Magnetization reversal in double-walled infinite nanotubes

微磁學無論是在基礎研究或工業上的應用都具有極大的重要性， 在本論文中我們將以微磁學為基礎來研究雙層鐵磁奈米管的翻轉模式，這當中我們僅討論"同步翻轉模式"(rotation in unison mode) 和 "螺旋翻轉模式"(curling mode) 此兩種翻轉模式。 從研究中可以發現，當我們的模型為內層同步翻轉模式且外層為螺旋翻轉模式時，不論在何種情下與另外三種模型比較下都不會有最小的成核場，我們也討論了內外兩層表面異相性對於成核場的影響，研究指出內層的表面異相性會比外層的表面異相性具有更大的影響力，提高成核場的數值較明顯，除此之外，藉由分析內層表面異相性和交互界面的強度在此模型之下將表現出與以往不同的成核場曲線，調整上述兩種參數可以使得成核場曲線具有一個或者是兩個區域最小值。而在某些特定的情況之下，給定內外層的材料係數後，可以藉由改變內外層的厚度得到不同的翻轉模式。最後，我們把四種可能的翻轉模式標示在同一張圖上，如此將可以非常的方便理解在何種條件之下對應到的翻轉模式。The study of magnetism is important both for fundamental research and industry applications. In this thesis I study magnetization reversal in double-walled infinite nanotubes on the basis of the micromagnetics theory. I consider two modes, the rotation in unison mode and the curling mode, but exclude the buckling mode from the system. In this investigations, the nucleation field with the rotation in unison mode in the first region, and the curling mode in the second region are never the smallest compared to the other three possible modes. I also investigate the nucleation field influenced by the inner-wall and outer-wall surface anisotropy, and find that the inner-wall surface anisotropy has a larger effect than the outer-wall surface anisotropy. Both the inner-wall surface anisotropy constant or the interfacial coupling constant play an important role on effecting the nucleation field. We can adjust these two constants to yield one or two local minimum values of the nucleation field. Under some special conditions, a critical reversal mode could be derived by just varying the inner-wall thickness or the outer-wall thickness. Finally, I plot a phase diagram which is a convenient picture to summarize all possible modes and their dependence on the system parameters