First-principles Study of Bias Effect on Magnetoresistance of Fe/MgO/Fe Tunnel Junctions
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First-principles Study of Bias Effect on Magnetoresistance of Fe/MgO/Fe Tunnel Junctions Ning Deng and Hongguang Cheng Institute of Microelectronics, Tsinghua University, Beijing 100084, P. R. China ABSTRACT We studied the transport properties of the Fe/MgO/Fe and Fe/Ag/MgO/Ag/Fe magnetic tunnel junctions (MTJs) with 13-layer MgO barrier under bias voltage based on first-principles calculations. Our results showed that two features determine the TMR value decreases with bias of Fe/MgO/Fe MTJ: (1) interfacial states lying at 1.06 eV in spin down channel (2) the energy level of the spin down Δ1 band of the Fe electrode. Our results showed that an inserted Ag monolayer at Fe/MgO interface can remarkably improve the TMR effect at a high bias voltage. INTRODUCTION Fe/MgO/Fe single-crystal magnetic tunnel junction (MTJ) has attracted much research interests in experimental and theoretical studies recently for its huge tunneling magnetoresistance (TMR)1,2 and potential device applications. Previous studies3,4 showed that the symmetry-filter property of the MgO barrier is the origin of the huge TMR effect. However, the TMR value of the Fe/MgO/Fe single-crystal MTJ decreases much rapidly with the applied bias voltage increases, which confirmed by both experimental2,5,6 and theoretical7,8 reports prevents obtaining a high output voltage Vout for device applications. Though theoretical studies of transport properties of Fe/MgO/Fe single-crystal MTJs with or without bias voltage have been reported,4,7,8 to our knowledge the studies of the bias dependence of the TMR of the Fe/MgO/Fe MTJ with a thick MgO barrier have not been reported yet. Recently, the first-principles modeling of Pt/LaAlO3/SrTiO3 capacitors under an external bias potential has been reported.9 In this paper, the state-of-the-art quantum transport technique which is based on nonequilibrium Green’s function (NEGF) formalism combined with density functional theory (DFT) was performed on the Fe/MgO/Fe and Fe/Ag/MgO/Ag/Fe single-crystal MTJs with 13-layer MgO barrier (about 2.9 nm). We studied the transport properties of the MTJs under bias voltage, our results showed that an inserted Ag mono-layer at Fe/MgO interface can remarkably improve the TMR effect and can get a high output voltage at a high bias. THEORY The electronic structure and quantum transport properties calculations of the MTJs were performed by the ATK code,10,11 which is based on density functional theory (DFT) combined with non-equilibrium Green’s function (NEGF) technique. The structural model of the Fe/Ag/MgO/Ag/Fe MTJ is schematically illustrated in Fig. 1. The 13-layer MgO barrier is sandwiched by two Fe(001) electrodes. The in-plane lattice constants of the junction were constrained to the experimental bulk value of 2.866 Å for Fe. This makes the MgO barrier become tetragonal distorted. The atomic positions were relaxed along the transport direction.
Figure 1. Schematic illustration of the Fe/Ag/MgO/Ag/Fe magnetic tunnel junction. For transport calculations, we used revised Perdew-Burke-Ernze
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