Finite-Element Capacitance Calculation and Spin-dependent Transport Modeling of Double Magnetic Tunnel Junctions

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We model the combined effects of Coulomb blockade (CB) and spin-dependent tunneling in a nanoscale planar-type Ni/NiO double-junction, based on the geometry of a proposed experimental structure. Rectangular and disk-shaped island electrodes, of a size range of w =10 to 50 nm are considered. Precise determination of the circuit capacitances is performed using finite-element (FE) methods. The calculated capacitance values differ significantly from those obtained using analytical formulae. For instance, the island self-capacitance C_s values are about five times larger, which means that the operational temperature for the CB device is seriously over-estimated by the analytical formulae. The capacitance calculations also show that an extended gate covering both the island and the contact electrodes, can improve the gate sensitivity by 95% with only a minimal increase (0.3%) of the selfcapacitance C_s. This avoids the need for an ultrasmall gate lithographed exactly below the island in the double junction device. The tunneling current is then obtained using the calculated capacitance values, and incorporates both single tunneling and cotunneling. Finally, we present the temperature dependence of the I-V and I-V_R characteristics and TMR values which, being based on actual fabricated dimensions, will be significant for comparison with experimental results.
社団法人日本磁気学会の論文
2004-02-01