In this paper, we examine the superconducting properties of MgB
2+X thin films prepared with various compositions. MgB
2+X thin films were prepared on polished silicon (100) single-crystal substrates using an electron beam evaporation technique without any post-annealing. The critical temperature of the MgB
2+X thin film decreased in accordance with increasing B ratio in the B-rich composition region. However, ⊿
Ms in the
M-H curves of the B-rich MgB
2 thin films were larger than that of the stoichiometric MgB
2 thin film and the B-rich composition resulted in a longer
c-axis length for the MgB
2 crystal. These results suggest that Mg vacancies or B substitution on Mg sites in the MgB
2+X crystals induced by the Mg-deficient composition act as effective pinning centers. Moreover, the chemical composition of the MgB
2 superconductor strongly affected the stability of the microstructure in the MgB
2 thin film. Therefore, we conclude that the slightly B-rich composition is preferable to the stoichiometric composition for the MgB
2 superconductor.
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