We have studied properties of the superconducting MgB 2 thin films made on sapphire substrates by radio frequency (rf) magnetron co-sputtering of Mg and B. The effects of annealing temperature and Mg:B ratio on the superconducting properties of the film were investigated. Films were prepared by room-temperature co-deposition of Mg and B using two guns and annealed in situ to form the superconducting MgB 2 phase. Ar sputtering pressure was 20 mtorr with 5% of hydrogen gas to trap remnant oxygen gas in the deposition chamber. Average rf power density of Mg target was varied from 2 to 4 W cm −2 with that of B fixed at 12 W cm −2 . An additional Mg layer was deposited on top to make up for Mg loss during in situ annealing. Films were heated up to 500-750 • C at the rate of 60 • C min −1 , annealed for 20 min, and naturally cooled down by turning off the heater. The best results were obtained for films deposited with the rf power of Mg target at 50 W and annealed at 700 • C. The mechanism for MgB 2 formation in low-pressure annealing is believed to be that during annealing the local Mg vapour pressure inside the film falls within the thermodynamic window.
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