Bias voltage and temperature dependence of magnetotunneling effect

We have studied systematically the magnetotunneling properties of several metallic magnetic-tunnel-junction systems (Ni80Fe20-insulator-Ni80Fe20,Ni80Fe20-I-Co,Co-I-Co, Ni40Fe60-I-Co). The room-temperature magnetoresistance MR value at zero-bias ranges between 16% and 27%, depending on the spin polarization of the electrodes. There seems to be a general bias dependence of MR in all of these systems. In particular, it requires a bias in the range of 0.22-0.23 V to suppress the maximum MR value by half. We have also measured the bias dependence of MR as a function of barrier parameters (thickness and oxidation time). At low temperature, a sharp cusplike feature appears near zero bias. In some cases, low-temperature MR values substantially exceed expectations from established spin-polarization. (C) 1998 American Institute of Physics. [S0021-8979(98)47311-0].