Dynamic conductance of Ni80Fe20/AlOx/Ni80Fe20 tunnel junctions

The shape of the dynamic conductance versus voltage of NiFe/AlOx/NiFe tunne l junctions is correlated with the intensity and duration of oxidation. A s hift of the conductance minimum towards positive voltage (up to 100 mV) ind icates that the Al layer is only partially oxidized. In contrast, a shift o f the conductance minimum towards negative voltage indicates oxidation of t he bottom electrode and/or damage to the top surface of the barrier. Anneal ing of the junctions makes the tunnel barrier effectively thinner and talle r, as implied by a fit to the parabolic conductance. These qualitative tren ds are observed for Al layers ranging from 10 to 35 Angstrom in thickness. In conjunction with these effects, we observe the highest magnetoresistance (up to 26.6%) and the best bias dependence of the magnetoresistance when t he conductance is symmetric. (C) 2000 American Institute of Physics. [S0021 -8979(00)54208-X].