耦合鐵磁層中自旋軌道轉矩驅動之電流翻轉

Zero Field Current Switching by Spin Orbit Torque in Coupled Ferromagnetic Layers

黃建舜(Jian-Shuen Huang)

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

在原子序較大的金屬元素中，如: Ta、Pt，根據自旋霍爾效應，在平行膜面方向通以電流，將會在垂直方向產生自旋流，此自旋流可使鄰近的磁性膜產生不穩定現象或使其磁矩產生翻轉。最近文獻中垂直式磁性薄膜結構(MgO/CoFeB/Ta、Pt/Cu)因自旋霍爾效應導致的磁翻轉，對未來元件的操控有很大的幫助。
本文將垂直式人工反鐵磁雙層結構MgO(1)/CoFeB(1.2)/Ta(1.0 and 2.3)/CoFeB(1.2)/MgO(1)（單位：nm)蝕刻成Hall bar樣品，並在樣品中通入電流來驅動磁矩翻轉。實驗的部分主要是霍爾電阻量測，包括固定磁場進行電流掃描以及固定電流進行磁場掃描，並作成H-J相圖和J-H相圖進行分析。相較於單層鐵磁層的翻轉，在雙層結構中還需要考慮耦合能的存在。耦合能的大小與中間非磁性金屬層厚度有關，從磁滯曲線分析Ta=1.0 nm和Ta=2.3 nm樣品分別呈現反鐵磁耦合與鐵磁耦合。
1.相圖特性
我們的實驗分為兩類:(1)固定電流，改變磁場，將所得到的電阻迴線中呈現的磁性態，作成H-J相圖。(2)固定磁場，改變電流，將所得到的電阻迴線中呈現的磁性態，作成J-H相圖。本論文中的兩個樣品(Ta=1 nm 及 Ta= 2.3 nm) 的相圖特性簡述如下:
Ta= 1.0 nm: 從H-J相圖中可以看到電流方向對翻轉曲線有所影響，與文獻報導的單層結構相比，在雙層結構中也看到了類似多重翻轉的現象。而在J-H相圖中，正負翻轉的臨界電流密度大小不一樣，造成上下不對稱，同時翻轉偏移量隨外加磁場增大而增加。當施加較小的外加磁場時，正負翻轉的臨界電流密度為J=±40 MA/cm2，但當外加磁場增加為H=±200Oe時，正翻轉電流密度仍為+40MA/cm2，但負翻轉電流密度卻變成-20MA/cm2，且相圖左右並非對H=0對稱。
Ta= 2.3 nm:在H-J相圖中，負電流的部分出現了多次翻轉，相圖呈現左右不對稱但上下對稱。而在J-H相圖中，也出現了上下不對稱的情形，當施加較小的外加磁場時，正負翻轉電流密度約為J=±36 MA/cm2，但當外加磁場增加為Hx=-200Oe時，正翻轉電流密度變成+25MA/cm2，但負翻轉電流卻仍為-36MA/cm2。
2.磁場翻轉與電流翻轉之差異
從電流翻轉的實驗裡看到磁性態翻轉是從反平行態翻向另一次穩態，此次穩態在單純磁場翻轉中並未出現，而是引入電流作用下才會出現，實驗結果顯示磁場翻轉屬於反轉對稱，而電流翻轉屬於鏡像對稱。
3.零場翻轉
在過去單層結構文獻中，電流翻轉實驗通常需要外加一磁場產生對稱性破壞，才能讓磁矩產生翻轉，而在本實驗中觀察到不用施加外加磁場，單純只施加電流就能引發磁矩翻轉，並且由翻轉電流密度40(MA/cm2)推算Ta自旋霍爾角約0.063，與文獻相近。

關鍵字:自旋霍爾效應、自旋轉矩、電流翻轉、人工反鐵磁、鈷鐵硼

關鍵字

人工反鐵磁 ； 電流翻轉 ； 自旋轉矩 ； 自旋霍爾效應 ； 鈷鐵硼

並列摘要

When a current passes through a heavy metal, such as Ta or Pt, a spin current may be generated along the transverse direction. This spin current has been observed recently and used for magnetic switching in MgO/CoFeB/Ta or Pt/Cu layered structures, which are perpendicularly magnetized.
In this thesis we fabricated Hall bars from thin film samples of synthetic antiferromagnetic structure MgO(1)/CoFeB(1.2)/Ta(1.0 and 2.3)/CoFeB(1.2)MgO(1) (unit: nm) and applied in-plane current to switch the magnetic moment. The Hall resistance measurements include current switch (fixed external field) and field switch (fixed current density) and phase diagrams of the magnetic states are mapped out for each sample.
Besides, we found that the switching process exists is influenced by the exchange coupling which depends upon the thickness of Ta. From the Hall resistance results, Ta=1.0 structure is classified as an antiparallel coupled condition while Ta=2.3 is classified as a weak parallel coupled condition.
Other major observations included (1) The loop of current switching has a shift that J-H magnetic phase diagram is not symmetric. In Ta=1.0, the positive and negative switching current density are J=±40MA/cm2 with a low magnetic field. When the magnetic field is increased, the positive switching current density is still J=+40MA/cm2 but the negative switching current density becomes J=-20MA/cm2 with the magnetic field Hx=±200Oe. In Ta=2.3,the positive switching current density becomes J=+25MA/cm2 but the negative switching current density keeps J=-36MA/cm2 when the magnetic field increases to Hx=-200Oe. (2) Current switching is different from conventional magnetic field switching, the former is mirror symmetry and the later is inversion symmetry. (3) More interestingly, in the past monolayer structure switching by current needed a external magnetic field to break symmetry,but we found that these coupled systems show zero-field switching indicating the up-down symmetry is intrinsically broken. We got the spin Hall angle is probably 0.063 which is calculated from the switching current density J=40MA/cm2coinciding with the references, and this feature may have application potential.

keywords: spin Hall effect、spin torque、current switch、synthetic antiferromagnetic structure、CoFeB

並列關鍵字

current switch ； spin torque ； spin Hall effect ； synthetic antiferromagnetic structure ； CoFeB

參考文獻

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[2] Luqiao Liu, O. J. Lee, T. J. Gudmundsen, D. C. Ralph, and R. A. Buhrman, “Current-Induced Switching of Perpendicularly Magnetized Magnetic Layers Using Spin Torque from the Spin Hall Effect” Phys. Rev. Lett. 109, 096602(2012)

[3] O. J. Lee, L. Q. Liu, C. F. Pai, Y. Li, H. W. Tseng, P. G. Gowtham, J. P. Park, D. C. Ralph, and R. A. Buhrman, “Central role of domain wall depinning for perpendicular magnetization switching driven by spin torque from the spin Hall effect” Phys. Rev. B 89, 024418(2014)

[4] Long You, OukJae Lee, Debanjan Bhowmik, Dominic Labanowski, Jeongmin Hong, Jeffrey Bokor, Sayeef Salahuddin, “Switching of Perpendicularly Polarized Nanomagnets with Spin Orbit Torque without an External Magnetic Field by Engineering a Tilted Anisotropy”

[7]吳忠陽, “Ta(Nb)/Co20Fe60B20/MgO異質結構中電流引起之等效場探討” 國立中正大學物理系研究所碩士論文