Resonance in tunneling through magnetic valve tunnel junctions

We present a quantum-statistical theory of electron tunneling in systems of the form MPO(P)M', where M and M' represent ferromagnetic transition metal layers, P are metallic non-magnetic layers and O is an oxide tunnel barrier one to a few nanometers thick. This system is a model of the magnetic tunnel junctions CoFe/Al/Al2O3/Co recently studied by Moodera (Moodera J. S. et al., Phys. Rev. Lett., 74 (1995) 3273; J. Appl. Phys., 79 (1996) 4724). We show that the presence of a thin non-magnetic layer between the magnetic electrodes and the barrier can lead to the formation of quantum well states adjacent to the barrier. For particular thicknesses, resonance effect can occur leading to much higher tunnel conductance and magnetoresistance than without the non-magnetic layer. The unidimensional character induced by the tunneling process increases significantly the sharpness of these resonances.