Abstract-Spin dependent quantum transport properties in twisted carbon nanotube and stretched carbon nanotube are calculated using density functional theory (DFT) and nonequilibrium green's function (NEGF) formulation. Twisting and stretching have no effect on spin transport in CNTs at low bias voltages. However, at high bias voltages the effects are significant. Stretching restricts any spin-up current in antiparallel configuration (APC) which results in higher magneto resistance (MR). Twisting allows spin-up current almost equivalent to the pristine CNT case resulting in lower MR. High spin filtration is observed in PC and APC for pristine, stretched and twisted structures at all applied voltages. In APC, at low voltages spin filtration in stretched CNT is higher than in pristine and twisted ones with pristine giving higher spin filtration than twisted CNT.Index Terms-Twisted carbon nanotube (twisted CNT), stretched carbon nanotube (stretched CNT), magneto resistance (MR), spin efficiency.
I. INTRODUCTIONSpintronics involves investigations associated with the electron's spin, magnetic moment, in addition to charge. Magnetic junction has usefulness in magnetic device applications, for example, magneto-resistive random access memory (MRAM), read head of hard disk drive, programmable logic and so on [1,2]. Two Probe magnetic junction is a three-layer device in which nonmagnetic material is sandwiched between two electrodes. Electrodes are basically made up of ferromagnetic materials. The ferromagnetic material is chosen because of its unique electronic properties, that is the spin-up and spin-down electrons pass from one electrode to other electrode through magnetic junction with different Fermi energies. Carbon nanotube (CNT) was discovered in 1990s [3]. CNT has high electrical conductivity and thermal conductivity. It can be metallic or semiconductor [4 -7], and it is one of the best nanomaterial that plays important role in electronics and spintronic based devices. The quantum transport properties of pristine CNT are reported in [8].It is understood that during fabrication process some defects may occur in CNTs. Defects such as twisting of CNT, stretching of CNT, bending of CNT, can inevitably occur. CNTs can also get radially expanded and axially elongated during transfer to the substrate. Due to twisting and stretching defects CNTs can undergo change in the transmission spectrum and band structure. It is therefore of interest to understand how twisting and stretching in CNTs affect its spin transport properties.In this paper, a comparative analysis of spin transport in twisted CNT, stretched CNT, and pristine CNT is done. Current-voltage characteristics, transmission spectrum, magneto résistance, and spin efficiency of twisted and stretched CNTs are calculated using DFT and NEGF. The results are compared with pristine CNT to understand the effect of twisting and stretching on magneto resistance and spin filtration in CNTs. Fig. 1 and 2 show the two-probe geometry of magnetic junction made up of twisted carbo...
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