As a microbe of the environment, 20–30 ppm of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Co}$ is substituted for Cu in the ${\mathrm{CuO}}_2$ planes of ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_4$. Mössbauer effect studies show that all the oxygenated and ‘‘deoxygenated’’ samples with x=0.00, 0.14, 0.16, and 0.18 contain large proportions of micro spin clusters. Even in the deoxygenated ceramic with x=0.16, superconducting regions coexist with about 53% of microscopic spin clusters of tyical size ∼25–250 Å. Spin clusters formation is induced by extraneous oxygen. 2D magnetic order in the electron-doped system is much stronger than that in systems hole-doped, with extremely long-lived (>${10}^{\mathrm{-}8}$ s) spin correlations far above the 3D Néel temperature.

• Received 17 February 1993

DOI:https://doi.org/10.1103/PhysRevLett.70.3355