利用自旋極化掃描穿隧電子顯微鏡解析反鐵磁錳薄膜以及鐵磁鈷奈米島的自旋結構

Abstract

隨著奈米磁儲存和自旋電子元件的尺寸不斷縮小，許多低維度下的量子磁性行為以及奇特現象，在近年來引起廣泛的關注與討論。藉由目前最尖端的自旋顯影技術，自旋極化掃描穿隧電子顯微鏡(SP-STM)，能夠提供表面原子尺寸下的磁區顯影和自旋極化能譜分析(SP-STS)，如此般的高空間解析度及多功能力對研究低維度下奈米磁性自旋結構而言是一大潛力。本論文的研究工作是分別在兩套不同的超高真空系統下建構並改進自旋極化掃描穿隧電子鏡的技術，以及成功地應用於研究反鐵磁錳薄膜和鐵磁鈷奈米島的自旋結構。 在本論文的第一部分，我們發現一維的奈米氧化鋁線能夠去成長奈米磁性鈷粒子以其操控其成長有序性，利用STM的表面高度偏壓依賴性和STS的能譜分析，確定奈米鈷粒子的成長行為是和此一維氧化鋁線的高能態密度分佈有密切的關聯性，並且對單電子的行為和不均勻的庫倫阻斷在單一顆奈米鈷粒子中，我們也有其深入解析和探討。 第二部份主要是研究反鐵磁錳薄膜的自旋結構，透過SP-STM解析反鐵磁的奈米自旋結構不但可以瞭解反鐵磁-鐵磁之間的交換偏耦合行為，並對奈米磁讀取頭的應用是有所幫助。我們利用鐵磁薄膜被覆的鎢環狀探針，藉由形狀磁異向能展現平行於表面的自旋解析度，進而得到層狀反鐵磁自旋結構於磊晶錳薄膜在鐵單晶體上。接著在低溫零下195.5度C的超高真空STM系統下，除了層狀反鐵磁自旋結構，自旋翻轉現象於同一層錳膜源自於底下鈷膜的階梯也被觀察到在錳-鈷的多層膜系統，這和我們利用微磁學模擬的結果是相吻合的。 在低溫零下271.5度C解析鈷奈米島的垂直於表面自旋結構是本論文的第三部份，我們利用軟鐵磁材料透過和鈷奈米島之間的磁作用力可達到重複去翻轉探針的磁軸方向，這擴展了SP-STM的技術領域於區分磁和電性訊號上不需使用外加磁場。此外，對於鈷奈米島之間相互競爭的磁性作用力，我們發現在如此小且近距離的尺寸下，磁偶矩間的作用力可以克服鐵磁有序性，使得鈷奈米島有著如反鐵磁般反向自旋排列。而被銅單原子層覆蓋的單一鈷奈米島，從自旋極化能譜分析上我們觀察到不但有著能量上的位移，自旋顯影上也顯示出非線性的自旋結構分布。最後，我們還把有機磁性分子成長於已知自旋結構的鈷奈米島上，結合自旋顯影和極化能譜分析，發現有機分子透過和底下的鈷奈米島的磁性作用，可以被自旋極化出不同的磁軸方向。

The state-of-the-art spin mapping technique of spin-polarized scanning tunneling microscopy (SP-STM) and spectroscopy (SP-STS) can provide information on magnetic domain and electronic structure with extremely high spatial resolution. This essentiality is desperate for the studying of interplay between structural, electronic and magnetic properties in various fascinating low dimensional magnetic systems within nanometer scale. In addition, the potentiality of this tool is possessed of high technological relevance in the respect of magnetic datastorage engineering and spintronic devices developing. In this dissertation work, the implementation with improved functionality of such versatile investigation tool has been realized in two different ultrahigh vacuum (UHV) STM chambers individually. Furthermore, the applications in the resolving spin structures of antiferromagnetic (AFM) manganese (Mn) ultrathin films and FM cobalt (Co) nanoislands have been demonstrated. In the first part of this dissertation, electronically patterning through one dimensional (1D) alumina nanostripes with high density of states (DOS) is presented. With low oxygen gas dosage on NiAl(001) substrate kept at high temperature, the single-crystalline Al2O3 domain with 1D nanostripes can be grown. From the bias dependent topographies and tunneling spectroscopy measurements, higher DOS of nanostripes than that of Al2O3 domain has been found. By using this electronic property, such nanostripes can construct an 1D electronically patterning template to have Co nanoparticles grown with self-alignment. In addition, single electron tunneling behavior of Coulomb blockade and nonuniform distribution of electronic structures within single Co nanoparticle have also been investigated accordingly. For the layered AFM spin structures of both bct and fct Mn with out-of-plane c-axis expansion resolved by SP-STM are included in the second part of this dissertation. With the ring shaped tip simply made by bending tungsten (W) wire,not only atomic resolution of different substrates in STM, but also the in-plane spin identification of magnetically coated ring shaped tip in SP-STM can be successfully achieved. After the room temperature SP-STM work on expanded bct (e-bct) Mn/Fe(001) substrate, the low temperature (77.5K) SP-STM work on e-fct Mn/Co/Cu(001) exchange-biased ultrathin films system is introduced. Due to epitaxially grown on and directly coupled with Co(001) films, not only the in-plane layered AFM spin structure of e-fct Mn ultrathin films, but also the spin frustration across same Mn layer due to the hidden steps of Co underlayers are investigated. The induced domain wall width from spin frustration phenomenon is fitted and in consistent with simulated micromagnetic OOMMF results. Triangular Co nanoislands with bilayer height can be grown on Cu(111) substrate and corresponding out-of-plane magnetic domain images have been resolved by SP-STM at 4.5 K. By using direct magnetic interactions with these single domain Co nanoislands in reduced tunneling distance, the in situ magnetization switching of most front soft magnetic tip end atoms is reliably controlled without applying external magnetic field. Besides, according to the spin-resolved tunneling spectroscopy measurements, not only the shift of spin-polarized surface state, but also non-collinear spin structures within single Cu covered Co island have been found. In addition, for the two connected Co nanoislands with antiparallel spin alignment, there is a competition between exchange coupling and dipolar interaction in the correlation with contact length and area size difference, respectively. After calculating by magnetic dipolar formula and simulation of corresponding magnetic field distribution, dipolar interaction is able to compete with ferromagnetic ordering energy and result in such dipolar antiferromagnetism of two Co nanoislands in close proximity. In the last, we evaporated organic molecule manganese phthalocyanine (MnPc) on these Co nanoislands and MnPc molecules show the preference of adsorption on magnetic Co nanoisland instead of the Cu(111) substrate at 4.5 K. The MnPc being spin-polarized through underneath Co nanoisland has been measured and major contribution to the magnetic asymmetry comes from the spin-polarized adsorption state near to the Fermi energy level.