發(fā)布時間：2018-05-02 11:40

  本文選題：鐵磁材料 + 自旋電子學��； 參考：《南京大學》2017年碩士論文

【摘要】：隨著互聯(lián)網(wǎng)+技術(shù)和量子計算的高速發(fā)展,存儲設(shè)備在數(shù)據(jù)存儲和讀取方面需具備越來越高的能力。自旋電子器件作為新一代應(yīng)用,在量子計算機、自旋晶體管等領(lǐng)域展現(xiàn)出誘人的前景,研究電子自旋相干的物理現(xiàn)象已成為國際上的前沿熱點,并形成了自旋電子學這一新興學科。同時,超短脈沖激光技術(shù)的發(fā)展使人們在極短時間尺度(100 fs)下研究光與物質(zhì)的相互作用和強場物理的微觀過程,進一步推動了超快自旋動力學的研究。然而,如何控制鐵磁系統(tǒng)的超快磁化強度進動的機制尚不明確,還需要更多的實驗來探究其中的微觀機制,因此,本論文選取CoFeB這一典型鐵磁材料作為研究對象,圍繞超快磁化強度進動來實現(xiàn)阻尼因子調(diào)控的想法,具體如下:第一,在Ta/CoFeB/MgO結(jié)構(gòu)中,通過交換Ta層和MgO層的位置分別作為覆蓋層和緩沖層,利用時間分辨磁光克爾效應(yīng)測量系統(tǒng)進行自旋弛豫動力學的研究。實驗結(jié)果表明,樣品的有效阻尼因子隨著磁場的增加而趨向于一常數(shù),這說明樣品的本征阻尼因子是固有的,不隨外界環(huán)境改變而變化。同時,兩組樣品的本征阻尼因子有著明顯的不同,這主要是因為CoFeB/Ta界面增強了混合電導,使得電子-聲子散射幾率增強。通過生長次序的改變我們在實驗上實現(xiàn)了 CoFeB薄膜本征阻尼因子的調(diào)控。第二,研究電流通過帶狀CoFeB薄膜時超快磁化強度進動的變化。通過電流依賴性實驗,我們發(fā)現(xiàn)電流的方向和大小都會對CoFeB薄膜的進動產(chǎn)生影響,這主要來源于樣品表面在電流的誘導下產(chǎn)生了一個面內(nèi)的有效場,使得最后的弛豫過程發(fā)生改變。同時,當電流消去后,CoFeB薄膜的本征阻尼因子與不加電流時改變了 70%,成功實現(xiàn)阻尼因子的調(diào)控,這為自旋器件實現(xiàn)自旋反轉(zhuǎn)提供了可能。第三,研究泵浦光功率對CoFeB薄膜超快磁化強度進動過程的影響。實驗結(jié)果發(fā)現(xiàn)不同泵浦光功率對樣品在時間尺度上的弛豫過程影響不同,并且當泵浦光功率達到一定數(shù)值時,本征阻尼因子會發(fā)生變化:本征阻尼因子隨著泵浦光功率的增加而減小,減小比例大約有23%。根據(jù)溫度模型和散射機制,我們進行了分析并得出合理解釋:泵浦光產(chǎn)生的熱效應(yīng)導致的樣品系統(tǒng)溫度與居里溫度的比值在這里不占主導因素,最終改變CoFeB薄膜本征阻尼因子的是其自身自旋軌道耦合效應(yīng)的減弱。這一研究結(jié)果表明熱功耗對存儲設(shè)備的讀寫會產(chǎn)生明顯作用。
[Abstract]:With the rapid development of Internet technology and quantum computing, storage devices need to have more and more high capacity in data storage and reading. As a new generation of applications, spin electronic devices have shown attractive prospects in the fields of quantum computers, spin transistors and so on. The research on the physical phenomena of electron spin coherence has become a hot spot in the world. Spin electronics, a new discipline, was formed. At the same time, the development of ultrashort pulse laser technology makes people study the interaction between light and matter and the microscopic process of strong field physics at a very short time scale of 100 fs, which further promotes the study of ultrafast spin dynamics. However, how to control the mechanism of ultrafast magnetization precession of ferromagnetic system is not clear, and more experiments are needed to explore the microscopic mechanism. Therefore, CoFeB, a typical ferromagnetic material, is chosen as the research object in this paper. The idea of realizing damping factor regulation around ultrafast magnetization precession is as follows: first, in Ta/CoFeB/MgO structure, the positions of Ta layer and MgO layer are used as overlay and buffer layer, respectively. The spin relaxation kinetics was studied using a time resolved magneto-optic Kerr effect measurement system. The experimental results show that the effective damping factor of the sample tends to be a constant with the increase of the magnetic field, which indicates that the intrinsic damping factor of the sample is inherent and does not change with the change of the external environment. At the same time, the intrinsic damping factors of the two groups are obviously different, which is mainly due to the enhancement of the mixed conductance at the CoFeB/Ta interface and the enhancement of the electron-phonon scattering probability. By changing the growth order, we have experimentally realized the regulation of intrinsic damping factor of CoFeB films. Secondly, the change of ultrafast magnetization precession when the current passes through the banded CoFeB thin film is studied. Through current-dependent experiments, we found that the direction and size of the current have an effect on the precession of the CoFeB film, which is mainly due to an in-plane effective field on the surface of the sample induced by the current. The final relaxation process changes. At the same time, when the current is eliminated, the intrinsic damping factor of CoFeB thin film is changed and the damping factor is adjusted successfully, which makes it possible for the spin device to realize spin reversal. Thirdly, the effect of pump power on the precession of ultrafast magnetization of CoFeB thin films is investigated. The experimental results show that different pump power has different effects on the relaxation process of the sample on a time scale, and when the pump power reaches a certain value, The intrinsic damping factor will change: the intrinsic damping factor decreases with the increase of pump power by about 23%. According to the temperature model and scattering mechanism, we have analyzed and got a reasonable explanation: the ratio of system temperature to Curie temperature caused by the thermal effect of pump light is not the dominant factor here. The ultimate change of the intrinsic damping factor of CoFeB films is the weakening of its own spin orbit coupling effect. The results show that thermal power consumption plays an important role in the reading and writing of memory devices.
【學位授予單位】：南京大學
【學位級別】：碩士
【學位授予年份】：2017
【分類號】：O484

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本文編號：1833714