臨場電子傳輸與X光能譜學於自旋電子學之研究

Abstract

本計畫將使用本團隊在同步輻射建立之臨場電子傳輸-X光吸收光譜技術，探討兩個自旋科技相關研究。此技術乃利用X光吸收光譜及磁圓偏振二向性的元素及電子軌域選擇性，搭配臨場電子傳輸分析，探究單一元素的電子組態對材料電子傳輸及磁性的影響，並突破傳統自旋電子學僅研究巨觀磁-電交互作用之限制。我們將利用此技術研究CoFeB-MgO磁穿遂介面之電驅動磁性質，以及Ti1-xCoxO2之傳輸性質。 我們欲探究CoFeB/MgO介面2p-3d混成軌域對元件垂直異向性以及電驅動磁性質的影響。電驅動磁性質大量地在鐵磁/高介電氧化物的元件觀察到。然而，目前對電驅動磁性質的瞭解仍停留在實驗發現以及理論的猜測。至於微觀磁學的研究，如外加電壓如何改變3d(2p)電子的自旋態以導致巨觀磁性質變化，在實驗上尚無先例。我們將利用此技術觀察在外加電場下，3d(2p)電子如何重組及極化，進而對過去幾年相關的研究做一總結性的解釋。 在Ti1-xCoxO2，我們旨在瞭解Ti1-xCoxO2與缺陷TiO2室溫鐵磁性的基礎差異。雖然研究指出空位缺陷是誘發TiO2室溫鐵磁性的機制，然而比起Ti1-xCoxO其居禮溫度較低，也不具備不尋常霍爾效應，故兩者機制截然不同。我們將探討Ti1-xCoxO2中Co(3d)-O(2p)交互耦合，缺陷，和Co濃度對於不尋常霍爾效應的影響。本計畫以電子組態為立足點研究自旋電子學，旨在於瞭解載子、電子自旋、電子軌域之間的相互關係，對自旋電子學有基礎上的貢獻。

This proposal is to use a combined electrical transport and x-ray spectroscopic technique (called in-situ E-S characterization henceforth) that we recently developed at NSRRC, to investigate two topics related to spintronics. The technique allows us to perform x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) with simultaneous transport characterizations, enabling research concerning electrical transport, element- and orbital-selective magnetization, with an in-situ fashion. It is a unique approach to real-time change of electronic state for a material/device exhibiting magneto-electric responses. Using this technique, we plan to investigate the electrical (E)-controlled magnetic properties of CoFeB-MgO based magnetic tunnel junction (MTJ), and spin-transport of Ti1-xCoxO2 dilute magnetic semiconductors. For CoFeB-MgO MTJ, we will focus on the effects of Co(Fe)-oxygen p-d hybridization on the perpendicular anisotropy and E-controlled magnetic properties, probing the nature of Co(Fe) spin-orbital coupling involving in the anisotropy reversal. Recent works speculated that the interfacial hybridization in CoFeB/MgO, as well as in other ferromagnetic/high-k oxide interfaces determines their E-controlled magnetic properties. However, the understanding of the phenomenon remains at the step of speculation, yet E-induced change in 3d (2p) electronic states leading to the magnetization change, hasn’t been experimentally observed. In this work, we plan to harness the in-situ E-S characterization to probe 3d (2p) electronic modifications of CoFeB (MgO) in response to applied E-field. This will point to whether the ferromagnetic d-band electron filling leads to magnetic manipulation in CoFeB via interfacial hybridization. Using CoFeB-MgO as an example, the work will provide a conclusive interpretation to the studies dealing with E-controlled magnetic properties over recent years. For Ti1-xCoxO2, we are aimed at understanding the fundamental difference between Ti1-xCoxO2 and defective TiO2, in terms of induced ferromagnetism persistent to room temperature. Although recent discoveries in defective TiO2 suggested that the ferromagnetism originates from the defects, a much lower Curie temperature and the absence of anomalous Hall effect (AHE) of the defective TiO2, in comparison to Ti1-xCoxO2, implying that their induced ferromagnetisms are fundamentally different. In this work we will take advantage of the technique to probe the AHE in the presence of Co 3d – O 2p exchange interactions. We will also investigate the Co-dependency of Co 3d – O 2p exchange interactions along with the AHE, to see the influence of Co concentration on the exchange and transport effects. For defective TiO2, we focus on understanding the Ti -O coupling originating from the Ti 3d – O 2p hybridization, which highly depends on the defect forming. The fundamental difference between Ti1-xCoxO2 and defective TiO2 could be understood by identifying the induced moment quantities, associated electronic states, and concentration/mobility of carriers. The success of the work will provide a deep understanding of interplay between charge, spin and orbital degrees of freedom in spintronics and bring considerable impact to the field.