Magnetic ordering of Nd3+ in single-crystal NdBa2Cu3O6+x

We describe a neutron-diffraction study of the low-temperature magnetic ord ering in single crystals of NdBa2Cu3O6+x. We investigated a superconducting crystal (T-c=95.5 K), and two nonsuperconducting crystals (x approximate t o 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 oth er (B) began to reorder into the AFII phase below 30 K. We observed long-ra nge 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 orderi ng in the nonsuperconducting crystals, and from this we place an upper limi t 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 in tensities agree broadly with those deduced previously from neutron powder d iffraction studies, but differ in some of the details. [S0163-1829(99)02826 -X].