- 學位論文
從鐵銠到三氧化二鉻:反鐵磁自旋相依傳輸之研究
Investigation of spin-dependent transport in antiferromagnets: From FeRh to Cr2O3
摘要
反鐵磁(antiferromagnets)被視爲是未來自旋電子元件的重要角色。最近，人們在反鐵磁上探索出許多與自旋相依(spin-dependent)的電性傳輸(electrical transport)性質。有研究指出，將一個反鐵磁絕緣體夾在自旋產生端與自旋探測端中間可以大大提高自旋電流(spin current)的訊號。除此之外，大量的研究指出電流中的自選軌道轉矩(spin-orbit torque)可以翻轉反鐵磁中的尼爾向量(N\'eel vector)。然而，蔣智傑發現所謂的反鐵磁尼爾向量翻轉(N\'eel vector switching)的訊號其實是來自於不均匀的焦耳熱(Joule heating)。因此，關於尼爾向量翻轉的議題如今面臨很大的挑戰。 在本論文中,我可以在鐵銠合金(FeRh alloys)中證實用電性或熱激發的方式實現自旋電流閥(spin current valve)。鐵銠具有的一個獨特性質是室溫下的反鐵磁-鐵磁一階相轉變。並且，鐵銠相轉變的溫度也可以用磁場來調節。因此自旋電流閥既可以用溫度調節也可以用磁場調節。在反鐵磁到鐵磁的相轉變過程中，磁阻變化率(magnetoresistance ratio)達到50%。此外，利用熱激發的自旋極化電流所造成的開關比率(ON/OFF ratio)幾乎接近無限大。然而，在鐡銠中我並沒有找到尼爾向量轉動的直接證據。爲了理清反鐵磁尼爾向量翻動的機制，我們轉而去研究反鐵磁三氧化二鉻(Cr2O3)的自旋轉向(spin-flop)。 首先我用單軸(uniaxial)反鐵磁材料三氧化二鉻展示自旋轉向的機制。借由超導量子干涉儀(SQUID)的量測，我在單晶三氧化二鉻塊材中觀測到了自旋轉向。接下來我首次演示了自旋霍爾磁阻(spin Hall magnetoresistance)可以用電性的方式來偵測自旋轉向，並且用電性量測到的行爲與磁性是一致的。在縱向電性傳輸中(longitudinal electrical transport)，我量測了磁場角度相依(magnetic field angular-dependent)的自旋霍爾磁阻。結果顯示尼爾向量與磁場角度并不是簡單的$\cos^2{\theta}$的關係，因爲單軸反鐵磁的晶體各向異性能量(crystal anisotropy energy)會與外加磁場相競爭。最重要地，我們證明了尼爾向量歷經自旋轉向躺在平面後，可以被一個很小的平面内的外加磁場操控且尼爾向量垂直於磁場。利用這一特性，我們的結果提供了清楚的證據來證明如何在反鐵磁中用電性的方式來實現“讀”與“寫”。這一成果將為未來的反鐵磁翻轉研究奠定重要的基石。
並列摘要
Antiferromagnet (AFM) is regarded as the key material for future of spintronic devices. Recently, numbers of spin-dependent electrical transport properties of AFM have been exploited. It was reported that an AFM insulator sandwiched between spin generator and spin detector can enhance the signal of spin current. Also, plenty of researchers have reported that they can switch AFM electrically. However, my colleague Chiang et al. found out that the so-called AFM current switching signal actually comes from non-uniform Joule heating. Therefore, the experimental realization of N\'eel vector switching remains challenge. In this work, we demonstrate spin current valve can be switched electrically and thermally in FeRh alloy. The first-order AFM-FM phase transition near room temperature is one of the unique features of FeRh. Also, the phase-transition temperature can be modulated by magnetic field. we could demonstrate the spin current valve either by tuning the temperature or by the magnetic field. A large magnetoresistance ratio 50% is obtained when switching between AFM and FM states. Furthermore, the ON/OFF ratio for thermally induced spin-polarized current is nearly infinite. However, we can't clearly identify the orientation of AFM N\'eel vector in FeRh. In order to figure out the mechanism of AFM N\'eel vector switching, we turn to investigate spin-flop transition in AFM Cr2O3. We first study the mechanism of spin-flop in uniaxial AFM material, Cr2O3. we observe the spin-flop transition in single crystal Cr2O3 slab by using SQUID. Then I demonstrate the electrical detection of spin-flop for the first time by spin Hall magnetoresistance (SMR) probe, which is consistent with magnetic property measurements. In the longitudinal electrical transport, we measure the magnetic field angle-dependent SMR. The result indicates that the angular dependence of N\'eel vector is not simple $\cos^2{\theta}$ due to competition between crystal anisotropy of uniaxial AFM material and external magnetic field. Most importantly, we demonstrate that N\'eel vector could be perpendicularly rotated by a small in-plane magnetic field after N\'eel vector lying down in-plane during spin-flop transition. Utilizing this property, our result provides the unambiguous evidence how to write and read in AFM by electrical means. This result may lay a foundation for further AFM-based switching exploration.