苝四甲酸二酐有機自旋閥在不同氧化鋁層組合下的介面效應

在以有機半導體為中間層的自旋閥研究中，在鐵磁層和有機半導體的介面上所形成的混成能階，預期會對自旋電子的傳輸有極大的影響。儘管理論模型對混成能階的行為已有一定的預測，但在相關的研究中始終沒有直接觀測到混成能階的存在，大多只是對所量測到的電流-電壓曲線，或是磁阻的變化，用混成能階的概念來解釋。在這篇論文中，我們在鐵磁層與有機層之間插入厚度僅0.6 奈米的氧化鋁絕緣層，用四種不同型態的元件來研究其效應: 鐵磁/有機/鐵磁，鐵磁/絕緣層/有機/鐵磁，鐵磁/有機/絕緣層/鐵磁，鐵磁/絕緣層/有機/絕緣層/鐵磁。由於氧化鋁絕緣層可以將鐵磁層與有機層隔開，使混成能階無法產生，因此經由比較系統中四種不同型態元件的磁阻表現，我們可以看出混成能階對傳輸現象的影響。在有機自旋閥的基本性質部份，我們成功做出了在室溫下有10%的磁阻元件。經由四點量測發現沒有插入絕緣層以及僅插入上層絕緣層的元件，即使在中間層超過20nm 仍然是短路，這很可能是上層鈷鐵層的原子在濺鍍時擴散進入有機層造成的。而在僅插入下層絕緣層以及上下都有插入絕緣層的元件，其電阻隨著有機層厚度增加呈現指數型上升，並且隨著所加的偏壓方向不同，所量到的磁阻量值有一定程度的差異。為了描述這個磁阻對偏壓的不對稱性，我們定義了兩種不同數學形式的D-factor，可用來描述中間層兩側自旋偏極化能階的不對稱性。經過比較我們認為D-factor 中對量測偏壓的修正項是不需要的。觀測D-factor 在不同有機層厚度下的趨勢，我們發現僅插入下層絕緣層的元件的D-factor 比上下都有插入絕緣層的元件的D-factor 來得大，這差異性指向了混成能階的存在性，並指出其對自旋偏極化電子的傳輸確實有相當程度的影響力。

關鍵字

有機自旋閥；磁性穿隧接面；偏壓相依性；混成軌域

並列摘要

Spin-dependent hybridization of orbitals at ferromagnetic/organic interface is predicted to play a crucial role on spin transport in organic spin valves. However, a direct evidence of observing hybrid states is still lacking. To study this critical issue, we insert aluminum oxide layer between ferromagnetic (FM) layer and organic layer (perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride, PTCDA) systematically with a nominal thickness of 0.6 nm, creating four types of device: FM/PTCDA/FM, FM/Al2O3/PTCDA/FM, FM/PTCDA/Al2O3/FM, FM/Al2O3/PTCDA/Al2O3/FM. The hybridization between ferromagnetic and organic is isolated by inserting insulators, and the effect of hybrid states is observed in varied aluminum oxide layer combination. For the basic electrical properties of organic spin valves, a megnetoresistance value about 10% at room temperature is achieved. As we extract electrical characteristic by 4-probe measurement, the devices with up Al2O3 layer and without Al2O3 layer are all shorting due to CoFe inter-diffusion, but in devices with down Al2O3 layer and two Al2O3 layers we observe some MR-bias asymmetry. An original D-factor with two different formulas is defined to describe the asymmetry. During comparing with conductance and resolving the physical meanings, we find the normalization term in D-factor is not necessary. Observing their tendency on varied PTCDA layer thickness the D-factor of devices with down Al2O3 layer seems larger than devices with two Al2O3 layers, which indicates the existence of hybrid states at interface.

並列關鍵字

organic spin valve ； magnetic tunnel junction ； bias dependence ； hybrid states

參考文獻

[30] Greg Szulczewski, Stefano Sanvito and Michael Coey, Nature Mater. 8, 693

[26] Stuart S. P. Parkin, Christian Kaiser, Alex Panchula, Philip M. Rice, Brian

Olivier, Robert Silbey, and Jean-Luc Bredas, Chem. Rev. 107, 926 (2007)

[9] Saroj P. Dash, Sandeep Sharma, Ram S. Patel, Michel P. de Jong and Ron

[31] Veaceslav Coropceanu, Jerome Cornil, Demetrio A. da Silva Filho, Yoann

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苝四甲酸二酐有機自旋閥在不同氧化鋁層組合下的介面效應

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