Ta/CoFeB/MgO與MgO/CoFeB/Ta上下結構的自旋霍爾效應驅動磁翻轉之比較與異向能對其影響之探討

Current Induced Spin Orbit Torque Switching of Perpendicularly Magnetized Ta/MgO/CoFeB and MgO/CoFeB/Ta Structures

游劉傳(Liou-Chuan Yu)

指導教授 ： 陳恭

國立中正大學/理學院/物理學系/碩士(2015年)

摘要

當施加平行於膜面的電流通過重金屬層(如Ta或Pt)，由於自旋霍爾效應，沿著橫向方向(水平y方向)會生成一自旋電流，此自旋電流產生的力矩用來可以翻轉鄰近的鐵磁層。值得注意的是在這個過程中，另外需要一個外加的磁場沿著縱向方向(水平x方向)以打破其磁狀態的對稱性。目前這個機制被用在具垂直磁化的MgO/CoFeB/Ta和Pt/Co結構，以及其他的磁性元件。
本論文主要在探討具垂直磁化之單層結構系統在通入電流時，因自旋霍爾效應 (Spin Hall effect)而導致磁翻轉的現象分析。我們選用的樣品結構分為兩類:
(1)上結構: Sub/Ta(5)/MgO(1)/CoFeB(1.4)/Ta(x)，x= 2 ,3 ,5 nm。
此系列是透過覆蓋層(Ta)的厚度的改變，觀察金屬層其厚度對自旋霍爾效應的影響。
(2)下結構: Sub/Ta(5)/CoFeB(y)/MgO(1)/Ta(5)，y = 0.9 , 1.1 , 1.3 nm。
此系列是探討不同厚度的鐵磁層對於自旋霍爾效應的影響性。
樣品製備方面我們透過微奈米蝕刻的方式將這些結構分別蝕刻成尺寸為 10 μm × 60μm及 15 μm × 150μm的Hall bar元件。蝕刻過後通入一外加電流於Hall bar的水平(x)方向，並同時施加一z方向(垂直)或x方向(平行)的外加磁場，並於Hall bar兩端的電極量測霍爾電阻。此外，磁滯曲線量測方式時改變的實驗參數如下:
磁場誘發磁矩翻轉-固定外加電流下掃磁場:在水平x方向通入一固定大小的電流(電流範圍在±15 mA之間)，改變外加磁場進行水平x方向的掃描(範圍為±2000 Oe)並量測其異常霍爾電阻的變化。
電流誘發磁矩翻轉-固定外加磁場下掃電流:在水平x方向通入一固定大小的磁場(磁場範圍在± 400 Oe之間)，改變外加電流進行水平x方向的掃描(範圍為±15mA之間)並量測其異常霍爾電阻變化。
實驗結果如下:
1.在實驗中我們觀察到上下結構因自旋霍爾效應導致磁矩翻轉的磁矩變化方向呈現相反的變化趨勢。此結果是由於因自旋霍爾效應的產生，在順磁金屬層平面x方向上的電流會產生垂直於膜面傳播的自旋流，其自旋極化方向平行於y方向，即(m ⃑×(z ̂×j ̂ ))。而由於上下結構其鐵磁層/重金屬層的介面方向相反，因而所產生出的等效場方向呈現相反的趨勢。
2.我們將上下結構在固定電流掃磁場的方式下，其施加不同電流密度所量測到的矯頑場繪成Hc-J相圖，由此相圖我們發現因外加電流方向的改變，所產生的自旋霍爾效應其等效場方向相反，因而造成正負電流相圖的不對稱。
3.上下結構固定外加磁場下的電流掃描實驗中，當Hx<200 Oe和Hx>-200 Oe會觀察到偏移的電流翻轉曲線。透過R-H相圖的分析可觀察到上下結構在正磁場下電流誘發磁矩翻轉由+Mz翻轉至趨向水平的磁性態；在負磁場下電流誘發磁矩翻轉由 -Mz翻轉至趨向水平的磁性態，此結果推測是因為垂直異向性減弱所造成的結果。
4.在上下結構固定外加磁場下的電流掃描實驗所整理出的Jc-H相圖中，我們觀察到外加磁場範圍在Hx=200到0Oe和0到-200 Oe之間，當上結構的Ta層厚度的增加，翻轉電流密度越容易往正值發生偏移，而當下結構CoFeB層的增加，其翻轉電流密度越容易往負值偏移。由此結果可得知當上結構Ta層與下結構CoFeB層厚度增加，皆會造成電流翻轉曲線出現明顯偏移的現象。

關鍵字: 自旋霍爾效應等效場、磁場誘發磁矩翻轉、上下結構比較、電流誘發磁矩翻轉

關鍵字

自旋霍爾效應上下結構比較 ； 磁場誘發磁矩翻轉 ； 自旋霍爾效應等效場 ； 電流誘發磁矩翻轉

並列摘要

When a current passes through thin films of heavy metals such as Ta or Pt a spin current is generated along the transverse direction due to spin Hall effect. This spin current may exert a torque on the adjacent ferromagnetic layer and causes a magnetic switching of the ferromagnetic layer. This mechanism introduces an alternative way to design a new process of magnetic devices, such as magnetic random access memory (MRAM), with a current controlling mode.
In this thesis, we fabricated a series of MgO/CoFeB/Ta, which is perpendicularly magnetized and is used as a core structure in the MRAM devices. We did Hall resistance measurements on these films. Samples are divided in two groups:
Top structure: Sub/Ta(5)/MgO(1)/CoFeB(1.4)/Ta(x), x= 2 ,3 ,5 nm. This series changes the thickness of the cap layer (Ta) and the effect on spin Hall resistance is studied.
Bottom structure: Sub/Ta(5)/CoFeB(y)/MgO(1)/Ta(5), y = 0.9 , 1.1 , 1.3 nm. This series changes the thickness the ferromagnetic layer (CoFeB) and the on spin Hall resistance is studied.
All samples were prepared by nanoetching in which these structures were etched into a size 10 μm × 60μm and 15 μm × 150μm cell, respectively. Current is then applied along the longitudinal (x) direction when applying a magnetic field either along the and the z-direction (perpendicular) or x direction (in plane). The experiments are carried under two different conditions:
(1). Current sweep with fixed external magnetic field: The current range varies between between ±15 mA and the change of the applied magnetic field (± 2000 Oe).
(2). Field sweep with fixed current density: The magnetic field varies in the range of ± 400 Oe) and the change of current density (± 20 mA/cm2).
The main results include:
Top structures: First, the opposite effect with respect to the polarity pair of (J, Hx) is confirmed. Since the direction of the effective field of spin Hall effect (Hshe) is defined by (m ⃑ x( j) ⃑ x z ⃑) the interface of top structure (+z) and bottom (-z) introduces an opposite effect on Hshe. The confirmation of this polarity dependence provides extra evidence of the interface effect. Second, the magnetic switching of the thicker Ta sample shows distorted loop in that the magnetic states tend to switch between Mz (-Mz) and in-plane direction at field Hx> 200 Oe (< - 200 Oe) due to probably weaker perpendicular magnetic anisotropy. However, this distortion is recovered by reducing the Ta thickness. Third, these distorted loops also shift with respect to the J=0 and the switching to in-plane direction occurs at lower current density relative to the current switching to the perpendicular direction.
Bottom structures: (1) the magnetic states deviate from the perpendicular direction toward to the in-plane direction and (2) the critical current also reduces as the magnetic state approaches to in-plane direction and results in asymmetrical loop with respect to the J= 0.

Keyword: Current sweep with fixed external magnetic field、Field sweep with fixed current density、Spin Hall effect

並列關鍵字

Field sweep with fixed current density ； Current sweep with fixed external magnetic field ； Spin Hall effect

參考文獻

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