Abstract
We study transport through a two-level quantum dot (QD) weakly coupled toferromagnetic electrodes, the mutual magnetization orientation of which can be changedfrom a parallel to an antiparallel configuration. Calculations are performed in a sequentialtunnelling regime taking into account an inter-level Coulomb and an exchange interactionon the QD with many-body electronic states. Our interests are mainly focused onthe role of singlet and triplet states on the tunnel magnetoresistance (TMR).We have found that TMR characteristics strongly depend on different local spinconfigurations in the QD, which originate from an anti- or ferromagnetic exchange couplingJ as well as its strength. A strong inter-channel Coulomb blockade (which influences theTMR) appears when the ground state is singly occupied. Activation of the singlet or thetriplet states and competition between various tunnelling rates are responsible for the spinaccumulation even in the parallel configuration. We have also found negative TMR andnegative differential resistance (NDR) effects in the system with strong couplingasymmetry.
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