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The Quantum Transport Properties of Magnetic and Nonmagnetic Tunnel Junctions and Devices: A First-principles Study

Author: ChengHongGuang
Tutor: LiuZuLi
School: Huazhong University of Science and Technology
Course: Materials Physics and Chemistry
Keywords: spintronics spin-polarized transport magnetic tunnel junction tunnel magne-toresistance rectifying behavior quantum device tunnel diode first principles noncquilibrium Green’s function
CLC: O469
Type: PhD thesis
Year: 2011
Downloads: 77
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Abstract


We study the quantum transport properties of magnetic and nonmagnetic tunnel junc-tions and devices by first principles calculations based on the state-of-the-art density func-tional theory combined with non-equilibrium Green’s function technique.In Fe/MgO/Fe magnetic tunnel junction, our calculations show there exists a large interface state at the energy about 1.06 eV above the Fermi level in the spin-down channel. In 2D Brillouin zone, the k|| of the interface state is very close to theΓpoint, so it possesses the smallest decay parameterκ, and can contribute to a large transmission even in a thick tunnel junction (3 nm), which reflected in the transmission spectrum is it has a peak around energy of 1.06 eV in the spin-down channel. This makes an abrupt increased spin-down current with the voltage applied around 1.1 V in anti-parallel magnetization configuration and a decreased magnetoresistance as a result. We suggest this may be one of the reasons of the rapid decreased magnetoresistance with bias which observed in experiments. On the other hand, the barrier height of MgO is only about 1.2 eV in Fe/MgO/Fe junction, this cause the rapid increased current in all spin channels with bias, we suggest this is the second reason of the rapid decreased magnetoresistance. In a Fe/Ag/MgO/Ag/Fe junction which a Ag monoatomic layer is inserted to the FeMgO interface, we find the transmission of spin-down electrons is suppressed greatly and the barrier height of MgO shifts up. Our calculations show Fe/Ag/MgO/Ag/Fe junction have a large magnetoresistance and Vout at high bias.In all-oxide La2/3Sr1/3 MnO3/MgO/SrRuO3 magnetic tunnel junction, our calculations show it exhibit a sizeable rectifying behavior in anti-parallel magnetization configuration, the rectification ratio can reach 100 at the bias of 0.5 V in the junction with 13 layers of MgO barrier. We prove this rectifying behavior arises from the symmetry-filtering proper- ties of the MgO barrier. There is a spin up△1 state at the Fermi level in LSMO electrode, which has the smallest decay parameter within the MgO barrier, and a spin up△1 state, a spin down△5 state at the Fermi level in ferromagnetic SRO electrode, respectively. When in anti-parallel magnetization configuration, the△1 state in LSMO can tunnel through the MgO barrier and enter the spin up△1 band of SRO more easily, but there is no△1 state at Fermi level in SRO, so the reverse current is much smaller than the positive current and lead to a rectifying behavior. In La2/3Sr1/3MnO3/SrTiO3/SrRuO3 magnetic tunnel junction, no such rectifying behavior is exhibited. We prove this rectifying effect can be enhanced by increasing the MgO barrier thickness, and can be controlled by changing the magnetiza-tion configuration of two electrodes. This rectifying property which is totally dominated by the quantum tunneling, could be used to design faster quantum devices compare to conven-tional semiconductor devices, e.g., tunnel diode and tunnel transistor for the future device applications.Finally, we propose a tunnel diode model and present its principle. In a tri-layer structure which a insulating barrier is sandwiched by two metallic electrodes, if one electrode (emitter) has the smallest decay state band across the Fermi level, they can tunnel through the barrier more easily compared to other states, and there is a smallest decay state band not across but above the Fermi level of the other electrode (acceptor), it can receive the smallest decay states come from the emitter, this tunnel junction will exhibit a rectifying behavior. We study the Ag/MgO/SrRuO3 tunnel junction as an illustration, the calculated I-V curve of the junction shows a good rectifying behavior, and a high rectification ratio in a wide bias range. Our calculations show this tunnel diode is available.

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