The introduction of various pinning center are examined as the effective means for improvement of J c of MgB 2 thin films. We have investigated the effects of introduction of oxygen during deposition on the superconducting properties of MgB 2 thin films.
MgB 2 thin films were prepared on polished sapphire C(0 0 0 1) single crystal substrates by using electron beam evaporation technique (EB) without any post-annealing. The background pressure was less than 1.3 β’ 10 Γ6 Pa. The evaporation flux ratio of Mg was set at 30 times as high as that of B, and the growth rate of MgB 2 film was 1 nm/s. The film thickness was typically 300 nm at 5 min deposition. The substrate temperature was 245 Β°C.
Under these conditions, we controlled the oxygen partial pressure (P O 2 ) within the range from 1.3 β’ 10 Γ6 to 1.3 β’ 10 Γ3 Pa by using a quadrapole mass spectrometer. Although T c of deposited thin film decreased in order of P O 2 , DM in the magnetization hysteresis loops measured from 0 to 6 T at 4.2 K increased up to 1.3 β’ 10 Γ5 . On the other hand, thin film prepared under P O 2 of 1.3 β’ 10 Γ3 Pa does not show superconducting transition. Between these films, there is no difference in the crystal structure from X-ray diffraction (XRD). These results suggest that the pinning center in the thin films increased by introduction of oxygen. Extremely small amount of oxygen introduction has enabled the control of growth of oxide.