自旋霍爾效應驅動之人工反鐵磁零場電流翻轉

Current-Induced Switching of synthetic antiferromagnet without magnetic field from the Spin Hall Effect

李甄棋(ZHEN-QI LI)

指導教授 ： 陳恭

國立中正大學/理學院/物理學系/碩士(2016年)

摘要

過去實驗發現利用電流誘發之等效場能間接或直接地操控磁矩翻轉，從文獻中提到，在重金屬材料(Ta、Pt)通入in-plane電流，利用鐵磁層結構斜面破壞，或在下層增加一天然反鐵磁層PtMn，造成對稱性破壞，且不需要外加磁場就能使鄰近鐵磁層產生等效場並驅動磁矩翻轉；文獻中的結構主要以探討單層鐵磁系統為主。若電流翻轉不須外加磁場，只需通入電流就能造成磁矩翻轉，此技術將其導入自旋電子元件中會是一項重大突破。
本實驗拓展電流誘發磁矩翻轉的研究，利用濺鍍方式製作反鐵磁耦合雙層結構MgO/CoFeB/Ru/CoFeB/MgO以及MgO/CoFeB/TaRu/CoFeB/MgO的薄膜此結構由於MgO/CoFeB界面效應，具備垂直磁異向性(PMA)特性。詳細的膜厚為：
Sub/ Ta(10)/ MgO(1)/ CoFeB(1.2)/ Ru(2.2)/ CoFeB(1.2) / MgO(1) / Ta(3)
Sub/ Ta(10)/ MgO(1)/ CoFeB(1.2)/Ta(0.3)Ru(0.7)/ CoFeB(1.2) / MgO(1) / Ta(3)單位是nm。
量測的方式是將電流注入Hall bar元件中，電流方向為x方向，施加外加磁場的方向分別為Hz(垂直模面)、Hx(平行電流)、Hy(垂直電流)，並在y方向進行異常霍爾電性量測。

實驗主要分為兩部分：
1.異常霍爾效應(AHE)量測

改變電流的方向及大小，進行磁場掃描，定義霍爾電阻曲線的翻轉形式，並根據霍爾電阻曲線的特徵以及磁性態畫出相圖。相圖的特點在於非典對稱圖形，而是呈現鏡像對稱的特徵。
2.電流驅動磁矩翻轉

施加固定磁場，進行電流掃描，當電流達到臨界電流時，磁矩發生翻轉。根據曲線的臨界電流大小以及施加之磁場，畫出相圖，利用相圖去討論磁性態；另外，此結構在不施加外加磁場的情況下亦能引發磁矩翻轉之現象，表示系統面內的對稱性破壞的特性，同時此零場翻轉的現象有具有未來製作元件的潛力。

關鍵字：自旋霍爾效應、人工反鐵磁、電流翻轉

關鍵字

電流翻轉 ； 人工反鐵磁 ； 自旋霍爾效應

並列摘要

Past experiments showed that the equivalent field induced by using the current can indirectly or directly control the magnetization reversal. It is mentioned in the literature that the structural slope damage of ferromagnetic layer can be utilized by applying the in-plane current into the heavy metal materials (Ta, Pt) or the symmetry damage can be resulted in by adding a natural antiferromagnetic layer (PtMn) on the bottom layer, which can enable the neighboring ferromagnetic layer to produce the equivalent field and drive the magnetization reversal without needing to add the additional magnetic field. The structures in the literatures mainly involve the exploration of the single-layer ferromagnetic system. If no additional magnetic field needs to be added for the current reversal and the magnetization reversal can be caused only by applying the current, it would be a major breakthrough if this technology is introduced into the spintronic devices.
This experiment expands the study on the current induced magnetization reversal and utilizes the sputtering method to produce the film of the antiferromagnetic coupled double-layer structure of MgO/CoFeB/Ru/CoFeB/MgO and MgO /CoFeB Ta/Ru/CoFeB/MgO.Due to the interface effect of MgO/CoFeB, this structure has the property of perpendicular magnetic anisotropy (PMA). The specific film thicknesses are:
Sub/ Ta(10)/ MgO(1)/ CoFeB(1.2)/ Ru(2.2)/ CoFeB(1.2) / MgO(1) / Ta(3)
Sub/ Ta(10)/ MgO(1)/ CoFeB(1.2)/Ta(0.3)Ru(0.7)/ CoFeB(1.2) / MgO(1) / Ta(3). The unit is nm.
The measurement method is to apply the current into the Hall bar component. The current direction is in the x direction; the directions of additional magnetic field application are Hz (vertical modular surface), Hx (parallel current) and Hy (vertical current) respectively; and the anomalous Hall electrical property is measured in the y direction.
This experiment expands the study on the current induced magnetization reversal and utilizes the sputtering method to produce the film of the antiferromagnetic coupled double-layer structure of MgO style/CoFeB/Ru/CoFeB/MgO style and MgO style/CoFeB TaRu/CoFeB/MgO.Due to the interface effect of MgO/CoFeB, this structure has the property of perpendicular magnetic anisotropy (PMA). The specific film thicknesses are:
Sub/ Ta(10)/ MgO(1)/ CoFeB(1.2)/ Ru(2.2)/ CoFeB(1.2) / MgO(1) / Ta(3)
Sub/ Ta(10)/ MgO(1)/ CoFeB(1.2)/Ta(0.3)Ru(0.7)/ CoFeB(1.2) / MgO(1) / Ta(3). The unit is nm.
The measurement method is to apply the current into the Hall bar component. The current direction is in the x direction; the directions of additional magnetic field application are Hz (vertical modular surface), Hx (parallel current) and Hy (vertical current) respectively; and the anomalous Hall electrical property is measured in the y direction.

The experiment is mainly divided into two parts:

1. Measurement of anomalous hall effect (AHE)
Change the current direction and size. conduct the magnetic field scanning to define the reversing mode of the Hall resistance curve and develop the phase diagram according to the characteristics of the Hall resistance curve and magnetic state. The characteristic of the phase diagram lies in that instead of showing the typically symmetric figure, it presents the characteristic of mirror symmetry.

2. Current driven magnetization reversal
Apply the fixed magnetic field and conduct the current scanning; and the magnetization reversal would occur when the current reaches the critical current. Develop the phase diagram according to the critical current size of the curve and the applied magnetic field and use the phase diagram to explore the magnetic state. This structure can also induce the magnetization reversal without applying the additional magnetic field, which indicates the characteristic of symmetry damage within the system plane; at the same time, the zero-field reversing phenomenon has the potential of component production in the future.

Keyword：spin Hall Effect ,synthetic antiferromagnet, current-Induced Switching

並列關鍵字

synthetic antiferromagnet ； Spin Hall Effect ； current-Induced Switching

參考文獻

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[4] L’ubom´ır Baˇnas “Numerical Methods for the Landau-Lifshitz-Gilbert Equation”

[5] PhysRevB.89.104421 ”Magnetization switching through spin-Hall-effect-induced chiral domain wall propagation”