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A successful attempt in obtaining magnetic tunnel junctions (MTJs) with resistance-area (RA) of 4.7 Ωm2 and magneto-resistance (MR) ratio of about 150% at room temperature is reported, in MTJs using CoFeB/MgO/CoFeB. The MTJs consisting of Ta/CuN/Ta/Pt50Mn50/Co70Fe30/Ru/Co60Fe20B20/MgO (x nm)/Co60Fe20B20/Ta/Cu/Ta/Ru were prepared on a thermally oxidized silicon wafer using an ANELVA C-7100 sputtering system. All the metallic films were deposited by using dc magnetron method. The insulating MgO layer was deposited by rf sputtering directly from a sintered MgO target, and the thickness was varied from 6 to 24 Å. After the film deposition, the MTJs were annealed at 360°C for 2 hours in a magnetic field of 8 kOe. It was found that as MgO thickness decreases, RA decreases exponentially, but MR ratio remains constant of about 230% until around 12 Å and gradually decreases for MgO thickness below 12 Å.