Effects of randomness on tunnel conductance and magnetoresistance in ferromagnetic tunnel junctions

The effects of randomness on the tunnel conductance and the tunnel magnetoresistance (TMR) are studied theoretically for ferromagnetic tunnel junctions. An emphasis is put on a combined effects of the interfacial randomness and the electronic structures of the metallic leads. The conductance is formulated by taking the randomness into account at T = 0 K and zero bias limit by using the Kubo formula and the coherent potential approximation in a single orbital tight-binding model. The vertex correction to the conductance is calculated so as to satisfy the current conservation law. The tunnel conductance and the TMR with and without randomness are found to depend on the Fermi energy E-F and the shape of the Fermi surface. The TMR is affected by the randomness in such a way that it can be increased by the randomness when E-F is close to the band bottom, while it is decreased when E-F is higher. The variation of the TMR with the barrier height and thickness is also calculated.