Impurities and Inelastic Processes in Magnetic Tunnel Junctions

We have studied tunnel magnetoresistance (TMR) in junctions with 3d ferromagnetic electrodes. Previously we predicted that defects in the barrier would result in reduced effective polarization P of the impurity assisted current. This is confirmed experimentally in the present work: introductions of defects into the barrier drastically decreases the TMR. The degradation of magnetoresistance with bias has also been studied and shows universal features, attributed to effects of tunneling assisted by magnons and phonons, whose different role is described. Details of the bias dependence of the TMR depend on preparation procedures and well described by the model which includes assisted tunneling. Non-linear features, seen at low biases, are related to excitation of bulk modes by tunneling electrons. The analysis of factors resulting in fall-off of the TMR with bias is presented.