二維材料於自旋電子學之應用

Abstract

二維材料由於其銳利的界面和原子厚度，是當今自旋閥元件中，金屬或金屬氧化物材質的間隔層最有希望的替代材料。本論文通過數值模擬和實驗研究了二維材料在自旋閥元件中的應用。 我們開發了Co/Gr/Co元件的製程，期間我們使用拉曼光譜學來確認石墨烯成功轉移。在電阻隨溫度變化的測量中，我們觀察到在低於150K的溫度下，隨著溫度降低，電阻上升，這對應了在波茲曼模型中一個0.1 meV的能隙，顯示了元件中能帶結構的扭曲。 為因應這現象，我們使用了密度泛函理論模擬軟件QuantumATK來研究Co/Gr/Co元件的能帶結構，觀察到鈷和石墨烯之間存在著強烈的耦合和軌道混合。我們還將這類計算擴展到其他二維材料，如二硫化鉬和六方氮化硼。此外，我們還透過模擬觀察到，氧化的鐵磁層會大大降低磁阻比。 在磁阻測量中，我們未能觀察到在平面磁場中，元件有顯著的磁阻信號。這可能是由於轉印二維材料的過程中，元件底部電極層的氧化。相比之下，我們觀察到在我們實驗室另一位同事開發的更先進的元件(其內沒有氧化層)中，觀察到顯著的磁阻信號，這證實了氧化層的確破壞了磁阻。這個結果與我們之前的數值計算結果一致。這個先進的元件還具有其他優勢，比如更低的阻面值（阻力乘以面積）和更好的介面面質量。 最後，我們在垂直磁場中的元件中觀察到了由塞曼效應和漢勒效應引起的磁阻信號。這樣的觀察結果表明在我們的元件中具有自旋極化，並有助於未來設計具有顯著自旋信號的自旋電子元件。

Two-dimensional material is a promising alternative to metal or metal oxides as the spacer in nowadays spinvalve devices due to its sharp interface and atomic thickness. This thesis investigates the application of two-dimensional material to spinvalve by means of numerical simulation and experiment. We developed the fabrication method to build up a Co/Gr/Co device, during which we used Raman spectroscopy to confirm a successful transfer of graphene. In temperature-dependent measurement of electrical resistance, we observed a rise of resistance as the temperature is lowered under 150K, corresponding to a band gap of 0.1 meV in Boltzman model, suggesting a distortion of bandstructure in the device. In response, we used QuantumATK, a DFT-simulation software, to investigate the bandstructure of Co/Gr/Co device and observed a strong coupling and orbital hybridization between cobalt and graphene. We also extended such calculation to other 2D materials such as molybdenum disulfide and hexagonal boron nitride. In addition, we also observed that oxidized FM layer can decrease the magnetoresistance ratio dramatically in the simulation. For the magnetoresistance measurement, we failed to observe significant magnetoresistance signals in the in-plane magnetic field. This may be due to the oxidized bottom electrode layer from the transfer process. In comparison, we observed significant magnetoresistance signal of a more advanced fabrication device, which is without oxidation layer and developed by another colleague in our laboratory, confirming that oxidation destroys the magnetoresistance. This result is consistent with our previous numerical calculation. The advanced device also has other advantages, such as lower resistance-area value (resistance×area) and better interface quality. Lastly, we observed magnetoresistance signals from Zeeman effect and Hanle effect in the devices in the perpendicular magnetic field. Such observation suggests that the spin polarization is retained in our device, and benefit future design of spintronic devices with strong spin signals.