近年，透過電流將磁化方向翻轉的技術有了顯著的進步，特別是在自旋軌道矩-磁性記憶體(SOT-MRAM)的應用上，透過自旋軌道矩(spin-orbit torques)翻轉磁化方向的方式，來實現低耗能，高密度的邏輯電路。自旋軌道矩最初認為是來自於塊材的自旋霍爾效應和界面的Rashba效應。目前為止，大部分針對這兩個現象的研究是透過重金屬/鐵磁層的系統來完成。當外加電流通過該系統時，重金屬提供自旋霍爾效應，在垂直方向上產生了自旋電流，而這個自旋電流對鐵磁層產生了自旋軌道矩，導致了磁性的翻轉。
本實驗使用的結構是重金屬/反鐵磁/鐵磁材料 (HM/AFM/FM)三層結構的系統。於此結構中，重金屬產生的自旋電子流會先穿過反鐵磁層再到達鐵磁層，這跟以往研究大部分使用(HM/FM/AFM)結構中，最重大的區別。在於這個複雜的反鐵磁結構，不僅能夠在未來提供另一個探討自旋軌道矩的系統，在跟MTJ的相容性方面，由於反鐵磁跟重金屬都在鐵磁層的同一側，因此將來能夠更順利的整合入SOT-MRAM。
透過重金屬/反鐵磁/鐵磁這樣的三層結構中，我們成功實現完全磁化翻轉以及垂直膜面的交換偏壓反轉。這表示了自旋軌道矩可以改變反鐵磁跟鐵磁界面中造成交換偏壓產生的未補償自旋(uncompensated spin)。另一個對於磁性記憶體來說重要的參數是鐵磁層的熱穩定性。這個參數決定了磁性記憶體的單一位元在常溫中受到熱擾動時，是否會產生訊號的識別錯誤，因此透過垂直交換偏壓的引進，能夠間接大幅提升鐵磁層的熱穩定性。此外，我們研究了在外加場下的自旋軌道矩翻轉磁化方向。我們展示了沿電流方向(x方向）或/和垂直於電流方向(z方向)的外場如何影響自旋軌道轉矩切換。我們還研究了反向結構FM/AFM/HM。反向結構(FM/AFM/HM)表現出獨特的自旋軌道扭矩切換行為，由於在這種結構中不施加任何外部場，我們可以實現完全磁化和交換偏壓翻轉。這個結果可以成為下一代SOT-MRAM的構建模塊，因為這個設計可以解決SOT-MRAM的兩個最重要的挑戰，第一個是沒有外場的自旋軌道扭矩切換，第二個是來源於垂直交換偏壓而有更高的熱穩定性。

Magnetization reversal from the electric current is an area which has made rapid advancements in recent time. In particular, SOT-MRAM, where magnetization is reversed by spin-orbit torques. Spin-orbit torque technique offers energy efficient magnetization switching, and also high scalability in logic circuits. Spin-orbit torques are originated from the bulk spin Hall effect (SHE) and interfacial Rashba effect. So far, these effects are mainly studied in heavy metal and ferromagnet bilayer system. Where the heavy metal exhibits bulk spin Hall effect (SHE). When charge current is injected in the HM/FM bilayer system, a spin current is generated in the transverse direction and this spin current exerts the spin-orbit torques on ferromagnet which results in magnetization switching.
Here, in this work, we have studied heavy metal, antiferromagnet, and ferromagnet (HM/AFM/FM) tri-layer structure. In this structure, the spin current generated by the heavy metal first injected in the antiferromagnet and then reaches to a ferromagnet, which is the major difference between the tri-layer structure researcher mainly studied in the past (HM/FM/AFM). Given the complex structure of the antiferromagnet opens an exciting path to explore the spin-orbit torques in this structure. Moreover, the HM/AFM/FM structure is also compatible with full MTJ structure since AFM and HM both are one side of FM. Therefore, this structure can be easily integrated into SOT-MRAM.
In the tri-layer structure, we demonstrated that the full magnetization reversal is achieved along with the perpendicular exchange-bias reversal. This shows that the spin-orbit torque can alter the interfacial uncompensated spin of AFM which leads to the exchange-bias reversal. For MRAM, thermal stability is a crucial parameter. It determines the stability of MRAM bit upon thermal agitation, the introduction of a perpendicular exchange-bias provides the unidirectional anisotropy to the ferromagnet. The unidirectional anisotropy enhances the thermal stability of FM enormously. Besides, we also studied the spin-orbit torque switching under the external fields. We showed how the external field along the current direction (x-direction), or /and perpendicular to the current direction (z-directional) can influence the spin-orbit torque switching. Along with this structure, we also studied reversed structure FM/AFM/HM. The reverse structure exhibits unique spin-orbit torque switching behavior, because in this structure without applying any external field we can achieve full magnetization and exchange bias switching. This result can be the building block for next-generation SOT-MRAM, because this design can tackle two most important aspects of SOT-MRAM, first, spin-orbit torque switching without an external field, and second, higher thermal stability from the perpendicular exchange bias.

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