Positive-muon spin-rotation and -relaxation measurements of the oxygen-deficient perovskite ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathrm{x}}$ have revealed local antiferromagnetic order for 6.0≲x≲6.4 with a Néel temperature $T_{\mathrm{N}}$ that decreases rapidly with increasing oxygen content x. For slowly annealed samples with 6.35≲x≲6.5 the superconducting transition temperature $T_c$ increases smoothly with x from 25 K at x=6.348 to 60 K at x=6.507. Two such samples with x=6.348 and x=6.400 appear to ‘‘switch’’ from superconductivity to antiferromagnetic order at lower temperatures.

• Received 31 December 1987

DOI:https://doi.org/10.1103/PhysRevLett.60.1073