Magnetoresistive properties of a double magnetic molecule spin valve in different geometrical arrangements

Spin-dependent transport through a spin valve consisting of two large-spin molecules, such as single molecular magnets, weakly coupled to external ferromagnetic electrodes is theoretically studied. The main focus is on the dependence of the tunneling current, its spin polarization, differential conductance and tunnel magnetoresistance on the arrangement of molecules embedded in the tunnel junction. These quantities are calculated by using the real-time diagrammatic technique in the lowest-order perturbation theory with respect to the coupling strength. When the molecules' geometry is parallel, both an enhanced as well as inverse tunnel magnetoresistance can develop depending on the molecule's occupation. On the other hand, if the two molecules form a serial geometry, a strong current-suppression occurs due to the Pauli spin blockade effect, resulting in negative differential conductance. In this transport regime we also find large tunnel magnetoresistance, which now exhibits strong asymmetry with respect to the bias voltage reversal. In addition, we show that an enhanced magnetoresistance and negative differential conductance can develop when a T-shaped molecular geometry is taken under consideration. These effects are explained by invoking particular occupation of molecular states and nonequilibrium spin accumulation that builds up in the case of the antiparallel magnetic configuration of the device.