Bias voltage dependence of inverted magnetoresistance on the annealing temperature in MgO-based magnetic tunnel junctions

MgO-based magnetic tunnel junctions (MTJs) with a layer sequence Ir(22)Mn(78) or Fe(50)Mn(50) (10 nm)/CoFe (2 nm)/Ru (0.85 nm)/CoFeB (0.5 <= t < 2 nm)/MgO (2.5 nm)/CoFeB (3 nm) have been fabricated. The bias voltage dependence of tunneling magnetoresistance (TMR) is given as a function of the annealing temperature for these MTJs, which shows the TMR ratio changes its sign from inverted to normal at a critical bias voltage (V(C)) when an unbalaned synthetic antiferromagnetic stack CoFe/Ru/CoFeB is used. V(C)s change with the thickness of the pinned CoFeB and annealing temperature, which implies one can achieve different VC(s) by artificial control. The asymmetric V(C) values suggest that a strong density-of-states modification occurs at bottom oxide/ferromagnet interface. (C) 2009 Elsevier B.V. All rights reserved.