垂直各向异性CoFeB薄膜超快磁动力学和阻尼因子的调控研究

发布时间：2018-05-02 11:40

本文选题：铁磁材料 + 自旋电子学　；参考：《南京大学》2017年硕士论文

【摘要】：随着互联网+技术和量子计算的高速发展,存储设备在数据存储和读取方面需具备越来越高的能力。自旋电子器件作为新一代应用,在量子计算机、自旋晶体管等领域展现出诱人的前景,研究电子自旋相干的物理现象已成为国际上的前沿热点,并形成了自旋电子学这一新兴学科。同时,超短脉冲激光技术的发展使人们在极短时间尺度(100 fs)下研究光与物质的相互作用和强场物理的微观过程,进一步推动了超快自旋动力学的研究。然而,如何控制铁磁系统的超快磁化强度进动的机制尚不明确,还需要更多的实验来探究其中的微观机制,因此,本论文选取CoFeB这一典型铁磁材料作为研究对象,围绕超快磁化强度进动来实现阻尼因子调控的想法,具体如下:第一,在Ta/CoFeB/MgO结构中,通过交换Ta层和MgO层的位置分别作为覆盖层和缓冲层,利用时间分辨磁光克尔效应测量系统进行自旋弛豫动力学的研究。实验结果表明,样品的有效阻尼因子随着磁场的增加而趋向于一常数,这说明样品的本征阻尼因子是固有的,不随外界环境改变而变化。同时,两组样品的本征阻尼因子有着明显的不同,这主要是因为CoFeB/Ta界面增强了混合电导,使得电子-声子散射几率增强。通过生长次序的改变我们在实验上实现了 CoFeB薄膜本征阻尼因子的调控。第二,研究电流通过带状CoFeB薄膜时超快磁化强度进动的变化。通过电流依赖性实验,我们发现电流的方向和大小都会对CoFeB薄膜的进动产生影响,这主要来源于样品表面在电流的诱导下产生了一个面内的有效场,使得最后的弛豫过程发生改变。同时,当电流消去后,CoFeB薄膜的本征阻尼因子与不加电流时改变了 70%,成功实现阻尼因子的调控,这为自旋器件实现自旋反转提供了可能。第三,研究泵浦光功率对CoFeB薄膜超快磁化强度进动过程的影响。实验结果发现不同泵浦光功率对样品在时间尺度上的弛豫过程影响不同,并且当泵浦光功率达到一定数值时,本征阻尼因子会发生变化:本征阻尼因子随着泵浦光功率的增加而减小,减小比例大约有23%。根据温度模型和散射机制,我们进行了分析并得出合理解释:泵浦光产生的热效应导致的样品系统温度与居里温度的比值在这里不占主导因素,最终改变CoFeB薄膜本征阻尼因子的是其自身自旋轨道耦合效应的减弱。这一研究结果表明热功耗对存储设备的读写会产生明显作用。
[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.
【学位授予单位】：南京大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O484

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