Two breakdown mechanisms in ultrathin alumina barrier magnetic tunnel junctions

Two breakdown mechanisms are observed in magnetic tunnel junctions having an ultrathin alumina barrier. The two breakdown mechanisms manifest themselves differently when considering large ensembles of nominally identical devices under different stress conditions. The results suggest that one type of breakdown occurs because of the intrinsic breakdown of a well-formed oxide barrier that can be described by the $E$ model of dielectric breakdown. The other is an extrinsic breakdown related to defects in the barrier rather than the failure of the oxide integrity. The characteristic of extrinsic breakdown suggests that a pre-existing pinhole in the barriers grows in area by means of dissipative (Joule) heating and/or an electric field across the pinhole circumference.