Giant tunnel magnetoresistance in multilayered metaloxide structures comprising multiple quantum wells

From a theoretical point of view, we have investigated the transport properties in metal/oxide multilayer structures of the form M/O/M/O/M/O/M where M represent ferromagnetic layers alternating with three insulating barriers (O = oxide). The two inner magnetic layers form two quantum wells, the depths of which are spin dependent. For particular thicknesses of these magnetic layers, resonances occur in the quantum wells which lead to strong increase in the electron transmission through the insulating barriers. We show that if the magnetization in the successive magnetic layers can be changed from parallel to antiparallel as in spin-valves, then, for particular values of the thicknesses of the two inner magnetic layers, very large magnetoresistance effects can be expected due to the interplay of resonance effects in the two neighbouring quantum wells. The conductivity and magnetoconductivity are calculated within a quantum theory of linear response (Kubo formalism) taking into account the scattering in the magnetic layers. We show that in such a structure, giant tunnel magnetoconductivity can arise not only from a difference between spin up and spin down Fermi wave-vectors in the magnetic layers but also from spin-dependent mean free paths. In the latter case, the effect of the scattering is to induce a spin-dependent broadening of the resonances in the quantum wells.