The voltage dependence of magnetoresistance in spin dependent tunneling (SD
T) junctions was studied experimentally and theoretically. Different magnet
oresistance (MR)-V dependence in various patterned junctions was observed a
nd correlated with other technologically important parameters, including th
e magnitude of the MR, linearity of the current-voltage characteristic, tem
perature dependence of the junction resistance, and the MR. A phenomenologi
cal model based on a spin-independent two-step tunneling via defect states
in the barrier, in addition to the spin-dependent direct tunneling, is prop
osed to account for the MR-V dependence. The MR ratio is determined by the
ratio of the two currents. The MR-V dependence results from a stronger volt
age dependence of the two-step tunneling current compared to that of the di
rect tunneling current. The same model also satisfactorily predicts other p
roperties of SDT junctions. A high quality barrier is required to minimize
the MR-V dependence and improve other junction properties. The approach to
achieving desirable junction impedance for data storage applications is dis
cussed. (C) 1998 American Institute of Physics. [S0021-8979(98)28311-3].