Spin-dependet tunnel magnetoresistance of Co/Al_2O_3/Co_x/NiFe was studied as a function of the thickness of Co_x. The thickness (X) of the doped Co was varied between 0.8nm and 2.0nm. An increase in tunneling magnetoresistance ratio from 3.5% to 9% was found for the spin-dependent tunnel junctions with 0.8 nm to 2.0 nm of doped Co_x. The enhanced tunneling magnetoresistance may be attributed to the increasing in the effective polarzation of the tunnel electron due to the spin-filtering effect from the doped material.
