五環素與鈷介面之磁區微結構與巨觀磁性質之探討

Micro-domain Structure and Magnetic Properties of the Interface between Pentacene and Cobalt Layer

王家豪(Chia-Hao Wang)

指導教授 ： 李志浩 ； 魏德新

國立清華大學/原子科學院/先進光源科技學位學程/碩士(2010年)

https://doi.org/10.6843/NTHU.2010.00266

摘要

本篇論文主要探討在不同製備參數下，有機自旋閥結構中之有機半導體分子五環素(Pentacene, Pc)與蒸鍍在其上層鐵磁層鈷(Cobalt, Co)介面的磁特性。 雙層結構的介面是經由製備超薄膜來模擬，而磁特性的量測則是透過軟X-光吸收光譜、顯像式光電子激發顯微鏡、磁光柯爾效應儀等工具來進行。本研究的實驗結果顯示，當蒸鍍於Cu(100)表面的Pc厚度為8 nm 時， 其上層之Co膜需要達到1.8 nm的厚度才能夠顯現出室溫鐵磁性。同時，Co膜層的磁影像在出現鐵磁性的初期具有多個方向磁化向量，但是隨著Co厚度的持續增加，其磁化向量的方向漸趨一致。類似的鐵磁性與膜厚的關係亦出現在巨觀磁性質的量測上；Co膜層的矯頑磁場隨著厚度而逐漸增加至13.3 Oe的飽和值。
另外，由於稍早的光電子能譜術研究已經指出Co/Pc的介面會出現化學反應，而在對Co的X光吸收光譜的強度進行分析後，我們認為蒸鍍初期的部分Co原子有可能滲入Pc膜層內。本篇論文進而討論是否透過將基板降低溫時蒸鍍Co以及在Co/Pc介面間增加一層阻擋層銅(Copper, Cu)可以降低Co和Pc間發生化學反應的機會。實驗結果證實，於低溫的基板上蒸鍍Co，可將Co在Pc上出現鐵磁性的臨界厚度縮短至1.44 nm，而利用加入2.7 nm Cu作為阻擋層也可以將臨界厚度縮短至0.9 nm。由於極化載子在介面傳輸時，將會穿過此層非磁性層，因此將樣品出現磁性的臨界厚度縮短，可以減少極化載子被散射的機會，對於有機自旋閥元件的效率有著直接的影響。

關鍵字

有機自旋閥 ； 磁區 ； 光電子發射顯微鏡 ； 磁光柯爾效應儀

並列摘要

In this thesis, different fabrication processes were employed to grow Co/Pc(Co on Pc) bilayers structure. Through Magneto Optic Kerr effect (MOKE) and X-ray Photoemission Electron Microscope (X-PEEM), the magnetic responses of ferromagnetic layer deposited on organic semiconductor film were studied. According to the experimental results, Co film depositing on an 8 nm Pc film begins to show its room temperature ferromagnetism at 1.8 nm, along with a complex magnetization patterns. Increasing the thickness of Co film transforms the lateral dimensions and magnetization directions of domains gradually to a larger and better aligned configuration. The coercivity of thick Co film eventually reached a stable value of 13.3 Oe.
According to the X-ray Photoemission Spectroscopy (XPS) study reported earlier, the interface of Co/Pc appears to be chemical reacted. In this study, we step further to investigate “where” this chemical reaction is likely to occur through analyzing the spectra of X-ray Absorption Spectroscopy (XAS). Based on the intensity analysis, the Co atoms are suggested to diffuse into Pc layer at the beginning of Co deposition. In order to prevent the occurrence of chemical reaction and diffusion at interface, two methods were adopted - inserting a 2.7 nm Cu layer as the blocking spacer between Pc and Co layers, cooling the substrate during Co deposition. The results show that both approaches can reduce the critical thickness that corresponds to the onset of room temperature ferromagnetism. Since spin transport in a vertical hybrid structure requires the polarized spins to transport through the interface, practical methods capable of reducing the magnetic critical thickness would lower the chance of polarized spins to be scattered (reduce the scattering cross section of polarized spins), and thus help to maintain the spin coherence among carriers.

並列關鍵字

orgnic spin-valve ； magnetic domain ； PEEM ； MOKE

參考文獻

16. 吳建文,“磁性顯微術”, 國立中山大學物理學研究所, 中華民國九十七年

1. G. Binasch, P. Grünberg, F. Saurenbach, and W. Zinn,“Enhanced magnetoresistance in layered magnetic structures with antiferromagnetic interlayer exchange”, Physical Review B, 39, 4828, 1989

4. V. I. Krinichnyi,“2-mm Waveband electron paramagnetic resonance spectroscopy of conducting polymers”, Synthetic Metals, 108, 173-222, 2000

5. F. J. Jedema, A. T. Filip, and B. J. van Wees,“Electrical spin injection and accumulation at room temperature in an all-metal mesoscopic spin valve”, Nature, 410, 345-348, 2001

7. Z. H. Xiong, D. Wu, Z. V. Vardeny, and J. Shi,“Giant magnetoresistance in organic spin-valves”, Nature, 427, 821-824, 2004