Resonant spin-dependent tunneling in spin-valve junctions in the presence of paramagnetic impurities

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 the impurity and the bulk magnetization of the neighboring magnetic electrode, and between the spin of the impurity and the spin of the tunneling electron. Consequently, the resonant levels of the system formed by a tunneling electron and a paramagnetic impurity with spin S =1 are a sextet, and 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 that 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-Rip 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) characteristic of impurity-assisted current. Two mechanisms responsible for the origin of this effect are identified. The first one is due to the excitation of spin-hip processes at low voltages and the second one arises from the shift of resonant levels inside the insulator layer under high applied voltages.