Theory of Tunnel Magnetoresistance

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We present a review of theories of tunnel magnetoresistance (TMR) putting an emphasis on the role of electron scattering due to randomness. We adopt the linear response theory and generalize the conductance formula to calculate the electrical conductance in layered structures. The effect of randomness is treated in the coherent potential approximation and⁄or direct numerical simulation. Because the momentum conservation parallel to the junction planes does not hold in the presence of randomness, number of tunneling path through the barrier increases. Using a simple tight-binding model, we demonstrate first how the tunnel conductance and TMR are influenced by the randomness and by the shape of the Fermi surface of the metallic electrodes. The theory is applied to epitaxial tunnel junctions of Fe⁄MgO⁄Fe. It is shown that the basic concept of tunneling in the presence of randomness is also hold in realistic junctions. Magnetoresistance of tunnel junctions with semimetals and those with manganites is studied to clarify the effect of their electronic structures and interaction on TMR.

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