磊晶奈米結構之磁性質

Magnetism in epitaxial nanostructures

廖聖傑(Sheng-Chieh Liao)

指導教授 ： 賴志煌

國立清華大學/工學院/材料科學工程學系/博士(2015年)

摘要

奈米結構的特色，在於它能以多樣的方式組合兩種以上的材料。同時，它的高介面¬—體積比提升了介面對於整體性質的影響力。而在磊晶奈米結構中，磊晶的引入使得介面與晶體的方向能夠被控制，使得介面現象能夠進一步的被探明。在本論文中，作者探討了在不同種類的磊晶奈米結構裡的磁性，以及這些磁性質與結構的關聯。其中作者特別著重於介面的磁耦合與它如何影響整體的磁異向性。
此論文包含三個議題：一、在多鐵性鐵酸鉍的109度疇壁中，磁矩的異向性為何？二、在核—殼（core-shell）奈米結構中，其異向性與介面上的交換耦合現象為何？此二者如何被結構幾何所控制？三、如何成長鐵磁金屬—氧化物之垂直柱狀磊晶奈米結構，以及控制其磁異向性？而本研究整合結構與磁性量測、以及巨磁學與微磁學分析，得到下面結論：一、應用鐵磁—反鐵磁交換耦合，推測在鉍酸鐵的109度疇壁中，平均的反鐵磁磁矩平行於疇壁，並有著垂直膜面的向量分量。而其未抵銷且固定之磁矩則並沒有顯著之異向性。二、應用微磁學分析與改變結構幾何，在磊晶核—殼奈米結構中，推測出¬結構幾何不僅決定整體的磁異向性，亦影響著介面異向性常數與交換偏壓。三、利用氧缺陷以及元素對於氧化能力的差異，可以用物理方式鍍製金屬鐵磁—氧化物柱狀磊晶奈米結構。而溫度與基板能控制晶粒的方向與尺寸，因此得以控制磁異向性並得到垂直膜面的磁易軸。從這些結論中，得以看出磊晶的引入能使得人們得以進一步探究與控制奈米結構中的介面現象以及其異向性。而在研究過程中提出的方法，亦有助於後續磁性奈米結構的研究。

關鍵字

磊晶 ； 奈米結構 ； 脈衝雷射濺鍍 ； 磁異向性 ； 交換耦合 ； 介面 ； 複雜氧化物

並列摘要

Nanostructures features variety of structures composed by two or more materials. High interface-to volume ratio raises the impact of interface to the properties of the nanostructures. The introduction of epitaxy to the nanostructures confines the direction of interfaces and crystallographic direction, bringing an insight to the interface phenomena. In this thesis, the author discusses the magnetism in different types of epitaxial nanostructures and their correlation to the structures. Interfacial coupling and its contribution to the magnetic anisotropy are especially addressed. Three problems of magnetism of epitaxial nanostructures are involved in this thesis: the anisotropy of magnetic moment in 109° domain walls (DWs) of multiferroic bismuth ferrite (BiFeO3), magnetic anisotropy and exchange coupling in epitaxial core-shell nanostructures and how they are affected by structure geometry, growth and control of magnetic anisotropy in the epitaxial metal-oxide nanostructures. The author applies comprehensive structural and magnetic analyses as well as the macro-spin/ micromagnetic modeling and deduces the following conclusions: 1. The average direction antiferromagnetic spin in 109°DWs is parallel to the DWs with a certain out-of-plane component. No obvious anisotropy is observed of pinned uncompensated spins. 2. With micromagnetic simulation and variation of structure geometry, the author deduce that the structure geometry not only determine the magnetic anisotropy but also effect the exchange coupling in epitaxial core-shell nanostructures. 3. The metal-oxide columnar nanostructures could be fabricated by physical deposition via oxygen vacancy and difference of the oxygen affinity. Growth temperature and substrate control the dimension and orientation of nano-columns and thus the magnetic anisotropy. These results confirm that the introduction of epitaxy allow further investigation and control of interface phenomena and their anisotropy in nanostructures. The methods proposed in this thesis also helps researches of magnetic nanostructures.

並列關鍵字

epitaxy ； nanostructures ； pulsed laser deposition ； magnetic anisotropy ； exchange coupling ； interface ； complex oxide

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