Epitaxial films of the Bi2Sr2Ca1Cu2Ox phase were prepared on MgO(100) substrates in situ by RF magnetron sputtering. The as-grown film showed a Tc onset of 92 K and a Tc end of 82 K. The superconducting properties and the c-axis lattice constant could be varied reversibly by annealing the film in various atmosphere at below 400°C. When the c-axis lattice constant was increased by about 0.2 Å, the Tc onset increased from 60 K to 93 K, suggesting that the Tc increased with decreasing oxygen content in the film. However further decrease of the oxygen content in the film caused the degradation of the Tc when the films were annealed in vacuum at above 250°C.
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