  In double spin filter (SF) tunnel junctions, the spin information is
generated and analyzed purely from the SF effect with nonmagnetic electrodes.
In this article we numerically evaluate the bias dependence of
magnetoresistance in such tunnel junctions (nonmagnetic metal / SF /
nonmagnetic insulator / SF / nonmagnetic metal), particularly in cases when
different SF materials are utilized. A large magnetoresistance with
nonmonotonic and asymmetric bias dependence is expected within the frame of WKB
approximation. We illustrate the systematic influence of tunnel barrier height,
tunnel barrier thickness, and exchange energy splitting on magnetoresistance,
particularly focusing on the asymmetric behavior of the magnetoresistance bias
dependence.
