Spin-Dependent Transport in Magnetic Tunnel Junctions

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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.***