Influence of s-d interfacial scattering on the magnetoresistance of magnetic tunnel junctions

We propose the two-band s-d model to describe theoretically a diffuse regime of the spin-dependent electron transport in magnetic tunnel junctions (MTJ's) of the form F/O/F where F's are 3d transition metal ferromagnetic layers and O is the insulating spacer. We aim to explain the strong interface sensitivity of the tunneling properties of MTJ's and investigate the influence of electron scattering at the nonideal interfaces on the degradation of the tunneling magnetoresistance (TMR) magnitude. The generalized Kubo formalism and the Green's functions method were used to calculate the conductance of the system. The vertex corrections to the conductivity were found with the use of "ladder" approximation combined with the coherent-potential approximation (CPA) that allowed us to consider the case of strong electron scattering. It is shown that the Ward identity is satisfied in the framework of this approximation that provides the necessary condition for a conservation of a tunneling current. Based on the known results of ab initio calculations of the TMR for ballistic junctions, we assume that exchange split quasifree s-like electrons with the density of states being greater for the majority spin subband give the main contribution to the TMR effect. We show that, due to interfacial interband scattering, the TMR can be substantially reduced even down to zero value. This is related to the fact that delocalized quasifree electrons can scatter into the strongly localized d subband with the density of states at the Fermi energy being larger for minority spins compared to majority spins. It is also shown that spin-flip electron scattering on the surface magnons within the interface leads to a further decrease of the TMR at finite temperature.