Oxygen dependence of the magnetic order of Nd in NdBa2Cu3O6+x

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Abstract
Neutron scattering and specific heat measurements have been carried out on a series of NdBa2Cu3O6+x powder samples with varying oxygen concentrations (x=0.94, 0.78, 0.45, 0.3, 0.13) in order to study the effects of oxygen on the magnetic order of Nd. For our highest oxygenated superconducting NdBa2Cu3O6.94 (Tc≂92 K), three‐dimensional (3D) long range order develops below TN≂0.53 K, with a spin configuration which is antiferromagnetic along all three crystallographic axes. For a small reduction of oxygen to x=0.78, we observe drastic effects on the Nd order; 3D order is inhibited and only short range 2D correlations are found at low temperature. However, the 2D correlations develop at a much higher temperature, near 1.5 K. At (nonsuperconducting) x=0.45, the 3D magnetic order is better developed at low temperature, yet long range order still does not occur. For x=0.3, 3D long range magnetic order of the Nd ions is reestablished, developing below TN≂1.5 K, three times that of the fully oxygenated material. All of these ordering temperatures are much higher than what is expected based on dipolar interactions alone, and this coupled with our experimental results indicates that exchange interactions play an essential role, and are strongly influenced by the chain layer oxygen.