Junction resistance, tunnel magnetoresistance ratio, and spin-transfer torque in Zn-doped magnetic tunnel junctions

We report an extensive theoretical investigation of impurity doping and alloying effects to spin-polarized quantum tunneling in Fe/MgO/Fe magnetic tunneling junctions. We find that with proper impurity atoms, an optimal device characteristic regime can be established where the junction resistance R-J is nonlinearly quenched while the tunnel magnetoresistance ratio (TMR) is only linearly reduced. As a consequence, the spin transfer torque is significantly increased by a factor similar to the R-J reduction. We search for this optimal device regime using first-principles calculations by doping MgO with different impurity atoms. We predict that Zn has the desired property due to a well-balanced effect of producing gap states that mediate charge transport, thus quenching R-J, and producing relatively weak interchannel coupling to alleviate the detrimental diffusive scattering effect on the TMR.