二維狄拉克材料自旋閥之自旋傳輸特性

Abstract

磁阻元件的主要應用於硬碟讀取磁頭、磁性隨機存取記憶體、磁性感測器與磁性場效電晶體等，目前主流以氧化鎂作為穿隧層的磁性穿隧接面元件，在實際應用中室溫下最高的磁阻值大約為600%。 本論文研究了多個上閘極電位障勢壘的石墨烯與拓撲絕緣體自旋閥，並觀察到優異的磁阻表現及自旋傳輸現象。在基於雙閘極勢壘石墨烯奈米帶的自旋閥中，其高達104 %的磁阻值約是傳統氧化鎂磁性穿隧接面磁阻的15倍，此現象是由反平行配置獨特的電流抑制效果所導致，如此巨大的反平行態電阻是歸因於結構中能在適當的入射能量窗口中調製共振和非共振態通道。此外，本研究發現通過週期性電位障勢壘能夠更進一步提高元件的磁阻性能表現，結果顯示，在具有10單元勢壘的自旋閥中，室溫下的磁阻值超過109 %，對於自旋閥或磁性穿隧接面而言此為非常高的性能表現。當考慮到現實的限制時，本研究採用3單元電位障勢壘的自旋閥元件，其在室溫下仍保持在105 %以上的磁阻值，而在低溫條件下，其磁阻值超過1010 %，通過此研究顯示，經由調整能量窗口中的適當導通帶和禁帶能夠引發超高磁阻的表現，然而，當元件設計不滿足上述討論的條件時，結果得到低於100%非常普通的磁阻。 對於研究中的另一種元件，基於拓撲絕緣體薄膜的自旋閥，在低溫近似費米能階附近顯示出有趣的磁阻振盪，結果顯示，其磁阻表現只有少數具高磁阻值的平坦帶能夠被應用於元件中。根據電流分析，在具有3單元上閘極電位障勢壘的自旋閥中，其室溫下的磁阻值能夠高於1000%，結果顯示平行與反平行配置之間的電阻差異非常顯著，且磁阻效應受到拓樸薄膜厚度、閘極電位、閘極尺寸和閘極分佈等因子的強烈影響。本論文詳盡的研究了二為狄拉克材料自旋閥中的自旋電子傳輸行為，以及超巨大的磁阻效應和其形成的原因，這樣的新穎元件能夠應用於新型儲存記憶體、感測器讀取裝置和自旋電晶體等高效自旋電子元件。

Well-performed magnetoresistance (MR) is a significant aim to develop cutting-edge spintronic devices for high-performance read heads of hard disk drives (HDDs), magnetoresistive random access memories (MRAM), magnetic sensors, and magnetic field-effect transistors. The representative device, magnetic tunnel junction (MTJ), possesses a room-temperature MR ratio about 600% in application using a high-quality (100)MgO tunnel barrier. Here, extraordinary MR effects and related phenomena in multiple top-gate-controlled potentials spin-valve built on a lateral two-dimensional (2D) Dirac material, graphene or topological insulator (TI), as a transport channel are studied in this dissertation. The giant MR value up to 104 % in the double-gate graphene-based spin-valve is approximately 15 times higher than the MR of traditional MgO-based MTJs. The result is produced by particular current suppression in the antiparallel (AP) configuration. Such huge AP-mode resistance is due to the modulation of resonance and non-resonance effect in an appropriate energy window. Furthermore, the MR performance is further enhanced by the periodic gate potentials in the graphene-based spin-valve. It is found that the MR value can easily exceed 109 % at room temperature in the 10-cell gate spin-valves, which is quite amazing performance. When the period number decreases to 3, the MR value still retains up to 105 % at room temperature and up to about 1010 % at extremely low temperature in experiments. Through the investigation, the ultra-giant MR is induced by appropriate allowed bands and forbidden bands in the energy window. Needless to say, the MR value is very ordinary (under 100%) while the device setup keeps away from the circumstances described above (resonance point). For TI-based spin-valves, the device reveals interest MR oscillations near the Fermi level of low-energy approximation. The results exhibit that only a few robust bands of high-MR properties are available in practical applications. A MR value higher than 1000% at room temperature is showed in a TI thin-film (TITF) spin valve with a 3-cell segment gate potential. The results reveal a very large resistance difference between the parallel and antiparallel configurations. The MR effect is strongly influenced by the thin-film thickness, the gate potential, the gate size, and the distribution. The research investigates the ultra-giant MR effect and its reason that could be applied to high-efficiency spintronics devices, including novel storage memories, read-head sensors, and spin transistors.