| Summary: | 博士 === 國立清華大學 === 材料科學工程學系 === 91 === This work has focused on the study of the exchange anisotropy in the Co/NiMn systems, especially on the correlation between the exchange anisotropy and NiMn crystal structure. Two main issues have been discussed in this dissertation. The first issue is the developments of the growth of epitaxial and textured Co/NiMn thin films using electron-beam evaporation system. By choosing the appropriate buffer layers, (001) and (111) high-quality epitaxial films can be grown on hydrogen-terminated Si (001) and (111) substrates, respectively. The (001) films were grown through a Cu buffer layer based on the MMES technique while the (111) films were grown by means of the Au/Cu buffer layers. Besides, for the (001) system it was found that the structure of NiMn strongly depends on the deposition temperature. The (001) epitaxial NiMn can be grown at 120°C whereas the (001) textured NiMn was formed if the growth temperature is lower than 120°C. In addition, it was found that there is a dramatic change of the structure of Co/NiMn films when an extra Au layer is inserted into the Cu buffer layer on hydrogen-terminated Si (001) substrates. We found that the Co/NiMn films still maintain the (001) epitaxial films when the Au thickness is smaller than 5Å. However, an unusual structure of (111) high-textured films constructed of two sets of (111) epitaxial grain rotated by 90° with respect to each other along [111] direction is obtained when the Au thickness is thicker than 50Å. Furthermore, by changing the thickness of the Au layer between 5 to 20 Å, we can manipulate the volume fraction of (111) grains. These results enable us to control the microstructure of Co/NiMn films, and provide an incentive to study the relationship between the exchange anisotropy and the crystal structure.
The second issue is the study of magnetic properties in Co/NiMn films with different crystal structures. In (001) epitaxial films, we found that the exchange coupling between Co and NiMn not only induces a unidirectional exchange anisotropy but also an extra unidirection anisotropy. In addition, unusual double-shifted hysteresis loops were observed when measured along the hard axis, which can be attributed to the induced uniaxial anisotropy. Furthermore, the anisotropy constants can be quantitatively determined in the (001) epitaxial films by numerical fitting the unusual double-shifted loops. This is the first report of the accurate determination of the exchange-bias-induced uniaxial anisotropy constant. We also study the temperature effect on anisotropy constants in (001) epitaxial films. Compared with the results of (111) textured films, we found that the crystalline anisotropy constant in NiMn drastically decreases with increasing temperature around 180°C. In (111) epitaxial and high-textured films, we found that the NiMn phase transformation is easier occurred in (111) planes than in (001) planes. In addition, our results indicate that the exchange field of Co/NiMn films increases with increasing (111) textured. However, for the (111) epitaxial films, the exchange field was suppressed due to the stabilizing effect at interfaces as will as the lack of nucleation sites for NiMn phase transformation. In the study of the effect of orientation transition from (001) to (111) on the exchange anisotropy, we found that the exchange anisotropy in Co/NiMn is very sensitive to the NiMn orientation, and is less sensitive to the interface roughness. Furthermore, both of the exchange field and the coercivity increase with increasing the volume fraction of (111) grains.
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