同步輻射磁性光譜學應用於3d金屬-氧化物奈米結構與自旋電子學之研究

Abstract

本論文主旨在利用同步輻射之X光磁性吸收光譜學研究無電鍍CoxNi1-x奈米結構、Cu摻雜之ZnO奈米顆粒與CoFeB/MgO磁穿隧介面中結構-磁性之偶合關係。在CoxNi1-x的奈米結構中，Co的引入(x~6 at%) 能誘發Ni奈米結構產生一奈米晶/超順磁→多晶/鐵磁的結構-磁相轉變，透過X光磁性吸收光譜的研究，我們了解透過Co摻雜所誘發之結構-磁的相轉變將伴隨著Ni的電子結構重組及Ni 3d導帶自旋極化增強的現象。而當Co的濃度增加接近x=0.7時，有一面心立方(FCC)→六方最密(HCP)的結構相變產生，且此結構的相轉變同時將伴隨著矯頑磁力(Hc)有近六倍的大幅提升，此結果歸因於CoxNi1-x在一球鏈的微結構下有著不同的磁翻轉機制所致；而X光磁性光譜的研究中顯示，Co-Ni間的3d電子交互作用隨著Co濃度的增加而增強，而呈現出巨觀Slater-Pauling行為。在一階迴轉曲線(First-order-reversal curves, FORC)的量測中發現，無論FCC或HCP的CoNi中都呈現出軟、硬兩種磁相，而彼此的消長將決定截然不同程度的磁化可逆性。

而為了解Co、Ni兩種元素對於不同製程條件所產生之電子-磁的響應，在控制Co含量(x=0.5)的條件下，我們分別對CoNi處以熱退火以及在製備時給予一外加磁場進行磁場無電鍍。實驗結果發現磁場無電鍍所製備的CoNi奈米結構透過磁晶異向性的效應能具備較高的電子-磁與結構的偶合效應，結合顯微鏡學，我們能知道此高度的電子-磁與結構的偶合效應為磁場能控制FCC磁晶易軸 [111] 的生長方向而強化CoNi；反之，熱退火處理因缺少磁晶異向性的輔助在結構上呈現均向性的發展，使CoNi呈現較弱的電子-磁與結構的偶合關係，此結果使Co產生多種電子價態而與Ni呈現截然不同的發展趨勢。 而在Cu摻雜之ZnO奈米顆粒的研究中，我們利用X光磁性光譜以及其它的同步輻射技術探測其真實的磁性來源，結果Cu的引入會佔據在ZnO的最密堆積面上形成一種[CuO4]平面四邊形配位的對稱性結構，此局部對稱使銅原子間具有反鐵磁偶合之特性，而[CuO4]在結構上易捕獲氧空缺(Vo)在周圍而成為一種很局部性的鐵磁在ZnO的基材中，此機制與單含氧空缺的ZnO不同，Vo所誘發的Zn、O局部自旋極化現象透過Vo離域軌道的媒合，而呈現長程鐵磁的特性。

最後，我們利用磁性光譜探索CoFeB/MgO的磁穿隧介面中，CoFe元素其電子結構對於其系統之垂直磁異向性(PMA)與穿隧磁阻(TMR)之競爭關係，透過改變X光入射角度可探索CoFe 體心立方(BCC)的單晶結構中，電子的佔有率在特定對稱性的軌域會直接影響垂直異相性與穿隧介面的電子傳輸性質，而此種競爭關係源自於CoFe的少數載子能帶，由於少數載子沿CoFe的[001]方向有優選的佔有率，而使CoFe沿膜面的垂直方向有較強的軌道磁矩，故有較強的自旋-軌道偶合，但此現象對於電子傳輸而言，卻減少了CoFe導帶上的自旋極化度，而使其穿隧性質的自旋相依性降低，故垂直異向性與磁穿隧機制的介面必須來自於不同的CoFeB介面必須在此CoFeB/MgO的相關元件設計中被考量。

In this thesis, synchrotron-based x-ray magnetic spectroscopy was intensively utilized to explore the inter-relations among electronic, magnetic and structural properties of nanostructured and spintronic materials. We first investigated the element-specific electronic behaviors of CoNi nano-arrays while the materials underwent a structural transition, where first-order-reversal-curves (FORC) analysis was also used to reveal the mechanism of magnetization reversal.

In second project, we also investigated the local symmetry of Cu-dopant and resultant structural imperfections in mediating Zn1-xCuxO nanoaprticles’ ferromagnetism (FM). Prepared by an antisolvent method, Cu appeared to preferably populate on the basal plane of ZnO with an antiferromagnetic local symmetry of [CuO4] in the Zn1-xCuxO nanoparticles. Such [CuO4] electronically and structurally coupled to surrounded oxygen vacancies (Vo) that yielded a localized FM. In contrast, the FM of the undoped but oxygen-deficient ZnO appeared to be itinerant and long-range, where Vo percolated the FM effectively and isotropically through oxygen’s delocalized orbital.

Third, we investigated the physical principles regulating the tunneling magneto-resistance (TMR) and perpendicular magnetic anisotropy (PMA) of the CoFeB/MgO magnetic tunnel junction (MTJ), by means of an angle-resolved x-ray magnetic spectroscopy. The angle-resolved capability was facilely achieved whereas it provided additional sensitivity to the symmetry-related d-band occupancy than traditional x-ray spectroscopy. This added degree of freedom successfully solved the long-time puzzle of this MTJ system renowned for the controllable PMA and excellent TMR. As a surprising discovery, these two physical characteristics interact in a competing manner because of opposite band-filling preference in space-correlated symmetry of the 3d-orbital. An overlooked but harmful superparamagnetic phase resulting from magnetic inhomogeneity was also observed. The finding is essential by revealing that simultaneously achieving a fast switching and a high tunneling efficiency at an ultimate level, is unlikely for this MTJ system due to its fundamental limit in physics. We suggest that the development of independent TMR and PMA mechanisms is critical towards a complementary relationship between two physical characteristics, as well as the realization of superior performance, of this perpendicular MTJ. Furthermore, this study provides a facile approach to foresee the futurity of any emerging spntronic candidates by electronically examining their magnetic anisotropy and transport relationship.