利用阻抗頻譜分析擷取介面及塊體在有機自旋閥傳輸性質的貢獻

Abstract

在有機自旋閥 CoFe/AlOx/PTCDA/AlOx/NiFe/CoFe 裡，鐵磁性層和有機層之間的介面扮演著很重要的關鍵因素，而使用阻抗頻譜加上等價電路模組的分析，並從有機自旋閥中截取出各層之間的電容電阻的貢獻，此為本實驗研究有機自旋閥介面及傳輸特性的分析方式。在整個實驗過程中，我們的有機自旋閥表現出兩個串聯的電容電阻並聯電路，並相較於原本推測的單一電容電阻並聯電路，其分析結果顯示為前者較為符合有機自旋閥在阻抗頻譜上的表現。而在這樣的分析結果下，有機自旋閥的等價電路結構可以分為兩部分，介面及塊體。而根據實驗結果顯示，介面和塊體的電容電阻並聯電路都顯現出會隨著有機層苝四甲酸二酐(PTCDA)厚度的增加而變化。關於磁阻(magmetoreistance)的表現，介面電阻表現出很大的 磁阻，但塊體電阻卻沒有表現出磁阻的特性，由此實驗結果可見，介面電阻的磁阻表現佔了整個有機自旋閥很大部分的磁阻貢獻。至於介面及塊體電容的部分，兩個都會隨著有機層 PTCDA 厚度增加而受影響。此實驗假設塊體電容可以視為一個平行板電容，隨著介電層(有機層)增加厚度而逐漸變小，而其分析出來的介電常數值也符合 PTCDA 介電常數的範圍。關於介面電容的部分，電容因為介面的粗糙度而受影響，然而隨著PTCDA 厚度的增加，其介面的粗糙度會更嚴重，因此進而影響介面電容使其小。在磁電容的變化上，介面及塊體對於 PTCDA 厚度的變化表現出不同的趨勢。最後本實驗的結果讓我們更了解和發現更多有機自旋閥其介面上的特性，其結果有助於我們未來有機自旋閥的發展。

The organic spin valves(OSVs) has been successfully demonstrated, with junction structure CoFe/AlOx/PTCDA/AlOx/NiFe/CoFe. The ferromagnetic/organic interface is predicted to play a crucial role on OSVs. Using impedance spectrum and fitting with an equivalent circuit to extract the interface capacitance from junction is a way to study interface of OSVs. During our experiment, our OSVs present a better fitting result with two RC circuits than one RC circuit, which indicates the configuration of organic junction should be separated into bulk and interface parts. According to the experimental result, RC circuit of interface and bulk both depend on PTCDA thickness. Interface resistance presents a huge magnetoresistance, but not bulk resistance, which indicates interface resistance is the major part for con- tributing magnetoresistance of OSVs. The interface and bulk capacitances both are affected by increasing PTCDA thickness. The bulk capacitance is considered as a parallel plate capacitor, which decreases with increasing thickness of spacer layer, and the dielectric constant of bulk capacitance agrees with the dielectric property of PTCDA. The interface capacitance suffers from interface roughness, which gets worse with increasing PTCDA thickness, causing the interface capacitance became smaller. Magnetocapacitance of bulk and interface show totally different behaviour. This experimental result discovers more behaviour about interface of OSVs, which helps us to improve the application of OSVs in the future.