The average staggered magnetization evaluated at room temperature is about 0.40${\mu }_B$ in the ${\mathrm{CuO}}_2$ planes while in the oxygen deficient (Cu-chain) layers it is about 0.04${\mu }_B$. The exchange coupling between the Cu atoms in the oxygen deficient layer containing Cu chains and its nearest-neighbor ${\mathrm{CuO}}_2$ layers (Cu planes) is thus ferromagnetic; this may be readily understood by consideration of the relevant ${\mathrm{Cu}}^{2+}$ orbitals.