Spin-Dependent Transport in Magnetic Tunnel Junctions

这笔赠款支持了探索磁隧道结中自旋极化输运性质的基本物理的理论研究。这方面的研究主要集中在两个方面：建立和推广涉及磁性电极的隧穿机制和隧穿公式的新方面，以及研究外部缺陷对现实隧道结的I-V特性的影响。这项研究将促进新型自旋电子学的发展，它将自旋自由度融入到高科技中。在研究的第一部分，将通过包括隧道效应的自旋极化性质来扩展先前的理论公式。这样的扩展并不像以琐碎的方式添加两个独立的自旋通道那么简单。相反，自旋极化隧穿的公式与电荷隧穿的公式有很大的不同。要解决的基本新特征包括界面附近的局域自旋电流变化，依赖于自旋的电场穿透到磁性电极，界面附近的局域与巡游状态，以及涉及极化隧道效应的临界长度尺度。我们还将研究电荷和自旋的本征电子弛豫，它们决定了理想结的自旋极化隧穿的温度依赖性。利用从头计算中已知电子结构的模型系统来验证自旋极化隧穿理论公式的有效性。在研究的第二部分中，这些公式将应用于各种实际的磁隧道结。主要的重点将放在预测磁传输特性的新特征和解释实验的I-V特性上。实验隧道结中缺陷的存在导致了与理想结完全不同的I-V特性和磁阻。将开发合适的哈密顿量来模拟这些缺陷。从这些模型哈密顿量出发，利用量子输运方程，可以推导出磁输运性质。在大多数情况下，首先会使用适当的微扰方法来简化问题，这样就可以使用标准的数值技术来推导物理量，直到可以与实验进行详细的比较。%这笔赠款支持理论研究，以探索磁隧道结中自旋极化输运性质的基本物理。这方面的研究主要集中在两个方面：建立和推广涉及磁性电极的隧穿机制和隧穿公式的新方面，以及研究外部缺陷对现实隧道结的I-V特性的影响。这项研究将促进新型自旋电子学的发展，即“自旋电子学”，它将自旋自由度融入高科技。*

This grant supports theoretical research to explore fundamental physics of spin-polarized transport properties in magnetic tunnel junctions. There are two focused areas to the research: to establish and generalize new aspects of tunneling mechanisms and tunneling formulations involving magnetic electrodes, and to investigate the influence of external defects on the I-V characteristics of realistic tunnel junctions. This research will facilitate the development of novel spin electronics, which merge spin degrees of freedom into high technologies.In the first part of the research, earlier theoretical formulations will be extended by including the spin-polarized nature of tunneling. Such an extension is not as simple as adding two independent spin channels in a trivial manner. Rather the formulations of spin-polarized tunneling differ from those of charge tunneling in a significant way. The fundamental new features to be addressed include the local spin current variation near interfaces, spin-dependent electric field penetrations into magnetic electrodes, local versus itinerant states near the interfaces, and critical length scales involving the polarized tunneling. Intrinsic electronic relaxations of charge and spin, which determines the temperature dependence of the spin-polarized tunneling of an ideal junction, will also be investigated. A model system with known electronic stuctures from ab initio calculations will be used to test the validity of the theoretical formulations for spin-polarized tunneling.In the second part of the research, applications will be made of these formulations to various realistic magnetic tunnel junctions. The main emphasis will be on predicting novel features of magneto-transport properties and on interpreting the experimental I-V characteristics. The presence of defects in experimental tunnel junctions leads to quite different I-V characteristics and magntoresistance compared to ideal junctions. Suitable Hamiltonians will be developed to model these defects. The magnetotransport properties will be deduced from these model Hamiltonians by using quantum transport equations. In most cases, appropriate perturbation methods will be first used to simplify the problem so that standard numerical techniques can be used to derive the physical quantities up to the point where detailed comparison with experiments can be made.%%%This grant supports theoretical research to explore fundamental physics of spin-polarized transport properties in magnetic tunnel junctions. There are two focused areas to the research: to establish and generalize new aspects of tunneling mechanisms and tunneling formulations involving magnetic electrodes, and to investigate the influence of external defects on the I-V characteristics of realistic tunnel junctions. This research will facilitate the development of novel spin electronics, i.e., "spintronics," which merge spin degrees of freedom into high technologies.***