The effects of annealing temperature, annealing time and film thickness on the properties of MgB_2 thin films prepared by ex-situ annealing of Mg/B muhilayer have been studied. The films with the thickness of 250 nm annealed at temperatures as low as 400 ℃ exhibited a superconducting transition. The optimal annealing condition of 250 nm thick MgB_2 films regarding to the transition temperature, T_c, was 750 ℃ for 20-30 rain. T_c became lower as the film thickness decreased and the T_c of thinner MgB_2 films was influenced more greatly by the annealing temperature. With reducing the thickness of each single layer, we were able to fabricate the 20 nm thick MgB_2 superconducting film with a T_c~(onset) of 31 K.%用Mg/B多层膜退火的方法制备了一系列MgB_2超导薄膜,研究了退火温度、退火时间和薄膜厚度对于MgB_2薄 膜性质的影响.厚度为250 nm的Mg/a多层膜经400℃低温退火后已经生成超导相,此厚度薄膜750℃下退火20-30 min实现最佳超导转变温度(T_c).前驱膜分层厚度相同时,随着薄膜厚度减小MgB_2薄膜T_c明显降低,而且较薄的膜T_c受退火温度影响更加明显.通过减小Mg/B多层膜分层厚度,制备的20 nm厚MgB_2超导薄膜在31 K左右出现超导转变.
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