層間反對稱交互作用: 物理行為以及三維磁元件的自旋工程

Abstract

近年來，Dzyaloshinskii-Moriya交互作用（DMI）因其在產生非共線鐵磁性方面的關鍵作用而受到了不少關注，它在手性磁域壁的形成以及穩定斯格明子等拓撲結構等情況中至關重要。科學家們已經探索了各種創新性的想法，嘗試通過電流來操縱這些具有DMI結構，以期創造可行的基於自旋的賽道/邏輯元件。不過，由於薄膜的堆疊順序可以自然產生垂直平面的對稱性破缺，DMI至今主要被限制於介面的情況中。在這篇論文中，我將展示如何通過引入一個平面內的對稱性破缺，將DMI擴充到一個新穎的（IL-DMI）的情況。在這個系統中，被一個非磁性的間隔層分開的兩個磁化量會因IL-DMI而傾向垂直排列。 首先，我在具有正交磁化量的磁多層膜系統中觀察到了不低的IL-DMI。通過利用特殊設計的變角度的磁場掃描，在實驗中觀察到了IL-DMI的反對稱性質及其飽和行為。利用反常霍爾效應（AHE）和橫向磁光克爾效應（L-MOKE）探測分別具有垂直磁化（PMA）和平面內磁化（IMA）的兩個鐵磁層，我在驗證了IL-DMI的對稱性的同時計算了IL-DMI的強度。透過同時觀測兩個鐵磁層並在實驗上證明兩者同時發生磁化翻轉的行為，我展示了如何通過IL-DMI將針對特定磁化的操縱轉移到與其正交的另一個鐵磁層上。 隨後，我對不同對稱性破缺要素的有效性進行了研究。首先，我將IL-DMI的重要性與PMA層中的傾斜磁各向異性進行了比較，這兩種效應由於在材料成長中使用了斜角沉積（OAD）而會共存於磁薄膜中。我接著通過IMA和PMA的磁化翻轉特性的比較確認了IL-DMI的重要性。其次，與OAD不同，我研究了實現平面內對稱性破缺的另一種方法：在外加的平面場Hext下進行樣品生長。我確認了雖然OAD和使用Hext都能在IMA層中產生磁各向異性，但基於磁異向性的的對稱性破壞對IL-DMI的強度和方向幾無影響，這強烈暗示了IL-DMI的起源是結構性的。 接著通過利用多個OAD膜層，我發現了IL-DMI的整體強度和方向是由各個楔形層的貢獻所決定。這些獨立貢獻與總體強度之間明顯的線性關係，以及觀察到的相反特徵方向都啟發了我，發展了基於Fert-Lévy三位元點模型的玩具模型。此一模型可以良好的整合實驗結果並解釋IL-DMI的物理起源。此外，通過適當的OAD設計策略，可以抑制由OAD引起的空間不均勻性的同時最小化對IL-DMI強度的降低，對元件的實用化有益。最後，我明確展示了當相互作用由Ru傳導時，IL-DMI強度有明顯的阻尼振盪行為。根據理論預測的擬和得出約1納米的振盪週期證實了理論預測，即DMI是RKKY相互作用的一個附加項。

In recent years, Dzyaloshinskii-Moriya interaction (DMI) has garnered enormous attention for its pivotal role in facilitating noncollinear magnetism, such as the formation of chiral domain walls and stabilizing topologically distinct structures like magnetic skyrmions. Various innovative proposals have been explored to manipulate these DMI mediated structures via electric currents, to create viable spin-based racetrack/logic devices. Thus far, DMI has primarily been confined to an interfacial case due to the out-of plane symmetry breaking naturally generated by the film’s stacking order. In this dissertation, I showcase that by introducing an in-plane symmetry breaking element, DMI can be extended to a novel interlayer scenario (IL-DMI). Here, perpendicular alignment of spins is mediated between two discrete magnetizations, separated by a nonmagnetic spacer. Firstly, sizable IL-DMI is observed within a multilayer system with orthogonal magnetic orientations. The antisymmetric nature of IL-DMI and its saturating behavior are both experimentally observed utilizing an angle-dependent field-scan protocol. The directionality of IL-DMI is confirmed to obey the symmetry pointed out by Moriya. By probing both perpendicularly magnetized (PMA) and in-plane magnetized (IMA) layers via anomalous Hall effect (AHE) and longitudinal magneto-optical Kerr effect (L-MOKE), respectively, the reciprocity of IL-DMI is verified, with the strength of IL-DMI estimated. Particularly, supported by the coherent switching of both IMA and PMA magnetizations, I demonstrate how manipulation of one magnetization can be transposed to an orthogonal axis through IL-DMI. The efficacy of different symmetry breaking factors are subsequently investigated. First, the importance of IL-DMI is compared to the tilted magnetic anisotropy in the PMA layer where both effects coexist due to the oblique angle deposition (OAD). Their relative magnitudes are evaluated, affirming the importance of IL-DMI by comparing the IMA and PMA switching characteristics. Secondly, in contrast to the OAD process, we investigate an alternative means of achieving in-plane symmetry breaking: sample growth under an external in-plane field Hext. It’s concluded that while both OAD and Hext can induce magnetic anisotropy in the IMA layer, magnetically based symmetry breaking factor has a negligible impact on IL-DMI’s strength and direction. This strongly suggests that the origin of IL-DMI is structural. Lastly, by exploiting multiple OAD opportunities, I unveil the overall strength and direction of IL-DMI to be governed by individual wedged layer’s contributions. The evident linearity, and the opposite characteristic directions observed among these individual contributions inspired the development of a toy model based on the Fert-Lévy three site model. This model can explain the physical origin of the IL-DMI while incorporating experimental findings. Furthermore, by implementing proper OAD design strategies, spacial inhomogeneity created by OAD can be suppressed while minimizing the reduction in IL-DMI strength. Finally, I definitively showcase a damped oscillation of IL-DMI when the interaction is mediated by Ru, a conventional Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction spacer. The identical oscillation period of ~ 1 nm confirms the theoretical prediction that DMI arises from an additional term in RKKY interaction.