X-ray Magnetic Spectroscopy Study on Interfacial Reconstruction of 3d-Spintronic Hetrostructures
|Keywords:||自旋電子;吸收光譜;異質介面;Spintronics;X-ray absorption spectroscopy;Heterostructure|
|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磊晶級CoxFe3-XO4/CoO、Ni80Fe20/Bi2Se3與Fe/ZnO異質結構之介面重構效應。在CoxFe3-XO4/CoO研究中利用脈衝雷射之相分離法成功在SrTiO3基板上使用兩種鉍前驅物沉積出磊晶級CoxFe3-XO4/CoO殼核異質結構並能有效控制其金字塔奈米形貌於。利用同步加速器X光磁性光譜技術探測出此異質結構為亞鐵/反鐵磁相，並具有垂直異相特性；從巨觀的磁滯曲線量測發現此特殊結構由於強烈的亞鐵磁-反鐵磁相互作用結果伴隨出磁滯曲線不對稱性、交換偏置(exchange bias)和矯頑磁場增大等特性，推測當中磁異質介面存在未抵銷自旋(uncompensated spin)，為了瞭解此介面特性所引發出的磁性行為，我們在維持金字塔形貌下改變殼核比例並發現在殼核交互作用下有易磁化軸翻轉行為; 其作為操縱奈米結構與磁晶異性和矯頑力的有效手段。該研究可以被認為是物理沉積和化學合成的一體化，成功實現具有高度可調諧的結構和磁性的新型磊晶結構的創造以及徹底的理解。此技術為當前的磁性氧化物技術打開了在原子尺度上追求可控性的最新方法。
而在拓撲絕緣體的表面引入鐵磁性可以實現具有自旋電荷現象相關技術的價值。這種令人著迷的原因在於其具有均勻鐵磁排列相有關。通過同步加速器X光譜和微結構分析，我們觀察到Ni80Fe20 / Bi2Se3異質結構的鐵磁層發生相分離。相分離是由於Ni顯著擴散進入Bi2Se3所引起的Ni:Bi2Se3三元磁相。 Ni的向內擴散導致FeSe相向外擴展，從而將原始的Ni80Fe20 / Bi2Se3轉變成具有雙磁相的FeSe / Ni:Bi2Se3 / Bi2Se三明治結構。儘管雙重磁性相的存在，拓譜特性仍然能被保護，這解釋了為什麼在以前的研究中常被忽視。通過自旋軌道能量計算，發現FeSe和Ni:Bi2Se3分別具有垂直和水平異向性。通過Bi2Se3厚度可以容易地調控異質結構的磁排列，從而干擾兩個磁相的磁排列。這種發現對於質量關鍵的自旋電荷現象至關重要，如自旋轉移轉矩，自旋軌道轉矩，自旋泵浦和量子異常霍爾效應等，在類似的材料組合中，鐵磁層由多元素組成。
最後，結合同步加速器X光吸收光譜(X-ray Absorption Spectrum，XAS)、X光磁圓偏振二向性光譜(X-ray Magnetic Circular Dichroism)與電子傳輸系統，以臨場電壓控制Fe/ZnO異質結構之介面電子結構組態，藉由調整X光能量範圍剖析元件結構中各層、元素與自旋特徵以達到研究自旋電子傳輸特性之研究。在施加臨場電壓之XAS光譜上，觀察到鐵氧鍵結經歷兩階段變化，第一階段隨著施加電場當施加臨場電場大於70 V，發生鐵-氧斷鍵並隨著氧空缺的形成使元件轉為高電阻態；伴隨此階段所發生的鐵氧介面重組現象，鐵氧元素間發生電荷轉移(charge transfer)與矯頑力增大等現象；第二階段在持續施加偏壓的情況下，發現鐵的自旋極化上升且頑磁力減弱，與第一階段不同的是此階段所發現的特徵皆居有可逆性(reversible control)。此研究工作目的在鐵磁/半導體自旋電子元件中提供臨場觀測技術直接的偵測元件在運作中自旋傳輸特性。
In this thesis, synchrotron-based x-ray magnetic spectroscopy was intensively utilized to explore the interface-driven magnetic properties in spintronic materials. , We first report on the successful fabrication of epitaxial-discrete Co1−XFe2+XO4/CoO magnetic nanostructures on a SrTiO3 substrate, where the cross-reactions of the two magnetic phases in the vicinity of the epitaxial junction were thoroughly investigated. These nanostructures were originally prepared as Fe3O4-CoO core-shell structures through the phase decomposition of bismuth perovskite precursors by pulsed-laser deposition. An antiphase boundary emerged during the structural/electronic transition from the CoO core to the Co1−XFe2+XO4 shell; thereby developing a ferrimagnetic/antiferromagnetic interface. Uncompensated spins (UCS) arose from the Co1−XFe2+XO4/CoO interface as a result of strong ferrimagnetic–antiferromagnetic interactions. A notable exchange bias as well as a significant exchange enhancement was observed owing to the UCS, which had a locking effect because of the decoupling of the Co1−XFe2+XO4/CoO reversal from the antiphase boundary. Control of the precursor ratio allowed for the fine-tuning of the Co1−XFe2+XO4 phase and the associated locking behaviors. This, in turn, allowed the anisotropy and coercivity of the nanostructures to be manipulated. Thus, we were able to create and thoroughly understand a complex epitaxial configuration with tunable structural and magnetic properties. This study should open new opportunities with regard to current magnetic oxide technology, which requires novel methods for pursuing extremity of controllable properties over an atomic landscape. In second project, we investigated the emerging magnetism at a ferromagnet/topological-insulator heterostructure. Introducing ferromagnetism at the surface of a topological insulator (TI) could enable fascinating spin-charge phenomena that possess great value for related technology. Such fascinating physics was presumed being associated with a homogeneous ferromagnetic (FM) layer with one type magnetic order. By synchrotron x-ray and microstructural analyzes we observed phase separation within the FM layer of a Ni80Fe20/Bi2Se3 heterostructure. The phase separation was caused by significant diffusion of Ni into Bi2Se3 that formed a ternary magnetic phase of Ni:Bi2Se3. The inward diffusion of Ni led to the development of FeSe phase outward, hence turning original Ni80Fe20/Bi2Se3 into a sandwiched structure of FeSe/Ni:Bi2Se3/Bi2Se3 featuring dual magnetic phase. The TI’s properties were well preserved despite the dual magnetic phase, which explained why it was overlooked in previous studies. By spin-orbit energy calculation FeSe and Ni:Bi2Se3 were found to possess in-plane and out-of-plane magnetic anisotropy, respectively. The heterostructure’s magnetic order could be readily tuned by Bi2Se3 thickness as compromising the magnetic orders of the two magnetic phases. The discovery is essential to the quantification of critical spin-charge phenomena, such as spin-transfer torque, spin-orbit torque, spin pumping, and quantum anomalous Hall effect, in similar material combinations, of which the FM layer is composed of multi-element. Third, using x-ray magnetic spectroscopy with in-situ electrical characterizations, we investigated the effects of external voltage on the spin-electronic and transport properties at the interface of a Fe/ZnO device. Layer-, element-, and spin-resolved information of the device was obtained by cross-tuning of the x-ray mode and photon energy, when voltage was applied. At the early stage of the operation, the device exhibited a low-resistance state featuring robust Fe-O bonds. However, the Fe-O bonds were broken with increasing voltage. Breaking of the Fe-O bonds caused the formation of oxygen vacancies and resulted in a high-resistance state. Such interface reconstruction was coupled to a charge transfer effect via Fe-O hybridization, which suppressed/enhanced the magnetization/coercivity of Fe electronically. Nevertheless, the interface became stabilized with the metallic phase if the device was continuously polarized. During this stage, the spin-polarization of Fe was enhanced whereas the coercivity was lowered by voltage. This stage is desirable for spintronic device applications, owing to a different voltage-induced electronic transition compared to the first stage. The study enabled a straightforward detection of the spin-electronic state at the ferromagnet-semiconductor interface in relation to the transport and reversal properties during the operation process of the device.
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