Magnetic order, spin correlations, and superconductivity in single-crystal ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4+\mathrm{\delta }}$

Measurements are reported of the static, instantaneous, and dynamic spin correlations in single crystals of ${\mathrm{Nd}}_{1.85}$ ${\mathrm{Ce}}_{0.15}$ ${\mathrm{CuO}}_{4+\mathrm{\delta }}$. The as-grown crystals show antiferromagnetic long-range order. However, the Néel temperatures and spin-stiffness constants are greatly reduced from their respective values in ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_{4+\mathrm{\delta }}$. After reduction and annealing, the crystals superconduct with ${\mathit{T}}_{\mathit{c}}$=23 K, but inclusions of the antiferromagnetic phase, albeit with reduced Néel temperatures, are always present. Based on these data, we suggest that the disorder due to excess oxygen in as-grown samples stabilizes the magnetic correlations. This leads to a speculative model for the effects of deoxygenation in which the removal of the excess oxygen causes the magnetic correlations to diminish and hence superconductivity to become possible.