| Summary: | 碩士 === 國立中興大學 === 材料科學與工程學系所 === 107 === The thin films of two systems, Ta（10 min, around 9 nm）/ MnBi（20 min, around 50 nm）/ Si wafer系統與SiO2（10 min, around 25 nm）/ MnBi（20 min, around 50 nm）/ Si wafer, were prepared by the ion beam assisted deposition system (IBAD). After preparing the samples, the vacuum thermal annealing process was performed by the rapid thermal annealing furnace (RTA), and total annealing time was 60 minutes. Ta and SiO2 films were deposited as capping layers to prevent Mn oxidation and Bi diffusion during the annealing process. Effects of capping layers on microstructural and magnetic properties of MnBi alloy thin films were investigated in this study. In the part of microstructure, the grain size distribution and crystal structure were observed by transmission electron microscope (TEM). The distributions of two systems were in the range of 23-63 nm, and the average sizes of the grains were 34-38 nm. It could be found that the films of both systems were polycrystalline from the diffraction pattern. From the diffraction pattern, the corresponding crystallographic planes were compared with JCPDS cards. In the two systems, corresponding crystallographic planes were estimated including α-Mn, Bi, O-MnBi, manganese oxide and LTP-MnBi. The lattice constants of LTP-MnBi of the two systems were calculated from the crystallographic planes to be a = 4.194 Å, c = 6.278 Å (Ta/ MnBi/ Si wafer) and a = 4.217 Å, c = 5.905 Å (SiO2/ MnBi/ Si wafer), the change in lattice constant might be caused by the low content of LTP-MnBi but and high in other elements and compounds of the thin film, then resulting in lattice deformation. In the magnetic properties, the M-H curve measured by vibrating sample magnetometer (VSM), and the hysteresis curve was not obvious. It might be caused by the formations of α-Mn, Bi, O-MnBi and manganese oxide. The LTP-MnBi ratio was lower, and the polycrystalline structure of LTP-MnBi might be the cause, too. Comparing the different capping layers, it was found that the oxygen contained in SiO2 might diffuse during the annealing process to form more manganese oxides. Ta was more suitable to be the capping layer than SiO2. The Ta layer could be adjusted density and thickness to make the Ta cap layer more effective. The formation of O-MnBi and the decomposition of Mn and Bi can be avoided by lowering the annealing temperature.
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