The spin-polarized transport through a coherent strongly coupled doublequantum dot (DQD) system is analyzed theoretically in the sequential andcotunneling regimes. Using the real-time diagrammatic technique, we analyze thecurrent, differential conductance, shot noise and tunnel magnetoresistance(TMR) as a function of both the bias and gate voltages for double quantum dotscoupled in series, in parallel as well as for T-shaped systems. For DQDscoupled in series, we find a strong dependence of the TMR on the number ofelectrons occupying the double dot, and super-Poissonian shot noise in theCoulomb blockade regime. In addition, for asymmetric DQDs, we analyze transportin the Pauli spin blockade regime and explain the existence of the leakagecurrent in terms of cotunneling and spin-flip cotunneling-assisted sequentialtunneling. For DQDs coupled in parallel, we show that the transportcharacteristics in the weak coupling regime are qualitatively similar to thoseof DQDs coupled in series. On the other hand, in the case of T-shaped quantumdots we predict a large super-Poissonian shot noise and TMR enhanced above theJulliere value due to increased occupation of the decoupled quantum dot. Wealso discuss the possibility of determining the geometry of the double dot fromtransport characteristics. Furthermore, where possible, we compare our resultswith existing experimental data on nonmagnetic systems and find qualitativeagreement.
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