  We calculate the tunneling magnetoresistance (TMR) of Fe|ZnSe|Fe|ZnSe|Fe
(001) double magnetic tunnel junctions as a function of the in-between Fe
layer's thickness, and compare these results with those of Fe|ZnSe|Fe simple
junctions. The electronic band structures are modeled by a parametrized
tight-binding Hamiltonian fitted to ab initio calculations, and the conductance
is calculated within the Landauer formalism expressed in terms of Green's
functions. We find that the conductances for each spin channel and the TMR
strongly depend on the in-between Fe layer's 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
layer's thickness.
