Magnetic ordering of Nd 3+ in single-crystal NdBa 2 Cu 3 O 6+x

We describe a neutron-diffraction study of the low-temperature magnetic ordering in single crystals of ${\mathrm{NdBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}.$ We investigated a superconducting crystal ${(T}_{c}=95.5\mathrm{K}),$ and two nonsuperconducting crystals $(x\ensuremath{\approx}0.1).$ The two nonsuperconducting crystals differed in their Cu sublattice magnetic ordering. One (A) exhibited the simple AFI antiferromagnetic spin structure down to the lowest attainable temperature (0.3 K), while the other (B) began to reorder into the AFII phase below 30 K. We observed long-range antiferromagnetic ordering of the Nd sublattice in all three crystals, but in crystal B the Nd ordering was of very short range in the c direction. The onset of Nd ordering produced no detectable changes in the Cu ordering in the nonsuperconducting crystals, and from this we place an upper limit of 0.05 meV on the strength of any Nd-Cu pseudodipolar magnetic coupling. The ordered arrangements of the Nd moments refined from the diffraction intensities agree broadly with those deduced previously from neutron powder diffraction studies, but differ in some of the details.