Alq3有機自旋元件之表面電子及磁特性研究

Abstract

有機自旋閥(OSVs)元件中，由於自旋極化載子必須穿過鐵磁-有機半導體-鐵磁(FM-OSC-FM)的多層膜結構，因此有機-鐵磁-有機的介面雜混結構扮演了影響自旋注入和傳輸效率的重要角色。本文中，我們研究上層Alq3/Co和下層Co/Alq3接觸電極中之界面電子結構、化學鍵結及磁耦合特性。實驗結果顯示，Alq3/Co 和 Co/Alq3的介面對稱性是經由tris(8-hydroxyquinolinato)-aluminium (Alq3) 和Cobalt (Co)之間的d電子和共軛鍵雜交的程度決定。我們證明了在上層Alq3/Co有更活潑的介面，因為上層Co 金屬團簇滲透擴散進入 Alq3層，d電子和共軛鍵的雜交誘導Alq3自旋極化其N軌道在LUMO能態和O的軌道在HOMO能態，因此在介面雜交層產生有效的自旋過濾性。此外，我們研究了Alq3在不同LSMO終止層上的界面電子結構和磁性特徴。在三種LSMO終止的最外層，我們觀察到不同量的SrO團聚，這可能與表面磁特性的變化和表面偶極矩形成在該混合LMSO/Alq3、MnO2-ter/Alq3, 和 LSO-ter /Alq3等介面中有關。因此了解在FM-OSC-FM異質界面的行為，對以Alq3為基礎的自旋閥元件的設計和操作是很重要的。

The interfacial regions at ferromagnetic-organic semiconductor-ferromagnetic (FM-OSC-FM) hybrid structures are critical to the spin injection and transport in organic spin valves (OSVs) since the spin-polarized charge carriers have to travel across these interfaces. In this thesis, I investigated the spin interface in terms of the electronic, chemical and magnetic properties at both top Alq3/Co and bottom Co/Alq3 contacts. An interfacial symmetry at Alq3/Co and Co/Alq3 interfaces were obtained through the degree of hybridization between -conjugated orbitals of π-conjugated tris(8-hydroxyquinolinato)-aluminium (Alq3) and d-electrons of Cobalt (Co). I demonstrated a more reactive interface at the top Alq3/Co due to the interdiffusion of Co clusters into Alq3 and thus enhanced orbital hybridization. This intermixed interface plays a spin filter role in which the induced spin polarization of non-magnetic Alq3 came mainly from the spin polarization of N orbitals at the LUMO state and O orbitals at the HOMO state. In addition, I examined the influence of LSMO termination layers on the interfacial electronic and magnetic properties of LSMO/Alq3 heterostructures. Different amount of SrO segregation at the outermost layer of three kinds of LSMO termination was observed, which may correlate to the variation of surface magnetic properties and the formation of surface dipole at the mixed LMSO/Alq3, MnO2-ter/Alq3, and LSO-ter/Alq3 interfaces. Understanding the interfacial behavior at FM-OSC-FM heterostructures is crucial for the design and operation of Alq3-based OSV devices.