Spin-flip scattering in magnetic junctions

Processes which flip the spin of an electron tunneling in a junction made up of magnetic electrodes are studied. It is found that (i) magnetic impurities give a contribution which increases the resistance and lowers the magnetoresistance, which saturates at low temperatures. The conductance increases at high fields. (ii) Magnon assisted tunneling reduces the magnetoresistance as $T^{3/2}$ and leads to a nonohmic contribution to the resistance which goes as $V^{3/2}.$ (iii) Surface antiferromagnetic magnons, which may appear if the interface has different magnetic properties from the bulk, gives rise to $T^2$ and $V^2$ contributions to the magnetoresistance and resistance, respectively. (iv) Coulomb blockade effects may enhance the magnetoresistance, when transport is dominated by cotunneling processes.