應用於自旋電子學的新穎磁性氧化物

Novel Magnetic Oxides for Spintronic Applications

巫啟男(Chi-Nan Wu)

指導教授 ： 郭瑞年

國立清華大學/理學院/物理學系/博士(2015年)

摘要

鐵磁氧化物被廣泛的研究，因其具有十分有趣的物理特性，以及有潛力應用於自旋電子學元件。本論文聚焦在研究應用於自旋電子學的新穎磁性氧化物，並根據主題包括：「無群集，鈷摻雜氧化釔（Co: Y2O3）高介電係數稀磁氧化物（DMO）」，「雷射分子束外延（Laser MBE）生長SrRuO3和Sr2RuO4用於實現自旋極化的二維電子氣（2DEG）和自旋極化超導電流」和「磁性氧化物釔鐵石榴石（YIG）結合拓撲絕緣體（TI）的自旋幫浦研究和自旋轉移矩鐵磁共振（ST-FMR）以開發自旋電子元件」。此外，在成長YIG異質結構之前，我們也進行了TI /鐵磁金屬（TI / FM）的自旋幫浦研究。在低溫沉積生長的無群集，Co: Y2O3，我們觀察到室溫（RT）鐵磁性，且通過退火過程調製氧空缺濃度，我們可以控制飽和磁矩。我們證明氧空缺在鐵磁有序中發揮了至關重要的作用，其為磁性極化子模型的缺陷中心，以解釋中能隙，低載子濃度DMO的磁性來源。對於Laser MBE生長的SrRuO3和Sr2RuO4，我們獲得SrRuO3膜的良好結晶性和理想的電阻率。我們也獲得Sr2RuO4膜的良好結晶性和非常低的殘餘電阻率，但沒有觀察到超導現象。可能由於Sr2RuO4超導對於缺陷很敏感，並容易被缺陷破壞。為了實現超導，我們需要付出更多努力來降低Sr2RuO4的剩餘電阻。此外，我們成功的成長出具有良好結晶度，而且是金屬性的SrRuO3 / SrTiO3超晶格，提供了一個基礎來研究自旋極化2DEG。在Bi2Se3 / Fe3Si和Fe / Bi2Te3高品質的薄膜的自旋幫浦研究方面，我們在室溫下觀察到，由於逆自旋霍爾效應產生的大電壓（VISHE），也進行了自旋霍爾角計算。 TI /FM產生的價電電流密度比Fe3Si /NM和Fe3Si /GaAs結果都還來的高，大約2-5倍，這可歸因於TI固有的強的自旋-軌道耦合特性，並表示以TI為基礎的元件可在自旋電子的應用上有很大的潛力。我們成長並分析了高品質的YIG薄膜。 YIG薄膜的優良結晶度和低磁阻尼確保了高效的自旋輸運和自旋幫浦量測，我們分別在Bi2Se3 / YIG，白金/ YIG結構上都進行了實驗。我們都觀察到了VISHE和大的共振場的負位移。此外，ST-FMR在鉑/ YIG結構已被我們觀察到，這提供了一個新方向，以開發用於自旋電子應用的新穎設備。

關鍵字

磁性氧化物 ； 自旋電子學 ； 稀磁性氧化物 ； 雷射分子束磊晶 ； 自旋幫浦 ； 拓樸絕緣體 ； 釔鐵石榴石

並列摘要

Magnetic oxides have been studied widely for the interesting physical properties and potential applications for spintronics. This thesis was focused on the novel magnetic oxides for spintronic applications, and based on the topics including: "cluster free, Co doped Y2O3 (Co: Y2O3) high  diluted magnetic oxides (DMO) ", "Laser MBE grown SrRuO3 and Sr2RuO4 for realizing the spin polarized 2 dimensional electron gas (2DEG) and the spin polarized supercurrent", and "magnetic oxide yittrium iron garnet (YIG) combined with topological insulator (TI) for the study of spin pumping and spin-transfer-torque ferromagnetic resonance (ST-FMR) for novel spintronic devices". Spin Pumping in TI/ferromagnetic metal (TI/FM) heterostructures were studied before YIG-based heterostructures researches. For the cluster free, DMO of Co: Y2O3 grown by low temperature deposition, room temperature (RT) ferromagnetism was observed, and the saturation magnetic moment was modulated by oxygen vacancy concentration through post annealing process. Oxygen vacancies are shown to play a crucial role in ferromagnetic ordering, as defect centers in the bound magnetic polaron model to account for this DMO of medium band gap with low carrier concentration. For Laser MBE grown SrRuO3 and Sr2RuO4, we obtained good crystallinity of SrRuO3 films and reasonable RT resistivity. We also obtained good crystallinity of Sr2RuO4 films and very low residue resistivity, but without superconductivity, which might be sensitive and destroyed by the defects. More efforts are needed to acquire lower residual resistivity for the superconductivity. The metallic SrRuO3/SrTiO3 superlattice with good crystallinity was successfully grown, providing a matrix to studied spin polarized 2DEG. The spin pumping effect in high quality films of Bi2Se3/Fe3Si and Fe/Bi2Te3 were studied. At RT, large voltages due to the inverse spin Hall Effect (VISHE) were detected in Bi2Se3/Fe3Si and Fe/Bi2Te3. The spin Hall angle was calculated. The charge current densities of TI/FM are about 2-5 times higher than the Fe3Si/normal metal and Fe3Si/GaAs results, attributed to strong spin-orbit coupling inherent of TIs, demonstrating the high potential of exploiting TI-based structures for spintronic applications. High quality YIG films have been grown and characterized. The excellent crystallinity and low magnetic damping of YIG films ensure efficient spin transport measurements and spin pumping have been performed in Bi2Se3/YIG, Pt/YIG structures. We observed VISHE and large negative resonance field shifts. Moreover, ST-FMR has been demonstrated in Pt/YIG structures, providing a direction to developing novel devices for spintronic applications.

並列關鍵字

Magnetic Oxide ； Spintronics ； Diluted Magnetic Oxide ； Laser MBE ； Spin Pumping ； Topological Insulator ； Yttrium Iron Garnet

參考文獻

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