Asymmetric bias voltage dependence of the magnetoresistance of Co/Al2O3/Comagnetic tunnel junctions: Variation with the barrier oxidation time

Recently it has been observed that the magnetoresistance (MR) of plasma oxi dized exchange biased Co/Al2O3/Co tunnel junctions can have a strongly asym metric bias voltage (V-bias) dependence. In this article we report on the d ependence of this phenomenon on barrier oxidation time t(ox). For junctions based on 1.5 nm Al, t(ox) was varied from 20 to 120 s. For t(ox) = 20 s, f or which the MR is approximately 20% at V-bias = 0, and for t(ox) greater t han or equal to 90 s symmetric MR(V-bias) curves are found, with the MR dec reasing monotonically with \V-bias\. A strong asymmetric bias voltage depen dence was observed for intermediate oxidation times, which correspond to es sentially full oxidation of the Al layer, but almost no formation of stoich iometric CoO at the bottom electrode. Samples with t(ox) = 60 s show even a n asymmetric double peak in MR(V-bias). Due to its strength, it has an impo rtant consequence for device applications: for a series of junctions with v ariable t(ox) the maximum signal voltage (at a fixed current) is not necess arily obtained for junctions which have the largest MR ratio at V-bias = 0. (C) 2001 American Institute of Physics.