利用同步輻射磁性光譜學研究Co2FeAl薄膜之相變磁學結構

Abstract

Heusler合金(X2YZ)為一種具有半金屬特性的材料，故能達到極高的自旋極化率而被大家所重視及關注，在這三元合金系統裡，其特殊多變的電子結構符合了現今磁性元件的需求。其中最為突出的應用之一即是結合了磁學及電子傳輸在自旋電子元件上，如磁穿隧結構（MTJ），在鐵磁性/氧化物的異質結構中呈現出一量子級的電子傳輸特性。在本論文中，我們大量使用同步加速器的X光技術探討類似磁穿隧結構之Co2FeAl/MgO多層膜在結構的轉變中(A2轉變為B2)，其磁結構之特性表現。此項技術賦予元素選擇性之獨特性，因此我們可以在氧化鎂(001)基板上探討複雜的Heusler三元合金系統，針對鈷原子及鐵原子觀察其中結構與磁性的交互作用。在結果中我們發現，Co2FeAl從A2變為B2的結構轉變中，由於鈷與鐵交互耦合作用的關係，誘發一電子轉移現象的發生從鈷原子至鐵原子，使得鈷與鐵在各自自旋極化率的表現上呈現競爭關係，鈷上升而鐵反而下降，儘管如此，由於Co2FeAl特殊原子比例之關係(鈷比鐵為2比1)，故整體磁性及自旋極化率仍隨著結構的改變而增加。換言之，結構的改變並不如以往受限於巨觀儀器量測的認知而直接影響自旋極化率的提升，反而是微觀上在X元素(鈷)與Y元素(鐵)中做競爭的關係，之後呈現出微幅的自旋極化率提升。因此我們認為X元素與Y元素在電子結構間的協調為此種材料適用於自旋電子元件技術的關鍵。

Heusler (X2YZ) compounds are a remarkable class of materials exhibiting half-metallicity carrying highly spin-polarized electrons. Their extremely flexible electronic structure offers a versatile toolbox allowing the realization of demanded but apparently contradictory functionalities within one ternary compound. One of the most prominent applications of Heusler is the combination of magnetism and exceptional transport properties in spintronic devices, such as magnetic tunnel junction (MTJ), of which the ferromagnetic/oxide heterostructure enables a novel quantum-level transport. In this thesis the phase-driven (A2→B2) magneto-structural properties of Co2FeAl (CFA)/MgO MTJ-like structure was intensively investigated by synchrotron-based x-ray techniques. Given the advantage of x-ray’s element-specificity, we were able to fingerprint the structural and magnetic cross-reactions between Co and Fe within the complex Heusler X2YZ interpenetrating FCC structure deposited the MgO (001) substrate. From x-ray results we discovered that the two magnetic elements interacted in a competing manner via a charge transfer effect, by which Co/Fe’s spin-polarization was promoted/suppressed while the CFA underwent a A2→B2 structural transition. The charge transfer effect had raised an undesired trade-off during the Co-Fe exchange interactions despite an overall increase of the CFA’s magnetization accompanied by anisotropy modifications, because of the twice number of the X site (Co) to the Y site (Fe) in the X2YZ formula. In other words, the CFA’s magnetic ordering is unfortunately regulated by compromising the enhanced X (Co) site and the suppressed Y (Fe) site, irrespective the development of the previously known, higher spin-polarization phase of B2. We suggest an electronic tuning between the X and Y sites is critical towards the realization for this material being applicable in spintronic technology.