  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.
