Magnetic tunnel junctions (MTJ), structures consisting of two ferromagnetic layers separated by a thin barrier of Al2O3, have recently been shown to exhibit large magnetoresistance (MR) at room temperature. The possible use of these structures in devices requires a thorough characterization and understanding of the transport properties of MTJs, which has motivated the present work. The junctions studied here were fabricated via de sputtering on silicon substrates at room temperature with a series of contact masks to define the junction area (80 x 80 mu m(2)), and plasma oxidation to produce the insulating barrier. A long standing problem of MTJs is the sensitivity of their MR to bias voltage. For example, in a recent work(1) it was reported that the peak MR of 11% exhibited by a CoFe/Al2O3/Co junction was decreased by half at a bias voltage of <200 mV. In this work, we will show a significant improvement of the MR voltage dependence, with peak MR values of 20%-25% near zero bias decreasing to half these values for 300-500 mV. The voltage falloff is typically asymmetric with regard to the sign of the applied voltage. For example, the peak MR of a Co/Al2O3/Ni85Fe15 MTJ, in which the Co is exchange biased with Mn54Fe46, decreases by half at a positive bias of 300 mV but requires a negative bias of 500 mV to attain the same MR decrease. There is significant structure in the resistance versus bias voltage curves with distinct differences at high and low magnetic fields for which the moments of the Co and Ni85Fe15 layers are parallel and antiparallel, respectively. A decrease in MR at high bias levels is found in all junctions independent of their resistance and the composition of the magnetic layers. However, the detailed dependence of MR on bias voltage varies with the detailed structure of the MTJ. Results will be presented for a wide range of magnetic materials and for junction resistances varying from 1 to 10 000 Omega. Possible explanations for this phenomena will be offered. (C) 1997 American Institute of Physics.
