Evolution of barrier asymmetry in magnetic tunnel junctions

Magnetic tunnel junctions usually consist of different layer stacks at the two sides of the tunneling barrier. The exposure of these junctions to high temperatures thus can induce large asymmetries in the electronic potential with respect to the barrier. Using the example of Co/Cu/Co/Al2O3/Co tunnel junctions, we show that the measured current/voltage characteristics devel op a large asymmetry upon annealing at a temperature larger than 230 degree sC. This is accompanied by a Cu enrichment at one side of the barrier and c annot be explained by simply using the work function differences between th e electrode materials. Thus, a Cu-Al2O3 intermixing zone at the barrier int erface must be taken into account, which leads to an asymmetric step-like b arrier shape. The interpretation is supported by numerical evaluation of mo del barriers which reproduce the experimental asymmetries if an intermixing zone of only 0.2 nm thickness is assumed. (C) 2001 American Institute of P hysics.