Bias Dependent TMR in Fe/MgO/Fe(100) Tunnel Junctions
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0941-Q01-03
Bias Dependent TMR in Fe/MgO/Fe(100) Tunnel Junctions Ivan Rungger1, Alexandre Reily Rocha1, Oleg Mryasov2, Olle Heinonen3, and Stefano Sanvito1 1 School of Physics, Trinity College, Dublin, 2, Ireland 2 Seagate Research, Pittsburgh, Pennsylvania, 15222 3 Seagate Technology, Bloomington, Minnesota, 55435
ABSTRACT We calculate from first principles the I-V characteristics of Fe/MgO/Fe(100) tunnel junctions. In particular we compare the zero-bias transmission with self-consistent calculations at finite bias. In the case the magnetizations of the two Fe layers are parallel to each other, at small bias there is a significant contribution to the transmission coming from the minority spin channel. This is due to a sharp resonance in the transmission coefficient close to the Fermi level, originating from a surface state. As a bias exceeding 25 mV is applied, the surface states get out of resonance and the current through the minority spin channel saturates, so that the current flows mainly through the majority channel. The same effect is not present for the antiparallel alignment of the magnetization with the net result of large TMR at low bias, which then saturates for a bias larger than 25 mV. INTRODUCTION Fe/MgO/Fe(100) tunnel junctions have a large tunneling magnetoresistance (TMR) of up to 180% at room temperature [1,2]. Therefore these are promising candidates for device applications as magnetic read heads and MRAM. Theoretical spin-transport calculations in the linear response limit predict extremely large TMR values [3,4,5]. The calculations have shown that for thin junctions with only 4 MgO monolayers resonant surface states contribute significantly to the current at zero bias. Calculations where the bias is introduced as a rigid shift of the density of states of the two magnetic electrodes predict a decrease of the TMR at very small bias [6]. This can be attributed to the surface states getting out of resonance. Here we investigate the current versus voltage behavior by calculating self-consistently the potential drop across the junction for an applied bias voltage. The dependence of the current through the surface states over the applied bias voltage is analyzed. THEORY The transport characteristics are studied with our newly developed code SMEAGOL [7]. SMEAGOL interfaces the non-equilibrium Green’s functions (NEGF) method with density functional theory (DFT) using the numerical implementation contained in the SIESTA code [8]. The NEGF method splits up a two-terminal device into three regions, a semi-infinite left lead, a scattering region and a semi-infinite right lead. The key aspect is that it is possible to absorb the effects of the leads over the scattering region by means of the so-called self-energies ΣL and ΣR,
respectively for the left and right lead. These are non-hermitian matrices. When added to the Hamiltonian for the scattering region HS , this describes the scattering region in the presence of the leads. The single particle Green’s function for the scattering for one dimensional sy
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