The influence of impurities within the barrier on tunneling magnetoresistance

Tunneling magnetoresistance (TMR) of a ferromagnetic tunnel junction is calculated in the presence of nonmagnetic metal impurities and disorder in the barrier layer within a single- band tight-binding model. We found that the introduction of impurities which have onsite atomic energies close to the Fermi level leads to a rapid drop in TMR as a function of the impurity concentration. This results from the increasing number of localized electronic levels close to the Fermi energy which promote the formation of highly conducting channels within the insulator. With increasing atomic energy of the impurities, tunneling via the impurity band results in higher values of TMR due to a higher effective potential barrier associated with this mechanism of tunneling. In this case introduction of impurities within the barrier can provide enhanced values of conductance without a significant reduction in TMR, which is required for MRAM applications of tunnel junctions. (C) 1999 American Institute of Physics. [S0021-8979(99)36608-1].