  The tunnel magnetoresistance (TMR) of F/O/F magnetic junctions, (F's are
ferromagnetic layers and O is an oxide spacer) in the presence of magnetic
impurities within the barrier, is investigated. We assume that magnetic
couplings exist both between the spin of impurity and the bulk magnetization of
the neighboring magnetic electrode, and between the spin of impurity and the
spin of tunneling electron. Consequently, the resonance levels of the system
formed by a tunneling electron and a paramagnetic impurity with spin S=1, are a
sextet. As a result the resonant tunneling depends on the direction of the
tunneling electron spin. At low temperatures and zero bias voltage the TMR of
the considered system may be larger than TMR of the same structure without
paramagnetic impurities. It is calculated that an increase in temperature leads
to a decrease in the TMR amplitude due to excitation of spin-flip processes
resulting in mixing of spin up and down channels. It is also shown that
asymmetry in the location of the impurities within the barrier can lead to
asymmetry in $I(V)$ characteristics of impurity assisted current and two
mechanisms responsible for the origin of this effect are established. The first
one is due to the excitation of spin-flip processes at low voltages and the
second one arises from the shift of resonant levels inside the insulator layer
under high applied voltages.
