Ferromagnetic tunnel junctions with sputtered Al₂O₃ barriers were fabricated. We found that the junction resistance increased exponentially as the Al₂O₃ barrier thickness increased, and that MR ratio of 3%-4% was achieved for a 14Å-20Å Al₂O₃ barrier. We obtained the barrier height and thickness by fitting the I-V curves to Simmons' formula. The value of the barrier height ranged from 0.2 to 0.7 eV. In addition, the barrier thickness dependence of the tunnel magnetoresistance was investigated on the basis of Slonczewski's theory.