We calculate the tunneling magnetoresistance (TMR) of Fe$mid$ZnSe$mid$Fe$mid$ZnSe$mid$Fe (001) double magnetic tunnel junctions as a function of the in-between Fe layers thickness, and compare these results with those of Fe$mid$ZnSe$mid$Fe simple junctions. The electronic band structures are modeled by a parametrized tight-binding Hamiltonian fitted to {it ab initio} calculations, and the conductance is calculated within the Landauer formalism expressed in terms of Greens functions. We find that the conductances for each spin channel and the TMR strongly depend on the in-between Fe layers thickness, and that in some cases they are enhanced with respect to simple junctions, in qualitative agreement with recent experimental studies performed on similar systems. By using a 2D double junction as a simplified system, we show that the conductance enhancement can be explained in terms of the junctions energy spectrum. These results are relevant for spintronics because they demonstrate that the TMR in double junctions can be tuned and enhanced by varying the in-between metallic layers thickness.
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