Effects of oxygen on the magnetic order of the rare-earth ions in RBa2Cu3O6+x (R=Dy, Er, Nd).

Abstract

Neutron diffraction has been used to study the effects of oxygen on the magnetic order of the rare-earth ions in R${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$ (R=Dy, Er, Nd). For fully oxygenated superconducting (${\mathit{T}}_{\mathit{c}}$=92 K) ${\mathrm{ErBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$, two-dimensional (2D) short-range magnetic correlations are observed just above the ordering temperature ${\mathit{T}}_{\mathit{N}}$=0.62 K, while at ${\mathit{T}}_{\mathit{N}}$ long-range correlations develop in the a-b plane, inducing 3D long-range order. Below ${\mathit{T}}_{\mathit{N}}$ the sublattice magnetization is found to obey Onsager's exact solution of the 2D S=1/2 Ising model. For oxygen-reduced (x\ensuremath{\approxeq}0.6) superconducting ${\mathrm{ErBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.6}$ the 3D ordering temperature is lowered to ${\mathit{T}}_{\mathit{N}}$\ensuremath{\simeq}0.48 K. For insulating ${\mathrm{ErBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$ (x\ensuremath{\approxeq}0) neither 2D nor 3D long-range order develops, as only 2D short-range correlations are observed. For ${\mathrm{DyBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ (${\mathit{T}}_{\mathit{c}}$\ensuremath{\simeq}92 K) 3D long-range order occurs at ${\mathit{T}}_{\mathit{N}}$\ensuremath{\simeq}0.93 K, with spins coupled antiferromagnetically along all three crystallographic axes. As we reduce the oxygen concentration the 3D magnetic ordering temperature decreases.In addition, for a concentration 0.54\ensuremath{\le}${\mathit{x}}_{\mathit{c}}$1, still in the superconducting phase, the spin configuration changes along the c-axis to ferromagnetic. In the insulating phase, x\ensuremath{\lesssim}0.4, we observe both 2D and 3D diffuse scattering developing below 2 K, but 3D long-range order never occurs. In our highest oxygenated ${\mathrm{NdBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.94}$ (${\mathit{T}}_{\mathit{c}}$\ensuremath{\simeq}92 K), 3D long-range order develops below ${\mathit{T}}_{\mathit{N}}$\ensuremath{\simeq}0.53 K. For a small reduction of oxygen to x=0.78, we observe drastic effects on the Nd order as only short-range 2D correlations are found at low temperatures. At nonsuperconducting x=0.45, 3D correlations develop at low temperatures, yet long-range order still does not occur. For x=0.3, 3D long-range order is reestablished, developing below ${\mathit{T}}_{\mathit{N}}$\ensuremath{\simeq}1.5 K, three times that of the fully oxygenated material. Though the oxygen dependence of the rare-earth magnetism varies between the three compounds we have studied, our results indicate there is significant coupling between the chain layer oxygen and the rare-earth ions in each case.