MAGNETIC ORDER, SPIN CORRELATIONS, AND SUPERCONDUCTIVITY IN SINGLE-CRYSTAL ND1.85CE0.15CUO4+DELTA

Measurements are reported of the static, instantaneous, and dynamic spin correlations in single crystals of Nd1.85Ce0.15CuO4+delta. The as-grown crystals show antiferromagnetic long-range order. However, the Neel temperatures and spin-stiffness constants are greatly reduced from their respective values in Nd2CuO4+delta. After reduction and annealing, the crystals superconduct with T(c) = 23 K, but inclusions of the antiferromagnetic phase, albeit with reduced Neel 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.