Antiferromagnetism and superconductivity in oxygen-deficient YBa2Cu3Ox

Positive-muon spin-rotation and -relaxation measurements of the oxygen-deficient perovskite YBa2Cu3Ox have revealed local antiferromagnetic order for 6.0≲x≲6.4 with a Néel temperature TN that decreases rapidly with increasing oxygen content x. For slowly annealed samples with 6.35≲x≲6.5 the superconducting transition temperature Tc 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.