磁性拓撲絕緣體是近年來凝態物理受歡迎的一個題目，這一種材料能在無磁場下的邊界有一維無耗散的手性邊界態，且內部為絕緣體，能使用在自旋電子學中有低功耗與低失真特性。目前主要實現的方法為參雜磁性元素在拓樸材料，或是藉由磁性材料的鄰近效應使得拓樸材料具有磁性，然而至今為止仍缺乏對二維磁性拓樸絕緣體的了解，因此引起我們對此系統研究興趣。
我們在先前的研究中[1]使用反鐵磁的鉻針，研究具有鐵磁性2 ML厚度的三角形Co島嶼在Cu(111)基板上，且會因為fcc與hcp堆疊方式不同，而島嶼具有不同的方向。再者，我們通過在低溫下成長Sn在鐵磁性Co島嶼上，用掃描穿隧顯微鏡(STM)觀察Sn在Co島嶼上與基板上的晶格長度都為0.51 nm的單層stanene(以錫排列成為蜂巢狀晶格結構)，再透過掃描穿隧能譜(STS)量測中發現stanene/Co上在+0.1 eV具有峰值，且這個峰值未在基板上的stanene發現。這個峰值可能是因Co島嶼的磁性鄰近效應導致stanene時間反演對稱性被破壞，因此這個系統有機會能成為二維磁性拓樸材料的研究。
在此篇研究中，從LEED討論Sn隨著鍍量增加在Co/Cu(111)系統上從P(2 x 2)到P(2 x2)與\beta-Sn同時存在，再到只有\beta-Sn晶格結構，根據不同的晶格結構上使用MOKE分析，證實了此系統具有鐵磁性且Sn會增強垂直磁性。再者，藉由自旋極化掃描的STM與STS(SP-STM/SP-STS)，量測到stanene/Co上有+0.1 V與-0.1 V的峰值，而未在基板上stanene 發現這一特徵，且這兩個峰值會隨著探針與樣品自旋極化方向平行與反平行，有對比度反轉的現象，顯示這兩個峰值為majority或minority主導，我們能再通過外加磁場翻轉針與樣品的磁性方向，觀察因穿隧電導隨著磁場變化的磁阻效應(magnetoresistance)，觀察島嶼與探針的磁性翻轉磁場的大小。再進一步透過理論的計算這個系統，得知stanene/Co島嶼具有自旋軌道藕荷能隙且在+0.1 V與-0.1 V具有majority與minority貢獻，我們能透過dI/dV asymmetry來觀察，stanene/Co島嶼與Co島嶼的自旋極化分布差異。

Magnetic topological insulators (Magnetic TI) have recently been a hot topic in condensed matter physics. Magnetic TI exhibits the quantum Hall effect without a need of external magnetic field and can be used in the application of spintronics with a low power consumption. Currently, magnetic TIs can be achieved by introducing magnetism into TI, such as by doping with magnetic impurities or by magnetic proximity effect next to the magnetic materials. However, magnetic TIs with dimension down to single atomic layer, i.e., two-dimensional limit, has not been reported yet, which is the main motivation to grow atomically-thin stanene on top of ferromagnetic Co nanoislands from this thesis work.

In our previous experiments [1], we investigated ferromagnetic 2 ML Co islands on Cu(111) by using SP-STM with an antiferromagnetic Cr tip. Two types of Co island orientations have been found, which is due to the hcp and fcc stackings on Cu(111) surface. In addition, we have successfully fabricated a monolayer stanene – the tin counterpart of graphene – on Co nanoislands by means of a low-temperature growth. The lattice constant of the stanene on Co nanoislands is 0.51 nm, which is the same as that of the stanene on the Cu(111) substrate. In the spectral curves, the stanene/Co islands has peaks at about ±0.1 V, which were neither found in stanene/Cu(111) nor in bare Cu(111). This system, i.e., stanene on ferromagnetic Co nanoislands is expected to provide an opportunity to investigate the interplay between magnetism and two-dimensional topological properties.

Apart from STM studies, the crystalline structures of Sn on Co/Cu(111) has also been studied by LEED, and the diffraction pattern that varies from p(2 x 2)-stanene to a higher-coverage-Sn phase has been resolved. By using MOKE measurements, magnetic hysteresis loop indicating an enhancement of perpendicular magnetic anisotropy has been observed for stanene covered ferromagnetic Co islands. From SP-STM and SP-STS experiments, the ferromagnetic spin contrasts have been resolved on stanene/Co/Cu(111), and two peaks at about
m0.1 V are responsible for the asymmetry of spin-polarized dI/dU signals. Furthermore, the spin-polarized dI/dU as a function of applied magnetic field and the resultant spatial mapping of spin polarization of stanene capped Co islands have been measured. Finally, our preliminary DFT calculations have found a spin-orbit coupling gap (~0.26 eV) on the stanene/Co/Cu(111) and the spin-polarized peaks at
m0.1 V are related to the both spin majority and minority contributions.

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